Kratochvil’s Fundamentals of Removable Partial Dentures 0867157976, 9780867157970

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Kratochvil’s Fundamentals of Removable Partial Dentures

Kratochvil’s Fundamentals of

REMOVABLE PARTIAL DENTURES Ting-Ling Chang, dds Clinical Professor Chair, Section of Prosthodontics Division of Advanced Prosthodontics School of Dentistry University of California, Los Angeles Los Angeles, California

Daniela Orellana, dds Assistant Clinical Professor Section of Prosthodontics Division of Advanced Prosthodontics School of Dentistry University of California, Los Angeles Los Angeles, California

John Beumer III, dds, ms Distinguished Professor Emeritus Division of Advanced Prosthodontics School of Dentistry University of California, Los Angeles Los Angeles, California

Berlin, Barcelona, Chicago, Istanbul, London, Milan, Moscow, New Delhi, Paris, Prague, São Paulo, Seoul, Singapore, Tokyo, Warsaw

Library of Congress Cataloging-in-Publication Data Names: Chang, Ting-Ling, author. | Orellana, Daniela, author. | Beumer, John, III, 1941- author. Title: Kratochvil’s fundamentals of removable partial dentures / Ting-Ling Chang, Daniela Orellana, and John Beumer III. Other titles: Fundamentals of removable partial dentures Description: Batavia, IL : Quintessence Publishing Co, Inc, [2019] | Includes bibliographical references and index. Identifiers: LCCN 2018032471 (print) | LCCN 2018033048 (ebook) | ISBN 9780867157970 (ebook) | ISBN 9780867157901 (hardcover) Subjects: | MESH: Denture, Partial, Removable | Denture Design Classification: LCC RK656 (ebook) | LCC RK656 (print) | NLM WU 515 | DDC 617.6/92--dc23 LC record available at https://lccn.loc.gov/2018032471

© 2019 Quintessence Publishing Co, Inc Quintessence Publishing Co, Inc 411 N Raddant Road Batavia, IL 60510 www.quintpub.com 5 4 3 2 1 All rights reserved. This book or any part thereof may not be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, or otherwise, without prior written permission of the publisher. Editor: Zachary Kocanda Design: Sue Zubek Production: Sue Robinson Printed in the USA

Dedication

To my mother, Te-Chih Wang; my father, Tien-Dow Chang; my husband, Felix Peng; and my daughter, Lillian, for their unwavering love and support. — Ting-Ling Chang To my mentor and father, Dr Eduardo Orellana, and my mother, Dr Maria Isabel Vasquez, for their love and support throughout my academic journey. — Daniela Orellana To Jan, for her continuing love and support. — John Beumer

Contents Preface viii Contributors

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1 Introduction to Removable Partial Dentures 2 Removable Partial Denture Rests

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3 The Tooth-Tissue Junction and Proximal Plate Design

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4 Major Connectors, Minor Connectors, and Denture Base Connectors 5 Retainers, Clasp Assemblies, and Indirect Retainers 6 Types of RPDs, Biomechanics, and Design Principles

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7 Partial Denture Design Principles and Design Sequence

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8 Surveying and Determining the Most Advantageous Treatment Position 67 9 Diagnosis, Treatment Planning, and Intraoral Preparation

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10 Impressions for the RPD Framework and Laboratory Instructions

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11 RPD Digital Design and Manufacturing

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12 Physiologic Adjustment of the RPD Casting and Altered Cast Impressions 121 13 Maxillomandibular Records and Occlusion for RPDs

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14 Optimizing Esthetics: Attachments and Rotational Path RPDs 15 Surveyed Crowns and Combined Fixed RPD Cases

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16 Overlay RPDs Using Retained Roots and Implants

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17 Using the RPI System for Defects of the Maxilla and Mandible 18 Treatment Removable Partial Dentures 19 Insertion and Maintenance of RPDs 20 Clinical Appointment Sequence Glossary 211 Index

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Preface Few people changed the practice of prosthodontics like Professor F. J. Kratochvil did. After a distinguished career in the US Navy, he joined the faculty of the University of California, Los Angeles (UCLA) School of Dentistry as Chair of the Section of Removable Prosthodontics in 1966. The school had been established in 1964, and Professor Kratochvil was charged with developing the predoctoral curriculum devoted to removable prosthodontics. This program was soon recognized as one of the best in the country and was copied by many schools throughout the United States, Europe, and Asia. Indeed, the school’s clinical identity was closely associated with the excellence of this training program. In the early 1970s, Professor Kratochvil also initiated the school’s postdoctoral residency program in advanced prosthodontics and served as its director for many years. Many of the residents he mentored became important contributors to the specialty of prosthodontics. However, Professor Kratochvil’s most notable contribution to his discipline was the development of the so-called “RPI system” of removable partial denture (RPD) design: a clasp assembly consisting of a rest, a proximal plate, and an I-bar retainer. He was one of the first to recognize the importance of biomechanics in RPD design and used these principles to develop a whole new design philosophy. His initial article in The Journal of Prosthetic Dentistry in 1963 (and later his textbook) forever changed the way dentists approach partial denture design. Before he developed this system, RPDs were thought to be a transitional dental treatment, with the assumption that RPD patients would inevitably become edentulous and be forced to wear complete dentures, forever compromising their chewing function. Professor Kratochvil’s research changed that thinking, and the RPI system is presently used throughout the world. Kratochvil’s Fundamentals of Removable Partial Dentures presents the basic philosophy of the RPI system as developed by Professor Kratochvil and is not intended as a reference book describing other philosophies. Throughout the book we have attempted to retain the flavor of Professor Kratochvil’s original text. Our prime objective was to convey to the reader the basic philosophy of the RPI system as Professor Kratochvil envisioned. After an introductory chapter, several short chapters follow that describe RPD components and their functions. The real distinctiveness of Professor Kratochvil’s RPI system begins in chapter 6,

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Professor F. J. Kratochvil, conferring with Dr Arun Sharma.

which describes his design philosophy in intimate detail as well as the basic principles of biomechanics upon which his design philosophy is based. This chapter is almost an exact duplicate of the same chapter in Professor Kratochvil’s original textbook, and from our perspective it is the most important chapter in the book. Readers who understand the basic principles outlined in this chapter will be able to design a biomechanically sound RPD framework for just about any dental configuration they encounter. Throughout the book we make several references to the rapidly emerging field of digital design and manufacturing of RPD frameworks. We have attempted to indicate to the reader the current limits of this new and exciting technology, and indeed chapter 11 is devoted to digital design and manufacturing of RPDs. We have added several more chapters that were not included in Professor Kratochvil’s original textbook, including chapters dedicated to esthetics and the proper use of attachments in edentulous extension RPDs, the design and fabrication of overlay RPDs and surveyed crowns, and the application of Kratochvil’s RPI design concepts for use in patients with maxillofacial defects. Finally, we have included an illustrated glossary because we recognize that prosthodontic terminology is confusing and constantly changing and as a result can bewilder the student and novice practitioner.

Acknowledgments We would like to acknowledge the special contribution that Professor Ted Berg has made to this book and to the teaching of RPDs at UCLA. Dr Berg was a very special clinician, mentor, and educator. He loved to teach and developed many creative tools to make the design and fabrication of RPDs interesting to his students. His students recognized his dedication and expertise and presented him with more than 25 teaching awards during his career. Many of his teaching slides and examples of his clinical cases are found in this book. The authors extend a special thanks to Dr Robert Duell for his support, advice, and counsel. Dr Duell was one of Professor Kratochvil’s original residents in the advanced prosthodontics training program. Upon completion of his Navy service, he established a prosthodontic practice in Laguna Woods, California, devoted primarily to removable prosthodontics. For the last 20 years, he has been a valuable part-time faculty member in the Division of Advanced Prosthodontics at UCLA, teaching courses in complete dentures to sophomore students and conducting a seminar series in removable prosthodontics to residents in the advanced prosthodontics program. He has generously provided slides of his clinical cases for use in this book and has reviewed the manuscript and made many useful suggestions. John Beumer would like to take this opportunity to thank Dr F. J. Kratochvil. Considered one of the giants of the discipline of prosthodontics, Dr Kratochvil recruited me to UCLA, and I was his first resident in the advanced prosthodontics residency

program. The opportunity to study and work with him was wonderful and laid the groundwork for everything that followed in my professional career. His commitment to excellence and his enthusiasm for his work have inspired me and countless others in our profession. Daniela Orellana would like to thank her program director and mentor, Dr Michael Razzoog, Professor of Prosthodontics, University of Michigan, for his professionalism, commitment, and heart. Dr Razzoog welcomed me into his family while mine was 5,000 miles away. On campus walks, his gentle soul, humor, and advice manifested his concern for my well-being beyond scholarly achievements. I also wish to thank Dr John Beumer for taking me under his wing. It has been an honor and privilege to work by his side. Dr Beumer is an exceptional mentor, and I am grateful beyond words. The fact that our paths have crossed will forever be a fortuitous event in my professional and academic career. Ting-Ling Chang wishes to thank her incredible mentors Dr Ted Berg and Dr John Beumer. Dr Ted Berg was a wonderful role model who has inspired me in my academic career. Another mentor who greatly shaped my professional life is Dr John Beumer. His love and generosity in knowledge dissemination and sharing is most inspiring. I feel fortunate and blessed to work with him. It was John’s vision, energy, and drive that made this book possible. Finally, the authors would like to thank Brian Lozano, senior artist, UCLA School of Dentistry, for his wonderful illustrations.

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Contributors Frederick C. Finzen, DDS Professor Emeritus Division of Prosthodontics School of Dentistry University of California, San Francisco San Francisco, California

Jay Jayanetti, DDS Assistant Clinical Professor Director, Maxillofacial Prosthetics Division of Advanced Prosthodontics School of Dentistry University of California, Los Angeles Los Angeles, California

Ryan Wallace, DDS Lecturer Section of Prosthodontics Division of Advanced Prosthodontics School of Dentistry University of California, Los Angeles Los Angeles, California

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Chapter 1 Introduction to Removable Partial Dentures John Beumer III | Ting-Ling Chang | Daniela Orellana

Professor F. J. Kratochvil was one of the first to recognize the importance of biomechanics in the design of removable partial dentures (RPDs) and used these principles to develop a whole new design philosophy. It is the purpose of this book to present this philosophy. His initial publication1 forever changed the way in which dentists approached RPD design. Although he is most often associated with the use of the I-bar retainer, the reader should understand that he stressed the totality of RPD design and recognized the important role of other major components in the successful use of the I-bar retainer. Obviously, the I-bar retainer was an important component of his design philosophy, but the design of the guide planes and proximal plates were also fundamental. Because the I-bar has a relatively low retentive value compared to other retainer designs, its effectiveness is dependent upon the horizontal stability provided by the minor connectors and the proximal plates, and these portions of the RPD are integral to his design philosophy. He believed that there was no such thing as a simple I-bar RPD, just as there is no one technique that serves as a panacea for all clinical situations. RPDs will continue to be one of the primary methods used to restore the missing dentition of partially edentulous patients in the foreseeable future, and consequently, it will continue to be important for dentists to be intimately familiar with the basic principles of RPD design and fabrication. The recent innovation in digital technologies will change the manner in which we design and fabricate RPDs, but the laws of biomechanics, and therefore the principles of RPD design that Kratochvil established, will not change. Treatment of partially edentulous patients with RPDs has become increasingly sophisticated in recent decades, and when

this treatment is planned and executed properly it will help to preserve the existing structures. In contrast, a poorly designed and fabricated RPD can trigger resorption of bony bearing surfaces and accelerate the loss of remaining dentition. Unfortunately, in recent years, curriculum time devoted to RPDs has been significantly reduced in many dental schools, and those directing the curriculum often lack appropriate training, experience, and educational resources. The result of this change has been startling. In recent surveys of dental laboratories in the United States, more than 90% of casts submitted lacked visible rests and RPD designs. Many students graduate from dental school without fabricating an RPD for a patient. In many studies, significant numbers of RPDs do not meet even half of the usual and customary design standards.2 The widespread perception that the health of the remaining teeth is compromised by RPDs as compared to other forms of treatment is not supported by the evidence. Studies comparing the outcomes of fixed dental prostheses (FDPs) and RPDs have indicated no differences in periodontal health of abutment teeth between the groups. The only differences noted in these studies were the higher levels of maintenance required by RPDs.3,4 The number of partially edentulous patients continues to increase as the population in most developed countries continues to age. Often times the only viable treatment option available to most patients is to restore the integrity of the dental arch and replace the missing dentition with an RPD. There are several reasons for this. In many patients, FDPs are not indicated, such as when the edentulous span is too great or in edentulous extension areas. Also, cost precludes the use of dental implants in most patients. 1

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Introduction to Removable Partial Dentures

Fig 1-1 (a) Bilateral extensionbase RPD. (Courtesy of Dr R. Faulkner, Cincinnati, Ohio.) (b) Bilateral extension areas restored with a single implant connected to a natural tooth abutment. The mastication efficiency of the RPD is equivalent to that obtained with the implantsupported FDP.

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b Fig 1-2 (a) Pneumatized maxillary sinus. (b) Resorption of bone over the inferior alveolar nerve. Both preclude implant placement in the absence of site enhancement.

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RPDs Versus Implants It is quite clear that the expanding need for tooth replacement cannot be met with osseointegrated implants. In the United States, the number of partially edentulous patients restored with dental implants is expected to plateau at 3% to 5% of those potentially in need of this service. Cost is an important factor, but there are several other reasons for this phenomenon. An interesting paper published several years ago by Bassi et al5 illustrates the impact of additional factors. Forty consecutive partially edentulous patients seeking implant therapy were screened at the dental clinic at the University of Turin. Only 1 out of the 40 patients was ultimately restored with osseointegrated implants. There were a variety of reasons why implant therapy was not delivered to the other 39 patients. Many patients were not suitable candidates because they lacked sufficient bone volume at the desired sites. Another group, upon questioning, were happy with their RPDs, while another, when described the nature of the surgery to place the implants and/or enhance the potential implant sites, declined to undergo the surgery. Another factor to consider is that the functional outcomes achieved with RPDs are comparable to those achieved with implant-supported FDPs. In the late 1980s and early 1990s, Kapur et al3,6–9 conducted a randomized clinical trial comparing the mastication efficiency of implant-supported FDPs 2

with extension base (tooth-mucosal borne) RPDs (Fig 1-1). Both treatments were equally effective in improving chewing function. A large number of patients in both groups expressed satisfaction with their prostheses, but as expected, the level of patient satisfaction was higher in the fixed implant-supported group. Similar outcomes were recently reported by Nogawa et al.10 Kapur et al3,6–9 concluded that despite the superiority of the implant-supported FDPs in terms of patient satisfaction, lack of functional differences and success rates do not support the selection of implant-supported FDPs over RPDs, without consideration of other factors. Moreover, implants cannot be used in many patients in need of tooth replacement in the posterior quadrants because of pneumatization of the maxillary sinuses or insufficient bone over the inferior alveolar nerve in the mandible (Fig 1-2). Sinus augmentation has become common in recent years, and the success rates of implants placed into these sites is quite good. However, the high cost of this procedure plus the cost of implant placement precludes most patients from selecting this option. In the mandible, most patients missing dentition in the posterior quadrant lack sufficient bone volume over the inferior alveolar nerve for implant placement, and the development of predictable procedures aimed at supplementing the vertical height of these bony sites has proved illusive.

Objectives of Treatment: The Partially Edentulous Patient

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Fig 1-3  (a) Implant-supported FDP at delivery. (b) Bone levels 2 years after delivery. (c) Bone levels 5 years after delivery.

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Fig 1-4  (a and b) Typical partially edentulous patient with multiple missing posterior teeth, lost occlusal vertical dimension, and loss of arch integrity. (Courtesy of Dr A. Davodi, Beverly Hills, California.)

Short, wide-diameter implants in these sites have not obtained an acceptable level of success.11,12 The reasons for this are now becoming clear and include not only the length of the implants but also the lack of width of the alveolar bone to enclose the implant (Fig 1-3). Lateralizing the inferior alveolar nerve enables the placement of implants of suitable length. However, the morbidity associated with injury to the nerve can be quite significant.13 A typical partially edentulous patient is shown in Fig 1-4. The patient is missing posterior dentition in both the maxilla and mandible. Numerous spaces and diastemata have developed, destroying the integrity of both arches. Multiple teeth exhibit erosion and wear. Occlusal vertical dimension has been lost, reducing the height of the face and compromising facial esthetics. With proper treatment this dentition can be saved, the integrity of the arch restored, missing teeth replaced, and occlusal function restored to reasonable levels. Restoring the occlusal vertical dimension will dramatically improve facial esthetics. The purpose of this text is to delineate a treatment approach and RPD design principles that will consistently lead to favorable long-term treatment outcomes in such patients.

Fig 1-5  If the progression of tooth loss and malposition persist, the dentition will become irretrievably lost. (Courtesy Dr A. Pozzi, Rome, Italy.)

Objectives of Treatment: The Partially Edentulous Patient When teeth are lost, the remaining dentition loses the interproximal contacts that permitted the intact arch to function as a continuous unit. Loss of integrity is one of the first steps toward disorganization of the dental arch, leading to progressive compromise and eventual loss of the remaining dentition (Fig 1-5). Individual teeth may supererupt or become mobile or displaced, altering the plane of occlusion and occlusal relationships. The relationship between centric relation and centric occlusion becomes unfavorable, disrupting the functional harmony of the temporomandibular joint and the muscles of mastication. Individual teeth may be displaced and tipped, resulting in the delivery of nonaxial forces and unfavorable leverages on the periodontal ligament and bone during function. The usual course of these events eventually reaches a turning point in the life of the dentition, and if this progression is not stopped, edentulism is the inevitable result.

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Introduction to Removable Partial Dentures

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Fig 1-6 (a) A removable orthodontic appliance to restore premolar contact prior to prosthodontic treatment. (b) Treatment by orthodontic movement to restore tooth position with bodily tooth movement.

Therefore, two treatment objectives for a partially edentulous patient are to do the following: 1. Stabilize the individual arch and protect remaining hard and soft tissues 2. Organize interarch functions (proper occlusal vertical dimension, occlusal plane, and centric occlusal contact) and esthetics A well-designed RPD should provide cross-arch support, unite the remaining teeth, restore function, and control the direction of force onto remaining teeth and edentulous bearing surfaces without violating the biomechanical equilibrium.

Methods of Restoring and Stabilizing the Partially Edentulous Arch Repositioning teeth In some situations it may be advantageous to consolidate individual arch segments by repositioning the teeth with orthodontic devices (Fig 1-6). The missing segments can then be restored with conventional FDPs, implant-supported FDPs, RPDs, or a combination of these.

Individual restorations When individual teeth are lost, teeth adjacent to the resultant edentulous space migrate out of position and lose interproximal contacts, disrupting relationships with the opposing occlusion. If the spaces are not excessive, mesiodistal contacts can be restored 4

with individual restorations. Re-establishing proximal contacts restores the integrity of the arch, allowing it to function as a unit as before (see Fig 1-7).

Fixed dental prostheses Sometimes an FDP is used to restore the integrity of the remaining dental arch or individual arch segments so it may function as a continuous unit, and an RPD is used to replace the teeth in the posterior and/or anterior extension areas (Figs 1-7 and 1-8). The degree of arch stability thus created is dependent upon the number of teeth involved in the restoration and the quality of the periodontal support provided by each of the abutments versus the value of cross-arch stabilization that could have been achieved if an RPD was employed. FDPs and individual restorations can also be used to unite individual arch segments and to idealize the occlusal plane; this practice is especially advantageous when the RPD opposes a complete denture. Good examples of these approaches are shown in Figs 1-7 and 1-8. The patient in Fig 1-7 presented with multiple missing teeth in the mandible opposed by an edentulous maxilla. The mandibular left molars and the incisors have also been lost. The residual dentition on the right side is disorganized with individual teeth tipped, disrupting the plane of occlusion. The FDP was used to restore the integrity of this arch segment and to idealize the occlusal plane before the mandibular RPD and maxillary complete denture were fabricated. Such an approach to treatment leads to more sustainable long-term clinical outcomes. The patient in Fig 1-8 presented with multiple spaces and diastemata secondary to tooth loss and migration of the remaining teeth (see Fig 1-4). The integrity of the maxillary arch has been restored with individual crowns and an FDP. Thus restored, the arch can function more like a continuous unit,

Methods of Restoring and Stabilizing the Partially Edentulous Arch

Fig 1-7  (a) Migrating teeth resulting in disorganization of the occlusion. (b) Contacts, occlusion, and stability restored with overcontoured restorations. (c) Following the loss of several teeth, those remaining have migrated and tipped. Note that the molar is tipped to the mesial and that the interproximal contact has been lost between the canine and the premolar. The patient has an edentulous extension area in the left posterior region, and the incisors have also been lost. (d) Before the RPD is fabricated, the integrity of this arch segment is restored with an FDP. Such practice leads to sustainable results with an RPD. (Parts c and d courtesy of Dr J. Kelly, Omaha, Nebraska.)

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Fig 1-8  (a) The dental arch of the patient shown in Fig 1-4. Integrity of the arch has been restored with crowns and an FDP. (b) An RPD was fabricated to restore the missing posterior teeth. (Courtesy of Dr A. Davodi, Beverly Hills, California.)

distributing the forces delivered during occlusal function more widely among the individual units as opposed to an isolated tooth or arch segment.

Osseointegrated implants Individual teeth and missing arch segments can be restored with dental implants given sufficient bone volume at the implant sites and an adequate number of implants14 (Fig 1-9). They can also be used in combination with an RPD to facilitate retention and improve the esthetic outcome. For example, in a patient with a large extension defect, the implants can be used as overdenture abutments to facilitate support (see chapter 16).

Fig 1-9  Dental implants have been used to replace the mandibular right second premolar and first molar but also serve to restore arch integrity, stabilizing the position of remaining teeth and allowing the arch to function as a unit. (Reprinted from Beumer et al14 with permission.)

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Introduction to Removable Partial Dentures

Fig 1-10 (a and b) In many instances, it is prudent to remove tori prior to RPD treatment.

a

b Fig 1-11 The maxillary molars have supraerupted, disrupting the plane of occlusion. This discrepancy must be addressed before the definitive RPD is fabricated. (Courtesy of Dr T. Berg, Los Angeles, California.)

Removable partial dentures In posterior edentulous extension areas and partially edentulous arches with long edentulous spans, RPDs continue to be the most cost-effective treatment. However, as noted above, often it is necessary to supplement this treatment with FDPs or individual full-coverage restorations to ensure sustainable clinical outcomes. An RPD can be designed to provide cross-arch support, to stabilize the position of the remaining dentition, and to restore the integrity of the arch as a continuous functioning unit. A properly designed and executed RPD restores a harmonious occlusion and controls and idealizes the direction of forces that are directed against remaining teeth and denture-bearing tissues during function.

Supporting Structures and Other Considerations Successful long-term treatment outcomes take into consideration the needs of the supporting structures of the residual dentition and the mucosa and bone of the edentulous bearing surfaces. A thorough evaluation of the health of the supporting structures should be undertaken and any pathologic conditions addressed prior to commencing treatment. This may include extraction of diseased teeth, endodontic therapy, periodontal therapy, and splinting periodontally compromised teeth together that are 6

adjacent to an edentulous extension area. If an RPD is planned, preprosthetic surgical procedures may need to be employed prior to treatment such as removal of mandibular or maxillary tori, tuberosity reduction, and maxillary osteotomies to reposition dentoalveolar segments (Fig 1-10) (see chapter 9).

Establishing a Proper Plane of Occlusion Restoring a proper plane of occlusion is likewise fundamental to long-term successful treatment outcomes with RPDs, especially when opposed by a complete denture. In some instances it may be necessary to remove teeth and their anchoring bone or perform endodontic procedures on selected teeth and restore them in order to develop a proper plane of occlusion (Fig 1-11).

Professional Responsibility It is the professional responsibility of the dentist to understand and develop all procedures associated with RPD treatment. Thorough treatment planning and design is the foundation upon which any successful therapy is based. It is the responsibility of the clinician to make these decisions, and they cannot ethically be delegated to other allied heath care personnel.

Components of an RPD and Their Functions

Proximal plate Minor connector

Denture base connector

Rest

Proximal plate

Proximal plate Retainer

Denture base connector

Rest

Proximal plate

Minor connector Retainer

Major connector

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Major connectors

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Fig 1-12  (a) Components of a mandibular RPD framework. (b) Components of a maxillary RPD framework.

Components of an RPD and Their Functions To provide a systematic approach to partial denture therapy, it is important to identify the parts of an RPD and their functions (Fig 1-12). Each part is presented individually and in the sequence in which it is designed. The parts of the RPD that provide support are considered first.

Rests A rest is a rigid extension of a partial denture that contacts a remaining tooth in a prepared rest seat to transmit vertical or horizontal forces.

Function Positive rests control the relationship of the prosthesis to the supporting structures and are contoured and positioned to direct occlusal forces along the long axis of the abutment teeth. As the occlusal force increases, the prosthesis should remain firmly seated in the rest seats prepared in the abutment teeth. The rest should be positioned insofar as it is possible in the center of the abutment tooth. They should never be placed on an inclined plane in such a way as to deliver lateral forces to the abutments. Where necessary, rests can also be used to restore the occlusal plane and provide reciprocation for retainers (see chapter 2).

Major connectors A major connector joins the components of the RPD on one side of the arch to those on the opposite side.

Function The major connectors are rigid and provide cross-arch stability (resistance to lateral forces) for the RPD and in some instances enhance support (resistance to occlusal forces). In the mandible, the prime example is the lingual bar. This rigid bar connects the components from one side of the arch to the other side, and its rigidity enhances stability. The prime example in the maxilla is the anteroposterior palatal strap (see chapter 4).

Minor connector A minor connector is the connecting link between the major connector of the RPD and the other units of the prosthesis, such as the clasp assembly, indirect retainers, occlusal rests, or cingulum rests.

Function The minor connectors are strong, rigid components of an RPD that provide stability (resistance to lateral forces) (see chapter 4). They can also be used to facilitate frictional retention when proximal surfaces, through which the minor connectors traverse, are recontoured to be parallel to the guiding surfaces. 7

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Introduction to Removable Partial Dentures

Fig 1-13 Proximal plates are plates of metal in contact with proximal surfaces of the abutment teeth. They should extend 2 mm onto the mucosa of the alveolar ridge (arrows).

Proximal plates A proximal plate is an extension of the minor connector in contact with the proximal surface of the abutment tooth (Fig 1-13).

Function

The retainers can provide both retention and stability (bracing action). A properly designed retainer also helps to control the position of the prosthesis in relation to the remaining teeth and supporting structures (see chapter 5).

Function The proximal plates maintain arch integrity by an anteroposterior bracing action. If the guide planes on the abutment teeth, which the proximal plates engage, are relatively parallel to one another, they also enhance retention by frictional contact. They can also be designed to provide reciprocation for a retainer (clasp). According to the Kratochvil philosophy, they are extended to cover the gingival margin and extend approximately 2 mm beyond the tooth-mucosa junction onto the edentulous area (see chapter 3).

Denture base connectors A denture base connector is the part of the RPD to which the resin denture base is connected.

Function The denture base connectors provide a strong rigid support structure for attachment of the acrylic resin portion of the prosthesis containing the teeth.

Retainers A retainer is the component of an RPD used to prevent dislodgment, usually consisting of a clasp assembly or precision attachment. 8

Denture base A denture base is the part of the denture that rests on the edentulous bearing surfaces and to which the denture teeth are attached.

Function The denture base engages the edentulous bearing surfaces. A properly extended denture base (eg, extending the denture base to cover the retromolar pad and buccal shelf in a mandibular extension-base RPD) will significantly enhance the support (resistance to the vertical forces of occlusion) for the RPD and limit the resorption of the underlying bone.

Impact of Digital Technologies on Design and Manufacture of RPD Frameworks Computer-aided design/computer-assisted manufacture (CAD/ CAM) systems are beginning to have a significant impact on the design and fabrication of RPD frameworks (Fig 1-14). Presently, the master cast is scanned and the RPD framework is designed and printed in a light-curing resin. The printed resin pattern is then invested and cast in the usual manner.

References

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Fig 1-14  (a) Digitized master cast. (b and c) Virtually designed RPD framework. (d and e) Cast framework seated on the stone master cast. (f) Completed prosthesis seated intraorally. (Courtesy of Dr J. Jayanetti, Los Angeles, California.)

However, it is not yet possible to fabricate RPD frameworks with CAM techniques with the accuracy and consistency necessary for clinical use. In the past, most of the techniques were “subtractive” (eg, three-dimensional milling), and this approach was made difficult by the lack of bulk and ease of deformation of portions of most RPD frameworks. However, recent advances in additive manufacturing techniques, specifically selective laser melting (SLM), have made it possible to fabricate RPD frameworks of reasonable accuracy.15 Conventional impressions have remained the most costeffective and accurate means of obtaining a full-arch master cast, although this method may also be displaced by intraoral scanners in the not-too-distant future. Presently, the master cast can be scanned and surveyed with available software (Dental System, 3Shape); a specific path of insertion can be identified; and undercuts can be identified, quantified, and blocked out virtually as needed. The RPD framework can then be designed consistent with the principles of RPD design (see chapter 11). The RPD design data can be transferred as an STL (standard triangulation language) file and imported into an SLM rapid prototyping system for fabrication in chrome cobalt. The frameworks are finished and polished in the usual fashion. Fit and finish have been shown to be nearly comparable to those ob-

tained with conventional fabrication methods.15 These methods are becoming increasingly cost-effective and nearly as accurate as conventional methods of design and fabrication, and the time is rapidly approaching when they will be.

References 1. Kratochvil FJ. Influence of occlusal rest position and clasp design on movement of abutment teeth. J Prosthet Dent 1963;13:114–124. 2. Hummel SK, Wilson MA, Marker VA. Nunn ME. Quality of removable partial dentures worn by the adult U.S. population. J Prosthet Dent 2002;88:37–43. 3. Kapur KK. Veterans Administration Cooperative Dental Implant Study—Comparisons between fixed partial dentures supported by blade-vent implants and removable partial dentures. Part II: Comparisons of success rates and periodontal health between two treatment modalities. J Prosthet Dent 1989;62:685–703. 4. Isidor F, Budtz-Jørgensen E. Periodontal conditions following treatment with distally extending cantilever bridges or  removable partial dentures in elderly patients. A 5-year study. J Periodontol 1990;61:21–26. 5. Bassi F, Schierano G, Lorenzetti M, et al. Oral conditions and aptitude to receive implants in patients with removable partial denture: A cross-sectional study. Part II: Aptitude. J Oral Rehabil 1996;23:175–178.

9

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Introduction to Removable Partial Dentures

6. Participants of CSP No. 86, Kapur KK. Veterans Administration Cooperative Dental Implant Study—Comparisons between fixed partial dentures supported by blade-vent implants and removable partial dentures. Part I: Methodology and comparisons between treatment groups at baseline. J Prosthet Dent 1987;58:499–511. 7. Kapur KK. Veterans Administration Cooperative Dental Implant Study—Comparisons between fixed partial dentures supported by blade-vent implants and removable partial dentures. Part III: Comparisons of masticatory scores between two treatment modalities. J Prosthet Dent 1991;65:272–283. 8. Kapur KK. Veterans Administration Cooperative Dental Implant Study—Comparisons between fixed partial dentures supported by blade-vent implants and removable partial dentures. Part IV: Comparisons of patient satisfaction between two treatment modalities. J Prosthet Dent 1991;66:517–530. 9. Garrett NR, Kapur KK, Hasse AL, Dent RJ. Veterans Administration Cooperative Dental Implant Study—Comparisons between fixed partial dentures supported by blade-vent implants and removable partial dentures. Part V: Comparison of pretreatment and post treatment dietary intakes. J Prosthet Dent 1997;77:153–161. 10. Nogawa T, Takayama Y, Ishida K, Yokoyama A. Comparison of treatment outcomes in partially edentulous patients with implant-supported fixed prostheses and removable partial dentures. Int J Oral Maxillofac Implants 2016;31:1376–1383. 11. Eckert SE, Meraw SJ, Weaver AL, Lohse CM. Early experience with wide-platform Mk II implants. 1. Implant survival. 2. Evaluation of risk factors involving implant survival. Int J Oral Maxillofac Implants 2001; 16:208–216.

10

12.

13.

14.

15.

Attard NJ, Zarb GA. Implant prosthodontic management of partially edentulous patients missing posterior teeth: The Toronto experience. J Prosthet Dent 2003;89:352–259. Krogh PH, Worthington P, Davis WH, Keller EE. Does the risk of complication make transpositioning the inferior alveolar nerve in conjunction with implant placement a “last resort” surgical procedure? Int J Oral Maxillofac Implants 1994;9:249–254. Beumer J III, Faulkner RF, Shah KC, Moy PK (eds). Fundamentals of Implant Dentistry: Volume 1—Prosthodontic Principles. Chicago: Quintessence, 2015. Ye H, Ning J, Li M, et al. Preliminary clinical application of removable partial denture frameworks fabricated using computer-aided design and rapid prototyping techniques. Int J Prosthodont 2017;30:348–353.

Suggested Reading McCracken WL. Differential diagnosis: Fixed or removable partial dentures. J Am Dent Assoc 1961;63:767–775. Silverman SI. Differential diagnosis: Fixed or removable prosthesis. Dent Clin North Am 1987;31:347–362.

Chapter 2 Removable Partial Denture Rests Ting-Ling Chang | Daniela Orellana

The primary function of the rest is to control the relationship between the teeth and the partial denture and to provide support for the prosthesis. Well-designed positive rests will help maintain the residual dentition and the mucosa and bone of the edentulous areas. Poorly designed and positioned rests may actually accelerate the loss of the remaining teeth and also accelerate the loss of alveolar bone of the edentulous areas, particularly in patients with edentulous extension defects (Fig 2-1). Excessive lateral forces are not well tolerated by the teeth and may compromise periodontal support, leading to their premature loss. Properly designed rests engage the dentition in a positive fashion and direct occlusal forces along the long axis of the teeth. Rests that are positioned properly also verticalize the occlusal forces in the edentulous extension area and help to distribute these forces more widely (see Fig 6-10). Photoelastic studies confirm clinical observations that the position of rests and their movement have a primary effect on stress distribution.1–3 Rests are prepared and positioned to control the direction of force delivered to the abutment teeth and the edentulous areas. To provide the most ideal support position, the rest is placed as close to the center of the tooth as possible (Fig 2-2). Teeth and edentulous areas are best maintained when occlusal forces are directed perpendicular to the edentulous bearing tissues and along the long axis of the abutment teeth. The design and placement of rests are the most important factors that control occlusal forces. The rest must not allow the partial denture to slide off the tooth or allow the tooth to move out of position during occlusal function. The greater the occlusal force, the more firmly the prosthesis should seat on the rest seats incorporated within the abutment teeth.

Fig 2-1 This rest has been positioned adjacent to the extension area. As a result, when occlusal forces are delivered to the extension area, the tooth is tipped to the distal.

Fig 2-2 The ideal location to support occlusal force on a tooth is directly in the center of the long axis of the tooth.

11

2

Removable Partial Denture Rests

a

b

Fig 2-3 Rests should not be placed on inclined Fig 2-4 (a and b) Crescent-shaped cingulum rests can sometimes be prepared on the maxillary canine surfaces of anterior teeth in such a way that the and central incisor if the cingula are prominent. (Part a courtesy of Dr R. Duell, Los Angeles, California. Part abutment tooth is exposed to lateral forces. This may b courtesy of Dr T. Berg, Los Angeles, California.) result in tooth displacement, damage to the periodontal ligament, and disruption of the occlusion.

The Concept of the Positive Rest

Anterior Rests

The most important requirements for a rest are that it must be positive and not allow the prosthesis to slide off the tooth or allow the tooth to move out of the existing relationship to other teeth as occlusal pressure is exerted and increased. To this end, all rests must provide a secure connection between the prosthesis and the tooth that does not allow the tooth and prosthesis to separate as masticatory forces are increased. Positive rests preserve the remaining oral structures by doing the following:

Given the differences in the anatomy of the anterior and posterior teeth, rests on these teeth are designed quite differently. It is difficult to obtain a positive seat for rests on anterior teeth because the lingual tooth surface is sloped and has no central fossa or marginal ridges. The most damaging situation is placement of the rest on an inclined surface (Fig 2-3), which makes the rest unable to positively engage the tooth. Engagement of an inclined surface applies a lateral torquing force on the tooth, which (1) causes tooth displacement and bone loss around the tooth, (2) allows the prosthesis to move out of position and displace soft tissues inappropriately, and (3) disrupts and disorganizes the occlusion. With the exception of the maxillary canine, it is difficult to prepare a positive anterior lingual rest in natural tooth structure because of the basic morphology of the anterior teeth. The enamel is quite thin near the gingiva, and an adequate rest of the required depth cannot be prepared without significant exposure of the dentin. Although it may be possible to prepare a properly contoured rest in maxillary canines and central incisors where there is a prominent cingulum of adequate enamel (Fig 2-4), if a positive rest is to be incorporated within anterior teeth, it may be necessary to fabricate a partial- or full-coverage crown.

• Controlling the position of the prosthesis in relation to the remaining teeth • Controlling the position of the prosthesis in relation to the mucosa of the edentulous areas • Controlling the amount and direction of movement of the abutment teeth The rest is the controlling factor in the triad of prosthesistooth-periodontium. If the rest is not properly designed and positioned and does not precisely engage the abutment teeth, these teeth will be displaced and subjected to undesirable torquing and tipping forces. The result may be premature loss of the abutment teeth. 12

Anterior Rests

a Fig 2-5  The rest is positioned as close to the center of the tooth as possible when viewed from all directions. The center of the rest is the deepest portion.

Fig 2-7  (a and b) When possible, the rest is prepared in line with the extension area. This will ensure an unencumbered rotation around the axis of rotation when occlusal forces are applied to the denture teeth, restoring the extension areas.

b

Fig 2-6  (a) Cingulum rests should be rounded in all aspects. (b) When an occlusal force is applied, the rest should engage more securely.

a

Crescent-shaped cingulum rests The crescent-shaped cingulum rest is designed to engage the tooth in a positive fashion and to direct occlusal forces along the long axis of the tooth. The characteristics of ideal cingulum rest are the following: • The center of the rest is deeper than the periphery (Fig 2-5). • The rest is rounded in all aspects; there are no sharp angles. • There is easy access for impression making and oral hygiene. • The rest is contoured to form a half circle, especially for extension-base removable partial dentures (RPDs), when the rest is located on the axis of rotation (fulcrum line). • There should be no undercuts. • The rest is placed as close to the gingiva and periodontal support area as possible to reduce the leverage on the abutment. There should be no interference with planned opposing occlusion. • The rest is contoured so that when the force to the prosthesis is increased, the rest will engage more securely to prevent separation between the rest and the abutment tooth (Fig 2-6).

b

• If possible, the rest is positioned in line with the residual ridge for an extension-base prosthesis (Fig 2-7). This will permit a more ideal rotation of the RPD in the rest when an occlusal force is applied in the edentulous extension area. The crescent-shaped cingulum rest is commonly used on maxillary canines and central incisors and often can be carved into a nonrestored maxillary central incisor or canine (see Fig 2-4). It is difficult to use on mandibular anterior teeth without preparing a partial- or full-coverage crown (see Figs 2-13 and 2-14).

Incisal rests Incisal rests are used when the abutments are sound and when a partial- or full-coverage restoration is not indicated or possible because of cost limitations. To be effective, it must restore the major portion of the incisal surface to provide contact and anterior guidance (when appropriate) with the opposing teeth in lateral and protrusive excursions. This type of rest extends over the incisal surface of the tooth and provides a positive rest. 13

2

Removable Partial Denture Rests

Fig 2-8 The incisal rest should extend onto Fig 2-9 The incisal rest restores the occlusal surface to pro- Fig 2-10 Incisal rests on mobile and periodonthe facial surface of the tooth in order to pre- vide anterior guidance during excursion when necessary. The tally involved anterior teeth provide stability and vent horizontal movement of the abutment appearance from the facial resembles a three-quarter crown. allow these teeth to function as an integrated unit. during function. (Courtesy of Dr G. King, Houston, Texas.)

a

b

c

Fig 2-11 (a and b) The incisal rest from buccal and lingual views. The incisal rest seat is concave mesiodistally and convex buccolingually and engages the labial surfaces. (Part b courtesy of Dr R. Duell, Los Angeles, California.) (c) The rest must engage the labial surface.

The ideal incisal rest fulfills the following requirements: • It provides a positive seat by extending over the incisal edge onto the labial surface of the tooth (Fig 2-8). • It restores the anatomy of the tooth and, when necessary, anterior guidance (Fig 2-9). • It stabilizes teeth with compromised periodontal support (Fig 2-10). The incisal rest is used primarily on mandibular canines, but in some instances, it is employed to help stabilize mandibular central incisors that are compromised periodontally. From a biomechanical perspective, the incisal rest is not as ideal as the cingulum rest because of increased leverage on the abutment tooth. The incisal rest is rarely used on maxillary teeth because of esthetic considerations and occlusal interference with the mandibular anterior teeth. The indications for use of the incisal rest include the following: • The need to provide a positive rest on a tooth that requires no restoration and when the lingual anatomy of the tooth is not appropriate for preparation of a cingulum rest. • The need to restore anterior guidance. • The need to stabilize periodontally compromised teeth. 14

• Financial limitations—In some instances, the patient cannot afford the cost of a partial- or full-coverage crown. • Geriatric considerations—In elderly patients, this type of rest will spare the patient the time and effort expended to fabricate a partial- or a full-coverage crown. In many instances, the incisal surfaces of the natural teeth have been abraded, reducing the anterior guidance and allowing the posterior teeth to engage inappropriately during excursions. The incisal rest, besides providing a positive rest seat and directing occlusal forces axially, restores anterior guidance when necessary (see Fig 2-9). To be effective, the rest must extend over the incisal edge onto the facial aspect of the tooth in a positive manner to prevent the RPD framework from sliding off the tooth and to keep the tooth from moving away from the RPD framework during function. It must cover at least half the width of incisal surfaces. The rest seat preparation must be of sufficient thickness (1 mm) to provide appropriate thickness of metal. The incisal rest seat should be concave mesiodistally and convex buccolingually (Fig 2-11). The incisal rest has two disadvantages: (1) Esthetics may be affected due to metal display, and (2) there will be greater leverage on the tooth because the incisal rest is further from the periodontal support than a cingulum rest.

Anterior Rests

Fig 2-12  (a) Circular concave rests have been prepared on the surveyed crown on the maxillary right canine and within the natural tooth structure on the maxillary left canine. Note the amalgam restoration on the lingual surface of the maxillary left canine. When preparing this type of rest, frequently the enamel is perforated. (b) The completed partial denture in position. Note the absence of anterior retainers. This is a rotational path of insertion RPD. (Courtesy of Dr T. Berg, Los Angeles, California.)

a

a

b

b

Fig 2-13  Surveyed crowns were fabricated for the mandibular left canine to the mandibular right canine. Note the contour of the cingulum rests. They are positive and positioned low on the teeth. (Courtesy of Dr A. Davodi, Beverly Hills, California.)

Fig 2-14  A cingulum rest incorporated with a partial-coverage crown fitted to a mandibular canine. (Courtesy of Dr R. Duell, Los Angeles, California.)

Circular concave rests In a few select situations, the circular concave rest can be considered (Fig 2-12). It is usually used when there is insufficient space for a cingulum rest because of the opposing occlusion. It can also be considered when an RPD rest is planned on the existing crown to minimize the risk of perforating the crown. Circular concave rests are tolerated by the tongue better than crescent-shaped cingulum rests. However, if the circular concave rest is not positive, it will create greater leverage on the tooth compared to cingulum rests. Preparation of a circular concave rest often results in perforation of the enamel. If so, the rest preparation is altered to accept either an amalgam or a composite resin restoration (see Fig 2-12).

Methods used to develop positive cingulum rests Partial- or full-coverage crowns As mentioned above, maxillary canines frequently present with sufficient contours in the cingulum area to develop a positive rest without perforating the enamel. However, maxillary incisors and most mandibular anterior teeth will require a full- or partial-coverage crown in order to create a positive cingulum rest (Figs 2-13 and 2-14). When preparing the tooth for either a full-coverage or partial-coverage crown, the lingual wall of the preparation should be contoured, and the chamfer

a

b

Fig 2-15  (a and b) Full or partial veneer crowns enable the development of teeth with ideal contours and rest position. (c) The cingulum rest incorporated within this metal-ceramic crown is in ideal position with appropriate contours.

c

should be widened sufficiently to develop a rest of proper position (low on the lingual surface) and contour that does not interfere with the opposing occlusion. The rest can then be placed in optimal position for support, and all portions of the tooth topography (proximal plates, undercut areas, etc) can be developed to accommodate the design of the partial denture (Fig 2-15). 15

2

Removable Partial Denture Rests

a

b

c

d

e

Fig 2-16 (a) Diagrammatic representation of a pin-retained inlay used as a cingulum rest. (b) The inlay Fig 2-17 Bonded cingulum rests. preparation. (c and d) The pin-retained inlay in position. (e) The canine in centric occlusion. Note that it does not interfere with the opposing occlusion and provides space for the RPD framework. (Courtesy of Dr R. Duell, Los Angeles, California.)

Pin-retained inlays

In many instances, an inlay with parallel pins to retain it can provide a positive rest when the remainder of the tooth does not require restoration (Fig 2-16). However, these inlays are challenging to prepare, and the pins must be carefully positioned to avoid perforation of the pulp.

Bonded rests With advancements in adhesive dentistry, metal rests can be bonded onto the lingual surface of the tooth to provide support for the RPD (Fig 2-17). The bonded cingulum rest has the potential to produce the same functional outcomes as full- or partial-coverage crowns when sufficient enamel is present. This approach has many advantages. One retrospective study reports on the success of the resin-bonded cingulum rests supporting RPDs.4 Of the 42 rests on 26 patients examined, none of the resin-bonded cingulum rests were altered, lost, worn significantly, or required rebonding. The minimum follow-up period was 11 years. Some clinicians have suggested that a rest can be created by building up the linguocervical portion of the tooth with lightcured composite resin when this surface is covered with enamel. This type of rest should be designed such that the seating area is in enamel, whereas the lingual and proximal areas are built up with the composite resin. This procedure is recommended 16

for use in tooth-supported partial dentures but not for rests that serve as the axis of rotation in edentulous extension-type RPDs. The long-term survivability of light-cured composite resin rests has yet to be determined.

Posterior Rests The posterior rests of RPDs provide support and stability for the prosthesis. Like anterior rests, posterior rests must be positive and direct occlusal forces along the long axis of the teeth (Fig 2-18). During occlusal function they should not be displaced; otherwise, the prosthesis may act as a fulcrum and exert unfavorable forces on undesired tooth surfaces such as the abutment teeth. Although the greatest occlusal force is in the vertical direction, rests may also be designed to help resist horizontal forces.

Functions of posterior rests The functions of posterior rests are the following: • Provide rigid prosthetic support • Restore occlusion • Direct occlusal forces along the long axis of the abutment teeth

Posterior Rests

Fig 2-18  (a and b) The posterior occlusal rest must be rigid, with sufficient bulk to prevent flexure. When possible, the rest should be extended to the center of the tooth. The center portion of the rest (arrow) is deepest and is contoured as a half sphere. (Part b courtesy of Dr R. Duell, Los Angeles, California.)

a

b

a

b

b

c

Fig 2-19  (a and b) The extended rest across the occlusal surface restores the occlusal plane and prevents further tooth migration. (b) Note the posterior balancing ramp. This ramp is used to ensure balanced articulation during excursions. (Courtesy of Dr T. Berg, Los Angeles, California.)

a

Fig 2-20  (a to c) Rests are often extended to cover two or more posterior teeth to restore the plane of occlusion and occlusal harmony. This type of rest is used when teeth are tipped or not fully erupted or to restore the occlusal vertical dimension. (Courtesy of Dr R. Duell, Los Angeles, California.)

• Provide reciprocation and stabilization • Splint periodontally compromised teeth

Provide rigid support The posterior rest must be rigid with sufficient bulk to resist flexure under occlusal loads. If the rest flexes, eventually it will fracture. The posterior rest preparation on the abutment tooth must be definitive (see Fig 2-18). The rest must be of sufficient dimension and strength to provide a constant, controlled relationship among teeth, prosthesis, and mucosa.

Restore occlusion

An important asset of the posterior rest is that it can be designed to restore the occlusion and/or the occlusal plane when (1) a tooth has tipped out of proper occlusal alignment, (2) teeth have not erupted to proper occlusal position (Figs 2-19 and 2-20), or (3) there has been a loss of occlusal vertical dimension due to wear or loss of teeth. Rests are often extended (so-called “extended rests”) over two or more teeth to accomplish these goals. The amount of occlusal restoration necessary is determined by an occlusal analysis and careful scrutiny of the mounted diagnostic casts. In many instances, it is necessary to extend the rest over the entire mesiodistal length of several teeth to restore the occlusal plane. The width of these rests is usually one-third to one-half the buccolingual width of the tooth (see Fig 2-22). 17

2

Removable Partial Denture Rests

a Fig 2-21 The occlusal rest controls the direction of force applied to the tooth. In tooth-supported situations, it is best to extend the rest to the center of the tooth.

Fig 2-22 (a and b) A buccal extension of the occlusal rest on the mandibular right second molar provides reciprocation of the lingual I-bar retainer in a 0.01-inch undercut. This rest also re-establishes the plane of occlusion. Note that the portion of the rest restoring the occlusal plane is approximately one-third the width of the occlusal surface of the molar.

Direct forces along the long axis of the teeth Posterior teeth tolerate the forces of occlusion best when they are delivered along the long axis of the teeth (Fig 2-21). If molars are employed as abutments, it is advisable to extend the rest at least to the center of the tooth (see Fig 2-18). If the rest is positioned on one side of the tooth, it places a majority of the occlusal load on that side; this can result in tipping of the tooth. By extending the rest to the middle of the tooth, force is transmitted equally to both the tooth roots and the periodontal ligament, and from there to the bone.

Provide reciprocation and stabilization Rests can perform a variety of functions in addition to support and restoring the occlusion. An equally important function is to stabilize the abutment tooth against torquing or twisting forces. An occlusal view of a posterior tooth (Fig 2-22) indicates that a tooth standing alone is subject to movement in any direction under function because of the lack of the positional bracing contact formerly provided by the missing adjacent teeth. The rest can provide positive positional stabilization, utilizing remaining teeth in the arch. Rests on remaining teeth can unify and stabilize the arch, thereby maintaining the position of the individual teeth and restoring or maintaining arch integrity. In some situations, it is advantageous to extend the occlusal rest onto either the buccal or lingual surfaces of the abutment. This extension prevents the tooth from moving, provides reciprocation, and minimally interferes with tooth contours and the tongue or buccal mucosa (see Fig 2-22). 18

b

Continuous rests and splinting periodontally compromised teeth When the remaining dentition is periodontally compromised, tipped, or displaced, the posterior continuous rest can serve as an effective stabilizing or unifying device. The rest can be designed to extend entirely across the occlusal surface of two or more teeth, and in some instances, across the entire arch. This type of rest can restore and stabilize the occlusion and prevent further migration of teeth (Fig 2-23; see also Figs 2-10 and 2-19). When an occlusal force is applied in any region, all remaining teeth act in unison to provide support and unify the arch by means of the continuous posterior rests. The continuous rest is often used for a single tooth when the tooth fossa is excessively deep and is engaged by an opposing cusp tip. By recontouring the fossa with the rest and reducing the length of the opposing cusp, a more favorable occlusal relationship is obtained, minimizing the lateral forces during function (Fig 2-24). This may be especially advantageous when the abutment tooth has lost periodontal support. As mentioned above, a built-up rest is often used to idealize the plane of occlusion (see Figs 2-19, 2-20, and 2-22). The width of the rest should be approximately one-third the width of the occlusal surface. A built-up rest contoured in this fashion has the following advantages over rests designed to restore the full contour of the occlusal surface: • There is minimal tooth-metal contact, allowing for easier cleaning of both the tooth and the casting.

Posterior Rests

a

b

Fig 2-23  (a and b) The continuous rests in combination with the proximal plates unite the arch segments and stabilize the periodontally compromised teeth. (Courtesy of Dr E. King, Houston, Texas.)

Fig 2-24  The continuous rest effectively reduces the depth of the fossa.

Fig 2-25  Teeth move out of the proper position with loss of tooth contacts and opposing occlusion. Extending a rest to the second molar will prevent this tooth from further supereruption.

• The rest provides positive occlusal contact in and around centric contact but can be more easily designed to avoid contact during lateral excursions than a full-coverage rest. • The minimal occlusal surface area of this rest reduces the force necessary to penetrate the food bolus. • Occlusal adjustment is simplified compared to a full-­coverage rest. • It is easier to fit the RPD casting to the rest preparation compared to a full-coverage rest. • It controls the position of unopposed teeth.

Control position of unopposed teeth Many patients present with a tooth that has lost its antagonist in the opposing arch, and yet this missing tooth does not need a replacement for mastication function. Under these circumstances, extending the rest not only gains support from that tooth but also holds it in position, preventing supereruption and eliminating the necessity of an additional prosthesis in the opposing arch. Elongation and tipping of such unopposed teeth may result in traumatic occlusal interference in excursions, which can trigger periodontal compromise and temporomandibular joint symptoms (Fig 2-25). 19

2

Removable Partial Denture Rests

Fig 2-26 Note the definitive posterior rest preparation. The rests must be of sufficient mass to avoid flexure under load.

a

Fig 2-27 Occlusal rests in tooth-supported partial dentures should extend to the center of the abutment tooth or beyond.

b

c

d

Fig 2-28 (a to d) These rests all blend harmoniously with the occlusal surfaces. (Part d courtesy of Dr R. Duell, Los Angeles, California.)

General requirements of posterior rests The anatomy of posterior teeth allows for more convenient placement of positive rests in a position that enables functional forces to be directed along the long axis of the tooth. The requirements of a rest are the following: • It provides rigid support with sufficient thickness and width (one-third of the buccolingual occlusal width and 1.0 to 1.5 mm in thickness) (Fig 2-26). • It extends to the center of the tooth in tooth-supported situations (Fig 2-27; see also Fig 2-18). • All aspects of the rests are rounded, with no sharp angles, for ease of cleaning and making impressions and to prevent tooth fracture. 20

• The center of the rest is slightly deeper and rounded, shaped like a spoon (see Fig 2-18). • There are no undercuts in the path of insertion. • The rest blends harmoniously with the contours of the tooth (Fig 2-28). • When necessary, it can be used to restore and idealize the occlusal plane and to establish harmonious occlusion. This is particularly advantageous when the RPD opposes a complete denture (see Fig 2-19b). • When needed, it can be designed to provide reciprocation (see Fig 2-22).

Suggested Reading

Methods used to create positive posterior rests Preparing rests in natural tooth structure The width of the rest for a tooth with normal occlusal contours and position is usually equal to the size of a #6 or #8 round bur. Special consideration is given to provide sufficient space for the development of proper contours and thickness of the rests where they cross marginal ridges and particularly where the rests join the proximal plates or minor connectors, because these areas are most susceptible to fracture due to inadequate metal bulk. A primary consideration is that the finished rest seat be smooth and rounded. The minimum metal thickness of a rest is 1 mm for chrome cobalt alloy. The rest must be placed so that it directs the force along the long axis of the tooth. The design is specific to the individual need and determined by factors such as the opposing occlusion, the need to restore the occlusal plane, the need for bracing, and to idealize support.

Partial- and full-coverage crowns It is often necessary to restore a tooth with a crown for a variety of reasons. When a crown is indicated, it should be integrated and compatible with the RPD design so as to idealize the contours of the guiding surfaces and rest seats. Planning the RPD design prior to the crown preparation can ensure proper tooth reduction and the creation of proper guiding surfaces, thereby creating adequate space and thickness of material in the rest seat area (see chapter 15).

References 1. Kratochvil FJ, Caputo AA. Photoelastic analysis of pressure on teeth and bone supporting removable partial dentures. J Prosthet Dent 1974;32:52–61. 2. Thompson WD, Kratochvil FJ, Caputo AA. Evaluation of photoelastic stress patterns produced by various designs of bilateral distal-extension removable partial dentures. J Prosthet Dent 1977;38:261–273. 3. Berg T Jr, Caputo AA. Anterior rests for maxillary removable partial dentures. J Prosthet Dent 1978;39:139–146. 4. Janus CE, Unger JW, Crabtree DG, McCasland JP. A retrospective clinical study of resin-bonded cingulum rest seats. J Prosthodont 1996; 5:91–94.

Suggested Reading Applegate OC. Evaluating oral structures for removable partial dentures. J Prosthet Dent 1961;11:882–885. Cecconi BT. Effect of rest design on transmission of forces to abutment teeth. J Prosthet Dent 1974;32:141–151. Cohn LA. The physiologic basis for tooth fixation in precision-attached partial dentures. J Prosthet Dent 1956;6:220–244. Frechette AR. Partial denture planning with special reference to stress distribution. J Prosthet Dent 1951;1:710–724. Frechette AR. The influences of partial denture design on distribution of force to abutment teeth. 1956. J Prosthet Dent 2001;85:527–539. Granger ER. Mechanical principles applied to partial denture construction. J Am Dent Assoc 1941;28:1943–1951. Ivanhoe JR. Alternative cingulum rest seat. J Prosthet Dent 1985;54:395–396. Kratochvil FJ. Influence of occlusal rest position and clasp design on movement of abutment teeth. J Prosthet Dent 1963;13:114–124. Leupold RJ, Faraone KL. Etched castings as an adjunct to mouth preparation for removable partial dentures. J Prosthet Dent 1985;53:655–658. Sansom BP, Flinton RJ, Parks VJ, Pelleu GB Jr, Kingman A. Rest seat designs for inclined posterior abutments: A photoelastic comparison. J Prosthet Dent 1987;58:57–62. Seto BG, Avera S, Kagawa T. Resin bonded etched cast cingulum rest retainers for removable partial dentures. Quintessence Int 1985;16:757–760. Steffel VL. Planning removable partial dentures. J Prosthet Dent 1962;12: 524–535. Toth RW, Fiebiger GE, Mackert JR Jr, Goldman BM. Shear strength of lingual rest seats prepared in bonded composite. J Prosthet Dent 1986;56:99–104.

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Chapter 3 The Tooth-Tissue Junction and Proximal Plate Design Ting-Ling Chang | Daniela Orellana

The effectiveness of the retention provided by I-bar retainers is largely dependent upon the stability of the removable partial denture (RPD) framework because of their relatively low retentive value. Resistance to lateral forces (stability) is provided by the minor connectors and the proximal plates, which engage the guiding surfaces, and this is fundamental to the success of the RPI design concept as envisioned by Kratochvil. Kratochvil encouraged the use of aggressive guide planes (the tooth surface from the marginal ridge to the gingival margins) and stressed the benefits of buttressing in the cervical area. Proximal plates designed in this way enhance retention and help to redistribute stresses across the arch, reducing the risk of an isolated abutment becoming overloaded. Therefore, issues that require special consideration during diagnosis and treatment planning include the contours of the proximal surfaces of the teeth adjacent to the edentulous spaces and the relationship of the prosthesis to the tooth-tissue junction. Maintaining and preserving the original position of the teeth, maintaining gingival health, and removing or correcting pathologic conditions are the prime objectives. Most RPD abutments present with bone and gingival recession, altering clinical crown contours (Fig 3-1). If these surfaces are not recontoured, food traps are often created when the prosthesis interfaces with the surfaces of these teeth, which may lead to further degradation of the periodontal apparatus.

Basic Problem When a single tooth or several teeth are removed, there is usually significant loss of bone and soft tissues at the extraction site as well as near the adjacent teeth. Periodontal disease leads to the same result. These phenomena can expose a concavity or an undercut on the proximal surfaces of the teeth adjacent to the edentulous space. If these proximal surfaces retain their original contours, there will be a space between the tooth and the partial denture in the gingival area (Fig 3-2). When these spaces are incorporated into the partial denture design, undesirable sequelae may result, including the following: • • • •

Impaction of food into this space Hypertrophy of the gingival tissues into this space (Fig 3-3) A reduced bracing effect between the remaining teeth Further degradation of the periodontal health of the abutment teeth

An important principle of the RPI concept is to eliminate these spaces by recontouring the proximal surfaces of the abutment teeth (by the creation of guide planes) and extending the metal framework (proximal plate) of the partial denture to the gingival margin and onto the edentulous area (Fig 3-4). There should be no voids between the guide planes of the abutment teeth and the proximal plates of the RPD framework. 23

3

The Tooth-Tissue Junction and Proximal Plate Design

Fig 3-1 When teeth are extracted, the proximal surfaces of the teeth adjacent to the extraction sites often exhibit concavities or undercuts, which will result in significant space between these surfaces and the RPD if left unaltered. (Courtesy of Dr T. Berg, Los Angeles, California.)

Fig 3-2 Upon extraction of teeth, concavities or undercuts are exposed on the proximal surfaces adjacent to the edentulous space, which will permit food impaction between these surfaces and the RPD (arrows) if left unaltered.

Fig 3-3 The gingival tissues hypertrophied into the space between the RPD and the proximal surface of the abutment (arrow), deepening the periodontal pocket on the mesial.

Proximal Plates The portion of the metal partial denture framework that contacts the guide planes and the tooth-tissue junction is referred to as the proximal plate. It is considered a minor connector with a special function. It should be thin and closely adapted to the guide plane prepared on the tooth and should extend onto the edentulous mucosa approximately 2 mm (see Fig 3-4). During function, the position of the proximal plate is maintained by the rests. The benefits of properly designed proximal plates include the following: 24

• Prevention of food impaction between the RPD and the abutment teeth • Prevention of hypertrophy of the tissues between the abutment teeth and the prosthesis • Enhanced retention of the RPD by frictional contact with the abutment teeth • Maintenance of arch integrity by an anteroposterior bracing action (Fig 3-5) • Enhanced reciprocation for retainers in concert with other connectors (Fig 3-6)

Proximal Plates

Fig 3-4  Properly contoured guide planes with well-adapted proximal plates enhance retention and stability and restore arch integrity when that side of the arch is tooth-supported. Preparation of the guiding surfaces permits the proximal plate to contact the tooth-­ tissue junction.

Fig 3-5  Tooth alteration will eliminate all spaces and voids, permitting the proximal plates to contact the tooth and mucosa with no space between the prosthesis and tissue. Guide planes and proximal plates distribute stress widely, reducing the risk of overloading isolated abutments. They also enhance retention.

Fig 3-6  Note how the proximal plates help provide reciprocation for the retainers, especially the proximal plate engaging the molar. (Courtesy of Dr T. Berg, Los Angeles, California.)

Fig 3-7  The proximal plate (arrow) follows the contours of the proximal surfaces. In combination with the minor connector, it also provides reciprocation.

Recontouring the designated proximal surfaces of the teeth forms a guide plane (guiding surfaces) that allows the prosthesis to slide into position and eliminates voids between the surface of the teeth and the partial denture. Guiding surfaces should extend vertically from the occlusal surface to the gingival margins (see Figs 3-4 and 3-5). As such, when the proximal plates engage the guide planes properly, they will provide positive bracing for the remaining teeth and restore and maintain the integrity of the arch segment. The prepared guiding surface should follow the buccolingual curvilinear contours of the abutment tooth (Fig 3-7). In tooth-borne situations, making the guide planes parallel will enhance the retention of the partial denture by

means of frictional contact (see Figs 3-4 and 3-5). They also determine the path of insertion of the RPD. Metal is preferred over acrylic resin for the proximal plate. Acrylic resin is porous and harbors microorganisms, which, if in constant contact with the surface of the tooth and tooth-tissue junction, will predispose these teeth to caries and periodontal compromise. In addition, acrylic resin is subject to wear, compromising the bracing effect of the proximal plates. Metal proximal plates can be cast to precisely engage the guiding surfaces of the tooth and are not as subject to wear. The metal surfaces are not porous and are easier to clean, reducing the risk of caries and periodontal compromise of the abutment teeth. 25

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The Tooth-Tissue Junction and Proximal Plate Design

Fig 3-8 (a) The metal proximal plate should extend 2 mm onto the edentulous mucosa. (b) The mucosal portion of the metal is designed to form a right angle. This will ensure a strong union between the acrylic resin and the metal framework.

a

b Fig 3-9 The labial extension of the proximal plates may need to be limited in the esthetic zone. (Courtesy of Dr R. Duell, Los Angeles, California.)

As mentioned previously, the metal proximal plate is designed to extend approximately 2 mm onto the mucosa (Fig 3-8). In addition, the internal finish line associated with the junction between the metal of the cast framework and the resin covering the edentulous area should be configured at right angles to the underlying mucosa, creating a butt joint between the metal partial denture framework and the acrylic resin covering the edentulous areas. When such a joint is created between the resin and metal framework, the acrylic resin is less subject to crazing and fracture (see chapter 4). The faciolingual extension of the proximal plate should extend to a self-cleansing area. In the posterior region, proximal plates usually extend from the buccal line angle to the lingual line angle. They can be extended onto the lingual surfaces of the abutment teeth to enhance bracing and reciprocation. In the esthetic zone of the maxilla, it may be necessary to limit the labial extension of the proximal plates to the lingual half of the abutment tooth because of esthetic concerns (Fig 3-9). When preparing the guiding surfaces on the teeth, the natural curvatures of the teeth should be carefully followed in order to minimize tooth reduction. From the occlusal view, the guiding surfaces and the proximal plate should follow the buccolingual curvilinear contours of the tooth (see Fig 3-7). In 26

some situations, when abutment teeth present with excessive bone loss and gingival recession, it may be difficult to create guiding surfaces with appropriate contours without excessive tooth reduction. Under these circumstances, the bracing provided for the residual dentition becomes most important for the long-term survival of these teeth. Therefore, in these situations it may be prudent to fabricate a surveyed crown in order to create suitable guiding surfaces that extend all the way to the gingival margin (see chapter 15).

Suggested Reading Berg T Jr. I-bar: Myth and countermyth. Dent Clin North Am 1979;23:45–75. Bezzon OL, Mattos MG, Ribeiro RF. Surveying removable partial dentures: The importance of guiding planes and path of insertion for stability. J Prosthet Dent 1997;78:412–418. Kratochvil FJ, Caputo AA. Photoelastic analysis of pressure on teeth and bone supporting removable partial dentures. J Prosthet Dent 1974;32:52–61. Krol AJ, Jacobseon TE, Finzen FC. Removable Partial Denture Design: Outline Syllabus, ed 5. San Rafael, CA: Indent, 1999. Thayer HH, Kratochvil FJ. Periodontal considerations with removable partial dentures. Dent Clin North Am 1980;24:357–368.

Chapter 4 Major Connectors, Minor Connectors, and Denture Base Connectors Ting-Ling Chang | Daniela Orellana | John Beumer III

Major Connectors The purpose of the major connector is to provide a rigid connection between all portions of the prosthesis. Connectors are designed in various configurations: bars, straps, or plates. Bars are narrow and thick and are usually employed in the mandible. Straps are wider and thinner and are usually used in the maxilla. Plates are thin and maximize coverage, such as when the entire palate is covered. Major connectors must be rigid in order to do the following: • Control the relationship of remaining teeth to each other • Control the direction of force against all remaining support structures and opposing occlusion • Utilize and unite the remaining dentition of the arch to optimize cross-arch stabilization so as to distribute functional forces as widely as possible Occlusal forces delivered to the prosthesis are transmitted through the occlusal surfaces of the prosthesis to the rests and then through the major connectors to be distributed to the remaining natural teeth and the edentulous mucosa (in extension situations). Failure to provide a rigid connection between these elements can result in the delivery of uncontrolled and destructive forces to abutment teeth, mucosa, and bone. Hence, metal is far superior to flexible materials.

Maxillary Major Connectors There are four types of major connector designs used for a maxillary removable partial denture (RPD): anteroposterior

palatal strap, single palatal strap, U-shaped palatal connector, and complete palatal coverage plate. Criteria considered for the design of the major connector are the following: • • • • • •

Location of the edentulous areas Required amount of support Required degree of rigidity Patient preference Anticipated loss of natural teeth Location of the axis of rotation

Anteroposterior palatal strap Anteroposterior palatal straps (Fig 4-1) provide optimal rigidity. Most maxillary RPDs are designed in this manner. This design provides maximum resistance to flexure and distortion when the forces of mastication are applied to the prosthesis. If the anterior palatal strap is used without the posterior component, it requires a significant increase in bulk and area coverage to ensure adequate rigidity. The posterior palatal strap should not extend beyond the vibrating line of the palate and, in many instances, can be positioned anteriorly to the vibrating line and terminate adjacent to but just short of the hamular notch region (Fig 4-2). The anterior strap is positioned according to the following factors: • If anterior teeth are to be replaced by the prosthesis, the strap will extend into this area (Fig 4-3). • The strap may extend onto the anterior teeth for bracing and lateral stabilization when necessary (Fig 4-4a).

27

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Major Connectors, Minor Connectors, and Denture Base Connectors

Fig 4-1 The circular anteroposterior strap is favored for most maxillary RPDs because it combines maximum rigidity and strength with minimum bulk. The rests on the canines provide indirect retention. (Courtesy of Dr A. Davodi, Beverly Hills, California.)

Fig 4-2 The position of the posterior strap should be anterior to the vibrating line and terminate adjacent to but just short of the hamular notch. As such, the acrylic resin of the denture base can cover the tuberosity and smoothly transition with the RPD framework.

Fig 4-3 (a and b) When anterior teeth are to be replaced, the anterior portion of the connector is extended into the area to be restored. (Courtesy of Dr G. King, Houston, Texas.)

a

a

b

b

Fig 4-4 (a) An anterior strap can be used for lateral stabilization. (Courtesy of Dr T. Berg, Los Angeles, Fig 4-5 The maxillary major connector is designed to California.) (b) The anterior strap is connected to positive rests on anterior teeth, which provide be at least 5 to 6 mm from the gingival margins. (Courtesy of Dr R. Duel, Los Angeles, California.) indirect retention and stabilization. (Courtesy of Dr R. Duel, Los Angeles, California.)

28

Maxillary Major Connectors

1.5 mm 8–12 mm

Fig 4-6 Cross section of a posterior palatal strap. Fig 4-7 A bead as shown (arrows) prevents debris The central portion is thickened to provide rigidity. from accumulating beneath the RPD casting.

a

Fig 4-8 Single palatal strap connector. The connector should be at least 10 mm wide, and the central portion should be at least 1.5 mm thick to ensure appropriate rigidity.

b

Fig 4-9 (a and b) Anterior palatal connectors are employed when restoring the anterior dentition, when Fig 4-10 An anterior palatal connector was used the patient presents with a palatal torus, or when the patient has an active gag reflex. (Courtesy of Dr J. because this patient presented with a palatal torus Jayanetti, Los Angeles, California.) and had a strong gag reflex.

• The strap may connect with a rest, which can provide indirect retention in specific situations (Fig 4-4b; see also Fig 4-1). • The connector may be used to stabilize a mobile tooth. If anterior coverage is not required, the connector is designed 5 to 6 mm away from the teeth (Fig 4-5). The borders of the connector terminate in the valley of the rugae to provide a smooth transition between the metal of the framework and the palatal mucosa. The posterior palatal strap is thickest in the central portion of the casting and tapers to blend smoothly into the mucosa at the margins (Fig 4-6). The width of the posterior strap should be 8 to 12 mm. The contours of all parts of the prosthesis should be smooth and blend into existing tissues, while at the same time reproducing the contours of the missing structures. It is imperative that the strap be in intimate contact with the tissues because any space between the strap and the mucosa can accumulate debris and cause discomfort to the patient. A light bead is designed along the borders of the major connector around the palatal opening and posteriorly to provide a seal to prevent accumulation of food and debris beneath the framework (Fig 4-7).

Single palatal strap Replacement of one or two teeth can be accomplished with a single palatal strap. The width of the strap is increased to provide the necessary rigidity, cross-arch stability, and control of the prosthesis (Fig 4-8). It is advised that it be at least 10 mm wide.

U-shaped palatal connector The U-shaped palatal connector is indicated in the following clinical situations: • Tooth-borne RPDs when anterior teeth are missing (Fig 4-9) • When a palatal torus precludes the use of an anteroposterior palatal strap • When the patient has a strong gag reflex and cannot tolerate a posterior palatal strap (Fig 4-10) When using this type of connector, a significant increase in bulk and surface area coverage is required to provide sufficient 29

4

Major Connectors, Minor Connectors, and Denture Base Connectors

Fig 4-11 Full palatal coverage is used when maximum support is desired. In this patient, there was insufficient tooth support on the left side, so full palatal coverage was employed. (Courtesy of Dr T. Berg, Los Angeles, California.)

a

b

Fig 4-12 (a and b) A complete palatal plate. (Courtesy of Dr R. Duell, Los Angeles, California.)

a

b

Fig 4-13 (a and b) A lingual bar. It should at least 3 mm from the gingival margins.

a

b

Fig 4-14 A lingual plate. Note the scalloping to mimic the contours of the underlying dentition.

30

Mandibular Major Connectors

At least 4 mm in height, 2 mm in thickness

a

b

Fig 4-15  (a and b) A minimum of 7 mm from the gingival crest and the elevated position of the floor of the mouth is required to fabricate a lingual bar with sufficient rigidity. (Part b is redrawn from Kroll et al.1)

rigidity for the connector. The central portion of the connector should be at least 1.5 mm thick.

Complete palatal coverage Complete coverage of the palate is indicated when maximum palatal support is required, such as when the abutments are not ideally positioned or are compromised periodontally (Figs 4-11 and 4-12). Complete palatal coverage can be accomplished with either metal or acrylic resin. Coverage with acrylic resin is advantageous because if necessary, the palatal portion can be relined. In addition, palatal coverage with acrylic resin will be better adapted to the palatal mucosa, enhancing support. Note the posterior palatal seal in the patient in Fig 4-11.

Mandibular Major Connectors The anatomy of the mandible makes it more difficult to fabricate a rigid partial denture framework. Rigidity of the prosthesis is further compromised by the limited distance between the mobile portion of the floor of the mouth and the residual dentition. As a result, the bulk of the partial denture framework must be increased. Two types of mandibular major connectors are favored: 1. The lingual bar (Fig 4-13) is the design of choice because of its simplicity and minimal tissue coverage. It is preferred when there is sufficient space between the gingival margin and the elevated position of the floor of the mouth. Accumulation of plaque and food debris is minimized with this design, it is simple to fabricate, and there is minimal interference with oral function.

Fig 4-16  The location of the extension of attached mucosa can be quickly measured with a periodontal probe.

2. The lingual plate (Fig 4-14) is used when there is insufficient space for a lingual bar of sufficient bulk and rigidity. It is particularly useful in the presence of tori. It is also the desired choice when it is anticipated that anterior teeth will be lost and in need of replacement without having to remake the partial denture framework.

Criteria for selection Space available As noted previously, the lingual bar is the design of choice because of its simplicity and minimal tissue coverage. However, sufficient space must be available between the floor of the mouth and the crest of the gingiva to allow placement of a sufficiently rigid connector. The lingual bar must not prevent normal elevation of the floor of the mouth during function and must not impinge upon the lingual frenum. The superior extent of the bar should be well below the gingival crest—usually 3 to 4 mm (Fig 4-15; see also Fig 4-13). If positioned too close to the gingival margin, the tissue tends to hypertrophy over the bar, and food and debris tend to become impacted between the bar and the mucosa, leading to further tissue irritation. If there is insufficient space for a lingual bar, a lingual plate is recommended. The tissue surface of the bar should be lightly relieved to permit rotation of the prosthesis around the fulcrum line without impinging on the underlying mucosa. The space available between the lingual gingival crest and the elevated position of the floor of the mouth is determined clinically. The patient is asked to touch the vermilion border of the upper lip with the tip of the tongue. This will elevate the floor of the mouth. A periodontal probe can then be used to measure the distance between the gingival crest and the elevated position of the floor of the mouth (Fig 4-16). This distance can 31

4

Major Connectors, Minor Connectors, and Denture Base Connectors

Fig 4-17 A lingual plate can be used to stabilize the anterior teeth when periodontal support is compromised. It also allows for the addition of denture teeth in the event of the loss of one or more of the anterior teeth.

Fig 4-18 The lingual bar should be pear shaped in cross section, and the top of the bar should be positioned at least 3 mm below the gingival crest. f, occlusal force.

also be determined on the master cast if the floor of the mouth contours have been recorded with a border-molded impression. A lingual plate is selected under the following circumstances.

Anticipated loss of natural teeth A lingual plate type of connector is preferred when the loss of anterior teeth is anticipated because it can be designed to allow for the addition of prosthetic teeth if required at a future date.

Periodontal status and mobility of the anterior teeth The lingual plate type of connector is preferred when the anterior teeth are mobile and periodontally compromised. This type of connector can be designed to splint these teeth together. Placement of lingual coverage can stabilize the teeth and buttress them against lateral and distal forces because the lingual surfaces of the maxillary anterior teeth contact the mandibular anterior teeth in protrusive and lateral excursions (Fig 4-17).

Gingival recession When a patient presents with significant gingival recession, the topography on the lingual side in the anterior region will predispose to food impaction and further compromise the health of the periodontium. Under these circumstances, a lingual plate should be considered. The lingual plate will cover these areas and prevent accumulation of food and debris in the region.

32

Design Considerations Lingual bar In cross section, the lingual bar is shaped like a pear, with a height of at least 4 mm and thickness of at least 2 mm (Fig 4-18). As mentioned previously (see section entitled “Space available”), the top of the bar should terminate at least 3 mm from the gingival crest (see Fig 4-13). Relief of the tissue surface of the bar may be necessary to avoid displacing and irritating the tissues during function. More relief is required if the RPD restores an edentulous extension area because of the movement of the prosthesis around the axis of rotation during occlusal function (see Fig 4-18). This relief (~0.020 inch) is created in the dental laboratory during the development of the framework. Teeth tipped lingually can pose significant challenges. If this phenomenon occurs and is left unattended, there will be a space between the connector and the tissues that will be predisposed to food impaction and tissue hypertrophy. This problem can be addressed in two ways: (1) by reshaping or restoring the dentition to remove the undercuts (Fig 4-19) or (2) by orthodontically repositioning the teeth.

Lingual plate The contours of the lingual plate are designed to be in intimate contact with the attached gingiva, including as much of the interdental papilla as possible (Fig 4-20). It should extend to contact the lingual surfaces of the teeth above the survey line

Minor Connectors

Fig 4-19  (a and b) In this patient, the mandibular left second premolar was lingually inclined, so the lingual surface was recontoured with an air rotor to eliminate the severe undercut.

a

b

Fig 4-20  The tissue surface of the casting is Fig 4-21  The contour of the lingual plate should be designed to be in intimate contact with the at- scalloped to mimic the contours of the underlying tached mucosa. dentition.

to avoid food entrapment. The inferior portion is shaped like a lingual bar to provide sufficient rigidity for the framework. If the teeth are excessively tipped facially, the framework should terminate near the junction between the cervical third and middle third of the teeth. The portion overlying the anterior dentition should be scalloped to mimic the contours of the underlying natural dentition (Fig 4-21; see also Fig 4-14).

Minor Connectors Minor connectors connect the major connector with the other parts of the RPD casting, including the clasp assembly, indirect

Fig 4-22 The minor connectors extending from the rests to the major connector should cross the tooth-tissue junction at right angles. The minor connectors should be 4 to 5 mm apart (arrows). Note that the minor connector crosses the canine in its center. These design features will minimize food entrapment.

retainers, proximal plates, and rests. Minor connectors should be designed to cross the tooth-tissue junction at right angles (Fig 4-22). In the anterior region, it is advisable to cross the tooth-tissue junction in the center of the tooth (see Figs 4-21 and 4-22). Minor connectors should be 4 to 5 mm apart to improve food flow patterns and reduce the incidence of food impaction. Additionally, minor connectors should be designed with sufficient bulk to maintain the rigidity needed to transmit forces effectively between the linked components. Therefore, they should be at least 1.5 mm thick in the area that connects with the rest.

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Major Connectors, Minor Connectors, and Denture Base Connectors

Fig 4-23 There should be a minimum of 1 mm of acrylic resin between the denture base connector and the tissue surface (arrow).

Fig 4-24 In the mandible, the denture base connectors should be positioned on the crest of the ridge and on the lingual surface. The buccal portion is left unobstructed so as to permit easy and proper positioning of denture teeth. (Courtesy of Dr A. Davodi, Beverly Hills, California.)

Fig 4-25 (a) In the maxilla, there is often little space between the tuberosity and the opposing dentition and/or retromolar pad. (b) Therefore, the denture base connectors are designed on the palatal side of the ridge crest.

a

b

a

b

Fig 4-26 The junction between the major con- Fig 4-27 (a and b) The addition of posts to the anterior denture base connector will enhance the retennector and the denture base connector should be tion of the denture teeth. (Courtesy of Dr R. Duell, Los Angeles, California.) shaped like a fan (arrows) and have sufficient bulk and rigidity to resist flexure and breakage.

Denture Base Connectors The denture base connectors are designed to do the following: • Provide a framework for the attachment of the acrylic resin of the denture base to the metal casting 34

• Provide the necessary strength to maintain rigidity throughout the prosthesis during occlusal function • Establish a positive finish line at the junction between the acrylic resin and the metal framework

Finish Lines

a

b

c

Fig 4-28  Finish lines are created to establish the junction between the acrylic resin of the denture base and the metal framework. A right-angle joint should be created to prevent crazing and fracture of the acrylic resin. (a) External finish line. (b and c) Internal finish line (arrows).

There are three types of denture base connectors: open lattice, mesh, and metal base. The open lattice type is preferred because superior retention between the resin and the framework is achieved with this design. The metal base is designed to directly contact the underlying mucosa and is used when replacing a single tooth. Denture base connectors should be designed so that there is at least 1 mm of acrylic resin between the tissue surfaces and the connector (Fig 4-23). Positioning of the connectors is influenced by (1) the amount of space available between the opposing arches and (2) the placement of the denture teeth. In the mandibular arch, the connectors are positioned on the crest and on the lingual surfaces of the residual ridge (Fig 4-24). This design leaves the labiobuccal portion clear for positioning the denture teeth without having to reduce their bulk unnecessarily. If the bulk of the denture teeth is overreduced, the esthetics of the prosthesis, as well as its retention in the acrylic resin, may be compromised. In the maxilla, there is often very little space between the maxillary tuberosity and the opposing dentition and/or the retromolar pad. Therefore, the denture base connectors are positioned well away from this region (Fig 4-25). Otherwise, the acrylic resin overlying the denture base connectors will be excessively thin and subject to crazing. They should not be placed on the buccal side, where they may interfere with the placement of the denture teeth.

Strength and rigidity The junction between the denture base connector and the major connector should be of sufficient strength and rigidity to resist flexure and/or fracture. The connector should be fan-shaped at the junction, and the bulk in this area should be increased (Fig 4-26). Note the rough surfaces of the denture base connector to enhance mechanical retention of acrylic. The design of the denture base connectors in anterior extension cases may require special consideration. The addition of posts placed in the tooth positions is recommended to help retain the denture teeth in the acrylic resin of the denture base (Fig 4-27).

Finish Lines The finish lines between the metal framework and the acrylic resin of the denture base should be made at a 90-degree angle (Figs 4-28a and 4-28b). If the acrylic resin of the denture base is too thin or tapers at the junction between the metal framework and the denture base, the resin tends to craze and fracture, allowing food and debris to collect in this area. As mentioned previously, the internal finish line should be positioned at least 2 mm away from the tooth-tissue junction (see Fig 3-8). The internal and external finish lines should be staggered, particularly in the maxilla, to avoid weakening the partial denture framework, and they should be positioned so that the portion of the alveolar ridge subject to resorption can be relined with acrylic resin (Fig 4-28c). 35

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Major Connectors, Minor Connectors, and Denture Base Connectors

Reference 1. Kroll AJ, Jacobson TE, Finzen FC. Removable Partial Denture Design: Outline Syllabus, ed 5. San Rafael, CA: Indent, 1999.

Suggested Reading Ben-Ur Z, Matalon S, Aviv I, Cardash HS. Rigidity of major connector when subjected to bending and torsion forces. J Prosthet Dent 1989;4:557–562. Ben-Ur Z, Mijiritsky E, Gorfil C, Brosh T. Stiffness of different designs and cross-sections of maxillary and mandibular major connectors of removable partial dentures. J Prosthet Dent 1999;81:526–532.

36

Campbell LD. Subjective reactions to major connector designs for removable partial dentures. J Prosthet Dent 1977;37:507–516. Carr AB, McGivney GP, Brown DT. McCracken’s Removable Partial Prosthodontics, ed 11. St Louis: Mosby, 2005. Eto M, Wakabayashi N, Ohyama T. Finite element analysis of deflections in major connectors for maxillary RPDs. Int J Prosthodont 2002;15:433–438. Frechette AR. The influences of partial denture design on distribution of force to abutment teeth. 1956. J Prosthet Dent 2001;85:527–539. Henderson D. Major connectors for mandibular removable partial dentures: Design and function. J Prosthet Dent 1973;30:532–548. Henderson D. Major connectors, united it stands. Dent Clin North Am 1973;17:661–678. LaVere AM, Krol AJ. Selection of a major connector for the extension-base removable partial denture. J Prosthet Dent 1973;30:102–105.

Chapter 5 Retainers, Clasp Assemblies, and Indirect Retainers Ting-Ling Chang | Daniela Orellana | Ryan Wallace

Retainer Design and Position Removable partial denture (RPD) retainers have previously been defined as “any type of device used for the stabilization or retention of a prosthesis.” There are two types of RPD retainers: direct and indirect. Direct retainers are flexible parts of the metal casting that are deliberately designed as the primary source for retention. They engage undercuts on abutment teeth to resist removal of the prosthesis and to prevent dislodgment during function. An indirect retainer is the component of an RPD that assists the direct retainer(s) in preventing displacement of a distalextension denture base; it functions through lever action on the opposite side of the fulcrum line when the denture base moves away from the tissues in pure rotation around the fulcrum line.

does not cross the survey line of the tooth when the prosthesis is in a seated position. The I-bar is the most common infrabulge retainer used today (Fig 5-2). The I-bar retainer has low retentive value compared to other types. This is due to minimal tooth engagement and purposeful design because support, stability, and control of tooth position are provided with positive rests, minor connectors, and proximal plates. The I-bar’s effectiveness of retention is dependent upon the stability (resistance to lateral displacement during function) of the RPD, which in turn is dependent on the presence of properly designed minor connectors, guide planes, and proximal plates. In particular, properly contoured guide planes and proximal plates are essential to successful use of I-bar retainers.

Indications and benefits of I-bar retainers

Direct Retainers There are two basic types of direct retainers (Fig 5-1): infrabulge and suprabulge.

Infrabulge retainers An infrabulge retainer (see Fig 5-1) is a direct retainer that approaches the crown of the tooth from an apical direction and

The advantages of the I-bar are that it allows for the following: Minimal tooth contact Exact placement of retention contact Minimal interference with natural tooth contour Maximum natural cleansing action Passive functional movement of an extension-base prosthesis when the rest is properly positioned (RPI concept) • Reduced display of metal for better esthetics (Fig 5-3) • Easier adjustment

• • • • •

37

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Retainers, Clasp Assemblies, and Indirect Retainers

Fig 5-1 The infrabulge retainer approaches, as seen on the mandibular left first premolar, from below the height of contour (red dotted lines). The suprabulge retainer approaches, as seen on the mandibular left second molar, from above the height of contour.

Fig 5-2 This infrabulge retainer is commonly called an I-bar. The retainer approaches the tooth from an apical direction. The I-bar retainer crosses the tooth-tissue junction at a right angle in a straight line until it reaches the unattached mucosa, where it turns to a horizontal position. The vertical portion of the retainer is roughly parallel to the long axis of the tooth.

Fig 5-3 (a to f) I-bar retainers provide superior esthetic results with minimal metal display. Only the tips are visible in most patients.

a

b

c

d

e

f

38

Direct Retainers

Fig 5-4  The amount of tooth undercut engaged determines the amount of retention from the retainer. The undercut gauge is used to determine the amount of undercut. Height of contour 0.01"

Fig 5-5  (a and b) The retainer joins the metal casting between the proposed position of the denture teeth (arrow) so as not to interfere with the placement of the prosthetic teeth. (Courtesy of Dr J. Jayanetti, Los Angeles, California.)

a Fig 5-6  (a and b) The horizontal portion of the I-bar is placed on attached tissue whenever possible. Space is provided under the retainer where it crosses the tooth-tissue junction with the acrylic resin of the denture base. The space between the retainer and the tissue is approximately 0.010 inch. This space is necessary to prevent tissue irritation and hypertrophy. A properly designed I-bar should engage the undercut down to the 0.25-mm region, and the occlusal-incisal tip terminates at the height of contour (dotted line), preferably at the gingival third.

b

0.25 mm retention

a

Design principles of I-bar retainers

I-bars are designed to cross the tooth-tissue junction at right angles or in the same plane as the long axis of the tooth (Fig 5-4; see also Figs 5-1 and 5-2). This practice enables favorable food flow patterns and minimizes food impaction between the tooth and the retainer. From the point of contact, the retainer extends in a straight line to the unattached tissues (mucogingival junction). This positioning minimizes the amount of debris that collects on the facial surface of the abutment tooth. The size of the retention undercut used is 0.01 inch or 0.25 mm. The portion of the retainer that contacts the tooth (contact area)

b

is circular or oval in shape. Precise positioning of this portion of the retainer is essential and is determined with an undercut gauge (Fig 5-5). The horizontal portion of the retainer is placed on attached tissue whenever possible (Fig 5-6). The junction between the retainer and the denture base connectors should be positioned in the interproximal area between the proposed positions of the denture teeth. This practice allows for the proper placement of the denture teeth without compromising the esthetics or excessively shortening the teeth due to lack of space (see Fig 5-6). The portion of the retainer that traverses the gingival tissues before the horizontal portion begins should not 39

5

Retainers, Clasp Assemblies, and Indirect Retainers

a

b

c

Fig 5-7 I-bar retainers are contraindicated under the following circumstances: (a) Maxillary molars tilted to the buccal. (b) Mandibular molars with a minimal zone of attached gingiva. (c) High frenum attachments. Akers clasps are favored under these circumstances.

be in direct contact with the attached keratinized tissues. This area should be relieved (0.01 inch) to prevent tissue irritation and hypertrophy (see Fig 5-6). Ideally the I-bar retainer should terminate in the gingival third of the tooth. A properly designed I-bar engages the tooth from the height of contour and down into the undercut terminating at the 0.25-mm level (see Figs 5-5 and 5-6).

Contraindications for I-bar retainers There are several situations where an I-bar is not suitable (Fig 5-7): • Teeth with short clinical crowns. Guiding surfaces and proximal plates will be short, and under these circumstances, the I-bar may not provide acceptable retention. • High frenum attachments. This precludes proper positioning of the horizontal component of the retainer. • Severely tilted abutments. The I-bar will project into the cheek and may irritate the buccal mucosa. This is particularly a problem in the second molar region of the maxilla. • Lack of attached gingiva and shallow vestibule. For example, there is often a lack of attached gingiva on the buccal aspect of mandibular second molars. Moreover, the shallow vestibule in this region may preclude placement of an I-bar.

Suprabulge retainers A suprabulge retainer is a direct retainer that begins at the middle third of the tooth and extends onto the cervical third of the tooth, crossing the survey line and extending into a tooth undercut. These types of retainers provide bracing and retention for the RPD. An example of a suprabulge retainer is the circumferential retainer (Figs 5-8 and 5-9; see also Fig 5-1). Factors causing concern with use of the circumferential retainer include 40

a greater area of tooth contact, interference with tooth-tissue contours (Fig 5-10), and decalcification of the tooth structure beneath the area of tooth contact (Fig 5-11). Moreover, if the rest is inappropriately positioned, there is potential to torque abutment teeth adjacent to the edentulous extension area (see Figs 6-11 and 6-12). A circumferential retainer approaches the undercut from above the height of contour of the abutment tooth. Its flexible retentive arm originates from a minor connector or a proximal plate, and its terminal third crosses the height of contour into the undercut (see Fig 5-9a). The rigid reciprocating arm stays at or above the height of contour (see Fig 5-9b). Circumferential retainers are best used in the following situations: • With posterior tooth abutments • When additional bracing is needed: teeth with short clinical crowns in patients with long edentulous spans requiring additional bracing • For tipped teeth: maxillary molars (see Fig 5-7a) • In the presence of high frenum attachments (see Fig 5-7c) • When there is minimal attached gingiva (mandibular second molars) (see Fig 5-7b) Another suprabulge retainer commonly used is the embrasure clasp. When the teeth are sound and retentive areas are available, retention can be very effective with this design (Fig 5-12). Its use in unprotected abutments is based on caries index, oral hygiene, opposing occlusion, and tooth contours. When used in nonrestored teeth, frequently inadequate tooth reduction is made in the proximal area (3 mm wide and 1.5 mm deep). As a result, these retainers are susceptible to fracture because of lack of metal bulk, and it is difficult to obtain a nonporous casting throughout the clasp assembly. These problems can be avoided if the embrasure retainers are employed in association with cast restorations (Fig 5-13).

Direct Retainers

Fig 5-8  Suprabulge retainer.

a

b

c

d

Fig 5-9  (a and b) The circumferential retainer originates from above the height of contour of the abutment tooth, and its terminal third crosses the height of contour and engages the undercut area. The rest of the circumferential arms stays above the height of contour for reciprocation and bracing. (c and d) Circumferential retainers interfere with food flow patterns and may torque the tooth when there is movement of the prosthesis.

Fig 5-10  (a and b) Note the wear and erosion of the buccal surface where the retainer engages the tooth surface secondary to movement of the RPD during function.

a

b

a

b

Fig 5-11  (a and b) Note the decalcification where the circum­ ferential retainer engages the abutment tooth. (Courtesy of Dr R. Duell, Los Angeles, California.)

41

5

Retainers, Clasp Assemblies, and Indirect Retainers

Fig 5-12 Embrasure clasps are effectively employed for this mandibular unilateral extension RPD. However, this type of clasp is predisposed to a high rate of fracture if insufficient space is provided interproximally. Also note that additional bracing is provided by the lingual plating of the remaining molars.

a

b Fig 5-13 Embrasure clasps and rests created on the surveyed crowns.

Disadvantages of suprabulge circumferential retainers Disadvantages of the suprabulge retainers include the following: • They are difficult to adjust. • They disrupt normal food flow patterns, predisposing the space between the retainer and the tooth to food impaction. • They may torque abutments if the rest is inappropriately positioned.

Indirect Retention: Myth or Reality? The concept of indirect retention was first introduced by Cummer.1 An indirect retainer is a positive rest placed on the dentate side of the axis of rotation in an extension-base RPD to prevent lifting of the denture base around the fulcrum line (axis of rotation) (Fig 5-14). The fulcrum line is a line drawn through the rests most adjacent to the extension base that determines the axis of rotation (see Fig 5-14). When a patient occludes on the teeth associated with the extension area, the RPD may rotate around this line such that the posterior denture bases move toward the residual ridge and the anterior aspects of the denture slightly raise away from their respective bearing surfaces. Hence, it would 42

be inappropriate to place a cast direct retainer anterior to the fulcrum line; this retainer would elevate the abutment during function and place an extraction force on the tooth. Unlike direct retainers, indirect retainers are always placed anterior to the axis of rotation for distal-extension RPDs. When the distal extension bases are pulled away from the denturebearing surface around the fulcrum line, the anterior aspects of the denture will depress into their respective bearing surfaces. Therefore, an anterior hard tissue stop, designed as a positive rest, will resist this rotational lifting of the denture base. This rest indirectly acts as a retentive element. The classic example involves mastication of a sticky food bolus on the distal extension of a partial denture. The sticky food bolus may pull the extension base away from the residual ridge, around the fulcrum line. However, a rest placed anteriorly will remain seated and indirectly retain the prosthesis. The rests designed to function as indirect retainers are most ideally placed perpendicular to a point in the center of the fulcrum line. This position for the rest allows it to be farthest from the axis of rotation and centered along the arc of rotation (Fig 5-15). Theoretically, the effectiveness of the indirect retainer is believed to increase the further it is located from the fulcrum line because the mechanical advantage increases proportionately. The direct retainers must be effective if the indirect retainer is to function when the denture rotates; otherwise the partial denture will be dislodged.

Clasp Assemblies

e lin on) rumotati c l Fu of r is (ax

(ax Fulcr is o um f ro lin tat e ion )

a

b

Fig 5-14  (a and b) An indirect retainer is defined as the component of an RPD prosthesis that assists the direct retainer(s) in preventing displacement of the distal-extension RPD by functioning through lever action on the opposite side of the fulcrum line (red dotted line) when the denture base moves away from the tissues in pure rotation around the fulcrum line. The lingual plate and rests on the mandibular left first premolar serve as the indirect retainers on the mandibular RPD design. The cingulum rest on the maxillary left canine serves as the indirect retainer on the maxillary RPD design.

Fig 5-15  A patient with a left partial palatectomy defect. The mesial rest on the maxillary right premolar (arrow) acts as an indirect retainer. The forces of gravity tend to displace the obturator portion of the prosthesis, down and out of the defect. The indirect retainer resists this displacement. Reciprocation for the retainer engaging the molar is provided by the proximal plate and the lingual plate. Reciprocation for the retainer engaging the left pre­molar is provided by the proximal plate and the minor connector.

Clasp Assemblies A clasp assembly is a combination of several RPD components that engage an abutment tooth extracoronally for support, stability, and retention of the RPD. The components of the clasp assembly include the direct retainers, the reciprocating elements, the rests, the minor connectors, and the proximal plates (Fig 5-16). A well-designed clasp assembly provides the following functions: • Support through the positive rest • Retention through the direct retainer

Axis of rotation

• Reciprocation and encirclement • Bracing/stability through the proximal plates and minor connectors • Passivity when the prosthesis is fully seated

Reciprocation and encirclement Reciprocation is defined as the resistance to horizontal forces (primarily in the buccal and lingual direction) exerted on a tooth by an active retentive element. Reciprocation can be provided by the rigid reciprocating clasp arm opposite the retentive arm, minor connectors, proximal plates, and lingual 43

Retainers, Clasp Assemblies, and Indirect Retainers

5

Proximal plates

Direct retainer, circumferential clasp

Rests

Direct retainer, I-bar Fig 5-16 A clasp assembly provides support, stability, and retention for an RPD by partially encompassing an abutment tooth with the following components: a direct retainer, a reciprocating element, a rest, and a proximal plate.

a

b

c

d

Fig 5-17 (a) In the classic RPI system, reciprocation of the retentive I-bar (red arrow) is provided by the distal proximal plate (green arrow) and the mesial rest and minor connector (yellow arrow). The RPI assembly engages more than 180 degrees of the circumference of the abutment and fulfills the encirclement concept. Lateral forces are generated by a retentive retainer/clasp when it passes over the height of contour of the abutment. (b) Reciprocation for the retainer is provided by the mesial and distal proximal plates. (c and d) In part c, the retentive I-bar is reciprocated by an extended rest (d) that engages the buccal surface of the molar.

plates (Fig 5-17). Reciprocation can prevent tooth movement that may result from overadjustment of a retentive clasp arm (ie, making the I-bar or the retentive arm excessively retentive). Encirclement refers to a design concept that implies that the clasp assembly must engage more than 180 degrees of the circumference of the abutment tooth. Encirclement is achieved with a combination of retainers, proximal plates, minor connectors, and extended rests. Otherwise, the abutment tooth may be displaced away from the clasp assembly during function. Examples of the methods used to accomplish this task are shown in Fig 5-18. 44

Bracing/stability Bracing (stability) provides resistance to horizontal, lateral, or torsional components of force generated during mastication or eccentric movements of the mandible. It is provided primarily by the rigid portions of a clasp assembly, such as the rigid reciprocating clasp elements, minor connectors, proximal plates, lingual plates, and extended occlusal rests. Increased bracing through the use of lingual plates is necessary when teeth are present on only one side of the arch (see Figs 17-8, 17-15, and 17-16).

Suggested Reading

Fig 5-18  Guide planes of teeth engaged by proximal plates provide stress reduction in several areas, including buttressing.

Passivity Abutment teeth should not be exposed to an active force when the framework is fully seated and the retainers are engaged. Upon seating, retainers should only activate when a dislodging force is applied. During mastication, the occlusal forces are directed toward the edentulous areas, and the retainers should stay passive. The activation of the direct retainer should only be needed to resist removal or dislodgment of the RPD from the abutment teeth during function.

Reference 1. Cummer WE. Partial denture service. In: Anthony LP (ed). The American Textbook of Prosthetic Dentistry in Contributions by Eminent Authorities, ed 7. Philadelphia: Lea & Febiger, 1942:753.

Suggested Reading Ben-Ur Z, Aviv I, Cardash HS. A modified direct retainer design for distalextension removable partial dentures. J Prosthet Dent 1988;60:342–344. Benson D, Spolsky VW. A clinical evaluation of removable partial dentures with I-bar retainers. Part I. J Prosthet Dent 1979;41:246–254. Brudvik JS, Wormley JH. Construction techniques for wrought-wire retentive clasp arms as related to clasp flexibility. J Prosthet Dent 1973;30:769–774. Cecconi BT, Asgar K, Dootz E. Clasp assembly modifications and their effect on abutment tooth movement. J Prosthet Dent 1972;27:160–167.

Cecconi BT, Asgar K, Dootz E. The effect of partial denture clasp design on abutment tooth movement. J Prosthet Dent 1971;25:44–56. Chandler JA, Brudvik JS. Clinical evaluation of patients eight to nine years after placement of removable partial dentures. J Prosthet Dent 1984;51: 736–743. Chou TM, Caputo AA, Moore DJ, Xiao B. Photoelastic analysis and comparison of force-transmission characteristics of intracoronal attachments with clasp distal-extension removable partial dentures. J Prosthet Dent 1989;62:313–319. Clayton JA. A stable base precision attachment removable partial denture (PARPD): Theories and principles. Dent Clin North Am 1980;24:3–29. Clayton JA, Jaslow C. A measurement of clasp forces on teeth. J Prosthet Dent 1971;25:21–43. Demer WJ. An analysis of mesial rest-I-bar clasp designs. J Prosthet Dent 1976;36:243–253. DeVan MM. Preserving natural teeth through the use of clasps. J Prosthet Dent 1955;5:208–214. Eliason CM. RPA clasp design for distal-extension removable partial dentures. J Prosthet Dent 1983;49:25–27. Fisher RL, Jaslow C. The efficiency of an indirect retainer. J Prosthet Dent 1975;34:24–30. Frank RP. Direct retainers for distal-extension removable partial dentures. J Prosthet Dent 1986;56:562–567. Frank RP, Brudvik JS, Nicholls JI. A comparison of the flexibility of wrought wire and cast circumferential clasps. J Prosthet Dent 1983;49:471–476. Frank RP, Nicholls JI. An investigation of the effectiveness of indirect retainers. J Prosthet Dent 1977;38:494–506. Frank RP, Nicholls JI. A study of the flexibility of wrought wire clasps. J Prosthet Dent 1981;45:259–267. Harrop L, Javid N. Reciprocal arms of direct retainers in removable partial dentures. Dent J 1976;42:208–211. Hebel KS, Graser GN, Featherstone JD. Abrasion of enamel and composite resin by removable partial denture clasps. J Prosthet Dent 1984;52: 389–397.

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Johnson DL, Stratton RJ, Duncanson MG Jr. The effect of single plane curvature on half-round cast clasps. J Dent Res 1983;62:833–836. Kapur KK, Deupree R, Dent RJ, Hasse AL. A randomized clinical trial of two basic removable partial denture designs. Part I: Comparisons of five-year success rates and periodontal health. J Prosthet Dent 1994;72:268–282. Kapur KK, Garrett NR, Dent RJ, Hasse AL. A randomized clinical trial of two basic removable partial denture designs. Part II: Comparisons of masticatory scores. J Prosthet Dent 1997;78:15–21. Kotowicz WE, Fisher RL, Reed RA, Jaslow C. The combination clasp and the distal extension removable partial denture. Dent Clin North Am 1973;17: 651–660. Kratochvil FJ. Influence of occlusal rest position and clasp design on movement of abutment teeth. J Prosthet Dent 1963;13:114–124. Kratochvil FJ, Davidson PN, Guijt J. Five-year survey of treatment with removable partial dentures. Part I. J Prosthet Dent 1982;48:237–244. Krol AJ. Clasp design for extension-base removable partial dentures. J Prosthet Dent 1973;29:408–415. Krol AJ. RPI (rest, proximal plate, I bar) clasp retainer and its modifications. Dent Clin North Am 1973;17:631–649. Krug RS. Metal framework modifications to accommodate wrought wire clasps in distal extension removable partial dentures. J Prosthet Dent 2003;89:79–81. Langer A. Combinations of diverse retainers in removable partial dentures. J Prosthet Dent 1978;40:378–384. LaVere AM. Analysis of facial surface undercuts to determine use of RPI or RPA clasps. J Prosthet Dent 1986;56:741–743. Marei MK. Measurement (in vitro) of the amount of force required to dislodge specific clasps from different depths of undercut. J Prosthet Dent 1995; 74:258–263.

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Maroso DJ, Schmidt JR, Blustein R. A preliminary study of wear of porcelain when subjected to functional movements of retentive clasp arms. J Prosthet Dent 1981;45:14–17. Matheson GR, Brudvik JS, Nicholls JI. Behavior of wrought wire clasps after repeated permanent deformation. J Prosthet Dent 1986;55:226–231. Morris HF, Asgar K, Brudvik JS, Winkler S, Roberts EP. Stress-relaxation testing. Part IV: Clasp pattern dimensions and their influence on clasp behavior. J Prosthet Dent 1983;50:319–326. Morris HF, Asgar K, Roberts EP, Brudvik JS. Stress-relaxation testing. Part II: Comparison of bending profiles, microstructures, microhardnesses, and surface characteristics of several wrought wires. J Prosthet Dent 1981; 46:256–262. Snyder HA, Duncanson MG Jr, Johnson DL, Bloom J. Effects of clasp flexure on a 4-META adhered light-polymerized composite resin. Int J Prosthodont 1991;4:364–370. Spielberger MC, Lubow RM, Bange AA, Mayhew RB. Effect of retentive arm clasp design on gingival health: A feasibility study. J Prosthet Dent 1984; 52:397–401. Stern MA, Brudvik JS, Frank RP. Clinical evaluation of removable partial denture rest seat adaptation. J Prosthet Dent 1985;53:658–662. Stone ER. Tripping action of bar clasps. J Am Dent Assoc 1936;23:596–617. Tietge JD, Dixon DL, Breeding LC, Leary JM, Aquilino SA. In vitro investigation of the wear of resin composite materials and cast direct retainers during removable partial denture placement and removal. Int J Prosthodont 1992;5:145–153. Vig RG. Splinting bars and maxillary indirect retainers for removable partial dentures. J Prosthet Dent 1963;13:125–129. Waldmeier MD, Grasso JE, Norberg GJ, Nowak MD. Bend testing of wrought wire removable partial denture alloys. J Prosthet Dent 1996;76:559–565.

Chapter 6 Types of RPDs, Biomechanics, and Design Principles Ting-Ling Chang | Daniela Orellana | John Beumer III

Professor Kratochvil brought a unique approach to removable partial denture (RPD) design philosophy, one based on biomechanics as opposed to retention. His approach to partial denture design was driven by three factors: 1. The forces of greatest concern were occlusal forces. 2. The forces delivered to the abutment teeth should be controlled through the rest. 3. The forces delivered to the abutment teeth are best tolerated when they are delivered along the long axis of the teeth. He believed that the classification system used for partial edentulism should be simple. He argued that there were only two basic types of RPDs: tooth-borne and extension. Kratochvil focused most of his research on creating biomechanically ideal designs for extension-base RPDs.1–3

Tooth-Borne Partial Dentures In tooth-borne RPDs, the forces in function are borne primarily by the remaining natural teeth, which transmit these forces to the periodontal ligament and to the bone (Figs 6-1 and 6-2). This type of RPD is virtually synonymous with a fixed dental prosthesis (FDP) if basic RPD design concepts are employed. For the tooth-borne prosthesis, the rests can be placed in any

position as long as they deliver occlusal forces along the long axis of the tooth. This requires positioning the rest in the center of the tooth or on both sides of the tooth. In addition, the partial denture should incorporate the necessary bracing in the design to distribute lateral forces appropriately.

Extension Partial Dentures The extension RPD presents a unique situation in that it relies on two entirely different means of support. In one instance, the support is derived through the tooth from the periodontal ligament and bone; the other source of support is the mucosa that covers the bone of the extension areas (Fig 6-3). However, oral mucosa is not designed to act as a direct support tissue or to withstand the forces of occlusion. Its primary function is to cover the bone and the cervical area of the tooth while supplying necessary nourishment and acting as a protective mechanism for the tooth and surrounding structures. The oral mucosa is not structured to withstand a compressive force. When a denture base is placed over the mucosa and an occlusal force is applied, the mucosa is compressed between the hard surface of the denture base and the surface of the bone, and blood flow is inhibited (Fig 6-4a). If the nutrient supply to the bone and periosteum is compromised, a resorption-remodeling response may be triggered. 47

6

Types of RPDs, Biomechanics, and Design Principles

a

b

Fig 6-1 In a tooth-borne partial denture, the oc- Fig 6-2 (a) Maxillary tooth-borne situation: Support will be provided by existing teeth. (b) Mandibular clusal forces are borne by the abutment teeth. tooth-borne situation: The prosthesis will restore function within the confines of the remaining teeth. Rests should be located so as to direct these forces axially.

One factor that dentists and patients must accept is that the extension RPD, with its combination of tooth and mucosal support, moves or rotates during function. This occurs because the mucosal support allows more movement of the prosthesis during function than the tooth-supported portion of the prosthesis. This factor must be fully understood and analyzed when planning the RPD design. The amount of movement that will occur depends on several factors: • The surface area of mucosal support • The type and thickness of mucosa in the extension area • How effectively the edentulous area is used (ie, accuracy of the impression and the adaptation of the denture base to the tissues of the edentulous extension area) • Refinement of the occlusal factors during the delivery appointment • Physiologic response of the individual patient, which in turn is dependent upon many biologic factors The biologic consequences may be devastating if the denture base of the extension RPD is underextended and not properly adapted. During mastication or parafunction (clenching and bruxing), the mucoperiosteum is compressed, and a resorptive remodeling response may be provoked (Fig 6-4b), leading to resorption of the underlying alveolar bone. This phenomenon will be accelerated by the presence of natural dentition in the opposing arch. Therefore, the coverage of the edentulous area 48

must be maximized with fully extended impressions. This is best achieved by utilizing an altered cast impression (see chapter 12), making fully extended impressions with a custom tray, or relining the RPD at delivery. When planning the extension RPD, the first point to consider is the location of the axis of rotation (fulcrum line). This is determined by the position of the rests, specifically the part of the rest that is closest to the edentulous area (Fig 6-5; see also Fig 6-9). The most significant movement to consider is the movement that results during function rather than the movement that occurs during removal. The two types of RPDs (tooth-borne and extension) can also be found in patients who have lost anterior teeth. The RPD is considered tooth borne when it replaces central and lateral incisors with the canines still present (Fig 6-6a). If the tooth loss extends to the canine region, it is considered an anterior extension-base RPD (Fig 6-6b). The principles used to create biomechanically ideal designs for anterior extension-base RPDs are the same as those described for the distal extension-base RPDs. Kratochvil’s great contribution to the field of prosthodontics was his approach to partial denture design for extension RPDs. He carefully considered the movement patterns of the prosthesis during occlusal function and developed a system that accommodated this movement while simultaneously directing occlusal forces along the long axis of the abutment teeth. He believed that when an RPD was properly designed and fabricated, the

Extension Partial Dentures

a

b

c

Fig 6-3  (a) Unilateral posterior extension: A combination of teeth and mucosa will provide support. (b) Mandibular bilateral posterior extension: The RPD will utilize a combination of teeth and the edentulous area for support. (c) Anterior RPD: Support is shared between the dentition and the anterior extension.

Fig 6-4  (a) Mandibular posterior extension RPD: Mucosa provides denture support between bone and denture base. This is a most unnatural situation and tends to restrict blood flow to the mucosa. (b) Excessive movement of the denture base during mastication or parafunction is destructive to the underlying bone and soft tissue if the denture base fails to cover the buccal shelf and retromolar pad.

Prosthesis Mucosa Bone

a

b

Axis of rotation

a Fig 6-5  Mandibular unilateral extension RPD with rests adjacent to the edentulous area. Note the axis of rotation (black line).

b

Fig 6-6  (a) When the replacement of anterior teeth is no more than incisors, the RPD is considered a tooth-borne RPD. (b) If the replacement of missing teeth involves canine to canine or more teeth, the RPD is considered an anterior extension-base RPD.

life of the abutment teeth would be maximized if the patient was compliant, and the alveolar ridges of the extension areas would suffer little or no resorption. Kratochvil recognized that during occlusal function, all extension partial dentures were compressed into the edentulous denture foundation area. This movement occurred around the axis of rotation, and RPD designs that impaired or restricted this movement placed the abutment teeth at risk because these

teeth were exposed to off-axis loads. These loads were often magnified because of the long lever arms associated with the prosthesis. His designs, particularly the position of the rests in relation to the extension area, the type of retainer, and the exact location where the retainer engages the tooth surface of the abutments, permit free and unencumbered rotation of the partial denture around the fulcrum line (axis of rotation) during occlusal function and direct occlusal forces along the long axis 49

6

Types of RPDs, Biomechanics, and Design Principles

Fig 6-7 Occlusal force applied. All parts of the prosthesis on the edentulous side of the rotation axis move toward the mucosa in an arc around the axis. All parts of the prosthesis on the tooth-borne side of the axis move in an arc away from the occlusal surface of the teeth.

Fig 6-8 A view looking directly down the rotation axis, diagramming the movement of the parts of the RPD in function.

Fig 6-9 Maxillary posterior extension RPD. The actual rotation axis is a half-sphere to allow pure rotation. There should be no inclines that could cause sliding action in function.

of the abutments, limiting the bending moments (off-axis loads) applied to the abutments. Kratochvil’s principles of RPD design can be summarized as follows: • Occlusal rests must be positive and direct occlusal forces along the long axis of the teeth. • Extension RPD designs must anticipate and accommodate the movements of the prosthesis during function without exerting pathologic stresses on the abutment teeth. • Major connectors must be rigid. • Guide planes are employed to enhance stability and bracing. • Retention must be within the limits of physiologic tolerance of the periodontal ligament of the abutments. 50

• Maximum support is gained from the adjacent soft tissue denture foundation area. • Designs must consider the needs of cleansibility. These objectives can be accomplished with (1) the RPI system in combination with physiologic adjustment of RPD frameworks and (2) altered cast impressions to maximize the support in the edentulous extension area. When planning an RPD, the first point to consider is the location of the axis of rotation. This axis is determined by the position of the rests, specifically the rests closest to the edentulous areas (Fig 6-7). All parts of the partial denture on the edentulous side move in an arc or circle in the direction of the mucosa when an occlusal force is applied in the edentulous extension area.

Positioning the Axis of Rotation to Idealize Support in the Edentulous Area

a Fig 6-10  (a) The best edentulous area support is provided when forces are directed at right angles to mucosa and bone and in the long axis of the tooth. (b) Locating the rest and the rotation axis on the distal aspect of this abutment tooth results in more horizontal movement of the denture base, especially in the area adjacent to the tooth. (c) Positioning the rest and the rotation point anteriorly results in a more vertical direction of the force on the edentulous area. (d) Anterior positioning of the rest and the rotation axis results in more advantageous support from the denture base area. (e) Lowering of the rest and the rotation axis results in the most ideal condition of denture base support.

b

c

d

e

All parts of the partial denture on the tooth-borne side tend to move away from the occlusal surfaces of the teeth when an occlusal force is applied to the edentulous extension area. Once the axis of rotation is established, it is possible to determine the direction of movement of all parts of the prosthesis in function under occlusal load. When looking down the axis of rotation, an imaginary set of circles can be visualized extending outward from the axis, and the movement of all parts of the prosthesis can be visualized under function (Fig 6-8). Determination of the direction of movement is of paramount importance in placement of guide planes and retainers so that during function they do not torque, bind, or elevate the abutment teeth or other teeth in the arch. In some instances, these points of interference can become detrimental rotation points when the designs are incorrect and/or the RPD castings are not properly physiologically adjusted. Because the portion of the rest closest to the edentulous area becomes the axis of rotation, the rest preparation on the tooth at that point should be a perfect half-sphere to enable ideal rotation around the fulcrum line (Fig 6-9). This rotation area should be about the size of a #6 or #8 round bur and should be highly polished.

Positioning the Axis of Rotation to Idealize Support in the Edentulous Area A prime objective when planning treatment for a partially edentulous arch is to utilize all remaining tissues to their greatest potential. The edentulous area provides the best support when forces are directed at right angles to the surface of the mucosa and bone (Fig 6-10a). Placement of the rest in relation to the edentulous area in an extension situation determines the rotation axis, which in turn determines the direction of movement of the prosthesis against the mucosa and bone. Placement of the rest on the occlusal surface of the abutment tooth next to the edentulous area results in horizontal movement of the prosthesis at the gingival portion of the tooth (Fig 6-10b). Little or no vertical support is obtained from the tissue next to the abutment tooth. This movement is at an angle to the supporting tissues and tends to compress the mucosa between the bone and denture base (see Fig 3-3). As the rotation axis is moved anteriorly or away from the edentulous area, the arc of rotation becomes greater, and the forces are delivered in a more vertical direction in the edentulous extension area (Figs 6-10c and 6-10d). Conceptually, lowering of the axis of rotation toward the gingiva also improves the direction of force, and therefore it is advisable to lower the rotation point whenever possible (Fig 6-10e). 51

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Types of RPDs, Biomechanics, and Design Principles

a

b

Fig 6-11 Placement of the rest adjacent to the Fig 6-12 (a and b) The distal rest in combination with the Akers clasp has tipped the premolar to the edentulous extension area may produce a tipping distal, opening a diastema (arrows) between the canine and the premolar. (Courtesy of Dr R. Duell, Los force during function, which opens the contact be- Angeles, California.) tween the abutment tooth and the adjacent tooth. Fig 6-13 Moving the rest away from the edentulous area produces a direction of force that tends to maintain contact with adjacent teeth, resulting in multiple-tooth support and acceptable directions of force application.

Direct Effect of Rest Position on Abutment Teeth Placement of a rest immediately adjacent to the edentulous area of an extension RPD tends to pull or tip the tooth toward the edentulous extension area (Fig 6-11). This has the effect of placing a wrench on the tooth and producing a tipping force. Clinically, this is manifested by the development of a diastema (space) between the abutment tooth and the tooth adjacent (Fig 6-12). Placement of the rest on the surface of the tooth away from the edentulous area produces a tipping force in the opposite direction (Fig 6-13). The identical amount of force from occlusal function is delivered to the tooth-rest area; however, the direction of force tends to move the abutment tooth toward the remaining teeth, tightening the interproximal contact and utilizing adjacent teeth beside the single abutment tooth to help control the position of the teeth and the prosthesis. The wrench effect is reversed, and the direction of force is more favorable. If the abutment tooth contact is opened and the tooth moves to a different position, the prosthesis moves with the tooth. This 52

bodily movement disrupts the occlusion developed at the time of insertion and produces occlusal interferences, which in turn cause excessive force to be placed on the remaining teeth and the edentulous area.

Design and Positioning of the Retainer Movement of retainers on an edentulous extension RPD in function is determined by the rotation axis (Fig 6-14). Directional movement of the retainers must be controlled so that contact is diminished or eliminated when the RPD moves from a passive position to a functional position. Ideally, the engagement of the retainer should occur only in a removal action. For example, given a mandibular posterior extension situation (Fig 6-15) in which the rotation axis is on the mesial occlusal surface of the abutment tooth, the retentive tip of the I-bar is placed at the point of greatest mesiodistal curvature of the premolar on the facial surface (Fig 6-16). This permits downward and forward

Design and Positioning of the Retainer

B Axis

A

Fig 6-14  The direction of retainer movement is determined by the location of the rotational axis. Note the direction of movement on both sides of the rotation axis at points A and B.

Fig 6-15  Anterior positioning of the rest results in forward and downward movement of the retainer during occlusal force, preventing torque.

Fig 6-16 Occlusal and facial view. The retainer contact is placed at the point of greatest mesiodistal curvature of the tooth. The retainer will then disengage under the force of occlusion.

Fig 6-17 Moving the rotation axis forward provides quicker retainer disengagement and diminishes the possibility of tooth torque caused by the retainer.

Fig 6-18  Lowering the rotation axis produces the most ideal retainer movement.

Fig 6-19  Occlusal view. The retainer must not be placed behind the greatest curvature of the tooth in an extension situation because it could torque the tooth as the retainer moves forward in function. In addition, retention will be suboptimal.

movement of the tip of the retainer under occlusal load, allowing it to disengage from the tooth (see Fig 6-15). Conceptually, as the rest and the rotation axis are moved farther away from the extension area, the retainer tip disengages more rapidly during occlusal function (Figs 6-17 and 6-18). Note that it is not advisable to position the retentive tip of the retainer distal to the

center of the tooth or distal to the point of greatest mesiodistal curvature of the tooth because any forward movement of the retainer may torque the tooth and harm the supporting structures (Fig 6-19). Moreover, the retention will be compromised if the retainer is placed in this position.

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A

Fig 6-20 If the rotation axis or the rest is placed between the retainer and the extension denture base, the retainer moves forward and upward, engaging and torquing the tooth when the patient applies occlusal force.

Fig 6-21 All parts of the prosthesis forward of the rotation axis lift off the teeth when occlusal force is applied to the extension area (A). If a retainer is placed on the canine, it should not engage the undercut, or it will tend to lift the canine during function, which may compromise the health of its periodontium. Fig 6-22 Rest position on the mesial or distal location controls the rotation axis, which has a profound influence on the direction of movement of the tip of the retainer.

Retainer positioned forward of the axis of rotation The retentive portion of a retainer should not engage an undercut forward to the axis of rotation in an extension situation because that part of the prosthesis moves upward in function (Fig 6-20) and tends to torque the tooth. The periodontal ligament acts as a sling and exerts a pull on the bone when loaded occlusally. A lifting action on the tooth is abnormal and may trigger periodontal problems (Fig 6-21). If a retainer is placed anterior to the axis of rotation, it is placed at the height of contour of the tooth without engaging the undercut area. This will allow the retainer to disengage during function. In the example shown in Fig 6-21, a retainer placed on the canine serves two purposes: (1) The retainer enhances stability, and (2) contact with the tooth provides some retention because of parallelism and friction between the lingual rest and the retainer (such as the retention of a full gold crown provided by parallel preparation). Moreover, if there is a possibility of 54

posterior abutment tooth loss, a replacement tooth can be added to the prosthesis as an immediate procedure at the time of the loss. The retainer can then be altered and recontoured or replaced to engage an undercut area to provide the necessary retention. Attention must be given to the position and placement of the retainers on the opposite side of the arch from the extension side. Retainers on all teeth in all parts of the mouth move according to the basic laws of physics and must be designed to prevent tooth engagement, except against removal activity. In Fig 6-14, the lingual retainer on the molar engages a tooth undercut, but during occlusal function, it moves downward and forward, deeper into the undercut, disengaging from the tooth. During a removal motion, it engages and provides retention. Rest position directly influences the direction of movement of the retentive portion of the retainer. Moving the rest position from a distal to a mesial location on one tooth can change the direction of movement of the retainer 180 degrees (Fig 6-22). Retainer position must always be coordinated with rest position so that the retainer disengages from tooth contact during forceful closure.

Movement of Guiding Surfaces in Function

Fig 6-23  The proximal plates of the casting are designed to contact the surfaces of the teeth and cover the tooth-tissue junction.

a

Fig 6-24  Lingual view of a mandibular posterior extension prosthesis. The proximal plate and the minor connector between the premolars could bind and torque the teeth as the prosthesis moves in function.

b

Fig 6-25  (a) In function, the proximal plate could engage the guide plane, torquing the tooth. (b) Lowering the rotation axis greatly reduces the potential for binding of the distal guiding surface.

Movement of Guiding Surfaces in Function Other portions of the extension prosthesis are affected by functional movements around the axis of rotation. Of particular concern are the proximal plates, which contact the surface of the teeth next to the edentulous area (Fig 6-23), and the minor connectors, which extend from the major connectors to the rests (Fig 6-24). The rationale for coverage of the tooth-tissue junction with the metal casting has been presented previously (see chapter 3). However, it must be noted that when the RPD moves in function, these proximal plates may bind (Fig 6-25). To prevent

this binding of the prosthesis, it is necessary to physiologically adjust the casting in the mouth (see chapter 12). Each tooth moves slightly differently during function depending on its root configuration. It is not possible on a stone cast to engineer or design an RPD framework that allows for all teeth idiosyncrasies and functional movements, thus the need for physiologic adjustment. Extension-base RPD designs must anticipate and accommodate the movements of the prosthesis during function without exerting pathologic stresses on the abutment teeth. The physiologic adjustment process is used to ensure that the RPD framework rotates around the axis of rotation during function in a fashion consistent with the design. This process is covered in detail in chapter 12.

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Fig 6-26 The casting is designed so as not to engage the lingual tooth surface at point A. The minor connector (B) is placed parallel to the long axis of the tooth until it reaches unattached mucosa before continuing anteriorly or posteriorly as the lingual bar.

A

B

Diagonal Placement of the Axis of Rotation When analyzing movement patterns of retainers, proximal plates, and minor connectors while designing an RPD framework, most analysis is done in two planes. However, function in three planes should be considered. In cases when the rest on one abutment is close to the gingiva (eg, a cingulum rest on the canine) and the rest on the other abutment is higher on the occlusal surface, a high-low axis of rotation is the result. As a result, the direction of movement of the various parts of the RPD framework is more difficult to ascertain. Also, when the axis of rotation is anterior on one side and posterior on the opposite side, the direction of movement of the various parts of the metal framework is more difficult to analyze. Therefore, final assessment is made intraorally when the RPD framework is physiologically adjusted (see chapter 12). A special effort should be made to prevent any portion of the metal casting that might be contacting an inclined tooth surface from becoming a rotation point. This will affect the rotation axis and as a result affect the direction of movement of all parts of the partial denture. Under these circumstances, teeth may move out of position, disrupting the occlusion and perhaps compromising their periodontal apparatus.

Lingual Design Considerations for the Extension RPD The effects of RPD movement on teeth have been discussed as they relate to tooth contact on facial, mesial, and distal surfaces. The effect of such movement on the lingual aspect of the RPD also needs to be considered. 56

Care should be taken to avoid inappropriate contact of the RPD casting in function against the lingual surfaces of the remaining teeth. In ideal anatomical configurations, the casting does not contact the lingual surfaces of the posterior teeth. The minor connectors are exceptions; they should cross the tooth-tissue junction at right angles to the occlusal plane or along the long axis of the tooth to minimize food impaction between the prosthesis and the tissues and to idealize self-cleansing action of the tongue. The minor connectors should also be extended to the mucogingival junction, where they turn to follow the crest of the unattached mucosa, continuing anteriorly or posteriorly to unite with the major connector (Fig 6-26). With this design, the potential for teeth becoming torqued from the lingual aspect is minimized, and tooth-tissue junctions are kept open. In the mandible, the distance between the gingival margin and the major connector should be at least 3 mm. The mesiodistal dimension of this space should be maximized to minimize food impaction and plaque accumulation.

Biomechanics of Other Retainer Designs The most commonly used retainer in the past was the suprabulge retainer (Fig 6-27) attached to the RPD casting at the middle third of the tooth (see chapter 5). This design can be successfully employed if the rest is placed on the side of the abutment tooth away from the extension area. The important point to recognize regarding movement of the retainers during function is that the prosthesis moves as a unit. The basic factor determining the direction of the movement of the retainers is the relationship of the retainers and the axis of rotation, which is determined by the position of the rests.

Biomechanics of Other Retainer Designs

Fig 6-27  Mandibular extension RPD with a distal rest and a circumferential retainer. If the tip of the retainer engages the tooth undercut and an occlusal force is delivered posteriorly, the tooth may be torqued.

Fig 6-28  The RPA system. The rest is placed on the mesial side of the tooth (on the side away from the extension), and the suprabulge portion of the retainer is relieved.

a Fig 6-30  An RPA design. Note that the suprabulge portion of the retainer has been relieved. When properly relieved, a mylar strip can easily be pulled through the interface between the abutment and the retainer above the height of contour. (Courtesy of Dr T. Berg, Los Angeles, California.)

Fig 6-29  Anterior rest and rotation axis. The distal portion of the retainer moving forward may engage and torque the tooth unless the area above the height of contour is carefully relieved.

b

Fig 6-31  (a and b) Placing the rest on the mesial side will not eliminate the potential for torquing action of the distal portion of the T-shaped retainer as it engages the curvature of the tooth.

Suprabulge circumferential retainer (RPA concept) The circumferential retainer is commonly used to provide retention for an RPD. The retentive portion is the terminal end of the retainer, which engages the undercut on the tooth. If this retainer is used in combination with a rest placed on the distal side of the abutment tooth adjacent to a posterior extension area or on the mesial side of the abutment tooth adjacent to an anterior extension area, the tooth will be torqued significantly toward the extension area during occlusal function3 (see Fig 6-27). The authors believe it is inadvisable to employ this design in edentulous extension-base RPDs. Moving the rotation axis to the mesial aspect of the tooth changes the direction of movement of the tip of the retainer that is engaging the undercut more favorably (Fig 6-28). Moving the axis of rotation further forward (Fig 6-29) again changes the direction of retainer movement. However, the potential for torque is present because the distal portion of the retainer above the height of contour curves around the tooth, as seen when observed from an occlusal view. Any contact of the cast-

ing with the tooth on these surfaces tends to rotate the tooth. Therefore, if this type of retainer is employed, the portion of the retainer above the height of contour must be relieved. This can be accomplished during physiologic adjustment or can be incorporated into the design by the laboratory technician (socalled “RPA concept”) (Fig 6-30).

Infrabulge circumferential retainers It is difficult to design and place an infrabulge circumferential retainer in such a way that it disengages during application of an occlusal force in the extension area, so their use is not recommended. As stated previously, the direction of the movement of the retainer is controlled by the position of the axis of rotation. If the retainer wraps around the curvature of the tooth, the potential exists for applying an inappropriate torquing force to the abutment tooth during application of an occlusal force in the extension area. With this type of retainer, this is apparent even if the rest is moved to the side of the tooth away from the edentulous extension area (Fig 6-31).

57

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Types of RPDs, Biomechanics, and Design Principles

a

b

Wrought Wire Retainers Brudvik4 has been the principal proponent of the use of wrought wire retainers in edentulous extension situations. This type of retainer is very flexible, and therefore more undercut needs to be engaged (0.02 inch). It minimizes the torquing of the abutment when an occlusal force is delivered in the edentulous extension area. Similar to Kratochvil, he suggests that rests be positioned on the abutment on the side opposite from the extension area—on the mesial when restoring a posterior extension area and on the distal when restoring an anterior extension area. Similar to Kratochvil, he recommends maximizing the support provided by the edentulous extension area by making altered cast impressions (see chapter 12), and finally he also recommends that partial denture frameworks be physiologically adjusted to allow the prosthesis to rotate freely around the axis of rotation without binding on the abutments. This type of retainer is of particular value when retention is deemed necessary anterior to the fulcrum line in a patient to be fitted with a posterior extension-base RPD (Fig 6-32). A wrought wire retainer is also advised when extension-base RPDs cannot be retained with I-bar retainers because of excessive soft tissue undercuts and the presence of frena. However, these types of retainers are difficult to properly adapt to the facial surfaces of the abutment tooth, and consistently good results are often difficult to obtain from the dental laboratory. 58

Fig 6-32 Wrought wire retainers have been used on the left premolar and the right canine. (a) Occlusal view of the RPD. (b) Note the I-bar retainer and proximal plate engaging the premolar. (c) Note the wrought wire retainer engaging the left premolar.

c

Recommended Retainer Design and Rest Position When considering the factors involved in the movement of the extension RPD, the authors recommend the following approach to retainer design and rest position: • The axis of rotation should be positioned away from the edentulous extension area and as close to the bone as possible. • The I-bar infrabulge retainer is favored and should be positioned to disengage from the surface of the tooth when an occlusal force is applied to the edentulous extension area, and engage only against removal. • If the RPA system is employed, the suprabulge contact of the retainer should be eliminated. • If a wrought wire retainer is used, rests should be placed away from the extension area and the framework should be physiologically adjusted. • The proximal plates maintain positional contact with the tooth but should not engage or bind on the abutment tooth in function.

Suggested Reading

Summary of RPD Design Principles It is acknowledged that there is always more than one way to design an RPD for a patient. A well-designed partial denture must be biomechanically sound to protect and preserve the remaining hard and soft tissue structures. It is recommended to follow these seven design principles to attain long-term successful RPD therapy: 1. Extension-base RPD designs must anticipate and accommodate the movements of the prosthesis during function without exerting pathologic stresses on the abutment teeth. 2. Major connectors must be rigid. 3. Occlusal rests must direct occlusal forces along the long axis of the teeth. 4. Guide planes are employed to enhance stability and bracing. 5. Retention must be within the limits of physiologic tolerance of the periodontal ligament. 6. Maximum support is gained from the adjacent soft tissue denture-bearing surfaces. 7. Designs must consider the needs of cleansibility.

References 1. Kratochvil FJ. The influence of occlusal rest position and clasp design on movement of abutment teeth. J Prosthet Dent 1963;13:114–124. 2. Kratochvil FJ, Caputo AA. Photoelastic analysis of pressure on teeth and bone supporting removable partial dentures. J Prosthet Dent 1974;32:52–61. 3. Thompson WD, Kratochvil FJ, Caputo AA. Evaluation of photoelastic stress patterns produced by various designs of bilateral distal-extension removable partial dentures. J Prosthet Dent 1977;38:261–273. 4. Brudvik JS. Advanced Removable Partial Dentures. Chicago: Quintessence, 1999.

Suggested Reading Avant WE. Factors that influence retention of removable partial dentures. J Prosthet Dent 1971;25:265–270. Avant WE. Fulcrum and retention lines in planning removable partial dentures. J Prosthet Dent 1971;25:162–166. Ben-Ur Z, Gorfil C, Shifman A. Designing clasps for the asymmetric distal extension removable partial denture. Int J Prosthodont 1996;9:374–378. Berg T, Caputo AA. Comparison of load transfer by maxillary distal extension removable partial dentures with a spring-loaded plunger attachment and I-bar retainer. J Prosthet Dent 1992;68:492–499. Browning JD, Eick JD, McGarrah HE. Abutment tooth movement measured in vivo by using stereophotogrammetry. J Prosthet Dent 1987;57:323–328. Brudvik JS, Morris HF. Stress-relaxation testing. Part III: Influence of wire alloys, gauges, and lengths on clasp behavior. J Prosthet Dent 1981;46:374–379. Cecconi BT. Effect of rest design on transmission of forces to abutment teeth. J Prosthet Dent 1974;32:141–151.

Cecconi BT, Asgar K, Dootz E. Removable partial denture abutment tooth movement as affected by inclination of residual ridges and types of loading. J Prosthet Dent 1971;25:375–381. Cecconi BT, Asgar K, Dootz E. The effect of partial denture clasp design on abutment tooth movement. J Prosthet Dent 1971;25:44–56. Craig RG, Farah JW. Stresses from loading distal-extension removable partial dentures. J Prosthet Dent 1978;39:274–277. DeVan MM. The nature of the partial denture foundation: Suggestions for its preservation. J Prosthet Dent 1952;2:210–218. Eliason CM. RPA clasp design for distal-extension removable partial dentures. J Prosthet Dent 1983;49:25–27. Fisher RL. Factors that influence the base stability of mandibular distalextension removable partial dentures: A longitudinal study. J Prosthet Dent 1983;50:167–171. Goodkind RJ. The effects of removable partial dentures on abutment tooth mobility: A clinical study. J Prosthet Dent 1973;30:139–146. Kratochvil FJ. Maintaining supporting structures with a removable partial prosthesis. J Prosthet Dent 1971;25:167–174. Kratochvil FJ, Thompson WD, Caputo AA. Photoelastic analysis of stress patterns on teeth and bone with attachment retainers for removable partial dentures. J Prosthet Dent 1981;46:21–28. Leupold RJ, Kratochvil FJ. An altered-cast procedure to improve tissue support for removable partial dentures. J Prosthet Dent 1965;15:672–678. Lytle RB. Soft tissue displacement beneath removable partial and complete dentures. J Prosthet Dent 1962;12:34–43. MacGregor AR, Miller TP, Farah JW. Stress analysis of partial dentures. J Dent 1978;6:125–132. Maxfield JB, Nicholls JI, Smith DE. The measurement of forces transmitted to abutment teeth of removable partial dentures. J Prosthet Dent 1979;41: 134–142. Ogata K, Shimizu K. Longitudinal study on forces transmitted from denture base to retainers of lower free-end saddle dentures with Aker’s clasps. J Oral Rehabil 1991;18:471–480. Plotnick IJ, Beresin VE, Simkins AB. The effects of variations in the opposing dentition on changes in the partially edentulous mandible. Part I. Bone changes observed in serial radiographs. J Prosthet Dent 1975;33:278–286. Plotnick IJ, Beresin VE, Simkins AB. The effects of variations in the opposing dentition on changes in the partially edentulous mandible. Part II. Densitometric measurements. J Prosthet Dent 1975;33:403–406. Plotnick IJ, Beresin VE, Simkins AB. The effects of variations in the opposing dentition on changes in the partially edentulous mandible. Part III. Tooth mobility and chewing efficiency with various maxillary dentitions. J Prosthet Dent 1975;33:529–534. Stratton RJ, Weibelt FJ. An Atlas of Removable Partial Denture Design. Chicago: Quintessence, 1988. Taylor DT, Pflughoeft FA, McGivney GP. Effect of two clasping assemblies on arch integrity as modified by base adaptation. J Prosthet Dent 1982;47: 120–125. Tebrock OC, Rohen RM, Fenster RK, Pelleu GB Jr. The effect of various clasping systems on the mobility of abutment teeth for distal-extension removable partial dentures. J Prosthet Dent 1979;41:511–516. Thomspon WD, Kratochvil FJ, Caputo AA. Evaluation of photoelastic stress patterns produced by various designs of bilateral distal-extension removable partial dentures. J Prosthet Dent 1977;38:261–273. White JT. Visualization of stress and strain related to removable partial denture abutments. J Prosthet Dent 1978;40:143–151. Wills DJ, Manderson RD. Biomechanical aspects of the support of partial dentures. J Dent 1977;5:310–318. Wright KW, Yettram AL. Reactive force distributions for teeth when loaded singly and when used as fixed partial denture abutments. J Prosthet Dent 1979;42:411–416.

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Chapter 7 Partial Denture Design Principles and Design Sequence Ting-Ling Chang | Daniela Orellana

The basic philosophy of treatment is to plan the best partial denture design for a given arch and to prepare the mouth for an ideal prosthesis. A patient presenting with missing tissues requires preparation and specific treatment. Periodontal disease and unusual configurations of soft tissue and tooth positions can jeopardize long-term success. It is best to resist compromising basic design principles whenever possible by performing proper mouth preparation. It is necessary to have an organized, orderly, and definite design sequence when designing the removable partial denture (RPD) casting. To this end, the elements providing support (rests) that control movement of the partial denture are placed first because their design and position control placement and affect the design of other parts of the prosthesis. The design sequence is accomplished in this order: 1. Rests 2. Major connectors 3. Minor connectors 4. Denture base connectors 5. Retainers The rationale and principles for use and design of these five parts of the casting have been described and discussed in previous chapters. The design is now outlined on the diagnostic cast, which is a three-dimensional replica of the oral condition of the patient to be treated. The design should be precise and detailed. It will serve as a treatment plan for mouth preparation as well as for communication with other dental specialists involved in dental treatment, including dental laboratory technicians. It will also serve as a patient record for future reference.

Maxillary Design Sequence Occlusal rests The extension, exact position, and width of each rest is precisely and clearly outlined on each tooth that will be engaged by a rest (Fig 7-1). A molar rest on the tooth-borne side should extend at least to the center of the tooth to ensure that occlusal forces are directed along the long axis of the tooth. Further extension of the rest across the tooth is determined by the need to restore the occlusal plane and/or the occlusion. Diagnostic casts mounted with appropriate maxillomandibular records are necessary to determine proper rest design and position. The rest on the premolar on the extension side is placed on the mesial side of the tooth because of the rotational factors described and discussed in chapter 6. A rounded, ball-andsocket–type rest seat is prepared to allow pure rotational movement of the extension portion of the prosthesis during function. The canine rest is positioned where the most positive rest seat can be prepared, compatible with the opposing occlusion. The position of this rest is determined by the mounted diagnostic casts. The central portion of the rest should be open (see Fig 7-1b). This allows the clinician to easily determine whether the rest is properly seating in the rest seat preparation. This design also facilitates cleaning of this portion of the RPD in that the bristles of the denture-cleaning brush can protrude through the opening and keep the rest seat area clear of debris.

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Partial Denture Design Principles and Design Sequence

a

Fig 7-2 Maxillary design sequence: Step 2. Design of the major connectors. In this case, the maxillary major connector is an anteroposterior palatal strap design. This type of strap design ensures that the RPD metal framework is sufficiently rigid to provide cross-arch stabilization.

Fig 7-1 Maxillary design sequence: Step 1. (a) Place the occlusal rests in the exact positions. Note that the right side is tooth borne, with rests in the center of the tooth; the left side is an extension situation, with a mesial rest. (b) The anterior rest is open in the center to visualize complete seating of the rest and for easy cleaning.

b

Fig 7-3 Maxillary design sequence: Step 3. The next step is to design the minor connectors. The proximal plate portion extends and covers the side of the tooth-tissue junction for a minimum of 2 mm.

Fig 7-4 All parts of the prosthesis that contact the tooth and the tooth-tissue junction are covered with metal. On the palatal side, note that the RPD is designed to be 5 to 6 mm away from the gingival margin (arrow).

Major connectors

Minor connectors and proximal plates

The design joins all rests and edentulous areas with a rigid connection to ensure control and maintain the position of abutment teeth. The anterior strap of the major connector is placed in the valley of the ridge to reduce bulk and is blended with the rugae pattern. The posterior strap of the major connector should not extend beyond the vibrating line of the palate. The design carries the metal just short of the hamular notch area. The finish line on the tooth-borne side is positioned to permit easy placement of the prosthetic teeth and to allow sufficient bulk of acrylic resin to effectively secure the prosthetic teeth to the metal casting (Fig 7-2). On the edentulous side, the strap is positioned more toward the middle of the palate to increase the surface coverage of the edentulous ridge. This practice increases the support provided for the RPD and allows easy relining of the edentulous extension area. The anteroposterior palatal strap optimizes the rigidity of the framework, ensuring optimal cross-arch stabilization and distribution of lateral torquing forces during function.

The minor connectors connected with the proximal plates are extended to cover the tooth-tissue junction and should extend onto the tissue for at least 2 mm (Fig 7-3). The facial extension covers and protects any tissue depressions, and the facial portion extends over the curvature of the ridge. In some instances, such as in the molar region, the minor connector provides the connection between the rest and the major connector. The proximal plate portion of the minor connector, contacting the tooth at the tooth-tissue junction, idealizes the bracing (resistance to lateral torquing forces) provided for the prosthesis because this area is closest to the attachment of the tooth to the bone. Note that the casting is designed well away from the tooth-tissue junction on the palatal side of the abutment (Fig 7-4).

62

Maxillary Design Sequence

a Fig 7-5  Maxillary design sequence: Step 4. Design of the denture base connectors. They are positioned on the crest of the ridge or slightly to the lingual, especially on the tuberosity so as not to interfere with the placement of denture teeth.

b

Fig 7-6  Maxillary design sequence: Step 5. (a) The retention portion of the retainer is positioned in the undercut as appropriate. (b) The retainer is designed to join the denture base connectors so as not to impair positioning of the denture teeth.

Fig 7-7  The side view illustrates the retainer crossing the tooth-tissue junction in a vertical direction at right angles to the mucogingival junction before its horizontal course begins.

Fig 7-8  The retainers are joined to the denture base connectors at the interproximal area between the proposed position of the denture teeth.

Denture base connectors

Retainers

The denture base connectors are designed to enable appropriate positioning of the denture teeth. Connectors that interfere with tooth placement may lead to foreshortening of the premolar denture teeth, compromising the esthetic outcome. In addition, when the denture teeth are inappropriately reduced in size, it will be difficult to retain them within the acrylic resin of the denture base. To eliminate these problems, the connectors are placed on the crest and the lingual side of the ridge, away from the facial surface (Fig 7-5). Note that the location of the posterior portion of the connector on the edentulous side is well to the lingual side and away from the crest of the tuberosity because of the limited space in this area. The double finish lines (internal and external finish lines) on the cast indicate a finish line on the tissue side and the tongue side of the casting that will form a positive union of the acrylic resin and the metal.

The location of the retention portion of the retainer is determined by the abutment tooth contour at the retention areas and the pattern of movement of the prosthesis in function. The retention portion is positioned first and designed to cross the tooth-tissue junction at right angles and continues in the direction of the long axis of the tooth (Fig 7-6). This portion extends to the mucogingival junction, where it turns in a horizontal direction (Fig 7-7). The retainer is joined to the denture base connector so as not to impair positioning of the denture teeth (see Fig 7-6b). When viewed from the facial aspect, the tapered design of the retainer can be visualized, with the bulkier portion at the junction with the denture base connector and located at the interproximal area of the denture teeth (Fig 7-8). The junction at this location will not interfere with proper placement of the denture teeth and idealizes the esthetic result by allowing the positioning of teeth of appropriate length and contour (see Fig 7-6b). 63

7

Partial Denture Design Principles and Design Sequence

Fig 7-9 Mandibular design sequence: Step 1. Occlusal rests are placed on select teeth. In extension situations, their position controls the axis of rotation (fulcrum line).

a

b

Fig 7-10 Mandibular design sequence: Step 2. (a) Major connector. A lingual bar. Increased bulk is required to provide a rigid connection between both sides of the arch. (b) A lingual plate design. It covers the tooth-tissue junction and contacts the cervical third of the teeth. This will provide the most favorable leverage to brace against lateral displacement during function. It is used when the elevation of the floor of the mouth precludes use of a lingual bar.

Mandibular Design Sequence Occlusal rests The rests provide support and control points between the abutment teeth and the prosthesis, so they are positioned first. Where an incisal rest is required (eg, on the right canine), it is designed to cover and restore the entire functional area. The mesial and distal rests on the left premolar may be joined together for increased strength because this is a non-occluding area (Fig 7-9). The rest on the right premolar is placed on the mesial side because this tooth is adjacent to a posterior edentulous extension area. The rest on the molar is extended into the middle of the tooth to direct occlusal forces along the long axis of the tooth.

Major connectors Rigidity for the major connector is more difficult to provide in the mandible. Greater bulk is required to produce the rigidity 64

necessary between the rests and the edentulous area. When the lingual bar is the design of choice, it should be positioned on unattached mucosa wherever possible and well away from the tooth-tissue junction. The bar can be extended into the lingual area as far as functional anatomical structures permit to gain the required bulk for rigidity. The lingual bar design should be 3 to 4 mm away from the gingival margin, and the bar must be 4 mm in height (Fig 7-10a; see also chapter 4). In some instances, the elevation of the floor of the mouth does not permit the design of a lingual bar with sufficient bulk. In this instance, a lingual plate design is advised (Fig 7-10b). Where the casting crosses the tooth-tissue junction, it should be at right angles to the junction in order to minimize food impaction between the RPD framework and the tissues and should continue onto the unattached gingiva before the horizontal portion begins. The space between the crest of the mucosa and the lingual bar should be as open as possible (see Fig 7-11).

Mandibular Design Sequence

Fig 7-11 Mandibular design sequence: Step 3. Minor connectors. The lingual bar is extended onto unattached gingiva and should be positioned at least 3 to 4 mm away from the gingival margin. The minor connector crosses the tooth-tissue junction at right angles.

Fig 7-12  Mandibular design sequence: Step 4. Denture base connectors are located on the crest of the ridge and on the lingual side to provide sufficient space to position denture teeth.

Fig 7-13  A strong union at the junction of the denture base connector and the major connector is essential to prevent flexure and breakage. Note the posterior sweep of the external finish line well into the edentulous area.

Fig 7-14  The major connector forms the inferior border of the denture base and an external finish line on the tooth-borne side of the arch.

Minor connectors and proximal plates The proximal plate portions of the minor connectors are positioned to cover the side of the tooth and to extend onto the mucosa from the tooth-tissue junction for a minimum of 2 mm. The design should provide metal contact at all points where the prosthesis contacts the tooth or the tooth-tissue junction. Note that the casting is designed well away from the tooth-tissue junction on the lingual side of the abutment (Fig 7-11).

Denture base connectors Denture base connectors are positioned on the crest of the ridge and toward the lingual side to minimize interference with denture tooth placement (Fig 7-12). On the side with the

edentulous extension, an area of critical concern is the junction with the major connector (Fig 7-13). This area is subjected to flexure and fracture and must be designed with additional bulk. A fan-shaped union will provide maximum strength. Note the position of the external finish line in this area; it is placed well to the distal side of the tooth and curved into the edentulous portion, where there is extensive bone and tissue loss. Placement of the bulky acrylic resin–metal junction in this area restores the contour of lost tissues without infringing on the tongue space. Note that the connectors end abruptly in anticipation of the continuous extension of the acrylic resin of the denture base. On the tooth-borne side of the arch, the major connector forms the inferior border of the prosthesis to provide maximum strength with minimum bulk and a junction point for the denture base connector as well as the external finish line area (Fig 7-14). 65

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Partial Denture Design Principles and Design Sequence

Fig 7-15 Mandibular design sequence: Step 5. The retainers. The retainers are positioned for proper retention and are attached to the denture base connectors. The alveolar contour of the tissue undercut may influence the length of the vertical portion of the retainer.

Fig 7-16 The position and attachment of the buccal frenum and soft tissue topography may require the horizontal component to circumvent this area.

Retainers

Suggested Reading

The retainers are designed and positioned to provide retention as required and to allow the proper placement of the denture teeth. In the mandibular arch, the contour of the tissues on the facial surface can present extensive undercuts that require a shorter vertical extension of the retainer before the horizontal portion is located (Figs 7-15 and 7-16). A high attachment of the buccal frenum may require the use of a wrought wire retainer or a suprabulge retainer. A systematic approach with a step-by-step sequence develops a quick and practical design, which organizes treatment planning and provides for detailed communication with the laboratory.

Berg T Jr. I-bar: Myth and countermyth. Dent Clin North Am 1979;23:65–75. Kratochvil FJ. Influence of occlusal rest position and clasp design on movement of abutment teeth. J Prosthet Dent 1963;13:114–124. Krol AJ, Jacobson TE, Finzen FC. Removable Partial Denture Design: Outline Syllabus, ed 5. San Rafael, CA: Indent, 1999. Miller EL. Systems for classifying partially dentulous arches. J Prosthet Dent 1970;24:25–40. Potter RB, Appleby RC, Adams CD. Removable partial denture design: A review and a challenge. J Prosthet Dent 1967;17:63–68. Steffel VL. Current concepts in removable partial denture service. J Prosthet Dent 1968;20:387–395.

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Chapter 8 Surveying and Determining the Most Advantageous Treatment Position Daniela Orellana | Ting-Ling Chang

The removable partial denture (RPD) provides positional control, stabilizes the remaining teeth, and unites the entire arch. In order to sustain a predictable long-term clinical outcome, the clinician must abide by the basic principles of RPD design and construction consistent with the patient’s remaining oral structures and physical condition. The design of the prosthesis should not be compromised by existing pathologic conditions or by the malposition of teeth. The basic principle is to prepare the oral cavity to receive the prosthesis. It is important to follow sound design principles in a logical sequence to achieve the desired result. When preparing a blueprint, the draftsperson first establishes a baseline, and all other lines are drawn in relation to that one position or line. The authors use a similar design concept when designing an RPD. The first step in developing the definitive RPD design is to identify the path of insertion, or the baseline. The primary objective is to establish a position of the cast that places teeth and associated tissues in the most advantageous position (MAP) for treatment with the RPD. The MAP is determined by the following factors: • Guiding surfaces: Guiding surfaces determine the path of insertion of the prosthesis. These may be the mesial or distal surfaces of the teeth next to edentulous areas (Fig 8-1). Minor connectors and portions of the major connector can also impact the path of insertion (Fig 8-2). • Retention areas: Retention areas profoundly influence the selection of the MAP. The ideal retention area is located in the cervical third of the abutment tooth, close to the tooth-tissue junction (see chapter 5). Altering cast angulation in relation to the vertical

a

b Fig 8-1 (a and b) The primary guiding surfaces that determine the path of insertion for the RPD are the tooth surfaces adjacent to the edentulous areas, in this case the maxillary right first premolar, left canine, and left second molar and the mandibular left second molar, left first premolar, and right first premolar.

67

8

Surveying and Determining the Most Advantageous Treatment Position

Fig 8-2 (a and b) The path of insertion is determined by the guiding surfaces, the minor connectors, and portions of the major connector that engage the dentition, such as the lingual plate shown in part b.

a

b

a Fig 8-3 The dental surveyor is a basic instrument for comparing the parallelism of one surface to other surfaces at a given position.

Fig 8-4 (a and b) The analyzing rod attached to the vertical arm of the instrument demonstrates the parallelism of one tooth surface to another tooth surface at a given cast orientation.

arm of the surveyor will influence the presence or absence of retentive areas and their magnitude. A given cast position and angulation may indicate correct retention areas, lack of retention areas, or excessive retention areas.

Survey Instrument The surveyor is a basic paralleling device that demonstrates the parallelism or lack of parallelism of all surfaces to a fixed cast position. It consists of the following elements: • A survey instrument with a vertical arm extended at right angles to the base (Fig 8-3). • A small analyzing rod is attached to the arm, which visually demonstrates the relationship of any part of the cast to a selected path of insertion (Fig 8-4). • The surveying table is adjustable and holds the cast in a specific position and angulation (see Fig 8-3). 68

b

Establishing the MAP Secure the diagnostic cast on the surveying table 1. Establish a basic position with an “eye survey” by placing the surveying table with the cast in place on a bench and standing directly over the center of the cast (Fig 8-5). 2. Adjust the surveying table until the guiding surfaces and retention area (undercuts) are relatively equal and the guiding surfaces are as parallel as possible.

Place the surveying table with the cast in place onto the base of the survey instrument 1. With the analyzing rod in place, check the parallelism of the guiding surfaces (Fig 8-6). 2. Check the retention areas for necessary undercuts. When sufficient undercuts are lacking associated with key abutment teeth, reposition the cast in an attempt to idealize the retentive areas (Fig 8-7).

Survey Instrument

a

b

c

Fig 8-5  Make an “eye survey” by orienting the cast Fig 8-6  (a to c) Check the guiding surfaces of all teeth to be contacted by the RPD for parallelism in the MAP. Stand directly over the cast and make with the diagnostic rod. All guiding surface areas of each abutment tooth are checked for space at the the guiding surfaces as parallel as possible by ad- tooth-tissue junction, as illustrated in part c. justing the surveying table.

Fig 8-7  (a) The analyzing rod evaluates the position and location of retention areas on the abutment teeth. (b) A 0.01-inch undercut gauge is used to identify the location of the desired undercut. (c) The analyzing rod is used to evaluate soft tissue undercuts and determine the suitability for use of an infrabulge retainer. If the soft tissue undercut is excessive, determine whether it can be accommodated by a new MAP. If not, a suprabulge retainer may be favored.

a

b

c

3. Check for tissue undercuts (see Fig 8-7). If the soft tissue undercut is excessive, determine whether it can be accommodated by a new MAP. If not, a suprabulge retainer may be favored.

4. Retention areas can be altered and recontoured to remove excess undercut areas or to provide proper contours for retention (Fig 8-9).

Make adjustments of the cast position to establish the best possible MAP

Eliminating spaces and voids

1. Keep the occlusal plane as parallel to horizontal as possible. 2. Do not permit a single tooth that is out of normal alignment to dictate the path of insertion (Fig 8-8). 3. Position the cast to the advantage of the other teeth in the arch and adjust or restore the malpositioned tooth to conform. Alter the malpositioned tooth with a restoration or enameloplasty.

The cast should ideally be positioned so that the proximal plates will contact teeth and soft tissues with no space remaining to avoid a soft tissue reaction (Fig 8-10). Soft tissue reacts to a space or void in one of two ways: (1) It may hypertrophy into the space, creating deep pockets, or (2) it may recede, especially if irritated by food impaction (see chapter 3). The cast should be maneuvered to determine the MAP. Such a position will minimize tooth reduction and voids. 69

8

Surveying and Determining the Most Advantageous Treatment Position

Fig 8-8 Establish the best position using the majority of normally aligned abutment teeth. Do not permit the malposition of one tooth to dictate the MAP. The malpositioned tooth (in this case, the mandibular left second molar) should then be altered with a restoration or enameloplasty.

0.01"

HC

a

b

c

d

e

Fig 8-9 Retention areas that place the retainer too coronally on the tooth or too far away from the mucosa require tooth recontouring on the facial side to move the retainer cervically and in closer proximity to the mucosal surfaces. (a) The height of contour (HC) is unfavorably coronal. (b) The facial side of the abutment is recontoured with an air rotor. (c) The height of contour has been repositioned cervically. (d and e) The undercut is now in ideal position for engagement with the retainer. Fig 8-10 The ideal path of insertion allows seating of the prosthesis with complete elimination of all spaces and voids. To achieve parallel guiding surfaces requires planning, measurement, and tooth preparation. (a) Ideally, the mesial surface of the molar should be recontoured either with enameloplasty or fabrication of a full-coverage crown. (b) The guiding surfaces are ideal. There are no spaces between the proximal plates and the guide planes. (Part a courtesy of Dr R. Duell, Los Angeles, California.)

a

b

Recording the MAP of the cast Tripoding is the method used to record the determined cast position. This position can be used at a later date in the laboratory during fabrication of the RPD framework. The procedure for tripoding is as follows (Fig 8-11):

70

1. Place the surveying table with the cast in the desired position on the base of the surveyor. 2. Position the surveyor vertical arm with the undercut gauge in place so the tip touches the cast at three points as far removed from each other as possible. 3. Tightly secure the surveyor arm with the set screw and mark three points on the cast with a pencil. Circle the mark for easy location at a later date.

Survey Instrument

Fig 8-11  (a and b) With the cast fixed in the MAP on the surveying table, the vertical arm of the surveyor is positioned so the tip of the undercut gauge touches the cast in three widely separated areas. The tripod marks are circled in red and blue pencil for easy identification. This allows for the removal and repositioning of the cast in the identical MAP. (Courtesy of Dr T. Berg, Los Angeles, California.)

a

b

a

b

Fig 8-12  (a and b) With the cast fixed in the MAP on the surveyor table, the metal post is secured with acrylic resin.

a

b

c

Fig 8-13  (a to c) The RPD design is drawn in red. Areas requiring tooth modifications for guiding surfaces, rests, and idealization of retention areas are marked in blue, as determined by the MAP.

Tripoding allows for exact repositioning when the master cast is sent to the laboratory for fabrication of the metal framework. An alternative tripoding method is the metal post technique. A metal post is secured onto the cast with acrylic resin at the desired position and angulation. The metal post will facilitate the repositioning of the cast at the determined path of insertion (Fig 8-12). When the RPD is designed digitally, the cast is surveyed digitally, and the MAP is determined (see chapter 11 for details).

Analysis of the guiding surfaces With the analyzing rod in place, all portions of the teeth that will serve as guiding surfaces are inspected. Areas on the teeth that exhibit undesirable undercuts are marked with a blue pencil as a guide for tooth alteration during mouth preparation. The facial surfaces of abutment teeth that are to be altered to reposition the undercut area cervically are likewise marked with blue pencil (Fig 8-13). It is advantageous to make preparations on the cast before performing this procedure in the mouth to evaluate final tooth contours or to demonstrate the need for new restorations. 71

Surveying and Determining the Most Advantageous Treatment Position

8

Height of contour

Height of contour 0.01" undercut

0.01" undercut

a

b

c

Fig 8-14 (a to c) The exact location, amount, and position of the required retentive area is determined with the undercut gauge. Any contact on the retained tooth below the undercut gauge increases retention. The ideal amount of retention for cast retainers is 0.010 inch. Note that the shank and the tip of the gauge are in contact with the tooth surface. Also note that the tip of the I-bar retainer extends from the undercut to the height of contour.

Fig 8-15 Metal-ceramic full-coverage crowns were fabricated on the canines to reposition the retentive areas cervically. Cingulum rests and guiding surfaces were also incorporated within the crowns.

Analysis of retention areas The proximal plates of the metal casting in contact with the guiding surfaces of the teeth stabilize them in a controlled position in relationship to the retainers, which engage tooth undercuts and hold the prosthesis in place. A measuring instrument can be placed on the surveyor to accurately determine the position and placement of the retainer to achieve the necessary retention for the prosthesis. The undercut gauges are manufactured to measure a set amount of distance. They are available in three increments of retention (0.010, 0.020, and 0.030 inches). The ideal amount of retention required for cast retainers is 0.010 inch or 0.25 mm (Fig 8-14). Wrought wire retainers, given their flexibility, can engage undercuts of greater magnitude. Place the undercut gauge with its shank touching the tooth surface. Move the surveyor arm until the tip of the gauge touches the tooth surface. Both the shank and the tip must contact the tooth simultaneously. The point of contact with the tip of the 72

gauge identifies the position of the 0.010-inch undercut on the tooth surfaces in question (see Fig 8-14). If there is contact of the retainer against the tooth gingival to this point, retention will be increased. If retainer contact is superior or occlusal to this point, retention will be lessened.

Excessive retention areas It is desirable to keep the contact area of the retainer as close to the gingival portion of the tooth as possible. In some instances, tooth contour or position creates excessive space below the height of contour. In the esthetic zone, this can be esthetically displeasing because the retainer will be positioned too close to the incisal edges. Recontouring the teeth or making new restorations may be necessary to reposition the tips of the retainers cervically (Fig 8-15; see also Fig 8-9). All areas of alteration are marked in blue on the cast for easy identification during intraoral preparation.

Tooth Preparation Guide

a

b

Fig 8-16  (a and b) The circumferential contour of the tooth should be surveyed because retention areas may be found on the lingual surfaces.

Fig 8-17  A dimple is made using a #6 round bur to create an adequate retention area.

Fig 8-18  A surveyed crown can be made to idealize contours and retention areas.

Lack of retention areas

Tooth Preparation Guide

In some instances, tooth contours need to be analyzed around the circumference of the whole tooth because the retention areas may be found on the lingual surfaces of the tooth. This is particularly common on mandibular molars due to their lingual inclination (Fig 8-16). In teeth that lack retention areas, a “dimple” can be created by enameloplasty. A #6 round bur is used to create an undercut area of 0.01 inch (0.25 mm) (Fig 8-17). A surveyed crown can also be fabricated to address the lack of retention areas (Fig 8-18). In young patients, crown lengthening procedures may be necessary to expose suitable retention areas.

The purpose of the tooth preparation guide is to assist clinicians in recontouring the abutment teeth according to the predetermined path of insertion. The authors recommend the fabrication of this guide when clinicians are treating their first few RPD patients and/or when multiple abutments are to be engaged with proximal plates. The sequence of fabrication is as follows (Fig 8-19): 1. Block out undercuts with wax and fabricate a record base using acrylic resin on the diagnostic cast. 2. Adjust the surveying table to position the diagnostic cast in the predetermined MAP. 73

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Surveying and Determining the Most Advantageous Treatment Position

a

c

b

d

e

Fig 8-19 A tooth preparation guide can be fabricated after completion of surveying. (a and b) Metal posts are secured to the vertical arm of the surveyor and connected to the record base with acrylic resin. (c and d) Ensure that the metal posts are placed to allow sufficient mesiodistal space to access the surfaces to be recontoured. (e) A parallel-sided bur or diamond is placed parallel to the metal post guide, and guide planes are prepared. This technique will allow the clinician to prepare the guide planes consistent with the predetermined path of insertion. (Courtesy of Dr T. Berg, Los Angeles, California.)

3. Secure a metal post to the vertical arm of the surveyor. 4. Connect this metal post to the record base using acrylic resin and hold it in position until the resin cures. Allow sufficient mesiodistal space to permit access to the surfaces of the abutment teeth that are to be recontoured. 5. Inspect the record base for any sharp or rough edges. Adjust and polish. The finalized tooth preparation guide is inserted intraorally, and a parallel-sided bur is used for preparation of the guide planes. This technique will ensure that parallel guide planes are prepared consistent with the RPD design and the path of insertion.

74

Suggested Reading Applegate OC. Use of the paralleling surveyor in modem partial denture construction. J Am Dent Assoc 1940;27:1397–1407. Bezzon OL, Mattos MG, Ribeiro RF. Surveying removable partial dentures: The importance of guiding planes and path of insertion for stability. J Prosthet Dent 1967;78:412–418. Curtis DA, Curtis TA, Holmes JB. Use of a paralleling post for cast orientation when fabricating removable partial denture abutment crowns. J Prosthet Dent 1988;59:117–118. Dunn BW. Treatment planning for removable partial dentures. J Prosthet Dent 1961;11:247–255 Frechette AR. Partial denture planning with special reference to stress distribution. J Prosthet Dent 1951;1:710–724. Glann GW, Appleby RC. Mouth preparation for removable partial dentures. J Prosthet Dent 1960;10:698–706. Henderson D, Seward TE. Design and force distribution with removable partial dentures: A progress report. J Prosthet Dent 1967;17:350–364. Schorr L, Clayman LH. Reshaping abutment teeth for reception of partial denture clasps. J Prosthet Dent 1954;4:625–633. Zoeller GN. Block form stability in removable partial dentures. J Prosthet Dent 1969;22:633–637. Zoeller GN, Kelly WJ Jr. Block form stability removable partial prosthodontics. J Prosthet Dent 1971;25:515–519.

Chapter 9 Diagnosis, Treatment Planning, and Intraoral Preparation Daniela Orellana | John Beumer III

Diagnosis and treatment planning have been deliberately included in this part of the book so the principles of removable partial denture (RPD) design can be considered while treatment is being planned.

Attitudes and Objectives of the Clinician and the Patient General patient evaluation Appraisal of the patient begins when he or she walks into the dental operatory. General gait, appearance, coordination, and demeanor are especially important in patients who will be treated with an RPD. Patient compliance and reasonable hand-eye coordination are required if the patient is to achieve the oral hygiene levels necessary to maintain the health of the dentition and properly care for the prosthesis.

First appointment encounter The patient is approached and evaluated as an equal during this first meeting. When soliciting the chief complaint, the history of the present illness, and the medical and dental history, it is advised that the clinician sit in a chair so both clinician and patient are at the same eye level. Patients can feel threatened and dominated by the clinician standing over or above them. It is desirable to establish an equal yet professional relationship with the patient.

Psychologic factors DeVan1 suggested that the following factors need to be addressed and resolved to achieve a successful clinical outcome: • The patient must have the will to adjust to and use the removable prosthesis. – The patient may have the need for the removable prosthesis but not the will to adjust to it. – There may be the need and the will but not the physical ability to use a removable prosthesis. • The prosthesis must be accepted as part of the body. • The clinician must take the patient’s need and turn it into a want.

Developing the patient-clinician relationship The first impression is most important from the patient’s point of view. The patient is evaluating the clinician in much the same manner as the clinician is evaluating the patient. It is not advisable to stand over the patient during the first meeting because it may be intimidating, make the patient uncomfortable, or indicate that treatment is imminent, for which the patient may not be prepared. Patients are interested in knowing what is going to happen to them. In lay terms, carefully lay before them your proposed approach to treatment. Determine whether treating the patient would be enjoyable or even possible. Patient-clinician feelings can influence your attitude and possibly the success of treatment. Remember to allow patients to tell the story of their 75

9

Diagnosis, Treatment Planning, and Intraoral Preparation

Fig 9-1 Diagnostic casts mounted in centric relation at the correct occlusal vertical dimension are essential for proper diagnosis and treatment plan­ ning. The retromolar pad area is an essential landmark for establishing the occlusal plane, and the cast must include this area. The tentative restorative plan is noted on the diagnostic casts and mounts. (Courtesy of Dr R. Duell, Los Angeles, California.)

complaints and concerns. Listen to their dental and medical histories carefully because these can have a profound impact on the success or failure of the clinical outcome. Do not promise or imply that you can restore the patient to his or her original functional capabilities with the use of RPDs, as is often possible with individual tooth restorations. Do not give the patient the impression that you or anyone else can accomplish this feat, but give assurance that you will do your utmost to maintain the health of the remaining oral structures and to limit their further degradation. You may be able to make significant improvements in esthetics, but do not stress the ability to restore mastication to levels achieved prior to tooth loss.

Diagnosis When planning for any restoration, you must start at the foundation and then proceed from that point. You must appreciate the value of the structures available to support the RPD and the functional forces to be encountered that will challenge the support apparatus. There must be a systematic approach; your thoughts and methods must be organized with a logical flow, or you soon will become lost in minutiae. Every clinician develops his or her own methodology to best plan procedures, which is as it should be, but the method must be systematic. The workup begins by eliciting the patient’s chief complaint and its history, the dental history, and the medical history. Then a thorough oral examination (including an oral cancer screening examination) is performed, initially focusing on the presence of caries and periodontal disease associated with the remaining dentition. Complete-mouth radiographs are taken, casts are made and mounted with a facebow transfer, centric relation records and protrusive records are made, and an occlusal analysis is performed (Fig 9-1). 76

If a posterior abutment is present, a thorough assessment of this tooth, the anterior abutment, and the length of the span is made; if these teeth are immobile, the span and periodontal support are favorable, and the crown-root ratios are 1:1 or better, a fixed dental prosthesis (FDP) might be the most appropriate option. If the abutments are unrestored or minimally restored and the defect spans a single tooth, a single implant may be the best choice. If the distal molar abutment is angled in excess of 25 degrees in any direction, orthodontic treatment may be appropriate prior to proceeding with fabrication of the RPD. A careful assessment is made of the alveolar ridge contours and the overlying tissues. Following the extraction of posterior teeth, wound healing and contracture often lead to a collapse of the width of the alveolar ridge. The amount of attached keratinized mucosa on the crest of the ridge is of particular importance in patients with edentulous extension defects because much of the support of the RPD is derived from the coverage of these tissues.

Occlusion The occlusion of the patient is carefully evaluated. A stable and ideal occlusion for patients restored with residual natural dentition supplemented with RPDs is characterized by the following: • Stable stops on all teeth when the condyles are in their seated physiologic orthopedic position (centric relation). This may not be necessary in an asymptomatic patient who presents with a minimal slide between centric relation and maximal intercuspal position. • Anterior guidance that is in harmony with the border movements of the envelope of function. • Disocclusion of all posterior teeth during protrusive movements. • Disocclusion of all posterior teeth during laterotrusive movements.

Diagnosis

b

a Fig 9-2  (a to c) Treatment partial dentures can be used to test the feasibility of opening the OVD and to establish a treatment position for the definitive restorations and RPD. (Courtesy of Dr A. Davodi, Beverly Hills, California.)

c

When performing an occlusal analysis for a prospective RPD patient who is partially edentulous in any of the posterior quadrants, the clinician must assess the existing guidance of the patient and whether it is desirable to change the occlusal scheme. Patients who present with group function and show no discernable signs or symptoms of pathogenic occlusion may be best left functioning with group function as long as the wear to the remaining dentition is minimal. However, the patient should be free of nonworking (balancing) interferences, which may stimulate parafunctional activity in some individuals. If wear of the dentition is moderate to severe, restoration of the anterior guidance may be desirable but should be approached in a systematic fashion consistent with sound prosthodontic principles. When a distal-extension RPD is planned, a mutually protected occlusal scheme utilizing anterior guidance is preferable, and the guidance established should be compatible with the condylar guidance of the patient. A diagnostic wax-up may be necessary prior to the initiation of treatment to assess the types of restorations necessary and to determine what occlusal adjustments need to be made to the remaining teeth (see Fig 9-11). Treatment RPDs may be needed prior to treatment to establish a treatment position that is compatible with the envelope of function of the mandible. If the patient has lost occlusal vertical dimension (OVD) secondary to wear, erosion, or loss of posterior tooth stops, the

OVD can be restored with RPDs. A thorough evaluation and adherence to sound prosthodontic principles are mandatory before treatment can be initiated in these situations. Arbitrary opening of the OVD without careful testing should be avoided. The use of treatment partial dentures for a reasonable period is strongly advised if significant changes are to be made to the OVD (Fig 9-2) (see chapter 18). Patients who exhibit severe parafunctional activity, such as central nervous system bruxism, a history of multiple tooth fractures (especially previously unrestored teeth), and brachycephaly should be approached with caution. If the clinician does decide to provide treatment, it is advisable to inform the patient that if parafunctional activity persists, rapid wear and frequent breakage of portions of the prosthesis are inevitable and that every effort should be made to address this phenomenon. Such patients may be best restored with implant-retained and stabilized overlay RPDs (see chapter 16). The dentition of the opposing arches should be carefully scrutinized. The integrity of the opposing arches should be restored. Any discrepancies of the plane of occlusion secondary to supereruption of individual teeth or teeth segments must be noted (Figs 9-3 and 9-4), and plans must be made for their correction before the RPD is fabricated. The plane of occlusion is configured after the OVD has been determined. If occlusal plane discrepancies go uncorrected, interocclusal space may be 77

9

Diagnosis, Treatment Planning, and Intraoral Preparation

Fig 9-3 Examination reveals loss of multiple posterior teeth, supereruption of the remaining maxillary molars, and lack of interocclusal space posteriorly. The occlusal plane, the edentulous spac­ es, and the integrity of the maxillary arch must be addressed before the missing dentition in the mandible is restored with an RPD. (Courtesy of Dr T. Berg, Los Angeles, California.)

a

b

c

Fig 9-4 (a to c) Lost OVD. The integrity of the arch, the anterior guidance, and the plane of occlusion of both arches as well as the OVD are carefully evaluated, and plans are made for their correction before the RPD is made. (Parts b and c courtesy of Dr A. Davodi, Beverly Hills, California.) Fig 9-5 Diagnostic casts are mounted with a centric relation record. Note the hypertrophic tuberosity and the occlu­ sal plane discrepancies secondary to supereruption of the mandibular molars. Note that the RPD design is outlined and that restorative plans for key teeth are also noted on the cast. (Courtesy of Dr T. Berg, Los Angeles, California.)

lacking for the prosthesis, and it will be difficult to properly control the occlusal factors and prevent inappropriate posterior contacts of the prosthesis during protrusive and laterotrusive movements. The posterior landmarks for the occlusal plane are the retromolar pads, so these landmarks must be recorded with the preliminary impression and diagnostic casts. The anterior landmark of the occlusal plane is dictated by esthetics and phonetics and therefore by the position of the anterior teeth when the arches are positioned at the appropriate OVD. A lone standing posterior molar is the most common disrupter of the occlusal plane. It may be tilted, supererupted, and elongated, and in such situations, gross modifications (endodontic therapy, crown lengthening, and restoration with a full-coverage crown) are required to create a satisfactory plane of occlusion. Supereruption of single teeth in an unopposed space (Fig 9-5; see also Fig 9-3) may also require endodontic treatment, crown 78

lengthening, and fabrication of a full-coverage crown to mitigate occlusal plane discrepancies. Occlusal plane discrepancies can also be resolved by selective grinding and reshaping of the individual teeth, as well as new restorations (see chapter 15). Supererupted arch segments can be addressed with segmental osteotomies or by intrusion with orthodontic treatment.

Mounted diagnostic casts Prior to making impressions for diagnostic casts, a thorough prophylaxis is necessary to remove plaque and debris so the contours of the dentition and soft tissues can be recorded with precision.

The Sequence of Treatment—Intraoral Preparation

Fig 9-6 When the patient pre­ sents with multiple missing teeth and lack of posterior tooth contacts, it may be necessary to make record bases with wax rims to mount the diagnostic casts. (a) Record bases and wax rims. (b) The record is being made. (c) Mounted diagnostic casts in centric relation. (d) Mounted diag­ nostic casts. (Courtesy of Dr T. Berg, Los Angeles, California.)

a

b

c

d

Accurate, properly extended impressions of both arches are made and should include all teeth and soft tissues that will be engaged by the RPD, as well as strategic landmarks such as the buccal shelf, the retromolar pad, the hamular notches, and the maxillary tuberosities. Impressions should also record the contours of the floor of the mouth and extend facially to record the vestibular extensions. Facebow and maxillomandibular records are made in centric relation. The OVD is evaluated. The study casts are mounted on the articulator (see Figs 9-1 and 9-5) at the proper OVD and in centric relation. It may be necessary to construct record bases if there are numerous missing teeth (Fig 9-6). As indicated previously, an occlusal analysis is conducted, and the clinician determines whether to restore the patient in centric relation or maximal intercuspal position.

The Sequence of Treatment— Intraoral Preparation The usual sequence of treatment is the following: 1. Treatment of abnormal or inflamed soft tissues of the edentulous denture-bearing surfaces 2. Preprosthetic surgical procedures (eg, dental extractions, tuberosity reduction, frenectomy, alveoloplasty, tori removal, crown lengthening)

3. Diagnostic wax-up 4. Periodontal treatment as necessary 5. Endodontic treatment as necessary 6. Orthodontic treatment as necessary 7. Tooth modifications 8. Needed restorations 9. Procedures for prosthesis fabrication

Treatment of abnormal or inflamed soft tissues of the edentulous denture-bearing surfaces Inflammatory hyperplasia or inflammation of the tissues associated with the existing prosthesis can be triggered by the following: • Lack of positive rests of the existing prosthesis. • Fungal infections: These infections are commonly seen when patients wear treatment RPDs made of acrylic resin (Figs 9-7a and 9-7b). They are also associated with the tissues in direct contact with the major connector when the patient wears the prosthesis 24 hours per day (Fig 9-7c). • Poor adaptation of the existing prosthesis to the edentulous denture-bearing surfaces. • Hyperocclusion of the existing prosthesis.

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a

Diagnosis, Treatment Planning, and Intraoral Preparation

b

c

Fig 9-7 (a and b) Candidiasis beneath a provisional RPD made of acrylic resin restoring the missing teeth. (Courtesy of Dr R. Duell, Los Angeles, California.) (c) Candidiasis beneath the major connector of a definitive RPD. (Courtesy of Dr E. King, Houston, Texas.)

Resolution can be accomplished by removing the offending prosthesis, relining the existing prosthesis with a temporary denture reliner, or fabricating treatment partial dentures with positive rests (see chapter 18). The clinician should not proceed with master impressions until the bearing-surface mucosa has completely healed and stabilized. In the maxilla, candidiasis is commonly associated with the tissues beneath an acrylic resin denture base, especially if the patient wears the prosthesis at night while sleeping. Treatment is best accomplished with antifungal medications. Nystatin, in the form of oral lozenges or vaginal suppositories, remains the most clinically effective and cost-effective drug available. The contaminated acrylic resin of the denture base should be relieved and replaced with an intermediate denture reline material.

Preprosthetic surgical procedures Teeth Teeth that are not serviceable should be removed as soon as possible to allow for appropriate healing time prior to fabrication of the definitive RPD. Impacted or partially erupted teeth should be removed with caution because they may erupt and could eventually become useful RPD abutments for extension prostheses.

Bone Large mandibular and maxillary tori should be removed if their presence makes it difficult to fabricate a properly designed RPD (Fig 9-8). The presence of tori can make it problematic to properly contour and position major connectors of appropriate bulk and rigidity. On occasion, the bony portion of the tuberosity needs to be trimmed following supereruption of a second 80

or third molar (Fig 9-9; see also Fig 9-5). In younger patients with Class II or Class III jaw relations, it may be prudent to reposition the mandible and/or maxilla with osteotomies to produce more favorable ridge and occlusal relationships and to improve esthetic outcomes.

Mucosa Hypertrophy of the soft tissues associated with the maxillary tuberosity can lead to insufficient space to allow for proper coverage of this area for a maxillary extension-base RPD (see Figs 9-5 and 9-9). A wedge of tissue may need to be removed to provide space for the denture base extensions onto the retromolar pad and the maxillary tuberosity. It may be desirable to resect the frenum and reposition muscle attachments to permit more favorable positioning of infrabulge retainers and allow for more favorable denture base extensions. Surgical intervention may also be necessary to address residual infections and remove cysts, root fragments, and foreign bodies. It is advisable to be cautious when removing residual root tips that are asymptomatic. They may erupt after the prosthesis is delivered and then are removed more easily with less surgical trauma and therefore less loss of bone. When such root remnants are identified, an entry should be made into the chart describing their presence and location.

Crown lengthening Younger patients frequently present with a lack of sufficient undercuts in strategic locations of abutment teeth to properly retain the RPD. In such instances, crown lengthening is advised. Crown lengthening combined with endodontic therapy may also be necessary when the crown of a tooth is substantially shortened to address occlusal plane discrepancies secondary to supereruption of the teeth. In these situations, crown length-

The Sequence of Treatment—Intraoral Preparation

Fig 9-8  (a and b) In many in­ stances, it is prudent to remove tori prior to partial denture treatment to permit proper placement of major connectors of appropriate contour and ri­ gidity. (Part a courtesy of Dr R. Duell, Los Angeles, California.)

a

b

Fig 9-9  On occasion, an enlarged maxillary tu­ berosity needs reduction to create restorative space so the retromolar pad and maxillary tuberosity can be covered with the extension bases. (a) An enlarged tuberosity secondary to inflammatory fibrous hyperplasia. (b) An enlarged tuberosity secondary to supererup­ tion of the maxillary molars.

a

a

b

b

c

Fig 9-10  (a to c) Crown lengthening permits design of a preparation with appropriate retention and resistance form. (Courtesy of Dr P. Camargo, Los Angeles, California.)

ening makes it possible to design the tooth preparation with sufficient resistance and retention form (Fig 9-10).

Diagnostic wax-up In complex cases where the OVD is altered, the occlusal plane is in need of reconfiguring, or when several restorations are designed to be fabricated in concert with the RPD, a diagnostic

wax-up is a valuable tool that will help the clinician develop an appropriate and coordinated plan of treatment (Fig 9-11). Deflective occlusal contacts are identified for adjustment and equilibration before rests and guiding surfaces are prepared (Fig 9-12). Equilibration may alter the position and depth of the rest preparations besides removing interferences in centric closure and lateral excursions. It is advised to map out the sequence of tooth alterations required to conduct the equilibration on the mounted study casts before proceeding intraorally. 81

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Diagnosis, Treatment Planning, and Intraoral Preparation

a

b

c

Fig 9-11 (a to c) A diagnostic wax­up is useful in defining the issues inherent in the design and fabrication of RPDs. Fig 9-12 Casts are mounted in centric relation, and an occlusal analysis is per­ formed. Note the interferences in centric relation in the molar region. Equilibra­ tion is carried out preliminarily on the mounted casts and then intraorally.

Periodontal treatment The level of oral hygiene and dental compliance is carefully assessed during the initial consultation appointment. The clinician must be satisfied that the patient will commit to appropriately caring for the prosthesis and their residual dentition and will return for regular follow-up appointments before he or she embarks upon treatment. Pocket depths of all teeth are recorded. The status of the periodontal support and health of the teeth that will be contact with or in close proximity to the RPD casting are of particular interest. Teeth with excessive pocket depths will require periodontal therapy, and healing should be complete before making a master impression for the RPD. Otherwise, the prognosis of abutment teeth may be open to question, and the adaptation of the proximal plates at the tooth-tissue junction may be suboptimal. Mobility of teeth may be triggered by a combination of several factors, including the following: • • • •

Inflammation Occlusal disharmonies Lack of bone support Loss of arch integrity

Inflammation and occlusal disharmonies can be addressed immediately. However, loss of arch integrity may require the fabrication of provisional partial dentures, followed up by the 82

definitive RPD. In some instances, it may be necessary to restore edentulous spaces with FDPs (Fig 9-13) or splint key teeth together, especially those periodontally compromised and adjacent to an edentulous extension area (Fig 9-14). The goal is to restore arch integrity by reuniting the remaining dentition so the residual dentition in the arch can function as a unit. The objective of these treatments is to unite the arch, stabilize the teeth, and reduce tooth mobility. In some situations, it is advantageous to splint periodontally compromised teeth and unite the entire arch with an RPD framework (Fig 9-15). Periodontal treatment for teeth that will be engaged by the RPD framework is a joint decision based on the clinical judgments of the restorative dentist in consultation with the periodontist. If tooth mobility can be kept to an acceptable level by treatment, the tooth should be retained if possible. It often is advantageous to design the RPD framework to allow future replacement of the tooth or teeth in question and attach their prosthetic replacements to the existing RPD without making a new framework. For example, the clinician may select a lingual plate as opposed to a lingual bar when fabricating an RPD for the mandible when the incisors have a questionable long-term prognosis (Fig 9-16). This can also be accomplished by the use of multiple rests. A narrow zone of keratinized attached mucosa can cause difficulties where the RPD engages the dentition or beneath suprabulge retainers. Widening the zone of attached keratinized mucosa may be advantageous and can be accomplished with free palatal grafts (Fig 9-17).

The Sequence of Treatment—Intraoral Preparation

Fig 9-13  (a and b) The diastemata have been closed, and the missing teeth have been replaced with fixed restorations. In so doing, the integ­ rity of the arch is restored and these teeth, with their interproximal con­ tacts and spaces restored, can func­ tion as an intact unit to resist the forces generated during occlusal function with the distal-extension RPD. (Courtesy of Dr A. Davodi, Bev­ erly Hills, California.)

a

b

a

b

Fig 9-14  (a and b) The central and lateral incisors were periodontally compromised and slightly mobile. They were splinted together with metal-ceramic crowns with cingulum rests incorporated.

Fig 9-15 A patient presents with multiple missing teeth, spaces be­ tween existing teeth, widespread periodontal bone loss, and some tooth mobility. (a and b) The RPD framework was used as a periodon­ tal splint to unite the arch and fill the edentulous spaces. (Courtesy of Dr G. King, Houston, Texas.)

a

b

Fig 9-16  A lingual plate permits easy replacement of a lost incisor without having to remake the RPD framework.

a

b

c

Fig 9-17  (a) There is only a narrow zone of attached gingiva remaining. (b) Note the position of the frenum. (c) Free palatal grafts can be used to widen the zone of attached keratinized mucosa, enabling the use of an I-bar. (Courtesy of Dr T. Berg, Los Angeles, California.)

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a

b

c

Fig 9-18 (a) A root fragment is shaped to function as an overdenture abutment. (b) Note how the retention of the root fragment has retained the bony con­ tours and the attached keratinized mucosa. However, when designing the RPD framework, the root fragment must be entirely covered with the metal casting as opposed to acrylic resin, otherwise the risk of caries will be high. (c) A residual root has been treated endodontically and fitted with a gold coping. In both instances, support for the RPD has been dramatically enhanced. (Part a courtesy of Dr R. Duell, Los Angeles, California.) Fig 9-19 The mandibular molar has been uprighted prior to definitive treatment. (Courtesy of N. Aboujaoude, Beirut, Lebanon.)

Endodontic treatment Endodontically treated teeth are not good abutments for extension-base RPDs, particularly those teeth with rests that control the axis of rotation, and especially if the tooth has been restored with a post and core. The additional stresses delivered to abutments adjacent to the extension area will predispose these teeth to root fractures. However, endodontically treated root fragments are useful when strategically located (Fig 9-18) (see chapter 16). The root with its intact periodontium provides additional support for the RPD and preserves the underlying alveolar bone. When preparing the root fragment as an overdenture abutment or for a gold coping, extra reduction of the facial surface is necessary to permit the proper positioning of the denture teeth. When designing the RPD framework, the root fragment must be entirely covered with the metal casting as opposed to acrylic resin, otherwise the risk of caries will be high. In addition, a drop of fluoride gel should be applied to this area of the casting daily.

Orthodontic treatment Orthodontic treatment can be used to close spaces between teeth to consolidate arch segments, upright tilted teeth (Fig 9-19), improve occlusal relationships, and intrude teeth to idealize the 84

plane of occlusion prior to fabricating the definitive RPD. Such strategies can serve to restore arch integrity and create occlusal stability, the most important factors for achieving a predictable and long-lasting treatment outcome.

Recording the final plan of treatment It is useful to record the treatment plan and sequence of treatment on the mounted diagnostic casts (see Figs 9-1 and 9-5): • • • •

The RPD design is drawn on the casts in red pencil. Areas of tooth alteration are marked on the casts in blue. All restorative procedures are marked as appropriate. All restorative procedures are marked in the proper sequence.

Patient presentation and information A written report is presented to the patient detailing all clinical, radiographic, and diagnostic findings. Potential difficulties are described, methods of treatment are explained in detail, and the cost of treatment is itemized. A treatment presentation appointment is scheduled to explain and discuss treatment procedures, costs, and prognoses.

Alterations of Tooth Contours and Restorations

Fig 9-21  Note that the guiding surface follows the interproximal contours of the premolar. Also, the interproximal region between the premolars has been reduced to increase the bulk of the minor connector while permitting the development of contours that will not irritate the tongue.

Fig 9-20  Supererupted or rotated teeth can sometimes be recon­ toured to create a proper plane of occlusion and to idealize the contour of cusps and fossae. (Courtesy of Dr R. Duell, Los Angeles, California.)

a

b

c

Fig 9-22  (a) A set of burs and diamonds such as these are useful in preparing guiding surfaces and rest seats. (b and c) Tooth preparation guides are useful adjuncts in helping the clinician to parallel the guiding surfaces intraorally. (Courtesy of Dr T. Berg, Los Angeles, California.)

Alterations of Tooth Contours and Restorations The presence of amalgam restorations in a potential abutment tooth does not preclude its use as an RPD abutment so long as sufficient bulk of tooth structure remains. Teeth that are rotated or supererupted, however, may need to be recontoured to idealize the plane of occlusion, idealize cusp-fossa relationships (Fig 9-20), contour the guiding surfaces, and remove inappropriate interferences during lateral excursions. It also may be obligatory to restore abutment teeth with full- or partial-coverage crowns. Such restorations may be necessary to do the following: • Restore badly broken down clinical crowns • Reconfigure the occlusal plane • Reposition the clinical crown of a key abutment • Create rests of appropriate contour, especially on anterior teeth • Restore interproximal contacts and arch integrity Onlays and partial-coverage crowns are favored because such restorations conserve more tooth structure. If abutment teeth are to be prepared to receive partial- or full-coverage crowns, the guiding surfaces must be prepared first to establish the path

of insertion for the RPD before beginning the tooth preparation. If this practice is not followed, it may not be possible to configure the guiding surfaces in the most favorable manner without jeopardizing the definitive restoration (see chapter 15).

Tooth modifications Guiding surfaces (guide planes) are prepared first, as dictated by the path of insertion and determined by a thorough analysis of the diagnostic casts. The prepared guiding surfaces do not need to be flat planes, but they should follow the faciolingual curvature of the abutment tooth (Fig 9-21). The objective is to eliminate tooth undercuts. A large-diameter, cylindrically shaped diamond is used initially to develop the basic shape and contours. Large-diameter diamonds are preferred because there is less tendency to produce the uneven or wavy surfaces often created by smaller-diameter burs or diamonds. Special tooth preparation guides can be used to help parallel the guiding surfaces during their preparation (Fig 9-22). If modifications are needed on existing metal-ceramic crowns, the use of diamond burs combined with air-water spray will lessen the risk of porcelain chipping and cracking.

85

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Diagnosis, Treatment Planning, and Intraoral Preparation

a Fig 9-23 The rests prepared on the mesial side of the premolars are contoured as half­spheres so the extension­base RPD will rotate as a ball and socket when occlusal loads are delivered into the extension area and the prosthesis is compressed into the mucosa. (Courtesy of Dr A. Davodi, Beverly Hills, California.)

Fig 9-24 (a and b) In tooth­supported RPDs, occlusal rests should be extended into the center of the tooth. Rests are made lower in the center of the tooth. The minimum interocclusal space (thickness of the rest) for metal is 1 mm. (Part a courtesy of Dr R. Duell, Los Angeles, California.)

As mentioned previously, the rests are prepared after the guiding surfaces (guide planes) have been completed. This ensures that the rest will be properly positioned and contoured. The basic requirements for the preparation of rests are as follows: • All surfaces must be rounded and smoothed with no sharp angles (Fig 9-23). • A minimum thickness of 1 mm is required for strength and rigidity. • The center of the rest seat should be deeper in the middle of the tooth rather than at the marginal ridge (Fig 9-24a). • Rests on molars should be extended to the center of the tooth (Fig 9-24b). • The rest preparation should be wider at the marginal ridge area than toward the center of the tooth. • Junctions with other parts of the casting should be rounded to reduce the risk of fracture. • All parts of the rest must be located in sound tooth structure or on properly designed restorations. A #6 or #8 round bur, depending on the size of the occlusal surfaces, is used with a high-speed air rotor to make the rest preparations. If the rest is located on the axis of rotation of an extension-base RPD, the rest should be prepared in the shape of a half-sphere so a ball and socket joint is created between the RPD casting and the rest, allowing a pure unimpeded rotation when occlusal forces are generated in the extension area (see Fig 9-23). If the rest is prepared as a slope at the rotation point, the rest will tend to slide down the slope when occlusal forces compress the RPD into the denture-bearing surfaces in the extension area, exposing the abutment to lateral and/or torquing forces. In tooth-supported RPDs, occlusal rests should be extended into the center of the tooth (see Fig 9-24). The width of the rest preparation should be approximately one-third the width of the occlusal surface. 86

b

It may be difficult to prepare rests on anterior teeth in native tooth structure because of the anatomy of the lingual surfaces and the minimal thickness of enamel in this region. Rests prepared on anterior teeth are often inadequate and may not sufficiently engage the RPD framework to direct functional forces axially and provide the necessary stabilization and control of tooth position. Attempts to create such rests can also result in penetration of the enamel. Under these circumstances, a small preparation is completed and filled with either amalgam or composite resin (Fig 9-25). Failure to do so will predispose the teeth to the development of caries. Positive cingulum rests can be created in enamel in some maxillary canines (Fig 9-26), but if positive rests are required on incisors or mandibular canines, other methods such as partial- or full-coverage crowns or bonded rests must be employed (see chapter 2). In summary, when preparing rest seats for anterior teeth, it is imperative they are positive and definitive and they direct occlusal forces axially. Cingulum rests on anterior teeth are best prepared with a tapered fissure bur or tapered diamond with a rounded tip (Fig 9-27). The bur or diamond is positioned at an angle to the long axis of the tooth and the rest is prepared. The rest should be sufficiently positive so that when an occlusal force is applied, the rest will engage more securely to prevent separation between the rest and the abutment tooth. The placement of anterior rests is determined following a thorough evaluation of mounted study casts. There must be sufficient space available so the rest does not interfere with the planned scheme of occlusion in centric or eccentric positions. Only minimal tooth reduction is necessary when preparing incisal rests on anterior teeth because there is often extensive wear of the incisal surfaces. The rest is designed and prepared to cover most of the mesiodistal width of the tooth. The appearance from an anterior view is similar to the appearance of a thin metal veneer seen on the incisal edge associated with a three-quarter partial-coverage crown. It is imperative that the

Alterations of Tooth Contours and Restorations

Fig 9-25  The enamel was perforated during the prepara­ tion of these cingulum rests, so small tooth preparations were completed and filled with amalgam. (Courtesy of Dr R. Duell, Los Angeles, California.)

Fig 9-26 Frequently, positive cingulum rests can be created on canine teeth.

Fig 9-27  A cingulum rest is prepared with a tapered diamond with a rounded tip.

Fig 9-28  (a and b) The prepara­ tion of the incisal rest should en­ compass most of the mesiodis­ tal surface and extend over the incisal edge.

a

b

Fig 9-29  (right) The preparation of guide planes should follow the faciolingual contours of the proximal surface. In addition, it may be useful to reduce the prox­ imal surfaces to create additional space for the minor connector. Fig 9-30  (far right) Lingually in­ clined posterior teeth may need to be recontoured to create an appropriate path of insertion for the RPD.

rest extend onto the facial surface of the tooth so that it forms a positive rest (Fig 9-28). In many instances, all that is necessary to prepare the incisal rest is a sandpaper disk to smooth and remove the sharp linguoincisal angle. Interproximal surfaces may need reshaping to provide additional space for the minor connectors so they have sufficient bulk but also blend with tooth contours (Fig 9-29). This allows the minor connector to be recessed between the teeth so as to

minimize interference with the tongue. Lingual tooth surfaces of tilted teeth also may need to be recontoured so as not to interfere with the path of insertion for a lingual bar (Fig 9-30). Excessive undercuts on the facial side of the abutment may also need to be reduced so that the retainer is not positioned excessively away from the mucosa. Infrabulge retainers, which project excessively away from the surface of the alveolus, may irritate the buccal mucosa. 87

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Diagnosis, Treatment Planning, and Intraoral Preparation

Fig 9-31 A cast fabricated during the clinical ap­ pointment to verify parallelism and contour of the guiding surfaces and the depth and contour of the rests. Modifications are made as needed.

All prepared surfaces are rounded and smoothed to provide strength for the RPD casting and to facilitate its fabrication and cleansibility. These surfaces are smoothed and polished with sandpaper disks, rubber wheels, and pumice. If the integrity of the teeth has been jeopardized by the preparation, it may be necessary to restore these teeth with restorations. Frequently, it is advisable to verify the contours of the guiding surfaces and the depth and contours of the rests with an impression and a new cast poured with fast-setting plaster or stone. The cast is then surveyed, areas in need of correction are identified, and modifications are made as needed (Fig 9-31).

Reference 1. DeVan MM. The nature of the partial denture foundation: Suggestions for its preservation. J Prosthet Dent 1952;2:210–218.

Suggested Reading Applegate OC. Evaluating oral structures for removable partial dentures. J Prosthet Dent 1961;11:882–885. Applegate OC. The interdependence of periodontics and removable partial denture prosthesis. J Prosthet Dent 1958;8:269–281. Applegate OC, Nissle RO. Keeping the partial denture in harmony with bio­ logic limitations. J Am Dent Assoc 1951;43:409–419. Bergman B. Periodontal reactions related to removable partial dentures: A literature review. J Prosthet Dent 1987;58:454–458. Bergman B, Hugoson A, Olsson CO. A 25 year longitudinal study of patients treated with removable partial dentures. J Oral Rehabil 1995;22:595–599.

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Boero E, Forbes WG. Considerations in design of removable prosthetic devic­ es with no posterior abutments. J Prosthet Dent 1972;28:253–263. Dail RA, Kopczyk RA. Removable partial dentures and oral health: A literature review. J West Soc Periodontol Periodontal Abstr 1977;25:122–129. Fenner W, Gerber A, Mühlemann HR. Tooth mobility changes during treat­ ment with partial denture prosthesis. J Prosthet Dent 1956;6:520–525. Holmes JB. Preparation of abutment teeth for removable partial dentures. J Prosthet Dent 1968;20:396–406. Jordan LG. Treatment of advanced periodontal disease by prosthodontic procedures. J Prosthet Dent 1960;10:908–911. Kapur KK, Deupree R, Dent RJ, Hasse AL. A randomized clinical trial of two basic removable partial denture designs. Part I: Comparisons of five­year success rates and periodontal health. J Prosthet Dent 1994;72:268–282. Kapur KK, Garrett NR, Dent RJ, Hasse AL. A randomized clinical trial of two basic removable partial denture designs. Part II: Comparisons of mastica­ tory scores. J Prosthet Dent 1997;78:15–21. Lytle RB. Soft tissue displacement beneath removable partial and complete dentures. J Prosthet Dent 1962;12:34–43. McCall JO. The periodontist looks at the clasp partial denture. J Am Dent Assoc 1951;43:439–443. McKenzie JS. Mutual problems of the periodontist and prosthodontist. J Prosthet Dent 1955;5:37–42. Neurohr FG. Health conservation of the periodontal tissues by a method of functional partial denture design. J Am Dent Assoc 1944;31:59–70. Smith FW, Applegate OC. Roentgenographic study of bone changes during exercise stimulation of edentulous areas. J Prosthet Dent 1961;11:1086– 1097. Thayer HH, Kratochvil FJ. Periodontal considerations with removable partial dentures. Dent Clin North Am 1980;24:357–368. Waerhaug J. Justification for splinting in periodontal therapy. J Prosthet Dent 1969;22:201–208. Wiebelt FJ, Shillingburg HT Jr. Abutment preparation modifications for re­ movable partial denture rest seats. Quintessence Dent Technol 1985;9: 449–451.

Chapter 10 Impressions for the RPD Framework and Laboratory Instructions Daniela Orellana | Ting-Ling Chang | John Beumer III

Digital Methods

Conventional Materials

Scanning methods have been developed over the years to allow clinicians to construct a virtual dentition cast. This technology appears to be quite accurate when laboratory scanners are used to scan a master cast generated from a conventional impression. However, oral scanners have not proven to be sufficiently accurate for making the full-arch impressions that are necessary for the fabrication of a removable partial denture (RPD) metal framework. The scanning strategies used to make the scan are technique sensitive. For example, when scanning in between single-tooth defects, it may be difficult to position the scanning head (scanning wand) so as to accurately record the guiding surfaces. In addition, errors are introduced because the distance from the scanner to the object (the dentition) varies when making the scan. Moreover, only small areas can be scanned at one time, and consequently the data must be stitched together to create a full-arch scan. The process has the potential to introduce angular and distance errors when making full-arch digital impressions.1,2 Therefore, based on the data currently available, conventional impression methods remain the standard when making impressions used to fabricate RPD frameworks. However, as scanners and software improve, the day will soon come when accurate full-arch digital impressions can be made intraorally (see chapter 11). Presently, most clinicians prefer to use conventional impression methods and materials for such impressions.

Several materials can be used to make impressions for RPD frameworks. The most widely used is irreversible hydrocolloid (alginate). Other materials that have been used are the following: • Reversible hydrocolloid • Polysulfide • Polyvinyl siloxane Several factors influence the selection of the material used, including the following: • • • • • •

Convenience of use Surface detail Working and setting time Cost Need for custom impression trays The training and preference of the clinician

Impression trays have a significant impact on the accuracy of an impression. A properly extended and contoured impression tray will lead to more consistent results, particularly when using irreversible hydrocolloid or polysulfide impression materials. It is important to control the thickness of the impression material when making the impression. If the material is too thin in selected areas, the impression may distort upon removal or when 89

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Impressions for the RPD Framework and Laboratory Instructions

Fig 10-1 Irreversible hydrocolloid impression. It Fig 10-2 Polysulfide impression obtained with is desirable to capture key anatomical landmarks, a custom tray. such as the tuberosity or the retromolar pad.

stone is vibrated into the impression. Distortions can also occur if the material is excessively thick and not properly supported. The authors prefer to use irreversible hydrocolloid impression (alginate) material for RPD frameworks. Alginate impressions have several advantages. They can be used in the presence of saliva, they are hydrophilic, they have a pleasant taste and odor, they are inexpensive, and they pour well with stone. The disadvantages of this material (poor edge strength and less surface detail compared to polysulfides and silicones) are not clinically significant when making impressions for an RPD framework. Polysulfides require a custom tray, and the impression may become distorted upon removal when several teeth with significant undercuts are present. Polyvinyl siloxanes are considerably more expensive than alginates, and the superior accuracy and surface detail obtained with these materials is not clinically significant when fabricating RPD frameworks. The following sections outline the advantages and disadvantages of the various materials used for RPD framework impressions.

Irreversible hydrocolloid (alginate) impressions Irreversible hydrocolloid impressions capture key anatomical landmarks and have several other advantages (Fig 10-1).

Advantages • They can be used with stock trays. • The material is hydrophilic and can be used in the presence of saliva. • They have a pleasant taste and odor. • The material is inexpensive. • They enjoy a short snap set. • They pour well with stone. 90

Fig 10-3 Polyvinyl siloxane impression.

Disadvantages

• They have a low tear strength. • They provide less surface detail than other materials. • Their dimensional instability requires an immediate pour.

Polysulfide (rubber base) impressions Polysulfide impressions have better surface detail and tear strength than alginate, but this material also has its disadvantages (Fig 10-2).

Advantages • • • •

They provide better surface detail than alginate. They exhibit better tear strength than alginate. They are moderately inexpensive. They are hydrophilic.

Disadvantages • They require custom trays, and the thickness of the material must be carefully controlled. • Medium- and heavy-body impression materials are highly cross-linked and do not recover well from deformation, so they should not be used when large numbers of teeth display significant undercuts. This is especially true when lone-standing periodontally involved teeth are present. • They have a long setting time. The material must be held still during the impression making procedure because the material does not have a snap set. • The custom trays must be designed to ensure a relatively uniform thickness of the impression material. Otherwise, distortions may occur. • They possess an unpleasant odor.

Tray Selection

• The material stains clothing. • Their dimensional instability requires an immediate pour.

Additional modifications may be required to ensure a consistent thickness of alginate and to prevent distortions.

Polyvinyl siloxane impressions

Tray alterations—Mandible

Polyvinyl siloxane impressions are more expensive but have superior accuracy and surface detail, among other advantages (Fig 10-3).

Posterior extensions

Advantages • They provide superior surface detail. • There is less polymerization shrinkage. • There is low distortion. • The material exhibits high tear strength. • The material recovers rapidly from deformation. • They possess a relatively short working time (3 to 5 minutes for addition reaction types and 5 to 7 minutes for condensation types). • The material can be used in a stock tray. • They are available in both hydrophilic and hydrophobic forms. • They are available in automixing devices. • They possess excellent dimensional stability. Multiple pours are possible if poured within 1 week.

Disadvantages • The material is considerably more expensive compared to other materials. • It is difficult to remove the impression from the patient’s mouth if significant bone and soft tissue undercuts are present. • It is difficult to retrieve the cast from the impression without fracturing lone-standing abutments.

A properly extended tray should be 1 to 2 mm short of the retromolar pad when the mandible is in the closed position. The posterior limits of the tray can be extended with periphery wax or dental compound when needed.

Flanges and contour The tray should be adapted and extended to within 2 mm of the peripheral tissues. Lingual extensions of the tray often need to be lengthened, particularly in the premolar and molar regions, to ensure proper positioning of the tongue and floor of mouth when making the impression. When applying periphery wax to the border, make sure it does not obliterate the rim lock portion of the tray. Otherwise, the impression material may become detached from the tray during removal of the impression. When making the impression, the patient is instructed to elevate the tongue before the tray is seated into position. In edentulous areas in the presence of extensive bone and soft tissue loss and where excessive space exists between the tray and the tissues, it is advisable to fill in this space with periphery wax or dental compound (Fig 10-4). Under some circumstances, it is desirable to border mold the contours of the floor of the mouth in the anterior region. This process will enable the clinician to accurately assess the space available for the lingual bar (see chapter 4).

Tray alterations—Maxilla

Tray Selection The authors favor the use of metal stock trays. These trays are rigid yet somewhat adjustable. Avoid the use of plastic stock trays when making master impressions. They are not sufficiently rigid and may become distorted when making and/or removing the impression. The following sequence is recommended: 1. If a diagnostic cast is available, size the tray to the cast. 2. Test the tray intraorally. Upon seating the tray, move it side to side and anteroposteriorly to ensure the presence of at least 3 mm of space between the tray and the teeth and soft tissues to be recorded. 3. Check the length of the tray to ensure adequate coverage of teeth and relevant soft tissues.

Posterior extensions The tray must extend to the hamular notch and the posterior palatal seal area. If the tray is short, dental compound is added to extend the tray around the tuberosity and into the hamular notch region. This will serve to enclose the impression material and prevent it from flowing away from the distal surfaces of the maxillary molars. Periphery wax can also be used to bring the tray into contact with the posterior palatal seal region as shown in Fig 10-5. These extensions will ensure positive contact of the impression material with the palate and prevent excessive impression material from escaping posteriorly. When periphery wax additions are made, the wax should be heated in a water bath prior to insertion. This will permit the wax to mold and adapt to the tissues. Building up the palatal portion with wax

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Fig 10-4 Mandibular stock tray modified with pe- Fig 10-5 Maxillary stock tray modified with periphery riphery wax in the edentulous area to ensure even wax. Note the extension to the posterior palatal seal thickness of the impression material. region and in the palatal vault area to ensure proper coverage and even thickness of the impression material.

or dental compound is commonly needed to ensure an even thickness of impression material in this region and prevent slumping of the impression material away from the palatal vault.

excessively. Doing so will result in the alginate sticking to the tooth surfaces and reduce the water content of the impression material, distorting it. Proceed to make the impression. Apply a thin layer of a suitable adhesive to the tray and air-dry it.

Clinical Procedures for Alginate Impressions

Preparation of the impression material

Instructions for the patient Prior to making the impression, the patient must be informed about the procedure and provided instructions regarding the nature of his or her participation. Instructions include positioning of the mandible, tongue, lips, and cheeks; breathing instructions; and the time required to complete the impression. The patient must be relaxed. A trial run of the impression procedure is always helpful and reassuring to the patient. The patient should be instructed as follows: • The cheeks, lips, and tongue should be as relaxed as possible. • Open the mandible only wide enough to insert the tray. After the tray has been inserted, instruct the patient to close just short of tooth-tray contact. • Instruct the patient to breathe through the nose when making the impression. It is useful to remind the patient that he or she has a second airway through the nose that bypasses the oral cavity and the impression. • For mandibular impressions, instruct the patient to position the tongue gently forward on top of the lingual flange of the tray. Remove excess saliva from the surfaces of the teeth by placing gauze pads on the tooth surfaces, but do not dry them 92

Mix the material following the water-powder ratio recommended by the manufacturer. It may occasionally be necessary to reduce the water content slightly to increase viscosity for soft tissue displacement. Load the tray, being careful to avoid entrapment of air pockets or bubbles. Smooth the surface of the material with a moist gloved finger prior to insertion. Do not add material or water to the smoothed surface.

Making the impression 1. Remove the gauze pads from the mouth of the patient. Instruct the patient not to moisten the teeth. 2. Spread a thin layer of alginate on the surfaces of the teeth using the index finger, paying particular attention to rests and other areas to be engaged by the RPD framework. In areas of difficult access, it may be advantageous to use a disposable syringe. 3. Insert and position the tray and instruct the patient to close just short of tooth-tray contact. 4. For mandibular impressions, instruct the patient to place the tongue on the lingual flange of the tray, move the tip forward, and gently push against the operator’s thumb. 5. When the mouth is partially closed, the tongue is in proper position, and the patient is relaxed, gently seat the tray into position. In the maxilla, the posterior portion of the tray should be seated first and then the tray rotated into posi-

Clinical Procedures for Alginate Impressions

Fig 10-6  (a and b) Framework maxillary impression and opposing mandibular impression.

a

b

Fig 10-7  (a) The stone is poured onto the impression and allowed to set. (b) A second pour of stone is placed atop the workbench, and the first pour is inverted to create the base and land area.

a

tion. This will minimize the amount of impression material displaced posteriorly. 6. The tray must remain immobile during gelation, otherwise an inaccurate impression will be produced. 7. Do not allow the tray to come in contact with occlusal surfaces or impinge on the soft tissues. Be gentle. 8. Gently massage the peripheral areas for better adaptation in these regions.

Tray removal 1. Hold the tray in one position for the setting time recommended by the manufacturer. 2. Remove the tray with a firm, quick snap. 3. Check to ensure that the alginate has not separated from the tray. 4. Carefully examine the impression to ensure that all details have been reproduced and no voids are present (Fig 10-6). If impression material located interproximally between the teeth has been displaced, carefully reposition it. In some situations, it may be advantageous to trim these areas. Spray the impression with a suitable disinfectant and cover it with a moistened paper towel before taking it to the laboratory. 5. Pour the impression immediately.

b

Pouring the impression and fabricating the cast 1. Clean saliva and debris away from the impression with air and slurry water. Remove the slurry water with a gentle air spray. 2. Prepare type IV dental stone with the correct water-powder ratio according to the manufacturer’s recommendations. Use a vacuum-powered mixer if available. 3. Place a small amount of stone at one end of the impression. With low vibration against the handle of the tray, watch to ensure that the stone flows completely into every part of the impression. Avoid the use of excessive vibration because this will produce bubbles on the cusp tips. Continue to add stone slowly until the impression is filled to the periphery (Fig 10-7a). 4. Invert the tray into a circle of stone placed atop the workbench. Take care not to distort or displace the alginate that extends beyond the tray. Use light pressure to avoid such distortions. Note that some clinicians use an alternative technique. The first pour fills the impression to the periphery. The impression is allowed to set and is then inverted into a second base pour (Fig 10-7b). The base of the cast should be 10 to 15 mm thick. 5. When the initial set has taken place, trim the periphery of the cast with a lab knife. 93

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Fig 10-8 (a and b) Master cast is fabricated with a base height of 10 to 15 mm and land area of 2 to 3 mm wide.

a

b

6. Allow the cast to set for 1 hour prior to separation. 7. Trim the cast, being careful to preserve the peripheral roll. Before trimming, soak the cast in slurry water for 5 minutes. Failure to soak the cast in slurry water will result in sludge produced by the model trimmer adhering to and damaging the cast. The flange beyond the peripheral roll is trimmed at right angles to the base and to create a land area 2 to 3 mm wide (Fig 10-8).

Common problems encountered The most common problems encountered when making irreversible hydrocolloid impressions and casts are inaccurate casts and casts with soft or chalky surfaces.

Inaccurate cast Inaccurate casts result from the following: Impression material separated from the tray Shrinkage of the impression material secondary to dehydration Imbibition or syneresis Air inclusion in the impression that distorts when the stone is poured • Nonrigid tray used for the impression

• • • •

Soft or chalky cast surface Casts that have a soft or chalky surface are generally due to the following: • • • •

Saliva in the impression when the cast was poured Improper mixing of stone Improper water-powder ratio used for stone Water from rinsing the impression remains in the impression

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Impressions with Custom Trays In patients with unusual anatomical configurations, occasionally metal stock trays cannot be altered sufficiently to permit making a suitable impression, and custom trays must be employed. Also, some clinicians choose to develop the borders for the RPD in edentulous extension areas with a single impression as opposed to making an altered cast impression with the RPD framework. These trays are fabricated from tray resin or light-curing resin and are usually used when making impressions with elastomeric impression materials such as polysulfide or polyvinyl siloxane. A custom impression tray must possess the following features: • There should be uniform and adequate space for the impression material. • The tray should be rigid. • The tray should not warp from moisture contamination or temperature change. • There should be adequate retention for impression material. The proposed outlines of the tray are scribed on the preliminary cast with a pencil. The bone and soft tissue undercuts are blocked out with baseplate wax. A spacer in the form of baseplate wax is applied to the dentition. For elastomeric impression materials, this spacer should be 2 to 3 mm. For alginate, the spacer should be at least 3 mm, and the tray should be perforated to enhance mechanical retention. For additional retention, suitable tray adhesive should be used (Fig 10-9). The extension areas of the tray are developed with dental modeling plastic (dental compound). Following a cutback of the compound to account for the thickness of the impression material in the edentulous extension areas, the impression is made. Most clinicians prefer polysulfide over polyvinyl siloxane because polysulfide’s favorable viscosity and flow patterns permit the making of excellent impressions of the edentulous extension areas. The completed impression is boxed in a similar manner to complete denture impressions (Fig 10-10).

Impressions with Custom Trays

a

b

c

Fig 10-9  (a) Preliminary cast. The outlines for the tray are scribed on the cast, generally 1 to 2 mm short of the depth of the vestibule. (b) Bone and soft tissue undercuts are blocked out. (c) Additional layers of baseplate wax are applied to create a spacer of the desired thickness. (d) The completed tray is fabricated for polysulfide or polyvinyl siloxane. (e) If irreversible hydrocolloid impression material is planned, the blockout should include soft tissue and teeth and the custom tray should be perforated for mechanical retention. (Parts a to d courtesy of Dr K. Ochiai, Los Angeles, California.)

e

d

a

b

c

Fig 10-10  (a) Border molded custom tray. (b) Polysulfide impression boxed with periphery and rope wax. (c) Impression ready for pouring. (Courtesy of Dr K. Ochiai, Los Angeles, California.)

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a

b

Fig 10-11 (a) Compound cake and ZOE impression paste capture the cusp tips of the remaining teeth. (b) Occlusal index used to confirm accuracy of the master cast.

Occlusal Index After the impression is made, an occlusal index is fabricated intraorally to help the clinician and laboratory verify the accuracy of the master and refractory casts. It is especially helpful when posterior teeth or lone-standing abutments are separated by large edentulous areas. The occlusal index is fabricated with an impression compound cake and a thin layer of zinc oxide– eugenol (ZOE) impression paste that captures the cusp tips and positions of the remaining teeth (Fig 10-11a).

Technique used to obtain the occlusal index 1. The impression compound cake is placed into a water bath at 122°F to 124°F until softened. 2. The soft compound cake is adapted to the remaining teeth until all cusp tips are registered. 3. The compound cake is cooled with an air syringe and removed from the mouth. Cold water may be used to speed cooling. 4. A thin layer of ZOE impression paste is applied to capture the cusp tips of the remaining teeth. Allow the paste to set, remove from the mouth, and inspect. If excessively deep indentations were obtained, a scalpel may be used to remove the excess material. Before submitting the master cast to the laboratory, the clinician should confirm the accuracy of the master cast using the occlusal index (Fig 10-11b).

Master Cast Preparation All parts of the master cast are inspected for smooth, accurate reproduction of the anatomy of the mouth. Special attention 96

is directed to the places the RPD casting will contact, such as rest areas, guide surfaces, retention areas, and interproximal contact areas. Artifacts and bubbles are removed. The trimmed and dried master cast is checked for dimensional accuracy by placing the occlusal index, which was recorded from the patient, onto the occlusal surfaces of the cast (see Fig 10-11b). If there are spaces between the index and the cast or if there is a “rocking” of the index on the occlusal surfaces, this indicates there are inaccuracies in the cast and a new impression must be made. The technician can also use this index to ensure accurate duplication of the refractory cast. The master cast is placed on the survey table, and the same procedure used with the design cast is repeated to establish the most advantageous position (MAP, see chapter 8). When the MAP appears by visual survey to be correct, proceed with the following checks using the surveyor: (1) Use the diagnostic rod to verify the guiding surfaces are parallel to each other (Fig 10-12). (2) Use the diagnostic rod and then measure with the undercut gauge to verify that there are proper retention undercuts as planned (Fig 10-13).

Tripod the cast In order to be able to fabricate a casting according to the clinician’s prescription, the laboratory must be able to reproduce the identical MAP the clinician identified. When the cast is tripoded, this position can easily be reproduced by the laboratory. To accomplish this task, the undercut gauge is placed in the vertical arm of the surveyor. The vertical arm is then fixed in position so the tip of the undercut gauge touches the master cast in three widely separated places (Fig 10-14) as the survey table with the cast in place is moved around the base table of the surveyor. Without changing the position of the arm, the clinician places a mark on the master cast at each of these three points and circles them in red or blue pencil for easy identification (see Fig 10-14). The marks should be made beyond the

Laboratory Communication and Instruction

Fig 10-12  Use the diagnostic rod to verify the parallel guiding surfaces of the abutment teeth on the master cast.

Fig 10-13  The retention areas are confirmed with the 0.01-inch undercut gauge.

Fig 10-14  Three widely separated marks on the same horizontal plane are scored into the cast with the undercut gauge to tripod the cast. The marks should be made beyond the limits of the proposed RPD framework.

limits of the proposed RPD framework. This exact orientation of the master cast can now be reproduced by the laboratory by repeating the procedure with a surveyor (see chapter 8).

this information can be transmitted is by a 3D replica of the patient’s oral anatomy with the design and the master casts.

Written instructions

Laboratory Communication and Instruction Design of the removable partial prosthesis is a critical factor for proper treatment. It is the major factor in diagnosis and treatment planning. The design is entirely dependent on the amount, type, and condition of the remaining oral anatomical structures of the patient, so it is the clinician’s complete responsibility to control all of the procedures. Ethically, the design cannot be delegated to an individual other than the treating clinician. The dental laboratory technician is a trained, skilled individual who is capable of producing a prosthesis that can successfully replace a missing part of the oral cavity if given complete information, instructions, and proper casts. The most important factor is the transfer of detailed clinical information and instructions from the clinician to the laboratory. Communication must be complete, precise, and three-dimensional (3D). The only way

Basic information and additional instructions are written on the laboratory prescription that accompanies the occlusal index and the master casts (Fig 10-15). The following information is included: • Patient identification • Type of prosthesis requested • Type of metal to be used • An appropriate design prescription as shown in Fig 10-16, including the following: –Sharp, smooth outlines of the framework design proportionately drawn –Bead seals clearly marked –Retention areas indicated –Resin-metal finish lines clearly marked • Specific instructions as necessary • Doctor’s signature and license number

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Laboratory Communication and Instruction

b

c

Fig 10-15  The written laboratory prescription (a), the occlusal index record (b), and the master cast (c) are submitted to the laboratory to provide precise instructions for RPD framework fabrication.

a

b Fig 10-16  (a and b) An appropriate design prescription will have sharp and smooth RPD design outlines proportionately drawn with the retention areas, guide plane–tissue contact, finish lines, and bead seals clearly marked as shown on the maxilla and the mandible.

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References 1. Ahlholm P, Sipilä K, Vallittu P, Jakonen M, Kotiranta U. Digital versus conventional impressions in fixed prosthodontics: A review. J Prosthodont 2018;27:35–41. 2. Rutkūnas V, Gečiauskaitė A, Jegelevičius D, Vaitiekūnas M. Accuracy of digital implant impressions with intraoral scanners. A systematic review. Eur J Oral Implantol 2017;10(suppl 1):101–120.

Suggested Readings Anusavice KJ, Shen C, Rawls HR. Impression materials. In: Phillip’s Science of Dental Materials, ed 12. St Louis: Elsevier, 2013:151–181. Aragón ML, Pontes LF, Bichara LM, Flores-Mir C, Normando D. Validity and reliability of intraoral scanners compared to conventional gypsum models measurements: A systematic review. Eur J Orthod 2016;38:429–434.

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De Luca Canto C, Pachêco-Pereira, Lagravere MO, Flores-Mir C, Major PW. Intra-arch dimensional measurement validity of laser-scanned digital dental models compared with the original plaster models: A systematic review. Orthod Craniofac Res 2015;18:65–76. Herfort TW, Gerberich WW, Macosko CW, Goodkind RJ. Tear strength of elastomeric impression materials. J Prosthet Dent 1978;39:59–62. Müller P, Ender A, Joda T, Katsoulis J. Impact of digital intraoral scan strategies on the impression accuracy using the TRIOS Pod scanner. Quintessence Int 2016;47:343–349. Nassar U, Aziz T, Flores-Mir C. Dimensional stability of irreversible hydrocolloid impression materials as a function of pouring time: A systematic review. J Prosthet Dent 2011;106:126–133. Rudd KD, Morrow RM, Strunk RR. Accurate alginate impressions. J Prosthet Dent 1969;22:294–300. Skinner EW, Hoblit NE. A study of the accuracy of hydrocolloid impressions. J Prosthet Dent 1956;6:80–86.

Chapter 11 RPD Digital Design and Manufacturing Jay Jayanetti | Daniela Orellana | Ting-Ling Chang

There are three basic phases of the digital workflow when designing and/or fabricating removable partial denture (RPD) frameworks: (1) data acquisition, (2) digital design (computeraided design (CAD), and (3) computer-aided manufacturing

Intraoral scanner

Fixed Pros STL

Lab scanner

Acquisition

(CAM) (Fig 11-1). The bulk of this chapter is dedicated to the design steps used in this workflow, utilizing sample maxillary and mandibular casts.

RP

Implant Pros

Rem Pros

Designing

STL

SLM SLA

SLS 3DP

Manufacturing

Fig 11-1 Diagram of the sequence of phases in the digital workflow. Manufacturing has the generic name of rapid prototyping (RP). STL, standard triangulation language; Pros, prosthodontics; Rem, removable; SLM, selective laser melting; SLS, selective laser sintering; SLA, stereolithography; 3DP, three-dimensional printing.

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Fig 11-2 Master cast with the maxillary RPD design. Notice that for maxillary frameworks, the master cast must be beaded prior to scanning. This cannot be accomplished digitally. Also, if the master cast is designed as shown here, some scanners can image the surface texture, including the pencil markings. This can aid in the digital design process.

a

b

Fig 11-3 The various depths of undercut areas on the dentition and the soft tissue of the digitized master cast at a given path of insertion can be noted with multiple simultaneous views. The operator can rotate and zoom in on the digital model with great control.

Data Acquisition A master cast is obtained with a conventional impression as described in chapter 10. Note that intraoral scanners have not proven to be sufficiently accurate for making the full-arch impressions that are necessary for the fabrication of an RPD metal framework. A preliminary design should be outlined on the master cast (Fig 11-2). Before scanning the master cast, bead lines for maxillary major connectors must be inscribed. This is a limitation of digital design because any changes to the position of the major connector would require a new impression and fabrication of a new master cast. The cast is scanned 102

with a high-resolution laboratory scanner. The data obtained is stored as an STL (standard triangulation language) file. Unlike single-unit restorations that are designed with a die spacer thickness of 100 microns, more precision is necessary when making RPD frameworks because the rests and proximal plates must be in intimate contact with multiple abutment teeth across the arch. The accuracy of the three-dimensional (3D) representations obtained is determined by the size and density of the triangles of the STL formatted files. This specific issue should be considered when the clinician decides to acquire a scanner for his or her office.

Maxillary RPD

Digital Design CAD software enables the design of the RPD framework in 3D. Using geometric analysis tools that are designed to digitally survey the cast, the depth of the undercut at a given path of insertion can be visualized (Fig 11-3). Unwanted undercut areas are blocked out digitally. The undercuts to be engaged are exposed, and the RPD framework is then designed (see Fig 11-6). The design software explained in this chapter organizes the process into three sections: 1. Surveying and blocking out: The design tools used are “Blocking out” and “Wax trimmings” (Fig 11-4). 2. R PD design: The design tools used are “Retention grids,” “Major connector,” “Clasps,” “Sculpt,” and “Finishing line” (see Fig 11-7). 3. Finalize: The design tools used are “Sculpt RPD design” and “Pre-manufacturing” (see Fig 11-18). The operator is able to return to any previous step and modify the design without losing downstream manipulations. However, depending on the speed of the computer, there is a lag time for rendering the image, and it behooves the designer to move constantly downstream.

Maxillary RPD Surveying and blocking out The digital survey is relatively straightforward because it allows the operator to quickly visualize the digital cast from any perspective. Color-coded isodepth curves serve to delineate the depths of the tooth and soft tissue undercuts (Fig 11-5). While conventional survey lines are marked in carbon, the digital equivalent is the occlusal aspect of the yellow isodepth curve. The gingival limit of the yellow isodepth curve corresponds to a 0.25-mm (0.01-inch) undercut. The software uses millimeters to measure the depth of the undercut instead of tenths of an inch. While chapter 8 discussed gauge undercuts of 0.01 inch, 0.02 inch, and 0.03 inch, the digital software uses the millimeter equivalent: 0.25 mm, 0.50 mm, and 0.75 mm, respectively (Fig 11-6). Broad curves indicate gradual convergence, while tightly spaced isodepth curves indicate greater gingival convergence (the slope of the undercut region). The operator may review the surveying result using multiple views. Tilting the cast can be accomplished in a variety of degree increments (1 to 5 degrees). Specific areas of interest can be magnified and rotated at will for better visualization until surveying is complete (see Figs 11-3, 11-5, and 11-6). Once the path of insertion or most advantageous position (MAP) has been determined, click the “Next” arrow icon.

Fig 11-4  The first of three sections in the digital design process. It begins with “Surveying and blocking out.” See how blocking out and wax trimming are the two steps in the section.

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Wax trimming

Fig 11-5 Automated parallel blockout based on the chosen path of insertion. The operator can control whether the blockout is to be truly parallel (0 degrees) or have more divergence (1 degree, 2 degrees, or 3 degrees).

When the MAP has been determined, all undercuts, including the measured undercuts required for retention, are blocked out parallel to the path of insertion. The next step is to quantitatively expose the desired undercuts for the tips of the retainers. All other unwanted undercut areas remain blocked out consistent with the path of insertion (see Fig 11-6b). Specific for this design, 0.25-mm undercuts are desired for all four retainers. Enough wax is removed to expose the junction of the yellow with the light orange. This is performed on the midfacial area of the canines and on the distobuccal line angles of the molars (see Fig 11-6). If a sharp angle is formed when removing the wax blockout, it may be smoothed with the appropriate tool. Make certain that the appropriate amount of undercut is exposed. It is advisable to err on exposing a bit more of the undercut because increasing retention once the frame is cast is more difficult than reducing the retention. In the “Wax trimming” section, arbitrary wax relief may be added where indicated, such as underneath a mandibular bartype major connector (discussed in the mandibular example later in the chapter) or underneath an infrabulge retainer.

RPD design

a

b

c

Fig 11-6 (a and b) The operator can digitally remove the digital rendering of the blockout wax from the desired retention areas for the RPD retainers. (c) The yellow coloring denotes the retentive region of interest. The 0.25-mm undercut begins at the junction of the yellow and light orange region. For example, if an I-bar retainer is used, the tip of the retainer should terminate at the occlusal side of the region of the tooth with yellow coloring and should engage the tooth cervically to the light orange–yellow junction. If the clinician wishes to engage a 0.50-mm undercut, the cervical wax blockout must be eliminated and the tip of the retainer extended to the junction between the light orange and dark orange.

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This section of the design software is divided into five steps, beginning with “Retention grids” (Fig 11-7), where relief wax for open lattice or mesh-type denture base connectors are designed. “Major connectors” is the second step in the design process, followed by the design of clasp assemblies. The “Clasps” design tool is also used to design minor connectors and rests. The so-called “Sculpt” tool is next, which allows the clinician to thicken, thin, smooth, and blend specific areas of concern. The final step in the design section is referred to as “Finishing line,” a misleading term because only external finish lines (EFLs) are positioned with this tool. Internal finish lines are positioned during step one or when the outlines of the retention grids are established. The design software follows a different sequence than the one discussed for drawing the design on a physical cast. The previously suggested sequence helps avoid mistakes that would require erasing red pencil marks on a stone cast, but this concern is eliminated on a virtual cast. While the sequence from the design software differs from conventional techniques, the RPD design should be consistent with the principles of RPD design described previously (see chapters 6 and 7).

Denture base connectors (“Retention grids”) There are a variety of denture base connectors that can be designed: mesh-type connectors, open lattice–type connectors, and metal base connectors with retention features, such as posts. If a mesh-type denture base connector is used, it should be designed first. If a metal denture base connector is chosen, it is designed

Maxillary RPD

as an extension of the major connector. If an open lattice design is used, the wax relief (described in the software as “resin gap”) is designed during this step, while the struts are laid out along with minor connectors later in the design process. The placement of relief wax is dependent upon the type of denture base connector chosen. For example, if a mesh type is selected, the relief wax is configured automatically by the software program. If an open lattice design is selected, the wax relief is outlined, but the struts are designed during a subsequent design step (Fig 11-8). The gauge of the relief wax can be modified as needed. Finally, if a metal base connector is desired, it is designed as an extension of the major connector, and relief wax is not needed (Fig 11-9). Whether a mesh or open lattice connector is desired, the borders of the wax relief define the internal finish lines; therefore, the clinician should configure it based upon the need to cover the tooth-tissue junction with metal and properly position the internal finish line. Note that EFLs are designed during step five of the RPD design section. The sample design for this maxillary RPD will have all three types of denture base connectors. Begin by selecting the tool for designing the wax relief. Remember that this is needed for an open lattice connector. Begin with a series of points to outline an area. The last point in the series must be superimposed on the first point, which will close the loop that completes the layer of wax relief. The term spline is introduced here. In mathematics, a spline is a curve that approximates points on a chart or a graph; the same concept applies here. When points are placed to design a line or an area, the program curves the line as opposed to connecting each point with the shortest straight line. The program also automatically adapts to the surface contour of the virtual cast. If desired, each point on the spline can be repositioned to manipulate the line or area. When a spline is completed, the area will fill with the selected material, which in this case is a layer of relief wax. The open lattice will be laid over the wax relief during a later step. By selecting a mesh and repeating the previous steps, a completed spline will render a digital representation of the chosen denture base connector along with the wax relief beneath it (see Fig 11-8).

Major connector From a software design perspective, there are two types of connectors: (1) connectors defined by an “area” such as plates and straps and (2) connectors defined by a “line” such as bars. The design process for the lingual bar major connector is explained in the section devoted to the design of a mandibular RPD framework (see section entitled “Mandibular RPD”). The outline of area-type major connectors is defined by a series of points that cover the desired denture-bearing surface. The spline should extend to include the bead lines. It is critical to have an overlap with any previously designed denture base connector to avoid an error message or voids between compo-

Fig 11-7  The recommended sequence of design as required by the software. Note the icons for each step of design. Note the difference between the “RPD design” tools and the “Finalize” tools.

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Fig 11-8 Wax relief is placed in the maxillary right first molar to first premolar edentulous span in anticipation of designing an open lattice. The mesh retention grid overlying the left first premolar to first molar edentulous span is automatically laid with its own wax relief. The anterior edentulous site will have a metal base, which is an extension of the major connector and does not require wax relief. Notice also the bead lines that will define the major connector outline.

Fig 11-9 Anteroposterior palatal strap major connector is designed. (a) The outline of the perimeter is laid down first, making sure to overlap with the mesh connector. (b) A complete palatal plate is rendered. (c) A window spline is laid down inside the bead line. (d) The rendering of the pattern after the window is placed separates the anterior and posterior palatal straps.

a

b

c

d Fig 11-10 Note that the white spline laid for the major connector outline overlaps with the denture base connector.

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Maxillary RPD

1 4 2 3

a

6

5

b

Fig 11-11  (a) This illustrates the occlusal rest spline that begins at the depth of the spoon-shaped mesial occlusal rest and wraps over the mesial marginal ridge and onto the guide plane (1). A proximal plate spline overlaps the occlusal rest at the marginal ridge and continues gingivally and onto the tooth-tissue junction and mesially toward the edentulous space where the wax relief has been laid for the open lattice denture base connector (2). Lastly, the palatal plate spline is a horizontally placed minor connector spline that also overlaps with the occlusal rest and proximal plate splines (3). Notice that the overlapping splines occupy the same space without doubling or tripling the thickness. (b) This illustrates a rest spline used to create a cingulum rest that begins and ends on either marginal ridge (4). It overlaps with the distal proximal plate (5) and the palatal plate mesially (6). The width of the rest and palatal plate are controlled to expose a cingulum window (as discussed in chapter 2). Fig 11-12  Open lattice denture base connector. This image illustrates the creation of an open lattice denture base connector. In a prior step, the relief wax was laid, defining the internal finish lines. During the minor connector step, struts are positioned to create a network for retaining the acrylic resin base. This example shows a buccal strut that connects the proximal plate minor connectors of the canine and molar. It is positioned to allow room for setting the cervical ridge lap of the prosthetic teeth. For this small edentulous space, one cross strut is sufficient and creates an open design. This one cross strut will lead into the approach arm of the I-bar infrabulge retainer.

Buccal strut

Cross strut

nents (Fig 11-10). Each point defining the perimeter can be moved to extend or shorten the palatal coverage. When designing an anteroposterior strap major connector, the clinician should first define the perimeter of the palatal plate. Then a window is designed within the plate to separate the anterior from the posterior straps (see Fig 11-9).

Minor connectors, retainers, and rests (“Clasps”) This step provides tools for creating clasp assemblies, including rests, proximal plates, bracing components, and retainers. Minor connector splines are also utilized to form a latticework over the previously laid relief wax for connecting a denture base. The software allows the clinician to design these components in any sequence. The authors recommend designing the rests first.

Rests. Rests are designed with a single spline as opposed to an area. As described previously, at each point along the spline, the width and thickness can be controlled. With rests, when increasing the width, the algorithm of the software fills the concave surface area of the prepared rest seat. Therefore, it is best to place the first point of the spline at the deepest and most central point in the occlusal rest seat. Two or three subsequent points are sufficient to complete the spline: one point on the marginal ridge and one point on the guide plane as seen in Fig 11-11a. A cingulum rest with a window is also designed with the same occlusal rest tool and in combination with the minor connector as a short palatal plate (Figs 11-11 and 11-12). Minor connectors. Like the previous components, minor connectors are also defined by single splines. Once rests have 107

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Fig 11-13 This diagram illustrates a proper taper of a retainer. Half-round cross sections are seen for the shoulder and terminus. The base and height of the circle are indicated.

been positioned, the minor connectors serve to connect these components to the major connector. Additionally, proximal plates are designed where needed and extended to cover the tooth-tissue junction and connect to the retention grid. In situations where relief wax was designed for an open lattice denture base connector, a grid pattern of struts is arranged to form the connector. Particular to infrabulge retainers, one of the cross struts of the open lattice should align with the approach arm of the retainer to provide the necessary bulk and rigidity. This also allows for the cast metal to flow from the strut into the I-bar. When plating palatal or lingual tooth surfaces, it is necessary to use the minor connector tool instead of extending the major connector onto the tooth. This will ensure that the plate will possess a smooth and polished surface rather than a stippled one. The reason for this is that a subsequent step will stipple the surface of the palatal major connectors.

The I-bar spline should begin at a point in the denture base connector that will least interfere with setting of the prosthetic tooth, especially when space is limited. This means designing the I-bar so it lies at an embrasure. When originating from an open lattice, the approach arm of the I-bar should connect to a cross strut of the latticework (Fig 11-15). I-bar retainers are also half-round in cross section. The approach arm should have a base dimension of 2 mm. It will curve into the vertical arm approximately 3 mm from the free gingival margin and cross the tooth-gingiva junction at 90 degrees. It should taper from 2 mm to 1 mm at its terminus. The tip will make contact with the tooth at the exposed undercut and continue occlusally to the height of contour (the occlusal limit of the yellow isodepth curve). It is a common mistake by novices to end the I-bar at the undercut and not extend to the height of contour (see chapter 5).

Retainers. When designing retainers, a drop-down menu includes software-specific terminology that may confuse the novice. The choices define the cross-sectional shape, all of which are variations of the half-round. The advanced user can customize a preferred cross section and taper. All retainer options are designed with a single spline. Each point on the spline can be modified in thickness and height, hence defining the 3D taper. Half-round dimensions at the shoulder of circumferential clasps should have a base of 1.5 to 2 mm and a height of 0.75 to 1 mm. The dimensions at the tip should be half that of the shoulder. This will produce the appropriate taper and appropriate flexure without concentrating stresses (Figs 11-13 and 11-14). The retainer tip is placed in the previously determined undercut. A bracing clasp must be thicker and contact more surface area of the abutment tooth. It should be designed with less taper to enhance rigidity. The entire length of the clasp should lie at or above the height of contour.

When the clinician reaches this point in the design process, all previously designed components will automatically be fused into one contiguous structure. This step allows for increasing or decreasing the thickness of any area of the framework. There is a virtual toggle that determines a tolerance for thickness. When set to 0.5 mm, for example, anything thinner than 0.5 mm will show in red, providing a visual indicator of potentially weak areas. The thickening tool is used in strokes over areas of concern. The area is thickened with every stroke, and red areas change to yellow and then green when thickness reaches above the set tolerance (Fig 11-16). Another tool in this section allows the clinician to smooth the surface. Areas of concern are junctions between components (eg, rests to minor connectors, retainers to minor connectors, and minor connectors to major connectors). This latter feature is much like using an alcohol torch to flame over rough areas of wax. The degree and area size that is to be smoothed is controlled by a click of the mouse (see Fig 11-16b).

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Sculpt

Maxillary RPD

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Fig 11-14  (a) This circumferential retainer originates and overlaps with the proximal plate. A few points define the spline, with the last point positioned at the designated undercut. Notice that the parallel blockout was removed to expose the junction of the yellow and light orange isodepth curve, which indicates a 0.25-mm undercut. (b) Notice that at each point on the spline, the width and height of the half-round cross section can be modified. This allows the clinician to taper the retainer. In this example, the shoulder of the retainer (yellow arrows) is designed to be about 2 mm in diameter (1-mm radius). Not illustrated here are the dimensions at the terminus, which are a 1-mm diameter with a 0.5-mm radius. Fig 11-15  An I-bar is designed to engage the canine. Note that the blockout has been removed in the area of the tooth to be engaged by the tip of the retainer. Also note that the retainer arm is aligned with the strut of the open lattice denture base connector.

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Fig 11-16  (a) Based on the settings chosen for this framework, the red areas indicate portions of the framework that are thinner than 0.5 mm. (b) The thickening tool is used to thicken the areas of concern and when they are appropriately thickened, the color turns to green. A smoothing tool (green oval) can be used to smooth the interfaces between components.

External finish line

EFLs are also designed with a spline. Each point along the spline may be broadened, narrowed, shortened, or made taller depending on need and location. The first point is placed on

Fig 11-17  This image shows splines for the EFLs of the three edentulous segments. The spline begins at the distopalatal line angle of the anterior abutment on the proximal plate and ends at the mesiopalatal line angle of the distal abutment.

the palatal line angle of the proximal plate. It swings palatally to allow space for positioning the denture teeth and for festooning (Fig 11-17). This is the last step in the “RPD design” section. Click “Next” to proceed to the last section. 109

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Fig 11-19 A braided post has been added to the metal base. It will help to retain the lateral incisor.

Fig 11-20 Finished design. The last step is choosing between textures available for the palatal straps. The operator has control of the depth of the pattern. Keep in mind that the ability to magnify can lead novice operators to overestimate the depth of the stipple pattern.

Finalize This final section grants an additional opportunity to sculpt, add retentive posts where indicated, choose a stipple pattern for the palatal major connectors, and add casting sprues (Fig 11-18). The fourth option is not discussed here because it should be delegated to the technician who will cast the printed pattern. Fig 11-18 The recommended sequence of design as required by the software. Note the icons for each step of the design. Note the difference between the “RPD design” tools and the “Finalize” tools.

110

Second sculpt (“Sculpt RPD design”) There is a second opportunity to refine the thickness and smoothness of the pattern with this tool. The entire framework is carefully scrutinized, and changes in thickness and smoothness are made as necessary.

Mandibular RPD

Fig 11-21  (a) Master cast with the mandibular RPD design. (b) Virtual cast.

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Fig 11-22  The various depths of undercut areas on the dentition and the soft tissues of the digitized master cast at a given path of insertion can be noted with multiple simultaneous views. The operator can rotate and zoom in on the digital model with great control.

Posts and sprues (“Pre-manufacturing”)

When metal bases are designed, posts are necessary to retain the denture tooth and encompassing acrylic resin. Areas of increased horizontal leverage such as anterior teeth or excessive ridge resorption should include a vertical post to enhance the retention provided for the acrylic resin. Once the post is placed it may be tilted as necessary. It can also be stretched or shortened as needed (Fig 11-19). The laboratory may prefer to design their own sprues based on their specific casting technique.

Stipple There are several choices available for stipple patterns and control of pattern surface texture (Fig 11-20). When the clinician is satisfied with the design, the STL file is exported to the dental laboratory for 3D printing of the pattern, investing, and casting. The printed framework of light-curing resin must be invested quickly because exposure to ultraviolet light will distort the pattern.

Mandibular RPD Data acquisition A master cast is obtained with a conventional impression as described in chapter 10. A preliminary design should be outlined on the master cast (Fig 11-21). This allows the drawn design to be read by the scanner and viewed on the screen while digitally designing the RPD framework. The cast is scanned with a high-resolution laboratory scanner.

Surveying and blocking out Color-coded isodepth curves delineate the depths of the tooth and soft tissue undercuts (Fig 11-22). The operator may tilt the cast in any direction by increments of a degree, and the software immediately renders the survey line and associated isodepth curves. By rotating the cast, one can decide on the MAP. Once the MAP has been determined, parallel blockout is automatically applied. 111

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a

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Fig 11-23 (a) Automated parallel blockout based on the chosen path of insertion. The clinician can control whether the blockout is to be truly parallel (0 degrees) or have more divergence (1 to 10 degrees). The parallel blockout is shown completely eliminating all undercuts. (b) The wax trimming tool is used to expose the desired undercuts on the premolars where the terminus of the I-bar will engage. Fig 11-24 Wax relief for the mandibular lingual bar major connector is applied arbitrarily, and 0.3 mm is suggested.

Wax trimming

When the MAP has been determined, all undercuts are blocked out parallel to the path of insertion. The desired undercuts for the tips of the retainers are exposed (“trimmed”). In this example, 0.25-mm undercuts are desired for the two I-bar retainers. Enough wax is removed to expose the junction of the yellow with light orange, which corresponds to the 0.25-mm undercut. This is performed on the midfacial portion of the second premolars (Fig 11-23). Make sure the appropriate undercut is exposed. It is advisable to err on exposing a bit more of the undercut because achieving more retention once the frame is cast is more difficult than reducing the retention. Because mandibular major connectors require relief so as not to impinge on the gingiva during function, during this step the wax addition tool is used to arbitrarily add wax on the surface of the lingual mucosa where the lingual bar major connector will be located. By holding the left-click button on the mouse and moving the cursor over the desired areas, wax is added like a spray can. The maximal thickness can be adjusted to avoid over- or underapplication of wax. In conventional methods, 26- to 30-gauge wax is used. For this software, 0.25 to 0.4 mm is equivalent (Fig 11-24). 112

RPD design As described previously, the software design sequence begins with “Retention grids,” followed by “Major connector,” then “Clasps” (including retainers, minor connectors, proximal plates, and struts for open lattice denture base connectors), followed by “Sculpt,” and lastly “Finishing line.” Retentive posts, stippling, and casting sprues are designed in a subsequent section entitled “Finalize.”

Denture base connectors (“Retention grids”) The use of a variety of denture base connectors (mesh type, open lattice, and metal base with retention posts) was explained in the previous section. In this example, open lattice types were selected. Select the tool for designing the wax relief. Begin with a series of points to outline the desired area. The last point of the spline must be superimposed on the first point to complete the spline (ie, the loop). When the loop is completed, the area will fill in with a digital representation of a layer of relief wax (Fig 11-25). The open lattice will be laid over the wax relief during the “Clasp” step. Repeat this on the contralateral extension

Mandibular RPD

Fig 11-25  Wax relief for the open lattices are placed 2 to 3 mm from the tooth-tissue junctions.

4.10 mm

1.87 mm

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Fig 11-26  (a) Wax relief for the open lattices are placed 2 to 3 mm from the tooth-tissue junctions. (b) Notice that the clinician can control the half-pear–shaped cross section of the lingual bar. The height of the bar should be approximately 4 mm. The thickness of the inferior portion of the bar should be approximately 2 mm.

base. The borders of the wax relief define the internal finish lines. Make sure that this line extends 2 to 3 mm from the tooth-tissue junction so this area will later be covered with the minor connector. The cross-section measuring tool allows one to precisely measure this distance. The same tool is illustrated in Fig 11-27, where the distance from major connector to the tooth-tissue junction is evaluated.

Major connector Bar type connectors are designed by a series of points that define the length of the bar (Fig 11-26). Extend the bar about a tooth length beyond the last abutment adjacent to the edentulous extension area. This will allow proper connection to the open lattices (denture base connectors). Once in place, each point along the spline can be manipulated to control both the thickness and the width of the connector (see Fig 11-26b). The space between the points is averaged to produce a continuously smooth edge of the connector. The half-pear cross-sectional configuration of a typical lingual bar is predetermined by the software (Fig

11-27). Advanced users are able to customize the cross-sectional configuration by manipulating the program settings. The cross-section tool is utilized to accurately measure distances and thicknesses. The software reads a measurement between any two points along a straight line. Because a minimum 3 mm of clearance is desired from the major connector to the free gingival margin, this tool is utilized to accomplish this task (see Fig 11-27).

Minor connectors, retainers, and rests (“Clasps”) This step provides tools for creating clasp assemblies, including rests, proximal plates, bracing components, and retainers. Minor connector splines are also utilized to form a latticework over the previously laid relief wax for denture base connectors. The software allows the clinician to design these components in any sequence. The authors recommend designing rests first. Rests. The occlusal rest tool is selected, and three points are positioned to define the spline. It is best to place the first point 113

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Fig 11-27 (a) The cross-section tool requires the operator to position a disc that sections the cast, wax, and framework at any angle. (b) Two selected points are used to measure the distance between the free gingival margin and the superior margin of the major connector. Notice that wax is seen in yellow; parallel blockout is seen labially, and relief wax is seen between the major connector and the soft tissue.

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Fig 11-28 (a) When two rests are abutted as seen here on the premolars, a single spline is used. (b) Note that the minor connector connects the rest to the major connector. The window associated with the cingulum rest is created in the following manner: A cingulum rest is created with the occlusal rest tool. The window is created by combining the rest with a minor connector as a short lingual plate.

Fig 11-29 (a) The denture base connector outline is formed within the perimeter of the relief wax. (b) Cross struts are designed to complete the open lattice.

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of the spline at the deepest and most central portion in the occlusal rest seat. In situations of abutting rest seats, continue the spline from one abutment to the next (Fig 11-28a). A cingulum rest is designed with the same occlusal rest tool. The window is created by combining the rest with a minor connector designed as a short lingual plate (Fig 11-28b). 114

Minor connectors. Once rests have been positioned, the minor connectors serve to connect these components to the major connector. Additionally, proximal plates are designed where needed and extended to cover the tooth-tissue junction, also extending to the denture base connectors (see Fig 11-28b).

Mandibular RPD

a

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Fig 11-30  (a) An I-bar originates from a cross strut of the open lattice. (b) Note that the blockout has been removed in the area of the tooth to be engaged by the tip of the retainer.

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Fig 11-31  The “Sculpt” step allows for thickening, thinning, and smoothing. (a) Notice the irregularities caused by overlapping splines of multiple minor connectors. (b) A smoothing tool was used to smooth the interfaces between components.

The same minor connector tool is used to form the denture base connector over the previously laid relief wax. Begin with a spline forming the outline of the grid pattern, making sure to overlap with the proximal plate and the major connector (Fig 11-29a). In the example shown, two cross struts complete the open lattice (Fig 11-29b). The cross strut closest to the abutment tooth should be aligned with the approach arm of the I-bar retainer, which is designed in the next segment. Retainers. Select a retainer tool with a half-round cross section and place the first point of the spline overlapping with a cross strut of the open lattice denture base connector. Continue down the crest of the ridge about 3 mm and swing mesially toward the abutment tooth. Curve into the vertical arm and cross the tooth-gingiva junction. The vertical arm should be parallel to the long axis of the tooth. The tip will make contact with the tooth at the exposed undercut and continue occlusally to the height of contour (the occlusal limit of the yellow isodepth curve). Modify the half-round base and height to properly taper the vertical arm from 2 mm at the bend to 1 mm at its terminus (Fig 11-30).

Sculpt

When the clinician reaches this point in the design process, all previously designed components fuse into one contiguous structure. This step allows use of tools to smooth and increase or decrease the thickness of the framework. Irregularities are often noted at the junctions between components (Fig 11-31). These are smoothed with the “Sculpt” tool.

External finish line The first point of the EFL spline is placed on the distolingual line angle of the proximal plate (Fig 11-32a). The spline moves distogingivally at about a 45-degree angle until it reaches the inferior aspect of the major connector. Each point along the spline may be broadened, narrowed, shortened, or made taller depending on need and location (Fig 11-32b). To blend the EFL with the occlusal aspect of the proximal plate, the height and width of the first point on the spline is made as short and narrow as possible. The portion over the major connector is broad to make it less conspicuous to the tongue. The spline has a wavelike cross section (see Fig 11-32b) where the concave face 115

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Waveform

a

Fig 11-32 These images show the EFL. (a) The spline begins at the distolingual line angle of the abutment on the proximal plate and ends at the inferior border of the major connector. (b) A close-up view of a point on the EFL spline being manipulated. Notice that the cross section of the spline has a waveform (arrow), with the face of the wave pointing to the denture base connector. Like all spline points, the width and height may be controlled.

b

Fig 11-33 These images show the completed mandibular framework. (a) The completed framework on the virtual cast. (b) The cast has been made invisible to allow an unobstructed view of all surfaces of the completed framework.

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b

of the wave must face the denture base connector. If facing in the wrong direction, right-click on the spline and scroll to the option “reverse spline.” This will allow the wave to flip to the correct orientation.

Finalize This final section provides the clinician an additional opportunity to “sculpt,” add retentive posts where indicated, choose a stipple pattern for palatal major connectors, and add casting sprues. Casting sprues are delegated to the technician who will cast the printed pattern. Mandibular lingual bar major connectors are never stippled. The example shown does not require retention posts. 116

At any point in the design process, a sliding toggle may be used to render the virtual cast, blockout and relief wax, or the in-progress framework translucent or completely invisible (Fig 11-33). Doing so can help gain an unobstructed view of the intaglio surface of the framework. This is particularly useful for evaluating the juxtaposition of the internal finish line to the tooth-tissue junctions. When evaluating the finished design, portions of the framework that require recontouring may be found. The operator may jump back to any prior step and make adjustments, such as repositioning a spline or broadening a point on a spline. One may return as far back as surveying and change the MAP by a couple degrees. The computing power of this software will re-render the design of everything downstream from the alteration made.

Computer-Aided Manufacturing

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Fig 11-34  (a) Designed framework. (b) Printed pattern confirmed to fit the master cast. (c) Cast framework completed. (d) Printed framework remounted on an articulator, with occlusion adjusted and wax additions made (arrow) to improve centric contact. (e) Notice that the addition of wax to the printed framework produced the desired contour (arrow) of the cast framework.

Computer-Aided Manufacturing CAM of metal RPD frameworks has been slow to be adopted because the techniques used in early CAM systems were exclusively subtractive methods, such as milling from a solid block of material. Although this method is effective when milling materials like ceramics, waxes, and resins, milling a pattern as intricate as an RPD framework from a solid metal puck is neither practical nor cost-effective. In recent years, rapid prototyping (RP), a general term used for several additive layer manufacturing techniques, has been refined. The most commonly used techniques are stereolithography, selective laser melting (SLM), selective laser sintering, selective deposition modeling, and 3D printing. RP allows machines to address every element of a part no matter the complexity of its shape. The most common CAM technology used in the fabrication of RPD frameworks is the printing of a light-curing resin framework pattern. The pattern is printed with supporting struts and sprues. It is critical that the operator check the fit of the printed pattern on a master cast prior to investment. If

the printed pattern fails to seat as designed, the clinician must troubleshoot the situation (ie, decide whether the error was made during the scanning or the printing). Adding wax or trimming with a dental handpiece permits small contour changes. When adaptation is confirmed, the usual and customary process of fabrication is pursued, including investment, burnout elimination, and casting. Finishing and polishing is performed as usual to seat the RPD framework onto the master cast (Fig 11-34). A recent study on printed RPD frameworks indicated that printed patterns are subject to distortion if left exposed to light. Therefore, the laboratory must minimize exposure to light before investing the pattern.1 Pattern checks should be limited to the technician just prior to investment. When an adjustment of the occlusion on mounted casts is desired (see Fig 11-34d), the articulator and mounted opposing casts should be sent to the laboratory. The clinician should refrain from requesting a pattern check for intraoral trial because of the risk of distortion. Although a fully digital workflow is convenient, the laboratory must be provided a master cast to try in and verify the fit of the finished framework.1 Finally, a stone master cast is necessary when an altered cast impression is planned (see chapter 12). 117

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a

b

Fig 11-35 (a) A framework fabricated via SLM requires more finishing and polishing than cast frameworks. This can be a source of error as demonstrated in part b. (b) Note that the rest is not in intimate contact with the rest seat.

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Fig 11-36 (a) The measuring tool allows the clinician to scrutinize the cross-sectional dimension of any component. In this example, the I-bar retainer is evaluated for appropriate contour and diameter. The measured base diameter of the approach arm was planned to be 1.65 mm. (b and c) However, the finished and polished product measures 1.1 mm. The difference is due to the increased need for post-sintering finishing procedures.

Using CAD technology to design the RPD framework and mill it of cobalt-chromium (Co-Cr) alloy was first reported by Bibb et al2 and Han et al.3 A recent clinical study by Ye et al4 evaluated the precision of fit of CAD/RP-designed and fabricated RPD frameworks in 15 patients. Each patient receieved two RPDs: (1) One was manufactured using the conventional investment casting method (control group) and (2) the other was fabricated using CAD/RP technique with the Co-Cr alloy RPD frameworks directly manufactured using SLM. The study 118

concluded that the RPD frameworks manufactured with the CAD/RP method, although considered clinically acceptable, showed slightly larger gaps between the occlusal rests and the corresponding rest seats compared to that of the investment casting control group. The authors’ own experiences with the SLM technique have been mixed. The precision of fit achieved with this method has not yet matched the consistency seen with the conventional methods (Figs 11-35 and 11-36).

Suggested Reading

The Future CAD/CAM will continue to evolve, and the authors believe that a complete digital workflow will be possible for the design and fabrication of RPD frameworks in the near future. Clinicians must be mindful that all these evolving technologies, although exciting, are tools and cannot substitute for proper diagnosis and a thorough knowledge of the principles of RPD biomechanics and design. Regardless of who performs the digital design, it is the responsibility of the clinician to approve the design, and this cannot ethically be delegated to other allied heath care personnel.

References 1. Cagino C, Jayanetti J, Moshaverinia A. Dimensional accuracy of removable partial denture frameworks fabricated by rapid prototyping. Presented at the Pacific Coast Society of Prosthodontics 82nd Annual Meeting, Coeur d’Alene, ID, 29 June 2017. 2. Bibb RJ, Eggbeer D, Williams RJ, Woodward A. Trial fitting of a removable partial denture framework made using computer-aided design and rapid protyping techniques. Proc Inst Mech Eng H 2006;220: 793–797. 3. Han J, Wang Y, Lü P. A preliminary report of designing removable partial denture frameworks using a specifically developed software package. Int J Prosthodont 2010;23:370–375. 4. Ye H, Ning J, Li M, et al. Preliminary clinical application of removable partial denture frameworks fabricated using computer-aided design and rapid prototyping. Int J Prosthodont 2017;30:348–353.

Suggested Reading Campbell SD, Cooper L, Craddock H, et al. Removable partial dentures: The clinical need for innovation. J Prosthet Dent 2017;118:273–280. Gratton DG. Evolving technologies in implant prosthodontics. In: Sadowsky SJ (ed). Evidence-based Treatment Planning and Clinical Protocols. Ames, IA: Wiley-Blackwell, 2017:184–206. Lang LA, Tulunoglu I. A critically appraised topic review of computer-aided design/computer-aided machining of removable partial denture frameworks. Dent Clin North Am 2014;58:247–255.

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Chapter 12 Physiologic Adjustment of the RPD Casting and Altered Cast Impressions Daniela Orellana | John Beumer III

Inspection and Verification of the Casting Design compliance After the framework is returned from the dental laboratory, it is carefully inspected for compliance with the prescription design, adaptation to the master cast, and overall quality. The completed casting is compared with the design outlined on the design cast (Fig 12-1). As stated in chapter 10, the authors

a

b

strongly suggest that a design cast with the outlined design be submitted to the dental laboratory technician with the master cast. The technician can use the design cast for reference while fabricating the pattern for the framework. In turn, when the framework is completed and returned to the clinician, the clinician can compare the design submitted on the design cast with the final removable partial denture (RPD) casting. This practice will reduce the risk of miscommunication between the clinician and the laboratory technician. When the framework is designed digitally, the data can be transmitted via the Internet.

c

Fig 12-1 (a to c) The casting must accurately reflect the design outlined on the design cast. (Courtesy of Dr A. Davodi, Beverly Hills, California.)

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Physiologic Adjustment of the RPD Casting and Altered Cast Impressions

Fig 12-2 The intaglio of the casting should be carefully evaluated. Irregu- Fig 12-3 Rests must be intimately adapted larities can be removed with a high-speed air rotor. to the rest seat and the occlusal surfaces and properly contoured. Metal flash that extends beyond the rest seat should be removed.

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Fig 12-4 (a) The positions of the retainers and proximal plates are checked and should conform to the design on the design cast. (b) There should be no space between the proximal plates and the guide planes prepared on the abutment teeth. (c) Make sure the I-bar retainer contacts the tooth surface from the height of contour down to the 0.01-inch undercut.

a Fig 12-5 Minor connectors should cross the tooth-tissue junction at right angles to minimize food impaction.

Fig 12-6 (a) The external surface is highly polished to minimize plaque retention and aid in the patient’s comfort. (b) Tissue surface with a matte finish.

Adaptation of the RPD framework to the cast The adaptation of the RPD framework to the master cast is carefully evaluated. The authors suggest the following sequence: 1. Inspect the underside (intaglio) and tissue side of the casting for roughness and bubbles. This inspection should be con122

b

ducted with magnification. Irregularities are removed with a high-speed air rotor (Fig 12-2). 2. Rests must be closely adapted and in intimate contact with their rest seats. There should be no metal flash extending beyond the rest area. If there is, these areas should be removed (Fig 12-3). 3. The casting must not rock on the master cast when seating forces are applied to the rests. If this occurs, a new impression should be made, and the new master cast should be

RPD Framework Try-in

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Fig 12-7  (a) A mixture of gold rouge and chloroform is used as a disclosing medium to identify portions of the RPD casting that inappropriately bind and prevent seating. (b) The solution is applied with a brush and, when dry, a very thin film of rouge will be deposited on the undersurface of the RPD casting that engages the dentition. (c) Spray type disclosing media can also be used.

c

resubmitted to the dental laboratory for fabrication of a new RPD casting. 4. The retainers must be precisely positioned per the design, engaging the precise amount of undercut on the abutment teeth as prescribed (Fig 12-4). On occasion, these areas on the retainers are overpolished and do not engage the abutment as prescribed. If this occurs, retention will be compromised, and a new master cast should be obtained and resubmitted to the dental laboratory for fabrication of a new RPD casting. 5. The proximal plates must engage the guide planes prepared on the abutments as prescribed and extend at least 2 mm onto the mucosa. In ideal designs, there should be no space or voids between the proximal plates and the guide planes or the tissues (see Fig 12-4b). 6. The position and contours of the RPD framework as it traverses the tooth-tissue junction should be verified. Minor connectors should cross the tooth-tissue junction at right angles (Fig 12-5). 7. The tissue surface of the casting should be finished to a fine matte texture, whereas the external surface should be highly polished to decrease plaque adherence (Fig 12-6).

Quality of the RPD casting The quality of the casting is carefully scrutinized. Areas of interest include reproduction of normal anatomical contours,

smoothness, and polish. The following issues should be carefully addressed and scrutinized: • Porosity • Rough or irregular surfaces • Sharp edges, which may cause irritation to the tongue or adjacent soft tissues • Contour of the minor and major connectors • Proper bulk and thickness for strength and rigidity

RPD Framework Try-in The RPD framework is inserted into the mouth. All rests must be fully seated and in intimate contact with their rest seats. Inability to seat the casting may be due to artifacts on the underside of the casting, errors in fabrication, or movement of teeth since the impression was made. On occasion, the source of these errors can be identified with disclosing media and the casting adjusted (Fig 12-7). The most effective dislosing medium is rouge and chloroform. A small amount of gold rouge is placed in a dappen dish, and a drop or two of chloroform is deposited in the dish (see Fig 12-7a). A very thin mixture of the dissolved gold rouge is then painted onto the underside of the RPD casting that engages the dentition. The chloroform quickly evaporates, leaving a very thin layer of rouge (see Fig 123

12

Physiologic Adjustment of the RPD Casting and Altered Cast Impressions

Fig 12-8 A metal caliper is used to measure rest thickness after occlusal adjustment. Avoid undue thinning of the RPD components.

12-7b). The casting is then seated, applying seating forces on the rests, and adjustments are made as necessary as indicated by the rouge being rubbed off the casting in areas of inappropriate tooth contact. Adjustments are completed using carbide burs in a high-speed air rotor. However, if the framework does not fully seat after the first couple of relatively minor adjustments, it is prudent to make a new impression, master cast, and framework. Once the RPD framework is fully seated, it must be adjusted to the opposing occlusion. The framework must not interfere with normal occlusal contacts in centric or excursive movements. The patient should be guided to close the mandible in the desired position, and occlusal contacts should be evaluated without the casting. The existing occlusal pattern should be maintained when the framework is in position. Occlusal interferences are marked by articulating paper and adjusted with carbide burs in a high-speed air rotor. Care must be taken during framework adjustments to avoid undue thinning of the framework. A metal caliper is used to ensure that the framework is not overreduced (Fig 12-8). Ideally, the rest should be at least 1 mm in thickness. In some instances, enameloplasty of opposing teeth may be indicated to avoid excessive thinning of the framework. If this method is employed, the clinician must obtain the patient’s approval. Upon completion of the necessary adjustments, the framework is polished using carborundum-impregnated wheels and points.

Physiologic Adjustment of the RPD Casting When the prosthesis is an extension-base RPD, it is necessary to physiologically adjust the casting to permit free rotation around the axis of rotation (fulcrum line) when occlusal forces 124

are applied over the extension area. This adjustment will prevent guide planes and minor connectors from delivering lateral and off-axis forces to the abutments and other teeth engaged by the RPD framework (Fig 12-9). When this operation is completed, the RPD framework and ultimately the finished prosthesis will freely rotate around the rests on the abutment teeth bordering the edentulous extension areas while directing occlusal forces along the long axis of these abutment teeth. As mentioned previously, a very thin mixture of the dissolved gold rouge is painted onto the underside of the RPD casting that engages the dentition. The framework is fully seated into position and compressed with finger pressure in the extension area to simulate cyclic masticatory occlusal forces. If there are rests anterior to the axis of rotation, they will become slightly separated from the teeth when a force is applied in the extension area. During this procedure, the rests must be reseated as the physiologic adjustment is performed (Fig 12-10). Areas of the RPD framework that bind on the proximal plates and the minor connectors, preventing a pure rotation around the axis of rotation (fulcrum line), can be identified and adjusted with a high-speed air rotor (Fig 12-11). It is especially important to carefully evaluate contact areas between the teeth where the casting (minor connectors) extends to the rests. Contact in these areas during function may create a wedging action, which may force the teeth apart. Adjustment of the RPD casting continues until the casting rotates freely around the axis of rotation through the rests on the fulcrum line (Fig 12-12; see also Fig 12-11). Physiologic adjustment provides a safety factor for the abutment teeth because if there is excessive movement of the prosthesis secondary to bone resorption, poor adaptation of the extension base, or compromise of the support in the extension area, the occlusal forces generated during function will deliver torquing forces to the abutment teeth.

Altered Cast Impression Procedure

Fig 12-9  (a and b) If the proximal plate and minor connectors are in intimate contact with the surfaces of the teeth, when an occlusal force is applied in the edentulous extension area, off-axis torquing forces will be delivered to these teeth.

a

Fig 12-10  Finger pressure is applied in the extension area to mimic the forces delivered during occlusal function.

b

Fig 12-11  The rouge will be displaced in areas of binding and heavy contact. Common areas are proximal plates and minor connectors. These areas are relieved with a high-speed air rotor. The process is continued until the casting rotates freely around the axis of rotation (fulcrum line).

Altered Cast Impression Procedure The extension-base RPD is unique because its support is derived from two different entities: (1) the periodontal ligament of the dentition and (2) the mucosa overlying the edentulous extension area. The periodontal ligament exhibits limited displacement compared to the mucoperiosteum, depending upon its thickness and compressibility. In order to idealize the support from the edentulous extension areas when occlusal forces are applied, a dual impression technique is recommended. The first impression,

Fig 12-12  Lingual view of an extension-base RPD. The minor connector between the premolars binds and torques the teeth as the prosthesis rotates around the fulcrum line during occlusal function.

commonly made of irreversible hydrocolloid, captures teeth in their anatomical positions (see chapter 10). The cast framework is fabricated, and a secondary impression, in essence an altered cast impression, is made of the edentulous extension areas. Altered cast impressions maximize the support obtained from the primary support areas (eg, retromolar pad, maxillary tuberosity, buccal shelf ). The altered cast impression is primarily used in the mandible when fabricating unilateral or bilateral extension-base RPDs. In the mandible, the alveolar process will rapidly resorb unless the RPD is extended to properly 125

12

Physiologic Adjustment of the RPD Casting and Altered Cast Impressions

a

b

Fig 12-13 One layer of baseplate wax is applied to the extension area. The RPD casting is fully seated on the cast, and the wax is removed around the denture base connectors. (Courtesy of Dr J. Jayanetti, Los Angeles, California.)

Fig 12-14 (a) Tray resin is mixed, and when in the doughy stage, it is adapted to the extension area. The tray is trimmed to be 1 to 2 mm short of the depth of the vestibule (red line). (b) The borders are smoothed and rounded and should be 1 to 2 mm thick. (Courtesy of Dr J. Jayanetti, Los, Angeles, California.)

Fig 12-15 The borders are verified intraorally and shortened as necessary.

Fig 12-16 Pattern resin is used to secure the tray to the denture base connectors.

engage the primary support areas, namely the retromolar pad and the buccal shelf. In the maxilla, the altered cast impression is primarily used in anterior extension situations to define the thickness and contours of the denture flanges.

Making the impression tray for the extension areas 1. A line is scribed on the cast with a pencil approximately 2 mm short of the depth of the vestibule and onto the buccal shelf and the retromolar pad. This line indicates the outline of the tray. The cast is soaked in slurry water for 5 minutes. 2. Separating medium is applied to the cast. 3. A single layer of baseplate wax is placed over the edentulous extension area (Fig 12-13). 4. The RPD framework is heated slightly and seated onto the master cast, making sure that the rests are fully seated. The wax around the denture base connectors is removed to enable a mechanical locking of the tray resin onto the denture base connectors. The tray resin is mixed per the manufacturer’s recommended water-powder ratios. Once the resin reaches a doughy consistency, it is applied to the extension area (Fig 12-14). The resin is allowed to polymerize and when 126

complete, the tray is immersed in warm water and the wax spacer is removed. Light-curing acrylic resin can also be used. 5. The tray extension is trimmed to match the outline on the cast, and all borders are smoothed. The tray extensions should be 1.5 to 2.0 mm thick at the periphery (see Fig 12-14). 6. The casting with the tray extension attached is inserted intraorally, and the borders of the tray are verified, relieved, and shortened as necessary (Fig 12-15). If the tray is not securely attached to the denture base connectors, self-curing pattern resin is used to secure the tray to the framework (Fig 12-16.) 7. The border extensions of the tray are refined with dental modeling plastic (compound) with the aid of a water bath. During border molding, the casting must seat properly while the extensions are developed. It is important to verify that the impression extends to engage the primary support areas: the buccal shelf and the retromolar pad (Fig 12-17a).

Materials used to make the altered cast impression The authors have used several materials to make altered cast impressions. The most common materials are polysulfide and polyvinyl siloxanes. These cause minimal tissue displacement,

Altered Cast Impression Procedure

a

b

c

Fig 12-17  (a) A fully extended impression. Note the extensions onto the buccal shelf and the retromolar pad. (b and c) The compound is cut back approximately 0.5 mm, and numerous perforations are made along the finish lines. (Part c courtesy of Dr R. Duell, Los Angeles, California.) Fig 12-18  (a) When an elastomeric impression material is used, the border-molded extension base is coated with a suitable adhesive. Note that the adhesive extends right up to the finish line. (b) Note that the tray extension has been coated with a thin layer of impression material. (Courtesy of Dr R. Duell, Los Angeles, California.)

a

b

a

b

Fig 12-19  (a and b) Sample impressions. They are carefully inspected. Impression material that has flowed beyond the internal finish line and onto the major connector is carefully removed.

reproduce excellent tissue detail, set relatively quickly, and are easily manipulated in the laboratory when boxing and pouring the impression.

Impression procedures Clinical steps 1. Perforate the tray along the finish line with a round bur (either #6 or #8) to avoid inadvertent compression of the tissues and to direct the flow of the impression material away from the major connectors. 2. Cut back the compound extension to make space for the material used (approximately 0.5 mm) in creating the altered cast impression (Figs 12-17b and 12-17c). 3. If an elastomeric impression material is to be used, apply tray adhesive up to the finish line (Fig 12-18a).

4. When making the wash impression, apply a very thin layer of impression material along the finish line. Do not overload this area (Fig 12-18b). 5. Ensure complete seating of all rests, then while holding the framework in position, instruct the patient to go through usual muscular movements to capture proper denture extensions. 6. Hold the framework in position until the impression material has fully set. Do not apply seating pressure to the extension-­ base areas. 7. The multiple perforations will prevent the impression material from spilling excessively onto the major connector. If material does spill onto the major connector beyond the internal finish line, carefully remove it with a scalpel. Completed impressions are shown in Fig 12-19. These impressions are ready to be boxed and poured.

127

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Physiologic Adjustment of the RPD Casting and Altered Cast Impressions

a

b

Fig 12-20 (a) The edentulous extension areas are removed from the master cast, and the metal framework is fully seated. Note that only the metal should contact the original cast. All areas associated with the impression should be out of contact. (b) The altered cast impression beaded and boxed. All areas must be sealed with wax to prevent stone from leaking onto the occlusal surfaces of the original master cast. (Courtesy of Dr J. Oyama, Los Angeles, California.)

a

b

Fig 12-21 (a and b) Sample altered cast impressions. Altered cast impressions ensure that maximal support is provided for the definitive RPD. (Courtesy of Dr A. Long and Dr J. Oyama, Los Angeles, California.)

Alteration of the cast, boxing, and pouring the impression 1. The edentulous portion of the cast is cut away just beyond the internal finish line of the major connector, and retention grooves are prepared in the bottom of the master cast (Fig 12-20a). 2. The RPD framework is seated onto the cast. All rests must be fully seated, and there must be no contact between the cast and the altered cast impression material. 3. The RPD framework is luted to the cast with sticky wax. 128

4. The impression is beaded, boxed, and poured (Fig 12-20b). 5. The cast should be soaked for 5 minutes before pouring. 6. All surfaces must be carefully sealed to prevent stone from flowing onto the occlusal surfaces of the master cast. When the stone has set, the impression is immersed in a water bath at 140°F for a few minutes, and the impression is gently separated from the cast. Sample altered cast impressions are shown in Fig 12-21. Box 12-1 outlines problems associated with altered cast impressions.

Suggested Reading

Box 12-1  |  Problems associated with altered cast impressions Problem

Cause

Second pour separates from the master cast

Retention grooves cut into the original cast are inadequate; original cast at time of pour is too dry

Dental stone on master cast

Poor seal with the beading wax

Framework with impression does not seat completely on cast

Impression material under the rests and minor connector while making the impression; areas of the cast not trimmed properly, preventing seating of the framework

Difficulty removing the impression from the altered cast

Failure to heat the compound sufficiently before separation

Completed RPD rocks in the mouth

RPD framework not seated when impression made; impression material under rest when altered cast impression made; impression material under the major connector when altered cast impression made

Suggested Reading Becker CM, Kaiser DA, Goldfogel MH. Evolution of removable partial denture design. J Prosthodont 1994;3:158–166. Feit DB. The altered cast impression technique revisited. J Am Dent Assoc 1999;130:1476–1481. Frank RP, Brudvik JS, Noonan CJ. Clinical outcome of the altered cast impression procedure compared with use of a one-piece cast. J Prosthet Dent 2004;91:468–476. Hindels GW. Load distribution in extension saddle partial dentures. J Prosthet Dent 1952;2:92–100.

Holmes JB. Influence of impression procedures and occlusal loading on partial denture movement. J Prosthet Dent 1965;15:474–483. (Reprinted J Prosthet Dent 2001;86:335–341.) Holmes JB. The altered cast impression procedure for the distal extension removable partial dentures. Dent Clin North Am 1970;14:569–582. Kratochvil FJ, Caputo AA. Photoelastic analysis of pressure on teeth and bone supporting removable partial dentures. J Prosthet Dent 1974;32:52–61. Leupold RJ, Kratochvil FJ. An altered-cast procedure to improve tissue support for removable partial dentures. J Prosthet Dent 1965;15:672–678. Roche AF. Functional anatomy of the muscles of mastication: A critical review. J Prosthet Dent 1963;13:548–570. Silverman SI. Denture prosthesis and the functional anatomy of the maxillofacial structures. J Prosthet Dent 1956;6:305–331.

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Chapter 13 Maxillomandibular Records and Occlusion for RPDs Ting-Ling Chang | Daniela Orellana | Jay Jayanetti | John Beumer III

Maxillomandibular relations are variable, and it is necessary to determine the position of the mandible in relation to the maxilla at which the prosthesis will be fabricated (ie, the treatment position). The prosthesis can be designed and fabricated for the patient to function in centric relation (CR) or maximal intercuspal position (MIP) depending on the occlusal analysis. Other factors to consider are the occlusal vertical dimension (OVD) and the skeletal jaw relation of the patient. For example, a patient with a Class III jaw relation is usually restored at CR, whereas the occlusion of a patient with a Class II jaw relation is designed with a so-called “long centric,” which permits the patient to function in a number of positions anterior to CR.

Occlusal Vertical Dimension Assessment of the OVD First it is necessary to evaluate contact of the remaining occluding teeth in order to determine the proper OVD. The clinician must determine whether to restore or maintain the existing OVD (Fig 13-1). There are many methods used to evaluate OVD. All methods agree on one major principle: During speech there should be no contact between opposing teeth. An acceptable OVD is characterized by vertical stops (posterior tooth contacts) maintained without tooth displacement or horizontal interference. When an existing OVD is correct, there may or may not

be sufficient restorative space for placement of denture teeth positioned consistent with the proper occlusal plane (Fig 13-2). Loss of interocclusal space does not always imply loss of OVD. Supereruption of teeth commonly leads to loss of restorative space (Fig 13-3). In some patients, even though the existing OVD is appropriate, supereruption of teeth may need correction before the removable partial denture (RPD) is fabricated (Fig 13-4). This can be accomplished by enameloplasty, endodontic treatment in conjunction with crown lengthening and full-coverage crowns, segmental maxillary osteotomy, or orthodontic intrusion of the offending teeth. In some instances, extraction of severely supererupted teeth may be necessary in order to correct the occlusal plane discrepancy (see Fig 13-20). The patient shown in Fig 13-4 is typical. New crowns are intended for the maxillary premolars and mandibular molars, and a tuberosity reduction was planned. Endodontic treatment plus crown lengthening was required on the second molar in order to prepare the tooth for a full-coverage crown with appropriate resistance and retention form. Reduced OVD can be secondary to wear, tooth loss, and migration of teeth (Fig 13-5; see also Fig 13-1). An appropriate treatment plan will require re-establishment of an OVD compatible with speech, swallowing, and esthetics. When there is suspected loss of OVD or the opposing arch is edentulous, it may be necessary to provide the patient with a treatment partial denture to determine the OVD at which the definitive RPDs will be designed and fabricated (Fig 13-6).

131

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Maxillomandibular Records and Occlusion for RPDs

a

b

Fig 13-1 (a and b) The assessment of the OVD is the first requirement when establishing correct max- Fig 13-2 When an existing OVD is appropriate, there illomandibular relations. This patient presented with significant wear and erosion, resulting in a closed often exists sufficient interocclusal space for denture OVD. (Courtesy of Dr A. Davodi, Beverly Hills, California.) teeth and proper occlusal guidance.

Fig 13-3 Loss of restorative space does not always imply loss of OVD. In this patient, loss of OVD was due to supereruption of the maxillary molars. (Courtesy of Dr T. Berg, Los Angeles, California.)

a

Fig 13-4 The supereruption of teeth and arch segments must be corrected before proceeding with the definitive RPD. (Courtesy of Dr T. Berg, Los Angeles, California.)

b

Fig 13-5 (a and b) In both patients, the OVD has been closed. There are no posterior stops remaining in either patient. The treatment plan will need to re-establish an OVD compatible with speech.

A tentative OVD is determined by the measurement of the vertical dimension of rest (VDR) minus 2 to 4 mm interocclusal rest space. VDR is first established by measuring the distance of two selected marked points (usually one on the tip of the nose and the other on the chin) using phonetics and swallowing methods. The tentative OVD is subsequently determined by the deduction of 2 to 4 mm of the VDR measurement to account for the interocclusal rest space. The evaluation of the OVD established by the treatment partial denture continues until the patient is comfortable and without symptoms associ132

ated with the temporomandibular joint (TMJ) and associated musculature. Often the OVD must be increased in increments, and as a result this process may take several months. When there are severe occlusal interferences caused by malpositioned teeth, these interferences often need to be eliminated before the proper OVD can be determined (Fig 13-7). When there are minor discrepancies between CR and MIP and the patient is asymptomatic, the clinician may use MIP as the treatment position.

Occlusion

Fig 13-6  (a to c) Treatment partial dentures have been fabricated to restore both the OVD and the missing posterior dentition. (d) Note the occlusal platform on the maxillary treatment partial denture used to restore the OVD (arrow). When the patient is comfortable and free of symptoms, the definitive prostheses can be fabricated. (Courtesy of Dr A. Davodi, Beverly Hills, California.)

a

b

c

d

Fig 13-7  Migration of teeth and occlusal interferences in CR can lead to an incorrect OVD. If CR is to be used as the treatment position, such occlusal interferences must be eliminated by occlusal adjustment or the fabrication of new restorations before the RPD is fabricated.

Occlusion

CR as the treatment position

There are two basic treatment positions that can be used when fabricating RPDs:

CR is independent of tooth contact and guided by the condyle position when the mandible is at an unstrained and physiologic position. A strong rationale for use of this relationship is that CR can be repeated and reproduced by duplicate records. This factor is advantageous because it serves to verify clinical procedures and laboratory transfers, ensures reproducibility, and removes the possibility of interocclusal interference if the mandible moves to a retruded position.

1. Rehabilitate  the patient in CR. This position is preferred when an opposing complete denture is planned with the RPD, when only anterior teeth remain in both maxillary and mandibular arches, when opposing posterior teeth do not occlude, and when full-mouth rehabilitation is planned. 2. Fabricate the prosthesis and restore the occlusion using the MIP. This position is used when the discrepancy between CR and MIP is small and in the absence of symptoms associated with the TMJ and associated musculature.

133

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Maxillomandibular Records and Occlusion for RPDs

a

b

Fig 13-8 (a and b) MIP was only slightly ahead of CR, and the patient was asymptomatic, so MIP was used to fabricate an extension-base RPD for this patient.

MIP as the treatment position The use of MIP as the treatment position is based on the theory that functional activities and muscle action and development have established this jaw position. If the head and neck examination and occlusal analysis fail to uncover disease or adverse symptoms within the masticatory system, the existing maxillomandibular relationship is considered acceptable (Fig 13-8).

Maxillomandibular Registrations The purpose of maxillomandibular registrations is to record the relationship of the mandible to the maxilla and transfer this record to the articulator.

Articulator The articulator is a mechanical instrument that represents the TMJ and the jaws, to which maxillary and mandibular casts are attached to simulate some or all mandibular movements. Articulators can be classified as the following: • Class I (non-adjustable): A simple holding instrument capable of accepting a single static registration; vertical motion is possible. • Class II (semi-adjustable): An instrument that permits horizontal and vertical motion but does not orient the motion to the TMJ. • Class III (semi-adjustable): An instrument that simulates condylar pathways by using averages or mechanical equivalents for all or part of the motion; these instruments allow for orientation of the casts relative to the joints and may be arcon or nonarcon instruments.

134

• Class IV (fully adjustable): An instrument that will accept three-dimensional (3D) dynamic registrations; these instruments allow for orientation of the casts to the TMJs and simulation of mandibular movements. The accurate reproduction of the patient’s jaw movements by the articulator is entirely dependent on the accuracy of the registrations that are used and the precision with which the instrument is programmed. Clinical, laboratory, and technical errors can interfere with an accurate representation of the patterns of mandibular movement of the patient. It is essential that a second set of registrations be made in order to check all transfer records and eliminate potential errors. The Class III semi-adjustable articulator is an acceptable instrument for use when fabricating RPDs. An example of this type of articulator is shown in Fig 13-9. The following records are used to program this instrument: • Facebow transfer record • Maxillomandibular record (eg, CR or MIP records) • Protrusive relation record

Facebow transfer record A facebow is an instrument used to record the spatial relationship of the maxillary arch to an anatomical reference point or points and then transfer this relationship to an articulator. The facebow transfer record orients the maxillary dental cast in the same relationship of the patient to the opening axis of the articulator. Customarily the anatomical references are the mandibular transverse horizontal axis and one other selected anterior reference point (Fig 13-10). If this position is not recorded, there is a possibility of error in 3D when the casts are mounted on the articulator. The errors introduced may make the occlusal contacts developed on the articulator incompatible with mandibular movements of the patient.

Maxillomandibular Registrations

a Fig 13-9  A Class III semi-adjustable articulator is acceptable for fabricating RPDs.

b

Fig 13-10  (a and b) The anterior reference point is marked using a reference plane locator and marker. The reference point for this system is 43 mm above the incisal edge of the right central or lateral incisor. (b) The screw on the anterior reference pointer is loosened. The bow is adjusted so that the pointer aligns precisely with the anterior reference point marked earlier.

Fig 13-11  The occlusal plane is often tilted. In this patient, the right side of the occlusal plane is lower than the left side. An accurate facebow transfer record can record these discrepancies on the articulator, which permits the articulator to duplicate the mandibular movements of the patient.

The facebow transfer record orients the maxillary cast in 3D on the articulator in a way identical to the relationship of the maxilla to the condyles as in the patient. The articulator must reproduce the anatomical distance from the hinge axis to all occlusal contacts. If this distance is not a reasonable duplication of the patient (ie, if the radius of the articulator’s arc of closure is different from the patient’s radius or arc of closure), there is the possibility of inappropriate contact of teeth as the patient opens and closes the jaw. From the lateral view, the maxillary cast must also duplicate the proper position. Otherwise, the position of the casts on the articulator will not accurately represent the patient. When the occlusion is designed and the prosthesis is fabricated, the path of the cusp tips during excursions on the articulator may not mimic that of the patient, resulting in occlusal interferences during excursions. Most patients are asymmetric, and when viewed from the front, the maxilla is located to the right or the left of a center point placed on the hinge axis. In some situations, the discrepancy is quite marked. In addition, one side of the occlusal plane may be lower than the other side (Fig 13-11). A facebow transfer record allows the clinician to properly record and transfer these

discrepancies to the articulator, which then allows the articulator to duplicate the mandibular movements of the patient.

Clinical procedure A facebow may be used to transfer a comparable radius from arbitrary reference points or from hinge axis points. One of the commonly used facebows is the earbow. This instrument uses the external auditory meatus reference point for determining the arbitrary hinge axis location. It registers the relation of the maxillary dental arch to the external auditory meatus and a horizontal reference plane. 1. L  ocate the anterior reference point. Mark the anterior reference point on the patient’s right side using the reference plane locator and marker. For this system, the point is 43 mm above the incisal edge of the right central or lateral incisor. 2. Prepare the bite fork. Cover the occlusal surface of the bite fork with modeling compound or hard baseplate wax. Seat the softened index material on the maxillary occlusal surfaces, aligning the patient’s midline with the index notch of the bite 135

13

Maxillomandibular Records and Occlusion for RPDs

Orientation with remaining teeth when MIP is the treatment position When there is sufficient occluding dentition to establish a positive, stable interarch position and this is the treatment position determined by the clinician, it is acceptable to orient the mandibular cast to the mounted maxillary cast by hand (Fig 13-13). This method eliminates the interposed record material, which increases the risk of error (Fig 13-14). If the nature of the dentition does not permit direct apposition of the casts by hand, a maxillomandibular record needs to be made with record bases and wax rims (Fig 13-15).

Centric relation record Fig 13-12 The facebow record is secured to the lower member of the articulator with a bite fork transfer jig to mount the maxillary cast.

fork. Ask the patient to hold the bike fork firmly in position with the thumbs of both hands. 3. Place and secure the facebow onto the bite fork. The facebow is brought gently over the patient with the stem of the bite fork entering the loose bite clamp or the vertical shaft of the bite fork assembly. Adjust and slide the facebow frame so the earpieces fit tightly into the patient’s ears. 4. A third point of reference is recorded. Raise or lower the bow so that the anterior pointer aligns precisely with the anterior reference point. (Note: The end of the anterior reference pointer is rounded; however, the patient’s eye must be protected with the finger during placement of the pointer and removal of the facebow.) 5. Secure the facebow to the articulator. Secure the facebow using the manufacturer’s recommendations. Sometimes the facebow is secured to the lower member of the articulator with a bite fork transfer jig (Fig 13-12). 6. Mount the maxillary cast onto the articulator using the facebow transfer record. Prepare remount notches on the base of the maxillary cast, coat them with petroleum jelly, and place the cast onto the occlusal index of the facebow. Mount the upper cast with mounting stone or plaster.

Maxillomandibular record The treatment position chosen, either MIP or CR, is based on the diagnostic information. Methods of recording the chosen treatment position are greatly influenced by the teeth and tissue remaining. There are two methods to record the maxillomandibular position. 136

The purpose of this registration is to record the exact position of the maxilla to the mandible in CR. Centric relation is defined as the “maxillomandibular relationship, independent of tooth contact, in which the condyles articulate in the anterior-superior position against the posterior slopes of the articular eminences; in this position, the mandible is restricted to a purely rotary movement; from this unstrained, physiologic, maxillomandibular relationship, the patient can make vertical, lateral or protrusive movements; it is a clinically useful, repeatable reference position.”1 Methods of recording CR are greatly influenced by the teeth and remaining tissues. When there is sufficient dentition to support the interocclusal registration material, it is possible to obtain a CR record without a record base (see Fig 13-14). When an edentulous extension area is present, it may be necessary to use a record base in conjunction with an interocclusal registration material to properly record centric position and transfer this record to the articulator (Fig 13-16; see also Fig 13-15).

Use of record bases and occlusal rims In most cases involving RPDs, extensive edentulous areas are present. There may be a single edentulous area in just one arch or the extreme situation of a complete maxillary denture opposing six or fewer anterior mandibular teeth. Recording of the CR position under these conditions requires the following: • Record bases that are well adapted to the tissue surface • Occlusion rims and interocclusal registration material that are dimensionally stable Record bases. The reliability of the centric registration depends on support from the edentulous mucosa being equal to the support provided by the finished prosthesis. The record base is fabricated on the surfaces of the altered cast, which will form the tissue surface of the definitive prosthesis. This record base is attached to the denture base connectors of the RPD casting. The record base is fabricated by blocking out undercuts on the

Maxillomandibular Registrations

Fig 13-13  When there is sufficient occluding dentition to establish a positive maxillomandibular position, it is acceptable to orient the mandibular cast to the mounted maxillary cast by hand. (Courtesy of Dr T. Berg, Los Angeles, California.)

Fig 13-14  When there is sufficient dentition to support the interocclusal registration material, it is possible to obtain the CR record without a record base. Remount notches are cut in the base of the cast, and a thin layer of petroleum is applied to the notches. The two casts are secured firmly together with hard sticky wax for mounting. (Courtesy of Dr T. Berg, Los Angeles, California.)

a

b

c

d

Fig 13-15  This record was made with the aid of a record base and wax rim. (Courtesy of Dr T. Berg, Los Angeles, California.)

Fig 13-16  (a) A record base and wax rim have been fabricated on the master cast with the RPD. (b) The record has been obtained from the patient and secured to the master cast. (c) The mandibular cast has been mounted onto the articulator. (d) The mounted casts. Note that there is sufficient restorative space for the placement of the denture teeth.

altered cast, applying a separating medium, and sprinkling a thin layer of autopolymerizing acrylic resin onto the support surface to attach it to the casting. Occlusion rims. The occlusion rim is a ridge of material that fills the space between the record base and the opposing occlusion. This rim should be fabricated with a dimensionally stable material. A simple yet positive occlusion rim can be constructed

at the same time the record base is fabricated. A thin, V-shaped ridge of baseplate wax is formed on the record base up to the approximate level of the opposing occlusal surfaces (see Fig 13-16a). This provides a firm, rigid, and unchanging rim that is easily adjusted. Materials like hard waxes are often used for occlusal rims. Exceptional care should be taken during use because they can be distorted by temperature change and pressure.

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Fig 13-17 (a and b) Modeling plastic (dental compound) was used for the interocclusal registration material in this patient.

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b Fig 13-18 The accuracy of the interocclusal record is verified by observing the repeatability of the mandibular closure into the recording material. (Courtesy of Dr T. Berg, Los Angeles, California.)

Interocclusal registration materials. The material chosen should enable the clinician to make a positive and accurate record. The material should exert little or no opposition to closure. The primary requirement for the material is that when set, it undergoes no dimensional change or distortion associated with compression or temperature change. There are several materials available that fulfill these requirements, including wax, modeling plastic (compound), zinc oxide pastes, plaster, and silicone registration materials (Fig 13-17; see also Figs 13-14 to 13-16).

Clinical procedure Patient conditioning and cooperation are of the utmost importance. Therefore, the nature of the procedure should be explained to the patient. The clinician should practice together with the patient until the patient is relaxed and the clinician is able to reproducibly manipulate the mandible in the proper position. A recording should not be attempted until a consistent CR position can be achieved. Close observation is necessary to ensure that there is no movement of the mandible while the registration material is setting. When the material has set, the accuracy of the record should be checked by having the patient close in CR and observing the repeatability of jaw closure into the recording material (Fig 13-18). Remount notches are cut in the base of the cast, and a thin layer of petroleum jelly is applied to the notches. The two casts are secured firmly together with hard, sticky wax, and the mandibular cast is mounted on the articulator (see Fig 13-17b). If the OVD has been increased during the recording of CR, the amount must be measured, and the pin must be adjusted 138

accordingly. It is advisable to make a second set of interocclusal records and to check the mounted articulator casts to confirm the correctness of the transfer against the possibility of error in the mounting.

Protrusive record The purpose of the protrusive relation record is to record the condylar inclination. The record is obtained by making an interocclusal record when the anterior teeth are edge to edge or the mandible is approximately 4 to 6 mm forward of the CR position. The record is removed from the mouth of the patient and placed on the casts that had previously been mounted on the articulator, then the condylar elements are adjusted to determine the condylar inclination. The articulator can now reasonably duplicate the mandibular movements of the patient.

Developing Occlusion and Esthetics The occlusion should be designed to do the following: • Preserve and control remaining oral structures • Organize the occlusion (for correct OVD, CR/MIP, and excursions) • Provide masticating surfaces • Restore, preserve, or improve esthetics

Developing Occlusion and Esthetics

Fig 13-19  Supereruption of the maxillary posterior teeth and tilting of the mandibular molar to the mesial result in inappropriate occlusal contact in excursive movements as well as esthetic challenges for the maxillary RPD.

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Fig 13-20  The patient presents with an edentulous maxilla opposing a partially dentate mandible. (a) Note that the mandibular molar is tilted to the mesial and supererupted. (b) When the patient attempts to close in CR, note the premature contact with the posterior teeth of the maxillary complete denture. The removal of the mandibular molar allowed the occlusal plane to be idealized and balanced articulation to be designed into the occlusion. (c) The new RPD in position. The crowns were redone in order to position rests on the mesial sides of the abutments adjacent to the edentulous area and to idealize tooth contours so as to facilitate the development of balanced articulation. (d) The definitive prosthesis. (Courtesy of Dr R. Faulkner, Cincinnati, Ohio.)

Preserve and control remaining oral structures Controlling the position of the residual dentition and preserving the structures that remain is a prime objective in treating the partially edentulous patient. This concept, as presented by DeVan,2 is a basic objective of treatment with the RPD. The replacement of teeth and mastication surfaces in and of themselves may not be proper treatment if it jeopardizes the health or survival of the residual dentition and supporting structures. Improperly designed or traumatic occlusion associated with an RPD may accelerate the processes that lead to loss of abutment teeth or remaining dentition as well as resorption of the edentulous bearing surfaces.

Organize the occlusion Basic factors to be considered in developing occlusion when treating a patient with RPDs are the following: • Occlusal plane • Condylar guidance • Occlusal scheme

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Occlusal plane

The first consideration in developing occlusion is the evaluation and establishment of the correct plane of occlusion. Without controlling this baseline, it is impossible to develop organized and compatible interarch occlusal contacts during function and parafunction. This condition is most obvious when a posterior tooth is supererupted and extends beyond the occlusal plane (Fig 13-19; see also Figs 13-3 and 13-4). Such tooth migrations can create occlusal interference upon opening and closing the mandible and during lateral excursions. The most significant difficulty, however, develops when there is horizontal mandibular movement that results in unwanted occlusal contact. This is commonly seen when an isolated mandibular tooth becomes tipped to the mesial and supererupts. When the maxilla is edentulous and is to be restored with a complete denture, the position and contour of the occlusal surfaces of such teeth must be addressed. Several options are available, such as enameloplasty, restoring position and contour with a full-coverage crown, or by an extended occlusal rest. However, if the supereruption is excessive, such teeth often need to be extracted in order to create an appropriate occlusal scheme (balanced articulation) (Fig 13-20). Moreover, supereruption of mandibular anterior teeth can cause difficulties in occlusion in excursions when an edentulous maxilla is opposed by a partially dentate mandible and 139

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Fig 13-21 (a to c) The vertical overlap of the anterior teeth was excessive, and there are no posterior contacts in protrusive position. As a result, the maxillary denture tips anteriorly during parafunction. This eventually led to severe resorption of the incisive bone, leading to the development of so-called “combination syndrome.”

Fig 13-22 The location and number of the remaining natural anterior teeth determine anterior guidance. In this patient, because of the vertical overlap, there will be immediate separation of the posterior teeth during excursions. (Courtesy of Dr A. Pozzi, Rome, Italy.)

Fig 13-23 Anterior guidance is provided by the remaining natural dentition. This is the ideal situation. (Courtesy of Dr T. Berg, Los Angeles, California.)

can make it difficult to position the anterior maxillary teeth in such a fashion that satisfies the esthetic demands of the patient while at the same time enabling the creation of a balanced articulation. The inevitable outcome is excessive tipping of the maxillary denture during parafunctional activities (clenching and grinding), triggering a resorptive remodeling response and severe resorption of the edentulous maxilla (Fig 13-21). This is commonly referred to as combination syndrome. The ideal occlusal plane is an imaginary line that starts from the middle of the retromolar pad and extends to the incisal edges of the maxillary central incisors at the correct OVD. Once the occlusal plane is established, the occlusal surfaces of the remaining teeth are assessed and modified or restored to develop the occlusal scheme that the clinician has selected.

tooth guidance and (2) facilitate the development of balanced articulation (bilateral balanced occlusion) in all excursions when an RPD opposes a complete denture.

Condylar guidance The contour of the glenoid fossa is unique to each individual. Its inclination can be recorded and transferred to an articulator and has a profound influence on developing occlusion in protrusive and lateral positions. The condylar inclination is transferred to the articulator so that the prosthetic occlusion will (1) be harmonious with and not interfere with natural 140

Occlusal scheme There are three types of occlusal schemes that can be employed when restoring the dentition with RPDs: 1. Mutually protected occlusion 2. Group function 3. Balanced articulation (bilateral balanced occlusion) The scheme chosen depends on the nature of the residual natural dentition or lack thereof in the opposing arches. Patients with natural dentition present in both the mandible and the maxilla are generally restored with either mutually protected occlusion or group function. Those patients presenting with an edentulous arch opposing a partially dentate arch are restored with balanced articulation. Mutually protected occlusion. When anterior maxillary and mandibular teeth remain, their positions often determine the

Developing Occlusion and Esthetics

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Fig 13-24  (a to d) Fixed dental prostheses coordinated with an RPD provide the opportunity to establish ideal anterior guidance. (Courtesy of Dr A. Davodi, Beverly Hills, California.)

Fig 13-25  A diagnostic wax-up prior to initiating treatment helps to identify occlusal problems and assists in developing ideal occlusion.

path of movement of the mandible in protrusive and lateral excursions. In some situations, excursions are characterized by an immediate, vertical separation of the posterior teeth because of significant vertical overlap of the anterior dentition (Fig 13-22). In other instances, there can be considerable horizontal movement of the mandible before the anterior teeth induce an opening movement. The vertical and horizontal overlap of the anterior dentition (anterior guidance) influences cusp angulations and the position of teeth in the posterior quadrants. In patients with significant vertical overlap with a steep incisal angle, there is immediate vertical opening and therefore less possibility of posterior occlusal interference during excursions. Some patients lack significant vertical overlap of the anterior teeth (either acquired from wear and erosion or congenital), and as a result, posterior interferences can result during lateral excursions. Under these circumstances, more careful placement and adjustment of the occlusal surfaces of the prosthetic teeth is required to avoid these interferences.

It is desirable to have remaining anterior teeth in both jaws to provide guidance during excursions (Fig 13-23). When natural anterior tooth guidance is present in conjunction with posterior RPDs, it is desirable that posterior teeth separate in excursions (centric only contact). If wear of the dentition is moderate to severe, it may be desirable to restore the anterior guidance (Fig 13-24). However, this process should be approached in a systematic fashion consistent with sound prosthodontic principles. A mutually protected occlusal scheme utilizing anterior guidance is preferable, and the guidance established should be compatible with the condylar guidance of the patient. A diagnostic wax-up should be completed prior to initiating treatment to assess the types of restorations necessary and to determine if occlusal adjustments need to be made to the remaining posterior teeth (Fig 13-25). In complex cases, a treatment partial denture should be employed prior to treatment to establish a treatment position that is compatible with an appropriate OVD and with the patient’s condylar guidance and envelope of function (see Fig 13-6 and chapter 18). 141

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a Fig 13-26 The premolar and the canine are in simultaneous contact during laterotrusion with the incisors, thus limiting the forces delivered to the abutments during parafunction. (Courtesy of Dr T. Berg, Los Angeles, California.)

Fig 13-27 (a) A common clinical presentation is a maxillary complete denture opposing a mandibular RPD. Balanced articulation is the desired occlusal scheme to avoid rapid resorption of the edentulous arch resulting from the denture tipping during parafunction. (b) Note the extended rest on the premolars and the balancing ramp distal to the mandibular molar. This type of rest is designed to optimize the occlusal plane and achieve balanced articulation. (Courtesy of Dr T. Berg, Los Angeles, California.)

Group function. There are significant differences in the occlusal scheme used between posterior extension-base RPDs and anterior extension-base RPDs. Centric only contact of the posterior teeth is preferable for distal-extension RPDs, but this is not feasible for anterior extension-base RPDs if the canines are missing. When restoring anterior extension areas in the maxilla, the lengths of the prosthetic anterior teeth, the vertical and horizontal overlap, and therefore the nature of the esthetic outcome are limited by the jaw relation, the incisal angle, and the opposing dentition. The denture teeth in the edentulous anterior extension area and the posterior natural teeth should be in simultaneous contact during laterotrusive movements (Fig 13-26). If anterior guidance is employed by the anterior prosthetic teeth, the abutment teeth adjacent to the anterior edentulous extension area will be subjected to undesirable forces during parafunction. Group function also can be considered for patients with worn remaining anterior dentition in the absence of symptoms associated with the TMJ and associated musculature, or when the patient presents with a skeletal Class III jaw relation. If such a patient cannot afford the fixed prosthodontic treatment to ideally restore the anterior guidance, more than likely the clinician will be required to restore the patient with group function. Balanced articulation. A common clinical presentation is a maxillary complete denture opposing an RPD (Fig 13-27). This condition requires an entirely different occlusal scheme. Whenever one arch is edentulous, balanced articulation (bilateral balanced occlusion) is employed (see Figs 13-20 and 13-27). Otherwise, the denture will tip and move excessively during parafunction and the edentulous arch will rapidly resorb (see Fig 13-21). It is necessary to provide contacts in both centric and excursive movements bilaterally (balanced articulation).

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Provide masticating surfaces Lingualized tooth molds are ideal for restoring posterior teeth in extension RPDs (Fig 13-28). The modern lingualized denture teeth are designed to have only the lingual cusp of the maxillary tooth in function against the mandibular denture tooth. This lingualized occlusion design minimizes the size of the opposing occlusal surfaces during mastication and reduces and simplifies development and adjustment of the occlusion but still provides necessary occlusal stability and mastication efficiency. The occlusal width of the mandibular denture tooth is lessened to reduce the surface area, and the central fossa is contoured into a shallow curve that provides occlusal contact in and around CR. The cusp angles of the mandibular teeth are flattened, and the maxillary palatal cusp tips engage a single contact area in the fossa. In addition, unlike cusp embrasure denture tooth designs, this design enables the clinician freedom to position the individual teeth as needed mesiodistally. The arrangement of the prosthetic teeth must be consistent with the occlusal scheme selected. A specific sequence should be followed when setting posterior denture teeth for RPDs. The first one positioned is the prosthetic tooth adjacent to the RPD proximal plate. This tooth should be arranged and contoured so that it is compatible with the designed occlusal scheme in centric and lateral excursions (Figs 13-29 and 13-30). In the maxilla, alteration of the proximal plate and the denture teeth may be necessary to optimize the esthetic outcome. Sometimes it is advantageous to leave a proximal space between the prosthetic teeth and natural teeth in a nonesthetic area.

Natural teeth versus the residual ridge The development of occlusion in posterior extension RPDs against natural dentition requires special consideration. Two

Developing Occlusion and Esthetics

Fig 13-28  Lingualized teeth are preferred for RPDs.

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Fig 13-29  (a to c) Positioning the first prosthetic tooth adjacent to the proximal plate may require significant alteration of the tooth. In this case, the mesial surface of the premolar has been altered to accommodate the proximal plate. (d) The occlusal surface is then recontoured to create centric only contact.

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Fig 13-30  (a to e) If the denture setup is performed using the proper sequence, the remaining prosthetic teeth can be positioned ideally. Centric only contact is advised. The natural anterior teeth provide guidance during lateral excursions.

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Movement

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factors influence placement and occlusal form of the denture teeth: (1) the relationship of the remaining natural teeth to the opposing residual ridge and (2) the tooth’s periodontal ligament support opposing the mucosal support of the extension-base prosthesis. When opposing ridges position the lingual cusp of the maxillary natural tooth over the mandibular ridge (Fig 13-31a), the only contact will be the maxillary palatal cusp against the mandibular denture tooth (Fig 13-31b). The occlusal contact is small and is developed to touch in CR. When anterior guidance of natural teeth is present, it is advantageous to have the posterior occlusion disengage (Fig 13-31c). When discrepancies of opposing arches place the residual mandibular ridge opposite the central fossa of the maxillary natural tooth (Fig 13-31d), the same principles of occlusion described previously are achieved through recontouring the mandibular denture tooth by removing the lingual cusp and contouring and adjusting the buccal cusp into the central fossa of the natural tooth (Fig 13-31e).

Addressing occlusal plane discrepancies associated with the opposing natural dentition As seen from previous examples, many patients present with significant discrepancies associated with the occlusal plane. 144

c Fig 13-31 (a) The lingual cusps of the remaining natural teeth are often positioned directly over the opposing edentulous ridge, which is weaker from a support standpoint. (b) The mandibular prosthetic tooth is modified to reduce occlusal size, and the centric contact area is flattened. This occlusal scheme is referred to as lingualized occlusion. (c) When natural anterior tooth guidance is present, it is desirable to have posterior extension occlusion disengage after 2 to 3 mm of eccentric movement. (d) In some situations, the edentulous ridge is opposite the central fossa of opposing natural teeth. (e) The lingual cusp of the mandibular prosthetic tooth is modified at the contact area and still provides sufficient support and function.

These occlusal discrepancies make it difficult to fabricate a complete denture for the opposing arch with balanced articulation. Supereruption of the remaining teeth left unaddressed predisposes the patient to the tipping of the denture and the generation of lateral forces, which can lead to compression of the mucoperiosteum and disruption of the vasculature, triggering a resorptive remodeling process of the underlying bone (see Fig 13-21). These occlusal discrepancies can be corrected by orthodontic intrusion, recontouring or enameloplasty of the existing teeth (see Fig 13-34), new restorations (Fig 13-32), or sometimes the extraction of the offending tooth or teeth (see Fig 13-20). Ribbon or overlay rests can also be used to level the plane of occlusion when balanced articulation must be employed(Fig 13-33; see also Fig 13-27b).

Adjustment of the occlusal surface of natural dentition and the prosthetic teeth When the denture teeth of the RPD oppose natural dentition, occasionally the contours of the occlusal surfaces of the natural teeth must be altered in order to idealize the plane of occlusion and interocclusal relationships (Fig 13-34).

Developing Occlusion and Esthetics

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Fig 13-32  This patient presents with an edentulous maxilla opposing a partially edentulous mandible requiring an RPD. (a) Existing complete denture opposing the partially dentate mandible. (b) Protrusive position. Note the mesially tipped molar and the premature occlusal contact with the molar. The opposing denture will tip and be displaced during parafunction, leading to resorption of the maxilla. (c) Right laterotrusive position. (d) Tooth preparations. Note that the teeth have been prepared so that the guide planes will be parallel. (e) Completed prostheses in CR. (f) Right laterotrusive position. The occlusal scheme is balanced articulation. (Courtesy of Dr J. Kelly, Omaha, Nebraska.)

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Fig 13-33  (a) Patient presents with tipped and infraerupted teeth disrupting the plane of occlusion. (b) An overlay RPD with extended rests was designed to idealize the plane of occlusion. (c) The RPD in position. (d) The complete denture without the RPD. (e) The complete denture and RPD in position. The overlay RPD permits the clinician to design a complete denture with balanced articulation. d (Courtesy of Dr G. King, Houston, Texas.)

e

Fig 13-34  Selective recontouring of opposing natural teeth can help to idealize the occlusal plane. (Courtesy of Dr R. Duell, Los Angeles, California.)

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13

Fig 13-35 Note the proximal plate on the mesial surface of this maxillary canine. The proximal plate has been thinned, and the labial extension has been reduced, making it possible to position the lateral incisor in direct contact with the canine and hiding the proximal plate from view. (Courtesy of Dr R. Duell, Los Angeles, California.)

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Restore, preserve, or improve esthetics Esthetics in RPD treatment is often complicated by the difficulty of harmonizing prosthetic replacements with remaining natural teeth. The remaining teeth often present a variety of sizes, colors, and shapes within the same arch. Producing a compatible replacement often requires reshaping, recontouring, and characterizing the prosthetic replacement to blend with the existing natural dentition. Resin teeth can be readily altered to harmonize with the remaining dentition. They can also be characterized with stains and restorations. Portions of the metal casting of the partial denture framework adjacent to the edentulous space and where teeth are to be positioned are evaluated carefully. In the esthetic zone, the labial extension 146

b Fig 13-36 (a) The shape and contour of the denture base have a great influence on the esthetic outcomes, particularly in patients with a high smile line. The gingival outlines and contours should be consistent with the principles of smile design. The shape and contours are finalized during the try-in appointment. (b) Occlusal view. The contours on the lingual side of the prosthesis should mimic normal anatomy so as to provide appropriate tongue space and enable proper speech articulation. (c) The completed prosthesis in position. Note how the denture base blends with the adjacent mucosal surfaces.

of the proximal plate may need to be reduced to permit the placement of the denture tooth in direct contact with the natural tooth abutment (Fig 13-35). At the try-in appointment for evaluation of appearance and esthetics, the final contour, size, and shape of the finished denture base must be reproduced in wax for careful evaluation by the patient and the clinician. Necessary changes such as repositioning of teeth and contouring of the denture base continue to be made until there is mutual agreement between the patient and the clinician that the appearance is acceptable (Fig 13-36). The contour and color of the denture base are quite critical in RPD treatment when the denture base is visible during an animated smile. Contours and color that are not compatible with existing structures are quite obvious, and a blending of topography

Laboratory Instructions

Fig 13-37  Amalgam stops have been placed to reduce wear and maintain the OVD.

and color is necessary. Color guides for denture bases are used to select a suitable color. When an anterior extension RPD is fabricated, the denture teeth and gingival contours should be consistent with the principles of smile design (see chapter 14).

Preventing occlusal wear and loss of OVD Posterior teeth provide the majority of vertical dimension control and require a material that is resistant to wear or abrasion. The material must also be compatible with the opposing natural teeth or the materials used to restore them. The most commonly used denture teeth are made of acrylic and composite resins. Recent advancements in composite resins have lessened the occlusal wear of the denture teeth. However, the patient needs to be informed of the possibility that eventually the denture teeth will need replacement. Amalgam stops can be used in posterior teeth to maintain the OVD (Fig 13-37), although the denture teeth tend to wear around these stops and eventually need to be replaced.

Fig 13-38  Laboratory instructions should be detailed and include all necessary information.

Laboratory Instructions Upon completion, the prosthesis is returned to the laboratory for processing. Detailed instructions are required for the technician to understand and fulfill the desires of the clinician and the patient (Fig 13-38). These instructions include the following: 1. The necessary refinements of occlusion in centric and eccentric 2. Orders for tooth modifications, changes, staining, or restorations 3. Specific details of denture base contour and color 4. The date for completion 5. A copy of articulator information (facebow record, articulator settings, etc)

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References 1. The Glossary of Prosthodontic Terms: Ninth Edition. J Prosthet Dent 2017;117:e1–e105. 2. DeVan MM. The nature of the partial denture foundation: Suggestions for its preservation. J Prosthet Dent 1952;2:210–218.

Suggested Reading Occlusion Cohn R. The relationship of anterior guidance to condylar guidance in mandibular movement. J Prosthet Dent 1956;6:758–767. Colman AJ. Occlusal requirements for removable partial dentures. J Prosthet Dent 1967;17:155–162. Craddock FW. The accuracy and practical value of records of condyle path inclination. J Am Dent Assoc 1949;38:697–710. Henderson D. Occlusion in removable partial prosthodontics. J Prosthet Dent 1972;27:151–159. Hindels GW. Occlusion in removable partial denture prosthesis. Dent Clin North Am 1962;6:137–146. Jeffreys FE, Platner RL. Occlusion in removable partial dentures. J Prosthet Dent 1960;10:912–920. McCracken WL. Occlusion in partial denture prosthesis. Dent Clin North Am 1962;6: 109–119. Saunders TR, Gillis RE, Desjardins RP. The maxillary complete denture opposing the mandibular bilateral distalextension partial denture: Treatment considerations. J Prosthet Dent 1979;41:124–128. Wallace DH. The use of gold occlusal surfaces in complete and partial dentures. J Prosthet Dent 1964;14:326–333.

Esthetics DeVan MM. The appearance phase of denture construction. Dent Clin North Am 1957;1:255–268. Frush JP, Fisher RD. Introduction of dentogenic restorations. J Prosthet Dent 1955;5:586–595. Tilman EJ. Molding and staining acrylic resin anterior teeth. J Prosthet Dent 1955;5:497–507. Wolfson E. Staining and characterization of acrylic teeth. Dent Abstr 1958; 1:41.

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Maxillomandibular relations and registrations Beck HO. Selection of an articulator and jaw registrations. J Prosthet Dent 1960;10:878–886. Beckett LS. Accurate occlusal relations in partial denture design. J Prosthet Dent 1954;4:487–495. Block LS. Preparing and conditioning the patient for inter maxillary relations. J Prosthet Dent 1952;2:599–603. Emmert JH. A method of registering occlusion in semi edentulous mouths. J Prosthet Dent 1958;8:94–99. Freilich MA, Altieri JW, Wahle JJ. Principles of selecting interocclusal records for articulation of dentate and partially dentate casts. J Prosthet Dent 1992;68:361–367. Hall WA Jr. Variations in registering interarch transfers in removable partial denture construction. J Prosthet Dent 1973;30:548–553. Hughes GA, Regli CP. What is centric relation. J Prosthet Dent 1961;11:16–22. Lauritzen AG, Bodner GH. Variations in locations of arbitrary and true hinge axis points. J Prosthet Dent 1961;11:224–229. Maveli TC, Suprono MS, Kattadiyil MT, Goodacre CJ, Bahjri K. In vitro comparison of the maxillary occlusal plane orientation obtained with five facebow systems. J Prosthet Dent 2015;114:566–573. Reitz PV. Technique for mounting removable partial dentures on an articulator. J Prosthet Dent 1969;22:490–494. Silverman MM. Determination of vertical dimension by phonetics. 1956;6: 465–471. Stade EH, Hanson JG, Baker CL. Esthetic considerations in the use of facebows. J Prosthet Dent 1982;48:253–256. Teteruck WR, Lundeen HC. The accuracy of an ear face-bow. J Prosthet Dent 1966;16:1039–1046. Weinberg LA. An evaluation of basic articulators and their concepts: Part I. Basic concepts. J Prosthet Dent 1963;13:622–644. Weinberg LA. An evaluation of basic articulators and their concepts: Part II. Arbitrary, positional, semi adjustable articulators. J Prosthet Dent 1963; 13:645–663. Weinberg LA. An evaluation of the face-bow mounting. J Prosthet Dent 1961;11:32–42.

Chapter 14 Optimizing Esthetics: Attachments and Rotational Path RPDs Ting-Ling Chang | Daniela Orellana | Frederick C. Finzen

The esthetic outcome of patients fitted with removable partial dentures (RPDs) was once considered problematic when key abutments for the RPD were located in the esthetic zone, especially when conventional suprabulge retainers were used. However, because of I-bar retainers, resilient attachments, and the emergence of the rotational path of insertion concept of RPD design, this no longer needs to be the case. Using these methods, even patients with anterior edentulous extension defects can be restored with RPDs with excellent esthetic outcomes.

Integration of Fixed and Removable Prosthodontic Treatment Frequently, loss of multiple posterior teeth and loss of posterior support predisposes the patient to closure of the occlusal vertical dimension (OVD) and loss of facial height. Often these changes are accompanied by displacement of anterior teeth with the creation of unwanted diastemata (Fig 14-1). In these patients, integrating RPD treatment with full-coverage crowns and fixed dental prostheses (FDPs) not only replaces the posterior teeth and allows the OVD to be restored and with it the facial height, but also provides the clinician the opportunity to idealize the esthetic outcome. In these situations, the clinician may need to ask several questions to decipher the specific motivations and needs of the patient before embarking upon a course of treatment. For

instance, does the patient wish for restoration of only the teeth that have been lost or are about to be lost, or does the patient wish for the entire esthetic zone to be restored to ideal color and dimension? If the patient wishes to restore and idealize the esthetic zone, several issues need to be addressed. These types of treatments can be very complex and require careful planning and meticulous execution by the clinician and the laboratory. A diagnostic waxup/setup is necessary if the esthetic outcome is to be idealized (Fig 14-2). The diagnostic wax-up helps the clinician identify occlusal issues that need to be addressed as well as the proposed position and contours of the anterior teeth. It can also be used to help the patient understand his or her dental needs and visualize the intended outcome to some degree. This can be performed conventionally (see Fig 14-2), or a digital rendering can be presented to the patient. Integration of fixed and removable prosthodontic treatment phases must follow the proper sequence, and several issues need to be addressed to optimize the esthetic outcome (see also chapters 15 and 18): • The appropriate OVD must be determined. • The occlusal plane must be idealized. Canted occlusal planes may be especially displeasing. • A treatment position should be established. This may require the fabrication of treatment RPDs. • The contours of the upper and lower lip during a relaxed and high smile should be carefully assessed.

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Fig 14-1 (a and b) The patient presents with multiple missing teeth, multiple diastemata, and a closed OVD. (Courtesy of Dr A. Davodi, Beverly Hills, California.)

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Fig 14-2 (a) Pretreatment mounted diagnostic casts show lack of posterior occlusal support, a closed OVD, and labial flaring of the maxillary anterior teeth. (b and c) Ideal size and contour of the anterior teeth have been developed with the diagnostic wax-ups, and a tentative OVD has been established.

Fig 14-3 The patient depicted in Fig 14-1. Based on a diagnostic wax-up and following preparation of the teeth, provisional restorations and treatment RPDs are used to determine the OVD and the treatment position. The size and contours of the individual teeth and the tooth display of the anterior dentition are also established before fabricating the definitive restorations. (Courtesy of Dr A. Davodi, Beverly Hills, California.)

• Tooth size and proportions and their relation to one another must be determined. • The size and symmetry of incisal embrasures must be carefully determined. • The incisal edges must be configured to produce an appealing smile line. Convex incisal planes are more esthetically pleasing than flat or concave incisal planes. • When preparing the RPD abutments, rests and guiding surfaces for the RPD must be taken into account (see chapter 15). • The retentive area of the abutments to be engaged by retainers must be positioned on the cervical third of the abutment to minimize the display of the I-bar retainers (see chapter 15). 150

These issues must be carefully analyzed and addressed before fabricating the definitive prostheses. They are best addressed with the use of provisional restorations and treatment RPDs (Fig 14-3). Only when these issues are resolved should the clinician begin fabrication of the definitive restorations and RPDs (Fig 14-4). It is particularly important to establish the OVD and a treatment position. The foundation for the modern approach to esthetic dentistry and smile design was laid by some of the founding fathers of complete denture prosthodontics: Frush, Fisher, and Pound in the United States; Preti in Italy; and Gerber in Switzerland. They recognized that (1) there are great variations in patient

Integration of Fixed and Removable Prosthodontic Treatment

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Fig 14-4  (a to e) The patient depicted in Figs 14-1 and 14-3. Following the establishment of the OVD and treatment position with the treatment RPDs as well as an assessment of the esthetic display of the provisional full-coverage crowns, the definitive restorations and RPDs were fabricated. (Courtesy of Dr A. Davodi, Los Angeles, California.)

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perceptions and individual anatomical forms, and (2) what is esthetically pleasing for one patient may not work for all patients. They advocated for an approach that combined realism with esthetically pleasing outcomes consistent with the patient’s facial form, age, and personality type. In modern culture, smile design and esthetics have become more idealized, and attempts are often made to recapture an idealized youth at the expense of realism. In many ways, this is not entirely a positive development because some clinicians apply a “one system fits all” approach that may not always be in the best interest of the patient. The ultimate arbiters, however, in this delicate balance between what is esthetically “ideal” (the so-called “Hollywood smile” of straight, white teeth with perfect bilateral symmetry) and what is realistic given the patient’s age and oral anatomy are the patient and his or her family and close friends. Patients’ perceptions of ideal esthetics vary. Experienced clinicians are able to inject a healthy dose of both of these approaches during their presentation to the patient so that their expectations are based on a realistic assessment of what is feasible and predictable. Humans are sensitive

to the harmony or disharmony of forms, and disharmonies in symmetry are readily apparent to most observers. Nevertheless, most clinicians prefer to maintain some realism for each patient by introducing slight variations on each side of the midline. A comprehensive review and consideration of these factors must be undertaken whenever the esthetic zone is to be restored. A detailed discussion of these issues is beyond the scope of this book. The authors refer the reader to the excellent reviews by Chiche and Pinault as well as Morley.1,2

A word of caution Esthetic therapies and smile design often include additional restorative treatment, orthodontic tooth movement, periodontal treatment, and orthognathic surgery to reposition jaw segments. Even though smile design and esthetics are for the most part driven by prosthodontic needs, a comprehensive approach to smile design usually requires an interdisciplinary effort.

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Fig 14-5 Minimal metal display of the I-bar retainers offers a superior esthetic outcome in the esthetic zone compared to a circumferential clasp, as seen in this patient.

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Fig 14-6 (a) A unilateral distal-extension RPD. The canine has been fitted with a partial-coverage crown with a cingulum rest. Indirect retention is provided by the positive occlusal rest on the first premolar. (b) Facial view. (c) View with a high smile. Note the minimal display of the I-bar retainer. This is an 18-year follow-up photograph.

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Fig 14-7 An RPD restoring both anterior and posterior edentulous defects. (a) The canines have been fitted with metal-ceramic crowns with cingulum rests. (b) The RPD in position. Note that the facial contours of the crowns have been contoured so that the I-bars engage the abutments close to the gingival margin, limiting their visual display during a high smile (c). Note that the incisal edges have been configured to idealize the smile line. This is a 9-year follow-up photograph.

Restoration of Posterior Edentulous Extension Defects Use of I-bar retainers in the esthetic zone When restoring posterior edentulous extension defects with RPDs, the most frequent complaint is the visual display of the retainers engaging the anterior abutments in the esthetic zone. This display can be mitigated by the use of the I-bar retainer. One of the many advantages of the I-bar retainer is its minimal display compared to other types of retainers, such as circum152

ferential clasps (Fig 14-5). The I-bar retainer can be positioned close to the gingival margin and is difficult to see during speech or even when the patient elevates the lip during a high smile (Fig 14-6). In select patients, it may be desirable to idealize the retentive areas by enameloplasty or the fabrication of surveyed crowns. When the patient presents with a unilateral posterior edentulous extension, the use of indirect retention will preclude the use of a retainer in the anterior region on the side opposite the extension area (see Fig 14-6). If the anterior abutments require a full-coverage crown, the facial surfaces should be contoured so that the I-bar retainer engages the cervical third of the abutment (Fig 14-7).

Restoration of Posterior Edentulous Extension Defects

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Fig 14-8  (a) The two central incisors adjacent to the edentulous extension areas have been splinted together, and a cingulum rest has been incorporated within the patient’s right incisor. An ERA attachment has been attached to the splinted teeth. (b) The prosthesis in position. Additional bracing is provided by the extension of the framework through the canine-premolar embrasure. (c) The esthetic display during a high smile.

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Fig 14-9  (a and b) Note that the two abutments adjacent to the extension base are splinted. The rests are located on the mesial of the splinted abutments. This design will ensure that when an occlusal force is generated in the posterior region, the attachment will be displaced in a more vertical plane. This will minimize attachment wear and better direct occlusal forces axially. (Courtesy of Dr T. Berg, Los Angeles, California.)

Use of attachments Retainers can be eliminated altogether by the use of attachments. However, if an attachment is used to retain an extension-base RPD, it must allow the prosthesis to rotate freely around the rests when an occlusal force is delivered to the edentulous extension area so as to avoid exposing the abutments to lateral torquing forces.3,4 The authors recommend the use of a resilient attachment, such as an extracoronal resilient attachment (ERA) (Fig 14-8). It allows up to 0.4 mm of compression into the denture-bearing tissues before the nylon patrix bottoms out within the cast metal matrix portion of the attachment. When using this design, it is suggested that the two teeth adjacent to the edentulous extension defect be splinted together and that the rest be placed on the tooth distant from the extension area (Fig 14-9; see also Fig 14-8). This practice is more favorable biomechanically and also directs the occlusal forces in the edentulous extension area more vertically (see chapter 6). Splinting the two teeth adjacent to the defect also reduces the risk of overloading the natural tooth abutments if the adaptation of the denture base to the tissues becomes compromised, resulting in vertical movement of the prosthesis into the bearing surface in excess

of 0.4 mm. Given the limited amount of vertical compression tolerated by these attachments, the support gained from the edentulous extension must be maximized by making fully extended altered cast impressions of the edentulous extension areas so as to capture the primary support areas: the retromolar pad and buccal shelf in the mandible and the tuberosity and hard palate in the maxilla (see chapter 12). Although the use of attachments improves esthetic outcomes, clinicians must be mindful of the following disadvantages and risks: • Additional expense. • Attachment fatigue requires frequent changes of the matrix portion of the attachment apparatus. • The patient must be closely followed because if the extension base does not maintain proper adaptation to the extension area, the RPD will act as a cantilever and deliver torquing forces that could potentially destroy the natural tooth abutments. • Increased technical expertise required. • Less control of the distribution of occlusal forces.

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Fig 14-10 (a and b) A patient with a posterior bilateral distal extension defect. Rigid attachments were used inappropriately on a tooth-mucosa–borne RPD. (b) The extension base portion of the prosthesis delivered cantilever forces to the abutments, which soon led to the fracture of the abutment on the patient’s left canine.

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There are two cautionary notes when considering the selection of an attachment for use in an extension-base RPD: 1. The attachment must allow the RPD to rotate freely around the axis of rotation as dictated by the occlusal rests, otherwise the abutments will be subjected to torquing forces. 2. There must be sufficient restorative space available to accommodate the attachment and allow for proper positioning of the denture teeth. Attachments are classified as rigid (nonresilient) or resilient based on the stiffness of the resulting joint. For rigid attachments, there is no movement between the parts. Rigid attachments are best used in situations where the abutment teeth fully support the occlusal forces, such as a tooth-borne RPD. Resilient attachments allow movement to take place between the parts (the matrix and the patrix) when the prosthesis is fully seated, thus providing a stress-breaking function. Therefore, resilient attachments are mandatory when used to retain tooth-mucosa–borne RPDs (edentulous extension-base RPDs). There are several types of resilient attachments available. As mentioned previously, the authors favor the ERA because it has a low profile and requires minimal restorative space. If ERA attachments are used, the following is recommended: • The abutments adjacent to the extension areas should be splinted (see Figs 14-8 and 14-9). • If the patient requires an extension-base RPD, the rest should be distant from the attachment to allow the attachment to move in a more vertical plane when occlusal forces are generated in the extension area (see Figs 14-8 and 14-9). • When the ERA is used in extension-base RPDs, close follow-up is necessary to ensure that the extension base is properly adapted. Periodic relines may be needed to maintain proper adaptation of the denture base to the extension areas.

Complications associated with inappropriate use of attachments If attachments are used inappropriately, they can severely compromise the abutment teeth. For example, if a rigid attachment 154

is used in an edentulous extension-base RPD, they will not permit the RPD to rotate around the axis of rotation when occlusal forces are applied to the extension base, exposing the abutments to tipping forces that can eventually lead to their compromise and even their loss (Fig 14-10).

Restoration of Anterior Edentulous Extension Areas When restoring anterior edentulous extension defects, frequently the use of an RPD is the most prudent choice and in the best interest of the patient. RPDs may provide a more predictable esthetic outcome than implant-retained prostheses because the restorative dentist has more control of tooth forms, tooth position, and gingival contours. Furthermore, the cost of restoring a large anterior edentulous extension defect with implants may be prohibitive because of the cost of multiple surgeries required to enhance the bone and soft tissues of the implant sites in addition to the cost of the implants and associated components. Moreover, restoring long-span anterior edentulous extension defects with FDPs, particularly those associated with tapering arch forms, may be problematic because of unfavorable biomechanics.

Use of attachments and retained roots Retained roots can be effectively used to support and retain anterior edentulous extension overlay RPDs (see chapter 16). If the retained roots are strategically located and the angulation of the roots is favorable, a variety of types can be incorporated within the retained root. The attachment selected must exhibit a sufficiently low profile to allow proper positioning of the denture teeth. The design of the metal framework should ensure that all retained roots, including the tooth-tissue junction, are covered with metal (Fig 14-11). If they are covered with the porous acrylic resin of the denture base, rapid onset of caries and periodontal compromise will lead to rapid loss of these abutments. The patient is instructed to apply a drop of fluoride

Restoration of Anterior Edentulous Extension Areas

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Fig 14-11  (a) Patient presents with posterior molars bilaterally and several retained roots treated endodontically, two of which have been fitted with attachments. (b) The RPD design. (c) The RPD casting. Note the opening through the top of the casting so that the matrix portion of the attachments can be retained in the denture base resin. (d) The intaglio of the definitive prosthesis. Note that all retained roots are covered with metal and that the rests on the molars have been extended onto the buccal surfaces. (e and f) The prosthesis in position. Note that the acrylic resin of the denture base has been characterized. (Courtesy of Dr R. Duell, Los Angeles, California.) Fig 14-12  (a) A large anterior edentulous extension defect. (b) The anterior dentition was restored with a rotational path RPD. (Courtesy of Dr G. King, Houston, Texas.)

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gel daily into indentations made by the retained roots in order to reduce the risk of caries.

RPDs with a rotational path of insertion Rotational path RPDs are often used to restore anterior edentulous extension defects.5,6 They are esthetically pleasing because with this design, conventional retainers are not needed in the anterior region (Fig 14-12). The rotational path RPD works by incorporating a curved path of placement that allows one or more of the rigid components of the framework to engage an undercut area (Fig 14-13). In a patient with an anterior edentulous extension area, minor connectors/proximal plates engage undercuts on the mesial side of the anterior abutments (Fig 14-14). The anterior abutment teeth adjacent to the edentulous

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area must have positive rests in order to control tooth position and provide support during function (Fig 14-15). Fabrication of rotational path RPDs permits little tolerance for error. It requires appropriate knowledge on the part of both the clinician and the laboratory technician. The most critical elements of the rotational path RPDs are the following: • Well-designed and -prepared rests. The rest seat preparation must be of sufficient depth to provide reciprocation and direct occlusal forces axially. The rest should be 1.5 to 2.0 mm thick (see chapter 2). Rest seat preparations on the canines may require a restoration to achieve an adequate positive rest. The rest serves to direct occlusal forces along the long axis of the tooth and prevent migration of the abutment during function (Fig 14-16; see also Fig 14-15).

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Fig 14-13 Rotational path RPD is placed in an anteroposterior path of placement as shown. The anterior segment is placed first. Point A is the center of rotation around which the RPD rotates into final seating position following the arc A´.

Fig 14-15 Cingulum rests have been incorporated within the lateral incisors in preparation for fabrication of a rotational path RPD. (Courtesy of Dr R. Duell, Los Angeles, California.)

Fig 14-14 Rigid minor connectors/proximal plates engage the undercuts on the mesial surfaces of the canines to eliminate the conventional retainers. Positive rests allow control of tooth position during function.

Fig 14-16 The cingulum rest should be positive in order to control the position of the abutment and direct the occlusal forces along the long axis of the tooth during function. (Courtesy of Dr R. Duell, Los Angeles, California.)

Fig 14-17 It is particularly important to ensure an unobstructed path of insertion with proper blockout when additional edentulous spaces exist posteriorly. The amount of blockout on the distal surface of the canine is greater than the amount required on the mesial surface of the molar as shown. Point A represents the center of rotation of the rotational path RPD. A´, B´, C´, and D´ represent the arc of rotation during prosthesis placement.

• Intimate contact must be maintained between the minor connectors/proximal plates and the guiding surfaces of the abutment teeth. Caution must be taken during waxing, casting, finishing, and adjusting of the minor connectors/proximal plates. Lack of contact can render the retention of the prosthesis ineffective. 156

• Unobstructed path of insertion when rotating the prosthesis into position. This is particularly important when additional edentulous spaces exist posteriorly. Interferences (tooth or soft tissue) that may impair the rotational path of placement must be adequately blocked out (Fig 14-17).

Restoration of Anterior Edentulous Extension Areas

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Fig 14-18  (a) Maxillary diagnostic cast being considered for a rotational path RPD on the surveying table. (b and c) The cast is surveyed at approximately a 0-degree tilt initially, and the analyzing rod indicates undercut areas on the mesial surfaces of the maxillary right central incisor and left canine. (Courtesy of Dr G. King, Houston, Texas.) Fig 14-19  (a and b) The cast is tilted until the undercuts on the mesial surfaces of the anterior abutments are eliminated. The tilt defines the initial path of insertion. The analyzing rod indicates the absence of undercuts. (Courtesy of Dr G. King, Houston, Texas.)

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Fig 14-20  (a and b) The analyzing rod is used to determine whether access exists for insertion of the RPD onto proposed rest seats.

Laboratory procedures

1. The diagnostic cast is surveyed with a 0-degree tilt (the occlusal plane should be relatively perpendicular to the analyzing rod) to determine the adequacy of the undercut on the mesial surfaces of the anterior abutments and the distobuccal surfaces of the posterior abutments. The amount of available undercut anteriorly should be at least 0.02 inch (Fig 14-18). The type of clasp selected to attain optimal retention determines the amount of undercut needed on the posterior abutment. 2. The cast is tilted upward anteriorly until the undercuts on the mesial surfaces of the anterior abutments are eliminated (Fig 14-19). 3. The analyzing rod is used to determine if access still exists for the anterior rests to be seated (Fig 14-20). If access does not exist, additional tooth modification in addition to the rest

preparation is necessary. There must be no interference for the placement of the anterior segment of the casting along the determined initial path of insertion. 4. A preliminary outline is drawn on the diagnostic cast indicating rests areas and any additional tooth modifications. 5. Rests and other tooth modifications are completed on the patient, and a master cast is obtained. 6. The master cast is resurveyed as before and tripodized for each position (Fig 14-21). 7. The anterior portion of the casting is positioned first, then the posterior segment is seated (Fig 14-22). 8. Resistance to vertical displacement of the anterior segment may be increased by the use of proximal plates or other rigid components on the distal surfaces of the molars.

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Fig 14-21 (a) The master cast is resurveyed at a 0-degree tilt and tripodized. (b) The master cast is tilted upward until the undercut areas on the mesial surfaces of the maxillary right central incisor and left canine are no longer present. A second set of tripod marks is completed. (c) The rotational path RPD design is drawn on the master cast. Note the two sets of tripod marks on the cast indicating both paths of insertion.

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Fig 14-22 The anterior portion of the rotational path RPD is seated first. Then the RPD rotates around the point where the anterior minor connector/proximal plate contacts the mesial surface of the canine (A) and rotates around this point until the molar clasp assembly is completely seated. (Courtesy of Dr J. Jayanetti, Los Angeles, California.)

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Fig 14-23 A tapered arch compared to a square arch of similar dimension. The distance between the mesial surface of the first premolars and distal surface of the second molars is identical. An anterior cantilevered force exerted a dislodging force on the posterior clasps, which increases as the distance between line A and line B increases or the distance between line B and line C decreases. Note the difference in these proportions with the square arch: The distance between lines A´ and B´ is less than in the tapered arch, and the difference between lines B´ and C´ is greater than in the tapered arch. As a general rule, the more posteriorly the retentive undercut is located, the more favorable the leverage.

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Restoration of Anterior Edentulous Extension Areas

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e Fig 14-24  (a) RPD framework designed for a rotational path of insertion. (b) The final wax-up. (c) The anterior portion of the rotational path RPD is seated first. Then the RPD rotates around the point where the anterior guide plane contacts the mesial surface of the canine and rotates around this point until the molar clasp assembly is completely seated. (d) The seated RPD. (e) The esthetic outcome. (Courtesy of Dr T. Berg, Los Angeles, California.)

When the anteroposterior rotational path RPD is employed to replace anterior teeth, the distance between the fulcrum line and the incisal edges of the teeth will influence the amount of retention necessary on the molar abutments. When the patient incises anteriorly, an anterior cantilevered force exerts a dislodging force on the posterior clasps. In a tapered arch form, the distance between the fulcrum line and the incisal edges is often greater than the distance in a square arch (Fig 14-23).

This factor may dictate the amount of retentive areas that must be engaged by the molar clasps. In a tapered arch form, it may be desirable to add implants to aid support (see chapter 16). A typical clinical case is demonstrated in Fig 14-24. Note that circular concave rests were employed in the anterior abutments and that the denture base was carefully blended with the adjacent mucosal tissues. The occlusion was group function (see Fig 13-26). 159

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References 1. Chiche G, Pinault A. Replacement of deficient crowns. In: Chiche G, Pinault A (eds). Esthetics of Anterior Fixed Prosthodontics. Chicago: Quintessence, 1994:53–74. 2. Morley J. Smile design—Specific considerations. J Calif Dent Assoc 1997;25:633–637. 3. Berg T, Caputo AA. Load transfer by a maxillary distal-extension removable partial denture with cap and ring extracoronal attachments. J Prosthet Dent 1992;68:784–789. 4. Berg T, Caputo AA. Maxillary distal-extension removable partial denture abutments with reduced periodontal support. J Prosthet Dent 1993;70:245–250. 5. King GE. Dual-path design for removable partial dentures. J Prosthet Dent 1978;39:392–395. 6. Krol AJ, Finzen FC. Rotational path removable partial dentures: Part 2. Replacement of anterior teeth. Int J Prosthodont 1988;1:135–142.

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Suggested Reading Berg T, Caputo AA. Load transfer by a maxillary distal-extension removable partial denture with cap and ring extracoronal attachments. J Prosthet Dent 1992;68:784–789. Firtell DN, Jacobson TE. Removable partial dentures with rotational paths of insertion: Problem analysis. J Prosthet Dent 1983;50:8–15. Kim HR, Jang SH, Kim YK, et al. Microstructures and mechanical properties of Co-Cr dental alloys fabricated by three CAD/CAM-based processing techniques. Materials (Basel) 2016;9: 596. Krol AJ, Jacobson TE, Finzen FC. Removable Partial Denture Design: Outline Syllabus, ed 5. San Rafael, CA: Indent, 1999.

Chapter 15 Surveyed Crowns and Combined Fixed RPD Cases Daniela Orellana | Ting-Ling Chang

Tooth surfaces often need to be recontoured to establish a precise path of insertion, idealize and make parallel guiding surfaces, idealize the position and contour of rests and retainers, and enhance esthetics. In many instances, reshaping the enamel surfaces is insufficient, and partial- or full-coverage restorations are indicated. These restorations, known as surveyed crowns, should be planned and fabricated with appropriate undercuts, guide planes, and rest seats to serve as removable partial denture (RPD) abutments.

Indications for a Surveyed Crown • To restore a compromised abutment tooth due to caries, fracture, or defective restorations • Teeth treated with endodontic therapy • To re-establish a proper occlusal plane (ie, supererupted or tipped teeth, loss of occlusal vertical dimension [OVD]) • To improve the contours of abutment teeth (ie, proper rests particularly on anterior teeth, adequate undercuts, and guide planes) • To replace large mesio-occlusodistal (MOD) direct restorations, especially in premolars, to reduce the risk of fracture • Existing crowns when modifications could lead to metal failure or porcelain fracture

Objectives of a Surveyed Crown • To develop an acceptable path of insertion for the RPD (idealize the guide planes)

• To promote favorable biomechanical properties to idealize support, stability, and retention for the RPD • To improve esthetics

Treatment Sequence 1. Mount the maxillary cast with a facebow record and the mandibular cast with an interocclusal record at the predetermined treatment position. 2. Determine the most advantageous position (MAP) and the tooth modifications required. 3. Fabricate treatment RPDs as necessary (see chapter 18). 4. Create a diagnostic wax-up idealizing guide planes and occlusal rests consistent with the MAP. 5. Duplicate the diagnostic wax-up and generate a stone cast. 6. Fabricate preparation guides and matrices for provisional restorations with vacuum-formed matrices or silicone putty. 7. Mouth preparations: a. Prepare guide planes and rest seats on the RPD abutments that are not being prepared for surveyed crowns based on the established MAP and RPD design. b. Prepare the abutment teeth that are to receive surveyed crowns and fabricate provisional restorations. 8. Make a definitive full-arch impression. The impression is sent to the laboratory, and two casts are made. The first pour is a pindexed cast, and the second pour is a solid cast that retains the soft tissue contours. 9. Cement the provisional restorations and adapt the treatment RPDs as necessary. 10. Fabricate record bases and wax rims on the pindexed cast if an inadequate number of teeth remain. 161

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Fig 15-1 (a and b) Diagnostic mounted casts with denture teeth setup and wax-up of abutment teeth.

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11. Obtain a facebow record to mount the maxillary cast. 12. Make a centric relation (CR) or maximal intercuspal position (MIP) record to mount the mandibular cast. Select denture teeth of appropriate shade and mold. When replacing teeth in the esthetic zone, use a try-in appointment to guide the technician for the wax-up and casting of the surveyed crown. 13. Fabricate surveyed full-contour crown wax-ups with positive rests, guide planes, and the desired undercuts. 14. For metal-ceramic restorations, cut back the wax pattern to allow space for porcelain. 15. Cast the crowns and verify the fit on the dies. If necessary, perform a metal try-in to verify the fit of the casting and to adjust the occlusion. 16. Develop the facial contours of the crown in porcelain and survey them to achieve the desired retention. 17. Cement the surveyed crowns. 18. Make an alginate impression and fabricate a study cast. Then survey the cast, redetermine the MAP, and tripod the cast. 19. Refine the remaining RPD abutments as necessary (guide planes, rest seats, facial and lingual heights of contour for retainers) and make the definitive RPD impression framework. 20. Finish the RPD in the usual manner.

Clinical and Laboratory Procedures for Surveyed Crowns When a surveyed crown is indicated, accurately mounted diagnostic casts are the starting point of a successful treatment. To accomplish this, the casts should be mounted using a facebow transfer record and an interocclusal record at either CR or MIP (see chapter 13). The casts are surveyed and analyzed to determine the definitive RPD design and modifications of the abutments that may be required. During this analysis, problems such as the position and relationship of the remaining teeth become apparent. As stated previously, often the amount of tooth modification required to idealize contours or correct occlusal discrepancies cannot be achieved by enamel modification alone, and thus surveyed crowns are indicated. 162

Diagnostic wax-up A diagnostic wax-up is a valuable diagnostic tool, especially if multiple crowns or fixed dental prostheses (FDPs) are planned. The diagnostic wax-up allows the clinician to envision potential problems that may be encountered in establishing the desired occlusal relationships and crown contours (eg, crown preparations that may encroach upon pulpal tissues). It also serves as a template to guide tooth preparation and to fabricate the provisional restorations. The diagnostic wax-up should include the proposed rest seats, guide planes, and retentive areas. In cases with supererupted or tilted teeth, it is recommended to perform a denture teeth setup in conjunction with the diagnostic wax-up of the abutment teeth. This will help guide the technician when developing ideal contours of the surveyed crowns (Fig 15-1).

Fabrication of the template for the provisional restorations Once the diagnostic wax-up of the abutment teeth is finalized, it is duplicated by making an alginate impression of the wax-up and retrieving a stone cast. A vacuum-formed template is fabricated with a 0.02-inch thermoplastic resin sheet. This template is used as an aid during tooth preparation and fabrication of the provisional restorations (Fig 15-2).

Preparation of teeth for surveyed crowns From a clinical perspective, it is essential that intraoral preparations on the remaining abutment teeth (ie, rests and guide planes) be completed prior to the crown preparations. Preparation guides may be used to guide the clinician in the recontouring of the abutment teeth consistent with the RPD design. This technique will ensure that all guide planes (surveyed crowns and other abutments) are parallel (Fig 15-3); see chapter 8 for details. The crown preparation should follow the established guidelines for retention and resistance form. The clinician must provide additional clearance in areas where rests are planned.

Clinical and Laboratory Procedures for Surveyed Crowns

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Fig 15-2  (a) Diagnostic wax-up including rests and guiding planes. (Courtesy of Dr J. Jayanetti, Los Angeles, California.) (b) Alginate was used to duplicate the diagnostic wax-up. (c) Pouring of the impression. (d) Vacuum-formed template. (e) Finalized provisional template.

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Fig 15-3  (a to d) Preparation guides are used to facilitate tooth modifications consistent with the predetermined path of insertion. (Courtesy of Dr T. Berg, Los Angeles, California.)

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Fig 15-4 (a and b) Preparations for surveyed crowns. Note that the rest seats are accounted for in the preparation.

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Fig 15-5 (a) Polyvinyl siloxane surveyed crown impression. (b) Pindexed cast. (c) Metal post attached to the cast to mark the determined path of insertion (MAP). (Parts a and b courtesy of Dr A. Davodi, Beverly Hills, California; part c courtesy of Dr J. Jayanetti, Los Angeles, California.)

Fig 15-6 (a and b) Record bases and wax rims fabricated on the pindexed cast. (Courtesy of Dr A. Davodi, Beverly Hills, California.)

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This will allow suitable space for the development of rests of appropriate depth and dimension (Fig 15-4). The mesial surfaces of mesially tipped teeth are corrected by means of the tooth reduction template or the preparation guide prior to crown preparation.

Final impression for surveyed crowns and preparation of casts and dies After the preparations are completed, a full-arch definitive impression is made with a suitable elastomeric impression material. 164

All crown preparation margins should be readily visible (Fig 15-5a). The shade is selected, and a laboratory prescription for the surveyed crown wax-ups is sent to the technician specifying the position of the rests and guide planes as well as the depth and location of the undercuts. In cases of missing anterior teeth, it is suggested that the shade and mold for the denture teeth be selected during this appointment. The definitive impression is sent to the laboratory, and two casts are fabricated. The first cast is pindexed but not trimmed, as shown in Fig 15-5b. The second cast retains the soft tissue contours (Fig 15-5c).

Clinical and Laboratory Procedures for Surveyed Crowns

Fig 15-7  Denture teeth setup helps the technician to establish contours of the fixed restoration. (a) Denture teeth are set up to guide the technician during fabrication of the wax-ups. (b) The complete denture opposes the wax-up of surveyed crowns. (c) Anterior teeth being replaced by an RPD. (Part c courtesy of Dr J. Jayanetti, Los Angeles, California.)

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The pindexed cast is mounted, surveyed to determine the path of insertion (MAP), and tripoded. It is suggested that a metal post be attached to the master cast with acrylic resin to be used to facilitate the reorientation of the cast at the determined path of insertion (MAP) (see Fig 15-5c). If an insufficient number of teeth remains, record bases and wax rims are fabricated in order to obtain interocclusal records (Fig 15-6). During the next appointment, the facebow record and maxillomandibular records (in this case, a CR record was the treatment position chosen) are made, and the maxillary and mandibular casts are mounted. In situations where anterior teeth are to be replaced by the RPD or when the RPD opposes a complete denture, an esthetic try-in appointment is suggested to provide the technician with the following information (Fig 15-7): • OVD • Occlusal plane • Position of anterior teeth • Shade and contours for the fixed restorations

a

c

Surveyed crown wax-up A full-contour wax pattern is fabricated with the desired contours on the pindexed dies. To accurately assess the wax pattern, the cast must be attached to the surveying table and reoriented at the correct MAP. If a metal post was used, reorient the master cast, inserting the post on the vertical arm of the surveyor, and tilt the surveying table until the rod fits passively into the vertical arm. Secure the rod to the surveyor and then tighten the knob on the surveying table to secure the surveying table into position (Fig 15-8). The waxed contours of the surveyed crown should include parallel guide planes, positive rests, adequate heights of contour, and undercuts. The crown contours can be more easily visualized if a thin layer of a fine powder (zinc stearate, alginate powder) is applied to the wax pattern. Guide planes can be refined with a wax carver knife inserted into the vertical arm of the surveyor (Fig 15-9a). Necessary modifications are made to preserve the natural curvilinear contours of the abutment teeth. The height and width of the guide planes should be maximized to optimize 165

15

Surveyed Crowns and Combined Fixed RPD Cases

a Fig 15-8 A metal post was used to reorient the master cast.

Fig 15-9 (a) Recontouring guide planes using the wax carver knife. (b) Analyzing rod is used to evaluate parallelism of guiding planes.

a Fig 15-10 Note the cingulum rests on the canines and mesial rests on the premolars. These rests will be in metal, and the facial and remainder of the occlusal surfaces will be restored with porcelain.

b

Fig 15-11 (a) I-bar retainer positioned on the midfacial aspect of the abutment tooth. (b) A 0.01inch undercut gauge measuring the desired undercut for a circumferential clasp.

RPD stability. Care must be taken to ensure that these surfaces are parallel to the previously prepared guiding surfaces of the remaining natural teeth abutments (Fig 15-9b). Positive rests are prepared using a discoid carver or a #6 or #8 round bur. The depth and dimensions should follow the guidelines discussed in chapter 2 (Fig 15-10). The undercut depth is measured with the appropriate undercut gauge. A 0.01-inch undercut is recommended for cast retainers, and a 0.02-inch undercut is used for wrought wire retainers. The position of the undercut is based on the retainer being used. For I-bar retainers, the undercut should be positioned on the midfacial aspect of the abutment tooth (Fig 15-11a), and the wax-up pattern should be evaluated on the solid cast to check for soft tissue undercuts that may interfere with the design of the retainer. For circumferential retainers, the undercuts are located at either the mesial or the distal line angles (Fig 15-11b). For metal-ceramic restorations, a cutback is performed to allow space for the porcelain (see Fig 15-10). Rests, guide planes, 166

b

and areas contacting the minor connector should be in metal. Once the wax pattern is finalized, all contours are inspected, and the pattern is invested for casting.

Surveyed crown casting The surveyed crown is cast using standard techniques, and the porcelain is applied in the usual manner. The definitive crown is placed on the master die and returned to the surveying table. The height of contours and the guide planes are assessed using the analyzing rod. The position and depth of the undercuts are verified with the appropriate undercut gauge, and rests are evaluated to ensure adequate depth (Figs 15-12a and 15-12b). For crowns that are designed with an infrabulge retainer, it is advised to verify the contours on the solid cast to ensure proper contours in relation to the surrounding soft tissue (Fig 15-12c). If modifications are needed to idealize contours, carbide burs or stones are used to adjust metal surfaces, and diamond

Suggested Reading

a

b

c

Fig 15-12  (a) A 0.01-inch undercut gauge. (b) Note the positive cingulum and occlusal rests. (c) The surveyed crowns are checked on the solid cast to ensure that the soft tissue contours allow for use of infrabulge retainers. (Part c courtesy Dr T. Berg, Los Angeles, California.)

Fig 15-13  Surveyed crowns cemented. Ensure that excess cement is removed.

burs are used to adjust porcelain surfaces. The adjusted areas are polished and/or reglazed prior to cementation. Care must be taken when adjusting metal components of the surveyed crown to avoid perforation. The thickness of the metal surfaces should be no less than 0.5 mm when measured with a caliper. The surveyed crowns are tried in, the margins are checked, the occlusal and interproximal contacts are verified, and the crowns are cemented (Fig 15-13). A new alginate impression is obtained, and a study cast is fabricated. The cast is surveyed and tripoded, and modifications are marked if needed. Refinements are performed intraorally as necessary, a definitive impression is made for the RPD, and a master cast is obtained. The RPD is then finished in the usual manner.

Suggested Reading Burns DR, Unger JW. The construction of crowns for removable partial denture abutment teeth. Quintessence Int 1994;25:471–475. Carr A, McGivney G, Brown A. Preparation of abutment teeth. In: McCracken’s Removable Partial Prosthodontics, ed 11. St Louis: Elsevier Mosby, 2005: 255–270.

Carracho JF, Razzoog ME. Removable partial denture abutments restored with all-ceramic surveyed crowns. Quintessence Int 2006;37:283–288. Chandler HT, Brudvik JS, Fisher WT. Surveyed crowns. J Prosthet Dent 1973; 30:775–780. Curtis DA, Curtis TA, Holmes JB. Use of a paralleling post for cast orientation when fabricating removable partial denture abutment crowns. J Prosthet Dent 1988;59:117–118. Gardner FM. Alteration in tooth preparations for surveyed crowns. Gen Dent 1984;32:498–500. Johnston JF. Preparation of mouths for fixed and removable partial dentures. J Prosthet Dent 1961;11:456–462. Miller EL. The surveyed cast crown—Key to more successful partials. Dent Surv 1979;55:38–45. Phoenix RD, Cagna DR, DeFreest CF. Mouth preparation and master cast. In: Phoenix RD, Cagna DR, DeFreest CF (eds). Stewart’s Removable Partial Prosthodontics, ed 4. Chicago: Quintessence, 2008:279–310. Seals RR Jr, Schwartz IS. Successful integration of fixed and removable pros­ thodontics. J Prosthet Dent 1985;53:763–766. Seals RR Jr, Stratton RJ. Surveyed crowns: A key for integrating fixed and removable prosthodontics. Quintessence Dent Technol 1987;11:43–49. Smith BJ, Turner CH. The use of crowns to modify abutment teeth of removable partial dentures. 1. Introduction and partial denture design. J Dent 1979;7:52–56. Turner CH, Smith BJ. The use of crowns to modify abutment teeth of removable partial dentures. 2. Clinical and laboratory procedures. J Dent 1979; 7:98–104.

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Chapter 16 Overlay RPDs Using Retained Roots and Implants Ting-Ling Chang | John Beumer III

Overlay RPDs Using Retained Roots Overlay removable partial dentures (RPDs) are RPDs that cover one or more natural teeth, retained roots, or dental implants. The advantages of overlay RPDs using natural dentition or retained roots are the following: • Provides additional tooth support for the RPD. The support for the prosthesis is improved in the edentulous area by its resting on the teeth/retained roots compared to the support obtained from the mucosa (Fig 16-1). • Prolongs the useful life of a tooth as an RPD abutment that otherwise might be extracted because of compromised periodon-

tal support. Biomechanics are improved by shortening the clinical crown and using them as overlay abutments (Figs 16-2 and 16-3). • Maintains the alveolar bone and the gingiva surrounding the retained roots. The alveolar ridge is preserved by the retained root (Fig 16-4). In the event of the loss of the root, sufficient bone will be retained for placement of implants at a future date. • Improves esthetic outcomes. If the retained root is strategically located, it can be fitted with a gold coping and an attachment, eliminating the need for a conventional RPD retainer in that particular location.

169

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a

Overlay RPDs Using Retained Roots and Implants

b

c

Fig 16-1 (a) The right canine has been treated endodontically and fitted with a gold coping retained with a post and core. (b) The overlay RPD. Note that the coping and the tooth-tissue junction have been covered with the metal of the RPD casting. (c) The RPD in position. This is a 7-year follow-up.

Crown-root ratio

Fig 16-2 Teeth with advanced periodontal bone loss can be transformed into useful abutments for overlay RPDs.

Fig 16-3 This premolar is not a good candidate for use as an RPD abutment because of the unfavorable crown-root ratio. However, it is a good candidate for use as an overdenture abutment.

170

Fig 16-4 Roots retain the bone as well as the gingiva.

Overlay RPDs Using Retained Roots

a

b

c

Fig 16-5  (a) A patient with an incomplete repair of a bilateral cleft of the lip and palate. The remaining premolars and anterior teeth had been covered with gold copings more than 25 years previous. Note that implants have been used to provide additional support and retention in the right posterior maxilla, which prompted a remake of the overlay RPD. (b) The overlay prosthesis. Note that the overlay abutments are covered with the metal casting and that the casting extends to cover the tooth-tissue junctions. (c and d) The prosthesis in position.

d Fig 16-6  When acrylic resin is used to overlay the overdenture abutments, the plaque and microorganisms infiltrating the resin will trigger candidiasis and tissue hypertrophy. The attached gingiva encircling the abutments must be covered with metal.

Historical perspectives Overlay RPDs were introduced in the 1930s to habilitate patients born with cleft lip and palate. These large, bulky removable prostheses were designed to replace missing teeth, correct horizontal or vertical growth deficiencies, and obturate hard and soft palate defects (Fig 16-5). Based on the follow-up of these patients, many invaluable lessons were learned. When the abutments of the overlay RPDs were covered with the acrylic resin of the denture base, the risk of caries was very high. Many abutments were lost within a year of delivery of the prosthesis, particularly if the patients wore their prostheses at night during sleeping hours. In addition, the gingival tissues immediately adjacent to the retained roots were subject to inflammation and hypertrophy (Fig 16-6). These phenomena were triggered by microorganisms (ie, those associated with dental plaque and fungal organisms) adhering to and infiltrating the porous acrylic resin of the denture base.

From these experiences, clinicians learned to cover these teeth with gold alloy copings or the metal of the RPD casting. Moreover, the metal of the RPD framework was designed to extend beyond the tooth-tissue junction to cover much of the attached gingiva (see Fig 16-5). In addition, patients were instructed to place a drop of topical fluoride in the tooth indentations made by these teeth or the retained roots daily after brushing and before inserting the prosthesis. The patients were also instructed to refrain from wearing the prosthesis during sleeping hours.

Clinical applications The simplest way to use these teeth as overdenture abutments is to treat them endodontically and fill the root canal access with amalgam. The crown is partially amputated and recontoured into the form of a dome. It should project at least 2 mm above the tissue (Fig 16-7a). If the abutment is a terminal abutment 171

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Overlay RPDs Using Retained Roots and Implants

a

b

Fig 16-7 (a) Following endodontic treatment, the retained root is recontoured into the form of a dome. It should project at least 2 mm above the tissue. The root canal access opening is then filled with amalgam. Retaining the root helps to maintain the width of the alveolar ridge. (b) The margin of the gold coping is extended subgingivally to reduce the risk of caries. (Courtesy of Dr R. Duell, Los Angeles, California.)

a

b

Fig 16-8 (a and b) The contours of the gold copings must be compatible with the path of insertion of the overlay RPD. This is verified with the use of a surveyor. Note the centrally located metal post. This indicates the path of insertion as determined by the most advantageous position.

Fig 16-9 Four roots were retained to facilitate the support and retention provided for the overlay RPD. The roots of the canines were fitted with gold copings and attachments (see Fig 14-11 for completed case). (Courtesy of Dr R. Duell, Los Angeles, California.)

adjacent to an edentulous extension area, the contour of the overlay abutment should be shaped to allow the prosthesis to rotate freely around the axis of rotation. Retention of such roots enhances the support for the RPD and retains the width of the alveolar ridge (see Fig 16-4). Some clinicians prefer to cover the retained root with a gold coping anchored in position with a post (Fig 16-7b). This practice will reduce the risk of caries. 172

Fig 16-10 The labial portion of the coping is contoured to allow placement of the denture tooth. Note that an attachment has been incorporated within the coping.

When multiple gold copings are used, their contours should be verified with a surveyor so their contours are compatible with the path of insertion established by the guiding surfaces of the residual dentition (Figs 16-8 and 16-9). In addition, the labial surface of the coping must be contoured so as to permit proper positioning of the denture teeth (Fig 16-10; see also Fig 16-7).

Overlay RPDs Using Retained Roots

a

c

g

b

d

e

h

f

i

Fig 16-11  (a) The cast with the gold coping fitted to a residual third molar. (b) The RPD framework. (c and d) The attachment was configured so that insertion and removal was compatible with the path of insertion of the RPD and so that it attached to the splinted central incisors. (e and f) The completed coping with the attachment incorporated. (g) Completed RPD. (h and i) The prosthesis inserted. Note the bracing arm between the canine and the premolar.

If additional retention is required, an attachment can be incorporated with the gold coping in combination with a post and core (see Fig 16-10). When attachments are employed as part of the copings, a surveyor is used to ensure that the attachments and the guiding surfaces of the residual dentition are positioned and oriented to be on the same path of insertion as dictated by the guide planes of the RPD. Otherwise, the attachments will wear prematurely, and retention of the prosthesis will be compromised (Fig 16-11). There are several types of attachments available for use. A detailed discussion of attachments is beyond the scope of this

book. However, when attachments are used to facilitate the retention of an extension-base RPD, they must be designed to allow the RPD to rotate freely around the axis of rotation (see chapter 14). Patients with overlay RPDs must be on a strict recall schedule. They should be seen at least every 6 months, and their oral hygiene should be closely monitored. The retained roots are susceptible to caries, especially if the patient ceases use of the topical fluoride. Attachments are subject to wear, attachment fatigue, and loss of retention, and the matrix portion requires periodic adjustment or replacement depending on its design. 173

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Overlay RPDs Using Retained Roots and Implants

Implants and Overlay RPDs Most RPDs fabricated have at least one edentulous extension area.1,2 The extension-base partial denture is unique in that it relies on two different means of support: One source of support is derived from the abutment teeth and the periodontal ligament, while the other source is the mucosal bearing surface of the extension area. As a result, during occlusal function, an extension-base RPD moves or rotates around the fulcrum line (axis of rotation), and this movement may be a source of patient dissatisfaction. Also, extension-base RPDs are subjected to significant vertical, horizontal, and torsional forces, which may predispose the patient to compromise of the edentulous support areas and the tooth abutments. Since implants have been introduced, they have been used to supplement the stability, support, and retention of RPDs. Implants have been used under the following circumstances in conjunction with RPDs: • In extension-base RPDs to supplement the support, stability, and retention provided by the existing dentition • When their anchorage is questionable and their configuration has unfavorable biomechanics • When unanticipated implant failures in key locations require that the patient be restored with an RPD • As a replacement for the loss of a key natural tooth abutment • As a supplement when the existing dentition cannot provide sufficient support, stability, and retention for the removable prosthesis

Supplement support and stability for distal-extension RPDs In recent years, the most common application of this approach has been in patients with extension-base partially edentulous configurations. Implant-supported fixed dental prostheses (FDPs) may be preferred by many patients, but in the mandible there may be insufficient bone over the inferior alveolar nerve to allow placement of implants of sufficient length and number to predictably support and stabilize an implant-supported FDP (see Fig 1-3). As a result, many clinicians have placed single implants in the edentulous extension area to supplement the support and stability provided by the remaining dentition for the RPD. Most authors have suggested the placement of solitary implants in the extension area. Lengths vary, but in recent times some clinicians have reported successful outcomes when using implants as short as 6 mm when the implants are used as overlay RPD abutments.3 Some clinicians have used the implants primarily for support and to secure a healing abutment to the implant fixture. The extension base is then designed to positively engage the head of the healing abutment. Other clinicians have used the implants to enhance the stability and retention of the 174

extension RPD by securing attachments of various types to the implant fixture (Fig 16-12). Whether the implants are used to enhance support, stability, retention, or all three, the partial denture should be designed with positive rests, guide planes, and proximal plates. Direct and indirect retainers are used as necessary to facilitate retention. Although follow-up times are relatively short, implant loss rates and complications associated with implant-supported RPDs fabricated for patients with extension-base RPDs appear to be minimal.3 The most common complications are peri-implantitis, loosening of abutments, wear of attachments, and occasionally the loss of an implant. Although improvements in mastication performance have been hard to document, most patients prefer the implant-supported RPD because of improved comfort, retention, and stability.4,5 Implant survival rates for this application have been quite favorable. Grossmann et al6 reported a retrospective study of 35 patients treated with both unilateral and bilateral distalextension implant-retained RPDs ⁄ implant-assisted RPDs. The most common arch configuration was mandibular unilateral distal-extension RPDs (10 patients) followed by mandibular bilateral extension RPDs (8 patients). A total of 67 implants were placed to provide support using healing abutments or retention using resilient attachments. An overall survival rate of 97.1% was achieved during a mean follow-up period of 35.4 months. Several other clinicians have reported similar outcomes.7–9

Implant length, size, and location/position Theoretically, placement of implants in the molar region provides the most optimal biomechanical configuration, changing an extension base to a configuration similar to a tooth-borne configuration. However, frequently there is insufficient bone overlying the inferior alveolar nerve or below the maxillary sinus to allow placement of an implant in this location. Sinus augmentation is quite expensive and not worth the effort in most patients if a removable prosthesis is planned. Enhancing the vertical height of the mandibular alveolar ridge is quite unpredictable. As a result, implants are commonly placed in the premolar site or anterior to the mental foramen. Bortolini et al8 reported that the majority of implants were placed in the canine or first premolar position (Fig 16-13). Implant lengths range from 6 to 13 mm in most published reports. Implants placed in premolar sites have less impact than those placed in the molar region but may be quite advantageous when the canine is lost or when the remaining dentition is compromised periodontally.

Prosthodontic procedures The authors favor the use of resilient attachments in these types of patients, particularly when the implant is placed in the premolar position. Also, in the mandible, extending the denture to engage the retromolar pad and the buccal shelf will enhance support, reduce movement of the prosthesis, and prolong the

Implants and Overlay RPDs

a

b

c

Fig 16-12  (a to c) A solitary implant has been placed in the first molar area to provide additional support and enhance the stability and retention of the RPD. A resilient attachment has been employed. This attachment allows the prosthesis to move up to 0.4 mm in the direction of the tissues. Note that the partial denture has been extended to engage primary support areas (ie, the buccal shelf and the retromolar pad). Note the rest on the premolars and the engagement of both canines with a lingual plate to further enhance stability. Fig 16-13  Placement of an implant in the first premolar site is not as advantageous as in the molar area.

lifespan of the attachments. The RPD framework must be designed with positive rests, guide planes, and proximal plates to idealize the support and stability of the prosthesis. The partial denture must engage tooth abutments on both sides of the arch in order to provide cross-arch stabilization for the implant portion (see Fig 16-12). In most situations, the authors prefer to enhance the retention provided by the attachments with direct retainers. Some clinicians use the implants only for support. They attach a healing abutment to the implant and engage it with the partial denture. The authors do not favor this approach for a number of reasons. First, these healing abutments tend to wear, become pitted, and loosen with use. Second, in the mandible, the implant becomes the primary means of support posteriorly and does not take advantage of the other very effective structures providing support posteriorly, namely the buccal shelf and the retromolar pad. If the implant is short in length and solitary with a healing abutment secured to it, theoretically there is a greater chance of implant overload compared to the use of a resilient attachment, which is designed to share the support with the edentulous extension areas (see Figs 16-12 and 16-13).

RPDs for restoration of implants with questionable anchorage or unfavorable biomechanics Although a plan of treatment may anticipate restoring an edentulous area with an implant-supported FDP, occasionally significant bone loss occurs around the implants before the patient is referred for prosthodontic care, making them unsuitable for use as FDP abutments. In other patients, a key implant may be lost or the biomechanics of the proposed FDPs may be unfavorable because of undesirable implant positioning or the placement of implants of insufficient length. In other patients, the distance between the head of the implants and the opposing dentition may be excessive, making the crown-root ratios of the proposed implant-supported FDP unfavorable. Implants with a linear configuration do not tolerate lateral forces (bending moments) well, and if the clinician doubts whether the implants can withstand the multitude of forces delivered during occlusal function, it may be prudent to restore the edentulous segment with an overlay partial denture and use the partial denture to facilitate the stability and support for the implant portion of the prosthesis. The partial denture can be designed to engage remaining tooth abutments with rests, proximal plates, and minor connectors on the opposite side of the arch, thereby spreading the lateral forces more widely (see Figs 16-14f and 175

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Overlay RPDs Using Retained Roots and Implants

a

b

c

d

e

f

g

h Fig 16-14 (a) Patient suffered multiple facial fractures in an automobile accident and lost multiple teeth in the left maxilla. (b and c) Following a sinus augmentation and additional grafting, three implants were placed. Note that they are positioned palatally. (d and e) Implant connecting bar. Note the resilient attachment on the distal end. The Hader bar segment is aligned parallel to the axis of rotation. (f) Overlay RPD. (g and h) Prosthesis in position. Note that the rests on the anterior teeth are positive. (i) The attachments retain the prosthesis on the side of the defect, providing an excellent esthetic outcome.

i

16-14g). Extra support can be gained by strategic positioning of rests on natural tooth abutments. Direct retainers are used as necessary but in some instances can be reduced in number or eliminated altogether when the prosthesis extends into the esthetic zone. The patient depicted in Fig 16-14 demonstrates this approach very nicely. This patient suffered multiple facial fractures that resulted in the loss of the maxillary dentition and alveolar ridge on the left side. Following reduction of the jaw, fractures of the mandible and maxilla, and healing, a sinus augmentation was performed. An additional grafting procedure was performed in 176

order to allow placement of implants in the left posterior maxilla. The edentulous region in the mandible was easily restored with an implant-supported FDP. However, in the maxilla, only three implants were successfully placed, and the distance between the heads of the implants and opposing dentition was excessive (see Figs 16-14b and 16-14c).

Analysis of the major issues Several factors led to the decision to restore this patient with an overlay RPD. If an implant-supported FDP had been fabricated,

Implants and Overlay RPDs

a

d

b

e

c

f

Fig 16-15  (a) Surviving implants are in premolar sites. Note the cingulum rests and the resilient attachments connected to the splinted central incisors. (b) The plastic burnout pattern of a resilient attachment is attached to a UCLA abutment. (c) The RPD framework. The attachments secured to the implants are oriented so that their path of insertion is parallel to that of the RPD. (d) The finished prosthesis. (e and f) The prosthesis in position.

the biomechanics would have been quite unfavorable, predisposing to implant overload, loss of bone around the implants, and loss of the implants. The following factors compromised the biomechanics: • Unfavorable crown-root ratios. This was secondary to the loss of teeth and the alveolar fractures. Although sinus augmentation was successful, reconstruction of the alveolar ridge with grafting was not possible. • Unfavorable implant position and angulation. The implants were placed palatally because of anatomical necessity and were angled toward the buccal. This made it virtually impossible to design an FDP that directed occlusal forces along the long axis of the implants. • Implants were primarily anchored in grafted bone. The implants were anchored in poor-quality bone, and the authors felt they were insufficient in number to restore the edentulous defect with an FDP. Therefore, the authors favored designing a removable overlay partial denture that could provide additional stability by engaging abutment teeth on the opposite side of the arch with the RPD framework. • Buccal cantilever. Before the advent of sinus augmentation, the authors restored the posterior quadrants of several patients with implant-supported FDPs. Because of the anatomy of the posterior maxilla, frequently these implants were angled excessively to the buccal. In those patients with buccal implant angulations, the authors experienced an unacceptable rate of implant failure.

• Linear configuration and restoring the corner of the arch. Linear configurations lack the cross-arch stabilization that is achieved with curvilinear arrangements. When the other factors are favorable (sufficient numbers of implants of sufficient lengths used in favorable bone sites with ideal angulations), an FDP can be employed predictably. However, in this patient, the authors felt the implants may not have been able to resist lateral forces without provoking a resorptive remodeling response of the bone anchoring the implants. The overlay RPD was designed with positive rests on the anterior teeth. The cross-arch stabilization achieved with the partial denture reduced the magnitude of lateral forces delivered to the implants.

RPDs for restoration after unanticipated failure of implants in key locations On occasion, implants in key locations fail. In most instances it is possible to replace them, but some patients prefer not to undergo another surgical procedure and wish to be restored with an RPD. In these instances, implants can be used to supplement stability, support, or retention depending on the configuration of the remaining dentition and implants. The patient presented in Fig 16-15 is a good example. The original treatment plan anticipated placement of three implants into each of the edentulous spaces to support FDPs. However, 177

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Overlay RPDs Using Retained Roots and Implants

Fig 16-16 (a) The canine adjacent to the edentulous area was lost secondary to periodontal bone loss and was replaced with an implant. The implant crown was designed with a cingulum rest and a distal guiding surface. Implants were also used to restore the maxillary molars on the opposite side. (b) The RPD in position.

a

a

b

b

c

Fig 16-17 (a and b) Two implants have been placed in the premolar positions and splinted together with an implant connecting bar with an extracoronal resilient attachment secured to each end. Note the cingulum rest on the canine. (c) The overlay RPD in position. The opposing arch was restored with a complete denture, and the occlusion was designed as balanced articulation.

four implants failed to osseointegrate, two on each side. The two that remained were well-anchored in bone but had excessive buccal angulation. The patient decided against further grafting procedures and wished to proceed with an RPD. The following design was employed. The two central incisors were splinted together, and cingulum rests were placed on each. Resilient attachments were attached to the distal side of each of these crowns. Mesial rests were placed on the molars adjacent to the edentulous spaces. Custom abutments were fabricated for each of the implants, and resilient attachments were incorporated consistent with the path of insertion of the RPD. Note the design of the RPD framework. In this design, the implants were used to facilitate retention and stability but were not used for support. Support was provided by the positive rests on the remaining dentition and the full palatal coverage by the RPD. The implants were primarily used to facilitate retention. The retention they provided as well as the retention provided by the resilient attachments on the two central incisors eliminated the necessity of direct retainers in the anterior region.

Replacement for a key natural tooth abutment On some occasions, a key abutment tooth on one side of the arch used to support and retain an RPD is lost. If the remain178

ing teeth on that side of the arch are not suitable abutments for an RPD, the lost tooth abutment can be replaced with an implant. The authors have frequently employed this approach when the canine is lost and is the terminal abutment adjacent to an edentulous extension area. When the implant is adjacent to an edentulous extension area, it can be turned into an RPD abutment with idealized rests and guiding surfaces (Fig 16-16), or it can be fitted with a resilient attachment.

As a supplement when the existing dentition cannot provide sufficient support, stability, and retention for the removable prosthesis Some patients retain healthy teeth on one side of the arch, but there are no suitable abutments on the opposite side. Implants can be used to provide support and retention on the edentulous side of the arch. Rests on the residual dentition must be positive, and the primary support areas must be used to maximal advantage (Fig 16-17). This is a cost-effective option that many patients will prefer, particularly if they have functioned well and had a satisfactory experience with their previous RPD.

Suggested Reading

Summary of Implants and RPDs Implants can often be used to supplement the support, stability, and retention of RPDs. These situations vary considerably, and designs of the partial denture will vary accordingly. In most situations, the level of patient satisfaction is improved. The principles of RPDs should be the primary driving factors in design of the prosthesis, namely the following: • Occlusal rests must be positive and direct occlusal forces along the long axis of the teeth. • Major connectors must be rigid. • Guide planes are employed to enhance stability and bracing. • Retention must be within the limits of physiologic tolerance of the periodontal ligament. • Maximum support is gained from the adjacent soft tissue denture-bearing surfaces. • Extension-base RPD designs must anticipate and accommodate the movements of the prosthesis during function without exerting pathologic stresses on the abutment teeth or the implants. • Designs must consider the needs of cleansability.

References 1. Curtis DA, Curtis TA, Wagnild GW, Finzen FC. Incidence of various classes of removable partial dentures. J Prosthet Dent 1992;67:664–667. 2. Pun DK, Waliszewski MP, Waliszewski KJ, Berzins D. Survey of partial removable dental prosthesis (partial RDP) types in a distinct patient population. J Prosthet Dent 2011;106:48–56. 3. Gates WD 3rd, Cooper LF, Sanders AE, Reside GJ, De Kok IJ. The effect of implant-supported removable partial dentures on oral health quality of life. Clin Oral Implants Res 2014;25:207–213. 4. Payne A, Kuzmanovic DV, De Silvaav-Kumara R, van Staden IP. Mandibular removable partial dentures supported by implants: One-year prosthodontic outcomes. J Dent Res 2006;85:(article 2570). 5. Ohkubo C, Kobayashi M, Suzuki Y, Hosoi T. Effect of implant support on distal-extension removable partial dentures: In vivo assessment. Int J Oral Maxillofac Surg 2008;23:1095–1101.

6. Grossmann Y, Levin L, Sadan A. A retrospective case series of implants used to restore partially edentulous patients with implant-supported removable partial dentures: 31-month mean follow-up results. Quintessence Int 2008;39:665–671. 7. Mijiritsky E, Ormianer Z, Klinger A, Mardinger O. Use of dental implants to improve unfavorable removable partial denture design. Compend Contin Educ Dent 2005;26:744–746,748. 8. Bortolini S, Natali A, Franchi M, Coggiola A, Consolo U. Implant-­ retained removable partial dentures: An 8-year retrospective study. J Prosthodont 2011;20:168–172. 9. Mitrani R, Brudvik JS, Phillips KM. Posterior implants for distal extension removable prostheses: A retrospective study. Int J Periodontics Restorative Dent 2003;23:353–359.

Suggested Reading de Freitas RF, de Carvalho Dias K, da Fonte Porto Carreiro A, Barbosa GA, Ferreira MA. Mandibular implant-supported removable partial denture with distal extension: A systematic review. J Oral Rehabil 2012;39:791– 798. Jensen C, Meijer HJA, Raghoebar GM, Kerdijk W, Cune MS. Implant-supported removable partial dentures in the mandible: A 3-16 year retrospective study. J Prosthodont Res 2017;61:98–105. Jensen C, Raghoebar GM, Kerdijk W, Meijer HJ, Cune MS. Implant-supported mandibular removable partial dentures; patient-based outcome measures in relation to implant position. J Dent 2016;55:92–98. Kaufmann R, Friedli M, Hug S, Mericske-Stern R. Removable dentures with implant support in strategic positions followed for up to 8 years. Int J Prosthodont 2009;22:233–241. Shahmiri RA, Atieh MA. Mandibular Kennedy Class I implant-tooth-borne removable partial denture: A systematic review. J Oral Rehabil 2010;37: 225–234. Stansbury BE. A retentive attachment for overdentures. J Prosthet Dent 1976;35:228–230. Strohaver RA, Trovillion HM. Removable partial overdentures. J Prosthet Dent 1976;35:624–629.

179

Chapter 17 Using the RPI System for Defects of the Maxilla and Mandible Jay Jayanetti | John Beumer III

Developing partial denture designs that do not compromise the health of the abutment teeth and their supporting structures represents a unique challenge in this group of patients. The basic principles of partial denture design should be followed; namely, major connectors should be rigid, occlusal rests should direct occlusal forces along the long axis of the teeth, guiding planes should be designed to facilitate stability and bracing, retention should be within the physiologic limits of the periodontal ligament, and maximum support and stability should be gained from the residual soft tissue denture-bearing surfaces including the defect.

Partial Denture Design Concepts for Maxillary Defects When a patient undergoes a partial resection of the hard palate resulting in a direct communication with the paranasal sinuses, unless the defect is effectively obturated with a prosthesis, the patient will exhibit hypernasal speech, and food and liquids will escape through the nose during swallowing. The prime objective of the clinician is to design and fabricate a prosthesis that seals the defect with sufficient retention to restore speech and swallowing. Several factors complicate removable partial denture (RPD) design in patients presenting with these defects. The most obvious factor is the compromised support resulting from the loss of a portion of the hard palate and the associated dentition (Fig 17-1). The arrangement of the remaining dentition presents

with less curvature than that seen in conventional patients and in some cases offers an almost linear configuration, limiting the effect of indirect retention and resulting in the loss of the value provided by cross-arch stabilization to distribute lateral forces. In addition, long lever arms are created from the necessary extension into the defect; consequently, lateral displacement of the RPD associated with velopharyngeal function, movement of the mandible, or contracture of the muscles of facial expression may result in considerable forces exerted onto the abutment teeth. These forces must be taken into account when designing the RPD framework, otherwise retention will not be adequate, and the abutment teeth will be subject to excessive lateral torquing forces, leading to their premature loss. As with conventional patients, the diagnostic casts should be surveyed carefully for the location of undercuts, the location and contour of potential guide planes, and selection of the path of insertion. Often a compound path of insertion must be employed to adequately use the undercuts available in the defect. For example, if the lateral and posterior undercuts in the defect are to be engaged properly, the prosthesis must first be inserted into the defect and then rotated up into position to engage the teeth. Multiple rests are suggested in order to improve stability and support for the prostheses. The rest seats should be rounded and polished so the rests on the partial denture framework can rotate without torquing abutment teeth. Full- or partial-coverage crowns may be required on select teeth to establish ideal contours for retention, guiding planes, and occlusal or cingulum rests. In defects extending to or beyond the midline, additional bracing may be necessary to distribute lateral forces more widely among the remaining dentition. 181

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Using the RPI System for Defects of the Maxilla and Mandible

a

b

c

Fig 17-1 Palatal defects vary widely. The RPD design for part a will be rather conventional. However, the designs used for parts b and c will require much more bracing from the residual dentition because of the loss of cross-arch stability secondary to the surgical resection.

a

b

c

Fig 17-2 (a) On anterior teeth, rests should be positioned at the junction of the gingival and middle thirds of the tooth. (b and c) A retainer should be positioned on the tooth so that it disengages or rotates around the point of contact on the tooth during an incising force.

The so-called “swing-lock” RPD, which locks onto the remaining dentition, has been questioned by some clinicians. Retention can be excellent with this design, but it may subject abutment teeth to additional stress,1,2 resulting in wear of the labial and buccal surfaces of the abutment teeth and possible premature tooth loss.

Adjacent abutment teeth Teeth adjacent to the defect require special consideration. These teeth must absolutely have a positive rest and be engaged by a retainer if adequate retention is to be achieved for the obturator prosthesis (Fig 17-2). This anterior retainer and rest ensure proper orientation of the prosthesis. If this concept is not employed, the prosthesis will tend to rotate out of retentive areas posteriorly. These abutments are subject to greater vertical and lateral forces1,3 and are more frequently lost than abutments in other positions. This is due to a number of factors. First, the extension area immediately adjacent to this abutment (which is basically the defect) provides little or no support. Second, the lever arms can become quite long, amplifying the forces delivered to the 182

abutment. Therefore, the placement of a positive rest on this tooth is critical to its long-term survival. Anterior teeth adjacent to a maxillary defect must have positive cingulum rests so that occlusal forces are directed along the long axis of the abutment. If incisors are adjacent to the defect, consideration should be given to splinting them with metal-ceramic restorations and incorporating cingulum rests4 (Fig 17-3). If a canine is adjacent to the defect, cingulum rests can be created by recontouring the cingulum areas, fabricating a three-quarter crown, or incorporating the rest into a full-coverage metal-ceramic crown (Fig 17-4). Bonded cingulum rests hold much promise,5 but long-term clinical studies are not yet available to assess their longevity in this group of patients. Often, compromised bony support for the tooth adjacent to the defect does not permit its use as a partial denture abutment. Consequently, the next tooth or other adjacent teeth must be used for this purpose. Frequently, these compromised abutment teeth can be treated endodontically. The crown is then amputated, and the root will serve as an overdenture abutment, helping to retain its encompassing alveolar bone and attached gingiva. In some patients, it may be advantageous to remove this tooth and place an implant into this site.

Partial Denture Design Concepts for Maxillary Defects

Fig 17-3  (a and b) It is advisable to splint incisor teeth adjacent to the defect with metal-ceramic crowns. Splinting will prolong the lifespan of these teeth.

a

b

a

b

Fig 17-4  Canines are adjacent to both defects. (a) The cingulum has been recontoured to create a positive rest. (b) A full-coverage metal-ceramic crown was necessary to create a positive cingulum rest.

Fig 17-5  The fulcrum line varies with the point of application of load. Load point #1 corresponds to fulcrum line AB. Given the configuration of the defect and remaining teeth, this would be the most common point of load. Load point #2 corresponds to line CD and would be seen when the patient incises the bolus. Load point #3 generates the fulcrum line EF.

D F

E

#2 Load

X

A

X

C B #1 Load X

Fulcrum lines The fulcrum line or axis of rotation of the partial denture obturator prosthesis for patients with acquired defects of the maxilla is influenced by the position of the rests, the size and configuration of the defect, and the magnitude and location of masticatory forces. The fulcrum line for classical maxillectomy patients is dynamic in that it shifts or changes during mastication relative to the size and configuration of the defect, the position of the bolus, and the masticatory force employed to penetrate it. Thus, there may be multiple axes or fulcrum lines, including the classical defined fulcrum line related to the rests on the teeth adjacent to the defect. The defect depicted

#3 Load

in Fig 17-5 has both a surgical defect secondary to resection of a tumor and an edentulous extension area on the nonsurgical side. When an occlusal load is applied to the extension area (#1), the prosthesis will rotate around axis AB. When a load is applied in the anterior section on the defect side (#2), the prosthesis rotates around axis CD. However, when the load is applied at point #3, the axis of rotation shifts to EF. Because patients with such a defect are instructed to masticate on the dentate nonresected side, partial denture designs must consider only those axes that are generated when the bolus is incised (load point #2) or when placed in the posterior extension area on the unresected side (load point #1).

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a

b

c

Fig 17-6 (a to c) Partial palatectomy defect. RPD design for a defect where the premaxillary segment was retained. The fulcrum line (dotted line) will be identical to that of a nonsurgical patient with a similar pattern of tooth loss. Only two retainers were used to retain the obturator prosthesis.

a

b

Fig 17-7 (a and b) Although the defect extends to include the soft palate, the RPD design differs little Fig 17-8 This large resection crossed the midline and from that shown in Fig 17-6. The only difference is the additional retainer on the right canine. encompassed most of the anterior maxillary segment. The remaining teeth are arranged in a linear fashion. The RPD is designed with multiple rests. Additional bracing is provided by plating the lingual surfaces of all remaining teeth.

RPD design The nature of the defect and the residual palatal shelf available are the most important factors to consider when designing an RPD framework for these defects. If the incisor region and the canine along with it have been retained, the RPD design follows conventional guidelines (Fig 17-6). When the support, stability, and retention for the obturator prosthesis can be enhanced by engaging selected areas within and peripheral to the defect, the retention, stability, and support available for the partial denture will be enhanced, and retainer-to-tooth relationships will be easily maintained. There will be little movement of the prosthesis in and out of the defect. Thus, fewer retainers and less bracing will be required (see Fig 17-6) compared to the patient with a defect lacking these important physical characteristics (see Fig 184

17-8). Even if the defect extends posteriorly to involve the soft palate, if the incisor region is retained and the canine is present, the RPD design can be relatively conventional. In the patient in Fig 17-7, the presence of the canine on the side of the defect brings with it the benefit of cross-arch stabilization, as well as more palatal shelf for support for the prosthesis. Therefore, little additional bracing is necessary and the only difference between the RPD defect seen in Fig 17-6 is the additional retainer engaging the right canine. Contrast these designs to the design shown in Fig 17-8. When the resection crosses the midline, the residual dentition is configured in a linear fashion and little palatal shelf is available to provide support for the prosthesis. When occlusal forces are applied on the defect side, the fulcrum line will essentially be identical with the tooth alignment. Patients with linear tooth

Partial Denture Design Concepts for Mandibular Defects

Occlusal rests

Occlusal load

Axis of rotation Occlusal load

Fulcrum line

Proximal plate Anterior

Posterior Height of contour Buccal view

Fig 17-9  Suggested partial denture design for discontinuity defect of the mandible. Note the bracing arms on the buccal sides. The I-bars are in retentive areas, but the bracing arm stays at or above the height of contour.

and arch arrangements with large defects will tend to exhibit more movement around the fulcrum line. Such configurations of the dentition require more occlusal rests for support and additional bracing so as to distribute lateral forces as widely as possible among the remaining dentition (see Fig 17-8). These factors demonstrate the importance of saving as many teeth and as much of the incisive bone as possible on the defect side. This will result in a less linear arrangement of the dentition, improve the location of the fulcrum line, and increase the effectiveness of indirect retention. The presence of the incisive bone and the dentition retained within it provides the added benefit of cross-arch stabilization that is taken for granted in conventional patients. It also simplifies the RPD design and enables the obturator prosthesis to function more like a conventional unilateral edentulous extension RPD.

Partial Denture Design Concepts for Mandibular Defects Lateral discontinuity defects with edentulous extension areas These patients present a unique challenge to RPD design. When an occlusal force is directed in the extension area, the axis of rotation goes through the rest adjacent to the edentulous extension area as it does in a conventional patient, but it does not traverse through the rest on the abutment on the opposite side of the arch (Fig 17-9). This is because the occlusal forces are entirely unilateral and can only be generated on the nonresected

side. However, if the retainer on this tooth is not placed in a retentive area on the abutment, the retention of the prosthesis will be decidedly suboptimal. The arc of closure of the mandible is quite different from that seen in nonsurgical patients and impacts RPD design. Viewed from the frontal plane, closure is angular rather than vertical (Fig 17-10a). Moreover, rotation of the mandible in the frontal plane causes the resected side to drop down out of occlusion as the force of contracture on the unresected side is increased (Fig 17-10b).6 Hence, the location of the fulcrum line is not as easily determined, making it more difficult to predict movement patterns of the prosthesis during function. As mentioned previously, the forces of occlusion are unilateral, and consequently the axis of rotation (fulcrum line) of the partial denture deviates from the norm. The I-bar mesial rest concept works well on the normal unresected side. By positioning the rest appropriately, it is possible to position a retainer on the distal abutment of the nonresected side that disengages when an occlusal load is applied to the edentulous extension area. However, because the teeth on the resected side are anterior to the axis of rotation, a retainer placed in a retention area will not disengage during occlusal function. Therefore, the retainer on the abutment adjacent to the defect must possess adequate flexibility so that an occlusal load will not unduly expose this tooth to pathologic stresses (Fig 17-11). Lingual plates are favored over lingual bars to enhance bracing, and occasionally a nonretentive buccal bracing clasp is added anterior to the retentive retainer on the unresected side (see Fig 17-9). This retainer is suggested because the angular path of closure increases the magnitude of lateral forces, displacing the prosthesis lingually during the chewing cycle.

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Before surgery

After surgery IP IP

Right

Left Rotation point

Normal subject

Vertical axis

Lateral mandibulectomy subject

a

b

Fig 17-10 (a) The envelope of motion of the resected mandible as viewed from the frontal plane compared to that of a conventional patient with an intact mandible. Note the lateral path of closure. (b) Note the rotation of the mandible in the frontal plane when the teeth come in contact on the unresected side. As the force of closure increases, the remaining teeth on the resected side drop further out of occlusion. Fig 17-11 (a) A lateral mandibular discontinuity defect. Surveyed crowns have been used to idealize the guiding surfaces and the rest seats. (b) The mandibular RPD in position. Note that the retainer on the abutment adjacent to the resection is positioned in an undercut.

a

b

Defects where mandibular continuity is maintained or re-established Anterior defects Included in this category are patients with anterior inner table resections and patients with anterior composite resections in whom mandibular continuity has been re-established with reconstructive surgery. Both types of patients have posterior teeth and an extensive edentulous area anteriorly, creating the need for an anterior extension partial denture (Fig 17-12). The length of the edentulous area depends on the extent of the surgery and the number and location of posterior teeth remaining. Following these resections, the occlusion of the posterior teeth is sometimes altered, but the pattern of mandibular movements is usually normal. Patients with surgically restored anterior discontinuity defects may display occlusal abnormalities because of graft contracture or inaccurate positioning of the residual mandibular fragments at the time of surgical reconstruction. 186

Once bony continuity is re-established, the occlusion cannot be changed except with occlusal equilibration or placement of coronal restorations. The anterior edentulous segment for both types of patients will often display unusual soft tissue configurations and compromised bony support. There is considerable variation in the size and length of these defects. RPDs for these patients enhance esthetics, provide support for the lower lip and cheek, and frequently lead to improved articulation of speech and enhanced control of saliva. In smaller defects, when a canine has been retained, mastication is effectively restored. In larger defects (Fig 17-13), the primary benefit of the prosthesis is to provide lip support for the patient. Masticatory efficiency is compromised because of the length and movement of the anterior edentulous section of the partial denture due to compromised mucosal support and the length of the edentulous section. The placement of implants in the symphyseal region will provide the necessary support and should be considered for these defects (Fig 17-14).

Partial Denture Design Concepts for Mandibular Defects

Axis of rotation Occlusal load

Fulcrum line

Occlusal rests

Proximal plate Posterior

Anterior Finish line

a

Lingual view

b

Fig 17-12  (a) A diagrammatic representation of an anterior mandibular defect. Posterior teeth remain on both sides. (b and c) Under these circumstances, the rests are positioned on the distal side of the teeth adjacent to the anterior edentulous extension area, and proximal plates engage their mesial surfaces. Properly positioned retainers will then disengage when an occlusal force is applied in the anterior edentulous extension area.

Occlusal rests

Occlusal load

Proximal plate Posterior

Anterior

Axis of rotation

Buccal view

c

a

b

c

Fig 17-13  (a) Patient presented with an anterior mandibular defect. Both sides of the mandible are rotated lingually because of muscle pull. (b) The lingual plate of the RPD was designed to engage the undercuts on the lingual side of the molars in the right mandible. A solitary retainer engages an undercut on the buccal surface of the left premolar. Note the rest on the right first molar. It extends onto the buccal side of the tooth to provide reciprocation for the lingual plate. (c) The RPD rotated into position. This is a rotational path RPD.

Fig 17-14  (a and b) The placement of implants in the symphyseal region dramatically improves support and restores mastication to a reasonable level. (Courtesy of Dr J. Kelley, Omaha, Nebraska.)

a

b

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Using the RPI System for Defects of the Maxilla and Mandible

Fulcrum line

Fulcrum line

X Incisal load

X a

Posterior load on defect side

b

Fig 17-15 (a and b) These dental configurations, lacking the benefits of cross-arch engagement and stabilization, employ more bracing (via the lingual plate) in order to distribute lateral forces equitably among the remaining dentition.

Partial denture designs must consider the movement of the anterior segment of the prosthesis. A suggested design is presented in Fig 17-12. Note the axis of rotation. With a distal rest, the retainers will disengage during a forceful closure. The long mesial rests on the second molars provide some indirect retention. Particular care should be taken to relieve the proximal plates and the distal aspect of the minor connectors during physiologic adjustment to allow for the expected movement of the framework during occlusion. The edentulous areas are recorded with an altered cast impression. Thermoplastic waxes are especially well suited for the anterior edentulous segment because they record movable tissue beds more accurately than elastic impression materials do. At try-in, esthetics, occlusion, and speech should be verified with particular attention paid to establishing the contour of the lower lip. These prostheses are processed, polished, delivered, and adjusted following conventional prosthodontic guidelines, and the patient is placed on a recall system for periodic monitoring.

Lateral defects Lateral defects in which posterior dentition remains on only one side of the arch are particularly difficult to restore. Many of these patients have had large mandibular resections with the body of the mandible reconstructed with a free vascularized flap. The long lever arms and compromised edentulous bearing surfaces contribute to excessive movement of the prosthesis during function (Fig 17-15). Most patients have lost the lingual and inferior alveolar nerves, and consequently they cannot detect or manipulate the bolus on the resected side, even if the continuity of the mandible is restored with a bone graft. These patients present with unique partial denture design challenges. The fulcrum line is dynamic in that it changes depending on the point of force application on the extension base. Patients tend to confine the bolus to the dentate side during 188

the chewing cycle because of loss of sensory input on the defect side, but they may generate considerable load when incising the food bolus. The abutment adjacent to the edentulous space is subjected to the greatest stresses, and therefore the rests on these teeth should be positive. If this tooth is a canine, a cingulum rest is recommended. If a premolar or molar represents the most anterior abutment, the rest should be placed on the distal side of the abutment. The fulcrum lines run through this most anterior rest in a manner shown in Fig 17-15. The anterior and posterior proximal plates should move freely during function without torquing the abutments. This is ensured by physiologic adjustment (see chapter 12). The labial retainer on the canine is placed in a retentive area. It should be positioned to disengage when a food bolus is incised. When the edentulous bearing surface is unfavorable on the defect side, bracing with lingual plating is recommended to distribute lateral forces as widely as possible among the remaining dentition. Ribbon rests are often employed to idealize the occlusal plane or restore occlusal contacts. The authors recommend that the posterior retainer be a circumferential clasp because this design provides additional bracing in the horizontal plane. Maximum coverage of the edentulous bearing area is also suggested. If possible, the occlusion should be refined so as to achieve contact in centric occlusion only. The patient should be instructed to masticate on the nondefective side with the residual mandibular dentition. Two examples of RPD designs are shown in Figs 17-15b and 17-16. Both designs employ minimal retention but multiple rests, guide planes, and proximal plates, as well as additional bracing with lingual plating. The I-bar retainers on the teeth adjacent to the edentulous area are positioned so that they will rotate further into the undercut when the patient incises the food bolus. The tips of the posterior retainers are positioned in undercuts, but the portion above the height of contour should be sufficiently rigid to enhance bracing.

Suggested Reading

Fig 17-16  (a) RPD design. Note the lingual plating and the continuous rests. The Akers clasps engaging the second and third molars are placed in retentive areas, even though they are on the wrong side of the axis of rotation. (b) Prosthesis in position.

a

The patient in Fig 17-16 has a lateral mandibular defect reconstructed with a fibula free flap. The skin and subcutaneous tissue overlying the fibula is about 1 cm thick and compressible. The tongue is intact, but the lingual and hypoglossal nerves have been resected. The residual mandibular segment is in good position in relation to the opposing maxilla. The mesial rest on the first molar controls the axis of rotation when a force is applied anteriorly during incision of the food bolus. The axis will change depending on the point of load application. Because of loss of sensory input on the defect side, the patient will tend to masticate the bolus on the unresected side. The tips of both retainers on the molars are in retentive areas even though they are theoretically on the wrong side of the fulcrum line. It is the opinion of the authors that the potential extraction force delivered to these teeth when incising the food bolus is not clinically significant. The I-bar retainer on the first molar is positioned so that it will rotate further into the undercut when the bolus is incised by the anterior teeth. The lingual surfaces of all three molars are covered with a lingual plate, and guide planes and proximal plates are placed on the distal of the third molar and mesial of the first molar to facilitate bracing and unite the teeth of the remaining arch quadrant. The casting is carefully physiologically adjusted to account for the most common and clinically significant axes of rotation generated during mastication. An altered cast impression was made of the edentulous area to idealize support in the extension area.

References 1. Schwartzman B, Caputo AA, Beumer J 3rd. Occlusal force transfer by removable partial denture designs for a radical maxillectomy. J Prosthet Dent 1985;54:397–403.

b

2. Myers RE, Mitchell DL. A photoelastic study of stress induced by framework design in a maxillary resection. J Prosthet Dent 1989;61:590–594. 3. Schwartzman B, Caputo AA, Beumer J 3rd. Gravity-induced stresses by an obturator prosthesis. J Prosthet Dent 1990;64:466–468. 4. Lyons KM, Beumer J 3rd, Caputo AA. Abutment load transfer by removable partial denture obturator frameworks in different acquired maxillary defects. J Prosthet Dent 2005;94:281–288. 5. Brudvik J, Taylor TD. Resin bonding for maxillofacial prostheses. In: Taylor TD (ed). Clinical Maxillofacial Prosthetics. Chicago: Quintessence, 2000:53–62. 6. Beumer J 3rd, Marunick MT, Silverman S Jr, et al. Rehabilitation of tongue and mandibular defects. In: Beumer J 3rd, Marunick MT, Esposito SJ (eds). Maxillofacial Rehabilitation: Prosthodontic and Surgical Management of Cancer-Related, Acquired, and Congenital Defects of the Head and Neck. Chicago: Quintessence, 2011:88–89.

Suggested Reading Aramany MA. Basic principles of obturator design for partially edentulous patients. Part II: Design principles. J Prosthet Dent 1978;40:656–662. Beumer J 3rd, Marunick MT, Garrett N, et al. Rehabilitation of maxillary defects. In: Beumer J 3rd, Marunick MT, Esposito SJ (eds). Maxillofacial Rehabilitation: Prosthodontic and Surgical Management of Cancer-Related, Acquired, and Congenital Defects of the Head and Neck, ed 3. Chicago: Quintessence, 2011:155–212. Beumer J 3rd, Marunick MT, Silverman S Jr, et al. Rehabilitation of tongue and mandibular defects. In: Beumer J 3rd, Marunick MT, Esposito SJ (eds). Maxillofacial Rehabilitation: Prosthodontic and Surgical Management of Cancer-Related, Acquired, and Congenital Defects of the Head and Neck, ed 3. Chicago: Quintessence, 2011:61–154. Kratochvil FJ. Influence of occlusal rest position and clasp design on movement of abutment teeth. J Prosthet Dent 1963;13:114–124. Parr GR, Tharp GE, Rahn AO. Prosthodontic principles in the framework design of maxillary obturator prostheses. J Prosthet Dent 1989;62:205–212.

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Chapter 18 Treatment Removable Partial Dentures Daniela Orellana | Ting-Ling Chang

A treatment removable partial denture (RPD) is an acrylic-resin interim prosthesis that replaces one or more missing teeth for a short period of time. A treatment RPD may be used for a variety of purposes. In most instances, an interim removable prosthesis is indicated when multiple teeth or teeth in the esthetic area are to be extracted. The prosthesis serves to address the patient’s concerns regarding esthetics and function.

Use of Treatment RPDs A treatment RPD may be indicated under the following circumstances: • As immediate tooth replacement after extractions to restore esthetics and function and to serve as a protective barrier during healing

0.01" a

• To replace missing posterior teeth in order to restore occlusal relationships • As an interim prosthesis while restorative treatment (surveyed crowns) is being finalized • In patients requiring restoration of occlusal vertical dimension (OVD) to test the patient’s adaptation to the new OVD • To evaluate acceptance of a removable prosthesis (trial prosthesis) • To monitor the prognosis of questionable potential abutment teeth The treatment RPD design should emulate the proposed design of the definitive RPD. This is especially important during tooth modification to ensure consistency between the interim prosthesis and definitive prosthesis (Fig 18-1). Treatment RPDs must have rests.

0.01"

0.01"

0.01"

b

Fig 18-1 (a) Definitive RPD design. (b) Treatment RPD design consistent with the definitive design.

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a

b

c

Fig 18-2 (a) Stock-tray alginate maxillary impression. (b) Custom-tray mandibular polysulfide impression. (c) Master cast.

Fig 18-3 (a) Cast surgery following the buccolingual gingival outlines. (b) Plaster blockout carved out to eliminate undercuts and ensure parallelism.

a

b

Treatment Sequence for Treatment RPDs 1. Intraoral preparations are performed based on the design, creating positive rests, parallel guiding surfaces, and the desired retention. 2. The final impressions for the treatment partial dentures are made. For maxillary cases or tooth-supported treatment partial dentures, stock trays and irreversible hydrocolloid are indicated. When making a mandibular distal-extension treatment partial denture, a custom-tray, border-molded polysulfide impression is preferred (Figs 18-2a and 18-2b). 3. The impression is poured using type III dental stone to obtain the master cast. This cast is surveyed, the most advantageous position is determined, and the heights of contour of the abutment teeth and surrounding soft tissues are marked (Fig 18-2c). 4. The teeth that are planned for extraction are removed from the cast using laboratory burs in a low-speed handpiece. The ridge is contoured in a convex fashion following the buccolingual outlines as shown in Fig 18-3a. 192

5. Undesirable undercuts are blocked out with plaster. The retention areas that are going to be engaged by the retainers should not be blocked out. The undercuts that are blocked out are determined by the survey. 6. The cast is placed on the survey table and reoriented according to the tripod marks. A wax carver knife is used to remove excess blockout material and verify the parallelism of the blockout (Fig 18-3). 7. Orthodontic wrought wire (0.04 inch) is used for the rests. The wire is adapted to the rest preparations using two- or three-prong orthodontic pliers and should not interfere with the opposing occlusion (Fig 18-4). 8. A smaller-diameter orthodontic wrought wire (0.036 inch) is used for the retainers because they are more flexible. The wire is adapted to the outline drawn on the cast using two- or three-prong orthodontic pliers. Loops are created to provide mechanical retention within the acrylic resin denture base as shown in Fig 18-4. When fabricating treatment RPDs, the wire should maintain intimate contact with the tooth surface, but as it approaches the soft tissue, the wire should be embedded within the wax to allow for adjustment of the acrylic resin secondary to soft and/or hard tissue changes.

Treatment Sequence for Treatment RPDs

a

b

c

Fig 18-4  (a) Orthodontic wire is adapted to the rest to provide support. Sticky wax is used to secure the wire rest to the cast. (b) Circumferential clasp engaging the 0.01-inch undercut. (c) A retention loop is created to provide mechanical retention for acrylic resin.

Fig 18-5  (a and b) I-bar bending sequence.

d c

Height of contour .01" undercut

b e

a a

9.  The following bending sequence for the I-bar retainer is performed (Fig 18-5): a. Using a 0.036-inch wrought wire, start with a right angle bend with two-prong pliers. b. Adapt the wire to the gingival tissues as shown. c. Bend the wire to engage a 0.01-inch undercut as shown (see Fig 18-5). d. Bend the wire to follow the tooth outline above the height of contour. Cut, taper, and polish the tip of the retainer. e. Bend the retention loop and make it parallel to the alveolar ridge. f. Bend the loop end to finish on the lingual or palatal tissues so as not to interfere with the positioning of the denture teeth. 10.  Use sticky wax or autopolymerizing acrylic resin to secure the wires to the cast (Fig 18-6a). 11.  The denture teeth are set. The denture base in the maxillary arch can be U-shaped or have complete palatal coverage depending on the need for support. In the mandibular arch, the prosthesis extends slightly short of the floor of the mouth as determined by the border-molded impression. This extension can also be determined with disclosing wax

f

a

b

at the time of delivery. The treatment partial denture must be extended to contact the lingual surfaces of the residual natural dentition. The gingival contours are then festooned (Fig 18-6b). 12.  The prosthesis is sent to the laboratory for processing. 13.  The processed treatment partial denture is evaluated for rough areas, sharp projections, or gross undercuts. 14.  During the delivery appointment (Fig 18-6c), areas that may be interfering with placement are identified and adjusted with acrylic burs in a low-speed handpiece. The most common surfaces interfering with proper seating are the interproximal areas. 15.  A thin layer of pressure indicating paste (PIP) is applied, and the prosthesis is carefully seated (Fig 18-7). Upon removal, any areas of undesirable tissue displacement are adjusted. 16.  Disclosing wax is applied and border molded, and over­ extended areas are adjusted and polished as necessary. 17.  The occlusion is checked with articulating paper and adjusted as needed. 18.  Home care instructions are given to the patient. Follow-up appointments are scheduled. 193

Treatment Removable Partial Dentures

18

a

b Fig 18-6 (a) Wires bent and secured with sticky wax. (b) Festooning of the treatment partial denture. (c) Finalized treatment partial denture covering the lingual surfaces of the teeth. (Part b courtesy of Dr J. Jayanetti, Los Angeles, California; part c courtesy of Dr T. Berg, Los Angeles, California.)

c Fig 18-7 PIP is used to identify areas of tissue displacement. (Courtesy of Dr R. Duel, Los Angeles, California.)

Suggested Reading Carr AB, McGivney GP, Brown DT. Temporary removable partial dentures. In: McCracken’s Removable Partial Prosthodontics, ed 11. St Louis: Elsevier, 2005:391–396.

194

Frank RP. Fabrication of temporary and treatment partial dentures. J Prosthet Dent 1973;30:215–221. Reitz PV, Weiner MG. Fabrication of interim acrylic resin removable partial dentures with clasps. J Prosthet Dent 1978;40:686–688.

Chapter 19 Insertion and Maintenance of RPDs John Beumer III | Daniela Orellana | Ting-Ling Chang

The objectives to be achieved during insertion of the prosthesis are the following: • Refine intra-arch control • Refine interarch control • Deliver patient instructions Errors associated with the definitive prosthesis can be derived from the following: • Tissues not completely recovered from irritation or displacement from a poorly fitting prosthesis when final impressions were made • Errors associated with the surface detail of the impression • Changes or damage to the master cast • Poorly adapted record bases when the maxillomandibular records were made • Distortion of or dimensional changes associated with registration materials while making the record or mounting the record on an articulator • Inaccurate maxillomandibular relation records • Changes in the patient’s temporomandibular joint • Articulator mounting errors • Dimensional errors secondary to flasking and processing The potential for error during fabrication of the removable partial denture (RPD) requires that all steps be accepted as tentative until the definitive prosthesis is available for the final and most accurate fitting. This in no way suggests that all procedures up to the insertion steps should be less than as ideal as possible but that there should be a series of ongoing checks, evaluations, and corrective measures available to reduce the potential for introducing error into the finished prosthesis.

Intra-arch Control The objective of these procedures is to refine the tissue adaptation of the prosthesis. This refines the support derived from the remaining dentition and mucosa. The impression and fabrication procedures are subject to technical and procedural errors. Errors can be made during the making of impressions and casts, maxillomandibular registrations, or processing of the definitive prosthesis. Clinical procedures performed during delivery can also accommodate for minor soft tissue changes that may have occurred since the final impression was made. With the finished prosthesis, these discrepancies can be identified and corrected to provide the best possible adaptation and support for the definitive prosthesis.

Tissue surface refinement Before the delivery appointment, a thorough visual and digital examination is made of the finished prosthesis for spicules, rough or sharp areas, or processing irregularities. The delivery appointment begins by explaining the insertion procedures to the patient. Pressure indicating paste (PIP) is used to identify areas of excessive tissue displacement (Fig 19-1). PIP is applied to the tissue surfaces of the prosthesis with a PIP brush. The prosthesis is inserted intraorally, seated with finger pressure, then removed and examined for pressure areas where the paste is displaced or for lack of appropriate adaptation as demonstrated by little or no compression of the PIP brush pattern. The pressure areas are relieved with an acrylic bur, and the PIP process is repeated until there is evidence of even flattening of the PIP brush pattern.

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Insertion and Maintenance of RPDs

a

b Fig 19-1 PIP is used to identify areas of excessive pressure and tissue displacement. (a) A thin layer of PIP is applied. (b) The RPD is inserted, and the extension area is compressed with finger pressure. (c) Note the displacement of the PIP in specific areas. The mylohyoid ridge area (arrow) is a common site of unwanted tissue compression. (Courtesy of Dr R. Faulkner, Cincinnati, Ohio.)

c

a

b

Fig 19-2 Disclosing wax has been used to verify the length, thickness, and contour of the peripheral extensions. (a) The wax is applied to the periphery. (b) The border-molded pattern. Note that the RPD flange is overextended in the masseter groove area. This is a common site for overextension (arrow). (Courtesy of Dr R. Faulkner, Cincinnati, Ohio.)

Refinement of the peripheral extensions

Interarch Control

Disclosing wax is used to identify areas of overextension (Fig 19-2). The wax is adapted to the extensions, and the prosthesis is inserted. The patient is instructed to move the mandible and tongue, then manipulate the lips and cheeks. The prosthesis is removed. Areas observed where the wax has been displaced indicate areas of overextension. These areas are carefully adjusted. This process is repeated until the areas of overextension are eliminated. These procedures ensure that the adaptation of the denture to the tissue surfaces and the peripheral extensions will be idealized.

Interarch control is determined by the occlusal surface contacts between the maxillary and mandibular dentition. Adjustment and refinement of these surfaces must be coordinated and compatible with the anatomy and mandibular movements of each individual patient at the time the prosthesis is inserted. This process is primarily used when an RPD opposes a complete denture. The most positive and precise method of refining interarch occlusion is to transfer a record of the occlusal surfaces of each arch to the laboratory, where final adjustments can be accomplished. In the partially edentulous patient, this procedure

196

Interarch Control

Fig 19-3  If the RPD opposes a maxillary prosthesis with complete palatal coverage with a post dam, the prosthesis must be fully seated prior to making the record. This is accomplished by inserting both prostheses and instructing the patient to occlude on cotton rolls placed in the premolar area bilaterally for 5 minutes before making the record.

a

b

c

Fig 19-4  (a) An impression is made of the RPD and the dentition. The impression is removed with the RPD embedded in the impression. (b) RPD castings and undercuts associated with the denture base are blocked out. Pumice was used in this example. (c) Fast-setting dental stone is poured into the impression. (Courtesy of Dr R. Faulkner, Cincinnati, Ohio.) Fig 19-5  (a and b) Low-fusing metal was used to make this clinical remount cast.

a

requires that the finished prosthesis and a replica of the occlusal surfaces of the remaining teeth be transferred to the articulator. These teeth must retain their position in relation to the prosthesis as established at the final fitting of the finished denture to the individual arch. The clinical remount and equilibration is accomplished as follows: 1. A centric relation (CR) record can be obtained at this point or after the remount casts are made. If the record is made at this point, it should be made with a material that permits easy retrieval and storage of the record. If the RPD opposes a maxillary prosthesis with complete palatal coverage with a post dam, the prosthesis must be fully seated prior to making the record. This is accomplished by inserting both prostheses and instructing the patient to occlude on cotton rolls placed in the premolar area bilaterally for 5 minutes before making the record (Fig 19-3).

b

2. With the prosthesis secured in position, an alginate impression (occlusal surfaces only) is made of the prosthesis and the remaining dentition. 3. The so-called “pickup impression” is removed with the prosthesis embedded with the impression (Fig 19-4a). 4. Undercuts associated with the denture base, retainers, and RPD casting are blocked out (Fig 19-4b). Undercuts associated with the extension base are blocked out with wet pumice or another suitable material. 5. Fast-setting dental stone or low-fusing metal is poured into the impression to reproduce the natural teeth (Fig 19-4c). 6. The prosthesis and the stone or metal occlusal surfaces are removed from the impression. Low-fusing metal (Fig 19-5) is favored by many clinicians because it solidifies quickly and produces a hard remount cast that is resistant to abrasion and chipping.

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Fig 19-6 A facebow transfer jig in preparation for a clinical remount. This is prepared by the dental laboratory.

Fig 19-7 Making the CR record.

Fig 19-8 Dental compound is favored by the authors when making clinical remount records.

This procedure reproduces the occlusal surfaces of the entire arch, including the prosthesis itself, with the natural teeth in stone or metal. This creates a quick, effective, and durable reproduction of the relationship developed at the fitting of the prosthesis. When the arch has been restored with an RPD, the plaster mounting should engage only the peripheral extensions of the prosthesis and the base surface of the replica of the residual dentition. Such a mounting will permit easy removal and replacement of the prosthesis into an exact keyed position (see Figs 19-4 and 19-5).

• The patient must not touch the denture teeth together or exert pressure on the denture base while making the record. Premature occlusal contacts can trigger tissue displacement or alter the mandibular closure pattern of the patient. • As mentioned previously, if the RPD opposes a maxillary prosthesis with complete palatal coverage with a post dam, the prosthesis must be fully seated prior to making the record (see Fig 19-3). A maxillomandibular record, if not made previously, is made with the material of choice (Fig 19-7). The authors prefer dental compound (Fig 19-8). • The stone or low-fusing metal replica of the residual dentition is secured to the prosthesis with sticky wax and mounted on the articulator with the record. • A second record is made and secured to the mounted casts/ prosthesis, and then the accuracy of the first articulator mounting is confirmed. If the two records do not coincide, the mounting procedure is repeated until two compatible records are obtained.

Facebow record If the RPD is opposed by a complete denture, a facebow transfer remount jig may be requested and produced by the dental laboratory at the time of the processing procedure (Fig 19-6). If a facebow transfer jig is not available, a new facebow record is obtained. The maxillary prosthesis and remount cast are then remounted on the articulator using this record.

Maxillomandibular record If not made earlier, the final maxillomandibular registrations are made and transferred to the articulator, and the mandibular cast is mounted. The authors prefer to verify the record with a second set of records. The following precautions are taken in order to ensure the making of accurate records:

198

A variety of materials have been used for making interocclusal records, including modeling plastic (dental compound), plaster, wax, bite registration paste, and silicone registration materials. A registration material that sets to a hard, unchanging form is favored. The material should have minimal resistance to closure and have a quick setting time. Dental compound is ideally suited for this purpose (see Fig 19-8). The material can be quickly and uniformly softened in a water bath and securely adapted to the occlusal surfaces of the prosthesis. The compound hardens quickly, is easily trimmed so that only shallow cusp indentations remain, cannot be distorted when hardened, and can quickly be resoftened in the water bath to remake the record.

Interarch Control

a

c

b

d

Fig 19-9  (a) The casts are mounted on the articulator and ready for equilibration. (b) The equilibration should be conducted using a specific sequence: centric first, followed by excursions. (c) The working position. (d) The protrusive position. (Parts a and b courtesy of Dr R. Faulkner, Cincinnati, Ohio.)

Protrusive record A protrusive record is made with the mandible positioned 4 to 6 mm forward of the CR position. Dental compound is favored when making these records (see Fig 19-8). The record is then transferred to the mounted casts/prosthesis apparatus, the inclination of the condyles is determined, and the articulator is adjusted accordingly.

Occlusal refinement and equilibration The rationale for completing a clinical remount and occlusal equilibration is to obtain a definite, repeatable, and proven

relationship between the maxillary and mandibular arches prior to final adjustments of the opposing occlusal surfaces. The occlusion developed for the definitive prosthesis affects the freedom of mandibular movement and influences the forces delivered to the teeth and the edentulous bearing surfaces. Adjustments with the cast and prosthesis mounted on an articulator with accurate and reproducible records are easily accomplished, the movements of the articulator are repeatable and precise, and refinement of the occlusion can be accomplished quickly and precisely. Occlusal adjustments made intraorally are more difficult to accomplish precisely, especially when equilibrating extension-base RPDs (Fig 19-9).

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Insertion and Maintenance of RPDs

a

b

Fig 19-10 (a and b) The completed prostheses.

Basic rules for equilibration

Intraoral evaluation

• Adjust for CR first. The maxillary lingual cusp tips of the posterior teeth must contact the central fossa of the mandibular teeth. Contour the fossa to achieve positive centric contacts. • Adjust the mandibular tooth surfaces to permit smooth, unencumbered, eccentric movements in all excursions per the occlusal scheme. When an extension-base RPD opposes a complete denture, balanced articulation is employed. When the anterior natural dentition in both arches has been retained and the RPD restores the extension areas, anterior guidance is advised, with the edentulous extension areas restored with centric-only contact.

The adjusted and polished prosthesis is inserted intraorally, and the following checks are made:

Natural teeth opposing denture teeth • Make adjustments on the denture teeth opposing the natural teeth. • When it is necessary to alter the natural teeth, perform the needed alterations on the cast, mark these areas on the cast, and reproduce these alterations intraorally.

Finish and polish • Check for smooth, unimpeded movement of opposing occlusal surfaces during eccentric movements. Eliminate rough or bumpy movements. • Inspect and smooth all surfaces; restore anatomy as much as possible. • Lightly polish the denture teeth. • Smooth, round, and polish all metal edges. • Highly polish the peripheral extensions. Figure 19-10 shows the completed prostheses. 200

• Verify simultaneous, even contact of the denture teeth and natural dentition. • Confirm the planned anterior guidance of the natural dentition during excursions. Ensure that there are no interferences of the occlusal surfaces of the posterior teeth during lateral and protrusive movements. • When an RPD opposes a complete denture, verify that the occlusal scheme is balanced articulation. • Ask the patient if there are pressure areas or sharp edges that may irritate the tongue, cheeks, or lips. Make adjustments as necessary.

Maintenance and Patient Instructions Patient preparation and instruction are continuing, ongoing communications that are part of every appointment and procedure. However, the patient will retain more information for a longer period if it is written and repeated several times. It is important that patients be informed and conditioned as to what to expect with the use of RPDs, such as feelings of fullness or of a foreign object in the mouth. The importance of maintaining cleanliness of remaining teeth, tissues, and the prosthesis must be emphasized repeatedly. Explicit instructions for maintenance are essential. Patient education cannot be effectively performed during a single appointment; it is an ongoing challenge.

Maintenance and Patient Instructions

Fig 19-11  The prosthesis must be cleaned after each meal or snack. A tapered brush is best to clean proximal plates and the denture base areas. Over-the-counter denture cleansers can be used after mechanical cleaning.

When to instruct patients in dental care Patients are instructed, educated, and conditioned during the entire period of treatment. Note that little of what the clinician says will be retained if the instructions are given while treating the patient and performing a specific task (eg, making impressions and records). The patient is more interested in what the clinician is doing than in listening to instructions. This is especially true when a prosthesis or other material has just been placed in the mouth.

Why patient instruction is necessary Younger patients are not always willing to accept an artificial replacement as readily as more mature individuals, and this is especially true of removable prostheses. If irritation develops on the denture-bearing tissues or surfaces of the tongue, patients are likely to remove the partial denture, place it in a drawer, and not remember it until much later when they become “dentally conscious” again, usually because of dental pain or discomfort. Patients have no way of knowing how to care for their RPDs. It is your responsibility as the clinician to instruct them as part of their treatment.

Instructions for RPD insertion and maintenance Placement of the prosthesis Insert and seat the RPD using the fingers. Never instruct the patient to “bite” the prosthesis into place. This action could bend or break the prosthesis or cause harm to the remaining teeth. RPDs have a definitive path of insertion. Make sure that the patient practices placement and removal of the prosthesis in front of a mirror. If the prosthesis seems to bend or does not slide smoothly into place, check the direction of insertion.

Removal of the prosthesis

Instruct the patient to remove the prosthesis at night and store it in water. To avoid bending the prosthesis or damaging the dentition, instruct the patient to remove the prosthesis along the same path used during insertion.

Cleaning the prosthesis • Clean the prosthesis and the natural dentition after every meal or snack (Fig 19-11). • Keep a brush at work as well as at home. • Use care when cleaning the prosthesis. Avoid dropping the prosthesis and scrub it over a basin filled with water or a towel. • Hold the prosthesis carefully and avoid squeezing and bending flexible parts such as the retainers. • Effervescent denture cleansers specifically designed for use for RPDs can be used to supplement hygiene.

Denture compatibility It will be several days or weeks before the prosthesis no longer feels like a foreign object to the patient. Also, advise the patient not to expect an immediate improvement in mastication efficiency. It takes time and practice for the tongue, cheeks, and lips to accommodate to the contours of the prosthesis and manipulate the food bolus in concert with the prosthesis. Early phonetic difficulties are best resolved by instructing the patient to read aloud or practice words that are especially troublesome.

Follow-up examinations The patient is given an appointment 24 hours post-insertion. Areas causing irritation or discomfort should be relieved immediately. PIP is used to identify pressure areas on the denture-bearing surfaces, and disclosing wax is used to identify sections of the denture flanges that are overextended. If further difficulties are anticipated, the patient is given an appointment the following 201

19

Insertion and Maintenance of RPDs

The denture you have received is meant to be a replacement for your missing teeth and to assist you in keeping your oral cavity in good health. It will restore support to your jaw, increase your chewing ability, and improve your appearance. If your mouth is to remain healthy, there is certain information that should be called to your attention to assist you in adjusting to and caring for your new prosthesis. This is your responsibility. Learning to use your prosthesis • Please be patient and expect that it will take 7 to 10 days before your prosthesis will really feel as if it is a part of your body. • If you have difficulty pronouncing certain words, practice reading them aloud, and you will soon master the correct pronunciation. • Do not expect to immediately chew easily and effectively. You must learn to use the new prosthesis; this will take time. Start with softer types of food. Take small amounts of food and cut them into small pieces with your knife and fork. Learning to use the prosthesis effectively is part of the treatment, so take the time to eat meals slowly during this learning period. Placing and removing the prosthesis • Place and remove the prosthesis with your fingers—never “bite” it into place. • There is one definitive path of insertion and removal. If the prosthesis tends to “bind” or is difficult to place or remove, stop and make sure you are using the correct path of insertion. Do not force the prosthesis when placing or removing it. Cleaning dentures • Your mouth and your prosthesis must be kept as clean as possible all the time. Failure to do so may result in damage to your natural teeth and/or your gums. • Remove and clean your prosthesis and natural teeth after each meal and snack. At the very minimum, rinse your mouth and prosthesis clean of food debris. • Regular soap or toothpaste can be used for cleaning. Keep a brush at home and at your place of work. • A prosthesis is slippery when wet. Scrub it over a basin filled with water or over a towel. If you drop the prosthesis and bend any of the metal parts, do not attempt to straighten it yourself. See your dentist. • Every day after cleaning the prosthesis and brushing your remaining teeth, apply a fluoride gel to all parts of the prosthesis that touch your remaining teeth. Overnight care Always leave the prosthesis out of your mouth at night while sleeping. Your mouth, like the rest of your body, requires a period of rest. Always keep the prosthesis in water when it is not in your mouth. This prevents the plastic parts from drying and warping. Denture cleaners placed in the overnight water bath are good for use after the prosthesis has been scrubbed. Develop the habit of examining your mouth in the mirror. This is the best insurance against disease and damage to your tissues. Carefully examine your cheeks, tongue, and gums. Look for food debris or accumulation around your teeth and gums, especially at the gum line and behind teeth. Clean your mouth to keep teeth free of decay, stains, or brown deposits, especially at the gum line. Test your remaining teeth for excessive movement or looseness. Difficulties Be sure to return for a follow-up appointment with your dentist 24 hours after receiving your prosthesis. Sore or irritated teeth and soft tissues must be treated immediately. Your prosthesis is not a permanent treatment. It is subject to wear; your remaining natural teeth may decay, and your gums may change or recede, just as they did when you had all your natural teeth. Periodic examination and treatment by the dentist is very necessary. See your dentist for examination and prophylaxis treatment every 6 months. Schedule an examination if you experience the following difficulties: • Soreness or irritation of the teeth or gums • Evidence of decay • Excessive movement of the prosthesis • Calculus or staining on the prosthesis or natural teeth Home remedies for ill-fitting prostheses aggravate a bad situation and may cause greater bone and tissue loss. See your dentist if you have a problem. Fig 19-12 Instructions for RPD patients.

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Suggested Reading

day. Otherwise, the next follow-up appointment should be scheduled 1 week later. This will ensure that the patient will transition smoothly through the accommodation stage with minimum difficulty. Stannous fluoride gel is prescribed for daily application to all parts of the prosthesis that are in contact with tooth surfaces. The gel is applied in a thin layer with a cotton tip applicator after the prosthesis and the dentition have been thoroughly cleaned. Periodic follow-up examinations are mandatory. The patient should be impressed with the necessity for regular 6-month follow-up examinations. It is incumbent upon the dental practitioner to establish an efficient recall system for his or her patients.

Written instructions for the patient Figure 19-12 lists written instructions that are given to patients to take with them.

Relines Examination and evaluation of the RPD patient is scheduled every 6 months. Determining if there are tissue and bone changes of the edentulous support area under the extension portion of the prosthesis is a primary concern. Proper evaluation requires removal of the prosthesis for a 24-hour period prior to examination of the support tissues so they can return to their natural contours. The adaptation of the prosthesis to the mucosa can then be checked for even contact with PIP, and the alveolar ridge can be observed for possible changes or resorption. Further evaluation of changes in the ridge support are made by observing the amount of movement that occurs when force is applied in the extension portion of the prosthesis. Excessive denture movement is evidenced by lifting the indirect retainer rests off their rest seats. Another method of evaluating changes requiring relining is the use of articulating ribbon or celluloid strips between the occlusal surfaces. If there has been bone resorption or ridge support changes, the occlusal surfaces will not record or hold the strips when the patient is in a closed position.

Reline procedure

The reline procedure is basically the same as an altered cast impression: 1.  Instruct the patient to remove the prosthesis for 24 hours prior to the reline appointment or until the edentulous areas are healthy. 2.  Remove sufficient acrylic resin from the tissue surface of the denture base to allow at least 1 mm of space between the denture base and the tissue. 3.  Mold the periphery with tempered dental compound. 4.  Remove all dental compound that has flowed onto the tissue side of the denture base. 5.  Make a final closed-mouth impression with a suitable impression material. 6.  Flask the RPD and process with acrylic resin. 7.  Insert the RPD following the same procedure employed during delivery of the new prosthesis. 8.  Post-insertion instructions and follow-up appointments are identical to those described for a new prosthesis.

Suggested Reading Bauman R. Minimizing postinsertion problems: A procedure for removable partial denture placement. J Prosthet Dent 1979;42:381–385. Fiebiger GE, Parr GR, Goldman BM. Remount casts for removable partial dentures. J Prosthet Dent 1982;48:106–107. Gay WD. Laboratory procedures for fitting removable partial denture frameworks. J Prosthet Dent 1978;40:227–229. Hickey JC. Responsibility of the dentist in removable partial dentures. J Ky Dent Assoc 1965;17:70–87. Maison WG. Instructions to denture patients. J Prosthet Dent 1959;9:825–831. McCracken WL. Occlusion in partial denture prosthesis. Dent Clin North Am 1962;6:109–119. Schuyler CH. Fundamental principles in the correction of occlusal disharmony, natural and artificial. J Am Dent Assoc 1935;22:1193–1202. Wagner AG. Instructions for the use and care of removable partial dentures. J Prosthet Dent 1971;26:477–480.

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Chapter 20 Clinical Appointment Sequence Ting-Ling Chang | Daniela Orellana

First Appointment For a partially edentulous patient who requires a removable partial prosthesis, diagnostic casts are mounted on the articulator for treatment planning. Impressions for use in fabricating these diagnostic casts are made at the first appointment.

Armamentarium • • • • • • • •

Mouth mirror Radiographs if available Periphery wax strips 4 × 4 gauze squares Explorer Impression trays Alginate Forms as required

Clinical procedure 1. Proceed with history taking and diagnostic examination; fill out the necessary records. 2. Prior to making diagnostic cast impressions, perform basic prophylaxis with rubber cup and pumice so that better diagnostic impressions can be made. 3. Take a facebow record. 4. Make a centric relation record. 5. Mount the diagnostic casts for study. 6. Order radiographs.

Diagnostic procedure 1. Evaluate all information obtained: clinical information, history, diagnostic casts, and radiographs. 2. Plan your recommended treatment procedure. Outline the steps on paper and on the diagnostic casts. If necessary, discuss your proposed treatment with consultants prior to the second appointment with the patient. 3. Request consultations with specialists if necessary.

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Clinical Appointment Sequence

Second Appointment The purpose of this appointment is to present a final treatment plan for approval by the patient and the doctor.

Armamentarium • • • •

Radiographs Mounted diagnostic casts Treatment plan History records and laboratory tests as required

Clinical procedure 1. Show and explain to the patient the situations that exist in the mouth. Use the diagnostic casts for training aids. 2. Obtain consultation from all specialists involved in the treatment. Write down or enclose their proposed treatment in your records. 3. Formulate the final treatment plan. 4. Upon completion of the treatment plan, outline all steps in the patient’s record. 5. Draw the final removable partial denture (RPD) design on the diagnostic casts prior to beginning restorative procedures. The RPD design often influences the size and contour of the restorations. When the treatment plan and diagnosis are completed, make a case presentation to the patient. Explain in detail and be sure that the patient is informed of the cost. It is always desirable to have the treatment plan and cost in writing. Proceed with the surgical, periodontal, and restorative procedures as necessary.

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Third Appointment

Third Appointment The purpose of this appointment is to accomplish the final intraoral preparation and to make final impressions for fabrication of the RPD. All necessary operative and periodontal procedures have been completed.

Armamentarium • Diamond stones • Burlew rubber wheels • Pumice • Alginate and trays • #6 and #8 round burs • Sandpaper disks • Rubber cups • 4 × 4 gauze squares

Clinical procedure 1.  Refer to your diagnostic cast, which indicates where parallel guiding planes must be prepared. Make these guiding surfaces parallel first with a diamond stone, then smooth with sandpaper disks and rubber wheels. 2.  Alter other tooth surfaces as pre-planned where necessary. Prepare rests with a #6 or #8 round bur or a diamond for posterior teeth. Do not leave sharp edges or parallel sides. The teeth must be smoothly rounded with a minimum of 1 mm of space provided for the metal rest. 3.  Check anterior rests that have been placed in inlays or crowns to make sure that they have positive, rounded rest seats. Recontour and polish them if necessary. Prepare anterior rests on natural tooth surfaces as planned. 4.  Cleanse the mouth well before making final impressions. Make two final impressions with alginate in a stock tray. 5.  Pour these impressions immediately in vacuumed stone. 6.  Make an occlusal index for verifying the accuracy of the master cast(s). Use a cake of compound and place the registration paste on one surface of the matrix in the shape of the arch. Place it in position in the mouth to record all occlusal and incisal surfaces.

Laboratory procedure 1.  Inspect the casts closely for defects such as bubbles. Select the better one to be the master cast. 2.  Finalize the design on the design cast. 3.  Tripod and mark the master cast. 4.  Fill out the laboratory prescription form in duplicate. 5.  Submit both casts to the laboratory with the occlusal index and the prescriptions.

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Clinical Appointment Sequence

Fourth Appointment The basic objectives of this appointment are to do the following: • Try in the casting and make physiologic adjustments • Make an altered cast impression if necessary • Obtain maxillomandibular registrations for transferring a duplication of the patient’s oral anatomy to a laboratory situation using an articulator

Armamentarium • • • • • •

Gold rouge Brush Articulator Chloroform Facebow Carbide burs

For altered cast impression procedures, add the following: • • • • •

Baseplate wax Acrylic tray material Modeling compound stick Polysulfide impression material Water bath

Clinical procedure 1. Coat the guiding planes and the rests of the casting with gold rouge and chloroform. Dry with air. Place in the patient’s mouth and check for positive seating of the rests. If the rests are not seating or if the casting binds, inspect the gold rouged areas for interferences. Adjust with a high-speed bur where indicated. 2. In extension cases, coat the proximal plates and minor connectors with gold rouge, and produce extension movement of the casting in the patient’s mouth. Relieve the binding areas as disclosed by the gold rouge until the castings move freely. 3. In extension cases requiring altering of the master cast, proceed as outlined. 4. In a tooth-borne case, proceed with the following: a. Take a facebow record. b. Mount the maxillary cast on the articulator. c. Obtain a maxillomandibular record in the chosen treatment position. d. Mount the mandibular cast on the articulator. e. Verify the maxillomandibular records with a second set of records. f. Make a protrusive record and adjust the horizontal condylar inclination on the articulator. g. Select teeth. 5. In a tooth-borne case, proceed with the laboratory fabrication of the prosthesis. 6. In extension situations, see the fifth appointment, discussed next.

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Fifth Appointment

Fifth Appointment The basic objectives of this appointment are to obtain maxillomandibular registrations and to transfer a duplication of the patient situation to a laboratory situation using an articulator. Specific details and instructions are found in chapters 12 and 13. This appointment is necessary for extension prostheses only.

Armamentarium • Facebow • Record materials, such as modeling compound stick or registration paste • Articulator

Clinical procedure 1.  Use the altered cast impression to alter the master cast by repouring the edentulous area. On this refined master cast, adapt sprinkle-on acrylic record base material to the edentulous area to provide a stable base for records. 2.  Mount the maxillary cast on the articulator with the facebow record. 3.  Evaluate and mark the occlusal vertical dimension. 4.  Adjust the wax occlusion rim to allow space for the interocclusal record. 5.  Make the maxillomandibular record at the chosen treatment position. 6.  Mount the mandibular cast on the articulator. 7.  Verify the mounting with a second set of records. 8.  Make the protrusive record. 9.  Adjust the condylar inclination of the articulator. When anterior teeth are involved or if there are esthetic considerations, an additional appointment is necessary.

Laboratory procedure 1.  Refine the occlusion on the articulator. 2.  Set up the teeth. 3.  Process and finish the prosthesis.

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20

Clinical Appointment Sequence

Sixth Appointment The try-in appointment is necessary when there are missing teeth in the esthetic zone or extensive edentulous areas are involved.

Armamentarium • RPD framework with denture teeth set in wax • Registration paste or modeling compound stick for interocclusal records

Procedure 1. 2. 3. 4.

Verify the accuracy of the maxillomandibular record with try-in wax RPD and remount if necessary. Obtain the protrusive record and condylar inclination of the articulator. Check the esthetics and phonetics. Obtain the patient’s approval. Schedule appointments for insertion as well as 24-hour and 1-week checks.

Seventh Appointment The insertion appointment requires final adaptation of the following: • The casting to the remaining teeth • The denture base to the edentulous areas • Refinement of occlusion by a clinical remount procedure

Armamentarium • • • • • • • • •

Pressure indicating paste Impression trays Registration paste or modeling compound stick for interocclusal records Articulating paper Articulator Facebow Occlusion adjustment stones Disclosing wax Alginate

Procedure 1. Complete the clinical remount and occlusal equilibration procedure (see chapter 19). 2. Give the patient an RPD instruction sheet (see chapter 19). 3. Confirm 24-hour and 1-week post-insertion appointments.

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Glossary

3D printing A general term describing additive manufacturing processes that build 3D structures by depositing layers of material on top of each other until the final structure is achieved. 3D printing can produce objects made of single or multiple materials without being limited by undercuts or complexity.

Altered cast impression An impression of the edentulous denture-

bearing area made independent of and after the initial impression of the natural teeth. This technique uses a custom tray or trays attached to the framework of the RPD.

3D rendering Computer graphics process of automatically converting

3D wire frame models into 2D images with 3D photorealistic effects on a computer.

3D scanner A device that analyzes a real-world object to collect data on its shape and/or other attributes, such as color or texture. Abutment A tooth or dental implant that serves to stabilize a pros-

thesis.

Acrylic resin Any of a group of thermoplastic resins made by polymerizing esters of acrylic or methyl-methacrylate acids. Additive manufacturing (AM) The “process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies, such as traditional machining,” as defined by American Society for Testing Materials (ASTM).

Alloy A mixture of two or more metals or metalloids that are mutually soluble in the molten state. Anterior guidance (anterior protected articulation or canine protected articulation) A form of mutually protected articulation in

which the vertical and horizontal overlap of the anterior teeth disengage the posterior teeth in all mandibular excursive movements.

Anteroposterior palatal strap A major connector that consists of an anterior and posterior palatal connector.

Akers clasp See Circumferential clasp. Altered cast (corrected cast, modified cast) A final cast that is re-

vised in part before processing a denture base.

Arch form The geometric shape of the dental arch when viewed in the

horizontal plane (eg, square, tapering, ovoid).

Arcon articulator An articulator that contains the condylar path ele-

ments within its upper member and the condylar elements within the lower member.

Articulation The contact relationships of maxillary and mandibular teeth during function.

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A

Glossary

Articulator A mechanical instrument that represents the TMJs and

jaws, to which maxillary and mandibular casts may be attached to simulate some or all mandibular movements.

Axis A straight line around which a body may rotate or about which a structure would turn if it could revolve. Axis of rotation (fulcrum line) An imaginary line connecting occlusal

Border molding 1. The shaping of impression material along the border areas of an impression tray by functional or manual manipulation of the soft tissue adjacent to the borders. 2. Determining the extension of a prosthesis by using tissue function or manual manipulation of the tissues to shape the border areas of an impression material.

Bracing The resistance to horizontal components of masticatory force.

rests of an extension-base RPD around which the RPD tends to rotate under masticatory forces.

Bracing component Any component of an RPD that offers resistance

Bead line A slight indentation scribed into the cast to prevent food

Buccal shelf A primary support-bearing area that is comprised of cor-

Bilateral balanced articulation The bilateral, simultaneous, anterior, and posterior occlusal contact of teeth in centric and eccentric positions.

CAD/CAM dentistry Using computer technologies to design and pro-

impaction beneath the major connector.

to horizontal components of masticatory force.

tical bone and extends from the base of the residual ridge in the posterior part of the mandible to the external oblique ridge.

duce different types of dental restorations, including crowns, veneers, inlays and onlays, fixed prostheses, dental implant restorations, and orthodontic appliances. Computer-aided design (CAD) The use of computer programs to create 2D or 3D graphic representations of physical objects. CAD software may be specialized for specific applications.

Protrusive position

Right laterotrusive position

Biomechanics The application of mechanical laws to living structures. Blockout 1. The elimination of undesirable undercuts on a cast. 2. The process of applying wax or another similar temporary substance to undercut portions of a cast so as to leave only those undercuts essential to the planned construction of a prosthesis; a blocked-out cast may also include other surface modifications needed relative to the construction of the prosthesis.

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Computer-assisted manufacturing (CAM) The use of computer

software to control machine tools and related machinery in the manufacturing of work pieces. Its primary purpose is to create a faster production process and components and tooling with more precise dimensions and material consistency. In some cases, it uses only the required amount of raw material (thus minimizing waste), while simultaneously reducing energy consumption.

Centric relation (CR) A maxillomandibular relationship, independent

of tooth contact, in which the condyles articulate in the anterosuperior position against the posterior slopes of the articular eminences; in this position, the mandible is restricted to a purely rotary movement; from this unstrained, physiologic, and maxillomandibular relationship, the patient can make vertical, lateral, or protrusive movements; it is a clinically useful, repeatable reference position.

Glossary

Cingulum rest A portion of an RPD that contacts the prepared recess in the cingulum of the tooth or artificial crown.

Circumferential clasp A retainer that encircles a tooth by more than

180 degrees, including opposite angles, that generally contacts the tooth throughout the extent of the clasp, with at least one terminal located in an undercut area.

Clasp assembly The part of an RPD that acts as a direct retainer and/or stabilizer for a prosthesis by partially encompassing or contacting an abutment tooth. Components of the clasp assembly include the retentive clasp, reciprocal clasp, cingulum, incisal or occlusal rest, and minor connector.

C

Condylar guide inclination The angle formed by the inclination of a

condylar guide control surface of an articulator and a specified reference plane.

Coronoplasty (occlusal reshaping) Any change in the occlusion intended to alter the occlusal surfaces of the teeth or restorations to change their form.

Cross-arch stabilization Resistance against dislodging or rotational forces obtained by using an FPD or RPD design that uses natural teeth on the opposite side of the dental arch from the edentulous space to assist in stabilization.

Crown lengthening A surgical procedure designed to increase the

extent of supragingival tooth structure for restorative or esthetic purposes.

Complete palatal plate A palatal plate that covers the entire hard

palate.

Crown-root ratio The physical relationship between the portion of Condylar guidance The mechanical form located in the posterior region of an articulator that controls the movement of its mobile member.

the tooth not within the alveolar bone, as determined by a radiograph, compared with the portion of the tooth within alveolar bone.

Cusp-fossa articulation scheme An occlusal arrangement where the maxillary and mandibular centric cusps articulate with the opposing fossae in maximal intercuspal position.

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C

Glossary

Cusp–marginal ridge articulation scheme An occlusal arrangement where the mandibular second premolar buccal cusp and mandibular molar mesiobuccal cusp articulate with the opposing occlusal embrasures in maximal intercuspal position. Custom tray An individualized impression tray made from a cast recovered from a preliminary impression; it is used in making a final impression.

Definitive prosthesis Any dental or maxillofacial prosthesis designed

for long-term use.

Denture base The part of a denture that rests on the foundation tis-

sues and to which teeth are attached.

Denture flange The part of the denture base that extends from the

cervical ends of the teeth to the denture border.

Digital scan 1. Capturing the computer numerical format of raster images (bitmap) of a geometric object that can be viewed on a monitor screen (2D) or hologram (3D). 2. In dentistry, capturing the optical image directly of the patient’s anatomy or indirectly of a definitive cast of the anatomy.

Digital sculpting The use of software that offers tools to push, pull, smooth, grab, pinch, or otherwise manipulate a digital object as if it were made of a real-life substance, such as clay. When creating 3D models in an application, this includes manipulating vertices and edges to get the desired look. While this works, it can be hard to get the fine detail often required, especially in organic models. Digital sculpting works around this issue by allowing the user to create a 3D mesh in much the same way as a traditional sculptor would. By interactively pushing and pulling areas of the model out, details like texture, pits, and sharp transitions (eg, the cementoenamel junction [CEJ]) can be created without having to select an edge or vertex. Digital workflow Any workflow that occurs primarily through the use

Diagnostic cast A life-size reproduction of a part or parts of the oral cavity and/or facial structures for the purpose of study and treatment planning.

of converting physical or analog structures into a digital format to be manipulated using CAD software. Often, the digital process resembles the analog process in steps, but is accomplished virtually on a computer until the design is machine fabricated through automated milling or 3D printing methods.

Direct retainer The component of an RPD used to retain and prevent

dislodgment, consisting of a clasp assembly or precision attachment.

Earbow A type of facebow that indexes to the external auditory me-

atus and registers the relation of the maxillary dental arch to the external auditory meatus and a horizontal reference plane; this instrument is used to transfer the maxillary cast to the articulator; it provides an average anatomical dimension between the external auditory meatus and the horizontal axis of the mandible.

Diagnostic wax-up A dental diagnostic procedure in which planned

restorations are developed in wax on a diagnostic cast to determine optimal clinical and laboratory procedures necessary to achieve the desired esthetics and function.

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Encirclement The characteristic of the clasp assembly that prevents

movement of the abutment away from the associated clasp assembly during function. The clasp assembly must encompass more than 180 degrees of the tooth by continuous (circumferential clasp type of retainer) or broken contact (bar type retainer). If broken, it must contact at least three different areas of the tooth.

Glossary

I

Extended occlusal rest A rigid extension of a partial denture that

teeth on the surgical side of the mandible move away from the opposing maxillary teeth after initial contact on the nonsurgical side has been established.

Extension-base RPD An RPD that is supported and retained by natural teeth anterior to the denture base and in which a portion of the functional force vector of the base is carried by the residual ridge.

Gingival convergence The slope of the undercut region.

contacts the occlusal surface of a posterior tooth in a prepared rest seat that extends more than one-half the mesiodistal width of the tooth.

Group function Multiple contact relations between the maxillary and

mandibular teeth in lateral movements on the working side whereby simultaneous contact of several teeth acts as a group to distribute occlusal forces.

Facebow fork That component of the facebow used to attach the occlusion rim and/or natural teeth to the facebow. Guide planes (guiding surfaces) Two or more vertically parallel surfaces on abutment teeth and/or fixed dental prostheses oriented so as to contribute to the direction of the path of placement and removal of an RPD, maxillofacial prosthesis, and overdenture.

Finish line 1. The junction of prepared and unprepared tooth structure with the margin of a restorative material. 2. The planned junction of different materials.

Height of contour (survey line) A line produced on a cast by a sur-

veyor marking the greatest prominence of contour in relation to the planned path of placement of a restoration.

Framework Any metal or combination of metal or ceramic materials, with various forms including designed slots, incorporated corrective angulation patterns, etc, which provide rigidity to a dental prosthesis; such a framework can be made in whole or made of component parts; frequently used to anchor or support a prosthesis to natural teeth or dental implant abutments or both. Frontal plane rotation In patients whose mandibular continuity has not been restored, the absence of the attachments of the muscles of mastication on the surgical side results in a significant rotation of the mandible upon forceful closure. When viewed from the frontal plane,

Hypernasal speech A perceived voice quality in which the nasal cavi-

ty acts as a resonating cavity for non-nasal sounds; generally associated with palatopharyngeal inadequacy.

I-bar retainer An infrabulge retentive clasp that resembles an “I” as it approaches perpendicular to the facial height of contour.

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I

Glossary

Image resolution An umbrella term that describes the detail an image holds. Resolution quantifies how close lines can be to each other and still be visibly resolved. The term applies to raster digital images, film images, and other types of images. Higher resolution means more image detail. Image resolution can be measured in various ways. Resolution units can be tied to physical sizes (eg, lines per mm, lines per inch), to the overall size of a picture (eg, lines per picture height, also known simply as lines, TV lines, or TVL), or to angular subtenant. Imbibition The act or process of imbibing or absorbing; in dentistry, an example is the absorption of water in hydrocolloid impression materials when stored in water and the resultant dimensional change that occurs.

Irreversible hydrocolloid (alginate) A hydrocolloid consisting of a

sol of alginic acid having a physical state that is changed by an irreversible chemical reaction forming insoluble calcium alginate. Used as an impression material.

Laboratory work authorization (laboratory prescription) A signed written order provided by the dentist to the dental laboratory detailing the work to be performed; a component of the patient’s dental record specifying the design and materials to be used.

Incisal rest A rigid extension of an RPD that contacts a tooth at the

incisal edge.

Indirect retainer The component (ie, rest) of an RPD that assists the direct retainer(s) in preventing displacement of the distal-extension denture base by functioning through lever action on the opposite side of the fulcrum line when the denture base attempts to move away from the tissues in pure rotation around the fulcrum line.

Lateral discontinuity defect An unrestored defect of the mandibular body or ramus where the residual portion of the mandible functions with one condyle.

Laterotrusion Condylar movement on the working side in the horizontal plane; this term may be used in combination with terms describing condylar movement in other planes. Infrabulge retainer An RPD retentive clasp that approaches the re-

tentive undercut from a cervical or infrabulge direction.

Interocclusal record A registration of the positional relationship of the opposing teeth or arches; a record of the positional relationship of the teeth or jaws to each other.

Intraoral scanning The process of scanning and capturing the intra-

oral cavity for translation into a digital file format, such as STL.

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Lingual bar A major connector located lingual to the dental arch joining two or more bilateral parts of a mandibular RPD.

Lingual plate A major connector located lingual to the dental arch that covers the gingival tissue and contacts the lingual surfaces of the teeth joining two or more bilateral parts of an RPD.

Glossary

O

Lingualized occlusion This form of denture occlusion articulates the

Minor connector The connecting link between the major connector

Major connector The part of an RPD that joins the components on one side of the arch to those on the opposite side.

Model scanning The process of acquiring the 3D image of a dental model for translation into a digital file format, such as STL. The digital file can be stored for future reference or used in a CAD software program for the design and fabrication of a dental prosthesis.

maxillary lingual cusps with the mandibular occlusal surfaces in centric occlusion and working and nonworking mandibular positions.

Mandibular deviation In patients with mandibular discontinuity de-

fects with only one functional condyle remaining, the residual mandible deviates towards the surgical side.

or base of an RPD and the other units of the prosthesis, such as the clasp assembly, indirect retainers, occlusal rests, or cingulum rests.

Most advantageous position (MAP) The position of the cast established on the surveyor that places teeth and associated tissues in the path of insertion for the RPD. Mucogingival junction The junction of gingiva and alveolar mucosa. Mucosa A mucous membrane comprised of epithelium, basement

membrane, and lamina propria.

Mutually protected articulation An occlusal scheme in which the Master cast A replica of the tooth surfaces, residual ridge areas, and/or

other parts of the dental arch and/or facial structure used to fabricate a dental restoration or prosthesis.

posterior teeth prevent excessive contact of the anterior teeth in maximal intercuspal position, and the anterior teeth disengage the posterior teeth in all mandibular excursive movements.

Non-arcon articulator Any articulator design in which the condylar element (analog) is part of the upper member of the articulator and may be used to simulate the 3D motions of the left and right condylar compartments. Obturator prosthesis A maxillofacial prosthesis used to close a congenital or acquired tissue opening, primarily of the hard palate and/or contiguous alveolar/soft tissue structures.

Maxillomandibular relationship Any spatial relationship of the max-

illa to the mandible; any one of the infinite relationships of the mandible to the maxilla.

Maximal intercuspal position (MIP) The complete intercuspation of

the opposing teeth independent of condylar position, sometimes referred to as the best fit of the teeth regardless of the condylar position.

Metal-ceramic restoration An artificial crown or fixed complete or partial denture that uses a metal substructure and porcelain veneer.

Occlusal analysis A systematic examination of the occlusion with spe-

cial consideration to the interocclusal relations of mounted casts.

Occlusal disharmony A phenomenon in which contacts of opposing

occlusal surfaces are not in harmony with other tooth contacts and/or the anatomical and physiologic components of the craniomandibular complex.

Occlusal equilibration The modification of the occlusal form of the teeth with the intent of equalizing occlusal stress, producing simultaneous occlusal contacts, or harmonizing cuspal relations.

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O

Glossary

Occlusal index Impression compound cake and zinc oxide–eugenol impression paste that captures the cusp tips and positions of the remaining teeth to verify the accuracy of the master and refractory casts.

Occlusal interference 1. Any tooth contact that inhibits the remaining occluding surfaces from achieving stable and harmonious contacts. 2. Any undesirable occlusal contact.

Occlusal plane 1. The average plane established by the incisal and occlusal surfaces of the teeth; generally, it is not a plane but represents the planar mean of the curvature of these surfaces. 2. The surface of wax occlusion rims contoured to guide in the arrangement of denture teeth. 3. A flat or curved template used in arranging denture teeth.

Occlusal rest A rigid extension of an RPD that contacts the occlusal

surface of a tooth or restoration, the occlusal surface of which is commonly prepared to receive this rest.

Overlay prosthesis (overdenture or overlay denture) Any remov-

able dental prosthesis that covers and rests on one or more remaining natural teeth, the roots of natural teeth, and/or dental implants; a dental prosthesis that covers and is partially supported by natural teeth, natural tooth roots, and/or dental implants.

Palatal strap (single strap) A major connector of a maxillary RPD

with an anteroposterior dimension of at least 10 mm that directly or obliquely traverses the palate; it is generally located in the area of the second premolar and first molar.

Partial-veneer crown (partial-coverage crown) This is a nonspecific

term that does not distinguish between the partial-coverage crown restoration of the tooth and the partial veneering of an artificial crown.

Path of insertion (path of placement) The specific direction in which

a prosthesis is placed on the residual alveolar ridge, abutment teeth, dental implant abutment(s), or attachments.

Physiologic adjustment Adjustments on extension-base RPDs to

permit free rotation around the axis of rotation. These adjustments prevent guide planes and minor connectors from delivering lateral and off-axis forces to the abutment teeth.

Occlusal vertical dimension (OVD) The distance between two select-

ed anatomical or marked points (usually one on the tip of the nose and the other on the chin) when in maximal intercuspal position.

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Glossary

R

Pick-up impression An impression that incorporates a prosthesis, framework, copings, or attachments for the purpose of making a cast as a relationship record within the arch.

Reciprocation The mechanism by which lateral forces generated by a retentive clasp passing over the height of contour are counterbalanced by a reciprocal clasp, guide plane, lingual plate, minor connector, etc.

Polysulfide An elastomeric impression material of polysulfide polymer

Record base An interim denture base used to support the record rim

(mercaptan) that cross-links under the influence of oxidizing agents, such as lead peroxide.

material for recording maxillomandibular records.

Polyvinyl siloxane An addition reaction silicone elastomeric impression material of silicone polymers having terminal vinyl groups that cross-link with silanes on activation by a platinum or palladium salt catalyst. Pre-prosthetic surgery Surgical procedures designed to facilitate fab-

rication of a prosthesis or to improve the prognosis of prosthodontic care.

Primary and secondary support areas (primary and secondary stress-bearing areas) 1. The surfaces of oral structures that resist forc-

es, strains, or pressure during function. 2. The portion of mouth most capable of providing support for a denture.

The primary stress-bearing areas of the maxilla are the hard palate and maxillary tuberosity, and the secondary is the residual maxillary alveolar ridge. The primary support areas of the mandible are the buccal shelf, posterior ridge, and retromolar pads. The secondary support areas are the mandibular anterior ridge and all ridge slopes.

Relief 1. The reduction or elimination of undesirable pressure or force from a specific region under a denture base, artificial crown, or coping. 2. The creation of space in an impression tray for impression material. 3. Coating of a die with a paint-on material to produce an oversized die dimension that allows for the flow of luting agent during cementation and eliminates any undesirable binding at the intaglio surface.

Prosthesis An artificial replacement of part of the human anatomy

restoring form, function, and esthetics.

Protrusion A position of the mandible anterior to centric relation without lateral deviation. Proximal plate A plate of metal in contact with the proximal surface

of an abutment tooth.

Reline The procedures used to resurface the intaglio of a removable dental prosthesis with new base material, thus producing an accurate adaptation to the denture foundation area. Residual ridge (alveolar ridge) The portion of the residual bone and

its soft tissue covering that remain after the removal of teeth.

Resilient attachment An attachment designed to give a tooth, soft tissue, or implant-borne removable dental prosthesis sufficient mechanical flexion to withstand the variations in seating of the prosthesis as a result of deformations of the mucosa and underlying tissues to avoid placing excessive stress on the abutments. Rapid prototyping (RP) A generic term that describes many additive manufacturing processes. In its most general sense, rapid prototyping can refer to any automated, repeatable, CAD-based process that results in rapid production of a physical object.

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R

Glossary

Resorption (alveolar resorption) The loss of tissue substance by

RPI A clasp assembly consisting of a rest, a proximal plate, and an I-bar retainer.

Rest A rigid extension of an RPD that contacts the occlusal, incisal, cingulum, or lingual surface of a tooth or restoration to transmit forces, the surface of which is commonly prepared to receive it.

Rugae An anatomical fold usually used in the plural sense; the irreg-

physiologic or pathologic processes.

ular fibrous connective tissue ridges located in the anterior third of the hard palate.

Selective laser sintering (SLS) Additive manufacturing technique that

uses a high-power laser (eg, a carbon dioxide laser) to fuse small particles of plastic, metal (direct metal laser sintering), ceramic, or glass powders into a mass that has a desired 3D shape.

Stability Resistance to horizontal displacement of a prosthesis. Standard triangulation language (STL) File format native to the ste-

Retention The ability of an RPD to resist movement away from its

foundation area and/or abutments.

Retentive fulcrum line 1. An imaginary line connecting the retentive points of clasp arms on retaining teeth adjacent to mucosa-borne denture bases. 2. An imaginary line connecting the retentive points of clasp arms, around which the RPD tends to rotate when subjected to dislodging. Retromolar pad (area) A mass of tissue comprised of keratinized

mucosa located on the distal aspect of the denture-bearing surfaces. This is a primary support area comprised of the pear-shaped pad and retromolar pad.

reolithography CAD software created by 3D Systems. This file format is supported by many other software packages; it is widely used for rapid prototyping and CAM. An STL file describes only the surface geometry of a 3D object without any representation of color, texture, or other common CAD model attributes.

Stipple To make small pointed indentations that together produce an even or softly graded shadow. Stock tray A metal or plastic prefabricated impression tray typically available in various sizes and used principally for preliminary impressions. Subtractive manufacturing (SM) Conventional machining is a form

of subtractive manufacturing in which a collection of material-working processes uses power-driven machine tools, such as saws, lathes, milling machines, and drill presses. These are used with a sharp cutting tool to physically remove material to achieve a desired geometry.

Supereruption Movement of a tooth or teeth, along with the tooth-supporting structures, in discontinuity with the normal occlusal plane.

Rotational path RPD (dual path RPD) An RPD that incorporates a

curved or dual path of placement allowing one or more of the rigid components of the framework to gain access to and engage the undercut area.

Support The foundation area on which a dental prosthesis rests; with respect to dental prostheses, the resistance to vertical seating forces directed toward the basal tissue or underlying structures. Suprabulge retainer Any RPD retentive clasp that approaches the retentive undercut from an occlusal direction.

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Glossary

Surveyed crown Crown that is fabricated with ideal height of contour, undercuts, guide planes, and rest seats to serve as an RPD abutment.

Surveying An analysis and comparison of the prominence of intraoral contours associated with the fabrication of a dental prosthesis. Surveyor A paralleling instrument used in making a dental prosthesis

to locate and delineate the contours and relative positions of abutment teeth.

W

is to be replaced by a definitive dental prosthesis; often such prostheses are used to assist in determining the therapeutic effectiveness of a specific treatment plan or the form and function of the planned definitive prosthesis.

Tripoding Marks drawn on a cast in a single plane perpendicular to the survey rod to assist with repositioning the cast on a dental surveyor in a previously defined orientation.

Undercut The portion of the surface of an object that is below the height of contour in relationship to the path of placement.

Syneresis The separation of water from its component base (eg, a gel

or impression material).

Tissue conditioning A procedure in prosthodontics usually performed by relining a removable complete denture, RPD, or a maxillofacial prosthesis with a resilient resin, and allowing a short duration of time for the patient’s soft tissue to heal. Tooth preparation guide Guide that preserves the predetermined path of insertion of the prosthesis and aids the clinician in the recontouring of the abutment teeth.

U-shaped palatal strap (anterior palatal strap) A palatal major connector that covers a portion of the anterior palatal tissue.

Wrought wire clasp A suprabulge or infrabulge retainer for an RPD Tooth-borne (tooth-supported) A term used to describe a dental prosthesis or part of a prosthesis that depends entirely on the natural teeth for support.

or maxillofacial prosthesis that has a wrought wire retentive clasp with a reciprocating arm or plate; it has comparatively more flexibility and adjustability than a cast clasp.

Torque A twisting or rotary force; the movement of a system of forces producing rotation. Treatment position The maxillomandibular position chosen to restore the dentition. It can be either centric relation or maximal intercuspal position. Treatment RPD An interim RPD designed to enhance esthetics, stabilization, and/or function for a limited period of time, after which it

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Index Page references followed by “f” denote figures; “b” denote boxes.

A Abutment teeth active force on, 45 amalgam restorations in, 85 assessment of, 76 design considerations for, 182, 183f forces on, 182 gingival recession with, 23 guiding surface preparation affected by, 26 implant used as, 178 overlay removable partial dentures using retained roots, 171–172 physiologic adjustment, 124 rests on, 58 stabilizing of, using posterior rests, 18 tooth preparation guide for recontouring of, 73–74, 74f Acrylic resin proximal plates, 25 Akers clasp, 52f, 189f Alginate impressions advantages and disadvantages of, 90, 90f clinical procedures for, 92–94 stock-tray, 192f Altered impressions, 125–128, 126f–128f, 129b Alveolar bone resorption, 48 Alveolar ridge, 169, 170f Amalgam restorations, in abutment teeth, 85 Amalgam stops, 147f Analyzing rod, 69, 157f Anterior edentulous extension defects attachments for, 154–155 illustration of, 155f retained roots for, 154–155 rotational path removable partial dentures for. See Rotational path removable partial dentures. Anterior guidance incisal rests for restoring, 14, 14f in stable occlusion, 76 Anterior palatal connector, 29, 29f, 31 Anterior rests axis of rotation and, 57f description of, 12 on inclined surface, 12 types of, 12f–13f, 13–16

222

Anterior teeth guidance of, 141 mobility of, lingual plate for, 32, 32f Anteroposterior palatal strap, 27–29, 28f–29f, 106f Articulator, 134, 135f Attachments for anterior edentulous extension defects, 154–155 for overlay removable partial dentures using retained roots, 173 for posterior edentulous extension defects, 153f, 153–154 Axis of rotation description of, 50 diagonal placement of, 56 functional movements around, 55, 55f positioning of, for edentulous area support, 51, 51f retainer positioned forward of, 54, 54f

B Balanced articulation, 142 Bars. See also I-bar retainers. as connectors, 27 lingual, 30f, 31–32 Biomechanics, 1 Blocking out, 103–104 Bonded cingulum rests, 16, 16f Bracing clasp assembly for, 44 lingual plates for, 44 Built-up rests, 16, 18–19 Burs, 85f, 86

C CAD/CAM systems. See Computer-aided design/computer-assisted manufacturing systems. Candidiasis, 80f Casts. See also Diagnostic casts; Master casts. design compliance of, 121 digitized, 9f framework adaptation to, 122–123 impressions for, 9

Index

inspection of, 121 most advantageous position on description of, 69–71 elimination of spaces and voids, 69 recording of, 70–71, 71f tripoding of, 70–71, 71f physiologic adjustment of, 124, 125f quality of, 123 surveyed crown, 166–167, 167f verification of, 121 Central incisors, crest-shaped cingulum rest on, 13 Centric occlusion, 3 Centric relation as treatment position, 132 definition of, 136 description of, 3 diagnostic casts in, 76f maximal intercuspation position and, 132 occlusal interferences in, 133f rehabilitation of patient in, 132 Centric relation record, 78f, 197 Chloroform, 123, 123f Cingulum rests bonded, 16, 16f, 182 characteristics of, 12f–13f, 13 crest-shaped, 12f–13f, 13 development methods for, 15–16, 15f–16f full-coverage crown as, 15, 15f for metal-ceramic restorations, 166 partial crown as, 15, 15f pin-retained inlays as, 16, 16f for posterior edentulous extension defects, 153f preparation of, 86, 87f rotational path removable partial dentures, 156f Circular concave rests, 15, 15f, 159 Circumferential retainers biomechanics of, 57 description of, 40–42, 41f infrabulge, 57, 57f suprabulge, 57, 57f Clasp assembly bracing/stability provided by, 44 definition of, 43 functions of, 43 illustration of, 44f passivity of, 45 reciprocation and encirclement provided by, 43–44 Clinical procedures checklist fifth appointment, 209 first appointment, 205 fourth appointment, 208 second appointment, 206 seventh appointment, 210 sixth appointment, 210 third appointment, 207 Clinician attitudes of, 75–76 professional responsibility of, 6 Cobalt-chromium alloy, 118 Combination syndrome, 140, 140f Complete palatal coverage plate, 30f, 31 Complete-denture prosthodontics, 150 Computer-aided design/computer-assisted manufacturing systems, 8, 117–119 Condylar guidance, 140

D

Connectors configurations of, 27 denture base. See Denture base connectors. major. See Major connectors. minor, 7, 33, 33f Continuous rest, 18, 18f–19f CR. See Centric relation. Crest-shaped cingulum rests, 12f–13f, 13 Crown(s) full-coverage as positive cingulum rest, 15, 15f as posterior cingulum rest, 21 partial-coverage as positive cingulum rest, 15, 15f as posterior rest, 21 tooth structure preservation using, 85 surveyed. See Surveyed crown. Crown lengthening, 80–81, 81f Custom impression tray, 94, 95f

D Data acquisition, for digital design and manufacturing, 102, 102f Dental compound, 198, 198f Dental laboratory technician, 97 Dental surveyor, 68f Dentist. See Clinician. Denture base finish lines of, 35, 35f function of, 8 of extension removable partial denture, 48 Denture base connectors design sequence mandibular, 65, 65f maxillary, 62–63, 63f digital design of, 104–105, 112–113 function of, 8, 34 illustration of, 34f major connector and, junction between, 35 mandibular, 65, 65f, 112–113 maxillary, 62–63, 63f record bases attached to, 136 types of, 34f, 35 Design, of removable partial dentures axis of rotation, 56 computer-aided design/computer-assisted manufacturing systems in, 8, 9f digital design and manufacturing. See Digital design and manufacturing. Kratochvil’s contributions to, 48–50 for mandibular defects, 185–189, 185f–189f for maxillary defects, 181–185, 182f–185f principles of, 59, 179 retainers, 56–58, 57f–58f Design sequence mandibular denture base connectors, 65, 65f illustration of, 65f–66f major connectors, 64, 64f–65f minor connectors, 65, 65f occlusal rests, 64 proximal plates, 65, 65f retainers, 66, 66f maxillary denture base connectors, 62–63, 63f

223

D

Index

illustration of, 62f–63f major connectors, 62, 62f minor connectors, 62, 62f occlusal rests, 61 proximal plates, 62, 62f retainers, 63 Diagnosis occlusion evaluation, 76–78 workup for, 76 Diagnostic casts in centric relation, 76f fabrication of, 93f, 93–94 inaccurate, 94 mounting of, 78f, 78–79 for rotational path removable partial dentures, 157f soft or chalky surface of, 94 Diagnostic wax-up, 81, 82f, 141f, 162, 163f Diamonds, 85f Diastemata, 83f Digital design and manufacturing computer-aided design/computer-assisted manufacturing systems, 8, 117–119 data acquisition, 102, 102f design software used in denture base connectors, 104–105 external finish line, 109, 109f finalizing, 110–111, 116 major connectors, 105–107 minor connectors, 107–108 recommended sequence by, 105f, 110f removable partial denture design, 104–109 rests, 107, 107f retainers, 108, 108f–109f sculpt, 108–109 surveying and blocking out, 103–104 wax trimming, 104 mandibular removable partial denture data acquisition, 111 denture base connectors, 112–113 external finish line, 115–116 finish, 115, 115f major connectors, 113 minor connectors, 113–115 rests, 113–114, 114f retainers, 115, 115f surveying and blocking out, 111, 112f wax trimming, 112, 112f phases of, 101f results of, 117f Disclosing wax, 196, 196f

E Embrasure clasp, 40, 42f Encirclement from clasp assembly, 43–44 definition of, 44 Endodontic treatment, 83, 83f Esthetic zone description of, 149 I-bar retainers in, 152, 152f Esthetics in occlusion development, 146f, 146–147 optimizing of, 149–160

224

Extended rests, 17, 19, 19f Extension removable partial denture denture base of, 48 description of, 47–51 design principles for, 50 forces on, 174 illustration of, 2f Kratochvil’s design of, 48–50 lingual design considerations for, 56 lingual view of, 125f mandibular posterior, 49f movement of description of, 48–49 guiding surfaces, 55 retainer position effects on, 54 posterior teeth in, 142 rest position in, 52, 52f retainer design and positioning for, 52–54 support of, 125, 174 unilateral posterior, 49f External finish line, 109, 109f, 115–116 Extracoronal resilient attachment, 153

F Facebow, 134 Facebow transfer record, 134–136, 135f–136f, 198 FDPs. See Fixed dental prostheses. Fifth appointment, 209 Finish lines external, 109, 109f, 115–116 of denture base, 35, 35f First appointment, 75, 205–206 Fixed dental prostheses contraindications for, 1 implant-supported, 2, 3f, 175, 176f indications for, 76 in partially edentulous patients, 4–5, 5f removable partial dentures and, 1, 141f, 149–151 tooth-borne partial denture as, 47 Force occlusal posterior rests and, 16 transmission of, 27 on abutment teeth, 182 on extension removable partial denture, 174 on tooth-borne partial dentures, 48f rest position effects on, 52, 52f Fourth appointment, 208 Framework components of, 7f–8f, 7–8 computer-assisted manufacturing of, 117 denture base and, finish lines between, 35, 35f digitally designed, 117f disclosing media for, 123 for implant restoration, 178 rotational path removable partial denture, 159f try-in, 123–124, 146 Free palatal grafts, 83f Fulcrum lines, 48, 124, 183, 183f, 188 Full-coverage crowns as positive cingulum rest, 15, 15f as posterior cingulum rest, 21

Index

Functional outcomes, 2 Fungal infections, 79, 80f

G General patient evaluation, 75 Gingiva attached, 83f hypertrophy of, 23, 24f recession of, lingual plate for, 32 Gold copings, 172f Gold rouge, 123, 123f, 125f Group function, 142 Guiding surfaces/guide plates abutment teeth and, 26 contours of, 88, 88f description of, 25f, 25–26, 45f framework engagement of, 123 instruments for preparing, 85f most advantageous treatment position determined using, 67 movement of, 26 rest preparation after completion of, 86

H “Hollywood smile,” 151

I I-bar retainers advantages of, 37, 38f, 152 bending of, 193, 193f contraindications for, 40, 40f description of, 1, 37 design principles of, 39f, 39–40 in esthetic zone, 152, 152f horizontal portion of, 39f illustration of, 38f positioning of, 58 retention provided by, 23 spline of, 108 Implants as abutment tooth, 178 crown-root ratio, 177 distal extension removable partial denture support and stability using, 174–175 failure of, 177–178 fixed dental prostheses supported by, 2, 3f, 175, 176f length of, 174 osseointegrated, in partially edentulous patients, 5, 5f prosthodontic procedures, 174–175 removable partial dentures versus, 2f, 2–3 restoration of, using removable partial dentures, 175–178, 176f–177f solitary, 174, 175f support and stability provided by, 178, 178f survival rates for, 174 wide-diameter, 3 Impression(s) alginate advantages and disadvantages of, 90, 90f clinical procedures for, 92–94 stock-tray, 192f

L

altered, 125–128, 126f–128f, 129b conventional materials for, 89–90 custom trays for, 94, 95f, 192f description of, 9, 197f digital methods for, 89 irreversible hydrocolloid, 90, 90f occlusal index, 96, 96f polysulfide, 90–91 polyvinyl siloxane, 90f, 91 pouring of, 93–94, 128 procedures for, 127, 127f surveyed crown, 164f, 164–165 Impression trays custom, 94, 95f, 192f for extension areas, 126, 126f impression accuracy affected by, 89 mandible, 91, 92f maxilla, 91–92, 92f posterior extensions of, 91–92 removal of, 93 selection of, 91–92 Incisal rests description of, 13–14, 14f preparation of, 86, 87f Indirect retainers, 42, 43f Infrabulge retainers buccal mucosa irritation caused by, 87 circumferential, 57, 57f description of, 37–40, 38f–40f I-bar retainers. See I-bar retainers. Inlays, pin-retained, 16, 16f Insertion interarch control, 196–200, 197f–199f intra-arch control, 195–196, 196f intraoral evaluation of, 200 occlusal refinement and equilibration, 199–200 overview of, 195 patient instructions for, 201–203, 202f Interarch control, 196–200, 197f–199f Interocclusal record, 138f Interocclusal space, 131 Interproximal contact, 5f Interproximal surfaces, 87 Intra-arch control, 195–196, 196f Irreversible hydrocolloid impressions, 90, 90f

K Kratochvil, F. J., viii, 1, 8, 23, 47–50

L Laboratory communication and instruction, 97, 98f–99f, 147, 147f Laboratory prescription, 97, 98f–99f Lateral stabilization, anterior palatal strap for, 28f Light-cured composite resin buildup, as rest, 16, 18–19 Lingual bars, 30f, 31–32, 185 Lingual cusps, 144f Lingual plates, 30f, 31–33, 32f–33f, 44, 82, 185 Lingualized teeth, 143f

225

M

Index

M Maintenance, 200–203 Major connectors denture base connectors and, junction between, 35 design sequence mandibular, 64, 64f–65f maxillary, 62, 62f digital design of, 105–107, 113 function/purpose of, 7, 27 mandibular design of, 32–33, 113 lingual bars, 30f, 31–32 lingual plates, 30f, 31–33, 32f–33f selection criteria for, 31–32 types of, 31 maxillary anterior palatal connector, 29, 29f, 31 anteroposterior palatal strap, 27–29, 28f–29f, 106f complete palatal coverage plate, 30f, 31 description of, 27 single palatal strap, 29, 29f U-shaped palatal connector, 29, 29f, 31 rigidity of, 27 Mandible impression trays for, 91, 92f lateral discontinuity defects of, 185, 185f Mandibular canines, incisal rests on, 14 Mandibular defects, partial denture design for anterior, 186–188, 187f lateral, 188f, 188–189 lateral discontinuity defects, 185, 185f Manufacturing, digital. See Digital design and manufacturing. MAP. See Most advantageous position. Master casts framework adaptation to, 122–123 illustration of, 99f inspection of, 96 occlusal index for accuracy confirmations, 96, 96f preparation of, 96–97, 102 tripoding of, 96–97 Masticating surfaces, 142–145, 143f–145f Maxilla candidiasis in, 80 denture base connectors in, 34f, 35 edentulous, 145f impression trays for, 91–92, 92f Maxillary defects, partial denture design for abutment teeth, 182, 183f description of, 184–185 diagnostic casts, 181 fulcrum lines, 183, 183f overview of, 181 Maxillary incisors, crest-shaped cingulum rest on, 13 Maxillary tuberosity, 81f Maxillomandibular record, 198, 198f Maxillomandibular registrations articulator, 134, 135f clinical procedure for, 138, 138f facebow transfer record, 134–136, 135f–136f maximal interposition position, 136 occlusion rims, 137, 137f protrusive record, 138 record bases, 136–137, 137f Maxillomandibular relations, 131

226

Maximal intercuspation position, 131–132, 134, 134f, 136 Mesh denture base connectors, 104 Metal base denture base connectors, 34f Metal proximal plates, 25–26, 26f Minor connectors design of, 33, 33f design sequence mandibular, 65, 65f maxillary, 62, 62f digital design of, 107–108 function of, 7, 33 mandibular, 65, 65f, 113–115 maxillary, 62, 62f proximal plates and, contact between, 125f, 156 rigid, 156f MIP. See Maximal intercuspation position. Most advantageous position description of, 67 factors used to determine, 67–68 guiding surfaces used to determine analysis of, 71 description of, 67, 67f on cast description of, 69–71 elimination of spaces and voids, 69 recording of, 70–71, 71f tripoding of, 70–71, 71f retention areas used to determine analysis of, 72 analyzing rod, 69 description of, 67–68 excessive, 72 lack of, 73 measuring instrument for, 72, 72f survey instrument, 67f, 67–68 tooth preparation guide, 73–74, 74f Mucosa keratinized attached, 82, 83f preprosthetic surgical procedures for, 80, 81f Mutually protected occlusion, 140f, 140–141

O Occlusal forces posterior rests and, 16 transmission of, 27 Occlusal index, 96, 96f Occlusal interferences, 132, 133f Occlusal plane configuration of, 77 discrepancies of, 78, 144 establishing of, 6, 6f in occlusion development, 139–140, 139f–140f posterior molar as disrupter of, 78 tilting of, 135f Occlusal rests description of, 18, 18f, 20f design sequence mandibular, 64 maxillary, 61 spline, 107f Occlusal scheme, 140–142, 140f–142f Occlusal vertical dimension amalgam stops for, 147, 147f

Index

assessment of, 131–132, 132f–133f diagnostic casts in, 76f loss of, 77, 78f, 147 reductions in, 131, 132f Occlusion development considerations for, 138–147, 139f–147f condylar guidance, 140 esthetics, 146f, 146–147 masticating surfaces, 142–145, 143f–145f occlusal plane, 139–140, 139f–140f occlusal scheme, 140–142, 140f–142f occlusal vertical dimension loss prevention, 147 occlusal wear prevention, 147 oral structures, 139 evaluation of, 76–78 mutually protected, 140f, 140–141 plane of. See Occlusal plane. posterior rests for restoration of, 17, 18f refinement and equilibration of, 199–200 Occlusion rims, 137, 137f Onlays, 85 Open lattice denture base connectors, 34f, 105 Opposing arches discrepancies of, 144 evaluation of, 77 loss of integrity, 82 Orthodontic treatment, 83, 83f Osseointegrated implants, in partially edentulous patients, 5, 5f OVD. See Occlusal vertical dimension. Overdenture, implants as abutments for, 5 Overlay removable partial dentures using retained roots abutments of, 171–172 advantages of, 169 attachments for, 173 clinical applications of, 171–173 historical perspectives on, 171 illustration of, 170f recall schedule for patients with, 173

P Palatal connector anterior, 29, 29f, 31 U-shaped, 29, 29f, 31 Palatal defects, 182f Palatal straps anteroposterior, 27–29, 28f–29f, 106f single, 29, 29f Parafunctional activity, 77, 140 Partial palatectomy, 184f Partial-coverage crowns as positive cingulum rest, 15, 15f as posterior rest, 21 tooth structure preservation using, 85 Partially edentulous patients arch restoration and stabilization methods for, 4f–5f, 4–6 clinical findings of, 3f fixed dental prostheses in, 4–5, 5f implants in, 2 osseointegrated implants in, 5, 5f population increases of, 1 removable partial dentures in, 1, 6 restorations in, 4, 5f treatment planning in, 3–4, 51

P

Patient alginate impression instructions for, 92 first appointment with, 75, 205–206 first impression with, 75 general evaluation of, 75 instructions for, 200–201 psychologic factors of, 75 Patient-clinician relationship, 75–76 Periodontal treatment, 82, 83f Pin-retained inlays, as positive cingulum rest, 16, 16f Planning, of removable partial dentures axis of rotation considerations, 50–51 rest position on abutment teeth, 52 Plates as connectors, 27 complete palatal coverage, 30f, 31 lingual, 30f, 31–33, 32f–33f, 44, 82 Pneumatized maxillary sinus, 2f Pocket depths, 82 Polysulfide impressions, 90–91 Polyvinyl siloxane impressions, 90f, 91 Positive rests functions of, 11–12 indirect retainers as, 42, 43f Posterior edentulous extension defects attachments for, 153f, 153–154 I-bar retainers for, 152, 152f Posterior extensions, of impression trays, 91–92 Posterior palatal strap, 27–29, 28f–29f Posterior rests continuous, 18, 18f–19f creating of, 21 forces along long axis of teeth directed by, 18 full-coverage crowns as, 21 functions of, 16–17 in natural tooth structure, 21 partial-coverage crowns as, 21 position of unopposed teeth controlled using, 19 reciprocation and stabilization provided by, 18 requirements of, 20, 20f rigid support provided by, 17 Posts, 111 Preprosthetic surgical procedures, 80–81 Pressure indicating paste, 193, 194f, 195, 196f Professional responsibility, 6 Protrusive record, 138, 199 Proximal plates acrylic resin, 25 benefits of, 24, 25f confirmation of, 122f definition of, 24 design of, 23–26, 24f–26f design sequence mandibular, 65, 65f maxillary, 62, 62f function of, 8, 24, 25f, 58 guiding surfaces/guide planes for description of, 25f, 25–26, 45f, 123 movement of, in function, 55, 55f labial extension of, 26, 26f metal, 25–26, 26f minor connectors and, contact between, 125f, 156 on maxillary canine, 146f Psychologic factors, 75

227

R

Index

R Rapid prototyping system, 9, 117 Reciprocation, from clasp assembly, 43–44 Record bases, 136–137, 137f Relines, 203 Removable partial dentures elements of, 4 fixed dental prostheses and, 1, 141f, 149–151 implants versus, 2f, 2–3 in partially edentulous patients, 1 rests of. See Rest(s). Resilient attachments, 153, 174 Resin-bonded cingulum rests, 16 Rest(s) anterior axis of rotation and, 57f description of, 12 on inclined surface, 12 types of, 12f–13f, 13–16 as axis of rotation, 51 built-up, 16, 18–19 canine, maxillary, 61 cingulum bonded, 16, 16f, 182 characteristics of, 12f–13f, 13 crest-shaped, 12f–13f, 13 development methods for, 15–16, 15f–16f full-coverage crown as, 15, 15f for metal-ceramic restorations, 166 partial crown as, 15, 15f pin-retained inlays as, 16, 16f for posterior edentulous extension defects, 153f preparation of, 86, 87f rotational path removable partial dentures, 156f circular concave, 15, 15f, 159 continuous, 18, 18f–19f definition of, 7 extended, 17, 19, 19f function of, 7, 11 incisal description of, 13–14, 14f preparation of, 86, 87f indirect retainers as, 42, 43f mandibular removable partial denture, 113–114, 114f molar, maxillary, 61 occlusal, 18, 18f, 20f on inclined surface, 12, 12f overview of, 11 placement of abutment teeth affected by, 52 design considerations for, 52, 52f illustration of, 12f positioning of, 7, 11f, 58 positive functions of, 11–12 indirect retainers as, 42, 43f posterior continuous, 18, 18f–19f creating of, 21 forces along long axis of teeth directed by, 18 full-coverage crowns as, 21 functions of, 16–17 in natural tooth structure, 21

228

partial-coverage crowns as, 21 position of unopposed teeth controlled using, 19 reciprocation and stabilization provided by, 18 requirements of, 20, 20f rigid support provided by, 17 premolar, maxillary, 61 rotational path removable partial dentures, 155–156, 156f seats for, 122, 122f Restorations amalgam, in abutment teeth, 85 in partially edentulous patients, 4, 5f Retained roots anterior edentulous extension defects restored using, 154–155 overlay removable partial dentures using. See Overlay removable partial dentures using retained roots. Retainers axis of rotation and, 54, 54f circumferential biomechanics of, 57 description of, 40–42, 41f infrabulge, 57, 57f suprabulge, 57, 57f definition of, 37 design of description of, 52–54, 57f–58f, 57–58 sequence for, 63, 66, 66f direct, 37–43, 38f–43f embrasure clasp, 40, 42f for extension removable partial denture, 52–54 function of, 8 I-bar. See I-bar retainers. indirect, 42, 43f infrabulge buccal mucosa irritation caused by, 87 circumferential, 57, 57f description of, 37–40, 38f–40f I-bar retainers. See I-bar retainers. mandibular design of, 115 design sequence, 66, 66f maxillary design sequence, 63 positioning of, 52–54, 122f suprabulge biomechanics of, 57, 57f description of, 40–42, 41f–42f types of, 37 wrought wire, 58, 58f, 192 Retention areas analysis of, 72 analyzing rod, 69 confirmation of, 97f description of, 67–68 excessive, 72 lack of, 73 measuring instrument for, 72, 72f preparation of, 150 Rigid attachments, 153 Rotational path removable partial dentures anterior portion of, 158f anteroposterior, 159 elements of, 155–156 framework for, 159f indications for, 155

Index

laboratory procedures for, 157–159, 157f–159f maxillary diagnostic casts for, 157f rests for, 155–156, 156f Rouge, 123, 123f, 125f RPA concept, 57, 57f RPDs. See Removable partial dentures. RPI design description of, 23 illustration of, 44f objectives of, 23 principles of, 23, 25f Rubber base impressions. See Polysulfide impressions.

S Second appointment, 206 Selective deposition modeling, 117 Selective laser melting, 9, 117, 118f Selective laser sintering, 117 Sequence(s). See Design sequence; Treatment sequence. Seventh appointment, 210 Single palatal strap, 29, 29f Sixth appointment, 210 SLA. See Stereolithography. SLM. See Selective laser melting. SLS. See Selective laser sintering. Smile design and esthetics, 150–151 Soft tissue hypertrophy, 80 Spline, 105, 107f Sprues, 111 Standard triangulation language file, 9, 102 Stereolithography, 117 STL file. See Standard triangulation language file. Straps anteroposterior palatal, 27–29, 28f–29f, 106f as connectors, 27 single palatal, 29, 29f Supporting structures, 6 Suprabulge retainers biomechanics of, 57, 57f description of, 40–42, 41f–42f Survey instrument, 67f, 67–68 Surveyed crown clinical and laboratory procedures for casting, 166–167, 167f cementation, 167, 167f diagnostic wax-up, 162, 163f final impressions, 164f, 164–165 preparation guides, 163f provisional restoration template, 162, 163f teeth preparation, 162–164, 163f–164f wax-up, 165–166, 166f indications for, 161 objectives of, 161 treatment sequence for, 161–162

W

T Teeth abutment. See Abutment teeth. infraerupted, 145f lingualized, 143f mobility of, 82 preparation of, for surveyed crowns, 162–164, 163f–164f preprosthetic surgical procedures for, 80 supraeruption of, 131, 132f, 139 tipped, 145f Third appointment, 207 Three-dimensional printing, 117 Tooth modifications, 85–88, 85f–88f Tooth preparation guide, 73–74, 74f Tooth-borne partial dentures description of, 47 forces on, 48f Tooth-tissue junction description of, 23, 26, 33 metal casting coverage of, 55 Tori, 6f, 80 Treatment planning, 3–4, 51 Treatment position centric relation as, 132 maximal intercuspation position as, 134, 134f, 136 most advantageous. See Most advantageous position. types of, 133 Treatment removable partial dentures, 77, 77f, 133f, 191–193, 191f–193f Treatment sequence abnormal or inflamed soft tissues of edentulous denture-bearing surfaces, 79–80 diagnostic wax-up, 81, 82f endodontic treatment, 83, 83f orthodontic treatment, 83, 83f overview of, 79 periodontal treatment, 82, 83f preprosthetic surgical procedures, 80–81 recording the final plan of treatment, 83 surveyed crown, 161–162 tooth modifications, 85f–88f, 85–88 Tripoding of design cast, 70–71, 71f of master cast, 96–97 Try-in, of framework, 123–124

U Undercut gauge, 166, 167f U-shaped palatal connector, 29, 29f, 31

V Vertical dimension of rest, 132

W Wide-diameter implants, 3 Wrought wire retainers, 58, 58f, 192

229

University in 1990 and completed two years of general practice residency and two years of prosthodontics residency training at National Taiwan University Hospital. She subsequently completed her advanced prosthodontics training in 1997 and maxillofacial prosthetics training in 1998 at the University of California, Los Angeles (UCLA). Dr Chang is currently a Clinical Professor in the Division of Advanced Prosthodontics at the UCLA School of Dentistry. Dr Chang is a Diplomate of the American Board of Prosthodontics.

Chang / Orellana / Beumer

Ting-Ling Chang, DDS, received her DDS from National Taiwan

Daniela Orellana, DDS, received her DDS from the Universidad

California, San Francisco in 1967. He subsequently completed his postgraduate training in oral medicine there in 1970 before continuing with postgraduate training in prosthodontics at UCLA. He is a Distinguished Professor Emeritus in the Division of Advanced Prosthodontics at the UCLA School of Dentistry and was formerly chair of that division. Dr Beumer has published extensively in the scientific literature, including Maxillofacial Rehabilitation (Quintessence, 2011) and Fundamentals of Implant Dentistry (Quintessence, 2015–2016).

REMOVABLE PARTIAL DENTURES

John Beumer III, DDS, MS, received his DDS from the University of

Kratochvil’s Fundamentals of

Andrés Bello, Viña del Mar, Chile, in 2008. She subsequently completed a postgraduate training program in prosthodontics at the University of Michigan in 2013 and an implant fellowship at Louisiana State University in 2014. Dr Orellana is currently a Clinical Assistant Professor in the Division of Advanced Prosthodontics at the UCLA School of Dentistry.

Kratochvil’s Fundamentals of

REMOVABLE PARTIAL DENTURES Ting-Ling Chang, DDS Daniela Orellana, DDS John Beumer III, DDS, MS