High-Strength Ceramics: Interdisciplinary Perspectives [1 ed.] 9780867156393, 0867156392

Table of contents :
Ferencz_Frontmatter
Ferencz_Ch01
Ferencz_Ch02
Ferencz_Ch03
Ferencz_Ch04
Ferencz_Ch05
Ferencz_Ch06
Ferencz_Ch07
Ferencz_Ch08
Ferencz_Ch09
Ferencz_Ch10
Ferencz_Ch11
Ferencz_Ch12
Ferencz_Ch13
Ferencz_Index

Citation preview

Interdisciplinary Perspectives Edited by

Jonathan L. Ferencz, dds Clinical Professor of Prosthodontics New York University College of Dentistry Private Practice in Prosthodontics New York, New York

Nelson R.F.A. Silva, dds, msc, phd Professor Department of Restorative Dentistry Federal University of Minas Gerais Belo Horizonte, Brazil

José Manuel Navarro, dds, ms Private Practice in Periodontics, Prosthodontics, and Implant Dentistry Brånemark Osseointegration Center Spain Las Palmas, Spain

Quintessence Publishing Co, Inc Chicago, Berlin, Tokyo, London, Paris, Milan, Barcelona, Beijing, Istanbul, Moscow, New Delhi, Prague, São Paulo, Seoul, Singapore, and Warsaw

Library of Congress Cataloging-in-Publication Data High-strength ceramics : interdisciplinary perspectives / edited by Jonathan L. Ferencz, Nelson R.F.A. Silva, Jose M. Navarro. p. ; cm. Includes bibliographical references and index. ISBN 978-0-86715-639-3 (hardcover) I. Ferencz, Jonathan L., editor. II. Silva, Nelson R. F. A., editor. III. Navarro, Jose M., editor. [DNLM: 1. Dental Porcelain. 2. Biocompatible Materials. 3. Dental Prosthesis--methods. 4. Esthetics, Dental. 5. Oral Surgical Procedures--methods. 6. Reconstructive Surgical Procedures--methods. WU 190] RK655 617.6’95--dc23 2014007961

5 4 3 2 1

© 2014 Quintessence Publishing Co Inc Quintessence Publishing Co Inc 4350 Chandler Drive Hanover Park, IL 60133 www.quintpub.com 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: Bryn Grisham Design: Ted Pereda Production: Sue Robinson Printed in China

Contents Foreword  vii Preface  viii Contributors  ix

1 2 3 4 5 6

Challenges in Engineering and Testing of Bioceramics  1 Paulo G. Coelho and Timothy G. Bromage

The Role of Industry in Developing New Ceramics  

19

George W. Tysowsky and Robert Gottlander

Practice-Based Research on the Effectiveness of Ceramics   33 Van P. Thompson and Kenneth A. Malament

Posterior Partial-Coverage Restorations: Inlays and Onlays   47 Petra C. Guess

Individual Ceramic Crowns for Teeth  63 Estevam A. Bonfante and Stefano Gracis

Ceramic Veneers  101 Susana Morimoto, Marcelo A. Calamita, Christian Coachman, and Galip Gürel

7 8 9 10 11 12 13

Fixed Dental Prostheses for Anterior and Posterior Teeth  119 Irena Sailer

Ceramic Applications to Restore Implants  149 Joerg R. Strub and Michael V. Swain

Cementation Procedures for Ceramics  173 Matthias Kern

Monolithic Zirconia Complete-Arch Reconstructions  189 Esther Grob and Mario Sisera

Successful Ceramic Application on Various Substructures  205 Murilo Calgaro, Victor Clavijo, Rogerio Goulart da Costa, and Willy Clavijo

Ceramic Dental Implants  235 Ralf Kohal and Eric Van Dooren

Digital Workflow in Reconstructive Dentistry  261 Wael Att and Michael Girard

Index  279

Foreword All-ceramic restorations have had a significant impact on dental practice first and foremost because of their mechanical resistance and their esthetic properties. High-strength ceramics are now an integral part of everyday practice. The options for all-ceramic restorations offered to patients have essentially been narrowed down to a small group of materials, and this book discusses those materials and provides detailed clinical protocols that will enable the reader to achieve a higher level of predictability in dental practice. This book also provides an in-depth review of techniques that were traditionally restricted to metal technology: layered or monolithic single restorations, short- and long-span fixed partial dentures, Maryland bridges, implant abutments and restorations, and ceramic implants. The range of applications covered in this text makes it a comprehensive reference on all-ceramic technology. The techniques presented herein represent the wealth of clinical and laboratory experience of the panel of international experts gathered by Dr Ferencz, Dr Silva, and Dr Navarro. This select group of master clinicians and ceramists exemplifies many years of practical knowledge and provides invaluable historical perspectives. What makes this textbook special, however, is that the authors base their discussions on fundamental research of the biologic and mechanical properties of ceramic materials as well as on the accepted consensuses documented by thorough literature reviews. Indeed, the reader will enjoy the evidence-based format of this text presented in a logical and attractive manner and the very effective connection of fundamental knowledge to clinical application. The use of high-strength ceramics as an alternative to porcelain-fused-to-metal restorations has fully matured in response to several factors: patient esthetic demands, the availability of CAD/CAM technology in dentistry, and the high cost of precious metals. Thus, zirconia and lithium disilicate offer specific properties that are illustrated throughout the text to allow an educated choice between these materials, their indications and contraindications, and their respective advantages and disadvantages. The editors should be commended on putting together such a diversified panel of clinicians, dental ceramists, and researchers to produce a major contribution to the field of esthetic dentistry. It will have a definitive impact on the dental community. Gerard J. Chiche, dds Thomas P. Hinman Endowed Professor Director of the Center of Esthetics and Implant Dentistry College of Dental Medicine Georgia Regents University Augusta, Georgia

vii

Preface The inspiration for this book began about a decade ago when New York University (NYU) College of Dentistry built one of the strongest biomaterials teams in the world. Under the leadership of former NYU Dean Michael Alfano, Diane Rekow and Van Thompson were recruited to come to NYU and create a biomaterials research group that focused on answering some basic questions about the behavior of ceramic materials. Corporate support and participation became a hallmark of this successful research endeavor, and many leading academics and researchers around the world wanted to spend time learning and participating in the many ongoing research projects. After a few years, it also became obvious that clinicians and dental laboratory technicians had valuable experiences and ideas to share with the group, because they used these materials on a daily basis. It became apparent that only when we shared our challenges and ideas for improvements could major breakthroughs occur. The idea of writing a textbook that focused on this interdisciplinary collaboration started in the summer of 2011. The three of us believed that a text could be created that focused on the collaboration of biomaterials science, industry, clinical, and dental laboratory perspectives. We envisioned that most chapters could combine perspectives by asking biomaterials experts to work with clinicians to write chapters. We selected many of the same individuals who came to NYU to participate in research projects during this time period. From this idea emerged a text that features contributors from nearly every aspect of dentistry representing 4 continents and nearly 12 countries. This text is intended for dental students, dental technicians, dental faculty, and practicing dentists alike. We believe that to be a successful user of ceramic materials today, one must have an understanding of basic biomaterials. We thoroughly hope that this text introduces readers to new ceramic materials, highquality clinical procedures, and laboratory techniques with an underpinning of biomaterials science. As editors, we are extremely grateful to the contributors who have so graciously lent their time and expertise to this project, to our publishers at Quintessence, and to our families who have encouraged and inspired us along the way.

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Running A Head

Contributors Wael Att, DDS, Dr Med Dent, PhD

Willy Clavijo, CDT

Professor and Director of Postgraduate Studies Department of Prosthodontics School of Dentistry Albert-Ludwigs University Freiburg, Germany

Dental Technician Curitiba, Brazil

Estevam A. Bonfante, DDS, MSc, PhD Assistant Professor Department of Prosthodontics Bauru College of Dentistry University of São Paulo Bauru, Brazil

Timothy G. Bromage, PhD Professor and Director of Hard Tissue Research Unit Department of Biomaterials and Biomimetics New York University College of Dentistry New York, New York

Marcelo A. Calamita, DDS, MSD, PhD Private Practice São Paulo, Brazil

Murilo Calgaro, CDT Dental Technician Curitiba, Brazil

Victor Clavijo, DDS, MS, PhD Professor Advanced Program in Implantology and Restorative Dentistry Implante/Perio Institute São Paulo, Brazil

Christian Coachman, DDS, CDT Private Practice in Esthetics São Paulo, Brazil

Paulo G. Coelho, DDS, PhD Associate Professor of Biomaterials and Biomimetics Director for Research Department of Periodontology and Implant Dentistry New York University College of Dentistry New York, New York Affiliated Faculty Division of Engineering New York University Abu Dhabi Abu Dhabi, United Arab Emirates

Jonathan L. Ferencz, DDS Clinical Professor of Prosthodontics New York University College of Dentistry Private Practice in Prosthodontics New York, New York

Michael Girard, RDT President and CEO Modern Dental USA  Troy, Michigan

Robert Gottlander, DDS Vice President of Global Prosthetic Solutions Henry Schein Melville, New York

ix

Rogerio Goulart da Costa, DDS, MS

Matthias Kern, DMD, PhD

Professor of Dental Prosthodontics Department of Prosthetic Dentistry Federal Institute for Education, Science and Technology of Paraná Curitiba, Brazil

Professor and Chair Department of Prosthodontics, Propaedeutics and Dental Materials School of Dentistry Christian-Albrechts University at Kiel Kiel, Germany

Stefano Gracis, DMD, MSD Private Practice in Prosthodontics Milan, Italy Esther Grob, CDT Arteco Dental Technology Zürich, Switzerland

Petra C. Guess, DDS, Dr Med Dent, PhD Associate Professor Department of Prosthodontics School of Dentistry University Medical Center Freiburg Freiburg, Germany

Galip Gürel, DDS, MSD Visiting Professor New York University College of Dentistry New York, New York Private Practice Istanbul, Turkey

x

Ralf Kohal, DMD, Dr Med Dent, PhD Professor Department of Prosthodontics School of Dentistry University Medical Center Freiburg Freiburg, Germany

Kenneth A. Malament, DDS, MScD Clinical Professor Tufts University School of Dental Medicine Boston, Massachusetts Susana Morimoto, DDS, MSD, PhD Professor of Graduate Program School of Dentistry Ibirapuera University São Paulo, Brazil

José Manuel Navarro, DDS, MS Private Practice in Periodontics, Prosthodontics, and Implant Dentistry Brånemark Osseointegration Center Spain Las Palmas, Spain

Irena Sailer, Prof Dr Med Dent

Michael V. Swain, BSc, PhD

Chair Division of Fixed Prosthodontics and Biomaterials University Center for Dental Medicine University of Geneva Geneva, Switzerland

Professor of Biomaterials Faculty of Dentistry University of Sydney Sydney, Australia

Nelson R.F.A. Silva, DDS, MSc, PhD Professor Department of Restorative Dentistry Federal University of Minas Gerais Belo Horizonte, Brazil

Professor Department of Biomaterials, Biomimetics, and Biophotonics King’s College London Dental Institute Guys Hospital London, England

Mario Sisera, CDT

George W. Tysowsky, DDS, MPH

Arteco Dental Technology Zürich, Switzerland

Vice President of Technology Ivoclar Vivadent Amherst, New York

Joerg R. Strub, DMD, Dr Med Dent, Dr hc, PhD Professor and Chair Department of Prosthodontics Associate Dean for Clinical Affairs School of Dentistry University Medical Center Freiburg Freiburg, Germany

Van P. Thompson, DDS, PhD

Eric Van Dooren, DDS Private Practice Antwerp, Belgium

xi

Challenges in Engineering and Testing of Bioceramics

1

Paulo G. Coelho, dds, phd Timothy G. Bromage, phd

The clinical success of modern dental ceramics is predicated on a number of factors, such as the initial physical properties of these brittle materials, the fabrication and clinical procedures that unavoidably damage them, and the oral environment. Examining how these factors influence clinical performance has involved investigators from the dental, ceramics, and engineering communities. This chapter will first summarize the rationale behind engineering of biomimetic ceramic dental restorative materials and then critically describe the basic methodology and analytic results reported in recent literature.

Properties of Enamel and Dentine as Objectives for Bioceramic Development Tooth structure has several important properties that are desirable to replicate in biomimetic bioceramics. Paramount is mechanical efficacy, largely a function of tooth crown morphology, enamel thickness and microstructure, and root structure. Related to this is proprioception, the internal perceptions that discourage the individual from transmitting excessive loads through enamel to dentine and the supporting structures and thereby reduce the probability of catastrophic fracture. An additional consideration is the porosity of healthy enamel. Further, it is fair to say that people who take their oral care seriously enough to be receiving bioceramic implants will also be concerned with the esthetics of their natural teeth. Finally, if properly managed, enamel surfaces will support a biofilm characteristic of healthy oral flora.

1

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Challenges in Engineering and Testing of Bioceramics Fig 1-1 Linearly polarized reflected light image of human first molar mesiobuccal cuspal enamel cut in longitudinal section, obtained using a Leica MZ-APO stereo zoom microscope (Leica Microsystems). HunterSchreger bands appear as alternating light and dark bands that relate to enamel prisms occurring, respectively, in and out of the plane of section. Hunter-Schreger bands course from the EDJ outward, dissipating toward the outer surface enamel.

Mechanical efficacy Before the mechanical efficacy of enamel is considered, a bit of background is necessary. Secretory ameloblasts secrete proteins responsible for the nucleation, regulation, and growth of enamel crystallites. During early stages of amelogenesis, enamel contains a high proportion of protein, which is rapidly removed by the enzymatic activities of these same cells, eliminating the vast proportion of protein and facilitating further growth in diameter of long and slender crystallites. Enamel ultimately reaches roughly 95% by weight carbonated hydroxyapatite [Ca10(PO4)6(OH)2], 3% to 4% protein, and 1% to 2% water—a composition yielding unique mechanical properties that enable enamel to last the lifetime of most individuals. Reviews of the molecular, cellular, and physicochemical processes involved in enamel matrix formation, ion transport, and regulation of extracellular pH are available in the literature.1,2 The mechanical efficacy of a tooth—a natural functionally graded material—is a function of hierarchy of scale and material properties. Macroscopically, the mechanical loading and maximum stress distribution experienced by a tooth and its surrounding structures depend on the incursive, intercuspal, and excursive contacts that characterize an individual’s masticatory function.3 Finite element analysis of the deformation of the jaw reveals how stress and strain vary as a result of highly specific occlusal collisions when a load is transmitted through the enamel and coronal dentine, the roots, and periodontal ligament to the surrounding bone.4 Thus, an evaluation of opposing con2

tact facets must remain among the highest of priorities for customized bioceramic tooth design if an individual’s functional balance is to be achieved and maintained. At a level down, the hierarchy of scale is enamel thickness. The maximum cuspal thickness of human first molars is about 2 mm.5 Among primates, humans are deemed to have thick enamel, considerably thicker than, say, that of a gorilla or chimpanzee. This thick enamel reveals the evolutionary legacy of modern humans, which included a diet of hard and tough food items6 whose mechanical properties far exceed those of foods eaten by industrialized people today. The substance of enamel, harboring as it does compositional and microstructural characteristics that inhibit the initiation and propagation of cracks (which will be discussed presently), makes thick enamel an important geometric property that has a bearing on masticatory mechanics. Through the substance of human enamel and the immediately underlying dentine there exist compositional variations regarded as “graded structures.” Material density transitions (ie, of mineralization) that are spatially sharp may engender differential strain on either side of the border between phases, and such transitions are prone to fracture and separation on physical or thermal stress. Human molar enamel reveals an astonishing gradation in both hardness and elastic modulus, diminishing from outer surface enamel toward the enamel-dentine junction7 (EDJ). For instance, as stress reaches the EDJ, the enamel becomes more compliant. The EDJ is also graded on the dentine side; dentine is comparatively hard and stiff nearest the EDJ, with values

Properties of Enamel and Dentine as Objectives for Bioceramic Development

Desmosome Terminal bar/web

Mitochondria

Coated vesicle Dense granule

t en vem mo asts of l ion elob ect m Dir of a

Fig 1-2  Diagram of an ameloblast. Note the asymmetry in matrix protein secretion and crystallite mineralization characteristic of human enamel formation. (Reprinted from Boyde12 with permission.) SI, stratum intermedium; rER, rough endoplasmic reticulum; TW, terminal web.

SI

Stratum intermedium

Tight junction Nucleus

Lipid Golgi

TW Tomes process

rER Matrix protein

like those of the adjacent enamel, and becomes gradually softer and yielding several hundred microns away from the EDJ.8–10 A remarkable feature of mammalian enamel, particularly of those species whose diet comprises hard and tough food items, is a developmental pattern referred to as enamel decussation. In this pattern, enamel prisms, or rods, cross paths at some angle and are visible as Hunter-Schreger bands by light microscopy11 (Fig 1-1). Secretory ameloblasts displace themselves outward from the EDJ in a direction opposite to the secretory face of their Tomes process (Fig 1-2). A group of perhaps 10 cells thick takes a gently sinusoidal course outward at some angle (about 45 to 55 degrees, depending on the species) to an adjacent group of cells.13 In modern human teeth, Hunter-Schreger bands are always present in the inner enamel; they sometimes reach the outer enamel but sometimes dissipate before reaching the surface11 (Fig 1-3). Importantly, the discontinuity of prism orientation between groups of prisms functions to resist crack propagation.14 Because occlusal loads transmit through a stiff outer shell of enamel to a relatively compliant landscape of dentine, peak tension is generated in the lowermost enamel as it tends to push inward at the EDJ. This establishes the presence of Hunter-Schreger bands, particularly at inner enamel locations, as an anti–crack-propagating adaptation in humans and other species.15,16

Prism boundary

Enamel is also rich with additional discontinuities at the micron and nanometer scales,12,17 the complexities of which require some detailed explanation. Human enamel prisms are approximately the size of an ameloblast, roughly 5 µm in diameter (Fig 1-4). They are characterized by a specific packing of crystallites, roughly 20 to 40 nm in diameter (Fig 1-5), formed on protein scaffolds synthesized from two locations. The first location is immediately below the junctional complexes (see Fig 1-2), which ties the cells together and isolates their distal ends to a secretory compartment. This semicircular periphery forms a rim that, by its height difference, defines an inner pit. This so-called interpit enamel is formed of crystallites whose crystallographic c-axes run parallel to the prism orientation and effectively constitute a continuous phase from the EDJ to the outer surface enamel (Fig 1-6). The open end of this collar of enamel is oriented in the cervical direction. The second location is the secretory pole of the cell, called the Tomes process, which has an asymmetry, becoming flush with the interpit enamel on its cervical surface (ie, open end of the collar). Matrix secretions from only that portion of the Tomes process that faces the developing enamel surface form the floor of the pit and its gently sloping surface leading up to the top of the pit and abutting interpit enamel, which is formed by cells immediately cervical to it.

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Challenges in Engineering and Testing of Bioceramics

a

b

c

d

Fig 1-3  (a) Image of etched human mesiobuccal enamel in cross section 100 µm deep to the outer surface enamel, obtained using a backscattered electron detector (BSE) in the EVO 50 (Carl Zeiss) scanning electron microscope; (SEM; uncoated at 15-kV, 600-pA current, 50-Pa pressure). (b) BSE-SEM image shown in (a), after image analysis software (QWin 550, Leica Microsystems) was used to measure and color code enamel prism aspect ratios; (inset) aspect ratio distribution. (c) BSE-SEM image of human mesiobuccal enamel in cross section 800 µm deep to the outer surface enamel. (d) BSE-SEM image in (c), after it was subjected to image analysis for measuring and color coding enamel prism aspect ratios. Note the increase in enamel decussation and (inset) aspect ratios in midcuspal enamel. Field widths = 500 µm.

Because enamel crystallographic c-axes orient perpendicularly to the surface of the Tomes process, variable crystallite orientations exist within the pit; crystallites are parallel with the prism at its coronally located back wall and become cervically rotated through roughly 45 degrees at the junction of the prism with the interpit enamel formed 4

by cells cervical to it (see Fig 1-6). This complex arrangement renders discontinuities in enamel crystallites, represented by acute abutments between crystallites of different orientation, along which nanoscopic cracks may form, potentially generating sliding planes. Enamel crystallite discontinuities occur both between interpit and pit enamel

Properties of Enamel and Dentine as Objectives for Bioceramic Development

Fig 1-4  BSE-SEM image of etched human superficial mesiobuccal cuspal enamel (uncoated at 15-kV, 600-pA current, 50-Pa pressure). Brightness relates to mineralization density; brighter regions are more densely mineralized. Field width = 150 µm.

Fig 1-5  BSE-SEM image of needlelike enamel crystallites along etched enamel prisms, which are seen as the fine fibrillar fabric in this image (uncoated at 15kV, 100-pA current, 30-Pa pressure). Field width = 50 µm.

Head region Tail region Enamel crystallites

Secretory territories

Discontinuity

Fig 1-6  Cross sections of human enamel prisms reveal a confluence of crystallites packed into the shape of a rounded head region (rod) and, directly cervical to this, its interrod enamel (tail). This structure has been likened to a keyhole. Additional section planes are represented by black arrows overlain onto the prism cross-section diagram, and these arrows point to their respective crystallite orientation diagrams. A variety of enamel crystallite discontinuities can be observed (eg, see lower right diagram). (Adapted from Boyde12 with permission.)

arising from the same cell and between the pit enamel of one cell and the interpit enamel formed by cervically adjacent cells. Because formation of tooth structure takes place in such a complex and organized fashion, it is expected that subtle variations in ameloblast secretory rate and mineralization chemistry also occur over daily and near-weekly

time frames in human enamel, producing incremental lines in enamel12,18 (see dark bands along prisms in Fig 1-2). This compositional variability combines with subtle changes in crystallite orientation to produce another order of discontinuity. In addition, differences in mineralization density— low mineral density in interpit enamel and the central region of pit enamel contrasted with relatively higher mineral 5

1

Challenges in Engineering and Testing of Bioceramics density in the peripheral pit floor—generate another discontinuity (see Fig 1-4). Nanoscopic fractures will always prefer paths of least resistance and follow the discontinuities that have been described.19 Because of their heterogeneity, and because of the abutments created by discontinuities, such cracks tend to be arrested. These microdiscontinuities, combined with the macrodiscontinuity of decussating enamel, have the effect of situating crack-arresting abutments in all 360degree spherical rotations, making enamel a supremely competent material under biologic loading stress. Because enamel crystallites are hexagonal in cross section, their flat faces have the potential to slip at nanometer and subnanometer scales, without fracture. This permits a small degree of strain in bulk enamel under load. This load is taken up mostly by crystallites but, as in many natural materials, is transferred to protein-laced shear zones between crystals,20 which absorb stress21 and generate tethering ligaments between separating crystals.22 Nanoscopic cracks that do emerge are arrested at short lengths, effectively toughening the material. A brief mention of the importance of water in hard tissues is warranted. Very little research has been undertaken into the presence and location of mobile and bound (ie, structural) water in bone and tooth materials. Research on enamel is scarce, but the presence of bound water, presumably located within crystallite structure, and of mobile water has been documented.23,24 That mobile and bound water in bone have been shown to relate to both the modulus of elasticity and strength of the tissue, respectively, is evidence of water’s critical role in maintaining the mechanical integrity of enamel.25 Indeed, evidence from studies of nanoindentation creep in enamel demonstrates that dehydrated enamel has a significantly lower energy absorption than does sound enamel.21 Although water represents only 1% to 2% by weight of enamel, it has a profound influence on mechanical efficacy. Finally, the wear characteristics of human enamel should be mentioned. Prism orientation is important for both resistance to load26 and functional wear resistance.27 There is a fascinating diversity in prism orientation among mammals, some of which is designed to generate differential wear; among humans, prisms and their contained crystallites arrive at rather steep angles to wear facets, providing excellent wear resistance.17,28 This adaptation is clearly only one of a number of solutions taken by members of the human family,29 one that depends on the dietary characteristics pertinent to niches occupied by early human species. From the perspectives of engineering of materials and narrow mechanical efficacy, achieving biomimetic designs in bioceramics is an ultimate challenge and would ideally include engineering of materials that are three-dimensionally

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functionally graded at multiple length scales. In essence, enamel and dentine are hierarchically complex, with functionally graded structures of one length scale within functionally graded structures of another length scale, these having been finely tuned over millions of years of evolution. Current technologies enable a partial assembly of multiscale functionally graded structures. When materials assembly is possible at multiple length scales in three dimensions, the addition of components that will mimic the structural role of enamel’s biologic components will be necessary to mimic enamel’s unique ability to hinder crack propagation. Finally, even if biomaterials scientists moving forward are able to design materials that function mechanically like enamel, proprioception, esthetics, and other biocompatibility considerations are as important as the mechanical behavior of a newly developed material.

Proprioception Mechanoreceptors in some tendons and muscles of mastication, as well as in the periodontal ligament itself, provide feedback on jaw position and bite forces. Interruption to the normal rate and force applied during mastication may trigger an involuntary proprioceptive response. With respect to teeth, this response is highly adaptive, allowing the masticatory system to recoil from accidental application of an excessive and concentrated load capable of causing catastrophic fracture (eg, as when a person bites down on a grain of sand). That receptors in the periodontal ligament are particularly important in this regard is suggested by the significantly blunted response of osteointegrated dental implants to occlusal forces.30 Thus, a bioceramic biomimetic design must include some mechanism to provide natural feedback that will elicit avoidance of occlusion at unnaturally high forces, or the materials used must include safety factors well beyond those of natural teeth.

Porosity The mobile water in enamel passes through a system of small pores, representing 6% by volume of the space thought to reside in the centers of prism heads, and larger pores, representing 0.3% by volume at prism boundaries.31 Beyond the potential importance of this pore space and its contained water for the mechanical efficacy of enamel (as already discussed), the function of these spaces is also to participate in the diffusion of ions in and out of the tooth, such as has been reported for calcium 45 ions32 and phosphorus 32 ions.33 That enamel has this permeability indicates that the constant and relatively rapid exchange of ions in and out of enamel is a normal process and one that has some adap-

Properties of Enamel and Dentine as Objectives for Bioceramic Development tive value. The nature of this adaptation has not received attention by enamel biologists, however. Upsets of oral pH can perturb this process—for example, reduction in pH can lead to dissolution of enamel crystallites and caries formation34—but this is an infectious disease, not an adaptation. Conversely, the same mechanism may be useful for enamel healing under different conditions. The most sensible explanation for the porosity of enamel is that diffusion processes facilitate the self-healing of fractures. It is possible that the precipitation of organic and/or mineral ions behind crack tips increases the strength and toughness of enamel, retarding crack propagation, as has been described for mica and silicate glass in aqueous solution.35 Conceivably, fractured enamel crystallite surfaces could seed mineral ions that diffuse through the enamel to bridge cracks. That calcium phosphate is supersaturated in saliva at neutral pH36 is a rather significant piece of circumstantial evidence in support of this theory. However, remineralization phenomena are understood only from purely physicochemical kinetics and epidemiologic studies involving fluoride supplementation. Caries research, while focused on topical fluoride-doped toothpastes and varnishes, has nevertheless demonstrated that it is only by frequent exposure to fluoride and incorporation of calcium fluorapatite into enamel that the ion transport through enamel pores can be explained.37 Ideally, though not necessarily through the same process as enamel self-healing, a biomimetic bioceramic would ultimately be able to self-heal. This pursuit is an area of active research in the field of materials science and engineering, incorporating all classes of materials.

Esthetics Enamel is not normally brilliant white, in stark contrast to the claims of the monumental dental bleaching industry, fed by demands of people who want such teeth.38 Long wavelengths in the warm colors predominantly reflect from dentine, while shorter wavelengths in the blue range are contributed by enamel.39,40 This arrangement, combined with a linear increase of light from enamel surface reflections in the visible spectrum, renders what is more or less a white or off-white tooth appearance. That light reflections from dentine contribute to tooth color is an indication that enamel efficiently transmits light through its substance, as has been documented.41 It has been suggested that enamel crystallite surfaces are responsible for the short-wavelength reflections in enamel,40 but the crystal dimensions are so far below the wavelengths of the blue spectrum that this appears unlikely. However, enamel crystallite discontinuities do exist in the approxi-

mately 500-nm spatial domain, such as those in interpit enamel, which may be the source of the reflected light. Bioceramics that hope to achieve biomimetic design in their monolithic materials can accommodate the desire for whitish teeth by incorporating or generating a broad range of grain sizes, spanning 400 to 700 nm, to coincide with broad-spectrum reflections. A greater emphasis on the shorter wavelengths may allow for a brighter appearance.

Biofilm Naturally occurring microscopic communities of bacteria live on wet enamel surfaces. If allowed to ascend the hierarchy of this community, acidifying bacteria, such as Streptococcus mutans, can lead to enamel dissolution and caries. For this reason, much attention has been paid to these “bad” bacteria, but one fascinating—and likely relevant—aspect to its presence is its exclusive maternal transmission.42,43 More than 1,000 resident gut bacteria have been identified to date; these are fundamental to human biologic processes, from immunity to development, digestion, and metabolism, and, like S mutans, are maternally acquired at birth.44 More than 700 species have been identified from the proximal end of the gastrointestinal tract: the oral cavity.45 As in any complex ecologic community, some species will be freeloaders (commensal species), and others will consume resources without providing anything back (parasitic species). However, this ecologic system has a long evolutionary history, and many species in the oral cavity are likely working with human biology to contribute something to survival (mutualist species). Bioceramics that aspire to biomimetic design should provide the nooks and crannies that nurture and protect oral biofilms. Human permanent teeth have a near-weekly incremental feature observed histologically, one that manifests on the surface as undulations or tilelike features called perikymata, which are visible to the naked eye.17 Perikymata are safe havens for bacteria; no existing toothbrush can reach into these crevices and valleys. A biomimetic surface design that provides the necessary residences required by bacterial dwellers will be of considerable benefit to them and to the well-being of the host. Although substantial advances have been made in ceramic engineering for dental restorations over the past decades, biomaterials scientists are still far from able to reproduce the three-dimensional functionally graded structures, mechanical properties, and esthetic appearance of teeth. Nonetheless, advances in both materials and fabrication methods have resulted in more durable and esthetically pleasing ceramic restorative materials.

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Challenges in Engineering and Testing of Bioceramics

Mechanical Testing Methods for Dental Ceramics Mechanical testing is used to ensure the treatment success of ceramic restorative materials, which depends on the effective transfer of cyclic stress through prosthetic components, tooth structures, and supporting tissues, such as the periodontal ligament and alveolar bone. Mechanical testing methods for dental materials have evolved sufficiently to accelerate failures while simulating clinically observed failure modes. Although several studies have utilized single-load-tofailure (SLF) tests to obtain strength data, fatigue testing methods have proven to be the most adequate and clinically relevant modalities for evaluating mechanical responses to dental ceramic systems. Fatigue is partially or totally related to failure, and the incidence of failure usually increases the longer the prosthetic components are loaded. Thus failure following fatigue is a time-dependent phenomenon. A number of industrial devices are available for repeatedly subjecting a specimen to controlled stress conditions during fatigue testing. However, because test accuracy depends on the simulation of real clinical conditions, some parameters should be controlled: cycling frequency, stress amplitude, dry-wet environment, temperature, and the reproduction of multidirectional functional loading, for example. The testing environment for novel ceramic restorative materials should comprise not only anatomically correct geometries but also a dynamic masticatory environment. Ideally, mechanical testing would be conducted in an accelerated fashion while providing clinically relevant fracture modes, thereby providing an informed platform for the further development of future ceramic restorative systems. In the dental literature, several methods have commonly been used to test ceramic restorative materials under either static or dynamic conditions. Because modern testing techniques involve both setups, static and dynamic methods are both described in the following sections.

SLF testing (strength to failure) The SLF, or ultimate fracture strength, test is characterized by loading of the specimen through compression at a constant strain rate until failure. During the test, a force displacement curve is acquired for each specimen and the maximum load to failure is recorded. Considering that this test approach involves loading the samples at stress levels higher than use stress to facilitate failures in a timely manner, in vitro studies have applied the

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technique to evaluate the strength of a multitude of prosthetic components. The classic limitation of SLF methods relative to cyclic loading ones can be highlighted by comparing two studies from the same group of scientists. In the first study, Andriani et al46 used the SLF mechanical testing setup to evaluate the strength to failure and fracture mode of three indirect composite resin materials directly applied on titanium implant abutments and cemented porcelain-fused-to-metal crowns. In this study, specimens were loaded to failure in compression by applying a ramping force in the tip of one of the four cusps by means of an indenter at a rate of 1 mm/min in a universal testing machine. After failure, the fracture mode of the specimens was classified according to the direction of crack propagation. Overall, Andriani et al46 concluded that no differences in strength existed among the materials, and that failure modes were similar among the groups. Although the study’s methodology was sound from the mechanical and fractographic standpoints, the strength to failure of all materials far exceeded physiologic loading conditions, resulting in fracture patterns that were not representative of clinical failures; this limited clinical insight on the further development of restorative materials. Such discrepancies between in vitro SLF results and clinically observed failures highlight what is known: In an oral environment, restorations are generally subjected not to increasing forces until failure but rather to cumulative damage from high and low forces generated repeatedly during mastication. In the follow-up study, Suzuki et al47 performed step-stress accelerated life testing (SSALT), a fatigue testing method for shortening the life of materials (which will be discussed in more detail later), on the exact same specimen groups and using the same physical configurations as were used in the previous study.46 With the new testing protocol, the researchers obtained very different results. Crowns that had achieved a single-cycle load-to-failure strength exceeding 1,000 N presented a survival rate of only 40% when 200,000 cycles were applied at 200 N load (cumulative damage).47

Fatigue Fatigue is a mode of mechanical failure: Cracks are induced by subjecting a material or structure to repeated subcritical loads, which eventually leads to failure.48 The term fatigue was first proposed by Jean-Victor Poncelet in 1839, a time when the Industrial Revolution had long been a reality and rapidly moving parts had become increasingly common. The main line of thought explained fatigue fractures by “crystallization” of the material, which became brittle after continued use and thus more prone to fracture.48

Mechanical Testing Methods for Dental Ceramics Fig 1-7  Constant stress test. Result of a load of constant magnitude applied over a number of cycles.

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The fatigue resistance of a component is usually assessed by an S-N diagram, in which S is the stress and N is the number of cycles endured before failure. Most of the techniques suggested for this analysis are based on quantal data, that is, the number of “failure” or “survival” results at a given level of stress. In dentistry, fatigue failure results from the development of microscopic cracks in areas of stress concentration that, under continued loading, fuse into an ever-growing fissure that weakens the restoration.48 This initial step, termed nucleation, represents a mandatory stage of fatigue failure. When a fissure reaches its critical size, it will definitely progress at each loading cycle. This process, referred to as propagation, amounts to about 90% of fatigue life. At the end, catastrophic failure occurs when a final loading cycle exceeds the mechanical capacity of the remaining sound portion of the material.48 Current research suggests that ceramic restoration failures are associated with accumulation of damage generated by functional loading. Fatigue test methods have therefore become customary in dental materials testing laboratories, although different laboratories may vary in the fatigue method employed. Presently three such methods are often used for dental research: constant stress testing, staircase testing, and SSALT. Regardless of the in vitro method used, any implantsupported prosthesis requires substantial evaluation before its release to clinical trials. Methods capable of predicting clinical performance over time are therefore highly desirable. Understanding the advantages and disadvantages of each of the three methods is necessary for critical evaluation of the current dental literature.

Constant stress testing In a constant stress test, a material is cyclically tested under a constant load (Fig 1-7). To simulate human masticatory function, several in vitro studies of dental materials have exposed specimens to constant stress.49–54 For instance, Assunção et al54 subjected single implant-supported prostheses to mechanical cycling with constant loads of 130 N in an attempt to simulate load­ing conditions in the anterior region of the mouth. Similarly, the dynamic fatigue properties of the dental implant–abutment interface were simulated via constant stress by several other groups.49–53 According to the literature51,54 an individual is assumed to present three episodes of chewing per day, each 15 minutes long, at a frequency of 60 cycles/min (1 Hz), resulting in 2,700 cycles a day or almost 106 cycles during 1 year. However, to date, there is no consensus among in vitro study protocols regarding the number of test cycles in a constant stress test—it varies from 103 to 106 and up— making it difficult to compare results. Moreover, the frequency of load application has been reported to range from 2 to 15 Hz (cycles per second).53,54 Thus, further investigation is needed to assess the parameters employed in this method and the correlation of in vitro results with clinical failures.

Staircase testing The staircase method is a straightforward procedure in which a series of specimens is tested sequentially to determine the median value of a fatigue limit.55,56 The test is conducted so that each specimen is tested for a determined

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Challenges in Engineering and Testing of Bioceramics Fig 1-8  Staircase test. Result for 20 specimens subjected to constant fatigue until (red) failure or (blue) survival after a predetermined number of cycles. When a specimen fails, the next specimen is tested at a lower stress level; if the specimen does not fail, the following test employs a higher stress level.

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lifetime corresponding to the infinite life. Although a life range of 106 to 108 cycles has been suggested,55,56 the number of cycles for which the component’s fatigue strength will be determined can be guided by previous experience and based on the expected number of cycles to which the component is likely to be subjected during its intended life. In the staircase method, if the specimen fails prior to infinite life, the next specimen will be tested at a lower stress level. If the specimen does not fail within this life of interest, the subsequent specimen will be tested at a higher stress level. Therefore, specimens are tested one at a time, each test dependent on the previous result, and the stress level is increased or decreased by selected increments (1 to 2 standard deviations). It is recommended that the test be run with at least 15 specimens55–57 (Fig 1-8). For example, Wiskott and coworkers58 applied the stair­ case method to compare four abutment types to determine whether the connector’s antirotational mechanism participates in fatigue resistance. The goal of the experiment was to determine the median fatigue strength—the stress level at which 50% of the specimens would survive 106 cycles and 50% would fail. The specimens were loaded to their long axis for a maximum of 106 cycles. Thereafter the test was halted, and the specimen was examined to determine whether it was broken or intact. If it had survived 106 cycles, the specimen was said to have “run out,” and the next specimen was loaded to the previous magnitude plus 5 N. The same force (5 N) was subtracted from the former load magnitude if the previous specimen had failed. All told, 30 specimens were tested in sequence.

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Considering that median determination will properly begin only when the first turnaround (from lower to higher or vice versa) occurs, extra specimens may be necessary to set the initial stress amplitude for the procedure appropriately,55,57 which represents a cost increase for the experiment. However, an SLF test can previously be run as a pilot study to determine the entry stress level, thereby avoid fabrication of additional specimens for the staircase test. At the end of the staircase testing sequence, data reduction techniques, such as the method described by Dixon and Mood or by Zhang and Kececioglu, must be applied to determine the statistical distribution of the staircase test results.55 The Dixon-Mood method is based on the maximum likelihood estimation and calculates the mean and standard deviation of a fatigue limit that follows a normal distribution. This approach considers either only the failures or only the survivals to determine the statistical properties and is dependent on the least frequent event. The method was proposed at a time when computing power was a scarce resource and simple, approximate methods were correspondingly valuable.57 On the other hand, the ZhangKececioglu method considers the suspended items (ie, those removed from the test prior to failure) and the maximum likelihood estimation for the data. It is usually applied if the Dixon-Mood method is not indicated, that is, when the fatigue limit is not normal, the stress increments are not identical, and the stress increment is greater than twice the standard deviation.55 Although the staircase method is an easy, “up and down” sequential technique for estimating the fatigue strength of

Mechanical Testing Methods for Dental Ceramics a component or material, its accuracy and many sensitivity factors may be questioned. For instance, the test is not conducted in a wide force range, and the specimen is not submitted to extreme stress values, which could mask the maximum strength of the material. In addition, the number of cycles to determine the failure or survival of each specimen is predetermined, avoiding the simulation of material performance for a longer period.

Step-stress accelerated life testing Step-stress accelerated life testing is a mechanical (fatigue) test method for shortening the life of materials or hastening the degradation of their performance. It is intended to quickly obtain data that, properly modeled and analyzed, yield desired information on product life or performance under normal use.59 The SSALT method allows prediction, with confidence intervals (based on calculation of a master Weibull distribution), of the life expectancy of a given material under specified loading. Computer software is available for life expectancy calculations. In SSALT, a specimen is subjected to successively higher levels of stress. First, each specimen is submitted to constant stress over a predetermined length of time. The stress on this specimen is then increased step by step until its failure or survival (which is represented by no failure at the end of the step-stress profiles). SSALT has been widely applied for metals, plastics, dielectrics and insulations, ceramics, rubber and elastics, food and drugs, lubricants, protective coating and paints, concrete and cement, building materials, and nuclear reactor materials. A similar process has also been implemented for biomaterials used in dentistry, such as ceramic restorations, dental implants, and adhesive bond materials.47,60–84 Previous work47,60–84 using SSALT in dentistry has utilized SLF as the first step to determine step-stress accelerated life testing profiles, usually three: mild, moderate, and aggressive (Fig 1-9). The use of at least three profiles for this type of testing reflects the need to distribute failure across different step loads and allows better prediction statistics, narrowing confidence intervals. Mild, moderate, and aggressive refer to the increasingly stepwise rapidity with which a specimen is fatigued to reach a certain level of load; that is, specimens assigned to a mild profile will be cycled longer to reach the same load as a specimen assigned to either a moderate or aggressive profile. These profiles usually begin at a load that is approximately 30% of the mean value of SLF and end at a load roughly 60% of the same value. Several previous investigations47,60–84 have demonstrated that 18 specimens are usually enough to obtain good linear regression fits. Previous work in the dental literature has thus used 3 specimens subjected to initial SLF and 18 then

assigned to mild (n = 9), moderate (n = 6), and aggressive (n = 3) fatigue profiles, a ratio of 3:2:1 (although 4:2:1 has also been used; see Fig 1-9). Following the parameters of loading for each predetermined profile, the specimens are fatigued until failure or survival, where maximum loads are applied up to a limit previously established based on the mean value of SLF (β). Based on the step-stress distribution of the failures, the fatigue data are analyzed using a power law relationship for damage accumulation, and the use-level probability Weibull curves (probability of failure versus cycles) at a use stress load are determined for life expectancy calculations by using specific software (Alta Pro 7, Reliasoft). The use-level probability Weibull analysis provides a β value, which describes the failure rate behavior over time. If the value of β is less than 1, the failure rate decreases over time; this is commonly associated with “early failures” or failures that occur due to egregious flaws. If the value of β is equal to 1, the failure rate does not vary over time; this is associated with failures of a random nature. If the value of β is greater than 1, the failure rate increases over time; this is associated with failures related to damage accumulation. A reliability calculation (with two-sided confidence bounds that can be calculated by a variety of methods, including the maximum likelihood estimation) is then mathematically estimated for the completion of a given number of cycles (mission) at a specific load level (Fig 1-10). If, for example, crown-implant systems are tested and the use-level probability Weibull-calculated β value is less than 1 for any tested group, then the failure of the crown-implant system is primarily driven by system strength rather than damage accumulation from fatigue testing. In that case, a two-parameter Weibull distribution can be calculated (Weibull 7++, Reliasoft) using only the final load at failure or survival of specimens, that is, disregarding the number of cycles47,60–85 (Fig 1-11). An instructive graphic method to determine whether these data sets are significantly different (based on nonoverlap of confidence bounds) is the utilization of a twoparameter Weibull contour plot (Weibull modulus [m] versus characteristic strength; see Fig 1-11). Weibull modulus is an indicator of strength reliability and/or the asymmetric strength distribution as a result of flaws within the material. It is often used in evaluating ceramics and other brittle materials. A higher m value indicates smaller and/or fewer defects (greater structural reliability); a lower m value is evidence of greater variability of the strength, reflecting more flaws in the system and a decrease in reliability.85 In dentistry, especially for dental ceramics testing, the SSALT method can be employed with an all-electric servo system, in which the indenter contacts the specimen surface, applies the prescribed load within the step profile, 11

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Challenges in Engineering and Testing of Bioceramics Fig 1-9  Load versus cycles: step-stress accelerated life test. Results of (a) mild, (b) moderate, and (c) aggressive profiles used for an accelerated fatigue test. The overall slope (ie, load:number of cycles) increases from the mild to moderate and from the moderate to aggressive profiles.

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Challenges in Engineering and Testing of Bioceramics and lifts off of the material surface. However, the stepstress method can also be conducted with an electro­ dynamic testing machine to simulate mouth-motion fatigue (MMSSALT).65,68,79–81 Thus, specimens can be tested in either axial or off-axis loading orientation.47,60–85 The applicability of SSALT in the evaluation of dental ceramics has been demonstrated by a series of studies using different ceramic materials and geometric configurations.65,38,79–81 For this series of cited studies, all specimens presented standardized geometric configurations and thus may be compared with one another. In addition, dental ceramics testing performed by MMSSALT has thus far been shown to be the only testing method able to reproduce in vitro the fracture modes observed clinically.65,68,79–81 This provides dental ceramics developers with an informed platform, one that is currently being utilized to improve dental ceramic systems for the future. The main advantage of a step-stress test is that it quickly yields failures; the increasing stress levels ensure this. However, quick failures do not guarantee more accurate estimates. A constant fatigue test with a few specimen failures usually yields greater accuracy than a shorter step-stress test in which all specimens fail; it is the total time on test (summed over all specimens), not the number of failures, that determines accuracy.59 Moreover, under clinical conditions most specimens run at constant stress, not step stress. Thus, the tested model must properly take into account the cumulative effect of exposure at successive stresses. The model must also provide an estimate of life under constant stress that should not exceed 3 to 4 times the average number of cycles employed throughout the test for all groups.59

Conclusion There are several attributes of enamel that are relevant to biomimetic bioceramic design. While it may not be required to adopt the same microstructure, it may be important to manufacture discontinuities in all 360-degree spherical rotational positions at hierarchies of scale from nanolevel to microlevel. It may also be useful to design a sensing technology that elicits a neuromusculature response to overloading. Furthermore, because even nanoscopic fractures have the capacity to grow, bioceramic compositions that favor calcium carbonitic and phosphitic annealing should be developed and used to fill these cracks in lieu of organic materials. Esthetics will always be a major concern in bio­ ceramic design. However, this should not obviate the need for surface structures that protect the oral biota. At present, the impact on human health is not known, and until such time as it is known, it is wise to side with nature.

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A number of mechanical testing methods have been reviewed, but regardless of the method supported in a particular study, it is important that the cycling frequency, stress amplitude, environment (a wet environment is preferred), temperature, and reproduction of multidirectional functional loading are controlled in the experiments. Attention to anatomically correct geometries and dynamic masticatory environments will bear more relevance to in vivo conditions. In an oral environment, cumulative damage is the result of high and low forces generated repeatedly during mastication, leading to the development of microscopic cracks in areas of stress concentration and fatigue failure. While studies on the statistical distribution of forces are unavailable, it is suspected that, over reasonable time frames (eg, 1 week to 1 month), this distribution has structure and obeys a power law. In this respect, step-stress accelerated life testing is highlighted as a method that is relatively fast experimentally and may also prove more relevant than other methods in determining life expectancy calculations and in reproducing in vitro fracture modes observed clinically.

Acknowledgments Research support was provided by the 2010 Max Planck Research Award to Dr Bromage, endowed by the German Federal Ministry of Education and Research to the Max Planck Society and the Alexander von Humboldt Foundation in respect of the Hard Tissue Research Program in Human Paleobiomics. Aspects of this study were also supported by National Science Foundation grants in aid of research to Dr Bromage (BCS-1062680). The fatigue-related studies of Dr Coelho were funded either by National Institute of Dental and Craniofacial Research grant P01 DE01976 (Dr E. Dianne Rekow, primary investigator) or as part of a project sponsored by multiple private corporations (Ivoclar and 3M).

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Challenges in Engineering and Testing of Bioceramics 57. Grove D, Campean F. A comparison of two methods of analysing staircase fatigue test data. Quality Reliability Eng Int 2008;24:485– 497. 58. Wiskott HW, Jaquet R, Scherrer SS, Belser UC. Resistance of internal-connection implant connectors under rotational fatigue loading. Int J Oral Maxillofac Implants 2007;22:249–257. 59. Nelson W. Accelerated Testing: Statistical Models, Test Plans, and Data Analysis. New York: Wiley, 1990. 60. Almeida EO, Freitas AC Jr, Bonfante EA, Marotta L, Silva NR, Coelho PG. Mechanical testing of implant-supported anterior crowns with different implant/abutment connections. Int J Oral Maxillofac Implants 2013;28:103–108. 61. Almeida EO, Freitas Júnior AC, Bonfante EA, Rocha EP, Silva NR, Coelho PG. Effect of microthread presence and restoration design (screw versus cemented) in dental implant reliability and failure modes. Clin Oral Implants Res 2013;24:191–196. 62. Almeida EO, Júnior AC, Bonfante EA, Silva NR, Coelho PG. Reliability evaluation of alumina-blasted/acid-etched versus lasersintered dental implants. Lasers Med Sci 2013;28:851–858. 63. Bonfante EA, Coelho PG, Guess PC, Thompson VP, Silva NR. Fatigue and damage accumulation of veneer porcelain pressed on YTZP. J Dent 2010;38:318–324. 64. Bonfante EA, Coelho PG, Navarro JM Jr, et al. Reliability and failure modes of implant-supported Y-TZP and MCR three-unit bridges. Clin Implant Dent Relat Res 2010;12:235–243. 65. Bonfante EA, Rafferty B, Zavanelli RA, et al. Thermal/mechanical simulation and laboratory fatigue testing of an alternative yttria tetragonal zirconia polycrystal core-veneer all-ceramic layered crown design. Eur J Oral Sci 2010;118:202–209. 66. Bonfante EA, Sailer I, Silva NR, Thompson VP, Rekow ED, Coelho PG. Failure modes of Y-TZP crowns at different cusp inclines. J Dent 2010;38:707–712. 67. Bonfante EA, Suzuki M, Lubelski W, et al. Abutment design for implant-supported indirect composite molar crowns: Reliability and fractography. J Prosthodont 2012;21:596–603. 68. Coelho PG, Silva NR, Bonfante EA, Guess PC, Rekow ED, Thompson VP. Fatigue testing of two porcelain-zirconia all-ceramic crown systems. Dent Mater 2009;25:1122–1127. 69. Freitas AC Jr, Bonfante EA, Martins LM, Silva NR, Marotta L, Coelho PG. Reliability and failure modes of anterior single-unit implantsupported restorations. Clin Oral Implants Res 2012;23:1005–1011. 70. Freitas AC Jr, Bonfante EA, Rocha EP, Silva NR, Marotta L, Coelho PG. Effect of implant connection and restoration design (screwed vs. cemented) in reliability and failure modes of anterior crowns. Eur J Oral Sci 2011;119:323–330. 71. Freitas Júnior AC, Bonfante EA, Silva NR, Marotta L, Coelho PG. Effect of implant-abutment connection design on reliability of crowns: Regular vs. horizontal mismatched platform. Clin Oral Implants Res 2012;23:1123–1126.

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72. Freitas-Júnior AC, Almeida EO, Bonfante EA, Silva NR, Coelho PG. Reliability and failure modes of internal conical dental implant connections. Clin Oral Implants Res 2013;24:197–202. 73. Freitas-Júnior AC, Bonfante EA, Martins LM, Silva NR, Marotta L, Coelho PG. Effect of implant diameter on reliability and failure modes of molar crowns. Int J Prosthodont 2011;24:557–561. 74. Freitas-Júnior AC, Rocha EP, Bonfante EA, et al. Biomechanical evaluation of internal and external hexagon platform switched implant-abutment connections: An in vitro laboratory and threedimensional finite element analysis. Dent Mater 2012;28:e218– e228. 75. Guess PC, Bonfante EA, Silva NR, Coelho PG, Thompson VP. Effect of core design and veneering technique on damage and reliability of Y-TZP-supported crowns. Dent Mater 2013;29:307–316. 76. Guess PC, Zavanelli RA, Silva NR, Bonfante EA, Coelho PG, Thompson VP. Monolithic CAD/CAM lithium disilicate versus veneered YTZP crowns: Comparison of failure modes and reliability after fatigue. Int J Prosthodont 2010;23:434–442. 77. Martins LM, Bonfante EA, Zavanelli RA, et al. Fatigue reliability of 3 single-unit implant-abutment designs. Implant Dent 2012;21:67–71. 78. Sanon C, Chevalier J, Douillard T, et al. Low temperature degradation and reliability of one-piece ceramic oral implants with a porous surface. Dent Mater 2013;29:389–397. 79. Silva NR, Bonfante EA, Martins LM, et al. Reliability of reducedthickness and thinly veneered lithium disilicate crowns. J Dent Res 2012;91:305–310. 80. Silva NR, Bonfante EA, Rafferty BT, et al. Modified Y-TZP core design improves all-ceramic crown reliability. J Dent Res 2011;90:104– 108. 81. Silva NR, Bonfante EA, Zavanelli RA, Thompson VP, Ferencz JL, Coelho PG. Reliability of metalloceramic and zirconia-based ceramic crowns. J Dent Res 2010;89:1051–1056. 82. Silva NR, Coelho PG, Fernandes CA, Navarro JM, Dias RA, Thompson VP. Reliability of one-piece ceramic implant. J Biomed Mater Res B Appl Biomater 2009;88:419–426. 83. Silva NR, de Souza GM, Coelho PG, et al. Effect of water storage time and composite cement thickness on fatigue of a glass-ceramic trilayer system. J Biomed Mater Res B Appl Biomater 2008;84:117– 123. 84. Silva NR, Thompson VP, Valverde GB, et al. Comparative reliability analyses of zirconium oxide and lithium disilicate restorations in vitro and in vivo. J Am Dent Assoc 2011;142(suppl 2):4S–9S. 85. Ritter JE. Predicting lifetimes of materials and material structures. Dent Mater 1995;11:142–146.

The Role of Industry in Developing New Ceramics

2

George W. Tysowsky, dds, mph Robert Gottlander, dds

Development of dental technology and materials has advanced at a rapid pace in recent years. Developed and introduced for all segments of the profession, innovations have affected operative materials, equipment, and, most importantly, clinical and laboratory processes. Changes and advancements in these areas are influenced by the dental manufacturing industry’s constant assessment of new trends, market changes and needs, and patient care objectives. The challenge has been for dental product manufacturers to respond to collective professional demands, whether laboratory or clinical, and with faster and more predictable results. At the same time, industry strives to maintain a structured, systematic development plan that will ensure quality products within specific financial guidelines. A complex and multifaceted process, product development—including that for ceramic materials—spans years, from the time ideas are conceptualized until a prototype is created and from the time production methods are upgraded until the time validation studies are conducted. The sequential efforts involved in bringing new ceramic products and equipment to market represent significant undertakings by scientists, chemists, and engineers throughout the development cycle. These endeavors also must account for the perspectives of end users and academics, all of whom have a stake in how concepts are realized in practical applications, and, of course, patients. Further, meticulous studies by independent and in-house researchers are conducted to validate the efficacy and safety of new ceramic materials. All of this time and effort affects the cost of product research and development. Delivery of ceramic products, equipment, and processes that drive the profession demands innovation, fulfillment of specific needs and expectations, and a sufficient profit margin. Ultimately, it demands delivery of ceramic products and processes that are good for the progress of both the profession and industry and most importantly beneficial for patients. For these combined reasons, theoretically good and potentially useful innovations may not come to fruition. When the cost for research and development of a product cannot be justified by opportunities for financial and clinical benefits, then the choice to realize a different product or innovation is made.

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The Role of Industry in Developing New Ceramics Therefore, open relationships and discussions among industry, end users, and universities are essential to ensuring that the ceramic products that are conceptualized will be properly, efficaciously, and successfully used by professionals. While industry helps to identify and define the allceramic needs and wants of the profession, as well as to develop product and process concepts internally, it is most successful in these endeavors when it collaborates and cooperates with other stakeholders. During the initial stages of development of a ceramic product, and later to encourage diffusion through education (eg, workshops and dental schools), industry relies on universities, advisory boards, and key opinion leaders. In fact, the involvement of universities and professional organizations is key in executing education and clinical processes as well as providing training in how materials should be used to optimize function and esthetics. For example, translational research at the university level can be the basis for instructions and protocol for end users so they know how products will benefit them and their patients. A key to success in industry is keeping the health and well-being of patients at the forefront when establishing long-range plans. Education is an important aspect of taking new technology to the market. Unfortunately, many new innovations are not fully integrated into practice or undergraduate academic programs until they have been withdrawn from the market. In the end, the significant steps involved with research and development of ceramic products are those grounded in person-to-person interactions, knowledge transfer, and respect for established guidelines and best practices. The remainder of this chapter outlines and explains the manner through which successful manufacturers have changed dentistry by introducing new products along with techniques and processes for their application. This chapter discusses the importance of the industry in the development of the ceramic materials that are available now and provides future perspectives. Emphasis is given to the debate on new technologies that rapidly change the material business and how this may potentially affect business in the dental field.

Best Practices in Research and Development Dental companies spend significant resources on evaluating customer needs to fuel innovations that facilitate new techniques and procedures. A formal project/product management system is among the best practices for product research and development. It safeguards the likelihood that 20

a potential product concept can be developed, scaled up, and properly tested for appropriate application in the marketplace. Formal project/product management systems encompass systematic phases, along with the appropriate checks and balances to ensure the delivery and use of appropriate products in a clinical or laboratory environment.1–3

Project/product management systems Several opportunities and challenges are inherent to the process of product research and development. Many brilliant ideas encounter obstacles that prevent their final development or introduction to market. Project ideas must be initially validated, after which their ability to be produced on a significantly large scale in a profitable manner must be verified. Simultaneously, they must be evaluated to ensure compliance with all biologic, legal, and clinical requirements for appropriate application. A formalized project/product management system structures product and innovation developments in a systematic way, ensuring that each phase can be expanded while still maintaining the initial product design requirements. These management systems control product development processes so that a concept can progress through and complete its entire development cycle yet undergo design review at each phase with an emphasis on validating and verifying that the product fulfills the original goal. A successful system provides an appropriate structure for the entire development process, ultimately resulting in a successful product in the marketplace.

Product development phases Project/product management systems include several phases, from managing a product concept to full implementation of manufacturing to a successful market launch (Box 2-1).

Feasibility and definition In the feasibility and definition phase, which is the first step of a product development process, it is necessary to clearly define a product concept and assess all potential risks and barriers to its development and upscaling. A specific definition of the concept is created for use as a benchmark of all other upscaling phases. This ensures that the final developed product matches the original goal and market requirements. During the feasibility and definition phase, testing and manufacturing potential also are assessed, along with any expected biologic implications. In this phase, a patent as-

Best Practices in Research and Development

Box 2-1

Project management process

Phase 1: Feasibility and definition Phase 2: Product/process development Phase 3: Premanufacturing Phase 4: Manufacturing Phase 5: Marketing and sales launch

sessment is initiated to evaluate product protection or potential conflict of a new technology in the marketplace. Based on a successful review of these potentials, the concept can advance to the next phase.

the development of some of the currently available ceramic materials.

Product/process development

The premanufacturing phase is also critical, because it verifies that all relevant criteria of a product concept can be met. A pilot batch of the proposed product is manufactured under simulated conditions to demonstrate that the product can be manufactured under appropriate financial and practical conditions. Development of the appropriate pilot batches enables full testing, including physical property testing, preclinical verification, biologic certification, and patentability assessment. Reliable and reputable manufacturers involved in establishing testing protocols do so to ensure that products will be successful and perform as expected and intended when used in the dental laboratory or clinical environment. Therefore, their clinical or laboratory trials and studies are comprehensive. This critical phase verifies that a product concept can be commercially viable and produced for market launch in an efficient manner while still meeting all safety and regulatory requirements. As with all phases of development, a design review is conducted to reconfirm that validation and verification results support the original product concept and that the idea still has merit and relevancy to the profession.

Another significant phase is product/process development, which involves the first development and testing to demonstrate that a product idea has the potential to progress to the envisioned concept that will be required for market introduction. This phase may involve development of tests and creation of prototype formulas that can be tested with preclinical methods to demonstrate viability as a potential product. Manufacturing plays a critical role in this phase, because many ideas can be created in a laboratory setting but are difficult to manufacture to full capacity. It is therefore critical for industry to demonstrate not only that a product concept has merit but also that it also has the potential to be fully manufactured in a productive and profitable fashion. Based on the success of developed tests and produc­tion concepts, a formal design review is completed to ensure that, with each phase, the product concept is validated to meet the original concept. With appropriate completion of this phase, a project or product manager can describe the customer benefits, the manufacturing potential, the market opportunities and expectations, and the economic impact of the proposed invention. The manager also can reconfirm that the project is relevant to the original target or goal. Market research is necessary to define the market potential and relevance of a product, including a first assessment of training needs and marketing activities. It is at this time that the developer can explore possibilities for bundling the new product or innovative technique with others. For example, the applicability of ceramic materials to chair­ side computer-aided design/computer-aided manufacture (CAD/­CAM) processes and the potential for cooperative arrangements with dental laboratories were instrumental in

Premanufacturing

Manufacturing After successful validation of the preproduction phases, a full manufacturing upscaling takes place. This includes development of final packaging and instructions for use, final registration with the appropriate governmental bodies, and validation and verification that the produced product meets all requirements of the original product design. Final clinical studies are also completed utilizing formally manufactured products to ensure the appropriate safety and relevancy of the product for its final clinical applications. 21

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The Role of Industry in Developing New Ceramics Formal design review ensures that the final product meets its design requirements and can fulfill the intended market application. Based on the successful completion of this phase, the product can be released commercially.

Marketing and sales launch This critical phase acknowledges that all preparation activities have been completed in the final development of a product or innovation and that the new product fulfills design expectations and customer needs. The product is released commercially, and its fulfillment of product objectives is monitored. Customer feedback is essential during this phase to ensure that the product launch is successful and that the design has met its goal. Positioning of the product within the marketplace affects this phase, particularly the manner in which marketing communication is presented. Although the product, innovation, technology, and technique solutions developed may be applicable worldwide, messages may be tailored based on geographic area, financial considerations, and other factors. For example, although the indications and applications of a product may be the same, the characteristics that impact a professional’s treatment choices may be different. In the United States and Europe, choices may differ based on the procedures covered by insurance. In the United States, the esthetics and shading of ceramic materials differ from those in most of Europe. However, the material solutions required by the profession are the same. Therefore, corporate partnerships have been instrumental in developing marketing strategies and subsequently bringing new ceramic products to market. These collaborative arrangements help to determine how products should be positioned and used by the dental profession, which likewise helps to establish and define the corporate partnerships themselves.

Essential Steps for Transforming Ideas into Completed Products Of paramount importance to launching ceramic products that satisfy professional and patient requirements is collaborating with end users to assess trends and needs. Establishment of advisory groups, cooperation with dental schools, and information gleaned by company representatives through the sales process provide avenues by which manufacturers can collect and compile data about industry needs. Trend and market reports are compiled and repeat22

edly vetted to curtail high monetary costs and sensitive resource expenditures that are inherent to the product development process. To conserve investment of financial and human resources on the back end (ie, scientific research and development), deliberate energy is expended during initial planning and conceptualization stages. It is therefore not by coincidence that market leaders with sought-after and reputable ceramic products and equipment remain at the forefront throughout the years. Insightful and genuine interest in the end user helps to ensure that professionals and patients receive and benefit from the appropriate ceramic products and processes for the indication. Although a complex process, the ultimate motivation for producing high-quality ceramics is not complicated. Industry plays a key role in the treatment and health of patients. If this essential criterion is fulfilled, other requirements of the product research and development process—such as realizing a positive financial impact for all involved—can be achieved. When ideas and concepts are transformed into a completed product, it is an essential step to dedicate significant time to assessing which solutions might benefit patients. Successful product research and development should be based on adapting solutions to satisfy patient needs, not adapting patients to products that already exist that might not fulfill their requirements. True innovation and development of ceramics, therefore, are predicated on clear and honest input from various members and segments of the profession.

Relationships between industry, universities, and professional organizations The relationship among industry, professional organizations, and universities helps to define the future of the profession. The focus of these interactions ranges from conceptualizing and validating products to creating solutions for the manner in which innovations are used (eg, procedural techniques and processing). High-quality research is costly, and millions of dollars are involved. From the funding of research staff to in-house testing, and from external in vitro testing to clinical trials, much of the burden of funding ceramic product research is borne by manufacturers. To satisfy current and future expectations, dental product manufacturers seek partnerships with universities and research centers to perform testing and validation. This collaboration enables manufacturers to continue to drive new technologies and areas of product development.

Essential Steps for Transforming Ideas into Completed Products If one entity’s scope of involvement is limited, another group can bridge the gap. For example, while universities are invaluable for basic research contributions, industry and professional organizations can help to define future, longterm ceramic product and equipment needs and translational research requirements. To this end, manufacturers invest heavily in interacting directly with professionals to determine how they practice dentistry and what technologic and ceramic product solutions they need. Interestingly, the needs perceived by university-level researchers may differ from the needs of individuals in clinical and laboratory practice. Further, the role of each group in the research and validation process differs. The tests and studies needed to evaluate the safety and efficacy of a product depend on the product itself. Whereas products that do not directly impact patient health and safety can be studied in a purely scientific manner, others (eg, implants) require verification that they will not harm patients. Universities provide a means for testing of ceramic products that have not been approved but also require stringent review and oversight from a university-based human subjects committee to satisfy approval processes. Research on any level requires significant financial investment. Ideally, dental product manufacturers pursue relationships that, in the long term, are fiscally responsible and mutually beneficial to both the university and industry. The task is daunting, particularly considering the funding required by universities to undertake research projects and a manufacturer’s need to balance and control expenses. However, the cornerstone of productive and relevant industry-university relationships is the balance of relational capital; mutual trust and respect are the currency that helps to ensure unbiased and credible interactions and testing, regardless of financial or product support. The university and its faculty and researchers must maintain a comfort level with the ability of the relationship to foster, encourage, and support its objectives. These objectives include working with and testing new ceramic products in a manner that will facilitate dissemination of valid information through published work as well as provide a basis for academic thought, study, and professional career development (ie, master’s and doctoral level). It is therefore incumbent on industry to allow universities to publish their research findings and test results, regardless of the potential effect on the company or product. Equally important is the role of universities in working toward the acceptance and adoption of new technology throughout the profession. Dentists characteristically— although not always—remain faithful to the techniques and products they learned and used during dental school, even after they have been in practice for a few years. Be-

cause dentists may be adverse to change, product placement, research, and training at the university level can help drive innovations throughout the profession. Without question, cultivation and dissemination of know­ ledge about ceramic materials and processing techniques are fundamental responsibilities of academia. Industry has demonstrated numerous examples of financial and intellectual support for this role through sponsorship and partnership of university research, education and training programs, and professional development. Several high-ranking university leaders and administrators began their careers and established themselves throughout the dental industry as a whole as a result of combined academic-industry pursuits. Among them are deans and department chairs at universities and centers.

Corporate partnerships In the present day, it has become more difficult for dental product manufacturers to remain on the leading edge of technology and product innovations while simultaneously keeping all research, development, and manufacturing under one roof. As a result, in recent years the dental industry has witnessed consolidation among different manufacturers to best leverage their strengths and capabilities. In other instances, the landscape of dental product development has changed to include partnerships between like-minded but diversely capable—and regionally or globally present— companies. These changes acknowledge the fact that for manufacturers to maintain their leadership positions in any given area, they must focus on that category, leaving specialization in a complementary area to a different entity. It is through partnership between manufacturers of complementary products and technologies that each is better enabled to present the newest innovations to the marketplace. Additionally, when different dental product and equipment manufacturers—as well as distributors—focus on their core competencies, the overall costs of researching, developing, verifying, and distributing a product, whether monetary or in terms of human effort, can be minimized. For example, during the development of ceramic materials, manufacturers must consider how materials will be used clinically, how they will be processed in the laboratory or chairside, and how to direct end users to the other materials and technologies necessary for properly adopting the new materials. It is no longer possible for one company to produce and provide all of these components in a beneficial way. In the development and diffusion of such ceramic innovations as Ivoclar Vivadent’s lithium disilicate (Fig 2-1), 23

2

The Role of Industry in Developing New Ceramics Fig 2-1  Scanning electron microscopic (SEM) image of CAD-­processed lithium disilicate (IPS e.max CAD, Ivoclar Vivadent).

10 µm

a

c

b

d

Fig 2-2  E4D Dentist System for CAD/CAM restorations (E4D Technologies). (a and b) Design center, NEVO intraoral scanner, and laptop. (c) CAD model. (d) CAD restoration. (Courtesy of E4D Technologies.)

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Essential Steps for Transforming Ideas into Completed Products

a

b

Fig 2-3  (a and b) Omnicam system and intraoral scanner (Sirona). (Courtesy of Sirona.)

Fig 2-4  SEM image of leucite ceramic material.

combined with laboratory and chairside CAD/CAM systems, corporate partnerships have been instrumental. For example, E4D Technologies, a company that manufactures high-technology equipment for CAD/CAM restorations, focuses on the equipment and software for creating predictable and successful restorations (Fig 2-2). Likewise, Sirona’s CEREC CAD/CAM systems also provide a means for designing and processing ceramic restorations (Fig 2-3). However, the success and diffusion of either of these innovations are dependent on the availability of suitable ceramic materials that can be processed via CAD/CAM (Figs 2-4 to 2-6). The quality of daily technical support is also vital to acceptance of new products and processes, as dental professionals must develop skills to understand and work with the systems. By demonstrating and offering solutions

50 µm

that address multiple areas of end-user concerns, corporate partnerships such as those between Ivoclar Vivadent, Henry Schein, E4D, 3M ESPE, Planmeca, and Sirona facilitate product marketing, acceptance, and integration throughout the profession (Fig 2-7). Additionally, while there were once differences between the materials and technology used in Europe, the United States, and elsewhere, the dental marketplace is now global. Consolidation, conversion, and globalization of the dental marketplace have made it more expensive to develop new ceramic materials and innovative technologies. Corporate partnerships among dental companies help to resolve the challenges inherent to bringing complete solutions to users worldwide.

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The Role of Industry in Developing New Ceramics

10 µm Fig 2-5  SEM image of feldspathic ceramic material.

Fig 2-7  Ceramic CAD/CAM blocks.

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2 µm Fig 2-6  SEM image of zirconia material.

Quality Assurance

Quality Assurance

tions is required for manufacturers who intend to market their products on a global basis.

In recent years, a trend among some dental product and equipment manufacturers has been to introduce products to the marketplace before they have been thoroughly tested. However, major manufacturers of ceramic materials adhere to rigorous protocols to ensure that their products are tested and evaluated before release.4–7 Although there is always pressure to meet deadlines to realize a proposed market launch timetable, both the manufacturer and the professionals using its products are better served if ample and trustworthy information is available first. When manufacturers follow the previously described project/product management phases, checks and balances are inherently enforced with the objective of ensuring quality, efficacy, and safety. University and independent evaluation groups focus on the key criteria for material and equipment success. By conducting validation studies and providing input that guides the final development of a product, manufacturers and their partners establish product quality, relevance, and performance. Standardized in vitro tests and calibrated clinical trials conducted under strict conditions represent other mechanisms in the quality assurance process that demonstrate how products and equipment will perform in a generalized dental practice or laboratory environment. Even when funded by the manufacturers, research conducted at universities and independent facilities can be trusted, particularly when the nature of the sponsorship is fully disclosed. Once a product is introduced to the market, additional scientific and evidence-based research can be conducted. Of course, the availability of reliable and trustworthy information is predicated on appropriate use of the materials and equipment, in accordance with the manufacturer’s instructions.

Design control

Regulatory requirements Regulatory requirements help to facilitate quality assurance measures. Regulatory requirements for device manufacturers influence all stages of product development, from product design to production to postmarket surveillance. For medical devices intended for the US market, the good manufacturing practices of the US Food and Drug Administration (FDA) regulations (21 CFR part 820) apply.8 For most international markets, International Organization for Standardization (ISO) 13485 is the standard for quality management systems.9 The intent of both 21 CFR part 820 and ISO 13485 is similar, but the requirements contain some differences; therefore, a thorough understanding of both regula-

Although the research phase of product development begins with free-flowing ideas designed to address the problems and challenges faced by the industry, once a manufacturer commits to allocating resources for new product development, the need for design control begins. Compliance with regulatory requirements is evidence based; therefore, all documentation relating to the design must be structured to show that the intent of the regulations was met. This documentation encompasses a design history file inclusive of all reports, analyses, and reviews in accordance with regulations.

Design input Beginning with design input, which details all requirements for the finished device so that designers can create it as expected, the technical specifications and measurable characteristics of a product are outlined. Design input specifically identifies all of the standards, including international standards (eg, ISO), that apply to the device. Whether for test methods or work methods, or for defining quality management systems and laboratory practices, the standards are applied to ensure compliance and marketability worldwide. Designers should be very familiar with all of the standards pertaining to the device they are developing and be cognizant at an early stage of the markets in which the device is intended to be sold. Design input is reviewed by a panel representing all facets of the product development process (marketing, quality control, regulatory, and production).

Design verification and validation Standards and regulations for dental ceramic materials involve two phases in the design control process that are frequently misunderstood: (1) design verification and (2) design validation. Throughout the design process, which is usually conducted in stages, the design input is revisited to confirm that the designers are developing a product that meets the original requirements, that is, that design inputs are consistent with design outputs. This process is called design verification and must be documented in accordance with regulations.8 If amendments are required, they are incorporated into a revised design input, further testing is conducted, and subsequent design verification is performed.

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The Role of Industry in Developing New Ceramics Design validation, however, involves a more complex process and analysis of the finished product, manufactured under actual conditions to determine if the device will meet user needs. Design validation planning should occur at a very early stage in development and should be done by people with knowledge of the actual use of the device in a clinical or laboratory setting. Design validation testing should be conducted by persons representative of the user of the device on production units under actual or simulated conditions of use.8 For dental ceramics, validation testing is usually carried out by the dental technician, with some additional input provided by the clinician. The validation test protocol must provide enough detail to the evaluator so that the results can be compared to identified acceptance criteria to prove whether or not the device meets user requirements. In particular, the ceramic material must meet the indications for use and user requirements for the manner in which it will actually be used. Such questions examined during this process may include: •  Can and will users actually understand the instructions for use? Are instructions written in the proper language (perhaps with translations) and with the correct terminology? •  Can and will users understand the product labeling? •  Does the product perform the way the user expects it to? •  Do the fired shades match the shade guide? Design validation may also include clinical studies and/ or a review of the critical research literature to determine whether the device will perform as expected in a clinical situation. Dental ceramics are used in a variety of clinical applications, from single crowns to long-span fixed partial dentures. All indications should be thoroughly discussed in the design validation summary report, including proper justification from the literature if necessary.

Design review Design review is the process that identifies successes and failures encountered during the design process so that a “go” or “no go” decision can be made with respect to the design project. Design review is conducted by representatives of all functions concerned with the design stage, including an individual who does not have direct responsibility for the design stage. Once approved, the development and production process continues to the next stage, as previously described.

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Process validation Once a device design has been finalized, regulations also require that manufacturers control the transfer of the design to production.8 This is an extremely important step that cannot be taken lightly. Prior to the sales release of any dental ceramic material, the members of the production line must be fully trained and equipped to make the product. This includes the need to identify and validate those processes from which the output cannot be 100% tested. Likewise, any computer software used by the manufacturer that may impact the device must be fully validated. Process validation is a system of specialized testing and statistical analysis that provides the manufacturer with a high level of assurance that all of the output of any process that cannot be 100% verified will meet specification. It requires a manufacturer to understand and define the acceptance criteria for all aspects of production and develop in-process tests and controls to prove conformance to the specifications. Once the process is validated, manufacturers are required to control and monitor the process in order to revalidate if something changes.

Production process Production and measuring equipment must also be carefully qualified before it may be used for the production of medical devices. Installation qualification, operation qualification, and performance qualification are common practices wherein manufacturers identify specifications and parameters for equipment prior to purchase. On arrival of the equipment, testing is documented as evidence of proper installation and operation. Ceramic manufacturers may also collect ongoing process data to monitor the ongoing performance of the equipment. The qualification step also incorporates the equipment into the manufacturer’s calibration and preventive maintenance systems, which also are required by the regulations. From a production standpoint, regulatory requirements influence the raw materials incorporated in products, which are specified throughout the design process to meet identified standards. These standards may be published industrywide or reflect internal quality assurance measures established by the manufacturers themselves. Such standards dictate the quality, composition, and manner of processing for any raw materials used, including those purchased from outside parties or obtained in conjunction with selected vendors.

Role of Industry in New Product Training As a result, to ensure that raw materials and product components will satisfy the specified requirements, dental product manufacturers require vendors and suppliers to engage in rigorous qualification and evaluation processes to provide a high level of assurance that they can meet the needs of the manufacturer. Some dental product manufacturers perform this due diligence based on risk analysis or assessment of several key factors. These include the criticality of the product or material a vendor is supplying and how much control a manufacturer will need over it. Following vendor selection, a quality inspection may also occur on receipt of materials to ensure compliance with specifications, after which subsequent product-processing specifications are enforced and validated through ongoing testing. Regulatory requirements further mandate that evidence be provided to document that different steps in the entire production process, including device packaging and labeling, have been checked independently and released (ie, reviewed during production compared to the specifications for each component and production phase). This process safeguards the likelihood that all aspects of a product or material will meet all specifications prior to being introduced to commerce.

Approval and Certification Processes Product registrations are required before new ceramic materials are introduced to the marketplace. These requirements differ from country to country, although some countries offer reciprocity to devices cleared in the United States. In the United States, most device registration is governed by a section of the Food, Drug, and Cosmetic Act that affects premarket notification.10 When manufacturers plan to market a new medical device in the United States, they must request permission from the FDA to do so. This process involves proving that the product is substantially equivalent to an already cleared product or device; in areas where the new product is not completely identical or the new design raises new issues of safety and effectiveness, manufacturers must prove that the product has been tested, risks have been minimized, and the product will perform as expected. Given the scope of the FDA’s regulations, manufacturers of ceramic materials and innovative technologies must adhere to rigorous product development processes (as previously described) to receive clearance for commercialization. Additionally, manufacturers also can demonstrate their commitment to quality by receiving ISO 13485 certification.9 This distinction indicates that a company adheres to a quality management system, one that meets and maintains internationally accepted standards for ensuring con-

sistent quality in product design, development, production, installation, and service. In addition to ISO 13485 certification, the most reliable route to global product registration is adherence to product-specific international standards. ISO is a voluntary organization whose members are recognized authorities on standards. The ISO standard for dental ceramics is ISO 6872.11 Most standards are established in a format that outlines basic requirements for the device, such as minimum compressive strength, minimum flexural strength, maximum solubility, biocompatibility requirements, and labeling and instructions. The standards also describe specimen preparations and testing methodologies, which are important so that all manufacturers of the same type of products are compliant with the same standard based on the same criteria. In the end, it is important that manufacturers support all claims they make for the medical device with evidencebased testing. Device labeling encompasses more than the label on the device itself. Manufacturers are responsible for ethically representing their devices to the market and not overstating or recommending off-label uses. The registration process includes a careful review of all labeling and instructions for use, and once clearance is granted, manufacturers are expected to control their literature and marketing efforts so as to accurately represent the cleared device.

Role of Industry in New Product Training As previously noted, many dentists continue using the products and techniques they were introduced to in dental school, even many years into their professional careers. If they have experienced success with a product and the manufacturer has maintained a long-term relationship and commitment with the university, then it is likely that practicing dentists will remain loyal to products and materials offered by that particular manufacturer. This presents challenges for marketing new ceramic materials and high-technology innovations. Making inroads toward current and future practicing dentists requires brand recognition as well as education and training.12–14 Whereas dental students previously graduated and shortly thereafter established their own practices—thereby starting their dental product purchasing behavior—today’s world is different. Many dentists are now working a greater number of years in group or franchise practices where purchase decisions are made by someone else or at a corporate level. The gap between knowledge of the materials used in dental school and knowledge of emerging and innova29

2

The Role of Industry in Developing New Ceramics tive dental ceramics creates marketing, education, and training challenges for dental material manufacturers and distributors. Established and experienced dentists often learn about new ceramic materials and techniques at trade shows, seminars, or manufacturer-sponsored events. Key opinion leaders who have used the product introduce examples of successful cases and provide instruction regarding proper protocol and treatment indications. The caveat about manufacturer-sponsored education, however, is the real or perceived influence of dental companies on content that is presented as part of educational training. Today’s savvy dental professionals are more skeptical of marketing and education messages and avoid being pushed toward a specific product. They would rather focus on understanding the application of types of solutions to different clinical or laboratory problems. For instance, dental professionals may prefer education about ways in which different conditions can be treated using available solutions and based on the scientifically researched criteria rather than brand-specific instruction. New technology and ceramic material innovations achieve widespread adoption in the marketplace through education. However, the costs of designing, sponsoring,

30

and conducting training and education programs to ensure the proper use and application of ceramic materials and technologic innovations require financial and intellectual investment by the dental industry. Partnerships among the dental industry, universities, and professional organizations facilitate the allocation of resources for the creation of worthwhile education and training venues.

Conclusion The commercial development of ceramic products and technology is complex and requires a formalized process that ensures a systematic progression from a concept to the complete fulfillment of requirements as well as the ability to successfully manufacture and market a relevant product. New ceramic technologies are rapidly advancing, and the challenges of regulatory and appropriate design testing have become more complicated. An appropriate project/ product management system, combined with strategic, like-minded, and respectful partnerships, ensures the relevancy and quality of new ceramic products and their ability to appropriately fulfill customers’ needs and expectations.

References

References 1. Dinsmore P, Cabanis-Brewin J. The AMA Handbook of Project Management, ed 3. New York: Amacom, 2010. 2. Harvard Business Review. HBR Guide to Project Management. Boston: Harvard Business School, 2012. 3. Heldman K. Project Management JumpStart, ed 3. Alameda, CA: Sybex, 2011. 4. Kelly JR, Benetti P. Ceramic materials in dentistry: Historical evolution and current practice. Aust Dent J 2011;56(suppl 1):84–96. 5. Guess PC, Schultheis S, Bonfante EA, Coelho PG, Ferencz JL, Silva NR. All-ceramic systems: Laboratory and clinical performance. Dent Clin North Am 2011;55:333–352. 6. Rekow ED, Silva NR, Coelho PG, Zhang Y, Guess P, Thompson VP. Performance of dental ceramics: Challenges for improvements. J Dent Res 2011;90:937–952. 7. Höland W, Beall GH. Glass-Ceramic Technology, ed 2. Hoboken, NJ: Wiley, 2012.

8. Food and Drug Administration, Department of Health and Human Services. Quality system regulation. 21 CFR §820. Washington, DC: Government Printing Office, 2012. 9. International Organization for Standardization. Medical devices. Quality management systems. Requirements for regulatory purposes. ISO 13485:2003. Geneva: ISO, 2003. 10. Federal Food, Drug, and Cosmetic Act, 21 USC §301–399. Washington, DC: Government Printing Office, 2012. 11. International Organization for Standardization. Dentistry. Ceramic Materials. ISO 6872:2008. Geneva: ISO, 2008. 12. Jahangiri L, Choi M. A model for an integrated predoctoral implant curriculum: Implementation and outcomes. J Dent Educ 2008;72: 1304–1317. 13. Mattheos N, Ivanovski S, Sambrook P, Klineberg I. Implant dentistry in Australian undergraduate dental curricula: Knowledge and competencies for the graduating dentist. Aust Dent J 2010;55:333–338. 14. Jalbout Z, El Chaar E, Hirsch S. Dental implant placement by predoctoral dental students: A pilot program. J Dent Educ 2012;76:1342– 1346.

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Practice-Based Research on the Effectiveness of Ceramics

3

Van P. Thompson, dds, phd Kenneth A. Malament, dds, mscd

There continues to be a translational gap, that is, a lag (averaging 15 to 20 years) between pivotal technologic discovery or clinical trial and general clinical application and adoption.1 Both medicine and dentistry are struggling to accelerate the incorporation of advances or best practices into clinical patient care. Practice-based research is centered on determining the comparative effectiveness of different procedures, treatments, or materials in use. The intention is to find out what works best in the real world clinical situations of practitioners with diverse talent levels treating broad populations of patients. Practice-based research can be considered translational research when it compares emerging technologies or materials to those in common usage, for example, all-ceramic posterior crowns to metal-ceramic crowns. In US Food and Drug Administration (FDA) terms, these are deemed phase V studies.2 Currently in the United States about 150 US medical practice-based research networks (PBRNs) are listed by the Agency for Healthcare Research and Quality of the US Department of Health and Human Services; however, only three PBRNs represent dentistry.3 PBRNs provide an infrastructure in which practitioners can participate in clinical studies, assessing conventional and new technologies, to answer questions generated by the clinical community. These practitioners and their findings are likely to be better accepted by the clinical community at large and reduce the translational gap for best practices. This chapter describes the organization and activities of PBRNs, in particular as a feasible means to broadly compare the effectiveness of all-ceramic crowns and fixed prostheses to that of traditional metal-ceramic restorations.

History of PBRNs While no agreed-on definition of a PBRN presently exists, the Practitioners Engaged in Applied Research and Learning (PEARL) Network defines a PBRN as follows: “a collaboration between an academic health science center and community practitioners for conducting primarily clinical studies of mutual

33

3

Practice-Based Research on the Effectiveness of Ceramics interest to benefit and enhance patient care and delivery, systems assessment, quality assurance, and other factors affecting health care outcomes and policy.”4,5 First conceived more than 100 years ago in England, medical PBRNs took root in the United States in the 1970s.6,7 PBRNs are an important instrument of change, driven primarily by the recent health care reform legislation highlighting patient-centered outcomes research in the form of the Patient-centered Outcomes Research Institute8–11 and the Health Care and Education Reconciliation Act of 2010.12–14 Medical PBRNs were designed to bring together community physicians to conduct studies of interest and relevance to everyday clinical practice,7 and many conduct surveys to assess practitioner practice patterns. Comparative effectiveness studies are directed at comparing treatment outcomes and are the clinical focus of PBRNs. Health improvement network has been suggested as a new title for PBRNs.15 In 2005, the US National Institute of Dental and Craniofacial Research (NIDCR) funded three dental PBRNs as an experiment to initiate change in dentistry: the PEARL Network at New York University (NYU); the Dental PracticeBased Research Network at the University of Alabama; and the Northwest Practice-Based Research Collabora­ tive in Evidence-Based Dentistry, a collaboration between the University of Washington and Oregon Health Sciences School of Dentistry. The NIDCR-funded PBRNs were designed to implement change in the way dentistry is practiced. Basic research that remains in the laboratory, not translated to clinical practice, does not contribute to improved patient care. The infrastructure design of the dental PBRNs allowed practicing dentists to be part of dental research, which was historically limited to a small percentage of academic dentists and scientists. The three NIDCR-funded dental PBRNs were replaced in 2012 by the National Dental PBRN, which is coordinated from the University of Alabama. The PEARL Network continues as an independent organization.

Role of the Practitioner Dentists are highly educated but underutilized health care professionals. They take medical histories, spending more time with a patient than do their physician counterparts; yet there is no forum for collaboration and transformation of this information. The advent of electronic health records means that dental practitioners will inevitably be part of a larger system of health care, inclusive of patients’ health histories, through the “electronic medical home.”16 PBRNs foster familiarity for such data exchange. Overall the intention is to provide clinical solutions to optimize oral health treatment and foster prevention paradigms.17 34

Dentists are generally relatively isolated in their practices (70% solo).18 The initial objective of the dental PBRNs was to build a network of dental practitioners, utilize them in surveys and standard-of-care studies, and keep them engaged through annual meetings, newsletters, study-related meetings, monthly teleconference calls, and, when possible, monitoring visits to assess study progress. Participation in PBRNs is a venue for dentists to learn newer and advanced treatments and techniques in a guided, university-based environment that allows peer education, participation in study protocols, and an information dissemination process that ensures some quality control and practitioner feedback; all these components work to establish best practice outcomes.19 Transformation theory suggests that practitioners’ willingness to change depends on the ability of the network to establish credibility and trust,20 objectives requiring a long-term commitment typical of PBRN experience. Funding may allow reimbursement of dentists for a portion of their time spent participating in clinical studies; it generally does not subsidize patient care. Patients routinely visit the dental office, and any patient presenting with a clinical situation that fits a study’s clinical criteria is eligible for recruitment into that study. Funding is intended to cover the time the practitioner and staff spend on informed consent, data recording and transmission, and follow-up visits.

Structure and Function of PBRNs This chapter will describe the activities of the PEARL Network as a successful example of PBRN organization and function across a range of comparative effectiveness research studies. A PBRN requires an executive committee comprising practitioner-investigators (PIs), academics, a data coordinating center (usually academic based), and staff (some of whom are clinical research coordinators) to provide study support and logistics. In addition, the network must work closely with one (ideally) or more institutional review boards (IRBs). The executive committee is critical because it represents the interests of the PIs and, working with the academics and data coordination center, formulates the research questions and designs the comparative effectiveness research studies that can be realistically conducted in the offices of the PIs. A data-coordinating center is pivotal to design of questionnaires and data collection instruments, electronic collection and collation of data, and statistical analysis. The clinical research coordinators play a critical role in implementation, conduct, and closeout of the studies. They are the primary interface with the individual dental practices and their staff, responsible for educating the staff

Structure and Function of PBRNs TABLE 3-1 Research capabilities of PBRN investigators, based on training Investigator designation Nature of the study

Tier 1

Tier 2

Randomized clinical trial

X

Phase III trial development

X

Standard-of-care randomized clinical study*

X

Comparative effectiveness research (prospective)

X

X

Comparative effectiveness research (retrospective)

X

X

Standard-of-care phase IV trial (randomized clinical trial)

X

X

Patient surveys

X

X

Tier 3

X

*Products cleared by the FDA and approved by the American Dental Association.

about the study and participant recruitment and concerns. In addition, they troubleshoot any problems encountered in patient recruitment, training, data entry, data integrity, data anomalies, or study closeout. PEARL evolved over 7 years to include a registry of more than 519 dentists designated as PIs. Of these, 364 are credentialed for enrolling patients, and 311 are trained to participate in the network (Table 3-1). PEARL also includes 30 community centers. Credentialing is based on a number of parameters for IRB risk mitigation and regulatory compliance, which will be discussed later. PEARL has also created a schema for determining a practitioner’s appropriate participation level (see Table 3-1). This model is intended to capture the input of as many practitioners as possible and include a range of clinicians, from those who want to participate only in surveys to those who are interested in joining randomized comparative effectiveness research studies. PEARL’s surveys assess practitioner interest in a given study and/or the feasibility of a study in terms of patients, logistics, and dental practice ergonomics; the surveys are usually, but not always, followed by a study. Recruitment strategies are designed to attract practitioners who understand the long-term benefits of a PBRN, because it costs time and money to replace and train PIs. High practitioner turnover can stall clinical study progress. Motivated practitioners are screened by PEARL, based on criteria from various agencies (FDA, Office of the Inspector General of US Department of Health and Human Services, and state boards of dentistry), to help ensure data integrity (Box 3-1). The size of any network is limited by the cost of support. As noted, 311 of PEARL’s 364 credentialed practitioners are trained PIs, having fulfilled the requirements of tier 2 partici-

pation (see Box 3-1). A relatively large number of trained PIs is necessary, and only a limited number will be interested in participation in a given study.21 A rule of thumb is that a network must have three times more trained PIs than are required for any individual study in order to achieve sufficient PI participation for each study. In other words, it is best to recruit to the study and not assume that all trained PIs will consider recruitment of participants. PEARL was designed to support the generalizability of the obtained data. The term generalizability suggests some level of confidence to ensure robustness of the findings to the community at large. How can a clinical network take clinicians with no training in clinical research, have them conduct a study, and expect that the data will be generalizable to the profession? To address generalizability, PEARL follows the principles of good clinical practice (GCP) for investigator training and clinical operations. GCP creates an audit trail of the data to ensure a level of confidence in clinical study findings.22 The entire clinical process is supported with standard operating procedures and oversight from certified clinical research coordinators. The GCP process extends to the official closeout of a study, including final monitoring visit, queries resolution, and database lock that ensures the completeness of the archived study. PEARL developed an organizational structure at each dental practitioner’s site. In addition to the practitionerinvestigator, an essential member of the study team is the practice research coordinator, the key person through whom PEARL clinical research coordinators maintain liaison via monthly teleconference calls and whenever there is a study-related issue. The dental practice unit also must include a dental hygienist, a dental assistant, and an office 35

3

Practice-Based Research on the Effectiveness of Ceramics Box 3-1

Credentialing and training requirements for PBRN clinical investigators

Tier 1: Experienced and capable of randomized controlled trials • Participation in previous studies and demonstrated capabilities at the tier 2 level • Awareness of need for continuous safety monitoring Tier 2: Trained for standard-of-care studies (tier 3 requirements plus) • Signed curriculum vitae or résumé • Current certification in cardiopulmonary resuscitation • Financial disclosure and conflict of interest statements • Evidence of decision-making authority at practice site • IRB–approved human subjects training* plus: – Informed consent – Good clinical practice • Study-specific training: – Study manual review and test – Manual of operations review – Electronic data capture training – Data closeout requirements • Screened as qualified, without any censure from governmental bodies or professional societies Tier 3: Credentialed for surveys • Registered as a research practitioner • Active dental license • Trained in informed consent • Abbreviated human subjects training • Data capture training *May require official affiliation with an institution or a hospital.

manager—a configuration PEARL has found to be optimal for the conduct of office-based studies but one that understandably limits practitioner recruitment. A close working relationship with an IRB helps the PBRN reduce the burden of training and compliance placed on the PI and his or her staff. The official IRB of record for the PEARL Network is the NYU School of Medicine. PEARL is structurally obligated to advise its IRB of the risk potential of its studies, to ensure patient safety, and to maintain a level of quality assurance for the practitioners and their staff through education and training, thereby maintaining compliance with protocol and adherence to GCP. Each PEARL practitioner engaged in a study is appointed as an NYU College of Dentistry Research Associate, to come under the IRB’s umbrella. PEARL terms its undertakings as clinical studies, not clinical trials, a term reserved for the drug development pipeline (see Table 3-1). This mitigates the risk potential for the IRB, so network studies are mainly deemed low risk and standard of care. The PEARL Network views its educational component as paramount in sustaining the PBRN concept of foster-

36

ing change in dentistry. Study information is disseminated through annual meetings, publications, newsletters, social media, and online learning. Practitioners have been encouraged to learn the process of presenting clinical results. One motivational component for PIs to participate in the PBRN is benchmarking: Practitioners participating in a study receive from PEARL a report measuring their performance anonymously against that of their study peers. This feedback is unique for each dental practitioner and informs the participant about how he or she may improve delivery of care and/or treatment outcomes. The PEARL Network envisions educating dentists in clinical research as a means of generating potential clinical faculty to fill the vacancies in US dental schools. The education obtained from a PBRN is a real-time dental curriculum, revised as clinical study results become available, closing the gap between academics and practitioners.23 The clinical philosophy of the PEARL Network is to design studies that are of interest to the practitioners, are clinically relevant, and have the potential to change practice patterns and improve patient care. Studies are also designed to

Potential PBRN Study Designs for All-Ceramic Restorations

Endodontic treatment 3 to 5 years previously: Tooth identified Tooth present Clinical assessment

Radiograph

Tooth missing

10% of preoperative periapical radiographs evaluated through random selection

Pain and quality of life questionnaire History data entered

Database

History data entered

Analysis

Fig 3-1  Algorithm of study design to determine endodontic treatment and restoration outcomes.

evaluate the capability and robustness of the network, and to balance science with clinical relevance and the logistics of conducting the study in the practice. The studies also adhere to the recent guidelines proposed by the PatientCentered Outcomes Research Institute.24,25 Additionally, the broader perspective of professional research organizations is reflected in PEARL studies, which include findings on oral health–related quality of life outcomes of specific protocols.26,27

Potential PBRN Study Designs for All-Ceramic Restorations The PEARL clinical portfolio comprises a variety of studies that demonstrate the capability of PIs to collect detailed and in-depth information over the duration of the study. PBRN studies balance control and risk with office logistics. As study risk increases, so does the control of the study for patient safety and IRB compliance. A strength of PBRN stud-

ies is their participant recall rates, which can be as high as 99% over a 6-month period,28 indicating the commitment of PIs and their participating patients to these studies. PBRN methods could be applied to evaluate the effectiveness of all-ceramic restorations. Such a study was planned for the PEARL Network, but funding limitations prevented its full development. Retrospective studies that involve patient visits to verify outcomes are ideal for PBRNs and could be applied to all-ceramic restoration outcomes. Review of recent PEARL studies suggests how this could be accomplished. For example, PEARL PIs wanted to compare the outcomes of implant restoration and endodontic treatment of compromised posterior teeth. To support the decision-making process in treatment planning, PEARL conducted two overlapping retrospective studies, one to assess endodontic outcomes and one to assess implant outcomes. The findings from the endodontic outcome study have been published.29,30 The study design is shown as a flow­chart in Fig 3-1. PI staff identified from their records patients in whom endodontic therapy and restoration were complet37

3

Practice-Based Research on the Effectiveness of Ceramics ed 3 to 5 years previously. At the recall appointment the patient was approached to be enrolled as a study participant. Following enrollment, the participant was evaluated by the dentist. If the study tooth was not present, the history of the loss was recorded. When the tooth was present, the PI performed a clinical evaluation of the tooth and restoration and reviewed a recent or immediate radiograph. Evaluation included, among other items, evaluation of the restoration for replacement or repair, pain in the general area, and pain on percussion. The patient completed questionnaires related to pain and quality of life. Data were collected from 64 PI offices and 1,312 participants. The data can be interpreted as endodontic failure (19.1%)29 or restorative failure (13.9%).30 Based on these patient-centered outcomes, the total failure rate (with all reasons mutually exclusive) across the practices was 33% at 3.9 ± 0.6 years in function. The study data for implant outcomes are still undergoing analysis; however, thus far the retention rate for the implants is very high, but bone loss is a concern. This type of retrospective study across a number of practices could provide outcomes data on a large number of restorations. Key to the validity of the findings is the clinical evaluation of the restorations, which is missing from sources such as insurance claims databases. Clinical evaluation would require agreement on what constitutes clinical success and failure for all-ceramic restorations, as discussed in a recent review.31 A prospective study design applicable to evaluation of all-ceramic crowns is a randomized clinical effectiveness study conducted by PEARL on treatment outcomes for hypersensitive noncarious cervical lesions28 (Fig 3-2). For this study, 17 PIs were trained in identification of, evaluation of, and three treatments for these lesions. The treatments were use of a dentifrice containing potassium nitrate, application of a dentin bonding agent covered with a sealant layer, or use of the same bonding agent and a flowable resin-based composite restoration. On enrollment, participants were randomly assigned by the coordinating center to one of the treatments. Participants were recalled at three time points to report their hypersensitivity and undergo clinical evaluation for hypersensitivity. At the 6-month recall the sealant and restoration were equally effective, while the dentifrice had positive effects but fell short of the other treatments at all recalls. This type of study design could be considered to compare short-term and even long-term results between adhesive cementation and luting of all-ceramic posterior resto-

38

rations. Such PRBN studies could provide meaningful data and avoid the expense associated with large randomized controlled trials that have recently been recommended.32 Meaningful clinical outcomes data can be gleaned from individual or group dental practices. The generalizability of the data becomes more acceptable if the results of several practices can be combined or if the practices have similar results. Both of these instances depend critically on agreement as to the data to be collected and on data definitions (curation) as well as rankings and scoring methods. Deidentification of the data for patient safety and compliance with privacy regulations is also of importance. Provided that a clinician pays careful attention to detail and maintains GCP, practice findings are a helpful source of information on treatment outcomes. However, most studies of all-ceramic crowns from individual practices or dental schools include a limited number of restorations or have a mean duration of less than 5 years.33–36 The German Society for Ceramic Dentistry sponsors a registry for single-tooth all-ceramic restorations and has collected data from more than 200 dental practices over almost 12 years. There is one publication to date on outcomes.37 Placement data have been collected for more than 5,000 ceramic restorations, predominately inlays and onlays and only about 15% complete crowns. The registry has collected recall data on 3,096 of these restorations, with times in service of up to 11.8 years. Reports from individual practices have contributed at least 50 restorations to the database. The report allows comparison of the placement techniques of individuals with the average across the group. Overall the German findings indicate that the all-ceramic restorations in the study are performing reasonably.37 The Kaplan-Meier probability of survival at 12 years was 81.6% for all teeth but only 60% for nonvital teeth. The registry recorded 2% of the restorations as failures; of the failures, 50% were fractures and 20% endodontic failures. Failure rates were reported to be higher for crowns than for inlays and onlays, but the values were not provided, nor were differences in ceramic systems discussed. Unfortunately to date there have been no subsequent publications regarding this German PBRN study. The dental profession should seize the opportunity to fully utilize the patient information that practitioners gather and to have a PBRN to help capture and relate that information to best practices. PBRNs offer the chance to expand the domain of interests and responsibilities for dentists at a time when they may be challenged by midlevel providers.

Potential PBRN Study Designs for All-Ceramic Restorations

• Establish subject eligibility • Obtain subject’s informed consent

• Hypersensitivity (HS) measure • Randomize to treatment arm • Impression • Sleep bruxism evaluation • HS survey • Quality of Life (QoL) survey

Treatment 1: Dentifrice dispensed, reviewed

Treatment 2: Self-etch DBA and sealant applied

Study baseline

Treatment 3: Self-etch DBA and flowable composite resin applied

Treatment

• HS measure • HS survey • Impression

Treatment baseline

• Pain medication usage • HS measure • HS survey • QoL

1-month recall

• Pain medication usage • HS measure • Impression • HS survey • QoL

3-month recall

• Pain medication usage • HS measure • Impression • HS survey • QoL

6-month recall

Fig 3-2  Flowchart of study design to compare the effectiveness of three different treatments for hypersensitive noncarious cervical lesions. DBA, dentin bonding agent.

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3

Practice-Based Research on the Effectiveness of Ceramics

Example of Individual Practice Data Collection One instance in which individual practice results have provided broadly applicable data should be noted. Glassceramics in a cast or pressable form (Dicor, Corning and later Dentsply) were advocated for complete-crown restorations based on their physical properties, which were reportedly similar or superior to tooth enamel.38 A data collection instrument was developed to document the performance of all-ceramic crowns; the original instrument comprised a 27-field data set for each restoration placed.39 This data set has now been reduced to 23 fields (Box 3-2). In addition to information on patient demographics, the instrument records the ceramic material, the margin design, and material thickness in six locations. Details of the supporting structure, cements employed, and opposing dentition are also recorded. This documentation is continued at each recall or when an adverse event occurs. Resulting from this dedicated individual clinical effort is a highly informative series of articles elucidating the factors leading to failure or survival of these glass-ceramic restorations over time39–41 and defining risk factors associated with failure.42 These results provided clinical verification of predictions from studies on layered ceramic models of crowns.43–45 Surprisingly, crown thickness and two different margin designs (shoulder or chamfer) did not play a significant role in outcomes. The culmination of this work is a 20-year report of outcomes of different combinations of variables and the risk factors associated with those combinations, derived from data on more than 1,400 restorations in 417 patients.46 The elastic modulus of the supporting structure (gold versus dentin), molar versus premolar, and gender are all compounding risk factors. Even comprehensive meta-analyses of clinical reports on all-ceramic crowns cannot provide such detail, because most studies do not have such comprehensive databases and do not include a sufficient number of restorations to allow survival analysis with appropriate statistics or to investigate compounding variables.32,47 The long-term clinical data46 cast doubt on extrapolation from short-term data, because the predicted survival from the short-term data and the actual survival rates of Dicor restorations in the long-term study differed considerably. The data strongly suggest that the only reliable method to evaluate the survival of prosthetic materials is long-term clinical testing in human subjects. Future studies of allceramic restorations should:

40

•  Include a baseline of at least 500 restorations, the majority of which are posterior crowns. •  Have a study duration of at least 5 years for all restorations. •  Require data to be updated at each recall examination. •  Include survival analysis with confidence intervals. •  Account for all participants entered into the study. During the time of data analysis and calculation of survival statistics for the 20-year report, Malament expanded the materials in his practice in 2006 to include lithium disilicate glass-ceramic restorations (e.max Press, Ivoclar Vivadent). Table 3-2 contains the latest summary of all-ceramic crown performance data from May 2006 and includes more than 1,700 patients and more than 5,550 restorations. Covered is the entire suite of all-ceramic materials employed for restoration in this practice, including partial-coverage restorations and veneers. Failures in Tables 3-2 and 3-3 are defined as ceramic fracture requiring replacement. In Box 3-2, the category “Reason for replacement” includes 18 different codes. This is the basis for the listings “Failures,” ”Chipping,” and “Replaced” in Tables 3-2 and 3-3. Not all failed restorations were replaced. In this summary, the failure rates are generally low, given the extended period in service for most of the materials, less than 1% per year for all except Dicor. Most of the failures for Dicor occurred in the first 5 years in service, as has been pointed out previously.46 While e.max restorations have been in service for a limited time in Malament’s practice (now more than 6 years for the earliest restorations), only one failure has occurred to date; this was a full crown on an implant abutment. This is the lowest failure rate of all the materials investigated in this practice. Unfortunately, there are no zirconia-based restorations in the database for comparison. The outcomes for all-ceramic crowns from March 1993 to date (June 30, 2013) are summarized in Table 3-3. Almost three-quarters of the lithium disilicate crowns (884 of 1,158) are located in the posterior region, and there have been no failures to date (service duration of up to 74 months). If highly motivated individual dentists were to employ the data set developed by Malament (or a modification thereof ) in their practices, important information about the outcomes of all-ceramic restorations could be gained more rapidly. Alternatively, individual dentists can develop their own database to answer a clinical question; guidance in this regard can be provided by most dental colleges. What is required are individual and group initiatives to gather and interpret clinical data.

Example of Individual Practice Data Collection

Box 3-2

All-ceramic restoration database fields and codes used by Malament

1 Subject identifier 2 All-ceramic material 1 Dicor 2 Inceram 3 Empress 4 Empress 2 5 Procera 6 Eris 7 e.max: Pressed and fluorapatite 8 e.max: Pressed 9 e.max: Pressed and Creation feldspathic 10 e.max: CAD/CAM and fluorapatite 11 e.max: CAD/CAM 12 Feldspathic porcelain 3 Gender (male = 1; female = 2) 4 Patient date of birth 5 Date completed 6 Last recall date 7 Tooth (number 1 to 32) 8 Procedure 1 Posterior complete coverage 2 Anterior complete coverage 3 Posterior partial-coverage inlay 4 Anterior partial-coverage inlay 5 Core 6 Posterior partial-coverage onlay 7 Veneer 8 Dowel core 9 Fixed partial denture (FPD) 10 Dicor luted to metal FPD 11 Zirconia post fused to Empress (dowel core) 12 Zirconia implant abutment 13 Crown and cantilever 14 Splinted crowns 15 e.max Pressed to gold dowel core 16 e.max Pressed to gold implant abutment 17 e.max endodontic core inlay 18 e.max luted to titanium implant abutment 9 Failure (yes = 1; no = 0) 10 Failure or replaced date 11 Replaced but no failure (yes = 1; no = 0) 12 Reason for replacement 1 Caries 2 Periodontics 3 Endodontics 4 Sensitivity 5 Esthetics 6 Loosening 7 New treatment plan 8 Fractured root 9 Dowel and/or core failure 10 Increasing mobility needing splinting 11 Open contacts 12 Poor fit 13 Restoration lost 14 Root resorption 15 Fractured cusp (tooth) 16 Internal cracking (ceramic) 17 Large ceramic chip 18 Fractured implant abutment 13 Chipping (yes = 1; no = 0) 14 Laboratory (create own code) 15 Preparation structure 0 None 1 Dentin

2 Gold and dentin 3 Empress and dentin 4 Dicor and dentin 5 Feldspathic porcelain and dentin 6 Composite resin and dentin 7 Enamel 8 Enamel and dentin 9 Alumina Ceradapt implant abutment 10 Porcelain-fused-to-gold implant abutment 11 Zirconia abutment 12 Silver amalgam and dentin 13 Gold abutment 14 e.max and dentin 15 Gold implant abutment 16 e.max implant abutment 17 Titanium implant abutment 18 e.max implant abutment 16 Margin design (shoulder = 1; chamfer = 2) 17 Luting agent 1 Zinc phosphate 2 Glass ionomer 3 Dicor light-activated resin 4 G Cera bis-GMA resin 5 Enforce UDMA resin 6 Dual resin 7 Sono Cem resin 8 Helio Link resin 9 Compspan resin 10 Calibra UDMA resin 11 Variolink 12 Appeal 13 Multilink 14 Resin-reinforced glass ionomer (RMGI) 15 Polycarboxylate (Duralon) with silicone 16 Zinc oxide and eugenol (Temp Bond) 18 Ceramic etch on internal (yes = 1; no = 0) 19 Restoration thickness (to 0.01 mm) 1 Mesial occlusal 2 Middle occlusal 3 Distal occlusal 4 Labial 5 Lingual 6 Mesial 7 Distal 20 Opposing dentition material 1 Natural tooth enamel 2 Silver amalgam 3 Edentulous 4 Gold 5 Composite resin 6 Feldspathic ceramic 7 Glass-ceramic 21 Dentin surface preparation 1 Dentin bonding agent 2 Varnish 3 Polyacrylic acid cleaning 4 None 22 Number of teeth in mouth 23 Biologic failures 1 Tooth requires endodontic therapy 2 Fractured tooth 3 Advancing periodontal disease 4 Increased mobility 5 Tooth sensitivity without endodontic 6 Psychosomatic pain

41

3

Practice-Based Research on the Effectiveness of Ceramics TABLE 3-2 All-ceramic restoration data from the Malament prosthodontic practice Ceramic Parameter Start date

Dicor

In-Ceram

Empress

e.max Press*

Totals

March 1993

February 1990

June 1992

May 2006

June 2013

Maximum service (mo) Patients (No.) Restorations (No.)

364

281

253

74

364

415

137

692

491

1,736

1,504

331

2,130

1,586

5,556

240

49

100

1

390

Failures (No.) †

Failures (%)

19.8

Chipping (No.) Replaced‡ (No.)

15.7

7.5

900

10.5

2

Procera Zirconia

210

14

6

1,200

10.4

2

Prettau Zirconia

210

12.5

NA

1,000

10

2

Dentin

16

0.6

3.1

—

11–14

—

Enamel

94

3.2

0.3

—

2–8

—

Material

Thermal conduction (Wm–1 K–1)

Veneering ceramic

IPS e.max Ceram IPS e.max ZirPress (fluorapatite) Glass-ceramic

Alumina

Zirconia

Tooth

NA, not applicable. Manufacturers: Vitablocks, Vident; Lava materials, 3M ESPE; IPS e.max materials, Ivoclar Vivadent; In-Ceram materials, Vident; Procera, Nobel Biocare; Cercon, Dentsply; DC-Zirkon, DSC Dental; Prettau Zirconia, Zirkonzahn.

materials have improved over time, but the failure rates did not decrease proportionally until a better understanding of the limitations of each material was acquired. This became possible thanks to recently published information from industrial, academic, and materials science sources, including specialized technical and clinical handling instructions. Second, the overall mechanical properties reported for high-strength ceramic materials are substantially higher

70

than those found for natural teeth and, although intact teeth may also crack,56 enamel has equivocally been considered a brittle structure. Comprehensive work has shown that enamel has a stress-strain response comparable to that of predominantly base metal alloys57 and exhibits viscoplastic and viscoelastic behavior closely matching those of bone, which is relevant for stress redistribution during loading.58 Also, a remarkable R-curve behavior (ie, an increase in crack growth resistance, as mentioned previously

Improving the Long-Term Performance

Box 5-1

Possible causes of chipping of the veneering ceramic over zirconia frameworks related to fabrication method and materials employed

• Insufficient support of the veneering material by the framework design60–62 • Mismatch of coefficient of thermal expansions between veneering and framework ceramics63 • Rapid cooling of the veneering porcelain64,65 • Unfavorable surface and heat treatment of the zirconia framework and associated phase transformation (high-pressure air abrasion with aluminum oxide)66 • Strength degradation of ceramics67

for zirconia) has been described for crack propagation from the outer to the inner enamel.59 Such characteristics may explain the strength and fatigue-resistant nature of human teeth.

Improving the Long-Term Performance Chipping, porcelain veneer cohesive fracture, and other nonstandardized terminology have been used to describe failures in single-unit zirconia crowns and also in conventional fixed dental prostheses, which have been far more investigated clinically than any other type of prostheses. Because a tremendous amount of recently acquired information has increased the profession’s understanding of the multifactorial nature of zirconia porcelain veneer failure (Box 5-1), it is hoped that present and future laboratory and clinical studies will lead to reduced fracture rates. Current manufacturer guidelines for laboratory handling of porcelain-fused-tozirconia ceramics have changed since the first version because of the unexpected early failures, almost exclusively limited to the porcelain veneer. Although numerous factors have been considered responsible for veneer failure, an important aspect that has been reviewed is the cooling rate.64 In the last few years, most companies have been suggesting a slow cooling regimen as part of the porcelain veneer firing protocol. Given that very few clinical studies of single crowns are available,45,49 the effect of changes in protocol and design were first evaluated in the laboratory setting. A significant contribution from multiple research groups has resulted in a number of publications in the field. To address the topic from a sequentially logical and informed platform, most of the studies described in this section will be those performed at the Department of Biomaterials and Biomimetics

at New York University College of Dentistry in collaboration with Dr Van P. Thompson, Dr E. Dianne Rekow, Dr Paulo G. Coelho, Dr Nelson Silva, Dr Petra Guess, Dr Yu Zhang, and several visiting scholars and clinical or research collaborators. A method capable of simulating clinically observed failures in the laboratory was developed and applied to several anatomically relevant prosthetic materials, especially in the molar region.68,69 One surprising and remarkable finding of a systematic review was that metal-ceramic crowns, in spite of their more than 50 years of use, have been sparsely investigated with controlled, prospective studies, unlike all-ceramic materials. Because metal-ceramic restorations are con­ sidered the gold standard, high-strength ceramics have been compared to them. In essence, zirconia molar crowns fabricated with early porcelain veneer firing protocols (and evaluated in 2008) presented significantly lower reliability (probability of survival when subjected to fatigue testing) than did metal-ceramic crowns.70 Therefore, reliability and characteristic strength targets were set based on the values observed for metal-ceramics and desired for high-strength ceramics. Some of the fatigue testing was mainly performed on the buccal cusp of the mandibular molar crown, where the occlusal contacts are normally expected to occur. However, observations of clinically failed zirconia prostheses showed that porcelain cohesive fractures also occurred in the lingual cusp of mandibular posterior teeth.50,71–73 For this reason, the probabilities of failure of the lingual and buccal cusps were compared; although no difference in reliability was observed between cusps, the finding that porcelain cohesive failures were much more extensive (virtually unrepairable) on the lingual than on the buccal cusp was of clinical significance.60 It became clear that frameworks for prostheses fabricated by CAD/CAM, when designed as default with even thicknesses, led to discrepancies in porcelain veneer thicknesses after the final anatomy was

71

5

Individual Ceramic Crowns for Teeth Fig 5-7  To reduce or eliminate the chance of porcelain chipping, zirconia frameworks should be shaped to support an even and reduced thickness of veneering ceramic, similar to metal-ceramic frameworks. In this single crown, note the lingual collar and the proximal ledge.

completed for function and esthetics. Therefore, framework design modifications that incorporate changes to minimize such effects have been suggested.60 The rationale for a specific framework design for an allceramic crown has been, in general, empirically suggested or simplistically derived from metal-ceramic frameworks when issues observed with the porcelain veneer compromised the survival of these restorations and a similar learning curve was required. Although several framework design modifications have been proposed since 1962 for metalceramic restorations,74 they mainly comprise a lingual collar of varied heights that extends proximally75–78 or a supportive framework for the final crown anatomy, known as anatomical design.61,79,80 Fatigue testing of porcelain-fused-to-zirconia crowns with uniform framework thickness compared to frame­ works where a lingual collar extending proximally was used for porcelain support showed that, in the latter, not only was reliability significantly improved, but also an important trend toward reduced porcelain veneer fracture sizes was consistently observed.62,81 Therefore, the benefits of improved characteristic strength and of reduced fracture sizes seem to be of clinical significance when framework design modification, including the lingual collar extending proximally, is considered (Fig 5-7). The main drawback is that such design modification provides changes only at the lingual aspect of the crown. Also, because these porcelains were fired according to protocols recommended in 2008, it is not surprising that, despite the greater reliability of the modified framework compared to the conventionally designed even-thickness framework, it was still significantly lower than that of the metal-ceramic crowns.70,82 Perhaps the most clinically relevant framework design modification for high-strength ceramics is anatomical design, involving use of a coping that is similar to the final crown anatomy, assuring an even and reduced thickness 72

of the veneering material compared to a coping of even thickness. For zirconia fixed dental prostheses, at a time where failure rates were commonly high, two clinical studies using this design demonstrated reduced compli­cations of porcelain veneer fracture.79,80 The use of an anatomically designed framework for zirconia crowns was investigated under fatigue testing and compared to a conventional framework design with uniform thickness, with either hand-layered or pressed veneering porcelain. The lowest reliability and strength were observed for the pressed veneer porcelain zirconia crown over the con­ventionally designed framework; a significant improvement was found if the same porcelain was pressed on the anatomically designed framework. Reliability was not significantly improved when a hand-layered veneering porcelain was fired onto the anatomically designed framework, because probability of survival was already at its highest levels, even in the conventionally designed coping. How­ever, irrespective of porcelain veneering method (pressed or hand-layered), the anatomical frame­work, besides improving the characteristic strength and reliability, always resulted in reduced porcelain veneer fracture size; repair or, sometimes, repolishing would return them to function. Such benefits were extended to all cusps.61 A proposal for a further evolution of the anatomical coping design has been advanced recently by a dentist–dental technician team who have been using zirconia as a prosthetic material for more than a decade.83 The modifications have been introduced for both posterior and anterior prostheses to improve their reliability while maintaining the esthetic advantages of a veneered restoration over a monolithic zirconia prosthesis. For posterior units, the zirconia coping is enriched with ridges and fins that support and reinforce the veneering ceramic at the cusp tips and circumferentially around the axial surfaces. The location and height of these ridges and fins are determined with the aid

Improving the Long-Term Performance Fig 5-8  Aesthetic functional area protection (AFAP) concept for the prevention of ceramic chipping, as applied to posterior zirconia frameworks. (arrows) Fins that extend inside the cusps almost to the surface provide a wall-contained area where the veneering ceramic is always placed under compression, even if the opposing teeth were to guide on the cusps’ inner inclines. The ridges on the axial surfaces of the coping support the veneering ceramic, which is no longer loaded with shearing forces but is placed in compression instead. This is important especially proximally, where the ridges are prone to fracture, even when the thickness of the veneering ceramic is not excessive, because of the absence of a supporting ledge. (Courtesy of Dr Mauro Broseghini and Cristiano Broseghini, CDT, Trento, Italy.)

a

b

c

d

e

f

g Fig 5-9  (a to c) The zirconia framework for a molar crown is shown after sintering. (d to f ) Different firings of the veneering ceramic on the corrugated coping. (g) Completed molar crown. (Courtesy of Dr Mauro Broseghini and Cristiano Broseghini, CDT, Trento, Italy.)

of a complete-contour wax-up. These additions to the basic coping design are shaped in such a way that they are just short of the surface of the completed restoration, so that they are entirely covered by the veneering material (Figs

5-8 and 5-9). Because zirconia is an esthetic material due to its color and relative translucency, in the critical areas, it can be brought almost to the surface without compromising the esthetic outcome of the restoration. 73

5

Individual Ceramic Crowns for Teeth

a

b

c

d

Fig 5-10  (a to d) The ceramic at the incisal margin of several anterior crowns of this metal-ceramic rehabilitation has chipped after 2 years in function.

In anterior restorations, the most vulnerable area, that is the one with a higher incidence of chipping, is the incisal edge (Fig 5-10). For this reason, this functional area of the crown and, possibly, the entire palatal surface of maxillary crowns ideally should be made of the stronger substructure material with little or no veneering ceramic layered on it. Once again, zirconia allows the technician to satisfy this requirement without negatively affecting the esthetic appearance of the completed crown. A very precise complete-contour wax-up of the crown is made. When the wax-up is cut back to create space for the veering ceramic, the incisal margin is kept intact, but a window is created below it (Fig 5-11). In this way, light is allowed to pass through, mediated by the more translucent veneering ceramic instead of being reflected by the framework material, which is much more opaque. Both posterior and anterior framework concepts have been named the aesthetic functional area protection (AFAP) frame­work designs.83 For both configurations, properly designed in vitro studies are needed to compare the fracture and chipping resistance of the veneering ceramic on this 74

framework design with that on other commonly used configurations. Comprehensive studies aimed at describing the reasons for cohesive porcelain fractures of layered- or pressedzirconia restorations have provided an enormous contri­ bution to the understanding of the role of porcelain thickness, coefficient of thermal expansion mismatches, and cooling rates on the resulting internal residual stresses within porcelain. The poor thermal conductivity of zirconia (2 Wm–1 K–1; see Table 5-2) compared to metal (200 Wm–1 K–1 for gold alloy) along with thick layers of porcelain veneer and rapid cooling of a layered zirconia restoration present a favorable scenario to generate internal residual stresses in the porcelain that can be exposed during occlusal adjustments or contacts, eventually resulting in the porcelain fractures observed clinically.64 Several studies incorporating a slow cooling protocol have shown that the reliability of zirconia crowns can be significantly improved, and specific firing protocols including this modification have been adopted by manufacturers.65,84–86 Within this context, combining framework design modification (such as AFAP, or an

Improving the Long-Term Performance

a

b

e

g

c

d

f

h

Fig 5-11  (a and b) Full wax-up of an anterior crown. Once the position, height, and volume of the restoration have been defined, a selective cutback can be carried out, either manually or digitally after the wax-up has been scanned. (c and d) The framework of the crown has been made out of a presintered zirconia blank using a pantographic milling system. The intact incisal margin is connected to the crown coping, and there is a wide open window between the two. (e and f ) The same framework is shown after the sintering process. (g) The veneering ceramic is applied. (h) The finished crown is placed. The incisal margin is entirely in zirconia without any overlying ceramic. (Courtesy of Dr Mauro Broseghini and Cristiano Broseghini, CDT, Trento, Italy.)

75

5

Individual Ceramic Crowns for Teeth

a

c

f

b

d

e

g Fig 5-12  (a) Complete wax-up of a single mandibular molar. A special wax has been used to capture the margin of the preparation. (b) Occlusal view of the wax-up. A handle has been added. (c) The wax-up is sprued and connected to the sprue former. (d) After the investment has hardened in the cylinder, the wax is burned in the heating oven, and the cylinder is then placed in the special oven, where the chosen ingot of lithium disilicate is heated and pressed. (e) The casting is shown after removal of the investment material with glass beads under air pressure. (f ) The crown is positioned and adapted on the master cast. (g) Occlusal view of the crown. The handle has been reproduced by the glass-infiltrated ceramic. (h) Stains to be used for coloring the surface of the restoration.

h

76

Improving the Long-Term Performance

i

j

k

l

Fig 5-12  (cont) (i) The finished crown is shown after the crystallization phase. (j) Occlusal view of the finished crown. (k) The crown has been cemented adhesively with a dual-cured composite resin. Use of rubber dam was made possible by the supragingival margins. (l) Occlusal view of the finished crown in situ. (m) Radiograph of the cemented crown. (Figures 5-12a to 5-12j courtesy of Luca Vailati, CDT, Tronzano Vercellese, Italy.)

m

anatomically designed framework) with slow cooling will result in improved support and a more even porcelain veneer–core thickness ratio; the consequences will be minimal residual stress and likely reduced fracture sizes, should they be caused by fatigue or parafunction.

To simplify the fabrication of high-strength ceramic posterior restorations, the exclusion of the porcelain veneer layer has been attempted in lithium disilicate crowns (Fig 5-12) as well as in zirconia crowns (Fig 5-13). Monolithic (complete-contour) high-strength all-ceramic crowns eliminate the weaker porcelain, resulting in improved 77

5

78

Individual Ceramic Crowns for Teeth

Fig 5-13  Monolithic zirconia crown. The surface has been stained and glazed in the oven.

Fig 5-14  Scalloped preparations present a challenge for the dental technician when fabricating metal-ceramic crowns. The larger the difference between the buccal and the proximal levels of the preparation, the more difficult it is to manufacture a metal-ceramic crown that maintains circumferentially acceptable marginal adaptation throughout the fabrication phases.

strength.87,88 However, the final thickness of the restoration plays an important role in crown reliability. Completecontour lithium disilicate molar crowns of 2.0-mm thickness at the occlusal surface and 0.5-mm thickness at the buccal surface have shown characteristic strength levels even higher than those of metal-ceramic restorations when subjected to fatigue. When the lithium disilicate crown was reduced to a 1.0-mm thickness to simulate limited occlusal clearance, the characteristic strength was not significantly different from that of the 2.0-mm crown but sufficiently lower to become similar to that of a metal-ceramic crown.89 Thus far, although clinical results are short-term, promising survival rates have been observed with monolithic lithium disilicate crowns.90,91 Similarly favorable results have been observed in vitro for complete-contour zirconia crowns, especially if glazed; similar translucency, contact wear of the opposing tooth, and an additional significant gain in strength have been observed compared to the properties of a porcelainlayered zirconia crown92 (see Fig 5-13). Although phase transformation (tetragonal-monoclinic) has been detected in a layer only 6 µm below the zirconia surface, mechanical properties have shown to be compromised by hydrothermal degradation, resulting in a 30% reduction of Young modulus of elasticity and hardness.67 Therefore, future clinical studies are warranted to ascertain if low-temperature degradation and aging of zirconia affect its long-term performance.

Clinical Criteria for Material Selection for Single Crowns To make a rational material selection between metal-­ ceramic crowns (with a metal margin or with a porcelain butt margin) and the different all-ceramic systems, the clinician and the dental technician can take into consideration a number of criteria. Material properties, as explained earlier, are the basis for understanding whether a new material has the potential to withstand the mechanical and thermal stresses of the oral environment. However, other clinical criteria can also influence the selection: •  Circumferential position of the preparation margin: Does the preparation margin follow the gingival margin, and thus is it scalloped, or does it follow the cementoenamel junction, and is it at a relatively horizontal level circumferentially? •  Appearance of the abutment: Is the color of the abutment within a normal range, or is it dark, or does it have a metal post? •  Position in the arch: Is the tooth to be restored an incisor, canine, or premolar, or is it a molar?

Clinical Criteria for Material Selection for Single Crowns

Porcelain

Porcelain

Porcelain

Metal Metal

Metal

Unstable

Stable a

b

Shrinkage during casting, high friction, and adaptation

Unstable Distortion during porcelain baking, marginal opening

c

Opening at the withdrawal of the wax pattern, distortion during baking

Fig 5-15  (a) In the fabrication of a metal-ceramic crown, having the margins approximately at the same level circumferentially helps the casting to remain stable during the firing of the veneering porcelain. Therefore, it is easier to produce a well-fitting crown. (b and c) With scalloped preparations, the crown is subjected to distortions during both casting and porcelain baking, and thus it is much more difficult to obtain a crown with well-adapted margins circumferentially. (Reprinted from Yamamoto96 with permission.)

Fig 5-16  Microscopic observation reveals the lack of marginal adaptation of a completed metal-ceramic crown with a disappearing metal margin.

Circumferential position of the preparation margin Marginal adaptation The work of a restorative dentist often consists in replacing previously made restorations. In those instances, the old preparation design and position can limit the freedom of choice as far as the selection of the material for the new crown is concerned. This is true especially when the margin is purposely “hidden” in the sulcus. An intrasulcular position of the crown margin often affects the restoration’s marginal precision and integrity in several ways: (1) it complicates the capture of an accurate impression of the finish line, therefore increasing session chair time; (2) it may hinder the operator in obtaining a satisfactory marginal seal during the cementation procedure; and (3) if the margin has followed the scallop of the gingiva, the preparation is scalloped as

well; this can make it difficult to fabricate metal-ceramic crowns with well-adapted margins around the circumference of the abutment. Several authors,93–96 recognized masters in the fabrica­ tion of metal-ceramic restorations, have pointed out the technical difficulties associated with obtaining and main­ taining optimal marginal adaptation throughout the manu­facturing process of the restoration. This is especially true when the finish line of a preparation has a pronounced scallop, that is, a marked difference between the buccal and proximal levels of the preparation (Fig 5-14). This scalloped preparation produces a crown with unsupported margins that are prone to distortion during the different firings in the porcelain oven (Figs 5-15 and 5-16). The request to fabricate a metal-ceramic crown with a porcelain butt margin increases further the level of difficulty for the technician, thus making circumferential marginal adapta­tion an almost impossible-to-reach goal or at least one that requires a high

79

5

Individual Ceramic Crowns for Teeth

a

b

c

d

Fig 5-17  (a) The fit of an alumina crown is observed on the stone die of a scalloped preparation of a mandibular premolar. (b) The fit of the crowns is demonstrated by a thin, uniform film of silicone disclosing paste. (c) The finished alumina crowns have been cemented with a glass-ionomer cement. (d) The same alumina crowns are shown after 10 years in function. There is slight discoloration at the margins.

level of competency and skill. When a relatively horizontal course is developed for the preparation margin, in contrast, metal-ceramic crowns demonstrate a more consistently satisfactory fit. Scalloped preparations for single crowns, therefore, may suggest the use of all-ceramic materials instead of metalceramic systems, certainly those fabricated through a heatpressed technique (leucite and lithium disilicate ceramics) but also those milled through CAD/CAM tech­nology (lithium disilicate ceramics, polycrystalline alumina, and zirconia). A potential advantage of polycrystalline ceramics is that they demonstrate good dimensional stability during all firing cycles of the veneering ceramic. Several studies97–101 have examined CAD/CAM crowns and have pointed out the factors that, beside preparation design and cementation procedure, may influence marginal adaptation: •  Software and hardware limitations •  Scanner type •  Machining technology 80

•  Single crown versus multiple-unit fixed partial denture •  Sintered versus nonsintered material The evidence published so far for many CAD/CAM systems on the market demonstrates a level of marginal precision that is clinically acceptable.102–104 Therefore, if the CAD/ CAM system utilized by the dental technician is able to produce a coping with clinically acceptable marginal adaptation, then the application of the veneering ceramic should not alter that relationship (Fig 5-17). This can represent a savings in time, which is an advantage for the dental technician, dentist, and patient. When a tooth is prepared for a crown for the first time, the approach of the clinician may be different. If the color of the abutment matches the adjacent and remaining natural teeth or prosthesis and no proximal spaces have to be closed, the dentist has the choice to keep the preparation margin supragingival and to follow the cementoenamel junction as the guide to the preparation instead of the gingival margin. The use of all-ceramic materials potentially al-

Clinical Criteria for Material Selection for Single Crowns

a

b

d

c

e

Fig 5-18  (a) This vital maxillary second premolar is prepared for a complete crown because of the fracture of the palatal cusp and the presence of a large mesio-occlusodistal composite resin restoration. (b and c) Because the cervical area on the buccal side is not involved, the preparation margin is kept at the level of the cementoenamel junction circumferentially. (d and e) The stone cast poured from the final impression of the tooth reveals the supragingival margins. (f ) Finished monolithic lithium disilicate crown.

f

lows the clinician to avoid intrasulcular placement of the preparation margins to hide the prosthetic margins, as is traditionally done with metal-ceramic crowns because of the opacity of the cervical area. These decisions not only can generate significant time savings during relining of a provisional restoration, impression taking, try-in, and cementation of the definitive crown, but also contribute to the preservation of tooth structure, especially the enamel around the margin, greatly benefiting the quality of the

marginal seal obtained with the cementation procedure and avoiding traumatic damage of the gingiva during preparation, impression, and cementation. Furthermore, if synthetic glass ceramics or the particle-filled glass ceramics are used, the prosthetic margins can be almost undetectable to the eye, enhancing the esthetic outcome. Therefore, whenever possible, preparations should follow the cementoenamel junction and avoid intrasulcular margin placement (Fig 5-18). 81

5

82

Individual Ceramic Crowns for Teeth

g

h

i

j

k

l

m

n

Clinical Criteria for Material Selection for Single Crowns

o

p

q

r Fig 5-18  (cont) (g and h) The crown is positioned on the stone cast. (i) The tooth is easily isolated with rubber dam. A waxed floss helps to keep the dam tucked cervically. (j) Because the tooth is isolated and the mouth protected, it is possible to clean the tooth with a light, short spray of aluminum oxide powder. (k) Cleaned tooth. (l and m) Although a self-adhesive cement is going to be used, the enamel margin is acid etched with 37% orthophosphoric acid for 30 seconds. (n) The abutment has been rinsed with water and dried. (o) The dual-curing resin cement has been light polymerized for a few seconds on both the buccal and the palatal sides. (p) The excess cement has been removed, and final light polymerization has been completed. (q) Buccal view of the finished crown. (r) After 4 years in situ, slight discoloration is visible at the margin.

83

5

Individual Ceramic Crowns for Teeth TABLE 5-3 Possible choices of cement types for complete crowns made of different substrates Prosthesis substrate

Cement RMGIC DC-CR

ZnP

GIC

✓ ✓

✓ ✓

✓ ✓

✓ ✓

✓ ✓

Densely sintered alumina

✗ ✓

✗ ✓

Densely sintered zirconia

✓

✓

High-noble alloys Noble alloys Predominantly base alloys Titanium and titanium alloys Glass-ceramics

LC-CR

CC-CR

✗ ✗

✗ ✗

✓ ✓

✓ ✓

✗ ✗

✗ ✗

✓ ✓

?

✓

?*

✓

?

✗

✓ ✓

✓

?

✗

✓

ZnP, zinc phosphate; GIC, glass ionomer; RMGIC, resin-modified glass ionomer; DC, dual-cured; CR, composite resin; LC, light-cured; CC, chemically cured. ✓, indicated; ✗, contraindicated; ?, possible, but not ideal. *If the thickness of the restoration is not more than 1 to 1.5 mm.

Cementation of single crowns Although cementation procedures and materials for highstrength ceramics are discussed in chapter 9, it is important to emphasize key aspects of the cementation procedures for single-unit, high-strength ceramic restorations. Cement selection and the ability to isolate the abutment to perform the cementation procedure properly are related factors that strongly influence the selection of crown material105,106 (Table 5-3). When a prosthesis has to be permanently cemented to a natural abutment, it is important to provide the proper conditions for the luting agent to set optimally. For example, a clinician interested in taking advantage of the superior translucency of glass-ceramics, aware that proper adhesive cementation is necessary to enhance their strength so that they can perform well clinically,107 may be in doubt whether to select such a material if the preparation margin is located apical to the gingival margin and the gingiva is not optimally healthy. At a time when composite resin cementation has become so popular and prescribed for any prosthetic substrate because of its superior retention, convenient packaging, and reduced setting time compared with zinc phosphate and glass-ionomer cements, the issue of selecting the best luting agent for intrasulcular preparations still is a dilemma for many clinicians. In principle, a luting agent should fulfill, among others, some basic requirements: provide the prosthesis with the necessary retention; be resistant to dissolution; present high strength in tension, shear, and compression; be user

84

friendly (adequate working and setting times); be biologically acceptable; and be adequate to seal the interface between prosthesis and tooth to avoid bacterial infiltration.7,9,108 For the last requirement to be fulfilled, the cement should not come in contact with saliva or blood during setting because part of the cement may be contaminated before it has had the chance to set, and the seal at the margin may be compromised. If the preparation margin is supragingival or at the gingival margin, keeping it isolated is fairly simple (see Fig 5-18i). If the margin is intrasulcular, rubber dam isolation may be a very time-consuming procedure and one that still may not guarantee total fluid control (Fig 5-19). Traditionally, metal-ceramic crowns have been cemented with zinc phosphate or glass-ionomer cements. These same agents have been recommended for the nonetchable ceramics, alumina and zirconia. The main drawbacks of these cements are their relatively high solubility in the early stages of setting and their prolonged setting times. Thus, in the presence of fluids, it is difficult to protect their integrity for the time needed to reach a sufficiently insoluble state. The advantage of the resin cements is that they set faster; in case of the dual-cured resin cements, their light-curing portion is within the control of the operator. Some clinicians feel that resin cements, therefore, can provide a more reliable marginal seal even in less than ideal conditions. Although there is no scientific evidence that an optimal seal is necessary to increase the longevity of a restoration, the lack of such a seal may increase the susceptibility of the

Clinical Criteria for Material Selection for Single Crowns

a

b

c

d

e

f

g

h

Fig 5-19  (a and b) Despite the healthy gingival tissues, the intrasulcular placement of the preparation margins makes it difficult for ideal isolation of the field to allow the cement to set in optimal conditions. In these instances, the use of a composite resin cement that has a light-curing component may be advantageous because the cement can be stabilized as soon as the placement of the restoration is deemed satisfactory. (c to e) Zirconia crowns have been cemented with a self-adhesive, dual-curing cement after a retraction cord was placed in the sulcus of each abutment. (Prosthetic work by Luca Vailati, CDT, Tronzano Vercellese, Italy.) (f to h) Periapical radiographs of the teeth show the definitive restorations in situ.

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Individual Ceramic Crowns for Teeth TABLE 5-4 Surface treatment of different prosthetic substrates to be luted with a composite resin cement* Substrate High-noble alloy Noble alloy Predominantly base alloy Titanium and titanium alloy

Surface treatment Air-particle abrasion

With what 50-μm alumina particles

Pressure

Activator

Sample product name

~3 bar

Specific for alloys or universal

Alloy Primer Signum Metal Bond or Scotchbond Universal† Monobond Plus†

30-μm silica-coated alumina particles Rocatec Soft or CoJet

~3 bar

Silane coating

RelyX Ceramic Primer Clearfil Ceramic Primer

or

Lithium disilicate glass-ceramic Leucite glass-ceramic Feldspathic ceramic

Acid etching

5% hydrofluoric acid: 20 s for disilicate, 60 s for leucite, and 120 s for feldspathic

NA

Silane coating

RelyX Ceramic Primer Clearfil Ceramic Primer

Densely sintered alumina

Air-particle abrasion

50-μm alumina particles

~3 bar

Silane coating

RelyX Ceramic Primer Clearfil Ceramic Primer

Densely sintered zirconia

Air-particle abrasion

50-μm alumina particles

~1.5 bar

Specific for ceramics or universal

RelyX Ceramic Primer Clearfil Ceramic Primer or Scotchbond Universal† Monobond Plus†

~1.5 bar

Silane coating

RelyX Ceramic Primer Clearfil Ceramic Primer

or 30-μm silica-coated alumina particles Rocatec Soft or CoJet

NA, not applicable. *Do not use orthophosphoric acid for the surface cleaning and decontamination of alloys, etched ceramics, and polycrystalline ceramics tried in the oral cavity just before final cementation with a composite resin cement, because orthophosphoric acid deactivates the surfaces of the crown substrates for adhesive resin cementation. † Contains methacryloyloxydecyl dihydrogen phosphate (MDP). Manufacturers: Rocatec Soft or CoJet, 3M ESPE; Alloy Primer, Kuraray; Signum Metal Bond, Heraeus Kulzer; Scotchbond Universal, 3M ESPE; Monobond Plus, Ivoclar Vivadent; RelyX Ceramic Primer, 3M ESPE; Clearfil Ceramic Primer, Kuraray.

patient to a failure because of the recurrence of secondary caries or loss of retention. An optimal marginal seal is enhanced whenever the crown’s substrate has been treated appropriately109 (Table 5-4), a proper field isolation has been provided, and the correct procedure has been carried out for the luting agent selected (Table 5-5). Ideally, rubber dam should be applied, but, if the preparation margin is intrasulcular and the gingiva is healthy, the placement of a retraction cord may be clinically acceptable as well.

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In addition, appropriate selection of the adhesive system is of utmost importance, irrespective of manufacturer recommendation. Incompatibility and permeability issues between simplified adhesive systems (one-step self-etch and two-step etch and rinse systems) and self- and dualcure resin cements have been extensively reported in the literature.110–113 Instead, conventional total-etch three-step or two-step self-etch adhesive systems are indicated for cementation with dual- or self-cure resin cements.108,114

Clinical Criteria for Material Selection for Single Crowns TABLE 5-5 Examples of resin cements, their categorization, and products for the pretreatment of the dental surfaces*

Cement with separate adhesive Total or selective etching (etch and rinse)

Self-etching (etch and dry)

✓

✓

Scotchbond Universal*

Clearfil Esthetic Cement

✓

ED Primer†

Panavia F2.0*

✓

ED Primer†

Product

Self-adhesive cement

Name of adhesive

Dual-cured RelyX Unicem2

✓

Clearfil SA Cement

✓

SpeedCem

✓

RelyX Ultimate

Variolink II

✓

Excite F DSC or Syntac Classic

Enacem

✓

Enabond

Light-cured Variolink Veneer

✓

RelyX Veneer

✓

Excite F DSC or Syntac Classic ✓

Scotchbond Universal*

Multilink‡

✓

Multilink Primer A/B

Panavia 21†

✓

ED Primer†

Self-cured

*Self-adhesive and self-cured cements are indicated for luting metal or metal-ceramic, alumina, and zirconia crowns. † Contains MDP. ‡ Can also be light-cured but, according to the manufacturer, only for an easier removal of excess cement. Manufacturers: RelyX materials, 3M ESPE; Clearfil materials, Kuraray; SpeedCem, Ivoclar Vivadent; Panavia materials, Kuraray; Variolink materials, Ivoclar Vivadent; Enacem, Micerium; Multilink, Ivoclar Vivadent.

Appearance of the abutment Because of the differing relative translucencies of different all-ceramic materials (Fig 5-20), the color of the underlying structure may be of relevance in the selection of the proper material. If the abutments are of normal color, the best material to use may be the more translucent ones, that is, the feldspathic and synthetic glass ceramics, especially if residual enamel is preserved, at least around the preparation margin. With these materials, it is possible to obtain an invisible margin between restoration and tooth substrate (Figs 5-21 and 5-22).

Discolored teeth, or teeth with metal posts or dark preprosthetic restorations (Fig 5-23a), on the other hand, may need to be masked by an opaque core. Studies on core translucency16,17 have pointed out the inadequacy of the glass-ceramics to mask underlying dark-colored substrates. However, the outcome depends also on the amount of space that the technician has available on the buccal side for the crown, especially in the marginal area. If the space exceeds 1.3 mm, there is a better chance to mask dark backgrounds with the all-ceramic materials. Moreover, in recent years, a number of opaque cores have been introduced in the disilicate family; these can reach the same degree of

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Fig 5-20  All-ceramic materials may exhibit different translucencies at the same thickness. Dental technicians must know the masking abilities of different materials when there is a request to fabricate an all-ceramic restoration on a discolored or metal substrate. This image highlights the different translucencies of four lithium disilicate samples of the same thickness (0.8 mm) but of increasing opacities. (Courtesy of Prof Daniel Edelhoff, Munich, Germany.)

a

b

c Fig 5-21  (a) A 19-year-old patient requested the replacement of composite resin restorations applied about 10 years earlier after a traumatic fracture of the two central incisors as well as closure of the diastema. (b) The composite resin has been removed, and the teeth have been prepared in enamel for porcelain veneers. The preparation margin is supragingival in both teeth. (c) After adhesive cementation of the two feldspathic ceramic veneers, the interface between the supragingival restorations and the remaining tooth structure is hardly visible. (Prosthetic work by Marco Cossu, CDT, Lugano, Switzerland.)

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Clinical Criteria for Material Selection for Single Crowns

a

b

c

d

Fig 5-22  (a) A 30-year-old woman had a lemon-sucking habit for several years. When eating and drinking started to cause severe sensitivity of her teeth, despite some conservative attempts to cover the eroded tooth structure, the patient asked for a more comprehensive approach. (b) To keep the mandibular incisors vital, after the removal of the existing composite resin restorations, a very conservative circumferential preparation of the incisors was carried out. The preparations end about 1 mm coronal to the cementoenamel junction. (c) The leucite-based ceramic crowns are shown shortly after cementation with a light-cured composite resin cement. (Prosthetic work by Fabrizio Tordi, CDT, and Roberto Nobili, CDT, Milan, Italy.) (d) The same restorations remain indistinguishable from remaining tooth structure after 14 years in service.

masking as the polycrystalline ceramics of similar thickness17 (Figs 5-23b and 5-23c). It is extremely helpful for the technician to see the color of the prepared teeth. The technician can be provided with photographic images of the prepared abutments to allow him or her to evaluate their color to aid in the selection of the material for the restoration. In esthetically demanding areas, it is strongly suggested that the copings be tried in prior to the application of the veneering ceramic (see Fig 5-23b).

When the abutment is very dark and the discoloration extends down the root, often the esthetic result is inadequate even when a totally opaque material is used, because the gingiva, especially if it is of a thin biotype, is unable to conceal the grayish appearance115 (Fig 5-24). At times, when opaque cores are used to raise the value of the restorations on endodontically treated teeth, even if the color of the residual tooth structure seems to be in a normal range, a shadow is generated in the cervical area (Fig 5-25). A possible solution may be to cut back the core in the area of the buccal margin to allow more passage of light. 89

5

Individual Ceramic Crowns for Teeth

a

c

a

b

Fig 5-23  (a) The right central incisor presents a cast gold post and core and moderately discolored cervical dentin, while the other three incisors are vital and have a normal-­colored substrate. This abutment has lost more tooth structure on the buccal side than has the contralateral tooth; therefore, more space is available for this crown. A glass-ceramic material will be used for all four crowns. (b) The try-in of the all-ceramic copings reveals the difference in color of the coping for the right central incisor compared to those for the other incisors. A more opaque and higher value blank has been used for the right central incisor. (c) The finished lithium disilicate crowns have been cemented with a self-adhesive composite resin cement. The color match at the crown is quite satisfactory, but at the gingival level, a grayish look remains. (Prosthetic work by Luca Vailati, CDT, Tronzano Vercellese, Italy.)

b

Fig 5-24  (a) The left central incisor of this young woman not only has a cast gold post and core but also presents an extremely dark dentin both at the supragingival and subgingival levels. (b) Two alumina crowns have been manufactured for these abutments. Despite the fact that the left crown is thicker on the buccal side and has been cemented with an opaque luting agent, the gingival area, as expected, displays a grayish shadow.

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Clinical Criteria for Material Selection for Single Crowns

a

b

c

d

e

f

g Fig 5-25  (a) A 37-year-old woman requested the replacement of her anterior crowns. (b) The four incisor metal-ceramic crowns, inserted about 10 years earlier, are opaque looking and have a visible dark margin. (c) The palatal view of the metal-ceramic crowns reveals areas of exposed opaquer and the high metal collars. (d) All the incisors had already been treated endodontically. The treatment seemed incomplete, but no periapical lesion is visible. (e) A slit has been created in the middle of each crown to allow them to be pried open with an instrument. (f ) After removal of the crowns. (g) The lateral incisor crowns display signs of marginal infiltration, but no caries is detected on the teeth.

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Individual Ceramic Crowns for Teeth

h

i

j

k Fig 5-25  (cont) (h) The preparations have been finalized. (i) All the teeth have been endodontically retreated. New direct preprosthetic reconstructions have been made with composite resin and the placement of glass-fiber posts. (j) Lithium disilicate crowns. (k) On the palatal aspect, the lithium disilicate coping has been left exposed because of the lack of space between the preparation and the opposing dentition.

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Clinical Criteria for Material Selection for Single Crowns

l

m

n Fig 5-25  (cont) (l) The definitive crowns have been cemented with a self-adhesive, self-curing composite resin cement. (m) Despite the use of a metal-free material, a slight gray shadow is visible in the marginal area extending apically. (Prosthetic work by Luca Vailati, CDT, Tronzano, Vercellese, Italy.) (n) Periapical radiographs of the teeth with the definitive restorations in situ.

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Position in the arch When metal-ceramic systems were the main material available for crown fabrication, the location in the arch of the tooth to be restored, whether an anterior tooth, a premolar, or a molar, was a variable that sometimes influenced the extension of the visible metal collars. On the other hand, with all-ceramic materials, this information may influence the clinician’s choice of the specific product.6 As has already been mentioned, the forces that can be generated in the mouth, especially in the posterior area, can be high; therefore, it is important to choose the most reliable materials to withstand long-term use. Different studies and reviews have pointed out that, for anterior teeth, any all-ceramic material can achieve a 5-year survival rate comparable to that of metal-ceramic restorations.6 Even at the premolar level, the performance seems to be just as satisfactory. However, some researchers have stated that, for glass-ceramics to be this successful, adhesive bonding is a prerequisite.60,103 The controversy today relates to the choice of material for the restoration of molars. On one hand, some clinicians recommend the polycrystalline ceramics, and zirconia in particular, as the best solution to reliably withstand loads. If a veneered zirconia crown is to be manufactured, however, then the preparation has to have depths similar to those for a metal-ceramic crown. Monolithic zirconia crowns may be an alternative that not only prevents chipping but also allows for a less invasive preparation.116 There are, however, questions about the wear induced by this material on the opposing dentition and on the procedures to properly adjust and polish the zirconia occlusal surface in the oral cavity,92 with information currently restricted to in vitro studies.108 The use of lithium disilicate in the monolithic configuration is acquiring more acceptance even in the molar region, because a properly performed adhesive cementation procedure should increase the already high reliability of the material,8,89 especially if some enamel is still present. Even in cases with abutment height of more than 4 mm and an angle of convergence less than 10 degrees, and when isolation or humidity control was not feasible, use of conventional glass-ionomer cement did not influence the failure or complication rate of lithium disilicate crowns in an up to 9-year prospective clinical study when compared to adhesive bonding.117 This may become a strong driving force in allowing and promoting less invasive preparation techniques. Furthermore, this material is undoubtedly easier to adjust and polish.

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High-Strength Ceramic Posts and Cores When endodontically treated teeth with significant loss of coronal dentin have to be restored with crowns, the placement of posts and cores to improve retention of the preprosthetic reconstructions and the definitive restorations has been recommended. Traditionally, cast posts have been used. However, prefabricated metallic (eg, stainless steel, titanium) and tooth-colored posts (eg, glass- or polyethylene fiber–reinforced composite resin or zirconia) associated with a direct restoration have become the preferred choice because their use eliminates one clinical appointment.118 A variety of prefabricated post materials and designs are available, but this discussion focuses on tooth-colored posts, specifically on zirconia (Fig 5-26). When zirconia posts are used, the restorative possibilities for core purposes are composite resin core, direct ceramic core heat pressing, and indirect ceramic core processing. Beside being tooth colored, an ideal post material should ideally present some of the following characteristics already shown to be important when used in combination with allceramic crowns119: good bond strength to resin cements; resistance to flexural forces; modulus of elasticity similar to that of the root dentin (16 GPa); user friendliness during insertion and removal, should endodontic retreatment be necessary; easy adaptation to the root canal without requiring excessive tooth structure removal; and biocompatibility. Although zirconia posts do not fulfill all of these characteristics, some of them can be improved, such as their bond strength to resin cements by silica deposition techniques; others simply cannot, such as ease of removal and modulus of elasticity. Zirconia posts are virtually impossible to grind, and their removal along with root dentin preservation becomes a challenge, if required.116 Only two clinical studies are available concerning the survival rates of zirconia posts, both with follow-up periods of less than 5 years. In one study, 30 zirconia posts (only 5 were in the posterior region) that had heat-pressed ceramic cores and were bonded with glass-ionomer cement were followed for 29 months. No complications such as fracture or loss of retention were reported.120 In another clinical study, the authors evaluated the survival rates of 34 zirconia posts with pressed glass-ceramic cores and 79 zirconia posts with direct composite resin cores after a follow-up period of up to 5 years. No failures were observed for the

High-Strength Ceramic Posts and Cores

a

b

c

d

Fig 5-26  (a) Two vital maxillary lateral incisors exhibit a considerable loss of coronal tooth structure in an elderly patient with severely calcified canals. (b) After tooth reduction and successful endodontic treatment, a post space has been prepared, prefabricated zirconia posts have been fitted in the canals, and a core has been molded with self-polymerizing resin applied with the salt and pepper technique. (c) The cores have been fabricated in a pressable ceramic on the zirconia post and adhesively luted in the canal with a composite resin cement. (d) The definitive pressable leucite-based glass-ceramic crowns have been cemented with a composite resin cement. (Courtesy of Dr Jonathan Ferencz, New York, New York.)

zirconia with composite resin core group, but three failures due to loss of retention were reported in the pressed glass-ceramic group.121 Although results seem promising, the overall evidence from these studies suggesting that zirconia posts are a scientifically validated and safe treatment method should be interpreted with caution. Longer followup periods in well-designed clinical studies are necessary. An interesting theoretical point concerns the relevance of having posts with similar modulus of elasticity to those of the root dentin. Remarkably, the available clinical studies fail to show any root fractures and, although such a finding may have commonly been found in in vitro studies (mainly observed in single-load-to-failure tests or “crunch

the sample tests” sometimes associated with previous cyclic fatigue),116,122,123 the authors believe that such results should also be interpreted with caution. If root fractures have not been described in clinical studies, but are commonly observed in laboratory studies, it may be appropriate to critically evaluate the information of such studies and reconsider the relevance of the methodologies being used. Because the majority of studies on zirconia posts are in vitro, limited recommendations can be provided in light of the existing evidence-based information.116 Therefore, future long-term randomized clinical trials comparing the survival rates and complications of zirconia posts with fiberreinforced composite resin and cast posts are warranted.

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Conclusion High-strength all-ceramic materials used for tooth-­ supported single crowns are a reliable treatment option for restoring anterior and posterior teeth. However, for a successful outcome, a multitude of factors must be carefully addressed, including patient selection, clinical procedures, and laboratory techniques. Future incorporation of new ceramic materials and processing technologies into esthetic dentistry must be carefully weighed and should definitely be preceded by relevant laboratory testing prior to clinical use. Science, industry, clinical, and laboratory expertise on high-strength ceramics have evolved to an extent that current knowledge and evidence are abundantly available to promote their successful use. This collaborative effort remains absolutely critical to the future development and adoption of new materials.

Acknowledgments Dr Gracis would like to thank Luca Vailati, CDT, and Marco Cossu, CDT, for their invaluable collaboration and expertise in the delivery of highquality, natural-looking restorations. Dr Bonfante would like to acknowledge grants from Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) (No. 4695/06-2) and Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) (No. 2010/06152-9). The authors would like to specially acknowledge Dr Van P. Thompson and Dr Elizabeth Dianne Rekow for their invaluable leadership and mentorship on ceramics and research.

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8. Guess PC, Schultheis S, Bonfante EA, et al. All-ceramic systems: Laboratory and clinical performance. Dent Clin North Am 2011;55: 333–352. 9. O’Brien WJ (ed). Dental Materials and Their Selection, ed 4. Chicago: Quintessence, 2008:212–229. 10. Denny IL. Restorative materials—Ceramics. In: Sakaguchi RL, Powers JM (eds). Craig’s Restorative Dental Materials, ed 13. Philadelphia: Elsevier Mosby, 2012:253–275. 11. Kelly JR, Benetti P. Ceramic materials in dentistry: Historical evolution and current practice. Aust Dent J 2011;56 Suppl 1:84–96. 12. Green DJ, Hannink RHJ, Swain MV. Transformation Toughening of Ceramics. Boca Raton, FL: CRC Press, 1989. 13. Garvie RC, Hannink RHJ, Pascoe RT. Ceramic steel? Nature 1975; 258:703–704. 14. Piconi C, Maccauro G. Zirconia as a ceramic biomaterial. Biomaterials 1999;20:1–25. 15. Chevalier J, Gremillard L, Virkar AV, Clarke DR. The tetragonal‐ monoclinic transformation in zirconia: Lessons learned and future trends. J Am Ceram Soc 2009;92:1901–1920. 16. Damestani Y, Reynolds CL, Szu J, et al. Transparent nanocrystalline yttria-stabilized-zirconia calvarium prosthesis. Nanomedicine 2013; 9:1135–1138. 17. Heffernan MJ, Aquilino SA, Diaz-Arnold AM, et al. Relative translucency of six all-ceramic systems. 1. Core materials. J Prosthet Dent 2002;88:4–9. 18. Heffernan MJ, Aquilino SA, Diaz-Arnold AM, et al. Relative translucency of six all-ceramic systems. 2. Core and veneer materials. J Prosthet Dent 2002;88:10–15. 19. Agrawal KR, Lucas PW, Bruce IC, Prinz JF. Food properties that influence neuromuscular activity during human mastication. J Dent Res 1998;77:1931–1938. 20. Hiiemae K. Mechanisms of food reduction, transport and deglutition: How the texture of food affects feeding behavior. J Texture Stud 2004;­35:171–200. 21. van der Bilt A. Assessment of mastication with implications for oral rehabilitation: A review. J Oral Rehabil 2011;38:754–780. 22. Goodacre CJ, Bernal G, Rungcharassaeng K, Kan JY. Clinical complications in fixed prosthodontics. J Prosthet Dent 2003;­90:31–41. 23. Ahlberg JP, Kovero OA, Hurmerinta KA, et al. Maximal bite force and its association with signs and symptoms of TMD, occlusion, and body mass index in a cohort of young adults. Cranio 2003;21:248– 252. 24. Bakke M, Michler L, Han K, Moller E. Clinical significance of isometric bite force versus electrical activity in temporal and masseter muscles. Scand J Dent Res 1989;97:539–551. 25. Bakke M, Holm B, Jensen BL, Michler L, Moller E. Unilateral, isometric bite force in 8–68-year-old women and men related to occlusal factors. Scand J Dent Res 1990;98:149–158. 26. Braun S, Bantleon HP, Hnat WP, et al. A study of bite force. 1. Relationship to various physical characteristics. Angle Orthod 1995; 65:367–372. 27. Ferrario VF, Sforza C, Serrao G, Dellavia C, Tartaglia GM. Single tooth bite forces in healthy young adults. J Oral Rehabil 2004;­31:18– 22. 28. Gibbs CH, Mahan PE, Lundeen HC, et al. Occlusal forces during chewing and swallowing as measured by sound trans­mission. J Prosthet Dent 1981;46:443–449. 29. Hagberg C. The amplitude distribution of electromyographic activity of masticatory muscles during unilateral chewing. J Oral Rehabil 1986;13:567–574. 30. Haraldson T, Karlsson U, Carlsson GE. Bite force and oral function in complete denture wearers. J Oral Rehabil 1979;6:41–48. 31. Helkimo E, Carlsson GE, Helkimo M. Bite force and state of dentition. Acta Odontol Scand 1977;35:297–303. 32. Ikebe K, Nokubi T, Morii K, Kashiwagi J, Furuya M. Association of bite force with ageing and occlusal support in older adults. J Dent 2005;33:131–137. 33. Miyaura K, Morita M, Matsuka Y, Yamashita A, Watanabe T. Rehabilitation of biting abilities in patients with different types of dental prostheses. J Oral Rehabil 2000;27:1073–1076.

References 34. Shinogaya T, Bakke M, Thomsen CE, Vilmann A, Matsumoto M. Bite force and occlusal load in healthy young subjects—A methodological study. Eur J Prosthodont Restor Dent 2000;8(1):11–15. 35. Thompson DJ, Throckmorton GS, Buschang PH. The effects of isometric exercise on maximum voluntary bite forces and jaw muscle strength and endurance. J Oral Rehabil 2001;28:909–917. 36. Tortopidis D, Lyons MF, Baxendale RH, Gilmour WH. The variability of bite force measurement between sessions, in different positions within the dental arch. J Oral Rehabil 1998;25:681–686. 37. van der Bilt A, Tekamp A, van der Glas H, Abbink J. Bite force and electromyography during maximum unilateral and bilateral clenching. Eur J Oral Sci 2008;116:217–222. 38. Waltimo A, Könönen M. A novel bite force recorder and maximal isometric bite force values for healthy young adults. Scand J Dent Res 1993;101:171–175. 39. Roldan S, Buschang PH, Isaza Saldarriaga JF, Throckmorton G. Reliability of maximum bite force measurements in age-varying populations. J Oral Rehabil 2009;36:801–807. 40. Rekow D, Thompson VP. Engineering long term clinical success of advanced ceramic prostheses. J Mater Sci Mater Med 2007;18:47– 56. 41. Rekow D, Zhang Y, Thompson V. Can material properties predict survival of all-ceramic posterior crowns? Compend Contin Educ Dent 2007;28:362–368. 42. Swain MV, Rose L. Strength limitations of transformation‐toughened zirconia alloys. J Am Ceram Soc 1986;69:511–518. 43. Groten M, Huttig F. The performance of zirconium dioxide crowns: A clinical follow-up. Int J Prosthodont 2010;23:429–431. 44. Marchack BW, Futatsuki Y, Marchack CB, White SN. Customi­zation of milled zirconia copings for all-ceramic crowns: A clinical report. J Prosthet Dent 2008;99:169–173. 45. Ortorp A, Kihl ML, Carlsson GE. A 3-year retrospective and clinical follow-up study of zirconia single crowns performed in a private practice. J Dent 2009;37:731–736. 46. Poggio CE, Dosoli R, Ercoli C. A retrospective analysis of 102 zirconia single crowns with knife-edge margins. J Prosthet Dent 2012; 107:316–321. 47. Scherrer SS, Quinn JB, Quinn GD, Wiskott HW. Fractographic ceramic failure analysis using the replica technique. Dent Mater 2007; 23:1397–1404. 48. Scherrer SS, Quinn GD, Quinn JB. Fractographic failure analysis of a Procera AllCeram crown using stereo and scanning electron microscopy. Dent Mater 2008;24:1107–1113. 49. Cehreli MC, Kokat AM, Akca K. CAD/CAM Zirconia vs. slip-cast glass-infiltrated Alumina/Zirconia all-ceramic crowns: 2-year results of a randomized controlled clinical trial. J Appl Oral Sci 2009;17: 49–55. 50. Sailer I, Gottnerb J, Kanelb S, Hammerle CH. Randomized controlled clinical trial of zirconia-ceramic and metal-ceramic posterior fixed dental prostheses: A 3-year follow-up. Int J Prosthodont 2009;22:553–560. 51. Koenig V, Vanheusden AJ, Le Goff SO, Mainjot AK. Clinical risk factors related to failures with zirconia-based restorations: An up to 9-year retrospective study. J Dent 2013;41:1164–1174. 52. Rafferty BT, Bonfante EA, Janal MN, et al. Biomechanical evaluation of an anatomically correct all-ceramic tooth-crown system configuration: Core layer multivariate analysis incor­porating clinically relevant variables. J Biomech Eng 2010;132­(5):051001. 53. Malament KA, Socransky SS. Survival of Dicor glass-ceramic dental restorations over 16 years. 3. Effect of luting agent and tooth or tooth-substitute core structure. J Prosthet Dent 2001;­86:511–519. 54. Kelly JR. Perspectives on strength. Dent Mater 1995;11:103–110. 55. Koutayas SO, Vagkopoulou T, Pelekanos S, Koidis P, Strub JR. Zirconia in dentistry. 2. Evidence-based clinical breakthrough. Eur J Esthet Dent 2009;4:348–380. 56. Lubisich EB, Hilton TJ, Ferracane J. Cracked teeth: A review of the literature. J Esthet Restor Dent 2010;22:158–167. 57. He LH, Swain MV. Nanoindentation derived stress-strain properties of dental materials. Dent Mater 2007;23:814–821.

58. He LH, Swain MV. Nanoindentation creep behavior of human enamel. J Biomed Mater Res A 2009;91:352–359. 59. Bajaj D, Arola DD. On the R-curve behavior of human tooth enamel. Biomaterials 2009;30:4037–4046. 60. Bonfante EA, Sailer I, Silva NR, et al. Failure modes of Y-TZP crowns at different cusp inclines. J Dent 2010;38:707–712. 61. Guess PC, Bonfante EA, Silva NR, Coelho PG, Thompson VP. Effect of core design and veneering technique on damage and reliability of Y-TZP-supported crowns. Dent Mater 2013;29:307–316. 62. Silva NR, Bonfante EA, Rafferty BT, et al. Modified Y-TZP core design improves all-ceramic crown reliability. J Dent Res 2011;90:104– 108. 63. Bonfante EA, Rafferty BT, Silva NR, et al. Residual thermal stress simulation in three-dimensional molar crown systems: A finite element analysis. J Prosthodont 2012;21:529–534. 64. Swain MV. Unstable cracking (chipping) of veneering porcelain on all-ceramic dental crowns and fixed partial dentures. Acta Biomater 2009;5:1668–1677. 65. Tholey MJ, Swain MV, Thiel N. Thermal gradients and residual stresses in veneered Y-TZP frameworks. Dent Mater 2011;27:1102– 1110. 66. Tholey MJ, Swain MV, Thiel N. SEM observations of porcelain Y-TZP interface. Dent Mater 2009;25:857–862. 67. Cattani-Lorente M, Scherrer SS, Ammann P, Jobin M, Wiskott HW. Low temperature degradation of a Y-TZP dental ceramic. Acta Biomater 2011;7:858–865. 68. Coelho PG, Bonfante EA, Silva NR, Rekow ED, Thompson VP. Laboratory simulation of Y-TZP all-ceramic crown clinical failures. J Dent Res 2009;88:382–386. 69. Coelho PG, Silva NR, Bonfante EA, et al. Fatigue testing of two porcelain-zirconia all-ceramic crown systems. Dent Mater 2009;25: 1122–1127. 70. Silva NR, Bonfante EA, Zavanelli RA, et al. Reliability of metalloceramic and zirconia-based ceramic crowns. J Dent Res 2010;89: 1051–1056. 71. Guess PC, Strub JR, Steinhart N, Wolkewitz M, Stappert CF. Allceramic partial coverage restorations—Midterm results of a 5-year prospective clinical splitmouth study. J Dent 2009;37:­627–637. 72. Raigrodski AJ, Chiche GJ, Potiket N, et al. The efficacy of posterior three-unit zirconium-oxide-based ceramic fixed partial dental prostheses: A prospective clinical pilot study. J Prosthet Dent 2006; 96:237–244. 73. Sailer I, Feher A, Filser F, et al. Five-year clinical results of zirconia frameworks for posterior fixed partial dentures. Int J Prosthodont 2007;20:383–838. 74. Shelby DS. Practical considerations and design of porcelain fused to metal. J Prosthet Dent 1962;12:542–548. 75. Bonfante EA, da Silva NR, Coelho PG, et al. Effect of framework design on crown failure. Eur J Oral Sci 2009;117:194–199. 76. Marker JC, Goodkind RJ, Gerberich WW. The compressive strength of nonprecious versus precious ceramometal restora­tions with various frame designs. J Prosthet Dent 1986;55:560–567. 77. Miller LL. Framework design in ceramo-metal restorations. Dent Clin North Am 1977;21:699–716. 78. Shoher I, Whiteman AE. Reinforced porcelain system: A new concept in ceramometal restorations. J Prosthet Dent 1983;50:489– 496. 79. Molin MK, Karlsson SL. Five-year clinical prospective evaluation of zirconia-based Denzir 3-unit FPDs. Int J Prosthodont 2008;21:223– 227. 80. Tinschert J, Schulze KA, Natt G, et al. Clinical behavior of zirconiabased fixed partial dentures made of DC-Zirkon: 3-year results. Int J Prosthodont 2008;21:217–222. 81. Bonfante EA, Rafferty B, Zavanelli RA, et al. Thermal/mechanical simulation and laboratory fatigue testing of an alternative yttria tetragonal zirconia polycrystal core-veneer all-ceramic layered crown design. Eur J Oral Sci 2010;118:202–209. 82. Silva NR, Bonfante E, Rafferty BT, et al. Conventional and modified veneered zirconia vs. metalloceramic: Fatigue and finite element analysis. J Prosthodont 2012;21:433–439.

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Individual Ceramic Crowns for Teeth 83. Broseghini C, Broseghini M, Gracis S, Vigolo P. Aesthetic functional area protection (AFAP) concept for prevention of ceramic chipping with zirconia frameworks. Int J Prosthodont 2014;27:174–176. 84. de Paula V, Lorenzoni F, Bonfante E, Silva NR, Bonfante G. Influence of cooling and core design on Y-TZP fatigue life. Presented at the 90th General Session and Exhibition of the IADR, 20–23 Jun 2012, Iguaçu Falls, Brazil. https://iadr.confex.com/iadr/2012rio/ webprogram/Paper166082.html. Accessed 2 December 2013. 85. Guess P, Silva NR, Bonfante EA, Coelho PG, Thompson VP. Cooling rate effect on fatigue reliability of zirconia-based all-ceramic crowns. Presented at the 89th General Session and Exhibition of the IADR, San Diego, 16–19 Mar 2011. 86. Guess P, Zhang Y, Thompson V. Thermal treatment effect on damage and reliability of veneered Y-TZP. Presented at the 87th General Session and Exhibition of the IADR, Miami, 1–4 Apr 2009. 87. Guess PC, Zavanelli RA, Silva NR, et al. Monolithic CAD/CAM lithium disilicate versus veneered Y-TZP crowns: Comparison of failure modes and reliability after fatigue. Int J Prosthodont 2010;23:434– 442. 88. Zhao K, Pan Y, Guess PC, Zhang XP, Swain MV. Influence of veneer application on fracture behavior of lithium-disilicate-based ceramic crowns. Dent Mater 2012;28:653–660. 89. Silva NR, Bonfante EA, Martins LM, et al. Reliability of reducedthickness and thinly veneered lithium disilicate crowns. J Dent Res 2012;91:305–310. 90. Fasbinder DJ, Dennison JB, Heys D, Neiva G. A clinical evaluation of chairside lithium disilicate CAD/CAM crowns: A two-year report. J Am Dent Assoc 2010;141(suppl 2):10S–14S. 91. Reich S, Fischer S, Sobotta B, Klapper HU, Gozdowski S. A preliminary study on the short-term efficacy of chairside computer-aided design/computer-assisted manufacturing–generated posterior lithium disilicate crowns. Int J Prosthodont 2010;23:214–216. 92. Beuer F, Stimmelmayr M, Gueth JF, Edelhoff D, Naumann M. In vitro performance of full-contour zirconia single crowns. Dent Mater 2012;28:449–456. 93. Kuwata M. Theory and Practice for Ceramo-Metal Restorations. Chicago: Quintessence, 1980:13–34. 94. Martignoni M, Schönenberger A. Precision Fixed Prosthodontics: Clinical and laboratory Aspects. Chicago: Quintessence, 1990:263– 336. 95. Massironi D, Pascetta R, Romeo G. Precision in Dental Esthetics: Clinical and Laboratory Procedures. Chicago: Quintessence, 2007: 524–563. 96. Yamamoto M. Metal Ceramics. Chicago: Quintessence, 1985:203– 218. 97. Balkaya MC, Cinar A, Pamuk S. Influence of firing cycles on the margin distortion of 3 all-ceramic crown systems. J Prosthet Dent 2005;93:346–355. 98. Bindl A, Mormann WH. Marginal and internal fit of all-ceramic CAD/ CAM crown-copings on chamfer preparations. J Oral Rehabil 2005; 32:441–447. 99. Nakamura T, Tanaka H, Kinuta S, et al. In vitro study on marginal and internal fit of CAD/CAM all-ceramic crowns. Dent Mater J 2005; 24:456–459. 100. Reich S, Wichmann M, Nkenke E, Proeschel P. Clinical fit of allceramic three-unit fixed partial dentures, generated with three different CAD/CAM systems. Eur J Oral Sci 2005;113:174–179. 101. Stappert CF, Denner N, Gerds T, Strub JR. Marginal adaptation of different types of all-ceramic partial coverage restorations after exposure to an artificial mouth. Br Dent J 2005;199:779–783. 102. Colpani JT, Borba M, Della Bona A. Evaluation of marginal and internal fit of ceramic crown copings. Dent Mater 2013;29:174–180.

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103. Matta RE, Schmitt J, Wichmann M, Holst S. Circumferential fit assessment of CAD/CAM single crowns—A pilot investigation on a new virtual analytical protocol. Quintessence Int 2012;43:801–809. 104. Moldovan O, Luthardt RG, Corcodel N, Rudolph H. Threedimensional fit of CAD/CAM-made zirconia copings. Dent Mater 2011;27:1273–1278. 105. Blatz MB, Sadan A, Kern M. Resin-ceramic bonding: A review of the literature. J Prosthet Dent 2003;89:268–274. 106. Vargas MA, Bergeron C, Diaz-Arnold A. Cementing all-ceramic restorations: Recommendations for success. J Am Dent Assoc 2011; 142(suppl 2):20S–24S. 107. Heintze SD, Rousson V. Fracture rates of IPS Empress all-ceramic crowns—A systematic review. Int J Prosthodont 2010;23:129–133. 108. Manso AP, Silva NR, Bonfante EA, et al. Cements and adhesives for all-ceramic restorations. Dent Clin North Am 2011;55:311–332. 109. Tripodakis AP, Gracis S, Blatz M, Eliades G. Material interfaces in esthetic dentistry. 2. Cementing, supporting and veneering prosthetic dental materials. Eur J Esthetic Dent 2012;7:215–241. 110. Itthagarun A, Tay FR, Pashley DH, et al. Single-step, self-etch adhesives behave as permeable membranes after polymerization. 3. Evidence from fluid conductance and artificial caries inhibition. Am J Dent 2004;17:394–400. 111. Tay FR, Pashley DH, Garcia-Godoy F, Yiu CK. Single-step, self-etch adhesives behave as permeable membranes after polymerization. 2. Silver tracer penetration evidence. Am J Dent 2004;17:315–322. 112. Tay FR, Pashley DH, Suh B, Carvalho R, Miller M. Single-step, selfetch adhesives behave as permeable membranes after polymerization. 1. Bond strength and morphologic evidence. Am J Dent 2004; 17:271–278. 113. Tay FR, Pashley DH, Suh BI, Carvalho RM, Itthagarun A. Single-step adhesives are permeable membranes. J Dent 2002;30:371–382. 114. Peumans M, Kanumilli P, De Munck J, et al. Clinical effectiveness of contemporary adhesives: A systematic review of current clinical trials. Dent Mater 2005;21:864–881. 115. Takeda T, Ishigami K, Shimada A, Ohki K. A study of discoloration of the gingiva by artificial crowns. Int J Prosthodont 1996;9:197–202. 116. Goracci C, Ferrari M. Current perspectives on post systems: A literature review. Aust Dent J 2011;56 Suppl 1:77–83. 117. Gehrt M, Wolfart S, Rafai N, Reich S, Edelhoff D. Clinical results of lithium-disilicate crowns after up to 9 years of service. Clin Oral Investig 2013;17:275–284. 118. Ozkurt Z, Iseri U, Kazazoglu E. Zirconia ceramic post systems: A literature review and a case report. Dent Mater J 2010;29:233–245. 119. Sailer I, Thoma A, Khraisat A, Jung RE, Hammerle CH. Influence of white and gray endodontic posts on color changes of tooth roots, composite cores, and all-ceramic crowns. Quintessence Int 2010; 41:135–144. 120. Nothdurft FP, Pospiech PR. Clinical evaluation of pulpless teeth restored with conventionally cemented zirconia posts: A pilot study. J Prosthet Dent 2006;95:311–314. 121. Paul SJ, Werder P. Clinical success of zirconium oxide posts with resin composite or glass-ceramic cores in endodontically treated teeth: A 4-year retrospective study. Int J Prosthodont 2004;17:524– 528. 122. Naumann M, Sterzenbac G, Alexandra F, Dietrich T. Randomized controlled clinical pilot trial of titanium vs. glass fiber prefabricated posts: Preliminary results after up to 3 years. Int J Prosthodont 2007;20:499–503. 123. Dietschi D, Duc O, Krejci I, Sadan A. Biomechanical considerations for the restoration of endodontically treated teeth: A systematic review of the literature. Part II (Evaluation of fatigue behavior, interfaces, and in vivo studies). Quintessence Int 2008;39:117–129.

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Ceramic Veneers Susana Morimoto, dds, msd, phd Marcelo A. Calamita, dds, msd, phd Christian Coachman, dds, cdt Galip Gürel, dds, msd

Dental ceramics have individual characteristics with regard to structure, properties, manufacturing processes, and, consequently, clinical indications. In view of the increasing number of material options available on the market, the dental practitioner needs to understand some essential concepts and classifications. Extensive information about the composition and structure of porcelain and glass-ceramic materials is presented in other chapters (see, for example, chapters 1, 4, 5, and 9). Therefore, this chapter is focused on the methods of fabrication and clinical aspects of ceramic veneer restorations in order to guide clinicians in their choice of materials, treatment approaches, and clinical procedures.

Materials Selection When the dental treatment plan includes the placement of veneers, the ultimate goal is the fabrication of restorations that provide the most esthetic appearance with minimal reduction of the dental structure. Therefore, this discussion focuses on only two types of ceramic for the fabrication of adhesively bonded veneers: porcelains and glass-ceramics. These ceramics have a balanced vitreous and crystalline structure that results in exceptional esthetics and permits acid etching. This characteristic makes it feasible to effectively bond the veneers to the tooth and to perform more conservative tooth preparations. After bonding, both materials show excellent clinical results.1,2

Methods of fabrication By definition, all ceramics are initially fabricated by means of treatment at extremely high temperatures (casting), when the main chemical reactions occur. This makes the thermal process fundamental in determining the final properties of the ceramic.3–6 The raw components from nature, such as feldspar, kaolin, and quartz, must be subjected to extremely high temperatures, leading to the fusion of these components and thereby creating a new product, labeled ceramic, with a melting point lower than that of the original raw components.5,6 This reduction in melting point facilitates the use of ceramics in dental laboratories by allowing the utilization of simpler techniques.

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Ceramic Veneers Manufacturers make ceramics available to dental laboratories in the form of powder, paste (for retouching color), ingots, and blocks. Therefore, ceramic restorations may be fabricated using the powder/liquid technique, pressed, or machined by the dental laboratory.3,4,7 With powder/liquid ceramics, the vitreous phase undergoes softening (pyroplastic flow), acquiring a more pasty consistency due to the presence of various materials with different melting points. Approximation of the particles (sintering) occurs, but there is no melting or complete union of the components, and there are no important chemical reactions.7,8 The porcelain is modeled during the fabrication of a restoration but undergoes sinterization shrinkage, which is largely related to the loss of water and densification of particles and may generate marginal deficiencies and porosities.7 Nevertheless, technical and technologic improvements will compensate for this misfit, such as using the stratification process, glazing, a vacuum furnace, and the pressable technique. Ceramics fabricated by the powder/liquid technique are porcelains, glass-ceramic (to re-cover a glass-ceramic or metal coping), and some crystalline ceramics. Veneers may be fabricated using the platinum foil technique, built up on a refractory die, or layered on metal or ceramic substructures (copings). With pressable ceramics, melting or fusion—the passage from the solid to liquid state at high temperatures— takes place. The ceramic components undergo a process of complete interaction, frequently forming new chemical elements. With machinable ceramics, porcelain, glass-ceramic, or crystalline ceramic blocks are milled by means of a computerized system (computer-aided design/computer-assisted manufacture [CAD/CAM]) to produce the final veneer restoration. Most CAD/CAM–fabricated veneers are made of glass-ceramic systems, although polycrystalline ceramics have been proposed as a potential material.4–8

Methods to increase ceramic strength The strength of ceramics is a relevant factor for the longevity of veneer restorations, and the resurgence in the use of ceramics in dentistry is largely attributable to improvements in this property. The strength of ceramics is determined by (1) intrinsic factors that are inherent to ceramics and (2) extrinsic factors that are not inherent to the composition of ceramics or the treatments these undergo to increase their own strength. Extrinsic factors are coadjuvant but essential.

Intrinsic strength The introduction of new, more fracture-resistant ceramics and strengthening techniques has improved the final 102

strength of these materials, culminating in the creation of high-strength ceramics that can be used for veneers.3,9–11 Incorporation of metal oxides. The inclusion of metal oxide in a ceramic enhances its hardness and fracture toughness, so that cracks do not propagate between crystals as they do in glass materials. Ceramic strengthening techniques. Ceramics may be strengthened by means of various processes: •  Compressive stress generation by means of exchanging smaller ions (eg, sodium ions) for larger ions (eg, potassium or rubidium ions; potassium is approximately 25% larger than sodium) on the surface of feldspathic porcelains. This process, which is called chemical tempering, increases flexural strength by approximately 43%.12,13 •  Sequential polishing, which increases strength by approximately 22%.13 •  Different rates of cooling contraction between the glaze and internal layers will generate compressive stress, which can result in appreciable strengthening by inhibiting crack growth from the surface through the body of the porcelain.13 •  Crystal growth (ceramification) within ceramified glasses. •  Improvements in fabrication techniques, such as vacuum furnaces that result in an increase in the final strength of ceramic parts. Machinable and pressable systems have much higher fracture resistance than powder/liquid systems.10 Impurities and porosities have an influence on the translucence and strength of the restoration. Porosities of 10% by volume can diminish the flexural strength by one-half. Defects such as cracks, fissures, and very deep indentations or preparations that have sharp irregularities may initiate fractures, as they act as points for the concentration of stresses. Abrupt alterations in shape, lack of rounded angles, and inadequate thickness of restorations are also points of stress. The concentration of stress at these points could be reduced by simply rounding off irregularities or filling them with a chemical glaze or resin cement.7,10 However, if ceramics are strengthened only by alterations to the surface, the improvements may be diminished or eliminated by wear.3,7,9,11

Extrinsic strength For many years, when the intrinsic strength of veneer ceramics was limited, attempts were made to develop external means of improving the final strength of the veneer restoration. Even today these adjuncts are of great importance.

Clinical Procedures Metal substructure. For many years, adding a metal substructure was the only predictable method to obtain a strong ceramic restoration. Metal-ceramic restorations are considered to offer high mechanical resistance with satisfactory esthetics.6–8,11,14 Bonding. The chemical-mechanical bond obtained with the use of acids, adhesives, and silane has became so strong and stable that it frequently exceeds the intrinsic (cohesive) strength of the ceramic itself as well as that of the tooth. When attempts are made to separate porcelain from resin, fracture of the resin or porcelain occurs, but the bond interface will frequently be maintained. The use of resin cement with insoluble, adhesive characteristics ensures the retention, marginal sealing, and reinforcement of the tooth and restoration; reduces postoperative sensitivity; and contributes to the final esthetic appearance.6,7,14,15 Ceramic, known to be a material with a high modulus of elasticity (rigidity), becomes less friable after the bonding procedures. Adhesive bonding is well suited for laminates, allowing thin ceramic restorations to become extremely resistant to fracture or dislodgment.

Selection parameters For clinicians who are planning to place veneer restorations, this question is fundamental: Which ceramics perform better, specifically, for veneers, and why? It is our opinion that two parameters, with an equal degree of importance, must be considered when a ceramic material is selected for veneers: (1) esthetics and (2) adhesion. Strength of the material can be ranked third in importance for several reasons: •  Generally, teeth where veneers are indicated should have adequate dental structure, particularly enamel, to maintain good fracture strength. If this condition cannot be met, complete-coverage restorations might be the preferred treatment alternative. •  The tooth-ceramic interface becomes very strong after adhesive cementation. Bonding reinforces the ceramic and restores the strength of the tooth.16 •  Ceramics that allow bonding require less tooth reduction. The larger the quantity of enamel preserved, the less the tooth will flex in function and therefore the lower the failure rate (debonding, chipping, fractures, and leakage) of the ceramic veneers and teeth will be.

Clinical Procedures As Gürel has observed previously, “Esthetic dentistry is a delicate combination of scientific principles and artistic

abilities.”14 An understanding of the appropriate choice of material for ceramic veneers is part of the treatment planning. However, the clinician also must properly design a desirable smile.

Preclinical consultation Various predetermining factors play important roles in the evaluation and decision-making process of treatment planning for each case. Where porcelain laminate veneers (PLVs) are planned, many factors should be thoroughly assessed before the actual treatment begins. These details must be carefully analyzed to minimize difficult situations that may arise during the treatment process and to avoid possible postoperative complications or complaints from patients.17 The dentist’s perception of a desirable smile should be considered along with the patient’s personal needs, wishes, and thoughts on his or her appearance.14 It is paramount that patients be fully informed about the possibilities, limits, and prognosis of their particular case.

Indications The following situations are indications for placement of PLVs: •  Changes in shape, contour, or position of teeth, such as correction of conical teeth, rotated teeth, and poor positioning as well as closure of diastemas •  Esthetic correction of congenital or acquired structural defects, such as extensive enamel caries, multiple restorations, fractures, size discrepancies, imperfect amelogenesis, erosion, abrasion, and abfraction •  Color changes for pigmented teeth that do not respond satisfactorily to dental bleaching, such as teeth affected by fluorosis, pigmentation by tetracycline, enamel dysplasia, and iatrogenic discoloration from endodontic treatment •  Functional restoration, such reestablishment of contacts on occlusal surfaces and incisal guidance

Limitations or contraindications The following situations are not ideal for placement of PLVs: •  Absence of enamel and structural integrity. The presence of tooth enamel assures more effective bonding procedures and provides a rigid substrate ideal for veneers.1,2,12,18–25 •  Poor tooth positioning for misalignment. Tooth preparation is often necessary to provide an adequate thickness of ceramic veneers.20,26 Thus, generally speaking, darker, 103

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Ceramic Veneers labially positioned teeth, eroded teeth, extruded teeth, and teeth needing alterations in contour will require less conservative preparations that extend into dentin. This type of preparation compromises the tooth structure and bonding procedures in the long term. •  Occlusal concerns. Patients with parafunctional habits and inadequate maxillomandibular relationships are poor candidates for PLVs.

such as the interpapilary, interalar, and intercomissural lines.29,35 •  Maxillary gingival plane. The height of gingival contours must be symmetric, continuous in shape, and parallel to the outline of the upper lip, and the interdental papillae must fill the interdental spaces. Gingival surgery, alone or in association with orthodontic traction, may be indicated for the correction of some irregularities.36

Esthetic evaluation

The same sequence must be used for evaluation of the mandibular occlusal plane when this arch requires treatment. After initial evaluation, the photographic record is made, and pretreatment impressions are taken. The diagnostic casts are mounted on an articulator, and the planned alterations are made on the casts by means of diagnostic waxing (Fig 6-1f ). The wax-up is transferred to the mouth using a silicone index, which is esthetically and functionally tested in the mouth. Once approved by the restorative team and the patient, these mock-ups (aesthetic pre-evaluative temporaries [APTs])14 are used as a precise guideline to prepare the tooth structure, based on the intended final tooth contour (Fig 6-1g).

The authors recommend that the esthetic evaluation be performed “from the whole to the parts” or “from the outside in.” The dentist should start the examination by evaluating the patient’s face, observing the balance between the different thirds of the face and noting if there are relevant asymmetries (Figs 6-1a to 6-1c). Furthermore, the clinician should analyze how the horizontal reference lines, such as the interpupillary (Fig 6-1d), interalar, and intercomissural lines, are related to the horizontal plane. This assessment is critical to evaluating the incisal plane and the plane of occlusion. Next, the dentist should locate the midline, since it can interfere with the design of the new restorations.27–29 These analyses are more accurate when performed with the aid of standardized pictures and movies. The use of the Digital Smile Design30,31 facilitates the evaluation process and provides predictability and consistency to the treatment planning process (Fig 6-1e). Analysis must focus on: •  The three-dimensional position of the incisal edges of maxillary central incisors in relation to the face and lips. This is the focal point that guides the entire planning process. The position of the incisal edges of the maxillary central incisors must be observed with the patient’s lips at rest, in a posed smile, and in a spontaneous smile. These incisal edges must be positioned in accordance with established esthetic parameters.32,33 However, these parameters are individualized for each patient according to his or her characteristics, desires, and expectations. When reestablishment of the incisal edge of maxillary teeth is indicated, this can be previously tested by means of an immediate mock-up,34 performed directly in the mouth using composite resin, to guide a more precise waxing. •  Position of the incisal edges of the maxillary lateral incisors, canines, premolars, and molars (maxillary occlusal plane). The incisal edges of central and lateral incisors and canines and the buccal cusps of premolars and molars should be parallel to the lower lip when this is symmetric. In cases of patients with an asymmetric facial pattern or irregular occlusal plane, the image should be digitally aligned according to horizontal references 104

Tooth preparation Basically, there are two different approaches to tooth preparation for veneers: The traditional approach, which is based on following the contour of the existent tooth structure,20 and APT, which is guided by the planned final contour of the restoration, with the use of indexes and mock-ups.14,37 Meticulous tooth preparation is required to maximize esthetics, improve fracture resistance, optimize laboratory procedures, and maintain soft tissue health for PLVs.38 The use of the APT technique may guide diagnosis, communication with the patient and dental technician, and the tooth preparation to facilitate a predictable final result. Use of controlled depth-cutting burs through the APTs ensures that the tooth structure will undergo only the minimal preparation necessary, perhaps even needing no preparation in certain areas, according to the preestablished goals. Although PLVs can be applied with very little tooth preparation, this type of restoration should not be misconstrued as a simple procedure; in fact, it actually requires great skill. It is beyond the scope of this chapter to provide all the steps associated with correct tooth preparation for veneers, but it is important to highlight some parameters (Fig 6-2): •  Each tooth should be properly reduced according to its structural integrity, position in the arch, desired final contour, color, gingival level, and occlusion.

Clinical Procedures

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Fig 6-1  (a to c) Preoperative clinical situation. The patient is not satisfied with her profile, tilted plane of occlusion, tooth alignment, or overall esthetic appearance. (d and e) After orthognathic surgery, the new incisal plane and esthetic design of the restorations are planned with the aid of the Digital Smile Design. (f ) After the two-dimensional information gathered from the Digital Smile Design is transferred to the cast, a three-dimensional representation of the planning is fabricated in wax. (g) The new smile design is tested in vivo. This mock-up will serve as a “test drive” for esthetic and functional parameters, such as the position of the incisal edge of the maxillary central incisor, the plane of occlusion, lip support, and phonetics, as well as a precise guide for tooth preparations. (FIgs 6-1d to 6-1f reprinted from Coachman and Calamita30 with permission.)

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a

b

Fig 6-2  (a and b) Teeth are reprepared according to the APT technique. The controlled depth-cutting burs are applied through the mock-up, providing minimally invasive preparations according to preoperative goals. The final color and tooth contours determine the exact amount of tooth reduction to reach the esthetic and functional results desired. Removal of the APTs reveals the minimal amount of tooth structure that has been removed. (Fig 6-2a reprinted from Coachman and Calamita30 with permission.)

•  Preparations should be of uniform thickness with rounded angles. •  Well-defined, cervical chamfered finish lines should be created to facilitate the technical procedures. •  Placement of margins in areas of stress concentration should be avoided. There is no consensus in the literature about whether or not the incisal edge should be included in the preparation. Laboratory studies have evaluated incisal coverage with regard to longevity and failure of veneers. In one study, tooth preparation without incisal overlap (window preparation) showed better results than the preparation with the incisal edge overlapped.39 If incisal coverage is indicated for occlusal or esthetic reasons, in vitro studies have shown that a shoulder with a palatal chamfer margin design increased the fatigue failure cycle count.40 On the other hand, a clinical study demonstrated that the overlapped incisal edge had a significantly positive effect on the survival rate.20 Another study reported that no significant differences could be observed between the outcomes of veneers with incisal porcelain coverage and those designed with an uncovered incisal edge.25

Impression After tooth preparation has been completed, completearch impressions should be taken with polyvinyl siloxane or polyether material or taken digitally. Tissue management is key, and it is recommended that small-diameter retraction cords be used to gently expose the margins. Alginate impressions can be made for the provisional restorations and opposing arch. 106

Bite registration If the patient does not present any sign or symptom related to occlusal or temporomandibular joint disorders, the casts can be mounted in centric occlusion or maximum intercuspation position. In most cases where PLVs are placed to address esthetic concerns, only the maxillary anterior teeth are treated with PLVs; because the tooth reductions are limited to the facial surfaces, almost all the teeth will be in contact when the two stone casts are brought into contact. The casts are then mounted on the articulator with the use of a facebow aligned with a reliable horizontal reference line on the face. In cases of digital impressions, the arch relationship can also be assessed digitally.

Laboratory communication Communication with the dental laboratory professionals is discussed in the section on laboratory procedures (Fig 6-3).

Provisional restoration Depending on the extent of tooth preparation, it may not be necessary to make provisional restorations. Whenever necessary, they can be made directly in the mouth, with the use of the silicone index derived from the diagnostic wax-up and bisacrylic resins, or indirectly on the stone casts in the laboratory, using acrylic or bisacrylic resins. It is important that the provisional restorations have appropriate contours, so that the patient can experience the appearance of the “new smile” and to allow adequate hygiene in the area (see Fig 6-1g).

Clinical Procedures

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b

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d

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Fig 6-3  (a and b) Substrate color and definitive restoration color are required. It is crucial to adequately communicate color to the technician. Good-quality photographs with different shade tabs can improve the quality of communication. Use of different photography angles can also communicate the correct surface texture to the technician. (c to e) Wax-up for the veneers. This wax-up is reduced to provide space for layered veneering ceramics. This kind of bilayered restoration provides superior esthetics, excellent clinical fit, and good strength. (f ) Wax-ups are sprued, invested, and burnt out prior to the ceramic pressing. (g) Ceramic veneers after pressing.

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Clinical Procedures

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Fig 6-3  (cont) (h to p) Sequence of ceramic buildup over the lithium disilicate structure. (q and r) The veneers are 0.2 to 0.3 mm thick. The strength and esthetic properties of the material allow the fabrication of conservative “contact lenses” (IPS e.max, Ivoclar Vivadent). (s to u) Veneers on the cast. Before the restorations are tried in, the clinician should evaluate them to confirm that all the esthetic and functional parameters are in accord with the treatment plan.

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Ceramic Veneers Fig 6-4  During the try-in, the need for some characterization is evaluated. This material permits extrinsic and intrinsic characterization in a very straightforward way.

Try-in After the provisional restorations, cement, and adhesive remnants are removed, the prepared teeth are first cleaned with fine pumice and water. The PLVs must be checked, individually and collectively, to evaluate their fit, contours, interproximal contact points, and color (Fig 6-4). The use of try-in pastes helps to keep the veneers in place and enables the clinician to evaluate the color match. If possible, this tryin is best performed without anesthesia so that the patient retains sensation to feel the relationship of the new veneers to the lips and face.

Cementation After the PLVs are approved, small-diameter retraction cords must be inserted in the gingival sulcus to withdraw the gingival margins, preventing the flow of crevicular fluid in the direction of the margins and preventing the flow of cement to intrasulcular areas.

Preparation of the veneers The intaglio surface of the veneers must be etched in accordance with manufacturer specifications. Stangel et al41 suggested an optimum etching time of 2.5 minutes, using 20% hydrofluoric acid (HF); however, porcelains may be etched with 9% to 10% HF for 60 to 120 seconds, depending on the concentration of the etching liquid and fabrication of the porcelain restoration.14,42 The speed of the HF reaction on the ceramics also depends on the situation of the surface to be etched; glazed surfaces or those fabricated on platinum foil are very smooth (requiring etching for approximately 4 to 5 minutes), whereas surfaces that have been fabricated on a lining and subsequently abraded with airborne particles 110

provide a broader contact area for the action of the acids.5 Leucite- and lithium disilicate–reinforced ceramics should be etched with 10% HF for 60 and 20 seconds, respectively. Porcelain and glass-ceramic have highly retentive surfaces after being etched with HF, favoring resin cementation. After acid etching, the surface is rinsed and dried; however, scanning electron microscopic studies have shown that even after the etched surface is rinsed with copious amounts of water, a large number of acid crystals remain deposited on the etched surface, which may affect the bond strength.5 To eliminate these crystals, the veneers should be put in the ultrasonic cleaner. After ultrasonic cleaning the porosities are exposed, and penetration of adhesive in these porosities facilitates bonding. The sheer bond strength that has been improved from the mean 600 to 3,000 MPa by acid etching can be further increased with the application of a silane coupling agent. The silane coupling agent is the second component of the classic conditioning methods for ceramic restorations and must be applied for approximately 60 seconds.14,43 This coupling agent makes the retention of the bonded ceramictooth interface possible because of its high wettability and chemical contribution to bonding. Once a dry surface is obtained after silanization, the adhesive of choice is applied inside the veneer with the help of a brush.

Preparation of the tooth surface Luting procedures require meticulous attention to every detail. The tooth has to be thoroughly cleansed after the try-in stage, using pumice stone, water, and a Robinson brush kept at low speed. After a transparent strip is placed to protect the adjacent teeth, 37% phosphoric acid is applied to the prepared area for 15 to 20 seconds on dentin and for 20 to 60 seconds on enamel. The tooth is then washed thoroughly and dried

Clinical Procedures

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b

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Fig 6-5  (a to c) Final characterization and surface texture of the high-strength PLVs. Note the positive biologic response of the gingival tissue to the fit of the restorations.

without dehydrating the dentin. If a three-step system has been selected, the primer is applied on the exposed dentin area, left in place for 30 seconds, and gently dried. The bonding agent can be applied to both dentin and enamel, dried, and light polymerized according to manufacturer instructions. The adhesive cement of choice is applied on the internal surface of the previously prepared veneer, the veneer is seated on the tooth, and the restoration is light polymerized for 3 seconds. Gross excess cement is removed, and all the veneers are light polymerized one tooth at a time, on each surface, for 20 seconds. The cementation procedures are addressed in more detail in chapter 9.

Occlusal adjustment, refinishing, and polishing Occlusal adjustment must be carefully performed, because excursive protrusive movements are critical for veneers, particularly when there has been an increase in the incisal edge length. Surface irregularities make the restoration prone to fractures; in addition, roughness accelerates wear of the opposing tooth.5 Veneers designed and fabricated with meticulous attention to detail achieve excellent biologic and esthetic integration in terms of form, texture, and color (Fig 6-5).

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d

e

f Fig 6-5  (cont) (d to f ) Final view of the restorations in place. The patient is very satisfied with the results. Incisal edge position, plane of occlusion, tooth morphology, and color were all correctly addressed in a systematic way, surpassing the patient’s expectations.

Laboratory Procedures Communication Close cooperation between the dentist and laboratory is essential to achieve the desired esthetic and functional results. To enable the highest level of communication, both the dentist and the technician should be knowledgeable with regard to current trends and keep the line of communication between them open at all times. In addition, the restorative team of dentist and technician must work together to develop common goals, values, abilities, and

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desires; to this end, the dentist should work to achieve an improved understanding of the indications, limitations, and stages of the laboratory phase, and the technician should have some knowledge of the clinical aspect of dentistry.44 It is paramount to communicate the patient’s preferences or any critical data to the technician in an adequate manner. The use of photographs substantially improves the quality and precision of communication. In this way, the technician can observe “in vivo” all the information needed. At present, with the use of innovative technologies, such as digital images, films, and Digital Smile Design, information can be shared at a distance without significant loss of quality.30,31 All this information may be available digitally to

Evidence-Based Literature Review enable access at any time. Meetings may be scheduled at a distance as well, with the help of technologies such as Skype (Microsoft), Windows Live Messenger (Microsoft), or similar software programs. Esthetic judgment is not an entirely objective process; the dentist must also consider the subjective concerns and the lifestyle of the individual patient when designing a natural smile. The dentist and the dental team must then integrate these criteria, the clinical needs and conditions, and their own personal artistic abilities and subjective feelings to create a smile for the patient. The creativity of the procedure makes each case unique and the dentist’s job pleasingly varied and rewarding.14,45 Thus, to fabricate highly personalized veneers, the technician must have images of the patient’s face, know about the patient’s desires and expectations, and have a visual rec­ord of the shade of the tooth preparations as well as the color and texture of the final veneers and the mock-up or provisional restorations approved in the mouth (see Figs 6-3a and 6-3b). There are three laboratory techniques that are most often used for the fabrication of veneers: (1) stratification, (2) pressing and stratification, and (3) machining and stratification.

Monochromatic glass-ceramic ingots are heated to allow the material to flow under pressure into a mold formed by using a conventional lost-wax technique4 (see Figs 6-3c to 6-3f ). This symbiosis between pressed and stratified glass can produce esthetics (see Figs 6-3g to 6-3p) equal to that achieved with the previously described stratification process and has the advantages of better adaptation and strength.46,47 These properties might provide the dentist greater facility in manipulation by the laboratory technician during try-in. These restorations also permit a minimum thickness of 0.2 mm (see Figs 6-3q to 6-3u).

Machining and stratification Machined veneers may be fabricated from various blocks of glass-ceramics (IPS e.max CAD, Ivoclar Vivadent; Authen­ tic, Ceramay) or porcelain (Vitablocks Mark II, Vident). The blocks are milled to build a substructure that is stratified with compatible porcelains or glass-ceramics. Moreover, the restoration may be characterized by external stains.4 How­ever, the esthetics of monolithic restorations do not equal those achieved by stratification, and the external stain­ing might be removed by wear or toothbrushing over time.

Stratification In the stratification process, powder/liquid porcelain and glass-ceramics are used in the application of sequential layers of ceramics that are taken to the furnace for sinterization, application of a new layer, and finalization with glaze. Ceramic layers can be applied with different degrees of translucence, opacity, and effects (opaque, fluorescent, opalescent, translucent, and high- or low-fusing ceramics, among other characteristics). Stratification makes it possible to fabricate esthetic ceramic restorations that display excellent naturalness. However, this is a technique-sensitive procedure because these restorations are very fragile before cementation. The laboratory technician may work with feldspathic porcelains or ceramified glasses, by means of a paintbrush or powder-liquid technique, on refractory dies or on platinum foil.4

Pressing and stratification In this method of fabricating glass-ceramic restorations, a substructure is constructed by means of the pressable system (ingots); glass-ceramic (powder/liquid) is then layered on this substructure. The restoration may be fabricated with injected ceramic only; however, this type of restoration would have to be superficially stained, and this might not allow as high a degree of esthetics as that achieved with stratification and might be removed by wear and brushing.

Evidence-Based Literature Review Porcelain laminate veneers provide minimally invasive14,24,34,48 esthetic restoration with a high rate of longterm success.18–25 To achieve the best results with these restorations, however, the clinician must understand some of the essential issues involved in their fabrication and apply a clinical protocol that ensures reliable esthetics and longevity. A critical issue for the long-term success of these restorations is the adhesive cementation.49 A stable and longlasting bond does not depend exclusively on the resin cement but rather on an understanding of the complex relationships among three factors at the bond interface. The first factor is the dental substrate. Currently, contemporary adhesive systems have allowed many concepts to be changed, particularly by enabling minimal tooth preparation.15,50,51 The second factor is the ceramic selected. Ceramics that contain a high percentage of vitreous phases, which can be acid etched and silanized, have the best bonding behavior.46,47,52 They also allow more conservative preparations and superior esthetics because of their translucency and color reflectance. Crystalline ceramics do not have a vitreous phase and, therefore, cannot be etched. They are indicated when significant tooth structure is missing, there 113

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Ceramic Veneers is an unfavorable risk of flexure and stress distribution, and it is impossible to obtain and maintain the bond and seal.4 The third factor is the resin cement that is interposed between the ceramic and tooth structure; this factor is interdependent on the other two factors.

Success and failure rates Variations among materials, dentists, technicians, and patients contribute to clinical failures. Thus, clinical studies are important to evaluate the performance of restorative materials and to determine what issues are strongly related to failures, even when certain intraoral conditions cannot be reproduced in the laboratory.22 The problems that occur during the first year after placement of PLVs are generally related to failure of adhesive cementation; these issues appear to occur most frequently in the first 6 months and afterward decline or stabilize to low rates.53 These bond failures may have an influence on marginal staining, leakage, and ceramic fractures, because incomplete impregnation or polymerization of the adhesive/cement may accelerate the process of hydrolysis in the short term.49 Over time, failures may be more related to fatigue at the bond interface or crack propagation within the ceramics, resulting from masticatory forces, dissolution of the resin matrix in the oral medium, or development of gaps due to hydrolysis of the bonds between the components of the ceramics.18,49,54 Longitudinal evaluations of porcelain veneers have shown excellent results over a period of 5 to 12 years, showing success rates ranging between 85% and 98%.18–22,24,25 In the longest follow-up, which evaluated 3,500 porcelain veneers for 15 years, Fradeani et al22 found a failure rate of only 7%; two-thirds of these were fractures (22%) or leakage and debonding (11%). In a different study,1 580 porcelain laminate veneers were cemented in 66 patients and followed up for a period of 12 years. Data revealed that 42 laminate veneers (7.2%) failed in 23 patients. Of these failed veneers, 20 (48%) fractured, 12 (29%) debonded, 7 (17%) showed microleakage, and the other 3 (7%) presented secondary caries, sensitivity, or indications for root canal treatment. Clinical follow-up of ceramic veneers for long periods has been carried out to obtain more reliable data on the longevity of the ceramic-silane–resin cement–adhesive-tooth interface, and the results have been considered encouraging.48 Etched porcelain veneers offer a predictable and safe treatment modality that preserves a maximum amount of sound tooth structure,21 and they have proved to be one of the most successful treatment modalities that modern dentistry has to offer.55

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Factors affecting the bond interface Dental substrate Several clinical factors may interfere with the success of restorations. However, variations in tooth preparation may explain many of these differences.20 Traditional approaches to veneer preparation can lead to major dentin exposure since the recommended preparation thickness values are frequently close to the average measurements of enamel thickness.37 Tooth preparation should always be as minimal as possible because this will preserve a larger amount of remaining enamel,1,2 providing greater strength, as flexion of the tooth may be related to fractures and debonding.33 Nevertheless, some cases make procedures less straightforward depending on the color of the substrate, type of substrate, position of the tooth, and contour required. For example, teeth with color alterations may demand more tooth reduction, often in dentin, to mask their color. Teeth affected by attrition, erosion, and abrasion may have only a thin layer of enamel, as they have lost some of their original volume and may not permit the entire preparation to be made in enamel. Enamel preservation can still be achieved with bonded porcelain veneer restorations.23,25,34,37,49 Although some studies20 have found no differences in the success rates of veneers placed on teeth with dentin exposure and those with preparations completely confined to enamel, others21,23,49 have emphasized that there is an increased risk of failure when veneers are bonded to large amounts of exposed dentin or on an existing restoration. Nevertheless, more conservative preparations undoubtedly help to preserve tooth vitality and reduce postoperative sensitivity.26 Previous research56 has reported fractures of veneers bonded on teeth with large composite resin restorations. The presence of composite resin restorations had a negative influence on the overall clinical performance but did not increase the loss of veneers in that study. This finding is in agreement with that of Gürel et al,2 who observed failure rates of 10.6% for veneers in teeth with restorations compared with failure rates of 6.6% in teeth without composite resin. Deeper preparation into dentin and a substrate with a lower modulus of elasticity than porcelain result in a less rigid base for restoration placement than enamel. This approach has resulted in higher fracture rates than are found for other previous enamel-supported restorations.1,2,21,57 The residual dentin thickness after preparation may, therefore, influence the life expectancy of the restoration.26,32,55

Evidence-Based Literature Review Several reports have demonstrated that fractures may occur if the surface of the tooth has not been prepared sufficiently to create space for the PLV buildup.1,2,55 On the other hand, deep preparations that expose dentin will increase the risk of microleakage and adhesive fractures.1,2,20,21 By taking into consideration the final volume of the restoration after approval of an additive mock-up, the APT technique14 optimizes the tooth preparation, allowing a higher percentage of dental preparations to be completely confined to enamel (80.5%), thus maintaining the rigidity of the structure, reducing postcementation sensitivity, improving support of the ceramic restoration, and avoiding endodontic intervention.24,40 Without this guide, the dentist resorts to freehand preparation, often exposing dentin20,24 and having difficulties in keeping a uniform thickness. When PLVs are executed according to the previously mentioned optimum guidelines, the least common problems associated with PLVs are marginal discoloration and loss of color stability because all margins are in areas in which hygiene is easy to maintain, the porcelain is often easily finished and polished, and its glazed surface is mostly impervious to extrinsic staining.55 Supragingival preparations also had a positive effect on the survival rate of porcelain veneers.20 In this study, marginal adaptation was considered good or very good (100%), and there was minimal microleakage (1.2%), probably because the preparations were situated at the gingival level, which facilitated impressions and cleaning of the margins. These factors may also have added to the low rate of gingival recession. No gingival recession was observed in 85.7% of PLVs.1

Ceramic material High-strength ceramics of medium and high resistance, such as lithium disilicate glass-ceramics, also provide bonding and esthetics and are an excellent choice at present. However, because the crystalline ceramics do not allow bonding to tooth structures, they may require more aggressive preparation approaches; this does not seem, to the authors, to be in keeping with the main philosophy regarding indications for a veneer. Recently, preparations for veneers have taken increasingly conservative lines, with the fabrication of “contact lenses,” partial veneers, or fragments. The authors believe this to be the better path: stronger ceramics that allow bonding and provide an extremely high degree of esthetics.

Resin cement Clinical follow-up comparing self-etch and total-etch adhesive systems in PLVs showed a similar behavior over the period of 5 years, but a phosphoric acid agent was applied to enamel in all the samples.19 Bonding at the level of dentin is based on a mechanical and chemical bond and has reached bond strength values of approximately 14 to 27 MPa with the use of fourthgeneration adhesives. The bond to enamel fundamentally is a more stable mechanical bond, and its value reportedly ranges from 18 to 31 MPa.58 Examination of contemporary adhesives revealed that the three-step etch-and-rinse adhesives remain the gold standard in terms of durability.59 Because the authors approach envisages interlocking adhesive with enamel, the use of total-etch adhesives is highly recommended. The polymerization and the hardness values of cements are the result of the nature of the cement (dual-, chemical-, or light-polymerizable types), the thickness and opacity of the porcelain, light intensity, exposure time, and distance between the source and restoration.60 A light-polymerized luting composite resin is preferred for cementation of ceramic veneers, because it provides longer working time and greater color stability than dual-cured or chemically polymerized systems; nevertheless, any eventual staining appears to be related to incorrect cement polymerization, when free camphorquinone remains, rather than to the nature of the cement (dual, chemical, or light polymerizable type).37,61,62 Although the usual ceramic thickness for the PLV of 0.5 to 1.0 mm has no significant influence on the hardness of a light-polymerized composite, the use of a dual-cure composite might be preferable in certain cases, especially in ceramics with the use of more opaque substructures.14 The use of a resin cement with adhesive features and insolubility not only ensures retention of the restoration but also contributes to marginal sealing, strengthening of the tooth and restoration, a reduction in postoperative sensitivity, and final esthetics.

Complications The occurrence of complications, such as secondary caries and periodontal disease, has not been reported in many studies,19,57 but it could become a significant factor depending on the patient’s hygiene.24 Despite major advances in materials and techniques, some other clinical factors may be responsible for failures. Occlusal factors and concerns related to the tooth-cementceramic interface have been the most frequently mentioned in the literature.1,2,18–20,22,24 115

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Conclusion Properly diagnosed, planned, and executed PLV applications have always been satisfactory restorations. When the esthetic perception of the dentist is enhanced by up-todate knowledge and then combined with the artistic skill and devotion of the dental technician, creation of a naturallooking smile is routinely achieved. When details such as anterior-posterior color gradation, translucent areas of the incisors, higher chroma of the gingival third, surface texture, and luster are meticulously applied, the PLVs will be undetectable from the other natural teeth.

Acknowledgments The authors would like to thank Marcos Pitta, DDS, MSC, for the excellent results obtained with the orthognathic surgery and Adriano Shayder, CDT, for the high-quality laboratory work.

References 1. Gürel G, Morimoto S, Calamita MA, Coachman C, Sesma N. Clinical performance of porcelain laminate veneers: Outcomes of the esthetic pre-evaluative temporary (APT) technique. Int J Periodontics Restorative Dent 2012;32:625–635. 2. Gürel G, Sesma N, Calamita MA, Coachman C, Morimoto S. Influence of enamel preservation on failures rates of porcelain laminate veneers. Int J Periodontics Restorative Dent 2013;33:­31–39. 3. Kelly JR. Dental ceramics: What is this stuff anyway? J Am Dent Assoc 2008;139(suppl):4S–7S. 4. McLaren EA, Whiteman YY. Ceramics: Rationale for material selection. Compend Contin Educ Dent 2010;31:666–672. 5. Morimoto S. Análise morfológica de diferentes cerâmicas antes e após o tratamento de superfície: Estudo ao microscópio eletrônico de varredura [thesis]. University of São Paulo, School of Dentistry, 1998. 6. Phillips RW. Skinner’s Science of Dental Materials, ed 9. Philadelphia: Saunders, 1991. 7. Anusavice KJ, Shen C, Rawls HR (eds). Phillips Science of Dental Materials, ed 12. St Louis: Elsevier Saunders, 2013:345–366. 8. Craig RG. Restorative Dental Materials, ed 9. St Louis: Mosby, 1993:473–490. 9. Giordano RA 2nd, Pelletier L, Campbell S, Pober R. Flexural strength of an infused ceramic, glass ceramic, and feldspathic porcelain. J Prosthet Dent 1995;73:411–418. 10. Giordano R, McLaren EA. Ceramics overview: Classification by microstructure and processing methods. Compend Contin Educ Dent 2010;31:682–688. 11. Piddock V, Qualtrough AJE. Dental ceramics—An update. J Dent 1989;18:227–235. 12. Denry IL, Rosenstiel SF, Holloway JA, Niemiec MS. Enhanced chemical strengthening of feldspathic dental porcelain. J Dent Res 1993;72:1429–1433. 13. Giordano RA 2nd, Campbell S, Pober R. Flexural strength of feldspathic porcelain treated with ion exchange, overglaze, and polishing. J Prosthet Dent 1994;71:468-472. 14. Gürel G. The Science and Art of Porcelain Laminate Veneers. Chicago: Quintessence, 2003.

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15. Fusayama T, Nakamura M, Kurosaki N, Iwaku M. Non-pressure adhesion of a new adhesive restorative resin. J Dent Res 1979;58:1364– 1370. 16. Cobankara FK, Unlu N, Cetin AR, Ozkan HB. The effect of different restoration techniques on the fracture resistance of endodonticallytreated molars. Oper Dent 2008;33:526–533. 17. Chiche GJ, Pinault A. Esthetics of Anterior Fixed Prosthodontics. Chicago: Quintessence, 1994. 18. Aristides GA, Dimitra B. Five-year clinical performance of porcelain laminate veneers. Quintessence Int 2002;33:185–189. 19. Aykor A, Ozel E. Five-year clinical evaluation of 300 teeth restored with porcelain laminate veneers using total-etch and a modified selfetch adhesive system. Oper Dent 2009;34–35:­516–523. 20. Çötert HS, Dündar M, Öztürk B. The effect of various preparation designs on the survival of porcelain laminate veneers. J Adhes Dent 2009;11:405–411. 21. Dumfahrt H, Schäffer H. Porcelain laminate veneers. A retrospective evaluation after 1 to 10 years of service. 2. Clinical results. Int J Prosthodont 2000;13:9–18. 22. Fradeani M, Redemagni M, Corrado M. Porcelain laminate veneers: 6- to 12-year clinical evaluation—A retrospective study. Int J Periodontics Restorative Dent 2005;25:8–17. 23. Friedman MJ. A 15-year review of porcelain veneer failure—A clinician’s observations. Compend Contin Educ Dent 1998;19:­625–636. 24. Granell-Ruiz M, Fons-Font A, Labaig-Rueda C, Martínez-González A, Román-Rodriguez JL, Solá-Ruiz MF. A clinical longitudinal study 323 porcelain laminate veneers. Period of study from 3 to 11 years. Med Oral Patol Oral Cir Bucal 2010;15:e531–e537. 25. Smales RJ, Etemadi S. Long-term survival of porcelain laminate veneers using two preparation designs: A retrospective study. Int J Prosthodont 2004;17:323–326. 26. Edelhoff D, Sorensen JA. Tooth structure removal associated with various preparation designs for anterior teeth. J Prosthet Dent 2002;87:503–509. 27. Fradeani M. Esthetic Rehabilitation in Fixed Prosthodontics, vol 1. Esthetic Analysis: A Systematic Approach to Prosthetic Treatment. Chicago: Quintessence, 2004. 28. Rufenacht CR. Fundamentals of Esthetics. Chicago: Quintessence, 1990. 29. Spear FM, Kokich VG, Mathews DP. Interdisciplinary management of anterior dental esthetics. J Am Dent Assoc 2006;­137:160–169. 30. Coachman C, Calamita M. Digital Smile Design—A tool for treatment planning and communication in esthetic dentistry. Quintessence Dent Technol 2012;35:103–111. 31. Coachman C, Van Dooren E, Gürel G, Landsberg CJ, Calamita MA, Bichacho N. Smile design: From digital treatment planning to clinical reality. In: Cohen M (ed). Interdisciplinary treatment planning, vol 2. Comprehensive case studies. Chicago: Quintessence, 2012:119– 174. 32. Calamita MA, Coachman C, Sesma N. The decisive role of the dental technician in the interdisciplinary treatment [in Portuguese]. In: 11th International Meeting of São Paulo Dental Technician Association (APDESP). São Paulo: Altana, 2009:13–34. 33. Magne P, Belser U. Bonded Porcelain Restorations in the Anterior Dentition: A Biomimetic Approach. Chicago: Quintessence, 2002. 34. Gürel G. Predictable, precise, and repeatable tooth preparation for porcelain laminate veneers. Pract Proced Aesthet Dent 2003;15:17– 24. 35. Spear FM. The maxillary central incisor edge: A key to esthetic and functional treatment planning. Compend Contin Educ Dent 1999; 20:512–516. 36. Joly JC, Silva RC, Carvalho PFM. Aesthetic Tissue Reconstruction: Plastic and Regenerative Periodontal and Peri-implant Procedures. São Paulo: Artes Médicas, 2009. 37. Magne P, Belser UC. Novel porcelain laminate preparation approach driven by diagnostic mock-up. J Esthet Restor Dent 2004;16:7–18. 38. Chalifoux PR. Porcelain veneers. Curr Opin Cosmet Dent 1994:­58– 66.

References 39. Hekimoglu C, Anil N, Yalçin E. A microleakage study of ceramic laminate veneers by autoradiography: Effect of incisal edge preparation. J Oral Rehabil 2004;31:265–270. 40. Chaiyabutr Y, Phillips KM, Ma PS, ChitSwe K. Comparison of loadfatigue testing of ceramic veneers with two different preparation designs. Int J Prosthodont 2009;22:573–575. 41. Stangel I, Nathanson D, Hsu CS. Shear strength of the composite bond to etched porcelain. J Dent Res 1987;66:1460–1465. 42. Calamia JR. Clinical evaluation of etched porcelain laminate veneers. Am J Dent 1989;2:9–15. 43. Fischer J, Kuntze C, Lampert F. Modified partial-coverage ceramics for anterior teeth: A new restorative method. Quintessence Int 1997;28:293–299. 44. Gwinnett AJ. Interactions of dental material with enamel. Trans Am Acad Dent Mater 1990;3:30–35. 45. Paolucci B, Calamita M, Coachman C, Gurel G, Shayder A, Hallawell P. Visagism: The art of dental composition. Quintessence Dental Technol 2012;35:187–200. 46. Ceramic restorations. Dent Advisor 1993;10(3):1–8. 47. Yüksel E, Zaimog˘lu A. Influence of marginal fit and cement types on microleakage of all-ceramic crown systems. Braz Oral Res 2011; 25:261–266. 48. Dietschi D, Spreafico R. Adhesive Metal-Free Restorations: Current Concepts for the Esthetic Treatment of Posterior Teeth. Chicago: Quintessence, 1997. 49. Piemjai M, Arksornnukit M. Compressive fracture resistance of porcelain laminates bonded to enamel or dentin with four adhesive systems. J Prosthodont 2007;16:457–464. 50. Bowen RL. Properties of a silica-reinforced polymer for dental restorations. J Am Dent Assoc 1963;66:57–64. 51. Buonocore MG. A simple method of increasing the adhesion of acrylic filling materials to enamel surfaces. J Dent Res 1955;34:849– 853.

52. Picard B, Jardel V, Tirlet G. Ceramic bonding: Reliability. In: Degrange M, Roulet JF (eds). Minimally Invasive Restorations with Bonding. Chicago: Quintessence, 1997:103–129. 53. Chen JH, Shi CX, Wang M, Zhao SJ, Wang H. Clinical evaluation of 546 tetracycline-stained teeth treated with porcelain laminate veneers. J Dent 2005;33:3–8. 54. Addison O, Fleming GJ, Marquis PM. The effect of thermocycling on the strength of porcelain laminate veneer (PLV) materials. Dent Mater 2003;19:291–297. 55. Calamia JR, Calamia CS. Porcelain laminate veneers: Reasons for 25 years of success. Dent Clin North Am 2007;51:399–417. 56. Peumans M, De Munck J, Fieuws S, Lambrechts P, Vanherle G, Van Meerbeek B. A prospective ten-year clinical trial of porcelain veneers. J Adhes Dent 2004;6:65–76. 57. Land MF, Hopp CD. Survival rates of all-ceramic systems differ by clinical indication and fabrication method. J Evid Base Dent Pract 2010;10:37–38. 58. Dentin bonding agents. Dent Advisor 1995;12(2):1–8. 59. Boksman L, Tousignant G, Boushell LW, Santos Jr GC. Adhesives: Newer is not always better. 2. Inside Dent 2012;8(3). http://www. dentalaegis.com/id/2012/03/adhesives-newer-is-not-always-better. Accessed 15 January 2014. 60. Strang R, McCrosson J, Muirhead GM, Richardson SA. The setting of visible-light-cured resinds beneath etched porcelain veneers. Br Dent J 1987;163:149–151. 61. Magne P, Belser UC. Porcelain versus composite inlays/onlays: Effects of mechanical loads on stress distribution, adhesion, and crown flexure. Int J Periodontics Restorative Dent 2003;­23:543– 555. 62. Schulze KA, Marshall SJ, Gansky SA, Marshall GW. Color stability and hardness in dental composites after accelerated aging. Dent Mater 2003;19:612–619.

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Fixed Dental Prostheses for Anterior and Posterior Teeth

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Irena Sailer, prof dr med dent

Fixed dental prostheses (FDPs) were traditionally constructed as a metallic framework veneered with a tooth-colored ceramic. In recent years, heightened esthetic expectations of both clinicians and patients have created the demand for prostheses fully constructed from ceramics. The first all-ceramic fixed partial dentures exhibited promising outcomes in anterior regions. However, in the posterior region all-ceramic prostheses suffered from high fracture rates within a few years of clinical service.1 It became obvious that the ceramics used at that time, more specifically glass-ceramics and glassinfiltrated ceramics, did not exhibit sufficient strength for this kind of application. The replacement of posterior teeth by means of ceramic FDPs was not recommended until the development of the high-strength ceramic zirconia.2 Today the replacement of both anterior and posterior teeth can be accomplished successfully with different types of zirconia-based reconstructions as long as the material-specific requirements of zirconia are fulfilled. Zirconia has excellent material properties (strength and toughness) that exceed the properties of other types of ceramics. Still, it is a brittle material, as are most ceramics, and must be handled with care and attention to detail. The success of the zirconia-based FDPs is highly dependent on selection of appropriate clinical indications and careful attention to the clinical and technical procedures best suited for this material. If these factors are considered and the user has knowledge of the physical properties of this material, both resin-bonded and conventional FDPs with zirconia frameworks can be considered a very good treatment option in a variety of clinical situations.

Cantilevered All-Ceramic Resin-Bonded FDPs Resin-bonded FDPs are a minimally invasive treatment option for the replacement of single teeth. Originally, resin-bonded prostheses were developed for the replacement or splinting of periodontally compromised anterior teeth.3 Subsequently, application of this type of restoration was extended to the premolar and molar regions.4

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Fixed Dental Prostheses for Anterior and Posterior Teeth

a

c

The first kind of resin-bonded FDP was constructed utilizing metal-ceramic technology. Unfortunately, debonding of these FDPs occurred frequently; this type of restoration, therefore, was solely considered to be an interim restoration for short to medium terms of clinical service times, usually up to 2 years. During the last 40 years, significant modifications of the materials and the design have improved outcomes, resulting in increased use of the resin-bonded prosthesis as a medium- to long-term treatment alternative. New resin cements with improved bonding capacity to various types of materials have reduced the debonding rates. In addition, the development of a minimally invasive preparation design encompassing an increase in surface area for the bonding and vertical grooves for retention has led to significant improvement in the survival and success rates of resin-bonded prostheses.5 Most interestingly, the use of ceramics instead of metal as framework material has greatly improved the clinical outcomes of resin-bonded prostheses. Long-term studies of all-ceramic resin-bonded FDPs have demonstrated very good outcomes with low or no incidence of debonding.6 The ceramic frameworks, in addition, have dramatically improved the esthetic outcomes of the prostheses (Fig 7-1). 120

b Fig 7-1  (a) Metal-based framework for a resin-bonded FDP. Note the grayish discoloration of the mesial part of the abutment tooth and the small dimensions of the retainer (thickness of 0.5 mm) and the connector (cross section of 6.0 mm2). (b) Glass-ceramic resin-bonded FDP framework (Empress I, Ivoclar Vivadent). No discoloration of the abutment tooth occurs. Note the pronounced framework dimensions needed in the connector area (cross section of 16.0 mm2). The minimum retainer thickness for this kind of FDP is 0.8 mm. (c) Zirconia resinbonded FDPs combine the best of both types of resinbonded FDP shown in a and b; they share the toothlike color of glass-ceramic frameworks and the minimal dimensions of retainers (thickness of 0.5 mm) and connectors (cross section of 6.0 mm2) in metal-ceramic frameworks.

Because of these improvements, all-ceramic resin-bonded FDPs may be considered a permanent solution in a number of different clinical situations today. Several factors influence the choice among single-tooth implants, conventional fixed partial dentures, and all-ceramic resin-bonded FDPs for the replacement of single missing teeth: •  Age and general medical health of the patient •  Health of the teeth adjacent to the edentulous area (presence or absence of restorations, caries, etc) •  Position of the teeth adjacent to the edentulous area (straight or tilted, convergent or divergent roots) •  Anatomical situation of the edentulous ridge (presence or absence of supporting bone and soft tissues, ridge defects) •  Compliance of the patient •  Patient’s interest in surgical procedures and financial situation Clinical success rates for all-ceramic resin-bonded FDPs can be expected to be similar to those for other treatment options, provided that the clinical situation and site are indicated for this type of reconstruction.

Cantilevered All-Ceramic Resin-Bonded FDPs

Clinical indications General indications, advantages, and limitations Resin-bonded prostheses are mostly indicated for patients in the following categories: •  Adolescents and young adults with congenitally missing teeth or teeth lost to trauma, which cannot be replaced with single implants due to the age of the patient •  Patients with narrow single-tooth edentulous spaces not suitable for the placement of implants •  Patients with a missing single tooth and healthy adjacent teeth who are not willing to undergo implant surgery •  Patients with medical contraindications to implant surgery To select the best restoration for any given clinical situation, the advantages and disadvantages of the various treatment options need to be considered. The main advantage of the resin-bonded FDP is the minimal invasiveness. Significantly less tooth structure is removed for a resin-bonded prosthesis than for other types of tooth-borne restorations.7 Another important advantage of the resin-bonded FDP is the low patient morbidity associated with the clinical treatment, most specifically compared to implants. Finally, the treatment time is shorter and the costs are lower for resin-bonded prostheses than for conventional fixed partial dentures or implants.8 This specific advantage is increasingly important for patients today. However, the resin-bonded FDP has some limitations that have to be considered. The literature indicates that the survival rates of metal-ceramic resin-bonded FDPs are lower than those of single-tooth implants or conventional FDPs. Systematic reviews of the literature estimated that resin-bonded prostheses have a 5-year survival rate of only 87.7%,9 whereas the corresponding survival rates of singleimplant crowns and conventional FDPs were much higher, 97.2%10 and 94.4%,11 respectively. However, these low survival rates seem only to be valid for the traditional metalceramic resin-bonded FDPs. The outcomes of all-ceramic resin-bonded prostheses are considerably better.6 As an example, cantilevered resinbonded FDPs made out of glass-infiltrated alumina exhibited a survival rate of 94.4% after 10 years.6 This specific ceramic is not in routine use any more due to the development of improved ceramics such as zirconia. Cantilevered zirconia resin-bonded FDPs performed even better than the ones constructed with glass-infiltrated alumina. Excellent results for the zirconia resin-bonded FDPs

were recently published, showing a survival rate of 100% at 3 and 4 years of service.12,13 Although these first reports are very positive, more clinical data are needed to support the findings.

Specific indications: Anterior region Another factor to be considered in the choice between different treatment options is the region of the jaw in which a tooth needs to be replaced. Replacement of incisors. All-ceramic resin-bonded pros­ theses are mainly indicated in anterior regions for the replacement of maxillary and mandibular incisors. In these regions the forces occurring during function are rather low. The maximal loading forces reported for incisor regions range between 108 and 382 N.14,15 For reconstructions in these regions, ceramics with a fracture resistance exceeding the maximal load are needed. The fracture resistance of leucite-reinforced glass-ceramic reconstructions (eg, Empress 1, Ivoclar Vivadent) ranges from 160 to 330 N, while the fracture resistance of lithium disilicate–reinforced glass-ceramic (eg, Empress 2, Ivoclar Vivadent) ranges from 260 to 280 N.16 Reconstructions made of a lithium disilicate glass-ceramic (ie, e.max Press and e.max CAD, Ivoclar Vivadent) exhibit a fracture resistance of 907 to 986 N.17 The fracture resistance of zirconiabased reconstructions can reach 1,620 N.18 Furthermore, laboratory studies of cantilevered resin-bonded prostheses made of glass-infiltrated alumina or zirconia showed fracture resistance values ranging from 270 to 290 N.19,20 With respect to stability, all of the types of ceramics discussed so far are appropriate for anterior resin-bonded FDPs. For weaker types of ceramics, such as glass-ceramics, the desired stability of the reconstruction has to be provided by increasing the framework thickness in the retainer and connector areas. This increase in thickness may cause overcontouring and, as a consequence, esthetic and hygienic issues. Zirconia framework dimensions (retainer and connector) can be designed more similarly to the dimensions of metallic frameworks.21 The choice of the ceramic for an anterior resin-bonded FDP is based on the following factors: •  Amount of vertical and horizontal interocclusal space in the abutment tooth and pontic regions •  Minimally required dimensions for the framework and the connector for the different types of ceramics •  Color and shade of the teeth adjacent to the area to be restored

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Fixed Dental Prostheses for Anterior and Posterior Teeth

a

b

c

d

Fig 7-2  (a) Palatal view of a canine in a model. The red area indicates the region of the central occlusal contact and the region loaded during canine guidance; the blue area indicates the region below the load during occlusion and function, the area that is useful for the retention of the prosthesis. (b) Contact point in centric occlusion. In this area adequate space for the retainer and connector of the prosthesis must be provided by preparation design. (c) A minimum of 0.5 mm of preparation is recommended in the region of the centric occlusion contact area, if frameworks have to be extended to that area. (d) Fracture of a zirconia ceramic resin-bonded prosthesis. This occurred because there was insufficient space for the framework in the region of the centric occlusion contact point, leading to the need for adjustments and thereby weakening of the zirconia framework.

Replacement of canines. In contrast to the incisor area, the replacement of canines with all-ceramic resin-bonded prostheses may be critical because of the type of load that occurs during function in the canine regions. During the occlusal and lateral movements of the jaws, high tensile load is transmitted to the retainer and/or the connector area of the resin-bonded FDP.19,22 This may increase the risk for fracture or debonding of the resin-bonded FDP during canine guidance (Fig 7-2). Consequently, the dimensions of the retainer and connector have to be increased in the canine region, and sufficient space has to be provided by a more invasive abutment tooth preparation. If there is a lack of space and/or enamel, a metal-ceramic resin-bonded prosthesis should be preferred. 122

Site-specific requirements. In general, the clinical success of the all-ceramic resin-bonded FDPs is highly dependent on careful selection of the appropriate patient and site. Anterior resin-bonded FDPs have the following sitespecific clinical prerequisites: •  Overjet greater than 0.5 to 1.0 mm to allow sufficient space for a retainer •  Overbite less than 1.0 to 1.5 mm to provide sufficient area for the bonding •  Centric occlusal contacts located in incisal third, to leave sufficient area for the retainer The choice of the most appropriate abutment tooth for the anterior resin-bonded prosthesis is based on the pala-

Cantilevered All-Ceramic Resin-Bonded FDPs

a

b

Fig 7-3  (a) The palatal surface of the central incisors is rather flat. (b) The palatal surface of the canine is rounded. Canines, therefore, allow for better retention of the framework because of the wraparound shape of the palatal surface.

Fig 7-4  Posterior metal-ceramic resin-bonded prosthesis. Note the visibility of the gray metal framework.

tal or lingual space offered in centric occlusion and on the shape and size of the palatal or lingual surface (Fig 7-3). Two factors have to be evaluated in order to choose the most appropriate abutment tooth: •  Size of the palatal or lingual surface area that can be used for the bonding •  Shape of the palatal or lingual surface—ideally oval or round to allow a wraparound design of the retainer (see Fig 7-3b) For example, for the replacement of a lateral incisor, a canine may be more suitable as abutment tooth than a central incisor. The flat palatal surfaces of central incisors may compromise the minimally invasive abutment tooth preparation design (see the section, “Clinical procedures”) and may complicate the positioning of the FDP during the cementation (see Fig 7-3a).

Specific indications: Posterior region Because of the high load occurring in posterior regions, posterior metal-ceramic resin-bonded FDPs were used exclusively until recently23 (Fig 7-4). Metal frameworks provide a high level of stability; however, their gray color may result

in esthetic compromises in posterior regions (see Fig 7-4). Furthermore, the difficulties associated with the bonding of metal to the tooth have a negative impact on posterior resin-bonded prostheses. The debonding rates of posterior resin-bonded FDPs (22.8%) have been shown to be higher than those of anterior prostheses (18.4%).9 In general, the published outcomes of posterior metalceramic resin-bonded FDPs are controversial. While some studies report posterior resin-bonded FDPs to have low rates of complications,24–26 others show the contrary.23,27 Altogether a meta-analysis of the published data indicated that more problems were associated with posterior than anterior metal-ceramic resin-bonded FDPs.9 This type of restoration was and still is only recommended as a longterm provisional solution. To date, very little information is available on the clinical outcomes of posterior all-ceramic resin-bonded FDPs. A recent retrospective study of cantilevered resin-bonded prostheses of lithium disilicate glass-ceramic showed very promising outcomes in the posterior region and a 5-year survival rate of 100%.28 Fractures did not occur in this investigation. Furthermore, no debonding of the posterior glassceramic resin-bonded FDPs occurred in this study. Still, a number of open questions have to be answered before this type of reconstruction can be recommended as 123

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Fixed Dental Prostheses for Anterior and Posterior Teeth Fig 7-5  (a and b) Possible design of a glass-ceramic resin-bonded prosthesis, including a mesial inlay to restore the tooth defect.

a

b

Fig 7-6 (a) Design of a zirconia ceramic resin-bonded prosthesis fabricated according to the techniques developed for metalceramic resin-bonded FDPs. (b) Fracture and debonding have occurred 9 months after placement of the FDP. This type of prosthesis, therefore, cannot be recommended.

a

b

a long-term solution. One question is whether or not all-ceramic resin-bonded FDPs provide sufficient stability for use in posterior regions for longer periods of time. The mean occlusal force at posterior teeth was found to be approximately 400 N.29 Considering the aforementioned fracture resistance values of the different types of ceramics, it seems that both lithium disilicate glass-ceramics and zirconia ceramics might be indicated as framework materials (Figs 7-5 and 7-6). Still, long-term data for this type of restoration are very limited. Randomized clinical studies are needed to test whether or not posterior all-ceramic resin-bonded FDPs 124

can be recommended as a long-term, predictable solution in the future. Furthermore, the most suitable ceramic and the ideal design for posterior resin-bonded prostheses need to be elucidated.

Relative contraindications Resin-bonded FDPs can be problematic because of lack of space or malpositioning of the abutment teeth in patients with deep bite or a crowded dentition. In these situations, orthodontic pretreatment should be performed to provide the conditions required for resin-bonded FDPs (see Fig 7-16).

Cantilevered All-Ceramic Resin-Bonded FDPs

a

b

Fig 7-7  (a) Two-retainer metal-ceramic resin-bonded prosthesis replacing the maxillary right central incisor, shown at the 10-year recall visit. The patient had observed a recently increasing discoloration of the maxillary left lateral incisor. The loosening of the retainer on the right lateral incisor, however, was not noticed at the recall visit. (b) Brownish discoloration of the abutment tooth, indicating the presence of secondary caries beneath the retainer. (c) Radiograph showing secondary caries in the marginal areas of the metallic retainer.

c

Other relative contraindications to all-ceramic resinbonded FDPs are parafunctional habits and bruxism. Affected patients need to be informed about the increased risk for fracture or debonding of the all-ceramic resin-bonded prostheses. Finally, if the esthetics of the abutment tooth are poor (ie, it is discolored or has an unesthetic shape or size) a complete crown with a cantilever may be a better treatment option than a resin-bonded prosthesis because the complete-coverage design can positively influence the esthetics of the abutment tooth.

Absolute contraindications The following clinical situations represent absolute contraindications to all-ceramic resin-bonded prostheses: •  Absence of more than one tooth (multiple pontics) •  Enamel deficiency (eg, amelogenesis imperfecta) •  All other types of enamel defects (eg, severe erosions or abrasions) •  Caries or extensive restorations on abutment teeth

Design of the FDP: Multiple versus single retainer The FDPs designed for the first patients treated with allceramic resin-bonded prostheses had two or more abutment teeth. This design was recommended to increase the bonding area and the geometric retention. Today, though, it has been well documented that the outcomes of singleretainer, cantilevered resin-bonded prostheses are significantly better than those of prostheses with two or more retainers.30 The major problem with the multiple-retainer prostheses is that unnoticed loosening of one retainer may occur, leading to high risk for secondary caries beneath the loose retainer (Fig 7-7). The most likely cause of this problem is the differing periodontal mobility of the abutment teeth that are splinted by the prosthesis. As the splinted teeth move in different directions, shear tensile strain occurs in the areas of the retainer and connector. In the case of metal frameworks, the most vulnerable area is the bonding interface between the framework and the tooth. As a consequence, debonding is the most likely problem to occur. Multiple125

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Fixed Dental Prostheses for Anterior and Posterior Teeth retainer resin-bonded prostheses showed higher rates for debonding and lower survival rates than cantilevered metal-ceramic resin-bonded FDPs.30 In all-ceramic resin-bonded prostheses, the most vulnerable area is the connector area. Under tensile load occurring during the differing movement of the splinted abutment teeth, tensile stress cumulates in the connectors, inducing an increase in risk for fracture of one of the connectors. In clinical investigations, multiple-retainer anterior all-ceramic resin-bonded FDPs suffered from fractures of one the connectors, whereas cantilevered resin-bonded FDPs exhibited no fractures and had a very high 10-year survival rate of 94.4%.6 Hence, the number of retainers has a significant effect on the outcomes of all-ceramic resin-bonded FDPs. The change in design from two or more retainers to cantilevered might also be reason for the lower complication rates reported for posterior resin-bonded prostheses in more recent studies.24–26 Furthermore, no fractures were reported for cantilevered posterior all-ceramic resinbonded FDPs.28 Thus, metal-ceramic and all-ceramic resin-bonded FDPs should be designed as one-retainer, cantilevered prostheses.

•  No or low risk for discoloration of the abutment teeth by the retainer •  Highly esthetic outcomes

Choice of materials

Advantages of glass-ceramic •  A large number of different shades are available. •  Color selection is relatively easy. •  Procedures for the adhesive cementation are well established in daily clinical practice.

All-ceramic versus metal-ceramic resin-bonded FDPs Ceramic frameworks for FDPs have several advantages over metal frameworks. Metal-ceramic resin-bonded FDPs offer excellent material stability; however, a number of problems have been associated with the metal. Most important, the retention of the metal-ceramic resin-bonded retainer to the tooth continues to be a clinical problem. A number of techniques for chemical or morphologic improvement of the bonding were introduced, such as the use of nonprecious alloys, chemical etching, or silica coating.5 Despite these improvements, debonding rates remain a real clinical problem. Furthermore, in the anterior region, the esthetics of metal-ceramic resin-bonded prostheses is problematic because of the possible display or shadowing of the metallic retainers on the abutment teeth. Finally, ceramics are biologically advantageous over metals because the metals used for the resin-bonded FDPs most frequently are nonprecious alloys.31,32 Ceramics offer the following advantages over metals as a choice of framework material: •  Higher biocompatibility •  Lower corrosion rates •  Lower allergenic potential 126

Metal-ceramic resin-bonded FDPs are indicated in situations where the FDP will be subjected to a very high load or there is very little interocclusal space for the retainer. In all other situations, all-ceramic resin-bonded FDPs should be preferred.

Clinical criteria for choice of ceramic Different types of ceramics have been used for the fabrication of resin-bonded prostheses, including reinforced glassceramics, glass-infiltrated alumina, and densely sintered zirconia.6,28 The choice of the ideal ceramic for the individual patient is mainly based on the space available for the retainer and the connector and the color of the adjacent teeth. The literature indicates that all previously mentioned types of ceramics may be used with good success; hence, the choice of the ceramic can be based on factors other than just the material stability.

Disadvantage of glass-ceramic •  Larger dimensions of the framework are needed for clinical stability. Advantages of zirconia •  Material stability is very high. •  Framework dimensions are similar to those of metallic frameworks. •  Different framework shades are available. Disadvantages of zirconia •  Adhesive cementation may be more difficult; specific resin cements are needed.33 •  Fabrication procedures are costly; computer-aided design/ computer-assisted manufacture (CAD/CAM) tech­nology is needed.

Technical criteria for choice of ceramic As mentioned previously, the relevant dental technical criteria for the choice of the material for the resin-bonded FDP are the color and translucency of the teeth and the space available for the retainer and the connector. Other impor-

Cantilevered All-Ceramic Resin-Bonded FDPs Fig 7-8  (a) Clinical situation of a patient with very translucent, slightly chromatic teeth. Glass-ceramic resin-bonded FDPs may be used for the replacement of the right and left lateral incisors. (b) Glass-ceramic resin-bonded FDPs (e.max Press, Ivoclar Vivadent). Note the pronounced thickness in the retainer and connector areas.

a

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Fig 7-9  (a) Clinical situation in a patient with bright and rather opaque teeth. A zirconia ceramic resin-bonded prosthesis is indicated for the replacement of the maxillary right central incisor. (b) The zirconia framework can be fabricated with thinner dimensions in the retainer and connector areas.

a

b

tant factors in the choice are the access to fabrication technology (eg, CAD/CAM) and the personal experience of the dental laboratory technician. In general,

•  Zirconia ceramic resin-bonded FDPs are superior to glassceramics for patients with limited interocclusal space and with opaque bright teeth (Fig 7-9).

•  Glass-ceramic resin-bonded FDPs are indicated for patients with large vertical and horizontal space and translucent and/or chromatic teeth (Fig 7-8).

One important technical requirement for zirconia ceramic resin-bonded prostheses is the adequate preparation of the abutment tooth to fulfill the demands of the CAD/CAM fabrication of the frameworks. 127

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a

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Fig 7-10  (a) Analysis of horizontal space in centric occlusion. A minimum of 0.5 mm is required for zirconia. (b) Analysis of vertical space in centric occlusion. A minimum of 3.0 mm is required for zirconia.

Clinical procedures Analysis of the clinical situation and diagnostics Meticulous treatment planning encompasses the choice of the appropriate patient and indication with respect to the load, the amount of space for the FDP retainer/connector (Fig 7-10), and the choice of the most appropriate abutment tooth with respect to the area available for the bonding.

Conditioning of the pontic site For the conditioning of the soft tissues of the pontic site, flowable composite resin can be applied to the provisional prosthesis. The composite resin is applied primarily to the palatal/lingual part of the provisional tooth, leaving space in the labial and interproximal regions for optimal soft tissue shaping. After the application of the resin, the removable provisional prosthesis is placed back in the mouth. Slight blanching of the surrounding tissues is expected with the application of localized pressure. After 1 to 2 weeks, additional resin should be applied for further conditioning to achieve the desired gingival form in the pontic site (Fig 7-11).

Abutment tooth preparation The goal of treatment with a resin-bonded prosthesis is to be as minimally invasive as possible. However, clinical experience clearly shows that a minimal retentive preparation of the abutment tooth is advantageous for long-term success.23,34 Various types of preparations have been suggested over the years with the aim to provide a path of insertion and a geometric resistance form of the resin-bonded FDP abutment tooth. Most of the preparation designs encompassed vertical grooves, boxes, or slots, occlusal rests, and reduction of

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the palatal or lingual surfaces to provide a path of insertion.5 With the introduction of all-ceramic resin-bonded prostheses, a completely new design for the preparation was presented.34 This design included a proximal veneer preparation, which was adapted to the needs of the glassinfiltrated ceramics. More recently, a minimally invasive preparation adapted to the properties of zirconia and its CAD/CAM fabrication procedures was introduced.14 The minimally invasive preparation design for anterior zirconia-based resin-bonded FDPs includes preparation of a mesial and a distal vertical groove plus a tiny slot at the palatal or lingual cingulum region (Fig 7-12). Proximal grooves provide mechanical resistance of the framework to dislodgment, reducing the stress at the cement-substrate interface when under functional load. Clinical studies have shown that the clinical incidence of debonding was reduced when resin-bonded FDPs included abutment tooth preparation.35 Unfortunately, there is still no agreement on the preparation design for posterior zirconia-based all-ceramic resin-­bonded FDPs. As mentioned earlier, the posterior all-ceramic resin-bonded FDP as such is still considered experi­mental. A significant clinical benefit of the minimally invasive preparation with grooves and ball-shaped rest is the simplification of adhesive cementation. With the defined path of insertion, the FDP can securely be positioned during cementation. For long-term provisional resin-bonded pros­ theses involving no preparation of the abutment tooth, hooks made of resin or metal can be used as an aid for the positioning of the prosthesis during cementation (Fig 7-13). To summarize, a minimally invasive retentive preparation is recommended for resin-bonded FDPs that are planned as a definitive treatment option. Pretreatment diagnostic planning should encompass use of the study casts for thorough planning of the least invasive yet most retentive tooth preparation for each patient’s situation.

Cantilevered All-Ceramic Resin-Bonded FDPs

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e

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Fig 7-11  (a) Clinical site for resin-bonded prosthesis to replace the maxillary lateral incisor, prior to the conditioning of the pontic area. (b and c) For conditioning the pontic, flowable composite resin is applied to the provisional prosthesis, primarily to the gingival part of the provisional tooth. Adequate space is left in the labial and interproximal regions for optimal soft tissue health. (d) Removable provisional prosthesis in place, after the application of the resin. The slight blanching of the surrounding tissues indicates the application of localized pressure. (e) Incisal view of the preparation and the shaped pontic area prior to the impression. Conditioning has been achieved within two visits over 3 weeks. (f ) Virtual model of the prosthesis area (Cadent iTero, Align Technology). The shape of the pontic area enables the ideal virtual design of the framework.

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Fig 7-12  (a and b) Demarcation of mesial and distal grooves by means of a separating diamond bur (Universal Prep Set, diamond D3, Intensiv) within the enamel. The mesial groove is located slightly behind the proximal contact zone and is defining the path of insertion. (c and d) Finalization of the mesial and distal grooves in enamel with a veneer prep diamond (Universal Prep Set, diamond D 18 GS), ensuring the appropriate size and taper for CAD/CAM zirconia frameworks. (e) Preparation of the ball-shaped rest in the area of the cingulum with a round diamond bur (Universal Prep Set). (f ) Digital impression of the finalized preparation for the design of the CAD/CAM zirconia framework. (g) Zirconia ceramic resin-bonded prosthesis displaying the grooves and the ball-shaped rest.

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Cantilevered All-Ceramic Resin-Bonded FDPs

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d Fig 7-13  (a and b) Composite resin retainer for the positioning of the no-preparation resin-bonded prosthesis. (c and d) Metallic retainers for the positioning of the no-preparation resin-bonded prosthesis.

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e Fig 7-14  (a) The digital impression of a tooth preparation using Cadent iTero provides the STL file. (b) The framework shape and extension are defined. (c) The virtual framework designed using CARES (Straumann). (d) The milled white-stage zirconia framework is sintered. (e) The framework is checked on the CAD/CAM digital resin master cast.

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Cantilevered All-Ceramic Resin-Bonded FDPs

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Fig 7-15  (a) Isolation of the area of interest with rubber dam. The pontic zone is covered, which may cause problems with the seating of the prosthesis. Alternatively, rubber dam may be cut out in the pontic zone. (b) Thorough cleaning of palatal surface of the abutment tooth with pumice.

Technical fabrication procedures Technical fabrication begins with a digital impression of the tooth preparation, which will provide a standard tessellation language (STL) file that can be sent to the dental laboratory technician via the Internet (Fig 7-14a). In the laboratory, the framework shape and extension are defined before the retainer, connector, and pontic of the zirconia framework are designed (Fig 7-14b). The virtually designed framework is checked and approved by the technician before the data are sent to the centralized milling center (Fig 7-14c). After the milling of white-stage zirconia, the framework is sintered to full density in a sintering furnace (Fig 7-14d). The fit of the framework is checked on the CAD/ CAM digital resin master cast (Fig 7-14e). Subsequently the framework is veneered with conventional zirconia veneering ceramics.

Cementation of resin-bonded FDPs Clinical procedures for lithium disilicate glass-ceramic For the cementation of lithium disilicate glass-ceramic resinbonded FDPs, dual-curing bisphenol glycidyl methacrylate (bis-GMA)–based resin cements (eg, Variolink II, Ivoclar Vivadent) are recommended.36 Preparation of the clinical site •  Placement of rubber dam •  Thorough cleaning of the palatal or lingual surface with pumice •  Etching of the palatal or lingual surface of the abutment tooth with phosphoric acid •  Application of bonding agent in accordance with the manufacturer’s instructions

Preparation of the resin-bonded FDP •  Etching with hydrofluoric acid •  Silanization •  Application of bonding agent •  Application of dual-curing resin cement Positioning of the resin-bonded prosthesis, removal of excess cement, and light curing are performed according to the manufacturer’s recommendations.

Clinical procedures for zirconia For the cementation of zirconia ceramic resin-bonded FDPs, specific phosphate monomer–containing resin cements (eg, Panavia 21, Kuraray) and specific silanes (eg, Clearfil Porcelain Bond Activator, Kuraray) have to be used, because conventional bis-GMA resin cements do not chemically bond to zirconia.33 Preparation of the clinical site •  Placement of rubber dam •  Thorough cleaning of the palatal or lingual surface with pumice •  Etching of the palatal or lingual surface of the abutment tooth with phosphoric acid •  Application of ED Primer (Kuraray) Preparation of the resin-bonded FPD •  Cleaning with alcohol and sandblasting (30-µm aluminum oxide, 2-bar pressure, from a 10-cm distance) •  Silanization •  Application of a chemically curing resin cement The resin-bonded FDP is positioned, excess cement is removed, and glycerin (Oxyguard, Kuraray) is applied until setting is completed (Fig 7-15). 133

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c

d Fig 7-15  (cont) (c) Etching of the abutment tooth by means of phosphoric acid. Subsequently primer (ED Primer) corresponding to the resin cement (Panavia 21 TC) will be applied. (d) Application of the resin cement on the framework. Pretreatment of the framework involves silanization with a corresponding silane (Clearfil Porcelain Bond Activator). (e) Removal of excess cement while the prosthesis is securely held in place and application of glycerin gel (Oxyguard).

e

Clinical example A 16-year-old adolescent girl is missing her maxillary right central incisor because of posttraumatic endodontic complications (Figs 7-16a and 7-16b). Because of her age, a resinbonded prosthesis is considered the treatment of choice. Crowding of the anterior teeth and the deep, steep bite contribute to the complication that insufficient space is available for a resin-bonded prosthesis (Fig 7-16c). Orthodontic pretreatment is planned to provide the ideal clinical condition for either a resin-bonded prosthesis or a single implant (Fig 7-16d). When the tooth is removed, an extensive bone and soft tissue defect is revealed (Fig 7-16e). Bone

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and soft tissue augmentation are performed for ridge preservation (Fig 7-16f ). After healing of the ridge, orthodontic treatment is initiated. The crowding of the teeth is corrected, and sufficient horizontal and vertical space is provided for placement of a resin-bonded prosthesis on the maxillary left central incisor (Figs 7-16g and 7-16h). The soft tissue is conditioned into the shape of an ovate pontic by means of a removable provisional prosthesis (Figs 7-16i and 7-16j). After tooth preparation and impression procedures, a zirconia ceramic resin-bonded prosthesis is made, and nopreparation veneers made of resin are used to improve the shape and esthetic appearance of the right lateral incisor (Figs 7-16k to 7-16o).

Cantilevered All-Ceramic Resin-Bonded FDPs

a

b

Fig 7-16  (a and b) Initial situation showing the affected maxillary right central incisor and posttraumatic endodontic complications. (c and d) Crowding of the anterior teeth and a deep, steep bite can be managed with orthodontic pretreatment.

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e

f

g

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Fig 7-16  (cont) (e and f ) After removal of the tooth, the extensive bone and soft tissue defect is managed with bone and soft tissue augmentation. (g and h) Orthodontic treatment corrects the crowding of the teeth and provides sufficient horizontal and vertical space for the resin-bonded restoration. (i and j) The removable provisional prosthesis is used to condition and shape the soft tissue for an ovate pontic. (k) The zirconia ceramic resin-bonded prosthesis.

k

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Cantilevered All-Ceramic Resin-Bonded FDPs

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o Fig 7-16  (cont) (l) No-preparation veneers. (m) Adhesive cementation of the resin-bonded prosthesis with Panavia 21 TC and of the no-preparation resin veneers with a bis-GMA resin (Tetric Classic, Ivoclar Vivadent). (n and o) Appearance after finalization of the restorations.

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Conventional All-Ceramic FDPs Conventional all-ceramic fixed partial dentures have become a viable treatment alternative to metal-ceramic FDPs and are utilized in anterior and posterior regions in daily practice today. The interest in all-ceramic FDPs was greatly affected by the introduction of zirconia as a framework material in the early 1990s. In contrast to ceramic FDPs utilizing glass-ceramic or glass-infiltrated alumina, reconstructions constructed of zirconia are able to withstand very high loads and are, therefore, applicable in different clinical indications. Because zirconia cannot be processed with traditional dental technical techniques, new CAD/CAM procedures developed over the last few years have made zirconia a viable alternative to metal-ceramic and weaker all-ceramic systems. These computerized methods of fabricating frame­ works, including the digital design of frameworks and the milling of prefabricated zirconia blanks, have overcome a number of initial challenges and have significantly evolved since their introduction. Today, zirconia ceramic FDPs and the CAD/CAM procedures required to fabricate them have become part of daily practice.

Indications and clinical research outcomes Zirconia ceramic FDPs have been in routine clinical use for more than 10 years, and their outcomes have been the subject of numerous studies. When zirconia was first introduced, zirconia ceramic FDPs exhibited very good clinical stability, but there were significant problems with the accuracy of the frameworks. As a consequence, a number of early clinical studies reported marginal discrepancies of the reconstructions and subsequent high incidence of secondary caries. High rates of loss of the reconstruction, caused by biologic complications, were therefore reported.37 The problems with accuracy were mainly associated with the early CAD/CAM procedures, which were either in a prototype stage38 or needed further refinement of the software or the processing.39 Another, clinically related explanation was that, initially, knowledge about the ideal type of tooth preparation for the new computerized manufacturing was lacking. The preparation guidelines for metal-ceramic reconstructions first had to be adapted and modified to conform to the specific needs of CAD/CAM and zirconia. These issues seem to be greatly reduced or even overcome today. Awareness of the requirement for adequate abutment tooth preparation by the clinician along with steady improvements in CAD/CAM systems have led to an

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increase in the accuracy of fit of zirconia frameworks.39 As a consequence, fewer biologic complications of zirconia ceramic FDPs are being reported today. Today, the clinical survival rates of anterior and posterior zirconia ceramic FDPs are similar to those of traditional metal-ceramic FDPs. The mean cumulative 5-year survival rate of zirconia ceramic FDPs was 94% in a recent systematic review.37 The same rate reported for metal-ceramic FDPs was 94%.11 More recent randomized controlled clinical trials of zirconia ceramic and metal-ceramic FDPs also show no differences in the survival rates of the two types of reconstruction.40 These positive results are valid both for anterior and posterior regions of the jaws. However, they are limited to short-span FDPs (three- to four-unit). It has been shown that long-span zirconia ceramic FDPs (five units or more) have a higher risk for failure due to catastrophic fracture.37

Technical complications: Chipping of the veneering ceramic Recently concerns have been raised with respect to chipping of the zirconia veneering ceramic (Figs 7-17 and 7-18). This complication occurs significantly more often with zirconia ceramic than with metal-ceramic FDPs41 and is found in all types of available zirconia framework materials, systems, and veneering ceramics.37 Numerous basic studies have been performed with the aim to solve the problem of chipping of zirconia veneering ceramic. Yet despite improvements of the framework support, the physical properties of the zirconia veneering ceramics, and the firing procedures, this significant clinical challenge with the zirconia ceramic FDPs has not been solved yet. More research and further modifications will be necessary to reduce rates of chipping in the future. Otherwise, zirconia ceramic FDPs will only be applicable as an alternative to metal in very specific clinical situations. In a number of investigations, chipping of the zirconia veneering ceramic was found at nonfunctional cusps (eg, the lingual cusps of mandibular molars).40,42 In this region, no or only minimal load should be expected, because these cusps are not in centric occlusion. In the presence of excursive contacts during function, however, shear loading of the veneering ceramic may occur at the nonfunctional cusps. It may be assumed that one additional factor influencing the risk for chipping is the type and direction of occlusal or functional load on the veneering ceramic. Occlusion and function lead to wear of the veneering ceramic, which significantly increases the risk for chipping, as has been shown in clinical studies.28,43 Scanning electron microscopic evaluations of zirconia-based ceramic

Conventional All-Ceramic FDPs

a

b

Fig 7-17  (a) Ceramic chipping at the buccal cusps of zirconia ceramic maxillary first and second premolar crowns. (b) There is a close relationship between the chipped areas and the occlusal contacts in centric occlusion.

2 mm

Fig 7-18  Ceramic chipping at the distolingual cusp of a pontic on a mandibular posterior zirconia ceramic prosthesis (original magnification ×100). (Reprinted from Sailer et al40 with permission.)

FDPs with chipping demonstrate the association between chipped and fractured areas and the occlusal and functional contact areas. Chipping of the zirconia veneering ceramic remains a significant clinical problem. Basic and clinical studies are needed to analyze the aforementioned observations in more detail. This is a significant research challenge because

there are so many variables involved, including preparation design, design of the zirconia framework, laboratory procedures, and chairside procedures to adjust the occlusion and polishing. Until this issue is solved, patients have to be informed about the risk of chipping, and the clinical indications for this type of reconstruction have to be weighed against other options. 139

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Clinical and technical considerations in the selection of material Anterior regions In the anterior region, different types of ceramics can be used as framework material, because the load in this area is rather low. In this region, the choice of restorative material can mainly be based on esthetic factors. The first generation of veneering ceramics for zirconia exhibited poor esthetics because their high translucency led to show-through of the bright white framework (see the article by Sailer et al38 on zirconia veneering ceramics). In the past few years, zirconia veneering ceramics have significantly improved.44 Furthermore, zirconia can be colored to match the shade of the reference teeth before veneering or can be delivered with different grades of translucency. The choice of the ideal restorative material is based on: •  The color of the substrate (dentin or core material). •  The space available as a result of the tooth preparation. •  Preferences in cementation techniques. Fixed partial dentures constructed of glass-ceramics are indicated when: •  The dentin of the abutment teeth is not discolored. •  The color of adjacent (reference) teeth is warm and chromatic. •  Sufficient vertical and horizontal space is available for the connectors (three-unit FDP: cross section of 16.0 mm2). •  Clinical prerequisites for good adhesive cementation (good dentin quality and low amount of saliva) are present. •  Low loads will be transmitted at occlusion and function. Fixed partial dentures constructed of zirconia ceramic are indicated when: •  Dentin is not discolored or lightly discolored. •  The color of adjacent teeth is bright and whitish. •  Sufficient vertical and horizontal space is available for the core (minimal thickness of 0.5 mm) and the connectors (three-unit FDP: cross section of 6.0 to 9.0 mm2). •  Clinical prerequisites for adhesive cementation are present. Alternatively, self-adhesive cements or glass-ionomer cements may be used. •  Low to medium loads will be transmitted at occlusion and function. In patients with high occlusal loads (parafunctions or bruxism), metal-ceramic FDPs should be preferred. 140

Posterior regions In posterior regions where high loading forces occur, zirconia is the “ceramic of choice” today for patients or clinicians desiring all-ceramic rehabilitations. Other types of reinforced ceramics, like lithium disilicate glass-ceramics (eg, e.max Press or CAD), may alternatively be considered in the future; however, the incidence of fracture of these prostheses is reportedly higher than that associated with zirconia.6 Furthermore, the dimensions of the framework have to be adapted to the requirements of this glass-ceramic to provide sufficient strength. Therefore, each posterior site has to be evaluated with respect to the expected load and available space, and connector sizes recommended by the manufacturers must be followed. Furthermore, the site needs to be judged with respect to the CAD/CAM requirements. For CAD/CAM frameworks, specific requirements for the shape and taper of the abutment teeth need to be considered at the abutment tooth preparation.45 In situations where high loads are expected, metal-ceramic FDPs are more typically indicated.

Clinical procedures Analysis of the clinical situation and diagnostics The restorative team of clinician and the dental technician has to evaluate each individual clinical situation with respect to the different framework materials. With respect to CAD/CAM zirconia frameworks, the team should consider the following factors (Fig 7-19): •  The length of the FDP: Is it suitable for the available size of the ingots? •  The axial height of the abutment walls: Do they allow for the required dimensions of the FDP connectors? •  The condition of the abutment teeth: Can undercuts be avoided? •  Occlusion and function: Is there a risk for chipping of the veneering ceramic?

Universal tooth preparation The design of the abutment tooth preparation has a significant influence on the outcomes of all-ceramic reconstructions. Most specifically, the accuracy of the CAD/CAM zirconia framework is influenced by the preparation.45 To provide the ideal prerequisites for all types of materials and fabrication techniques, use of a universal tooth preparation

Conventional All-Ceramic FDPs

Fig 7-19  Clinical situation after abutment tooth preparation, displaying the factors to consider with respect to CAD/CAM zirconia frameworks.

a

Fig 7-20  Universal tooth preparation of nondiscolored and discolored abutment teeth, allowing for both all-ceramic and metal-ceramic crowns.

b

Fig 7-21  (a and b) The labiolingual width of the incisal edge is crucial for CAD/CAM feasibility. Sharp-edged preparations might not be transferrable to the frameworks because of the large size of the milling instruments.

(Universal Prep Set, Intensiv) is recommended for crowns and FDPs. The universal tooth preparation fulfills the following criteria (Fig 7-20): •  Marginal shoulder: 1.0-mm, located equigingivally or 0.5 mm or less subgingivally •  Total occlusal convergence: 10 to 12 degrees •  Axial reduction: 1.5 mm or greater

•  Incisal or occlusal reduction: 1.5 to 2.0 mm •  Minimal labiolingual width of the incisal edge of anterior teeth: 1.0 mm (Fig 7-21) •  Buccolingual reduction of the occlusal surface: 30 degrees •  Height of axial walls: 3.0 to 4.0 mm •  Smoothed and rounded line angles and edges •  No undercuts in splinted multiunit reconstructions

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Impression techniques: Conventional versus digital Recently, multiple optical and digital impression systems have been introduced as alternatives to the conventional impression. With these systems, the intraoral condition is digitized, and the scan is sent to the dental laboratory via the Internet. Based on these data, the dental technician can order a master cast, which is fabricated from resin in a centralized production facility. Alternatively, based on the same data, a zirconia framework is virtually designed and sent to another centralized production facility for milling. Today veneering of the frameworks is performed manually. It is very likely, though, that in the future the veneering will be done by CAD/CAM techniques as well using prefabricated glass-ceramic ingots. Furthermore, monolithic zirconia reconstructions may also be recommendable in the future. The entirely digital method of fabricating zirconia ceramic FDPs is still very new, and the benefits and limitations of the optical and digital impressions compared to those of traditional impressions are currently being analyzed. CAD/ CAM fabrication of the frameworks, however, is well established.

Clinical example A patient presented with a posterior prosthesis extending from the mandibular left first premolar to first molar (Fig 7-22a). Secondary caries at the first molar abutment required removal of the prosthesis (Fig 7-22b). After removal of the caries, a composite resin core is built up on the first molar, and the teeth are prepared for a digital impression (Fig 7-22c). Retraction cords are placed according to the techniques used for conventional impressions, and the site is pretreated with a scanning powder (Figs 7-22d and 7-22e). A digital impression is performed, and the virtual

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model is delivered to the dental technical laboratory via the Internet (Figs 7-22f and 7-22g). In the laboratory, a virtual framework is designed by means of specialized software, combining crown copings with a preshaped pontic chosen from a virtual tooth library (Fig 7-22h). One significant advantage of CAD/CAM manufacturing is the inherent quality control. Before the framework data are sent for production, the desired thickness of the framework and the connectors have to be approved as defined by the manufacturer (Figs 7-22i to 7-22k). The zirconia framework and a centrally fabricated resin master cast are delivered to the dental laboratory for veneering and finalization (Fig 7-22l). The zirconia ceramic prosthesis is completed and cemented in place (Figs 7-22m and Fig 7-22n).

Cementation of high-strength ceramic FDPs Specific phosphate monomer–containing primers and cements are needed for adhesive cementation of zirconia ceramic FDPs.46 The resin cement best documented in the literature for use with zirconia is Panavia 21.47 This resin cement exhibits very high bond strengths but is technique sensitive because of its hydrophobic nature. More recently, less technique-sensitive self-adhesive resin cements with phosphate monomers were introduced; among these, RelyX Unicem (3M ESPE) performs very well in combination with zirconia.48 Interestingly, in clinical studies, conventional cementation of zirconia ceramic FDPs with resin-reinforced glassionomer cement did not negatively influence the outcomes compared to the results of studies testing with adhesive cementation.43,49 As a consequence, in situations in which adhesive cementation may be impaired (in the posterior region or in the presence of high saliva flow), conventional cementation of zirconia ceramic FDPs can be recommended.

Conventional All-Ceramic FDPs

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Fig 7-22  (a and b) The posterior prosthesis extending from the mandibular left first premolar to first molar must be removed because of secondary caries at the first molar abutment. (c) The caries is removed and a composite resin core is built up on the first molar. (d) Retraction cords are placed in preparation for the digital impression. (e) The site is pretreated with a scanning powder. (f ) The digital impression is completed with a Lava Chairside Oral Scanner (3M ESPE). (g) The virtual model.

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n

References

Conclusion All-ceramic FDPs are a widely used alternative to metalceramic prostheses for different indications. Resin-bonded prostheses fabricated of glass-ceramic or zirconia ceramic are a recommendable, minimally invasive treatment option for the replacement of missing incisors. The clinical requirements for these FDPs, however, have to be considered for each individual patient, and meticulous pretreatment diagnostics and planning must be performed. All-ceramic full-arch prostheses can only be recommended if zirconia is used as ceramic framework material. The high rates of chipping of the veneering ceramic are still a clinical problem. For this reason, zirconia ceramic full-arch prostheses are not an equally successful alternative to metal-ceramics today. These all-ceramic prostheses should, therefore, only be recommended in specific clinical situations or for patients who are willing to accept this risk. Future developments and improvements are needed to improve the outcomes of zirconia ceramic FDPs.

Acknowledgments Master Dental Technician Vincent Fehmer, chief dental technician for the Clinic for Fixed and Removable Prosthodontics and Dental Material Science of the University of Zurich, is gratefully acknowledged for his high expertise and important contribution of sections on technical criteria and technical fabrication procedures.

Fig 7-22  (opposite page) (h) The virtual framework is designed in the laboratory. (i to k) Quality control ensures the thickness of the framework and the connectors are approved as defined by the manufacturer prior to production. (l) The zirconia framework and a centrally fabricated resin master cast. (m and n) The finalized zirconia ceramic prosthesis is cemented in place.

References 1. Raigrodski AJ, Chiche GJ, Swift EJ Jr. All-ceramic fixed partial dentures. 3. Clinical studies. J Esthet Restor Dent 2002;14:313–319. 2. Raigrodski AJ, Hillstead MB, Meng GK, Chung KH. Survival and complications of zirconia-based fixed dental prostheses: A systematic review. J Prosthet Dent 2012;107:170–177. 3. Rochette AL. Attachment of a splint to enamel of mandibular anterior teeth. J Prosthet Dent 1973;30:418–423. 4. Livaditis GJ. Cast metal resin-bonded retainers for posterior teeth. J Am Dent Assoc 1980;101:926–929. 5. El-Mowafy O, Rubo MHM. Resin-bonded fixed partial dentures—A literature review with presentation of a novel approach. Int J Prosthodont 2000;13:460–467. 6. Kern M, Sasse M. Ten-year survival on anterior all-ceramic resinbonded fixed dental prostheses. J Adhes Dent 2011;13:407–410. 7. Edelhoff D, Sorensen JA. Tooth structure removal associated with various preparation designs for anterior teeth. J Prosthet Dent 2002;87:503–509. 8. Brägger U, Krenander P, Lang NP. Economic aspects of single-tooth replacement. Clin Oral Implants Res 2005;16:335–341. 9. Pjetursson BE, Tan WC, Tan K, Brägger U, Zwahlen M, Lang NP. A systematic review of the survival and complication rates of resinbonded bridges after an observation period of at least 5 years. Clin Oral Implants Res 2008;19:131–141. 10. Jung RE, Zembic A, Pjetursson BE, Zwahlen M, Thoma D. Systematic review of the survival rate and the incidence of biological, technical and esthetic complications of single crowns on implants. Clin Oral Implants Res 2012;23(suppl 6):2–21. 11. Sailer I, Pjetursson BE, Zwahlen M, Hämmerle CH. A systematic review of the survival and complication rates of all-ceramic and metal-ceramic reconstructions after an observation period of at least 3 years. 2. Fixed dental prostheses. Clin Oral Implants Res 2007; 18(suppl 3):86–96. 12. Sasse M, Eschbach S, Kern M. Randomized clinical trial on single retainer all-ceramic resin-bonded fixed dental prostheses: Influence of the bonding system after up to 55 months. J Dent 2012;40:783– 786. 13. Sailer IA, Hämmerle CHF. Zirconia-ceramic single-retainer resinbonded fixed dental prostheses (RBFDPs) after a mean follow-up of 4 years—A retrospective clinical and volumetric study. Int J Periodontics Restorative Dent (in press). 14. Helkimo E, Carlsson GE, Helkimo M. Bite force and state of dentition. Acta Odontol Scand 1977;35:297–303. 15. Waltimo A, Kononen M. Maximal bite force and its association with signs and symptoms of craniomandibular disorders in young Finnish non-patients. Acta Odontol Scand 1995;53:254–258. 16. Al-Wahadni AM, Hussey DL, Grey N, Hatamleh MM. Fracture resistance of aluminium oxide and lithium-disilicate-based crowns using different luting cements: An in vitro study. J Contemp Dent Pract 2009;10(2):51–58. 17. Abou-Madina MM, Özcan M, Abdelaziz KM. Influence of resin cements and aging on the fracture resistance of IPS e.max Press posterior crowns. Int J Prosthodont 2012;25:33–35. 18. Donnelly TJ, Burke FJ. In vitro failure of crowns produced by two CAD/CAM systems. Eur J Prosthodont Restor Dent 2011;19: 111–116. 19. Koutayas SO, Kern M, Ferraresso F, Strub JR. Influence on framework design on fracture strength of mandibular anterior all-ceramic resin bonded fixed partial dentures. Int J Prosthodont 2002;15:223– 229. 20. Rosentritt M, Kolbeck C, Ries, S, Gross M, Behr M, Handel G. Zirconia resin-bonded fixed partial dentures in the anterior maxilla. Quintessence Int 2008;39:313–319. 21. Komine F, Blatz MB, Matsumura H. Current status of zirconia-based fixed restorations. J Oral Sci 2010;52:531–539.

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Fixed Dental Prostheses for Anterior and Posterior Teeth 22. Pospiech P, Rammelsberg P, Goldhofer G, Gernet W. All-ceramic resin-bonded bridges. A 3-dimensional finite-element analysis study. Eur J Oral Sci 1996;104:390–395. 23. De Kanter RJAM, Creugers NHJ, Verzijden CWGJM, Van’t Hof MA. A five-year multi-practice clinical study on posterior resin-bonded bridges. J Dent Res 1998;77:609–614. 24. Botelho MG, Nor LC, Kwong HW, Kuen BS. Two-unit cantilevered resin-bonded fixed partial dentures: A retrospective, preliminary clinical investigation. Int J Prosthodont 2000;13:25–28. 25. Briggs P, Dunne S, Bishop K. The single unit, single retainer, cantilever resin-bonded bridge. Br Dent J 1996;181:373–379. 26. Aggstaller H, Beuer F, Edelhoff D, Rammelsberg P, Gernet W. Longterm clinical performance of resin-bonded fixed partial dentures with retentive preparation geometry in anterior and posterior areas. J Adhes Dent 2008;10:301–306. 27. Creugers NH, Snoek PA, van’t Hof MA, Kayser AF. Clinical performance of resin-bonded bridges: A 5-year prospective study. 1. Design of the study and influence of experimental variables. J Oral Rehabil 1989;16:427–436. 28. Sailer I, Bonani T, Brodbeck U, Hämmerle CHF. Retrospective clinical study of single-retainer cantilever anterior and posterior glassceramic resin-bonded fixed dental prostheses at a mean follow-up of 6 years. Int J Prosthodont 2013;26:443–450. 29. Körber KH, Ludwig K. Maximale Kaukraft als Berechnungsfaktor zahntechnischer Konstruktionen. Dent Lab 1983;31:55–60. 30. Van Dalen A, Feilzer AJ, Kleverlaan CJ. A literature review of two-unit cantilevered FPDs. Int J Prosthodont 2004;17:281–284. 31. Livaditis GJ, Tate DL. Gold-plating etched-metal surfaces of resinbonded retainers. J Prosthet Dent 1988;59:153–158. 32. Wirz J, Schmidli F, Steinemann S, Walli R. Fired alloys in the crevice corrosion test. Schweiz Monatsschr Zahnmed 1987;97:­571–573. 33. Kern M, Swift EJ Jr. Bonding to zirconia. J Esthet Restor Dent 2011;23:71–72. 34. Kern M. Clinical long-term survival of two-retainer and single-retainer all-ceramic resin-bonded fixed partial dentures. Quintessence Int 2005;36:141–147. 35. Rammelsberg P, Pospiech P, Gernet W. Clinical factors affecting adhesive fixed partial dentures: A 6-year study. J Prosthet Dent 1993; 70:300–307. 36. Blatz M, Sadan A, Kern M. Resin-ceramic bonding: A review of the literature. J Prosthet Dent 2003;89:268–274.

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37. Schley JS, Heussen N, Reich S, Fischer J, Haselhuhn K, Wolfart S. Survival probability of zirconia-based fixed dental prostheses up to 5 yr: A systematic review of the literature. Eur J Oral Sci 2010;118:443– 450. 38. Sailer I , Fehér A, Filser F, Gauckler LJ, Lüthy H, Hämmerle CHF. Five-year clinical results of zirconia frameworks for posterior fixed partial dentures. Int J Prosthodont 2007;20:383–388. 39. Abdou J, Lyons K, Swain M. Fit of zirconia fixed partial denture: A systematic review. J Oral Rehabil 2010;37:866–876. 40. Sailer I, Gottner J, Känel S, Hämmerle CHF. Randomized controlled clinical trial of zirconia-ceramic and metal-ceramic posterior fixed dental prostheses: A 3 year follow-up. Int J Prosthodont 2009;22: 553–560. 41. Heintze SD, Rousson V. Survival of zirconia-and metal-supported fixed dental prostheses: A systematic review. Int J Prosthodont 2010;23:493–502. 42. Tinschert J, Schulze KA, Natt G, Latzke P, Heussen N, Spiekermann H. Clinical behavior of zirconia-based fixed partial dentures made of DC-Zirkon: 3-year results. Int J Prosthodont 2008;21:217–222. 43. Sax C, Hämmerle CH, Sailer I. 10-year outcomes of fixed dental prostheses with zirconia frameworks. Int J Comput Dent 2011;14: 183–202. 44. Sailer I, Holderegger C, Jung RE, et al. Clinical study of the color stability of veneering ceramics for zirconia frameworks. Int J Prosthodont 2007;20:263–269. 45. Beuer F, Edelhoff D, Gernet W, Naumann M. Effect of preparation angles on the precision of zirconia crown copings fabricated by CAD/CAM system. Dent Mater 2008;27:814–820. 46. Lehmann F, Kern M. Durability of resin bonding to zirconia ceramic using different primers. J Adhes Dent 2009;11:479–483. 47. Keul C, Liebermann A, Roos M, Uhrenbacher J, Stawarczyk B, Ing D. The effect of ceramic primer on shear bond strength of resin composite cement to zirconia: A function of water storage and thermal cycling. J Am Dent Assoc 2013;144:1261–1271. 48. Mirmohammadi H, Aboushelib MNM, Salameh Z, Feilzer AJ, Kleverlaan CJ. Innovations in bonding to zirconia based ceramics. 3. Phosphate monomer resin cements. Dent Mater 2010;26:786–792. 49. Raigrodski AJ, Yu A, Chiche GJ, Hochstedler JL, Mancl LA, Mohamed SE. Clinical efficacy of veneered zirconium dioxide-based posterior partial fixed dental prostheses: Five-year results. J Prosthet Dent 2012;108:214–222.

Ceramic Applications to Restore Implants

8

Joerg R. Strub, dmd, dr med dent, dr hc, phd Michael V. Swain, bsc, phd

Fixed implant reconstructions such as single implant-supported crowns and multiple-unit fixed dental prostheses (FDPs) are well documented in the literature and are fully accepted as a treatment option for the replacement of single or multiple missing teeth.1,2 The osseointegration of dental implants has been thoroughly investigated and found to be highly predictable.3,4 Implant-supported reconstructions exhibit excellent clinical survival rates. In a recent systematic review, implant-supported crowns and FDPs also had high survival rates. resembling those of tooth-borne reconstructions, amounting to 95% at 5 years.1 However, the clinical success of implant reconstructions depends not only on the survival rates but also on the extent of biologic and technical complications occurring during clinical function. To improve the clinical success, the materials and techniques for implant-supported reconstructions are constantly analyzed to determine which provide the most predictable outcomes.1,2 In addition, the ideal connection between the implant and the reconstruction is also frequently questioned.5,6 The purpose of this chapter is to address the current level of knowledge regarding available ceramic materials and their properties, clinical outcomes of various ceramic materials, and the ideal connection to restore implants. In addition, alternative materials and fabrication techniques are presented and future perspectives discussed.

Available Ceramic Materials A wide range of ceramic materials have been developed for clinical restorative purposes. These range from nearly 100% glass to dense, fine-grained, sintered ceramics. Traditionally, the former were the basis of the veneer and the latter the supporting coping material. However, the situation today has become more diverse as both high-strength zirconia and glassy materials are being used in situations where direct contact with the opposing dentition occurs. This section briefly outlines the various types of materials available with a specific focus on their microstructure and resultant properties.

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a

2 µm

b

2 µm

Fig 8-1  (a) Scanning electron microscopic (SEM) image of the fracture surface of a somewhat more opaque porcelain, showing the presence of different crystallographic inclusions. (b) SEM image of the fracture surface of a more translucent glassy porcelain, showing the presence of pores and the underlying structure of zirconia substrate.

Feldspathic porcelains have had a long association with dentistry.7 In fact, they were part of the initial development of crowns by Dubois de Chémant in 1789.8 These materials have traditionally been obtained from naturally occurring orthoclase feldspars and subjected to various melting, fritting, blending, and pigmenting processes. More recently, chemically derived sources of porcelains have become available; they offer a more consistent composition and are generally more translucent. The mechanical, thermal expansion, and optical properties of feldspathic porcelains may be adjusted by utilizing additives and adjusting their composition. All these factors are important because the porcelain must match the thermal expansion of the substrate to which it is bonded, hide the underlying color of the substructure, and match the shade of the adjacent dentition. Matching the color and form of the adjacent teeth has traditionally been achieved through the art and creativity of the dental technician. With the advent of computer-aided design/computer-assisted manufacture (CAD/CAM) milling systems, more complex, multilayer shaded blocks are available to achieve this outcome without the manual build-up process associated with conventional powder-based porcelains. Typical observations of the microstructure of dentin and incisal feldspathic porcelains are shown in Fig 8-1. The porcelain in Fig 8-1a shows the presence of numerous inclusions containing various additives, including tin oxide, titanium oxide, and zirconium oxide. These inclusions have higher refractive indexes than the feldspathic glasses, thereby scattering the light and imparting a degree of opacity to 150

the porcelain to hide the underlying substructure. In the incisal edge porcelain (Fig 8-1b), which has much higher translucency, there are limited inclusions that must closely match the optical properties of the natural teeth and impart a degree of fluorescence similar to that of the natural teeth. Porcelains are now available in a range of forms for prosthetic use, from those that involve traditional powder build-up and sintering to CAD/CAM milling and glazing to pressing. The mechanical properties of porcelains can be altered by the composition, the nature of the inclusions, the extent of air bubble entrapment, and the extent of residual stresses developed during cooling. This issue is taken up in more detail later in the chapter. Leucite (K2O∙Al2O3∙4SiO2) has long been used as an additive to porcelain. Its refractive index is very close to that of feldspathic porcelain, but it has a much higher coefficient of thermal expansion. It has traditionally been added to feldspathic porcelains to adjust their thermal expansion to match that of the underlying substructure. Porcelains used for bonding to noble metals require a relatively high percentage of leucite; however, it has been shown that the size of the leucite particles is critical for both the resultant strength and the opacity of the porcelain.9 Coarse leucite grains, with their much higher coefficient of expansion, undergo twinning and circumferential cracking on cooling. The presence of leucite within feldspathic porcelain was found to improve toughness but reduces strength.9 About 25 years ago, a pressable leucite-containing glass-ceramic with 20% to 25% leucite by volume was developed; the material showed greater strength and fracture toughness than feldspathic porcelains.9

Available Ceramic Materials

a

b

Fig 8-2  (a) SEM image of IPS e.max Press material (Ivoclar Vivadent). (b) SEM image of IPS e.max CAD material prior to ceramming. (c) SEM image of IPS e.max CAD material after ceramming. (Courtesy of Ivoclar Vivadent.)

c

Lithium disilicate–based glass-ceramics have been available for more than 40 years. However, they have been used in dentistry only for the past 20 years. These materials were developed by Hölland et al,10 and the associated processing technology enabled them to be pressed directly into the desired form in a manner similar to the lost wax casting process, resulting in a net shape-forming capability. These materials have very a high percentage (65% to 70%) of needle-shaped lithium disilicate crystals that impart substantial strength and toughness. A specific feature of the pressed version of this material was the alignment of the crystals as they flowed through the sprue and narrow regions for formation of the tooth restoration.9 The high strength and toughness along with associated difficulty in milling lithium disilicate–based materials have more recently led to the release of a precrystallized block that can be readily machined. After milling, the restoration is cerammed so that the lithium disilicate crystals are coarsened, resulting in a two- to threefold increase in strength and toughness.10 Typical examples of the microstructure of these materials are shown in Fig 8-2.

Aluminum oxide (Al2O3), or alumina, is a high-strength technical ceramic that has a wide range of applications, including hip prostheses. This material is typically a finegrained powder that is pressed and then sintered at 1,300°C to 1,500°C to achieve high density. It has high elastic modulus and hardness values and exhibits reasonably high strength and fracture toughness values. In addition, alumina has a slightly higher refractive index than porcelain. The strength of an alumina ceramic is dependent on the grain size and the density achieved after sintering. In the case of orthopedic hip prostheses, strengths of 500 to 600 MPa are typical with grain sizes between 2 and 4 μm. It is possible to achieve much finer-grained sintered alumina materials, and these have been developed for ceramic orthodontic brackets. These materials have a submicron grain size, increased translucency, and strengths up to 1 GPa. Despite the use of alumina for orthopedic prostheses, the adoption of alumina for dental restorative purposes has been hampered by the significant shrinkage of alumina

151

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Ceramic Applications to Restore Implants

a

b Fig 8-3  (a) SEM image of the typical coarse microstructure of MgO– partially stabilized zirconia (Mg-PSZ), showing minor presence of a monoclinic phase at the grain boundaries. (inset) There is a high density of ellipsoidal fine tetragonal precipitates of the highest strength or peaked age (PA) material within the coarse cubic matrix that are metastable and undergo tetragonal-monoclinic transformation on grinding or at a crack tip. A crack passing through this microstructure shows multiple branching as well as phase transformation and follows a highly tortuous path because of the fine tetragonal precipitates. (b) SEM image of Y-TZP. (c) SEM image of In-Ceram Alumina-Zirconia. (Courtesy of Dr Richard Hannink, Melbourne, Australia.)

c

during sintering and the difficulty of machining it. During the past decade, with the advent of scanning systems and electronic transfer of files, as well as the development of CAD/CAM, this limitation has been overcome, and blocks of partially sintered material can be machined oversized and then sintered to full density with resultant shrinkage in size. Prior to the availability of dense alumina structures and abutments for implants, porous alumina (as well as spinel [MgO∙Al2O3] and alumina-zirconia) materials were available; these were produced by slip casting or copy milling from presintered porous blocks that were subsequently infiltrated with a lanthanum-based glass. These innovative materials, developed by Sadoun,11 have been used to fabricate very successful all-ceramic restorations with clinical follow-up of almost 20 years.12 Zirconium oxide (ZrO2), or zirconia, is a material that has attracted considerable attention during the past 30 years. For the past decade or so, it has experienced increased utilization and there has been considerable debate over its role

152

in dentistry. In its pure state, this material is unsuitable for structural applications because it undergoes phase changes on heating and cooling. There are substantial changes in volume (3% to 4%) as the monoclinic phase undergoes a transformation to the tetragonal phase at 1,100°C. However, the addition of divalent and trivalent metallic ions such as magnesium (Mg), calcium (Ca), and yttrium (Y) stabilizes the high-temperature tetragonal or cubic phases. The most critical discovery leading to greater acceptance of this material was that, when the tetragonal phase is stabilized, it is possible to achieve a stress-induced transformation around the crack tip. This results in a highly significant increase in the fracture toughness and strength. This finding was reported by Garvie et al,13 and since then there has been considerable development of these phase-transforming materials.14 These materials are generally grouped into three principal classes: partially stabilized zirconia (PSZ), stabilized tetragonal zirconia polycrystals (TZP), and composite mate-

Available Ceramic Materials

a

b

c

d

e

f

g

h

i

j

k

l

Fig 8-4  (a to c) Pretreatment condition. The patient is missing several permanent teeth and thus retains some primary teeth. The canine, first and second premolars, and third molars are missing bilaterally in the maxilla. In the mandible, the first and second premolars and both third molars are missing bilaterally. (d) NobelReplace implants placed in the areas of the maxillary canine and first and second premolars bilaterally. (e) NobelProcera Zirconia modified abutments. (f ) NobelProcera Zirconia cemented crowns. (g) NobelReplace implants, placed in the areas of the first and second premolars bilaterally with NobelProcera Zirconia regular abutments. (h) Panoramic radiograph. (i) NobelProcera Zirconia cemented implant-supported crowns. (j to l) Restorations after 4 years.

rials such as alumina-zirconia. The PSZ materials generally have a relatively coarse cubic grain structure containing Mg or Ca ions with tetragonal precipitates (Fig 8-3a), while the TZP materials have a fine-grained, primarily tetragonalgrained structure containing yttrium ions (Fig 8-3b), and

the alumina-zirconia contains ceria-zirconia (white grains) and alumina porous network structure infiltrated with glass (Fig 8-3c). A clinical example of the use of zirconia-based cement-retained crowns to replace missing teeth is shown in Fig 8-4. 153

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Ceramic Applications to Restore Implants

Biologic and Mechanical Properties of Zirconia Ceramics Recent studies and critical reviews of plaque accumulation on zirconia ceramics have been presented by a number of authors. Hisbergues et al15 explored the biocompatibility of zirconia ceramics and stated that zirconia has the ability to reduce plaque accumulation. Degidi et al16 investigated soft tissue attachment to zirconia and titanium healing caps and observed that zirconia resulted in a reduced inflammatory response and less blood microvessel development than titanium; there were also implications that the amount of bacterial colonization was lower around zirconia than around titanium. In another study, Bianchi et al17 investigated a transmucosal zirconia collar and found it provided better tissue stabilization than titanium. Rimondini et al18 and Scarano et al19 investigated the in vivo responses of zirconia and titanium disks of the same surface roughness attached to the molar area of patients for 24 hours. They found that zirconia had fewer bacteria attached than titanium. More recently, Scotti et al20 evaluated the effect of glazing and polishing of zirconia on early dental plaque formation and found no differences in bacterial presence at polished and glazed surfaces. The flexural strength of ceramics and brittle materials is a property that is frequently discussed, but often the consequences of various typical clinical procedures on such values are not addressed. Most brittle materials, including glass, can be made to have extremely attractive properties provided that the presence of defects is minimized. However, the processes of fabrication, such as shaping, grinding, or grit blasting followed by acid etching, may result in strengths far different from those obtained with the pristine polished surfaces that are often the basis for the ranking of materials. A recent study by Scherrer et al21 investigated the strengths of a variety of zirconia (yttrium-stabilized tetragonal zirconia polycrystal [Y-TZP]) ceramics after sintering and found that both the strength and variation in strength, as addressed using the Weibull analysis, varied significantly. Weibull analysis has a very important function, because in principle it enables the effect of volume under test to be appreciated and explains why three-point bending results in higher strengths than four-point bending.22 In the case of Y-TZP, the presence of phase transformation toughening generated by grit blasting can result in a strength increase, provided that the compressive stresses generated by grit blasting are not released by heating the ceramic above 600°C. Such heating transforms the monoclinic phase back to the tetragonal variant. The latter transformation process generally results in reduced strength, because the defects 154

and flaws generated by the grit blasting are now bigger and more prevalent than for the as-sintered or polished surface. Also, the firing to porcelain-sintering temperatures does not heal cracks, although the presence of porcelain may result in glass diffusing into the flaws generated. An area of particular interest at present is the consequence of grinding and adjusting of the surface of Y-TZP on the resultant strength of the material. Recent studies by a number of authors have suggested that the extent of the tetragonal-monoclinic phase transformation is relatively limited. Instead, a significant rhombohedral phase forms, which has a much reduced volumetric dilation and is a phase that has not been observed in high-resolution transmission electron microscopy of Y-TZP ceramics.23 Fracture toughness (K1c) is another area about which there has been much discussion in the literature. It is possible to generate significantly different K1c values, depending on the technique used to measure it. For instance, the values generated by indentation tests are notoriously optimistic, especially at lower loads, because of the compressive transformation stresses generated in zirconia-containing ceramics by indentation. Other tests that necessitate notching of a specimen may also generate phase transformations around the notch tip that develop localized compressive stresses, resulting in very optimistic values for toughness being reported. The most realistic values of toughness are those generated using far more expensive stable crack extension sample geometries, such as the double cantilever beam and chevron notch tests. Typically, values obtained for the toughness of zirconia when these approaches are used are approximately 5 MPa · m1/2, but slow crack growth in moist conditions occurs at values well below this.24 The transformation toughening mechanism involves tensile stress–induced transformation around the tip of a loaded crack. In the case of zirconia, the volume dilation associated with the metastable tetragonal-monoclinic phase change results in a volume dilation of 3% to 4%. The extent of the toughness increase associated with this mechanism is dependent on both the volume fraction of the tetragonal phase that transforms as well as the size of the transformed zone. The size of the transformed zone is dependent on the critical stress to trigger the tetragonal-monoclinic transformation. This outcome results in a compromise between the strength and toughness of transformation-toughened ceramics. This is a very tough transformation-toughened ceramics “yield,” whereas high-strength ceramics are flaw-size dependent and have a relatively lower fracture toughness.25 Another feature of transformation-toughened ceramics that is often not appreciated is that their strength and toughness decline with increasing temperature because of the stability of the tetragonal phase at elevated temperatures.26 Low-temperature degradation (LTD) is a unique

Current Issues phenomenon that involves the slow transformation of the metastable tetragonal phase to monoclinic phase in the presence of moisture. This was appreciated more than 30 years ago.27 It is far more prevalent in yttria (Y-TZP) ceramics and follows a classic time-temperature-transformation response in that the process has an activation energy and the maximum rate takes place at about 200°C but still occurs at body temperatures. The process is also highly dependent on the microstructure and composition of the Y-TZP; lower yttria content, porosity, and larger grain size contribute to enhanced metastability. There is limited evidence currently available to suggest that the phenomenon of LTD has been instrumental in the failure of dental prosthodontic systems; however, a decade ago it caused a major catastrophe in the orthopedic community, when more than 500 hip prostheses failed in vivo from inattention to the microstructure and porosity that resulted from sintering and hot isostatic pressing.28 To limit the extent of LTD, manufacturers have used various additives such as alumina and other stabilizers such as ceria (CeTZP) and alloys.29

Current Issues Despite the fact that zirconia ceramics, and in particular Y-TZP compositions, have been available for more than 30 years, there are some perennial issues that are still important and other specific issues that have arisen with dental usage of these materials.

Aging of zirconia Ceramic materials, because of their highly inert nature, are generally considered very stable in moist environments. Some glasses, typically more alkaline porcelains, may exhibit minor solubility issues and release various ions. However, the metastability required so that transformation toughening can occur in zirconia ceramics results in aging or the LTD process described earlier. This feature was also belatedly appreciated by the orthopedic community with the result that polymeric acetabular cups had higher wear rates than alumina ceramics because of the enhanced roughness of Y-TZP that resulted from grain uplift and surface microcracking. The influence of the LTD process results in the presence of monoclinic nucleation at the surface, which gradually extends deeper into the Y-TZP ceramic. Initially there is an increase of strength of the ceramic as the volume dilation associated with this transformation induces compressive stresses at the surface. However, as the depth of the transformation increases, microcracking becomes more dominant and there is a decrease in strength

as well as a decrease in hardness and surface elastic modulus. These detrimental effects lead to enhanced wear of not only the ceramic but also the materials with which it is in contact. In the case of Y-TZP materials in the dental environment, a number of clinical and laboratory procedures may contribute to aging and initiation of the LTD process. For instance, it is common for materials to be steam cleaned prior to use, which would assist in the more rapid LTD nucleation of the transformation. Also, with the baking of porcelain on various frameworks, it is typical to grit blast, steam clean, and then place a wet porcelain layer on the surface; this layer is dried in an accelerated manner sitting beneath the open furnace prior to sintering. All of these processes will contribute to the presence of monoclinic zirconia on the surface of Y-TZP ceramics.30 Another factor that is anticipated to result in enhanced aging is the high-temperature sintering of Y-TZP materials that do not contain alumina. This procedure has become popular to enhance the translucency of Y-TZP ceramics and thereby to attain more esthetic outcomes.

Veneer failure During the past 5 years or more, researchers have reported clear evidence that veneer fracture occurs at a higher rate on zirconia cores than on most other materials.31,32 The extent of this failure has in some instances been as high as 50%. As a consequence, there has been intense research and speculation regarding the possible causes of this behavior. Some of the mechanisms suggested for such failure include core design, the presence of residual stresses associated with thermal expansion mismatch between the veneering material and the zirconia core, and cooling procedures that induce so-called tempering stresses following the sintering or final glaze firing cycle. Fischer et al33 investigated a range of porcelains used for veneering to zirconia and showed that there was a weak correlation between the shear bond strength and thermal expansion mismatch. Swain,34 on the other hand, suggested that the major contributor was the presence of tempering stresses associated with final cooling; he contended that the very low thermal conductivity of zirconia resulted in large temperature gradients between the surface and interior parts of the crown. This was particularly the case with thick sections of porcelain, such as at the cuspal regions of crowns, where chipping was most prevalent. Other researchers have argued that the cause of the problem is the limited support offered by the traditionally shaped core, which is typically 0.5 mm thick. They recommended use of an anatomical design framework with a much thinner layer of veneering porcelain.35,36 This ap155

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Ceramic Applications to Restore Implants

a

b

2 mm

Fig 8-5  SEM images of veneer failure after occlusal adjustment. (a) Occlusal view of veneer failure (circle). (b) Detail of failure (arrows). (Courtesy of Dr Irene Sailer, Geneva, Switzerland.)

proach certainly results in less extensive chipping, although the outcome of such crowns may be less esthetic. These issues and the advances possible with CAD/CAM systems have led to the development of newer systems in which the porcelain or glass-ceramic veneer is separately milled and bonded with either a low temperature–fusing glass or resin to the milled substrate. This approach allows lower sintering temperatures or even elimination of a final firing to generate the crown. Choi et al37 showed that substantial residual stresses were developed as a consequence of faster cooling, and these could be directly associated with the temperature difference between the surface and core of the porcelain during cooling through the glass transition temperature.

zirconia ceramic, a monoclinic phase will be observed on the treated surface. This monoclinic transformation will increase the strength of the restoration.40,41 However, if a crack initiates in that area, there will be no transformation toughening mechanism available anymore to oppose crack propagation, because the tetragonal phase has already transformed.42 Therefore, special attention has to be paid to the static and dynamic occlusion of zirconia-based, implant-supported restorations.43 Occlusal adjustment should be performed with fine-grain diamonds followed by a polishing sequence (Fig 8-5).

Handling

The majority of implant manufacturers offer zirconia abutments for esthetic implant-supported restorations because it was reported that metallic restorations can cause dis­ coloration of the mucosa.44 Several authors have shown that all-ceramic restorations on ceramic abutments provide a better color match to unrestored adjacent teeth than porcelain-fused-to-metal restorations.45–47 Zirconia abut­ ments are available in prefabricated or customizable forms

Clinicians and laboratory technicians should follow precise treatment steps when fabricating zirconia-based restorations.38 Zirconia as a framework material is highly susceptible to surface modifications resulting from improper laboratory and clinical handling.39 If any subtractive procedure (sandblasting or grinding) is performed after sintering of 156

Zirconia Abutments

Zirconia Abutments TABLE 8-1 In vitro fracture strengths of zirconia abutments Study

Implant system

Abutment

Restoration/tooth

Restorative material

Yildirim et al52

Brånemark standard

Wohlwend ZrO2

Zirconia

SC/maxillary incisor

Glass-ceramic

No

738

Butz et al53

Osseotite

ZiReal

Zirconia with titanium insert

SC/maxillary incisor

Nonprecious metal

TCML

NR

Att et al54

NobelReplace

Aesthetic Zirconia

Zirconia

SC/maxillary incisor

Procera Alumina

TCML

470

NobelReplace

Aesthetic Zirconia

Zirconia*

SC/maxillary incisor

Procera Zirconia

TCML

593

Gehrke et al

XiVE XiVE

Cercon Cercon

Zirconia Zirconia

Spherical caps/ maxillary incisor

TCML No

269 672

Canullo et al57

ProUnic

Custom-made zirconia

Zirconia

NR

NR

No

436

Att et al

55

56

Wiskott et al58

Fatigue loading

Mean fracture load (N)

Abutment material

Replace Select

Aesthetic Zirconia

Zirconia

NR

NR

TCML

59

Kerstein and Radke

Brånemark standard

Atlantis

Zirconia

NR

NR

740

830

60

Sundh and Sjögren

Straumann Straumann

Denzir M Denzir

Magnesia-zirconia Zirconia

Ceramic copy

Ceramic

No

430 470

Sailer et al61

Straumann standard Brånemark standard NobelReplace Straumann standard

CARES Procera Procera Zirabut

Zirconia Zirconia Zirconia Zirconia

SC/maxillary incisor

All-ceramic

No

378 416 490 246

Adatia et al62

Astra Tech

Y-TZP (reduction)

Zirconia

NR

NR

NR

429–547

Albrecht et al63

Straumann

Prototype

Zirconia

NR

NR

NR

705

Att et al64

NobelReplace

Aesthetic Zirconia (reduction)

Zirconia

SC/maxillary incisor

Nonprecious metal

TCML

451–491

Nothdurft et al65

OsseoSpeed

ZirDesign (reduction)

Zirconia

SC/maxillary incisor

Nonprecious metal

TCML

269–355

Koutayas et al

OsseoSpeed

ZirDesign (reduction)

Zirconia

SC/maxillary incisor

Glass-ceramic

No

294–384

66

56

SC, single crowns; TCML, thermocycling and mechanical loading; NR, not reported. Manufacturers: Brånemark standard, Nobel Biocare; ZrO2, Wohlwend Innovative; Osseotite, Biomet 3i; ZiReal, Biomet 3i; NobelReplace, Nobel Biocare; Aesthetic Zirconia abutments, Nobel Biocare; Procera, Nobel Biocare; XiVE, Dentsply; Cercon, Dentsply; ProUnic, Impladent; Custom-made zirconia abutments, Zirkonzahn; Replace Select, Nobel Biocare; Atlantis, Dentsply; Straumann implants, Straumann; Denzir, Decim; CARES, Straumann; Zirabut, Wohlwend Innovative; Astra Tech, Dentsply; OsseoSpeed, Dentsply; ZirDesign, Dentsply.

and can be prepared in the dental laboratory by hand or by CAD/CAM techniques. Zirconia abutments are successors to the densely sintered high-purity alumina abutments. Compared with the latter, zirconia abutments are more radiopaque and demonstrated significantly higher fracture resistance.48 It is well known that ceramics, including zirconia, are highly biocompatible and are less prone to plaque accumulation than metal substrates.16,18,19 It is commonly agreed that ceramic abutments should show proper resistance to masticatory forces generated during chewing or swallowing.49 Researchers have reported a mean loading force of 206 N and maximum biting forces of up to 290 N in the esthetic zone.50,51 Several laboratory studies have evaluated the fracture strength values of different zirconia abutments (Table 8-1). No implantsupported FDPs with zirconia abutments were tested; all

studies identified used implant-supported single crowns. The resistance-to-fracture values ranged from 246 to 737 N for specimens not subjected to fatigue loading and from 56 to 593 N for specimens subjected to fatigue loading.52–66 In four studies, the wall thicknesses of zirconia abutments were reduced by grinding and compared with unmodified abutments.62,64–66 In three of the four studies, there were no statistically significant differences in the fracture strength values between the test and control groups. Nevertheless, there is a need to explore the effect of grinding procedures on the resistance of zirconia abutments as well as to identify the minimal wall thickness that guarantees long-term stability. The lack of knowledge about the outcome of zirconia abutments in restorative systems other than single crowns, as well as the effect of the abutment’s design on its resistance, highlights the necessity for further evaluation of 157

8

Ceramic Applications to Restore Implants TABLE 8-2 Clinical outcome of zirconia abutments Survival rate Observa- AbutRestoration period ments tions

Study design/ restoration/ material

No. of restorations

Resin cement

Prospective/ SC/press ceramic

54 abutments/ NR

4y

100%

NR

NR

Zirconia abutments with titanium connection

Resin cement

Prospective/ SC/zirconia

30

3.3 y

100%

100%

NR

XiVE

Zirconia (Cercon)

Resin modified glass-ionomer cement

Prospective/ SC/ posterior Y-TZP

40

6 mo

100%

NR

Zembic et al69

Brånemark

Zirconia (Procera)

Resin cement

RCT/SC/ glass-ceramic, alumina, or zirconia

18

3y

100%

100%

NR

Ekfeldt et al70

Replace Select

Zirconia (Procera)

Different cements

Retrospective/SC/ alumina or zirconia

185

3–5 y

99%

100%

NR

Implant system

Abutment

Cement

Glauser et al67

Brånemark

Zirconia

Canullo68

TSA

Nothdurft and Pospiech43

Study

Complications

Chipping of veneering ceramic: 7.5%

SC, single crowns; NR, not reported; RCT, randomized controlled trial. Manufacturers: Brånemark, Nobel Biocare; Zirconia abutment, Nobel Biocare; TSA, Impladent; XiVE, Dentsply; Cercon, Dentsply; Procera, Nobel Biocare; Replace Select, Nobel Biocare.

TABLE 8-3 In vitro fracture strengths of implant-supported zirconia-based crowns and FDPs Restoration/ framework material

Veneering material

Fatigue loading

Mean fracture load (N)

VM9

TCML

593

Posterior 3-unit FDPs/ Creation Procera Zirconia Three different framework/bar designs

No fatigue

Initial crack/ final fracture load Straight bar: 644/1,292 Occlusal curve: 476/1,398 Gingival curve: 722/1,040

Posterior 3-unit FDPs/Cercon Base Three different framework/bar designs

Cercon Ceram S

No fatigue

Initial crack/ final fracture load Regular pontic: 682/916 Occlusal curve: 439/1,691 Gingival curve: 945/1,516

Resin cement

Posterior 3-unit FDPs/Lava

Lava Ceram

Accelerated step-stress fatigue

Prior to fatigue: 693 Load at which 63.2% would fail: 497

Glass-ionomer cement

Posterior 3-unit FDPs/Cercon Base

No veneer

No fatigue TCML

Standard abutment/ Individual abutment No fatigue: 473/424 TCML: 647/556

Study

Implant system Abutment

Cementation

Att et al55

Nobel Replace or Nobel Select

Y-TZP Abutments (Aesthetic Zirconia)

Resin cement

Anterior SC/Procera Zirconia

Kokubo et al71

NR, Straumann

NR

Resin cement

Tsumita et al72

Brånemark Mk III

Titanium (Procera)

NR

Bonfante et al73

NR, Nobel Biocare

Abutment Replicas (Replica Snappy Abutment)

Nothdurft et al74

XiVE S

Y-TZP (Cercon) Standard or individualized

SC, single crowns; NR, not reported; TCML, thermocycling and mechanical loading. Manufacturers: NobelReplace, Nobel Biocare; Nobel Select, Nobel Biocare; Aesthetic Zirconia Abutments, Nobel Biocare; Procera Zirconia, Nobel Biocare; VM9, Vident; Straumann implants, Straumann; Creation, Geller; Brånemark Mk III, Nobel Biocare; Cercon, Dentsply; Replica Snappy Abutment, Nobel Biocare; Lava, 3M ESPE; XiVE S, Dentsply.

158

Implant-Supported Zirconia-Based Fixed Restorations TABLE 8-4 Clinical performance of implant-supported zirconia-based crowns and FDPs Study

Implant system

Abutment

Cementation

Study design/ restoration/ material

No. of restorations

Observation period

Incidence of fracture (%) Abutment Framework Veneer fracture fracture fracture

Larsson et al75

Titanium (Astra Tech Standard)

Zinc phosphate Titanium cement Abutments (Astra Tech ST)

Prospective/posterior 2- to 5-unit FDPs/ Denzir with Esprident Triceram

13 FDPs

12 mo

0

0

53

Kohal et al76

Y-TZP one-piece

NA

Glass-ionomer cement

Prospective/SC and 3-unit FDPs/ Procera Zirconia

65 SC 27 FDPs

15.3 mo 13 mo

NA

0

18.5 41

Nothdurft and Pospiech43

Titanium (XiVE S)

Y-TZP Abutments (Cercon)

Resin-modified glass-ionomer cement

Prospective/posterior SC/Cercon

40 SC

6 mo

0

0

7.5

Larsson et al77

Titanium (Astra Tech)

Titanium Abutments (Astra Tech)

Zinc phosphate cement

Prospective/ 2- to 5-unit FDPs/ Denzir with Esprident Triceram or In-Ceram with Vitadur Alpha

25 FDPs

60 mo

0 0

0 0

69 17

Larsson et al78

Titanium (Astra Tech)

Titanium Abutments (Astra Tech)

Resin cement

Prospective/ 9- to 10-unit FDPs/ Cercon

10 FDPs

36 mo

0

0

34

Hosseini et al79

Titanium (Astra Tech Standard)

ZirDesign (Astra Tech)

Zinc phosphate cement or resin cement

RCT/posterior SC/ KaVo Zirconia or Procera Zirconia

38 SC

12 mo

0

0

0

Schwarz et al80

Titanium (Tissue Level, Bone Level, or NobelReplace)

NR

Different cements Retrospective/ SC/Cercon

53 SC

24 mo

NR

0

24.5

SC, single crowns; RCT, randomized controlled trial; NA, not applicable; NR, not reported. Manufacturers: Astra Tech, Dentsply; Denzir, Decim; Esprident Triceram, Dentaurum; Procera Zirconia, Nobel Biocare; XiVE S, Dentsply; Cercon, Dentsply; ZirDent, Zirkonzahn; In-Ceram, Vident; Vitadur Alpha, Vident; KaVo Zirconia, KaVo Dental; Tissue Level implants, Straumann; Bone Level implants, Straumann.

these parameters under laboratory conditions before these devices receive widespread clinical application. Less information is available on the clinical outcome of zirconia abutments (Table 8-2). Over observation periods between 6 months and 4 years, the survival rates of zirconia abutments were 100%.43,67–70 A systematic review estimated a 5-year survival rate of 99.1% for zirconia abutments, which was similar to that estimated for metal abutments (97.4%).49 Despite encouraging short-term data, there is a need for long-term data concerning the treatment outcomes of zirconia abutments.

Implant-Supported Zirconia-Based Fixed Restorations The fracture strength of implant-supported zirconia-based restorations has been evaluated in a small number of laboratory studies (Table 8-3). Only five investigations with vari-

ous testing protocols and study designs could be identified.55,71–73 For zirconia-based implant-supported single crowns, the resistance-to-fracture values amounted to 593 N. For zirconia-based FDPs, the resistance-to-fracture values ranged between 424 N and 1,691 N. Initial restoration failure was caused by failure of the veneering ceramic. Clinical data concerning treatment outcomes of zirconiabased implant-supported restorations are still scarce. Apart from case reports, only seven short-term clinical studies on zirconia-based implant-supported crowns and FDPs could be identified43,75–80 (Table 8-4). Fracture rates within the veneering ceramic of implant-supported zirconia-based single crowns ranged from 0% to 18.5% after 6 and 24 months. Even higher failure rates (17% to 69% after 12 to 60 months) were reported with implant-supported zirconia-based FDPs. None of the studies revealed fractures of zirconia frameworks or implant-supported single crowns or FDPs. In summary, implant-supported zirconia-based crowns and FDPs exhibited an unacceptable number of veneer chipping failures (Fig 8-6; see Table 8-4). Impaired proprio159

8

Ceramic Applications to Restore Implants

a

b

Fig 8-6  Chipping of veneering materials on zirconia frameworks. (a) NobelRondo crown. (b) Three-unit Procera Zirconia FDP.

ception and rigidity of osseointegrated implants, resulting in higher functional impact forces, might further exacerbate porcelain fractures.

Connection Between Implant and Reconstruction An implant-supported reconstruction can be screw retained on the implant or abutment or cemented on abutments. Initially, screw retention was used for multiple-unit completearch FDPs in edentulous patients.81 Single crowns were generally cemented on prefabricated abutments.82 Both types of reconstruction exhibited satisfactory clinical longterm outcomes.81,83 Both cementation and screw retention have their benefits and shortcomings in clinical application.5,6 Cemented implant reconstructions are easier to fabricate and manipulate in the patient’s mouth. One shortcoming of cementretained crowns and FDPs is the difficulty with the removal of the excess cement. In vitro investigations have shown that excess cement always remains at the tested specimens, irrespective of the submucosal position of the crown margin.84,85 Wilson86 showed that excess cement causes peri-implantitis. Another shortcoming of cement-retained reconstructions is that, if complications arise, they are very difficult or virtually impossible to remove without damage or destruction of the reconstruction. The major benefit of screw-retained reconstructions is their retrievability.87,88 Tech­nical complications such as loosening of retaining screws or fracture of the veneering ceramic have been reported.89 To date, very little scientific information is available com­ par­ing the advantages and disadvantages of cement re160

tention and screw retention.5,90 Sailer et al91 showed in a systematic review that neither of the fixation methods is clearly advantageous relative to the other. Cement-retained reconstructions exhibited more biologic complications (implant loss), while screw-retained reconstructions exhibited more technical problems. Because screw-retained reconstructions are retrievable, however, technical problems can be solved. These reconstructions therefore seem to be better from the biologic perspective. High costs of technical maintenance have not been taken into account, however.

Alternative Materials Aluminum ceramics The clinical data for implant-supported aluminum oxide ceramic restorations are very limited. Current evidence consists of reported 100% framework survival and 2% veneer fractures of Procera Alumina crowns after 3 years70 and 100% framework survival and 17% veneer fractures of InCeram Zirconia FDPs after 5 years.77

Monolithic CAD/CAM lithium disilicate (fatigue behavior) Monolithic CAD/CAM–fabricated, full anatomical lithium disilicate glass-ceramic crown restorations have recently been explored with promising results.92 The in vitro strength values of CAD/CAM lithium disilicate crowns on zirconia abutments were published by Albrecht et al.63 Short-term clinical experiences are promising (Figs 8-7 and 8-8).

Alternative Materials

a

b

d

c

e

Fig 8-7  (a to e) Straumann implant- and tooth-supported cemented CAD/CAM lithium disilicate crowns made using an intraoral digital scanner. They replace the mandibular left second premolar and first molar and restore the second molar.

161

8

Ceramic Applications to Restore Implants

a

b

c

d

e

f

Fig 8-8  (a to c) Extraction of the hopeless maxillary left central incisor. (d to f ) NobelActive implant-supported all-ceramic restoration.

162

Alternative Materials

CAD/CAM materials sintered to zirconia To overcome chipping fractures of veneered zirconia restorations, Beuer et al93 have suggested a novel approach. They asserted that sintering a CAD/CAM lithium disilicate veneer cap on the zirconia coping can significantly increase the mechanical strength of crowns and FDPs and represents a cost-effective way of fabricating all-ceramic restorations. To date, no published clinical studies have looked at this method, and further in vitro studies are needed before this type of restoration can be utilized clinically.

Zirconia cores and CAD/CAM veneers Studies have shown that CAD/CAM–guided application of lithium disilicate ceramic veneer on zirconia cores resulted in fatigue-resistant crowns (2,699 N), whereas pressed (1,507 N) and hand-layered veneer crowns (1,195 N) were prone to veneer chipping after exposure to the artificial environment.93,94 Single load to failure of pressed and layered veneer crowns resulted in veneer chipping, whereas CAD/CAM veneer crowns showed bulk fractures exposing the resin preparation.94 No clinical data have been published on the outcome of CAD/CAM application of lithium disilicate ceramic veneer on zirconia cores.

Composite resin abutments and bonded CAD/CAM veneers There are some concerns about the use of zirconia abutments, such as difficulty bonding to this substrate, the risk of propagating fractures while trimming prefabricated abutments, and the absence of shock absorption during occlusal loading. The aim of a study published by Magne et al95 was to assess in vitro the fatigue resistance and failure mode of type III veneers (porcelain versus composite resin) bonded to CAD/CAM composite resin implant abutments. The researchers were able to show that porcelain veneers bonded to custom composite resin implant abutments presented a higher survival rate than composite resin veneers. The survival probability of composite resin abutments did not differ from that of zirconia ones.

Monolithic zirconia restorations Complete anatomical zirconia restorations with subsequent surface characterization and glazing have been developed (Zirkonzahn).96 Monolithic zirconia was used to decrease the risk of chipping or fracture. Monolithic zirconia restorations are claimed to have the following advantages:

•  There is no issue with chipping. •  There is no need for veneer application. •  Thermal mismatch between core and veneer is avoided. •  Only color glazing is needed. •  There is no wear on zirconia. Monolithic zirconia also has several disadvantages: •  Esthetic characterization options are limited. •  Aging of zirconia is a possibility. •  In vivo data on outcomes are limited. Rosentritt et al97 compared the wear of dental porcelain and substructure oxide ceramics after exposure to enamel in a chewing simulator. They did not find any wear of the enamel with glass-infiltrated ceramics and zirconia in the as-polished condition. From the point of wear testing, zirconia may be used for the fabrication of FDPs without veneering; however, the influence of surface adjustment with burs, intraoral polishing, and food abrasion is unknown. Similarly, little experience has been reported for application of monolithic zircona in removable dentures. Bühler et al98 presented examples of the clinical and technical fabrication of zirconia bars on implants and of the corresponding zirconia complete denture. To date, no clinical data regarding the performance of monolithic zirconia restorations on implants are available. Monolithic zirconia restorations have several potential issues. They have esthetic limitations, because the very high refractive index and relatively high opacity of zirconia may cause issues when the clinician is attempting to match the new restoration to existing restorations or natural teeth. Moreover, aging of zirconia or LTD in the oral cavity, as mentioned earlier, may cause surface roughness, especially in some of the current monolithic materials, because they are being sintered at higher temperatures and without alumina additives. These two factors result in larger tetragonal grain sizes, which are more metastable and more likely to experience LTD at a faster rate. The result is a rougher occlusal surface that will cause greater wear of the opposing surface. Currently, data are limited because these materials are relatively new to the marketplace.

Polymethyl methacrylate–based CAD/CAM materials Table 8-5 gives an overview of the polymethyl methacrylate (PMMA) products that are available on the market. To date clinical information regarding the longevity and survival rates of these products is lacking. However, their reported 163

8

Ceramic Applications to Restore Implants TABLE 8-5 PMMA-based CAD/CAM materials PMMA material

CAD/CAM system

artBloc TEMP (Merz Dental)

Cerec (Sirona)

Biotec CP (Teamziereis)

Datron D5 (Datron) TZ450i (imes-icore) MDX40a (Roland)

CAD-TEMP (Vident)

Cerec (Sirona) Everest (KaVo)

CARA-PMMA Prov (Heraeus Kulzer)

CARA System (Heraeus Kulzer)

Ceramill TEMP units (Amann Girrbach)

Ceramill motion (Amann Girrbach)

Cercon Base PMMA (Dentsply)

Cercon brain expert (Dentsply)

BeCe Temp (Bego)

Bego CAD/CAM (Bego)

New Outline CAT (Anaxdent)

Organical (R+K) Zenotec (Wieland)

Polycon AE (Straumann)

CAD/CAM (Straumann)

Quattro Disc Eco PMMA (Goldquadrat)

Quattro Mill Comfort/Maxi5X/Easy (Goldquadrat)

SHERA eco-Disc PM (Shera Werkstoff Technologie)

SHERA-eco Mill 40/50/80 (Shera)

Telio CAD (Ivoclar Vivadent)

Procera (Nobel Biocare) Cerec (Sirona) Planmeca PlanScan (E4D Technologies) PlanMill (E4D Technologies)

TEMP Basic (Zirkonzahn)

5-Tec (Zirkonzahn)

Zenotec PMMA (Wieland)

All Zenotec (Wieland)

Tizian PMMA Blanks (Schütz)

Tizian CAD/CAM (Schütz)

flexural strength ranges from 50 to 130 MPa, and their modulus of elasticity ranges from 2,000 to 3,200 MPa.99,100 The advantages of PMMA materials are their high fracture resistance and low susceptibility to aging. They are available in six shades. These materials are indicated for use as provisional restorations for up to 6 months (Fig 8-9).

Composite resin–based CAD/CAM materials Composite resin dental restorations represent a unique class of biomaterials with severe limitations related to biocompatibility, curing behavior, esthetics, and ultimate mechanical properties. The use of these materials is limited by shrinkage and polymerization-induced shrinkage stress, limited toughness, the presence of unreacted monomer that remains following polymerization, and several other factors. In recent years, the performance of these restorations has been improved by changes in the initiation system, monomers, fillers, coupling agents, and polymerization strategies.101 Prefabricated blocks can be used, in combination with the CAD/CAM systems, for the fabrication of dental restorations. 164

Andriani et al102 tested the strength to failure and fracture mode of three indirect composite resin materials (Tescera, Bisco; Ceramage, Shofu; and Diamond crown, DRM) applied directly to titanium implant abutments and compared the data with those of cemented porcelain-fused-to-metal crowns. All crowns were loaded to failure by an indenter placed at one of the cusp tips. The single loads to failure recorded were between 1,155 and 1,081 N. The three indirect composite and porcelain-fused-to-metal systems fractured at higher loads than those typically associated with normal function. No significant differences in single-loadto-fracture resistance were found among composite resin systems and porcelain-fused-to-metal crowns. In an in vitro study, Santing et al103 analyzed the fracture strength and failure mode of maxillary implant-supported screw-retained composite resin single crowns on polyetheretherketone (PEEK) and titanium abutments. The researchers concluded that provisional crowns on PEEK abutments showed fracture strengths similar to those observed with titanium abutments, except for central incisors. Maxillary central incisor composite resin crowns on PEEK abutments fractured below the mean masticatory loading force.

Alternative Materials

a

b

Fig 8-9  (a to c) CAD/CAM implant-supported, threeunit PMMA provisional restoration replacing the mandibular left first, second, premolars, and the first molar.

c

Suzuki et al104 used two different composite resin materials (Ceramage and Diamond Crown) to restore molars on titanium abutments. The crowns were then loaded until failure occurred. The fracture strength values were high (1,099 and 1,155 N, respectively), but no significant differences could be observed between the two materials. Failure modes comprised composite veneer chipping.

Novel ceramic–composite resin materials Innovative ceramic–composite resin compound materials may combine the esthetic features of ceramic with the favorable load-bearing mechanical properties of the composite resin component, according to He and Swain.105 They investigated the properties of an interpenetrating network material in which a porous ceramic structure is infiltrated with composite resin rather than glass, as occurs in the InCeram system (Vident).

There have been other developments in this area, with the release of a higher-percentage ceramic-loaded composite resin: Paradigm MZ 100 Block (3M ESPE) and Enamic (Vita) for Cerec (Sirona). This is available as a CAD/CAM composite for inlay, onlay, veneer, and crown applications. Early clinical results are promising (Fig 8-10). Advantages associated with these materials over allceramic systems include more toothlike properties: The compound materials are resilient and shock absorbent, not brittle, and have a dentinlike elastic modulus of 12 to 20 GPa. They offer good esthetics with 12 available shades and a toothlike fluorescence. Studies of mechanical properties105,106 have shown that the properties of interpenetrating network composite materials are superior to those of composite resin materials; the former have higher strength and toughness values than porcelain materials, an elastic modulus that is midway between that of enamel and dentin (25 to 30 GPa), and hardness that is less than that of enamel. Because of their lower

165

8

Ceramic Applications to Restore Implants

a

b

c Fig 8-10  (a to e) Permanent hybrid ceramic implant-supported Enamic crowns replacing the mandibular left first and second premolars.

d

e

hardness and higher toughness values, these materials can be more rapidly machined with sharper margins. As yet, no reports of clinical trials have been presented. An in vitro study by Guess et al94 showed that although the strength of the interpenetrating structure was comparable to that of existing glass-ceramic materials, the interpenetrating structure exhibited no cracking during chewing simulator

166

studies, whereas the glass-ceramics exhibited more than 40% incidence of cracking. Preliminary interpretation of these results suggests that the lower stiffness contributed to the better outcome of the interpenetrating ceramic– composite resin material. Figure 8-11 summarizes the clinical data that are available for different materials used to restore dental implants.

Conclusion

Ceramic implants and abutments

Zirconia bilayer

Zirconia core and CAD/CAM veneer

Monolithic lithium disilicate and zirconia

Permanent: resin nanoceramic/hybrid

✓ Reliable and esthetic Mid-term data

✓ Reliable core High veneer failure

✓ CAD/CAM veneer No clinical data

✓ Reliable construction No clinical data

✓ Favorable properties Short-term data

Fig 8-11  Summary of the current status of ceramic applications for dental implants.

Future Perspectives

Conclusion

Inkjet printing of zirconia prostheses

Implant-supported crowns and multiple-unit FDPs have proven to be a reliable treatment option for the replacement of missing teeth. Over the years, a variety of ceramic materials have been used with great success to fabricate esthetic and functional implant-supported restorations. Nevertheless, the profession is always striving to improve outcomes through the development of new materials and techniques. In recent years, the potential of zirconia ceramics for use as implants, abutments, and restorative materials has been investigated. Fracture of veneering ceramics and the susceptibility of zirconia to aging are major concerns for the clinical long-term success of zirconia in fixed implant prosthodontics. Presently, there are very limited clinical data evaluating the performance of zirconia abutments and implant-supported fixed restorations. Therefore, like other novel materials and devices, zirconia implants can only be recommended for use in daily private practice with caution.

CAD/CAM milling systems provide a rapid and individual method for the manufacture of zirconia dental restorations. However, the disadvantages of these systems include limited accuracy, possible introduction of microscopic cracks, and a waste of material because of the subtractive process. Ebert et al107 and van Noort108 showed that direct inkjet printing of zirconia prostheses has the potential to produce cost-effective, all-ceramic dental restorations with high accuracy, good physical properties, and a minimum of wasted material during manufacturing. These zirconia-based materials have the following characteristics: novel generative manufacturing procedures, the ability to tailor properties and shades, a characteristic strength of 763 MPa, and a fracture toughness of 6.7 MPa ∙ m1/2.

Solid freeform fabrication of threedimensional structures In the future, robocasting technology that generates threedimensional, custom-made layered structures may be a promising fabrication method for zirconia in dental applications. A variety of structures with changing or graded configurations can be produced by using colloidal pastes, slurries, or inks with different compositions of alumina and zirconia to control the specific mechanical and esthetic characteristics of the final product.109

Acknowledgments The authors acknowledge the support of Dr Petra Guess, Mrs Ulrike Soldat, and Mr Cumhur Yörük, Freiburg, Germany.

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Cementation Procedures for Ceramics

9

Matthias Kern, dmd, phd

Restorations made from weaker silica-based ceramics need to be adhesively luted to obtain sufficient strength.1 In contrast, restorations made from reinforced silicate ceramics such as lithium disilicate or those made from oxide ceramics can be cemented either with adhesive luting resins2,3 or with conventional cements.4,5 There is no evidence to date that adhesive luting of crowns made from reinforced silicate ceramics or high-strength oxide ceramics will improve their clinical outcome.5–7 However, in cases presenting limited abutment retention, adhesive cementation can be assumed to be advantageous. In addition, adhesive luting techniques are required for restoration types that present limited or no mechanical retention, such as labial or occlusal veneers (“table tops”),8 partial-coverage restorations, or Maryland-type fixed dental prostheses (FDPs).9,10 Moreover, luting resins present good to excellent translucency, not leaving the opaque cementation lines that are exhibited when conventional cements (eg, zinc phosphate and glass-ionomer cements) are used. These resins also minimize microleakage when the correct dentin adhesives are used.11 The purpose of this chapter is to describe conventional and adhesive cementation techniques for high-strength ceramics, elucidating their specific requirements as well as their possible sources of error.

Conventional Cementation Due to their strength, complete-coverage restorations made from reinforced silicate ceramics or high-strength oxide ceramics can be cemented with conventional cements such as zinc phosphate cement or glass-ionomer cement.5,12,13

Abutment preparation Conventional cementation techniques are less time-consuming and less technique sensitive under clinical conditions than most of the adhesive

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Cementation Procedures for Ceramics luting techniques. However, conventional cementation requires complete-coverage restorations and adequate retentive preparation of the abutment teeth. Loss of retention is one of the most frequent causes for clinical failure of posterior metal-based FDPs.14–17 The dislodgment of cast posterior restorations was strongly associated with a lack of resistance form in the preparations; ie, dislodged restorations came mostly from preparations with tapers that did not provide resistance form.18 Provided that an adequate finishing line such as a shoulder or chamfer is used for all-ceramic restorations,19 it has been recommended that the abutment tooth preparation for complete-coverage all-ceramic restorations made from lithium disilicate or high-strength oxide ceramics follow the already established preparation guidelines for metal-based crowns20,21 in order to achieve an adequate retention and resistance form that will resist the dislodgment of the restorations from their abutments. In addition, abutment surface roughness is needed to retain conventional cements. Therefore, abutment preparation should be finished with the use of fine-grain diamond instruments (International Organization for Standardization standard 504), which produce a surface roughness of about 15 µm. Use of finer diamond instruments might reduce cement retention.22 To succeed in conventional cementation, the clinician should protect the dental pulp. To protect the pulp of vital abutment teeth, a freshly mixed suspension of calcium hydroxide can be applied with a rotating rubber cup to block the dentinal tubules.23–25 Alternatively, resin-based sealing systems can be used.26 However, their effect on crown retention is not fully understood.27,28 In general, unfilled or low-filled sealers seem to be more suitable than highly filled sealers, because they create a thicker sealing layer and will smooth the retentive surface. Because long-term clinical results regarding the combination of resin-based sealers and conventional cements are unknown, the routine application of calcium hydroxide prior to conventional cementation is still most often recommended.21,25

Restoration surface preparation Like the abutments, the inner surfaces of completecoverage restorations made from lithium disilicate and high-strength oxide ceramics should be roughened prior to conventional cementation to obtain optimal cement retention. Lithium disilicate ceramic restorations should be etched with 5% hydrofluoric acid for 20 seconds5 (Fig 9-1a), while high-strength oxide ceramics should be air-abraded with 50-µm alumina particles used at 2.5-bar pressure or less.4 However, whether airborne particle abrasion alters the strength of high-strength ceramic restorations on a 174

clinically significant level is still controversial. While some studies have shown a strengthening effect of airborne particle abrasion on oxide ceramics,29–32 others have reported a strength-reducing effect.33–35 The pressure used for air abrasion might be clinically significant. While surface roughness of zirconia ceramic increases with increasing air pressure, the flexural strength decreases with increasing pressure, which has been tested at pressures of 2, 4, and 6 bars.36 Therefore, use of 50-µm alumina particles with a pressure of 2.5 bars or less is recommended to minimize subsurface damage of the ceramic but still provide adequate retention for conventional cements.37 To date there are no controlled clinical studies showing whether airborne particle abrasion influences the clinical outcome of high-strength oxide ceramic restorations positively or negatively. This topic is discussed separately, later in the chapter, because of its controversy and relevance.

Cementation procedures For patients with vital abutment teeth with increased sensitivity, administration of local anesthesia prior to the cementation procedure is sometimes recommended. The area of cementation is isolated with cotton rolls, a saliva evacuator, and, when indicated, retraction cords if the preparation margin is subgingival. The abutments are cleaned thoroughly using a rotating rubber cup with a slurry of fine pumice followed by water spray (Fig 9-1b). The teeth should be dried gently with cotton rolls or special absorbent strips. The teeth should not be overdried, because this may lead to postoperative sensitivity. The cement for any conventional cementation is mixed in accordance with the manufacturer’s instructions. The author prefers glass-ionomer cement in capsules for trituration mixing in a high-speed mixing machine. The capsules can be stored in a refrigerator and taken out directly prior to mixing, which prolongs the clinical working time considerably. A disposable brush is used to coat the clean but conditioned inner surfaces of the all-ceramic restorations with a thin layer of the freshly mixed cement. The complete internal surface and margins of the restoration should be coated. The restoration is seated with firm, gradually increasing pressure until complete seating is achieved. Correct seating of the restoration is checked with an explorer on several easily accessible marginal locations. When posterior restorations are cemented, the patient can be asked to bite on cotton rolls to keep the restoration in the correct position. In the anterior region, the operator should retain the restoration in its correct position using firm finger pressure until the cement has fully set.

Adhesive Cementation

a

b

c

d

Fig 9-1  (a) Etching of a lithium disilicate crown with 5% hydrofluoric acid for 20 seconds (for etching pattern, see Fig 9-2a). (b) Cleaned abutment tooth prior to conventional cementation. (c) Complete hardening of conventional glass-ionomer cement prior to removal of excess cement. (d) Conventionally cemented lithium disilicate crown.

During setting, the cement should be protected from drying out and from excessive moisture (Fig 9-1c). When it has fully set, the excess cement at the restoration margins is removed with an explorer and dental floss with a small knot. Optional retraction cords are removed, and the occlusion is checked in both centric occlusion and excursive movements and corrected when necessary (Fig 9-1d). After any occlusal adjustment, the ceramic surface must be polished meticulously following the recommendations of the manufacturer.19

Adhesive Cementation Suggested bonding methods When adhesive cementation is used, the created bonding interfaces should not only provide mechanical stability but also prevent microleakage, which is achieved through

physicochemical bonding systems. As a first step, the ceramic bonding surface is chemically or micromechanically roughened and therefore enlarged. Simultaneously, the surface is cleaned thoroughly and chemically activated. In a second step, the activated surface is conditioned with a primer (adhesive monomer), which promotes chemical bonding between the surface oxides and the double bonds of the luting resin. Some resins already contain adhesive monomers effective on oxide ceramics, so that no additional primer is needed; the Panavia product group (Kuraray), for example, contains 10-methacryloyloxydecyl dihydrogen phosphate (MDP). Without adequate ceramic surface roughening and activation, bonded specimens usually debonded spontaneously during artificial aging.38–43 For predictable and durable resin bonding, it is essential to condition the ceramic surface restoration only after all try-in procedures with the definitive restoration have been completed, because typical dental cleaning methods are 175

9

Cementation Procedures for Ceramics TABLE 9-1 Typical composition of popular high-strength ceramics (wt%)* Product Oxide

Lithium disilicate ceramics e.max Press

e.max CAD

Glass-infiltrated oxide ceramics In-Ceram Alumina Classic

In-Ceram Zirconia Classic

Glass-free oxide ceramics In-Ceram 2000 AL Procera Alumina

Cercon IPS e.max ZirCAD In-Ceram 2000 YZ Lava Procera Zirconia Zerion

Silicon dioxide (SiO2)

57–80

57–80

4–5

3–4

—

—

Lithium oxide (Li2O)

11–19

11–19

—

—

—

—

Potassium oxide (K2O)

0–13

0–13

—

—

—

—

Phosphorus pentoxide (P2O5)

0–11

0–11

—

—

—

—

Aluminum oxide (Al2O3)

—

0–5

82

57

> 99.9

—

Zirconium dioxide (ZrO2)

0–8

0–8

—

26

—

95–97

Yttrium oxide (Y2O3)

—

—

—