Hip Replacement : Approaches, Complications and Effectiveness [1 ed.] 9781608767458, 9781606923269

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Copyright © 2009. Nova Science Publishers, Incorporated. All rights reserved. Aoi, T., and A. Toshida. Hip Replacement : Approaches, Complications and Effectiveness, Nova Science Publishers, Incorporated, 2009. ProQuest Ebook Central,

Copyright © 2009. Nova Science Publishers, Incorporated. All rights reserved. Aoi, T., and A. Toshida. Hip Replacement : Approaches, Complications and Effectiveness, Nova Science Publishers, Incorporated, 2009. ProQuest Ebook Central,

HIP REPLACEMENT: APPROACHES, COMPLICATIONS AND EFFECTIVENESS

Copyright © 2009. Nova Science Publishers, Incorporated. All rights reserved.

No part of this digital document may be reproduced, stored in a retrieval system or transmitted in any form or by any means. The publisher has taken reasonable care in the preparation of this digital document, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained herein. This digital document is sold with the clear understanding that the publisher is not engaged in rendering legal, medical or any other professional services.

Aoi, T., and A. Toshida. Hip Replacement : Approaches, Complications and Effectiveness, Nova Science Publishers, Incorporated, 2009. ProQuest Ebook Central,

Copyright © 2009. Nova Science Publishers, Incorporated. All rights reserved. Aoi, T., and A. Toshida. Hip Replacement : Approaches, Complications and Effectiveness, Nova Science Publishers, Incorporated, 2009. ProQuest Ebook Central,

HIP REPLACEMENT: APPROACHES, COMPLICATIONS AND EFFECTIVENESS

T. AOI AND

A. TOSHIDA

Copyright © 2009. Nova Science Publishers, Incorporated. All rights reserved.

EDITORS

Nova Biomedical Books New York

Aoi, T., and A. Toshida. Hip Replacement : Approaches, Complications and Effectiveness, Nova Science Publishers, Incorporated, 2009. ProQuest Ebook Central,

Copyright © 2009 by Nova Science Publishers, Inc. All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic, tape, mechanical photocopying, recording or otherwise without the written permission of the Publisher. For permission to use material from this book please contact us: Telephone 631-231-7269; Fax 631-231-8175 Web Site: http://www.novapublishers.com NOTICE TO THE READER The Publisher has taken reasonable care in the preparation of this book, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained in this book. The Publisher shall not be liable for any special, consequential, or exemplary damages resulting, in whole or in part, from the readers’ use of, or reliance upon, this material. Independent verification should be sought for any data, advice or recommendations contained in this book. In addition, no responsibility is assumed by the publisher for any injury and/or damage to persons or property arising from any methods, products, instructions, ideas or otherwise contained in this publication. This publication is designed to provide accurate and authoritative information with regard to the subject matter covered herein. It is sold with the clear understanding that the Publisher is not engaged in rendering legal or any other professional services. If legal or any other expert assistance is required, the services of a competent person should be sought. FROM A DECLARATION OF PARTICIPANTS JOINTLY ADOPTED BY A COMMITTEE OF THE AMERICAN BAR ASSOCIATION AND A COMMITTEE OF PUBLISHERS.

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Library of Congress Cataloging-in-Publication Data Hip replacement : approaches, complications, and effectiveness / editors, T. Aoi and A. Toshida. p. ; cm. Includes bibliographical references and index. ISBN 978-1-60876-745-8 (E-Book) 1. Total hip replacement. I. Aoi, T. II. Toshida, A. [DNLM: 1. Arthroplasty, Replacement, Hip. WE 860 H66781 2009] RD549.H535 2009 617.5'810592--dc22 2009003391

Published by Nova Science Publishers, Inc.  New York

Aoi, T., and A. Toshida. Hip Replacement : Approaches, Complications and Effectiveness, Nova Science Publishers, Incorporated, 2009. ProQuest Ebook Central,

Contents Preface Chapter 1

Chapter 2

Chapter 3

Chapter 4

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Chapter 5

vii Counteracting Reliability Problems in Advanced Hip Prostheses: Lessons from the Past and New Technologies for the Future Giuseppe Pezzotti

1

Ultra Smooth Nanostructured Diamond Coatings for Biomedical Implants in Total Hip Replacement Applications S. Chowdhury, Yogesh K. Vohra and William. C. Clem

43

Uncemented Total Hip Prosthesis with Hydroxyapatite Ceramic (HAC) Coating A. A. Shetty, A. J. Tindall, K. D. James and G. Bhatnagar

105

Materials Development and Latest Results of Various Bearings for Total Hip Arthroplasty Ingrid Miloševi, Rihard Trebše and Simon Kovač

159

New Insights into Aseptic Loosening of the Total Hip Arthroplasty: An Emerging Role of Genetics and Proteomics Robert Kolundžić, Vladimir Trkulja and Dubravko Orlić

233

Chapter 6

Complications in Total Hip Replacement: Dislocation—Biomechanical and Clinical Aspects 249 Martin Ellenrieder, Carmen Zietz, Daniel Kluess, Wolfram Mittelmeier and Rainer Bader

Chapter 7

Direct Lateral Approach for a Primary THR V. S. Pai

Index

Aoi, T., and A. Toshida. Hip Replacement : Approaches, Complications and Effectiveness, Nova Science Publishers, Incorporated, 2009. ProQuest Ebook Central,

267 283

Copyright © 2009. Nova Science Publishers, Incorporated. All rights reserved. Aoi, T., and A. Toshida. Hip Replacement : Approaches, Complications and Effectiveness, Nova Science Publishers, Incorporated, 2009. ProQuest Ebook Central,

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Preface Hip replacement as well as hip arthroplasty, is a surgical procedure in which the hip joint is replaced by a prosthetic implant. Such joint replacement orthopedic surgery generally is conducted to relieve arthritis pain or fix severe physical joint damage as part of the hip fracture treatment. This book provides an overview of different types of hip replacements and success and complications that may occur. The growing number of hip replacement surgeries demonstrates the scientific advancement and success in this field. Chapter 1 - Looking back over 100 years span of history in hip arthroplasty, one can realize how the hip replacement surgery represents from both a scientific and an economical aspect a continuously growing field and a quite successful practice. However, during its historical evolution, hip arthroplasty have encountered and still occasionally encounters enormous obstacles, which mainly deal with the quality of the used prosthetic materials. Since the first attempts using prostheses made of primitive materials at the end of the nineteenth century, many different materials have been experimented and improved surgical approaches undertaken. Hip prostheses have been conceived according to different design criteria, but they all have been following the wide underlying principle of low-friction arthroplasty. Such a basic concept yet represents the standard in total hip arthroplasty. In this chapter, an analytical overview is given, mainly from a materials science perspective, of the major problems encountered in the recent history of hip replacement. An outline is also given of emerging technologies aimed at the improvement of the structural and environmental reliability of advanced biomaterials. Chapter 2 - Total hip replacements represent a majority of joint replacement procedures; many of these are revision surgeries to replace damaged or loosened implant components as a result of excessive wear. The number of total hip and knee replacements performed each year in the U.S. is 198,000 and 245,000, respectively. Revision surgeries account for 17 percent of all hip replacement and eight percent of all knee replacement surgeries, for a combined total of nearly 54,000 revision surgeries each year. A primary problem with current designs is the generation of wear debris particles at the articulating surface that causes local pain and inflammation. Large debris are normally sequestered by fibrous tissue, while small debris is taken up by macrophages and multinucleated giant cells which may release cytokines that result in inflammation. This inflammation cascade damages surrounding bone, ultimately resulting in osteolysis, loosening, and implant failure. The proposed solution for the problem

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T. Aoi and A. Toshida

of osteolysis caused by wear debris is to develop ultra hard materials for the articulating surfaces that are more wear resistant, which would reduce the number of debris particles generated. In our on going research we have developed ultra smooth nanostructured diamond (USND) coatings that will have optimal proprieties and can survive in the harsh environment in the body and will last longer as wear resistant biomaterials. USND coatings were deposited by microwave plasma chemical vapor deposition (MPCVD) technique using He/H2/CH4/N2 gas mixture on Ti–6Al–4V medical grade substrates. We were able to deposit diamond coatings as smooth as 6 nm (root-mean-square), as measured by an atomic force microscopy (AFM) scan area of 2 x 2 micron. The characterization of the coatings was performed with AFM, scanning electron microscopy (SEM), x-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), tribometer and nanoindentation techniques. XRD and Raman results showed the nanocrystalline nature of the diamond coatings. The surface morphology imaged by nano SEM at 300,000× also confirmed the nanocrystalinity of the diamond coatings. Nanoindentation demonstrated that the hardness and Young’s modulus of the coatings are around 60 GPa and 380 GPa, respectively. The plasma species during deposition were monitored by optical emission spectroscopy. All of the diamond (USND) coated Ti-6Al-4V disks had better wear performance against polyethylene compare to CoCrMo and polyethylene wear couple. In vitro biocompatibility tests on diamond coatings showing no sign of toxicity. Adhesion and spreading of human mesenchymal stem cells (MSCs) were found on the deposited coatings after culturing up to 2 weeks. Preliminary results confirm that these diamond coated implants are as good as standard Ti-6Al-4V and CoCrMo as orthopedic implant applications. When coupled with wear studies, the in vitro cell study and in vivo animal study results suggest that USND has the potential to reduce debris particle release of biomedical implants without compromising osseointegration, thus minimizing the possibility of implant loosening over time. Chapter 3 - Total Hip arthroplasty (THR) is the most commonly performed adult reconstructive hip procedure. Over 40,000 THR are performed in England every year in England and Wales. Cemented THR, has been, and still is the traditional method used since its evolution (Swiss Arthoplasty /Norwegian Arthroplasty Register). The procedure fulfils its primary purpose i.e. relieving incapacitating pain and improving mobility. Infection was the main concern in early years of joint replacement however; today bone loss without infection, leading to implant loosening and loss of bone stock is our greatest challenge. It is not a new problem, as experienced by Sir Charnley with his disastrous experience with Teflon Arthroplasty in 1960. Since 1960 numerous efforts have been made to improve the results of Total Hip Arthroplasty, not only in its technique and materials, but also reducing its complications such as aseptic loosening of its components. Work by the late Sir Charnley has greatly improved our understanding of all aspects of total hip replacement including the concept of low frictional torque arthroplasty, surgical alteration of hip biomechanics, lubrication, design and operating theatre enviroment. A major advancement was the use of cold curing-cement (polymethylmethacrylate) for fixation of components.

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Preface

ix

Long-term follow-up studies show the primary problem is aseptic loosening of the cemented prostheses characterized by fragmented cement and bone resorption. Investigation has proceeded along two major paths, one to eliminate the use of cement and the other to include the cemented hip. Cementless hip rely on implant design for primary fixation, and osseous integration for secondary fixation. Ever since the introduction of the cementless THR systems, there has been a healthy debate on the relative merits of cemented versus uncemented prostheses. Hydroxyapatite Ceramic-coated Total Hip Replacement (HAC) brings a new dimension to this debate. HAC-Coated THR has been developed during the last two decades and experimental studies have established its basic efficacy. Early results demonstrate that in every respect HAC-Coated THR prosthesis matches up to the “Gold Standard” of the long established Cemented Charnley Hip Prostheses. Chapter 4 - Bearing surface represents a critical point of the hip prosthesis. The development of metallic, polymer and ceramic materials used in the total hip arthroplasty is reviewed. Materials which are in use today are described in more detail. Furthermore, the history of different bearing combinations is described – metal-on-polyethylene (MOP), ceramic-on-polyethylene (COP) metal-on-metal (MOM) and ceramic-on-ceramic (COC). Several issues are described in detail for each bearing: wear, wear debris particles, tissue response to wear debris particles and current long-term results. Today surgeons are confronted with a variety of bearings options for hip arthroplasty. Currect clinical outcomes of these bearings are reviewed and patient selection is discussed as well. Finally, some new trends in hip arthroplasty are presented including resurfasing arthroplasty and ceramic-onmetal bearings. Chapter 5 - Introduction of the total hip arthroplasty (THA) techniques has revolutionized the treatment of a number of states resulting in hip dysfunction by conveying considerable improvements in patients’ ability and quality of life. As in the case of any medical procedure, patients undergoing THA may experience complications - early (e.g., infections, dislocations) or late. Aseptic loosening of the stem or the acetabular cup, or both, is the major late complication of THA. It is a consequence of the so-called particle disease: implant wear debris accumulating at the prosthesis interface due to friction activates phagocytes and induces an aseptic inflammation mediated by a number of cellular and humoral factors that eventually results in bone demineralization, prosthesis instability, dysfunction and pain. Whilst the early complications of THA are largely preventable by adherence to the principles of good clinical practice, attempts to prevent aseptic instability by suppressing inflammation and/or by preventing/improving bone loss have not been successful. The actual incidence of aseptic instability and “survival” of THA implants has not been uniformly estimated. It is more likely to occur earlier after arthroplasty with certain types of prosthetic devices/materials (e.g., uncoated cups or stems), with less experienced surgeons, in patients suffering from developmental hip dysplasia or complications of the femoral neck facture (vs. primary osteoarthritis), in obese patients, in men vs. women, in younger patients, in cases with more pronounced debris accumulation. However, all these factors cumulatively explain only a minor portion of variability of occurrence (and timing) of aseptic instability after THA – it may occur with any type of prosthetic material, in any age group, irrespective of surgeon's skill or underlying disease and with minimum accumulated

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debris (whereas THA may be perfectly stable despite a considerable debris accumulation). Consequently, “individual susceptibility” to aseptic loosening determined by factors other than demographic or morbidity characteristics has been well recognized. During the past five years several smaller studies appeared that tried to define the “genetic background” of “individual susceptibility” to aseptic loosening. They all followed the same logic – genes encoding for mediators of inflammation and/or bone remodeling, and particularly those with known polymorphisms affecting expression/activity, appear to be good candidates in this respect. So far, several significant associations have been found and the approach apparently has a great potential – at least theoretically, it might result in ability of reliable individual risk predictions that might influence the choice of prosthetic materials, post-surgical recovery programs or pharmacological treatments. However, the work is at very early stages and future efforts should: a) define the best genetic predictors (strength of association); b) provide evidence of consistency (different populations, different combinations of genetic, morbidity, demographic characteristics and prosthetic materials); c) define functional links between genetic markers and aseptic instability (biological plausibility). The rapidly developing techniques of proteomic analysis are likely to be useful in this respect – they may help to identify the best candidate genes and to recognize functional relationships. Chapter 6 - Dislocation of the artificial joint is a serious complication of total hip replacement (THR). The probability of dislocation ranges from 2% to 5% in primary THR, and up to 16% to 20% in revision and tumor surgery. Dislocation ranks second as a cause for revision of THR, following aseptic loosening. The most common risk factors include insufficient pseudo-capsular tissue, muscle weakness and malpositioning of the implant components. Approximately one third of these patients suffer recurrent dislocations and consequently require revision surgery. An insufficient range of motion can lead to impingement of the prosthetic neck on the acetabular cup. If impingement occurs, the centre of rotation moves from the head centre to the rim of the cup. Further motion leads to subluxation of the femoral head and lever out In addition, recurrent impingement can cause material failure of implant components, such as excessive wear of polyethylene liners as well as brittle fracture of ceramic components. Furthermore, the clinical problem of THR instability includes further patient-related factors. Advanced age, female gender, post-traumatic arthrosis, high-grade dysplasia, posterior surgical approach and previous hip operations are known to enhance the risk of dislocation. To evaluate the dislocation stability of different implant designs, multiple studies have been conducted. In our research group we performed computer-based range of motion studies and we established an experimental testing device to analyse the range of motion of THR, the resisting moments during subluxation and the stability against dislocation. Furthermore, we developed a three-dimensional finite element (FE) model capable of predicting dislocation stability of various designs of the prosthetic head, neck and liner. Thereby, the effects of different implant positions on the stress distribution during impingement and subluxation were analysed. In preventing dislocation of THR, correct positioning and adequate design of the components are required. Moreover, intraoperative monitoring of the mobility and the stability of the artificial hip joint as well as sufficient postoperative care can help to reduce

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Preface

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the risk of dislocation. However, the treatment of recurrent dislocation or instability is a demanding procedure, in particular after revision arthroplasty when the bone stock is damaged and enhancement of the soft-tissue tension is limited. Chapter 7 - Multiple approaches to total hip arthroplasty have been described in the literature: Posterolateral, trans-trochanteric, direct lateral and Watson Jone’s, recently miniincision and minimally invasive hip approaches. Of all the approaches, the posterolateral [Moore’s] and the direct lateral approaches are commonly used. The posterior approach is considered to be easy to perform and easy to train residents, however, increased rates of dislocation have been reported. The direct lateral approach diminishes risk of hip dislocation and the risk of injury to the sciatic nerve. However, there is an increased risk of limp. Dislocation of a hip prosthesis is a clinically important complication after THA, in terms of morbidity implications and costs. To minimize, risk of dislocation, the capsule and short external rotators in the posterolateral approach is sutured to the trochanter [24] and dislocation rate of 5-8% has been brought down to 1-3%. However, post-operative dislocation is still a problem in a posterolateral approach, more so when performed for fractured neck of femur [33], patients with neurologic problem like Parkinsonism It is well reported that the incidence of dislocation is much lower in the lateral approach compared to other approaches [13,24,29]. More recently, Masonis [18] analyzed 260 clinical studies including 4 prospective studies of which only 14 studies involving 13,203 Primary THR met the inclusion criteria. The combined dislocation rate was 1.27% for the transtrochanteric approach, 3.23% for the posterior approach 3.95% without posterior repair and 2.03% with posterior repair, 2.18% for the antero-lateral approach, and 0.55% for the direct lateral approach. It has been suggested15 that the available information is insufficient to make a firm conclusion on the optimum choice of surgical approach, a large prospective trial is required to prove usefulness of lateral approach.The use of smaller surgical incisions has become popularized for total hip arthroplasty (THR) because of the potential benefits of shorter recovery and improved cosmetic appearance. Using incisions typically less than 10 cm in length, surgeons can achieve adequate visualization of the surgical site while minimizing trauma to deep soft tissues [17,32]. Small incision surgery is associated with a learning curve and requires specialized instruments for favourable outcomes. However, an increased incidence of serious complications has been reported [1]. These procedures should be reserved for selective specialised centres A comparative study by Woolson [37], 50 mini posterior incision and 85 standard posterolateral incision, suggested that there was no evidence that the mini-incision technique resulted in less bleeding or less trauma to the soft tissues of the hip, factors that would have produced a quicker recovery and a shorter hospital stay, than did the standard technique. A modified lateral approach of Hardinge, which allows adequate access for orientation of the implant, has been described. Although this approach is more difficult than a posterior approach, and there is a learning curve, once mastered it definitely reduces the incidence of dislocation. In the Author’s opinion, this approach should be used routinely for total hip arthroplasty for fractured neck of femur where the incidence of dislocation is unacceptably high using the posterior approach.

Aoi, T., and A. Toshida. Hip Replacement : Approaches, Complications and Effectiveness, Nova Science Publishers, Incorporated, 2009. ProQuest Ebook Central,

Copyright © 2009. Nova Science Publishers, Incorporated. All rights reserved. Aoi, T., and A. Toshida. Hip Replacement : Approaches, Complications and Effectiveness, Nova Science Publishers, Incorporated, 2009. ProQuest Ebook Central,

In: Hip Replacement Editors: T. Aoi and A. Toshida

ISBN 978-1-60692-326-9 © 2009 Nova Science Publishers, Inc.

Chapter 1

Counteracting Reliability Problems in Advanced Hip Prostheses: Lessons from the Past and New Technologies for the Future Giuseppe Pezzotti Ceramic Physics Laboratory and Research Institute for Nanoscience Kyoto Institute of Technology, Sakyo-ku, Matsugasaki, 606-8585 Kyoto, Japan Department of Orthopaedics, Orthopaedic Research Center, Loma Linda University, Loma Linda, CA 92354, USA

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Abstract Looking back over 100 years span of history in hip arthroplasty, one can realize how the hip replacement surgery represents from both a scientific and an economical aspect a continuously growing field and a quite successful practice. However, during its historical evolution, hip arthroplasty have encountered and still occasionally encounters enormous obstacles, which mainly deal with the quality of the used prosthetic materials. Since the first attempts using prostheses made of primitive materials at the end of the nineteenth century, many different materials have been experimented and improved surgical approaches undertaken. Hip prostheses have been conceived according to different design criteria, but they all have been following the wide underlying principle of lowfriction arthroplasty. Such a basic concept yet represents the standard in total hip arthroplasty. In this chapter, an analytical overview is given, mainly from a materials science perspective, of the major problems encountered in the recent history of hip replacement. An outline is also given of emerging technologies aimed at the improvement of the structural and environmental reliability of advanced biomaterials.

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Giuseppe Pezzotti

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Introduction A careful examination of decisional trends, research directives and business schemes involved with the field of joint arthroplasty reveals the existence of quite peculiar trends as compared to other emerging fields. Although surgical procedures are statistically quite successful and already very effective from the point of view of market efficiency, companies continue to invest large capitals in research and innovation. In other words, despite excellent performances are systematically reported for implant systems, continuous efforts are made in the development of new implants and in the refinement of those already existing in the market, according to goals of improved functionality and lifetime elongation [1]. The reason for such a peculiar situation arises from two concurrent factors: on the positive side, is the belief in the progress of human welfare which continuously triggers the development of improved implants (with inherently high profits involved); on the negative side, there is always the risk of a dramatic drop in market shares when companies do not timely accomplish their market strategies with incorporating the latest cutting edge technologies. Total hip arthroplasty (THA) represents a very spread and effective surgical procedure. Surgeons and technologists make daily efforts in improving the outcomes of THA, with the ultimate goal of creating a prosthesis that reliably lasts at least as long as a human lifetime. While the results of primary hip arthroplasty are generally very good, revision surgeries might score variable success with regards to their clinical outcomes [2]. In addition, they invariably represent an expensive procedure and a severe burden to the patients. Thus, a reduction of the failure rates of only a few percents can, due to the large number of patients involved, have a vast influence on the accumulated costs and patient suffering. In other words, the key issue in hip arthroplasty resides in the improvement of the prostheses with regard to their long-term in vivo reliability. These circumstances clearly explain the peculiar market situation in this field and amply justify a search for new hip prostheses with improved structural/functional characteristics, and elongated lifetimes. Most recent innovative trends in THA have focused on the improvement of the tribological behavior of hip joints and challenged the achievement of a longer durability, with the potential for a service-life spanning several decades [3]. Such trends have naturally led to an increase in the use of ceramic materials, either as ceramic femoral heads yet coupled with advanced acetabular cups made of polyethylene (i.e., with improved molecular structure and quality), or as ceramic hip components for both acetabular and femoral bearing surfaces. The greater driving force in using ceramic bearings is a systematic reduction in periprosthetic osteolysis (i.e., mainly arising from polyethylene wear debris [4, 5]), which in turn can greatly reduce the number of surgical revisions. The high inertness and biocompatibility of ceramic materials may also reduce to a minimum the collateral effects on the human body, as possibly observed with metallic prostheses (e.g., contamination by metal ions [6], hypersensitivity [7], etc.). Despite those advantages, chipping and fracturing have severely limited the popularity of ceramic components [8, 9]. As a further issue, it should be noted that ceramic-on-ceramic articulations strongly require high precision in setting the orientation of the components during surgery (in order to avoid excessive impingement on the ceramic surface). Partly fractured ceramic bearings necessarily dictate revision. The main reason is that the ceramic remnants in the articulation would give

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Counteracting Reliability Problems in Advanced Hip Prostheses

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rise to severe third-body wear, especially in the presence of softer bearing counterfaces. Clearly, ceramic components offer a very high potential for further improving both structural performance and lifetime of hip joints but, being made of fragile materials, they also require significant progress in surgery technique, further advancements in joint design and materials manufacturing processes, as well as a peer non-destructive control of their structural reliability. In the slow historical evolution towards the perfect THA, a small number of patients, who are the voluntary participants in controlled trials, are subjected to novel, yet un-proven principles. Except for those patients, hip surgeries are made according to the existing knowledge with prostheses that have been known for their good long-term results. However, this ideal social scheme is not always successful and disputations are seen across the world. Nowadays, statistics teach us that wear and osteolysis are the most important issues that need to be counteracted in order to further improve the clinical outcome of THA. However, historical disputations have been dealing with a significantly wider spectrum of reasons, among which we selected the following as the most significant: (i) premature wear failure of polyethylene liners; (ii) catastrophic fracture of defective ceramic zirconia femoral heads; and, (iii) in vivo fracture or rim chipping of alumina liners. From the one hand, the strenuous efforts made towards the reduction of the number of cases of premature polyethylene wear failure have led to the development of advanced polyethylene acetabular cups with ultra-high molecular weight and highly cross-linking structural characteristics [10]. On the other hand, ceramics have been seen as the ideal candidates for building low-wear components, given their superior sliding behavior and the high inertness of their debris [11]. It should be noted that, provided wear rate be significantly reduced, larger femoral heads could be used with a higher safety margin. This circumstance also involves several additional advantages, such as allowing the patient a greater range of movements, less impingement of the femoral neck on the liner, a greatly reduced incidence of dislocation, and an increased safety margin during surgery in correctly positioning the acetabular component [12]. Therefore, a clear future trend can be recognized in developing ceramic hip components of a larger size. Comprehensive reports on the long-term clinical performance of advanced polyethylene materials with improved molecular structures are available in the recent medical literature [13-15]; however, such polyethylene structures may intrinsically differ from each other and, more remarkably, even from piece to piece in the same production batch. This is due to a somewhat sluggish technological development of quantitative methods for the control of the polyethylene surface structure after irradiation by gamma rays or after being subjected to other types of sterilization process. In addition, a concern remains that, despite a clear reduction in the amount of worn volume upon cross-linking, the amount of polyethylene debris particles with fine sizes comprised in the osteolytic potential range may remain high [16]. Thus, the potential for a reduction in clinical cases of osteolysis could be possibly lower than theoretically expected from the lowered volumetric wear rate. Furthermore, poor control of the sterilization process may also trigger the presence of free residual radicals on the irradiated surface of the polyethylene structure after cross-linking, thus causing local oxidation gradients with subsequent material embrittlement [17, 18]. In this latter context, retrieval studies of explanted highly cross-linked acetabular cups after short-term in vivo

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service have eventually shown trends that were not predicted by long-term simulator studies [19]. The high fracture strength of ceramic components and the low percentage of their reported fracture incidence are encouraging; however, this assertion involves the assumptions that the ceramic component in its unused state is conspicuously flawless, properly installed and that the acetabular cup is placed with the proper orientation [20]. The use of ceramic hip components has increased dramatically over the past few years, particularly after their approval in the US. As already mentioned, however, ceramic components are brittle and thus less “user friendly” than polyethylene and metal components. Such an inherent behavior may involve additional guidelines during surgery in order to reduce the risk of post-operative complications and revision. Several intraoperative factors may have impact on the clinical outcome and play a crucial role on the survival time of a ceramic artificial implant. For this reason, until quite recently, ceramic femoral heads were mainly coupled with polyethylene acetabular cups, early ceramic acetabular components being less popular due to fixation problems. Advanced alumina, zirconia and their optimized composite materials are nowadays of remarkably higher quality as compared to ceramic materials manufactured in the past. This improvement is due to concurrent progresses in design, manufacturing, and nondestructive proof testing. Undoubtedly, the wear characteristics of ceramic-on-ceramic articulations are extremely favorable and, in most of the published reports [11, 21-23], mechanical failures of modern ceramic bearings have been found to be exceedingly rare. However, chipping of acetabular liners due to in vivo edge loading and/or to impingement, as well as damages due to manipulation during surgery yet remain a problem [24, 25]. In this chapter, we shall first review some statistics on hip-joint surgery and have a brief survey on the main cases of failure in the recent history of hip prostheses. Then, a detailed description of the most advanced and recent technological approaches in material preparation, reliability control and non-destructive analysis of hip components will also be given. The main aim of this chapter is to drive the attention of the international orthopaedic community on the need for a highly interdisciplinary approach to the study of hip joint arthroplasty. In this context, we provide here some vivid examples of how newly developed technologies may provide final solutions to historical problems related to the chemical and the structural reliability of materials employed in total hip arthroplasty.

Lessons from the Past A joint surgery is referred to as primary when it is the first time a hip joint is replaced. Revision of a hip prosthesis is defined as a surgery in which part of or the entire prosthesis is exchanged or removed from the patient. On the other hand, a re-operation (other than a revision) is an operation following either a primary or a revision operation that does not require any joint implants to be removed or replaced, for example, if an implant needs to be re-aligned or has become loose. Figure 1 (A) shows an example of a statistical plot [26] of the number of hip operations by type as retrieved for the period of time 2003-2007 (note that in this statistical plot the number of re-operation was not collected for the period 2003/04). Two main features can be envisaged in the plot of Figure 1 (A): (i) the number of hip surgeries is

Aoi, T., and A. Toshida. Hip Replacement : Approaches, Complications and Effectiveness, Nova Science Publishers, Incorporated, 2009. ProQuest Ebook Central,

Counteracting Reliability Problems in Advanced Hip Prostheses

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quickly increasing year after year; and, (ii) although the ratio of revisions (and of reoperations) to the total number of primary surgeries appears to be approximately constant, clearly the absolute number of patients undergoing revision surgery is increasing year after year.

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Figure 1. (A) number of hip primary surgeries, revision and re-operations from 2003 to 2007, according to Ref. [26]; (B) statistics on the causes of revision surgeries in hip arthroplasty for the year 2006 (from Ref. [26]); n represents the total number of revision surgeries from which the shown fractions are calculated.

The reasons for revision in total hip arthroplasty are quite various and differentiate (for a recent statistics, cf. Figure 1 (B) [26]); they mainly include aseptic implant loosening, osteolysis, pain, deep infection, instability and wear damage, while fracture of the prosthesis or of the periprosthetic bone usually cover a quite low fraction of cases. Clinical outcomes after revision surgery involve a higher degree of uncertainty and are generally inferior to the outcomes of a primary joint replacement. Uncemented acetabular cups have been most commonly revised due to substantial wear of the liner, or owing to other (polyethylene) debris-related problems such as osteolysis, or aseptic loosening. However, the scientific support to different strategies in revision is sparse and the available literature consists mainly of moderately sized patient series and reports on a specific technique or implant. Due to the considerable heterogeneity of patients and operative procedures, as well as to a substantial lack of massive comparative trials, proofs to support a specific procedure above another in any specific trend of patient may be hardly found. However, in the most recent history of hip arthroplasty, there have been exceptionally clear patterns of failure, which certainly represent for all of us precious lessons from the past and should serve henceforth as experimental proofs to guide surgeons, scientists and technologists working in this field through the development of better specific surgery procedures, advanced prosthesis design, and new materials.

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Premature Wear Failure of Polyethylene Liners

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The polyethylene part of hip prostheses, referred to as a liner, is responsible for absorbing the impacts experienced by the implant during its lifetime and may become subjected to extensive wear (i.e., resulting in the release of foreign debris into the patient) (e.g., cf. acetabular cup shown in Figure 2).

Figure 2. Highly oxidized polyethylene acetabular cup and its surface spectroscopic analysis. The cup, whose surface was γ-irradiated in air with a dose of 33 kGy, was implanted in human body for about 12 y; the cause of revision was aseptic loosening. Typical Raman spectra collected from the non-wear zone (location 2) and from the main wear zone (location 1) are shown in (A) and (B), respectively. From such spectra, local maps of the oxidation index were constructed (i.e., using the relative intensities of the bands located by arrows in (A) and (B); cf. Ref. [62]). Oxidation maps, which help to visualize the surface oxidation effect on a microscopic scale, are displayed in (C) and (D); the maps are associated with locations 1 and 2, respectively.

Oxidation of polyethylene liners unavoidably takes place in vivo (cf., for example, the analytical results after retrieval shown in Figure 2) and greatly affects the structural integrity of the liner. Extensive wear of the liner will result in the need for a revision surgery. Modern liners are manufactured from ultra-high molecular weight polyethylene (UHMWPE), a material with extreme durability in its non-oxidized state [27]. Orthopaedic implant industry usually employs a sterilization method involving the exposure of the liner to gamma radiation. However, when this method is applied in air or in a non-completely controlled inert atmosphere, it may cause severe oxidation of the polyethylene surface, with the liner

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becoming highly brittle, defective and with a surface highly prone to wear [28]. These negative effects may be strongly amplified when irradiation in presence of air is coupled with an extended shelf life. In 1987, a modified polyethylene developed to reduce wear in total joint replacement components, including hip, knee, shoulder, and elbow joint replacements, was introduced in the market. Initially, a gamma-ray process in air was used to sterilize all components, while subsequently the components were sterilized by different processing methods including gamma-sterilization in nitrogen environment and gas plasma. In the late 1990s, all defective products were withdrawn from the market. In 2001, the British Government's Medical Devices Agency issued a voluntary recall of liners sterilized by gamma irradiation in air. Following a review of hip patients at sites with exceptionally high failure rate (up to more than thirteen times higher than the expected rate), a warning was also issued and patients instructed to proceed to a peer check of their implanted liners. It is now well known that gamma-ray sterilization of UHMWPE can induce oxidative degradation and that this effect increases during shelf storage and during implantation, significantly shortening the clinical life span of the prosthesis [27, 28]. Analytical studies of polyethylene wear debris has been recently published for a statistically meaningful number of patients who had received defective polyethylene implants sterilized by gamma rays in air (i.e., of the same type of those recalled by the British Government) and had suffered prosthetic loosening [29]. A comparison was carried out with implants made of traditional polyethylene and sterilized by the same method. Interestingly, the frequency distribution of globular and fibrillar particles was similar in both groups, but the globular particles of the defective samples had a mean surface area significantly lesser than that of the implants used for comparison. In other words, the two materials, despite undergoing the same type of sterilization, produced wear debris with significantly different morphologies. Such a difference can be regarded as the origin of a more intensive biological response, namely of an earlier and more massive osteolysis. In other words, experience teaches us that oxidative degradation of UHMWPE joint components is a complex phenomenon, which is strongly affected by the intrinsic molecular structure of the material. In addition, along with increased wear resistance, there can be deleterious changes in ductility and fracture resistance in UHMWPE. Nowadays, while some manufacturers prefer to sterilize their implants using ethylene oxide, gas plasma or electron beam irradiation, others continue to use gamma irradiation, but with the implant sealed in a low-oxygen package [30]. The reason for yet selecting sterilization by gamma rays resides in the understanding that crosslinking, induced as a by-product of radiation sterilization, can improve the wear resistance of UHMWPE. Clearly, enormous progress has been made in material processing technologies since the disclosure of the failures of liners sterilized by gamma irradiation in air. However, clinical reports continue to show that the extent of oxidation scatters markedly, depending on type of resin, atmosphere in which the component was irradiated and subsequently stored, duration of shelf storage, and duration of use in vivo [31]. Some of the radiation-sterilized UHMWPE implants show little or no oxidation, even after long-term shelf storage or implantation. In other words, an open issue yet remains on how can we be assured on a deterministic basis, namely with respect to each very piece of polyethylene implanted in the human body, about safety and lifetime concerns. These concerns are particularly true if one thinks that the numbers of total hip replacements in the

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world are increasing. Therefore, even assuming that (as shown, for example, in Figure 1 (A)) the revision rate has not significantly increased during the last decades, there is an overall increase in the total number of revisions. In other words, we are not improving in avoiding our mistakes. Clearly, an even minimum reduction of the failure rates by only a few percents can have a vast influence on the reduction of the number of patient suffering. This observation should represent the main driving force for improvements to be continuously pursued by surgeons and technologists.

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Fracture of Defective Ceramic Zirconia Femoral Heads The most widely known failure of a series of zirconia ceramic femoral heads happened in 2001 with a French manufacturer recalling batches of femoral heads fracturing at a higher rate than expected. This manufacturer recall applied to nine batches of zirconia heads produced since early 1998 after the company changed part of its manufacturing process. The Food and Drug Administration in the US subsequently issued a voluntary recall for these zirconia components, advising surgeons not to continue implantation of artificial hips with zirconia femoral heads manufactured by this French maker between early 1998 and 2001. Patients who already had these hip implants were also recommended to contact their surgeons for any question or concern, although no surgery was recommended to replace hip implants that have not fractured. An external panel of zirconia experts was then formed to study femoral head fractures in the US. According to those experts, a conclusion was reached that the majority of the fractures were from two lots produced early in 1988 and that fractures generally originated near the upper taper-crown regions of the machined bore (cf. fracture analysis of one of such balls shown in Figure 3). The femoral heads fractured into many small fragments and their fracture surfaces showed numerous large flaking structures and multiple cracks through poorly consolidated areas. Significant phase instability, namely phase transformation from the tetragonal to the monoclinic polymorph, was found nearby the fracture origin and on the ball surface (cf. analysis of fracture origin and other analytical results also shown in Figure 3). Post-mortem analyses showed numerous potential fracture origins, which were also observed on the ground taper surface. Autoclaving unused femoral heads of the same type resulted in a very high degree of monoclinic phase transformation, with monoclinic volume fractions up to 70 % being measured; in other words, these femoral heads were found to be in a highly ‘metastable’ state. It should be noted that the first twelve years of clinical experience with zirconia femoral heads provided an excellent safety record, with very small incidence of fracture. This result could be achieved when a batch furnace was used for production, in which an oven was set through one complete heating and cooling cycle to sinter the ceramic femoral heads. Only twenty-eight fracture cases were reported for this sintering process over that time and represented a fracture rate as low as 0.0085 % (i.e., 1 per 11,000 units). With the use of the proof test on every head produced, the fracture rate fell down further to 0.002 %. In January 1998, a crucial change was made to the manufacturing process with the introduction of a ‘tunnel’ furnace in order “to optimize” the production process. After

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thorough consultations, the expert study group concluded that the combination of the tunnelsintering and subsequent machining operations had probably negatively interacted to make the femoral heads extremely vulnerable to transformation into monoclinic phase in patients. In a recent review paper, Clarke et al. [32] noticed that, since their inception eighteen years ago, the performance of zirconia ceramic balls has been both confusing and controversial. Grouping all zirconia materials as the same product, in the absence of a rigorous scientific clarification of individual chemical and physical properties, leads to confusion and to misleading generalizations extremely worrying to orthopaedic surgeons.

Figure 3. Raman spectroscopic analysis of a broken piece of a defective femoral head retrieved after fracture in vivo. The femoral head was impinging on a UHMWPE cup and mounted on a Ti64 femoral stem. It fractured after 4 y implantation. Typical Raman spectra collected in selected areas of the fractured piece are also shown (in the spectra, plasma lines are only instrumental and are used as internal spectroscopic reference). High fractions of monoclinic phase were systematically found at taper-cone (location 1) impinging areas and on the taper edge (location 3) (cf. the presence of bands belonging to the monoclinic phase in the spectra (A) and (C), respectively), while no significant internal transformation was noticed (cf. location 2 and spectrum (B)). Catastrophic fracture originated at location 1, for which a monoclinic phase transformation map is also shown in (D).

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As in the case of oxidized polyethylene, the degree of metastability of zirconia materials as well as the involved probability of fracture strongly depends on the microstructural characteristics of the very individual material, and thus strongly on the manufacturing process. For example, the clinical history of yttria-stabilized, zirconia ceramic has been quite controversial. In vivo, a combination of hydrothermal and mechanical exposure may selectively trigger detrimental changes on the surface of femoral heads that have inferior phase stability [33, 34]. Local transformation from the tetragonal to the monoclinic phase may be influenced by the effect of body fluid and impingement conditions. Hydrothermal processes are the most salient to material stability, thus making the implant very sensitive to the adopted lubrication mode; for example, for zirconia ceramic-on-ceramic bearing systems tested in a hip simulator, early catastrophic failure was found when run in water but implants survived as long as 20 million cycles when run with calf serum lubrication [35]. This observation suggests that tribological conditions under the cup are the trigger for a metastable behavior. Most of the early implanted zirconia femoral heads, independent of their bearing counterpart, showed transformation areas after retrieval, especially in areas of contact. However, some femoral heads were found to be completely stable and untransformed, showing that yet little is understood of the actual hydrothermal conditions operating in the hip joints in vivo. Despite the numerous queries and worries on zirconia implants, it should be noted that all cases of fracture and premature retrievals in such implants refer to early generations of this material. In particular, the above-mentioned fracture recall, although representing a catastrophic experience, was contained and unique to the defective manufacturing process. But, again, what is of considerable importance here is that there is little (or nothing) of predictive in the present approach to the analysis of hip failures; quality control in this field, however accurate, is yet of a statistical and not of a deterministic type in its ultimate nature. This represents a very unfortunate circumstance and calls for a better understanding of critical material parameters and of their actual role in the lifetime elongation of prosthetic implants. Concurrently, we also need to emphasize here the urgency in the development of improved analytical methods, which can clarify and help makers and surgeons to predict with high exactness and in a non-destructive manner the expected lifetime of each individual hip component to be implanted in human body, before it actually fails.

In Vivo Fracture/Chipping of Alumina Liners The incidence of fracture among the causes for revision of alumina ceramic hip joints has been historically low. Heros and Willmann [36] published a milestone report on alumina fracture incidence in 1998. According to their publication, clinical data published in the early 1990s (representing the 1980s) gave a fracture incidence ranging from 0 to 0.8 %, namely a maximum of 8 fractured femoral head per 1000 cases. Statistical data for the early 1990s showed already a fracture incidence reduced to 2 balls per 10000 cases. Most recent clinical statistics, according for example to the National Joint Registry for England and Wales and pertaining to the cause of hip revision in 2006, report a percent of head and acetabulum fractures of 0.27 % and 1.14 %, respectively (on a number of revision surgeries n=8554) (cf.

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Figure 1 (B), [26]). Among the cases of acetabular fracture, surgeons in Europe, in Asia and in the US, have concurrently reported rim chipping and fracture of ceramic inserts [24, 37, 38]. However, it should be noted that the polycrystalline materials, of which modern alumina implants are made, are strongly improved over the previous generations, being treated by hot isostatic pressing, laser engraved, having a smaller grain size, fewer impurities, and being subjected as individual components to proof-testing after manufacturing. One of the early reported issues, dealing with ceramic acetabular inserts, involved the intraoperative chipping of ceramic inserts as the surgeon seats into a metal shell the alumina acetabular insert during total hip arthroplasty. These kinds of problem can be analyzed in a quite straightforward manner from a biomechanical viewpoint: if not properly handled or aligned with the metal shell, a piece of ceramic can chip off in correspondence of the insert edge. However, the problem is clearly technique-related since it mainly occurs if the surgeon, unaware of the brittleness of ceramics, impacts an eccentrically placed insert into a metal shell. While chipping likely damages only a small part of the edge, significant stress intensification can be created in the material, and the chipped insert must be mandatorily replaced with a new one [39]. Figure 4 shows a typical case of partial chipping of an alumina liner and the related notching effect.

Figure 4. Alumina acetabular cup retrieved from an Asian patient and belonging to a poorly designed ceramic-on-ceramic hip joint. The cup was implanted for 3 y and partly fractured due to strong and systematic impingement of the femoral head on the liner rim. Raman spectroscopic analyses showed significant tensile stress intensification on the fracture surface of the material (electron micrograph (B) and related residual stress map (C) taken at location 2). (A) and (D) represent an optical image and a stress map, respectively, which were collected nearby location 1. Notching and cracking effects can be clearly observed at such location, which can trigger catastrophic fracture of the entire liner.

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As seen from a microscopic residual stress analysis, significant stress magnitudes, tensile in nature, are piled up on the damaged parts. Such locations of tensile stress intensification may trigger catastrophic fracture of the entire liner. With special emphasis placed on Asian patients, design concerns have recently arisen about ceramic-on-ceramic bearings including a sandwich-type ceramic liner, given the pronounced attitude to squatting, which in turn results in hyperflexion and wide hip abduction. Early fracture of ceramic liners has been thus associated with impingement from excessive anteversion of the acetabular cup. However, unlike the cases of polyethylene and zirconia materials described above, the origin of these failures should be mainly identified with the choice of a defective design rather than dealing with the poor structural quality of the selected material. Unlike material defective cases, solutions for such kinds of surgical failures should become promptly available upon shifting the hip design concept toward more advanced perspectives and/or appropriately changing the design parameters, as for example allowing larger head diameters [40-45].

New Technologies for the Future

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Despite the unfortunate circumstances occurred in the past, as described in the previous section, the field of total hip arthroplasty has recently demonstrated brilliant technological improvements and collected outstanding clinical results. The functionality of a hip joint, composed of the femoral head and of the acetabular cup, is however very complex and the intricacies of the coupled motions of the joint components have made development of new materials and joint designs a challenge. There have been several design attempts to create a hip joint that recapitulates normal motion while providing significant longevity, a low incidence of complications and no possibility of catastrophic failure. Better understanding of the biomechanics of the hip joint and improvements in implant materials have made successful hip replacement a likely reality, and now several new materials and designs are ready for clinical trials. In this section, a selection of advanced technologies based on the most recent progress in materials processing and their non-destructive analysis is presented. An extensive discussion is also attempted in the following by considering material stability, strength and structural reliability issues.

Non-Destructive Raman Analysis of Polyethylene Liners From a materials science point of view, the general classification of UHMWPE refers to a polymeric substance whose repeating unit is the hydrocarbon ethylene group (-C2H4-) and whose molecular weight exceeds 1.75 million g/mol. The UHMWPE materials used for joint implants are generally in the order of 3–6 million g/mol. The increase in molecular weight is generally accompanied by an improvement of mechanical and functional properties, including a low coefficient of friction and a high resistance to wear. In particular, these latter two characteristics make UHMWPE suitable for use in bearing surfaces of joint implants [46, 47]. Since sterilization is needed in medical devices, the material needs to be further treated

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after manufacturing and Co60 γ-sterilization is nowadays the most widely used industrial method for sterilization. The main advantages of this method reside in its reliability, flexibility, cost-effectiveness, and in the total absence of chemical residues after sterilization. The effects of sterilization on the structure of polyethylene components for joint implants has been deeply investigated [48-51]; unfortunately, the results have not so far produced a fully conclusive verdict on how these structural changes affect the success or failure rates of the joint implant. However, all studies agree on the mechanism that produces the change in mechanical properties, which is the rupture of the molecular chain bonds through the mechanism of chain scission. Broken chains allow free radicals to form within the polymeric structure, thus creating potential sites of bonding to neighboring molecules. This phenomenon is usually referred to as cross-linking, namely the structural process through which adjacent linear chains are joined to each other by covalent bonds. Cross-linked (namely, γ- or electron-irradiated or, more in general, surface treated) polyethylene materials, would be expected to be stronger but also to possess decreased ductility and increased susceptibility to fatigue damage and wear [52]. As a matter of fact, concurrent to cross-linking, irradiation-driven chain scission can also lead to oxidative degradation, in which the free ends of the structural lamellas react with oxygen present in the surroundings. The degree to which oxidation occurs depends on the amount of oxygen diffusion and thus on the material structure on a mesoscopic scale. It should be noted that medical grade UHMWPE must be a pure polyethylene because of the highly required level of biocompatibility; accordingly, it must not contain any stabilizers and, as a result, it may be more prone to oxidative degradation than other commercially available grades. In addition, as already mentioned in the previous section, oxidation may take place during irradiation (i.e., immediate oxidative degradation), shelf life and in vivo operative life (i.e., long-term oxidative degradation) [27, 28, 31]. Immediate oxidative degradation can be minimized by elevating the radiation dose rate and/or by irradiating in inert atmosphere (e.g., nitrogen). On the other hand, long-term oxidative degradation can be delayed by carefully storing the samples in the dark at low temperature and in nitrogen atmosphere. In spite of all these efforts, even sophisticated methods of irradiation, storage, remelting, etc., have not been proved so far capable of prevent oxidation completely. Accordingly, it becomes clear how the method of sterilization becomes a primary concern with regard to total joint replacements. Some manufacturers favor the method of γ-sterilization in the presence of an inert gas since oxidation is minimal and the additional cross-linking improves abrasive wear resistance [5357]. Conversely, some other companies prefer EtO [57] or gas plasma [58] sterilization claiming that fatigue wear, causing pitting and delamination, is more likely to occur in γradiated materials (i.e., owing to its increased oxidation state). The extent to which structural changes contribute to failure rates of joint implants has been a source of conflict in recent years, many of these conflicts mainly dealing with a dualism between wear and deformation properties [59]. Two types of mechanism significantly contribute to hip joint degradation: creep deformation and abrasive wear [60, 61]. Creep deformation refers to a permanent deformation that occurs under loading and does not completely recover after load release. In this type of mechanism, no mass is lost from the sample, but the polyethylene molecules pack and adjust their reciprocal positions under pressure. Conversely, abrasive wear is

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accompanied by an irreversible loss of mass from the material. Molecules from the surface of the polyethylene body become partly delaminated and can be peeled away, involving volumetric changes, which in turn result in polyethylene debris. The degree to which the dimensional change is caused by these two mechanisms is difficult to determine quantitatively. Although somewhat competitive with respect to the design of an ideally efficient polymeric structure, creep and wear resistances may lead to the same disadvantages, namely increased femoral head penetration and, ultimately, joint loosening. In addition, as previously discussed, debris from abrasive wear is a direct cause of osteolysis. It is beyond the scope of this chapter to provide conclusive statements on how sterilization-related structural changes in UHMWPE can affect the in vivo lifetime of hip joint implants. While we consider a full rationalization of the effect of different sterilization techniques as a final goal to be thoroughly sought until an unequivocal clarification is reached, we shall drive here the reader’s attention on the new possibilities in non-destructive analysis offered by recent advances in confocal vibrational spectroscopy [62-64]. Such new spectroscopic techniques allow visualizing in a fully non-destructive manner and with microscopic spatial resolution (both in-depth and in-plane) the salient structural and micromechanical characteristics of biomedical UHMWPE grades. In past years, Fourier-transform infrared (FT-IR) spectroscopy has been extensively used to characterize the hydroperoxide concentration and distribution in UHMWPE biomedical samples [65, 66]. According to such a spectroscopic method, different oxidation behaviors and mechanisms have been identified and discussed in correlation with processing, sterilization, and packaging conditions. Raman spectroscopy enables one to make the same quantitative assessments of UHMWPE but in a nondestructive manner [62]. Figure 5 shows two drafts giving a comparison between FT-IR and confocal Raman analytical techniques.

Figure 5. Schematics showing a comparison between IR and confocal Raman analytical techniques for quantitatively assessing in-depth oxidation profiles in polyethylene acetabular cups.

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Raman spectra can identify crystalline and amorphous phase fractions, oxidation state and residual strain in the material. In other words, through Raman spectroscopic experiments, it is possible to verify the actual structural changes that occur during sterilization by γradiation in UHMWPE; but, more importantly, by taking advantage of its fully contactless character, its high scanning flexibility, and non-destructiveness, it is possible to scrutinize each single UHMWPE component according to a fast procedure before implantation. According to this practice, a new possibility is opened to introduce an advanced quality control procedure for biomedical implants of deterministic nature, thus fully eliminating the inability of randomized controlled trials to detect infrequent events and unexpected side effects. In Raman microprobe spectroscopy, a research grade optical microscope is coupled to an excitation laser and to a spectrometer producing a platform capable of obtaining conventional visible images and, in addition, to generate hyperspectral Raman images from selected sample areas [67]. The combination of a confocal Raman microscope, a motorized three-axis stage, and an autofocusing device enables the generation of 2D and 3D maps that can yield straightforward information on the distribution of different crystallographic phases, stress/strain fields, chemical species, and oxidation gradients in heterogeneous samples over selected areas. In recently published papers [62-64], we have described the analytical basis and made a review of the main applications of Raman spectroscopy/imaging in the biomaterials field. In this chapter, we emphasize the importance of the Raman spectroscopic technique for the non-destructive oxidation analysis of UHMWPE In order to characterize oxidation patterns not only on the surface but also along the material sub-surface, in-depth Raman spectroscopic analyses can be performed, according to the confocal method. In our investigations, we have scrutinized a large number of virgin UHMWPE cups as received from the makers, before exposure to atmosphere. Figure 6 shows a comparison between two acetabular cups of the same type fabricated by the same maker (both treated nominally in an inert atmosphere with the same amount of γ-radiation and under exactly the same nominal irradiation conditions: 33 kGy) and belonging to the same batch. The confocal configuration of the probe adopted in the experiment corresponded to a ×100 objective lens; numerical objective aperture and confocal pinhole diameter of the objective lens were fixed as: NA=0.9 and Φ=100 µm, respectively. Through confocal experiments on a large number acetabular cups obtained from various makers, the existence of a weak oxidation profile along the sub-surface has been typically observed (as shown in the series of sub-surface profiles in Figure 6(A)); however, we could also identify a small fraction of new (i.e., unused) cups (i.e., typically comprised between 1 and 3 %, depending on the investigated batch), in which the immediate sub-surface region experienced a relatively high oxidation gradient (as in the in-depth profile series for the acetabular cup shown in Figure 6(B)). It can be thought that the structural degradation occurring in vivo in the near-surface region may be directly affected by a pre-existing oxidation state in the acetabular cup, thus shortening the lifetime of the acetabular component, leading to the early formation of polyethylene debris upon wear contact in vivo, and triggering osteolysis.

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Figure 6. (A) and (B) show series of near-surface maps of the in-depth state of oxidation as detected in two new acetabular cups fabricated by the same maker, sterilized by γ-ray (33 kGy) and belonging to the same batch. Despite the common processing and surface irradiation treatment, a clear difference in the in-depth oxidation gradients can be noticed.

The cycle of oxidation products and radical species formation has been shown to occur over several years as oxidation levels continuously increase in components implanted in vivo [68, 69]. The resulting formation of chain scission products creates shorter molecular chains, degrading the mechanical properties and the wear performance of the surface slab of material, which thus represents the most critical part for the in vivo lifetime of the joint [69, 70]. Figures 7(A) and (B) show typical maps of sub-surface oxidation profiles as measured in the

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main wear zone of UHMWPE acetabular cups produced by the same maker and both retrieved from female patients after 2.8 and 12.2 y, respectively. Maps of in-depth profiles for non-wear areas belonging to the same cups are given in Figures. 7(C) and (D), respectively. It should be noted that both these implants were used in successful surgeries from the viewpoint of the relatively small amounts of volumetric wear detected.

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Figure 7. In-depth oxidation profiles from two different retrieved hip cups. (A) and (B) show series of typical maps of sub-surface oxidation profiles as measured in the main wear zone of two UHMWPE acetabular cups produced by the same maker and both retrieved from female patients after 2.8 and 12.2 y, respectively. Series of in-depth profiles for non-wear zones belonging to the same cups are shown in (C) and (D), respectively.

The reasons for the revision surgery were dislocation and aseptic loosening for the shortterm and the long-term in vivo exposed acetabular cups, respectively. As of a comparison among the four profiles shown in Figure 7, it is clear that, in the same implant, the wear zone undergoes a greater near-surface oxidation than the non-wear zone. In addition, a time elongation of the exposure in vivo clearly increases the amount of oxidation along the material subsurface. Unfortunately, data showing the oxidation state of the very same cup before implantation (i.e., in its unexposed status) and after its retrieval from the patient are not available at the present time. One may attempt to grasp the oxidation state of an explanted acetabular cup from its oxidation profile in the non-wear zone, although this practice might lead to quite approximate assessments since other factors may also influence the overall oxidation rate of the cup. Acetabular cups with a stronger near-surface oxidation state in the wear zone were found to usually experience higher oxidation profiles also in their non-wear zones. A puzzling

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observation for environmentally degraded UHMWPE components is the occurrence of the most severe oxidation typically below the exposed surface of the component [71-75]. Spectroscopic analyses for ketones, which are the primary oxidation products in polyethylene materials, revealed maximum concentrations of this species several millimeters below the material surface [74]. This circumstance has been explained by considering that alkyl radicals, which are uniformly generated in polyethylene during irradiation, react with peroxyl radicals to form ketones. Ketones levels are low both at the very surface and in the bulk of the material because high oxygen concentration depletes alkyl radicals (creating an excess of peroxyl radicals) at the surface and low oxygen concentration inhibits ketones formation (creating a lack of peroxyl radicals) in the bulk. As a result, a maximum ketones concentration, and thus oxidation rate, is noticed in the material sub-surface. However, the depths extensively reported so far for such a maximum are of the order of several mm [74, 75]. Recent advances in confocal Raman spectroscopy, including the development of precise mathematical deconvolution procedures [62], have enabled us to locate an additional oxidation maximum at depths of the order of the tens of microns. Typical oxidation profiles, as revealed by the Raman confocal probe in unused, short-term, and long-term in vivo oxidized acetabular cups (produced by the same maker and subjected to the same amount of γ-radiation dose), are shown in Figure 8. The existence of two distinct oxidation maxima of similar intensity in the sub-surface is clearly seen. At the present time, however, it is not clear whether or not the chemical mechanism leading to the oxidation maximum closer to the material surface is of the same nature of that observed farther below the surface, which was already reported in previous literature papers. Nevertheless, we have newly shown here that the former oxidation maximum is already non-negligibly present in a small fraction of (defected) new acetabular cups in their as-received state. Nowadays, national registers have developed in many countries for follow-up and surveillance of joint replacement surgery. Registers are primarily intended to function as quality control systems, their main objective being that of detecting inferior prosthetic implants, bone cements, routines and procedures as early as possible after their introduction to the market. The intrinsic quality of joint devices and replacement surgeries can be promptly assessed owing to hospital-specific results that are in turn reported yearly to all hospitals involved with joint replacement surgery. Statistical methods are commonly used in the evaluation of the data, which typically include survival analyses and sample regression. The end-point in these analyses usually is the revision of prosthetic components. However, it should be noted that all those kinds of “quality control” methods only work a posteriori and strongly rely on a statistical sampling rather than on a control of deterministic nature. It is evident that inferior as well as defected implants have been used, still are used, and will continue to be used in the lack of an obligatory system not only for the introduction but also for the a priori quality control of each individual prostheses implanted in the clinical practice. Severe lessons from the past are teaching us that regulations should not only allow future revisions in the same hip of the same patient to be linked to the corresponding primary procedure, but also the initial quality of the implanted components to be systematically measured, followed up and evaluated in terms of reliability parameters and residual durability.

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Figure 8. Linear assessment of in-depth oxidation profiles, as non-destructively revealed by a confocal Raman probe in unused, short-term, and long-term in vivo oxidized acetabular cups of the same type.

Therefore, there is a clear hint that health authorities should engage in the development of fully deterministic quality control procedures, including advanced spectroscopic screening especially for those components subjected to high risk of degradation (e.g., polyethylene). Concurrently, the research community together with the orthopaedic associations should strive to widely spread understanding and to influence this process with clear research outputs and outline suggestions for the selection of more advanced evaluation methods.

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Nanocrystalline Zirconia for Hip Implants with High Environmental Stability Polycrystalline zirconia ceramics stabilized with 3 mol. % of yttrium oxide have been extensively used in load-bearing biomedical components due to their excellent mechanical properties (e.g., high fracture strength and fracture toughness), high wear resistance and superior biocompatibility [76-79]. However, official warning was published about resterilization of zirconia joints because it may involve an increase in surface roughness [80]. In addition, high phase instability in biological environment was reported for retrieved zirconia femoral heads [81]. Although the wear properties of the monoclinic polymorph of zirconia are not intrinsically different from those of its stabilized tetragonal counterpart [82], a partial transformation in vivo of the tetragonal phase into the monoclinic one may lead to wear degradation of a ceramic hip joint and, for this reason, the usage of zirconia materials for joint prostheses is nowadays the object of great concerns among surgeons.

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As a result of extensive research, it is now well established that yttria-stabilized zirconia polycrystal may suffer spontaneous transformation from its tetragonal crystal structure into the more stable (i.e., at room temperature) monoclinic polymorph [83-85]. Spontaneous t→m transformation, which might occur as a consequence of embedment in hydrothermal environment such as human body, is also referred to as ‘low temperature degradation’. Such a kind of metastable behavior may have a deep impact on the degradation of mechanical properties on the material surface. Unfortunately, the kinetics of surface phase transformation in zirconia femoral heads when embedded in human body and its relationship with wear mechanisms (especially against UHMWPE sockets) have not been fully clarified yet. Nevertheless, it is known that phase stability in zirconia femoral heads may conspicuously vary depending on manufacturers and manufactured era. Zirconia heads belonging to earlier generations were shown to have a poorer performance as compared to nanostructured zirconia heads belonging to the latest generation [86]. Since many of the early implants are yet embedded in the bodies of patients, those old zirconia femoral heads actually represent the object of concerns. In this context, it has been suggested [87] that in vitro phase transformation on the surface of zirconia femoral heads has little influence on the wear rate of UHMWPE sockets. This conclusion has been reached from a comparison between the results of hip-joint simulator testing on virgin and aged zirconia femoral heads against conventional UHMWPE sockets. A recent report [87] showed that, in femoral heads, monoclinic fractions of 0 and 10 mol. % increased slightly during simulator testing, but samples already containing 50 and 80 mol. % did not change their crystalline form during testing in hip simulator. However, the kinetics of phase transformation needs to be further and precisely evaluated in order to ultimately clarify how phase transformation progresses and may differ when evaluated in a hip-simulator test, in vivo, and during aging test in hydrothermal environment. We have no choice but accept that spontaneous phase transformation in zirconia will sometime occur in water environment or upon embedment into human body [88, 81]. The question is how long it will take for such a phenomenon to take place and, more importantly, will this time elongation exceed the lifetime required from a ceramic hip joint? To properly answer those questions, one needs to clearly understand the rules governing the kinetics of phase transformation. Such an understanding should enable us to precisely predict the evolution with time of surface phase fraction. Nevertheless, the kinetics of phase transformation on the zirconia surface is a quite complex phenomenon. Nucleation of the monoclinic phase occurs at first on isolated grains belonging to the material surface; then, the monoclinic phase may spread towards neighboring grains [89-91]. Microscopic and spectroscopic results showing the kinetics of surface phase transformation in monolithic zirconia are shown in Figure 9. It is clear from this figure that, at its initial stage, spontaneous phase transformation discontinuously and selectively occurs nearby the material surface (Figure 9(A)); in addition, it also involves a conspicuous increase in roughness (cf. left-side inset in figure 9(A)). T→m phase transformation then spread toward the bulk of the material as the nuclei grow and ultimately link to each other (cf. Figure 9(B)). It should be noted that t→m transformation is accompanied by significant volume expansion, which may in turn induce a lack of stoichiometry and large residual stresses in the neighborhood of the transformed near-surface area.

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Figure 9. (A) Early stage of surface nucleation occurring during spontaneous t→m phase transformation in zirconia exposed to in vitro autoclave aging. Insets show roughness and monoclinic phase fractional maps collected in the squared area located on the optical micrograph; (B) cross-section of a zirconia sample after long-term in vitro exposure to moist environment. Insets show microscopic details of grown monoclinic nuclei linking to each other, and the steep in-depth profile of monoclinic phase fraction (also visible owing to a clear darkening in the laser micrograph).

It is thus clear that, when considering polycrystalline zirconia as a candidate material for load-bearing joint surfaces, one must necessarily take into consideration two negative aspects of its phase metastability: (i) the volume expansion that accompanies t→m transformation may lead to an enhanced surface roughness; and, (ii) highly localized residual stresses can be developed in yet untransformed tetragonal crystals upon partial phase transformation. Such stresses are highly graded and tensile in nature [92]; therefore, they may promote further transformation and also become preferential locations for microcracks and grain pulling-out when the surface bears a load. Enhancement of phase stability in polycrystalline zirconia has been mainly pursued by the addition of various oxide phases and their related mixtures (e.g., alumina, magnesia, ceria and calcia) [93-95]. However, concurrent to the use of suitable oxide stabilizers, we newly show in this chapter that substantial refinement (i.e., down to a nanometer scale) and thorough control of grain size can significantly delay t→m phase transformation. Evidence for substantiating this new concept has been obtained with the aid of photo-stimulated confocal spectroscopy techniques based on the analysis of Raman bands in commercially standardized zirconia ceramics [90]. In particular, a quantitative evaluation of phase stability can be pursued in water vapor environment as a function of time and grain size, in order to simulate (and severely accelerate) the hydrothermal effects occurring in the human body. Evolution of the amount of t→m phase transformation on the material surface (and subsurface) can be then quantitatively monitored with increasing grain size and used to estimate the expected lifetime in vivo of the joint. Nowadays, advances in ceramic processing allow the preparation of dense zirconia polycrystals with a quite fine microstructure. As an example, we show here zirconia materials most recently prepared by Japan Medical Materials (JMM) Co. The zirconia powder, which contained 3 mol. % of yttrium oxide and a small amount of aluminum oxide, was sintered at temperatures in the range between 1300 and 1500°C for 2 h in air. Then, the samples were

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subjected to a hot isostatic press (HIPing) treatment, after which they could reach a full density. Figures 10(A)-(H) show the typical microstructures of such dense materials (sintering temperatures, Ts, are explicitly reported in each micrograph, while the HIPing temperature was the same for all the investigated materials).

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Figure 10. (A)-(H) typical electron micrographs of microstructures from sintered and HIPed monolithic zirconia polycrystals. Ts values represent the sintering temperature.

Independent of sintering temperature, the grain morphology observed on the plane of polish was equiaxed. However, the average grain size of the materials increased with increasing sintering temperature, with average values typically ranging between 150 and 350 nm. The dependence of the average grain size measured in the dense zirconia polycrystals as a function of sintering temperature is reported in Figure 11. The dense samples were then evaluated with regard to their phase stability by keeping them for increasing times (i.e., ranging between 0 and 150 hr) into an autoclave operating at 121°C under a vapor pressure of 0.1 MPa (accelerating test) in order to simulate the effect of environmental aging in human body. Raman spectroscopic analyses showed no detectable contents of monoclinic phase in the samples before exposure to acceleration test in autoclave, independent of sintering temperature. Therefore, the as-sintered samples always consisted of the tetragonal and of a minor fraction of the cubic phases. The results of acceleration tests (shown in Figure 12) are given as a function of autoclave aging time and sintering temperature. The remarkable result here is that the samples sintered at temperatures below about 1375°C showed amounts of t→m transformation 1) between c and a lattice parameters of a tetragonal cell (i.e., c/a=1 in a cubic structure). A confirmation of the change in tetragonality with presintering temperature can be obtained according to the results of a Rietveldt analysis on Xray diffraction data, as shown in Figure 13.

Figure 13. Dependence of crystal lattice tetragonality ratio, c/a, on sintering temperature for the series of polycrystalline zirconia materials shown in Figure 10.

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A comparison between data in Figures 12 and 13 clearly indicates that as the tetragonality of the polycrystal decreases approaching unity, phase stability increases. A c/a ratio close to unity arises from a larger amount of cubic phase in the polycrystalline network. Yashima et al. [98] reported that tetragonality approaches the value 1 with increasing the amount of the oxide stabilizer added during sintering to the zirconia polycrystal. Moreover, Kim et al. [99] reported that small amounts of niobium oxide, when added to yttria-doped zirconia, have a detrimental effect on phase stability due to an increase of polycrystal tetragonality. The results newly shown in this chapter demonstrate that lowering sintering temperature and, thus, refining the grain size of yttria-doped zirconia polycrystal can effectively bring the tetragonality ratio closer to unity, so that phase stability can be greatly improved. The advantages, in terms of strength, of producing a polycrystal with a fine grain size have been deeply discussed in a recent paper by Eichler et al. [100]. In the analysis of those researchers, in the case of nanocrystalline zirconia, three different contributions of grain size to the mechanical behavior were considered, according to previous literature: (i) as a general pattern for ceramic materials, an increase in strength was foreseen as arising from a decrease in grain size; (ii) below a critical grain size, a change in the fracture criterion needed to be considered to explain an increase in strength of the material; and, (iii) a decrease in transformation-induced toughening, which was directly related to the effect of hydrothermal degradation (and thus in turn depended on grain size) could be recognized. The salient outcomes of the investigation, mainly focused on the effect of hydrothermal aging on strength, revealed that up to a critical value of 200 nm in size for the zirconia grains, hydrothermal degradation of strength was found to be limited, while materials with larger grain sizes exhibited either premature failure or an increase in strength. The decrease in fracture strength caused by hydrothermal degradation for grain sizes up to 200 nm was small and the scatter was low, according to a phase-stabilizing effect of small grain sizes. Fracture strength at higher grain sizes was affected by a surface transformation zone that formed during the autoclave treatment and that caused either a shift of the fracture origin toward the interior of the sample (i.e., in the case of an increase in strength) or the formation of a macroscopic crack (i.e., in the case of a decrease in strength). This body of evidences confirms the positive effect of small grain sizes on the resistance of the material to hydrothermal environment and links such a positive effect to the mechanical properties of the material. The past, dramatic failure events encountered by zirconia femoral heads in the year 2001 have opened strong and controversial issues on the suitability of zirconia as a biomaterial. Chevalier [34] has recently reviewed and analyzed the current knowledge on ageing process and its effect on the long-term performance of implants with the strenuous and constructive aim of distinguishing between scientific facts and speculation. That review has pointed out the current state of the art in the literature and demonstrated a strong variability of different zirconia grades to in vivo degradation, as a consequence of the strong influence of processing parameters on ageing kinetics. Different zirconia from different makers may have different microstructures and there is an impellent need to assess their ageing sensitivity with using advanced and accurate techniques. A full understanding of the physics and chemistry behind hydrothermal transformation of zirconia polymorphs can

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only be beneficial to material manufacturers and open for them the path to a fully exploitation of this unique material. In addition, standards should be modified in order to ensure the structural reliability of the hip components made of zirconia, thus obtaining full confidence from clinicians. In this context, the clarification, as shown in this chapter, of the role of grain size on the hydrothermal stability of polycrystalline zirconia may represent an initial and fundamental step in future research. The practice of pushing the grain size of polycrystalline biomaterials toward a nanoscopic scale, as successfully shown by JMM Co., is likely to lead to outstanding results in terms of reliability in the near future. It should be also considered that nanostructured materials possess unique capabilities for specific interactions with biological entities. The processing methods for obtaining ceramics can thus be tailored to control fundamental properties of the joint surfaces, such as wettability, surface morphology and charge, that directly affect interactions with biological entities (e.g., ions, lubricant bio-macromolecules and cells) [101]. Such interactions are indeed already known to directly depend on the nanostructure of the ceramic surfaces. Thus, it is expected that, through a full understanding and control of the interactions between nanostructured ceramics and biological entities, major developments could be soon achieved.

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Statistical and Deterministic Scrutiny of Reliability in Implants Made of Advanced Composite Advanced studies on the mechanics of the human hip joint have revealed the complexity of the load-distribution mechanisms on the femoral head [102-105]; for example, it has been shown that the inter-cartilaginous space is variable in size and location [106, 107], and that it changes in shape with different positions of the joint [108]. In addition, the pressure distribution in the hip at the cartilage interface has been measured during load bearing, either according to direct or indirect methods. Measurements have mostly involved the insertion of piezo-electric transducers in the acetabulum [109, 110], as well as the use of pressuresensitive films [103, 104, 111, 112] or instrumented endoprostheses [102, 105, 113, 114], in order to obtain the pressure contours on the cartilage when the hip was loaded. An area of high pressure could be identified in the anterosuperior segment of the joint, a site that coincides with the most usual location of the progressive cartilage loss seen in osteoarthritis. Most joints of the older population show this pattern, with a 3 % prevalence of osteoarthritis of the hip and with a detrimental factor being the pressure gradient around the high-pressure sites. Experimental contour maps usually show tortuous shapes, which may be influenced by surface irregularities; an alternative explanation has been given that, when a spherical head mates with a spherical acetabulum, unusual contour patterns arise through local differences in stiffness of the cartilage or underlying cortical bone. A combination of both phenomena is responsible for the observed patterns. The contact pressures experimentally measured are steeply graded and represent the radial stresses perpendicular to the contact surface between femoral head and acetabular cup. The application of external load also generates secondary stresses acting parallel to the joint surface, which are difficult to measure. Such secondary stresses can be of greater or smaller

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magnitude than the radial ones, depending on local geometry and stiffness of the cartilage. The most relevant issue here is that pressure differences within the area of steep pressure gradients would set up forces within the cartilage, thus producing tensile stresses in the collagen fibers running in the direction of the pressure gradient (i.e., lateromedial in the case of the hip joint). Afoke et al. [111] have estimated that the tensile stress induced in the collagen fibers experiences a magnitude of about 12 times the applied pressure (peak value of about 10 MPa); thus, the maximum tensile stress the collagen fibers would have to sustain is about 120 MPa. Although at present it is not possible to relate this stress to the ultimate tensile strength of collagen fibers, it would seem that the structure of cartilage could cope with pressure gradients in the lateromedial direction since the collagen fibers in its superficial layer are predominantly aligned in the lateromedial direction. While advanced biomechanics studies seem to unequivocally remind us how much even the most advanced designs of hip joints are yet far behind in reproducing the natural working conditions of hip bearing surfaces, they also provide us with basic parameters for evaluating new synthetic materials. The intrinsic strength of a synthetic ceramic crystal can be estimated as more than one order of magnitude larger than the maximum tensile stress piled up in the collagen fibers of a femoral head [115, 116]. However, in contrast to the high intrinsic strength of a synthetic ceramic crystal, the strength of a ceramic polycrystalline body typically shows significantly lower values and a high scatter [115]. It is important to understand that failure in a ceramic polycrystal is always triggered by imperfections of the microstructure [117]. Ceramics with high structural performance can only be achieved when the size of inherent defects in the material is very small. Typical relevant imperfections in advanced ceramics are comprised within a range of 5– 50 µm in size. Accordingly, the available strength can be lowered by at least one order of magnitude with respect to the inherent strength of the corresponding (unflawed) ceramic crystal. The scatter in size of inherent defects directly matches the scatter in strength, which can be in turn described according to a statistical parameter referred to as the Weibull modulus [118]. A high value of the modulus indicates low scatter in strength. For highperformance structural biomaterials, a Weibull modulus of ≥ 7 is tolerated in the specification; however, the Weibull modulus measured in the most advanced structural biomaterials can be pushed towards significantly larger values [76, 119]. Nowadays, quite extensive and systematic efforts are made in statistically analyzing the strength properties of advanced biomaterials for hip joints, batch by batch from the production line. The strength properties should be determined according to the standard ISO 6474, which is applicable for pure alumina and currently being extended to alumina-based composite materials (ISO 6474-2). In general, the performance of a hip joint system depends not only on the intrinsic material properties, but also on design and manufacturing quality of its components, on external load, and on environmental conditions (i.e., leaving aside the quality of surgical operations like as joint mounting and installation). The adoption of high strength materials can greatly ameliorate the performance of a hip system; however, other factors may be even more decisive for the success of a system; therefore, design analyses, modeling, simulations, non-destructive evaluations and risk analyses must be necessarily used to evaluate these complex factors and their correlations. The most basic assessments of materials and systems for hip joints rely on the setup of a 4-point bending test (e.g., as

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recommended in ISO 6474-2) and of the burst strength test (i.e., according to ISO 7206), according to the drafts shown in Figures 14 (A) and (B), respectively.

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Figure 14. Drafts showing the procedures for structural assessments of materials and systems for hip joints: (A) 4-point bending test (ISO 6474-2); and, (B) burst strength test (ISO 7206).

The bending test reveals the intrinsic (statistical) strength of the material whereas the burst test is designed in order to simulate the in vivo loading conditions of a ceramic ball head. Unlike early ceramic acetabular components, which were not subject to widespread use due to their poor structural reliability and fixation problems [120], contemporary aluminabased ceramic composites guarantee an improved quality due to progress in material and system design, manufacturing, and proof testing [38, 121]. Accordingly, ceramics on both the acetabular and the femoral bearing surfaces are becoming increasingly popular. In most statistical reports, mechanical failure of the ceramics is exceedingly rare with these modern ceramics (cf. also recent statistics as shown in Figure 1(B)). How such a remarkable statistical reduction in the number of fracture events in ceramic hip joints has been possible in the last decade? The answer is certainly complex and leads to the identification of several key-factors. Among those factors, the contributions of two should be mainly considered: (i) the significant improvement achieved in the last decade for ceramic manufacturing processes, including sintering and machining techniques [38]; and, (ii) the introduction of a deterministic proof-testing procedure, according to which each individual component is tested with respect to its structural reliability before being released to the market [122, 123].Despite pioneering efforts from the point of view of both design and manufacturing issues, early alumina materials for hip joints were based on polycrystalline ceramics that were developed for completely different industrial applications. Some of the materials had insufficient purity, low density, and a coarse grained microstructure, which unavoidably led to a poor mechanical strength of the joint. Since, as discussed above, mechanical strength is correlated to reliability and fracture rate, old grade alumina materials were inadequate for biomedical applications. In order to counteract this problem, the ISO

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standard 6474-2, was set up to qualify the ceramics used in hip prostheses. In other words, there is now a standard available which mandatorily requires much better materials than old alumina grades. Three main stages can be recognized in the development of medical-grade alumina materials, usually referred to as its three generations (the first generation being the oldest). Main advancements in material processing can be located in purity enhancement, grain-size refinement and density improvement, leading to an enhanced mechanical behavior. A close correlation in fact exists between mechanical strength and these microstructural characteristics. Control of porosity (usually localized at grain boundaries) and grain size in polycrystalline alumina plays an important role in the enhancement of mechanical strength and in the reduction of wear rate [124]. Therefore, eliminating pores and minimizing grain size have been considered to be two crucial targets in improving medical-grade alumina materials. Modern alumina grades are hot isostatically pressed (HIPed), thus being sintered with the help of high pressure, in order to minimize grain growth and concurrently to achieve full densification with fine grain structure. A comparison between the microstructure of early generation and modern alumina materials is given in Figure 15 (in (A) and (B), respectively). Table I summarizes the processing conditions and the salient parameters of alumina-based materials belonging to different generations [38, 121]. As can be seen, the improvement in structural properties is remarkable and it is clearly accompanied by an improved degree of microstructural control. The most recent challenge in microstructural design of alumina-based ceramics has been that of developing an improved material, which not only maintains all the advantageous properties of the third-generation alumina materials, but also allows new applications that require high mechanical loading bearing capability. CeramTec AG has launched since the year 2000 a new composite material, consisting of an alumina matrix added with fine zirconia dispersoids and with a selection of additional mixed oxides additives (in other words, a zirconia-toughened alumina), which has shown the potential for greatly enhancing the mechanical properties and the reliability of the third-generation monolithic alumina ceramics.

Figure 15. Comparison between the microstructures of alumina-based materials belonging to different generations: (A) first (i.e., the oldest) generation monolithic alumina; (B) third generation monolithic alumina (Biolox®forte, from CeramTec AG); and, (C) zirconia-toughened alumina composite (Biolox®delta, from CeramTec AG). Micrographs displayed in (B) and (C) are courtesy of Dr. M. Kuntz.

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Table I. Processing conditions and the salient microstructural and structural parameters of alumina-based materials belonging to different generations. “Advanced composite” refers to Biolox®delta, from CeramTec AG GENERATION Hardness HV Bending Strength (MPa) Average Grain Size (μm) Density (g cm-3) HIP Proof-tested

I

II

III

ADVANCED COMPOSITE

1800 450 ≤4.5 3.86 No No

1900 500 ≤3.2 3.94 No No

2000 620 1.75 3.97 Yes Yes

1700 1400 0.54 4.37 Yes Yes

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In Table I, a comparison is given between the mechanical properties of this new composite material and the previous generations of monolithic alumina materials. The unique properties of this new composite from CeramTec AG rely on its fine microstructure, which is mainly achieved through the combined effect of a sintering process (including a HIPing cycle) and of solid-state reactions between different ceramic phases occurring during sintering. A comparison among the microstructure of the new composite and those from monolithic alumina materials belonging to different generations is shown in Figure 15. Zirconia covers a fraction of about 17 % of the total volume of the composite material, mainly as a tetragonal polymorph. The tetragonal zirconia phase of the composite is partially stabilized both chemically and mechanically. The high strength and toughness reported for the material (cf. Table I) can be mainly achieved through a transformation toughening mechanism, which is also responsible for the excellent reliability properties, systematically reproduced batch by batch with a very low scatter. A Raman spectroscopic visualization of the toughening mechanism, occurring on the microstructural scale at the tip of a propagating crack, is shown in Figure 16, as obtained according to advanced in situ spectroscopic techniques.

Figure 16. (A) monoclinic phase transformation map and (B) equilibrium stress field in the neighborhood of the tip of a crack propagating in zirconia-toughened alumina composite (Biolox®delta, from CeramTec AG). The plus and minus signs applied to the stress magnitudes represent tensile and compressive stresses (red and blue colored locations in the map in (B)).

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Raman maps clarify that phase transformation occurs in the neighborhood of the crack tip (Figure 16(A)) and contributes to shield the propagating crack by means of a compressive (equilibrium) stress field (Figure 16(B)). In other words, the microstructure of the zirconiatoughened alumina composite is specially designed in order to provide an optimum of resistance against fracture initiation and crack extension. Figure 17 shows a comparison between the Weibull moduli of the newest monolithic alumina and of the advanced alumina-zirconia composite fabricated by CeramTec AG. The plots clearly show the significance of the improvement in the reliability level achieved in the composite material with respect to the monolithic one. Similar to the results of intrinsic strength, also the results of burst strength tests (according to ISO 7206) showed for the newly developed composite a remarkable improvement above the most recent generation of alumina femoral heads. Table II shows a comparison between intrinsic strength and burst loads (for different diameters of the femoral head) in the most recently developed monolithic alumina and in the zirconia-toughened alumina composite. Burst critical loads were found in both materials always far above the average value of 46 kN required by regulation (i.e., the FDA’s “Guidance document for the preparation of pre-market notifications for ceramic ball hip systems”), and, in the case of the composite, were systematically higher than in the monolithic material, independent of the diameter of the femoral head tested. It should be also noted that the maximum load applied in vivo on a femoral head could be estimated as approximately 10 kN, where the lower limit for the critical load for a ball head in the abovementioned FDA’s guidance document is 20 kN.

Figure 17. Plots of failure probability (Weibull plots) in 4-point bending test (ISO 6474-2) for monolithic alumina (Biolox®forte, from CeramTec AG) and zirconia-toughened alumina composite (Biolox®delta, from CeramTec AG).

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Table II. Comparison between intrinsic strength and burst loads (for different diameter, d, of the femoral head) recorded for the most recently developed monolithic alumina composite (Biolox alumina (Biolox®forte, from CeramTec AG) and for an advanced zirconia-toughened ®delta, from CeramTec AG) Parameter

Test/Design

Unit

BIOLOX®forte

BIOLOX®delta

Improvement ratio

Strength

4-point bending

MPa

620

1400

2.3

Burst Load

d= 28 mm

kN

54

85

1.6

Burst Load

d= 36 mm

kN

110

131

1.2

Due to its statistical nature, the strength also depends on specimen size and on stress distribution (i.e., on the design of the joint); while size effects can be mathematically balanced, the loading geometry should be fixed if one wants to obtain comparable results among different materials. The data of burst tests given in Table II are obtained from femoral heads with identical geometry and tested with exactly the same setup. Thus, the higher burst load of femoral heads made of the composite with respect to those made of the monolithic alumina material arises from the higher intrinsic strength of the material. It is also seen that the burst load strongly depends on the ball head size; the larger the diameter of the ball head the higher the burst load. It could be concluded that the use of larger sizes always increases the in vivo safety margin of the component. However, it should be noted that, although the intrinsic strength of the composite is improved by 2.3 times above that of the monolithic material, the composite ball heads show an improvement in burst strength of only 1.6 and 1.2 times as compared to monolithic ball heads with the same geometry in the case of small and large diameters, respectively (cf. the column “improvement ratio” in Table II, which refers to the strength or load ratio between the composite and the monolithic components). In other words, the achievable benefit in terms of structural properties in using the composite instead of the monolithic material strongly depends on the specific design adopted. Besides the above-documented intrinsic reinforcing mechanism operating on the microstructural scale of the material, it is also necessary to examine if environmental exposure leads to any damage or loss in bulk strength. For this purpose experiments can be designed in order to combine accelerated aging in an aggressive environment and mechanical loading at high stress level. Samples can be tested in 4-point bending configuration (cf. testing configuration depicted in Figure 14(A)) after exposure to accelerated aging and then to loading-unloading fatigue cycles under high stress. The residual strength of the material can be then monitored after aging and cyclic loading. Table III shows the results of fatigue tests, which were conducted at two maximum stress levels (300 MPa and 600 MPa) under cyclic loading. The lower and the higher stress levels were applied for 20 and 5 million cycles, respectively. Accelerated aging was simulated by 5 and 100 h treatment in standard autoclaving conditions (134°C, 2 bar water steam). The outstanding performance of the new composite material is clearly expressed in the fact that the yield of specimen surviving in all tests was 100 % for all investigated conditions.

Aoi, T., and A. Toshida. Hip Replacement : Approaches, Complications and Effectiveness, Nova Science Publishers, Incorporated, 2009. ProQuest Ebook Central,

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33

Table III. Results of residual strength (4-point bending test, ISO 6474-2) after fatigue and autoclaving tests. Two maximum stress levels for fatigue loading were monitored (300 MPa and 600 MPa) Autoclaving duration 0h 5h 100 h

Strength (MPa) Monoclinic phase content (%) Strength (MPa) Monoclinic phase content (%) Strength (MPa) Monoclinic phase content (%)

no cyclic load

300 MPa, 20x106 cycles

600 MPa, 5x106 cycles

1346 18 1332 22 1234 30

1433 33 1248 33 1308 33

1284 44 1361 42 1300 47

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This result is even most relevant if one considers that the higher stress level selected for cyclic fatigue represents four times the highest loading level experienced by a femoral head during in vivo loading. From a statistical point of view, it is thus possible to conclude that the reliability of advanced load-bearing ceramics for arthroplastic use has been pushed towards values that exceed by far the standard requirements needed by reliable surgical components. In order to ensure on a fully deterministic scale the structural reliability of the femoral heads made either of monolithic alumina or of the zirconia-toughened alumina composite, systematic proof testing in burst loading geometry are systematically performed on each of the femoral heads produced, before they are released in the market. In a proof test, a static load of a sub-critical level is applied to the femoral head, which is comparable in magnitude but higher than the expected maximum load under physiological conditions. Femoral heads with internal flaws, and thus with a low strength, fracture and can be thus systematically eliminated. Such a test represents a quite severe structural check. In particular, it allows eliminating from the production batches the minimal fraction of pieces that were already flawed since the manufacturing procedure. We can thus conclude that, at the present stage of development in manufacturing and in non-destructive evaluation, new ceramic materials appear to have achieved their maturity as structural components to be fully and safely employed in hip arthroplasty.

Conclusion In summary, from a critical review of the key-events and of the most recent research achievements in materials for hip prostheses, we have noticed that with the proliferation of several types and classes of new materials, the screening, evaluation and integration of these new materials into the biomaterial market require more comprehensive and effective approaches. Statistical and deterministic evaluations of the properties of new materials are necessarily required in order to reduce the number of failures and, accordingly, their negative economic impact on the society [125]. New evaluation tools involve multiple complementary metrology approaches, while correlating results from these analyses has traditionally been very time consuming and has led in the past to ambiguous conclusions. To overcome these

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Giuseppe Pezzotti

limitations, we have suggested here the use of advanced Raman spectroscopic tools that have a potential to rapidly screen and to evaluate chemical, crystallographic and mechanical properties of new materials. The spectroscopic approach could greatly help to spread new trust in advanced materials, through the process of their integration process in the market. The end results could be to obtain safer products while shortening both material screening and characterization time. Due to surface oxidation issues and to the related osteolysis problems caused by polymer wear debris, the interest in ceramic materials as bearing components for total hip implants continues to grow. The environmental stability and the mechanical performance of ceramic materials are strongly related to their grain size, and thus to the manufacturing process. A fine and controlled microstructure can significantly enhance the mechanical strength of the material and its reliability as well. Significant improvement of medical-grade alumina and zirconia materials has been recently achieved and the modern materials for hip prostheses not only experience rather low wear rates but also a minimal amount of fracture events. It has been emphasized that ceramic-on-ceramic articulations necessarily rely on the precise orientation of the components in order to avoid impingement; and, modularity in terms of femoral neck length is somewhat restricted. However, it appears that concerns arising from the brittleness of ceramic materials and, thus, from their vulnerability to fracture due to unexpected load situation, can be successfully counteracted by exploiting synergistic effects in alumina-zirconia ceramic composites. Such an approach has recently enabled the obtainment of a new composite material, whose fracture resistance is greatly enhanced by a crack-shielding effect induced by phase-transformation processes. Component fractures will nevertheless remain a concern, but the mechanisms of fracture have been for the most part fully identified and controlled, learning our lessons from the past. With the aid of modern technologies, non-destructive evaluations, deterministic proof testing and meticulous intraoperative techniques, ceramic joint components will become available that can be used with a significantly improved degree of confidence. With the reduction in wear rate and fracture events a significantly durable hip replacement with the potential for a service life spanning several decades now may be possible.

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References [1] [2]

[3] [4]

Learmonth I, Young C, Rorabeck C. The operation of the century: total hip replacement. The Lancet 2007; 370(9597): 1508-1519. Kurtz S, Mowat F, Ong K, Chan N, Lau E, Halpern M. Prevalence of primary and revision total hip and knee arthroplasty in the United States from 1990 through 2002. J. Bone Joint Surg. Am. 2005; 87(7): 1487-1497. Jones CP III, Kelley S. Cementless fixation of the femoral stem. Hip reconstruction. Current Opinion in Orthopedics 2001; 12(1): 52-56. Oparaugo PC, Clarke IC, Malchau H, and Herberts P. Correlation of wear debrisinduced osteolysis and revision with volumetric wear-rates of polyethylene: A survey of 8 reports in the literature. Acta Orthopaedica 2001; 72(1): 22-28.

Aoi, T., and A. Toshida. Hip Replacement : Approaches, Complications and Effectiveness, Nova Science Publishers, Incorporated, 2009. ProQuest Ebook Central,

Counteracting Reliability Problems in Advanced Hip Prostheses [5] [6]

[7]

[8] [9]

[10]

[11]

[12] [13] [14]

[15]

Copyright © 2009. Nova Science Publishers, Incorporated. All rights reserved.

[16]

[17]

[18] [19]

[20]

35

Zhu YH, Chiu KY, Tang WM, Review article: Polyethylene wear and osteolysis in total hip arthroplasty. J. Orthopaedic Surg. 2001; 9(1): 91-99. Jacobs JJ, Skipor, Patterson LM, Hallab NJ, Paprosky WG, Black J, Galante JO. Metal release in patients who have had a primary total hip arthroplasty. A prospective, controlled, longitudinal study. J. Bone Joint Surg. Am. 1998; 80(10): 1447-1458. Willert HG, Buchhorn GH, Fayyazi A, Flury R, Windler M, Koester G, Lohmann CH. Metal-on-metal bearings and hypersensitivity in patients with artificial hip joints. A clinical and histomorphological study. J. Bone Joint Surg. Am. 2005; 87(1): 28-36. Hasegawa M, Sudo A, Hirata H, Uchida A. Ceramic acetabular liner fracture in total hip arthroplasty with a ceramic sandwich cup. J. Arthroplasty 2003; 18(5): 658–661. Park YS, Hwang S-K, Choy W-S, Kim Y-S, Moon Y-W, and Lim S-J, Ceramic Failure After Total Hip Arthroplasty with an Alumina-on-Alumina Bearing. J. Bone Joint Surg. Am. 2006; 88(4): 780-787. Santavirta S, Boehler M, Harris WH, Konttinen YT, Lappalainen R, Muratoglu O, Rieker C, Salzer M. Alternative materials to improve total hip replacement tribology. Acta Orthop. Scand. 2003; 74(4): 380-388. Clarke IC, Good V, Williams P, Schroeder D, Anissian L, Stark, Oonishi H, Schuldies J, Gustafson G. Ultra-low wear rates for rigid-on-rigid bearings in total hip replacements. Proc. Inst. Mech. Eng., Part H: Journal of Engineering in Medicine 2000; 214(4): 331-347. Garino JP. Modern ceramic-on-ceramic total hip systems in the United States: early results. Clin. Orthop. Relat. Res. 2000; 379: 41-47. Gordon AC, D’Lima D, Colwell Jr. CV. Highly cross-linked polyethylene in total hip arthroplasty. J. Am. Acad. Orthop. Surg. 2006; 14(9): 511-523. Jacobs CA, Christensen CP, Greenwald AS, McKellop H. Clinical Performance of Highly Cross-Linked Polyethylenes in Total Hip Arthroplasty. J. Bone Joint Surg. Am. 2007; 89(12): 2779-2786. Gordon AC, D'Lima DD, Colwell Jr. CW. Highly Cross-linked Polyethylene in Total Hip Arthroplasty. J. Am. Acad. Orthop. Surg. 2006; 14(9): 511-523. Endo MM, Barbour PS, Barton DC, Fisher J, Tipper JL, Ingham E, Stone MH. Comparative wear and wear debris under three different counterface conditions of crosslinked and non-crosslinked ultra high molecular weight polyethylene. Biomed. Mater. Eng. 2001; 11(1): 23-35. Premnath V, Harris WH, Jasty M, Merrill EW. Gamma sterilization of UHMWPE articular implants: an analysis of the oxidation problem. Biomaterials 1996; 17(18): 1741-1753. Heisel C, Silva M, Schmalzried TP. Bearing Surface Options for Total Hip Replacement in Young Patients. J. Bone Joint Surg. Am. 2003; 85(7): 1366-1379. Bradford L, Baker DA, Graham J, Chawa A, Ries MD, Pruitt LA. Wear and surface cracking in early retrieved highly cross-linked polyethylene acetabular liners. J. Bone Joint Surg. Am. 2004; 86-A(6): 1271-1282. D’Antonio J, Capello W, Manley M, Bierbaum B. New experience with alumina-onalumina ceramic bearings for total hip arthroplasty. J. Arthroplasty 2002; 17(4): 390397.

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[21] Rahaman MN, Yao A, Sonny Bal B, Garino JP, Ries MD. Ceramics for Prosthetic Hip and Knee Joint Replacement. J. Am. Ceram. Soc. 2007; 90(7): 1965–1988. [22] Bizot P, Banallec L, Sedel L, Nizard R. Alumina-on-Alumina Total Hip Prostheses in Patients 40 Years of Age or Younger. Clin. Orthop. Rel. Res. 2000; 379: 68–76. [23] Boehler M, Plenk Jr. H, Salzer M. Alumina Ceramic Bearings for Hip Endoprostheses: The Austrian Experiences. Clin. Orthop. Rel. Res. 2000; 379: 85–93. [24] Allain J, Roudot-Thoraval F, Delecrin J, Anract P, Migaud H, Goutallier D. Revision Total Hip Arthroplasty Performed after Fracture of a Ceramic Femoral Head. A Multicenter Survivorship Study. J. Bone Joint Surg. Am. 2003; 85(5): 825–830. [25] Hannouche D, Nich C, Bizot P, Meunier A, Nizard R, Sedel L. Fractures of Ceramic Bearings: History and Present Status. Clin. Orthop. Rel. Res. 2003; 417: 19–26. [26] National Joint Registry of England and Wales (NJR): 4th Annual Report. 2008. Available from: URL: www.njrcentre.org.uk. [27] Sutula LC, Collier JP, Saum KA, Currier BH, Currier JH, Sanford WM, Mayor MB, Wooding RE, Sperling DK, Williams IR. Impact of gamma sterilization on clinical performance of polyethylene in the hip. Clin. Orthop. 1995; 319: 28-40. [28] Oonishi H, Kadoya Y, Masuda S. Gamma-irradiated cross-linked polyethylene in total hip replacements—analysis of retrieved sockets after long-term implantation. J. Biomed. Mater. Res. Part A 2001; 58(2): 167-171. [29] Visentin M, Stea S, Squarzoni S, Reggiani M, Fagnano C, Antonietti B, Toni A. Isolation and characterization of wear debris generated in patients wearing polyethylene Hylamer inserts, gamma irradiated in air. J. Biomater. Applications 2005; 20(2): 103-121. [30] Bargmann LS, Bargmann BC, Collier JP, Currier BH, Mayor MB. Current sterilization and packaging methods for polyethylene. Clin. Orthop. 1999; 369: 49-58. [31] Fisher J, Hailey JL, Chan KL, Shaw D, Stone M. The effect of ageing following irradiation on the wear of UHMWPE. Trans. Orthop. Res. Soc. 1995; 20: 120-128. [32] Clarke IC, Manaka M, Green DD, Williams P, Pezzotti G, Kim Y-H. Current Status of Zirconia Used in Total Hip Implants. J. Bone Joint Surg. Am. 2003; 85(1): 73-84. [33] Tateishi T, Yunoki H. Research and development of advanced biocompatible materials and applications to the artificial hip joint. Bull. Mech. Eng. 1987; 45(1): 1-9. [34] Chevalier J. What future for zirconia as a biomaterial?. Biomaterials 2006; 27(4): 535546. [35] Brown SS, Green DD, Pezzotti G, Donaldson TK, Clarke IC. Possible triggers for phase transformation in zirconia hip balls. Journal of Biomedical Material Research B: Applied Biomaterials 2008; 85(2): 444-452. [36] Heros RJ, Willman G. Ceramic in total hip arthroplasty: history, mechanical properties, clinical results, and current manufacturing state of the art. Semin. Arthroplasty 1998; 9: 114-122. [37] Hasegawa M, Sudo A, Hirata H. Ceramic acetabular liner fracture in total hip arthroplasty with a ceramic sandwich cup. J. Arthroplasty 2003; 18: 658-661. [38] Tateiwa T, Clarke IC, Williams PA, Garino J, Manaka M, Shishido T, Yamamoto K, Imakiire A. Ceramic total hip arthroplasty in the United States: safety and risk issues revisited. Am. J. Orthop. 2008; 37(2): E26-31.

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Counteracting Reliability Problems in Advanced Hip Prostheses

37

[39] Garino J, Rahaman MN, Sonny Bal B. The reliability of modern alumina bearings in total hip arthroplasty. Semin. Arthro. 2006; 17: 113-119. [40] Kelley SS, Lachiewicz PF, Hickman JM, Paterno SM. Relationship of femoral head and acetabular size to the prevalence of dislocation. Clin. Orthop. 1998; 355: 163-170. [41] Hedlundh U, Ahnfelt L, Hybbinette CH, Wallinder L, Wecksreom J, Fredin H. Dislocations and the femoral head size in primary total hip arthroplasty. Clin. Orthop. Relat. Res. 1996; 333: 226-233. [42] Bader R, Scholz R, Steinhauser E, Zimmermann S, Busch R, Mittelmeier W. The influence of head and neck geometry on stability of total hip replacement: A mechanical test study. Acta Orthop. Scand. 2004; 75(4): 415-421. [43] Beaule P, Schmalzried T, Udomkiat P, Amstutz H. Jumbo femoral head for the treatment of recurrent dislocation following total hip replacement. J. Bone Joint Surg. Am. 2002; 84: 256-263. [44] Bartz RL, Noble PC, Kadakia NR, Tullos HS. The effect of femoral component head size on posterior dislocation of the artificial joint. J. Bone Joint Surg. Am. 2000; 82: 1300-1307. [45] Burroughs BR, Hallstrom B, Golladay GJ, Hoeffel D, Harris WH. Range of motionand stability in total hip arthroplasty with 28-, 32-, 38-, and 44-mm femoral head sizes. J. Arthroplasty 2005; 20(1): 11-19. [46] James SP, Blazka S, Merrill EW, Jasty M, Lee KR, Bragdon CR, Harris WH. Challenge to the concept that UHMWPE acetabular components oxidize in vivo. Biomaterials 1993; 14(9): 643-647. [47] Affatato S, Zavalloni S, Taddei P, Di Foggia M, Fagnano C, Viceconti M. Comparative study on the wear behaviour of different conventional and cross-linked polyethylene’s for total hip replacement. Tribology International 2008; 41(8): 813-822. [48] McKellop HA, Shen FW, Yu YJ, Lu B, Salovey R. Effect of sterilization method on the wear rate of UHMWPE acetabular cups in a hip simulator. In: Transactions of the 43rd Annual Meeting, Orthopedic Research Society 1997. San Francisco: 94-99. [49] Premnath V, Harris WH, Jasty M, Merrill EW. Gamma sterilization of UHMWPE articular implants: an analysis of the oxidation problem. Biomaterials 1996; 17(18): 1741–1753. [50] Tabb DL, Sevcik JJ, Koenig JL. FTIR study of the effects of irradiation on polyethylene. J. Polym. Sci. Polym. Phys. 1976; 13: 815-824. [51] del Prever EB, Crova M, Costa L, Dallera A, Camino G, Gallinaro P. Unacceptable biodegradation of polyethylene in vivo. Biomaterials 1996; 17(9): 873– 878. [52] Fanconi BM. Chain scission and mechanical failure of polyethylene. J. Appl. Phys. 1983; 54(10): 5577–5582. [53] Bapst JM, Valentine RH, Vasquez R. Wear simulation testing of direct compression molded UHMWPE irradiated in oxygenless packaging. In: Transactions of the 23rd annual meeting of the Society for Biomaterials 1997; New Orleans: pp. 72-78; [54] St John KR, Takeuchi MJ, Poggie RA. Effect of radiation dose and packaging conditions on the oxidation of UHMWPE. In: Transactions of the 23rd annual meeting of the Society for Biomaterials 1997; New Orleans: pp. 46-51.

Aoi, T., and A. Toshida. Hip Replacement : Approaches, Complications and Effectiveness, Nova Science Publishers, Incorporated, 2009. ProQuest Ebook Central,

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38

Giuseppe Pezzotti

[55] Streicher RM. Influence of ionizing irradiation in air and nitrogen for sterilization of surgical grade PE for implants. Radiat. Phys. Chem. 1988; 31(4-6): 693– 698. [56] Sun DC, Wang A, Stark C, Dumbleton JH. The concept of stabilization in UHMWPE. In: Transactions of the Society for Biomaterials 5th World Congress 1996; Toronto: pp. 195-201. [57] Trieu HH, Avent R. Effect of sterilization on shelf-aged UHMWPE tibial inserts. In: Transactions of the 21st annual meeting of the Society for Biomaterials 1995; San Francisco: pp. 109-114. [58] Fisher J, Reeves EA, Isaac GH, Saum KA, Sanford WM. Comparison of the wear of aged and non-aged UHMWPE sterilized by gamma irradiation and by gas plasma. In: Transactions of the Society for Biomaterials 5th World Congress 1996; Toronto: pp. 971-976. [59] Li S, Burstein AH. Current concepts review: UHMW polyethylene. J. Bone Joint Surg. Am. 1994; 76(7): 1080-1090. [60] Schmalzreid TP, Callaghan JJ. Current concepts review: Wear in total hip and knee replacements. J. Bone Joint Surg. Am. 1999; 81(1): 115-136. [61] Edidin AA, Pruitt L, Jewett CW, Crane D, Roberts D, Kurtz SM. Plasticity-induced damage layer is a precursor to wear in radiation-crosslinked UHMWPE acetabular components for total hip replacement. J. Arthroplasty 1999; 14(5): 616-627. [62] Pezzotti G, Kumakura T, Yamada K, Tateiwa T, Puppulin L, Zhu W, Yamamoto K. Confocal Raman spectroscopic analysis of cross-linked ultra-high molecular weight polyethylene for application in artificial hip joints. J. Biomed. Opt. 2007; 12(1): 014011-1-14. [63] Pezzotti G. Stress microscopy and confocal Raman imaging of load-bearing surfaces in artificial hip joints. Expert Rev. Med. Devices 2007; 4(2): 165-189. [64] Pezzotti G. Raman piezo-spectroscopic analysis of natural and synthetic biomaterials. Anal. Bioanal. Chem. 2005; 381: 577-590. [65] Tabb DL, Sevcik JJ, Koenig JL. FTIR study of the effects of irradiation on polyethylene. J. Polym. Sci. Polym. Phys. 1975; 13: 815– 824. [66] Shen F-W, Yu Y-J, McKellop H. Potential Errors in FTIR Measurement of Oxidation in Ultrahigh Molecular Weight Polyethylene Implants. J. Biomed. Mater. Res. 1999; 48(3): 203-210. [67] Lewis IR, Edwards HGM. Handbook of Raman Spectroscopy: From the Research Laboratory to the Process Line. New York (USA) and Basel (Switzerland); Marcel Dekker Ltd: 2001. [68] Taddei P, Affatato S, Fagnano C, Bordini B, Tinti A, Toni A. Vibrational spectroscopy of ultra-high molecular weight polyethylene hip prostheses: influence of the sterilization method on crystallinity and surface oxidation. J. Mol. Struct. 2001; 613: 121-129. [69] Eyerer P, Ke YC. Property changes of UHMW polyethylene hip cup endoprostheses during implantation. J. Biomed. Mater. Res. 1984; 18: 1137-1151. [70] Costa L, Luda MP, Trossarelli L, Brach del Prever EM, Crova M, Gallinaro P. Oxidation in orthopaedic UHMWPE sterilized by gamma-radiation and ethylene oxide. Biomaterials 1998; 19(7-9): 659-668.

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Counteracting Reliability Problems in Advanced Hip Prostheses

39

[71] Yeom B, Yu Y-J, McKellop HA, Salovey R. Profile of oxidation in irradiated polyethylene. J. Polym. Sci.: Part A: Polym. Chem. 1998; 36: 329-339. [72] Blanchet TA, Burroughs BR. Numerical oxidation model for gamma radiationsterilized UHMWPE: Consideration of dose-depth profile. J. Biomed. Mater. Res. (Appl. Biomater.) 2001; 58: 684-693. [73] Buchanan FJ, Sim B, Downes S. Influence of packaging conditions on the properties of gamma-irradiated UHMWPE following accelerated ageing and shelf ageing. Biomaterials 1999; 20(9): 823-837. [74] Daly BM, Yin J. Subsurface oxidation of polyethylene. J. Biomed. Mater. Res. 1998; 42(4): 523-529. [75] O’Neill P, Birkinshaw C, Leahy JJ, Buggy M, Ashida T. The distribution of oxidation products in irradiated ultra-high molecular weight polyethylene. Polymer Degradation and Stability 1995; 49: 239-244. [76] Piconi C, Maccauro G. Zirconia as a ceramic biomaterial. Biomaterials. 1999; 20(1): 125. [77] Garvie RC, Hannink RHJ, Pascoe TR. Ceramic steel?. Nature 1975; 258: 703-704. [78] Gupta TK, Lange FF, Bechtold JH. Effect of stress induced phase transformation on the properties of polycrystalline zirconia containing tetragonal phase. J. Mater. Sci. 1978; 13: 1464-1470. [79] Kumar P, Oka M, Ikeuchi K, Shimizu K, Yamamuro T, Okumura H, Kotoura Y. Low wear rate of UHMWPE against zirconia ceramic (Y-PSZ) in comparison to alumina ceramic and SUS 316L alloy. J. Biomed. Mater. Res. 1991; 25(7): 813-828. [80] US FDA: Recall of zirconia ceramic femoral heads for hip implants (2001). www.fda.gov/cdrh/recalls/zirconiahip.html. [81] Haraguchi K, Sugano N, Nishii T, Miki H, Oka K, Yoshikawa H. Phase transformation of a zirconia ceramic head after total hip arthroplasty. J. Bone Joint Surg. (Br.) 2001; 83(7): 996-1000. [82] Blaise L, Villermaux F, Cales F. Aging of zirconia. Everything you always wanted to know. Bioceramics 2000; 13: 553-556. [83] Sato T, Shimada M. Transformation of yttria-doped tetragonal ZrO2 polycrystals by annealing in water. J. Am. Ceram. Soc. 1985; 68: 356-359. [84] Sato T, Ohtaki S, Endo T, Shimada M. Changes in crystalline phase and microstructure on the surface of yttria-doped tetragonal zirconia polycrystals (Y-TZP) by annealing in humid conditions. Adv. Ceram. 1988; 24: 501-508. [85] Chevalier J, Cales B, Drouin JM. Low-temperature aging of Y-TZP ceramics. J. Am. Ceram. Soc. 1999; 82: 2150-2204. [86] Djurado E, Dessemond L, Roux C. Phase stability of nanostructured tetragonal zirconia polycrystals versus temperature and water vapor. Solid State Ionics 2000; 136-137: 1249-1254. [87] Ueno M, Iwamoto M, Ikeda J. Influence of Complex Frictional Heating and Mechanical Stress Effect for Surface Crystalline Structure Change of Zirconia Femoral Head. Key Eng. Mater. 2006; 309-311: 1239-1242.

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Giuseppe Pezzotti

[88] Ohmichi N, Kamioka K, Ueda K, Matsui K. Phase Transfromation of Zirconia Ceramic by Annealing in Hot Water. J. Ceram. Soc. Jpn. 1999; 107(2): 128-133. [89] Guo X. On the degradation of zirconia ceramic during low-temperature annealing in water or water vapor. J. Phys. Chem. Solids. 1999; 60: 539-546. [90] Pezzotti G, Porporati AA. Raman spectroscopic analysis of phase-transformation and stress patterns in zirconia hip joints. J. Biomed. Optics. 2004; 9(2): 372-384. [91] Basu B, Vleugels J, Van der Biest O. Transformation behaviour of tetragonal zirconia: role of dopant content and distribution. Mater. Sci. Eng. A 2004; 366(2): 338-347. [92] Pezzotti G, Sakakura S, Porporati AA, Clarke IC, Green DD, Sugano N. Phase transformations and residual stresses in retrieved zirconia hip implant: A Raman microprobe spectroscopy study. Ceram. Transactions. 2003; 147: 141-154. [93] Garvie RC. Stabilization of the tetragonal structure in zirconia microcrystal. J. Phys. Chem. 1978; 82(2): 218-224. [94] Hirano M. Inhibitation of low temperature of tetragonal zirconia ceramics - A review. Br. Ceram. Trans. J. 1992; 91: 139-147. [95] Li P, Chen L-W, Penner-Hahn JE. Effect of dopants on zirconia stabilization - An Xray absorption study: 2, Tetravalent Dopants. J. Am. Ceram. Soc. 1994; 77(5): 12811288. [96] Gremillard L, Chevalier J, Epicier T, Deville S, Fantozzi G. Modeling the aging kinetics of zirconia ceramics. J. Eur. Ceram. Soc. 2004; 24: 3483–3489. [97] Deville S, Chevalier J, Dauvergne CH, Fantozzi G, Bartolome JF, Moya JS, Torrecillas R. Microstructural investigation of the aging behavior of (3Y-TZP)–Al2O3 composites. J. Am. Ceram. Soc. 2005; 88(5): 1273–80. [98] Yashima M, Yoshimura M. Phase formation in rapidly quenched rare earths doped zirconia melts. In Zirconia Ceramics 12: edited by T. Masaki and M. Yoshimura ; 1992: pp. 21-46. [99] Kim D-J, Park I-S, Lee K-Y. Subcritical crack growth resistance of low temperature degradation-free zirconia/alumina composite for medical applications. Key Eng. Mater. 2005; 284-286: 1007-1010. [100] Eichler J, Rödel J, Eisele U, Hoffman M. Effect of grain size on mechanical properties of submicrometer 3Y-TZP: Fracture strength and hydrothermal decradation. J. Am. Ceram. Soc. 2007; 90(9): 2830-2836. [101] Traykova T, Aparicio C, Pau Ginebra M, Planell JA. Bioceramics as nanomaterials. Nanomedicine 2006; 1(1): 91-106. [102] Macirowski T, Tepic S, Mann RW. Cartilage stresses in the human hip joint. J. Biomech. Eng. 1994; 116: 10-18. [103] von Eisenhart-Rothe R, Eckstein F, Muller-Gerbl M, Landgraf J, Rock C, Putz R. Direct comparison of contact areas, contact stress and subchondral mineralization in human hip joint specimens. Anat. Embryol. (Berl.) 1997; 195: 279-288. [104] von Eisenhart-Rothe R, Steinlechner M, Muller-Gerbl M, Eckstein F. Quantitative determination of joint incongruity and pressure distribution during simulated gait and cartilage thickness in the human hip joint. J. Orthop. Res. 1999; 7: 532-539.

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Counteracting Reliability Problems in Advanced Hip Prostheses

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[105] Brown TD, Shaw DT. In vitro contact stress distributions in the natural human hip. J. Biomech. 1983; 16: 373-384. [106] Rushfeldt PD, Mann RW, Harris WH. Improved techniques for measuring in vitro the geometry and pressure distribution in the human acetabulum - I. Ultrasonic measurement of acetabular surfaces, sphericity and cartilage thickness. J. Biomech. 1981; 14: 253-260. [107] Shepherd DE, Seedhom BB. Thickness of human articular cartilage in joints of the lower limb. Ann. Rheum. Dis. 1999; 58: 27-34. [108] Konishi N, Mieno T. Determination of acetabular coverage of the femoral head with use of a single anteroposterior radiograph. A new computerized technique. J. Bone Joint Surg. Am. 1993; 75: 1318-1333. [109] Adams MA, Kerin AJ, Bhatia LS, Chakrabarty G, Dolan P. Experimental determination of stress distributions in articular cartilage before and after sustained loading. Clin. Biomech. (Bristol, Avon) 1999; 14: 88-96. [110] Adams D, Swanson SA. Direct measurement of local pressures in the cadaveric human hip joint during simulated level walking. Ann. Rheum. Dis. 1985; 44: 658-666. [111] Afoke NY, Byers PD, Hutton WC. Contact pressures in the human hip joint. J. Bone Joint Surg. Br. 1987; 69: 536-541. [112] Sparks DR, Beason DP, Etheridge BS, Alonso JE, Eberhardt AW. Contact Pressures in the Flexed Hip Joint During Lateral Trochanteric Loading. J. Orthop. Res. 2005; 23: 359-366. [113] Rushfeldt PD, Mann RW, Harris WH. Influence of cartilage geometry on the pressure distribution in the human hip joint. Science. 1979; 204: 413-415. [114] Rushfeldt PD, Mann RW, Harris WH. Improved techniques for measuring in vitro the geometry and pressure distribution in the human acetabulum. II Instrumented endoprosthesis measurement of articular surface pressure distribution. J. Biomech. 1981; 14: 315-323. [115] Wiederhorn SM. Brittle fracture and toughening mechanisms in ceramics. Ann. Rev. Mater. Sci. 1984; 14: 373-403. [116] Gilman J. Electronic basis of the strength of materials. Cambridge (UK): Cambridge Univ. Press; 2003. [117] Peterlik H. Relationship of strength and defects of ceramic materials and their treatment by Weibull theory. J. Ceram. Soc. Jpn 2001; 109(8): 121-126. [118] Weibull W. A statistical distribution function of wide applicability. J. Appl. Mech. 1951; 18: 293-297. [119] Richter HG, Willmann G. Reliability of ceramic components for total hip endoprostheses. Br. Ceram. Trans. 1999; 98(1): 29-34. [120] Fritsch EW, Gleitz M. Ceramic femoral head fractures in total hip arthoplasty. Clin. Orthop. 1996; 328: 129–136. [121] Willman G. The evolution of ceramics in total hip replacement. Hip Int. 2000; 10: 193– 203. [122] FDA Guidance Document for the Preparation of Premarket Notifications for Ceramic Ball Hip Stems, 1995.

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[123] Pfaff H. Ceramic component failure and the role of proof testing. Clinical Orthop. Rel. Res. 2000; 379: 29-33. [124] Ortiz-Merino JL, Todd RI. Relationship between wear rate, surface pullout and microstructure during abrasive wear of alumina and alumina/SiC nanocomposites. Acta Mater. 2005; 53(12): 3345-3357. [125] Furnes A, Lie SA, Havelin LI, Engesaeter LB, Vollset SE. The economic impact of failures in total hip replacement surgery: 28,997 cases from the Norwegian Arthroplasty Register, 1987-1993. Acta Orthop. Scand. 1996; 67(2): 115-121.

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In: Hip Replacement Editors: T. Aoi and A. Toshida

ISBN 978-1-60692-326-9 © 2009 Nova Science Publishers, Inc.

Chapter 2

Ultra Smooth Nanostructured Diamond Coatings for Biomedical Implants in Total Hip Replacement Applications S. Chowdhury1∗, Yogesh K. Vohra1 and William. C. Clem2 1

Department of Physics, University of Alabama at Birmingham (UAB), Birmingham, Alabama 35294-1170 USA 2 Department of Biomedical Engineering, University of Alabama at Birmingham, Birmingham, AL, 35294-4440 USA

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Abstract Total hip replacements represent a majority of joint replacement procedures; many of these are revision surgeries to replace damaged or loosened implant components as a result of excessive wear. The number of total hip and knee replacements performed each year in the U.S. is 198,000 and 245,000, respectively. Revision surgeries account for 17 percent of all hip replacement and eight percent of all knee replacement surgeries, for a combined total of nearly 54,000 revision surgeries each year. A primary problem with current designs is the generation of wear debris particles at the articulating surface that causes local pain and inflammation. Large debris are normally sequestered by fibrous tissue, while small debris is taken up by macrophages and multinucleated giant cells which may release cytokines that result in inflammation. This inflammation cascade damages surrounding bone, ultimately resulting in osteolysis, loosening, and implant failure. The proposed solution for the problem of osteolysis caused by wear debris is to develop ultra hard materials for the articulating surfaces that are more wear resistant, which would reduce the number of debris particles generated. In our on going research we have developed ultra smooth nanostructured diamond (USND) coatings that will have optimal proprieties and can survive in the harsh environment in the body and will last longer as wear resistant biomaterials. USND coatings were deposited by microwave plasma chemical vapor deposition (MPCVD) technique using He/H2/CH4/N2 gas mixture on Ti–6Al–4V medical grade substrates. We were able to deposit diamond coatings as ∗

E-mail: [email protected]; [email protected]

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S. Chowdhury, Yogesh K. Vohra and William C. Clem smooth as 6 nm (root-mean-square), as measured by an atomic force microscopy (AFM) scan area of 2 x 2 micron. The characterization of the coatings was performed with AFM, scanning electron microscopy (SEM), x-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), tribometer and nanoindentation techniques. XRD and Raman results showed the nanocrystalline nature of the diamond coatings. The surface morphology imaged by nano SEM at 300,000× also confirmed the nanocrystalinity of the diamond coatings. Nanoindentation demonstrated that the hardness and Young’s modulus of the coatings are around 60 GPa and 380 GPa, respectively. The plasma species during deposition were monitored by optical emission spectroscopy. All of the diamond (USND) coated Ti-6Al-4V disks had better wear performance against polyethylene compare to CoCrMo and polyethylene wear couple. In vitro biocompatibility tests on diamond coatings showing no sign of toxicity. Adhesion and spreading of human mesenchymal stem cells (MSCs) were found on the deposited coatings after culturing up to 2 weeks. Preliminary results confirm that these diamond coated implants are as good as standard Ti-6Al-4V and CoCrMo as orthopedic implant applications. When coupled with wear studies, the in vitro cell study and in vivo animal study results suggest that USND has the potential to reduce debris particle release of biomedical implants without compromising osseointegration, thus minimizing the possibility of implant loosening over time.

Keywords: Hip replacement, Chemical vapor deposition; Nanostructure; Diamond coatings.

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1. Introduction For more than 40 years, hip replacement surgery has provided improved quality of life and led to dramatic changes in lifestyle for patients who suffered from arthritic conditions of thehip. Approximately 300,000 total knee replacements and 200,000 total hip replacements are performed each year in the United States [1]. Other joints, such as shoulders, elbows, and temporomandibular joints can also be replaced. Osteoarthritis is the primary cause of joint pain, but may be primary or secondary. Previous trauma, infection, rheumatoid arthritis, crystal deposition diseases, avascular necrosis and the rare bone dysplasias may result in cartilage deterioration or osteoarthritis. As long as implants are positioned correctly and infection is avoided, they will generally last for many years. Improvements in implant design and surgical technique have improved the 10-year success rate for hip prostheses to >90% and 20-year success to >80% [1]. However, a joint composed of metals, polymers, or ceramics lacks the ability to repair and remodel itself, and will eventually fail. In the majority of commercial joint prostheses shown in Figure 1.1 available today, the approach is to use a polished metallic cobalt alloy (CoCrMo) surface which articulates with much softer polyethylene (PE) surface. The rationale is with a metal on polyethylene bearing, almost all of the wear debris generated polyethylenewill be the softer and relatively less toxic polyethylene debris. This approach effectively reduces the production of toxic metallic ions; however, even in ideal conditions, the wear rate of the polyethylene cup in a hip implant is expected to be 0.1 - 0.2 mm/year.

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Figure 1.1. Biomedical implants for hip replacements (www.zimmer.com).

Dividing the thickness of the cup by this wear rate will give the maximum expected life, but there are many instances where scratches on the CoCrMo surface cause acceleration of this wear rate. Now that implants are routinely reaching near the maximum life expectancy, requiring surgery to replace the polyethylene cup, a more permanent solution to wear debris is needed. Osteolysis (bone loss) shown in Figure 1.2 around a bearing surface is caused by an immune reaction against wear particles. The greater the wear particle production, the higher the risk of osteolysis and subsequent failure of the prosthesis. For this reason, there is increasing interest in developing metal-on-metal or ceramic-on-ceramic designs. Both of these designs produce much less wear particles compared to metal-on-polyethylene, which should result in fewer failures in theory. The single greatest concern with the use of a metalon-metal bearing is an elevated level of metal ions in a patents’ blood and urine following implantation. Table 1.1 shows cobalt and chromium ions levels in metal-on-metal implants. The long-term consequences of very high cobalt and chromium ion levels are unknown, although some patients develop metal hypersensitivity and some evidence indicates an increase in cancer risk [2-4]. The greatest concern with ceramic materials is fracture [5-7]. For a ceramic-on-ceramic bearing, the components must be substantially larger compared to other designs in order to prevent fracture. Among total hip arthroplasties (THA) worldwide, approximately 20% include femoral heads are made from ceramic. Ceramics hip replacement components are shown in Figure 1.3. Sixty percent of ceramic heads produced in 2001 were composed of alumina and 40% were zirconia [8-10]. Alumina ceramics were introduced into the clinical orthopedic setting in the late 1960s (Boutin, France; Griss, Germany). Early failures of ceramic heads were significantly reduced following the introduction of the Morse taper in 1970. The alumina femoral head has proven to be safe and useful over a long periods, through several tenthousands of clinical application. TZP femoral heads were introduced in 1985 as an alternative to alumina, because of zirconia’s superior hardness and resistance to fracture. Zirconia heads have been shown to

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S. Chowdhury, Yogesh K. Vohra and William C. Clem

have approx. 1.5 times higher strength than alumina heads. The bearing between zirconia ceramics and UHMWPE has revealed low friction and wear characteristics, and reduced polyethylene wear is expected by adopting zirconia ceramics as the material of femoral components of total knee replacements. Compared with Co-Cr-Mo alloy, zirconia ceramics have proven to reduce polyethylene wear - mean polyethylene wear rates were an order of maginitude lower for zirconia heads as compared to cobalt-chromium heads [11]. Thus, zirconia was recommended as the material of choice for femoral heads [12].

Figure 1.2. Osteolysis after total hip replacement.

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Table 1.1. Cobalt and chromium ions levels in metal-on-metal implants

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Figure 1.3. Ceramics hip replacement components.

Zirconia ceramics have three phases of crystalline structure, which vary with processing temperature. The monoclinic phase transforms into a tetragonal phase at 1100oC; whereas the tetragonal phase transforms into the cubic phase at 2370oC. The tetragonal phase is metastable and zirconia has the greatest mechanical strength in this phase [13]. The tetragonal phase can undergo transformation, however, into the monoclinic phase under stress and/or with aging [11,14-16]. This phase transformation involves a volume expansion of 34%, which halts crack propagation, sealing growing cracks [14,15]. Once the material undergoes extensive transformation into a monoclinic phase, however, this advantage is lost and the implant may be more susceptible to surface damage and to increasing surface roughness [16]. Over 500,000 TZP have been implanted since 1985 [12]. It has been postulated, however, that in an in vivo environment (water or body fluid), TZP ceramic femoral heads undergo aging with increasing conversion of their surface layers into monoclinic phase [10, 14, 16]. Over time, this phase transformation could lead to increasing crack formation, decreased surface hardness, and increased surface roughness, which may accelerate polyethylene wear in total joint replacements. Increased wear may cause premature loosening leading to early clinical revision. One recognized loss rate of these products was 1 in 10,000 [11], but recent reports indicate loss rates up to 8.8% for some specific batches of products [11]. The U.S. Food and Drug Administration (FDA) and the Australian Therapeutic Goods Administration (TGA) announced that firms making orthopedic implants were recalling series of TZP hip prostheses due to an instance of head fractures [10, 11]. This recall followed the action by the French Agency for the Medical Safety of Health Products (AFSSAPS) and the United Kingdom Medical Devices Agency suspending the sales of TZP ceramics heads. According to the manufacturer of the zirconia ceramics, the fracture origin was an accelerated tetragonal-tomonoclinic phase transformation of zirconia in particular batches due to thermal processing [16].

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2. Accelerating Aging of Zirconia Femoral Head Implants: Change of Surface Structure and Mechanical Properties We have evaluated the changes in the mechanical properties as well as microstructure due to the phase transformation as a result of laboratory aging in different zirconia femoral head samples. The purpose of our study is to compare the control specimens to thermal aged samples that have been subjected to phase transformation and determine the extent of degradation, if any, which occurs over time during aging process.

2.1. Methods TZP ceramics femoral heads without Hot Isostatic Press (HIP) treatment were aged at 121ºC in saturated water vapor. Four zirconia femoral heads of 22.22 mm diameter, three aged at different time range and one non-aged sample directly from the production line were taken for analysis. The tetragonal and monoclinic phase compositions of the aged samples were examined using glancing angle X-ray diffraction (XRD; PW3050, Philips Japan, Tokyo, Japan). Glancing angle of 3-degree incident beam was directed along the topmost surface of the ball. The topmost surface corresponded to the area where cyclic loading would be expected. Peaks from the XRD output were compared to library data and the tetragonal/monoclinic zirconia ratio was determined using the integrated intensity (measuring the area under the peaks) of the tetragonal (101) and two monoclinic (-111) and (111) peaks as describe by Garvie et Nicholson [17] and revised by Toroya et al. [18, 19]. According to the Garvie and Nicholson equation, monoclinic mole fraction can be expressed as:

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⎡ ⎤ I m (−111) + I m (111) Fm (mole%) = ⎢ ⎥ *100 ⎣ I t (101) + I m (−111) + I m (111) ⎦

where Fm is the monoclinic fraction in mole%; Ij(hkl) is the intensity of the (hkl) reflection bond of the plane j (t and m) for the tetragonal and monoclinic phase respectively as observed from the XRD pattern. Thermally aged zirconia femoral head samples with monoclinic fraction (mol%) 0 (control), 10, 50 and 78 were selected for further study. Surface morphology was examined by laser scanning microscope (OLYMPUS OPTICAL co., Tokyo, Japan). The monoclinic to tetragonal phase transformation was imaged by Scanning Electron Microscope (SEM; JSM-6500F, JEOL, Ltd., Tokyo, Japan). Zirconia femoral heads after thermal aging were cut into two pieces and cross-sections of that were polished by 1 µm diamond pastes and colloidal silica. In order to measure the hardness and Young’s modulus of zirconia femoral head samples nanoindentation measurements were carried out using a Nanoindenter XP (MTS Systems, Oak Ridge TN) system. The system was calibrated by using silica samples for a range of operating conditions. Silica modulus and hardness were calculated as 70 GPa and 9.1 GPa before indentation on samples. A Berkovich diamond indenter with total included angle of

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142.3° was used for all the measurements. The loading/unloading rate was 0.3 mN/s. A 10 second hold time at a maximum load and 50 second at 10 percent of max load during unloading was used in order to minimize thermal drift. All the measurements were done using 500 nm penetration depth. The data was processed using proprietary software to produce load-displacement curves and the mechanical properties were calculated using the Oliver and Pharr method [20].

2.2. Results The progress model of phase transformation during the aging test in the hydrothermal environment is simulated in the Figure 2.1. Laser microscope observation in Figure 2.2 confirmed the observation.

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Figure 2.1. The progress model of phase transformation during the aging test in the hydrothermal environment.

Figure 2.2. Laser microscope observation of the phase transformation during the aging test.

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In the 10mol% sample, the change in the surface morphology due to the phase transformation was clearly observed. In the 50mol% and 78mol% sample, it appeared that the surfaces of these samples were covered with the monoclinic layer. The cross-sectional images of different aged zirconia femoral heads are shown in Figure 2.3 and 2.4.

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Figure 2.3. SEM images of aged (a) 10mol% (b) 50% zirconia femoral heads sample.

Figure 2.4. SEM images of aged 78 mol% zirconia femoral head sample showing the transformation from tetragonal to monoclinic phase. (Arrows show nano-crack at grain boundary).

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In the 10 mol% sample, transformed monoclinic regions shown by the cross section plane were observed. In the 50mol% sample the monoclinic region was from 2 to 3 μm along the edge, and the boundary between tetragonal and monoclinic layers was not clearly defined. In the 78mol% sample, the monoclinic region was a greater depth with the transformed layer approximately 8 μm. Also, some nano-micro-cracks were observed at grain boundary located with in the monoclinic zone. Typical load and displacement data collected during indentation measurement on high tetragonal (10mol%) and high monoclinic phase content (78mol%) aged zirconia head samples are shown in Figure 2.5. More resistance to penetration was observed for 10mol% implant at same indentation depth as shown in Figure 2.5. 80 70 60

10mol%

Load (mN)

50 40 30 78mol%

20 10 0 0

100

200

300

400

500

600

Displacement into surface (nm)

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Figure 2.5. Nanoindentation load-displacement curve of 10 mol% and 78 mol% aged zirconia implants showing more resistance of penetration for10 mol% sample at same indentation depth.

The surface hardness and modulus was measured in different aged zirconia head samples and correlated with their monoclinic mole%. The results are showed in Figure 2.6. Both hardness and Young’s modulus were decreased with the progress of phase transformation from tetragonal to monoclinic due to aging. The control sample (zirconia with only tetragonal phase) showed a hardness (H) and Young’s modulus (E) of 16.6 GPa and 254 GPa respectively (Figure 2.6). With an increase of monoclinic phase up to 78 mol%, H and E values were decreased to 9.7 GPa and 185 GPa respectively. Hardness variation at different displacement into surface for control and 78% aged zirconia femoral head samples are shown in Figure 2.7. In zirconia sample with lower amount of monoclinic phase content (10mol%) decreasing in the hardness corresponding to the surface relief was observed at an indentation depth of 150 nm. Figure 2.8 demonstrates that low values of hardness and Young’s modulus were found when we selectively indents on the monoclinic phase in tetragonal matrix.

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Figure 2.6a. Surface hardness values of different aged zirconia femoral head samples.

Figure 2.6b. Young’s modulus values of different aged zirconia femoral head samples.

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30 78 at%

25

control

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20 15 10 5 0 0

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100

150

200

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350

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Figure 2.7. Hardness variation at different displacement into surface for control and 78% aged zirconia femoral head samples.

20

300

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18

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14

200

12

monoclinic phase

10

150

8 100

6 Hardness

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50

Young's modulus

2

Young's modulus (GPa)

250

16

0

0 0

2

4 6 Different test locations

8

10

Figure 2.8. Hardness and Young’s modulus variation on 10 mol% aged zirconia femoral heads sample (max indentation depth 150 nm).

It is known that the monoclinic phase in zirconia is the most stable phase. On the other hand tetragonal phase is normally metastable at room temperature and may transform into a monoclinic phase under some thermodynamic conditions [21]. It has been shown in detail by Lange [22] that tetragonal to monoclinic transformation rate in Y-TZP ceramics is

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proportional to the free energy change of the tetragonal to monoclinic transformation. Also, the phase transformation in zirconia occurs by nucleation and growth first on isolated grains at the surface which then propagates to the neighboring grains as a result of various stresses and the accumulation of microcracks [23]. The transformation penetrates step by step within the bulk of the material, providing there is physical contact between the zirconia grains. Our current result demonstrates that in vivo degradation of zirconia can be simulated by accelerating aging treatment in the laboratory without any mechanical stress. It is also possible to closely monitor the transformation and measure the mechanical and structural characterization over time. Other study also shows that the phase transformation on the surface of zirconia femoral heads had little influence to the wear rate of UHMWPE (Ultra High Molecular Weight Polyethylene) sockets from the result of hip simulator test of aged zirconia femoral heads and conventional UHMWPE sockets shown in Figure 2.9. Further, measurement of mechanical properties of zirconia femoral heads after hip simulator test using Nanoindenter is needed to define relationship between mechanical properties and progress of phase transformation. The changes in the mechanical properties as well as microstructure due to the phase transformation as a result of laboratory aging in different zirconia femoral head sample was evaluated by SEM, laser microscope, XRD and Nanoindentation technique. We found monoclinic to tetragonal phase transformation in zirconia prostheses with the amount of aging undertaken artificially in the laboratory. Cross sectional image taken by SEM and laser microscope images confirm the progress of transformation from tetragonal phase to monoclinic phase during the aging process. Thickness of the monoclinic layer increased as a function of mole%. In the 50mol% sample the monoclinic region was found to be 2 to 3 μm along the edge and the thickness of this region increased up to 8 μm in 78mol% zirconia aged sample. Mechanical properties mainly hardness (H) and Young’s modulus (E) values were measured by nanoindentation technique on the surface of these implants.

Figure 2.9. Cumulative worn weight of UHMWPE cups.

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The results show that both H and E values decreased with increase of monoclinic phase in zirconia and also confirm the phase transformation over time during aging process. In order to retain the quality of zirconia the phase transformation should be avoided. In vivo degradation of zirconia can be simulated by accelerating aging treatment in the laboratory without any mechanical stress. In this study it is possible to closely monitor the transformation and measure the mechanical and structural characterization over time.

3. Mechanical and Structural Transformation of Zirconia Femoral Head Implants In Vivo: A Retrieval Study We evaluated the mechanical and structural transformation of zirconia femoral head implants that may have undergone phase transformation in vivo, and to determine the extent of property changes, if any, which occurs over time in functional human patients with THA.

3.1. Methods Previously implanted fifteen zirconia femoral heads were collected at revision THA by the Orthopedic Retrieval Program at the University of Alabama at Birmingham (UAB) with full approval by the UAB Institutional Review Board. The femoral components were soaked in 10% buffered formalin and a mild chlorox solution for sterilization and processing prior to examination. The heads were separated from their femoral stems and washed in an acetone bath to remove organic residue. Patient data including age, gender and implant time in vivo, were obtained from patient records. Nonimplanted zirconia samples were also examined for comparison.

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3.2. Results The XRD results showed that the surfaces of the TZP explanted components underwent varying degrees of monoclinic phase transformation with a direct association with time of implantation. The XRD patterns from four zirconia femoral head samples are shown as example of the analysis in Figure 3.1. The monoclinic (-111) and (111) peaks are clearly defined in the XRD pattern at 28 and 31.2 degrees. The ratio of integrated intensities of the monoclinic (-111) and tetragonal (101) are shown by the arrowed numbers. The region of the surface from the sample retrieved after 60 months (black curve) revealed an increased amount of monoclinic phase with a low relative amount of tetragonal phase. The non-implanted (blue curve) and 2 months specimens (pink curve) showed lower amounts of monoclinic phase and a higher relative amount of tetragonal phase. The XRD peaks of the monoclinic phase at 31.3 and 34 (2-theta) were also prominent and are related to the increase of this phase at the specimen surface. The XRD analysis

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showed that the monoclinic to tetragonal phase ratio in zirconia implant increased with time of implant life in vivo, as shown in Figure 3.2. 20000 Tetragonal (101)

18000 16000

Intensity (a.u.)

14000 12000

Monoclinic (-111)

10000

1.95

8000 6000

0.39

4000 0.01

2000

0.02

0 26

27

28

29

30

31

32

33

34

35

36

2- Theta (degrees)

Figure 3.1. Glancing angle (3.5 degree) XRD patterns of explanted femoral heads for sample of different monoclinic contents. The diffraction peaks belonging to monoclinic phase (-111) (111) placed at around 28 degree, 31.2 degree (2-theta) and tetragonal phase 30 degree.

35

Monoclinic mole%

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30 25 20 15 10 5 0 0

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40

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120

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Implant life (months) Figure 3.2. Correlation of monoclinic to tetragonal phase ratio with time of implants life in vivo.

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Typical load and displacement data collected during indentation of explanted zirconia head samples with high tetragonal content (2 months) and higher monoclinic content (60 months) are shown in Figure 3.3. More resistance to penetration was observed for the 2 month implant at the same indentation depth. The variations of surface hardness and Young’s modulus for the different zirconia surfaces with time in vivo are shown in Figure 3.4 and 3.5, respectively. These figures illustrate that surface hardness and modulus decreased with increased time in vivo. The zirconia surface hardness decreased from 17.2 GPa to 11.2 GPa and Young’s modulus decreased from 267 GPa to 193 GPa, 120 months after implantation. 16 2 months

14

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10 8 6 4 2 0 0

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Figure 3.3. Nanoindentation load-displacement curve of 2 months and 60 month explanted implants showing more resistance of penetration for 2 months implant at same indentation depth.

20

Hardness (GPa)

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18 16 14 12 10 8 0

20

40

60

80

100

120

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Implant life (months)

Figure 3.4. Surface hardness of different zirconia femoral head surfaces compared with time of implants in vivo.

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58 300

Young's modulus (GPa)

280 260 240 220 200 180 160 0

20

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Figure 3.5. Young’s modulus of different zirconia femoral head surfaces compared with time of implants in vivo.

20

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Hardness (GPa)

18 16 14 12 10 8 0

10

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Figure 3.6. Correlation of surface hardness with monoclinic to tetragonal phase ratio.

The surface hardness and modulus were correlated with the monoclinic to tetragonal phase ratios for the different samples. The results showed a good correlation between increased amounts of the monoclinic phase and decreased surface hardness and modulus, as shown in Figure 3.6 and 3.7. The surface hardness decreased from 17.8 GPa to 10.7 GPa and

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the Young’s modulus decreased from 255 GPa to 203 GPa with the increase of monoclinic phase from 2% to 68%. AFM measurements confirmed the presence of small pits, voids and scratch marks on the retrieved zirconia surfaces (Figure 3.8).

Young's modulus (GPa)

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Figure 3.7. Correlation of Young’s modulus with the ratio of monoclinic to tetragonal phase ratio.

Figure 3.8. Zirconia femoral head surfaces morphology after (a) 20 months (RMS roughness 16.2 nm) and (b) 25 months in vivo (RMS roughness 22 nm).

The phase transformation in zirconia in the in vivo may follow localized damage to the femoral head component from severe mechanical stress such as rim impingement or 3-body abrasive wear [14]. An aging effect in the moist body environment is also possible, where the reaction between H2O and Y2O3 to form Y(OH)3 during aging contributes in the transformation process [24]. The fundamental role of internal stresses associated with water diffusion in the zirconia lattice has been demonstrated [25, 26]. The transformation may also result from the pressure-temperature shear effects of sliding on the surface of the opposing

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acetabular liner, or from sensitivity created by manufacturing and machining processes. Zirconia phase transformation is also sensitive to a number of micro-structural features. By modifying parameters such as purity, density, porosity, particle size and crystalline structure, the kinetics can be shifted and the performance of zirconia as a material for articulating bearings in orthopedic devices may be improved. We have investigated the changes in the mechanical and structural properties due to the phase transformation of explanted TZP femoral head implants by XRD, AFM and Nanoindentation techniques. Structural characterization with XRD showed that the component surfaces were altered to varying degrees of monoclinic phase transformation, with a direct association with time of in vivo implantation. Mechanical properties of hardness (H) and Young’s modulus (E) values measured by nanoindentation showed that both H and E values decreased with increased time in vivo and the amount of the monoclinic phase which altered surface microtopography. In order to retain the quality of zirconia and its optimal performance as a femoral head in total hip replacement, such phase transformations, property changes and increased surface roughness should be avoided.

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4. Ultra Smooth Nanostructured Diamond Coatings for Orthopedic Implant Applications Complications arising from wear include component loosening, deleterious biological responses, osteolysis, mechanical instability, decreased joint mobility, increased pain, and ultimately implant failure [27]. A major goal is to develop smooth and wear resistant coatings on the articulation surfaces in order to reduce the friction and wear in mating total joint replacement components. Hard, ultra-smooth and wear resistance diamond films on metal surface can serve this purpose very well. Diamond coatings possess several properties, particularly hardness, smoothness, low wear rate, and biocompatibility, which make them highly desirable for biomedical implants. Diamond coating on the metal implants may also prevent metal ion release in to the blood urine and end organ tissue and eliminate concern for long time use of metal bearings. A common method of producing diamond coating is by using hydrogen and methane gases in the plasma mixture in chemical vapor deposition technique (CVD). These will results microstructured diamond with grain size 2-10 micron and roughness between 200 nm to 1 micron shown in Figure 4.1. Adding small amount of nitrogen in the conventional gas mixture can produce nanostructured diamond with smooth surface. Diamond films grown using gas mixtures such as hydrogen, nitrogen and methane have been investigated primarily in order to get smooth nanocrystalline diamond film [28-31]. The surface roughness value of 15-20 nm (RMS) and grain size of 13-15 nm was achieved shown in Figure 4.2. The film grown without nitrogen addition shows large, well defined crystalline facets indicative of high-phase-purity diamond [32]. In contrast, the films grown with added nitrogen exhibit a nanocrystalline appearance with weak agglomeration into rounded nodules of submicron size. It has also been observed that the transformation from microcrystalline to nanocrystalline diamond structure can occur by adding Ar in H2/CH4 feed gases with a total transformation observed at Ar/H2 volume ratio of 9 [33,34].

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Figure 4.1. AFM and SEM images of microstructured diamond using H2 and CH4 gas mixtures.

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Figure 4.2. AFM and SEM images of microstructured diamond using H2, N2 and CH4 gas mixtures.

Surface roughness as low as 18 nm and grain size of 3-50 nm was demonstrated on highly polished (area RMS ~ 1 nm) silicon (111) wafers [33]. We have reported the effect of helium addition to H2/CH4/N2 feedgas mixtures on growth of high quality ultra-smooth nanostructured diamond films on Ti-6Al-4V. The addition of He significantly reduced film roughness to 5-6 nm and grain size of diamond nanocrystals to 5-6 nm without deterioration of film hardness, or adhesive properties. We describe the synthesis of ultra-smooth nanostructured diamond films by using He/H2/CH4/N2 plasma with different N2/CH4 volume ratio (CH4 is fixed) in a microwave plasma chemical vapor deposition (MPCVD) reactor. Structural and mechanical properties were evaluated by XRD, Raman spectroscopy, AFM, SEM and nanoindentation techniques.

4.1. Methods Ti–6Al–4V alloy disks with 25.4 mm diameter and 3.4 mm thickness were punched from Ti-6Al-4V sheets supplied by Robin Materials (Mountain View, CA). They were polished to a root-mean-square (RMS) roughness of 3-4 nm using a mechanical polisher with SiC paper,

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followed by a chemical–mechanical polish with a 0.06 μm colloidal silica solution containing 10% hydrogen peroxide. The polished disks were cleaned by ultrasonic agitation in a 1 micron diamond powder/water solution after a series of detergent solution, methanol, acetone, and finally deionized water. Cleaned substrates were placed in a Wavemat MPCVD reactor, equipped with a 6 kW, 2.4 GHz microwave generator shown in Figure 4.3. Chamber pressure was 65 Torr and the substrate temperature, as measured by ‘‘Mikron M77LS Infraducer’’ two-color IR pyrometer, was kept in the range 690–720 oC by adjusting microwave power in the range 0.93–1.1 kW. This two color pyrometer provided accurate measurement of the substrate temperature without requiring correction of the emissivity of the surface during growth. Total flow rate of He, H2, and CH4 gases was fixed at 336 sccm (71% He in He + H2, 36 sccm of CH4) and the ratio of N2/CH4 gas flow changed from 0 to 0.6.

Figure 4.3. Schematic of Microwave Plasma Chemical Vapour Deposition (MPCVD) reactor.

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Optical emission spectroscopy (OES) was performed to qualitatively determine the activated species present in the plasma. All the measurements were taken with 3000 points in the range of 350-700 nm wavelength and integration time of 250 ms. The crystallinity of the diamond films was analyzed by micro-Raman spectroscopy. The Raman spectra were taken using the 514.5 nm line of an argon-ion laser focused onto the film at a laser power of 100 mW. The Raman scattered signal was analyzed by a high resolution spectrometer (1 cm-1 resolution) coupled to a CCD system. XRD patterns on the diamond sample were examined using glancing angle XRD (X’pert MPD, Philips, Eindhoven, The Netherlands). XRD was performed using a glancing angle of 3-degree incident beam directed at the topmost surface of the coating surface. Spectra were taken from 30 to 90 (2-theta) at a scan speed of 0.012o min-1 and a step size of 0.005o as well as from 40 to 47 (2- theta) in order to clearly document the intensity and Full Width at Half Maximum (FWHM) of the diamond (111) diffraction peak. Structure and surface morphology of the diamond surfaces was imaged by, a TopoMetrix Explorer® AFM. The images were collected in non-contact imaging mode. Roughness was measured from a 2 μm2 scan area consistently for all samples. Surfaces of the diamond film were also imaged by FEI Nova NanoSEMTM. The hardness and elastic modulus of the diamond films was measured using a Nanoindenter XP (MTS Systems, Oak Ridge TN) system with a continuous stiffness attachment such that the loading and unloading displacement rates were constant. This provided continuous hardness/modulus measurements with increasing depth into the film [13,14].

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4.2. Results It was found that with the introduction of helium gas in H2/CH4/N2 plasma, the transformation from microcrystalline to nanocrystalline occurred and roughness decreased dramatically. The roughness decreased from 19-20 nm to 9-10 nm with the introduction of helium up to 71% (in He/H2 with fixed N2 and CH4 ratio 0.1) shown in Figure 4.4. Helium gas was also introduced in H2/CH4 feedgas without N2 and interestingly it was found that the roughness and grain size of the diamond films also decreased with increase of helium flow rate. The RMS roughness was as low as 20 nm and grain size 16-18 nm at helium flow up to 71% (in He/H2 with fixed CH4 content), as shown in Figure 4.5a and 4.5b. It was found that the combined effect of He and N2 played the vital role of decreasing the roughness and grain size and producing ultra smooth nanocrystalline diamond films. We have investigate the effect of N2 (N2 /CH4 ratios 0.05-0.6) with fixed He, CH4 and H2 feed gases. According to Afzal et al.[30] higher levels of gas phase CN radicals reduce the CH3 concentration and thus reduce growth rate. Figure 4.6 shows the drop of the growth rate from 0.37 μm/hr to 0.22 μm/hr by changing N2/CH4 ratios from 0.05 to 0.6. In the insert of Figure 4.6, an optical interference pattern collected from the interferometer is shown for the film deposited at N2/CH4 of 0.4.

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Figure 4.4. Diamond films grown in He/H2/CH4/N2 plasma at different He contents: (a) FWHM of the (111) diamond XRD peak and calculated average diamond grain size. (b) RMS surface roughness of the films calculated from 2×2 μm AFM images.

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Figure 4.5. Diamond films grown in He/H2/CH4 plasma at different He contents and no N2 (a) FWHM of the (111) diamond XRD peak and calculated average diamond grain size. (b) RMS surface roughness of the films calculated from 2 ×2 μm AFM images.

Optical emission spectroscopy was used to monitor the changes in plasma chemistry upon N2 addition during diamond growth. The optical emission spectra of the He/H2/CH4/N2 microwave plasmas with N2/CH4 ratios of 0.05 and 0.4 are shown in Figure 4.7. The ratios of OES intensities of plasma species (CN/Hα, C2/Hα and Hβ/Hα) as a function of N2/CH4 ratio is also shown in Figure 4.8. It was found that upon an increase of N2/CH4 ratio, the Hβ/Hα remained practically constant, indicating only minor changes in plasma temperature. The ratio C2/Hα remained almost constant as well. Interestingly, the CN/Hα ratio increased from 1 to 5 up to the N2/CH4 ratio of 0.4, and then decreased afterwards. Experimental results indicated that with increase of He addition up to 71% (in He+H2) and N2/CH4 gas concentration ratio 0.1, there was a 3 fold increase of CN/Hα. With the same 71% of He (in He+H2 gas mixture) addition but N2/ CH4 gas concentration ratio of 0.4 we had 2.5 times increase of CN/Hα. It decreased beyond a N2/CH4 ratio of 0.4.

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Growth Rate (micron/hr)

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Figure 4.6. Growth rate of diamond films at different N2/CH4 volume ratio in He/H2/CH4/N2 plasma. In the insert optical interference pattern collected from interferometer is shown for film deposited at N2/CH4: 0.4.

Figure 4.7. The optical emission spectra of the He/H2/CH4/N2 microwave plasmas with different ratio of N2/CH4. (a) N2/CH4: 0.05 and (b) N2/CH4: 0.4. It is to be noted that the intensity scale in the two spectra are different, CN peak increase in intensity by a factor of ten.

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66 6

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Figure 4.8. Normalized optical emission intensities of Balmer Hβ (1,486.14 nm), C2 (2,516.5 nm), and CN (3,386 nm) lines versus N2/CH4 content in He/H2/CH4/N2 plasma. Lines were normalized to Balmer Hα line (656.3 nm) intensity. Diamond (111) TiC

1200 Ti

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Figure 4.9. The x-ray diffraction patterns of the He/H2/CH4/N2 microwave plasmas with different ratio of N2/CH4. The insert show the close-up view of TiC and diamond (111) peaks showing the change due to the change of N2/CH4 volume ratio.

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Figure 4.9 shows the glancing angle XRD patterns for the nanostructured diamond films grown on Ti–6Al–4V alloy using the He/CH4/H2/N2 feedgas mixture with different N2/CH4 ratios. Characteristic of these patterns are the cubic diamond (111) and (220) reflections as well as several peaks attributed to interfacial titanium carbide phases. The diamond peaks (shown in the insert) were significantly broadened as compared to those obtained from the conventional CVD process. The average grain size as calculated from the diamond (111) peak width using the Scherrer formula was between 4-8 nm. The grain size decreased to around 4-5 nm as N2/CH4 ratio increased up to 0.4 and then increased again. It was also found that as the N2 content increased the intensity of the (200) diffraction peak from the TiC phase decreased. At N2/CH4 of 0.4 there was no TiC peak and the intensity again increased as the ratio N2/CH4 increased. The average grain size of diamond films with different N2/CH4 concentration ratios was estimated from the full width at half maximum (FWHM) of the (111) diamond peak using Scherrer equation (after correction for instrumental broadening) and presented in Figure 4.10. Scanning Electron Microscope (SEM) image at 300,000X on the surface of diamond sample grown in He/H2/CH4/N2 plasma at N2/CH4 ratio of 0.4 is shown in Figure 4.11. The grain size is as low as 5.7 nm and confirms the diamond nanostructure. 1.7

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Figure 4.11. SEM image at 300,000X of the diamond film surface grown in He/H2/CH4/N2 plasma at N2/CH4 flow ratio of 0.4.

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A lattice resolution TEM image taken of USND diamond film deposited on Titanium Alloy substrate is shown in Figure 4.12. The image is showing a group of diamond grains slightly as well of highly misoriented with respect to each other. Individual member of the grain appears to lie in the range of 5 to 10 nm.

Figure 4.12. A lattice resolution image of USND diamond film deposited on titanium alloy substrate.

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An indexed polycrystalline diffraction pattern of the diamond thin film deposited on Titanium Alloy substrate is also shown in Figure 4.13. Sharp Bragg reflections are visible, indicating good crystallinity. No appreciable scattering intensity from either graphite crystallites or amorphous carbon (glossy carbon) suggesting that the film is made of pure diamond only.

Figure 4.13. An indexed polycrystalline diffraction pattern of the diamond thin film deposited on titanium alloy substrate.

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An indexed Nano diffraction pattern (Figure 4.14) with incident electron beam parallel to the [001] direction of a typical single grain present in the microstructure of the USND diamond film deposited on Titanium Alloy substrate.

Figure 4.14. An indexed Nano diffraction pattern of the diamond thin film deposited on Titanium Alloy substrate substrate.

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Analyses of the pattern yields a diamond cubic lattice for the unit cell with a cell parameter that belongs to a diamond crystal only. The pattern, therefore indicates the existence well defined crystallinity in the diamond particles present in the thin film. The micro-Raman spectra for each of the nanostructured diamond films on Ti–6Al–4V alloy are shown in Figure 4.15. The diamond band at 1332 cm-1 is significantly broadened, and Raman scattering intensity in the 1400-1600 cm-1 region is pronounced. This band is usually associated with the “G-band” of disordered graphite which is downshifted from 1580 cm-1 and therefore likely involves scattering from amorphous sp2 and sp3 bonded carbon domains [35]. The Raman spectra have another peak at 1150 cm-1 in addition to the main diamond and graphite bands. It is suggested that this band is due to the nanocrystalline nature of the diamond films [36]. The films produced consist of diamond nanocrystallites imbedded in amorphous carbon matrix with a relatively small amount of graphitic carbon.

1300

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Figure 4.15. Micro-Raman spectra for high density plasma processed nanostructured diamond films on Ti-6Al-4V alloy at different N2/CH4 feed gas fraction. The spectra were normalized from the as received one shown in the insert.

The plan view AFM images in Figure 4.16 of the as-grown diamond films show the morphological change due to change in different deposition conditions. Roughness measurement by AFM in 2 µm2 scan area for diamond films deposited in CH4/H2 plasma

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without N2 (Figure 4.16-a) and with N2 addition (Figure 4.16-b) was found to be 41 nm and 17 nm respectively. The AFM images of diamond films with different N2/CH4 ratios in He/H2/CH4/N2 plasma are shown in Figure 4.16 (c-h). We could clearly observe the morphological change with change in the N2 concentration in the feed gases. Earlier roughness measurement showed that by adding He in CH4/H2 feedgas (with no N2) changed the roughness of the diamond films dramatically (shown in Figure 2b). We also reported 9-10 nm RMS roughness value from diamond films deposited in He/H2/CH4/N2 with 71% He in He+H2 and N2/CH4 ratio of 0.1. By increasing N2/CH4 ratio up to 0.4 in the same gas mixture roughness decreased even further to 6 nm (RMS) and produced ultra smooth diamond surfaces.

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(a)

(e)

(b)

(f)

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(g)

(d)

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Figure 4.16. The plan view AFM images of the as-grown diamond films prepared from microwave plasma showing the morphological change due to change in different deposition conditions (a) CH4/H2 plasma without N2 (b) CH4/H2 plasma with N2 (c-h) He/H2/CH4/N2 plasma with different N2/CH4 ratios (c) N2/CH4: 0.05 (d) 0.1 (e) 0.2 (f) 0.3 (g) 0.4. (h) 0.5.

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These roughness values increased again with the increase of N2/CH4 ratios as shown in Figure 4.17. The nanoindentation load–displacement curve and hardness values for N2/CH4 ratio of 0.4 are given in Figure 4.18. Nanoindentation measurement revealed that the hardness (H) and modulus (E) of diamond films with different N2/CH4 ratios were in the range of H = 50-60 GPa and E = 330-380 GPa respectively USND(RMS)

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Figure 4.18. Nanoindentation hardness vs depth for high density plasma processed ultra smooth nanostructured diamond coating at N2/CH4 ratio 0.3 in He/H2/CH4/N2 plasma. Nanoindentation loaddisplacement curve for same sample is shown in the insert.

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The effect of He addition is not a simple effect of plasma dilution, but it based on a more complex mechanism. Diamond films grown in H2/CH4/N2 plasma without He at corresponding high CH4/H2 ratio of 0.6 produced poor quality films with high content of graphitic phase [37]. Helium addition reduced the diamond grain size and this suggests that the rate of secondary nucleation/renucleation increases in He/H2/CH4/N2 plasma, terminating the growth of large diamond nanocrystals. The effect of He addition was simply to increase the effective CH4/H2 ratio in He/H2/CH4/N2 plasma, which should reduce the effect of hydrogen on suppressing secondary nucleation by regasifying nondiamond carbon. Helium has also a strong influence on the CN radical on the degree of diamond nanocrystallinity [2931]. Small amounts of CN and HCN in conventional mixtures (CH4/H2) effectively abstract adsorbed H atoms, creating vacant growth sites and thereby reducing the carbon supersaturation [37-39]. The use of large N2 additions (N2 /CH4 ratios greater than 0.05) resulted in a reduction of diamond phase purity, a more nanocrystalline structure, and a smoother film surface. The larger amounts of CN and HCN resulted in excessive abstraction of adsorbed H which leaves the surface open to further adsorption by CN or other nitrogen species that are not able to stabilize the diamond structure efficiently [39]. Therefore, higher CN species promote higher nanocrystallinity and more CN species form in N2 and He gas mixture. Apart from causing the nanocrystallinity of the diamond component in the film, the addition of nitrogen in the gas phase also resulted in amorphous carbon content in the film with a corresponding increase in the Raman 1550 cm-1 peak intensity [40]. Higher CN levels also induced increased twinning and stacking faults resulting in the nanocrystalline structure. Our previous OES measurements taken from He/He/CH4/N2 plasma reflected that at 71% He content and N2/CH4 ratio of 0.1, both the CN/Hα and C2/Hα were maximized. The present results indicated that at the same 71% He content and 0.4 N2/CH4 gas flow ratio there has been 2.5 times increase of CN/Hα ratio. There was no significant change in C2/Hα values. The lowest roughness and smaller grain size values were achieved in the diamond films at the N2/CH4 ratio of 0.4. Thus, CN has a strong influence in formation of smooth nanocrystalline diamond films and the activity of CN radical is highly effected by the addition of He. In summary, we have synthesized ultra smooth nanostructured diamond films on Ti-6Al4V medical grade substrates by adding helium in H2/CH4/N2 plasma and by changing the N2/CH4 gas flow from 0 to 0.6. We were able to deposit diamond films with 6 nm (RMS) roughness in 2 μm2 area and grain size 4-5 nm. Roughness decreased from RMS 22 nm to 6 nm from N2/CH4 gas flow ratio of 0.05 to 0.4 (CH4 is fixed) and then increased again up to 13 nm at N2/CH4 ratio of 0.6. Raman spectra were typical for nanostructured diamond films and did not show significant changes with varying N2/CH4 ratio. Nanoindentation demonstrated that the hardness and Young’s modulus of the films are in the range of 50–60 GPa and 330–380 GPa, respectively. X-ray diffraction showed that all the spectra have broad diamond (111) peaks characteristic of nanostructure diamond and the grain size was calculated between 4-8 nm. The grain size decreased and drop to around 4-5 nm as the N2/CH4 ratio increased up to 0.4, and then again increased. The surface morphology imaged by nano SEM at 300,000X also confirms the nanocrystalinity of the diamond films. It was also found that as the N2 content increased the intensity of the TiC peak decreased. At a N2/CH4 ratio of 0.4 there was no (200) TiC peak and the intensity again increased as the N2/CH4 ratio increased beyond 0.4. It was previously described that reducing diamond grain

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size and film surface roughness by He addition is attributed to plasma dilution, enhanced fragmentation of carbon containing species, and enhanced formation of CN radical. From optical emission data we found that CN/Hα relative intensity was highest at a N2/CH4 gas concentration ratio of 0.4, which resulted in the smoothest nanostructured hard diamond films. Therefore it can be concluded that CN radical has a strong influence in formation of smooth nanocrystalline diamond films.

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5. Growth Mechanism and Mechanical Wear Studies of Ultra Smooth Nanostructured Diamond (USND) Coatings Though many research groups have been producing nanocrystalline diamond (NCD) films, there have been different views as to what is responsible for growing such thin films. The traditional growth route was based on high H2 content (99%) with very low CH4 content (1%) [34]. However, this leads to microcrystalline diamond films with high surface roughness [41]. In an attempt to grow smooth diamond films, a new plasma chemistry was introduced by Gruen and co-workers. This is either a CH4/Ar mixture or a C60/Ar mixture, both with very little hydrogen content [34,42]. Strong C2 Swan band optical emission was observed using optical emission spectrometry (OES) and from this it was proposed that C2 may be the growth species for NCD. Ab initio calculations supported the hypothesis that C2 could insert directly into the C–H bonds which terminate the growing diamond surface. Similar results were obtained using Ar/H2/CH4 gas compositions in microwave plasma. These high noble gas content mixtures provided new routes for diamond growth. Researchers have found that high noble gas content with some carbon precursor allows for secondary nucleation to occur [34]. This leads to nanocrystalline diamond with crystallite sizes 3-15 nm [34]. However, later research was aimed at determining whether C2 dimer was responsible for nanocrystalline diamond. Rabeau et al [43] grew nanocrystalline diamond films in Ar/H2/CH4 and He/H2/CH4 plasmas. They detected via OES strong C2 emission in the Ar plasma, yet in the He plasma C2 could not be detected. However, their conclusion was based on the fact that for both noble gas containing plasmas the growth rates were similar. This along with the fact that nanocrystalline diamond film was grown in the presence of the He plasma, which contained undetectable C2 levels, led them to the conclusion that C2 dimer is not the key growth species [43]. Also, there was no correlation between the amount of C2 and the growth rate in the Ar plasma [43]. Since it was accepted that the addition of noble gas to the plasma greatly enhances the quality by reducing the roughness and grain size of the NCD, research was directed at optimizing the other parameters involving diamond growth. Catledge and Vohra [37] studied the effect of N2 on the way to produce nanostructured diamond films. They found that N2 addition produced smooth nanocrystalline diamond coatings with root-mean-squared (rms) surface roughness of 18 nm. They concluded that the addition of N2 results in a smoother film [37]. Our results indicate that the addition of He to the plasma gas mixture H2/CH4/N2 produced nanostructured diamond with RMS surface roughness of 9-10 nm with average grain size of 5-6 nm. These results were obtained under the optimal condition when He occupied 71% of the gas mixtures. We also found a difference

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in C2 intensity when using He instead of Ar. In Ultra Smooth Nanostructured Diamond (USND) coating roughness as low as 5 nm and grain size less than 6 nm was obtained by varing the N2/CH4 from 0.05 to 0.6 in H2/CH4/N2/He and the optimal value of low roughness and grain size was achieved at the N2/CH4 of 0.4. Our results also imply that nanocrystalline diamond can be grown without high C2 content. Here we demonstrate that the CN species in the H2/CH4/N2/He plasma might be responsible for producing nanocrystallinity in the USND coatings. Characterization of the deposited coating were done by using Raman spectroscopy, glancing angle XRD, atomic force microcopy (AFM), optical emission spectroscopy (OES), nanoindentation technique etc. We also evaluated the wear performance of the USND coated titanium alloy against standard polyethylene that is the common material for orthopedic and dental implants.

5.1. Methods USND coatings were deposited by microwave plasma chemical vapor deposition (MPCVD) technique using He/H /CH /N gas mixture. Optical emission spectroscopy (OES)

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2

4

2

was performed to qualitatively determine the activated species present in the plasma. All the measurements were taken with 3000 points in the range of 350-700 nm wavelength and integration time of 250 ms. Three different sets of samples were subjected to wear testing. 1. USND coated Ti–6Al–4V disks against polyethylene and compared with medical grade cobalt chromium alloy (CoCrMo). 2. USND coated Ti–6Al–4V disks against USND coated pins 3. Multilayer diamond (defined as a four layers structure composed first layer of nanostructured diamond using H2/CH4/N2 plasma then second layer of micro-structured diamond using H2/CH4 plasma, third layer of nano-structured diamond and final layer of USND coatings) coated Ti–6Al–4V disks against multilayer diamond coated Ti–6Al–4V pins. The wear test was performed using the OrthoPOD® (AMTI, Watertown, MA) following the guidelines set by the American Society for Testing and Materials (ASTM Standard F 732) [44]. For the test lubricant we used bovine calf serum that was mixed with 80 mL of 250 mM ethylenediaminetetraacetic acid (EDTA) that binds the calcium, and mixed with 2g of sodium azide, an antibacterial agent that stops the formation of calcium phosphate on the components or on its surfaces [45,46]. The temperature of the serum was held constant at 37oC. Since, the OrthoPOD® is a six station pin-on-disk apparatus; we used one station for a soak control pin that had no contact to a disk. It was used for any fluid absorption and in the calculation of mass loss for each pin, which accounted for any mass change of the soak control pin during the wear test [45]. The CoCrMo disk was polished by following the same procedure as for Ti–6Al–4V. We then placed the disks into the stages where stations 1-4 were the USND coated Ti–6Al–4V disks (USND 1 to USND 4), station 5 was the CoCrMo disk, and station 6 was used for the soak control pin. Before starting the wear test the contacting surfaces of each UHMWPE pin were shaved (approximately 180 to 270 µm) using a Microtome (Polycut S Reíchert-Jung BE16023). This gave it a smooth finish, which was free of any machining marks. Before starting the wear tests the pins had been soaked for at least two weeks in distilled water [45]. Prior to starting the wear test, the pins were cleaned according to ASTM Standard F 732 in 1% LiquiNox® solution that was sonically agitated for 15 minutes, then

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rinsed with distilled water and sonically agitated again in distilled water for five minutes. The pins were dried with a lint free tissue and then placed in ethyl alcohol for about three minutes then dried under a dust-free vacuum for 30 minutes. After each cleaning the pins were weighed on a Mettler Toledo AG245 microbalance with five separate measurements to 0.00001g, and then averaged. All of the pins were weighed prior to testing and after every 500,000 cycles. We ran the wear test for two million cycles at 500,000 cycles per interval where this amount of cycles showed wear of the UHMWPE pins. At the end of every 125,000 cycles, the OrthoPOD® was programmed to record the loads and coefficient of friction of every pin. In finding the wear factor (k) we used Achard’s Law, which can be calculated by finding the volumetric wear rate or the volume loss (Q) and the product of the average vertical force (F) with the total sliding distance (d), and then dividing the two to give the equation where k = Q/(F*d). This wear test ran under applied forces that ranged from 15 to 130 N. The UHMWPE pins were articulating on the disks in a square shaped pattern, where the surface of the polyethylene will change based on the multidirectional motion of the pin. This prevents the polymer chain in the polyethylene to align itself and present a practical simulation [46, 47]. The wear test was also performed for diamond coated Ti–6Al–4V disks and diamond coated Ti–6Al–4V pin couples. The tests were done for each pin and disk for 1000 cycles at 5 Newtons.

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5.2. Results In the previous section it has been reported that with the introduction of helium gas in H2/CH4/N2 plasma, the transformation from microcrystalline to nanocrystalline occurred and roughness decreased dramatically. The roughness decreased from 19-20 nm to 9-10 nm with the introduction of helium up to 71%. We again found that the roughness goes down to 5 nm and grain size of 6 nm using H2/CH4/N2/He plasma where N2/CH4 = 0.4. In this section we tried to correlate the roughness and grain size with the amount of CN species present in the plasma. It was found that with the increase of He addition in the H2/CH4/N2/He plasma the CN/Hα species increased. In order to investigate the amount of CN species in the plasma different gases in several proportions have been introduced in the CVD reactor and OES spectra was obtained during the experiment. Since multiple gases were introduced into the CVD reactor, the plasma chemistry can become quite complicated. CN is one fragment in the plasma that has a strong detection peak in OES. Different flow rates (sccm) of H2, CH4, N2, and He were studied and CN species normalized by Balmer H line were monitored. Several α

diamond coatings were deposited using different amount of CN concentration in plasma (from high to low). After that correlations were made with film quality like roughness, hardness and nanocrystallinity with the relative amount of CN species in the plasma. Among all the gas combinations H2/CH4/N2/He: 100/36/21.6/250 sccm produced CN/H of 7.6, H2/CH4/N2/He: α

100/36/14.4/250 sccm produced 6.4 where as H2/CH4/N2/He: 200/36/14.4/300 sccm

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produced 2.8 and H2/CH4/N2/He: 500/88/8.8/0 sccm of 0.87. The optical emission spectra of the He/H2/CH4/N2 microwave plasmas with CN/H of 6.4 and 2.8 are shown in Figure 5.1. α

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Figure 5.1. The optical emission spectra of the He/H2/CH4/N2 microwave plasmas with different ratios of CN/Hα in the plasma.

Interestingly it was found that there was a very small amount of C2 emission in the plasma compare to CN and other species. Therefore, C2 dimer is not contributing for the growth of nano-structured diamond coatings. On the other hand there was a strong peak from CN emission in the OES spectra and it changed with the small change of plasma chemistry. Also, an experiment was performed to determine if changing the absolute flow rates of the gases while keeping the ratios the same would affect the results. It was concluded that the flow rate does not affect results as long as the ratio is unchanged. Further analysis via AFM, Raman spectroscopy, thin-film X-ray diffraction (XRD) and nanoindentation were used to characterize the film grown with this experimentally optimized plasma chemistry containing low and high values of CN/H . α

Figure 5.2 shows the diamond growth rate in relation to CN/H ratios. The growth rate α

dropped as the CN/H ratios increased and it stayed constant with the increase of the CN α

concentrating in the plasma. Higher levels of CN radicals reduce the CH3 concentration and thus reduce the growth rate [30].

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Temperature (degree C)

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Figure 5.2. Growth rate of diamond films at different CN/Hα ratios. In the inset, the optical interference pattern collected from interferometer is shown for film deposited at CN/Hα of 7.6.

TiC

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1200

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1000

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800

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0 30

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Figure 5.3. The glancing angle XRD patterns for the nanostructured diamond films grown on Ti–6Al– 4V alloy using the He/CH4/H2/N2 feed gas mixture where CN/Hα in the plasma is 7.6. The insert show the close-up view of TiC and diamond (111) peaks showing the change due to the change of CN/Hα.

Figure 5.3 shows the glancing angle XRD patterns for the nanostructured diamond films grown on Ti–6Al–4V alloy using the He/CH4/H2/N2 feed gas mixture where CN/H in the

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α

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plasma is 7.6. Characteristic of these patterns are the cubic diamond (111) and (220) reflections as well as several peaks attributed to interfacial titanium carbide phases. The diamond peaks shown in the insert were significantly broadened as compared to those obtained from the conventional CVD process. The diamond coating deposited with high CN/H (7.6) produced broader peak than the diamond coating deposited with CN/H α

α

value of 2.8. The results demonstrated that the plasma containing more species of CN produced broader diamond (111) peak due to the production of more nano-structured diamond. The average grain size as calculated from the diamond (111) peak width using the Scherrer formula was between 4-6 nm. Scanning Electron Microscope (SEM) image at 400,000X on the surface of diamond sample grown in He/H2/CH4/N2 plasma containing CN/H ratio of 5.1 is shown in figure 5.4. The grain size is as low as 5.7 nm and confirms the α

diamond nanostructure. The micro-Raman spectra for the nanostructured diamond coatings deposited in the plasma containing different CN species concentration are shown in Figure 5.4. The Raman spectra show the significantly broadened diamond peak at around 1332 cm-1 due to the sp3 bonded carbon [19]. Again slightly broaden peak at around 1450-1600 cm-1 region is assigned to graphite (sp2 bonded carbon, G band). The Raman spectra have another peak at 1150 cm-1 in addition to the main diamond and graphite bands. It is suggested that this band is due to the nanocrystalline nature of the diamond films [36]. The films produced consist of diamond nanocrystallites imbedded in amorphous carbon matrix with a relatively small amount of graphitic carbon. The Raman spectra collected from different diamond coatings show significant difference in their 1550 cm-1 and 1150 cm-1 bands. These bands appear to sharpen as the CN/H ratio decreases.

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α

Figure 5.4. Raman spectra of nanostructured diamond coatings deposited in different CN/Hα in the plasma.

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AFM images revealed that with the increase of CN species concentration in the plasma it was possible to produce smoother diamond coatings. Figure 5.5 shows the relation between the CN species concentration (normalized by Balmer Hα line) with the roughness of the diamond surfaces. As the CN species concentration increased the roughness was decreased. RMS roughness value as low as 4.3 nm (2 µm2 scan area) was achieved using H2/CH4/N2/He of 100/36/21.6/250 sccm containing CN/H of 7.6 in the plasma. AFM images shown in α

Figure 5.6 clearly demonstrate the morphological change due to the change of CN species concentration in the plasma. The hardness and Young’s modulus of USND coating deposited with CN/H value of 5.1 α

is 65 ± 5 GPa and 400 ± 24 GPa. The hardness and nanoindentation modulus variation over depth is shown in the Figure 5.7 along with typical load-displacement curve. Generally hardness has been regarded as a primary material property, which defines wear resistance. There is a strong evidence to suggest that the elastic modulus can also have an important influence on wear behavior, in particular, the elastic strain to failure, which is related to the ratio of hardness (H) and elastic modulus (E). It has been shown by a number of authors [48] that the elastic strain to failure could be a more suitable parameter for predicting wear resistance than hardness alone. It is also significant that the ratio between H and E described in terms of ‘plasticity index’ or ‘elastic strain to failure’ are widely quoted as a valuable parameter in determining the limit of elastic behaviour in a surface contact, which is clearly important for the avoidance of wear [47,49]. We calculated the H/E value as 0.16 for USND coatings where the typical value for biomedical grade polyethylene is 0.07, Ti-6Al-4V is 0.03 and CoCrMo is 0.04. 18

RMS roughness (nm)

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16 14 12 10 8 6 4 2 0 0

1

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CN/Hα Rel. Intensity Figure 5.5. A plot of the surface roughness measured in 2x2 μm area of different as-grown diamond films with different CN/Hα in the plasma.

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(a)

(b)

(c)

(d)

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Figure 5.6. AFM images of the as-grown diamond films prepared from microwave plasma showing the morphological change due to change in different CN/Hα in the plasma (a) CN/Hα of 0.87 (b) CN/Hα of 1.8 (c) CN/Hα of 2.8 (d) CN/Hα of 7.6. 800

100 Hardness

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Displacement into surface (nm) Figure 5.7. Nanoindentation hardness and modulus vs depth for ultra smooth nanostructured diamond coating at CN/Hα = 5.3 in He/H2/CH4/N2 plasma. Nanoindentation load-displacement curve for same sample is shown in the insert.

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Two types of wear studies were performed. First we studied diamond coated Ti-6Al-4V surfaces against polyethylene (Figure 5.8-a) and then diamond coated pins against diamond coated disks (Figure 5.8-b).

Figure 5.8. (a) Polyethylene-diamond coated titanium alloy disk system. (b) Diamond coated pin and disk system.

All four of the USND coated metal surfaces against polyethylene pins exhibited lower wear than CoCrMo on polyethylene. The wear of the UHMWPE pins were graphed on up to 2 million cycles in Figure 5.9. 10 USND-1 USND-2

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CoCrMo

USND-3

3

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8

USND-4

6

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4 2 0 0

0.5

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1.5

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-2

Number of cycles (Mc) Figure 5.9. The wear rate of four USND coated titanium alloy articulating against polyethylene. The wear rate of CoCrMo against polyethylene is also shown for comparison.

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The average value of k for USND was 7.41 mm3/Nm and the value for CoCrMo was more than twice the USND, at 15.1 mm3/Nm, shown in Figure 5.10

Figure 5.10. The wear factor k for USND coated titanium alloy and CoCrMo articulating against polyethylene.

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In the diamond against diamond wear, virtually no wear was seen in the pin and disk combinations coated with multilayered coatings. SEM images in Figure 5.11 show that diamond contact virtually polishes the diamond surfaces.

Figure 5.11. SEM images showing the wear pattern (polished surface) on multilayer diamond coating after wear test. Raman spectra on the diamond coating showing the evidence of diamond layer both in the wear track (point A) and away from the wear track (point B). Multilayer geometry (combined with micro, nano and USND diamond layers) is also shown.

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The evidence of diamond coating in the polished area was analyzed by Raman spectroscopy. It was found that both in the wear track (polished area) and in the area away from it, the characteristic diamond Raman spectra remain proving that no coating delamination occurred after the wear test. Some evidence of wear was found only in the Ti6Al-4V pins but not in the disks when we use only USND layer to coat both the pins and disks. These results confirm earlier wear study with multilayer coatings (micro diamond using H2/CH4 and nanodiamond using H2/CH4/N2) approach and proved the hypothesis that multilayer diamond coated Ti-6Al-4V wear couple might have better performance when we consider diamond against diamond articulating surfaces. It has been previously claimed that C2 species are responsible for producing nanocrystalline diamond. In this work we found that we can grow nanostructured diamond with a very low concentration of C2 dimers proving that C2 is not responsible for growing nanostructured diamond. CN radical in the plasma was the dominant species in our effort to produce the nanostructured diamond coating using H2/CH4/N2/He feed gases. With the change of plasma chemistry we can control CN species concentration in the plasma. Our results show that with the increase of CN species concentration in the plasma we can grow ultra small grain sized diamond. Small amount of CN and HCN in conventional mixture (H2/CH4) effectively abstract absorbed H atoms, creating vacant growth sites, thereby reducing carbon supersaturation. The larger amounts of CN and HCN resulted in excessive abstraction of adsorbed H which leaves the surface open to further adsorption by CN or other nitrogen species that are not able to stabilize the diamond structure efficiently [24]. Therefore, higher CN species concentration promotes higher nanocrystallinity. Higher CN species concentration also promotes poisoning of diamond growth resulted nanostructured diamond. Helium also has a strong influence on increase of the CN radical which control the degree of diamond nanocrystallinity. Helium addition reduced the diamond grain size and this suggests that the rate of secondary nucleation/renucleation increases in He/H2/CH4/N2 plasma, terminating the growth of large diamond nanocrystals. Diamond coatings deposited with different CN concentration in the plasma were analyzed by AFM, Raman, glancing angle XRD, nanoindentation technique etc. Films grown with higher CN species concentration in the plasma produced smoother and nanostructured diamond coating without compromising the mechanical properties. Therefore CN might be responsible for producing nanostructured diamond coating using H2/CH4/N2/He feed gases. We demonstrate that with the increase of CN species (normalized by Balmer Hα line) in the plasma we can deposit ultra smooth diamond coatings. Roughness as low as 4 nm and grain size of 5-6 nm were achieved using the plasma which gives highest CN species concentration. It has been previously claimed that C2 species is responsible for producing nanocrystalline diamond. In this work we found that we can grow nanostructured diamond with a very low concentration of C2 dimers proving that C2 is not responsible for growing nanostructured diamond. Diamond coatings deposited with different CN concentration in the plasma were analyzed by AFM, Raman, glancing angle XRD, nanoindentation technique. The characterization results proved that the films grown with higher CN species concentration in the plasma produced smoother and nanostructured diamond coating without compromising the mechanical properties. Therefore CN might be responsible for nanocrystallinity in the diamond coating using H2/CH4/N2/He feed gases. Wear properties of

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these USND coating were also promising. All of the diamond (USND) coated Ti-6Al-4V disks had better wear performance against polyethylene compare to CoCrMo and polyethylene wear couple. In diamond-on-diamond wear tests, multilayer diamond coated Ti6Al-4V wear couple (both pin and disk) have better wear performance than single layer USND coated wear couple (pin and disk) proving the hypothesis that multilayer diamond coated Ti-6Al-4V wear couple might have better performance when we consider diamond against diamond articulating surfaces.

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6. Biocompatibility Study of Nanostructured Diamond for Orthopedic Implant Applications Diamond coatings have the advantage of chemical inertness, high electrical resistivity, and impermeability that is expected to reduce the crevice corrosion that is commonly seen on conventional metallic implants. These properties have led many groups to consider carbon coatings for applications that require osseointegration. Extensive literature is available that describes the biocompatibility of some carbon-based hard materials, such as pyrolytic carbon and diamond-like carbon (DLC) (reviewed in [50-53]. However, the nature of the carbon forms used in almost all of the previous studies is either amorphous (with primarily sp2 bonding) or turbostratic pyrolytic carbon (similar to graphite, but with disordered layer structure). In comparison to other carbon coatings, very few biocompatibility studies have been performed on diamond produced by chemical vapor deposition (CVD diamond). One indicator of biomaterial performance in orthopaedic applications is the interaction of the material with osteogenic cells. In the current study, we evaluated the behavior of human mesenchymal stem cells (MSCs) on USND discs that were modified with H, O, or F surface treatments. We found that O and F-treated USND substrates did not support any cell adhesion or survival, suggesting that these surfaces were essentially inert. In contrast, H-terminated substrates were highly cytocompatibility. More specifically, MSC adhesion, proliferation, and osteoblastic differentiation on H-terminated USND appeared comparable to cell responses elicited by CoCrMo and Ti-6Al-4V, two biocompatible metals currently utilized in most commercially available implant designs. Collectively, these results suggest that USND is a very versatile material which may have utility for both the articulating and anchoring regions of implants.

6.1. Methods The surface atoms of the USND coating were replaced with either H, O, or F. Immediately following film deposition in the CVD reactor, the standard practice for our previous studies has been to slowly cool the sample in a 100% hydrogen plasma by gradually reducing the microwave power over a 10-min period. This practice produces a H-terminated USND lattice that is very hydrophobic. A 10-min treatment with plasma composed of 10 sccm O2 and 100 sccm He at 450°C produces an O-terminated USND lattice that is very hydrophilic. Additionally, F-terminated USND (hydrophobic) was produced by introducing

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F2 gas into a closed chamber containing the H-terminated USND films for 48 hours at 100°C. These surface termination states were confirmed by x-ray photoelectron spectroscopy (XPS). Elemental composition of the surface atoms was determined from XPS measurements on H-terminated USND, O-terminated USND, and F-terminated USND. Spectra for the Fterminated sample were obtained at a 90° take-off angle using a Kratos Axis 165 system (Kratos Analytical, Chesnut Ridge, NY) and monochromatic Al Kα x-rays at 210 W (14 kV, 15 mA). A charge neutralizer at a nominal bias of -1.15 V was used to compensate for peak shifting. Pass energies of 80 eV and 20 eV were used for survey and high-resolution scans, respectively. Spectra from the other two samples were obtained at 90° take-off angle using a Kratos XSAM 800 system with Mg Kα x-rays at 225 W (15 kV, 15 mA) and with the same pass energies. Compositional information was taken from survey and high-resolution via standard calculations that involve peak areas and component sensitivity factors. The RMS roughness was measured on 2 × 2 µm scan areas by AFM (TopoMetrix Explorer). Surface wettability of the substrates was determined by the half angle method using a CAM-MICRO model contact angle meter (Tantec Inc., Schaumburg, IL), with deionized water as the probe liquid. Two spots were measured from each of the three samples and averaged. Human mesenchymal stem cells (MSCs) were isolated from bone marrow donations. Briefly, cells were pelleted by centrifugation, resuspended in Dulbecco’s Modified Eagle Medium (DMEM), and then applied to a Histopaque-1077 column (Sigma, St. Louis, MO). A density gradient was generated by centrifugation at 500 g for 30 min. Cells from the DMEM/Histopaque interface were extracted with a syringe and seeded onto tissue culture dishes and cultured in DMEM containing 10% fetal bovine serum. Cells had a homogenous and fibroblast-like appearance, and no osteoclasts or adipocytes were present, as measured by Tartrate Resistant Acid Phosphatase (TRAP) and Oil-O-red staining, respectively. Bone marrow samples were obtained with prior approval from the University of Alabama Institutional Review Board. As a first assessment of cytocompatibility of USND with hydrogen, oxygen, or fluorine surface treatments, MSC morphology was evaluated following 1 hour culture by reflected light microscopy. Mesenchymal stem cells were seeded at a density of 5×104 MSCs/disc onto H, O, or F-terminated USND discs and cultured in serum-free DMEM at 37°C for 1 h. Unattached cells were removed by washing with PBS. Digital images of the discs were taken following fixation in 3.7% formaldehyde, using reflected light microscopy (Fisher Micromaster light microscope equipped with top-mounted light source, objective lens, and digital camera). Cells could be visualized without staining on the highly polished surfaces. SEM images were taken in order to observe the spread morphology observed on the Hterminated USND surface. For these images, MSCs were cultured for 24h, rinsed with PBS to remove unattached cells, and fixed in 2.5% glutaraldehyde in PBS. The attached cells were dehydrated in a gradient of ethanol in water, followed by a gradient of hexamethyldisilazane (HMDS) in ethanol, and then sputter coated with Au/Pd for imaging. Images were obtained using a Philips 515 SEM with an accelerating voltage of 10 kV. Fluorescent images were taken to observe the morphology of MSCs cultured for extended times on H-terminated USND. Mesenchymal stem cells were seeded at a density of 5×104 cells/disc onto H-terminated USND and cultured in DMEM + 10% FBS at 37°C for 1

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h, 24 h, 7 days, and 14 days. Unattached cells were removed by washing with PBS, and attached cells were fixed in 4% formaldehyde in PBS. Cells were treated with 0.2% Triton-X 100 (Sigma T9284) in PBS, blocked in 2% denatured bovine serum albumin (dBSA), and probed for actin with Alexa 488-phalloidin (Invitrogen A12379, 1:200) in 25 mM Tris buffer containing 2% dBSA for 45 min at 37°C. Nuclei were labeled with 20 μg/mL DAPI (Invitrogen D21490) in PBS for 4 min at room temp, followed by rinsing in Tris-buffer. Fluorescent images were taken with an 80i Nikon Eclipse microscope. Adhesion of MSCs to H-terminated USND was compared to Ti-6Al-4V and CoCrMo. Three samples of each material were coated with fetal bovine serum overnight at 4°C, and three samples of each were left uncoated. Following serum coating, samples were washed with PBS to remove loosely-bound proteins. Mesenchymal stem cells were added at a concentration of 5×104 cells/disc in serum-free DMEM and allowed to adhere for 90 min at 37°C. Unattached cells were then removed by three washes with phosphate-buffered saline (PBS) on a mechanical shaker. Attached cells were lysed by ultrasonic agitation in 10 mM Tris, 1 mM EDTA buffer at pH 8 (TE buffer) containing 1% Triton X-100. The DNA content of the attached cells was assayed by addition of Picogreen reagent (Molecular Probes) according to manufacturer’s protocol. Absorbance was read on a spectrometer at 612 nm, compared to a DNA standard curve, and normalized to CoCrMo. Two independent experiments were performed, with each surface tested in triplicate. The Ti-6Al-4V, CoCrMo, and H-terminated USND discs were coated with fetal bovine serum (FBS) overnight at 4°C. They were washed twice with PBS to remove unattached proteins. The remaining proteins adsorbed to the surfaces were removed by shaking for 5 min in 100 µL of boiling 25 mM Tris buffer containing 5% 2-mercaptoethanol and 20% SDS. Gel loading on each lane was on an equal volume basis, with 50 µL/lane of each desorbed protein solution resolved on a 6% polyacrylamide gel. For western blot analyses, proteins were transferred to PVDF membrane, blocked in 5% dry milk, and probed with polyclonal antifibronectin primary antibody (Chemicon AB1954, 1:2000), followed by an HRP-conjugated secondary antibody (ECL NA9340V, 1:2000). Proteins were detected by enhanced chemiluminescence (Immobilon, Milipore, Billerica, MA). Proliferation of MSCs on H-terminated USND was compared to Ti-6Al-4V and CoCrMo. Mesenchymal stem cells were seeded at low density (7,500 cells/disc) and cultured in DMEM containing 10% FBS. After 3 days or 7 days of culture, the media was replaced with 200 µL of DMEM (free of phenol red) containing 100 µg MTT. The viable cells were allowed to convert the MTT to formazan for 4 hours before lysing cells with SDS in 0.01 M HCl. Absorbance of formazan was read on a spectrometer at 570 nm and normalized to CoCrMo. Two independent experiments were performed, with each surface tested in triplicate. MSCs were added at a high density (5×104 cells) to USND, Ti-6Al-4V and CoCrMo discs, and were cultured for two days in DMEM containing 10% FBS to form a confluent cell layer on the surface. The media was then replaced every 2-3 days with an osteogenic media (except for the negative control) composed of DMEM + 10% FBS, supplemented with 0.05 mM aspartic acid, 10 mM 2-glycerolphosphate, and 100 nM dexamethasone. A mineralized matrix could begin to be visualized on the highly polished surfaces at approximately 3 weeks (data not shown). After 4 weeks, cells were lysed and the mineralized matrix was solubilized

Aoi, T., and A. Toshida. Hip Replacement : Approaches, Complications and Effectiveness, Nova Science Publishers, Incorporated, 2009. ProQuest Ebook Central,

S. Chowdhury, Yogesh K. Vohra and William C. Clem

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by shaking for 24 h in 0.5 M HCl. Supernatants were analyzed for calcium by addition of phenolsulphonephthalein dye (Quantichrom, Bioassay Systems, Hayward, CA), which forms a stable blue colored complex specifically with free calcium. The intensity of the color, measured at 612 nm, was compared to a CaCl2 standard curve and normalized to CoCrMo. Three independent experiments were performed, with each surface tested in triplicate. Data sets were assessed using one-way ANOVA. If significant differences were found, Fisher’s Protected Least Significant Differences post hoc test was used to determine the level of significance. A 95% confidence level was considered significant.

6.2. Results The surface layer of an implant determines essentially the biocompatibility whereas the bulk material imparts its mechanical properties. In the current study USND, Ti-6Al-4V, and CoCrMo substrates were polished in an attempt to minimize differences in topography between samples. The RMS roughness of all samples was very similar; the RMS values were 6 nm for Ti-6Al-4V, 4 nm for CoCrMo, and 5 nm for USND. Samples were also analyzed by XPS, which reveals the atomic concentrations of elements in the topmost 10 nm of the surface. The XPS survey scans from H-terminated, Fterminated and O-terminated USND are shown in Figure 6.1. Compositional ratios for O/C and F/C calculated from these spectra are given in Table 6.1. The increases in O/C and F/C, when compared to the values from the H-treated sample, represent a direct increase in the surface number density of oxygen or fluorine atoms respectively. The H-terminated and O-terminated samples showed Si peaks, while the F-terminated samples showed S peaks. Component fits were also done on high resolution scans of the C 1s and O 1s peaks for each sample (not shown). The ratio of hydroxyl to carbonate oxygen and carbonyl to aliphatic carbon were determined from these fits and are included in Table 6.1.

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Table 6.1. Relative composition ratios determined by XPS (mol%/mol%) and water contact angles determined on samples for each type of treatment. The O/C and F/C ratios were measured from survey scans in Figure 6.1. The hydroxyl to carbonate and carbonyl to aliphatic ratios were measured from high resolution scans of oxygen and carbon, respectfully Termination

O/C

F/C

OH/COO

C=O/C-C

Contact angle

H

0.08

--

0.10

0.19

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O

0.21

--

1

1