This book addresses the background and significance of the factors potentially influencing the clinical and biological outcomes of metal-on-metal hip implants. Metal-on-metal bearings were introduced and evaluated as an alternative to other bearing couples, particularly metal-on-polyethylene, due to their enhanced wear resistance as determined in laboratory testing. Initially, reports of short-term clinical outcomes were favorable and an increasing number of metal-on-metal prostheses were implanted. Subsequently, isolated case findings describing adverse tissue responses around the articulation became the harbinger of an increasing number of reports describing pseudotumors and other significant lymphocytic-based responses associated with metal-on-metal prostheses. Questions have been raised as to whether this is an implant, design, or patient-specific response. The reasons why some patients have a negative biological response and pathology while others do not remain to be determined, but tens of thousands of patients in the US, the UK, and around the world are considered to be at risk. Leading researchers and clinicians describe the issues related to the nature of the biological and pathological responses and the protocols that should be followed to determine if an adverse response is occurring. This book is essential reading for researchers, engineers, and orthopaedic surgeons who are involved in the design, evaluation, and implantation of metal-on-metal prostheses.

Over a million metal-on-metal hip prostheses have been implanted since 1996, according to the Hip Society [1]. Controversy arose with reports of adverse tissue reactions and increased rates of revision associated with some of the implant designs during the 2000s. Today (2013), while many orthopaedists have discontinued using metal-on-metal implants, others continue to implant prostheses with this hard-onhard bearing. This book will review the current understanding of the history and rationale for the development of metal-on-metal hips, the clinical experience, the basic science, and the future. As an outgrowth of several workshops on the topic, this book represents a collaborative effort between members of the Orthopaedic Research Society, theAmericanAcademy of Orthopaedic Surgeons (Biological Implants Committee, Biomedical Engineering Committee, and the Orthopaedic Device Forum), and the Society For Biomaterials.
The book is divided into five sections. Part one provides a historical review of metal-on-metal implants and poses the questions that have been raised concerning their use. The second part focuses on the clinical experience with modern metal-onmetal implants. It discusses the results of registries and outcome studies as well as the significance of testing patients for metal ion levels and hypersensitivity. Part three explores the biological response to metal-on-metal implants. Beginning with a discussion of the basic tenets of wound healing, inflammation, and immune responses, the implications of the adverse reactions seen around metal-on-metal implants are then analyzed by experts in the field. Part four discusses the possible contribution of the materials used in the manufacture of these implants, with special emphasis on wear mechanisms and tribocorrosion. The closing chapter of the book explores future directions.
We hope that this book will become a reference source for orthopaedic residents and fellows, orthopaedists, academics studying joint arthroplasty, and their colleagues in industry.

Part I Introduction

1 Overview of Metal-on-Metal Implants 3

Lynne C. Jones,Warren O. Haggard and A. Seth Greenwald

2 Bearing Surfaces for Joint Replacement: New Materials or New Problems 13

Stuart Goodman

Part II Clinical

3 Outcome Studies for Metal-on-Metal Bearings: What Evidence-Based Medicine Tells Us 23

Michael A. Mont and Robert Pivec

4 Metal Reactivity: Its Influence on Primary and Revision Outcomes 39

George Grammatopoulos, Hemant Pandit, Adrian Low and David Murray

5 Are Metal Ion Levels a Trigger for Surgical Intervention? 63

David Langton

6 Metal Sensitivity: Is It Possible to Determine Clinically? 83

Nadim J. Hallab and Paul H.Wooley

Part III Biology

7 Wound Healing, Chronic Inflammation, and Immune Responses 109

Paul H.Wooley and Nadim J. Hallab

8 Benign Responses to Orthopaedic Implants: Really? 135

H. John Cooper and Joshua J. Jacobs

9 What Does the Histology Tell Us? 153

Thomas W. Bauer

Part IV Biomechanics

10 Why Metal-on-Metal: What Laboratory Tests Have Shown Us 165

Peter Thomas, Burkhard Summer, Marc Thomsen, Veit Krenn and Jan Philippe Kretzer

11 What Do the Retrievals Really Tell Us? 173

Robin Pourzal, Robert M. Urban and Markus A.Wimmer

Future Directions 195

A. Seth Greenwald,Warren O. Haggard, and Lynne C. Jones

Index 199

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