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How have wear testing and joint simulator studies helped to discriminate among materials and designs?

Dr. McKellop is Professor, Department of Orthopaedic Surgery, UCLA–Orthopaedic Hospital, and Director, The J. Vernon Luck Orthopaedic Research Center, Los Angeles, CA. Dr. D’Lima is Director, Orthopaedic Research Laboratories, Shiley Center for Orthopaedic Research and Education at Scripps Clinic, La Jolla, CA.

*The Implant Wear Symposium 2007 Engineering Work Group included Donald L. Bartel, PhD, Thomas D. Brown, PhD, Ian C. Clarke, PhD, Roy D. Crowninshield, PhD, Darryl D’Lima, MD, PhD, A. Seth Greenwald, DPhil(Oxon), Steven M. Kurtz, PhD, Jack Lemons, PhD, Michael T. Manley, PhD, Harry A. McKellop, PhD, Orhun K. Muratoglu, PhD, Ebru Oral, PhD, Lisa Pruitt, PhD, Clare Rimnac, PhD, Peter S. Walker, PhD, and Timothy Wright, PhD.

Dr. McKellop or a member of his immediate family has received research or institutional support from DePuy, Wright Medical Technology, and Exactech; has received royalties and miscellaneous nonincome support, commercially derived honoraria, or other nonresearch-related funding from DePuy; and is a consultant to or an employee of DePuy. Dr. D’Lima or a member of his immediate family has received research or institutional support from Stryker, DePuy, Smith & Nephew, and Zimmer, and has received miscellaneous nonincome support, commercially derived honoraria, or other nonresearch-related funding from Stryker and DePuy.

Historically, hip joint simulators most often have been used to model wear of a bearing surface against a bearing surface. These simulators have provided highly accurate predictions of the in vivo wear of a broad spectrum of bearing materials, including cross-linked polyethylenes, metal-on-metal, ceramic-on-ceramic, and others in development. In recent years, more severe conditions have been successfully modeled, including jogging, stair climbing, ball-cup micro separation, third-body abrasion, and neck-socket impingement. These tests have served to identify improved materials and to eliminate some with inadequate wear resistance prior to their clinical use. Simulation of the knee joint is inherently more complex than it is for the hip. It is more difficult to compare the results of laboratory tests with actual clinical performance, due to the lack of accurate in vivo measures of wear. Nevertheless, knee simulators, based on force control or motion control, have successfully reproduced the type of surface damage that occurs in vivo (eg, burnishing, scratching, pitting) as well as the size and shapes of the resultant wear particles. Knee simulators have been used to compare molded versus machined polyethylene components, highly cross-linked polyethylenes, fixed versus mobile bearings, and oxidized zirconia and other materials, under optimal conditions as well as more severe wear modes, such as malalignment, higher loading and activity levels, and third-body roughening.