Originally posted on 12/6/2006

Presidential Guest Lecture: Tribology of Alternative Bearings
John Fisher, Zhongmin Jin, Joanne Tipper, Martin Stone, Eileen Ingham
Clinical Orthopedics and Related Research, Number 453, pp 25-34, December, 2006

Who is John Fisher?

The paper we’ll look at today is a review of much of the work of John Fisher, et al., from the Institute of Medical and Biological Engineering, University of Leeds, Leeds, United Kingdom. Much of Fisher’s career has been directed toward the study of the tribological behavior of total joint (hip) replacement bearing surfaces. Specifically, he has examined the wear behavior of these bearings and corresponding biological activity (osteolytic behavior) of wear debris produced by materials such as highly cross-linked polyethylene, alumina, cobalt-chromium alloy, and CrCN coatings.

John Fisher currently holds the following titles at the University of Leeds: Professor of Mechanical Engineering, Pro-Vice-Chancellor for Research, Director of the Institute of Medical and Biological Engineering, Director of BITE Centre for Industrial Collaboration, and Leader of the Biomedical Engineering Research Group in the School of Mechanical Engineering.

Alternative Bearing Surfaces: Why?

The interest in alternative bearing surfaces arises from the increasing number of relatively young and more active people receiving total joint replacements. This patient population has life expectancies following TJR of any where from 20 to 40+ years. The well-studied, conventional polyethylene has a maximum lifetime of approximately 15 years. The problem, therefore, is readily apparent, in that the wear of conventional bearing surfaces alone would cause some patients to have 2 or more revision surgeries, resulting in increased health care costs, income loss, and risk to the health of the patient.

Total Joint Wear Simulators

The tribological (or wear) behavior of implant materials is most easily discovered via the controlled environment of in vitro simulator testing where a machine is devised which (ideally) allows physiological loads, motion patterns, and lubricant conditions to be applied to implants. The resulting wear can then be studied at various stages, eliminating the need for animal or clinical studies until a (hopefully) optimized material and design are found. The problem with simulator studies lies in the ability to design machines with appropriate motion simulation, and lubrication. Also, a wide variety of simulators and study protocols have been used, making grand comparisons between various laboratories’ studies difficult.

What Fisher Found

In the past 10+ years of work reviewed in this paper, John Fisher and his research group has found, through the careful use of simulator studies, the following for these alternative bearing surfaces.

Highly cross-linked polyethylene [bearings] show a four-fold reduction in functional biological activity.
Ceramic-on-ceramic bearings have the lowest wear rates and least reactive wear debris. The functional biological activity is 20-fold lower than with highly cross-linked polyethylene. Hence, ceramic-on-ceramic bearings address the tribological lifetime demand of highly active patients.
Metal-on-metal bearings have substantially lower wear rates than highly cross-linked polyethylene and wear decreases with head diameter. Bedding in wear is also lower with reduced radial clearance.
Differential hardness ceramic-on-metal bearings and the application of ceramic-like coatings reduce metal wear and ion levels.

How Fisher’s Work Differs from Others

Fisher performed these studies with several protocol alterations as compared to most other simulator studies. First, the fluid in which he ran the simulations contained a physiologically relevant concentration of 15mg/L protein (25% v/v new born calf serum). The majority of other reports and standard practice has been to use 90% serum. One study even suggests no less than 20 mg/mL be used. This most probably led to his finding more wear for highly cross-linked polyethylene compared to similar works published by others. Second, Fisher’s group collected wear debris from their simulation studies and obtained size distributions from the material, then exposed the materials to macrophage cells and analyzed the cells’ subsequent viability and osteolytic potential. Third, he used a variety of motion patterns, whereas most published work uses only patterns that simulate walking.

Do these protocol differences lead to a more accurate prediction of the wear behavior of these materials in vivo? John Fisher believes so. Apparently so does the simulator manufacturing industry (including companies such as AMTI, Endolab, Instron, and Prosim Simulation Solutions), which is currently producing more intricate, variable machines for simulation studies. The groups that suggest study protocols, ASTM and ISO, are also developing new and improved standards for simulation testing of total joints.

Comments on Funding

Prof. Fisher’s group, like most others, was funded in part through industry grants, specifically from DePuy International, UK, Stryker Howmedica, Switzerland, and Ceramtec, Germany.

Your Turn

Now, tell me what you think about this paper or others by John Fisher!

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