Accelerated Life Cycle Testing and Modeling of UHMWPE Bearings in Knee Prostheses

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9908096KennedyWear of ultra-high molecular weight polyethylene (UHMWPE) used as the bearing material in joint prostheses is a major problem in the long-term survival of replacement joints. To counteract this wear, methods of crosslinking the polymer have been developed. The new highly crosslinked materials are designed to have improved resistance to adhesive wear and they have shown nearly zero wear in recent hip simulator tests, in which the polymer bearings are subjected to oscillatory sliding contact conditions. This research will address the effects of newly introduced fabrication processes and sterilization techniques on long-term durability of UHMWPE knee joint prosthesis bearings. The loading conditions in the knee are substantially different than in the hip; they are primarily oscillatory rolling/sliding contact and contact fatigue can be a more important mode of knee bearing failure than is sliding wear as in the hip prostheses. The work will apply novel accelerated testing techniques, which are now being developed to study the changes observed in conventional UHMWPE following gamma irradiation in air, to assess the time dependent chemical and mechanical property changes in these newly developed crosslinked materials. Accelerated aging of bearing material will be done to simulate, within a few weeks, the long-term changes that previously have been documented in joint prosthesis bearings in use for 5 to 10 years or more. Numerical modeling of both the chemical oxidation process and the stress distribution in the bearings under cyclic rolling / sliding contact will be carried out to allow prediction of clinical impacts of material changes. Cyclic wear and contact fatigue testing using a knee simulator and a rolling/sliding contact tester will test the performance of unaged and aged bearing material. Correlation between the stress analyses and the test conditions will ensure accurate modeling of the complex geometry and stress states in joint prostheses. A large, established archive of joint bearings explanted from patients will be used to confirm the clinical wear mechanisms that are being modeled for predictive capability.***