Improved wear performance with crosslinked UHMWPE and zirconia implants in knee simulation

Background Suggestions for improved wear performance of total knee replacements have included replacement of standard CoCr femoral components with ceramic and replacement of 3.5-Mrad ultra-high molecular weight polyethylene (UHMWPE) inserts with 5- or 7-Mrad UHMWPE inserts. The ceramic materials used clinically have included alumina, zirconia ceramic and oxidized zirconium. Patients and method We compared both CoCr and zirconia versions of the Bi-Surface knee replacement in a 6-station knee simulator using alpha calf serum for lubrication (20 mg protein per mL) to evaluate the relative bearing performance. Results We studied the 4-way knee simulation of implant materials: zirconia ceramic, CoCr, 3.5-Mrad UHMWPE, and 7-Mrad UHMWPE. With CoCr femoral components, the 7-Mrad UHMWPE resulted in a 5- to 8-fold reduction in wear compared to the 3.5-Mrad insert. With the 3.5-Mrad insert, the zirconia bearing provided approximately 4-fold wear reduction compared to CoCr. These wear rates with standard UHMWPE were similar to published wear studies on entire knees. With the exception of the CoCr 7-Mrad and ZrO2 3.5-Mrad combinations, the wear differences were statistically significant. Interpretation The ZrO2 7-Mrad UHMWPE combination gave the best performance, with no measurable wear over the 5.5 million cycle test duration.