WEAR OF POLYETHYLENE ACETABULAR CUPS AGAINST ZIRCONIA FEMORAL HEADS STUDIED WITH A HIP-JOINT SIMULATOR

Five zirconia (ZrO2) femoral heads 32 mm in diameter were articulated against ultra-high molecular weight polyethylene (UHMWPE) acetabular cups, for five million walking cycles in a five-head hip joint simulator designed for wear studies on five total replacement hip joints at once. The test environment was at body temperature and distilled water was used as lubricant. The wear was measured by weighing the cups. The average wear rate of the cups was found to be very low, 4.1 mg per one million cycles (range from 0.4 to 7.1 mg per one million cycles). It was estimated to correspond to a clinical wear rate of 5 mum year-1. The average wear factor was calculated to be 8 X 10(-8) mm3 N-1 m-1. Notable amounts of UHMWPE wear debris were nevertheless produced. Low wear rate is important in total replacement hip joints to avoid adverse tissue reactions that cause loosening of the prosthesis fixation. A wear rate of 5 mum year-1 would hardly cause problems clinically.