Evaluation of wear in an all-polymer total knee replacement. Part 1: laboratory testing of polyethylene on polyacetal bearing surfaces.

The friction and wear characteristics of a prototype, all-polymer hip prosthesis were evaluated using a multi-station hip-joint simulator. The components consisted of 41 mm diameter polyacetal copolymer balls in combination with acetabular cups of UHMW polyethylene, and were compared to the conventional combination of cobalt-chromium alloy balls against UHMW polyethylene cups. The all-polymer combination exhibited about 20% lower friction and 39% lower wear of the polyethylene cups. The total volume of polymer wear from the all-polymer combination (i.e. including the wear of the polyacetal ball) was 23% less than the volume of polyethylene wear from the metal-polymer components. The appearance of the worn surfaces of the polyethylene cups in the scanning electron microscope were similar for the two combinations of materials, and the surfaces of the polyacetal balls showed only mild wear. Formaldehyde, a product of the degradation of polyacetal, was present in traces in some samples of lubricant from the test chambers with polyacetal balls. In this laboratory model, the friction and wear properties of a polyacetal copolymer ball in combination with an UHMW polyethylene demonstrated good potential for clinical application.