Quantification of the effect of cross-path motion on the wear rate of ultra-high molecular weight polyethylene

Ultra-high molecular weight polyethylene (UHMWPE) is used worldwide as a bearing material in orthopaedic implants. Multi-directional motion or “cross-shear” motion has been identified as one of the most significant factors affecting the wear rate of UHMWPE in total hip joint replacement prostheses. The loci or trajectory of motion at the point of contact between a femoral head and an acetabular cup takes a general quasi-elliptical or rectangular shape during a gait cycle. Due to variations in gait patterns, some patients have either more elongated or square motion patterns than others. It is postulated that differences in motion pattern affect the in vivo wear rates of UHMWPE cups in patients where other factors such as age, weight and body proportion are similar. In this study, UHMWPE pins were articulated against cobalt chromium disks in diluted calf serum using an OrthoPOD™ multi-directional wear tester under physiological loading conditions. Five different rectangular wear path geometries and linear tracking, all with identical path lengths per cycle, were tested. Gravimetric weight loss was converted into both volumetric wear data and wear factor values, k, in order to determine the effects of motion pattern on the wear rate of UHMWPE. The results supported the hypothesis that wear rate is dependent upon the wear path geometry that is in turn dependent upon gait cycle.