Tribology and wear of metal-on-metal hip prostheses: Influence of cup angle and head position

Background: Clinical studies have indicated that the angular position of the acetabular cup may influence wear in metal-on-metal total hip bearings. A high cup angle in comparison to the anatomical position may lead to the head being constrained by the superior lateral surface and rim of the cup, thus potentially changing the location of the contact zone between the head and the cup. The aim of this study was to test the hypothesis that both a steep cup angle and a lateralized position of the head can increase head contact on the superior rim of the cup, with the consequence of increased wear. Methods: Hip-joint simulator studies of metal-on-metal bearings were undertaken with cup angles of 45 degrees and 55 degrees. The femoral head was either aligned to the center of the cup or placed in a position of microlateralization. Wear was measured gravimetrically over 5 million cycles. Results: A steep cup angle of 55 degrees showed significantly higher long-term steady-state wear than a standard cup angle of 450 (p < 0.01). The difference was fivefold. Microlateralization of the head resulted in a fivefold increase in steady-state wear compared with a centralized head. The combination of a steep cup angle and a microlateralized head increased the steady-state wear rate by tenfold compared with a standard cup angle with a centralized head. Conclusions: These studies support the hypothesis that both an increased cup angle and a lateral head position increase wear in metal-on-metal hip prostheses. Clinical Relevance: These experimental studies support the recent clinical observations of increased wear with increased cup angle. Furthermore, the experimental studies indicate that the position and alignment of the femoral head may also be an important determinant of wear in vivo.