The Influence of Cup Inclination Angle and Head Position on the Wear of Metal-on-Metal Bearings in Total Hip Replacements

Retrieval studies of metal-on-metal bearings have shown large variations in wear rates and wear mechanisms. High wear rates have been associated with high cup inclination angle. However, increasing the cup inclination angle in in vitro studies has not replicated wear levels seen in retrievals. Clinically relevant wear rates, wear patterns and wear particles have been observed in vitro by ceramic-on-ceramic bearings when microseparation (head offset deficiency) was introduced to the gait cycle. Size 28mm diameter metal-on-metal bearings in total hip replacement was tested on the Leeds II hip simulator under standard gait and microseparation conditions with cup inclination angles of 45 degrees (n=3) and 65 degrees (n=3). Increasing the cup inclination angle from 45 degrees to 65 degrees resulted in 2.6-fold increase in wear rate. The introduction of microseparation to the gait cycle resulted in 4.6-fold increase in wear rate. Under microseparation conditions, the increase in cup inclination angle did not influence the wear rate in this study. When microseparation condition was introduced to the gait cycle, edge loading occurred and stripe-like wear scar was formed on the femoral head with a corresponding area on the acetabular cup. High magnification images which were taken using a scanning electron microscopy showed excessive damage to the surface of the components due to edge loading. The results suggest that cup inclination angle and head position affect the wear rate of metal-on-metal bearings in total hip replacements. Edge loading due to microseparation had the dominant effect on wear rate than cup inclination angle. It can be concluded that high in vivo wear rate could be associated with head-rim contact and edge loading due to high cup inclination angle, head offset deficiency or the combination of both conditions.