Influence of cup orientation on the wear performance of metal-on-metal hip replacements

There are a number of factors that determine the overall outcome of total hip replacement (THR) surgery, some of which appear to be related to the surgical procedure. In particular, the inclination angle at which the acetabular component is placed has been reported to influence the long-term successful performance of THR. The present study assessed the influence of cup orientation on the wear of 40 mm diameter metal-on-metal (MoM) hip bearings tested in a hip simulator. The bearings had a mean radial clearance of 150 pm; the cups oriented at 35 degrees, 50 degrees, and 60 degrees to the horizontal were loaded for up to 6 x 10(6) cycles. In each test the wear rates during the run-in phase were higher than in the steady state phase; the wear rates during the run-in phase were not significantly different for each cup orientation. However, at cup angles of 50 degrees and 60 degrees, the steady state wear rates were 0.69 mm(3)/10(6) cycles and 1.7 mm(3)/10(6) cycles respectively, significantly higher than at 35 degrees (0.24 mm(3)/ 10(6) cycles). The results indicated that larger cup inclination angles not only move the position of the wear scar but also, more significantly in MoM bearings, increase the wear rates and total wear volume generated.