Effects of acetabular abduction on cup wear rates in total hip arthroplasty.

This study was designed to determine the effect of acetabular abduction on the polyethylene wear rates of the acetabular component. The hypothesis of this study is that acetabular placement, in particular abduction, effects contact forces and therefore polyethylene wear. A total of 364 total hip arthroplasties that were performed between 1974 and 1976 were included in this study. The cemented all-polyethylene acetabular components were the same for each case and came from a single supplier. The polyethylene wear rates were calculated by measuring the shortest radius from the center of the prosthetic femoral head to a point on the outer surface of the acetabular cup. An immediate postoperative radiograph was compared with a follow-up radiograph at least 9.5 years later. A standardized radiograph was used to prevent differences due to magnification. The abduction or inclination of the acetabular cups was measured in all of the patients. The amount of acetabular cup abduction was measured relative to the ischial tuberosity line. The mean abduction was 44.1 degrees with a standard deviation of 9.2 degrees and a median of 44 degrees. The range of abduction was from 0 degrees to 85 degrees. This analysis failed to show a correlation between the amount of acetabular abduction and polyethylene wear rates (Pearson's correlation coefficient = 0.0679; P = .20). With a sample size of 364, there is over a 95% change (statistical power) of detecting an underlying true correlation between wear and abduction that is greater than or equal to 0.20. We were unable to demonstrate a difference in wear that would correlate with the differing degrees of abduction of the acetabular socket. We felt that the radiographic measurements of wear were quite accurate. This method of determining acetabular abduction has also been documented and supported in previous literature and has allowed us to accurately determine acetabular abduction. The results of our study demonstrate that within the normal ranges described by our study, polyethylene wear did not significantly increase with increased abduction of the acetabular component.