EFFECTS OF ACETABULAR ABDUCTION ON CUP WEAR RATES IN TOTAL HIP-ARTHROPLASTY

This study was designed to determine the effect of acetabular abductio n on the polyethylene wear rates of the acetabular component. The hypo thesis of this study is that acetabular placement, in particular abduc tion, effects contact forces and therefore polyethylene wear. A total of 364 total hip arthroplasties that were performed between 1974 and 1 976 were included in this study. The cemented all-polyethylene acetabu lar components were the same for each case and came from a single supp lier. The polyethylene wear rates were calculated by measuring the sho rtest radius from the center of the prosthetic femoral head to a point on the outer surface of the acetabular cup. An immediate postoperativ e radiograph was compared with a follow-up radiograph at least 9.5 yea rs later. A standardized radiograph was used to prevent differences du e to magnification. The abduction or inclination of the acetabular cup s was measured in all of the patients. The amount of acetabular cup ab duction was measured relative to the ischial tuberosity line. The mean abduction was 44.1 degrees with a standard deviation of 9.2 degrees a nd a median of 44 degrees. The range of abduction was from 0 degrees t o 85 degrees. This analysis failed to show a correlation between the a mount of acetabular abduction and polyethylene wear rates (Pearson's c orrelation coefficient = 0.0679; P = .20). With a sample size of 364, there is over a 95% change (statistical power) of detecting an underly ing true correlation between wear and abduction that is greater than o r equal to 0.20. We were unable to demonstrate a difference in wear th at would correlate with the differing degrees of abduction of the acet abular socket. We felt that the radiographic measurements of wear were quite accurate. This method of determining acetabular abduction has a lso been documented and supported in previous literature and has allow ed us to accurately determine acetabular abduction. The results of our study demonstrate that within the normal ranges described by our stud y, polyethylene wear did not significantly increase with increased abd uction of the acetabular component.