Purpose of the study: Wear of polyethylene acetabular components is an important issue in total hip arthroplasty. This study was designed to evaluate differences in polyethylene wear rates between machined and direct compression molded acetabular cups.
Methods: Two hundred thirty-nine prostheses underwent radiographic evaluation using the technique of Chevrot-Kerboull. One hundred thirty-one were all-polyethylene cups machined from extruded bar stock, and one hundred eight were all-polyethylene cups direct compression molded. Both groups, all operated on in 1988, were similar in the acetabular and femoral components, were all cemented, and the acetabular components were all-polyethylene, non metal backed. The femoral components were all Charnley-Kerboull MK III type. Patient weight, average weight, the post-operative PMA score and the duration of the follow-up were similar for the two groups (mean 7.4 years).
Results: Results showed a mean linear wear rate of 0.06 mm per year for compression-molded polyethylene and 0.08 mm per year for machined polyethylene. Results were not significantly different. The number of acetabular radiolucencies in zone I (5.5 versus 5.3 p. 100) and the amount of lysis of the proximal part of the femur did not differ between the two groups. The number of excessive wears did not differ either.
Discussion and conclusion: Although polyethylene wear was lower with compression molded acetabular cups than with ram-extruded acetabular cups, results of this study suggest a non significant difference between the two types of components. A more controlled experiment would have to be performed to attribute a difference between two different types of UHMPE processing.