Comparison of polyethylene wear in machined versus molded polyethylene in total hip prosthesis

Purpose of the study Wear of polyethylene acetabular components is an important issue in total h ip arthroplasty. This study was designed to evaluate differences in polyeth ylene 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-polyethy lene cups machined from extruded bar stock, and one hundred eight were all- polyethylene cups direct compression molded. Both groups, all operated on i n 1988, were similar in the acetabular and femoral components, were all cem ented, and the acetabular components were all-polyethylene, non metal backe d. The femoral components were all Charnley-Kerboull MK III type. Patient w eight, 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 p art of the femur did not differ between the two groups. The number of exces sive wears did not differ either. Discussion and conclusion Although polyethylene wear was lower with compression molded acetabular cup s than with ram-extruded acetabular cups, results of this study suggest a n on significant difference between the two types of components. A more contr olled experiment would have to be performed ro attribute a difference betwe en two different types of UHMPE processing.