EFFECT OF DIRECTLY MOLDING ACETABULAR CUPS ON THE CLINICAL AND HIP SIMULATOR WEAR RATES OF UHMWPE

Abstract

Bankston et. al. reported that the clinical wear rates of molded acetabular cups was 50% less than a group of machined UHMWPE cups. However, due to covariables between groups including different femoral stems, cement technique, polyethylene resins and surgeons, unequivocal attribution of the low wear rates to direct molding could not be made.

In order to more directly assess the benefits of directly molded acetabular cups vs. machined cups, we report the comparison of hip simulation wear rates of machined and directly molded cups. These simulator results will then compared to two recent clinical reports on molded and machined cups of the same hip stem and cup design. The molded cups were made from 1900 resin and gamma sterilized in an inert atmosphere. The machined cups were made from HSS reference UHMWPE (4150) and gamma sterilized in air. The molded 1900 cups had a 55% lower wear rate after 5 million cycles on the hip simulator (14 v. 31mg/million cycles). Ranawat reported the average linear head penetration rate for 235 direct-lymolded, all polyethylene, cemented cups at a mean follow-up time of 6 years was .075mm/year. This is 56% lower than the rate of .17 mm/year he reported previously for the machined, uncemented metal-backed cups of the same design. These results provide further evidence that directly molding acetabular cups can provide wear rates over 50% less than machined cups both in both clinical and hip simulator evaluations. It is interesting to note based on other reports, that there is no osteolysis at 10 years of follow-up when the wear rates are < 1mm. The clinical and simulation wear rates reported here for the directly molded cups are within this performance range.