Background: Reduced implant survivorship due to aseptic loosening has prompted research into alternative bearing materials. Simulator testing is useful, but clinical studies are the gold standard to evaluate the wear characteristics of new bearing materials. Net compression molded polyethylene has clinically reported improved wear characteristics over traditionally used RAM extruded UHMWPE 1,4,17,22. Machining of the compression molded acetabular component however may be detrimental to its wear properties 29. We report a 23–29% increase in 2D and 3D linear and volumetric wear in a cohort of patients in which machined compression molded acetabular components were used.
Methods: Ninety-two patients matched for gender, body mass index, primary pathology, Charnley grade, and length of follow up underwent uncemented total hip replacement using an identical acetabular and femoral implant. Group 1 (52 patients) had a RAM extruded polyethylene liner (GUR 4150 HP) and Group 2 (40 patients) had a machined compression molded polyethylene liner (Montell H 1900). UHMWPE sterilisation regimes were identical. Antero-posterior and lateral radiographs were analysed for acetabular wear using the Martell technique at a minimum follow up of five years and a maximum mean follow up of 87.4 months for group 1 (SD=8.7) and 84.9 months for group 2 (SD 7.7).
Results: Both 2D and 3D linear and volumetric wear rates in patients with a RAM extruded polyethylene acetabular component were 23–29% less compared to patients that received a machined compression molded acetabular liner. There was a statistical difference in age between to the two groups (P=0.007). Looking at acetabular wear in patients over 55 years, machined compression poly was still 16–31 % worse then RAM extruded polyethylene although statistical difference could not be reached for 3D linear and volumetric wear. The incidence of acetabular osteolysis on review of radiographs at maximum follow up was similar in both groups (group 1 =16.3% versus group 2 =15%).
Conclusions: Despite favourable reports of improved wear characteristics of net compression molded UHMWPE, this study shows a 23– 29% increase in 2D and 3D linear and volumetric wear in machined compression molded acetabular components. It appears that machining of compression molded polyethylene bar stock, to obtain the final component, is detrimental to the wear properties of the acetabular liner.
