Assessment of wear in extensively irradiated UHMWPE cups in simulator studies

Higher levels of UHMWPE crosslinking currently are being advocated for improved wear resistance of acetabular cups. Pioneering Japanese studies, begun in 1971, have achieved good clinical results with UHMWPE irradiated to 1000 kGy for use with a cemented-cup design. The objective of our study was to use contemporary simulator techniques to determine the in vitro wear performance of such high-dose irradiated cups. Extruded UHMWPE cups were processed with 500, 1000, and 1500 kGy of gamma-radiation doses under vacuum, annealed, and machined to shape. The cups were mated with 26-mm alumina heads and run in a multidirectional simulator with bovine serum. Over a 6-million cycle (Mc) study, the weight loss of the nonirradiated control cups averaged 52.8 mg/Mc + 1.4% (wear = 57.2 mm(3)/Mc). In contrast, the irradiated wear cups had a consistent weight gain. Thus cups with irradiation of 500-1500 kGy had no detectable wear in this study. The original machining marks still were partially evident in the wear zones, along with some macrofissures in the 1000- and 1500-kGy cups. Areas adjacent to the fissures showed delaminating plaques of 100-300 microm in size. It also was noted that the wear cups systematically gained more weight than their corresponding soak controls. Each 200-kGy radiation gain increased the fluid sorption ratio by 10%. The increased fluid sorption and evidence of some surface deterioration may indicate that such high-dose irradiated cups are more susceptible to mechanical damage. This indicates that we should take care to ensure that our desire to reduce the wear debris to a zero amount does not result in a modified UHMWPE that lacks the necessary mechanical properties for contemporary metal-backed cup designs.