Wear of '100 Mrad' cross-linked polyethylene: effects of packaging after 30 years real-time shelf-aging

Studies have shown that gamma-irradiation of polyethylene (PE) generally results in degradation by surface oxidation. However, from 1970 to 1978 Oonishi et al. used ultra-high-molecular-weight polyethylene (UHMWPE) cross-linked and sterilized by 100 Mrad of gamma-irradiation in air (100 Mrad PE) for total hip prostheses, and obtained excellent clinical results extending for 30 years. In the present study, we used a hip joint simulator to investigate the wear characteristics of 100 Mrad PE cups which had been shelf-aged for an extremely long period (30 years). The PE cups, aged in an air-containing triple polyethylene package for 30 years (packaged 100 Mrad PE), showed low wear with 3.4 mg of weight loss, even after 5 x 10(6) cycles. In contrast, non-packaged 100 Mrad PE showed considerable wear: 47.0 mg at run-in ((0-0.25) x 10(6) cycles) and 114.1 mg at the end of 5 x 10(6) cycles. The substantially, lower wear even in the presence of an oxidized surface layer for the packaged 100 Mrad PE, was comparable to the low wear seen on retrieved 100 Mrad PE after 30 years of clinical use. The long-term shelf-storage conditions, which affect the surface oxidative degradation of PE, are assumed to be the key factor in the wear-resistance of gamma-irradiated UHMWPE.