Different studies have shown that gamma-irradiated polyethylene generally results in degradation by oxidation. On the other hand, we clinically used ultra-high molecular weight polyethylene sterilized by gamma-irradiation of 1,000kGy dose in the air (100Mrad cross-link polyethylene) for total hip prostheses from 1970 to 1978, and excellent clinical results extending for 30 years were shown. In the present study, the wear characteristics were evaluated by hip simulator on 100Mrad cross-link polyethylene cups which were available after shelf-aging for extremely long term (about 30 years). The results are compared with those ofthe explanted cup after 30 years of clinical use as well as the cups aged by acceleration in order to review the influence of aging environments and themechanism of degradation.

The 100Mrad polyethylene cups were manufactured by Mizuho Medical Instruments in the 1970s and they have been in stock on the shelf in air-containing package or without being packed for 30 years. These cups were tested with alumina heads (36mm in diameter) by the AMTI hip-joint simulator.

The 100Mrad polyethylene cup aged for 30 years without a package showed considerable initial wear. In case of the same cup aged in an air-containing package, however, a significant reduction of wear was observed even with the presence of the oxidized surface layer. This observation agreed with the low wear of explanted 100Mrad polyethylene after 30 years of clinical use. These results suggest that wear properties of this gamma-irradiated polyethylene are significantly affected by the environmental conditions it was kept for long period of time.