Abstract
Various studies have shown that gamma-irradiated polyethylene generally suffers degradation by oxidation. On the other hand, we clinically used ultra-high molecular weight polyethylene crosslinked and sterilized by 100 Mrad of gamma-irradiation in air (100 Mrad PE) for total hip prostheses from 1970 to 1978, and excellent clinical results lasting some 30 years were shown. In the present study, the wear characteristics of two 100 Mrad PE cups were tested by hip simulator after shelf-aging for around 30 years. The results were compared with those of an explanted cup after 30 years of in vivo use, by accelerated simulation, in order to assess the influence of aging environments and the pattern of degradation. The 100 Mrad PE cups were manufactured by Mizuho Medical Instruments in the 1970s and they had been in stock on a shelf, two in air-containing package, or two unpacked cups, for about 30 years. These cups were tested with alumina heads (36 mm in diameter) on an AMTI hip-joint simulator. The polyethylene cup aged in the air-containing package showed wear with 3.4 mg of weight loss after 5 million cycles. On the other hand, the run-in wear (0-0.25 million cycles) of the unpacked cup was 47.0 mg in weight loss, rising to 114.1mg at the end of 5 million cycles, which was considerable initial wear. In the case of the similar cup aged in an air-containing package, however, a relative reduction of wear was observed despite the presence of the oxidized surface layer. This observation agreed with the low wear of the explanted 100 Mrad PE cup after 30 years of in vivo use. These results suggest that the wear properties of gamma-irradiated polyethylene are significantly affected by long-term environmental conditions.
