Abstract
It has generally been said that gamma-irradiated polyethylene (PE) is liable to oxidative deterioration. On the other hand, we started clinical trials of UHMWPE with gamma-irradiation of 100 Mrad in air (100 Mrad PE) in the 1970s, and found excellent clinical results over 30 years. In the present study, we evaluated the oxidative deterioration of 100 Mrad PE stored on a shelf as long as 30 years, and compared those data with those from material used clinically for a long period. The 100 Mrad PE was sealed into a poly-package with air, or not sealed, and then stored on a shelf for about 30 years. After simulation tests of the 100 Mrad PE against a 36 mm alumina ball, it was found from microscopic FT-IR that the oxidation index gradually decreased from the surface (4.0-6.0) to the inside (ca. 1.5) . The oxidation index at the surface of the non-packaged PE was about 7.0 before the simulation test. The oxidation index was higher in the shelf-aged PE than in the clinically used PE (ca. 0.5) . The progress of oxidation in the shelf-aged PE is assumed to depend on the concentration of oxygen diffused through the surface. Because of higher oxygen concentration in the shelf-aged PE than in clinically used PE, the oxidative deterioration would proceed more quickly in the former than in the latter. The surrounding oxygen concentrations should be decisive for the progress of oxidation, since the ESR results showed little difference in the residual free radical concentrations between the shelf-aged PE and clinically used PE. The 100 Mrad PE suffered less oxidative deterioration in clinical use in spite of high concentrations of residual free radicals, since oxygen concentration is lower in body environments than in air.
