Fatigue strength of polyethylene after sterilization by gamma irradiation or ethylene oxide.

The oxidation level of ultrahigh molecular weight polyethylene specimens sterilized by gamma irradiation in either air or Ar gas was compared with that of unsterilized and ethylene oxide sterilized ultrahigh molecular weight polyethylene. The fatigue strength of ultrahigh molecular weight polyethylene specimens sterilized by gamma irradiation in air was compared with that of unsterilized and ethylene oxide sterilized ultrahigh molecular weight polyethylene. At the specimen surface, oxidation was highest for ultrahigh molecular weight polyethylene gamma irradiated in air, lower for ultrahigh molecular weight polyethylene gamma irradiated in Ar gas, and absent in unsterilized and ethylene oxide sterilized ultrahigh molecular weight polyethylene. At a depth of 3.5 mm below the specimen surface, oxidation levels were equivalent for ultrahigh molecular weight polyethylene gamma irradiated in either air or Ar gas whereas unsterilized and ethylene oxide sterilized specimens were again unoxidized. Thus, even in an inert atmosphere, oxidative degradation of gamma irradiated ultrahigh molecular weight polyethylene occurs. The 10 million cycle fatigue strength was similar for unsterilized and ethylene oxide sterilized ultrahigh molecular weight polyethylene whereas the fatigue strength of gamma irradiated in air ultrahigh molecular weight polyethylene was lower. Results of this study show that ethylene oxide gas does not degrade ultrahigh molecular weight polyethylene whereas gamma radiation in air causes changes in the polymer that adversely affect its mechanical properties. Ethylene oxide gas is a viable alternative to gamma radiation in air that avoids oxidation and fatigue strength degradation known to accompany irradiation of ultrahigh molecular weight polyethylene polymer bearing surfaces in total joint implants.