FATIGUE-STRENGTH OF POLYETHYLENE AFTER STERILIZATION BY GAMMA-IRRADIATION OR ETHYLENE-OXIDE

The oxidation level of ultrahigh molecular weight polyethylene specime ns sterilized by gamma irradiation in either air or Ar gas was compare d with that of unsterilized and ethylene oxide sterilized ultrahigh mo lecular weight polyethylene. The fatigue strength of ultrahigh molecul ar weight polyethylene specimens sterilized by gamma irradiation in ai r was compared with that of unsterilized and ethylene oxide sterilized ultrahigh molecular weight polyethylene, At the specimen surface, oxi dation was highest for ultrahigh molecular weight polyethylene gamma i rradiated in air, lower for ultrahigh molecular weight polyethylene ga mma irradiated in Ar gas, and absent in unsterilized and ethylene oxid e 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 specimen s were again unoxidized. Thus, even in an inert atmosphere, oxidative degradation of gamma irradiated ultrahigh molecular weight polyethylen e occurs, The 10 million cycle fatigue strength was similar for unster ilized and ethylene oxide sterilized ultrahigh molecular weight polyet hylene whereas the fatigue strength of gamma irradiated in air ultrahi gh molecular weight polyethylene was lower. Results of this study show that ethylene oxide gas does not degrade ultrahigh molecular weight p olyethylene whereas gamma radiation in air causes changes in the polym er that adversely affect its mechanical properties, Ethylene oxide gas is a viable alternative to gamma radiation in air that avoids oxidati on and fatigue strength degradation known to accompany irradiation of ultrahigh molecular weight polyethylene polymer bearing surfaces in to tal joint implants.