Investigation of free radicals in ultra-high molecular weight polyethylene (UHMWPE) materials following irradiation and aging

The ultra-high molecular weight polyethylene (UHMWPE) used in total joint arthroplasty has been sterilized commonly by gamma radiation. Irradiation and subsequent aging in air reportedly cause extensive oxidation in this material. The original properties of UHMWPE, including wear resistance, can be altered significantly as a result of oxidation. In this study, long-lived free radicals in gamma-sterilized and subsequently shelf-aged UHMWPE tibial components were investigated. UHMWPE bar stock, resin powder, and fibers that had been irradiated with electron beam and then stored in air for three years were also characterized for long-lived free radicals. Significant concentrations of free radicals were found in all irradiated UHMWPE materials. Peroxy radicals are predominant in the UHMWPE materials that are highly accessible to oxygen, while more primary radicals remain in the UHMWPE materials that are isolated from oxygen. Based on this study, it is expected that irradiation of UHMWPE in an inert environment would result in long-lived primary radicals, which could be a potential cause of long-term oxidation