In vivo change of elastic property in polyethylene acetabular components

Polyethylene is an elastic material. It is known that oxidative degradation of polyethylene occurs after sterilization by means of gamma irradiation. However, there have been few detailed reports with regard to the effects of this degradation on the mechanical property, especially in total hip prostheses. The purpose of this study was to examine the change in mechanical property of irradiated and non-irradiated polyethylene cups after implantation. Fifty-six ultra-high molecular-weight polyethylene (UHMWPE) cups retrieved at revision surgery were evaluated. Thirty-two cups were sterilized by gamma irradiation in air and 24 by ethylene oxide gas (EtO). To evaluate the mechanical property of the cup and its regional distribution, Vickers hardness was measured at nine points at the cross-section of the cups. In the irradiated cups, the hardness increased in proportion to the time from sterilization. This phenomenon was not found in the cups sterilized by EtO. Less change of hardness was observed in the cups sterilized by EtO than in those sterilized by irradiation. The gamma-irradiation in air actually affected the elastic property of cup polyethylene in vivo, although any difference in the wear rate was not detected between two sterilization methods. In cases with accelerated wear of the acetabular cup, other factors affecting wear should also be considered.