The Otto Aufranc Award. Impact of gamma sterilization on clinical performance of polyethylene in the hip

Despite studies to determine their causes, significant variations in polyethylene acetabular component wear rates, radial cracking of component rims, and occasional delamination cannot be explained. A subsurface white band frequently occurs in such damaged components. These damaged components often are gamma sterilized. To date, the origin of the band and its effect on polyethylene chemical and mechanical properties, and hence, clinical performance, have not been confirmed, and correlations between radiation sterilization and clinical wear have not been made. By developing techniques for polyethylene retrieval testing and rating, chemical analysis, and mechanical analysis, this research has determined that gamma sterilization in air alters the chemical and mechanical properties of polyethylene over time, resulting in high subsurface oxidation, reduced ductility, and reduced strength. Gamma sterilization-induced oxidation is found to be most severe in the subsurface region of components, and coincides with zones of significantly reduced strength and ductility. This chemical and mechanical property degradation is time dependent and is not typically visible until after 3 years' postirradiation. The presence of the subsurface white band significantly correlates with clinical cracking and delamination observed in retrieved components. Wear of the retrieved components often is observed to have progressed into this heavily oxidized, weakened, and embrittled zone. A method for accelerated aging shows that irradiating in air causes oxidation damage in polyethylene components that is not seen with other sterilization methods. Modifications of gamma sterilization techniques to minimize this damage are discussed.