GAMMA-STERILIZATION OF UHMWPE ARTICULAR IMPLANTS - AN ANALYSIS OF THEOXIDATION PROBLEM

Gamma irradiation of Ultrahigh Molecular Weight Polyethylene (UHMWPE) leads to long-lived free radicals which react with oxygen. Diffusion o f oxygen, occurring over months or years, controlled by the permeabili ty characteristics of the polymer, results in progressive oxidation, b reaking of polymer chains, alteration of the crystalline portion of th e polymer, and deterioration of the mechanical properties of the polym er. This paper reviews the observations in the literature on this issu e and then presents a conceptual model concerning the interplay of rad ical diffusion, oxygen diffusion, non-uniform permeability, and free-r adically driven chain reactions in order to explain these observations . The suggested model is based on literature that is available on the oxidation of linear polyethylenes during and after irradiation. The mo del directs the attention of researchers in the field of orthopaedic i mplants to the complexity of the process and the variety of issues and parameters to be considered while studying the long-term effects of r adiation sterilization on UHMWPE.