POSTIRRADIATION AGING OF ULTRA-HIGH-MOLECULAR-WEIGHT POLYETHYLENE

A study was performed to determine the time-course of oxidative degradation and the extent to which the degradation proceeded through the bulk of ultra-high molecular weight polyethylene joint components that had been irradiated and stored on a shelf. Standardized cylindrical samples, taken from a single batch of extruded polyethylene, were cleaned, packaged, and sterilized according to protocols used for commercial joint-replacement components. After sterilization, the samples were stored in the packages for time-periods of one day to more than one year. At each interval studied, thin sections were cut as a function of depth into the bulk of the sample and were used to determine the density and the infrared spectra. Marked alterations in the density and the infrared spectra consistent with continuing oxidative degradation occurred throughout the year of storage on the shelf. The alterations were most severe near the surface of the samples. CLINICAL RELEVANCE: The results of this study support the hypothesis that sterilization of polyethylene joint components with gamma irradiation followed by storage on a shelf contributes to oxidative degradation. The degradative changes continue with time and occur not only on the surface but also through the bulk of the component. Alterations in the material properties associated with the degradation increase the stresses on the surface of and within the polyethylene total joint-replacement components once these components have been implanted and are subjected to loading in vivo. Thus, the wear resistance of a polyethylene component that has been stored on a shelf for an extended time may be diminished before the component is implanted.