POSTIRRADIATION AGING OF ULTRA-HIGH-MOLECULAR-WEIGHT POLYETHYLENE

A study was performed to determine the time-course of oxidative degrad ation and the extent to which the degradation proceeded through the bu lk of ultra-high molecular weight polyethylene joint components that h ad been irradiated and stored on a shelf. Standardized cylindrical sam ples, 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 st ored 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 de pth into the bulk of the sample and were used to determine the density and the infrared spectra. Marked alterations in the density and the i nfrared spectra consistent with continuing oxidative degradation occur red throughout the year of storage on the shelf. The alterations were most severe near the surface of the samples. CLINICAL RELEVANCE: The r esults of this study support the hypothesis that sterilization of poly ethylene joint components with gamma irradiation followed by storage o n a shelf contributes to oxidative degradation. The degradative change s continue with time and occur not only on the surface but also throug h the bulk of the component. Alterations in the material properties as sociated with the degradation increase the stresses on the surface of and within the polyethylene total joint-replacement components once th ese components have been implanted and are subjected to loading in viv o. Thus, the wear resistance of a polyethylene component that has been stored on a shelf for an extended time may be diminished before the c omponent is implanted.