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.
Department of Biomechanics, Hospital for Special Surgery, New York, NY 10021.