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.