Abstract
The depth profile of oxidation index and gel fraction has been measured for two silane crosslinked poly(ethylene) (SXLPE) acetabular cups (one gamma irradiated in air, and one non‐irradiated, both with a shelf‐life of 13 years) and for two UHMWPE components (one gamma irradiated in air and one non‐irradiated, with shelf‐lives of 13 and 7 years, respectively). Only the irradiated UHMWPE exhibited any variation in these properties with depth. The oxidation profile (maximum 1 mm below surface) has been explained to result from reduced levels of diffused oxygen with depth, giving rise to a balance of alkyl and peroxyl radicals (and hence maximum carbonyl production) just below the surface. The gel fraction profile (maximum 4 mm below surface) is also attributed to the lower levels of diffused oxygen with depth, causing crosslinking to dominate in the bulk and chain scission to dominate at the surface. The resistance to oxidative degradation in the non‐irradiated SXLPE was attributed to the use of antioxidants in the polymer processing.