Effect of terminal sterilization with gas plasma or gamma radiation on wear of polyethylene liners

Background: Although terminal sterilization with gamma radiation in air promotes cross-linking, which improves wear resistance, it also results in free radicals, which can oxidize and degrade the mechanical properties of polyethylene liners used for total hip arthroplasty. For this reason, non-cross-linked polyethylene components have also been sterilized with chemical surface treatments, such as gas plasma. In this study, we tested the hypothesis that conventional polyethylene liners cross-linked by sterilization with gamma radiation in air had better in vivo wear performance than non-cross-linked liners sterilized with gas plasma. Methods: We retrospectively reviewed the wear rates in a series of hips treated with a Duraloc 100 cup, a 28-mm femoral head, and an Enduron liner that had been sterilized with either gamma radiation (sixty-one hips followed for a mean of 5.2 years) or gas plasma (sixty-three hips followed for a mean of 3.9 years). The irradiated liners had been stored with access to ambient oxygen for an average of 1.0 year (range, 0.05 to 5.72 years) prior to implantation. Multiple linear regression was used to assess the effect of the sterilization method on the wear rate while accounting for the possible influence of other factors, including liner geometry, femoral head material, patient gender, cup abduction angle, and age at surgery. Results: The polyethylene liners that had been sterilized with gamma radiation in air had a significantly lower wear rate than did the gas-plasma-sterilized liners (0.097 compared with 0.19 mm/yr, p < 0.001). The sterilization method (p < 0.001) and age at surgery (p = 0.001) were the only factors that we analyzed that correlated with the wear rate. Conclusions: The in vivo wear of conventional polyethylene liners that had been sterilized with gamma radiation in air was, on the average, 50% less than that of non-cross-linked liners sterilized with gas plasma. Level of Evidence: Therapeutic study, Level III-2 (retrospective cohort study). See Intructions to Authors for a complete description of levels of evidence.