Shelf life and in vivo duration. Impacts on performance of tibial bearings

Polyethylene has been used for more than 30 years as an orthopaedic bearing material; however, recently concern has been focused on the early failure of some polyethylene bearings. The damage seen in some bearings has been linked to gamma radiation sterilization performed in an air environment. Gamma sterilization in air has been documented to cause an increase in oxidation and degradation of mechanical properties that continue with time. However, not all retrieved bearings that are gamma sterilized in air exhibit the elevated oxidation and mechanical property degradation that lead to early component failure. Bearings that are gamma sterilized in air oxidize while sitting in inventory before implantation. Shelf oxidation rate was estimated based on analysis of a series of never implanted tibial bearings. This shelf oxidation rate allowed estimation of in vivo oxidation for retrieved tibial bearings of known sterilization date. Bearings with less than 1 year of shelf life after gamma sterilization in air had lower in vivo oxidation and better in vivo performance than did those with longer shelf life before implantation. Shelf time before implantation appears to be a significant factor in the success or failure of bearings that are gamma sterilized in air.