A new mechanism of oxidation in ultrahigh molecular weight polyethylene caused by squalene absorption

Although synovial fluid lipids were found to absorb in ultrahigh molecular weight polyethylene (UHMWPE) total joint implants in vivo, their effect on the oxidation of the polymer was not known. Current understanding of the oxidation and oxidative stability of UHMWPE joint implants is focused on the presence or elimination of radiation-induced free radicals, which are long-lived and can react with oxygen over the long term. Recently, we found unexplained oxidation in irradiated and melted UHMWPE components that were exposed to bodily fluids then stored on the shelf despite being free of detectable free radicals at the time of implantation. Thus, we hypothesized that lipids absorbed from the synovial fluid in vivo could initiate and accelerate oxidation of UHMWPE even in the absence of detectable residual free radicals. We found that squalene, a precursor in cholesterol synthesis and a synovial fluid lipid with unsaturated bonds, accelerated oxidation in irradiated and melted UHMWPE under in vitro accelerated aging conditions. This result represents a paradigm shift in our understanding of oxidative stability of UHMWPE and prompts further investigation of in vivo oxidation mechanisms as well as the development of relevant in vitro aging models.