Oxidative degradation of ultrahigh molecular weight polyethylene (UHMWPE) bearings is one of the main factors that deteriorates their mechanical properties and abrasive wear resistance, which further results in the failure of total joint replacements. Absorption and diffusion of synovial fluids have been considered as causes of the oxidation of UHMWPE bearings. However, the role of synovial fluids in oxidation of UHMWPE remains elusive. In this work, we aimed to reveal the oxidation mechanism of UHMWPE joints with respect to squalene (a representative component of the synovial fluid). The UHMWPE doped in squalene showed slight oxidation, while severe oxidation was observed when squalene doped UHMWPE was exposed to oxygen atmosphere. Squalene manifested lipid peroxidation with notable oxidation products and oxygen-derived free radicals during thermo-oxidative aging. Lipid peroxidation was deduced to follow two routes including autoxidation and thermal oxidation. The aggressive free radicals of squalene abstracted hydrogen atoms from the UHMWPE chains, initiating the oxidation of UHMWPE artificial joints. These findings not only provide evidence for comprehending the process of squalene-induced UHMWPE oxidation but also are instructive to develop highly oxidative-resistant UHMWPE.
Keywords: UHMWPE; accelerated aging; lipid; oxidation mechanism; squalene; total joint replacement.