The Novel Use of Nitroxide Antioxidants as Free Radical Scavengers in Ultra-High Molecular Weight Polyethylene (UHMWPE) for Total Joint Replacements

Abstract

Ultra-high molecular weight polyethylene (UHMWPE) has been the standard load-bearing material used in total joint replacements since the 1960s. However, oxidative degradation can lead to premature aging and wear of UHMWPE, requiring implant revision. The novel use of nitroxide antioxidants to prevent oxidation in UHMWPE was proposed and the resulting structure and property changes were evaluated in this work. Standard sterilization and crosslinking methods of Co-60 gamma or high energy electron beam radiation produce alkyl free radicals on the polymer chain. Alkyl radicals react to form bimolecular crosslinks and long-lived allyl radicals at high dose rates; at low dose rates they tend to react with oxygen to form peroxyl radicals. The peroxyl radicals further interact with the polymer chain producing hydroperoxides and more free radicals, leading to oxidative degradation. As an alternative to post-irradiation remelting, which allows radical recombination but reduces fatigue strength, antioxidants can be introduced into UHMWPE to scavenge residual radicals. Nitroxides are stable organic compounds that have a strong paramagnetic signal and are very efficient in preventing lipid peroxidation and in providing radioprotection in biological tissues. The nitroxides used are 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and 4-hydroxy-TEMPO (Tempol). Through radical-radical interactions, radiolytically-produced alkyl radicals in UHMWPE are scavenged by the nitroxide radical. This is demonstrated through Electron Paramagnetic Resonance (EPR) spectroscopy where the paramagnetic nitroxide signal decays as it interacts with carbon-centered radicals in UHMWPE. Pulse radiolysis kinetics studies also show that alkyl radicals in UHMWPE preferentially react with nitroxides in the absence of oxygen. Controlled infiltration of UHMWPE with nitroxides is also observed using EPR. The resulting crosslink densities were investigated using Thermomechanical Analysis. It was observed that the addition of nitroxides after irradiation does not alter the crosslink density. Tensile testing of crosslinked and nitroxide-doped UHMWPE demonstrates increased ultimate tensile strength and toughness and the material exhibits an increase in crystallinity. Additionally, accelerated aging of specimens containing trace levels of nitroxide show very low oxidation levels when placed in an aggressive oxygen environment. Consequently, low concentrations of nitroxides diffused into UHMWPE after crosslinking produce an oxidation-resistant and highly crosslinked material for improved implant performance.