α-Tocopherol-doped irradiated UHMWPE for high fatigue resistance and low wear

Longevity of total joints has been compromised by wear and fatigue of ultrahigh molecular weight polyethylene (UHMWPE) components. Crosslinking reduces UHMWPE wear, but combined with postirradiation melting, also reduces its fatigue strength, therefore limiting its use in high-stress applications. We hypothesized that a lipophilic antioxidant (α-tocopherol, α-T) can protect UHMWPE against oxidation eliminating the need for postirradiation melting of crosslinked UHMWPE and improve its fatigue strength. To test these hypotheses, 65- and 100-kGy irradiated, α-T-doped and subsequently γ-sterilized UHMWPE were used. (I) α-T-doped irradiated UHMWPEs showed significantly lower oxidation levels (0.48±0.25 and 0.44±0.06) compared to 100-kGy irradiated UHMWPE (3.74±0.16) after 5 weeks of accelerated aging at 80°C in air. (II) Wear rate of α-T-doped irradiated UHMWPE (1.9±0.5, and 0.9±0.1 mg/million cycles (MC) for 65- and 100-kGy irradiated UHMWPE, respectively) were comparable to that of 100-kGy irradiated/melted UHMWPE (1.1±0.7 mg/million cycles). (III) The stress intensity factor at crack inception (ΔKi) of 100-kGy irradiated UHMWPE increased significantly upon doping with α-T from 0.74 to 0.87 MPa m1/2 (p<0.01). The ΔKi for the 100-kGy irradiated and melted UHMWPE, currently in clinical use, was 0.55 MPa m1/2. Doping with α-T eliminated the need for postirradiation melting to protect irradiated UHMWPE against long-term oxidation. The fatigue strength was improved by 58% for α-T-doped 100-kGy irradiated UHMWPE compared to irradiated and melted UHMWPE. The increase in oxidative stability of α-T-doped UHMWPE is attributed to the ability of α-T to react with peroxy free radicals on lipid chains and arrest the oxidation reactions. The improved fatigue strength is attributed to the increase in plasticity of UHMWPE due to the lipophilic nature of α-T.