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 (
alpha-tocopherol, alpha-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, alpha-T-doped and subsequently gamma-sterilized
UHMWPE were used. (I) alpha-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 degrees C in air. (II) Wear rate of alpha-T-doped irradiated
UHMWPE (1.9+/-0.5, and 0.9+/-0.1mg/million cycles (MC) for 65- and 100-kGy irradiated
UHMWPE, respectively) were comparable to that of 100-kGy irradiated/melted
UHMWPE (1.1+/-0.7mg/million cycles). (III) The stress intensity factor at crack inception ( DeltaKi) of 100-kGy irradiated
UHMWPE increased significantly upon doping with alpha-T from 0.74 to 0.87MPam(1/2) ( p<0.01 ). The DeltaKi for the 100-kGy irradiated and melted
UHMWPE, currently in clinical use, was 0.55MPam(1/2). Doping with alpha-T eliminated the need for postirradiation melting to protect irradiated
UHMWPE against long-term oxidation. The
fatigue strength was improved by 58% for alpha-T-doped 100-kGy irradiated
UHMWPE compared to irradiated and melted
UHMWPE. The increase in oxidative stability of alpha-T-doped
UHMWPE is attributed to the ability of alpha-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 alpha-T.