Use of vitamin E to protect cross-linked UHMWPE from oxidation

Wear and oxidative degradation may limit the life span of UHMWPE implants. Cross-linking and stabilisation by vitamin E are proposed to overcome wear and degradation. The present investigation takes a close look to the oxidative behaviour of cross-linked and stabilised UHMWPE. First, the consolidated vitamin E stabilised UHMWPE was qualified in terms of microstructure and homogeneity of the distribution of the additive to be suitable for oxidation profiles over the entire section. Then cross-linked samples with five different concentrations of vitamin E (nil to 1.0%) underwent two different ageing protocols. The first was under pressurized oxygen at 70 °C, as defined in the ASTM F 2003 standard with a prolonged period of 60 days, the second was in 5% aqueous hydrogen peroxide solution with iron (III) chloride as catalyst at 50 °C. The first accelerated ageing protocol showed that a vitamin E concentration as low as 0.05% is effective to protect irradiated highly cross-linked UHMWPE against oxidation when exposed direct to oxygen. Vitamin E stabilised, highly cross-linked UHMWPE exhibits therefore no oxidation potential origination from the irradiation treatment. Analysis of samples treated by the second chemical ageing yielded, that vitamin E is effective to prolong initial stability against a supplementary attack of hydrogen peroxide and reactive radicals. The time period of stability against the aggressive hydrogen peroxide solution increases with increasing vitamin E content. However, even 0.05% have a marked stabilisation effect. Therefore, such small additions of vitamin E are effective to protect the UHMWPE material against a supplementary exposure to in vivo oxidation after the irradiation treatment. In conclusion, vitamin E shields cross-linked UHMWPE for orthopaedic application against oxidation in the heat of consolidation, during irradiation treatment and finally while implanted in the human body.