The anti-oxidative properties of α-tocopherol in γ-irradiated UHMWPE with respect to fatigue and oxidation resistance

Although addition of an antioxidant (αα-tocopherol) is reported to prevent delamination in ultrahigh molecular weight polyethylene (UHMWPE) knee components, contribution of αα-tocopherol as an antioxidant to the improvement of long-term fatigue performance of UHMWPE is an unknown mechanism. To solve this problem, bi-directional sliding fatigue tests were performed for γγ-irradiated (25 kGy), γγ-irradiated (25 kGy) with 0.3 wt% αα-tocopherol added, and γγ-irradiated (25 kGy) with 0.3 wt% tocopheryl acetate added UHMWPE specimens. Internal defect initiation was quantified with scanning acoustic tomography (SAT). Also, oxidation index and crystallinity were obtained from infrared absorption spectra measured using Fourier transform infrared (FT-IR) microscopy. Only γγ-irradiated UHMWPE specimens resulted in severe fatigue fractures. αα-Tocopherol-added UHMWPE specimens showed significantly lower projected area ratio of defects (1.80±0.82) than did γγ-irradiated (7.0±2.29) and tocopheryl acetate-added ones (8.50±2.01). The oxidation index of γγ-irradiated UHMWPE specimens (0.111±0.0052) was extremely higher compared to those of doped ones; 0.0179±0.0026 and 0.0144±0.0069 for αα-tocopherol-added and tocopheryl acetate-added ones, respectively. The crystallinity of γγ-irradiated UHMWPE specimens (57.5±1.16) was lower compared to those of doped ones; 60.3±0.72 and 60.4±1.38 for αα-tocopherol-added and tocopheryl acetate-added ones, respectively. The incorporation of αα-tocopherol significantly improves the long-term fatigue performance of γγ-irradiated UHMWPE with oxidation stability. Also, the addition of αα-tocopherol controls macromolecular structures resulting in the improvement of fatigue performance of UHMWPE.