WTC2005-63306 A SECOND GENERATION HIGHLY CROSSLINKED UHMWPE: VITAMIN E STABILIZATION DOES NOT ADVERSELY AFFECT THE WEAR RATE OF IRRADIATED ACETABULAR LINERS WTC2005-63306

ABSTRACT We developed a second generation highly cross-linked UHMWPE by doping and stabilizing irradiated UHMWPE with the antioxidant vitamin E. With this stabilization method, the crystallinity of the irradiated polyethylene is not decreased as with melting, and hence mechanical properties and fatigue strength can be preserved. Vitamin E-doped, irradiated second generation highly crosslinked UHMWPE showed wear comparable to and higher strength and fatigue resistance than that of 100-kGy irradiated and melted first generation highly cross-linked UHMWPE. INTRODUCTION Cross-linking by ionizing radiation increases the wear resistance of ultra-high molecular weight polyethylene (UHMWPE) but also generates residual free radicals, precursors of long-term oxidative embrittlement. To eliminate these residual free radicals, in the first-generation highly crosslinked UHMWPEs, oxidative stability was achieved by postirradiation melting. While effective in increasing oxidative stability, melting reduced the crystallinity of UHMWPE; resulting in a decrease in fatigue strength Vitamin E, a potent antioxidant, has been suggested as a stabilizer to gamma-sterilized (25-40 kGy) UHMWPE by blending with UHMWPE powder prior to irradiation The elevated temperature doping and homogenization is to ensure that Vitamin-E is present throughout the thickness of the liners. However, this may result in dimensional changes beyond the machining tolerances of the liners. γ-Sterilization is proposed to ensure that the Vitamin-E in the polyethylene is sterile. Finally, it is not known if the acetabular liners thus fabricated will be as wear resistant as a 100-kGy irradiated and melted polyethylene. We hypothesized that the dimensional stability will not be compromised by doping and homogenization, and that the 85-kGy irradiation combined with γ-sterilization will effectively result in the wear resistance of 100-kGy irradiated and melted polyethylene. In addition, we hypothesize that the fatigue strength and mechanical properties of irradiated, Vitamin-E doped, and γ-sterilized UHMWPE will be better than those of irradiated and melted UHMWPEs. MATERIALS AND METHODS Hot isostatically pressed GUR1050 UHMPWE stock was used in all experiments. Conventional UHMWPE and irradiated and Vitamin-E doped UHMWPE were prepared. All test samples of the cross-linked/Vitamin-E group were machined out of annealed UHMWPE stock to minimize potential dimensional changes during high temperature doping. The Vitamin-E group samples were then γ-irradiated to 85-kGy in inert gas packaging. These samples were then doped with Vitamin-E at 120ºC for 2 hours followed by annealing at 120ºC in argon for 24 hours. Samples of the conventional group were machined from the consolidated stock. All samples from both groups were packaged in inert gas and γ-sterilized. In the Vitamin-E group, all samples were analyzed by IR to determine the diffusion profile of Vitamin-E in UHMWPE. The samples were cut in half and sectioned (150 µm) using a