Our hypothesis was that cross-linked UHMWPE stabilized with vitamin-E would be wear and fatigue resistant. Acetabular liners were radiation cross-linked, doped with vitamin E and γ-sterilized. Hip simulator wear rate of vitamin E-stabilized UHMWPE was approximately 1 and 6 mg/million-cycles in clean serum and in serum with third-body bone cement particles, respectively; a four to ten-fold decrease from that of conventional UHMWPE. The ultimate strength, yield strength, elongation-atbreak and fatigue resistance of vitamin E-stabilized UHMWPE were significantly higher than that of 100-kGy irradiated and melted UHMWPE and were unaffected by accelerated aging. Rim impingement testing with 3.7 mm-thick acetabular liners up to 2 million-cycles showed no significant damage of the cross-linked liners compared to conventional, gamma-sterilized in inert UHMWPE vitamin-E stabilized liners. The data indicate good wear properties and improved mechanical and fatigue properties for vitamin-E stabilized cross-linked UHMWPE.

Adhesive/abrasive wear in ultra-high molecular weight polyethylene (UHMWPE) has been minimized by radiation cross-linking. Irradiation is typically followed by melting to eliminate residual free radicals that cause oxidative embrittlement. Irradiation and subsequent melting reduce the strength and fatigue resistance of the polymer. We determined the radiation dose dependence and decoupled the effects of post-irradiation melting on the crystallinity, mechanical properties and fatigue crack propagation resistance of room temperature irradiated UHMWPE from those of irradiation alone. Stiffness and yield strength, were largely not affected by increasing radiation dose but were affected by changes in crystallinity, whereas plastic properties, ultimate tensile strength and elongation at break, were dominated at different radiation dose ranges by changes in radiation dose or crystallinity. Fatigue crack propagation resistance was shown to decrease with increase in radiation dose and with decrease in crystalline content. Morphology of fracture surfaces revealed loss of ductility with increase in radiation dose and more detrimental effects on ductility at lower radiation doses after post-irradiation melting.

Vitamin E-stabilized, highly cross-linked ultrahigh molecular weight polyethylene (UHMWPE) is a promising oxidation and wear resistant UHMWPE with improved mechanical strength in comparison with the first generation, irradiated and melted UHMWPE. One approach of incorporating vitamin E in UHMWPE is through blending of vitamin E in UHMWPE powder followed by consolidation and radiation crosslinking. However radiation crosslinking efficiency of UHMWPE decreases in the presence of vitamin E. Therefore an optimum vitamin E concentration and radiation dose level needs to be determined to achieve a cross-link density comparable to 100-kGy irradiated and melted UHMWPE, which has shown excellent wear properties in vivo. We investigated the cross-link density and mechanical properties of vitamin E-blended UHMWPEs as a function of vitamin E concentration in the blend and gamma irradiation doses up to 200 kGy. We found that 0.3 wt% vitamin E-blended UHMWPE could not be cross-linked above a cross-link density achieved at a radiation dose of 65 kGy for virgin UHMWPE and 1.0 wt% vitamin E-blended UHMWPE could not be cross-linked above a cross-link density achieved at a radiation dose of 25 kGy for virgin UHMWPE even when the former were irradiated to a radiation dose of 200 kGy. In addition, higher plasticity at vitamin E concentrations at and above 0.3 wt% indicated that increased chain scissioning may be prevalent. Since the wear resistance of this irradiated UHMWPE would be expected to be low, vitamin E concentrations equal to or above 0.3 wt% are not recommended for subsequent irradiation to achieve a wear resistant cross-linked UHMWPE. The long-term oxidative stability of irradiated blends with low vitamin E concentrations has yet to be studied to determine an optimum between cross-link density and long-term oxidative stability.

Adhesive/abrasive wear in ultra-high molecular weight polyethylene (UHMWPE) has been minimized by radiation cross-linking. Irradiation is followed by melting to eliminate residual free radicals and avoid long-term oxidative embrittlement. However, post-irradiation melting reduces the crystallinity of the polymer and hence its strength and fatigue resistance. We proposed an alternative to post-irradiation melting to be the incorporation of the antioxidant α-tocopherol into UHMWPE prior to consolidation. α-Tocopherol is known to react with oxygen and oxidized lipids, stabilizing them against further oxidative degradation reactions. We blended GUR 1050 UHMWPE resin powder with α-tocopherol at 0.1 and 0.3 wt% and consolidated these blends. Then we gammairradiated these blends to 100-kGy. We characterized the effect of α-tocopherol on the cross-linking efficiency, oxidative stability, wear behavior and mechanical properties of the blends. (I) The crosslink density of virgin, 0.1 and 0.3 wt% α-tocopherol blended, 100-kGy irradiated UHMWPEs were 175±19, 146±4 and 93±4 mol/m 3 , respectively. (II) Maximum oxidation indices for 100-kGy irradiated UHMWPE previously blended with 0, 0.1 and 0.3 wt% α-tocopherol that were subjected to accelerated aging at 80 °C in air for 5 weeks were 3.32, 0.09, and 0.05, respectively. (III) The pinon-disc wear rates of 100-kGy irradiated UHMWPE previously blended with 0.1 and 0.3 wt% α-tocopherol that were subjected to accelerated aging at 80 °C in air for 5 weeks were 2.10±0.17 and 5.01±0.76 mg/million cycles, respectively. (IV) Both accelerated aged, α-tocopherol-blended 100-kGy irradiated UHMWPEs showed higher ultimate tensile strength, higher yield strength, and lower elastic modulus when compared to 100-kGy irradiated, virgin UHMWPE. These results showed that α-tocopherol-blended 100-kGy irradiated UHMWPEs were not cross-linked to the same extent as the 100-kGy irradiated, virgin UHMWPE.

Eliminating post-irradiation melting and stabilizing the residual free radicals of radiation crosslinked ultra-high molecular weight polyethylene (UHMWPE) with Vitamin-E resulted in improved fatigue crack propagation resistance without compromising wear resistance. We designed a cantilever post bending test to determine the bending fatigue resistance of vitamin E-doped, irradiated UHMWPE (α-TPE) in comparison to conventional UHMWPE. The bending fatigue behavior of α-TPE was comparable to conventional UHMWPE. Upon accelerated aging the fatigue resistance of α-TPE was substantially better than that of conventional UHMWPE. α-TPE has shown improved wear and oxidation resistance, migration stability of vitamin E, and improved mechanical properties. The use of this material may be beneficial in total knee arthroplasty where its improved fatigue properties may be an advantage under high stresses.

Higher crystallinity and extended chain morphology are induced in ultra-high molecular weight polyethylene (UHMWPE) in the hexagonal phase at temperatures and pressures above the triple point, resulting in improved mechanical properties. In this study, we report the effects of the presence of a plasticizing agent, namely vitamin E (α-tocopherol), in UHMWPE during high pressure crystallization. We found that this new vitamin E-blended and high pressure crystallized UHMWPE (VEHPE) has improved fatigue strength and wear resistance compared to virgin high pressure crystallized (HP) UHMWPE. This suggested different mechanisms of wear reduction and fatigue crack propagation resistance in UHMWPE.

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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 cross-linked 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.