Oxidation and Oxidation Potential of Radiation Cross-linked Vitamin E/UHMWPE Blends

Introduction: A Vitamin E-stabilized, radiation cross-linked UHMWPE was developed to maintain oxidative resistance while eliminating the need for post-irradiation melting, which can result in lowered fatigue strength [1]. While diffusion after irradiation is used for introducing Vitamin E into UHMWPE, blending Vitamin E with UHMWPE resin and subsequently irradiating the consolidated mixture is an alternative method. Due to Vitamin E reacting with free radicals during the radiation cross-linking, blended UHMWPE has a lowered crosslink density compared to irradiated virgin UHMWPE [2]. Low concentration of Vitamin E is necessary to obtain sufficient cross-link density equal to that of 100kGy irradiated and melted UHMWPE, shown to have high in vivo wear resistance. However, the vitamin E in low concentration blends could be exhausted during irradiation, leading to decreased oxidative stability. Our aim was to study the relative oxidative resistance of vitamin E blended, then radiation cross-linked UHMWPEs with low Vitamin E concentrations subjected to accelerated and realtime aging. Materials and Methods: GUR1050 UHMPWE resin powder was blended with Vitamin E at three concentrations, 0.02 wt%, 0.05 wt% and 0.1wt%, and then compression molded. Virgin UHMWPE and blended blocks were then γ-irradiated to 100kGy, 150kGy and 200kGy. From the irradiated blocks, 25x25x5mm blocks were machined for accelerated aging for 2 weeks at 70°C, 5atm in a pure O2 and real-time aging in 40°C water [3]. Dogbones stamped out of each unaged sample (3.2 mm-thick) were tested in accordance with ASTM D-638. Thin sections were cut from the inner surface of each block (150 μm) using a microtome. Spectra were collected by an infrared microscope as a function of depth. Oxidation indices were calculated as the ratio of the areas under 1680 cm-1 – 1780 cm-1 to the absorbance over 1370 cm-1 – 1390 cm-1. Average surface oxidation index (SOI) over the first 1.5mm from the surface was calculated. Additional thin sections were suspended within a flask purged with nitric oxide over 16 hours and analyzed using IR microscope. Nitrate level was quantified by normalizing the height of the nitrate peak at 1630 cm-1 to 1895 cm-1, after subtracting corresponding baselines. 3mm cubes were tested in a dynamic mechanical analyzer (Perkin Elmer) as previously described [1]. Results: Figure 1 and 2 and Table 1 summarizes the findings of this study. Discussion: Crosslink density was used to determine if irradiated Vitamin E blends maintained high cross-linking efficiency. The blended samples irradiated at 150 and 200kGy showed a crosslink density similar to or higher than that of 100kGy irradiated UHMWPE (Table 1). Therefore, these blends are expected to have similar or lower wear compared to 100kGy irradiated UHMWPE. Also, except for the 200kGy irradiated 0.02 wt% vitamin E blend, the ultimate tensile strength (UTS) of irradiated blends were similar to 100-kGy irradiated UHMWPE, therefore they are expected to have similar mechanical behavior in their unaged state. Accelerated aging is used to determine the comparative oxidative stability of UHMWPEs in a high pressure, high temperature, pure oxygen environment. In contrast, real-time aging subjects UHMWPE to an aqueous environment with similar temperature and oxygen levels to those in vivo. Accelerated aging of vitamin E-blended and subsequently irradiated UHMWPE resulted in no detectable oxidation (Fig 1a). In contrast, real-time aging for 10 months showed detectable oxidation in blends with 0.02 wt% vitamin E (Fig 1b). Real-time aging resulted in greater differentiation in irradiated blends with low vitamin E concentration and may be a more suitable method for ascertaining the relative oxidative stability of vitamin E containing irradiated materials. Hydroperoxides are intermediate oxidation products, whose breakdown is detected as oxidation. By quantifying hydroperoxides using NO staining, the relative oxidation potential of different materials can be determined. Real-time aged samples revealed similar oxidation potentials in irradiated, 0.1wt% and 0.05wt% Vitamin E blends, while irradiated 0.02wt% Vitamin E blends showed high oxidation potential at 10 months compared to their unaged state (p<0.01; Fig 2). The oxidation potential and oxidation of these samples suggested that the vitamin E may have been exhausted during irradiation. References: 1.Oral Biomaterials 25:5515 2004. 2.Oral Biomaterials 26:6657 2005. 3.Oral Biomaterials 27:558