THE EFFECTS OF OXIDATION ON RADIATION CROSS-LINKED UHMWPE WITHOUT RESIDUAL FREE RADICALS

Introduction Inradiation cross-linked and melted ultrahigh molecular weight polyethylene (UHMWPE) total joint implants, the oxidation potential is afforded to the material by by post-irradiation melting. The resulting cross-linked UHMWPE does not contain detectable free radicals at the time of implantation and was expected to be resistant against oxidation for the lifetime of the implants. Recently, analysis of long-term retrievals revealed detectable oxidation in irradiated and melted UHMWPEs, suggesting the presence of oxidation mechanisms initiated by mechanisms other than those involving the free radicals at the time of implantation. However, the effect of oxidation on these materials was not well studied. We determined the effects of in vitro oxidation on the wear and mechanical properties of irradiated and melted UHMWPEs. Materials and Methods Medical grade slab compression molded UHMWPE (GUR1050) was irradiated using 10, 50, 75, 100, 120 or 150 kGy. The irradiated and melted UHMWPEs were accelerated aged at 70°C for 2, 3, 4, 6 and 8 weeks at 5 atm of oxygen. Oxidation profiles were determined by first microtoming 150 μm cross sections; these were then extracted by boiling hexane for 16 hours and vacuum dried for 24 hours. They were then analyzed on an infrared microscope as a function of depth away from the surface. An oxidation index was calculated per ASTM 2102 as the ratio of the area under the carbonyl peak at 1740 cm-1 to the area under the crystalline polyethylene 1895 cm-1 peak. The cross-link density was calculated as previously described (Oral 2010). The wear rate was determined using a custom-designed pin-on-disc wear tester against CoCr polished discs at 2 Hz and a rectangular path of 5 × 10 mm in undiluted bovine serum (Bragdon 2001). Tensile mechanical properties were determined using Type V dogbones according to ASTM D638. Results and Discussion Oxidation increased as a function of aging duration for all UHMWPE samples. The cross-link density decreased non-linearly with increasing oxidation and the wear rate increased non-linearly. The dependence of wear on cross-link density was different for freshly irradiated, unoxidized samples in contrast to aged and oxidized samples (Figure 1). The elongation at break and the ultimate tensile strength decreased with increasing oxidation (Figure 2) and the modulus increased with increasing oxidation. There was an increase in the oxidation rates and oxidation levels of irradiated and melted UHMWPEs with increasing radiation dose (Figure 1), which suggested that regardless of the presence of residual free radicals, increased cross-linking made the material more prone to oxidation and oxidative degradation. The wear rate was not very sensitive to oxidation with an increase only observed at an oxidation index of 1 (Figure 3), suggesting a significant level of degradation and oxidative damage only at this level of oxidation. In contrast, the tensile strength and elongation-at-break were very sensitive to oxidation, showing severe degradation at low oxidation levels. Significance This is the first study exploring the effects of simulated oxidation in irradiated and melted UHMWPEs without detectable free radicals known to cause oxidation. We have shown that when oxidation occurs, severe degradation may occur in irradiated and melted UHMWPEs.