Vitamin E-blended UHMWPE with a Gradient Vitamin E Concentration

INTRODUCTION: Vitamin E-stabilized, radiation cross-linked ultra-high molecular weight polyethylene (UHMWPE) is used in total joint arthroplasty as a bearing surface with low wear and improved mechanical properties compared to irradiated and melted UHMWPE [1]. Vitamin E stabilizes the residual free radicals trapped in the material after irradiation and ensures oxidative resistance, thus resulting in the retention of mechanical properties in the long-term. In one approach, vitamin E-blended UHMWPE is radiation crosslinked. When present during irradiation, Vitamin E causes a decrease in cross-link density compared to irradiated virgin UHMWPE [2]. Obtaining a cross-link density equal to that of 100 kGy irradiated and melted UHMWPE, which has shown good wear resistance in vivo [3], requires a low concentration of Vitamin E with a high radiation dose; but concentration levels should be above 0.05 wt% to not leave irradiated UHMWPE vulnerable to oxidation [4]. We proposed a gradient vitamin E concentration to cross-link the surface preferentially, while leaving the bulk of the material at a low cross-link density. This will result in wear and oxidation resistance at the surface, while retaining mechanical properties equivalent to that of conventional UHMWPE in the bulk. In this study, we present different methods of creating a gradient of vitamin E concentration in UHMWPE. METHODS: Sample preparation for extraction: GUR1050 UHMPWE powder was blended with Vitamin E (D, L-α-tocopherol) at 0.5 wt% and compression molded (Orthoplastics, Lancashire, UK). From this block, 30mm cubes were machined (n=3 each). (1) Cubes were placed in boiling hexanes under reflux for 1, 2, 4, 6, 24, and 40hrs. Blocks were removed and dried under vacuum. (2) Cubes were placed in a pressure vessel filled with a 10% solution of Tween 80 in water. The vessel selfpressurized at 135°C for 2 or 6 hrs. Sample preparation for compression molding: Compression molding was done using GUR1050 powder blended with 0.05 and 0.5 wt% vitamin E (Zimmer, Inc.):