A. Rufner, Zimmer, Warsaw, IN; R. A. Pals, D. L. Pletcher, R. A. Gsell, H. E. Brinkerhuff, M. E. Hawkins, Zimmer, Inc, Warsaw, IN
Recent studies have shown the feasibility of using Vitamin E, a free radical scavenger, to stabilize highly crosslinked ultra-high molecular weight polyethylene (UHMWPE) for orthopedic implants, foregoing the need for elevated temperature annealing of the material1. This investigation assessed (1) the oxidative stability of UHMWPE blended with Vitamin E and (2) the effect, if any, the Vitamin E has on the amount of crosslinking in the material. Material was prepared by blending GUR 1050 and GUR 1020 resins with 0.2, 0.5, and 1 wt% Vitamin E (d/l-α-tocopherol). Each resin mix was then compression molded into a slab and cut into bars. The bars were preconditioned and e-beam irradiated following a Design of Experiment (DOE) using the following five factors: 1. material (GUR 1050, GUR 1020), 2. Vitamin E concentration (0.2, 0.5, 1 wt%), 3. preheat (40 °C, 120 °C), 4. e-beam irradiation dose (90, 150, 200 kGy), 5. dose rate (75, 155, 240 kGy-m/min). Crosslinking levels were evaluated on microtomed films by measuring the trans-vinylene index (TVI) by FTIR. Swell ratio was measured per Shen2. Oxidation levels were investigated on accelerated aged films. After hexane extraction, the films were analyzed for oxidation index (OI). Eight DOE standards (all GUR 1050) were also accelerated aged as blocks to examine depth of oxidation into the bulk. Results show that TVI data correlates well with dose, while the swell ratio (crosslink density) was mainly affected by irradiation temperature. When the material was cold irradiated, higher concentrations of vitamin E inhibited crosslinking. OI was also slightly elevated in the cold irradiated material. Warm irradiated material had a high crosslink density at high vitamin E concentrations and also showed oxidative stability with as low as 0.2% vitamin E.
Summary: In this study, highly crosslinked ultra-high molecular weight polyethylene (UHMWPE) was stabilized using Vitamin E. A design of experiments (DOE) approach was used to investigate the level of crosslinking and the oxidation resistance of the Vitamin E-blended UHMWPE.
