Oxidative stability of commercially available highly cross linked ultra-high molecular weight polyethylene (UHMWPE) implants is commonly achieved through post-irradiation heat treatments. Recent studies prove the feasibility of Vitamin E stabilization without post irradiation melt annealing¹.  The effects of Vitamin E concentration and e-beam irradiation conditions on UHMWPE cross linking were investigated using a multifactor fractional factorial design of experiments approach.  In this study, GUR 1050 and GUR 1020 UHMWPE resins were blended with 0.2 wt%, 0.5 wt% and 1 wt% Vitamin E (d/l-α-tocopherol).. Each combination of resin and Vitamin E concentration was compression molded into slabs and cut into bars. The bars were thermally preconditioned and consequently e-beam irradiated using a design of experiments (DOE) approach.  Design-Expert® software was used to generate an experimental model with high statistical correlation (p<0.0001) to the measured data. The factors varied in the experiment include the following: (1) bar preheat temperature prior to irradiation (40 – 120 °C); (2) electron beam dose level (90 – 150 kGy); (3) dose rate (75-240 kGy-m/min.); (4) resin type (GUR 1050 and GUR 1020); and (5) vitamin E level (0.2 -1.0 weight %). For each experimental condition, ten Type V tensile specimens were prepared and tested using a crosshead speed of 0.05 in/min. The responses, tensile yield strength and elongation, were measured according to ASTM D638. Analysis of the response data shows tensile properties are predominantly impacted by preheat temperature, dose level and Vitamin E concentration. Tensile yield strength increases with decreasing preheat temperatures or increasing dose levels while elongation increases with increasing Vitamin E concentrations or decreasing dose levels.

1. (Oral, E., et. al. J Arthoplasty, 2006 21(4): p.580-591.)