M. S. Peiserich, A. Rufner, R. Gsell, Zimmer, Inc., Warsaw, IN; D. Pletcher, H. Brinkerhuff, M. Hawkins, Zimmer, Warsaw, IN
Irradiation cross linking of polyethylene has been shown to significantly reduce wear in total joint replacement components. During irradiation, free radicals are generated, which are stabilized by subsequent heating processes. This study examined the temperature and time during post-irradiation thermal treatment at which free radicals are reduced to below detectable levels. GUR1050 ultrahigh molecular weight polyethylene was e-beam irradiated to 110 kGy, followed by thermal treatment in a convection oven at various temperatures both below and above the melt temperature of polyethylene (130oC – 155oC). Thermocouples were placed in the center of each bar to monitor the internal bar temperature throughout the thermal treatment process. Bars were removed from the oven over a range of times from 1 minute to 24 hours after the center of the bar reached the desired temperature. Electron spin resonance (ESR) samples were then machined from the center of each bar and sent for analysis using a Varian E-4 ESR Spectrometer to determine the residual free radical concentration (FRC). Regression analysis, which was performed on the free radical data for each temperature and time, showed that FRC is inversely proportional to the amount of time the polyethylene has been held at that temperature. This study also showed that when thermal treatment is performed below the melt temperature, free radicals reduce over time but still remain after 24 hours; however, above the melt temperature, free radicals are reduced to below detectable levels after 1 minute at temperature. Reduction of free radicals to below detectable levels minimizes the risk of oxidation and potential material embrittlement.
Summary: During irradiation of polyethylene, free radicals are generated, which are stabilized by subsequent heating processes. This study examined the temperature and time during post-irradiation thermal treatment at which free radicals are reduced to below detectable levels.
