The influence of gamma irradiation and aging on degradation mechanisms of ultra-high molecular weight polyethylene

The aging behavior of ultra-high molecular weight polyethylene (UHMWPE) has been studied following gamma irradiation in air. Accelerated aging procedu res used elevated temperature (70 degreesC), pressurized oxygen (5 bar) and applied stress. Shelf and in vivo aged components have also been investiga ted. The variation in polymer properties with depth into the polymer was de termined using density measurements, infra-red spectroscopy and differentia l scanning calorimetry. Accelerated aging in pressurized oxygen resulted in peaks in polymer density and degree of oxidation up to 500 mum below the p olymer surface. Shelf and in vivo aging was also found to result in increas ed density at or below the component surfaces. Changes in density were main ly due to changes in crystallinity within the UHMWPE and, to a smaller exte nt, due to oxygen incorporation within the polymer. The application of stre ss did not appear to influence the accelerated aging of UHMWPE. A method for estimating the residual stress distribution in the UHMWPE usin g the measured changes in density is proposed. This study has indicated tha t oxidation of UHMWPE may lead to the development of tensile residual stres ses, near the component surface, in the region of 1.7 MPa. These stresses m ay contribute to the failure mechanism of UHMWPE acetabular cups or knee ti bial trays during service. (C) 2001 Kluwer Academic Publishers.