DEGRADATION RATE OF ULTRA-HIGH-MOLECULAR-WEIGHT POLYETHYLENE

Ultra-high molecular weight polyethylene components for total joint re placement chemically degrade before and after implantation, and the de gradation is associated with an increase in density. The goal of this study was to determine the average rate of density change in these com ponents following sterilization by gamma radiation in air as a functio n of shelf age and implantation time. Using the density gradient colum n method, density profiles were obtained through the thickness from lo aded and unloaded regions of 10 retrieved Insall-Burstein/Posterior-St abilized II tibial components and one operating-room inventory compone nt for which the initial density profile and patient history (if appli cable) were known. The average density of the components increased at a constant rate of 0.000186 g/cc/month during the first 50 months afte r sterilization (r(2) = 0.54) but was not significantly affected by lo ading (p > 0.05). The quantitative degradation rates may be useful to help verify kinetic models to predict bulk degradative changes on the basis of microstructural and chemical processes. This research also su ggests the hypothesis that degradation of ultra-high molecular weight polyethylene can be modeled in terms of changes in bulk or average pro perties.