Total joint replacement is an established method of treatment for severely impaired joints. However, significant complications exist which may limit the long-term success of total joint replacements. Several of these complications are associated with mechanical failure of ultra high molecular weight polyethylene, which may be affected by chemical degradation of the polyethylene. The purpose of this research is to determine the extent of chemical degradation occurring in polyethylene components in total joint arthroplasty. The degradation is affected by both the chemical environment and the loading history to which polyethylene is exposed in vivo. Chemical properties such as molecular weight and percent crystallinity are changed with time of implantation and there is an associated change in physical properties such as density and in mechanical properties such as elastic modulus. The dependence of the chemical, physical, and mechanical properties of polyethylene on degradation and loading history will be determined by exposing specimens to a fluid which simulates the in vivo environment, to cyclic loading, and to a combination of both as a function of time. The in vivo changes in the chemical and physical properties of polyethylene will be determined as a function of time and of location (in loaded and unloaded areas) in tibial components retrieved from a prospective study of total knee replacements. The course of degradation of mechanical properties for the retrieved tibial components will be predicted by performing a sequence of stress analyses in which the mechanical properties are varied as dictated by degradation and loading history utilizing the determined relationship between chemical and mechanical properties from the in vitro test. Parametric studies will then be performed to determine variations in stresses associated with degradation due to small changes in geometry of the femoral and tibial total knee components. The goal will be to determine the geometry which minimizes the effects of loading history on degradation and, therefore, improves the performance of polyethylene components while maintaining appropriate kinematic constraints for the total knee replacement.
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