CHEMICAL AND MECHANICAL DEGRADATION OF UHMWPE - REPORT OF THE DEVELOPMENT OF AN IN-VITRO TEST

Ultra high molecular weight polyethylene (UHMWPE), used in total joint replacement implants, undergoes oxidative degradation due to gamma radiation sterilization and to exposure to oxidizing agents in the body environment. UHMWPE components also experience large stresses both on and near the articulating surface. It is hypothesized that there is a causal relationship between chemical degradation and mechanical loading of UHMWPE joint components. This report describes the development of an in vitro test to examine the combined effects of chemical environment and cyclic loading history on the chemical, physical, and mechanical properties of UHMWPE. The test apparatus consists of modular load trains, in which strings of UHMWPE test specimens can be subjected to cyclic tensile and compression load ranges, while being exposed to either control or degrading environments. Following cyclic loading for specified time intervals, the specimens can be statically loaded to determine the effect of cyclic loading and environment on the monotonic tensile and compressive stress-strain behavior of UHMWPE. To determine the appropriate control and degrading environments, gamma radiation sterilized UHMWPE specimens were maintained (unloaded) for up to 6 months in aqueous environments of distilled water and three concentrations of hydrogen peroxide (0.01, 0.1, and 1.0%) in distilled water at 37 degrees C. Specimens were evaluated with depth from the surface for changes in density. Infrared spectroscopy (IR) analysis was conducted on selected surface sections. No significant changes in density or surface IR spectra occurred for specimens in distilled water as compared with the UHMWPE in the initial radiation sterilized condition. Specimens exposed to hydrogen peroxide solutions demonstrated changes in density and IR spectra consistent with oxidative degradation and comparable to the changes previously observed on retrieved UHMWPE components. Based on these results, distilled water was selected for the control environment. A 0.5% hydrogen peroxide solution was selected for the test environment, to produce moderate changes at a rate compatible with the maximum 12-month time period of the experiment. (C) 1994 John Wiley and Sons, Inc.