Wear Mechanisms of Untreated and Gamma Irradiated Ultra-High Molecular Weight Polyethylene for Total Joint Replacements

Modification of the surface microstructure of ultra-high molecular weight polyethylene (UHMWPE) is essential for improving the wear resistance of orthopedic implants. A common approach is to cross-link the polymer by gamma irradiation. The objective of this study was to examine the tribological behaviors of untreated and gamma irradiated UHMWPE under physiologically relevant contact conditions. Emphasis was placed on the identification of the dominant wear mechanisms in the early stage of polymer wear. The irradiation dose exhibited a strong effect on the tribological properties of UHMWPE sliding against Co–Cr alloy in a bath of bovine serum. Transmission electron microscopy (TEM) and environmental scanning electron microscopy (ESEM) were used to examine the microstructure and morphology of the worn surfaces. Regularly spaced folds with average spacing depending on the irradiation dose (i.e., cross-link density) formed on the wear tracks. Surface folding was related to plastic flow and the degree of mobility of the crystalline lamellae. SEM and TEM results elucidated the roles of the cross-link density and crystalline lamellae in the wear process. Based on the experimental evidence, a deformation model was obtained that provides explanation for the dependence of surface folding on the cross-link density and lamellae reorientation during sliding.