An improved theoretical model of orientation softening and cross-shear wear of ultra high molecular weight polyethylene

A theoretical model was previously proposed by Wang et al. and Turell et al. to explain the molecular reorientation and wear mechanisms of ultra high molecular weight polyethylene (UHMWPE). This model has been used to explain observed wear behavior in orthopaedic devices using UHMWPE as a bearing surface. Frictional energy exerted into the UHMWPE is dissipated in the form of heat, wear, and also in reorientation of the molecular chains of the polymer. While the previous model addresses the wear portion of this energy, it fails to account for the active molecular reorientation that occurs due to the cumulative stresses experienced by the polymer. Multiple physical tribological studies have shown a discrepancy with the predictions of the original model. By accounting for the molecular reorientation, wear, and orientation softening effects of the total energy exerted into the UHMWPE, the current model improves upon the simplified assumptions of the previous model. This study correctly predicts the cross shear ratio that generates the maximum wear as demonstrated in the physical tribological studies.