Enhanced computational modelling of UHMWPE wear in total hip joint replacements: The role of frictional work and contact pressure

Hip joint replacements using highly cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) as a bearing material offer an effective treatment to arthritis. However, the bearing wear may affect the longevity of implants which requires more rigorous investigations. UHMWPE wear is highly dependent on cross-shear (CS) as a result of the multidirectional motion of bearings. Previously, a wear model has been developed with the potential to provide accurate simulation, of which the wear was determined as an exponential function of sliding distance with CS at the crossing angle of 90 degrees, and the effect of CS for an arbitrary angle was quantified to be proportional to the perpendicular component of sliding distance. In the present study, the wear was proposed as the exponential function of frictional work. In particular, contact pressure was incorporated into the calculation of frictional work by considering that the friction coefficient is a function of contact pressure. In addition, the CS quantification for arbitrary crossing angles was considered by coupling the sliding distance and frictional force. The computational formulations of the present and previous models were compared by applying the models to a 36 mm diameter metal-on-polyethylene total hip prosthesis with highly cross-linked UHMWPE at 5 Mrad irradiation under simulated walking conditions. The simulation was also validated against the published results of hip simulator tests with respect to the influence of motion/loading condition on wear. The study showed markedly improved computational predictions with wear rate discrepancies in the range of 8-25% for two kinematic conditions either with the fixed or with varying direction of load. The results show the important role of both the frictional work and contact pressure, and they should be both incorporated appropriately to enhance the computational modelling.