Thermomechanical analysis of ultra-high molecular weight polyethylene-metal hip prostheses

In order to predict the frictional heating and the contact stresses between the polyethylene cup and the metallic ball-head forming the articulation of a hip prosthesis a three-dimensional finite element model was developed and calculated. The non-linear model includes a fully coupled thermomechanical formulation of the mechanical properties of the ultra-high-molecular-weight polyethylene, and a large-sliding Coulomb frictional contact between the two components. The model predicts the temperature of the polyethylene with an accuracy that was tested by comparing the model predictions with the temperature measurements. The temperature measurements were taken by thermocouples placed on the cup surface, the head surface and the inside of the thermostatic bath, during a complete test within a hip joint wear simulator. The model was found to be very accurate, predicting the measured temperatures with an accuracy better than 2 per cent. The temperature peak (51 degrees C) was predicted at the contact surface. The model results indicate that frictional heat is mostly dissipated through the metallic ball-head. The full coupling between the thermal and the mechanical conditions used in this study appears to be necessary if accurate predictions of the polyethylene deformation are required.