THE INFLUENCE OF CONTACT STRESS ON THE WEAR OF UHMWPE FOR TOTAL REPLACEMENT HIP PROSTHESES

The generation of wear and wear debris in total replacement hip prostheses is one of the main limitations on the successful long term performance of these devices. The actual contact stresses occurring in the UHMWPE acetabular cup are effected by the design of the prosthesis and the loads that are applied. The design can control the size of the femoral head and the degree of conformance between the head and the cup. However, the load is generated by inertia and muscle forces and even during normal walking can reach peaks of four times the patient body weight. Therefore the actual contact stress in the UHMWPE can vary significantly during normal patient activity. This study combines pin-on-plate style wear testing with elastic-plastic finite element analysis in order to study the effects of a range of constant loads and contact stress (3.4-28.3 MPa nominal) on the wear factor of isotropic UHMWPE in conditions representative of those occurring in total replacement hip prostheses. The results show that the wear factor (volume lost due to wear per unit load per unit sliding distance) has a tendency to decrease with increasing nominal contact stress. The finite element study analyses the stress and strain occurring in the polymer pins at loads in the same range as those used in the wear tests. The results show that the actual contact stress varies across the contact face reaching a peak at the edge. The analysis also predicts that yielding of the UHMWPE will occur at the edge of the contact face at nominal stresses lower than yield, this correlates with previous experimental studies of high residual strain in this area.