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 prosth eses is one of the main limitations on the successful long term perfor mance of these devices. The actual contact stresses occurring in the U HMWPE acetabular cup are effected by the design of the prosthesis and the loads that are applied. The design can control the size of the fem oral head and the degree of conformance between the head and the cup. However, the load is generated by inertia and muscle forces and even d uring normal walking can reach peaks of four times the patient body we ight. Therefore the actual contact stress in the UHMWPE can vary signi ficantly during normal patient activity. This study combines pin-on-pl ate style wear testing with elastic-plastic finite element analysis in order to study the effects of a range of constant loads and contact s tress (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 decre ase with increasing nominal contact stress. The finite element study a nalyses the stress and strain occurring in the polymer pins at loads i n the same range as those used in the wear tests. The results show tha t the actual contact stress varies across the contact face reaching a peak at the edge. The analysis also predicts that yielding of the UHMW PE will occur at the edge of the contact face at nominal stresses lowe r than yield, this correlates with previous experimental studies of hi gh residual strain in this area.