A PARAMETRIC ANALYSIS OF THE CONTACT STRESS IN ULTRA-HIGH-MOLECULAR-WEIGHT POLYETHYLENE ACETABULAR CUPS

It is well known that the wear factor for ultra-high molecular weight polyethylene (UHMWPE) sliding on metallic or ceramic counterfaces is largely independent of contact stress for modest loading conditions and sliding distances. However, it is note, recognized that under more severe stress levels and with sliding distances comparable to those encountered in current replacement synovial joints, subsurface fatigue contributes to the volume of wear debris. Since the fatigue process is influenced by surface stress levels it is becoming increasingly important to limit the contact stress through design in order to minimize the volume of UHMWPE wear debris in implants. The contact pressure in UHMWPE acetabular cups has been predicted using both the simple elasticity analysis and the finite element method. It has been shown that the radial clearance between the femoral head and the socket is the dominant parameter in determining the contact stress. Thus, the radial clearance should be controlled so the contact half width is close to the femoral head radius (a total included angle of contact of 120 degrees) to minimize the contact pressure. There is little benefit to be gained by, increasing the contact half width greater than the femoral head radius. This is consistent with the geometrical constraint of the anatomical position and the direction of loading. It has been shown that the radius of the femoral head has the most significant effect on the maximum contact pressure for these closely conforming contacts where the contact half width is close to the femoral head radius. The effect of the elastic modulus and the thickness of UHMWPE is relatively small under these contact conditions. However, an increase of the elastic modulus and a decrease of the layer thickness both result in a decrease of the radial clearance required to minimize the contact stress and this may prove to be impractical. This places a further constraint on the design of the contact. This paper demonstrates that with careful selection of the tribological design parameters, contact stress on acetabular cups can be reduced and this is more easily achieved with larger diameter femoral heads.