Analysis of Contact Stress on the Ultra-High Molecular Weight Polyethylene Cup for Different Hip Implants Dimensions and Materials

Abstract

The subsurface fatigue that occurs in the Ultra-High Molecular Weight Polyethylene (UHMWPE) hip joint cup has been shown to be correlated with the contact stress at that cup. This cup stress is known to be affected by the implant design, dimensions and materials. In this study, 3D finite element modeling has been used to investigate the effects on the cup contact stress when using Function Graded (FG) Material as a femur head. Also the effects on the cup contact stress due to using different sizes of femur heads, presence of metal backing shell and presence of radial clearance (gap) between cup and femur head are studied. The finite element results show that the using of FG femur head with low stiffness at the outer surface and high stiffness at the core results in significant reduction in the cup contact stress compared with Titanium (Ti), Stainless Steel (SS) and Cobalt Chromium (Co Cr Mo) femur heads. The presence of metal backing shell results in a slight reduction in the cup contact stresses especially for small femur heads. Finally, the presence of radial clearance results in remarkable increase in the cup stress especially for small femur heads. The present results suggested that rather minor changes in design and geometrical parameters of the hip joint have significant consequences (e.g. fatigue, wear, loosening) in the long term use of the joint and should be taken into consideration during the design of the hip joint.