OBSERVATIONS OF RESIDUAL SUBSURFACE SHEAR STRAIN IN THE ULTRAHIGH MOLECULAR-WEIGHT POLYETHYLENE ACETABULAR CUPS OF HIP PROSTHESES

Analysis of bearing surfaces of explanted cups can help to determine t he wear mechanisms that are responsible for generation of wear debris. In this study a microscope polariscope was used to detect residual su bsurface shear strains, deformation and subsurface cracks in explanted Charnley acetabular cups. The wear surfaces were compared to an aceta bular cup from a hip joint simulator test. The six explanted cups that were studied had all failed after long periods of implantation, with penetrations ranging from 2.1 to 3.8 mm. The explanted and simulator c ups both had a smooth, high-wear region. High residual subsurface shea r strains were found in the high-wear region of most cups, with certai n cups possessing subsurface cracks running parallel with the surface 5-10 mum deep, close to the areas of high residual subsurface shear st rain. This was caused by plastic deformation and subsurface fatigue of the polymer surface.