Influence of microscopic surface asperities on the wear of ultra-high molecular weight polyethylene in a knee prosthesis.

The wear of ultra-high molecular weight polyethylene (UHMWPE) in knee and hip prostheses is one of the major factors restricting the longevity of these implants. A number of microscopic scratches caused by various factors were observed on the metallic femoral components of the retrieved knee prostheses with an anatomical design. It appears that microscopic surface asperities caused by this surface damage contribute to increasing and/or accelerating the wear of the UHMWPE tibial insert. In this study, in the first step, microscopic observations and surface roughness measurements of several retrieved metallic femoral components were performed in order to produce simplified two-dimensional (2D) finite-element method (FEM) models of a microscopic surface asperity using roughness parameters. Next, a three-dimensional (3D) microscopic surface profile measurement of the damaged surface of a retrieved metallic femoral component and the reproduction of the femoral component surface were performed in order to produce 3D FEM models of a microscopic surface asperity based on actual measurement data. 2D and 3D elastoplastic contact analyses between a metallic microscopic surface asperity and UHMWPE were also performed in order to investigate the mechanical state and microscopic wear of UHMWPE caused by a metallic microscopic surface asperity. The analytical findings of this study suggest that the aspect ratio, shape ratio, and indentation depth of the microscopic surface asperity have significant influence on increasing and/or accelerating the wear of UHMWPE. Higher aspect ratios, shape ratios, and indentation depths cause higher contact stresses and plastic strains in UHMWPE.