Permanent deformation in the Ultra High Molecular Weight Polyethylene tibial component has been reported in more than 60% of retrieved components (T.Wright 1968). Under conditions of elastic and plastic deformation, the femoral condyle can penetrate into the polyethylene component such that the smaller contact radii existing at the edges of the primary contact radii in the frontal plane can come into contact with the polyethylene surface (Figure 1). Under these conditions Hertzian contact theory or finite element models that do not include both elastic and plastic deformation would not apply. The aim of this study was (1) to calculate the penetration of the femoral condyle into the polyethylene component as a function of the frontal plane radii, and (2) to predict the altered contact stress distribution in the polyethylene component under conditions where penetration exceeds the primary bearing surface.

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