The effect of conformity on stresses in dome-shaped polyethylene patellar components

Polyethylene stresses were examined in patellar components with convex-shaped articulating surfaces in contact with convex metallic surfaces. Two finite element models were used: 1 with geometry of a newly manufactured component and 1 with geometry of a retrieved patellar component that had worn and deformed so that the contact area was concave. The range of maximum principal stress and the maximum shear stress were more severe in the newly manufactured model than in the model of the retrieved component. These stresses were also more severe than determined previously for models of femorotibial contact. For both patellar models, the von Mises stress was at or near the polyethylene yield stress in most of the contact area, consistent with the large amount of permanent deformation observed on many retrieved patellar components. This suggests that deformation may continue, even when the component's surface has deformed and worn into a concave shape.