Wear and deformation of patellar components in total knee arthroplasty.

Clinical experience has shown that deformation of patella components can lead to loosening or excessive particulate debris, requiring revision. Factors affecting patella performance are the degree of congruity, the shear forces occurring due to tracking variations, and the excessive patellofemoral forces, particularly at higher flexion angles. A method was devised to load the patella with a constant force against the femoral component. The load was applied through a range of flexion. The deformation and wear patterns resembled those of retrieved clinical specimens. All-plastic components suffered from local deformation and underlying bone failure, whereas metal backing led to eventual penetration of the metal onto the femoral flange. Increased conformity of the patella component onto the femoral flange improved the situation for all-plastic and metal-backed designs by increasing the contact area and the available thickness of plastic.