Failure mechanisms of metal-backed patellar components.

A specific type of failure of a metal-backed patella occurred, consisting of a separation of the polyethylene from the metal backing. This dissociation is followed by wearing of the metal backing against the femoral component. This produces clinical signs and symptoms that usually require revision. This failure occurred in 16 knees of 14 patients. Average time to failure was 14 months. Failure was associated with patellar implant design, surgical technique, patient characteristics, and femoral component design. The plastic of the metal-backed patellae was thin at the periphery. The metal plate did not extend to the edge of the plastic and acted like a sharp edge that cut the thin plastic. The plastic dome was not bonded to the plate and became separated from the plate once the plastic tore. The plate became deformed. Failure was more likely with a subluxated, tilted, or excessively thick patella or flexed femoral component. The typical patient was a large, active male who had achieved more than 115 degrees of flexion. A prominent edge on the lateral intercondylar ridge of the femoral component resulted in a concentrated load being applied to the plastic dome and favored rapid wear of the patella. Until a greater understanding of patello-femoral loading is acquired, caution in the use of metal-backed patellae is recommended.