Effect of patella thickness on patella strain following total knee arthroplasty.

Patellar complications following total knee arthroplasty (TKA) have begun to emerge as a major cause of failure. In an effort to understand some of the mechanical factors that might contribute to patellar component failure, a biomechanical study was performed. Quadriceps force and anterior patellar strain were measured during dynamic flexion in 10 fresh, paired human cadaver knee joints. First, tests were performed in the intact knee, followed by either posterior cruciate ligament (PCL) retention or sacrifice of TKA without patellar resurfacing. Tests were then performed following patellar resurfacing with an overly thick, optimum and thin, bony patella. Patellar strain increased in each specimen (with flexion angles of up to 80 degrees), was most pronounced as the bony patella became thinner, was closest to the intact knee when the patella was not resurfaced, and was unaffected by PCL retention or sacrifice. Patellar osteotomy, resulting in a bony patellar thickness of less than 15 mm, resulted in significantly increased strain. TKA systems should include instrumentation that allows precise restoration of overall patellar thickness while maintaining a bony patellar thickness of at least 15 mm.