Patellar Overstuffing in Total Knee Arthroplasty Results in Decreased Extensor Mechanism Efficiency

Patellofemoral offset is influenced by patellar thickness in total knee arthroplasty (TKA), with effects on extensor mechanism efficiency (EME), patellar contact moment arm (PCMA), and patellar contact force (PCF). This study utilized a computational model to better quantify these measures during knee range of motion (ROM). A computational model of a cruciate-sacrificing, posterior-stabilized TKA design was used to quantify moments at knee flexion angles from 0° to 135° when patella thickness was increased ("overstuffed") or decreased ("understuffed") by 2 mm and 5 mm from normal. Outcomes included changes in EME, PCMA, PCF, tibial tubercle moment arm (TTMA), and the quadriceps moment arm (QMA). Overstuffing of the patellofemoral articulation by 2 mm and 5 mm decreased EME up to 12 and 28%, respectively, with an 8% and 16% increase in EME with understuffing that peaked at 25° knee flexion. The QMA increased up to 5 and 12% with overstuffing by 2 mm and 5 mm and decreased up to 5% and 14% with understuffing, respectively. There was an increase in the PCMA/PCF, which opposed the QMA, up to 8%/17% and 16%/39% with overstuffing by 2 mm and 5 mm, respectively. The PCMA/PCF decreased up to 3%/12% and 8%/33% with understuffing by 2 mm and 5 mm, respectively. In this computational model, increased patellofemoral offset resulting in overstuffing of the patella resulted in decreased EME and increased PCMA and PCF during knee ROM. This may contribute to abnormal patellofemoral mechanics and a potential etiology for anterior knee pain and quadriceps weakness after TKA.