The unstable patella after total knee arthroplasty: etiology, prevention, and management.

In total knee arthroplasty, most complications related to the extensor mechanism are caused by patellar maltracking or instability. Patellar maltracking may result from component malpositioning and limb malalignment, prosthetic design, improper patellar preparation, or soft-tissue imbalance. Patellofemoral instability likely results most frequently from internal malrotation of the femoral or tibial components. Although a patellofemoral radiograph may display the lateral subluxation of the patella, only computed tomography can quantify rotational malalignment of the femoral or tibial component. Nonsurgical treatment is generally unsuccessful; major malposition of components is best managed by implant revision. In the absence of component malposition, proximal realignments (lateral patellar retinacular release with lateral advancement of the vastus medialis obliquus muscle) or tibial tubercle transfers have been used. Surgical procedures on the patellar tendon itself may risk rupture of the extensor mechanism.