Patellar Instability

Clinical photo of a woman with recurrent lateral patellar dislocation

A patellar dislocation occurs when the patella shifts laterally and “comes out” of the trochlea or femoral groove. This can occur as a result of trauma or can be atraumatic. Atraumatic patellar instability commonly occurs with a twisting of the knee with a fixed/planted foot. The femur rotates internally, turning the femoral groove with it. The patella has to follow the pull of the quadriceps or the attachment of the patellar tendon. The propensity to dislocate the patella is increased if the knee also has a valgus or “knock kneed” force as well. People with congenital laxity are more susceptible. There are also some anatomic factors that can predispose people to patellar instability, including a high riding patella, rotational and alignment variations of the femur, and a shallow femoral groove.

MRI of a laterally dislocated patella and complete absence of the femoral groove

Following an initial patellar instability episode, most patients are treated conservatively with immobilization, bracing, medications, and eventual physical therapy. Patellar stabilization braces and/or patellar taping can provide some stability for patients during more strenuous activities and sports. Recurrent patellar instability occurs in anywhere from 15-50%, depending on risk factors. Adolescent women have the highest incidence of recurrence. Residual symptoms of pain and impaired function ( difficulty kneeling, squatting, stairs, and sports ) are seen in up to 60% of initial patellar dislocators at 6 months post-injury.

For those with recurrent patellar instability or chronic symptoms, surgical intervention can be considered. The procedures depend on predisposing factors and can include reconstruction of the medial patellofemoral ligament ( stabilizing ligament of the patella ), realignment of the patella by shifting the tibial tubercle, and on occasion, deepening the femoral trochlea.

Image of a medial patellofemoral ligament reconstruction
Lateral x-ray of a tibial tubercle osteotomy

Watch a video of a medial patellofemoral ligament reconstruction demonstration