Knee

Patellar dislocation

Mechanism

The patella shifts laterally out of the trochlea, usually in flexion with valgus and external rotation of the lower leg. A shallow trochlea, a high-riding patella and a large Q-angle make it more likely.

Recognition

It often reduces spontaneously on extending the knee, leaving the patient with only a markedly swollen knee. Tenderness over the medial retinaculum and a positive apprehension test point to it.

Course

After a first dislocation about one in three dislocates again, and in young patients with an abnormal trochlea that figure is higher. The medial patellofemoral ligament almost always tears with it.

Structures

• Medial and lateral patellar retinacula (MPFL & LPFL)
• Kneecap
• Vastus Medialis

Joints involved

• Patellofemoral joint (art. patellofemoralis)

Clinical tests

• Fluctuation test for effusion

Sources

• Magee DJ, Manske RC. Orthopedic Physical Assessment, 7e editie. Elsevier. 2021