This condition usually results from a twisting injury or a direct blow, with the knee in slight flexion.
The force required to cause dislocation may vary according to the patellar morphology e.g. - in patients with normal patellar morphology dislocation may occur when they are exposed to direct high energy forces while in patients with abnormal anatomy, indirect forces may result in dislocation (1,2)
Sometimes, the patella reduces itself but more commonly, is observed as a prominent bulge on the lateral margin of the knee. The medial condyle is uncovered and may be mistaken for the displaced patella. However, medial displacement is extremely uncommon and usually, a complication of surgery in recurrent dislocation.
The knee is swollen and neither active nor passive movement is possible.
X-ray of the knee confirms the displacement and in about 5% of cases, reveals an associated osteochondral fracture.
Last reviewed 01/2018