This condition usually results from a twisting injury or a direct blow, with the knee in slight flexion.
- the injury is painful and may cause the patient to fall to the ground
- commonly seen in the second and third decade of life and often in women (1)
- a retrospective study of 270 patients and 284 knees with acute patellar dislocations revealed that
- 21% were recurrent dislocations,
- 16% had a family history of dislocation
- 42% were from athletic participation (2)
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)
- patellar instability occurs when the patella fails to engage securely in the trochlea at the start of flexion; it slips laterally and either dislocates completely or slips back medially to its correct position as flexion continues. In some patients this happens because the trochlear groove is too shallow or uneven (trochlear dysplasia) (3)
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.
- (1) Sillanpää PJ, Mäenpää HM. First-time patellar dislocation: surgery or conservative treatment? Sports Med Arthrosc. 2012;20(3):128-35
- (2) Wiler JL. Diagnosis:patellar dislocation. Emergency medicine news 2007;29:6-14
- (3) NICE (January 2014). Arthroscopic trochleoplasty for patellar instability
Last reviewed 09/2021