primary (first time) patellar (patella) dislocation

Last reviewed 01/2018

primary (first-time) patellar dislocation

Primary patellar dislocation occurs when there is traumatic disruption of the previously uninjured medial peripatellar structures (1).

• lateral dislocation of the patellar  results in injury to medial patellar restrains (specially the medial patellofemoral ligament MPFL) (2)
• usually seen in the young physically active population and results from twisting, bending, rotating, or valgusing movement of the lower limb
• the incidence range from 6 to 112 per 100,000 persons (the figure varies with the age of the population) (2)

In adults, primary dislocations (without prior knee complaints) results in soft tissue damage and medial restrain injury with acute hemarthrosis. Additionally osteochondral fractures are also frequent in traumatic primary dislocations (2)         

Risk factors for primary patellar dislocation include tall height and excess weight (1).

Clinical finding in patellar dislocation include:

• patient may describe the incidence as “kneecap was out of place”
• severe pain at the patellar area (3)
• acutely swollen knee and hemarthrosis
• there can be slight or no knee effusion in children with dysplasia of the patellorfemoral joint (due to absence of excessive soft tissue damage)  

Diagnosis is usually based on history and clinical findings.

• clinical examination may reveal
• knee effusion
• a laterally displaced patellar but in majority the patellar is spontaneously reduced
• limited range of motion
• difficulty in bearing weight
• tenderness at the tibiofemoral junction, especially over the medial side
• patellar apprehension test (when gentle laterally directed pressure is applied to the patellar, the patient expresses fear of patella dislocation) is positive in most cases
• plain radiographs - always necessary to evaluate the position of the patellar and to check for any osteochondral fractures (2)
• MRI
• recommended to confirm diagnosis, check for additional injuries and to describe the anatomic factors of the patellofemoral joint
• should be done immediately after injury due to high prevalence of osteochondral fractures (2)  

Reference:

• (1) Tsai CH et al. Primary traumatic patellar dislocation. J Orthop Surg Res. 2012;7(1):21.  
• (2) Sillanpää PJ, Mäenpää HM. First-time patellar dislocation: surgery or conservative treatment? Sports Med Arthrosc. 2012;20(3):128-35  
• (3) Wiler JL. Diagnosis:patellar dislocation. Emergency medicine news 2007;29:6-1