What is the diagnosis?

Patellar dislocation

Image 2. AP and lateral x-ray of the knee with shown patella dislocation in the superolateral location. Case courtesy of Maulik Patel, radiopaedia.org, rID: 55061

What is the mechanism of injury?

The most common mechanism is flexion with a valgus stress, this is the cause in up to 93% of all injuries [1]. This typically leads to a lateral displacement and tearing of the medial patellofemoral ligament (MPFL). 

• Pearl: This injury occurs most frequently in young active patients (<20 years old) [1]. 

What physical exam findings are expected?

If the patella is still dislocated, the knee will often be in a flexed position and the patella will likely be located in a superolateral position. 

If the patella has already been reduced the patient may have a positive patellar apprehension test. This is when the patient feels that they may dislocate their patella again if continued pressure in a lateral direction is applied. 

• Pearl: Superior dislocation with patella stuck on femoral condyle is possible as well, but very uncommon. 

What is the management in the ED?

Reduce the patella if it is dislocated. This is done by applying a medially directed pressure to the patella while straightening the knee into full extension. The patella should easily glide back into place. This procedure generally does not require sedation, however, analgesia and anxiolysis should be administered as necessary on a case by case basis. Symptoms often resolve or significantly improve once the patella is reduced. This case is one of the few reasons why the patient should be placed in a knee immobilizer. 

• Pearl: Once reduced the knee should remain in extension. If they flex the knee again they are at risk of repeat dislocation. 

• Pearl: Pre-reduction plain film in the ED with clinical suspicion for patellar dislocation is institution dependent, but often performed. The most common complication of a dislocation is an osteochondral fracture, especially in the pediatric population. 

When do you consult Orthopedics?

Orthopedic consult is not typically required. They can follow up as an outpatient with Orthopedics or sports medicine in 1 to 2 weeks. If they are a repeat dislocator they would likely benefit from surgical intervention and should follow up with Orthopedics.