Patellar Instability/Extensor Mechanism Injury
What is it? Extensor mechanism problems can range from a true dislocation of the patella to recurrent subluxation or instability of the patella leading to symptoms consistent with patellofemoral syndrome. A patellar dislocation is the displacement, usually laterally, of the patella from the femoral trochlea. A subluxation is a transient partial displacement of the patella from the trochlea.
How does it present? A true dislocation is often a traumatic event and the patella usually remains dislocated until it is reduced, often by extending the knee. This occurs with a twisting of the knee with a strong quadriceps contraction. Often the patient will feel a "pop" and the abrupt onset of pain. The patella may reduce spontaneously when the knee is extended. In these cases there is usually swelling and effusion of the knee. Patellar subluxation may occur with a similar, although less dramatic mechanism of injury. They may have experienced a true dislocation in the past. Swelling is usually much less prominent.
Common exam findings: With a true dislocation, if not reduced, you may notice the patella over the lateral femoral condyle. Often the patella is already reduced and you may see significant swelling and effusion and tenderness over the patella, particularly the medial side. The patient often has a positive Patellar Apprehension Test and hypermobility (the Patellar Apprehension Test is also available in Spanish; click on the "CC" icon at the bottom of the video to select the language). With a patient that has recurrent subluxation the patellar apprehension and hypermobility tests may be positive. Usually the swelling is not as prominent as with dislocation.
Tests and treatment: Knee x-rays should be done to evaluate for bony abnormalities, particularly to rule out osteochondral fractures or loose bodies. Treatment may involve a short period of immobilization to alleviate pain followed by a rehabilitative program to try to prevent symptoms of further instability and recurrent injury. Patellar bracing may also be used during portions of rehabilitation.