Patellar dislocations can result from either direct trauma or powerful quadriceps contraction with knee flexion. Commonly, the patella dislocates laterally. Patients who present after a patellar dislocation may state their knee dislocated and spontaneously reduced. A hemarthrosis can be present. Common complaints include pain, swelling, and deformity. The patellar apprehension test (gently displacing the patella laterally or medially while in extension) is often positive in spontaneously reduced patellar dislocations. The patient will become apprehensive as you attempt to reproduce the dislocation. Radiography will confirm the dislocation and help rule out associated fracture. Fractures of the patella or lateral femoral condyle occur in 5% of patients.
Patellar Dislocation. An obvious lateral deformity of the right patella in a patient with dislocation. (Photo contributor: Cathleen M. Vossler, MD.)
Patellar Dislocation. Lateral deformity of the left patella in a patient with dislocation. (Photo contributor: Suzanne Dooly-Hash, MD.)
Management and Disposition
Lateral dislocations are reduced by flexing the hip, extending the knee, and gently directing pressure medially on the patella. Reduction is easily accomplished and results in immediate relief of pain. Postreduction films should be obtained. After successful reduction, patients require a knee immobilizer in full extension for 4 to 6 weeks. Outpatient referral to a musculoskeletal specialist should be obtained for long-term follow-up.
A dislocated patella may reduce spontaneously prior to presentation; it is a possibility in any patient who presents with knee pain. This may be elucidated by inquiring about a knee deformity at the time of injury that is no longer present.
Complications of patellar dislocation include degenerative arthritis, recurrent dislocations, and chondral or osteochondral loose bodies.