The patella is a flat triangular bone that lies anterior to the tibiofemoral junction. It is considered a sesamoid bone, which develops from within the tendon of the quadriceps femoris. The proximal attachment is the quadriceps tendon, and the distal attachment is the patellar tendon. Patellar dislocations may occur when patients with normal anatomy are exposed to direct high-energy forces, but most studies find that it occurs more often when patients with abnormal anatomy are exposed to indirect forces. (Prim Care 2004;31:909.)
Patellar dislocations occur most often in the second and third decades of life (Am J Sports Med 2000;28:472), and occur more commonly in women. (Am J Sports Med 2004;32:1114.) The annual incidence of acute patellar dislocation in children under 16 is calculated to be 43 in 100,000, with osteochondral fractures occurring in 39 percent of these cases. (J Pediatr Orthop 1994;14:513.)
Depending on the study, 30 percent to 72 percent of dislocations occur from sports-related activities. (Prim Care 2004;31:909.) A retrospective study of 270 patients and 284 knees with acute patellar dislocation found that 21 percent of cases were recurrent dislocations, 16 percent had a family history of dislocation, and 42 percent resulted from athletic participation. (Br J Sports Med 1995;29:239.)
Patients with patella alta (high-riding patella), generalized joint laxity, a history of patellar subluxation, genu valgum, increased femoral anteversion with compensatory external tibial torsion (angular difference between axis of femoral neck and transcondylar axis of the knee), tear of medial patellofemoral ligament, and below-the-knee amputations also are at increased risk of recurrent patellar dislocation. (Acta Orthop Scand Suppl 1983;201:1; Am J Orthop 2005;34:246.)
Acute traumatic patellar dislocations tend to be lateral, typically with disruption of the medial patellofemoral ligament at the femoral attachment. (Wheeless' Textbook of Orthopaedics; www.wheelessonline.com/ortho/medial_patellofemoral_ligament.) The usual mechanism of injury is a sudden internal rotation of the femur or valgus stress on the knee while the foot is fixed, causing the patella to be displaced laterally. The knee is frequently held in 20 to 30 degrees of flexion with the patella palpable laterally. (Henretig RM, King C (eds). Textbook of Pediatric Emergency Procedures. Baltimore: Williams and Wilkins, 1997.) Ninety percent of patellar dislocations, however, reduce spontaneously as the patient extends the knee. (CPEM 2007;8:31.)
An acute patellar dislocation is usually clinically obvious unless the patella has spontaneously reduced. The patient will typically complain of severe pain at the patellar area, and may claim to have heard a “pop” at the time of dislocation. Physical examination will reveal a knee effusion, limited range of motion, and difficulty in bearing weight. There is often tenderness at the tibiofemoral junction, especially over the medial facet, medial retinaculum, and adductor tubercle of the medial femoral epicondyle. A patellar apprehension test (when the patient expresses fear that the patella may dislocate as the examiner applies gentle laterally directed pressure) is almost always positive. (CPEM 2007;8:31.)
If the patella does not spontaneously reduce, manual reduction is required, but it should not be attempted if a fracture is suspected or verified by radiography. If the injury is recent or the patient is cooperative, immediate manual reduction can be attempted. Patients with significant muscle spasm and pain may require analgesics and procedural sedation before reduction is successful. Radiographs are not necessary prior to reduction unless a patellar fracture is suspected.
Manual reduction is achieved by placing the patient in the supine position with the hip on the affected side slightly flexed to relax the quadriceps muscle. Firm but gentle continuous pressure is place on the medical aspect of the laterally dislocated patella, while the affected leg is extended.
Post-reduction radiographs should be performed to identify associated fractures. Ligamentous stability of the knee also should be determined. Other associated traumatic injuries should be identified by standard protocols. Large hemarthrosis can be evacuated by standard atherocentesis technique to decrease discomfort.
After reduction, a compression wrap should be placed on the knee with a knee immobilizer and the patient instructed to elevate, ice, and limit walking on the affected leg by using crutches. Analgesic medications, including nonsteroidals, should be recommended unless contraindicated. Patients also should be instructed to see an orthopedist within one week. Athletes with acute patellar dislocation can expect to return to play within four to six weeks with appropriate strengthening exercises and the use of a knee brace. (Kibler, WB. American College of Sports Medicine's Handbook for the Team Physician, Kibler, WB (ed), Baltimore, MD: Williams and Wilkins, 1996.)
The first episode of traumatic lateral patellar dislocation or subluxation is treated nonoperatively unless there is a loose fracture fragment in the joint. (Am J Sports Med 1988;16:244.) Overall, surgery has been shown to have redislocation rates of zero to 17 percent and good-to-excellent results in 50 percent to 100 percent of patients. Most nonsurgical protocols report redislocation rates of 13 percent to 52 percent and good-to-excellent results in 47 percent to 85 percent of patients. (Acta Orthop Scand 1997;68:415.)