Dislocation of the patella generally results from a traumatic event.1,2 It is most commonly due to a direct blow to the flexed knee. It may also occur from a forceful quadriceps contraction while the femur is internally rotated on the tibia with the foot planted (e.g., baseball, football, and soccer). Many patients may not notice the dislocation as it may spontaneously reduce immediately after the injury. There are numerous theories as to the predisposition to a patella dislocation (e.g., adolescents, age 10 to 30, anteverted femur, contracted iliotibial groove, excessive patellar lateral tilt, family history of patella dislocation, females, flat intercondylar groove, joint laxity, “knock-knees” or genu valgus, hypoplastic femoral condyle, large Q-angles, ligamentous laxity, obesity, patella alta, patellar hypermobility, physical activity, shallow intercondylar groove, and vastus medialis muscle atrophy).3-6 This condition is most commonly seen in adolescents and females.7,8
ANATOMY AND PATHOPHYSIOLOGY
The knee consists of the patellofemoral and the tibiofemoral joints. The patellofemoral joint is a gliding joint. The patella is an oval-shaped sesamoid bone that develops in the tendon of the quadriceps muscle. It is suspended between the quadriceps superiorly and the tibial tuberosity inferiorly. The patella articulates between the femoral condyles. It is held in place by the vastus medialis muscle, the medial retinaculum, the medial and lateral patellofemoral ligaments, and the patellotibial ligament.
The patella may dislocate in numerous directions (Figure 109-1). Lateral dislocations are the most common type (Figure 109-2).8 The patella usually dislocates laterally due to its asymmetrical shape and the normal upward and lateral pull of the quadriceps muscle. The patella may dislocate intraarticularly, medially, and superiorly in rare instances (Figure 109-1).8-13
Types of patellar dislocations.
Anatomy of a lateral patellar dislocation. A. Anteroposterior view. B. Lateral view.
The clinical determination of a lateral patellar dislocation is usually simple and obvious (Figure 109-3A).8 The knee is held in partial flexion. The patella can be seen and palpated on the lateral surface of the knee. This may be accompanied by edema and/or ecchymoses over the anterolateral knee.8
The lateral patellar dislocation. A. The presentation is often clinically dramatic. B. Radiograph. (Used with permission from www.lifeinthefastlane.com.)
Pain over the parapatellar ligaments may be the only clinical sign in patients whose patellar dislocation has spontaneously reduced. The physical examination usually reveals mild edema in the parapatellar recesses. There is often laxity in the tendons and ligaments surrounding the patella. A patellar apprehension test ...