Dislocation of the patella generally results from a traumatic event.1–9 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. Many patients may not notice the dislocation as it may spontaneously reduce immediately after the injury. There are numerous theories as to the predisposition, if any, to a patella dislocation.1,2 These include adolescents, females, flat intercondylar groove, joint laxity, “knock-knees” or genu valgus, large Q-angles, obesity, and vastus medialis muscle atrophy. This condition is most commonly seen in adolescents and females.
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 87-1). Lateral dislocations are the most common type (Figure 87-2). The patella usually dislocates laterally due to its asymmetrical shape and the normal upward and lateral pull of the quadriceps muscle. The patella may also dislocate superiorly, medially, and intraarticularly in rare instances.3,4
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 quite obvious (Figure 87-3). 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.
The lateral patellar dislocation. The presentation is often clinically dramatic. (Photograph courtesy of Dr. Robert R. Simon.)
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 is generally positive. The knee joint is usually stable.
The pathophysiology of this dislocation may include abnormalities secondary to malalignment, laxity, and hyper-elasticity of the joint. Osteochondral fractures are common but seen only on arthroscopy.1,2,5 Magnetic resonance imaging, bone scans, and arthroscopy are considerations for further evaluation and diagnosis of the patellofemoral joint by the Orthopedic Surgeon.