Patellar Dislocation

Anatomy

The patella, or kneecap, is a thick triangular piece of bone that covers and protects the knee joint. It lies within a vertical groove on the femur (thighbone). This vertical groove is called the trochlear groove and is located between the medial and lateral femoral condyles. The condyles are two bulbous ends of the femur through which our weight passes. The patella is covered by a fibrous capsule and is attached above to the tendon of the quadriceps [Page 1033]femoris (the main extensor muscle on the front of the thigh). It is attached below to the patellar tendon, which then connects to the tibial tuberosity (an oblong elevation of the tibia).

The quadriceps muscle group consists of the vastus lateralis (outer portion of the thigh muscle), vastus medialis (inner portion of the thigh muscle), and vastus intermedius (middle portion of the thigh muscle), which originate on the femoral shaft and have muscle tendons that insert on the superior (upper) pole of the patella.

The medial patellofemoral ligament (MPFL) originates at the medial femoral condyle on the inner aspect of the knee and attaches to the medial (inside) border of the patella. The MPFL has been found to be the major soft tissue restraint to lateral patellar dislocation (kneecap dislocating to the outside of the knee). The MPFL and two other ligaments comprise the medial patellar retinaculum.

The lateral aspect of the patella is stabilized by the lateral patellar retinaculum. The lateral patellar retinaculum is the confluence of several fibrous structures, including the lateral patellofemoral ligament (LPFL), which connects from the lateral femoral condyle to the outer portion of the patella.

Causes

Patellar dislocation can result from minor trauma in those with risk factors or can result from a high-impact force in a patient with normal anatomy. A common mechanism is dislocation of the right patella when the torso rotates left and the right foot is planted on the ground. The patella then dislocates laterally (to the outside of the knee). Lateral instability is much more common than medial instability. Risk factors for patellar dislocation include the following:

• Any abnormality that increases the lateral (outside) force on the patella
• Weakness of the vastus medialis muscle
• Hypertrophy of the vastus lateralis muscle, thus pulling the patella laterally
• Patella alta—an abnormally high-riding patella with a long patellar tendon
• Shallow trochlear groove, which makes it easier for the patella to slide out of this groove
• Foot pronation, where the foot turns outward at the ankle, causing walking to be done on the inner side of the foot
• Pes planus, or flat feet
• Genu valgum, also called “knock-knees,” where the knees angle inward and the lower legs angle outward
• Femoral anteversion, where the femoral neck (the top of the femur near the hip joint) leans forward with respect to the rest of the femur, causing the knees to rotate inward
• External tibial torsion, where the tibia is rotated outward

Symptoms

An acute patellar dislocation often occurs after direct contact or a sudden change in direction. Symptoms include rapid swelling of the knee joint, intense knee pain, and inability to extend the knee joint. Sometimes patients will report having heard a pop when the injury occurred. The knee is usually maintained in a flexed position. Patients who have had one episode of dislocation are more likely to have another because the tissues have been stretched out by the initial episode. Such patients usually have some of the anatomic predisposing factors listed above.

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