The patella, or kneecap, is one of three bones, along with the tibia (shin bone) and femur (thigh bone), that make up the knee joint. All of these bones are covered with a layer of cartilage at points where their surfaces come into contact. Furthermore, the patella is enveloped by a tendon. This tendon connects the quadriceps muscle of the thigh to the shin bone (tibia) below the knee.
The kneecap slides up and down a groove on the end of the femur as the knee bends. This groove is called the patellofemoral articulation. The kneecap is designed to fit in the center of this groove, and slide evenly within the groove--this is called patellar tracking. In some patients, the kneecap is pulled towards the outside of the knee. As this happens, the kneecap does not track centrally within the groove.
Also called patellar subluxation, patients who experience an unstable kneecap have a patella that does not track evenly within its groove on the femur. Depending on the severity, this improper tracking may not cause the patient any problems, or it may lead to dislocation of the patella (where the kneecap fully dislocates out of joint). Most commonly, the tracking problem causes discomfort with activity, and pain over the edges of the kneecap. Patellar subluxation and dislocation are conditions that usually affects adolescents, and sometimes younger children.
Diagnosis and Treatment
There are several causes of patellar subluxation. This problem is usually attributed the quadriceps muscle, specifically the vastus medialis obliquus (or VMO). This muscle pulls the kneecap towards the middle of the knee joint, maintaining its position in the joint. If this muscle is not as well developed as other muscles around the knee, or if its muscle fibers are not adequately oriented to control the kneecap, then patellar subluxation may result. The most common cause of kneecap pain is chondromalacia, or an irritation of the cartilage on the undersurface of the kneecap. Patellar subluxation and chondromalacia can go hand in hand, but they should be considered separate entities. That said, if chondromalacia is being caused by subluxation, then the instability of the kneecap must be addressed for treatment to be successful. Other causes of kneecap pain include osteoarthritis, patellar tendonitis (Jumper's knee), and plica syndrome.
Treatment of the unstable patella is first to ensure that the patella is not dislocated. If it is, your doctor will need to properly 'reduce,' or reorient, the position of the kneecap. Early treatment of patellar subluxation includes:
Some patients are not cured by conservative therapy, and it may be determined that surgery is needed, especially in patients who have significant pain or recurrent dislocation. By looking into the knee with an arthroscope, the surgeon can assess the mechanics of the knee joint to ascertain if there is an anatomic malalignment that could be corrected. One common malalignment is the result of too much lateral tension that pulls the kneecap out from its groove; this can place increased pressure on cartilage and lead to dislocation. For this problem, a procedure known as a lateral release can be performed. This procedure involves cutting the tight lateral ligaments to allow the patella to resume its normal position.