Kneecap Instability - Patellar Subluxation

What to do About an Unstable Kneecap

knee exam
Rehab may be an important treatment of patellar subluxation. DNY59 / Getty Images

The patella, or kneecap, is one of three bones that forms the knee joint. Each of these bones has a protective layer of cartilage where their surfaces come into contact. The patella is also enveloped by a tendon. This tendon connects the quadriceps muscle of the thigh to the shinbone (tibia) below the knee.

Patellar Subluxation

The kneecap slides up and down a groove on the end of the thigh bone as the knee bends.

This groove is called the trochlea. The kneecap is designed to fit in the center of the trochlear groove, and slide evenly within the groove. In some people, the kneecap is pulled towards the outside of the trochlear groove. As this happens, the kneecap does not slide centrally within its groove--we call this patellar subluxation.

Depending on the severity of the patellar subluxation, the improper tracking may not cause the individual any symptoms, or it could lead to dislocation of the patella (where the kneecap fully comes out of the groove). Most commonly, patellar subluxations cause discomfort with activity, and pain around the sides of the kneecap, called patellofemoral pain syndrome, or PFPS. 

Many people with symptoms of kneecap pain are diagnosed with patella subluxation or maltracking of the patella. These issues with the mechanics of how the knee joint bends are thought to be a common cause of symptoms that range from knee pain to dislocation of the kneecap.

Causes of Patellar Subluxation

There are dozens of factors implicated in the cause of patellar subluxation. The bottom line is that it is probably the contribution of several factors that lead to instability of the kneecap. Possible factors include a wider pelvis, a shallow groove for the kneecap, and abnormalities in a person's gait.

There has been tremendous interest in recent years of the way our muscles help guide the kneecap as it bends in the knee joint. In particular the muscles around the hip are thought to be the most important muscles in controlling the mechanics of the joint and how the kneecap moves as the joint bends.

Typically the pain associated with patellar subluxation is attributed to PFPS.  Other causes of kneecap pain include knee arthritis, patellar tendonitis (Jumper's knee), and plica syndrome. It is important to know that not every cause of kneecap pain is simply the result of patellar subluxation, and other factors may also be important to determine the cause of pain, and the treatments that should be recommended.

Treatment of Patellar Subluxation

Treatment of patellar subluxation should first ensure that the patella is not dislocated. Your doctor can determine by examining your knee and obtaining x-rays to see if the kneecap is outside of its groove. In patients with a kneecap dislocation, the kneecap may need to be repositioned, or "reduced."

Treatment of patellar subluxation includes traditional physical therapy, used to strengthen the quadriceps muscle and realign the kneecap. Some research has shown that this is probably not the critical factor in eliminating kneecap problems.

Focusing instead on strengthening of the hip abductors and hip flexors (so-called pelvic stabilization exercises) offers better control of the kneecap. The kneecap position depends not only on the the muscles directly around the knee joint, but also the muscles that control the position of the entire extremity. For example, when the femur, thigh bone, internally rotates, the kneecap will tend to pull to the outside of the trochlea. This is precisely why improving the stability of the core and the hips is so critical to improving the mechanics of the knee joint and relieving patellar subluxation.

Bracing and taping of the kneecap is also a controversial topic in the rehabilitation of kneecap problems. These often provide symptomatic relief, but are certainly not a long-term solution. However, if a patient has symptomatic relief with a brace or tape, it is certainly appropriate to continue with this as a treatment. Footwear contributes to the gait cycle. Motion control running shoes may help control your gait while running and decrease the pressure on the kneecap.

Surgery for Patellar Subluxation

Some patients are not cured by simple treatments, and  surgery may be 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 issue that can be corrected. There are several treatment options to correct a patellar subluxation, and the most appropriate option depends on the severity of the condition and the cause of the abnormal positioning of the kneecap.

While surgery can be a useful tool to help manage kneecap problems, it is important to clearly understand what the goal of surgery is, and how the procedure will help to correct the underlying problem. For a number of years, arthroscopic surgery was performed, as well as a procedure called a lateral release, for vague kneecap problems. While some patients improved, others didn't get better with surgery.  Simply performing a surgery, without understanding the specific problem being targeted for correction, can lead to unsatisfactory results.

Sources:

Smith, T. Donell, S. Song, F., Hing, CNational Institutes of Health National Library of Medicine "Surgical versus non‐surgical treatment after kneecap dislocation."  Oct. 2014

Koh JL, Stewart C. "Patellar instability" Orthop Clin North Am. 2015 Jan;46(1):147-57.

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