Kneecap Instability - Patellar Subluxation

An Unstable Kneecap

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 knee. 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 patient 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.

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
  • Abnormalities in gait

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.

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:

  • Physical Therapy
    Traditionally, patients were sent to physical therapy to strengthen their VMO (part of the quadriceps muscle) to realign the pull on the kneecap. More recent 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.
  • Bracing and Taping
    Bracing and taping of the kneecap are also a controversial topic in the rehabilitation of kneecap problems. These often provide symptomatic relief, but are certainly not a long-term solution. Certainly, if symptomatic relief is found with a brace or tape, it is certainly appropriate to continue with this as a treatment.

Surgery for Patellar Subluxation

Some patients are not cured by simple treatments, and it may be determined that surgery is needed, especially 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.


Post WR. "Anterior Knee Pain: Diagnosis and Treatment" J Am Acad Orthop Surg December 2005 vol. 13 no. 8 534-543.

Tyler TF, et al. "The Role of Hip Muscle Function in the Treatment of Patellofemoral Pain Syndrome" Am J Sports Med. April 2006: 34 630-636.

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