Subluxation is another word for partial dislocation of a bone. Patellar subluxation is a partial dislocation of the kneecap (patella). It’s also known as patellar instability or kneecap instability.

The kneecap is a small protective bone that attaches near the bottom of your thigh bone (femur). As you bend and straighten your knee, your kneecap moves up and down in a groove at the bottom of the thigh, called the trochlea.

Several groups of muscles and ligaments hold your kneecap in place. When these become injured, your kneecap may move out of the groove, causing pain and difficulty flexing the knee.

The extent of the dislocation determines whether it’s called a patellar subluxation or a dislocation.

Most injuries push the kneecap toward the outside of the knee. This can also damage the ligament on the inside of the knee, known as the medial patello-femoral ligament (MPFL). If the MPFL does not heal properly, it can set the stage for a second dislocation.

You may experience the following symptoms with patellar subluxation:

  • buckling, catching, or locking of the knee
  • slipping of the kneecap to the outside of the knee
  • pain after extended sitting
  • pain at the front of the knee that worsens after activity
  • popping or cracking in the knee
  • stiffness or swelling of the knee

Although you might be able to self-diagnose, you will need to see a doctor for treatment.

Any extreme activity or contact sport can cause a patellar subluxation.

Patellar subluxations and dislocations mainly affect young and active people, especially between the ages of 10 to 20 years. Most first-time injuries occur during sports.

After an initial injury, the chances of a second dislocation are very high.

To diagnose a patellar subluxation, your doctor will bend and straighten the injured knee and feel the area around the kneecap.

X-rays may be used to see how the kneecap fits into the groove at the bottom of the patella and to identify any other possible bone injuries.

Magnetic resonance imaging (MRI) may be used to visualize the ligaments and other soft tissue around the patella. Children and adolescents are sometimes not aware that they’ve had a patellar dislocation. The MRI can help confirm it.

Nonsurgical treatment is recommended for the majority of people with a first-time patellar subluxation or dislocation.

Nonsurgical treatment includes:

  • RICE (rest, icing, compression, and elevation)
  • nonsteroidal anti-inflammatory drugs (NSAID), such as ibuprofen (Advil, Motrin)
  • physical therapy
  • crutches or a cane to take weight off the knee
  • braces or casts to immobilize the knee
  • specialized footwear to decrease pressure on the kneecap

After a patellar subluxation, you have about a 33 percent chance of a recurrence.

In 2007, a systematic review of 70 previous studies found little difference in long-term outcomes between those who had surgery for their patellar dislocation and those who didn’t. Those who had surgery were less likely to have a second dislocation but more likely to develop arthritis in the knee.

A 2015 study found a lower rate of recurrence of full dislocation of the kneecap in people who had surgical treatment. But the rate of recurrence of patellar subluxation was almost the same (32.7 versus 32.8 percent), whether the person had surgery or not.

Most cases of first-time patellar subluxation are treated conservatively, without surgery. Surgical treatment is recommended if you have a repeat episode or in special cases.

Some common types of surgery for repeat episodes of patellar subluxation or dislocation are:

Medial patellofemoral ligament (MPFL) reconstruction

The medial patellofemoral ligament (MPFL) pulls the kneecap toward the inside of the leg. When the ligament is weak or damaged, the kneecap can dislocate toward the outside of the leg.

MPFL reconstruction is an arthroscopic surgery involving two small incisions. In this operation, the ligament is reconstructed using a small piece of tendon taken from your own hamstring muscle or from a donor. It takes about one hour. You usually return home the same day wearing a brace to stabilize your knee.

The brace keeps your leg straight while walking. It’s worn for six weeks. After six weeks, you begin physical therapy. Most people can resume sports and play activities four to seven months after MPFL reconstruction.

Tibial tuberosity transfer

The tibia is another name for your shin bone. The tibial tuberosity is an oblong elevation, or bulge, in the tibia just below your knee.

The tendon that guides your kneecap as it moves up and down in the trochlear groove attaches to the tibial tuberosity. An injury that has caused the kneecap to dislocate may have damaged the connection point for this tendon.

Tibial tubercle transfer operation requires an incision about three inches long above the shin bone. In this operation, your doctor transfers a small piece of the tibial tuberosity to improve the attachment of the tendon. This then helps the kneecap to move properly in its groove.

The surgeon will place one or two screws inside your leg to secure the piece of bone that is transferred. The operation takes about one hour.

You’ll be given crutches to use for six weeks following surgery. After that, physical therapy begins. Most people are able to return to work or school two weeks after surgery. It takes about nine months before you can return to sports.

Lateral release

Until about 10 years ago, lateral release was the standard surgical treatment for patellar subluxation, but it’s rare nowadays because it increases the risk of recurrence of instability in the kneecap.

In this procedure, ligaments on the outside of the knee are partially cut to prevent them from pulling the kneecap to the side.

Without surgery

If you don’t have surgery, your recovery will begin with the basic four-letter treatment known as RICE. This stands for

  • rest
  • icing
  • compression
  • elevation

Initially, you shouldn’t push yourself to move around more than is comfortable. Your doctor may prescribe crutches or a cane to take the weight off your knee.

You’ll likely see your doctor again within a few days of the injury. They’ll tell you when it’s time to start increasing activity.

You’ll probably be assigned physical therapy two or three times a week for the first six weeks. Your physical therapist will help evaluate when you are ready to get back into sports and other strenuous activity.

With surgery

If you’ve had surgery, the recovery is a longer process. It may take four to nine months before you’re able to resume sports, though you should be able to resume light activities within two to six weeks.

Certain exercises may help strengthen your leg muscles and reduce the chance of knee injuries, including patellar subluxation. To reduce your risk for this type of injury, add some of the following exercises to your routine:

  • exercises that strengthen your quadriceps, such as squats and leg lifts
  • exercises to strengthen your inner and outer thighs
  • hamstring curl exercises

If you’ve already had a kneecap injury, wearing a brace can help prevent recurrence.

Wearing proper protective gear in contact sports is another important way to prevent all types of kneecap injuries.

Patellar subluxation is a common injury for children and adolescents, as well as some adults. The first occurrence does not normally require surgery. If surgery is needed, a number of new techniques make it likely that you’ll regain all or most of your previous strength and activity.