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What is kneecap subluxation?

A subluxing patella (kneecap) is a temporary, partial dislocation of the kneecap from its normal position in the groove in the end of the thigh bone (femur). This groove is located between two bumps at the end of the thigh bone called the femoral condyles.

How does it occur?

This temporary dislocation of the kneecap usually happens during forced leg straightening, with the kneecap moving out of the groove to the outer side of the knee.

The cause is usually abnormal structure in the leg. The inner thigh muscle may be underdeveloped or the outer thigh muscle may be overdeveloped. Your kneecap may be higher in the leg than most people’s. You may be knock-kneed or the outer side of the femur bone may be underdeveloped.

What are the symptoms?

You may feel the kneecap moving out of position. You may have swelling and pain behind the kneecap. You may have pain when you bend or straighten your leg.

How is it diagnosed?

Your healthcare provider will ask about your symptoms and examine your knee. He or she may be able to feel the kneecap slipping to the outside as you bend and straighten your leg. An X-ray may show underdevelopment of the lateral femoral condyle.

How is it treated?

To treat this condition:

  • Put an ice pack, gel pack, or package of frozen vegetables, wrapped in a cloth on the area every 3 to 4 hours, for up to 20 minutes at a time.
  • Raise the knee on a pillow when you sit or lie down.
  • Take an anti-inflammatory medicine such as ibuprofen, or other medicine as directed by your provider. Nonsteroidal anti-inflammatory medicines (NSAIDs) may cause stomach bleeding and other problems. These risks increase with age. Read the label and take as directed. Unless recommended by your healthcare provider, do not take for more than 10 days.
  • Wear a brace prescribed by your healthcare provider to keep your kneecap in place.
  • Follow your provider’s instructions for doing exercises to help you recover.

Some people need surgery to keep the kneecap from subluxing.

While you are recovering from your injury you will need to change your sport or activity to one that will not make your condition worse. For example, you may need to bicycle instead of run.

When can I return to my normal activities?

Everyone recovers from an injury at a different rate. Return to your activities depends on how soon your knee recovers, not by how many days or weeks it has been since your injury has occurred. The goal of rehabilitation is to return you to your normal activities as soon as is safely possible. If you return too soon you may worsen your injury.

You may safely return to your activities when, starting from the top of the list and progressing to the end, each of the following is true:

  • Your injured knee can be fully straightened and bent without pain.
  • Your knee and leg have regained normal strength compared to the uninjured knee and leg.
  • Your knee is not swollen.
  • You are able to bend, squat, and walk without pain.

If you develop pain, swelling, or the feeling that your kneecap is moving out of place again, you need to contact your healthcare provider.

How can I prevent a subluxing kneecap?

A subluxing kneecap is best prevented by keeping your thigh muscles strong, especially the group of muscles on the inner side of the thigh.

Written by Pierre Rouzier, MD, for RelayHealth. Published by RelayHealth.
Last modified: 2011-02-08
Last reviewed: 2010-10-11 This content is reviewed periodically and is subject to change as new health information becomes available. The information is intended to inform and educate and is not a replacement for medical evaluation, advice, diagnosis or treatment by a healthcare professional. References
Pediatric Advisor 2011.4 Index
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