Merck Manual

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Kneecap Dislocations

(Patellar Dislocation)


Danielle Campagne

, MD, University of California, San Francisco

Reviewed/Revised Jan 2023

Kneecap (patellar) dislocations occur when the kneecap and the ligaments that hold it in place slide sideways and to the outside of the knee.

  • The kneecap can slide out of place when people, usually adolescent girls with a knee abnormality, suddenly try to change direction.

  • The kneecap looks out of place, and people feel pain under the kneecap and cannot straighten the knee.

  • Doctors can diagnose a dislocated kneecap by examining the knee.

  • Doctors move the kneecap gently back in place, then apply a knee immobilizer or hinged brace.

Kneecap dislocations are common. They often occur in adolescent girls who have an anatomic abnormality of the knee joint, such as loose ligaments or slightly misaligned bones.

Usually, the kneecap slides out of place when people suddenly try to change direction. Kneecap dislocations may occur in sports that involve sudden twisting of the knee and/or impact (such as soccer, gymnastics, baseball when swinging a bat, and ice hockey). They can also occur when people trip or slip.

Sometimes the force that caused the dislocation fractures the cartilage on the back of the kneecap or causes a small piece of the end of the thighbone to break off.

Symptoms of a Dislocated Kneecap

The dislocated kneecap looks out of place. People feel pain under the kneecap. The joint is tender and often swollen. People cannot straighten the knee and can often move the kneecap from side to side. They may be unable to walk.

Diagnosis of a Dislocated Kneecap

  • A doctor's evaluation

  • X-rays to check for fractures

A dislocated kneecap may slide back into place on its own before people can see a doctor. However, people should still see a doctor so that the doctor can check for fractures that may have occurred during the injury.

Did You Know...

  • If a kneecap moves out of place, people should see a doctor regardless of whether the kneecap slides back in place on its own.

Doctors can diagnose a dislocated kneecap when they examine the knee. If the kneecap has already slid back in place, they may suspect that it was dislocated because they detect blood within the joint and the joint is tender.

Doctors take x-rays from several angles to check for fractures.

Treatment of a Dislocated Kneecap

  • Manipulation to move the kneecap back in place

  • Usually an elastic bandage and crutches or sometimes a knee immobilizer

  • If the kneecap remains unstable, surgery

Doctors move the dislocated kneecap gently back in place (called reduction). Usually, no sedative or analgesic is necessary. Then the knee is checked for stability by moving it through its range of motion (flexing and extending the knee). If the knee is stable, it is wrapped in an elastic bandage, and people are given crutches to use. If the knee is unstable, a knee immobilizer is applied. This device supports the knee and keeps it from bending. The immobilizer is worn for several days. After it is removed, physical therapy is needed to restore muscle strength and to improve the knee's range of motion.

The joint may need to be repaired surgically by an orthopedic surgeon if any of the following occur:

  • A bone is fractured.

  • Cartilage in the joint is injured.

  • The knee continues to be unstable after initial treatment.

Surgery may be done using a small viewing tube inserted in the joint through a tiny incision (arthroscopic surgery Surgery Through a Keyhole Surgery Through a Keyhole ), or open surgery (requiring a larger incision) may be done.

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