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, but surgery is sometimes needed later.
Kneecap dislocations differ from knee dislocations (which are much more serious—see Knee Dislocations).
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.
The 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.
Sometimes, if the kneecap cartilage was fractured, osteoarthritis develops eventually. Also, the kneecap may become dislocated again, particularly in people who have an abnormal joint.
The kneecap may slide back into place on it own before people can see a doctor. However, people should still see a doctor because such an injury damages the knee joint.
Doctors can diagnose a dislocated kneecap when they examine the knee. If the kneecap has already slid back in place, they may be able to tell that it was dislocated because they detect blood within the joint and the joint is tender.
Doctors may take x-rays from several angles to check for fractures.
Doctors move the kneecap gently back in place (called reduction). Then a knee immobilizer (a device that supports the knee and keeps it from bending) or a hinged brace is applied. It is worn for several weeks. After the immobilizer or brace is removed, physical therapy is needed to restore muscle strength and to improve the knee's range of motion.
If the knee remains unstable after such treatment or if a bone is fractured, the joint may be repaired surgically to prevent future dislocations. Surgery may be done using a small viewing tube inserted in the joint through a tiny incision (arthroscopic surgery—see Surgery Through a Keyhole), or open surgery (requiring a larger incision) may be done.