Kneecap dislocation occurs when the triangle-shaped bone covering the knee (patella) moves or slides out of place. The problem usually occurs toward the outside of the leg.

Dislocation - kneecap; Patellar dislocation or instability

Kneecap (patella) dislocation is often seen in women. It usually occurs after a sudden change in direction when your leg is planted. This puts your kneecap under stress.

Dislocation may also occur as a direct result of injury. When the kneecap is dislocated, it can slip sideways and around to the outside of the knee.

• Knee appears to be deformed
• Knee is bent and cannot straighten
• Kneecap (patella) dislocates to the outside of the knee
• Knee pain and tenderness
• Knee swelling
• "Sloppy" kneecap -- you can move the kneecap too much from right to left (hypermobile patella)

The first few times this occurs, you will feel pain and be unable to walk. However, if dislocations continue to occur and are untreated, you may feel less pain and have less immediate disability. This is not a reason to avoid treatment. Kneecap dislocation damages your knee joint.

If you can, straighten out the knee. If it is stuck and painful to move, stabilize (splint) the knee and get medical attention.

Your health care provider will examine the knee, which could confirm that the kneecap is dislocated.

A knee x-ray and, sometimes, MRIs should be done to make sure that the dislocation did not cause a bone to break or cartilage to be damaged. If tests show that you have no such damage, your knee will be placed into an immobilizer or cast to prevent you from moving it for several weeks (usually about 3 weeks).

After this time, physical therapy can help build back your muscle strength and improve the knee's range of motion.

If the knee remains unstable, you may need surgery to stabilize the kneecap. This may be done using arthroscopic or open surgery.

Call your health care provider if you injure your knee and have symptoms of dislocation.

Call your health care provider if you are being treated for a dislocated knee and you notice:

• Increased instability in your knee
• Pain or swelling return after they went away
• Your injury does not appear to be getting better with time

Also call if you re-injure your knee.

Use proper techniques when exercising or playing sports. Keep your knee strong and flexible.

Some cases of knee dislocation may not be preventable, especially if physical factors make you more likely to dislocate your knee.

De Carlo M, Armstrong B. Rehabilitation of the knee following sports injury. Clin Sports Med. 2010;29:81-106.

Steiner T, Parker RD. Patella: subluxation and dislocation. 2. Patellofemoral instability: recurrent dislocation of the patella. In: DeLee JC, Drez D Jr., Miller MD, eds. DeLee and Drez's Orthopaedic Sports Medicine. 3rd ed. Philadelphia, Pa: Saunders Elsevier;2009:chap 22;sect C.