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Kneecap dislocation

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

Causes

Kneecap (patella) often occurs after a sudden change in direction when your leg is planted. This puts your kneecap under stress. This can occur when playing certain sports, such as basketball.

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

Symptoms

Symptoms of kneecap dislocation include:

  • Knee appears to be deformed
  • Knee is bent and cannot be straightened out
  • 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. If you continue to have dislocations, your knee may not hurt as much and you may not be as disabled. This is not a reason to avoid treatment. Kneecap dislocation damages your knee joint. It can lead to cartilage injuries and increase the risk of developing osteoarthritis at a younger age.

First Aid

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

Your health care provider will examine your knee. This may confirm that the kneecap is dislocated.

Your provider may order a knee x-ray or an MRI. These tests can show if the dislocation caused a broken bone or cartilage damage. If tests show that you have no damage, your knee will be placed into an immobilizer or cast to prevent you from moving it. You will need to wear this for about 3 weeks.

Once you are no longer in a cast, physical therapy can help build back your muscle strength and improve the knee's range of motion.

If there is damage to the bone and cartilage, or if the kneecap continues to be unstable, you may need surgery to stabilize the kneecap. This may be done using arthroscopic or open surgery.

When to Contact a Medical Professional

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

Call your 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 your provider if you re-injure your knee.

Prevention

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

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

Alternative Names

Dislocation - kneecap; Patellar dislocation or instability

References

Mascioli AA. Acute dislocations. In: Azar F, Beaty JH, Canale ST, eds. Campbell's Operative Orthopaedics. 13th ed. Philadelphia, PA: Elsevier; 2017:chap 60.

Naples RM, Ufberg JW. Management of common dislocations. In: Roberts JR, Custalow CB, Thomsen TW, eds. Roberts and Hedges' Clinical Procedures in Emergency Medicine and Acute Care. 7th ed. Philadelphia, PA: Elsevier; 2019:chap 49.  

Sherman SL, Hinckel BB, Farr J. Patellar instability. In: Miller MD, Thompson SR, eds. DeLee and Drez's Orthopaedic Sports Medicine. 5th ed. Philadelphia, PA: Elsevier; 2020:chap 105.

Review Date 11/5/2018

Updated by: C. Benjamin Ma, MD, Professor, Chief, Sports Medicine and Shoulder Service, UCSF Department of Orthopaedic Surgery, San Francisco, CA. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.