Your kneecap (patella) sits over the front of your knee joint. As you bend or straighten your knee, the underside of your kneecap glides over a groove in the bones that make up your knee joint.
A kneecap that slides out of the groove partway is called a subluxation.
A kneecap that moves fully outside the groove is called a dislocation.
More about Your Injury
A kneecap can be knocked out of the groove when the knee is hit from the side.
A kneecap can also slide out of the groove during normal movement or when there is twisting motion or a sudden turn.
Kneecap subluxation or dislocation may occur more than once. The first few times it happens will be painful, and you will be unable to walk.
If subluxations continue to occur and are not treated, you may feel less pain when they happen, but these episodes can further damage your knee joint.
What to Expect
You may have had a knee x-ray or an MRI to make sure your kneecap bone did not break and there was no damage to the cartilage or tendons (other tissues in your knee joint).
If tests show that you do not have damage:
Your knee may be placed in a brace, splint, or cast for several weeks.
You may need to use crutches at first so that you do not put too much weight on your knee.
You will need to follow up with your primary care provider or a bone doctor (orthopedist).
Most people recover fully within 6 - 8 weeks.
If your kneecap is damaged or unstable, you may need surgery to repair or stabilize it. Your health care provider will usually refer you to an orthopaedic surgeon.
Sit with your knee raised at least 4 times a day. This will help reduce swelling.
Ice your knee. Make an ice pack by putting ice cubes in a plastic bag and wrapping a cloth around it.
For the first day of injury, apply the ice pack every hour for 10 - 15 minutes.
After the first day, ice the area every 3 - 4 hours for 2 or 3 days or until the pain goes away.
Pain medicines such as acetaminophen, ibuprofen (Advil, Motrin, and others), or naproxen (Aleve, Naprosyn, and others) may help ease pain and swelling.
Be sure to take these only as directed. Carefully read the warnings on the label before you take them.
Talk with your health care provider before using these medicines if you have heart disease, high blood pressure, kidney disease, liver disease, or have had stomach ulcers or internal bleeding in the past.
You will need to change your activity while you are wearing a splint or brace. Your health care provider will advise you about:
How much weight you can place on your knee
When you can remove the splint or brace
Bicycling instead of running while you heal, if your usual activity is running
Many exercises can help stretch and strengthen the muscles around your knee, thigh, and hip. Your health care provider may show these to you or may have you work with a physical therapist to learn them.
Before returning to sports or strenuous activity, your injured leg should be a strong as your uninjured leg. You should also be able to:
Run and jump on your injured leg without pain
Fully straighten and bend your injured knee without pain
Jog and sprint straight ahead without limping or feeling pain
Be able to do 45- and 90-degree cuts when running
When to Call the Doctor
Call your health care provider if:
Your knee feels unstable
Pain or swelling returns after having gone away
Your injury does not seem to be getting better with time
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:chap22;sect C.
Hing CB, Smith TO, Donell S, Song F. Surgical versus non-surgical interventions for treating patellar dislocation. Cochrane Database Syst Rev. 2011;11:CD008106.
C. Benjamin Ma, MD, Assistant Professor, Chief, Sports Medicine and Shoulder Service, UCSF Department of Orthopaedic Surgery. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M. Health Solutions, Ebix, Inc.