Patellar instability means the patella (kneecap) slips out of the femoral groove in the thighbone. An unstable kneecap can lead to a dislocated knee. Physical therapy and leg braces can help. Some people have chronic (ongoing) patellar instability. This condition increases the risk of dislocated knees, ACL tears and arthritis in the knee.
Patellar instability means an unstable kneecap. It occurs when the patella (kneecap) moves out of the groove at the end of the thighbone (femur) that holds it in place.
When you bend and straighten your knee, the kneecap moves up and down in a V-shaped notch called the trochlear groove. With patellar instability, the kneecap doesn’t track in the groove the way it should.
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Patellar instability can lead to a dislocated kneecap. You may have:
You use your knees constantly throughout the day. This makes them more prone to problems like patellar instability. Experts estimate that 50 to 77 out of 100,000 Americans have some degree of patellar instability.
The kneecap is part of the skeletal system. Connective tissues (muscles, tendons and ligaments) in the front of the thighbone (femur) go over the kneecap and connect to the shinbone (tibia). These muscles pull the kneecap up through the trochlear groove when you straighten your leg and down the groove when you bend it. When the kneecap is unstable, it moves outside of this groove.
Causes of patellar instability include:
Anyone can develop patellar instability. Females tend to have looser ligaments that make them more prone to patellar instability.
You may have higher risk if you play high-impact sports like football or do activities that require a lot of quick pivoting, like basketball, cheer or soccer.
Certain health conditions can cause loose connective tissue that contributes to patellar instability. These include:
When your kneecap slips out of the trochlear groove, your knee may buckle. Your knee and leg may not be able to support your weight or keep you standing upright. You may not be able to straighten the knee or walk.
Other signs of patellar instability include:
Even if your kneecap moves back into place on its own, you should see your healthcare provider for a diagnosis and potential treatment. Your provider will perform a physical exam to assess symptoms, including your range of motion.
You may also get:
If the kneecap doesn’t move back to the trochlear groove on its own, seek medical attention. A healthcare provider will gently push the kneecap back into place. This process (reduction) can be uncomfortable. You may receive pain medicine, but you shouldn’t need anesthesia.
For partial knee dislocations and patellar instability, your provider may recommend:
If you have chronic patellar instability or a complete kneecap dislocation, your healthcare provider may recommend surgery. Most knee surgeries take place arthroscopically. The procedure uses small incisions and a tiny camera (arthroscope). Recovery can take six to 12 months.
If an injury causes patellar instability, you may have loose cartilage or bone fragments in the knee. Your provider will surgically remove them.
Surgical options include:
An unstable kneecap can damage connective tissue. That damage increases your risk for repeat kneecap dislocations.
Physical therapy exercises can strengthen muscles and connective tissue that keep the kneecap in the femoral groove. Cycling on an exercise bike or outside on an actual bike is also a good knee strengthener. Your provider may also recommend wearing a knee brace during certain activities.
Nearly half of people who complete nonsurgical treatments for patellar instability will dislocate the knee again. This means the kneecap is still unstable.
Chronic patellar instability increases your risk of developing arthritis and ligament damage. Your healthcare provider may recommend surgery.
You should call your healthcare provider if you experience:
You may want to ask your healthcare provider:
A note from Cleveland Clinic
Patellar instability increases your risk of dislocating a knee. You may tear ligaments and develop arthritis. Most people feel better without surgery, by wearing a brace and doing physical therapy exercises. If the kneecap remains unstable (chronic patellar instability), or you completely dislocate the kneecap, you may need surgery.
Last reviewed by a Cleveland Clinic medical professional on 05/21/2021.
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