Patellar Instability

What is patellar instability?

Patellar instability (also known as knee instability or patellofemoral instability) means your kneecap (patella) is unstable when your knee moves. It occurs when your patella comes out of the groove at the end of your thighbone (femur) that holds it in place.

Normally, when you bend and straighten your knee, your kneecap moves up and down in a V-shaped notch called the trochlear groove. With patellar instability, your kneecap doesn’t track in the groove the way it should.

You use your knees constantly throughout the day. They help you stand, sit, walk, run and move. This makes them more prone to problems like patellar instability.

An unstable kneecap can lead to a dislocated kneecap. You may have a complete dislocation or a partial dislocation (subluxation).

What are the symptoms of patellar instability?

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 your knee or walk.

Other signs of patellar instability include:

• Knee pain, stiffness and swelling
• Cracking or popping sounds in your knee when you climb stairs or bend your knee
• Feeling like your kneecap is catching on tissue or moving from side to side

What causes patellar instability?

Causes of patellar instability include:

• Shallow or uneven trochlear groove
• Loose knee ligaments or extremely flexible joints
• Sharp blow to your kneecap during a fall, sports injury or another accident

What are the risk factors for patellar instability?

Anyone can develop patellar instability. People assigned female at birth tend to have looser ligaments that make them more prone to patellar instability.

You may have a higher risk if you play high-impact sports, like football, or participate in activities that require a lot of quick pivoting, like basketball, cheerleading or soccer.

Certain health conditions can cause loose connective tissue that contributes to patellar instability. These include:

• Cerebral palsy
• Down syndrome
• Ehlers-Danlos syndrome

What are the complications of patellar instability?

Without treatment, recurrent (recurring) patellar instability can lead to:

• Kneecap dislocation
• Limited functioning of your knee
• Chronic pain
• Runner’s knee
• Severe knee arthritis

How is patellar instability diagnosed?

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 ask how the injury occurred or what you were doing when your kneecap moved out of the groove. They’ll also perform a physical exam to assess your symptoms. They’ll look at the appearance of your knee, check for tenderness and evaluate your range of motion. They may ask you to walk or straighten and bend your knee.

What tests will be done to diagnose patellar instability?

To diagnose the condition, your healthcare provider may request imaging tests, including:

• Knee X-rays to see if your kneecap is out of place and check for broken bones
• MRI to check for injuries like anterior cruciate ligament (ACL) tears, meniscal tears or loose bone fragments
• CT scan to check the alignment of your knee joint

What are nonsurgical treatments for patellar instability?

If your kneecap doesn’t move back to the trochlear groove on its own, seek medical attention. A healthcare provider will gently push your kneecap back into place. This process (closed reduction) can be uncomfortable. You may receive pain medicine, but you shouldn’t need anesthesia.

Your provider may recommend:

• A knee brace to immobilize your knee and keep your kneecap in place — you may need to wear a brace for several weeks and use crutches during this time
• Nonsteroidal anti-inflammatory drugs (NSAIDs) to ease pain and swelling
• Physical therapy to strengthen muscles that hold your kneecap in place and improve your range of motion

What are surgical treatments for patellar instability?

If you have chronic patellar instability, your healthcare provider may recommend surgery. Most knee surgeries take place arthroscopically. The procedure uses small cuts (incisions) and a tiny camera (arthroscope). Recovery can take six to 12 months.

If an injury caused patellar instability, you may have loose cartilage or bone fragments in your knee. Your provider will surgically remove them.

Surgical options include:

• Medial patellofemoral ligament (MPFL) repair to strengthen and repair the ligaments that hold your kneecap in place
• MPFL reconstruction to replace a damaged ligament with a hamstring tendon taken from a donor or elsewhere in your body
• Knee osteotomy (tibial tubercle transfer) to realign your shinbone, thighbone, kneecap and connective tissues — this open surgical procedure requires a larger incision and longer recovery than arthroscopic surgery

How can I prevent patellar instability?

An unstable kneecap can damage the connective tissue around your knee. That damage increases your risk for further injury.

Physical therapy exercises can strengthen the muscles and connective tissue that keep your kneecap in the femoral groove. Cycling is also a good knee strengthener. Your provider may recommend wearing a knee brace during certain activities.

What is the outlook for people with patellar instability?

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.

When should I call my healthcare provider?

You should call your provider if you experience:

• An inability to bend or straighten your knee, put weight on your leg or walk
• Pain or limited range of motion in your knee that interferes with daily life
• Unusual redness or swelling in your knee joint

What questions should I ask my healthcare provider?

You may want to ask your provider:

• What caused patellar instability?
• What treatments can help?
• Can physical therapy or other exercises help?
• Would I benefit from surgery?

Why can I move my kneecap up and down?

When you straighten or bend your leg, you normally pull your patella up and down through the trochlear groove. If you have patellar instability, it means the tendons and ligaments that hold your kneecap in place are already loose and unstable. A previous injury or another preexisting anatomical condition might cause this. An unstable kneecap will dislocate more easily.