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Patellar Instability: Causes, Symptoms and Treatment

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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.

Patellar Instability

Patellar instability can lead to a dislocated kneecap. You may have:

  • Complete dislocation: The ligaments that hold the kneecap in place slide to the outside of the knee, taking the kneecap with them. The ligaments may tear or stretch. The kneecap is entirely out of place.
  • Partial dislocation (subluxation): The kneecap slips partially out of the groove.

How common is patellar instability?

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.

What causes 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:

  • Shallow or uneven trochlear groove.
  • Loose ligaments or extremely flexible joints.
  • Sharp blow to the kneecap during a fall, sports injury or other accident.

What are the risk factors for patellar instability?

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:

  • Cerebral palsy.
  • Down syndrome.
  • Ehlers-Danlos syndrome.

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 the knee or walk.

Other signs of patellar instability include:

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

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 perform a physical exam to assess symptoms, including your range of motion.

You may also get:

  • X-rays to see if the kneecap is out of place and check for broken bones.
  • MRI to check for injuries like anterior cruciate ligament (ACL) tears, meniscal tears and loose bone fragments.

What are nonsurgical treatments for patellar instability?

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:

  • Knee brace to immobilize the knee and keep the kneecap in place. You may need to wear a brace for several weeks and use crutches during this time.
  • Nonsteroidal anti-inflammatory drugs (NSAIDs), rest, elevation and ice packs to ease pain and swelling.
  • Physical therapy to strengthen muscles that hold the kneecap in place and improve range of motion.

What are surgical treatments for patellar instability?

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:

  • Medial patellofemoral ligament (MPFL) repair to strengthen and repair the ligaments that hold the 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 the shinbone, thighbone, kneecap and connective tissues. This open surgical procedure requires a larger incision and longer recovery than arthroscopic surgery.
  • Knee replacement to treat severe arthritis or recurrent dislocations.

How can I prevent patellar instability?

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.

What is the outlook for people with patellar instability?

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.

When should I call the doctor?

You should call your healthcare provider if you experience:

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

What questions should I ask my doctor?

You may want to ask your healthcare provider:

  • What caused patellar instability?
  • Do I have a partial or complete knee dislocation?
  • What treatments can help?
  • Can physical therapy or other exercises help?
  • Would I benefit from surgery?
  • How can I prevent future knee problems or dislocations?
  • Should I look out for signs of complications?
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