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It can occur because of a fall while playing sports or, sometimes, for no particular reason at all. The knee buckles and feels unstable. There may be an accompanying popping sound. If this happens to you, it's possible that your patella (or kneecap) has slipped out of place. Even if slips back and all seems well it's important to see a doctor. 

Treating kneecap  instability can be complex. Yale Medicine Orthopedics & Rehabilitation surgeons work with the most difficult cases. They draw upon their experience to provide patients with the best outcomes possible, especially when surgery is involved. “That’s what we offer at Yale Medicine.” says knee specialist Michael Medvecky, MD, chief of Yale Medicine Sports Medicine.

What causes kneecap instability?

According to Dr. Medvecky, when the leg bends, the kneecap should slide smoothly into an indentation (like a valley) in the femur (leg bone). There are several potential reasons why this doesn't always work as it should, and why the problem becomes recurrent. 

“Some people have mechanical factors that predispose them to having a loose kneecap,” Dr. Medvecky says, “while others have instability that is caused by a traumatic injury, such as in sports, that distorts the knee and makes it unstable.”

Patellar (or kneecap) instability most commonly occurs in people in their teens and 20s. Because of their wider hips, women are more likely to experience knee instability due to misalignment. In a person with a tendency toward misalignment, the problem can develop from overuse and/or the stress the knees experience when a person is overweight or obese. A kneecap that has popped out of place at least once is more likely to do so again than it is in a person who has never had a problem.

What does kneecap instability feel like?

People with chronic instability may or may not have soreness or pain as they climb up or down stairs. Sometimes there is no real discomfort, other than the unease and anxiety that come with worrying about a knee that may give out at any time. When a traumatic injury causes a dislocation, it’s typically painful and accompanied by swelling.

How is kneecap instability diagnosed?

Based on their description of symptoms and the clinical exam, patients with chronic kneecap  instability often have a straightforward diagnosis, says Dr. Medvecky. But when a traumatic injury distorts the knee in a way that leads to dislocation or instability, diagnosis can be trickier.

“It can easily be confused with a ligament tear because the two injuries occur and present almost identically,” Dr. Medvecky says, adding that such an injury is more typically caused by a sudden twist (as in tennis or playing ultimate Frisbee) than a collision. “The dislocation causes stretching or tearing of the inner side ligament, which is what then predisposes the patella to slip to one side,” Dr. Medvecky says. Imaging, including X-ray or magnetic resonance imaging (MRI), may be necessary to make the diagnosis.

How is kneecap instability treated?

The first line of treatment is usually nonsurgical. Rest and nonsteroidal anti-inflammatories, taken orally, are prescribed to heal the ligament, reduce the swelling and allow for the natural healing of the ligament to occur. Physical therapy may follow to get back range of motion and strength. “Most patients can then return to prior level of activity,” says Dr. Medvecky.

When do people need surgery for kneecap instability?

If the kneecap keeps shifting, either fully or partially, surgery may be necessary to allow people to get back to the activities they enjoy. “That’s when we talk about changing the anatomy of the knee to improve stability, so the kneecap maintains its alignment,” says Dr. Medvecky.

Surgery can involve the ligament, the bone or both. “Based on the structure of the patient’s knee, we may need to tighten the inner ligament and reconstruct it,” Dr. Medvecky says, “or the bone may need to be cut and slid toward the inner part of the body to improve the vertical mechanics of the knee.”

The surgery may be done on an outpatient basis, or some patients may need to stay in the hospital overnight. Patients are typically on crutches for eight to 10 weeks. Physical therapy helps them to regain range of motion and strength, with full recovery taking three to six months. “The success rate for this surgery is very high,” Dr. Medvecky says.

What makes Yale Medicine’s approach to the treatment of kneecap instability unique?

Yale Medicine Orthopedics & Rehabilitation offers extensive experience in the treatment of knee instability, says Dr. Medvecky. “The anatomy of this part of the knee is relatively under-appreciated, especially because the injury occurs somewhat infrequently,” he says. “Knee instability can be confused with other conditions of the knee, particularly if it’s not a visible, full dislocation.”

Because Yale New Haven Hospital is a tertiary center, complex cases are often referred there. The corrective surgery is complex, so a surgeon’s expertise is invaluable to maximize success and reduce the risk of complications. “So if a patient needs surgery, it can be in a patient’s best interests to have it done by a specialist with extensive experience,” Dr. Medvecky says.