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 Orthopaedics & 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.

John Fulkerson, MD, a patella instability specialist at Yale and president of the Patellofemoral Foundation, agrees and says that 3D imaging and printing has added greatly to our ability to understand these complex problems at Yale.

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 stress from being overweight in some cases. 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. 

Some patients have abnormalities of the knee cap groove (trochlea) that can predispose to instability and make recovery more difficult. Yale Orthopaedics has the most comprehensive program in the country currently for understanding the specific structural problem of each patient. It uses advanced 3D imaging technology, innovative computer programs, and expertise at the Yale School of Engineering in some cases.

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.

Based on their description of symptoms and the clinical exam, patients with chronic kneecap instability often have a straightforward diagnosis. 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.”  

Selective advanced imaging including magnetic resonance imaging (MRI), weight bearing 3D computerized tomography, and gait lab analysis may be necessary to optimize planning in some patients.

The first line of treatment is usually nonsurgical. Rest and nonsteroidal anti-inflammatories are encouraged to reduce the swelling and allow for the natural healing of the ligament to occur. Physical therapy is performed to get back range of motion and strength. Most patients can then return to prior levels of activity.  

“With 3D imaging, we are improving our understanding of this dynamic and complex anatomy and aim to better predict now who will get better without surgery,” Dr. Fulkerson says. “If surgery is needed, this added information of 3D imaging will hopefully lead to more successful surgery and better long-term outcomes.”

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. Some patients need to have a new ligament added to stabilize the patella and some may need to have the patella tendon moved to correct excessive lateral tracking.

The surgery can almost always be done on an outpatient basis. Patients are typically on crutches for six to 10 weeks. Physical therapy is necessary to regain range of motion and strength, with full recovery taking three to six months. 

Drs. Medvecky and Fulkerson agree that recovery is typically very successful, particularly now with advanced understanding of relevant anatomy and safer surgical techniques. Dr. Fulkerson has developed an improved, anatomic method for patella stabilization called medial quadriceps tendon-femoral ligament (MQTFL) reconstruction that is less traumatic, anatomically precise, and safer (no drill holes in the patella). 

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.” 

Yale Orthopaedics is at the forefront nationally and internationally in the precise diagnosis and treatment of knee instability currently, adds Dr. Fulkerson. 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 interest to have it done by a specialist with extensive experience,” Dr. Medvecky says.