Whenever you bend or stretch your leg, the kneecap (patella) moves along a groove on your thigh bone (femur). An awkward twist of the joint or a bump from the side can knock it out of this groove. The kneecap is then “dislocated.” The medical term for this is patellar dislocation. It happens most often during sports, and is most common in teenagers and young adults.

A dislocated kneecap often moves back into the correct position on its own. But even then it’s important to have a doctor look at the knee because there could be damage to the bones, cartilage or ligaments. The kneecap is less stable after a dislocation, and it could become dislocated again.

The kneecap may become dislocated if the knee joint is slightly bent and turned inward while the foot is flat on the ground.

Dislocated kneecaps are often caused by a sports-related accident – typically while dancing, doing gymnastics, or playing handball or soccer. That type of dislocation is also referred to as an acute or traumatic dislocation. It is the most common type, making up 80% of dislocated kneecaps.

Partial kneecap dislocation (patellar subluxation) is less common: This happens when the kneecap moves from side to side in the groove, without becoming fully detached. This can result from a previous knee injury or ligaments that have become too loose, for instance.

Kneecap dislocation is most common in young people between the ages of 10 and 20. Girls and young women are at greater risk because they typically have more flexible ligaments and less muscle tissue than boys do.

The risk of kneecap dislocation is also higher in people who have

• Joints that stretch farther than normal (hypermobility) or weak ligaments
• A kneecap that is higher than normal (patella alta)
• A deformed trochlear groove (trochlear dysplasia) or other anatomical abnormalities
• Deformities of the leg bones (such as bow-leggedness)
• Weak inner thigh muscles

These factors also increase the risk of the kneecap becoming dislocated again.

A dislocated kneecap is extremely painful. When it happens, the kneecap almost always comes out on the outer (lateral) side of the knee. Usually it causes a tear in the joint capsule and the ligament that stabilizes the kneecap on the inner (medial) side of the knee, known as the medial patellofemoral ligament or MPFL. These injuries make the knee swell up.

When a kneecap is dislocated, it might make a popping sound. The dislocated kneecap can usually be seen clearly through the skin, which can be frightening.

If you have already dislocated your knee in the past, it might hurt less and not swell.

A kneecap that has been dislocated is less stable afterwards. After a dislocation, it may feel like the kneecap no longer has enough support. The front of the knee may hurt, especially when more stress is put on the joint.

In 15 to 45% of people who have a kneecap dislocation, it happens again afterwards. Sometimes this happens after just a few weeks, but it can also happen several months or years later.

Athletes who have had a dislocated kneecap don't necessarily have to give up their sport, though. In studies, about half of the participants continued to do their sport after finishing treatment.

There's not much good research on the longer-term effects of kneecap dislocation so less is known about them. But it can increase the risk of knee osteoarthritis. That is particularly true if the kneecap has been dislocated several times, which usually damages the cartilage more.

A dislocated kneecap is typically easy to diagnose. A doctor looks at the knee and feels it.

After the kneecap is pushed back into place, an X-ray is taken to see whether the bones have been damaged or chipped.

Various physical examinations and magnetic resonance imaging (MRI) can be used to find out whether the ligaments or meniscuses have been injured. The MRI image will also show if there are any anatomical deformities or cartilage damage.

A kneecap dislocation is typically quite a frightening experience. If the kneecap doesn’t move back into its normal position on its own, you should call an ambulance quickly (in Germany and most European countries: 112, in the U.S.: 911). If the kneecap returns to its normal position, being driven to the emergency room is enough. First aid for a dislocated kneecap includes:

• Remaining as calm as possible.
• Sitting or lying down in order to take the weight off the leg.
• Keeping the knee joint as still as possible to prevent further injury.
• Cooling the knee to reduce swelling and relieve the pain a little (to avoid harming the skin, wrap cool packs or bags of ice in a towel before using them).

At the hospital, a doctor will give you a fast-acting painkiller and then push the kneecap back into its normal position. Often it already moves back to its normal position by slowly stretching the leg. Sometimes a special manual technique is used to help:

• This involves lying on your back and relaxing the muscles in your leg so that all of the muscles, tendons and ligaments are as loose as possible.
• Then the doctor places their hands around the kneecap, putting their thumbs right on the kneecap. An assistant holds your leg by the ankle.
• While the doctor pushes your kneecap back into its correct position, the assistant pulls your leg to extend it.

Once everything is back in place, you wear a special brace or bandage to stabilize the kneecap. It may also help to use crutches at first in order to put less weight on the injured knee.

The following is recommended for the first few days after a kneecap dislocation:

• Rest the knee (stand and walk as little as possible, and avoid bending or extending the joint).
• Put up the leg regularly and cool it several times a day for 15 to 20 minutes.
• Take anti-inflammatory painkillers like ibuprofen if you need to.

Surgery is usually considered if this is already the second time your kneecap has been dislocated. It may also be considered if the kneecap is likely to be dislocated again or if there are major cartilage or bone injuries.

Rehabilitation starts after the kneecap has been moved back into place or following surgery. Physical therapy is a good idea after the knee has been immobilized for several days. The first goal is to start moving the knee joint more again. Swelling is treated with lymphatic drainage massages.

After that, it’s important to do exercises to strengthen the muscles in the leg so that they can support the kneecap better. It's especially important to strengthen the inner thigh muscle. This muscle is connected to the ligament that stabilizes the kneecap.

Strong muscles in the hip, pelvis, ankle and torso also help to stabilize the knee. If certain muscles are shortened, which can put unequal strain on the joint, special stretching exercises can help.

The amount of time it takes to recover from a dislocated kneecap depends on how severely the knee was injured and how it is treated.

If there was no major damage, it takes about six weeks before you can return to your normal daily activities. It’s usually possible to start doing sports again after 3 to 4 months.

If there is major damage and the knee needs to be operated on, it can take much longer to resume sports activities – sometimes over one year.

When and how intensively you can start doing sports again depends on your individual circumstances and the type of sport. Various medical tests can help you to see how the healing process is going.

The knee isn’t always as strong as it was before the treatment. It’s important to do the rehabilitation exercises on your own and regularly, and not to put too much strain on the knee too soon. That would increase the risk of dislocating the kneecap again during sports. Before you resume your sports activities or increase the strain on the knee, it's a good idea to talk with your physical therapist and doctor.