Dislocated kneecap (patellar dislocation)

The kneecap can pop out of its groove as a result of an unfortunate twisting motion or a bump. Sometimes it pops back again by itself. But even then it is important to have a doctor examine the knee. This is because it is possible for the surrounding bones and ligaments to be injured.

At a glance

  • A kneecap is usually dislocated during sports (patella(r) dislocation).
  • Patellar dislocation is most common in adolescents and young adults.
  • Sometimes a dislocated kneecap slides back again by itself.
  • Whatever happens, the knee needs to be examined by a doctor.
  • Surgery is only needed in some cases or if there are additional injuries.
  • The affected knee must be rested for a few weeks to several months depending on how serious the injury is and how it is treated.

Note: The information in this article cannot and should not replace a medical consultation and must not be used for self-diagnosis or treatment.

Dislocation of the knee: young woman lying on a treatment table. A physiotherapist is palpating her bent knee with both hands.

What is patellar dislocation?

Medical practitioners talk of patellar dislocation when the kneecap is dislocated.

When the knee bends or extends, the kneecap (patella) slides through a groove in the thighbone. An unfortunate twisting motion or a bump from the side can knock it out of its groove (dislocation). This often happens during sports, especially in adolescents and young adults.

A dislocated kneecap often pops back into its normal position by itself. Nevertheless, the knee should still be examined by a doctor even then, as it is possible for bones, cartilage and ligaments to be injured as a result of the dislocation.

It may be that the kneecap pops out again at a later date because it is more unstable after being dislocated.

What are the signs of patellar dislocation?

Patellar dislocation usually causes a lot of pain. It is usually very obvious that the kneecap has slipped out of position. Sometimes a pop is heard as the knee dislocates and it feels as though the knee is out of place.

It can also swell up because the joint capsule and the ligament that stabilizes the kneecap on the inside – also called medial patellofemoral ligament (MPFL) – tear. This happens when the kneecap pops out to the outside of the knee, which is almost always the case. There is often no swelling when the kneecap is repeatedly dislocated.

What causes patellar dislocation?

The kneecap can pop out when the knee joint is turned inwards in a slightly bent position, while the foot lies flat on the ground. This often happens during sports and is called acute or traumatic dislocation. Typical types of sport where this injury is particularly likely to happen include football, handball, dancing and gymnastics.

Sports where dislocation of the knee is particularly likely to happen include football, handball, dancing and acrobatics.

At 80 percent, acute patellar dislocation is the most common form. Patellar subluxation can also occur, but this is less common. With subluxation, the kneecap moves sideways in the groove back and forth, without popping out completely. This can be as a result of a previous knee injury or ligaments that are too loose, for example.

What are the risk factors for patellar dislocation?

There are a few factors that increase the risk of the kneecap dislocating. These include:

  • malformations of the legs such as knock knees
  • a deformed groove or other anatomical abnormalities
  • a higher kneecap
  • a weak inner thigh muscle
  • hypermobile joints or weak ligaments 

How common are patellar dislocations?

Dislocation of the knee mainly occurs in young people between 10 and 20 years of age. Girls and young women are more commonly affected because they usually have more flexible ligaments and weaker muscles than boys.

Dislocation of the knee mainly occurs in young people between 10 and 20 years of age. Incidence is higher in girls and young women.

How does patellar dislocation go on to develop?

How long it takes for the knee to recover completely depends on how seriously it has been injured and how it is treated.

If there has been no major damage to the knee, it takes about 6 weeks for the person to be able to carry out their everyday activities again with ease.

Playing sport is normally only possible again after about 3 to 4 months. This can take much longer if surgery is needed because of serious injury – sometimes over a year.

But patellar dislocation does not mean that the person has to give up their sport forever. Studies have shown that about half of those affected can continue to practice their sport again as before after they have had a break. But before the person affected starts again, it is a good idea for them to talk to a physiotherapist or a doctor.

The kneecap is more unstable after it has been dislocated than it was before. Sometimes it feels a bit wobbly. The person may also experience pain in the front of the knee, particularly with greater stress on the knee.

In 15 to 45 percent of people, the kneecap pops out again after it is dislocated the first time. This can happen after a few weeks or only after months or even years.

There has not been enough research into the long-term effects of kneecap dislocation and so it is difficult to gauge this. But there is evidence that patellar dislocation can increase the risk of arthritis of the knee.

How is patellar dislocation diagnosed?

A dislocated kneecap is usually apparent from the outside. The examination involves the doctor looking at the knee and feeling it. Once it has been manipulated back into place, an x-ray is taken to see whether bones have been injured.

The doctor can determine whether there are any injuries to the ligaments and menisci with various physical examinations and using magnetic resonance imaging (MRI). The MRI images can also identify whether there are any anatomical malformations and cartilage damage.

How is patellar dislocation treated?

Sometimes the kneecap pops back into its normal position by itself. If this doesn’t happen, the person should call for an ambulance as soon as possible (emergency number 112). If the kneecap does pop back, the person can simply be driven to the emergency department.

First aid measures immediately after the accident are:

  • keep calm
  • sit down or lie down to take the weight off the leg
  • keep the knee still so as not to further injure it
  • cool the knee to reduce swelling and ease the pain a little

In the hospital, the person affected first of all receives fast-acting pain relief. Then the kneecap is manipulated back into place. Often slowly extending the leg is all that’s needed. Sometimes the doctor has to help by performing a specific maneuver. Once the kneecap has been manipulated back into place, the person wears a special bandage for a few weeks to stabilize the kneecap. In the beginning, crutches can also be helpful to take the weight off the injured knee.

An operation can be considered if the kneecap has popped out yet again. Surgery may be necessary even if further dislocations are likely or there has been major damage to the cartilage and bone.

How does rehabilitation work with patellar dislocation?

After being treated in hospital, the knee should be immobilized for a few days. This is then followed by rehabilitation with physiotherapy. The first thing is to get the knee moving again. Swelling can be treated by lymph drainage.

Then the leg muscles are strengthened through targeted training. This gives the kneecap more stability. Strengthening the inner thigh muscle is especially important. This is connected to the ligament that stabilizes the kneecap. But well-trained hip, pelvis, ankle and torso muscles also stabilize the knee. It is a good idea to do special stretching exercises if some muscles are shortened and could lead to one-sided strain.

Further information about physiotherapy for patellar dislocation can be found at gesundheitsinformation.de.

In cooperation with the Institute for Quality and Efficiency in Health Care (Institut für Qualität und Wirtschaftlichkeit im Gesundheitswesen) (IQWiG). As at:

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