Dislocated kneecap (patella luxation)

  • In a patella dislocation, the kneecap pops out of its track.

Introduction

Photo of two women with soccer ball

When the knee is bent or extended, the kneecap (patella) slides through a guide groove on the femur. It can pop out of its groove due to an unfortunate twisting motion or a lateral impact. This is called patella luxation and happens most often during sports, mostly in teenagers and young adults.

Often, a dislocated kneecap slides back on its own. Even then, it is important to have the knee examined by a doctor because bones , cartilage and ligaments may be injured. After a dislocation, the patella is more unstable and it is possible that it will pop out again later on.

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A patella luxation is very painful. Almost always the kneecap pops out to the outside of the knee. This usually tears the joint capsule and the ligament that stabilizes the kneecap on the inside of the knee (medial patellofemoral ligament, MPFL). The knee swells due to the injuries.

A popping sound may be heard during dislocation and it feels like the knee is "dislocating". Usually the slipped patella is clearly visible, which can be frightening.

With repeated dislocations, pain may be limited to the patella, swelling may be absent.

Graphic: normal anatomy of the knee (left) and patella luxation (right)

normal anatomy of the knee (left) and patella luxation (right)

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The kneecap can pop out if the knee joint is rotated inward in a slightly flexed position while the foot is flat on the floor.

Patella luxation is often the result of a sports accident – typically dancing, gymnastics, handball or soccer. This is then also referred to as an acute or traumatic dislocation. This is the most common form with 80.

Patella subluxation is rarer: in this case, the kneecap moves back and forth laterally in the guide groove without popping out completely. This can occur, for example, after a previous knee injury or due to ligaments that are too loose.

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Patella luxations occur mainly in adolescents and young adults between 10 and 20 years of age. Girls and young women are more often affected because they generally have more flexible ligaments and weaker muscles than boys.

The risk of patella luxation is increased in people with:

  • overmobile joints or weak ligaments
  • of a higher standing kneecap (patella alta)
  • a deformed guiding groove (trochlear dysplasia) or other anatomical peculiarities
  • Deformities of the legs (for example, knock knees)
  • a weak inner thigh muscle

These factors also increase the risk of recurrent dislocation.

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Once the kneecap pops out, it is more unstable afterward. After a dislocation, it can feel as if the kneecap no longer has sufficient support. Anterior knee pain may also occur, especially with more intense exercise.

In 15 to 45% of people, the kneecap pops out again after an initial dislocation. Sometimes this happens after a few weeks, sometimes only after months or years.

For athletes, however, a patella luxation does not necessarily mean that they have to give up their sport. In studies, about half of the participants continued to play their sport as before after completing treatment.

The longer-term effects of patella dislocations are not well studied and therefore difficult to assess. However, they can increase the risk of knee osteoarthritis. This is especially true for repeated dislocations, which usually also cause more damage to the cartilage.

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A popped-out kneecap is usually clearly visible. During the examination, the doctor looks at the knee and palpates it.

After the kneecap has been set, an X-ray is taken to determine whether any bones have been injured or splintered off.

Various physical exams and a magnetic resonance imaging ( MRI ) scan can determine whether there have been any injuries to ligaments or menisci. An MRI image also reveals anatomical deformities and cartilage damage.

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When the kneecap pops out, it’s usually pretty scary. If the kneecap does not spring back into its normal position by itself, an ambulance should be called quickly (emergency call 112). If the kneecap pops back out, it’s enough to get a ride to the emergency room. First aid measures are:

  • Keep as calm as possible.
  • Sitting down or lying down to relieve the leg.
  • Keep the knee as still as possible to avoid further injuring it.
  • Cool the knee to reduce swelling and relieve some of the pain. (Wrap cold packs or ice packs in a cloth before applying them so as not to injure the skin.)

In the clinic, the doctor first gives a fast-acting painkiller and then sets the kneecap back in place. Often it slides back into normal position just by slowly stretching the leg. Sometimes it is necessary to help with a specific action:

  • To do this, lie on your back and relax the leg muscles so that all muscles, tendons and ligaments are as loose as possible.
  • Then the physician places his or her hands around the kneecap and applies the thumbs to the patella. An assistant grasps the leg at the ankle.
  • As the doctor pushes the kneecap back into place, the aide pulls the leg into extension.

After setting, wear a special orthosis or brace for a few weeks to stabilize the kneecap. Initially, walking aids can also be useful to relieve the injured knee.

In the first few days after the injury, it is recommended,

  • to spare the knee (as little as possible standing and walking, avoid bending and stretching).
  • Elevate the leg regularly and cool it several times a day for 15 to 20 minutes.
  • To take anti-inflammatory painkillers such as ibuprofen when needed.

If the kneecap has already popped out for the second time, surgery is usually considered. Other possible reasons for intervention include risk factors for further dislocation or major cartilage or bone injury.

More knowledge

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After the knee is set or after an operation, rehabilitation follows. After the knee has been immobilized for a few days, physiotherapy is advisable. The initial goal is to get the knee mobile again. Swelling can be treated by lymphatic drainage.

Afterwards, it is important to strengthen the leg muscles through targeted training to give the kneecap more support. Strengthening the inner thigh muscle plays a particularly important role here. This is connected to the ligament that stabilizes the kneecap.

Well trained hip, pelvic, ankle and trunk muscles also ensure knee stability. If certain muscles are shortened, which can lead to one-sided strain, special stretching exercises may also be considered.

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How long the recovery takes depends on how badly the knee was injured and how it is treated.

If the knee has not suffered any major damage, it will take about six weeks before you can resume your normal daily activities. Sports are usually possible again after 3 to 4 months.

If the knee is more severely injured and requires surgery, it can take much longer to return to sports – sometimes more than a year.

When and how intensively one can train again depends on the personal conditions, but also on the type of sport. Various (sports-)medical examinations can help to assess the healing process.

After treatment, the knee is not always as resilient as it was before. It is important to do the rehabilitation exercises independently and regularly and not to put too much stress on the knee too soon. Otherwise, the risk of the kneecap popping out again during sports increases. Consultation with physiotherapists and physicians is recommended before resuming sports or increasing the load.

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The family doctor’s office is usually the first place to go when you are sick or need medical advice for a health problem. We provide information on how to find the right practice, how to best prepare for a visit to the doctor and what is important in the process.

Frosch S, Balcarek P, Walde TA, Schuttrumpf JP, Wachowski MM, Ferleman KG et al. Therapy of patellar luxation: a systematic review of the literature . Z Orthop Unfall 2011; 149(06): 630-645.

Vetrano M, Oliva F, Bisicchia S, Bossa M, De Carli A, Di Lorenzo L et al. I.S.Mu.L.T. first-time patellar dislocation guidelines . Muscles Ligaments Tendons J 2017; 7(1): 1-10.

IQWiG health information should help to understand the advantages and disadvantages of important treatment options and health care offerings.

Whether one of the possibilities described by us is actually useful in the individual case, can be clarified in a conversation with a doctor. Health information.de can support the discussion with physicians and other specialists, but not replace. We do not offer individual advice.

Our information is based on the results of high-quality studies. They are written by a team of authors from the medical, scientific and editorial communities and peer-reviewed by experts outside IQWiG. How we develop our texts and keep them up to date, we describe in detail in our methods .

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Created on 29. July 2020
Next planned update: 2023

Institute for Quality and Efficiency in Health Care (IQWiG)

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