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Baker Cyst

Alternate Names

  • popliteal cyst

Definition

A Baker's cyst is an abnormal collection of joint fluid inside a sac that is located behind the knee.

What is going on in the body?

In some people, an abnormal outpouching from the knee joint may form in the back of the knee. This outpouching is known as a Baker's cyst.
Fluid in the knee may move from the knee joint causing accumululation in the Baker's cyst. A one-way valve mechanism prevents fluid from returning to the knee joint.

Risks

What are the causes and risks of the condition?

In children, this condition seems to occur for no reason and is not related to knee irritation. In adults, Baker's cysts are commonly due to:
  • a common type of arthritis, known as osteoarthritis. Other forms of arthritis, such as rheumatoid arthritis (RA), may also cause this condition.
  • increased fluid in the knee. This is called an effusion, and can be due to infection and other causes,
  • injury to the knee. This is usually a cartilage tear

Prevention

What can be done to prevent the condition?

Most cases cannot be prevented. Sports safety guidelines should be followed for children, adolescents, and adults.

Diagnosed

How is the condition diagnosed?

Diagnosis is primarily made by the medical history and physical exam of the knee. Fluid may be withdrawn from the cyst. This can often confirm the diagnosis and reduce or stop symptoms. The cyst may also be studied with imaging tests, such as ultrasound, MRI scans, or joint X-rays. These tests may confirm the diagnosis, look for arthritis or injury to the knee, and help rule out other causes for the bump in the back of the knee.

Long Term Effects

What are the long-term effects of the condition?

The cyst can rupture, causing pain and swelling in the calf - even mimicking a condition called thrombophlebitis.
The cyst can also slowly get bigger, which may worsen symptoms. This is especially common in people with rheumatoid arthritis.
The cyst is not a cancer and is not life-threatening.

Other Risks

What are the risks to others?

There are no risks to others, as this condition is not contagious.

Treatments

What are the treatments for the condition?

Treatment is not usually needed for the cyst.
If symptoms become severe, a needle can be inserted through the skin and into the cyst to drain it and, in some cases, inject corticosteroids into the cyst to reduce inflammation. A number of medicines may be given to reduce inflammation and damage to the joint.
In severe cases, surgery can be used to remove the cyst.
If a cause for the Baker's cyst can be found, the cause may need treatment. For example, a knee injury that causes a cartilage tear may need surgery.
A person with rheumatoid arthritis may be given an injection of cortisone into the knee to reduce fluid formation.

Side Effects

What are the side effects of the treatments?

The cyst might come back if fluid is withdrawn from it or after surgery. Surgery may cause bleeding, nerve injury, knee stiffness, and infections. Medicines used for rheumatoid arthritis may cause stomach upset, allergic reactions, and other effects.

After Treatment

What happens after treatment for the condition?

Most people are able to continue or return to normal activities after treatment. Those who have no further symptoms need no further therapy. If symptoms return or continue, further treatment may be advised. Other treatment may be needed for the underlying cause, such as arthritis or knee injury.

Monitor

How is the condition monitored?

A rapidly growing mass, increased pain, knee or calf swelling, or trouble moving the knee should be reported to the healthcare provider. Any other new or worsening symptoms should also be reported to the doctor.

Sources

Emergency Medicine, 1998, Rosen et al.

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