A Baker's cyst is an abnormal collection of joint fluid inside a sac that is located behind the knee.
Bursae are sacs located in and around joints. They are normal and they contain fluid. These sacs and the joint fluid help to reduce friction and wear on the bones and muscles. In some people, an abnormal outpouching from these sacs may form in the back of the knee. This outpouching is known as a Baker's cyst. These cysts usually form due to some type of knee irritation or injury and may cause symptoms.
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 person may have no symptoms at all. When signs and symptoms do occur, they may include:
Symptoms are usually mild and often come and go. However, if the cyst breaks or ruptures, severe symptoms may occur suddenly.
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 Fluid may be withdrawn from the cyst.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.
Most cases cannot be prevented. Sports safety guidelines should be followed for children, adolescents, and adults.
The cyst can rupture, causing pain and swelling in the calf. 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.
There are no risks to others, as this condition is not contagious.
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. 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 any of a number of medicines to reduce damage to the joints.
The cyst might come back if fluid is withdrawn from it or after surgery.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.
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.
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.
Author:John A.K. Davies, MD
Editor:Crist, Gayle P., MS, BA
Reviewer:Adam Brochert, MD
Emergency Medicine, 1998, Rosen et al.