An accumulation of joint fluid (synovial fluid) that forms behind the knee.

Alternative Names

Popliteal cyst

Causes, incidence, and risk factors

A cyst is a fluid collection and a Baker's cyst, also called a popliteal cyst, is a fluid collection behind the knee. This cyst may formed by the connection of a normal bursa (a normal lubricating fluid sac) with the knee joint (more common in children) or by the herniation of the knee joint capsule out into the back of the knee (more common in adults). The latter type of Baker's cyst is commonly associated with a tear in the meniscal cartilage of the knee. Nearly one half of Baker's cysts are found in children, where it appears as a painless swelling behind the knee that is more obvious with the knee fully extended. A large cyst may cause some discomfort or stiffness but generally is asymptomatic . Baker's cysts usually disappear spontaneously after a variable length of time.

Signs and tests

Physical exam will often demonstrate a soft mass in the back of the knee. If the cyst is small, comparing one knee to the normal knee can be helpful. There may be limitation in range of motion from pain or from the size of the cyst. In some cases there will be signs and symptoms of a meniscal tear. Transillumination, or shining a light through the cyst, can demonstrate that the mass is fluid filled. If the mass demonstrates any abnormal signs, like rapid growth, night pain, severe pain, or fever, a more involved work-up is indicated to rule out non-cystic tumors that can grow in the back of the knee. X-rays will not show the cyst or a meniscal tear but will show other abnormalities that may be present including arthritis. MRI can be helpful to visualize the cyst and to demonstrate any meniscal injury.

Treatment

Often no treatment is necessary and the practitioner can observe the cyst over time. Arthroscopic surgery to decompress the cyst and treat any meniscal tear may become necessary if the cyst is extremely large or painful. Aspiration , or draining the cyst with a needle, will decrease cyst size but generally the cyst recurs.

Expectations (prognosis)

A Baker's cyst is a benign lesion that will not cause any long-term harm, but can be annoying and painful. Long term disability is very rare as most cases improve with time or arthroscopic surgery.

Complications

Complications are unusual but include:

Calling your health care provider

Call for an appointment with your health care provider if there is a swelling behind the knee that becomes large or painful; (infection is not associated with a Bakers cyst).

Prevention

There is no known way to prevent the development of a Baker's cyst.