What is it?

Popliteal cyst or Baker's cyst is a cyst that occurs in the back of the knee. The cyst consists of a sac distended with clear thick fluid often arising from the tendon sheath in the back of the knee. Frequently it arises from the bursa just deep to the Semimembranosus tendon, so it is also called a Semimembranosus cyst.

What causes it?

No one knows why it occurs in children. In adults, a popliteal cyst communicates with the knee joint, and reflects a problem in the knee. In children, the popliteal cyst usually does not communicate with the knee, and does not reflect a knee problem.

What are the symptoms?

The child could present at any age, and is often detected by a parent accidentally. It is usually painless, and does not interfere with knee function.

What does your doctor do about it?

Your doctor may do an X-ray to rule out other knee problems, but it is usually negative. The location of the cyst and it characteristic feel is diagnostic, and other tests are usually unnecessary. Usually no treatment is necessary except for periodic observation. Most popliteal cysts in children disappear within 1 to 2 years. In the rare case where the cyst gets larger and causes symptoms, excision can be performed.