A Baker's cyst is a fluid-filled cyst that causes a bulge and a feeling of tightness behind your knee. The pain can get worse when you fully flex or extend your knee or when you're active.
A Baker's cyst, also called a popliteal (pop-luh-TEE-ul) cyst, is usually the result of a problem with your knee joint, such as arthritis or a cartilage tear. Both conditions can cause your knee to produce too much fluid, which can lead to a Baker's cyst.
Although a Baker's cyst may cause swelling and make you uncomfortable, treating the probable underlying problem usually provides relief.
In some cases, a Baker's cyst causes no pain, and you may not notice it. If you do have signs and symptoms, they might include:
Your symptoms may be worse after you've been active or if you've been standing for a long time.
If you have pain and swelling behind your knee, see your doctor. Though unlikely, a bulge behind your knee may be a sign of a condition more serious than a fluid-filled cyst.
A lubricating fluid called synovial (sih-NO-vee-ul) fluid helps your leg swing smoothly and reduces friction between the moving parts of your knee.
But sometimes the knee produces too much synovial fluid, resulting in buildup of fluid in an area on the back of your knee (popliteal bursa), causing a Baker's cyst. This can happen because of:
A Baker's cyst can often be diagnosed with a physical exam. However, because some of the signs and symptoms of a Baker's cyst mimic those of more-serious conditions, such as a blood clot, aneurysm or tumor, your doctor may order noninvasive imaging tests, including:
Rarely, a Baker's cyst bursts and synovial fluid leaks into the calf region, causing:
These signs and symptoms closely resemble those of a blood clot in a vein in your leg. If you have swelling and redness of your calf, you'll need prompt medical evaluation to rule out a more serious cause of your symptoms.
If your doctor determines that arthritis is causing the cyst, he or she may advise you to take some or all of the following steps: