Swelling on the back of one knee may be a Baker's cyst. A Baker's cyst can form when joint-lubricating fluid fills a cushioning pouch (bursa) at the back of your knee.
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.
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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:
- Swelling behind your knee, and sometimes in your leg
- Knee pain
- Stiffness and inability to fully flex the knee
Your symptoms may be worse after you've been active or if you've been standing for a long time.
When to see a doctor
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:
- Inflammation of the knee joint, such as occurs with various types of arthritis
- A knee injury, such as a cartilage tear
Rarely, a Baker's cyst bursts and synovial fluid leaks into the calf region, causing:
- Sharp pain in your knee
- Swelling in the calf
- Sometimes, redness of your calf or a feeling of water running down your calf
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.