A Baker cyst is a fluid-filled growth behind the knee. It causes a bulge and a feeling of tightness. Also called a popliteal (pop-luh-TEE-ul) cyst, a Baker cyst sometime causes pain. The pain can get worse when with activity or when fully straightening or bending the knee.

A Baker cyst is usually the result of a problem with the knee joint, such as arthritis or a cartilage tear. Both conditions can cause the knee to produce too much fluid.

Although a Baker cyst may cause swelling and discomfort, treating the underlying problem that is causing it usually provides relief.


In some cases, a Baker cyst causes no pain, and you may not notice it. If you do have symptoms, they might include:

  • Swelling behind the knee, and sometimes in the leg
  • Knee pain
  • Stiffness and inability to fully flex the knee

The symptoms may be worse after you've been active or if you've been standing for a long time.

When to see a doctor

Seek medical attention if you have pain and swelling behind your knee. Though unlikely, these symptoms may be a sign of a blood clot in a leg vein.


A lubricating fluid called synovial (sih-NO-vee-ul) fluid helps the leg swing smoothly and reduces friction between the moving parts of the knee.

But sometimes underlying conditions can cause the knee to produce too much synovial fluid. When this happens, fluid can build up in the back of the knee, leading to a Baker cyst.

This can be caused by:

  • Inflammation of the knee joint, which can occur with various types of arthritis
  • A knee injury, such as a cartilage tear


Rarely, a Baker cyst bursts and synovial fluid leaks into the calf region, causing:

  • Sharp pain in the knee
  • Swelling in the calf
  • Sometimes, redness of the calf or a feeling of water running down the calf