Definition : 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 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.
Symptoms
In some cases, a Baker's cyst causes no pain, and you may not even notice it. If you do experience signs and symptoms, you may notice:
- Swelling behind your knee, and sometimes in your leg - Knee pain - Stiffness - Texture similar to a balloon filled with water
When to see a doctor
If you're experiencing pain and swelling behind your knee, see your doctor to determine the cause. Though unlikely, a bulge behind your knee may be a sign of a more serious condition, such as a tumor or a popliteal artery aneurysm, rather than a fluid-filled cyst.
Causes
The cartilage and tendons in your knee rely on a lubricating fluid called synovial (si-NO-vee-ul) fluid. This fluid helps your leg swing smoothly and reduces friction between the moving parts of your knee.
Synovial fluid circulates throughout your knee and passes in and out of various tissue pouches (bursae) throughout your knee. A valve-like system exists between your knee joint and the bursa on the back of your knee (popliteal bursa).
This regulates the amount of synovial fluid going in and out of the bursa. But sometimes the knee produces too much synovial fluid, resulting in buildup of fluid in the bursa and what is called a Baker's cyst.
This can be caused by:
- Inflammation of the knee joint, such as occurs with various types of arthritis - Knee injury, such as a cartilage tear
(Source: mayoclinic.com)