The knee joint is encased in a thick tissue called the articular capsule. This capsule surrounds the knee joint. Inside this capsule is a fluid called synovial fluid, which helps lubricate the joint.

Sometimes this capsule becomes ruptured. This means that a hole develops in the capsule. This may be caused from an injury and in some case may possibly be a complication of a surgery to the knee. When this hole develops in the back side of the capsule, the synovial fluid leaks out and into the area behind the knee. This is called a Baker's cyst. Symptoms of a Baker's cyst are swelling behind and slightly below the knee joint, and tenderness in the area.

Treatment involves modalities to reduce the swelling. Therapeutic exercise may help with strength and conditioning if chronic swelling has reduced the function of your knee. A Baker's cyst may heal on its own but typically does not ever heal completely. If the condition remains very extreme, surgery may be advised to remove the cyst and attempt to correct the defect in the capsule.