The synovial fluid found in the bone joints. Whenever joint is traumatized, large amount of fluid is secreted by the synovium, which makes its way out of a weakest part and leads to the formation of so called cyst. The popliteal cyst is the same type of cyst and is found in the back of the knee joints. They are benign in nature and never are a cause of cancer. Feeling of a little bump behind the knee is a sign of the condition. These cysts require no treatment and in due course of time the fluid goes back ito the knee joint.