The synovium is a thin membrane that lines the inner surfaces of joints and produces synovial fluid, which lubricates and protects joints from wear and tear. This fluid can sometimes build up and form a synovial cyst. Synovial cysts are benign (non-cancerous). Although their cause is not fully understood, they are often associated with osteoarthritis. Ganglion cysts are non-cancerous, fluid-filled lumps that can develop around joints and tendons in any part of the body, particularly the hand and wrist. Unlike synovial cysts, ganglion cysts do not have a synovial cell lining. Symptoms, if any, depend on the size and location of the synovial or ganglion cyst, and can include pain, numbness, tingling, weakness or trouble with joint movement.