Synovial chondromatosis is a type of  non-cancerous tumor that arises in the lining of a joint. The knee is most commonly affected; however it can affect any joint. The tumors begin as small nodules of cartilage. These nodules can separate and become loose within the joint. Some tumors may be no larger than a grain of rice. Synovial chondromatosis most commonly occurs in adults ages 20 to 50. Signs and symptoms may include pain, swelling, a decreased range of motion, and locking of the joint. The exact underlying cause of the condition is unknown. Treatment may involve surgery to remove the tumor. Recurrence of the condition is common.