Synovitis is due to the synovial membrane hyperplasia, hypertrophy, oozing increased swelling around the joints, pain, normal joints have joint fluid nutrients joints, when the increase in joint fluid is called joint effusion, joint effusion will also be swollen, pain. Take the knee joint as an example, the difference between synovitis and joint effusion is that joint effusion increases when there is more exudation from the proliferating synovial membrane, and the difference between the two is that synovitis is the basis for the formation of joint effusion. Regardless of the cause of synovitis, when the synovial membrane produces more than normal joint fluid, joint effusion is formed.