Whether the knee joint effusion is serious depends on the specific situation. The details are as follows: 1. The knee joint itself has lubricating fluid. As a kind of movable joint, the lubricating fluid can help reduce the friction between the meniscus joints and play a good lubricating role, so it is normal to have a small amount of joint effusion. If there is synovitis of the joint, this will produce a small amount of effusion, the joint has swelling, at this time rest, do braking and other simple treatment, joint cavity effusion will gradually reduce, is not a very serious problem; 2, but when the osteoarthritis is very serious, synovial crepitus hyperplasia is heavier, the formation of serious effusion, will show joint swelling, can not squat, walk, affect the normal activities, this will cause a relatively large Impact; 3. Internal diseases, especially rheumatism, rheumatoid, ankylosing spondylitis and such diseases, the cause of joint cavity effusion is because of internal antigenic antibody reactions or immune system problems. This results in wandering pain in the extremities, including other joints, and can severely limit normal life. Therefore, whether a knee effusion is mild or severe depends on the cause and the severity of the disease.