Knee effusion is actually an accumulation of fluid in the joint cavity of the knee joint, because the knee joint is a very large movable, weight-bearing joint with very many synovial folds, and the synovial membrane itself is very prone to exudation if there is inflammation, and there will be an accumulation of fluid. There are many causes of effusion, as follows: 1) joint instability, such as osteoarthritis, which can cause joint effusion; 2) internal diseases, such as rheumatism, rheumatoid, and even ankylosing spondylitis; 3) antigenic antibody reactions of the immune system, which can cause increased synovitis exudation and joint effusion; 4) traumatic arthritis, infection, and tuberculosis, which can also cause joint effusion; 5) a small child’s joint can cause joint effusion. 5. Transient viral infections in children can also cause joint effusion. Therefore, there are many causes of joint effusion, and a local examination is needed to see if there is joint effusion. If there is joint effusion, it is important to look for the cause.