Many patients with knee problems often experience joint swelling and pain, along with “joint effusion”. If you go to the hospital, the doctor will say you have “synovitis”, or the doctor will tell you that you have “fluid in the joint”. Patients are often confused and often ask me, “Which doctor is right? Are synovitis and hydrocele the same thing? Yes, synovitis is synovial fluid, which is one of the most important manifestations of arthritis. Synovitis is the medical diagnosis that describes this phenomenon. Let me explain: The synovium is a layer of membranous tissue that surrounds the joint. It not only protects the joint, but also produces joint fluid, which provides “lubrication” for the joint to move. At the same time, the synovial fluid also needs to be reabsorbed through the synovial membrane. For a normal organism, the production and absorption of joint fluid is a “dynamic balance”. When arthritis strikes, the synovial membrane is stimulated by inflammation and undergoes inflammatory changes, resulting in brittle and thickened tissues that produce large amounts of joint fluid, as well as impaired resorption of joint fluid. At this time, the dynamic balance between the production and absorption of joint fluid is disrupted, and the production of joint fluid is greater than its reabsorption, resulting in “joint effusion”. Therefore, it can be said that the appearance of joint fluid and synovitis is an important manifestation of an arthritic attack.