What is synovitis? Synovitis is a multifaceted, persistent and common condition. It can be seen in all age groups, but is more prevalent in younger people and usually lasts between the ages of 16 and 25. It develops mainly in the knee joint. This is because the knee is the joint with the most synovial membranes, the largest articular surface and the most complex structure with the highest activity, and because the synovial membranes of the knee are extensive and located in the shallow part of the limb surface, there are more chances of damage, infection and mechanical stimulation. The synovial membrane is mainly located in the inner layer of the joint capsule and is composed of smooth, shiny, pink, thin and soft loose connective tissue. The synovial membrane is located immediately on the inner surface of the fibrous layer of the joint capsule and is attached to the periphery of the articular cartilage. In fact, the synovial membrane covers all tissues within the joint except for the articular cartilage, the articular disc and the central part of the fibrocartilaginous meniscus. There are two layers of synovial tissue, the inner synovial layer near the joint and the subsynovial layer. The inner synovial layer, also known as the synovial lining, is composed of polymorphic synovial cells and synovial interstitial cells, which contain no fibrous components and no blood vessels or lymphatic vessels. The subsynovial layer, however, is rich in blood vessels and lymphatic vessels, forming a rich vascular network. It secretes lubricating fluid to lubricate the joints. The production and absorption of joint fluid is a “dynamic equilibrium”. When the reabsorption of joint fluid is impaired, the dynamic equilibrium between the production and absorption of joint fluid is broken, and the production of joint fluid is greater than the reabsorption, then “joint effusion” occurs. Synovitis is an aseptic inflammation caused by poor microcirculation due to various pathogenic factors, and the main symptom is fluid accumulation. Generally speaking, the presence of synovial inflammation is evidenced by the presence of exudative fluid in the joint, which is characterized by congestion and swelling of the synovial membrane, increased exudation, joint effusion, joint pain, difficulty in moving and squatting, and functional limitations. Strictly speaking, “synovitis” is not a diagnosis, but a symptom or a sign. It is not a diagnosis, just as “cough” is not a diagnosis. Common conditions that can cause synovitis include: osteoarthritis, rheumatoid arthritis, pigmented villous synovitis, gouty arthritis, hemophilic arthritis, reactive arthritis, tuberculous synovitis, psoriatic arthritis, Wright’s syndrome, systemic sclerosis, common disease arthritis, septic arthritis, etc. Therefore, it is important to confirm the diagnosis of synovitis, neither to be afraid nor to take it seriously and take medication indiscriminately to cover up the condition. If necessary, joint fluid examination or arthroscopic subsurface membrane biopsy should be performed to confirm the diagnosis, and then the different causes should be treated promptly and correctly.