There are many different causes of synovitis in the knee, but overall, it is due to synovial membrane damage. Because the knee joint itself is a large joint that carries weight and has a lot of movement and synovial membrane, synovitis occurs when the synovial membrane is damaged. There are many causes of knee synovitis, such as common osteoarthritis, rheumatoid or rheumatoid arthritis of the immune system, and cold irritation, all of which can cause synovitis. In children, synovitis can also be caused by a transient viral infection. The treatment may vary depending on the cause. For synovitis of the knee, regardless of the type of synovitis, rest and braking may be needed to minimize synovial damage.