Synovitis. Arthroscope (endoscope) view of a knee with synovitis, inflammation of the synovium (synovial membrane) that lines the joint capsule, due to osteoarthritis. Osteoarthritis is joint inflammation due to the loss of its friction- reducing cartilage. Osteoarthritis, along with other forms of arthritis, is a major cause of synovitis. In synovitis, the inflamed synovium secretes too much lubricating fluid into the capsule that encloses the joint, causing pain and swelling. Treatment is with rest, joint support, painkillers, anti-inflammatory drugs or, in severe cases, through the surgical removal of the synovium.