Synovitis. Arthroscope (endoscope) view of a shoulder with synovitis, inflammation of the synovium (synovial membrane) that lines the joint capsule. This is inhibiting the movement of the shoulder, a condition known as frozen shoulder (adhesive or retractile shoulder). In synovitis, the inflamed synovium secretes too much lubricating fluid into the capsule that encloses the joint, causing pain and swelling. Synovitis commonly results from arthritis, although it can also be caused by infection, injury or joint overuse. Treatment is with rest, joint support, painkillers, anti-inflammatory drugs or, in severe cases, by the surgical removal of the synovium.