A thin layer of tissue—only a few cells thick—also known as the synovial membrane, lines the joints and tendon sheaths. It acts as a semipermeable membrane to control what substances can pass into the joint space and what substances remain outside it. The cells within the synovium produce synovial fluid, a transparent, viscous fluid that lubricates and nourishes cartilage and bone within the joint capsule.
In RA, the white blood cells of the immune system travel to the synovium and cause inflammation, a condition known as synovitis. This inflammation, the resulting thickening of the synovium and overproduction of synovial fluid cause the traditional symptoms of RA—warmth, redness, swelling and pain in the joint. As RA progresses, the inflamed synovium infiltrates and destroys cartilage and bone within the joint.
Synovitis also is a hallmark of several other rheumatic conditions, including spondyloarthritis (a family of rheumatic conditions that affect the spine), psoriatic arthritis (arthritis associated with the skin condition psoriasis), systemic lupus erythematosus (a chronic inflammatory condition involving many systems in the body) and gout (a disorder in which increased levels of uric acid cause painful swelling in joints). Synovitis can be treated with nonsteroidal anti-inflammatory drugs (such as ibuprofen or naproxen sodium) and injections of cortisone. In some cases, the synovium must be surgically removed, a procedure known as a synovectomy.