Arthritic hand. X-ray of the hand of a patient with severe rheumatoid arthritis, particularly in the thumb, at right. Rheumatoid arthritis is an autoimmune disorder, where the immune system attacks the body's own tissues, causing progressive joint and cartilage destruction. As the cartilage is worn away, new bone grows as part of the repair process. This causes stiffness and deformity of the fingers. Treatment is with anti-inflammatory drugs and physiotherapy.