Boutonnière deformity. A, There are boutonnière deformities of joints, including the thumb of the right hand and the second to fifth digits of the left hand. Swan neck deformity of the index finger of the right hand and multiple rheumatoid nodules are also present. These deformities develop as the disease becomes established. Wrist swelling, tenosynovitis of the extensor tendon of the third digit of the right hand, and profound interosseous muscle wasting are additional features of the hand in rheumatoid arthritis. The skin of the digits may appear atrophied. There is chronic synovial thickening of multiple joints, especially the wrists. B, The boutonnière (buttonhole) deformity is characterized by hyperextension at the metacarpophalangeal, flexion at the proximal interphalangeal and hyperextension at the distal interphalangeal joints. Synovitis causes relaxation of the dorsal slip, with the proximal interphalangeal protruding through the radial and ulnar ligaments of the dorsal aponeurosis, which are below the usual axis of motion of these bands. The proximal interphalangeal joint passes through the defect much as a button is passed through a buttonhole.