Common deformities of the hand in RA:
Fusiform swelling: synovitis of PIP joints, causing them to appear spindle-shaped.
Boutonnière deformity: flexion of the PIP and hyperextension of the DIP joint, caused by weakening of the central slip of the extrinsic extensor tendon and a palmar displacement of the lateral bands. This deformity resembles a knuckle being pushed through a buttonhole.
Swan-neck deformity: results from contraction of the flexors (intrinsic muscles) of the MCPs, resulting in flexion contracture of the MCP joint, hyperextension of PIP, and flexion of the DIP joint.
Ulnar deviation of fingers: with subluxation of MCP joints. This results from weakening of the extensor carpi ulnaris leading to radial deviation of the wrist causing all finger tendons to pull the fingers ulnarly with power grasp.
Hitchhiker thumb: hyperextension of IP joint with flexion of MCP and exaggerated adduction of first metacarpus. Causes inability to pinch.
“Piano key” ulnar head: secondary to destruction of ulnar collateral ligament leading to a floating ulnar styloid.
Arthritis der Hand - Rheumaliga.ch (German)