The thumb accounts for 60% of hand function. When thumb movement is limited by pain, instability, or deformity, hand function is greatly reduced. The rheumatoid arthritis thumb deformity shown here is characterized by metacarpophalangeal joint flexion and interphalangeal hyperextension. When marked, this deformity is commonly known as a 90/90. References: * Ruby LK, Cassidy C. Hand clinics: rheumatoid arthritis of the hand and wrist. Vol. 12. No. 3. Philadelphia: W.B. Saunders Company; 1996. * Harrell PB. Splinting of the hand. In: Wegener ST, Belza BL, Gall EP, editors. Clinical care in the rheumatic diseases. Atlanta: American College of Rheumatology; 1996. p. 83-8.