Understanding the Kinematics and Dynamics of the Digit

Abstract

The flexor tendon system of the hand and the relationship between the flexor digitorum superficialis and flexor digitorum profundus is complicated and has been the subject of much research1-3. Treatment recommendations for diseases and injuries involving the flexor tendon system have evolved substantially over the past thirty years as a better understanding of its anatomy, function, healing capacity, and rehabilitation protocols has emerged. Both muscles originate in the forearm, and distally they form the flexor tendons of the digits. The flexor tendons are oval-shaped spiral bands composed of tenocytes and type-I collagen. Distally, beginning at the level of the metacarpal neck, the flexor tendons are enclosed in the flexor sheath, a fibro-osseous sheath with multiple functions. As has been well described, the sheath has thickened regions that form the pulley system. The pulley system is composed of the anular and cruciform pulleys. The A1, A3, and A5 pulleys arise from the volar plate of the metacarpophalangeal, proximal interphalangeal, and distal interphalangeal joints, respectively. The A2 and A4 pulleys arise from the proximal and middle phalanges. The cruciform pulleys are located between the anular pulleys and are less rigid. The flexor digitorum superficialis tendon of each digit is located volar to the flexor digitorum profundus tendon until the level of the flexor sheath, at which point the flexor digitorum superficialis tendon divides into two slips. One slip passes radial and the other passes ulnar to the flexor digitorum profundus tendon before rejoining dorsal to the flexor digitorum profundus tendon and inserting into the middle phalanx (Fig. 1). At this location, the flexor digitorum superficialis appears, anatomically, to have the capacity to act as an additional pulley for the flexor digitorum profundus. This anatomic layout prompted our investigation. Fig. 1 Diagram of the flexor tendon system of the digit. (Reprinted, …