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Understanding the Kinematics and Dynamics of the Digit
Jennifer Connor, MD1; Farid Amirouche, PhD1; Mark Gonzalez, MD1
1 Department of Orthopaedic Surgery, University of Illinois at Chicago, 835 South Wolcott, Chicago, IL 60612. E-mail address for J. Connor: jennifer.connor@gmail.com
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Disclosure: The authors did not receive any outside funding or grants in support of their research for or preparation of this work. One or more of the authors, or a member of his or her immediate family, received, in any one year, payments or other benefits of less than $10,000 or a commitment or agreement to provide such benefits from a commercial entity (DePuy).

The Journal of Bone and Joint Surgery, Inc.
J Bone Joint Surg Am, 2009 Nov 01;91(Supplement 6):74-78. doi: 10.2106/JBJS.I.00560
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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.
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    These activities have been planned and implemented in accordance with the Essential Areas and policies of the Accreditation Council for Continuing Medical Education (ACCME) through the joint sponsorship of the American Academy of Orthopaedic Surgeons and The Journal of Bone and Joint Surgery, Inc. The American Academy of Orthopaedic Surgeons is accredited by the ACCME to provide continuing medical education for physicians.
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