Flexor Tendon Lacerations

Abstract: Since initial reports suggesting primary tendon repair as possible and even desirable emerged in the 1960s, significant advancements in the understanding of flexor tendon anatomy, biology, mechanisms of response to injury, and methods of repair, have been made. Recent research highlights enhanced improvements in operative techniques and rehabilitative care that have made primary flexor tendon repair a preferred operative approach for lacerations and can successfully achieve a reliable flexor tendon repair site, optimizing digital motion. The formative goals of surgical treatment for lacerated flexor tendons have remained constant: accurate smooth coaptation of tendons ends to allow application of a postoperative rehabilitation protocol that encourages tendon gliding, prevents peritendinous adhesion formation without gapping, stimulates gliding surface restoration while optimizing opportunity for primary site healing, and ultimately, achieves satisfactory strength to allow early range of motion to the finger. Flexor digitorum profundus (FDP), flexor digitorum superficialis (FDS), and flexor pollicis longus (FPL) muscles power flexion of the fingers and thumb. Within the forearm, FDS tendons share a common muscle belly, while each FDP tendon has its own individual muscle belly. At the metacarpal head (Camper’s chiasma), FDS tendons divide into two halves, where then each head rotates laterally (180 degrees) around the associated FDP tendon. FDS then slips dorsally to rejoin the opposite head deep to the FDP tendon at the distal aspect of the proximal phalanx, prior to volarly and laterally inserting on the middle phalanx as two separate slips. : FDS tendons flex the proximal interphalangeal (PIP) joints. FDS originates from the medial epicondyle, coronoid process of the ulna, and proximal shaft of radius and inserts on the middle phalanx. The median nerve innervates FDS; its vascular source is from the radial and ulnar arteries.  . FDP originates on the proximal ulna and interosseous membrane and inserts on the volar base of the distal phalanx. FDP tendons flex the distal interphalangeal (DIP) joint. While the FDP tendons of the index and middle fingers are innervated by the anterior interosseous branch of the median nerve, the ring and small finger FDP tendons are innervated by the ulnar nerve. Blood supply to FDP is mainly from the ulnar artery. FPL flexes the thumb interphalangeal (IP) joint. FPL originates from the proximal radius, radial head of the interosseous membrane, and medial epicondyle or accessory head of the coronoid process. It inserts on the volar base of the thumb distal phalanx. FPL is innervated by the anterior interosseous nerve branch of the median nerve. Blood supply is predominantly from the radial artery. In the distal forearm, the most superficial FDS tendons to long and ring fingers overlay the FDS tendons to index and little fingers. In the deeper layers remain 4 FDP tendons and FPL. The relationship between these nine digital flexors remains relatively constant in their orientation and relationship as they enter the proximal aspect of carpal tunnel. Each of the tendons mentioned above lies within a tendon sheath, subsequently reinforced by thickened areas known as pulleys, which hold tendons close to the phalanges at all positions through extension and flexion. Pulleys permit tendon excursion while maximizing mechanical competence and improving the overall efficiency of the flexor apparatus. Each layer of the pulley system has a strategic purpose: the innermost secrete hyaluronic acid is designed to facilitate gliding, the middle is rich in collagen to resist palmar translation, and the outer facilitates nutrition of the pulley system. There are five annular (A) pulleys and three cruciate (C) pulleys. Odd-numbered A pulleys are at the joint level: A1 at the metacarpophalangeal (MP) joint, A3 at the PIP joint, and A5 at the DIP joint. The A2 pulley is at the proximal portion of the proximal phalanx, and the A4 pulley lies at the middle portion of the middle phalanx. A2 and A4 pulleys are the most critical components for proper flexor function; injury to either of these precludes bowstringing of the flexor tendon. The pulley system within the thumb is unique in that it only contains two annular pulleys (A1, A2) and an intervening oblique pulley. Injury to the oblique pulley within the thumb can lead to bowstringing of the FPL tendon, as it is an extension of the adductor pollicis aponeurosis. The fingers and thumb flexor tendon zones can be subdivided via universal nomenclature called Verdans, initially developed by Kleinert and colleagues, and Verdan; repair techniques and prognoses vary within each zone. : Five zones for fingers: Zone 1 - distal to FDS insertion; only the FDP resides here. Zone 2 - from A1 pulley (proximally) to FDS insertion (distally) (within the sheath = “no man’s land”); contains both FDS and FDP. Zone 1 & Zone 2 are described by the fibro-osseous digital sheath. Within this sheath, the tendons are covered by a layer of flattened fibroblasts termed epitenon - a crucial gliding surface that must be restored for flexor tendon repair to be successful. Zone 3 - from the distal end of carpal tunnel to A1 pulley; denotes the origin of lumbricals from FDP. Zone 4 - within the carpal tunnel, under the flexor retinaculum. Zone 5 - proximal to the carpal tunnel. Five zones for thumb: Zone T1 - distal to interphalangeal (IP) joint. Zone T2 - from A1 pulley to IP joint. Zone T3 - over the thenar eminence. Zone T4 - within the carpal tunnel. Zone T5 - proximal to carpal tunnel.