Kinesiology for Occupational Therapy, Third Edition by Melinda Rybski;

Author:Melinda Rybski; [F. RYBSKI, MELINDA]
Language: eng
Format: epub
ISBN: 9781630914721
Publisher: SLACK Incorporated

Figure 8-11. Extensor compartments of the hand. Compartment 1: EPB (extensor pollicis brevis) and APL (abductor pollicis longus). Compartment 2: ECRB (extensor carpi radialis brevis) and ECRL (extensor carpi radialis longus). Compartment 3: EPL (extensor pollicis longus). Compartment 4: EDC (extensor digitorum communis) and EIP (extensor indicis proprius). Compartment 5: EDM (extensor digiti minimi). Compartment 6: ECU (extensor carpi ulnaris).

The extensor mechanism is made up of the extensor digitorum, connective tissue, and expansion fibers from the dorsal interossei, volar interossei, and lumbricals (Figure 8-12). The extensor digitorum tendons pass through the first dorsal compartment with the extensor indicis.

At the proximal phalanx, each tendon divides into three components. One portion, the central slip, inserts into the dorsum of the proximal end of the middle phalange. This central slip acts to extend the proximal phalange and to stabilize the proximal finger joint so that the lumbricals and interossei can extend the DIP and PIP joints and laterally move the digit. Two lateral bands pass on either side of the central tendon, cross the proximal phalange, reunite as a single terminal tendon on the distal phalange, then unite with the lumbricals and interossei. The fascia extends laterally from the extensor tendon that forms a hood that encircles the intrinsics (interossei and lumbricals).

If only the PIP joint is flexed, the entire trifurcated extensor assembly is pulled distally following the central slip, which is taut while a distal pull occurs at the middle phalanx. The lateral bands are slack. This creates a tension force at the PIP joint, so flexion of this joint is unavoidable. If the DIP is actively flexed, the entire assembly is displaced distally as the central slip relaxes, creating increased tension on the retinacular ligaments. This creates a flexion force at the PIP joint that is important in grasp and pinch (Nordin & Frankel, 2012). In addition, simultaneous flexion of the MCP, PIP, and DIP joints forces the extensor digitorum to stretch to its fullest. Full flexion of the wrist can only occur if fingers are allowed to uncurl.

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