Extensor Indicis Muscle Side, Lateral View

Description

Arising from the posterior surface of the distal ulna and adjacent interosseous membrane, the extensor indicis muscle appears as a deep, narrow belly in the posterolateral forearm, tucked beneath the extensor digitorum. From this lateral vantage, its tendon can be followed distally as it passes deep to the extensor retinaculum in the fourth dorsal compartment, lying ulnar to the extensor digitorum tendon to the index finger. The radius remains lateral (thumb side) with the ulna medial, and the carpal bones, metacarpals, and phalanges provide distal bony landmarks for the tendon’s course onto the dorsal expansion of digit II. Small, deep, and easy to miss. Independent index finger extension depends on extensor indicis, so its anatomy becomes clinically relevant when patients lose isolated index extension after posterior interosseous nerve palsy or after deep dorsal forearm laceration. This lateral, posteriorly oriented presentation also clarifies why the tendon shares the fourth compartment with extensor digitorum, a setup that can contribute to dorsal wrist tenosynovitis and can confuse tendon identification during operative repair. For tendon transfer planning, extensor indicis proprius is a common donor for restoring thumb extension in chronic extensor pollicis longus rupture, and you need to appreciate its oblique, deep trajectory to harvest it safely. Hand and upper-limb anatomy courses, orthopedic and plastic surgery texts, and operative hand therapy references can all use this view to teach compartmental anatomy at the wrist and tendon relationships on the dorsum of the hand. It also suits clinical education on posterior interosseous nerve lesions and extensor tendon repair protocols for zone VI to VII injuries. Anatomical accuracy verified by SciePro's Medical Advisory Board.