Extensor indicis muscle

Origin and insertion

Extensor indicis is a narrow muscle that originates mainly from the ulna, arising from the posterior two-thirds of its distal surface, distal to extensor pollicis longus muscle. Some fibers also stem from the adjacent interosseous membrane. It extends inferiorly and narrows into a tendon that passes deep to the extensor retinaculum. Beneath the retinaculum, the tendon is invested within a tendinous sheath with the tendons of extensor digitorum muscle.

As it courses in the hand, the tendon of extensor indicis passes obliquely over the carpal bones and joins the ulnar side of the tendons of extensor digitorum, opposite to the head of the second metacarpal. Here, the tendon of extensor indicis inserts at the extensor expansion on the back of the proximal phalanx of the index finger.

At times, extensor indicis gives off several accessory slips to the extensor tendons of other digits. In addition, it can contain an additional muscle belly on the dorsum of the hand called extensor indicis brevis manus, although this occurs rarely.

Relations

Extensor indicis is situated in the deep extensor compartment of the forearm situated deep to the extensor digitorum, medial to the extensor pollicis longus, and lateral to the extensor carpi ulnaris.

Deep to the extensor retinaculum, the extensor indicis tendon lies in the fourth dorsal (extensor) compartment of the wrist. It shares this compartment with the tendons of extensor digitorum muscle, sitting medial to them in their common tendinous sheath. Medially to their sheath is that of the extensor pollicis longus (third extensor compartment), while laterally is the tendinous sheath of extensor digiti minimi (fifth extensor compartment). In addition, the posterior interosseous nerve passes deep to the shared compartment of the extensor indicis and extensor digitorum.

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Innervation

Extensor indicis receives its nervous supply from posterior interosseous nerve, a branch of the radial nerve derived from spinal roots C7 and C8. The skin overlying the muscle is supplied by the same nerve, with fibers that stem from the spinal roots C6 and C7.

Blood supply

The superficial surface of the extensor indicis receives arterial blood supply from posterior interosseous branch of the ulnar artery, whereas its deep surface receives blood from perforating branches of the anterior interosseous artery.

Function

Extensor indicis acts at the metacarpophalangeal and interphalangeal joints to extend the index finger. Unlike most of the other fingers of the hand, the index finger has its own separate extensor, which enables it to extend independently from other fingers. In addition, as it crosses the wrist, this muscle produces a weak extension of this joint.