Extensor Indicis  Muscle – Anatomy, Nerve Supply, Functions

Extensor Indicis  Muscle is a narrow, elongated muscle found in the posterior compartment of the forearm. It belongs to the deep extensors of the forearm, together with supinator, abductor pollicis longus, extensor pollicis longus, and extensor pollicis brevis muscles. Extensor indicis can be palpated by applying deep pressure over the lower part of the ulna while the index finger is extended.

The main function of extensor indicis involves the extension of the index finger at the metacarpophalangeal and interphalangeal joints. As the index finger is one of the few fingers that have their own separate extensor muscle, it is able to extend independently from other fingers. Additionally, extensor indicis muscle produces a weak extension of the wrist.

Extensor indices is a narrow, elongated skeletal muscle in the deep layer of the dorsal forearm, placed medial to, and parallel with, the extensor pollicis longus. Its tendon goes to the index finger, which it extends.

Structure of Extensor Indicis

It arises from the distal third of the dorsal part of the body of ulna and from the interosseous membrane. It runs through the fourth tendon compartment together with the extensor digitorum, from where it projects into the dorsal aponeurosis of the index finger. ^[rx]

Opposite the head of the second metacarpal bone, it joins the ulnar side of the tendon of the extensor digitorum which belongs to the index finger.

Like the extensor digiti minimi (i.e. the extensor of the little finger), the tendon of the extensor indicis runs and inserts on the ulnar side of the tendon of the common extensor digitorum.^[rx] The extensor indicis lacks the juncturae tendinum interlinking the tendons of the extensor digitorum on the dorsal side of the hand. ^[rx]

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.

Extensor indicis

• Origin: ulna and interosseous membrane
• Insertion: phalanx of first digit
• Action: extension of the wrist and extension thumb at IP, MCP, and CMC joints
• Innervation: posterior interosseous nerve

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