Extensor Indicis

The extensor indicis is a deep, slender muscle in the posterior compartment of the forearm. It functions to extend the index finger independently of the other digits. It lies medial to the extensor pollicis longus and shares the same extensor hood system as the extensor digitorum but is anatomically and functionally distinct.

Location

The extensor indicis is located in the distal third of the posterior forearm, deep to the extensor digitorum and extensor carpi ulnaris. Its tendon passes beneath the extensor retinaculum and joins the extensor expansion of the second digit (index finger).

Structure

The muscle originates from the ulna and interosseous membrane and becomes tendinous as it approaches the wrist.

• Origin: Posterior surface of the distal third of the ulna and adjacent interosseous membrane.
• Insertion: Extensor expansion of the second digit (index finger), typically joining the ulnar side of the extensor digitorum tendon.
• Tendon Path: Passes through the fourth dorsal compartment of the extensor retinaculum alongside the extensor digitorum tendons.

Function

The extensor indicis provides:

• Independent Extension of the Index Finger: Enables isolated extension at the metacarpophalangeal, proximal interphalangeal, and distal interphalangeal joints of the index finger.
• Assistance in Wrist Extension: Contributes to wrist extension when acting with other extensors.
• Fine Motor Control: Enhances precision grip and dexterity of the index finger.

Innervation

Supplied by the posterior interosseous nerve (C7–C8), which is the continuation of the deep branch of the radial nerve.

Blood Supply

• Posterior interosseous artery: The primary vascular supply.

Relations

The extensor indicis lies medial to the extensor pollicis longus and deep to the extensor digitorum. Its tendon merges into the dorsal digital expansion of the index finger, often appearing slightly medial to the extensor digitorum tendon of the same digit.

Surface Anatomy

Though not individually visible on the surface, the extensor indicis contributes to the tendon group visible on the dorsal hand during active index finger extension, particularly during isolated extension of the index finger.

Development

The extensor indicis arises from the dorsal muscle mass of the forearm during limb development. It represents a specialization of the extensor system that allows finer, independent digit control in humans.

Anatomical Variations

• The tendon may split or have an accessory slip joining adjacent extensor tendons.
• In rare cases, the muscle may be duplicated or absent.
• Occasionally shares a common belly with the extensor pollicis longus proximally.

Clinical Significance

• Tendon Injury: Though rare, injury to the extensor indicis tendon can impair isolated extension of the index finger and may require surgical repair.
• Tendon Transfer Surgery: Often used as a donor in reconstructive tendon transfer procedures due to its independent action and redundancy with extensor digitorum.
• De Quervain-like Symptoms: Although not classically involved, inflammation around the fourth compartment can occasionally affect this tendon.

Comparative Anatomy

In other primates and mammals, this muscle is often absent or not distinctly developed. In humans, it has evolved to enhance independent control of the index finger, contributing to complex tool use and fine hand function.