First dorsal interosseous muscle

The first dorsal interosseous muscle (a.k.a. the abductor indicis) is the largest of the dorsal interossei. It occupies the first metacarpal space, situated between the thumb and index finger..

Origin

This flat, triangular muscle arises by two distinct heads from the adjacent sides of the first and second metacarpal bones.

• The lateral head originates from the proximal half of the ulnar border of the first metacarpal bone.

• The medial head arises from nearly the entire length of the radial border of the second metacarpal bone.

A fibrous arch separates these two heads, providing a passage for the radial artery as it travels from the dorsum of the hand into the palm.

Insertion

The muscle's tendon inserts into the radial side of the base of the proximal phalanx of the index finger and its extensor hood.

Action

Functionally, the primary role of the first dorsal interosseous muscle is to abduct (spread) the index finger away from the midline of the hand (which runs through the middle finger). Furthermore, due to its connection to the dorsal aponeurosis, it also assists in flexion at the metacarpophalangeal (MCP) joint and extension at the interphalangeal (IP) joints of the index finger.

Innervation

Like all dorsal interossei muscles of the hand, it is innervated by the deep branch of the ulnar nerve (derived from spinal segments C8 and T1).