Fourth dorsal interosseous muscle

Origin

The fourth dorsal interosseous muscle occupies the fourth metacarpal space, situated between the ring and little finger. It arises by two heads from the adjacent sides of the fourth and fifth metacarpal bones.

• The lateral head originates from the ulnar border of the fourth metacarpal bone.

• The medial head arises from the radial border of the fifth metacarpal bone.

Insertion

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

Action

Functionally, the primary role of the fourth dorsal interosseous muscle is to abduct (spread) the ring finger medially (ulnar deviation), moving it away from the hand's central axis. Additionally, via its connection to the dorsal aponeurosis (extensor hood), it assists in flexion at the metacarpophalangeal (MCP) joint and extension at the interphalangeal (IP) joints of the ring 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).