Dorsal interossei abduct the fingers at the MCP joints and assist in MCP flexion and IP extension.
The dorsal interossei muscles are four bipennate intrinsic muscles located between the metacarpal bones on the dorsal side of the hand. They function primarily to abduct the fingers away from the midline (the third digit) and also assist in flexion at the metacarpophalangeal joints and extension at the interphalangeal joints. These muscles are essential for coordinated finger movement, particularly during object manipulation and fine motor tasks.
The dorsal interossei lie in the dorsal intermetacarpal spaces, positioned deep within the hand between adjacent metacarpal bones. Despite their location on the dorsal aspect, their insertions and actions are exerted on the palmar side of the fingers through the extensor expansions.
There are four dorsal interossei muscles. Each arises by two heads from adjacent sides of neighboring metacarpals and inserts into the extensor expansion and base of the proximal phalanx.
The dorsal interossei muscles have the following actions:
All dorsal interossei are innervated by the deep branch of the ulnar nerve (C8–T1).
These muscles lie between the metacarpals and are deep to the extensor tendons on the dorsal surface. Palmar to them are the interosseous spaces, lumbrical muscles, and flexor tendons.
Though not distinctly visible, the 1st dorsal interosseous is palpable in the web space between the thumb and index finger, especially during finger abduction. The others contribute to the fullness and depth of the dorsal hand between the metacarpals.
The dorsal interossei arise from the dorsal mesenchymal mass of the developing limb. Their bipennate structure reflects their role in producing balanced lateral movement and abduction.
In non-human primates and quadrupeds, dorsal interossei may be present but are less specialized. In humans, their development supports the refined abduction and coordination needed for advanced hand functions like writing, typing, and musical instrument playing.