The four dorsal interossei are bipennate muscles filling the four metacarpal spaces on the dorsal hand, producing finger abduction (spreading apart from the middle finger) and MCP flexion with IP extension through the extensor expansion. Their visible wasting produces the characteristic dorsal guttering between the metacarpals that is one of the most striking physical signs of advanced ulnar nerve palsy and rheumatoid arthritis.
| Origin | Adjacent sides of adjacent metacarpal bones — each muscle has two heads, one from each metacarpal flanking the space |
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| Insertion | Bases of the proximal phalanges and extensor expansions of fingers 2, 3, and 4 |
| Nerve Supply | Deep branch of the ulnar nerve (C8, T1) |
| Blood Supply | Dorsal metacarpal arteries |
| Actions | Abduction of the fingers away from the middle finger axis (DAB mnemonic); Flexion of the MCP joints; Extension of the IP joints through the extensor expansion |
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The DAB mnemonic (Dorsal interossei ABduct) distinguishes them from the palmar interossei which adduct. Together with the lumbricals they are the intrinsic hand muscles that produce the intrinsic plus posture of MCP flexion with IP extension required for precise finger control.
Interosseous muscle wasting producing visible dorsal hand guttering is a hallmark of advanced ulnar nerve palsy and is also seen in rheumatoid arthritis, motor neurone disease, and Charcot-Marie-Tooth disease. The first dorsal interosseous is the most visible and most easily tested, forming the muscle bulk between the thumb and index metacarpals that should be rounded and firm in a healthy hand. Its wasting produces a hollow first web space.
The first dorsal interosseous is palpable as the rounded muscle bulk in the first web space between the index metacarpal and thumb metacarpal, becoming firm during index finger abduction from the thumb.
Progressive dorsal interosseous atrophy from ulnar nerve palsy, rheumatoid arthritis, or anterior horn cell disease producing dorsal metacarpal guttering and loss of finger abduction strength with clawing.
