The first dorsal interosseous is the largest interosseous muscle, forming the visible bulk of the first web space on the dorsal hand. It is the primary index finger abductor and a key lateral pinch muscle. Its visible wasting in ulnar nerve palsy produces the characteristic hollowing of the first web space. The first DI is the most commonly biopsied intrinsic muscle for neuromuscular disease assessment.
| Origin | Radial surface of the second metacarpal shaft; Ulnar surface of the first metacarpal shaft |
|---|---|
| Insertion | Radial base of the index proximal phalanx and index extensor expansion radially |
| Nerve Supply | Deep branch of the ulnar nerve (C8, T1) |
| Blood Supply | First dorsal metacarpal artery (dorsal branch of radialis indicis) |
| Actions | Index finger abduction (away from the middle finger — radially); Index MCP flexion with IP extension (intrinsic-plus action through the lateral band) |
|---|
First dorsal interosseous atrophy is the first and most visible sign of ulnar nerve motor involvement — the web space hollowing is visible before weakness is reported. Wartenberg's sign (persistent little finger abduction due to interosseous weakness) is the complementary sign of intrinsic weakness. The first DI is preserved in radial nerve palsy (ulnar nerve supply) and thenar paralysis (median nerve territory).
The first DI is prominently palpable in the first web space between the thumb and index metacarpals on the dorsal hand, becoming firm during resisted index finger abduction.
First DI web space hollowing in ulnar neuropathy indicating intrinsic denervation managed with ulnar nerve decompression.
