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First Dorsal Interosseous (Detail)

musculus interosseus dorsalis primus manus detail

The first DI is the largest dorsal interosseous and the first intrinsic to show wasting in ulnar nerve palsy. Its belly is visible between the first and second metacarpals.

Nerve: Deep branch of ulnar nerve (C8, T1) Blood Supply: First palmar metacarpal artery Region: Hand & Wrist

Anatomical Data

Origin, Insertion & Supply

Origin	Lateral first metacarpal (radial head); Medial second metacarpal (ulnar head)
Insertion	Radial base of index proximal phalanx and extensor expansion
Nerve Supply	Deep branch of ulnar nerve (C8, T1)
Blood Supply	First palmar metacarpal artery

Biomechanics

Function & Actions

Actions	Abducts the index finger; Assists index MCP flexion with IP extension via the extensor expansion

Clinical Relevance

Clinical Notes

First DI wasting — flattening of the dorsal first web space — is the earliest clinical sign of ulnar neuropathy. Froment sign co-occurs with first DI wasting. EMG of the first DI is the standard test for ulnar nerve integrity.

Palpation

Visible on dorsal hand between thumb and index metacarpals during resisted index abduction.

Pathology

Common Injuries & Conditions

First DI Wasting in Ulnar Neuropathy

Flattening of the first web space from ulnar denervation, the earliest visible sign of ulnar neuropathy, confirmed by EMG.