Upper limb

First Dorsal Interosseous

Musculus interosseus dorsalis I manus

Origin

Adjacent sides of the 1st and 2nd metacarpal bones (bipennate)

Insertion

Radial side of the base of the proximal phalanx of the index finger, with extension into the extensor hood

Function

Abduction of the index finger (away from the middle finger axis); assists MCP flexion and IP joint extension

Innervation

Ulnar nerve, deep (motor) branch

Movement & chain

Works together with the other dorsal interossei as a finger spreader, for example when fanning out the hand; during grip movements it stabilizes the MCP joint in flexion together with the lumbricals while extending the IP joint.

Palpation

Clearly visible and palpable as the rounded muscle belly in the first web space between thumb and index finger, which bulges during active abduction of the index finger against resistance.

Clinical

In progressive ulnar neuropathy, for example cubital tunnel syndrome, this muscle is often the first visibly affected and is routinely included in electromyographic examination of the ulnar nerve.

Segmental innervation

Ulnar nerve, C8-T1.

In clinical practice

The muscle belly between the thumb and index finger (first web space) is a sensitive indicator of ulnar nerve palsy: atrophy here is often the first visible sign.

Region in the model

Upper limb