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Muscular system · Upper limb

Dorsal interossei !

The dorsal interossei are four muscles between the palm bones; they spread the fingers apart from the middle finger.

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What it is

Four muscles between the metacarpals, felt on the back of the hand.

What it does

Spread the fingers apart and help the lumbricals.

Key facts

Origin
Adjacent sides of two metacarpals (two heads each)
Insertion
Extensor expansions and bases of the proximal phalanges of digits 2–4
Action
Abduct the fingers (DAB); flex MCP, extend IP
Nerve
Deep branch of the ulnar nerve (C8–T1)
Blood supply
Dorsal metacarpal arteries, deep palmar arch

Relations

Behind
The extensor digitorum tendons with the juncturae, the dorsal fascia of the hand and the dorsal venous network
In front
The palmar interossei, then the deep palmar arch and the deep branch of the ulnar nerve; the adductor pollicis in front of the first
Contents
Between the two heads of the first, the radial artery on its way from the back of the hand into the palm

How to find it

The first is seen and felt as the bulge on the back of the first web space when the index is pushed toward the thumb against resistance; the others lie between the metacarpals on the back of the hand.

Examination

  • Resisted index abduction (first dorsal interosseous): weakness or a hollow first web points to the ulnar nerve, or to the C8–T1 roots or lower trunk
  • Egawa sign: with the hand flat, the middle finger cannot be rocked from side to side, indicating weakness of the 2nd and 3rd dorsal interossei (ulnar palsy)
  • Bunnell–Littler test: the PIP joint bends less when the MCP joint is held straight than when it is bent, indicating intrinsic tightness

On imaging

On MRI and ultrasound they fill the spaces between the metacarpals, seen best from the back of the hand. They are not imaged on their own for injury. The classic finding is wasting with ulnar nerve palsy, most visible in the first web space; MRI shows it as oedema early and fatty atrophy later. Electrodiagnostic studies, not imaging, confirm the nerve lesion.

Clinical notes

  • Ulnar claw hand reflects loss of the interossei and ulnar lumbricals: MCP joints hyperextend and IP joints flex
  • Intrinsic tightness (after crush injury, swelling or rheumatoid arthritis) pulls the fingers toward the intrinsic-plus position and can lead to swan-neck deformity
  • The interosseous compartments of the hand are released through dorsal incisions over the 2nd and 4th metacarpals in hand compartment syndrome

Anatomical variants

  • Classically all bipennate, but about a quarter were unipennate in one dissection study
  • With a Martin–Gruber anastomosis, median fibres reach the first dorsal interosseous through the ulnar nerve, so it may be spared in a high ulnar lesion

Development

The fingers separate in weeks 6–7 as cells between the digital rays die; the interossei form in the tissue left between the metacarpals. Their C8–T1 supply means a lower plexus birth injury (Klumpke palsy) wastes them and claws the hand.

Memory aid

DAB: Dorsal interossei ABduct, measured from the middle finger, which has two of them because it moves both ways.

Test yourself

Think of the answer, then tap the question to see it.

What do the dorsal interossei do, and around which axis?

They abduct the fingers from the axis of the middle finger; the middle finger has two, the thumb and little finger none.

What is the most visible early sign of an ulnar nerve lesion in the hand?

Wasting of the first dorsal interosseous in the first web space.

What is the Egawa sign?

With the hand flat, the middle finger cannot be rocked from side to side: weak 2nd and 3rd dorsal interossei (ulnar palsy).

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Educational information, not medical advice. For any symptom, see a health professional.

Its names

Official Latin (TA2)
musculi interossei dorsales manus TA2 2533
Official Spanish (DeCS)
Músculos interóseos dorsales de la mano
English (TA2)
Dorsal interossei

Sources: Terminologia Anatomica (TA2); Anatomy, Shoulder and Upper Limb, Hand Dorsal Interossei Muscle; 8.5 Development of the Appendicular Skeleton (OpenStax (Rice University), 2022) · adapted, CC BY 4.0.

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Representative model, for educational purposes only.

Dorsal interossei highlighted on the Durgical 3D model