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Fourth dorsal interosseous muscle of hand

The fourth dorsal interosseous muscle is the most ulnar of the four dorsal interossei in the hand. It occupies the fourth intermetacarpal space between the fourth and fifth metacarpal bones. This muscle functions primarily to abduct the ring finger (fourth digit) away from the middle finger and assists in flexion of the metacarpophalangeal (MCP) joint and extension of the interphalangeal joints.

As part of the intrinsic hand musculature, it contributes to fine motor control and grip stabilization. The dorsal interossei are bipennate muscles, each originating from adjacent metacarpal shafts and inserting onto the extensor expansions and bases of proximal phalanges.

Synonyms

  • Fourth dorsal interosseus

  • Dorsal interosseous of the ring finger

  • Intermetacarpal abductor muscle

Origin, Course, and Insertion

  • Origin: Adjacent sides of the fourth and fifth metacarpal shafts.

  • Course: Fibers converge toward the dorsal aspect of the ring finger, passing beneath the extensor tendons and lumbrical muscles.

  • Insertion: Lateral side (radial aspect) of the base of the proximal phalanx of the ring finger and into the dorsal digital expansion.

Relations

  • Dorsally: Extensor tendons of the ring and little fingers.

  • Palmar aspect: Palmar interossei and deep palmar arch.

  • Laterally: Third dorsal interosseous muscle.

  • Medially: Fifth metacarpal bone and hypothenar muscles.

  • Deep surface: Interosseous membrane and deep palmar metacarpal vessels.

  • Superficial surface: Dorsal fascia and subcutaneous tissue of the dorsum of the hand.

Tendon Attachments

  • The tendon merges with the extensor expansion over the proximal phalanx of the fourth digit.

  • It contributes to the extensor hood mechanism, enabling coordinated finger flexion at the MCP joint and extension at the PIP and DIP joints.

Nerve Supply

  • Deep branch of the ulnar nerve (C8–T1), derived from the lower trunk of the brachial plexus.

Arterial Supply

  • Dorsal metacarpal arteries (especially the fourth dorsal metacarpal branch) arising from the dorsal carpal arch.

  • Contributions from the deep palmar arch.

Function

  • Primary action: Abducts the ring finger away from the middle finger (axis of the hand).

  • Secondary actions:

    • Assists in flexion of the MCP joint.

    • Aids extension of the PIP and DIP joints via the extensor expansion.

  • Stabilization: Maintains proper finger spacing and alignment during grip and fine manipulative tasks.

  • Synergy: Works in coordination with lumbricals and other interossei for precise digital movements.

Clinical Significance

  • Ulnar nerve palsy: Paralysis of dorsal interossei causes loss of finger abduction and weakened grip strength.

  • Intrinsic hand wasting: Flattening of dorsal intermetacarpal spaces seen in chronic neuropathies.

  • Overuse or strain: May occur in musicians, typists, or manual workers, leading to dorsal hand pain.

  • Compartment syndrome: The interosseous compartment may become affected in hand trauma.

  • Imaging relevance: Important landmark in assessing ulnar neuropathy, muscle atrophy, and soft-tissue masses of the hand.

MRI Appearance

  • T1-weighted images:

    • Muscle belly: intermediate signal intensity, sharply defined between metacarpal bones.

    • Tendon: low signal (dark).

    • Perimuscular fat: bright, delineating muscle borders.

    • Atrophy or fatty infiltration: increased intramuscular brightness on T1.

  • T2-weighted images:

    • Normal muscle: intermediate-to-low signal, darker than on T1.

    • Tendon: low signal.

    • Edema or myositis: bright hyperintense foci within the muscle.

    • Chronic denervation: muscle appears darker and smaller with fatty replacement margins.

  • STIR:

    • Normal muscle: intermediate-to-dark signal intensity.

    • Pathology: bright hyperintensity representing edema, inflammation, or acute denervation.

    • Useful for detecting early neuropathic changes in ulnar nerve palsy.

  • Proton Density Fat-Saturated (PD FS):

    • Normal muscle: intermediate-to-dark, uniform signal.

    • Pathologic: bright focal signal areas in edema, strain, or partial tear.

    • Highlights soft-tissue edema and small peritendinous fluid collections.

  • T1 Fat-Sat Post-Contrast:

    • Normal muscle: mild uniform enhancement.

    • Inflammation or infection: marked enhancement with indistinct borders.

    • Chronic atrophy or fibrosis: minimal enhancement with low-signal fibrotic strands.

CT Appearance

Non-Contrast CT:

  • Muscle: soft-tissue density, seen between fourth and fifth metacarpals.

  • Bone margins: clearly defined; muscle appears as a wedge-shaped structure.

  • Useful for detecting calcifications, ossifications, or post-traumatic changes in intermetacarpal spaces.

  • Chronic atrophy: reduced muscle bulk with increased perimuscular fat attenuation.

Post-Contrast CT (standard):

  • Normal muscle: homogeneous enhancement.

  • Inflammatory or infectious processes: localized hyperenhancement.

  • Mass lesions or fibrosis: irregular enhancement or attenuation changes.

  • Helpful for differentiating soft-tissue tumors, abscesses, or fibrotic scars near intermetacarpal spaces.

MRI image

Fourth dorsal interosseous muscle of hand of hand axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00001

MRI image

Fourth dorsal interosseous muscle of hand of hand coronal cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

CT image

Fourth dorsal interosseous muscle ct axial image