First Palmar Interosseous Muscle of Hand: Anatomy, Function, Imaging

Topic

First plantar interosseous muscle of hand

The first palmar interosseous muscle is one of the three palmar (plantar) interossei located in the deep layer of the hand, between the metacarpal bones. It lies on the ulnar side of the index finger, acting primarily to adduct the index finger toward the middle finger.
Although sometimes small or variable in size, it plays an important role in fine motor control, grip strength, and precision movements of the hand, functioning synergistically with the lumbricals and dorsal interossei.

Synonyms

First palmar interosseous muscle
First volar interosseous muscle
Ulnar interosseous of the index finger

Origin, Course, and Insertion

Origin: Arises from the ulnar (medial) side of the first metacarpal shaft, facing the middle finger’s metacarpal.
Course: Fibers pass distally, running deep to the flexor tendons and lumbricals, converging toward the base of the index finger.
Insertion: Inserts into the medial (ulnar) side of the base of the proximal phalanx and the extensor expansion of the index finger.

Tendon Attachments

The tendon contributes to the extensor hood and joins the medial slip of the extensor expansion of the index finger.
It assists both in adduction and extension at the interphalangeal joints through the extensor mechanism.

Relations

Dorsally: Dorsal interosseous muscles and metacarpal bones
Ventrally: Deep palmar arch and lumbricals
Medially: Second palmar interosseous muscle
Laterally: First dorsal interosseous muscle
Superiorly: Deep branch of the ulnar artery and deep palmar branch of the ulnar nerve

Nerve Supply

Deep branch of the ulnar nerve (C8–T1)

Arterial Supply

Deep palmar arch (branch of the radial artery)
Perforating branches of the palmar metacarpal arteries

Function

Adduction: Brings the index finger toward the middle finger (axis of the hand).
Assists extension: Helps extend interphalangeal joints through the extensor hood.
Flexion support: Weakly assists in flexion of the metacarpophalangeal (MCP) joint.
Precision grip: Stabilizes the index finger during fine motor tasks.

Clinical Significance

Ulnar nerve palsy: Leads to weakness or paralysis of palmar interossei, resulting in loss of finger adduction and characteristic “claw hand” deformity.
Muscle atrophy: Chronic neuropathy or disuse may cause wasting of interosseous spaces on the dorsum of the hand.
Hand trauma: Laceration or deep compartment injury can compromise interosseous integrity and dexterity.
Electromyography: Used to assess intrinsic hand muscle function in peripheral neuropathies.
Surgical importance: Deep palmar interossei are vital landmarks during ulnar nerve decompression and intrinsic hand flap procedures.

MRI Appearance

T1-weighted images:

Muscle belly: intermediate signal intensity with clear fascicular pattern.
Surrounding fat: bright, delineating the muscle margins.
Atrophy: decreased bulk, replaced by bright fatty signal.
Acute injury: focal or diffuse intermediate-to-bright signal from edema or hemorrhage.

T2-weighted images:

Normal muscle: intermediate-to-low signal, slightly darker than T1.
Tendon insertions: low signal (dark lines) along phalanx base.
Pathology: tears, edema, or denervation changes show bright hyperintense signal.

STIR:

Normal muscle: intermediate-to-dark signal intensity.
Pathology: bright hyperintense regions representing edema, inflammation, or denervation.
Useful for detecting subtle muscular strain or neuropathic changes.

Proton Density Fat-Saturated (PD FS):

Normal: intermediate-to-dark, uniform muscle texture.
Abnormal: bright hyperintense foci in acute myositis, strain, or nerve injury.
Distinguishes active inflammation from chronic fatty atrophy.

T1 Fat-Sat Post-Contrast:

Normal: homogeneous mild enhancement.
Inflammation or myositis: diffuse enhancement.
Denervation or chronic fibrosis: faint or peripheral enhancement with central low-signal fat infiltration.

CT Appearance

Non-Contrast CT:

Muscle: soft-tissue density with clear borders between metacarpals.
Chronic atrophy: visible volume loss and increased fat attenuation.
Calcification or ossification may appear in post-traumatic or myositis ossificans cases.

Post-Contrast CT (standard):

Normal muscle: mild, uniform enhancement.
Inflamed or hyperemic muscle: shows stronger focal or diffuse enhancement.
Helpful in identifying abscesses, foreign bodies, or soft-tissue mass lesions within the deep palmar space.

MRI image

CT image

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