First Dorsal Interosseous Muscle: Anatomy, Function, Imaging

Topic

First dorsal interosseous muscle of hand

The first dorsal interosseous muscle (FDI) is the largest and most powerful of the four dorsal interossei of the hand. It lies between the thumb and index finger metacarpal bones, forming the muscular web space at the base of the thumb. It plays a crucial role in abducting the index finger, assisting in flexion at the metacarpophalangeal joint, and extension at the interphalangeal joints.

The FDI is a key landmark for hand surgeons, electromyographers, and radiologists, as it is prominently involved in fine motor control and is a sensitive indicator of ulnar nerve function. Its bulk forms the characteristic fullness of the first web space on the dorsum of the hand.

Synonyms

First dorsal interosseus
Abductor of index finger
Radial interosseous muscle of index finger

Origin, Course, and Insertion

Origin: Adjacent sides of the first and second metacarpal bones (thumb and index).
Course: Muscle fibers converge laterally toward the index finger, forming a strong tendon that passes toward the base of the proximal phalanx.
Insertion: Lateral side of the base of the proximal phalanx of the index finger and into the extensor expansion.

Tendon Attachments

The tendon blends with the dorsal digital expansion of the index finger.
Reinforces the extensor hood, allowing coordinated extension at interphalangeal joints and flexion at the metacarpophalangeal joint.
Forms part of the lateral digital slip contributing to index finger abduction.

Relations

Dorsally: Skin and dorsal fascia of the first web space.
Palmarly: Adductor pollicis muscle (oblique head).
Medially: Second dorsal interosseous muscle.
Laterally: First metacarpal bone (thumb).
Proximally: Deep palmar arch passes deep to the muscle.
Distally: Base of proximal phalanx of the index finger.

Nerve Supply

Deep branch of the ulnar nerve (C8–T1).

Arterial Supply

Deep palmar arch and radial artery perforating branches.

Function

Abduction: Abducts the index finger away from the midline of the hand (toward the thumb).
Flexion: Assists in flexion at the metacarpophalangeal joint.
Extension: Aids in extension at both interphalangeal joints via extensor expansion.
Grip stabilization: Provides lateral stability and power during pinch and grasp.
Proprioception: Contributes to fine positional control of the index finger.

Clinical Significance

Ulnar nerve palsy: Wasting of FDI causes hollowing of the first web space and weak index abduction.
Entrapment neuropathy: Commonly evaluated in cubital tunnel and Guyon’s canal syndromes.
Muscle injury: Rare, but can occur in penetrating trauma or post-surgical fibrosis.
Electromyography (EMG): FDI is a key diagnostic muscle for assessing ulnar nerve integrity.
Imaging relevance: MRI identifies denervation changes, atrophy, or mass lesions (ganglion cysts, vascular malformations) affecting the muscle.

MRI Appearance

T1-weighted images:
- Normal muscle: intermediate signal intensity with fine internal fascicular pattern.
- Surrounding fat: bright, outlining the muscle belly.
- Chronic denervation or atrophy: increased fatty infiltration (hyperintense on T1).
- Tendon insertions: low-signal linear bands into the extensor expansion.
T2-weighted images:
- Normal muscle: slightly darker than on T1, intermediate-to-low signal.
- Acute denervation or myositis: bright hyperintense signal within muscle fibers.
- Tendon: low signal (dark) extending toward the index phalanx.
STIR:
- Normal muscle: intermediate-to-dark signal.
- Pathology: bright hyperintensity in edema, acute denervation, or inflammation.
- Useful for detecting subtle soft-tissue swelling or traumatic injury in first web space.
Proton Density Fat-Saturated (PD FS):
- Normal: intermediate-to-dark muscle signal with smooth contours.
- Pathologic: focal or diffuse bright areas representing edema or partial tear.
- Denervation edema: uniform high signal without fiber disruption.
T1 Fat-Sat Post-Contrast:
- Normal muscle: mild, uniform enhancement.
- Myositis or inflammation: patchy or diffuse enhancement.
- Tumor or vascular lesion: intense heterogeneous enhancement.
- Chronic denervation: minimal enhancement with fatty replacement.

CT Appearance

Non-Contrast CT:

Muscle belly: homogeneous soft-tissue density in first web space.
Adjacent fat: low density, clearly defining muscle borders.
Chronic atrophy: reduced bulk with replacement by fat (lower density).
Calcification or ossification: rare, but may appear after trauma or chronic inflammation.

Post-Contrast CT (standard):

Muscle enhances uniformly.
Areas of inflammation, neoplasm, or vascular malformation show focal increased enhancement.
CT defines osseous involvement, foreign bodies, or post-surgical changes in first web space region.

MRI image