Fourth Lumbrical Muscle of Hand: Anatomy, Function, Imaging

Topic

Fourth lumbrical muscle of hand

The fourth lumbrical muscle is the most medial and smallest of the four lumbrical muscles of the hand. It is a slender, elongated intrinsic muscle located in the palm, deep to the palmar aponeurosis and flexor tendons. The lumbricals link the flexor and extensor systems of the fingers, enabling simultaneous metacarpophalangeal (MCP) flexion and interphalangeal (IP) extension—a key movement pattern in hand function and precision grip.

The fourth lumbrical arises from the tendons of the flexor digitorum profundus (FDP) for the ring and little fingers and inserts into the extensor expansion of the little finger, functioning as a fine motor coordinator during finger extension.

Synonyms

Lumbrical IV
Medial lumbrical of the hand

Origin, Course, and Insertion

Origin: Adjacent sides of the tendons of the flexor digitorum profundus for the ring and little fingers (bipennate origin).
Course: The muscle passes distally and radially across the palm, superficial to the interossei and deep to the flexor tendons.
Insertion: Lateral side of the extensor expansion of the fifth digit (little finger), dorsal to the metacarpophalangeal joint.

Relations

Superficially: Palmar aponeurosis and flexor tendons of the little and ring fingers.
Deeply: Palmar interosseous muscles and metacarpal bones.
Medially: Fifth metacarpal and hypothenar muscles.
Laterally: Third lumbrical and deep palmar arch.

Nerve Supply

Deep branch of the ulnar nerve (C8–T1), which innervates the third and fourth lumbricals.

Arterial Supply

Superficial and deep palmar arches (branches of ulnar and radial arteries).
Small twigs from common palmar digital arteries.

Function

Flexion at MCP joint of the little finger.
Extension at proximal and distal IP joints via the extensor expansion.
Fine digital control: Coordinates precision grip and release actions.
Synergistic function: Works with interossei to balance MCP/IP joint activity.
Postural maintenance: Stabilizes the little finger during small object manipulation.

Clinical Significance

Ulnar nerve palsy: Results in weakness or paralysis of the fourth lumbrical, contributing to the characteristic claw hand deformity (hyperextension at MCP, flexion at IP).
Overuse strain: May occur in repetitive gripping or fine motor activities.
Tenosynovitis: Inflammation of adjacent flexor tendons may cause pain or dysfunction of lumbricals.
Intrinsic tightness: Leads to limited finger flexion when MCP joints are extended (positive Bunnell–Littler test).
Surgical importance: Recognized as an anatomical landmark in carpal tunnel release and ulnar tunnel decompression.

MRI Appearance

T1-weighted images:

Muscle belly: intermediate signal intensity with fine fascicular architecture.
Surrounding fat: bright, delineating muscle margins clearly.
Atrophy (denervation or chronic palsy): increased fatty replacement, appearing hyperintense on T1.

T2-weighted images:

Normal muscle: intermediate-to-dark signal, slightly darker than T1.
Pathology (strain, edema): bright hyperintense areas within the muscle.
Chronic denervation: overall muscle volume loss with fat infiltration (mixed high and intermediate signal).

STIR:

Normal: intermediate-to-dark signal.
Pathology: bright hyperintense regions indicating edema, myositis, or injury.
Denervation edema: uniform bright signal in early neuropathy stages.

Proton Density Fat-Saturated (PD FS):

Normal: intermediate-to-dark homogeneous signal.
Abnormal: focal or diffuse bright hyperintensity in acute injury or inflammation.
Detects subtle interstitial edema and early denervation changes.

T1 Fat-Sat Post-Contrast:

Normal: mild homogeneous enhancement.
Inflammation or myositis: focal or diffuse enhancement.
Denervation: early mild enhancement; chronic stages show poor enhancement due to fatty replacement.

CT Appearance

Non-Contrast CT:

Muscle: soft-tissue density with smooth contours.
Adjacent tendons and fascia clearly defined in high-resolution images.
Chronic atrophy: muscle volume reduction and increased perimuscular fat density.

Post-Contrast CT (standard):

Muscle: mild homogeneous enhancement.
Inflammation or infection: focal or diffuse increased enhancement.
Useful for evaluating postoperative fibrosis, mass lesions, or calcific tendinitis.

MRI image

CT image

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