Second Lumbrical Muscle of Hand: Anatomy, Function, Imaging

Topic

Second lumbrical muscle of hand

The second lumbrical muscle is one of the four small intrinsic muscles of the hand that flex the metacarpophalangeal (MCP) joints and extend the interphalangeal (IP) joints. It is located on the palmar aspect of the hand, deep to the palmar aponeurosis and superficial to the interossei. The second lumbrical lies between the tendons of the index and middle fingers and plays a crucial role in fine digital movements and grip coordination.

It is unipennate, arising from the radial side of the flexor digitorum profundus (FDP) tendon of the index finger, and inserts into the extensor expansion of the middle finger. It is supplied by the median nerve, distinguishing it from the ulnar-innervated third and fourth lumbricals.

Synonyms

Lumbricalis II
Second lumbrical of the hand
Middle finger lumbrical

Origin, Course, and Insertion

Origin: Radial (lateral) side of the flexor digitorum profundus tendon to the index finger.
Course: Passes distally and radially, superficial to the deep palmar arch and deep transverse metacarpal ligament.
Insertion: Lateral side of the dorsal digital expansion (extensor hood) of the middle finger.

Relations

Anteriorly: Palmar aponeurosis and superficial palmar arch
Posteriorly: Deep palmar arch and interossei muscles
Medially: Third lumbrical
Laterally: First lumbrical (associated with index finger)
Superiorly: Flexor tendons of the fingers
Inferiorly: Deep palmar fascia

Nerve Supply

Median nerve (digital branch), roots C8–T1

Arterial Supply

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

Venous Drainage

Superficial palmar venous network draining into the cephalic and basilic veins

Function

Flexion of MCP joint: Assists in flexing the MCP joint of the middle finger.
Extension of IP joints: Extends both PIP and DIP joints through the extensor expansion.
Fine motor control: Coordinates precise finger positioning during writing and gripping.
Synergistic action: Balances flexor digitorum profundus and extensor digitorum forces for smooth digital motion.
Grip stability: Contributes to controlled grip release and digital alignment during manipulation tasks.

Clinical Significance

Intrinsic muscle paralysis: Median nerve lesions (e.g., carpal tunnel syndrome) cause weakness of the first and second lumbricals, leading to impaired precision grip.
Lumbrical hypertrophy: Rare cause of carpal tunnel compression.
Overuse strain: May occur in repetitive finger flexion activities (musicians, typists).
Tenosynovitis: Inflammation of adjacent flexor tendon sheaths may irritate the lumbrical origin.
Surgical importance: Identified during carpal tunnel release and flexor tendon repair.

MRI Appearance

T1-weighted images:

Muscle belly: intermediate signal intensity, sharply defined between flexor tendons and interossei.
Tendons: low signal (dark linear bands).
Surrounding fat: bright, providing contrast.
Atrophy (from denervation): hyperintense fatty replacement.

T2-weighted images:

Normal muscle: intermediate-to-low signal, slightly darker than on T1.
Tendons: persistently dark.
Pathology: bright hyperintense areas in myotendinous junction from strain, inflammation, or edema.

STIR:

Normal lumbrical: intermediate-to-dark signal.
Pathology: bright hyperintense regions indicate edema, inflammation, or denervation.
Excellent for early detection of intrinsic hand muscle injury.

Proton Density Fat-Saturated (PD FS):

Normal: intermediate-to-dark homogeneous signal.
Pathologic: bright focal or diffuse signal in muscle fibers (strain, tear, or neuritis).
Shows subtle perimuscular fluid or fascial plane inflammation clearly.

T1 Fat-Sat Post-Contrast:

Normal lumbrical: mild homogeneous enhancement.
Myositis or inflammatory change: diffuse or patchy enhancement.
Postoperative scar: peripheral enhancement with low-signal fibrotic center.

CT Appearance

Non-Contrast CT:

Muscle belly: homogeneous soft-tissue density in the palmar hand.
Distinct from interossei and flexor tendons by its position and shape.
Chronic atrophy: reduced bulk and increased perimuscular fat density.
Calcification or fibrosis may appear as fine linear or punctate densities.

Post-Contrast CT (standard):

Normal muscle: mild homogeneous enhancement.
Inflammation or infection: focal or diffuse increased enhancement.
Post-surgical fibrosis: irregular enhancement with surrounding soft-tissue thickening.
Useful for identifying space-occupying lesions, fibrosis, or intrinsic muscle wasting.