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Third lumbrical muscle of hand

The third lumbrical muscle is one of four slender intrinsic muscles of the palm, situated deep to the palmar aponeurosis and along the radial sides of the flexor digitorum profundus tendons. It is a unipennate muscle that originates from the radial side of the tendon of the flexor digitorum profundus to the middle finger and inserts into the extensor expansion of the ring finger.

The lumbricals act as important link muscles between the flexor and extensor compartments, simultaneously flexing the metacarpophalangeal (MCP) joints and extending the interphalangeal (IP) joints. This unique action is critical for fine motor control, grip precision, and coordinated hand movements such as writing or typing.

Synonyms

  • Third lumbrical of the hand

  • Lumbricalis III

Origin, Course, and Insertion

Origin: Radial side of the flexor digitorum profundus tendon to the middle finger
Course: The slender muscle belly passes distally, superficial to the interossei and deep to the deep palmar arch, coursing along the radial side of the fourth metacarpal
Insertion: Lateral side of the extensor expansion on the dorsal surface of the ring finger (fourth digit)

Relations

Anteriorly: Palmar aponeurosis and superficial palmar arch
Posteriorly: Deep palmar arch and interosseous muscles
Medially: Fourth lumbrical and tendon of flexor digitorum profundus to ring finger
Laterally: Second lumbrical and flexor tendons to middle finger

Tendon Attachments

  • The lumbrical tendon inserts into the radial side of the dorsal digital expansion of the ring finger

  • Interdigitates with fibers of interosseous muscles and the extensor digitorum tendon, forming part of the extensor hood

  • Functions as a dynamic link between flexor and extensor systems, balancing finger posture during fine motor activity

Nerve Supply

  • Median nerve (C8, T1) — the first and second lumbricals are typically supplied by the median nerve; the third lumbrical shows variation

  • In most individuals, the third lumbrical receives dual innervation from both median and deep branch of the ulnar nerve

Arterial Supply

  • Superficial and deep palmar arches, with small muscular branches from the common palmar digital arteries

Function

  • Flexes metacarpophalangeal joints of the ring finger

  • Extends interphalangeal joints, balancing flexor and extensor forces

  • Stabilizes fingers during grip and precision tasks

  • Contributes to coordinated digital motion, particularly in writing, pinching, and object manipulation

Clinical Significance

  • Lumbrical weakness or paralysis: Causes imbalance between flexion and extension at MCP and IP joints

  • Entrapment or injury: May occur in carpal tunnel syndrome due to proximity of the muscle origin to the flexor tendons

  • Overuse or strain: Seen in repetitive fine-motor tasks or musicians, presenting with palmar pain and fatigue

  • Surgical relevance: Important for flap dissection and tendon transfers in intrinsic hand repair

  • Imaging importance: Evaluated in trauma, nerve lesions, or compartment syndromes affecting hand function

MRI Appearance

T1-weighted images:

  • Normal muscle: intermediate signal intensity with clear fascicular pattern

  • Tendon: low signal (dark), continuous from muscle belly to extensor expansion

  • Fat between lumbricals and interossei: bright, providing contrast for easy identification

  • Denervation or atrophy: increased fatty replacement, appearing hyperintense

T2-weighted images:

  • Normal muscle: intermediate-to-dark signal, slightly darker than on T1

  • Tendon: low signal

  • Pathology: bright hyperintense areas in muscle fibers indicating edema, strain, or inflammation

  • Chronic denervation: loss of muscle bulk with intermediate signal and fatty infiltration

STIR:

  • Normal lumbrical: intermediate-to-dark signal

  • Pathology: bright hyperintense signal consistent with edema or myositis

  • Useful for detecting early inflammation or compartmental changes in palm

Proton Density Fat-Saturated (PD FS):

  • Normal: intermediate-to-dark, homogeneous muscle signal

  • Pathologic: bright hyperintense foci in acute strain or neuritic inflammation

  • Ideal for subtle tendon or myofascial interface abnormalities

T1 Fat-Sat Post-Contrast:

  • Normal muscle: mild uniform enhancement

  • Active inflammation or myositis: diffuse or patchy enhancement

  • Postoperative or traumatic fibrosis: peripheral rim enhancement with central low-signal scarring

CT Appearance

Non-Contrast CT:

  • Muscle: soft-tissue density within palmar compartment, between flexor tendons and metacarpal shafts

  • Identified by its slender, oblique orientation lateral to flexor tendons

  • Pathology: muscular swelling or loss of definition in trauma or infection

Post-Contrast CT (standard):

  • Muscle enhances uniformly

  • Inflammation, abscess, or postoperative scarring shows focal enhancement

  • Helpful in evaluating space-occupying lesions, foreign bodies, or deep palmar infections

MRI image

Third lumbrical muscle of hand of hand coronal cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

MRI image

Third lumbrical muscle of hand of hand coronal cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00001

MRI image

Third lumbrical muscle of hand of hand axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

MRI image

Third lumbrical muscle of hand of hand axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00001

MRI image

Third lumbrical muscle of hand of hand axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00002

CT image

third lumbrical muscle of hand ct axial image