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Third plantar interosseous muscle of hand

The third plantar (palmar) interosseous muscle of the hand is one of three palmar interossei located in the deep layer of the hand’s anterior compartment. It is the most ulnar (medial) of the palmar interossei and is primarily responsible for adduction of the little finger toward the ring finger, as well as assisting in flexion at the metacarpophalangeal (MCP) joint and extension at the interphalangeal joints through its connection with the extensor expansion.

The muscle is small and triangular, lying deep to the flexor tendons and lumbricals, and forming part of the intricate musculature that stabilizes and controls fine finger motion.

Synonyms

  • Third palmar interosseous muscle

  • Interosseous adductor of the little finger

  • Deep adductor of the fifth digit

Origin, Course, and Insertion

  • Origin: Ulnar (medial) side of the shaft of the fifth metacarpal bone.

  • Course: Fibers run obliquely distally and dorsally toward the base of the proximal phalanx.

  • Insertion: Medial side of the base of the proximal phalanx of the little finger and into the extensor expansion.

Relations

  • Superficial: Flexor tendons and lumbrical of the little finger.

  • Deep: Dorsal interosseous muscles and metacarpal shaft.

  • Medial: Hypothenar muscles (especially the deep head of flexor digiti minimi brevis).

  • Lateral: Fourth metacarpal and adjacent interosseous spaces.

  • Posterior: Interosseous membrane and deep palmar arch.

Tendon Attachments

  • The tendon blends with the extensor hood and fibrous digital sheath of the little finger.

  • Contributes to both MCP joint flexion and IP joint extension by its dual attachment (palmar and dorsal fibers).

  • Works synergistically with the lumbrical of the fifth digit and hypothenar group.

Nerve Supply

  • Deep branch of the ulnar nerve (C8, T1).

Arterial Supply

  • Deep palmar arch (branch of the radial artery).

  • Palmar metacarpal arteries supplying the interosseous spaces.

Function

  • Adduction: Draws the little finger toward the ring finger (midline of the hand).

  • Flexion: Aids in flexing the metacarpophalangeal joint.

  • Extension: Through extensor expansion, assists in extending the interphalangeal joints.

  • Stabilization: Helps maintain alignment of the fifth digit during grasp and fine motor actions.

  • Synergistic action: Works with hypothenar muscles to support precision grip and object manipulation.

Clinical Significance

  • Ulnar nerve injury: Causes weakness or paralysis of palmar interossei, leading to impaired finger adduction and weakened grip strength.

  • Intrinsic minus (claw hand) deformity: Results from chronic ulnar nerve palsy affecting interossei and lumbricals.

  • Atrophy: Chronic disuse or neuropathy results in visible hollowing of the intermetacarpal space.

  • Muscle strain or fibrotic change: Rare but may occur due to repetitive hand use in musicians or manual workers.

  • Imaging importance: MRI used to assess interosseous muscle atrophy, edema, denervation, or mass lesions.

MRI Appearance

  • T1-weighted images:

    • Normal muscle: intermediate signal intensity with distinct fascicular pattern.

    • Surrounded by bright fat outlining the intermetacarpal space.

    • Chronic denervation or fatty infiltration: bright signal within muscle.

  • T2-weighted images:

    • Normal muscle: intermediate-to-low signal, slightly darker than T1.

    • Acute denervation or edema: bright hyperintense appearance.

    • Fat planes remain well defined.

  • STIR:

    • Normal muscle: intermediate-to-dark signal intensity.

    • Pathologic muscle (edema, inflammation, or strain): bright hyperintense signal.

  • Proton Density Fat-Saturated (PD FS):

    • Normal: intermediate-to-dark, homogeneous appearance.

    • Denervation or inflammation: bright hyperintense regions with loss of fascicular detail.

    • Excellent for identifying early intramuscular edema or mass effect.

  • T1 Fat-Sat Post-Contrast:

    • Normal muscle: uniform mild enhancement.

    • Inflammatory or neoplastic lesions: focal or diffuse enhancement.

    • Chronic fibrotic change: minimal enhancement, central low-signal areas.

CT Appearance

Non-Contrast CT:

  • Muscle seen as a soft-tissue density within the intermetacarpal space.

  • Distinct from surrounding fat and tendons.

  • Useful for evaluating calcifications, foreign bodies, or post-traumatic changes.

Post-Contrast CT (standard):

  • Muscle shows mild homogeneous enhancement.

  • Inflamed or infiltrated muscle: increased attenuation and regional enhancement.

  • Helpful in detecting soft-tissue tumors, infections, or deep space abscesses within the palm.

MRI image

Third plantar (palmar) interosseous muscle of hand of hand axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

MRI image

Third plantar (palmar) interosseous muscle of hand of hand axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00001

CT image

Third plantar (palmar) interosseous muscle ct axial image