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Second plantar interosseous muscle of hand

The second palmar interosseous muscle (PI2) is one of three palmar (plantar) interossei located in the anterior compartment of the hand, deep to the lumbricals and flexor tendons. It lies along the ulnar side of the ring finger (fourth digit) and functions primarily to adduct the ring finger toward the midline (middle finger) at the metacarpophalangeal (MCP) joint.

The palmar interossei are small, triangular intrinsic muscles that play a crucial role in fine motor control, grip stability, and coordination of finger movement. The second palmar interosseous assists lumbricals and flexor muscles in flexing the MCP joint and extending the interphalangeal joints.

Synonyms

  • Second palmar interosseous

  • Second volar interosseous

  • Ulnar interosseous of the ring finger

Origin, Course, and Insertion

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

  • Course: Fibers run anteriorly and distally between the fourth metacarpal and the tendon sheath of the ring finger flexors

  • Insertion:

    • Into the extensor expansion (dorsal digital aponeurosis) on the ulnar side of the proximal phalanx of the ring finger

    • Some fibers blend with lumbrical and deep transverse metacarpal ligament fibers

Tendon Attachments

  • The tendon merges with the extensor expansion and the fibrous digital sheath of the ring finger

  • Reinforces the deep transverse metacarpal ligament, stabilizing the MCP joint during adduction and flexion

  • Acts with lumbricals for combined MCP flexion and IP extension

Relations

  • Superficially: Flexor tendons and lumbrical muscle of the ring finger

  • Deeply: Third dorsal interosseous muscle and metacarpal shaft

  • Medially: Fifth metacarpal and third palmar interosseous

  • Laterally: Fourth metacarpal and extensor tendons of the ring finger

  • Anteriorly: Deep branch of the ulnar artery and nerve (in the intermetacarpal space)

Nerve Supply

  • Deep branch of the ulnar nerve (C8–T1), part of the intrinsic hand muscle group

Arterial Supply

  • Deep palmar arch via palmar metacarpal arteries and small branches from the deep branch of the ulnar artery

Function

  • Adduction: Draws the ring finger toward the middle finger (hand’s midline)

  • MCP joint flexion: Assists lumbricals and flexor digitorum profundus in flexing the MCP joint

  • IP joint extension: Works with extensor expansion to extend proximal and distal interphalangeal joints

  • Grip control: Stabilizes fingers during precision grasp and object manipulation

  • Synergistic action: Functions with dorsal interossei for balanced finger positioning

Clinical Significance

  • Ulnar nerve palsy: Paralysis results in loss of finger adduction and impaired grip strength

  • Intrinsic minus hand: Seen in severe ulnar nerve lesions, leading to clawing deformity

  • Overuse injury: Muscle fatigue or strain may cause intermetacarpal space pain in repetitive manual work

  • Surgical importance: Landmark during ulnar nerve decompression or deep palmar arch reconstruction

  • Imaging importance: MRI aids in diagnosing muscle denervation, atrophy, and space-occupying lesions in the intermetacarpal spaces

MRI Appearance

  • T1-weighted images:

    • Muscle belly: Intermediate signal intensity with clear fascicular pattern

    • Tendinous portion: Low signal (dark)

    • Fat planes: Bright signal separating interossei from adjacent lumbricals and metacarpals

    • Atrophy or denervation: Increased fatty infiltration with bright signal on T1

  • T2-weighted images:

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

    • Pathology: Bright signal in cases of inflammation, edema, or acute denervation

    • Tendon: Uniformly dark; high signal indicates tendinopathy or partial tear

  • STIR:

    • Normal muscle: Intermediate-to-dark signal intensity

    • Pathologic muscle: Bright hyperintensity in edema, infection, or nerve injury

    • Highly sensitive for acute denervation or inflammatory myositis

  • Proton Density Fat-Saturated (PD FS):

    • Normal muscle: Intermediate-to-dark homogeneous signal

    • Abnormal muscle: Focal or diffuse bright areas representing edema, strain, or fascial inflammation

    • Useful for subtle muscle tears and perimuscular fluid collection

  • T1 Fat-Sat Post-Contrast:

    • Normal muscle: Homogeneous mild enhancement

    • Inflammation or infection: Focal enhancement of muscle and fascia

    • Chronic denervation: Minimal enhancement with fatty replacement

    • Tumor or mass lesion: Irregular or nodular enhancement

CT Appearance

Non-Contrast CT:

  • Muscle: Soft-tissue density within intermetacarpal space between 4th and 5th metacarpals

  • Tendon: Linear soft-tissue structure inserting onto base of proximal phalanx

  • Adjacent structures: Easily differentiated by bone density of metacarpals and phalanges

  • Pathology: Calcification, muscle atrophy, or foreign bodies may be visualized

Post-Contrast CT (standard):

  • Muscle: Homogeneous enhancement

  • Inflammation or infection: Enhancing fascial planes and perimuscular soft tissues

  • Mass lesion: Irregular enhancement and expansion of intermetacarpal space

MRI image

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

MRI image

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

CT image

Second plantar (palmar) interosseous muscle ct axial image