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Second plantar interosseous muscle of foot

he second plantar interosseous muscle is a small, deep intrinsic muscle of the foot, located in the fourth (deepest) layer of the plantar musculature. It lies on the medial side of the fourth metatarsal bone, occupying the intermetatarsal space between the third and fourth metatarsals.

It functions primarily to adduct the fourth toe toward the second toe, assisting in metatarsophalangeal joint (MTP) flexion and interphalangeal joint (IP) extension through its connection with the extensor expansion. Like the other plantar interossei, it contributes to stabilization of the transverse arch and aids in fine balance during gait.

Synonyms

  • Second plantar interosseous of the foot

  • Plantar interosseus II

  • Adductor of the fourth toe

Origin, Course, and Insertion

  • Origin: Medial surface and base of the fourth metatarsal bone

  • Course: Fibers run distally and slightly obliquely along the medial aspect of the fourth metatarsal, deep to the flexor tendons and lumbricals in the fourth layer of the sole

  • Insertion: Medial side of the base of the proximal phalanx of the fourth toe, blending with the medial portion of its extensor expansion

Tendon Attachments

  • The tendon blends with the extensor expansion of the fourth toe to assist coordinated toe movement

  • Anchored distally by the deep transverse metatarsal ligament, maintaining metatarsal spacing and stability during flexion

Relations

  • Superiorly (dorsal): Shafts of the third and fourth metatarsals, deep plantar arch, and plantar metatarsal vessels

  • Inferiorly (plantar): Deep transverse metatarsal ligament and plantar aponeurosis

  • Medially: First plantar interosseous muscle

  • Laterally: Third plantar interosseous muscle

  • Anteriorly: Flexor tendons and lumbrical to the fourth toe

  • Posteriorly: Interosseous fascia and deep plantar vessels

Nerve Supply

  • Deep branch of the lateral plantar nerve (S2–S3), branch of the tibial nerve

Arterial Supply

  • Fourth plantar metatarsal artery (branch of the deep plantar arch)

  • Supplementary supply from dorsal perforating branches of the dorsal metatarsal arteries

Venous Drainage

  • Plantar venous arch and accompanying veins of the fourth plantar metatarsal artery, draining into the posterior tibial vein

Function

  • Adduction: Draws the fourth toe medially toward the second toe (axis of the foot)

  • Flexion: Assists flexion at the fourth MTP joint

  • Extension: Contributes to extension at the interphalangeal joints via the extensor expansion

  • Stabilization: Helps maintain the transverse arch and digital alignment

  • Balance control: Provides fine adjustment of forefoot position during stance and propulsion

Clinical Significance

  • Atrophy: Seen in neuropathies, chronic immobilization, or intrinsic muscle wasting from nerve injury

  • Nerve injury: Damage to the lateral plantar nerve causes weakness of toe adduction and collapse of the transverse arch

  • Denervation changes: Common in diabetic neuropathy or tarsal tunnel syndrome

  • Compartmental pathology: May become edematous in plantar interosseous compartment syndrome

  • Imaging importance: MRI is key for assessing intrinsic muscle atrophy, ischemia, or nerve-related changes

MRI Appearance

  • T1-weighted images:

    • Normal muscle: intermediate signal intensity with clear fascicular pattern

    • Surrounded by bright plantar and intermuscular fat

    • Tendon: low signal (dark line) at insertion

    • Chronic denervation: muscle atrophy with bright fatty infiltration

  • T2-weighted images:

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

    • Pathology: bright hyperintense foci indicating edema, inflammation, or microtear

    • Denervation: diffuse mild hyperintensity with reduced fascicular definition

  • STIR:

    • Normal: intermediate-to-dark signal intensity

    • Pathologic: bright hyperintense signal representing edema or myositis

    • Very sensitive to early interstitial or fascial fluid changes

  • Proton Density Fat-Saturated (PD FS):

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

    • Pathology: focal or diffuse bright signal corresponding to strain, inflammation, or ischemia

    • Ideal for detecting subtle perimuscular edema and compartmental involvement

  • T1 Fat-Sat Post-Contrast:

    • Normal: mild uniform enhancement

    • Inflammation or trauma: focal or patchy enhancement

    • Chronic denervation: minimal enhancement with fatty infiltration

CT Appearance

Non-Contrast CT:

  • Muscle: small, homogeneous soft-tissue density located along the medial aspect of the fourth metatarsal

  • Tendon: thin, low-density structure inserting into the base of the proximal phalanx

  • Chronic denervation: decreased muscle volume and increased fat density

  • Calcific myopathy or fibrosis may appear as focal dense deposits

Post-Contrast CT (standard):

  • Normal: homogeneous enhancement

  • Inflamed or hypertrophic regions: focal hyperenhancement

  • Excellent for visualizing bony margins, intermetatarsal lesions, or entrapment pathology in the fourth interosseous space

MRI image

Second Plantar Interosseous Muscle of foot axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

CT image

Second Plantar Interosseous Muscle ct axial image