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Third plantar interosseous muscle of foot

The third plantar interosseous muscle is one of the three plantar interossei located in the fourth (deepest) layer of the sole of the foot. It lies on the medial side of the fifth metatarsal bone, occupying the intermetatarsal space between the fourth and fifth metatarsals. Its primary role is adduction of the fifth toe toward the fourth toe, along with flexion at the metatarsophalangeal (MTP) joint and extension at the interphalangeal (IP) joints via its connection to the extensor expansion.

This small but functionally significant muscle contributes to toe stability, balance, and maintenance of the transverse arch of the foot during stance and propulsion. It acts in coordination with other plantar interossei to provide fine motor control of the toes and enhance forefoot stability during gait.

Synonyms

  • Third plantar interosseous of the foot

  • Plantar interosseus III

  • Adductor of the fifth toe

Origin, Course, and Insertion

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

  • Course: Fibers run obliquely forward and distally along the medial aspect of the fifth metatarsal, deep to the flexor tendons and lumbricals, within the fourth muscular layer of the sole

  • Insertion: Medial side of the base of the proximal phalanx of the fifth toe and the medial slip of the extensor hood of the same digit

Tendon Attachments

  • The terminal tendon attaches to the medial side of the proximal phalanx of the fifth toe, blending with the extensor expansion

  • Supported by the deep transverse metatarsal ligament, which maintains intermetatarsal stability

  • Works synergistically with the fourth dorsal interosseous for balanced control of the fifth toe

Relations

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

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

  • Medially: Fourth dorsal interosseous muscle

  • Laterally: Fifth metatarsal and abductor digiti minimi muscle

  • Anteriorly: Flexor tendons of the toes

  • 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

  • Plantar metatarsal artery (from the deep plantar arch)

  • Minor contribution from perforating branches of the dorsal metatarsal arteries

Venous Drainage

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

Function

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

  • Flexion: Assists in flexion at the fifth toe’s MTP joint

  • Extension: Helps extend interphalangeal joints through the extensor expansion

  • Arch support: Contributes to maintenance of the transverse and lateral longitudinal arches

  • Balance and propulsion: Provides dynamic stabilization of the lateral forefoot during walking and push-off

Clinical Significance

  • Atrophy or weakness: May occur in lateral plantar nerve injury or chronic foot neuropathy (e.g., diabetes, tarsal tunnel syndrome)

  • Intrinsic muscle strain: From overuse or excessive toe gripping during gait

  • Denervation changes: Seen in chronic nerve compression or radiculopathy

  • Biomechanical relevance: Essential for preventing excessive toe splay and maintaining forefoot balance

  • Imaging importance: MRI evaluates intrinsic muscle denervation, ischemia, or inflammation in neuropathic foot conditions

MRI Appearance

  • T1-weighted images:

    • Normal muscle: intermediate signal intensity with distinct fascicular pattern

    • Surrounding intermuscular fat: bright signal

    • Tendon: low signal (dark) inserting into the proximal phalanx

    • Chronic denervation: muscle atrophy with bright fatty infiltration

  • T2-weighted images:

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

    • Acute strain or inflammation: bright hyperintense areas within muscle belly or tendon insertion

    • Denervation: diffuse mild hyperintensity with reduced fascicular detail

  • STIR:

    • Normal: intermediate-to-dark signal

    • Pathology (strain, edema, myositis): bright hyperintense signal with loss of fascicular texture

    • Useful for detecting early edema and subtle soft-tissue inflammation

  • Proton Density Fat-Saturated (PD FS):

    • Normal: intermediate-to-dark homogeneous signal

    • Pathology: focal or diffuse bright signal representing edema, microtears, or inflammation

    • Excellent for identifying perimuscular fluid and intermetatarsal fascial thickening

  • T1 Fat-Sat Post-Contrast:

    • Normal: mild homogeneous enhancement

    • Inflammation or post-traumatic changes: show diffuse or patchy enhancement

    • Chronic denervation: minimal enhancement with fatty infiltration pattern

CT Appearance

Non-Contrast CT:

  • Muscle: small, uniform soft-tissue density along the medial side of the fifth metatarsal

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

  • Chronic denervation: muscle thinning with increased surrounding fat density

  • Calcifications: uncommon, may occur with chronic fibrosis or trauma

Post-Contrast CT (standard):

  • Normal: uniform mild enhancement

  • Pathologic muscle: focal or diffuse hyperenhancement indicating inflammation or hypertrophy

  • Useful for identifying osseous changes, nerve entrapment, or intermetatarsal space lesions

MRI image

Third plantar interosseous muscle of foot axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

CT image

Third plantar interosseous muscle ct axial image