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First plantar interosseous muscle of foot

The first plantar interosseous muscle is a small, deep intrinsic muscle of the foot situated in the fourth (deepest) layer of the sole. It lies on the medial (inner) side of the third metatarsal bone, between the second and third intermetatarsal spaces. Its primary function is adduction of the third toe toward the second toe, which represents the axis of the foot. It also assists in flexion of the metatarsophalangeal (MTP) joint and extension of the interphalangeal (IP) joints via its connection with the extensor expansion.

This muscle is an important stabilizer of the forefoot and transverse arch, helping maintain proper toe alignment during gait. Despite its small size, it contributes to fine motor control, balance, and medial-lateral stability of the toes during stance and propulsion.

Synonyms

  • First plantar interosseous of the foot

  • Plantar interosseus I

  • Adductor of the third toe

Origin, Course, and Insertion

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

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

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

Tendon Attachments

  • The tendon joins the medial slip of the extensor expansion of the third toe

  • May blend with the fibers of the lumbrical to the same digit near the MTP joint

  • Anchored by the deep transverse metatarsal ligament, which stabilizes the intermetatarsal space

Relations

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

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

  • Medially: Second dorsal interosseous muscle and second metatarsal shaft

  • Laterally: Third dorsal interosseous muscle

  • Anteriorly: Flexor tendons and lumbrical to the third 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

  • 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: Moves the third toe medially toward the second toe (axis of the foot)

  • Flexion: Aids flexion at the third toe’s metatarsophalangeal joint

  • Extension: Assists in extension at interphalangeal joints through the extensor expansion

  • Arch stability: Contributes to maintenance of the transverse arch of the foot

  • Fine motor control: Assists in balance and medial-lateral stabilization of the toes during stance and propulsion

Clinical Significance

  • Intrinsic muscle atrophy: May occur due to neuropathy, chronic disuse, or prolonged immobilization

  • Lateral plantar nerve injury: Weakness or paralysis causes loss of toe adduction and instability of the transverse arch

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

  • Biomechanical importance: Provides forefoot stability and prevents splaying of toes during gait

  • Imaging relevance: MRI helps identify denervation, ischemia, or inflammation within the interosseous compartment

MRI Appearance

  • T1-weighted images:

    • Normal muscle: intermediate signal intensity with visible fascicular architecture

    • Surrounded by bright intermuscular and plantar fat

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

    • Chronic denervation: muscle atrophy with bright fatty infiltration

  • T2-weighted images:

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

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

    • Denervation: diffuse mild hyperintensity with loss of normal striation

  • STIR:

    • Normal: intermediate-to-dark signal intensity

    • Edema or myositis: bright hyperintense signal with poorly defined borders

    • Sensitive for early ischemia, strain, or compartmental fluid

  • Proton Density Fat-Saturated (PD FS):

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

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

    • Excellent for detecting subtle perimuscular fluid or fascial inflammation

  • T1 Fat-Sat Post-Contrast:

    • Normal: mild, uniform enhancement

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

    • Chronic denervation: minimal enhancement with increased fatty infiltration

CT Appearance

Non-Contrast CT:

  • Muscle: small, homogeneous soft-tissue density along the medial side of the third metatarsal

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

  • Chronic denervation: muscle thinning and increased intermuscular fat density

  • Calcifications are rare but may occur in chronic fibrosis or myositis ossificans

Post-Contrast CT (standard):

  • Normal: homogeneous mild enhancement

  • Inflamed or hypertrophic muscle: focal hyperenhancement

  • Useful for evaluating osseous lesions, entrapment, or space-occupying masses in the intermetatarsal space

MRI image

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

CT image

First plantar interosseous muscle of foot axial ct image