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Flexor hallucis brevis muscle

The flexor hallucis brevis (FHB) is a short intrinsic muscle of the sole of the foot located in the third muscular layer, deep to the flexor digitorum brevis and tendon of the flexor hallucis longus. It plays a vital role in flexing the proximal phalanx of the great toe, providing stability to the first metatarsophalangeal (MTP) joint during gait and push-off.

The muscle has two distinct bellies (medial and lateral heads), each giving rise to a tendon that contains a sesamoid bone before inserting into the proximal phalanx of the great toe. This anatomical configuration enhances mechanical leverage and protects the flexor hallucis longus tendon that passes between the two sesamoids.

Synonyms

  • Short flexor of the great toe

  • Brevis flexor of the hallux

Origin, Course, and Insertion

  • Origin: Plantar surface of the cuboid and lateral cuneiform bones; lateral part of the tendon of the tibialis posterior

  • Course: Fibers pass forward on the sole, dividing into medial and lateral bellies separated by the flexor hallucis longus tendon

  • Insertion:

    • Medial head: Inserts into the medial side of the base of the proximal phalanx of the hallux via a tendon containing the medial sesamoid bone

    • Lateral head: Inserts into the lateral side of the base of the proximal phalanx via a tendon containing the lateral sesamoid bone

Tendon Attachments

  • Each head forms a tendinous insertion containing a sesamoid bone, located beneath the first metatarsal head

  • The flexor hallucis longus tendon passes between the two sesamoids, sharing a fibrous tunnel

  • Acts in concert with abductor hallucis (medial) and adductor hallucis (lateral) to stabilize the first MTP joint

Relations

  • Superiorly: Tendon of flexor hallucis longus

  • Inferiorly: Plantar aponeurosis

  • Medially: Abductor hallucis muscle

  • Laterally: Adductor hallucis (oblique and transverse heads)

  • Posteriorly: Plantar nerves and vessels

  • Anteriorly: Proximal phalanx of the great toe

Nerve Supply

  • Medial plantar nerve (branch of tibial nerve, roots S1–S2)

Arterial Supply

  • Medial plantar artery (branch of the posterior tibial artery)

  • Contributions from first plantar metatarsal artery

Venous Drainage

  • Medial plantar vein and deep plantar venous plexus, draining into the posterior tibial vein

Function

  • Flexion of great toe: Flexes the proximal phalanx at the first metatarsophalangeal joint

  • Support of longitudinal arch: Assists in maintaining the medial arch during stance

  • Stabilization: Provides medial-lateral balance of the hallux during toe-off

  • Synergistic role: Works with flexor hallucis longus for powerful toe flexion during push-off

Clinical Significance

  • Tendinopathy: Overuse in runners and dancers may cause plantar medial forefoot pain

  • Sesamoiditis: Inflammation around sesamoids can affect FHB tendon mechanics

  • Muscle strain: Occurs with repetitive toe flexion or hyperextension injuries

  • Nerve entrapment: Medial plantar nerve compression may affect FHB function

  • Surgical relevance: Important in hallux valgus correction and sesamoid excision procedures

MRI Appearance

  • T1-weighted images:

    • Muscle belly: intermediate signal intensity with distinct fascicular texture

    • Tendons: low signal (dark linear bands) inserting onto base of proximal phalanx

    • Sesamoids: low signal cortex with intermediate marrow signal

    • Surrounding fat planes appear bright

  • T2-weighted images:

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

    • Tendons: low signal, continuous fibers

    • Pathology: bright hyperintense areas in tendon or myotendinous junction indicating inflammation or partial tear

    • Perisesamoid edema or sesamoiditis: focal hyperintensity adjacent to sesamoid

  • STIR:

    • Normal muscle: intermediate-to-dark signal intensity

    • Pathology: hyperintense regions correspond to edema, muscle strain, or inflammation

    • Useful for detecting early soft-tissue edema or interstitial tears

  • Proton Density Fat-Saturated (PD FS):

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

    • Pathologic: focal or diffuse bright hyperintensity in tendon or muscle indicating tendinitis or tear

    • Accurately depicts peritendinous fluid and reactive changes

  • T1 Fat-Sat Post-Contrast:

    • Normal muscle: uniform mild enhancement

    • Inflamed tendon sheath or sesamoid region: shows focal enhancement

    • Chronic tendinopathy: peripheral rim enhancement with central low-signal fibrosis

CT Appearance

Non-Contrast CT:

  • Muscle: homogeneous soft-tissue density beneath plantar fascia

  • Tendons: thin soft-tissue bands inserting at base of proximal phalanx

  • Sesamoid bones: clearly visualized as dense ovoid ossicles under the first metatarsal head

  • Chronic stress or inflammation: cortical irregularity or sclerosis of sesamoids

Post-Contrast CT (standard):

  • Muscle: homogeneous enhancement pattern

  • Inflamed or hypertrophic FHB tendon and sesamoid regions may show focal enhancement

  • Useful for detecting calcifications, ossifications, or chronic scar tissue around MTP joint

MRI image

Flexor hallucis brevis muscle axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

MRI image

Flexor hallucis brevis muscle coronal cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

MRI image

Flexor hallucis brevis muscle SAGITTAL cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

CT image

Flexor hallucis brevis muscle ct axial image