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Lateral head of flexor hallucis brevis muscle

The lateral head of the flexor hallucis brevis (FHB) is one of two terminal divisions of the FHB muscle, located in the medial plantar compartment of the foot. It arises from the cuboid and lateral cuneiform bones, and passes forward along the plantar surface to insert on the lateral side of the base of the proximal phalanx of the great toe.

The tendon of the lateral head contains the lateral sesamoid bone, which lies beneath the head of the first metatarsal. This sesamoid acts as a pulley to increase the mechanical advantage of the tendon during flexion and to protect the underlying flexor hallucis longus (FHL) tendon, which passes between the medial and lateral heads of FHB.

The lateral head works synergistically with the adductor hallucis to flex and stabilize the great toe at the metatarsophalangeal (MTP) joint during the push-off phase of gait.

Synonyms

  • Lateral portion of flexor hallucis brevis

  • Outer head of flexor hallucis brevis

Origin, Course, and Insertion

  • Origin: Plantar surface of the cuboid bone and lateral cuneiform bone, and from the lateral slip of the tendon of the tibialis posterior

  • Course: Fibers run anteriorly on the plantar aspect of the foot, lateral to the flexor hallucis longus tendon, forming a distinct tendon that passes beneath the first metatarsal head

  • Insertion: Lateral side of the base of the proximal phalanx of the great toe via a tendon that contains the lateral sesamoid bone

Tendon Attachments

  • The lateral sesamoid bone is embedded within the tendon before it inserts into the base of the proximal phalanx

  • Acts as a pulley system, redirecting the force of contraction for efficient toe flexion

  • Works in coordination with the adductor hallucis (lateral) and abductor hallucis (medial) to stabilize the MTP joint

  • The flexor hallucis longus tendon passes between the two sesamoids shared by the FHB tendons

Relations

  • Superiorly: Flexor hallucis longus tendon

  • Inferiorly: Plantar aponeurosis

  • Medially: Medial head of FHB and abductor hallucis

  • Laterally: Adductor hallucis (oblique and transverse heads)

  • Posteriorly: Medial plantar artery and nerve branches

  • Anteriorly: Base of the proximal phalanx of the hallux

Nerve Supply

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

Arterial Supply

  • Medial plantar artery (from the posterior tibial artery)

  • Additional supply from first plantar metatarsal artery

Venous Drainage

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

Function

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

  • Stabilization: Works with adductor and abductor hallucis to stabilize the great toe during push-off

  • Arch support: Helps maintain the medial longitudinal arch of the foot

  • Force transmission: Through its sesamoid pulley, transmits muscle force effectively during locomotion

Clinical Significance

  • Tendinopathy or strain: Overuse during repetitive plantar flexion or toe-off in runners and dancers

  • Sesamoiditis: Inflammation or stress injury around the lateral sesamoid affecting tendon mechanics

  • Hallux valgus surgery relevance: Lateral head often visualized or retracted during corrective procedures

  • Nerve entrapment: Medial plantar nerve injury may weaken FHB and cause hallux instability

  • Imaging role: MRI used to assess sesamoid injury, tendinopathy, or muscular atrophy secondary to nerve lesions

MRI Appearance

  • T1-weighted images:

    • Muscle belly: intermediate signal intensity with visible fascicular architecture

    • Tendon: low signal (dark linear band) extending to lateral side of base of proximal phalanx

    • Lateral sesamoid: low-signal cortex with intermediate marrow signal

    • Bright surrounding fat planes enhance contrast with adjacent structures

  • T2-weighted images:

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

    • Tendon: dark, continuous fibers extending under the first metatarsal head

    • Pathology: bright hyperintense signal within tendon or near sesamoid insertion in tendinitis or partial tear

    • Perisesamoid inflammation or edema shows localized hyperintensity adjacent to the lateral sesamoid

  • STIR:

    • Normal muscle: intermediate-to-dark signal intensity

    • Pathology: bright hyperintense areas indicating edema, inflammation, or strain

    • Sensitive for detecting soft-tissue edema around the lateral sesamoid and MTP joint

  • Proton Density Fat-Saturated (PD FS):

    • Normal: intermediate-to-dark homogeneous muscle signal

    • Pathologic: focal or diffuse bright hyperintense signals in tendon or muscle due to inflammation or tear

    • Clearly delineates peritendinous fluid, bursitis, or reactive sesamoid changes

  • T1 Fat-Sat Post-Contrast:

    • Normal muscle: uniform mild enhancement

    • Inflamed tendon or sesamoid region: focal enhancement at insertion site or tendon sheath

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

    • Useful for differentiating acute vs chronic sesamoiditis and identifying fibrotic scarring

CT Appearance

Non-Contrast CT:

  • Muscle: soft-tissue density beneath plantar surface of the first metatarsal head

  • Tendon: thin low-attenuation band inserting into base of proximal phalanx

  • Lateral sesamoid: clearly visualized as a dense oval ossicle beneath the first metatarsal head

  • Chronic sesamoid stress: cortical thickening, sclerosis, or fragmentation

Post-Contrast CT (standard):

  • Muscle enhances uniformly

  • Inflamed tendon sheath or sesamoid region: shows focal enhancement

  • Useful in detecting calcifications, postoperative changes, or sesamoid osteonecrosis

MRI images

lateral head of flexor hallucis brevis muscle coronal cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

MRI images

Lateral head of flexor hallucis brevis muscle axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

CT image

lateral head of flexor hallucis brevis muscle ct axial anatomy