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Flexor hallucis longus muscle

The flexor hallucis longus (FHL) is a powerful deep muscle of the posterior compartment of the leg. It lies lateral to the tibialis posterior and flexor digitorum longus, originating from the fibula and inserting onto the distal phalanx of the great toe. It serves as the primary flexor of the hallux, playing a key role in push-off during walking, running, and jumping.

The FHL tendon runs a long course from the posterior leg, beneath the sustentaculum tali of the calcaneus, through the tarsal tunnel, and onto the plantar surface of the hallux. Because of its long gliding pathway, the FHL tendon is particularly vulnerable to tendinopathy, entrapment, and stenosing tenosynovitis.

Synonyms

  • Long flexor of the great toe

  • Hallucis flexor longus

Origin, Course, and Insertion

  • Origin: Inferior two-thirds of the posterior surface of the fibula and adjacent interosseous membrane

  • Course: Fibers run downward and medially, forming a tendon that passes posterior to the talus and inferior to the sustentaculum tali, within the tarsal tunnel, before entering the plantar foot

  • Insertion: Base of the distal phalanx of the great toe on the plantar surface

Tendon Attachments

  • Runs within a synovial sheath through the tarsal tunnel beneath the flexor retinaculum

  • Crosses beneath the flexor digitorum longus tendon (the "knot of Henry")

  • Inserts firmly into the distal phalanx of the hallux, enabling strong toe flexion

Relations

  • Superiorly: Soleus and posterior tibial vessels

  • Inferiorly: Talus, calcaneus, sustentaculum tali, and plantar structures

  • Medially: Tibialis posterior tendon

  • Laterally: Peroneal muscles and fibula

  • Anteriorly: Interosseous membrane

Nerve Supply

  • Tibial nerve (S2–S3), branch of the sciatic nerve

Arterial Supply

  • Posterior tibial artery and its muscular branches

  • Contribution from fibular (peroneal) artery

Venous Drainage

  • Posterior tibial veins and fibular veins, draining into the popliteal vein

Function

  • Flexion of hallux: Flexes distal and proximal phalanges of the great toe

  • Push-off power: Provides propulsion during gait, running, and jumping

  • Plantarflexion assistance: Aids in plantarflexion of the ankle

  • Medial longitudinal arch support: Helps maintain foot stability

  • Clinical marker: Weakness indicates tibial nerve dysfunction or posterior compartment syndrome

Clinical Significance

  • Tendinopathy: Overuse injuries common in runners, dancers, and athletes (“dancer’s tendon”)

  • Entrapment: Stenosing tenosynovitis within the tarsal tunnel or beneath sustentaculum tali

  • Tears: May result from acute trauma or chronic degeneration, leading to painful push-off weakness

  • Surgical relevance: FHL tendon often harvested for grafting in Achilles tendon reconstruction

  • Imaging importance: MRI critical for assessing tendon tears, entrapment, or synovitis

MRI Appearance

  • T1-weighted images:

    • Normal muscle: intermediate signal intensity

    • Tendon: low signal (dark band) along posterior ankle and plantar foot

    • Tears or tendinopathy: intermediate-to-bright focal signal in tendon substance

    • Surrounding fat planes: bright, providing contrast for tendon visualization

  • T2-weighted images:

    • Normal muscle: intermediate but slightly darker than T1

    • Normal tendon: very low signal (black)

    • Pathology: hyperintense tendon signal in partial tears or tendinitis

    • Fluid in tendon sheath: bright signal, suggesting tenosynovitis

  • STIR (Short Tau Inversion Recovery):

    • Normal muscle: intermediate to dark

    • Pathology: bright hyperintense signal in muscle (edema) or along tendon sheath (inflammation)

    • Highlights subtle synovitis, cysts, or surrounding edema

  • Proton Density Fat-Saturated (PD FS):

    • Normal muscle: intermediate to dark signal

    • Normal tendon: dark and homogeneous

    • Tears: bright linear or focal signal indicating fluid-filled defect

    • Excellent for detecting subtle partial-thickness tears and peritendinous inflammation

  • T1 Fat-Sat Post-Contrast:

    • Normal tendon: minimal or no enhancement

    • Tendinitis or synovitis: enhancing thickened tendon sheath

    • Partial tears: peripheral rim enhancement; complete rupture shows fluid gap with enhancing edges

CT Appearance

Non-Contrast CT:

  • Muscle: soft-tissue density, less defined than MRI

  • Tendon: linear soft tissue density coursing along posterior ankle

  • Chronic pathology: tendon calcification or ossification in chronic tendinopathy

Post-Contrast CT (standard):

  • Muscle: homogeneous enhancement with contrast

  • Inflamed tendon sheath or tenosynovitis: shows contrast enhancement

  • Less sensitive than MRI but useful for detecting calcification or bony entrapment

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

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

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Flexor hallucis longus muscle ct axial

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