Flexor Hallucis Longus Tendon: Anatomy, Function, MRI & CT

Topic

Flexor hallucis longus tendon

The flexor hallucis longus (FHL) tendon is the long distal extension of the flexor hallucis longus muscle, one of the deep muscles of the posterior compartment of the leg. It is the primary flexor of the great toe and plays an essential role in push-off during gait and athletic activities.

The tendon runs in a fibro-osseous tunnel behind the ankle, passes beneath the sustentaculum tali of the calcaneus, and continues along the plantar surface of the foot to insert into the distal phalanx of the hallux. Because of its long, narrow course through multiple fibro-osseous sheaths, it is prone to tendinopathy, stenosis, and tears, particularly in athletes such as ballet dancers, runners, and soccer players.

Synonyms

Tendon of the flexor hallucis longus
Great toe flexor tendon

Origin, Course, and Insertion

Origin of muscle belly: Posterior surface of the fibula, lower two-thirds, and interosseous membrane
Course of tendon:
- Descends in the posterior compartment, traveling deep to the soleus and gastrocnemius
- Passes posterior to the medial malleolus within a synovial sheath, in the tarsal tunnel
- Runs beneath the sustentaculum tali of the calcaneus in a groove
- Crosses the flexor digitorum longus tendon (known as the knot of Henry) on the plantar foot
Insertion: Base of the distal phalanx of the great toe

Tendon Attachments

Enclosed in a synovial sheath at the ankle and foot
Forms a fibrous tunnel beneath the sustentaculum tali
May send slips to the flexor digitorum longus tendon and proximal phalanx of the hallux
Firmly attached to the plantar plate of the great toe

Relations

Superiorly: Muscle belly in the deep posterior compartment of the leg
At ankle: Lies within the tarsal tunnel posterior to the talus and medial malleolus
Under sustentaculum tali: Runs in a fibro-osseous groove stabilized by a fibrous sheath
In plantar foot: Crosses with the flexor digitorum longus tendon (knot of Henry)
Distally: Courses to the hallux, inserting into the distal phalanx

Function

Toe flexion: Primary flexor of the hallux at both interphalangeal and metatarsophalangeal joints
Push-off in gait: Critical in propulsion during walking, running, and jumping
Ankle stabilization: Assists in plantarflexion and supports the medial longitudinal arch
Balance: Provides strength and coordination during tip-toe stance and athletic activities

Clinical Significance

Tendinopathy: Common in ballet dancers (“dancer’s tendonitis”) and runners due to repetitive push-off
Stenosing tenosynovitis: Occurs at fibro-osseous tunnels, especially under sustentaculum tali
Tears or ruptures: May follow trauma, overuse, or degenerative changes; cause loss of hallux flexion strength
Entrapment syndrome: Painful entrapment within the tarsal tunnel
Post-surgical issues: FHL may be harvested for tendon grafting; care is required to avoid morbidity

MRI Appearance

T1-weighted images:
- Normal FHL tendon: low signal (dark, fibrillar structure)
- Muscle belly: intermediate signal
- Tears or tendinopathy: intermediate-to-bright signal, tendon thickening, or discontinuity
- Fat surrounding tendon sheath: bright, outlining tendon course
T2-weighted images:
- Normal tendon: very low signal (black cord-like structure)
- Normal muscle: intermediate-to-low, darker than on T1
- Pathology: partial tears or tendinopathy appear as bright hyperintense areas within or around tendon
- Fluid or edema in tendon sheath appears hyperintense, suggesting tenosynovitis
STIR:
- Normal tendon: dark
- Pathology: hyperintense signal in tendon substance or surrounding sheath (inflammation, fluid, tear)
Proton Density Fat-Saturated (PD FS):
- Normal: tendon remains uniformly dark
- Pathology: bright irregular signal within tendon or sheath
- Very sensitive for subtle partial-thickness tears and tenosynovitis
T1 Fat-Sat Post-Contrast:
- Normal tendon: minimal or no enhancement
- Pathology: enhancement of tendon sheath in tenosynovitis or inflamed tendon
- Tears: peripheral enhancement may outline fluid-filled tendon defects