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Flexor digitorum longus tendon

The flexor digitorum longus (FDL) tendon is the long distal continuation of the flexor digitorum longus muscle, located in the deep posterior compartment of the leg. It plays a vital role in flexion of the lateral four toes, supports the longitudinal arches of the foot, and contributes to plantarflexion of the ankle.

The FDL tendon has a long oblique course, passing behind the medial malleolus within the tarsal tunnel, and divides into four slips that insert onto the distal phalanges of the second through fifth toes. Because of its long intra-sheath path, it is vulnerable to tendinitis, entrapment in tarsal tunnel syndrome, and lacerations.

Synonyms

  • Long flexor tendon of the toes

  • Tendon of flexor digitorum longus

Origin, Course, and Insertion

  • Origin (muscle belly): Posterior surface of tibia (middle third), below soleal line

  • Course (tendon): Descends posterior to tibia → passes posterior to medial malleolus within tarsal tunnel → runs beneath the sustentaculum tali of calcaneus → enters plantar foot

  • Insertion: Divides into four tendinous slips that insert onto the plantar surface of the distal phalanges of the second, third, fourth, and fifth toes

Tendon Attachments

  • Passes through a fibrous sheath within the tarsal tunnel

  • Crossed by the tendon of the flexor hallucis longus (forming the "knot of Henry") in the sole, where fibers may interconnect

  • Each slip is stabilized by fibrous digital sheaths on the plantar aspect of the toes

Relations

  • Superiorly: Tibialis posterior tendon and flexor hallucis longus tendon in deep posterior leg

  • Inferiorly: Plantar aponeurosis and digital sheaths

  • Medially: Medial malleolus, flexor retinaculum (tarsal tunnel)

  • Laterally: Flexor hallucis longus tendon (at knot of Henry)

  • Posteriorly: Tibia and sustentaculum tali of calcaneus

Function

  • Toe flexion: Flexes the distal phalanges of the lateral four toes

  • Arch support: Maintains medial and lateral longitudinal arches of the foot

  • Ankle plantarflexion: Assists in plantarflexion with gastrocnemius, soleus, and tibialis posterior

  • Gait: Provides push-off power during walking and running

Clinical Significance

  • Tendinopathy: Overuse or inflammation in athletes, especially runners and dancers

  • Entrapment: Can be compressed within the tarsal tunnel causing pain and toe weakness

  • Trauma: Lacerations or ruptures compromise toe flexion and push-off strength

  • Knot of Henry pathology: Cross-interaction with FHL tendon may cause pain and synovitis

  • Surgical relevance: FDL tendon often harvested for tendon grafts in reconstructive surgery

MRI Appearance

  • T1-weighted images:

    • Normal FDL tendon: low signal intensity (dark band)

    • Normal muscle belly: intermediate signal

    • Tears: discontinuity or linear hyperintense cleft within tendon

    • Adjacent fat planes: bright, highlighting tendon course

  • T2-weighted images:

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

    • Normal tendon: very low signal (black)

    • Pathology: bright hyperintense signal within tendon substance (tendinopathy, partial tear)

    • Surrounding edema or sheath effusion shows hyperintense signal

  • STIR:

    • Normal tendon: dark

    • Pathology: bright hyperintense signal representing edema, tenosynovitis, or partial rupture

  • Proton Density Fat-Saturated (PD FS):

    • Normal tendon: uniformly dark, sharply defined

    • Tears: focal bright signal disruption of tendon fibers

    • Subtle peritendinitis or sheath fluid well delineated

  • T1 Fat-Sat Post-Contrast:

    • Normal tendon: minimal or no enhancement

    • Tendinitis/synovitis: enhances due to inflammation

    • Partial tears: peripheral enhancement; complete rupture shows fluid-filled gap with enhancing margins

CT Appearance

Non-Contrast CT:

  • Muscle belly: soft-tissue density, indistinct from surrounding structures

  • Tendon: linear dense band crossing ankle and plantar foot

  • Chronic pathology: calcifications or thickening in chronic tendinopathy

Post-Contrast CT (standard):

  • Muscle belly enhances homogeneously

  • Inflamed tendon sheath may enhance subtly

  • Acute tendon tears may appear as irregular contour or thickening

MRI image

Flexor digitorum longus tendon axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00002

MRI image

Flexor digitorum longus tendon axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00003

MRI image

CT image

Flexor digitorum longus tendon ct axial image 1

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Flexor digitorum longus tendon ct axial image 2

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Flexor digitorum longus tendon ct axial image 3