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Flexor digitorum brevis tendons

The flexor digitorum brevis (FDB) tendons are the terminal tendinous extensions of the flexor digitorum brevis muscle, a key component of the first muscular layer of the sole of the foot. These tendons lie deep to the plantar aponeurosis and superficial to the flexor digitorum longus (FDL) tendons.

Their main function is to flex the middle phalanges of the lateral four toes (2nd–5th) at the proximal interphalangeal joints. Each tendon exhibits a unique bifurcated structure — splitting into two slips that allow the FDL tendon to pass through — a configuration that enhances toe flexibility and strength during push-off.

Synonyms

  • Short flexor tendons of the toes

  • Flexor digitorum brevis of the sole

Origin, Course, and Insertion

  • Origin: Medial process of the calcaneal tuberosity, deep surface of the central portion of the plantar aponeurosis, and intermuscular septa of the sole

  • Course: Muscle fibers form four tendons that travel anteriorly through the sole toward the lateral four toes; each tendon splits near the base of the proximal phalanx to allow passage of the FDL tendon, then reunites distally

  • Insertion: Onto the sides of the middle phalanx of the 2nd to 5th toes

Tendon Attachments

  • Each tendon splits into two slips at the base of the proximal phalanx (known as the Camper’s chiasm)

  • The flexor digitorum longus tendon passes between these slips to reach the distal phalanx

  • Enclosed in fibro-osseous digital sheaths with synovial linings for smooth gliding

  • Tendons blend distally with the plantar plate and fibrous digital sheath at the PIP joint

Relations

  • Superiorly: Flexor digitorum longus tendons

  • Inferiorly: Plantar aponeurosis and skin of the sole

  • Medially: Flexor hallucis brevis and abductor hallucis

  • Laterally: Abductor digiti minimi

  • Posteriorly: Plantar nerves and vessels (medial and lateral plantar branches)

Nerve Supply

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

Function

  • Toe flexion: Flexes the middle phalanges of the 2nd to 5th toes at the PIP joints

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

  • Propulsion: Assists in toe grip and ground push-off during gait

  • Balance: Provides postural stability during stance and locomotion

  • Synergy: Works with the flexor digitorum longus for coordinated toe flexion

Clinical Significance

  • Tendinopathy: Overuse or repetitive strain may cause inflammation, thickening, or partial tears

  • Plantar fasciitis association: Tight or hypertrophied FDB can contribute to plantar heel pain

  • Nerve entrapment: Compression of the medial plantar nerve affects FDB tendon performance

  • Muscle strain: May occur with sudden forced dorsiflexion or excessive loading

  • Surgical relevance: FDB tendons may be used for grafts or tendon transfers in foot reconstructive surgery

MRI Appearance

  • T1-weighted images:

    • Muscle belly: intermediate signal intensity, distinct fascicular pattern

    • Tendons: low signal (dark linear structures) coursing toward toes

    • Plantar fat: bright background delineating tendon course

    • Tears: discontinuity or irregularity with focal bright signal

  • T2-weighted images:

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

    • Tendons: very low signal intensity (dark)

    • Tendinopathy or strain: bright hyperintense foci within tendon or at myotendinous junction

    • Peritendinous edema: hyperintense signal surrounding the tendon

  • STIR:

    • Normal muscle: intermediate-to-dark signal intensity

    • Normal tendons: very low signal (dark)

    • Pathology: bright hyperintense signal within or around the tendon indicating inflammation, edema, or strain

  • Proton Density Fat-Saturated (PD FS):

    • Normal muscle: intermediate-to-dark homogeneous signal

    • Normal tendons: low (dark), sharply marginated

    • Pathology: bright hyperintense areas along tendon path or within muscle, indicating edema, small tears, or peritendinitis

  • T1 Fat-Sat Post-Contrast:

    • Normal tendon: minimal or no enhancement

    • Inflamed or scarred tendon sheaths: show smooth, linear enhancement

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

CT Appearance

Non-Contrast CT:

  • Tendons: thin, soft-tissue density strands under the plantar fascia

  • Muscle belly: uniform soft-tissue attenuation in the plantar compartment

  • Chronic inflammation: may show tendon thickening, fibrosis, or small calcific deposits

  • Plantar fascia: visualized as dense fibrous structure superficial to tendons

Post-Contrast CT (standard):

  • Muscle: homogeneous enhancement pattern

  • Inflamed or thickened tendons: mild-to-moderate enhancement

  • Useful for evaluating postsurgical changes, fibrosis, or calcific tendinopathy

MRI image

Flexor digitorum brevis tendons of the Foot axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

MRI image

Flexor digitorum brevis tendons of the Foot axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00001

MRI image

Flexor digitorum brevis tendons of the Foot axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00002

MRI image

Flexor digitorum brevis tendons of the Foot axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00003

MRI image

Flexor digitorum brevis tendons of the Foot coronal cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

MRI image

Flexor digitorum brevis tendons of the Foot coronal cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00001

MRI image

Flexor digitorum brevis tendons sfov of the Foot coronal cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

CT image

Flexor digitorum brevis tendons ct axial