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Flexor digiti minimi brevis muscle

The flexor digiti minimi brevis (FDMB) is a small intrinsic muscle located on the lateral side of the sole of the foot, within the third muscular layer. It lies medial to the abductor digiti minimi and lateral to the flexor digitorum brevis. The FDMB primarily functions to flex the proximal phalanx of the fifth toe at the metatarsophalangeal (MTP) joint and assists in stabilizing the lateral longitudinal arch during gait.

It is a short, fusiform muscle that contributes to the fine motor control and balance of the foot. Though small, the FDMB is important for toe-off during walking and provides lateral stability of the foot, especially when standing on uneven surfaces.

Synonyms

  • Short flexor of the little toe

  • Flexor digiti quinti brevis (variant term)

Origin, Course, and Insertion

  • Origin: Base of the fifth metatarsal bone and sheath of the fibularis (peroneus) longus tendon

  • Course: Fibers run distally and slightly medially along the lateral border of the foot, forming a short tendon

  • Insertion: Lateral side of the base of the proximal phalanx of the fifth toe

Tendon Attachments

  • The tendon inserts directly into the base of the proximal phalanx of the fifth toe

  • It may be connected by fibrous slips to the abductor digiti minimi or flexor digitorum longus tendon in some variants

  • No sesamoid bone is typically present in association with this muscle

Relations

  • Superiorly: Lateral plantar vessels and nerve

  • Inferiorly: Plantar aponeurosis

  • Medially: Flexor digitorum brevis and quadratus plantae

  • Laterally: Abductor digiti minimi

  • Posteriorly: Tendon sheath of fibularis longus

Nerve Supply

  • Superficial branch of the lateral plantar nerve (branch of tibial nerve, roots S2–S3)

Arterial Supply

  • Lateral plantar artery (branch of posterior tibial artery)

  • Occasional contributions from fourth plantar metatarsal artery

Venous Drainage

  • Lateral plantar vein and accompanying venae comitantes

  • Drains into the posterior tibial vein

Function

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

  • Lateral stability: Supports the lateral longitudinal arch during stance and walking

  • Assistive function: Aids in propulsion and balance, especially on uneven terrain

  • Synergistic role: Works with abductor digiti minimi for coordinated movement of the fifth toe

Clinical Significance

  • Muscle strain: Common in athletes due to overuse during push-off or sprinting

  • Atrophy: May occur secondary to tibial or lateral plantar nerve lesions

  • Pain syndrome: Can contribute to lateral plantar foot pain or mimic peroneal tendinopathy

  • Surgical relevance: Important during lateral approach to the fifth metatarsal and plantar nerve decompression procedures

  • Imaging role: MRI best demonstrates FDMB morphology and pathology in cases of pain, trauma, or denervation

MRI Appearance

  • T1-weighted images:

    • Muscle belly: intermediate signal intensity with clear fascicular pattern

    • Tendon: low signal (dark) inserting into the base of the fifth proximal phalanx

    • Adjacent fat planes appear bright and well defined

    • Chronic denervation: high T1 signal due to fatty infiltration

  • T2-weighted images:

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

    • Tendon: uniformly dark (low signal)

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

    • Chronic fibrosis: low-signal bands replacing normal muscle tissue

  • STIR:

    • Normal muscle: intermediate-to-dark signal intensity

    • Pathology: bright hyperintense regions indicating edema, myositis, or partial tear

    • Helps identify denervation edema or early inflammatory changes

  • Proton Density Fat-Saturated (PD FS):

    • Normal: intermediate-to-dark, homogeneous appearance

    • Tendon pathology: focal bright signal at insertion or along tendon course (tendinitis, tear)

    • Muscle strain or edema: patchy hyperintense signal within belly or along fascial planes

  • T1 Fat-Sat Post-Contrast:

    • Normal muscle: mild, uniform enhancement

    • Inflamed tendon sheath or perimysial tissue: shows focal enhancement

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

CT Appearance

Non-Contrast CT:

  • Muscle: appears as uniform soft-tissue density lateral to flexor digitorum brevis

  • Tendon: seen as fine linear density inserting onto the base of the fifth proximal phalanx

  • Chronic changes: muscle atrophy, fatty replacement, or calcification in old injuries

  • Adjacent bone: may show enthesophytes or cortical thickening at insertion site

Post-Contrast CT (standard):

  • Muscle enhances homogeneously

  • Inflamed peritendinous tissue or fascia may enhance more intensely

  • Useful for detecting chronic inflammatory changes, soft-tissue mass, or postoperative scarring around the lateral plantar region

MRI images

Flexor digiti minimi brevis muscle axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

MRI images

Flexor digiti minimi brevis muscle axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00001

MRI images

Flexor digiti minimi brevis muscle axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00002

MRI images

Flexor digiti minimi brevis muscle coronal cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

MRI images

Flexor digiti minimi brevis muscle coronal cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00001

CT image

Flexor digiti minimi brevis muscle ct axial image