Flexor Digitorum Brevis Muscle: Anatomy, Function, Imaging

Topic

Flexor digitorum brevis muscle

The flexor digitorum brevis (FDB) is a superficial intrinsic muscle of the sole of the foot, located within the first (superficial) muscular layer. It lies just deep to the plantar aponeurosis and plays an essential role in flexing the lateral four toes at the proximal interphalangeal joints.

The FDB is a key muscle for stabilizing the foot during standing and walking, contributing to the medial longitudinal arch. It is often involved in plantar fasciitis, overuse injuries, and muscle strains in athletes and runners.

Synonyms

Short flexor of the toes
Plantar flexor brevis

Origin, Course, and Insertion

Origin: Medial process of the calcaneal tuberosity, plantar aponeurosis, and intermuscular septa
Course: Muscle fibers extend anteriorly within the sole, dividing into four tendons that pass toward the toes
Insertion: Each tendon splits into two slips to allow passage of the flexor digitorum longus tendon, then inserts on the sides of the middle phalanges of toes 2–5

Tendon Attachments

Four tendons pass anteriorly along the plantar aspect
Each tendon splits into two slips at the proximal phalanx, forming a tunnel for the flexor digitorum longus tendon
Inserts bilaterally on the middle phalanges of the lateral four toes

Relations

Superficially: Plantar aponeurosis
Deeply: Tendons of flexor digitorum longus and quadratus plantae muscle
Medially: Abductor hallucis
Laterally: Abductor digiti minimi

Nerve Supply

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

Arterial Supply

Medial and lateral plantar arteries (branches of posterior tibial artery)

Venous Drainage

Medial and lateral plantar veins → posterior tibial vein

Function

Toe flexion: Flexes the proximal interphalangeal joints of toes 2–5
Arch support: Maintains the medial longitudinal arch of the foot
Stabilization: Provides dynamic support during stance and push-off in gait
Assistance: Aids flexor digitorum longus in toe flexion

Clinical Significance

Plantar fasciitis: Overuse of FDB contributes to pain in the heel and medial arch
Strains: Overuse injuries in athletes, especially runners or dancers
Tears: Rare but may occur in trauma or forceful extension of the toes
Surgical relevance: Important in reconstructive procedures and plantar fascia release

MRI Appearance

T1-weighted images:
- Normal muscle: intermediate signal
- Fat planes between tendons: bright
- Tears or fatty atrophy: areas of higher signal intensity
T2-weighted images:
- Normal muscle: intermediate to dark signal, slightly darker than T1
- Tendons: very low (dark) signal
- Pathology (strain, tear, inflammation): focal bright hyperintense areas within muscle belly or tendon sheath
STIR:
- Normal FDB: intermediate to dark
- Pathology (edema, myositis, strain): bright hyperintense signal
Proton Density Fat-Saturated (PD FS):
- Normal: intermediate to dark muscle signal
- Pathology: bright signal changes in partial tears or strains
- Excellent for detecting subtle fascial involvement or peritendinous edema
T1 Fat-Sat Post-Contrast:
- Normal muscle: minimal homogeneous enhancement
- Pathology: enhancement in inflamed or scarred regions
- Abscess or neoplasm: irregular or nodular enhancement