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Medial sesamoid bone

The medial sesamoid bone is one of two small ovoid bones embedded within the tendinous insertions of the flexor hallucis brevis (FHB) beneath the first metatarsal head. It lies within the tendon of the medial head of FHB on the plantar surface of the great toe and articulates with a groove on the plantar aspect of the first metatarsal head.

It functions as a pulley to increase the mechanical advantage of the flexor hallucis longus (FHL) and FHB tendons, improves load distribution during toe-off, and protects tendons and metatarsal cartilage from stress. The medial sesamoid typically lies slightly larger and more prominent than the lateral sesamoid and bears a greater proportion of load during gait.

Synonyms

  • Tibial sesamoid bone

  • Medial hallux sesamoid

  • Medial sesamoid of the great toe

Location and Structure

  • Located beneath the head of the first metatarsal, within the medial tendon of flexor hallucis brevis

  • Lateral sesamoid lies adjacent within the lateral FHB tendon

  • The flexor hallucis longus tendon passes between the two sesamoids in a fibro-osseous tunnel

  • The bone has a smooth dorsal (articular) surface that glides against the metatarsal head and a rough plantar surface for tendinous attachment

Relations

  • Superiorly: Plantar surface of the first metatarsal head

  • Inferiorly: Tendon of the medial head of FHB and plantar aponeurosis

  • Medially: Abductor hallucis muscle

  • Laterally: Lateral sesamoid and flexor hallucis longus tendon

  • Posteriorly: Plantar nerves and vessels at the level of the first MTP joint

Arterial Supply

  • Branches of the medial plantar artery (from posterior tibial artery) supply the sesamoid and surrounding tendons

Venous Drainage

  • Medial plantar venous plexus, draining into the posterior tibial vein

Nerve Supply

  • Medial plantar nerve, providing sensory innervation to the surrounding soft tissues and capsule

Function

  • Mechanical leverage: Acts as a fulcrum to improve the force transmission of FHL and FHB tendons during toe flexion

  • Shock absorption: Distributes weight across the first metatarsal head during walking and running

  • Joint protection: Reduces friction and wear on tendons and articular cartilage

  • Stabilization: Assists in maintaining alignment and stability of the first MTP joint during push-off phase

Clinical Significance

  • Sesamoiditis: Overuse inflammation causing pain under the first metatarsal head, often in runners and dancers

  • Fracture or stress reaction: Due to direct trauma or repetitive loading

  • Bipartite sesamoid: Normal variant that can mimic fracture on imaging

  • Osteonecrosis (Trevor’s disease): Rare cause of chronic pain and sclerosis

  • Tendon entrapment: FHL or FHB tendons may become impinged between fractured sesamoid fragments

  • Surgical relevance: Sesamoid preservation is critical during hallux valgus correction to prevent postoperative instability

MRI Appearance

  • T1-weighted images:

    • Cortical bone: low signal (dark)

    • Marrow: bright signal intensity

    • Tendons (FHB) surrounding the sesamoid: low signal bands

    • Adjacent fat: bright, providing good contrast for cortical margins

  • T2-weighted images:

    • Marrow: bright signal intensity, normally hyperintense compared to muscle

    • Cortex: remains dark (low signal)

    • Edema, stress reaction, or fracture: increased bright signal in bone marrow and surrounding soft tissues

    • Joint effusion or sesamoiditis: perisesamoid hyperintense fluid signal

  • STIR:

    • Normal marrow: intermediate to dark signal

    • Pathology (edema, fracture, inflammation): bright hyperintense signal extending into surrounding soft tissue

    • Useful for differentiating active inflammation from chronic sclerosis

  • Proton Density Fat-Saturated (PD FS):

    • Normal marrow: intermediate-to-dark

    • Bone stress or inflammation: bright focal or diffuse signal within the sesamoid and soft tissues

    • Clear visualization of peritendinous fluid and sesamoiditis changes

  • T1 Fat-Sat Post-Contrast:

    • Normal: mild, uniform marrow enhancement

    • Active inflammation or osteitis: intense, patchy enhancement

    • Chronic sclerosis or infarction: low or absent enhancement

CT Appearance

Non-Contrast CT:

  • Sesamoid appears as a small, dense ovoid ossicle beneath the first metatarsal head

  • Cortex: thick, well-defined outer rim of high density

  • Marrow cavity: lower attenuation center corresponding to cancellous bone

  • Pathology: fracture line, fragmentation, or cortical irregularity

  • Bipartite sesamoid: smooth corticated margins between two bone parts, differentiating it from fracture

Post-Contrast CT (standard):

  • Enhances surrounding soft tissues, not the cortex itself

  • Inflamed or edematous regions show mild enhancement

  • Useful for identifying stress fractures, osteonecrosis, or postoperative changes

MRI image

Medial sesamoid bone axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

MRI image

Medial sesamoid bone coronal cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

CT image

Medial sesamoid bone CT axial image

CT VRT image

Medial sesamoid bone

MRI image

Medial sesamoid bone mri anatomy labelled image-img-00000-00000

X ray image

Medial sesamoid bone