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Lateral sesamoid bone

The lateral sesamoid bone is one of two small oval bones embedded within the tendons of the flexor hallucis brevis (FHB) beneath the head of the first metatarsal bone. It lies on the lateral (fibular) side of the first metatarsophalangeal (MTP) joint, within the lateral head of the FHB tendon, while the flexor hallucis longus (FHL) tendon passes between the medial and lateral sesamoids.

The sesamoids act as pulley-like structures, reducing tendon friction, improving mechanical leverage, and absorbing load during gait. The lateral sesamoid bears a significant portion of forefoot pressure during push-off, especially during toe-off and balance phases.

It articulates superiorly with the first metatarsal head via the lateral sesamoid groove and contributes to the stability of the first MTP joint.

Synonyms

  • Fibular sesamoid of the hallux

  • Lateral hallux sesamoid

  • Outer sesamoid of the great toe

Relations

  • Superiorly: Articulates with the plantar surface of the first metatarsal head (lateral sesamoid groove)

  • Inferiorly: Embedded within the lateral tendon of the flexor hallucis brevis

  • Medially: Flexor hallucis longus tendon passes medially to it

  • Laterally: Fibrous capsule of the first MTP joint and adductor hallucis tendon

  • Anteriorly: Proximal phalanx of the great toe

  • Posteriorly: Plantar plate and medial plantar vessels

Development and Ossification

  • Type: Sesamoid (accessory) bone formed by intratendinous ossification

  • Ossification: Begins between ages 8–10 years

  • May present as bipartite (congenital variant) or multipartite sesamoid

  • Separate ossification centers may mimic fracture on imaging

Function

  • Load transmission: Absorbs weight-bearing stress during the toe-off phase of gait

  • Mechanical leverage: Increases the efficiency of the flexor hallucis brevis and flexor hallucis longus tendons

  • Friction reduction: Acts as a smooth fulcrum for tendon gliding

  • Joint protection: Distributes compressive forces at the first MTP joint

  • Stabilization: Contributes to medial–lateral balance of the hallux during motion

Arterial Supply

  • Medial and lateral plantar arteries (branches of posterior tibial artery) via sesamoid branches of the first plantar metatarsal artery

  • Perforating branches from the plantar arterial arch

Venous Drainage

  • Plantar venous plexus draining into the medial and lateral plantar veins, then into the posterior tibial vein

Nerve Supply

  • Medial plantar nerve (for plantar aspect)

  • Digital branches supplying the joint capsule and perisesamoid soft tissues

Clinical Significance

  • Sesamoiditis: Inflammation from repetitive stress or overuse, common in athletes and dancers

  • Fracture: May occur from acute trauma or chronic stress; often confused with bipartite sesamoid on imaging

  • Osteonecrosis (Avascular necrosis): Chronic pain and sclerosis from compromised vascularity (seen in “Renander’s disease”)

  • Bipartite variant: Congenital separation may mimic fracture but has smooth corticated margins

  • Arthritis and degeneration: Seen in hallux valgus, sesamoid arthritis, and first MTP joint arthrosis

  • Surgical relevance: Important in sesamoidectomy, bunion correction, and plantar forefoot surgery

MRI Appearance

  • T1-weighted images:

    • Lateral sesamoid: low-to-intermediate signal (cortical bone low, marrow bright )

    • Surrounding FHB tendon: low signal bands

    • Normal fat planes around sesamoid appear bright

    • Fracture or avascular necrosis: low signal in marrow with surrounding soft-tissue edema

  • T2-weighted images:

    • Normal sesamoid: low signal cortex, marrow bright , darker than on T1

    • Joint cartilage and fluid: bright hyperintense

    • Sesamoiditis: high T2 signal in adjacent bone marrow and soft tissues

    • Fracture: linear bright line or cortical disruption with bone marrow edema

  • STIR:

    • Normal sesamoid: intermediate-to-dark

    • Pathology: bright hyperintensity due to edema or inflammation (sesamoiditis, fracture, AVN)

    • Bipartite sesamoid: smooth, well-corticated margins without surrounding edema

  • Proton Density Fat-Saturated (PD FS):

    • Normal: intermediate-to-dark cortical outline with clean surrounding tendons

    • Pathology: hyperintense bone marrow and peritendinous fluid in inflammation or fracture

    • Excellent for detecting subtle marrow edema, perisesamoid bursitis, and early AVN

  • T1 Fat-Sat Post-Contrast:

    • Normal: minimal enhancement

    • Sesamoiditis or bursitis: diffuse enhancement of bone and surrounding soft tissues

    • Chronic AVN: peripheral enhancement with central low-signal necrosis

CT Appearance

Non-Contrast CT:

  • Lateral sesamoid: dense ovoid or bean-shaped bone beneath the first metatarsal head

  • Cortical margins: smooth, continuous in normal

  • Bipartite sesamoid: well-corticated two-part bone with narrow lucent line

  • Fracture: sharp, irregular lucency through the bone, often with sclerosis or fragmentation in chronic cases

  • AVN: increased density, fragmentation, or collapse

Post-Contrast CT (standard):

  • Normal bone: mild homogeneous enhancement

  • Inflammation or osteonecrosis: heterogeneous or peripheral enhancement

  • Useful for assessing sclerosis, cortical irregularity, and bone morphology

MRI image

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

MRI image

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

CT image

lateral sesamoid bone TOE CT AXIAL

CT VRT image

Lateral sesamoid bone

MRI image

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

X ray image

Lateral sesamoid bone