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Ulnar sesamoid bone of thumb

The ulnar sesamoid bone of the thumb is a small, rounded ossicle embedded within the tendon of the adductor pollicis and flexor apparatus at the ulnar (medial) side of the first metacarpophalangeal (MCP) joint. It plays a role in altering tendon mechanics and stabilizing the thumb during pinch and grip.

It is an important anatomical landmark in hand anatomy, trauma evaluation, sports injuries, and imaging of the thumb.

Synonyms

  • Medial sesamoid of the thumb

  • Ulnar sesamoid at the first MCP joint

Location

  • Situated on the ulnar (medial) side of the first metacarpophalangeal joint

  • Embedded within the volar plate and tendon complex

  • Inferior (volar) to the head of the first metacarpal

  • Medial to the flexor pollicis longus tendon

  • Opposite the radial sesamoid of the thumb

Anatomical components

  • Small ovoid sesamoid bone

  • Composed of cortical shell with central cancellous bone

  • Embedded in the tendon of adductor pollicis and volar plate

  • Part of a paired sesamoid system (ulnar and radial sesamoids)

Relations

Anteriorly (volarly):

  • Skin and subcutaneous tissue of the palm

Posteriorly (dorsally):

  • Head of the first metacarpal

Medially (ulnarly):

  • Adductor pollicis tendon

Laterally (radially):

  • Flexor pollicis brevis tendon and radial sesamoid

Inferiorly:

  • Volar plate of the MCP joint

Superiorly:

  • First metacarpal head

X-ray appearance

Plain radiographs (AP, oblique, and lateral thumb views):

  • Ulnar sesamoid: Small, round or ovoid radiopaque structure

  • Position: Volar and ulnar to the first metacarpal head

  • Margins: Smooth and corticated

  • Best visualized on: Oblique and lateral views

CT appearance

Non-contrast CT:

  • Sesamoid bone: Well-defined ossicle with cortical shell and trabecular center

  • Location: Clearly seen embedded in the volar soft tissues adjacent to MCP joint

  • Shape: Ovoid or slightly flattened

  • Utility: Excellent for detecting subtle cortical irregularities or multipartite morphology

MRI appearance

T1-weighted images:

  • Cortical bone: Low signal intensity

  • Marrow (if present): Intermediate to high signal

  • Adjacent tendons: Low signal

T2-weighted images:

  • Cortex: Low signal

  • Marrow: Intermediate signal

  • Joint fluid: High signal outlining the sesamoid

STIR:

  • Fat suppression: Suppresses marrow fat signal

  • Sesamoid margins: Well delineated against bright surrounding fluid and soft tissue

Proton density fat-saturated (PD FS):

  • Bone marrow: Suppressed fat signal

  • Joint fluid and soft tissues: High signal

MRI image

Ulnar sesamoid bone of thumb mri anatomy labelled image-img-00000-00000

X-Ray image

Ulnar sesamoid bone of thumb x ray anatomy labelled image-img-00000-00000

MRI image

Ulnar sesamoid bone of thumb mri

MRI image

Ulnar sesamoid bone of thumb sag mri