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Radial sesamoid bone of thumb

The radial sesamoid bone of the thumb is one of two small sesamoid bones embedded within the tendon of the flexor pollicis brevis at the level of the metacarpophalangeal (MCP) joint of the thumb. It lies on the radial (lateral) side of the joint and plays an important role in modifying tendon mechanics and stabilizing the thumb MCP joint.

It is a key anatomical structure in hand anatomy, thumb biomechanics, trauma assessment, and imaging interpretation.

Synonyms

  • Radial thumb sesamoid

  • Lateral sesamoid of the thumb MCP joint

Location

  • Situated at the volar aspect of the thumb MCP joint

  • Embedded within the flexor pollicis brevis tendon

  • Radial (lateral) to the first metacarpal head

  • Anterior to the articular surface of the thumb MCP joint

  • Deep to the palmar plate

Anatomical components

  • Small ovoid ossicle

  • Cortical shell with cancellous center

  • Articular surface:

    • Faces the volar aspect of the first metacarpal head

  • Soft tissue associations:

    • Flexor pollicis brevis tendon

    • Palmar plate

    • Collateral ligament complex

Relations

Anteriorly:

  • Flexor pollicis brevis tendon

Posteriorly:

  • Head of the first metacarpal

Medially:

  • Ulnar sesamoid bone

Laterally:

  • Radial collateral ligament of the thumb MCP joint

Inferiorly:

  • Proximal phalanx of the thumb

Superiorly:

  • Distal first metacarpal

X-ray appearance

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

  • Radial sesamoid: Small, round or ovoid radiopaque structure at the volar radial aspect of the thumb MCP joint

  • Cortical margin: Smooth and well-defined

  • Relationship: Seen adjacent to the first metacarpal head

  • Often paired: Seen with the ulnar sesamoid

CT appearance

Non-contrast CT:

  • Sesamoid bone: Well-corticated ossicle with central cancellous bone

  • Shape: Ovoid or slightly flattened

  • Position: Clearly located at the volar radial aspect of the MCP joint

  • Useful for: Precise evaluation of size, shape, and spatial orientation

MRI appearance

T1-weighted images:

  • Cortical bone: Low signal intensity

  • Marrow: Intermediate to high signal within the sesamoid

  • Surrounding tendons: Low signal

T2-weighted images:

  • Cortex: Low signal

  • Marrow: Intermediate signal

  • Joint fluid: High signal outlining the sesamoid

STIR:

  • Fat suppression: Uniform suppression of marrow fat

  • Sesamoid outline: Well delineated against suppressed background

  • Adjacent soft tissues: Bright signal

Proton density fat-saturated (PD FS):

  • Marrow fat: Suppressed signal

  • Joint fluid: High signal

MRI image

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

X-Ray image

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

MRI image

Radial sesamoid bone of thumb mri coronal

MRI image

Radial sesamoid bone of thumb sag mri