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Sesamoid bone of interphalangeal joint

The sesamoid bone of the interphalangeal (IP) joint of the thumb is a small, rounded ossicle embedded within the soft tissues near the volar aspect of the thumb IP joint. Although less common than sesamoids at the metacarpophalangeal joint, it represents an important normal variant and anatomical structure that influences joint biomechanics and imaging interpretation.

It is clinically relevant in thumb pain evaluation, trauma, degenerative changes, and differentiation from fractures or loose bodies on imaging.

Synonyms

  • Thumb IP joint sesamoid

  • Interphalangeal sesamoid of the thumb

Location

  • Situated at the volar (palmar) aspect of the thumb interphalangeal joint

  • Embedded within the volar plate or flexor apparatus

  • Inferior to the head of the proximal phalanx

  • Superior to the base of the distal phalanx

  • Deep to the flexor pollicis longus tendon

Anatomical components

  • Small ovoid or round ossicle

  • Composed of cortical shell and cancellous core

  • Embedded within fibrous or tendinous tissue

  • Articular relationship:

    • Lies adjacent to the volar articular surface of the IP joint

  • Associated structures:

    • Volar plate

    • Flexor pollicis longus tendon sheath

Relations

Anteriorly (volarly):

  • Skin and subcutaneous tissue of the thumb pulp

Posteriorly (dorsally):

  • Thumb IP joint space

Proximally:

  • Head of the proximal phalanx

Distally:

  • Base of the distal phalanx

Superficially:

  • Flexor pollicis longus tendon sheath

Deep:

  • Volar plate of the IP joint

X-ray appearance

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

  • Sesamoid: Small, well-corticated round or oval ossicle at the volar aspect of the IP joint

  • Margins: Smooth and sharply defined

  • Position: Fixed relative to joint rather than free within joint space

  • Differentiation: Distinguished from fractures or loose bodies by uniform shape and cortication

CT appearance

Non-contrast CT:

  • Sesamoid bone: Well-defined ossified structure with cortical and cancellous components

  • Location: Precisely localized to the volar aspect of the IP joint

  • Relationship to joint surfaces: Clearly demonstrated

  • Utility: Excellent for differentiating from fractures, osteophytes, or intra-articular bodies

MRI appearance

T1-weighted images:

  • Cortex: Low signal intensity

  • Marrow: High signal within the sesamoid

  • Outline: Clearly visualized against surrounding soft tissues

T2-weighted images:

  • Cortex: Low signal

  • Marrow: Intermediate-to-high signal

  • Adjacent joint fluid: High signal outlining the ossicle

STIR:

  • Marrow: Suppressed fat signal

  • Sesamoid margins: Well delineated

  • Surrounding soft tissues: High contrast visualization

Proton density fat-saturated (PD FS):

  • Marrow fat: Suppressed

  • Joint fluid: High signal

  • Tendons and volar plate: Seen as low-signal structures adjacent to the sesamoid

MRI image

Sesamoid bone of interphalangeal joint mri anatomy labelled image-img-00000-00000

MRI image

Sesamoid bone of interphalangeal joint mri anatomy labelled image-img-00000-00000_00001