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Proximal phalanx of thumb

The proximal phalanx of the thumb is the first phalangeal bone of the thumb, located between the first metacarpal and the distal phalanx. It forms an essential part of the thumb’s skeletal framework, enabling powerful grasp, fine manipulation, and oppositional movements.

It is a key anatomical structure in hand anatomy, trauma evaluation, sports injuries, and imaging of the thumb.

Synonyms

  • First phalanx of the thumb

  • Proximal thumb phalanx

Location

  • Situated in the radial side of the hand

  • Between the first metacarpal proximally and the distal phalanx distally

  • Forms the bony core of the proximal segment of the thumb

  • Participates in the metacarpophalangeal (MCP) joint proximally

  • Participates in the interphalangeal (IP) joint distally

Anatomical components

  • Base:

    • Proximal expanded end

    • Articulates with the head of the first metacarpal

  • Shaft (body):

    • Cylindrical and slightly flattened

  • Head:

    • Distal articular surface for the distal phalanx

  • Cortical shell and cancellous core

Relations

Proximally:

  • Head of the first metacarpal

  • Metacarpophalangeal joint capsule

Distally:

  • Base of the distal phalanx

  • Interphalangeal joint capsule

Anteriorly (volar):

  • Flexor pollicis longus tendon (indirectly via distal phalanx)

  • Volar plate at MCP and IP joints

Posteriorly (dorsal):

  • Extensor pollicis longus and brevis tendons

Medially and laterally:

  • Collateral ligaments of MCP and IP joints

X-ray appearance

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

  • Bone outline: Well-defined cortical margins

  • Base: Broad proximal articular surface

  • Shaft: Uniform cortical thickness

  • Head: Smooth, rounded distal articular surface

  • Joint spaces: Preserved MCP and IP joint spaces

CT appearance

Non-contrast CT:

  • Cortex: Sharp, high-density cortical bone

  • Trabecular bone: Normal cancellous pattern

  • Articular surfaces: Clearly defined

  • Spatial assessment: Excellent for evaluating bone morphology and alignment

MRI appearance

T1-weighted images:

  • Cortical bone: Low signal intensity

  • Marrow: High signal intensity

  • Bone contours: Well delineated

T2-weighted images:

  • Cortex: Low signal

  • Marrow: Intermediate signal

  • Joint fluid: High signal outlining articular surfaces

STIR:

  • Fat suppression: Uniform suppression of marrow fat

  • Bone and soft tissue contrast: Clear differentiation of surrounding tendons and ligaments

Proton density fat-saturated (PD FS):

  • Marrow: Suppressed fat signal

  • Joint fluid: High signal

MRI image

Proximal phalanx of thumb MRI coronal

MRI image

Proximal phalanx of thumb MRI IMAGE

X-Ray image

Proximal phalanx of thumb x ray anatomy labelled image-img-00000-00000