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Interphalangeal joint of thumb

The interphalangeal (IP) joint of the thumb is a hinge-type synovial joint formed between the proximal and distal phalanges of the thumb. Unlike the fingers, which have two interphalangeal joints, the thumb contains only one. This joint plays a critical role in fine motor function, pinch, and grasp activities.

It is an essential anatomical landmark in hand anatomy, orthopedic evaluation, trauma assessment, and musculoskeletal imaging.

Synonyms

  • Thumb IP joint

  • Interphalangeal joint of the pollex

Location

  • Situated between the proximal and distal phalanges of the thumb

  • Distal to the metacarpophalangeal (MCP) joint

  • Proximal to the thumb tip (distal phalanx)

  • Forms the central articulation of the thumb digit

Anatomical components

  • Articulating bones:

    • Head of the proximal phalanx

    • Base of the distal phalanx

  • Joint type:

    • Synovial hinge joint

  • Articular surfaces:

    • Covered by hyaline cartilage

  • Capsule:

    • Fibrous capsule enclosing the joint

  • Supporting structures:

    • Medial and lateral collateral ligaments

    • Volar plate

    • Dorsal extensor mechanism

Relations

Anteriorly (volar):

  • Flexor pollicis longus tendon

  • Volar plate

Posteriorly (dorsal):

  • Extensor pollicis longus tendon

Medially and laterally:

  • Collateral ligaments

Proximally:

  • Proximal phalanx of the thumb

Distally:

  • Distal phalanx of the thumb

X-ray appearance

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

  • Joint space: Uniform radiolucent space between phalanges

  • Articular margins: Smooth cortical outlines

  • Alignment: Proper axial alignment of proximal and distal phalanges

  • Sesamoids: Not present at the IP joint (unlike MCP joint)

CT appearance

Non-contrast CT:

  • Bony anatomy: Clearly defined cortical and trabecular bone of both phalanges

  • Joint space: Well visualized as a thin gap between bones

  • Articular surfaces: Smooth and congruent

  • Utility: Excellent for subtle fractures and cortical irregularities

MRI appearance

T1-weighted images:

  • Cortical bone: Low signal intensity

  • Bone marrow: High signal in phalanges

  • Joint space: Low signal cartilage and capsule interface

  • Tendons: Low-signal linear structures

T2-weighted images:

  • Joint fluid: High signal within joint cavity

  • Cartilage: Intermediate signal

  • Surrounding soft tissues: Intermediate-to-high signal

STIR:

  • Fat suppression: Uniform suppression of marrow fat

  • Joint cavity: Bright fluid signal

  • Capsule and ligaments: Well delineated against suppressed marrow

Proton density fat-saturated (PD FS):

  • Joint fluid: High signal

  • Cartilage and ligaments: Intermediate-to-low signal

  • Tendons: Low signal

MRI image

Interphalangeal joint of thumb mri image

X-Ray image

Interphalangeal joint of thumb x ray anatomy labelled image-img-00000-00000 - Copy