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Metacarpophalangeal joint of thumb

The metacarpophalangeal (MCP) joint of the thumb is a synovial joint formed between the head of the first metacarpal and the base of the proximal phalanx of the thumb. It plays a crucial role in thumb stability and precision grip, allowing controlled flexion, extension, and limited lateral movement.

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

Synonyms

  • Thumb MCP joint

  • First metacarpophalangeal joint

Location

  • Situated at the junction between the first metacarpal and proximal phalanx

  • Forms the knuckle of the thumb

  • Distal to the first carpometacarpal joint

  • Proximal to the interphalangeal joint of the thumb

  • Superficial and easily palpable on the dorsal and palmar aspects of the thumb

Anatomical components

  • Articular surfaces:

    • Head of the first metacarpal

    • Base of the proximal phalanx

  • Joint capsule:

    • Fibrous capsule lined by synovium

  • Collateral ligaments:

    • Ulnar collateral ligament (UCL)

    • Radial collateral ligament (RCL)

  • Volar plate:

    • Thick fibrocartilaginous structure on the palmar aspect

  • Sesamoid bones:

    • Typically two embedded within the volar plate

Relations

Dorsally:

  • Extensor pollicis longus and brevis tendons

Palmarly:

  • Volar plate

  • Flexor pollicis longus tendon (in close proximity)

Medially (ulnar side):

  • Ulnar collateral ligament

Laterally (radial side):

  • Radial collateral ligament

Proximally:

  • First metacarpal head

Distally:

  • Proximal phalanx of the thumb

X-ray appearance

Plain radiographs (AP, lateral, oblique views):

  • Joint space: Well-defined and uniform

  • Articular margins: Smooth cortical outlines

  • Sesamoid bones: Seen on palmar aspect of the joint

  • Alignment: Normal congruity between metacarpal head and proximal phalanx

CT appearance

Non-contrast CT:

  • Bony anatomy: Clearly delineated articular surfaces

  • Joint congruity: Precise visualization of alignment

  • Sesamoids: Well visualized

  • Cortical margins: Smooth and intact

MRI appearance

T1-weighted images:

  • Cortical bone: Low signal

  • Marrow: High signal in metacarpal and phalanx

  • Joint capsule and ligaments: Low-to-intermediate signal

  • Sesamoids: Low signal cortical rim with central marrow signal

T2-weighted images:

  • Joint fluid: High signal outlining articular surfaces

  • Cartilage: Intermediate signal

  • Collateral ligaments: Low-signal linear structures

STIR:

  • Fat suppression: Uniform suppression of marrow fat

  • Joint space: Fluid appears bright

  • Ligaments and volar plate: Clearly visualized against suppressed background

Proton density fat-saturated (PD FS):

  • Joint fluid: High signal

  • Ligaments: Low-signal bands (UCL and RCL well delineated)

  • Cartilage and volar plate: Well visualized with high soft-tissue contrast

MRI image

Metacarpophalangeal joint of thumb mri anatomy labelled image-img-00000-00000

X-Ray image

Metacarpophalangeal joint of thumb x ray anatomy labelled image-img-00000-00000