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Base of proximal phalanx

The base of the proximal phalanx is the expanded proximal end of the proximal phalanx in each finger. It articulates with the head of the corresponding metacarpal to form the metacarpophalangeal (MCP) joint, a condyloid synovial joint permitting flexion, extension, abduction, and adduction.

The dorsal and palmar surfaces of the base serve as essential attachment sites for extensor mechanisms, collateral ligaments, and the volar plate, making it a key stabilizing structure for MCP joint motion. The base also provides the insertion point for intrinsic hand muscles such as interossei and lumbricals. Its cortical contour and ligamentous anchoring make it structurally robust yet vulnerable to avulsion injuries, MCP subluxation, and arthritic degeneration.

Synonyms

  • Proximal phalangeal base

  • MCP joint phalangeal base

  • Phalanx base (proximal segment)

Location and Structure

  • Position: Proximal end of the proximal phalanx, articulating with the metacarpal head.

  • Shape: Concave articular surface dorsally and palmar thickening contributing to volar plate attachment.

  • Surfaces:

    • Dorsal: For extensor expansion attachment

    • Palmar: Strong anchoring region for volar plate and flexor tendon sheath

  • Margins: Reinforced by collateral ligament attachment zones

Relations

  • Dorsally: Extensor expansion and extensor digitorum tendons

  • Palmar: Flexor tendon sheath (A1 pulley region), volar plate

  • Laterally: Collateral ligaments and lumbrical/interosseous tendon slips

  • Proximally: Metacarpal head (MCP articulation)

  • Distally: Shaft of proximal phalanx

Attachments

  • Tendons:

    • Extensor digitorum and intrinsic extensor hood attach dorsally

    • Lumbrical and interosseous tendons anchor laterally and partially dorsally

  • Ligaments:

    • Collateral ligaments insert along lateral margins

    • Volar plate attaches to palmar base

  • Fibrous sheath: A1 pulley complex attaches along palmar surface

Function

  • Forms the MCP joint’s phalangeal articular component

  • Provides stable foundation for finger flexion, extension, and lateral movements

  • Supports tendon gliding and force transfer through extensor and flexor tendons

  • Anchors collateral ligaments, allowing controlled MCP joint mobility

  • Contributes to precision grip and fine motor control

Clinical Significance

  • Common site of avulsion fractures from collateral ligament or volar plate tension

  • Important in MCP dislocations, especially dorsal dislocations

  • May demonstrate early erosive or degenerative changes in inflammatory arthritis

  • Crucial landmark for reconstruction in tendon and ligament repair

  • Frequently evaluated in trauma, sports injuries, and chronic MCP instability

MRI Appearance

T1-weighted images:

  • Cortex: Low signal intensity (dark)

  • Bone marrow: Bright, reflecting fatty marrow composition

  • Articular surface: Smooth intermediate-to-low signal

  • Clear identification of ligament and tendon attachments as thin low-signal bands

T2-weighted images:

  • Cortex: Low signal

  • Bone marrow: Bright, slightly less intense than on T1

  • Articular cartilage: Intermediate-to-bright signal

  • Flexor and extensor tendons: Dark linear structures crossing joint region

STIR:

  • Normal marrow: Intermediate-to-dark signal

  • Tendons and ligaments: Dark, sharply defined

  • Distinct visualization of soft-tissue contrast and phalanx–tendon interfaces

CT Appearance

Non-Contrast CT:

  • Cortex: High-attenuation dense cortical bone

  • Marrow: Lower-density cancellous center

  • Joint surface: Smooth concave articulating surface with metacarpal head

  • Excellent anatomic detail of ligament insertion zones, volar plate attachment, and subtle cortical contour variation

MRI image

Base of proximal phalanx MRI CORONAL image-img-00000-00000

X Ray image

Base of proximal phalan x ray anatomy labelled image-img-00000-00000 - Copy (2)