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Intermediate phalanx

The intermediate (middle) phalanx is the second bone in the fingers (index, middle, ring, and small fingers), located between the proximal phalanx and distal phalanx. It participates in the proximal interphalangeal (PIP) joint proximally and the distal interphalangeal (DIP) joint distally (except in the thumb, which lacks an intermediate phalanx).

Structurally, the intermediate phalanx consists of a base, shaft, and head, each with distinct cortical and cancellous bone patterns. It contributes to digital stability, fine manipulation, and force transmission during grip and pinch activities.

Synonyms

  • Middle phalanx

Location and Structure

  • Base: Proximal end, concave articular surface forming the PIP joint with the proximal phalanx

  • Shaft: Narrow cylindrical or triangular bone with cortical walls and central trabecular marrow

  • Head: Distal condylar end forming the DIP joint with the distal phalanx

  • Surrounding tissues: Enclosed by tendons, ligaments, and joint capsules contributing to dexterity and strength

Relations

  • Dorsally: Extensor digitorum and extensor indicis tendons (depending on digit)

  • Palmarly: Flexor digitorum superficialis (FDS) slips bifurcate around the shaft; flexor digitorum profundus (FDP) continues distally

  • Laterally: Proper collateral ligaments at PIP and DIP joints

  • Proximally: Proximal phalanx

  • Distally: Distal phalanx

Attachments

  • Collateral ligaments of PIP and DIP joints attach along the sides of the base and head

  • Volar plate attaches to ventral base at PIP joint

  • Flexor tendon sheath attaches along the shaft

  • Extensor mechanism (central slip) attaches dorsally near the dorsal base

Function

  • Forms the central part of the skeletal support of the finger

  • Enables flexion and extension at PIP and DIP joints

  • Provides rigid anchoring for flexor and extensor tendons

  • Participates in precision grip, pinch strength, and fine motor tasks

MRI Appearance

T1-weighted images:

  • Cortex: Very low signal (black)

  • Bone marrow: Bright signal (fatty marrow)

  • Articular cartilage: Thin intermediate-to-low signal at joint surfaces

  • Surrounding tendons: Low signal bands along dorsal and palmar aspects

  • Soft tissues: Moderate contrast with surrounding fat planes

T2-weighted images:

  • Cortex: Low signal

  • Marrow: Bright, though slightly less intense than fat on T1

  • Cartilage: Intermediate-to-bright

  • Joint space: High signal representing synovial fluid

  • Tendons: Low-signal linear structures

STIR:

  • Bone marrow: Intermediate-to-dark in normal bone

  • Cortex: Dark

  • Soft tissues: Clear contrast due to fat suppression

  • Joint fluid: Hyperintense

CT Appearance

Non-Contrast CT:

  • Cortex: High attenuation and sharply defined

  • Trabecular bone: Fine internal lattice

  • Joint surfaces: Smooth, well-circumscribed contours

  • Soft tissues: Surrounding tendons and ligaments visible as soft-tissue density

MRI image

Intermediate phalanx MRI CORONAL image-img-00000-00000