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Annular ligaments (annular pulleys)

The annular ligaments of the finger, also known as the annular pulleys, are strong fibrous bands forming part of the fibro-osseous tunnel that secures the flexor tendons to the phalanges. They are essential components of the digital flexor mechanism, preventing bowstringing of the tendons during finger flexion and ensuring precise, efficient biomechanics.

There are typically five annular pulleys (A1–A5) along each finger, each varying in thickness, strength, and location. Together with the cruciform pulleys, they maintain the intimate relationship between the flexor digitorum profundus (FDP), flexor digitorum superficialis (FDS), and the phalanges.

Synonyms

  • Annular pulley system

  • A1–A5 pulleys

  • Digital flexor sheath pulleys

Location and Structure

  • A1 pulley: Over the metacarpophalangeal (MCP) joint

  • A2 pulley: Over the proximal phalanx (one of the strongest and most important)

  • A3 pulley: Over the proximal interphalangeal (PIP) joint

  • A4 pulley: Over the middle phalanx

  • A5 pulley: Over the distal interphalangeal (DIP) joint

  • Structure:

    • Thickened transverse collagen bands, tightly adherent to the volar plates and periosteum

    • Part of the fibro-osseous tunnel containing FDS and FDP tendons

Relations

  • Deep: Flexor digitorum superficialis and profundus tendons

  • Superficial: Skin, subcutaneous tissue, volar plates

  • Lateral: Digital neurovascular bundles

  • Proximal/Distal: Adjacent cruciform pulleys (C1–C3)

Attachments

  • Firmly anchored to the volar plate, periosteum, and fibrous digital sheath

  • Create a rigid pulley system preventing tendon displacement during flexion

Function

  • Prevent flexor tendon bowstringing

  • Maintain smooth gliding of tendons during finger motion

  • Optimize force transmission for grip and fine motor control

  • Stabilize joints during dynamic hand movement

MRI Appearance

T1-weighted images:

  • Annular pulleys appear as thin, low-signal bands along the volar aspect of the phalanges

  • Flexor tendons show intermediate signal, easily distinguished from the low-signal pulley bands

  • Surrounding fat appears bright, providing clear contrast

T2-weighted images:

  • Pulley structures remain low signal, thin, and sharply defined

  • Flexor tendons show intermediate signal

  • The fibro-osseous tunnel and phalanges provide clear anatomic boundaries

  • Normal pulleys do not exhibit thickening or increased signal

STIR:

  • Pulley system appears as dark linear structures due to dense collagen composition

  • Excellent suppression of surrounding fat highlights the tendon–pulley interface

  • Normal pulleys show no bright signal or edema

T1 Fat-Saturated Post-Contrast:

  • Normal annular ligaments show minimal to no enhancement

  • Tendons enhance faintly or not at all

  • Contrast helps delineate the digital sheath and soft tissue surroundings

CT Appearance

Non-Contrast CT:

  • Annular ligaments are not directly visualized due to low soft-tissue contrast

annular ligaments(annular pulleys) of the finger MRI axial image-img-00000-00000