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Annular ligament (annular pulley)

The annular ligaments of the fingers, commonly referred to as the annular pulleys, are strong transverse fibrous bands that form part of the fibro-osseous tunnel enclosing the flexor digitorum profundus (FDP) and flexor digitorum superficialis (FDS) tendons. They maintain tendon alignment close to the phalanges, preventing bowstringing during finger flexion and allowing smooth, efficient digital motion.

There are five annular pulleys (A1–A5) located from the metacarpophalangeal (MCP) joint to the distal interphalangeal (DIP) joint. The A2 and A4 pulleys are biomechanically the most important, preventing tendon excursion away from bone during gripping.

Synonyms

  • Annular pulley

  • Flexor tendon pulley

  • A1–A5 pulley system

Location and Structure

  • A1 pulley: Located over the metacarpophalangeal (MCP) joint; thick and clinically significant in trigger finger

  • A2 pulley: Strong, located along the proximal phalanx

  • A3 pulley: Positioned at the proximal interphalangeal (PIP) joint

  • A4 pulley: Located mid–middle phalanx

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

  • Composition: Dense transverse collagen fibers forming part of the fibrous flexor sheath

  • Function: Keeps tendons closely applied to the bone during flexion; forms the tunnel for gliding of FDS and FDP tendons

Relations

  • Deep: Flexor digitorum profundus (FDP) tendon, flexor digitorum superficialis (FDS) tendon, phalanges

  • Superficial: Subcutaneous tissue and digital skin

  • Laterally: Digital nerves and vessels running parallel to the tendon sheath

  • Proximally/Distally: Interconnected with cruciform (C1–C3) pulleys forming continuous sheath

Attachments

  • Firmly attached to the periosteum of the phalanges

  • Completely encircles the flexor tendons forming a rigid fibro-osseous tunnel

  • Merges with cruciform pulleys between segments

  • Maintains tendon alignment against the phalanx throughout finger flexion

Function

  • Prevents bowstringing of flexor tendons

  • Ensures efficient finger flexion and grip strength

  • Provides a low-friction surface for tendon gliding

  • Maintains proper biomechanical moment arms for flexor tendons

  • Stabilizes flexor tendons during power grip and fine motor tasks

Clinical Significance

  • Trigger finger: Thickening or stenosis of the A1 pulley, causing tendon catching

  • Pulley ruptures: Common in climbers; A2 and A4 most frequently injured

  • Tenosynovitis: Inflammation of tendon sheath limiting motion

  • Trauma: Lacerations or crush injuries may disrupt pulley function

  • Surgical importance: Reconstruction required for multiple pulley injuries

MRI Appearance

T1-weighted images:

  • Annular pulleys appear as thin, low-signal (dark) fibrous bands surrounding flexor tendons

  • Flexor tendons show low signal, surrounded by intermediate-signal soft tissues

  • Bone marrow of phalanges appears bright due to fatty content

T2-weighted images:

  • Pulley remains low signal due to dense collagen

  • Flexor tendons: low signal

  • Synovial sheath appears intermediate to bright depending on fluid presence

  • Clear visualization of A2 and A4 pulleys in axial and sagittal planes

STIR:

  • Annular pulley: dark, maintaining low-signal fibrous appearance

  • Surrounding fat suppressed to dark, giving excellent contrast with tendons and pulleys

  • Highlights fluid within the tendon sheath or peritendinous tissues

MRI image

annular pulley HAND MRI CORONAL image-img-00000-00000