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Flexor pollicis longus tendon

The flexor pollicis longus (FPL) tendon is the distal extension of the flexor pollicis longus muscle, a deep muscle of the anterior compartment of the forearm. It serves as the primary flexor of the thumb, enabling flexion at the interphalangeal (IP) and metacarpophalangeal (MCP) joints.

The tendon passes through the carpal tunnel along with other long flexor tendons, within a separate synovial sheath, and continues into the thumb to insert on the base of its distal phalanx. It plays a crucial role in fine thumb movements, grip strength, and precision tasks. Due to its long, slender course through narrow fibro-osseous tunnels, the FPL tendon is vulnerable to tenosynovitis, attritional rupture, or entrapment, particularly in overuse syndromes or after distal radius fractures.

Synonyms

  • Tendon of the long flexor of the thumb

  • Tendon of flexor pollicis longus

Origin, Course, and Insertion

  • Origin (of muscle): Anterior surface of the radius (middle third) and adjacent interosseous membrane

  • Course of tendon:

    • Descends through the forearm deep to the flexor digitorum superficialis

    • Passes beneath the flexor retinaculum within the carpal tunnel in its own synovial sheath

    • Continues distally into the palm, crossing the thumb’s metacarpophalangeal joint

    • Passes between the two heads of the flexor pollicis brevis

  • Insertion: Base of the distal phalanx of the thumb (palmar surface)

Relations

  • Proximally: Lies deep to the flexor digitorum superficialis

  • Distally: Passes deep to flexor retinaculum, superficial to pronator quadratus

  • At wrist: Occupies the radial side of the carpal tunnel

  • In palm: Runs between the heads of flexor pollicis brevis

  • At thumb: Covered by synovial sheath and lies beneath the digital pulley system

Tendon Attachments

  • The tendon is ensheathed by a synovial sheath that extends from the carpal tunnel to the distal phalanx

  • Held in place by fibro-osseous pulleys along the thumb’s palmar surface

  • The annular pulleys (A1–A2) and oblique pulley stabilize the tendon, preventing bowstringing during flexion

Nerve Supply

  • Anterior interosseous nerve (branch of the median nerve, C7–C8)

Function

  • Flexion of the thumb: Flexes the interphalangeal and metacarpophalangeal joints of the thumb

  • Grip and pinch: Facilitates opposition and precision gripping

  • Dynamic stabilization: Maintains thumb position during fine manipulative movements

  • Clinical indicator: Weakness or loss of thumb flexion indicates anterior interosseous nerve palsy

Clinical Significance

  • Tenosynovitis (trigger thumb): Inflammation of the tendon sheath, often due to overuse or repetitive thumb flexion

  • Attritional rupture: Common following distal radius fracture or volar plate fixation (tendon rubs against hardware)

  • FPL entrapment: May occur after carpal tunnel surgery or trauma

  • Partial tear: Results in pain and weak thumb flexion

  • Complete rupture: Loss of interphalangeal flexion; may require surgical repair or tendon grafting

  • Post-surgical monitoring: MRI and ultrasound are key in assessing tendon integrity and gliding function

MRI Appearance

  • T1-weighted images:

    • Normal tendon: low signal (dark) continuous band extending to distal phalanx

    • Surrounding fat: bright, highlighting tendon margins

    • Muscle belly proximally: intermediate signal intensity

    • Pathology: partial tear appears as focal interruption or thickening with small intermediate-signal gap

  • T2-weighted images:

    • Normal tendon: low signal

    • Surrounding synovial sheath: thin, low-signal rim

    • Tendon inflammation or tear: bright hyperintense signal within or around the tendon

    • Synovial effusion or tenosynovitis: bright fluid outlining tendon within sheath

  • STIR:

    • Normal tendon: dark (flow void-like appearance)

    • Pathology: bright peritendinous or intratendinous hyperintensity in edema, partial tear, or infection

    • Excellent for early detection of tenosynovitis or postoperative inflammation

  • Proton Density Fat-Saturated (PD FS):

    • Normal tendon: uniformly dark with sharp borders

    • Pathologic: focal bright signal within tendon or sheath in partial tear or tendinopathy

    • Best for visualizing minor intrasubstance degeneration or synovial thickening

  • T1 Fat-Sat Post-Contrast:

    • Normal tendon: minimal or no enhancement

    • Tendinitis/tenosynovitis: circumferential or linear enhancement around sheath

    • Postoperative scar or adhesions: enhancing fibrotic tissue adjacent to non-enhancing tendon gap

CT Appearance

Non-Contrast CT:

  • Tendon: soft-tissue density extending across palmar aspect of thumb

  • Difficult to differentiate from surrounding tissues unless contrast or high-resolution protocol used

  • Calcifications (if chronic inflammation) appear as tiny high-density foci along tendon path

  • Hardware from previous wrist surgery can obscure or irritate tendon trajectory

Post-Contrast CT (standard):

  • Tendon sheath enhancement: indicates inflammation or synovitis

  • Tendon discontinuity: visible as a soft-tissue gap or displacement in complete rupture

  • Scar tissue or adhesions: show irregular enhancing tissue planes

  • Useful in assessing postoperative anatomy and hardware impingement on the tendon

MRI image

Flexor pollicis longus tendon axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

MRI image

Flexor pollicis longus tendon axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00001

MRI image

Flexor pollicis longus tendon axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00002

MRI image

Flexor pollicis longus tendon axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00003

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Flexor pollicis longus tendon coronal cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

MRI image

Flexor pollicis longus tendon coronal cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00001

CT image

Flexor pollicis longus tendon ct axial image

CT image

Flexor pollicis longus tendon ct axial