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Flexor pollicis longus muscle

The flexor pollicis longus (FPL) is a long, flat muscle of the deep anterior compartment of the forearm, responsible for flexion of the thumb. It is unique to humans among primates, allowing fine manipulation and precision grip. The muscle lies lateral to the flexor digitorum profundus and medial to the flexor carpi radialis in the forearm, forming a long tendon that passes through the carpal tunnel into the thumb.

It plays a vital role in flexing the interphalangeal and metacarpophalangeal joints of the thumb and contributes to hand dexterity and grip strength. Its tendon course through the carpal tunnel makes it susceptible to tenosynovitis and entrapment (e.g., in carpal tunnel syndrome).

Synonyms

  • Long flexor of the thumb

  • FPL muscle

Origin, Course, and Insertion

Origin:

  • Anterior surface of the radius (middle two-fourths)

  • Adjacent interosseous membrane

  • Occasionally from the medial epicondyle of humerus or coronoid process of ulna (accessory heads—“Gantzer’s muscle”)

Course:

  • Descends deep in the forearm between flexor digitorum profundus and flexor carpi radialis

  • Passes deep to the flexor retinaculum through the carpal tunnel within its own synovial sheath

  • Emerges in the palm to run toward the thumb

Insertion:

  • Base of the distal phalanx of the thumb, on its palmar aspect

Tendon Attachments

  • Travels through the carpal tunnel deep to the flexor retinaculum

  • Enclosed in a synovial sheath continuous with that of the radial bursa

  • Inserts into the distal phalanx of the thumb, forming the main tendon responsible for flexing the interphalangeal joint

Relations

  • Anteriorly: Flexor retinaculum and superficial flexor tendons

  • Posteriorly: Radius and interosseous membrane

  • Laterally: Flexor carpi radialis

  • Medially: Flexor digitorum profundus

  • Distally: Lies between the heads of flexor pollicis brevis before inserting on the thumb phalanx

Nerve Supply

  • Anterior interosseous nerve (branch of median nerve, roots C8–T1)

Arterial Supply

  • Anterior interosseous artery (branch of ulnar artery)

  • Additional contributions from radial artery branches in the distal forearm

Function

  • Flexes the interphalangeal joint of the thumb

  • Assists in flexion at the metacarpophalangeal and carpometacarpal joints of the thumb

  • Stabilizes the thumb during fine grip and pinch movements

  • Works synergistically with flexor digitorum profundus during gripping or writing

Clinical Significance

  • Tendinopathy: Overuse can lead to inflammation or partial tears, especially in repetitive thumb flexion

  • Rupture: May occur due to distal radius fractures (attritional rupture on volar plate) or rheumatoid arthritis

  • Entrapment: Tendon sheath involvement in carpal tunnel syndrome can cause anterior wrist pain and thumb weakness

  • Gantzer’s muscle: Accessory head may compress the anterior interosseous nerve, leading to weakness of FPL

  • Surgical relevance: Important in tendon grafting and reconstructive thumb surgeries

MRI Appearance

T1-weighted images:

  • Muscle belly: intermediate signal intensity, homogeneous texture

  • Tendon: low signal (dark linear band)

  • Surrounding fat planes: bright for contrast

  • Chronic atrophy or fatty infiltration: increased T1 signal within muscle

T2-weighted images:

  • Normal muscle: intermediate-to-dark signal, slightly darker than on T1

  • Tendon: very low signal (black line)

  • Pathology: bright hyperintense signal in tendon or myotendinous junction indicating tendinitis or tear

  • Synovial sheath fluid appears hyperintense, outlining tendon

STIR:

  • Normal muscle: intermediate-to-dark signal intensity

  • Pathology: bright signal in inflammation, strain, or edema

  • Useful for detecting early tenosynovitis and subtle tears

Proton Density Fat-Saturated (PD FS):

  • Normal: intermediate-to-dark, uniform signal in muscle

  • Tendinopathy: focal or diffuse bright hyperintense signal in tendon or sheath

  • Highlights peritendinous fluid, early inflammation, and partial fiber disruption

T1 Fat-Sat Post-Contrast:

  • Normal muscle: mild, uniform enhancement

  • Tendinitis or tenosynovitis: shows linear or peritendinous enhancement

  • Tear or rupture: non-enhancing gap with peripheral rim enhancement

  • Post-surgical or chronic cases may show minimal enhancement with fibrosis

CT Appearance

Non-Contrast CT:

  • Muscle: uniform soft-tissue density along anterior forearm

  • Tendon: linear low-density structure in carpal tunnel

  • Chronic calcific tendinopathy: small high-attenuation foci within tendon sheath

  • Volar plate fractures may impinge on tendon path

Post-Contrast CT (standard):

  • Muscle: homogeneous enhancement

  • Inflamed tendon sheath or surrounding soft tissue: focal enhancement

  • Useful for evaluating traumatic tendon lacerations, sheath thickening, or post-repair complications

MRI image

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

MRI image

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

MRI image

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