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Extensor pollicis longus muscle

The extensor pollicis longus (EPL) is a slender, long muscle located in the deep layer of the posterior compartment of the forearm. It acts as the primary extensor of the thumb, enabling extension at the interphalangeal, metacarpophalangeal, and carpometacarpal joints.

Its long tendon passes around the Lister’s tubercle (dorsal tubercle of the radius), where it changes direction sharply before crossing the dorsum of the wrist to reach the thumb. The tendon forms the medial boundary of the anatomical snuffbox, an important clinical landmark.

The EPL is critical for thumb extension and precision grip, and its tendon is prone to rupture following distal radius fractures, rheumatoid arthritis, or chronic tenosynovitis.

Synonyms

  • Long extensor of the thumb

  • EPL muscle

Origin, Course, and Insertion

  • Origin: Posterior surface of the ulna (middle third) and adjacent interosseous membrane

  • Course:

    • Fibers run distally and laterally in the posterior forearm deep to the extensor digitorum and extensor carpi radialis brevis.

    • Tendon passes obliquely around the Lister’s tubercle, using it as a pulley to change direction toward the thumb.

    • Crosses the wrist joint in the third extensor compartment beneath the extensor retinaculum.

  • Insertion: Base of the distal phalanx of the thumb on its dorsal surface

Tendon Attachments

  • The tendon is enclosed in a synovial sheath within the third dorsal compartment of the wrist.

  • As it crosses the dorsum of the hand, it forms the ulnar border of the anatomical snuffbox.

  • Inserts into the base of the distal phalanx of the thumb, extending all three thumb joints.

Relations

  • Anteriorly: Radius and interosseous membrane

  • Posteriorly: Skin and superficial fascia

  • Medially: Extensor indicis and extensor digitorum tendons

  • Laterally: Extensor pollicis brevis and abductor pollicis longus tendons

  • At wrist: Crosses over the second extensor compartment tendons at the level of Lister’s tubercle

Nerve Supply

  • Posterior interosseous nerve (branch of the deep branch of the radial nerve, root value C7–C8)

Arterial Supply

  • Posterior interosseous artery and radial artery perforating branches

Function

  • Extension of thumb: Extends distal and proximal phalanges at interphalangeal and metacarpophalangeal joints

  • Extension at wrist: Assists in wrist dorsiflexion

  • Reposition of thumb: Brings the thumb back from flexion or opposition

  • Precision grip: Stabilizes the thumb during pinching or grasping actions

Clinical Significance

  • Tendon rupture: Common after distal radius fractures or chronic corticosteroid use

  • Tendinitis/Tenosynovitis: Inflammation at Lister’s tubercle due to repetitive use

  • Anatomical snuffbox pain: May indicate EPL tendinitis or partial tear

  • Surgical relevance: EPL tendon transfers are performed for thumb extension reconstruction

  • Imaging importance: MRI is the best modality for evaluating tendon continuity, inflammation, or rupture

MRI Appearance

  • T1-weighted images:

    • Muscle belly: intermediate signal intensity, well-defined fascicular pattern

    • Tendon: low signal (dark linear band) along posterior forearm and dorsum of wrist

    • Fatty tissues: bright, contrasting with tendon and muscle

    • Chronic tear: muscle atrophy with increased fatty infiltration (bright on T1)

  • T2-weighted images:

    • Muscle: intermediate-to-low signal, slightly darker than on T1

    • Tendon: uniformly dark in normal state

    • Pathology: bright hyperintense signal in partial tears, tendinopathy, or inflammation

    • Surrounding synovial sheath fluid appears bright hyperintense

  • STIR:

    • Normal muscle: intermediate-to-dark signal intensity

    • Abnormal: hyperintense signal in tendon sheath (tenosynovitis) or muscle (strain, edema)

    • Highly sensitive for early inflammatory changes and fluid accumulation

  • Proton Density Fat-Saturated (PD FS):

    • Normal tendon: dark linear structure with uniform low signal

    • Pathology: bright areas in partial tear, peritendinitis, or inflammation

    • Ideal for assessing subtle tendon fiber disruption or sheath effusion

  • T1 Fat-Sat Post-Contrast:

    • Normal muscle and tendon: mild homogeneous enhancement

    • Tendinitis: Focal or diffuse enhancement of tendon sheath and surrounding soft tissue

    • Tear or postoperative scar: peripheral or irregular enhancement around tendon defect

CT Appearance

Non-Contrast CT:

  • Muscle: homogeneous soft-tissue density in posterior forearm

  • Tendon: thin linear density crossing dorsum of wrist

  • Chronic calcific tendinitis: localized hyperdense foci along tendon path

  • Post-traumatic or degenerative changes: bony irregularities around Lister’s tubercle

Post-Contrast CT (standard):

  • Normal tendon sheath: may show minimal enhancement

  • Inflamed or thickened sheath: increased peritendinous enhancement

  • Helps differentiate tendon tears, calcific deposits, and peritendinous fibrosis

  • 3D reconstructions useful for evaluating Lister’s tubercle relationship and tendon displacement

MRI images

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

MRI images

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

MRI images

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

MRI images

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