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Extensor pollicis brevis muscle

The extensor pollicis brevis (EPB) is a slender muscle of the posterior compartment of the forearm, located deep to the superficial extensor group. It arises from the posterior radius and interosseous membrane, and extends the proximal phalanx of the thumb at the metacarpophalangeal (MCP) joint.

The EPB lies lateral to the extensor pollicis longus (EPL) and medial to the abductor pollicis longus (APL). Its tendon forms part of the lateral boundary of the anatomical snuffbox, making it a key landmark in wrist and thumb imaging. It functions synergistically with the EPL and APL in thumb extension and abduction.

Synonyms

  • Short extensor of the thumb

  • Brevis extensor pollicis

Origin, Course, and Insertion

Origin: Posterior surface of the radius, below the origin of the abductor pollicis longus, and from the adjacent interosseous membrane
Course:

  • Fibers pass obliquely downward and laterally

  • Form a tendon that travels in the first dorsal compartment of the wrist along with the abductor pollicis longus

  • Passes deep to the extensor retinaculum
    Insertion: Dorsal base of the proximal phalanx of the thumb; occasionally extends to the dorsal capsule of the MCP joint

Tendon Attachments

  • Shares a fibro-osseous tunnel with the abductor pollicis longus beneath the extensor retinaculum

  • Inserts into the base of the proximal phalanx of the thumb

  • Occasionally sends slips to the dorsal thumb capsule or extensor expansion

Relations

Superficial: Skin and fascia over the anatomical snuffbox
Deep: Radius and interosseous membrane
Medial: Extensor pollicis longus tendon
Lateral: Abductor pollicis longus tendon
Distally: Dorsal capsule of the metacarpophalangeal joint of the thumb

Nerve Supply

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

Arterial Supply

Posterior interosseous artery (branch of the ulnar artery via the common interosseous artery)
Minor contributions from radial artery perforators near the wrist

Function

  • Extension of thumb at MCP joint

  • Assists in thumb abduction at carpometacarpal (CMC) joint

  • Stabilizes thumb during grasping and pinching motions

  • Works in coordination with the extensor pollicis longus and abductor pollicis longus

  • Important for fine motor control and precision grip

Clinical Significance

  • De Quervain’s tenosynovitis: Inflammation of the EPB and APL tendons within the first dorsal compartment, causing pain over the radial wrist and snuffbox

  • Tendon rupture: Rare, often traumatic or post-surgical

  • Variant anatomy: Some individuals have a double or absent EPB tendon, influencing surgical approach and imaging interpretation

  • Impingement: Thickened retinaculum may compress the EPB tendon

  • Imaging importance: MRI best for evaluating tendon integrity and sheath inflammation

MRI Appearance

T1-weighted images:

  • Muscle belly: intermediate signal intensity

  • Tendon: low signal (dark) extending distally along the lateral radius

  • Surrounding fat: bright

  • Insertion and retinaculum margins appear sharply defined

  • Pathology (tenosynovitis or tear): thickened tendon with increased intermediate signal intensity

T2-weighted images:

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

  • Normal tendon: low signal (black line)

  • Tendinitis or partial tear: bright hyperintense regions within or around tendon

  • Effusion or sheath fluid: bright high signal outlining tendon in first dorsal compartment

STIR:

  • Normal muscle: intermediate-to-dark signal

  • Pathology: bright hyperintense signal indicating edema, inflammation, or sheath fluid

  • Useful for early detection of De Quervain’s tenosynovitis

Proton Density Fat-Saturated (PD FS):

  • Normal tendon: dark, continuous, sharply outlined

  • Tenosynovitis: bright linear or circumferential fluid signal around tendon sheath

  • Partial tear: focal bright intratendinous signal with fiber discontinuity

  • Ideal for evaluating peritendinous inflammation and retinacular thickening

T1 Fat-Sat Post-Contrast:

  • Normal tendon: mild uniform enhancement

  • Inflamed sheath or synovium: intense linear or ring enhancement

  • Partial tears or post-surgical scarring: focal or peripheral enhancement

CT Appearance

Non-Contrast CT:

  • Muscle: homogeneous soft-tissue density along posterior-lateral forearm

  • Tendon: fine dense band in first dorsal compartment

  • Bony landmarks: clearly visualize radial styloid and scaphoid surfaces of the anatomical snuffbox

  • Calcific tendinopathy: small focal hyperdensities along tendon sheath

Post-Contrast CT (standard):

  • Muscle enhances homogeneously

  • Inflamed tendon sheath or soft tissue shows focal enhancement

  • Useful in detecting calcifications, scar tissue, or post-traumatic changes

MRI image

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MRI image

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MRI image

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MRI image

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MRI image

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