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# EPB

The extensor pollicis brevis is a deep forearm muscle whose tendon reaches the thumb; it straightens the thumb knuckle.

Official name: Extensor pollicis brevis

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## What it is

A deep forearm muscle whose tendon goes to the thumb's first bone.

## What it does

Straightens the thumb knuckle.

## Key points

- Shares the first compartment with APL; in many people a septum gives it a separate subcompartment, which must be opened in De Quervain release
- Forms the palmar border of the anatomical snuffbox with APL; EPL forms the dorsal border
- It extends the thumb MCP joint only; extension of the tip (IP joint) is EPL's job, which helps sort out which tendon is cut

## Key facts

**Origin**
Posterior [radius](https://durgical.com/en/anatomy/radius/) and interosseous membrane

**Insertion**
Base of the [proximal phalanx of the thumb](https://durgical.com/en/anatomy/proximal-phalanx-of-the-thumb/)

**Action**
Extends the thumb at the MCP joint

**Nerve**
[Posterior interosseous nerve](https://durgical.com/en/anatomy/posterior-interosseous-nerve/) (C7–C8)

**Blood supply**
Posterior interosseous artery

## Relations

**In front**
[Abductor pollicis longus](https://durgical.com/en/anatomy/abductor-pollicis-longus/) (APL), its companion in the first compartment, lying on its palmar side

**Behind**
Across the anatomical snuffbox, the [extensor pollicis longus](https://durgical.com/en/anatomy/extensor-pollicis-longus/) (EPL)

**Deep**
Radial styloid; in the snuffbox the [radial artery](https://durgical.com/en/anatomy/radial-artery/), the [scaphoid](https://durgical.com/en/anatomy/scaphoid/) and the [trapezium](https://durgical.com/en/anatomy/trapezium/)

**Superficial**
Branches of the [superficial radial nerve](https://durgical.com/en/anatomy/superficial-branch-of-the-radial-nerve/) and the beginning of the [cephalic vein](https://durgical.com/en/anatomy/cephalic-vein/) over the snuffbox

## How to find it

With the thumb extended and abducted, a hollow (the anatomical snuffbox) appears at its base; EPB is the tendon on the palmar edge of the hollow, traceable to the base of the thumb's proximal phalanx.

## Examination

- Finkelstein test: pain over the radial styloid with the thumb folded and the wrist tilted toward the little finger supports De Quervain tenosynovitis
- Resisted MCP extension with the thumb tip kept bent isolates EPB; pain over the first compartment suggests EPB involvement or a separate subcompartment

## On imaging

Shares the first extensor compartment with APL. Ultrasound shows de Quervain tenosynovitis as thick tendons with sheath fluid and may show a septum giving EPB its own subcompartment, a reason injection or release fails. With APL it forms the radial border of the anatomical snuffbox, the landmark for examining the scaphoid, which radiographs then show.

## Clinical notes

- Tenderness in the snuffbox, between EPB and EPL, raises suspicion of a scaphoid fracture
- Persistent pain after De Quervain surgery often means an unreleased EPB subcompartment

## Anatomical variants

- Sometimes absent or fused with APL; extra slips can continue to the distal phalanx or join EPL

## Related conditions

- [De Quervain tenosynovitis](https://durgical.com/en/conditions/de-quervain-tenosynovitis/)

## Test yourself

Think of the answer, then tap the question to see it.

## Which thumb joint does the EPB (extensor pollicis brevis) extend, and how does that differ from the EPL?

Only the MCP joint; extension of the thumb tip (IP joint) is the EPL's job.

## Why can pain persist after de Quervain surgery, in relation to the EPB?

The EPB often has a separate subcompartment; if it is not released, pain persists.

## What does tenderness in the anatomical snuffbox, between EPB and EPL, raise suspicion of?

A scaphoid fracture.

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Educational information, not medical advice. For any symptom, see a health professional.

## Its names

**Official Latin (TA2)**
extensor brevis pollicis TA2 2518

**Official Spanish (DeCS)**
Músculo extensor corto del pulgar

**English (TA2)**
Extensor pollicis brevis

## Related

- [Posterior interosseous nerve](https://durgical.com/en/anatomy/posterior-interosseous-nerve/)
- [Interosseous arteries](https://durgical.com/en/anatomy/interosseous-arteries-of-the-forearm/)
- [Proximal phalanx, thumb](https://durgical.com/en/anatomy/proximal-phalanx-of-the-thumb/)
- [Radius](https://durgical.com/en/anatomy/radius/)
- [APL and EPB tendons](https://durgical.com/en/anatomy/abductor-pollicis-longus-and-extensor-pollicis-brevis-tendons/)

Sources: Terminologia Anatomica (TA2); [Anatomy, Shoulder and Upper Limb: Hand Anatomical Snuff Box](https://www.ncbi.nlm.nih.gov/books/NBK482228/); [Anatomy, Shoulder and Upper Limb, Wrist Extensor Muscles](https://www.ncbi.nlm.nih.gov/books/NBK534805/).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/anatomy/extensor-pollicis-brevis/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/anatomy/extensor-pollicis-brevis/