Extensor Pollicis Brevis: Origin, Insertion, Nerve, Action & De Quervain's

The Extensor Pollicis Brevis (EPB) is a deep muscle of the posterior forearm that works specifically on the thumb. It acts as the "little sibling" to the Extensor Pollicis Longus. It is famous for traveling with the Abductor Pollicis Longus and for its involvement in thumb pain known as De Quervain's Tenosynovitis.

[Image of Extensor Pollicis Brevis muscle anatomy]

Quick Anatomy Snapshot

Origin (Proximal)	Posterior surface of the distal third of the Radius and the Interosseous Membrane.
Insertion (Distal)	Dorsal surface of the base of the Proximal Phalanx of the Thumb (1st digit).
Nerve Supply	Posterior Interosseous Nerve (Deep branch of Radial Nerve) - C7, C8.
Blood Supply	Posterior Interosseous Artery.
Primary Actions	Thumb Extension: Extends the thumb at the MCP and CMC joints. Radial Deviation: Assists in deviating the wrist. Note: Does NOT extend the tip of the thumb (IP joint).

Deep Dive: The Snuffbox Border

The EPB is anatomically inseparable from its partner, the Abductor Pollicis Longus (APL).

1. The First Compartment

The EPB and APL tendons run together in the 1st Dorsal Compartment of the wrist. They pass through a tight fibro-osseous tunnel over the radial styloid. This tunnel is the site of friction in De Quervain's Tenosynovitis.

2. The Anatomical Snuffbox

Extend your thumb. You will see a triangular depression at the base of the thumb called the Anatomical Snuffbox.
• The Lateral (Radial) Border is formed by the EPB and APL tendons.
• The Medial (Ulnar) Border is formed by the Extensor Pollicis Longus (EPL).

Physio Corner: Clinical Relevance

💪 Functional Fact: The EPB extends the knuckle of the thumb (MCP joint) but leaves the tip (IP joint) relaxed. This is useful when you want to open your hand to grab a large object without hyperextending the thumb tip.

Palpation

Ask the patient to extend their thumb (make a "hitchhiker" sign). Locate the Anatomical Snuffbox. Palpate the radial (lateral) border of the snuffbox. You will feel two tendons close together: the ventral one is APL, and the dorsal one is EPB. The EPB tendon can be traced to the base of the proximal phalanx.

⚠️ Clinical Pathology: De Quervain's Tenosynovitis
Inflammation of the sheath surrounding the EPB and APL tendons causes severe pain on the thumb side of the wrist.
Finkelstein's Test: Have the patient tuck their thumb inside their fist and then ulnar deviate the wrist. Pain over the radial styloid suggests a positive test.

Manual Muscle Testing (MMT)

To isolate the EPB from the EPL, we focus on the proximal phalanx.

Testing Tip: Stabilize the 1st Metacarpal. Apply resistance only to the proximal phalanx. If you apply resistance to the tip of the thumb, you are testing the Longus (EPL), not the Brevis!

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Resistance)	Position: Forearm in neutral (mid-position), wrist neutral. Action: Patient extends the thumb at the MCP joint (lifts the knuckle) while keeping the IP joint relaxed. Resistance: Applied to the dorsal aspect of the Proximal Phalanx, pushing down into flexion. Grade 3: Full range against gravity. Grade 4/5: Holds against resistance.
Grade 2 (Gravity Eliminated)	Position: Forearm pronated (palm down), hand flat on table. Action: Patient slides the thumb out/extends laterally. Result: Full range of motion.
Grade 0, 1 (Palpation)	Action: Palpate the snuffbox tendons at the base of the thumb. Cue: "Try to lift your thumb knuckle." Grade 1: Tendon tension felt. Grade 0: No activity.

Frequently Asked Questions

What is the main difference between EPB and EPL?

EPB (Brevis): Inserts on Proximal Phalanx, acts on MCP joint, runs in 1st compartment.
EPL (Longus): Inserts on Distal Phalanx, acts on IP joint (tip), runs in 3rd compartment.

Does EPB originate from the Ulna?

No. The EPB originates from the Radius and interosseous membrane. Its partner, the APL, originates from both the Radius and Ulna.

Can you live without the EPB?

In rare cases, the EPB is absent or fused with the APL. Hand function is usually maintained by the APL and EPL, though independent MCP extension might be slightly weaker.

Test Your Knowledge: EPB Quiz

1. Which nerve supplies the Extensor Pollicis Brevis?

2. Where does the EPB insert?

3. Which dorsal compartment does EPB occupy?

4. Finkelstein's test is used to diagnose pathology in which muscles?

5. Does EPB form a border of the Anatomical Snuffbox?

6. Where does the EPB originate?

7. Which muscle travels immediately alongside the EPB tendon?

8. Does the EPB extend the interphalangeal (IP) joint of the thumb?

9. To test MMT Grade 5, resistance is applied to:

10. What is the primary joint action of EPB?

References

• Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2018). Clinically Oriented Anatomy. 8th ed. Philadelphia: Wolters Kluwer.
• Standring, S. (2016). Gray's Anatomy: The Anatomical Basis of Clinical Practice. 41st ed. Elsevier.
• Magee, D. J. (2014). Orthopedic Physical Assessment. 6th ed. Elsevier.