Extensor Pollicis Brevis - Structure, Diagram, Function

The extensor pollicis brevis (EPB) is one of the deep muscles of the posterior compartment of the forearm. It acts to extend the thumb at the metacarpophalangeal and carpometacarpal joints. Working in concert with the abductor pollicis longus and extensor pollicis longus, it plays a vital role in thumb movement and hand function.

Location

The EPB is located in the distal posterior forearm. It lies lateral (radial) to the extensor pollicis longus and medial to the abductor pollicis longus. Its tendon passes through the first dorsal compartment of the wrist along with the tendon of the abductor pollicis longus.

Structure

This small fusiform muscle originates from the radius and interosseous membrane and extends toward the thumb, forming a short tendon that inserts onto the base of the proximal phalanx.

Origin: Posterior surface of the distal third of the radius and adjacent interosseous membrane.
Insertion: Dorsal surface of the base of the proximal phalanx of the thumb.
Tendon Path: Passes beneath the extensor retinaculum through the first dorsal compartment, alongside the abductor pollicis longus tendon.

Function

The extensor pollicis brevis contributes to:

Extension of the Thumb: Extends the thumb at the metacarpophalangeal joint.
Carpometacarpal Joint Extension: Also assists in extension and slight abduction at the carpometacarpal joint of the thumb.
Wrist Movement: Provides weak assistance in radial deviation (abduction) and wrist extension.

Innervation

Innervated by the posterior interosseous nerve (C7–C8), the terminal branch of the deep radial nerve.

Blood Supply

Posterior interosseous artery: Main arterial supply.

Relations

The EPB lies between the extensor pollicis longus (medially) and abductor pollicis longus (laterally). At the wrist, the tendon contributes to the lateral border of the anatomical snuffbox, while the extensor pollicis longus forms the medial border.

Surface Anatomy

The tendon of the EPB can be visualized and palpated on the radial side of the wrist when the thumb is actively extended. It helps define the lateral boundary of the anatomical snuffbox, along with the abductor pollicis longus.

Development

The EPB arises from the deep layer of the extensor mass during forearm development. It shares embryological origin with other thumb extensors and is innervated by the radial nerve like the rest of the dorsal musculature.

Anatomical Variations

The EPB tendon may be fused partially or completely with the abductor pollicis longus.
May be absent in some individuals, with its function compensated by adjacent muscles.
Insertion may vary, sometimes attaching to the extensor expansion or base of the distal phalanx.

Clinical Significance

De Quervain’s Tenosynovitis: The EPB tendon is one of the two tendons affected in this condition, where inflammation of the first dorsal compartment leads to pain over the radial side of the wrist, especially with thumb movement.
Tendon Entrapment: Thickening of the tendon sheath or fibrosis can restrict motion and cause pain.
Steroid Injections and Splinting: Targeted interventions are often directed at the first extensor compartment containing EPB and APL tendons.

Comparative Anatomy

In many mammals, the extensor pollicis brevis is not well developed or may be absent, reflecting simpler thumb function. In humans, it has evolved to enable refined thumb extension and contribute to the dexterous use of the hand in tool manipulation and fine motor control.