Extensor Pollicis Brevis

What is Extensor Pollicis Brevis

The extensor pollicis brevis (EPB) is a slender, flat muscle in the deep posterior compartment of the forearm. Relatively short for a forearm muscle, it is one of the three muscles forming the boundaries of the anatomical snuff box. The abductor pollicis longus and extensor pollicis longus are the two other muscles contributing to this anatomical space in the hand.

Anatomy

Location and Attachments

Origin	The posterior surface of the radius’s distal one-third
Insertion	The posterior surface of the 1st proximal phalangeal base

Origin

The extensor pollicis brevis originates from the back of the radial shaft at its distal side. Some muscle fibers also originate from the interosseous membrane that runs from the radius to the ulna. The point of attachment on the radius lies just inferior to the point where the abductor pollicis longus, the other longer, deep posterior forearm muscle, originates.

Insertion

The muscle belly runs distally, narrowing into a tendon just before the wrist. The tendon runs between the superficial posterior muscle extensor carpi radialis longus and the abductor pollicis longus to pass deep to the extensor retinaculum. The muscle courses further distally, alongside the abductor pollicis longus, sharing the 1st extensor compartment of the wrist. It then enters the hand on its dorsal surface and inserts at the base of the 1st proximal phalanx (of the thumb).

Relations With Surrounding Muscles and Structures

The extensor pollicis brevis has the extensor digitorum lying superficial to it, while the abductor pollicis longus lies laterally. The extensor pollicis longus runs anterolateral to the EPB.

On the distal side, just before it reaches the wrist, the EPB tendon crosses the extensor carpi radialis brevis and longus tendons obliquely before passing underneath the extensor retinaculum.

Near the wrist, the tendon forms the radial border of the anatomical snuff box along with the abductor pollicis longus tendon. The anatomical snuff box is a triangular depression on the radial side of the hand at the base of the thumb. It is where the radial pulse is taken as the radial artery passes through this space.

Function

Action

Extension of the thumb at the carpometacarpal and metacarpophalangeal joints

Along with the extensor pollicis longus, this muscle primarily acts on the 1st metacarpophalangeal joint to extend or straighten the thumb. The muscle also acts on the carpometacarpal joint of the thumb to extend it.

Both these actions are essential for all movements and functions of the thumb. The muscle plays a vital role in opening a grip to let go of something held in your hand.

Another minor function of the extensor pollicis brevis is assisting the extensors and abductors of the wrist in their action.

Innervation

Nerve

Posterior interosseous nerve (C7 and C8)

The posterior interosseous nerve (C7, C8), continuing from the radial nerve’s deep branch, innervates the extensor pollicis brevis.

Blood Supply

Artery

Posterior interosseous artery

The posterior interosseous artery supplies the muscle’s primary vasculature, while the perforating branches of the anterior interosseous artery provide an additional supply. Both are branches of the ulnar artery.