Extensor pollicis brevis muscle

Origin and insertion

Extensor pollicis brevis originates from the posterior surface of the distal third of radius, inferior to the origin of extensor pollicis longus. The origin area extends to the adjacent interosseous membrane. From here, the muscle runs downwards towards the wrist, ending in a cord-like tendon proximally to the wrist.

The tendon passes deep to extensor retinaculum, between the tendons of abductor pollicis longus and extensor carpi radialis longus. Along with abductor pollicis longus, it is located in the first extensor (dorsal) compartment of the wrist. After passing under the retinaculum, the tendon runs laterally to insert onto the dorsal surface of the base of the proximal phalanx of thumb.

Relations

Extensor pollicis brevis is a deep extensor of the thumb that lies deep to extensor digitorum muscle. It sits directly medial to abductor pollicis longus and posterolateral to extensor pollicis longus muscle. Just above the wrist, extensor pollicis brevis obliquely crosses the tendons of extensor carpi radialis brevis and extensor carpi radialis longus muscles.

Together with the tendon of abductor pollicis longus, the tendon of extensor pollicis brevis comprises the lateral border of a triangular depression on the lateral aspect of the wrist, called the anatomical snuffbox. The tendon of extensor pollicis longus and the styloid process of radius make the medial and proximal borders of this space, respectively. Among other structures, the snuffbox is traversed by the radial artery. This is an important clinical point since it is the most common spot for the palpation of the radial pulse.

Innervation 

Extensor pollicis brevis is innervated by posterior interosseous nerve which is a continuation of a deep branch of radial nerve (root value C7 and C8).

Blood supply

Extensor pollicis brevis receives its blood supply by posterior interosseous artery and perforating branches from the anterior interosseous artery, which are the branches of common interosseous artery. The common interosseous artery arises immediately below the tuberosity of radius from the ulnar artery.

Function

Together with extensor pollicis longus, extensor pollicis brevis is in charge of extension of the thumb in the first metacarpophalangeal joint. It also extends the thumb in the carpometacarpal joint of the thumb. This movement is important in the anatomy of the grip, as it enables letting go of an object. As it crosses the wrist, extensor pollicis brevis also participates in the extension and abduction of this joint.