Extensor pollicis longus muscle

Origin and insertion

Extensor pollicis longus originates from the middle third of the posterior surface of ulna, mostly along its radial border. This attachment extends onto the adjacent interosseous membrane, and is situated proximal to the origin of extensor indicis muscle. From here, the muscle belly runs obliquely in a radial direction, towards the lateral aspect of the wrist joint. The muscle ends in a tendon that passes deep to extensor retinaculum and travels across the posterolateral aspect of the hand to insert onto the base of the distal phalanx of the thumb. 

More specifically, the tendon of extensor pollicis longus forms what is referred to as an extensor expansion (or dorsal aponeurosis) as it is joined by the tendon of abductor pollicis brevis laterally and adductor pollicis medially. Together, this aponeurosis forms the extensor mechanism of the thumb.

Relations

Extensor pollicis longus is a deep muscle of the posterior aspect of forearm extending over the distal aspect of interosseous membrane and distal radioulnar joint. It is initially deep to the extensor digitorum, medial to abductor pollicis longus and lateral to extensor indicis, with the posterior interosseous artery and nerve running across the posterior aspect of the muscle. The relatively long tendon of extensor pollicis longus traverses the tendons of extensor carpi radialis longus and extensor carpi radialis brevis in the distal aspect of hand.  

At the level of the radiocarpal joint, the tendon of extensor pollicis longus sits medial to that of extensor pollicis brevis, which collectively form a depression (when the thumb is extended) known as the anatomical snuffbox. 

Innervation

Extensor digitorum 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 from the posterior interosseous artery and perforating branches of the anterior interosseous artery.

Function

The main action of extensor pollicis longus is extension of the thumb at the metacarpophalangeal and interphalangeal joints. Extension at the metacarpophalangeal joint occurs in synergy with extensor pollicis brevis muscle. When the thumb reaches the full extension or abduction, extensor pollicis longus can also assist in adduction of the thumb. 

Owing to the fact that their tendons cross the distal radioulnar joint, the extensors of thumb may assist in the supination of the forearm, as well as extension and abduction of the wrist joint.