Extensor pollicis longus

At a glance

English

Extensor pollicis longus

Latin (TA2)

Extensor longus pollicis

System

Muscle

Region

Upper limb

Side

Paired, left and right

About the extensor pollicis longus

Extensor pollicis longus lies in the deep layer of the back of the forearm, between abductor pollicis longus on its radial side and extensor indicis on its ulnar side. It is the longest of the muscles that move the thumb from the forearm.

It arises from the middle third of the ulna on its posterior face and from the neighbouring interosseous membrane. The belly runs obliquely towards the radius and ends in a tendon that enters its own channel, the third compartment under the extensor retinaculum. There it turns sharply around the dorsal tubercle of the radius, which acts as a pulley and changes its line of pull towards the thumb. The tendon then crosses the tendons of the two radial wrist extensors and runs along the back of the thumb to reach its terminal phalanx at the base.

Because of that change of direction it does more than straighten the thumb. It extends the interphalangeal joint, and it also extends the metacarpophalangeal and carpometacarpal joints, pulling the thumb back and towards the index, a movement called retropulsion. It assists extension and radial tilt of the wrist. It is the only muscle that can extend the thumb tip strongly against resistance. Its nerve is the posterior interosseous nerve and its blood comes from the posterior interosseous artery.

The tendon forms the ulnar or back boundary of the anatomical snuffbox and stands out clearly when the thumb is lifted. It can fray and snap where it bends round the radial tubercle, either weeks after an undisplaced wrist fracture or in rheumatoid arthritis. The thumb tip then droops, and the usual repair is a transfer of the extensor indicis tendon.

Attachments, action and supply

Origin

Posterior face of the middle third of the ulna and the adjacent interosseous membrane.

Insertion

Dorsal side of the thumb's terminal phalanx, at its base.

Action

Extends the thumb at the interphalangeal, metacarpophalangeal and carpometacarpal joints and draws it backwards; assists wrist extension.

Nerve supply

Posterior interosseous nerve (C7, C8).

Blood supply

Posterior interosseous artery.