This arises below abductor pollicis longus from the radius and the adjacent interosseous membrane (Fig. 2.41). It spirals from the depths of the forearm around the radial extensors and brachioradialis, in contact with abductor pollicis longus, whose tendon is somewhat overlies on the radial border of the snuffbox. Its slender tendon lies with the tendon of abductor pollicis longus at the radial border of the snuffbox and then passes along the dorsal surface of the first metacarpal, and is inserted into the base of the proximal phalanx.
Nerve supply of extensor pollicis brevis:
By the posterior interosseous nerve (C7, 8) in the forearm.
Action of extensor pollicis brevis:
It extends the proximal phalanx of the thumb. It prevents flexion of the metacarpophalangeal joint when flexor pollicis longus is flexing the terminal phalanx, as in pinching index and thumb pads together (e.g. threading a needle).