Deep extensors of the forearm

Supinator muscle

The supinator muscle has a complex origin tendon arising from the lateral epicondyle of the humerus (superficial part) and the supinator crest of the proximal ulna (deep part). Furthermore, it also attaches to the radial collateral ligament and radial anular ligament.

From there, the muscle curls dorsally around the neck of the radius and inserts at the posterior,  lateral and anterior sides of the ulna, between the radial tuberosity and the insertion of the pronator teres.

Abductor pollicis longus muscle

This muscle originates at the posterior surface of the proximal half of the radius, ulna and the interosseous membrane. It inserts at the base of the metacarpal bone of the thumb. Often the insertion tendon splits into two and additionally attaches to the trapezium.

Extensor pollicis longus muscle

The EPL arises at the posterior surface of the middle third of the ulna and the interosseous membrane and inserts at the base of the distal phalanx of the thumb. There, it forms the extensor expansion/dorsal aponeurosis covering the thumb medially, laterally and dorsally. This sheet-like tendinous expansion serves mainly as an insertion point for muscles and ligaments (similar to the dorsal aponeurosis of the extensor digitorum).

Extensor pollicis brevis muscle

The EPB begins more distally at the posterior surface of the distal third of the radius and the interosseous membrane and courses to the posterior aspect of the proximal phalanx of the thumb.

Extensor indicis muscle 

The extensor indicis originates at the posterior surface of the distal third of the ulna and the interosseous membrane. It inserts at the extensor expansion/dorsal aponeurosis of the index finger.

Innervation of the deep extensors

Like all extensors of the forearm, these five muscles are innervated by the radial nerve (C6-C8).

The radial nerve divides into a superficial branch and deep branch at the level of the radial head. While the superficial branch runs along the brachioradialis, the deep branch continues between the two layers of the supinator. Thereupon, it penetrates the supinator muscle and branches off as the posterior interosseous nerve (C7-C8), which is responsible for the innervation of almost all deep extensors.

The supinator is the only deep extensor that receives supply directly from motor branches of the radial nerve.

The anatomical 'snuffbox'

A triangular depression lies at the radial side of the dorsum of the hand, which becomes even more prominent during hand extension. It is commonly referred to as the anatomical snuff box (tabatière anatomique) or radial fossa. The depression is framed by the radius (proximal) and both the tendons of the extensor pollicis longus (dorsal), and of the extensor pollicis brevis (palmar); its base is formed by the trapezium and scaphoid bone. The radial artery courses through the anatomical snuff box and, thus, can be easily felt here.