Deep extensors of the forearm

The deep extensors of the forearm consist of five muscles located at the posterior side of the forearm. These muscles are: the supinator muscle, abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus and extensor indicis muscle.

They act on interphalangeal joints causing certain fingers to extend, but also act on the wrist joint causing its dorsal flexion. They are all innervated by the radial nerve, which is the branch of the brachial plexus. Their muscle bellies and tendons form the surface of the distal forearm and the wrist where they can be easily palpated.

This article will discuss the anatomy of the deep extensors of the forearm.

Anatomy and supply

The following muscles are known to be the deep extensors.

Supinator muscle

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

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

Abductor pollicis longus muscle

This muscle originates at the posterior side of both the radius and ulna and the interosseous membrane and 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 brevis muscle

The EPB begins more distally at the posterior side of the radius and the interosseous membrane and courses to the proximal phalanx of the thumb.

Extensor pollicis longus muscle

The EPL arises more medially at the posterior side of the ulna and the interosseous membrane and inserts at the base of the distal phalanx of the thumb. There, it forms the 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 indicis muscle 

The extensor indicis originates at the posterior side of the ulna and the interosseous membrane near the wrist and inserts at the 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.

Functions

The main function of the deep extensors is to move the joints of the hand and fingers (except for the supinator muscle).

• The abductor pollicis longus pulls the thumb forward (abduction) at the 'saddle-like' caprometacarpal joint of the thumb. It also contributes to   to a lateral movement of the radius at the wrist joint (radial abduction).
• The extensor pollicis brevis extends the thumb at the caprometacarpal and metacarpophalangeal joints of the thumb. It too is involved in  radial abduction at the wrist.
• Its 'big brother', the extensor pollicis longus, can additionally extend the thumb in the interphalangeal joint of the thrumb due to its more distal attachment. Its contraction causes also a dorsal extension of the hand.
• The extensor indicis pulls the index finger straight by fulfilling extension at the metacarpophalangeal, proximal interphalangeal and distal interphalangeal joints. This leads to an extension of the hand at the wrist as well.
• In contrast, the function of the supinator is to pull the radius, causing a lateral rotation at the radioulnar joints (supination).