Deep extensors of the forearm
Anatomy and supply
The deep extensors of the forearm consist of five muscles located at the posterior side of the forearm. Their muscle bellies and tendons form the surface of the distal forearm and the wrist where they can be easily palpated. The following muscles are known to be the deep extensors:
- 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: 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: 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: 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: 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.
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 height 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.
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.
The main function of the deep extensors is to move the joints of the hand and fingers (except the supinator). The abductor pollicis longus pulls the thumb forward at the saddle joint (abduction), which leads to a lateral movement of the radius at the wrist joint at the same time (radial abduction). The extensor pollicis brevis extends the thumb in the saddle and metacarpophalangeal joints, which also leads to a radial abduction at the wrist. Its 'big brother', the extensor pollicis longus, can additionally extend the thumb in the distal interphalangeal joint due to its more distal attachment. Its contraction causes also a dorsal extension of the hand. The extensor indicis pulls the index finger by fulfilling an extension at the metacarpophalangeal, proximal interphalangeal and distal interphalangeal joints. This leads to an extension of the hand as well. In contrast, the function of the supinator is to pull the radius, causing a lateral rotation at the radioulnar joints (supination).
The narrow course of the deep branch of the radial nerve between the two layers of the supinator muscle carries a high risk of entrapment, this is a condition referred to as supinator syndrome. A predilection site for this condition is a tendinous arch located at the superficial part of the supinator - arcade of Frohse, whereas rare causes include muscle hypertrophy or space-occupying lesions. The guiding symptom is pain at the proximal forearm, which increases during turning movements and can radiate to the wrist. Since the deep branch of the radial nerve supplies the extensors of the forearm, the affected patients additionally exhibit a weakened ability to extend the hand, fingers and thumb.