Radial Nerve

Within the axilla, the radial nerve lies posterior to the axillary artery, and subsequently upon the subscapularis and latissimus dorsi muscles while anterior to the teres major muscle. Here it is vulnerable to compression injuries, such as inappropriate positioning of crutches. As the radial nerve exits the axilla, entering the arm, it lies between the brachial artery and the triceps brachii long head innervating the triceps long and medial heads as it passes deep to this muscle.

Proximal to the spiral groove, the posterior cutaneous nerve of the arm arises from the radial nerve. The primary radial nerve trunk (C5, 6) then passes distally, accompanying the brachial artery within the shallow radial (spiral) groove of the posterior humerus, where it is vulnerable to external compression with pressure against the humerus, potentially leading to wristdrop. More distally, the radial nerve takes a medial to posterolateral direction. The radial posterior muscular branches innervate the lateral and medial heads of the triceps muscle (C6, 7) as well as a long, slender branch to the distal anconeus muscle (C6, 7) at the elbow. This small forearm extensor muscle, lying adjacent to the lateral epicondyle of the humerus, sometimes helps localize the primary site of radial neuropathies. If electromyography (EMG) demonstrates anconeus denervation, the lesion site approximates the spiral groove, that is, midhumerus; if the anconeus is unaffected, the injury is distal to the radial groove. The posterior cutaneous nerve of the forearm crosses above the anconeus muscle.

Distal to the spiral groove, adjacent to the lower humerus, the radial nerve lies within the furrow between the medial brachialis (C5, 6) muscle and the brachioradialis (C5, 6) /extensor carpi radialis longus (C6, 7) laterally, innervating these muscles. There are three cutaneous radial nerve branches originating proximal to the elbow: (1) the posterior cutaneous and (2) lower lateral cutaneous nerves of the arm, and (3) the posterior cutaneous nerve of the forearm.

RADIAL NERVE IN FOREARM

As the radial nerve enters the forearm, piercing the lateral intermuscular septum while descending anterior to the lateral humeral epicondyle, it soon supplies lateral muscular branches, innervating the most proximal brachioradialis and extensor carpi radialis muscles. Here the radial nerve bifurcates into its superficial and deep terminal branches. The superficial sensory radial branch (SSRB) descends within the forearm deep to the brachioradialis and extensor carpi radialis muscles, eventually emerging in the distal forearm as the superficial terminal sensory radial branch (STSRB), descending along the anterolateral side of the forearm. In its upper third, the STSRB branch and the radial artery converge meeting midway down the lateral forearm, only again to diverge in the distal forearm as the STSRB inclines posterolaterally, deep to the brachioradialis tendon. Here it pierces the deep fascia, subdividing into a lateral branch supplying the radial side of the thumb and a medial branch splitting into four or five dorsal digital nerves. This cutaneous sensory innervation extends just to the distal interphalangeal joints, whereas the most distal phalanges are supplied, respectively, by the median (digits 1-3.5) and ulnar (digits 4.5 and 5) nerves. The cutaneous hand areas supplied by the radial, median, and ulnar nerves can have individual variations due to intrabranch communication and minor overlap.

The terminal deep muscular radial branch winds posteroinferiorly around the lateral side of the radius innervating the brachioradialis (C5, 6), extensor carpi radialis longus (C6, 7), and supinator (C5, 6, 7). It enters the supinator muscle through the arcade of Frohse between its superficial and deep heads, reaching the posterior forearm as the posterior interosseous nerve (PIN). This accompanies the posterior interosseous artery between the superficial and deep extensor forearm musculature innervating the extensor carpi radialis brevis (C6, 7), extensor digitorum and minimi (C7, 8), extensor carpi ulnaris (C7, 8), abductor pollicis longus (C7, 8), extensor pollicis longus (C7, 8) and brevis (C8), and extensor indicis proprius (C8). The radial nerve terminates as a small nodule, a pseudoganglion, sending filaments to the distal bones, joints, and ligaments.

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