Anatomy Review: The course of the Radial Nerve
One of the divisions of the posterior cord of the brachial plexus (along with the Auxillary nerve), the Radial nerve consists of neural fiber contributions from the C5-T1 nerve roots. When the posterior cord splits into these two main branches, it is located posterior to the brachial artery as it travels parallel to the lateral and medial cords, so named due to their relative position to the artery. The radial nerve continues following the split anterior to the insertion of the latissimus dorsi on the anterior and proximal aspect of the humerus (medial lip of the intertubercular groove). It then ‘dives’ posterior to the humerus to enter what is known as the ‘radial groove’ which courses inferiorly and laterally along the posterior surface of the bone. While in the groove, the nerve is covered superficially by the lateral head of the triceps brachii. (Note: the ‘radial groove’ is also the origin of the medial head of the triceps)
As the nerve continues along the groove on route to the lateral elbow, a single branch exits the nerve proper which quickly divides further into two branches subsequent branches: First is the ‘Inferior lateral cutaneous nerve of the arm’ which pierces the lateral head of the tricep on route to innervate the skin located on the lateral aspect of the arm inferior to the deltoid; the second branch, the ‘Posterior cutaneous nerve of the forearm,’ travels inferiorly past the elbow to innervate the skin of the posterior forearm. Of clinical note, numbing or tingling in the posterior forearm cannot be caused by irritation of the nerve distal to the elbow as the fibers innervating this region separates from the radial nerve proximal to the elbow.
The radial nerve proper continues into the lateral intermuscular septum, a fascial tunnel located just superior to the lateral epicondyle, on route to the anterior aspect of the elbow. The nerve surfaces from the septum anterior to the lateral epicondyle just lateral to the lateral edge of the brachialis, and medial to the brachioradialis. At this point, the ‘Superficial branch of the radial nerve’ separates from the nerve proper and courses inferiorly deep to the brachioradialis. In the inferior 1/3 of the forarm, the superficial branch surfaces in the posterior forearm between the tendons of the brachioradialis and the ECRL. It then courses superficial to the outcropper muscles (Abductor pollicis longus, EPL, & EPB) to innervate the dorsal aspect of the 1st, 2nd, 3rd, and lateral half of the 4th digit up to the DIP joints. The radial nerve proper, in the anterior aspect of the elbow (anterior to the lateral epicondyle) dives deep to the supinator muscle in the ‘arcade of frohse,’ to end up once again on the dorsal aspect of the forearm. At this point, the radial nerve is strictly a motor nerve, which serves the muscles of the posterior forearm.