The radial nerve (C5-T1) is the largest branch of the brachial plexus and the nerve of the extensor muscles of the arm and forearm. It winds around the back of the humerus in the spiral groove, where it lies directly on bone, then pierces the lateral intermuscular septum and divides in front of the lateral epicondyle into a superficial sensory branch and a deep motor branch (the posterior interosseous nerve). This division point separates a full radial nerve palsy from a posterior interosseous nerve palsy.
| Origin | Posterior cord of the brachial plexus (C5, C6, C7, C8, T1) |
|---|---|
| Course | Leaves the axilla through the triangular interval below teres major, winds around the back of the humerus in the spiral groove with the profunda brachii artery, pierces the lateral intermuscular septum, and divides anterior to the lateral epicondyle into superficial and deep (posterior interosseous) branches |
| Motor |
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| Sensory |
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The radial nerve is the nerve most often injured with humeral shaft fractures, especially at the junction of the middle and distal thirds, because it lies on the bone in the spiral groove. It is also compressed there when the arm hangs over a chair back during deep sleep (Saturday night palsy). A spiral groove lesion causes wrist drop, finger and thumb drop and numbness over the back of the hand, while elbow extension is usually preserved because most triceps branches arise above the groove. Most fracture-related palsies recover spontaneously, so management is usually splinting and observation, with EMG and exploration if there is no recovery by about 3-4 months. A lesion of the deep branch alone (posterior interosseous nerve) spares wrist extension by extensor carpi radialis longus and causes no sensory loss.
Humeral shaft fracture or compression in the spiral groove causes wrist and finger drop with dorsal hand numbness but usually spares triceps; most cases recover over weeks to months with splinting.
Entrapment of the deep branch beneath the arcade of Frohse causes finger and thumb extension weakness with preserved wrist extension and no sensory loss.
