radial nerve

The radial nerve arises from the posterior cord of the brachial plexus (roots C5-T1) and travels through the arm in the spiral (radial) groove of the humerus, alongside the profunda brachii artery, making it vulnerable to injury with mid-shaft humeral fractures. It innervates the triceps brachii (elbow extension), anconeus, brachioradialis, extensor carpi radialis longus/brevis, and supinator in the arm/proximal forearm. It then branches into the superficial (sensory) and deep (motor, becomes posterior interosseous nerve) branches. The posterior interosseous nerve innervates the extensor digitorum, extensor carpi ulnaris, extensor pollicis longus/brevis, abductor pollicis longus, and extensor indicis—muscles responsible for wrist and finger extension. Sensory distribution covers the posterior arm and forearm and the dorsal hand, especially the dorsal web space between the thumb and index finger.

Clinical significance: Radial nerve injury classically causes wrist drop due to loss of wrist and finger extensors, with sensory loss over the dorsal hand web space. Common causes include mid-shaft humeral fractures (nerve injury at the spiral groove), 'Saturday night palsy' (compression during deep sleep, often with arm draped over a chair, associated with alcohol use), crutch use (axillary compression), and improper use of a tourniquet. The posterior interosseous nerve (a distal branch) can be injured in radial head dislocations or elbow fractures, causing finger drop but sparing wrist extension (since it branches after supplying ECRL). Testing wrist extension against resistance and checking sensation in the dorsal web space are key exam maneuvers. This nerve is highly tested for its anatomic course and correlation with fracture sites, as well as differentiating radial nerve palsy from other upper extremity neuropathies (e.g., ulnar claw hand, median nerve carpal tunnel syndrome).