Radial Nerve

Introduction

Introduction to the Radial Nerve in the Arm

The radial nerve is the largest branch of the brachial plexus, arising from the posterior cord (C5-C8, T1). In the arm, it is the primary nerve of the posterior compartment, supplying the triceps brachii (all three heads) and providing cutaneous innervation to the posterior arm. It is unique because it exits the axilla, enters the posterior compartment by passing through the triangular interval, then runs in the radial (spiral) groove of the humerus, where it lies directly on the bone. After winding laterally, it pierces the lateral intermuscular septum to reach the anterior compartment (cubital fossa), where it divides into superficial (sensory) and deep (motor/posterior interosseous) branches. Its close relationship to the humerus makes it highly vulnerable to mid-shaft fractures, compression ('Saturday night palsy'), and iatrogenic injury.

Study

Course of the Radial Nerve in the Arm

The radial nerve enters the arm from the axilla, lying posterior to the axillary artery. It then passes through the triangular interval (borders: teres major superiorly, long head of triceps medially, humerus laterally). It then runs in the radial (spiral) groove on the posterior surface of the humerus, between the lateral and medial heads of the triceps. At the distal third of the humerus, it pierces the lateral intermuscular septum (approximately 10 cm above the lateral epicondyle) to enter the anterior compartment. It then descends between the brachialis (medially) and brachioradialis (laterally) and divides into superficial and deep branches at the level of the radial head.

Branches of the Radial Nerve in the Arm

1. Muscular branches to triceps brachii: Usually multiple branches – to long head (in axilla), to lateral and medial heads (in radial groove). The branch to the medial head also supplies the anconeus (via the nerve to anconeus). 2. Posterior cutaneous nerve of the arm: Arises in the axilla, supplies skin over posterior arm. 3. Inferior lateral cutaneous nerve of the arm: Arises in the radial groove, supplies skin over lower lateral arm. 4. Posterior cutaneous nerve of the forearm: Arises in the radial groove, supplies skin over posterior forearm. 5. Articular branches to the elbow joint (variable). Note: The radial nerve gives off its motor branches to the triceps BEFORE entering the radial groove; therefore, a lesion in the groove spares triceps function (elbow extension preserved).

Clinical Significance

Radial nerve injury is common due to its superficial position and bony relationship. - Mid-shaft humeral fracture (Holstein-Lewis fracture – distal third spiral fracture) can entrap or transect the radial nerve. - Saturday night palsy: Compression of the radial nerve in the spiral groove against the humerus (e.g., sleeping with arm over a chair back or hanging over a bar). - Radial nerve injury causes wrist drop (loss of wrist extensors), finger drop (loss of finger extensors), and thumb drop (loss of thumb extension). Sensory loss over dorsal hand (first web space) – the 'anatomical snuffbox' area sparing? Actually, radial nerve supplies dorsum of hand over thumb, index, middle (proximal phalanges), but the superficial branch is purely sensory. - High radial nerve lesion (axilla) causes triceps weakness (loss of elbow extension) in addition to wrist/finger drop. - Triceps reflex (C7) is lost in high radial nerve injuries.

Summary

Key Takeaways – Radial Nerve in the Arm

Origin: Posterior cord of brachial plexus (C5-C8, T1). Course: Axilla → triangular interval → radial groove (posterior humerus) → pierces lateral intermuscular septum → anterior compartment. Motor: Triceps brachii (all heads) – branches given off in axilla/proximal arm. Sensory: Posterior arm, lateral arm, posterior forearm. Clinical: Injury causes wrist drop, finger drop; high lesion also triceps weakness. Most vulnerable in mid-shaft humeral fractures and compression in spiral groove.