Radial nerve

Nervus radialis

Also known as: posterior interosseous nerve, superficial radial nerve

The largest branch of the brachial plexus (posterior cord, C5–T1) and the nerve of all the extensor muscles of the upper limb. It spirals around the humerus in the radial groove, which makes it vulnerable to fractures, and damage causes the classic "wrist drop".

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Overview

RootsC5–T1

Largest branch of plexusYes

Autonomous sensory zoneFirst dorsal web space

Key lesionWrist drop

The radial nerve continues the posterior cord behind the axillary artery. It supplies triceps in the axilla and arm and leaves the axilla through the triangular interval (below teres major, between the long head of triceps and the humerus) with the profunda brachii artery. It then winds around the back of the humerus in the radial (spiral) groove, between the medial and lateral heads of triceps. It pierces the lateral intermuscular septum and enters the anterior compartment, where it lies between brachialis and brachioradialis in front of the lateral epicondyle.

In the cubital fossa it divides into two branches:
• the superficial branch is purely sensory and runs under brachioradialis to the dorsum of the hand;
• the deep branch is motor; it passes through supinator (under the arcade of Frohse) and becomes the posterior interosseous nerve (PIN), which supplies the forearm extensors.

Because the branches to triceps arise proximal to the spiral groove, a mid-humeral injury usually spares elbow extension but causes wrist and finger drop.

Location: Posterior cord → posterior arm (radial groove) → anterolateral elbow → posterior forearm (PIN) and dorsum of the hand (superficial branch).

Parts & subdivisions

Superficial branch
Purely sensory. Its autonomous zone is the first dorsal web space. It can be compressed by tight watch straps or handcuffs (cheiralgia paraesthetica, or Wartenberg syndrome), causing numbness only.
Posterior interosseous nerve
The motor continuation of the deep branch through supinator. A PIN palsy (from radial head fracture or dislocation, a Monteggia injury, or the arcade of Frohse) causes finger and thumb drop without wrist drop, because ECRL arises proximally. Wrist extension deviates radially because ECU is lost. There is no sensory loss.

Anatomy

Innervation: All muscles of the posterior arm and posterior forearm, plus brachioradialis, and the skin of the posterior arm, posterior forearm and dorsolateral hand.
Nerve roots: C5, C6, C7, C8, T1 (posterior cord).
Course: Axilla (behind the axillary artery) → triangular interval with the profunda brachii artery → radial groove of the humerus → pierces the lateral intermuscular septum → between brachialis and brachioradialis anterior to the lateral epicondyle → divides in the cubital fossa. The superficial branch runs under brachioradialis to the dorsum of the hand. The deep branch passes through supinator to become the posterior interosseous nerve in the posterior compartment.
Branches: - Muscular: triceps (all heads), anconeus, brachioradialis, extensor carpi radialis longus, and a small part of brachialis.
• Deep branch / PIN: ECRB and supinator, then extensor digitorum, extensor digiti minimi, ECU, abductor pollicis longus, extensor pollicis brevis and longus, and extensor indicis.
• Cutaneous: posterior cutaneous nerve of the arm, inferior lateral cutaneous nerve of the arm, posterior cutaneous nerve of the forearm, and the superficial branch (dorsum of the lateral hand and the lateral 3½ digits proximal to the nail beds, including the first dorsal web space).

Function

Extension of the elbow (triceps, anconeus)
Extension of the wrist and digits; abduction and extension of the thumb
Supination (supinator) and elbow flexion in mid-pronation (brachioradialis)
Sensation of the posterior arm and forearm and the dorsolateral hand
Efferent limb of the triceps (C7) and brachioradialis (C6) reflexes

Clinical significance

Wrist drop (mid-shaft humeral fracture)

Fracture of the humeral shaft at the radial groove, or prolonged compression with the arm draped over a chair-back ("Saturday night palsy"), damages the nerve. The result is wrist drop (inability to extend the wrist and the MCP joints), weak grip because the flexors cannot act without wrist extension, and sensory loss over the first dorsal web space. Triceps and the triceps reflex are usually spared.

Axillary ("crutch") palsy

Compression high in the axilla, for example from poorly fitted crutches, affects the nerve before its triceps branches. This adds weak elbow extension and a lost triceps reflex to the wrist drop, with sensory loss over the posterior arm and forearm.

Mnemonics

Muscles supplied

"BEST": Brachioradialis, Extensors (wrist and digits), Supinator, Triceps.

Test yourself

A mid-shaft fracture of the humerus is followed by inability to extend the wrist and fingers, with sensory loss over the dorsum of the first web space. Elbow extension is preserved. Which nerve is injured?

Radial nerve in the radial (spiral) groove
Median nerve at the elbow
Ulnar nerve at the medial epicondyle
Musculocutaneous nerve

Answer: Radial nerve in the radial (spiral) groove

The radial nerve winds around the humerus in the radial groove and supplies the wrist and finger extensors, so injury there causes wrist drop. Branches to triceps usually leave proximal to the groove, so elbow extension survives.