Radial Nerve
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Direct answer
Wrist drop with the elbow still extending points to one nerve and one address: the radial nerve (C5–T1) in the spiral groove of the humerus. The continuation of the posterior cord supplies every extensor muscle of the upper limb; it leaves the axilla with the profunda brachii through the triangular interval, winds behind the humerus in the spiral groove, pierces the lateral intermuscular septum, and enters the cubital fossa between brachialis and brachioradialis, where it divides into the superficial radial and posterior interosseous nerves.
What you must remember
- In the axilla: lies behind the axillary artery; gives branches to the long and medial heads of triceps and the posterior cutaneous nerve of the arm — which is why triceps escapes a spiral groove lesion.
- Spiral groove: travels with the profunda brachii along the back of the humeral shaft, supplying the lateral head of triceps and anconeus; the humeral shaft fracture is its classic injury.
- Lower third of arm: pierces the lateral intermuscular septum, enters the cubital fossa between brachialis and brachioradialis, and supplies brachioradialis and extensor carpi radialis longus before dividing.
- Posterior interosseous nerve: enters supinator through the arcade of Frohse, supplying supinator and all remaining extensors of the forearm — a pure motor nerve.
- Sensory branches: posterior cutaneous nerves of arm and forearm, lower lateral cutaneous nerve of the arm, and the superficial radial nerve to the dorsum of the lateral three and a half digits.
- Saturday night palsy: compression in the groove during deep sleep; wrist drop with preserved elbow extension and sensory loss over the first dorsal web space — the earliest reliable sensory sign.
- Posterior interosseous syndrome: finger extension weak but the wrist still extends with radial deviation, because extensor carpi radialis longus is supplied above; no sensory loss.
Localising three radial lesions step by step
Take three patients. The first, waking after sleeping on an arm across a chair back, cannot extend the wrist or fingers and is numb over the first web space — but extends the elbow powerfully. The lesion is in the spiral groove: the long and medial heads of triceps were supplied back in the axilla, so they survive; brachioradialis and the wrist extensors, supplied below, do not. The second, injured by a crutch high in the axilla, also cannot extend the elbow — the triceps branches were caught before they left. The third, after a radial head fracture, cannot extend the fingers or thumb but extends the wrist with a radial tilt: the posterior interosseous nerve is caught at the arcade of Frohse, below the supply to extensor carpi radialis longus, and being a pure motor nerve causes no numbness at all.
Speak that ladder aloud in the viva: axilla (triceps plus everything), spiral groove (wrist drop, triceps spared), arcade of Frohse (finger drop, wrist preserved, no sensory loss). Test wrist extension with the forearm pronated and elbow flexed, so gravity cannot mimic brachioradialis, and chart the first web space.
Where students slip
Triceps is expected to die in every radial lesion; it dies only with axillary injury, because its branches leave proximal to the groove — the most tested fact here. Wrist drop is then attributed to the posterior interosseous nerve; that lesion spares wrist extension, and the discriminating sign is radial deviation. The superficial radial territory is misquoted as "one and a half digits" — that is the ulnar nerve; the radial serves the lateral three and a half on the dorsum to the proximal phalanges. Finally, brachioradialis gives the radial nerve its own tendon reflex — the supinator reflex, C5–C6.
Frequently asked questions
Which artery accompanies the radial nerve in the spiral groove?
The profunda brachii; together they leave the axilla through the triangular interval bounded by the lower border of teres major, the humeral shaft and the lateral head of triceps.
What is Saturday night palsy?
Compression of the nerve in the spiral groove during deep sleep, producing wrist drop with sensory loss over the first dorsal web space but preserved elbow extension — the signature of a lesion below the triceps branches.
Why is triceps spared in a spiral groove lesion?
Branches to the long and medial heads of triceps leave the nerve in the axilla, proximal to the groove; only the lateral head's branch, given within the groove, is lost.
What is the arcade of Frohse?
The fibrous arch at the upper border of supinator through which the posterior interosseous nerve enters; compression there weakens finger and thumb extension while the wrist still extends with radial deviation, and sensation is intact.
What is the sensory distribution of the superficial radial nerve?
The dorsum of the lateral three and a half digits and the corresponding radial part of the hand, stopping at the proximal phalanges.
Which tendon reflex tests the radial nerve?
The supinator (brachioradialis) reflex at C5–C6 — brisk tendon tap over the distal brachioradialis with the forearm relaxed in mid-pronation.