Recallab › Question bank › Anatomy › Upper Limb › #50501
Radial nerve in the spiral groove
A man falls asleep with his arm over the back of a chair and wakes with wrist drop. Sensation is lost over a small patch on the dorsum of the first web space, but the triceps is strong and the elbow extends normally. Where is the lesion?
- A. In the axilla, above the branches to triceps
- correctB. In the spiral groove, below the branches to triceps
- C. At the wrist, affecting the superficial radial nerve only
- D. At the elbow, affecting the posterior interosseous nerve only
Explanation
Key fact
The radial nerve supplies triceps before it enters the spiral groove. A strong triceps with a wrist drop places the lesion below those branches.
In the spiral groove, below the triceps branches. This is the classical Saturday night palsy.
Why
The nerve gives its branches to triceps high in the arm.
It then winds round the humerus in the spiral groove, where it is compressible.
Compression there spares elbow extension but paralyses the extensors below.
The result is wrist drop with weak finger and thumb extension.
The first web space patch is supplied by the superficial radial nerve.
Where a radial nerve lesion sits, and what each level spares
Why not the others
- A. In the axilla, above the branches to triceps — would also weaken elbow extension. The triceps is strong here, which excludes it. That is the crutch palsy.
- C. At the wrist, affecting the superficial radial nerve only — gives the sensory loss alone. There would be no weakness.
- D. At the elbow, affecting the posterior interosseous nerve only — gives finger drop with the wrist deviating radially, and no sensory loss at all, because the branch is purely motor.
Remember
Triceps working plus wrist drop means the spiral groove.
Go deeper
The nerve is also at risk in a mid-shaft humeral fracture, where it lies directly against the bone in the same groove.
Recovery from a compression neuropraxia is usually complete over weeks, and a cock-up splint maintains function meanwhile.
Brachioradialis is supplied above the elbow, so testing it separates a high radial lesion from a posterior interosseous one.
Gray's Anatomy for Students, 5th ed. (Elsevier, 2023); Snell's Clinical Anatomy by Regions, 11th ed. (Wolters Kluwer, 2024).