⚡ Nerve Injuries at Chokepoints
The four classic nerve–fracture pairings, their vascular running mates, wrist drop, foot drop, claw hand, Froment’s sign, the ulnar paradox, and every clinical sign the examiner will ask about.
The Anatomical Chokepoints
Peripheral nerves are vulnerable wherever they run through a tight, non-yielding space — a bony groove, a fibro-osseous tunnel, or a point where they wrap around bone close to the surface. When a fracture occurs at one of these chokepoints, the nerve is stretched, compressed, or severed alongside the bone it hugs.
Examiners love this topic because the pattern is predictable and anatomically logical — if you know where the nerve runs, you know what fracture threatens it, and from that you can derive every motor and sensory deficit. The four classic pairings are:
| Nerve | Chokepoint | Threatening Fracture/Injury | Chief Motor Deficit | Sensory Loss |
|---|---|---|---|---|
| Radial | Spiral (radial) groove of humerus | Mid-shaft humeral fracture | Wrist drop — can’t extend wrist/fingers | 1st dorsal web space / anatomical snuffbox |
| Axillary | Quadrangular space → wraps surgical neck of humerus | Surgical neck fracture / shoulder dislocation | Deltoid paralysis — can’t abduct 15–90° | “Regimental badge” — lateral proximal arm |
| Ulnar | Cubital tunnel (elbow) / Guyon’s canal (wrist) | Medial epicondyle fracture / wrist laceration | Claw hand (4th & 5th digits); Froment’s sign | Medial 1.5 digits (little + ulnar ring finger) |
| Common Peroneal | Subcutaneous at fibular neck | Fibular neck fracture / knee dislocation / tight cast | Foot drop (dorsiflexion) + loss of eversion | Dorsum of foot + 1st dorsal web space |
The Vascular Running Mates — Why They Matter
Each of the classic nerve chokepoints has a named artery travelling alongside the nerve. Knowing these pairings matters for two reasons:
1. A fracture can simultaneously injure both the nerve and its arterial companion → combined neurovascular injury requires urgent vascular repair before nerve repair
2. The examiner will ask about the artery when testing the fracture-nerve pairing
🔴 Radial nerve + Profunda brachii artery in the spiral groove
🔵 Axillary nerve + Posterior circumflex humeral artery around the surgical neck
🟣 Common peroneal nerve + Anterior tibial artery (at risk in high-energy fibular neck injuries with compartment syndrome)