A patient cannot extend the wrist and fingers (wrist drop) after a fracture of the mid-shaft of the humerus. Testing reveals preserved extension of MCP joints of index and middle fingers via the ulnar nerve, but loss of sensation on the dorsum of the hand in the distribution of the first dorsal web space. Which nerve is injured and at what level?

• A Radial nerve in the axilla
• B Radial nerve in the radial groove (spiral groove) ✓
• C Posterior interosseous nerve distal to the lateral epicondyle
• D Superficial radial nerve at the wrist

Explanation

Radial nerve injury in the spiral groove (mid-humeral shaft fracture) spares the branch to triceps (given off in the axilla and upper spiral groove) — triceps strength is preserved. The long head of triceps branch arises in the axilla; branches to medial and lateral heads arise in the spiral groove above the injury level, so triceps is usually intact. Wrist drop occurs due to loss of wrist/finger extensors. The superficial radial nerve (dorsum of hand, first web space) is also involved because it arises from the radial nerve proximal to the elbow. Posterior interosseous nerve injury spares wrist extension (ECRL preserved) and sensation.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.