A patient presents with inability to extend the wrist and fingers, with the hand hanging down ('wrist drop'). Sensory testing reveals loss only on the dorsum of the hand between the 1st and 2nd metacarpals. Where is the radial nerve most likely injured?

• A At the radial head (posterior interosseous nerve injury)
• B At the axilla
• C At the wrist
• D In the spiral groove of the humerus ✓

Explanation

Radial nerve injury at the spiral groove (midshaft humeral fracture) produces wrist drop (loss of wrist extensors and finger extensors) with preserved triceps (nerve to triceps leaves before the spiral groove) and sensory loss over the dorsum of the hand (posterior cutaneous nerve of forearm and superficial radial nerve). At the axilla, triceps is also paralyzed. Posterior interosseous nerve (deep branch, after dividing at the radial head) causes finger drop but NOT wrist drop and no sensory loss. The site in the spiral groove is the classic location for Saturday night palsy.

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

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Written and medically reviewed by the StethoPrep medical team.