Which nerve injury results in wrist drop after a midshaft humeral fracture?

• A. Median nerve
• B. Radial nerve
• C. Ulnar nerve
• D. Axillary nerve

Answer: (B)

Wrist drop is produced by loss of the extensor muscles of the wrist and fingers, which are innervated by the radial nerve. In a midshaft humeral fracture, the radial nerve travels along the humerus in the spiral groove and is easily injured there. When this nerve is compromised, the extensor muscles (such as the extensor carpi radialis longus and brevis and the finger extensors) can no longer extend the wrist, so the wrist rests in a flexed position and cannot be actively extended.

The other nerves produce different patterns: median nerve injury affects thumb opposition and finger flexion, leading to a different hand posture; ulnar nerve injury leads to intrinsic hand weakness and clawing; axillary nerve injury causes loss of shoulder abduction. Thus, the presentation after a midshaft humeral fracture most specifically points to radial nerve injury.