Which nerve is most commonly injured in a midshaft humerus fracture?

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

Answer: (C)

The key idea is how the radial nerve travels along the humerus and why its path makes it especially vulnerable at the midshaft. The radial nerve runs in the radial (spiral) groove along the midportion of the humerus, right where fractures commonly occur. It carries motor fibers to the wrist and finger extensors and provides sensation to the dorsum of the hand. When a midshaft fracture injures this nerve, you typically see wrist drop from inability to extend the wrist and fingers, along with sensory loss on the back of the hand. The nerve’s course in the midshaft region explains why damage here is the most common nerve injury with these fractures.

In contrast, the other nerves are more likely to be injured with fractures at different locations: the ulnar nerve is commonly affected with injuries around the medial elbow or at the hook of the hamate; the median nerve is often endangered with injuries near the elbow (like supracondylar fractures) or at the carpal tunnel; the axillary nerve is typically affected with fractures of the surgical neck of the humerus, impacting deltoid function.