A 6-year-old boy with a displaced supracondylar humerus fracture presents with weakness of thumb interphalangeal joint flexion and inability to flex the index distal interphalangeal joint, with no sensory loss. Pinch between thumb and index shows hyperextension of the thumb IP joint. Which structure is injured?

• A Radial nerve
• B Anterior interosseous nerve ✓
• C Ulnar nerve
• D Median nerve at the carpal tunnel

Explanation

Pure motor loss of flexor pollicis longus and flexor digitorum profundus to index and middle fingers with intact sensation defines anterior interosseous nerve palsy, the classic nerve injury in posterolaterally displaced supracondylar fractures. The positive pinch sign results from FPL and FDP index paralysis. Median nerve injury at the elbow would also affect thenar muscles and sensation, and radial or ulnar palsies produce different motor patterns.

Reference: Tachdjian's Pediatric Orthopaedics, 5th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.