A 38-year-old woman has difficulty buttoning clothes. On examination she makes a pinch between thumb and index pulp, but the index PIP joint hyperextends and the thumb IP joint flexes instead of the index DIP joint flexing. There is no sensory loss. Where is the lesion?

• A Recurrent motor branch of the median nerve
• B Anterior interosseous nerve ✓
• C Ulnar nerve at Guyon canal
• D Posterior interosseous nerve

Explanation

The abnormal pinch posture described is the positive pinch sign of AIN syndrome. The anterior interosseous nerve, a pure motor branch of the median nerve arising 5 to 8 cm distal to the elbow, supplies FPL, the radial half of FDP and pronator quadratus. Its lesion abolishes the index-thumb tip-to-tip pinch while sparing all thenar muscles and sensation, distinguishing it from carpal tunnel and recurrent motor branch lesions.

Reference: Campbell's Operative Orthopaedics, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.