A patient with a suspected low ulnar nerve lesion is asked to pinch a sheet of paper firmly between the thumb and index finger. The thumb IP joint visibly flexes strongly during the effort. Which muscle is compensating for the paralysed adductor pollicis?

• A Flexor pollicis longus ✓
• B Opponens pollicis
• C Flexor pollicis brevis
• D First palmar interosseous muscle

Explanation

This is Froment's sign. With adductor pollicis paralysed by the ulnar lesion, true adduction of the thumb fails, and the patient recruits flexor pollicis longus, a median nerve muscle acting on the interphalangeal joint, to grip the paper. Flexion of the thumb tip during key pinch therefore betrays the ulnar deficit. Flexor pollicis brevis and opponens pollicis are median supplied thenar muscles that do not flex the IP joint, eliminating them.

Reference: Snell's Clinical Anatomy by Regions, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.