During examination of a patient with suspected ulnar nerve palsy, you ask him to grip a piece of paper between his thumb and the radial side of his index finger. As you pull the paper away, he flexes the thumb's interphalangeal joint to hold on. This positive sign indicates weakness of which muscle?

• A Flexor pollicis brevis
• B Adductor pollicis ✓
• C First dorsal interosseous
• D Opponens pollicis

Explanation

This is Froment's sign. Adductor pollicis, supplied by the deep branch of the ulnar nerve, normally holds the thumb against the index finger in the plane of the palm. When it is paralysed, the patient substitutes flexor pollicis longus, a median nerve muscle, producing the telltale flexion of the thumb interphalangeal joint. The substitution by a median supplied muscle is precisely what makes the test specific for ulnar palsy.

Reference: Moore's Clinically Oriented Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.