Froment's sign indicates weakness of which muscle and nerve?

• A. Opponens Pollicis; Median nerve
• B. Adductor pollicis; Ulnar nerve
• C. Abductor pollicis brevis; Median nerve
• D. Flexor pollicis longus; Anterior interosseous nerve

Answer: (B)

Froment's sign tests the function of the adductor pollicis, a intrinsic hand muscle innervated by the deep branch of the ulnar nerve. When a patient is asked to pinch a piece of paper, a weak adductor pollicis cannot keep the thumb in an adducted position. To maintain grip, the flexor pollicis longus (innervated by the median nerve via the anterior interosseous nerve) flexes the thumb’s interphalangeal joint, causing a flexed IP joint of the thumb. This compensatory flexion is the hallmark of Froment's sign and points to ulnar nerve palsy affecting the adductor pollicis. If the issue were with the median-innervated muscles like Opponens pollicis or Abductor pollicis brevis, you’d expect problems with opposition or abduction rather than the distinctive IP flexion seen in Froment's sign.