Which nerve is tested by Froment’s sign, indicating a potential ulnar nerve issue?

• A. Median nerve
• B. Ulnar nerve
• C. Radial nerve
• D. Axillary nerve

Answer: (B)

Froment’s sign tests the function of the intrinsic hand muscles that adduct the thumb, which are primarily the adductor pollicis. This muscle is innervated by the ulnar nerve. When the ulnar nerve is compromised, adduction of the thumb weakens. To keep a pinch grip, the person then compensates by flexing the thumb’s interphalangeal joint via the flexor pollicis longus (which is median-nerve–innervated), producing a flexed IP joint instead of a true pinch. This compensatory flexion during a pinch is a positive Froment’s sign and points toward ulnar nerve injury.

The other nerves don’t produce this pattern: the median nerve handles the flexor pollicis longus and thenar muscles, so a problem there wouldn’t cause the same compensatory flexion in a pinch; the radial and axillary nerves aren’t involved in this thumb adduction/pinch mechanism.