A 43-year-old woman visits the outpatient clinic with a neurologic problem. Diagnostically, she cannot hold a piece of paper between her thumb and the lateral side of her index finger without flexing the distal joint of her thumb. This is a positive Froment sign and a diagnosis of ulnar neuropathy. Weakness of which specific muscle causes this sign to appear?
**Core Concept**
The Froment sign is a clinical test used to assess the strength of the adductor pollicis muscle, which is innervated by the ulnar nerve. This sign is positive when a patient is unable to hold a piece of paper between their thumb and the lateral side of their index finger without flexing the distal joint of their thumb, indicating weakness of the adductor pollicis muscle.
**Why the Correct Answer is Right**
The adductor pollicis muscle plays a crucial role in thumb adduction, which is the movement of the thumb towards the palmar surface of the hand. The ulnar nerve provides motor innervation to this muscle via the ulnar nerve's terminal branches. When the adductor pollicis muscle is weak due to ulnar neuropathy, patients often compensate by flexing the distal interphalangeal joint of the thumb to maintain grip strength. This is because the flexor pollicis longus muscle, which is innervated by the anterior interosseous nerve (a branch of the median nerve), assists in thumb flexion and provides a compensatory mechanism for the weakened adductor pollicis muscle.
**Why Each Wrong Option is Incorrect**
**Option A:** The flexor pollicis longus muscle is responsible for thumb flexion, not adduction. While it does assist in compensating for the weakened adductor pollicis muscle, it is not the primary muscle responsible for the Froment sign.
**Option B:** The opponens pollicis muscle is involved in thumb opposition, not adduction. Although it is also innervated by the ulnar nerve, it is not the muscle primarily responsible for the Froment sign.
**Option C:** The flexor digitorum profundus muscle is responsible for finger flexion, not thumb adduction. Although it is also innervated by the ulnar nerve, it is not the muscle primarily responsible for the Froment sign.
**Option D:** The lumbricals are a group of muscles involved in finger flexion and extension, not thumb adduction. They are innervated by the ulnar and median nerves, but are not the primary muscles responsible for the Froment sign.
**Clinical Pearl / High-Yield Fact**
The Froment sign is a useful clinical test for diagnosing ulnar neuropathy, particularly in patients with symptoms of hand weakness or numbness. It is essential to assess the strength of the adductor pollicis muscle in patients with suspected ulnar nerve damage to accurately diagnose and treat this condition.
**Correct Answer:** C.