Which pattern of weakness indicates anterior interosseous nerve entrapment?

• A. Weakness in flexor pollicis longus, lateral half of flexor digitorum profundus, and pronator quadratus
• B. Weakness in thenar muscles and wrist flexors
• C. Weakness in supinator and extensor pollicis longus
• D. Weakness in flexor carpi ulnaris and flexor digitorum profundus to the little finger

Answer: (A)

The pattern that signals anterior interosseous nerve entrapment is weakness of the flexor pollicis longus, the lateral half of the flexor digitorum profundus, and the pronator quadratus. The anterior interosseous nerve is a motor branch of the median nerve that supplies exactly these muscles, so when it’s compressed or injured you lose their function but not sensation. Clinically this shows up as a compromised ability to perform precise pinch movements, often described as an abnormal “OK sign” because you can’t flex the thumb’s interphalangeal joint or the index finger’s distal interphalangeal joint. This isolated motor pattern helps distinguish AIN entrapment from other nerve injuries: involvement of thenar muscles and wrist flexors points to a broader median nerve lesion; weakness of supinator and extensor pollicis longus suggests a radial/posterior interosseous nerve issue; weakness of flexor carpi ulnaris and the little finger side of FDP indicates ulnar nerve injury.