A patient is unable to oppose the thumb after a deep laceration of the palm. Testing reveals intact flexor pollicis brevis and adductor pollicis. Which specific muscle and its nerve supply are most likely damaged?

• A Opponens pollicis; median nerve (recurrent branch) ✓
• B Abductor pollicis brevis; median nerve (recurrent branch)
• C Flexor pollicis longus; anterior interosseous nerve
• D First dorsal interosseous; deep branch of ulnar nerve

Explanation

True opposition (rotating the thumb to face the little finger with the pad facing the palm) requires the opponens pollicis, which is innervated by the recurrent branch of the median nerve. Isolated opponens pollicis loss abolishes opposition while retaining abductor pollicis brevis and the superficial head of flexor pollicis brevis (also median-innervated) for limited movement. Adductor pollicis (ulnar nerve) performs adduction not opposition. The deep head of flexor pollicis brevis is ulnar-innervated. Opponens pollicis lies deep to abductor pollicis brevis and is the only thenar muscle that can be selectively damaged by deeper lacerations.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.