• Opposition of the thumb to the small finger.
• Thenar eminence muscle (an intrinsic hand muscle).
Muscle Function: Opposes (flexes and abducts with slight medial rotation) the carpo-metacarpal joint of the thumb. True opposition occurs when the thumb and small finger come together in contact.
Palpation: During thumb opposition, palpate thenar musculature and push the abductor pollicis brevis out of the way on the volar aspect of the first metacarpal and move ulnarly to palate.
Strength Testing: Position – the forearm in neutral. Stabilize – the wrist. Resist – against thumb opposition at the metacarpal of the thumb in the direction of extension and abduction.
Additional Examination Tips: Subtle changes in opponens pollicis strength can be detected with the squeeze test. The subject is asked to squeeze the examiner’s hand and if the opponens pollicis function is absent, the patient’s thumb metacarpal will move away from the examiner’s palm instead of surrounding it.
• Roots: C8, T1.
• Nerve: Median nerve, thenar branch.
• Innervation Route: C8, T1 → median nerve → recurrent (thenar) branch → opponens pollicis branch.
• In about 20% of the population, the opponens pollicis is innervated by the ulnar nerve.
Origins: Flexor retinaculum and tubercle of trapezium bone.
Insertion: Entire length of first metacarpal bone, radial aspect.