Opponens Pollicis (Musculus opponens pollicis)

Anatomy

The deep quadrilateral muscle of the thenar eminence, hidden beneath the abductor pollicis brevis. Like its hypothenar counterpart it inserts on the metacarpal — the radial border and palmar surface of the first metacarpal shaft — and rotates the thumb across the palm so its pulp meets the little finger's, the defining motion of human opposition.

Origin & insertion

From the trapezium and the flexor retinaculum, to the radial border of the shaft of the first metacarpal.

Action

Flexes and medially rotates the first metacarpal at the carpometacarpal joint, bringing the thumb pulp opposite the little finger.

Innervation

Recurrent motor branch of the median nerve (C8–T1).

Clinical notes

It is assessed by the thumb-to-fifth-metacarpal-head maneuver: the examiner hooks a finger under the patient's thumb and asks them to press down against resistance. Because it is the deepest thenar muscle, its function survives the earliest compressive lesions and is lost only in advanced carpal tunnel disease.

Related structures

• Trapezium (right)
• First metacarpal (right)

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