Flexor pollicis brevis

The flexor pollicis brevis is a thenar muscle of the hand, responsible for thumb flexion. It is typically described as having two distinct heads: a superficial and a deep portion.

Superficial Head

This more lateral and superficial part of the muscle originates from the flexor retinaculum and the tubercle of the trapezium bone. Its muscle belly runs along the radial side of the flexor pollicis longus tendon. As it becomes tendinous, it inserts onto the radial side of the base of the first (proximal) phalanx of the thumb. A sesamoid bone is characteristically embedded within its insertion tendon.

Deep Head

When present, the deep head is a smaller, more medial, and deeper portion of the muscle. It arises from the trapezoid and capitate bones.

Insertion

Both the superficial and deep heads course obliquely to converge and insert together onto the radial side of the base of the proximal phalanx of the thumb via a common tendon.

Innervation

The superficial head receives its supply from the recurrent branch of the median nerve. The deep head, when present, is innervated by the ulnar nerve.