Flexor Pollicis Brevis (Musculus flexor pollicis brevis)

Anatomy

The medial thenar flexor, classically described with a superficial and a deep head bridging the first metacarpal alongside the long flexor tendon. It is the only thenar muscle with a double nerve supply — the superficial head from the median nerve and the deep head from the ulnar nerve — which is why thumb flexion often survives an isolated median or ulnar lesion. Its tendon carries the radial sesamoid of the thumb.

Origin & insertion

Superficial head from the flexor retinaculum and trapezium, deep head from the ulnar side of the first metacarpal and capitate, to the radial base of the proximal phalanx of the thumb with the radial sesamoid.

Action

Flexes the metacarpophalangeal joint of the thumb and assists opposition.

Innervation

Superficial head: median nerve (C8–T1); deep head: deep branch of the ulnar nerve.

Clinical notes

The radial sesamoid within its tendon is the radiographic landmark of the thumb's metacarpophalangeal joint. Preserved thumb flexion with lost abduction indicates a proximal median lesion sparing the ulnar supply, while the reverse pattern with clawing implicates the ulnar nerve.

Related structures

• Trapezium (right)
• First metacarpal (right)
• proximal phalanx of the thumb (right)

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