FPB !

The flexor pollicis brevis is a thumb-pad muscle at the base of the thumb; it bends the thumb knuckle and gets nerve supply from two nerves.

Official name: Flexor pollicis brevis

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What it is

A thumb-pad muscle next to the APB.

What it does

Bends the thumb knuckle.

Key facts

Origin: Superficial head: flexor retinaculum, trapezium; deep head: trapezoid, capitate
Insertion: Base of the proximal phalanx of the thumb; lateral (radial) sesamoid of the MCP joint
Action: Flexes the thumb at the MCP joint
Nerve: Superficial head: recurrent median nerve; deep head: deep ulnar nerve (C8–T1)
Blood supply: Superficial palmar branch of the radial artery

Relations

Lateral: Abductor pollicis brevis (APB)
Medial: Oblique head of the adductor pollicis
Deep: Opponens pollicis and the FPL tendon
Superficial: Thenar fascia and skin; the superficial palmar branch of the radial artery crosses nearby

How to find it

Felt in the inner part of the thenar eminence, next to APB, when the thumb bends at the knuckle against resistance with the tip kept straight.

Examination

Resisted MCP flexion with the thumb IP joint straight (removing FPL): flexion preserved despite a median palsy suggests the ulnar deep head is compensating

On imaging

Usually not assessed alone: ultrasound and MRI show it in the thenar eminence beside and partly under abductor pollicis brevis. It inserts with the radial sesamoid of the thumb MCP joint, which radiographs show and which can fracture. With median nerve compression MRI may show thenar oedema or atrophy; its deep head may be spared (ulnar supply).

Clinical notes

Explains why thenar wasting can be partial and some flexion persists in carpal tunnel syndrome or median nerve injury
Its deep head adds to the loss of pinch strength in ulnar nerve palsy

Anatomical variants

The share supplied by the median and ulnar nerves varies widely; in some hands the whole muscle is median-supplied, in others ulnar-supplied

Test yourself

Think of the answer, then tap the question to see it.

What is the nerve supply of the FPB (flexor pollicis brevis)?

Dual: the superficial head from the recurrent median nerve, the deep head from the deep ulnar nerve.

Why can thumb MCP flexion persist after a median nerve lesion?

The deep head of the FPB, supplied by the ulnar nerve, keeps working.

Which tendon runs between the two heads of the FPB?

The flexor pollicis longus (FPL) tendon.

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Educational information, not medical advice. For any symptom, see a health professional.

Its names

Official Latin (TA2): flexor brevis pollicis TA2 2522
Official Spanish (DeCS): Músculo flexor corto del pulgar
English (TA2): Flexor pollicis brevis

Sources: Terminologia Anatomica (TA2); Anatomy, Shoulder and Upper Limb, Hand Thenar Eminence.

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Representative model, for educational purposes only.

FPB highlighted on the Durgical 3D model