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# 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 points

- Dual supply: its superficial head from the [median nerve](https://durgical.com/en/anatomy/median-nerve/) and its deep head from the [ulnar nerve](https://durgical.com/en/anatomy/ulnar-nerve/), so it may still work after a median lesion
- The [flexor pollicis longus](https://durgical.com/en/anatomy/flexor-pollicis-longus/) (FPL) tendon runs between its two heads
- Inserts with APB through the radial sesamoid of the thumb MCP joint

## Key facts

**Origin**
Superficial head: flexor retinaculum, [trapezium](https://durgical.com/en/anatomy/trapezium/); deep head: [trapezoid](https://durgical.com/en/anatomy/trapezoid/), [capitate](https://durgical.com/en/anatomy/capitate/)

**Insertion**
Base of the [proximal phalanx of the thumb](https://durgical.com/en/anatomy/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](https://durgical.com/en/anatomy/abductor-pollicis-brevis/) (APB)

**Medial**
Oblique head of the [adductor pollicis](https://durgical.com/en/anatomy/adductor-pollicis/)

**Deep**
[Opponens pollicis](https://durgical.com/en/anatomy/opponens-pollicis/) and the [FPL tendon](https://durgical.com/en/anatomy/flexor-pollicis-longus-tendon/)

**Superficial**
Thenar fascia and [skin](https://durgical.com/en/anatomy/skin/); the superficial palmar branch of the [radial artery](https://durgical.com/en/anatomy/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](https://durgical.com/en/anatomy/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

## Related

- [Recurrent motor branch](https://durgical.com/en/anatomy/recurrent-motor-branch-of-the-median-nerve/)
- [Ulnar nerve in the hand](https://durgical.com/en/anatomy/deep-and-superficial-branches-of-the-ulnar-nerve/)
- [1st metacarpal](https://durgical.com/en/anatomy/1st-metacarpal/)
- [Trapezium](https://durgical.com/en/anatomy/trapezium/)

Sources: Terminologia Anatomica (TA2); [Anatomy, Shoulder and Upper Limb, Hand Thenar Eminence](https://www.ncbi.nlm.nih.gov/books/NBK580533/).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/anatomy/flexor-pollicis-brevis/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/anatomy/flexor-pollicis-brevis/