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# Proximal phalanx, thumb

The proximal phalanx of the thumb is the thumb's first finger bone, beyond the knuckle; its base anchors the ligament that steadies pinch.

Official name: Proximal phalanx of the thumb

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## What it is

The proximal bone of the thumb.

## What it does

One of the finger bones; the joints between them let the finger curl.

## Key points

- An avulsion fragment from the ulnar base is a bony skier's (gamekeeper's) thumb: the thumb UCL tore off with a piece of bone
- The short thumb muscles end on its base: the [abductor pollicis brevis](https://durgical.com/en/anatomy/abductor-pollicis-brevis/) and [flexor pollicis brevis](https://durgical.com/en/anatomy/flexor-pollicis-brevis/) on the radial side, the [adductor pollicis](https://durgical.com/en/anatomy/adductor-pollicis/) on the ulnar side, through the [sesamoids](https://durgical.com/en/anatomy/sesamoid-bones-of-foot/) and extensor hood

## Key facts

**Joints with**
Metacarpal head (MCP joint), next phalanx (IP joint)

**Landmarks**
Base, shaft, head (condyles)

## Relations

**Above**
Head of the [1st metacarpal](https://durgical.com/en/anatomy/1st-metacarpal/) at the MCP joint, with the [thumb sesamoids](https://durgical.com/en/anatomy/sesamoid-bones-of-the-thumb/), the [thumb UCL](https://durgical.com/en/anatomy/ulnar-collateral-ligament-of-the-thumb-mcp-joint/) and the radial collateral ligament

**Behind**
EPB inserts on the back of the base; the [EPL tendon](https://durgical.com/en/anatomy/extensor-pollicis-longus-tendon/) passes over it to the distal phalanx

**In front**
[FPL tendon](https://durgical.com/en/anatomy/flexor-pollicis-longus-tendon/) in its sheath under the oblique pulley

## How to find it

On the back of the thumb between the MCP knuckle and the IP joint crease. Tenderness or a lump on the ulnar side of its base, just beyond the MCP joint, is where a skier's thumb or a Stener lesion is felt.

## Examination

- Ulnar collateral ligament stress test: the thumb MCP joint is pushed radially in extension and in about 30° of flexion; marked opening or no firm end point compared with the other thumb suggests a complete UCL tear

## On imaging

PA and lateral thumb views show avulsion fragments; ultrasound or MRI shows a Stener lesion, in which the torn UCL ends up above the adductor aponeurosis and cannot heal in place.

## Clinical notes

- A displaced bony gamekeeper's fragment or a Stener lesion usually needs surgical repair to restore pinch
- The radial collateral ligament on the other side of its base tears less often than the UCL but also leaves the thumb unstable in pinch and grasp

## Development

Like the other phalanges, it ossifies from a primary centre in the shaft before birth and a single secondary centre at its base, whose plate closes in adolescence.

## Test yourself

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

## What does an avulsion fragment from the ulnar base of the proximal phalanx of the thumb indicate?

A bony skier's (gamekeeper's) thumb: the thumb UCL tore off with a piece of bone.

## What is a Stener lesion?

The torn thumb UCL ends up above the adductor aponeurosis and cannot heal in place; it usually needs surgical repair.

## Which tendon inserts on the back of the base of the proximal phalanx of the thumb?

The extensor pollicis brevis; the extensor pollicis longus passes over it to the distal phalanx.

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

## Its names

**Official Latin (TA2)**
phalanx proximalis manus TA2 1277

**Official Spanish (DeCS)**
Falange proximal del pulgar

**English (TA2)**
Proximal phalanx of the thumb

## Related

- [A1 pulleys](https://durgical.com/en/anatomy/a1-pulleys-of-the-flexor-tendon-sheaths/)
- [Adductor pollicis](https://durgical.com/en/anatomy/adductor-pollicis/)
- [APL and EPB tendons](https://durgical.com/en/anatomy/abductor-pollicis-longus-and-extensor-pollicis-brevis-tendons/)
- [EPB](https://durgical.com/en/anatomy/extensor-pollicis-brevis/)
- [EPL tendon](https://durgical.com/en/anatomy/extensor-pollicis-longus-tendon/)
- [Finger joint capsules](https://durgical.com/en/anatomy/finger-joint-capsules/)
- [Flexor tendon sheaths](https://durgical.com/en/anatomy/flexor-tendon-synovial-sheaths/)
- [FPL tendon](https://durgical.com/en/anatomy/flexor-pollicis-longus-tendon/)

Sources: Terminologia Anatomica (TA2); [Anatomy, Shoulder and Upper Limb, Hand Bones](https://www.ncbi.nlm.nih.gov/books/NBK547684/); [8.5 Development of the Appendicular Skeleton](https://openstax.org/books/anatomy-and-physiology-2e/pages/8-5-development-of-the-appendicular-skeleton) (OpenStax (Rice University), 2022) · adapted, [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/anatomy/proximal-phalanx-of-the-thumb/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/anatomy/proximal-phalanx-of-the-thumb/