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# Trapezoid

The trapezoid is a small wrist bone in the second row, at the base of the index finger; it is wedged tightly and rarely breaks.

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## What it is

A small wedge-shaped wrist bone under the index metacarpal.

## What it does

One of the eight carpal bones that let the wrist bend and carry load to the forearm.

## Key points

- The least commonly fractured carpal bone: it is wedged tightly between the trapezium, capitate and the base of the 2nd metacarpal
- It locks the base of the 2nd metacarpal, so the index carpometacarpal joint is almost immobile and forms, with the 3rd, the rigid central pillar of the hand
- Carpal boss: a hard bump on the back of the hand at the 2nd–3rd carpometacarpal joints, often mistaken for a ganglion

## Key facts

**Joints with**
Scaphoid, trapezium, capitate, 2nd metacarpal

**Landmarks**
Small wedge-shaped bone

## Relations

**Above**
[Scaphoid](https://durgical.com/en/anatomy/scaphoid/)

**Below**
Base of the [2nd metacarpal](https://durgical.com/en/anatomy/2nd-metacarpal/)

**Lateral**
[Trapezium](https://durgical.com/en/anatomy/trapezium/)

**Medial**
[Capitate](https://durgical.com/en/anatomy/capitate/)

**Behind**
Dorsal [CMC ligaments](https://durgical.com/en/anatomy/carpometacarpal-and-metacarpal-ligaments/); the [ECRL](https://durgical.com/en/anatomy/extensor-carpi-radialis-longus/) tendon inserts on the 2nd metacarpal base just distal to it

## How to find it

Hard to feel on its own: on the back of the hand it lies just proximal to the base of the 2nd metacarpal, which is followed back from the index knuckle.

## On imaging

Overlaps its neighbours on standard views; the rare fracture or dislocation (usually dorsal) is best shown by CT.

## Clinical notes

- Isolated fractures are rare and come from an axial blow through the index metacarpal, often with a carpometacarpal dislocation; CT is usually needed to see them
- Arthritis between the scaphoid, trapezium and trapezoid (STT joint) is a frequent pattern of wrist arthritis and often coexists with thumb-base arthritis

## Development

Ossifies around age 5–7, after the trapezium.

## Test yourself

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

## Which carpal bone is the least commonly fractured?

The trapezoid, because it is wedged tightly between the trapezium, capitate and base of the 2nd metacarpal.

## Why is the index carpometacarpal joint almost immobile?

The trapezoid locks the base of the 2nd metacarpal, forming with the 3rd the rigid central pillar of the hand.

## What is a carpal boss over the trapezoid region?

A hard bump on the back of the hand at the 2nd–3rd carpometacarpal joints, often mistaken for a ganglion.

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

## Its names

**Official Latin (TA2)**
os trapezoideum TA2 1257

**Official Spanish (DeCS)**
Hueso trapezoide

**English (TA2)**
Trapezoid

**Also called**
lesser multangular

## Related

- [CMC ligaments](https://durgical.com/en/anatomy/carpometacarpal-and-metacarpal-ligaments/)
- [Dorsal intercarpal ligament](https://durgical.com/en/anatomy/dorsal-intercarpal-ligament/)
- [Midcarpal ligaments](https://durgical.com/en/anatomy/midcarpal-and-intercarpal-ligaments/)
- [Wrist extensor tendons](https://durgical.com/en/anatomy/wrist-extensor-tendons/)

Sources: Terminologia Anatomica (TA2); [Anatomy, Shoulder and Upper Limb, Hand Carpal Bones](https://www.ncbi.nlm.nih.gov/books/NBK535382/).

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Representative model, for educational purposes only.

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