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# Extensor tendons

The finger extensor tendons run over the back of the hand to the fingers; they straighten the fingers at the knuckles.

Official name: Extensor digitorum tendons

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## What it is

Four tendons across the back of the hand to the fingers, linked by small bands.

## What it does

Straighten the fingers at the knuckles.

## Key points

- Extensor zones run from I to IX, with the odd numbers over the joints: I DIP, III PIP, V MCP, VII wrist
- A tooth wound over a knuckle (zone V, 'fight bite') is assumed to have entered the MCP joint
- The index and little fingers have a second extensor (extensor indicis, extensor digiti minimi) on the ulnar side of the ED tendon, so each can extend alone
- Sagittal bands keep each tendon centred over its knuckle; their tear lets the tendon slip into the groove between knuckles

## Key facts

**Attaches**
Four tendons in the 4th dorsal compartment (with the extensor indicis) to the extensor expansions of fingers 2–5; linked on the back of the hand by juncturae tendinum

**Function**
Extend the MCP joints, and the interphalangeal joints through the expansion; the juncturae can hide a single cut tendon

**Tight in**
Finger and wrist flexion

## Relations

**Course**
From the [extensor digitorum](https://durgical.com/en/anatomy/extensor-digitorum/) belly the four tendons pass under the [extensor retinaculum](https://durgical.com/en/anatomy/extensor-retinaculum-of-the-wrist/) in the 4th dorsal compartment, fan out over the back of the hand and widen into the extensor hood over each knuckle

**Deep**
[Extensor indicis](https://durgical.com/en/anatomy/extensor-indicis/) tendon in the same compartment, with the end of the [posterior interosseous nerve](https://durgical.com/en/anatomy/posterior-interosseous-nerve/) on its floor; on the hand, the metacarpals and dorsal interossei

**Superficial**
Extensor retinaculum at the wrist, then the [dorsal venous network of the hand](https://durgical.com/en/anatomy/dorsal-venous-network-of-the-hand/) and the dorsal branches of the superficial radial and ulnar nerves

**Lateral**
[EPL tendon](https://durgical.com/en/anatomy/extensor-pollicis-longus-tendon/) in the 3rd compartment, crossing obliquely toward the thumb

**Medial**
[Extensor digiti minimi](https://durgical.com/en/anatomy/extensor-digiti-minimi/) in the 5th compartment, over the distal radioulnar joint

## How to find it

With the fingers extended against resistance, the four tendons stand out on the back of the hand; the linking bands can be seen as oblique cross-bands near the knuckles. Each tendon should stay centred over its knuckle as the fist closes.

## Examination

- MCP extension against resistance, one finger at a time: weakness or a lag points to a cut tendon; a linking band may keep some extension, so strength matters more than movement
- Tenodesis effect: wrist flexion normally extends the MCPs; if it does in a person who cannot extend actively, the tendons are intact and a posterior interosseous nerve palsy is more likely

## On imaging

Ultrasound is the first choice: in real time it shows tendon continuity, the gap of a cut and sagittal band tears, with the tendon slipping off the knuckle as the fist closes. MRI shows the same on axial images. Radiographs look for a fragment or a tooth in a fight bite over the MCP joint and for a bony mallet fracture at the DIP.

## Clinical notes

- Rheumatoid arthritis: attrition rupture over a prominent ulnar head (Vaughan-Jackson syndrome) starts with the little finger and moves radially
- Sagittal band rupture, usually the radial band of the middle finger ('boxer's knuckle'), makes the tendon snap ulnarly as the fist closes
- Zone V cuts from a fight bite carry bacteria into the MCP joint and are explored and washed out

## Anatomical variants

- The tendon to the little finger is often thin, absent or replaced by a linking band from the ring finger tendon
- Duplicated slips are common, and the extensor indicis origin and slips vary

## Related conditions

- [Rheumatoid arthritis](https://durgical.com/en/conditions/rheumatoid-arthritis/)
- [Mallet finger](https://durgical.com/en/conditions/mallet-finger/)
- [Boxer fracture (fifth metacarpal neck)](https://durgical.com/en/conditions/boxer-fracture/)
- [Rheumatoid arthritis of the hand and wrist](https://durgical.com/en/conditions/rheumatoid-arthritis-of-the-hand-and-wrist/)

## Test yourself

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

## In which compartment do the extensor digitorum tendons run, and what links them on the back of the hand?

The 4th dorsal compartment, with the extensor indicis; they are linked by the juncturae tendinum.

## Why is a tooth wound over a knuckle (extensor zone V) explored?

A fight bite is assumed to have entered the MCP joint, carrying bacteria in; it is explored and washed out.

## Why can the index and little fingers extend independently?

Each has a second extensor (extensor indicis, extensor digiti minimi) on the ulnar side of its extensor digitorum tendon.

## What happens to an extensor tendon when its sagittal band tears?

It slips off the knuckle into the groove, usually the middle finger snapping ulnarly as the fist closes.

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

## Its names

**Official Latin (TA2)**
tendines musculi extensoris digitorum TA2 2500

**Official Spanish (DeCS)**
Tendones del músculo extensor de los dedos

**English (TA2)**
Extensor digitorum tendons

## Related

- [Capitate](https://durgical.com/en/anatomy/capitate/)
- [Hamate](https://durgical.com/en/anatomy/hamate/)
- [Lunate](https://durgical.com/en/anatomy/lunate/)
- [2nd metacarpal](https://durgical.com/en/anatomy/2nd-metacarpal/)
- [3rd metacarpal](https://durgical.com/en/anatomy/3rd-metacarpal/)
- [4th metacarpal](https://durgical.com/en/anatomy/4th-metacarpal/)
- [5th metacarpal](https://durgical.com/en/anatomy/5th-metacarpal/)
- [Middle phalanx, index finger](https://durgical.com/en/anatomy/middle-phalanx-of-the-index-finger/)

Sources: Terminologia Anatomica (TA2); [Anatomy, Shoulder and Upper Limb, Hand Compartments](https://www.ncbi.nlm.nih.gov/books/NBK532942/); [Anatomy, Shoulder and Upper Limb, Wrist Extensor Muscles](https://www.ncbi.nlm.nih.gov/books/NBK534805/).

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Representative model, for educational purposes only.

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