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 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 belly the four tendons pass under the extensor retinaculum 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 tendon in the same compartment, with the end of the 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 and the dorsal branches of the superficial radial and ulnar nerves
Lateral: EPL tendon in the 3rd compartment, crossing obliquely toward the thumb
Medial: 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

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

Sources: Terminologia Anatomica (TA2); Anatomy, Shoulder and Upper Limb, Hand Compartments; Anatomy, Shoulder and Upper Limb, Wrist Extensor Muscles.

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Representative model, for educational purposes only.

Extensor tendons highlighted on the Durgical 3D model