Extensor indicis !

The extensor indicis is a deep forearm muscle with a tendon to the index finger; it lets the index finger straighten on its own to point.

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What it is

A deep muscle that straightens the index finger on its own.

What it does

Lets you point with the index finger.

Key facts

Origin: Distal posterior ulna and interosseous membrane
Insertion: Extensor expansion of the index finger
Action: Extends the index finger independently
Nerve: Posterior interosseous nerve (C7–C8)
Blood supply: Posterior interosseous artery

Relations

Lateral: Extensor pollicis longus (EPL) in the deep extensor layer
Superficial: The extensor digitorum tendons, which cover it in the fourth compartment
Deep: The ulna and interosseous membrane; at the wrist the terminal posterior interosseous nerve in the floor of the compartment
Course: Through the fourth compartment deep to extensor digitorum, then across the back of the hand to join the extensor mechanism of the index

How to find it

Making a fist and then pointing the index finger brings its tendon up on the back of the hand, just on the ulnar side of the extensor digitorum tendon to the index.

Examination

Isolated index extension with the other fingers in a fist: preserved pointing after an extensor digitorum injury shows extensor indicis is intact; it is checked before using it as a donor

On imaging

On ultrasound its tendon runs in the fourth extensor compartment with extensor digitorum and, at the index knuckle, lies ulnar to the extensor digitorum tendon of the index. Seldom imaged alone; it matters as a donor tendon, for example to replace a ruptured EPL. Ultrasound or MRI when a transfer is planned or a tear is suspected.

Clinical notes

Extensor indicis transfer is used for EPL rupture and, rerouted round the ulna, as an opponensplasty
Posterior interosseous nerve neurectomy through the floor of the fourth compartment is part of wrist denervation for chronic wrist pain
Taking it as a donor leaves index extension to extensor digitorum; most people keep useful independent pointing, though a small loss of strength is reported

Anatomical variants

Its origin and tendon slips vary; doubled tendons and a separate tendon to the middle finger (extensor medii proprius) occur

Test yourself

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

Why is the extensor indicis a good donor tendon for the EPL?

The index still extends through the extensor digitorum after it is moved.

How is the extensor indicis tendon identified at the knuckle?

It lies on the ulnar side of the extensor digitorum tendon to the index.

In which compartment does the extensor indicis run, and what lies in its floor?

The fourth compartment, deep to the extensor digitorum; the end of the posterior interosseous nerve runs in its floor.

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

Its names

Official Latin (TA2): extensor indicis TA2 2515
Official Spanish (DeCS): Músculo extensor del índice
English (TA2): Extensor indicis

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 indicis highlighted on the Durgical 3D model