Extensor indicis

Where is the muscle located and how is it structured?

Extensor indicis muscle belongs to the deep layer of muscles in the posterior forearm compartment. Almost all muscles of this layer originate from the forearm bones and the interosseous membrane.

• Origin (Origo): Posterior surface of the lower (distal) third of the ulna and the interosseous membrane.
• Insertion (Insertio): On the dorsum of the hand, the tendon blends into the dorsal tendinous (aponeurotic) expansion of the index finger (digit II).

Topography and adjacent structures

The diagram of the deep layer of the posterior forearm compartment visualizes:

• Extensor pollicis longus muscle (m. extensor pollicis longus), which originates from the middle third of the ulna.
• Supinator (m. supinator).
• Abductor pollicis longus (m. abductor pollicis longus).
• Extensor pollicis brevis muscle (m. extensor pollicis brevis).
• Extensor carpi ulnaris and extensor carpi radialis muscles.
• Anconeus muscle (m. anconeus).
• Bony landmarks: radial and ulnar styloid processes, lateral and medial epicondyles, olecranon (olecranon).

How does the tendon run?

The tendon of the extensor indicis passes through the 4th compartment together with the tendons of the extensor digitorum.

• Compartment contents: The tendons are enclosed in a common triangular synovial sheath — vagina tendinum mm. extensoris digitorum et extensoris indicis.
• Sheath features: The base of this synovial triangle faces distally toward the fingers.
• Boundaries: Proximally, the sheath extends 10 mm above the extensor retinaculum. Distally, it ends blindly at the level of the mid-metacarpals.

Function, blood supply, and innervation

• Function (Actio): The muscle provides independent extension of the index finger. Additionally, it assists in wrist extension.
• Innervation: Supplied by the radial nerve — n. radialis (motor spinal cord segments C7–C8).
• Blood supply: The muscle is supplied by the posterior interosseous artery — a. interossea posterior.

Clinical significance

During a neurological consultation, to objectively assess trophic changes and muscle strength, it is recommended to examine the extensor indicis (m. extensor indicis) along with other muscles:

• triceps brachii muscle (m. triceps brachii);
• brachioradialis muscle (m. brachioradialis);
• supinator muscle (m. supinator);
• extensor carpi radialis longus and brevis muscles (mm. extensores carpi radiales longus et brevis);
• extensor digitorum muscle (m. extensor digitorum);
• extensor carpi ulnaris muscle (m. extensor carpi ulnaris);
• abductor pollicis longus muscle (m. abductor pollicis longus);
• extensor pollicis brevis and longus muscles (mm. extensores pollicis brevis et longus).

It is also recommended to assess superficial and pain sensation in the zones innervated by the radial nerve: the posterior surface of the arm, the posterior surface of the forearm, and the posterior-lateral surface of the hand (Level of recommendation evidence — C, Reliability level of evidence — 5).

Class and exam questions

What else to study on this topic

• Extensor digitorum (m. extensor digitorum)
• Extensor retinaculum (retinaculum mm. extensorum)
• Radial nerve (n. radialis)
• Deep layer of muscles of the posterior forearm compartment

Other topics in the "Anatomical Structures" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.