Anatomy and Location
Extensor indicis belongs to the deep layer of the posterior forearm compartment. Almost all muscles in 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 digital expansion (aponeurosis) of the index finger (digit II).
Topography and Related Structures
In cross-section or deep dissection of the posterior forearm compartment, the following structures are visualized:
- Extensor pollicis longus (m. extensor pollicis longus), originating from the middle third of the ulna.
- Supinator (m. supinator).
- Abductor pollicis longus (m. abductor pollicis longus).
- Extensor pollicis brevis (m. extensor pollicis brevis).
- Extensor carpi ulnaris and extensor carpi radialis muscles.
- Anconeus (m. anconeus).
- Bony landmarks: radial and ulnar styloid processes, lateral and medial epicondyles, and the olecranon (olecranon).
Tendon Pathway
The tendon of the extensor indicis passes through the 4th extensor 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 triangular synovial sheath 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 (spinal cord segments C7–C8).
- Blood Supply: Supplied by the posterior interosseous artery — a. interossea posterior.
Clinical Significance
During a neurological examination to assess trophic changes and muscle strength, it is recommended to test the extensor indicis (m. extensor indicis) alongside other muscles:
- Triceps brachii (m. triceps brachii);
- Brachioradialis (m. brachioradialis);
- Supinator (m. supinator);
- Extensor carpi radialis longus and brevis (mm. extensores carpi radialis longus et brevis);
- Extensor digitorum (m. extensor digitorum);
- Extensor carpi ulnaris (m. extensor carpi ulnaris);
- Abductor pollicis longus (m. abductor pollicis longus);
- Extensor pollicis brevis and longus (mm. extensores pollicis brevis et longus).
It is also recommended to evaluate superficial and pain sensation in the innervation zones of the radial nerve: the posterior arm, posterior forearm, and dorsolateral hand.