Extensor indicis

The extensor indicis is a deep muscle located in the extensor compartment of the forearm, specifically dedicated to controlling the index finger.

Origin

It originates from the distal third of the posterior surface of the ulna and the adjacent interosseous membrane, lying medial and slightly distal to the origin of the extensor pollicis longus.

Course and insertion

The muscle belly of the extensor indicis lies deep to the extensor digitorum and extensor digiti minimi muscles. As it approaches the wrist, its tendon enters the fourth dorsal compartment, passing beneath the extensor retinaculum (dorsal carpal ligament). Within this compartment, it runs alongside the tendons of the extensor digitorum communis.

The extensor indicis tendon then travels along the ulnar side of the extensor digitorum tendon, specifically heading to the index finger. It ultimately inserts into the ulnar aspect of the extensor expansion (dorsal digital expansion) of the index finger, typically joining the extensor digitorum tendon at the level of the metacarpophalangeal joint..

Actions

The extensor indicis is crucial for the independent extension of the index finger. It extends the index finger at all its joints: the metacarpophalangeal (MCP), proximal interphalangeal (PIP), and distal interphalangeal (DIP) joints. Additionally, it contributes to the overall extension of the wrist.