Extensor Indicis Proprius Muscle Anatomy

The extensor indicis proprius muscle has a narrow, elongated belly that lies medial to and alongside that of the extensor pollicis longus. It’s located in the deep layer of posterior compartment of the forearm.

This muscle confers independence to the index finger in that it may act alone or together with the extensor digitorum to extend the index finger at the proximal interphalangeal joint, as in pointing. It also helps extend the hand.

Extensor Indicis Proprius Muscle Anatomy

The extensor indicis proprius muscle origins from from the posterior surface of Ulna and interosseous membrane, and inserts onto the base of the second proximal phalange and into the tendon of extensor digitorum.

See Also: Forearm Muscles Anatomy & Function

It’s innervated via the posterior interosseous nerve which is a deep branch of Radial nerve.

Its blood supply comes from the posterior interosseous branch of the Ulnar artery and perforating branches of the anterior interosseous artery.

The primary function of extensor indicis Ppropius is the extension of second digit at metacarpophalangeal and interphalangeal joints. It may assist in wrist extension. It assists in abducting the index finger. It may assist in slight supination of forearm.

Variations of the extensor indicis were classified into four types:

1. Type 1, an additional tendon slip from the extensor indicis tendon;
2. Type 2, an extensor indicis radialis or extensor pollicis et indicis accessorius;
3. Type 3, an extensor medii proprius with or without extensor medii brevis;
4. Type 4, an extensor indicis radialis and extensor medii proprius.

The extensor medii proprius was the most common variation.

References & More

1. Cael, C. (2010). Functional anatomy: Musculoskeletal anatomy, kinesiology, and palpation for manual therapists. Philadelphia, PA: Wolters Kluwer Health/Lippincott, Williams & Wilkins.
2. Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2014). Clinically Oriented Anatomy (7th ed.). Philadelphia, PA: Lippincott Williams & Wilkins.
3. Clinically Oriented Anatomy – 8th Edition
4. Komiyama M, Nwe TM, Toyota N, Shimada Y. Variations of the extensor indicis muscle and tendon. J Hand Surg Br. 1999 Oct;24(5):575-8. doi: 10.1054/jhsb.1999.0239. PMID: 10597935. Pubmed