Extensor Indicis

What is Extensor Indicis

The extensor indicis, also known as the extensor indicis proprius, is a long narrow muscle in the forearm that is an extrinsic hand muscle, working as an extensor of the 2nd digit. It is one of the 5 deep muscles in the posterior compartment of the forearm, along with the supinator, abductor pollicis longus, extensor pollicis longus, and brevis.

Anatomy

Location and Attachments

Origin	The posterior surface of the ulna on the distal one-third of its shaft
Insertion	The extensor hood of the 2nd digit (index finger)

Origin

The thin muscle rises from the posterior surface of the distal ulnar shaft or body. More specifically, from the distal third of the bone’s posterior surface. A part of the muscle also originates from the interosseous membrane of the forearm that connects the ulna with the radius.

It then travels distally down the forearm, with the muscle belly soon narrowing into a tendon that passes underneath the dorsal carpal ligament.

Insertion

As it travels down to enter the hand, the tendon crosses the wrist bones over to the ulnar side, opposite the second metacarpal head. Here it joins the extensor digitorum tendons that run down the ulnar side before inserting at the extensor hood (extensor expansion) on the posterior side of the 2nd digit’s proximal phalanx.

Sometimes, it may be connected to the extensor digitorum tendons of one or more of the other digits via thin tendinous slips.

On rare occasions, the muscle may have an additional belly, referred to as extensor indicis brevis manus, traveling down the back of the hand.

Relations With Surrounding Muscles and Structures

The elongated muscle lies in the deep posterior compartment, with the extensor digitorum immediately superficial. The extensor pollicis longus lies parallel and lateral to the extensor indicis, while the extensor carpi ulnaris lies medially.

On the distal side, the extensor indicis tendon lies deep to the extensor retinaculum. It shares the 4th extensor compartment with the tendons of the superficial muscle in the posterior forearm, the extensor digitorum. The 3rd extensor compartment containing the extensor pollicis longus runs medially, and the 5th extensor compartment containing the extensor digiti minimi runs laterally.

The posterior interosseous nerve travels deep to the 4th extensor compartment.

Function

Action

Extension of the index finger

Like most other muscles in the body, the name for extensor indicis comes from what it does, which is extending (‘extensor’) the 2nd digit or index finger (‘indicis’).

It is the extensor muscle dedicated to the index finger so we can extend it independently to make signs, like when you say ‘no’ with your finger or hold the finger up for someone to wait a moment. It extends the metacarpophalangeal and interphalangeal joints in the index finger.

The extensor indicis is also one of the muscles that do not attach to the dorsal surface of the hand via oblique connective tissue bands. As a result, it can move free of the other hand extensors.

An additional secondary function of the muscle is helping extend the wrist joint.

The main antagonists of the extensor indicis are the parts of the flexor digitorum superficialis and flexor digitorum profundus that insert at the base of the 2nd middle phalanx.

Innervation

Nerve

Posterior interosseous nerve (C7 and C8)

The posterior interosseous nerve that innervates the muscle is a branch of the radial nerve, with nerve roots in spinal nerves C7 and C8.

Blood Supply

Artery

Posterior interosseous artery

The posterior interosseous artery supplies the superior surface of the extensor indicis. The anterior interosseous artery provides vasculature to the inferior or deep surface via its perforating branches.