Extensor digitorum muscle

Origin and insertion

Extensor digitorum is a superficial muscle of the posterior compartment of the forearm. Like the majority of the muscles in this compartment, it originates via common extensor tendon that arises from the lateral epicondyle of humerus. This tendon serves as a proximal attachment for extensor digiti minimi, extensor carpi radialis brevis and extensor carpi ulnaris muscles. 

From its origin, the muscle descends superficially down the posterior aspect of the forearm. At the approximately distal third of the forearm, the muscle belly ends in four tendons that enter the dorsum of the hand, passing deep to extensor retinaculum. In the hand, the tendons diverge towards the medial four fingers. Some of the tendons are connected by the oblique intertendinous connections that are variable among individuals. 

The tendons insert onto the dorsal surface of digits 2-5, respectively.  Each tendon attaches to its respective digit via extensor expansion (extensor hood). This is a triangular aponeurotic expansion formed by the inserting tendons of digital extensors, lumbricals and interossei muscles. It covers the dorsal surfaces of digits 2-5, extending from the metacarpophalangeal joint all the way to the proximal interphalangeal joint of each digit. When it reaches the distal end of proximal phalanx, each extensor hood divides into three separate slips; the central part and two collateral (lateral) parts. The central part inserts onto the dorsal aspect of the base of the middle phalanx, while the two collateral parts merge with the intrinsic muscles of the hand to form a conjoint tendon that inserts onto the base of distal phalanx.

Relations

Being located superficial to abductor pollicis longus and extensor pollicis longus muscles, extensor digitorum is the most superficial muscle of the posterior forearm. These three muscles enclose a tunnel through which posterior interosseous artery and nerve pass. 

In the forearm, extensor digitorum is situated medial to extensor carpi radialis brevis muscle and lateral to extensor digiti minimi and extensor carpi ulnaris muscles. 
Deep to extensor retinaculum, its tendons are placed between the tendons of extensor digiti minimi on its medial side, and extensor pollicis longus on its lateral side. Along with extensor indicis, the tendons of extensor digitorum occupy the fourth extensor (dorsal) compartment. In the dorsum of the hand the tendons of extensor digitorum run superficial to dorsal interossei muscles.

Innervation

Extensor digitorum is innervated by posterior interosseous nerve which is a continuation of a deep branch of radial nerve (root value C7 and C8).

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Blood supply

Extensor digitorum is vascularized by the branches of three different arteries;

• posterior interosseous artery
• radial recurrent artery
• anterior interosseous artery

Anterior and posterior interosseous arteries are the branches of the common interosseus artery that arises from the ulnar artery. The radial recurrent artery is a branch of the radial artery.

Function

As its name suggests, the main function of extensor digitorum is the extension of four medial fingers in metacarpophalangeal and proximal and distal interphalangeal joints. It's important to highlight that the action of this single muscle opposes the actions of two flexors of the fingers; flexor digitorum superficialis and flexor digitorum profundus. This muscle participates in the anatomy of the grip as it contributes to the opening of the hand and letting go of an object. As it crosses the wrist, the contraction of extensor digitorum participates in the extension of this joint.

Don't forget to quiz yourself on the extensor digitorum and other muscles of the forearm to consolidate your knowledge!