Extensor digiti minimi muscle

Origin and insertion

Extensor digiti minimi originates from the lateral epicondyle of humerus via the common extensor tendon that it shares with extensor digitorum, extensor carpi radialis brevis and extensor carpi ulnaris muscles. The muscle courses inferiorly, however, its proximal fibers are usually indistinguishably fused with those of the extensor digitorum muscle.

From approximately the middle of the forearm, the fibers of extensor digiti minimi are clearly separated from extensor digitorum. Here, it can be recognized as an independent muscle sitting just medial to extensor digitorum.

In the lower half of the forearm, the muscle continues as a long tendon that passes deep to the extensor retinaculum. The tendon sits within its own sheath, forming the fifth extensor (dorsal) compartment of the wrist as it crosses the posterior side of the inferior radioulnar joint. The tendon then enters the dorsal aspect of the hand, slightly deviating medially towards the fifth digit as it splits into two slips.

The lateral slip fuses with a few tendinous fibers of extensor digitorum, but ultimately, both slips insert onto the extensor expansion of digit 5. Extensor expansions, or extensor hoods, are aponeurotic sheets on the dorsal surfaces of digits 2-5 that serve as inserting sites for the tendons of digital extensors, lumbricals and interossei muscles.

Relations

Extensor digiti minimi sits medial to, and is sometimes partially fused with, the extensor digitorum muscle. It is found lateral to extensor carpi ulnaris.

At the extensor retinaculum level, dorsal branches of the ulnar artery and ulnar nerve cross the superficial surface of the tendon of extensor digiti minimi, being separated from it by the extensor retinaculum.

Innervation

Extensor digiti minimi is innervated by the posterior interosseous nerve (C7, C8), which is a continuation of the deep branch of the radial nerve. The radial nerve is a branch of the posterior cord of the brachial plexus.

Blood supply

Blood supply to the extensor digiti minimi muscle comes from two sources;

• Radial artery, via the radial recurrent artery
• Ulnar artery, via the anterior and posterior interosseous arteries

Function

Extensor digiti minimi functions primarily to extend the fifth digit at its metacarpophalangeal joint. It also contributes to extension of the wrist and all the fifth finger joints from the ulnar side of the hand, via it’s attachment into the extensor expansion and by acting along with extensor carpi ulnaris muscle.

Having its own extensor muscle, the little finger can extend independently of digits 2-4. This is because the digits 2-4 are primarily extended by a single muscle that inserts to all of them; the extensor digitorum.