Extensor digitorum brevis manus is an extra or

accessory muscle An accessory muscle is a relatively rare anatomical variation where duplication of a muscle may appear anywhere in the muscular system. Treatment is not indicated unless the accessory muscle interferes with normal function. Examples are the stern ...

on the backside (dorsum) of the

hand A hand is a prehensile, multi-fingered appendage located at the end of the forearm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs. A few other vertebrates such as the koala (which has two opposable thumbs on each " ...

. It was first described by Albinus in 1758. The muscles lies in the fourth extensor compartment of the

wrist In human anatomy, the wrist is variously defined as (1) the carpus or carpal bones, the complex of eight bones forming the proximal skeletal segment of the hand; "The wrist contains eight bones, roughly aligned in two rows, known as the carpal ...

, and is relatively rare. It has a prevalence of 4% in the general population according to a meta-analysis. This muscle is commonly misdiagnosed as a ganglion cysta, synovial nodule or

cyst A cyst is a closed sac, having a distinct envelope and division compared with the nearby tissue. Hence, it is a cluster of cells that have grouped together to form a sac (like the manner in which water molecules group together to form a bubble ...

Structure

The extensor digitorum brevis manus usually originates from the dorsal aspect (backside) of the wrist, either from the joint capsule, the distal end (the most distant end) of the radius, the metacarpal, or from the radiocarpal ligament in the area of the fourth extensor compartment. Many variations of the muscle have been described in the literature. It could have up to four tendons with a single tendon inserting to the index or the middle finger being the two most common variations. At the insertion the tendon of the extensor digitorum brevis manus often joins the

extensor indicis proprius In human anatomy, the extensor indicis roprius'' is a narrow, elongated skeletal muscle in the deep layer of the dorsal forearm, placed medial to, and parallel with, the extensor pollicis longus. Its tendon goes to the index finger, which it exte ...

, although it also occurs when the extensor indicis proprius is absent. It was also reported to coexist with the

extensor medii proprius The extensor medii proprius (so called the ''extensor digiti medii'') is a rare anatomical variant in the extensor compartment of the forearm. The aberrant muscle is analogous to the extensor indicis with the insertion being the middle finger inst ...

, another anatomical variation in the extensor compartment of the hand. The muscle is supplied the

posterior interosseous nerve The posterior interosseous nerve (or dorsal interosseous nerve) is a nerve in the forearm. It is the continuation of the deep branch of the radial nerve, after this has crossed the supinator muscle. It is considerably diminished in size compared ...

and posterior branch of the

anterior interosseous artery The anterior interosseous artery (volar interosseous artery) is an artery in the forearm. It is a branch of the common interosseous artery. Course It passes down the forearm on the palmar surface of the interosseous membrane. It is accompanied ...

Variation

The extensor digitorum brevis manus was found to be present in 4% of the population, and there was no correlation between gender, side or ancestry. it inserted to the index finger and the middle finger in 77% and 23% of the cases, respectively. It occurred bilaterally in 26% the total cases.

Development

At some point during the

embryonic development An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...

, the precursor extensor mass differentiates into three layers: radial, superficial, and deep. The extensor digitorum brevis manus may have been originated from the deep layer, a typical location where most of the variations take place. Some authors believe that this muscle may represent a failure of proximal migration of ulnocarpal elements of the extensor muscle mass in humans.

Functions

It extends the index or the middle finger. It is believed to be a substitute for the extension of the index finger when the extensor indicis proprius is absent.

Clinical significance

Only a few clinical cases have been reported among more than 300 clinical and cadaveric dissections. This implies that the presence of this muscle is usually asymptomatic, although the extensor digitorum brevis manus might cause a painful swelling which can potentially be misdiagnosed as other

pathology Pathology is the study of the causes and effects of disease or injury. The word ''pathology'' also refers to the study of disease in general, incorporating a wide range of biology research fields and medical practices. However, when used in ...

such as synovial cyst and

lipoma A lipoma is a benign tumor made of fat tissue. They are generally soft to the touch, movable, and painless. They usually occur just under the skin, but occasionally may be deeper. Most are less than in size. Common locations include upper back, ...

References