The posterior compartment of the forearm (or extensor compartment) contains twelve muscles which primarily extend 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 digits. It is separated from the anterior compartment by the

interosseous membrane An interosseous membrane is a thick dense fibrous sheet of connective tissue that spans the space between two bones, forming a type of syndesmosis joint. Interosseous membranes in the human body: * Interosseous membrane of forearm * Interosseo ...

between the

radius In classical geometry, a radius (: radii or radiuses) of a circle or sphere is any of the line segments from its Centre (geometry), center to its perimeter, and in more modern usage, it is also their length. The radius of a regular polygon is th ...

and

ulna The ulna or ulnar bone (: ulnae or ulnas) is a long bone in the forearm stretching from the elbow to the wrist. It is on the same side of the forearm as the little finger, running parallel to the Radius (bone), radius, the forearm's other long ...

Structure

Muscles

There are generally twelve

muscle Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to muscle contra ...

s in the posterior compartment of the forearm, which can be further divided into superficial, intermediate, and deep. Most of the muscles in the superficial and the intermediate layers share a common origin which is the outer part of the

elbow The elbow is the region between the upper arm and the forearm that surrounds the elbow joint. The elbow includes prominent landmarks such as the olecranon, the cubital fossa (also called the chelidon, or the elbow pit), and the lateral and t ...

, the lateral epicondyle of

humerus The humerus (; : humeri) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius (bone), radius and ulna, and consists of three sections. The humeral upper extrem ...

. The deep muscles arise from the distal part of the

and the surrounding

. The

brachioradialis The brachioradialis is a muscle of the forearm that flexes the forearm at the elbow. It is also capable of both pronation and supination, depending on the position of the forearm. It is attached to the distal styloid process of the radius by way ...

, flexor of the

, is unusual in that it is located in the posterior compartment, but it is actually a muscle of flexor / anterior compartment of the forearm. The anconeus, assisting in extension of the elbow joint, is by some considered part of the posterior compartment of the arm. The majority of muscles found in the posterior compartment are

extrinsic In science and engineering, an intrinsic property is a property of a specified subject that exists itself or within the subject. An extrinsic property is not essential or inherent to the subject that is being characterized. For example, mass i ...

, meaning that their origin has some distance from the part moved. The brachioradialis and the anconeus are considered intrinsic muscles because they both arise within the forearm and they both move the forearm.

Extensor tendon compartments

Extensor tendons pass through the extensor retinaculum at wrist joint in 6 synovial sheaths, also referred to compartments. The supinator and the anconeus are the two extensor muscles in the posterior compartment of the forearm that do not pass through wrist extensor compartments. # The first compartment locating the most radial is occupied by the extensor pollicis brevis and the abductor pollicis longus to insert to the thumb. # The second compartment is occupied by the two radial wrist extensors, the extensor carpi radialis longus and the extensor carpi radialis brevis. # The third compartment exclusively accommodates the extensor pollicis longus, which hooks around Lister's tubercle of radius and inserts to the thumb. # The fourth compartment is the largest of all. It is occupied by the extensors of the digits, the extensor digitorum communis and the extensor indicis proprius. The extensor indicis proprius usually runs and inserts onto the ulnar side of the extensor digitorum communis of the index finger. # The fifth compartment is occupied by the extensor digiti minimi, the extensor of the little finger. # The extensor carpi ulnaris passes through the sixth compartment to insert to the base of the fifth metacarpal bone.

Innervation

The muscles of the posterior compartment of the forearm are innervated by the radial nerve and its branches. The radial nerve arises from the posterior cord of the plexus. The somatomotor fibers of the radial nerve branch from the main radial nerve at the level of the radial groove of the

Development

In the early stage of development, the extensor precursor divides into 3 layers namely, superficial layer, radial layer and deep layer.Straus WL (1941) The phylogeny of the human forearm extensors. Hum Biol 13, 23–50. The superficial group develops to become the extensor digitorum communis, the extensor carpi ulnaris and the extensor digiti minimi. The radial layer forms the extensor carpi radialis longus, the extensor carpi radialis brevis and the brachioradialis. The deep layer differentiates to become the abductor pollicis longus, the extensor pollicis longus and the extensor pollicis brevis.

Variations

The deep layer of the precursor extensor mass is known to be phylogenetically unstable and is undergoing evolution as high variability is seen in non-human primates. In humans, anomalous or additional muscles can be seen in small portion of population. Anomalous muscles in human extensor compartment are listed as follow: *

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 ins ...

* Extensor indicis et medii communis * Extensor pollicis et indicis communis * Extensor carpi radialis tertius * Extensor digitorum brevis manus

Clinical significance

Tennis elbow

Tennis elbow or lateral epicondylitis is a chronic or an acute

inflammation Inflammation (from ) is part of the biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. The five cardinal signs are heat, pain, redness, swelling, and loss of function (Latin ''calor'', '' ...

of the tendons that arise from the outer part of the elbow. The affected tendons are the tendons of extensor muscles which originate from the lateral epicondyle of humerus. It is caused by the repetitive movements and overuse. It damages the tendons which results in pain and tenderness on the outer part of the elbow.

De Quervain's syndrome

De Quervain's syndrome is a medical condition when the synovial sheath surrounding tendons in the first extensor tendon compartment becomes inflamed, so called tenosynovitis. The tendons of the abductor pollicis longus and the extensor pollicis brevis run narrower due to the thickening of the synovial sheath, which causes pain when extending and moving the thumb outward.

Fourth compartment syndrome

The presence of an additional tendon may result in a condition called fourth compartment syndrome. Supernumerary tendons are common in the fourth extensor tendon compartment. Supernumerary tendons can refer to the additional tendons of normal structures or tendons of rare anatomical variants such as 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 ins ...

or the extensor digitorum brevis manus. The increased pressure in the synovial sheath is known to directly or indirectly compress the posterior interosseous nerve of

radial nerve The radial nerve is a nerve in the human body that supplies the posterior portion of the upper limb. It innervates the medial and lateral heads of the triceps brachii muscle of the arm, as well as all 12 muscles in the Posterior compartment of the ...

. Also, the extra pressure causes

synovitis Synovitis is the medical term for inflammation of the synovial membrane. This membrane lines joints that possess cavities, known as synovial joints. The condition is usually painful, particularly when the joint is moved. The joint usually swells ...

which results in pain in the dorsal part of the wrist.

Misdiagnosis of rare anatomical variants

Anatomical variants are often encountered in the extensor compartment of the forearm. Clinical expressions of the extensor digitorum brevis manus are often mistaken for a

ganglion A ganglion (: ganglia) is a group of neuron cell bodies in the peripheral nervous system. In the somatic nervous system, this includes dorsal root ganglia and trigeminal ganglia among a few others. In the autonomic nervous system, there are ...

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 bubb ...

tumour A neoplasm () is a type of abnormal and excessive growth of tissue (biology), tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tiss ...

In other animals

In the superfamily

hominoidea Apes (collectively Hominoidea ) are a Family (biology), superfamily of Old World simians native to sub-Saharan Africa and Southeast Asia (though they were more widespread in Africa, most of Asia, and Europe in prehistory, and counting humans ...

or apes, configurations of the muscles of the posterior compartment of the forearm share similar characteristics. However, the anconeus is usually not present in the hylobates (gibbons). Also, the extensor pollicis brevis is only present in the genus

homo ''Homo'' () is a genus of great ape (family Hominidae) that emerged from the genus ''Australopithecus'' and encompasses only a single extant species, ''Homo sapiens'' (modern humans), along with a number of extinct species (collectively called ...

(humans) and the genus hylobates because the extensor pollicis brevis and the abductor pollicis longus exist as a single muscle in other genera.

Additional images

File:Gray421.png, Transverse section across distal ends of

and

. File:Carpal-Tunnel.svg, Transverse section across the

and digits.

References

{{Authority control Muscles of the upper limb