|Posterior compartment of the forearm|
Extensor compartment of the forearm and hand
|Artery||radial artery, radial recurrent artery, profunda brachii, posterior interosseous artery|
|Nerve||radial nerve,posterior interosseous nerve|
|Latin||compartimentum antebrachii posterius|
The posterior compartment of the forearm (or extensor compartment) contains twelve muscles which are chiefly responsible for extension of the wrist and digits, and supination of the forearm. It is separated from the anterior compartment by the interosseous membrane between the radius and ulna.
There are generally twelve muscles in the posterior compartment of the forearm, which can be further divided into a superficial, intermediate, and deep layer. Most of the muscles in the superficial and the intermediate layers share a common origin which is the outer part of the elbow, the lateral epicondyle of humerus. The deep muscles arise from the distal part of the ulna and the surrounding interosseous membrane.
The brachioradialis, flexor of the elbow, 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, meaning its origin has some distance from the part that it moves. The brachioradialis and the anconeus are considered intrinsic muscles because they both arise within the forearm and they both move the forearm.
|superficial||extensor carpi radialis longus||extrinsic||radial nerve|
|superficial||extensor carpi radialis brevis||extrinsic||radial nerve (deep branch)|
|superficial||extensor carpi ulnaris||extrinsic||radial nerve (as posterior interosseous nerve)|
|intermediate||extensor digitorum||extrinsic||radial nerve (as posterior interosseous nerve)|
|intermediate||extensor digiti minimi||extrinsic||radial nerve (as posterior interosseous nerve)|
|deep||abductor pollicis longus||extrinsic||radial nerve (as posterior interosseous nerve)|
|deep||extensor pollicis longus||extrinsic||radial nerve (as posterior interosseous nerve)|
|deep||extensor pollicis brevis||extrinsic||radial nerve (as posterior interosseous nerve)|
|deep||extensor indicis||extrinsic||radial nerve (as posterior interosseous nerve)|
|deep||supinator||intrinsic||radial nerve (deep branch)|
The supinator and the anconeus are the two muscles in the posterior compartment of the forearm that do not pass through wrist extensor compartments.
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 humerus.
In the early stage of development, the extensor precursor divides into 3 layers namely, superficial layer, radial layer and deep layer. 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.
|Phylogenetic origin||Embryologic origin||Representatives in humans|
|Brachioantebrachial group||Radial layer||Brachioradialis|
|Extensor carpi radialis longus|
|Extensor carpi radialis brevis|
|Superficial layer||Extensor digitorum communis|
|Extensor carpi ulnaris|
|Extensor digiti minimi|
|Antebrachiomanual group||Deep layer (radial)||Extensor pollicis brevis|
|Abductor pollicis longus|
|Deep layer (ulnar)||Extensor pollicis longus|
|Extensor indicis proprius|
|Extensor digitorum brevis manus|
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:
Tennis elbow or lateral epicondylitis is a chronic or an acute inflammation 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 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.
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 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. Also, the extra pressure causes synovitis which results in pain in the dorsal part of the wrist.
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, cyst or tumour.
In the superfamily hominoidea 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 (humans) and the genus hylobates because the extensor pollicis brevis and the abductor pollicis longus exist as a single muscle in other genera.