The extensor carpi radialis brevis is one of the seven superficial extensor muscles of the posterior forearm, along with brachioradialis, extensor carpi radialis longus, extensor digitorum, extensor digiti minimi, extensor carpi ulnaris and anconeus. Four of these seven have a tendon origin with a lateral epicondyle: Extensor carpi radialis brevis, Extensor carpi ulnaris, Extensor digiti minimi and Extensor digitorum. Most often, the pronator insertion (PT) is moved into the ECRB to restore the extension on the wrist [32, 33, 38, 39], which acts as an internal splint. The released PT muscle is guided deep into the brachioradial and intertwined with sutures on the ECRB tendon [32]. Studies at PT have shown that it is very similar to ECRB in architecture and mechanical properties, with a force potential similar to ECRB and a low differential index [38, 40]. Although this tendon transfer is the usual method and successfully restores wrist extension, a resulting loss of muscle strength and pronation capacity has been described in detail [32]. The ECRB muscle is found deep in the carpi radialis longus (ECRL) and EDC extensor [2] and superficially in the supinator. It has been shown to be innervated either by the deep posterior interosseous nerve (PIN) or by the superficial sensory branch of the radial nerve, which are the bifurcations of the actual radial nerve at the lateral epicondyle [2,5]. It has also been reported that a direct branch of the actual radial nerve feeds the ECRB [5]. ECRL is also innervated by the main radial nerve. It supports ECRB in the extension of the wrist by its origin in the lower third of the lateral supracondylar crest of the humerus and the lateral intermuscular septum and by its introduction into the posterior base of the second metacarpal bone. It originates from the lateral epicondyle of the humerus, through the common extensor tendon; radial collateral ligament of the elbow joint; a strong fascia covering its surface; and intermuscular septa between it and adjacent muscles. [2] The tendon of the extensor carpi radialis brevis passes under the abductor pollicis longus and the extensor pollicis brevis and then under the extensor retinaculum to introduce to the dorsal surface of the base of the third metacarpal.
Like all muscles in the posterior forearm, ECR brevis is powered by a branch of the radial nerve. At the level of the retinaculum, the tendon is medial to that of the long extensor muscle carpi radialis. Another physiological variant identified involves the union of the extensor carpi radialis brevis and extensor carpi radialis longus muscles. In one case report, the extender carpi radialis longus separated from its attachment to the supracondylar crest of the humerus into a lateral, intermedial and medial head. The lateral and intermediate heads are inserted at their typical insertion site on the second metacarpal base, but the medial head is placed on the carpi radialis brevis extensor muscle. [1] Carpi radialis longus and brevis extensor tendons sometimes also have bifid tendons that can unite with each other and insert into the base of the metacarpals. [2] The carpi radialis brevis extender receives its vascular supply mainly from the radial artery, an additional supply from the radial collateral branch of Profunda brachii. From the radial artery, the carpi radialis brevis extender first receives a branch of the radial artery. Then, further down in the forearm, the muscle gets an extra branch of the radial artery. Reports of origins other than the lateral epicondyle have been reported, including deep capsuloligaly structures (i.e., radial collateral ligament or lateral ulnar collateral ligament of the elbow joint) and adjacent intermuscular septum [1, 2].
Differentiation is often difficult even with distal to proximal dissection, as the ECRB tendon fuses proximal with the tendon of the extender digitorum communis (EDC) [2,3]. The tendons of the carpi ulnaris extensor (ECU) and the digiti minimi extender (EDM) also target in the environment [4]. The use of autologous nerve grafts is another practical way for treating radial nerve paralysis, especially transfer from the median nerve to the radial nerve [36, 41, 42]. For example, Ray and Mackinnon (2011) investigated the use of redundant branches of the median nerve as donor nerves, transferring a median nerve branch to the digitorum superficialis flexor (FDS) to the ECRB nerve and the carpi radial flexor nerve to the PIN to restore wrist lengthening [36].