Introduction

The brachioradialis is a superficial forearm muscle located in the lateral forearm. The brachioradialis primarily flexes the forearm at the elbow but also functions to supinate or pronate depending on the rotation of the forearm. The muscle originates along the proximal two-thirds of the lateral supracondylar ridge of the humerus and distally inserts into the lateral surface of the styloid process of the radius. The brachioradialis has unique attachment points compared to other muscles in the body because it originates from the distal end of one bone and inserts into the distal end of another bone. [1][2]

Structure and Function

The brachioradialis muscle can be palpated in the anterolateral forearm. The brachioradialis originates proximally from the lateral supracondylar ridge of the humerus and inserts distally to the radial styloid process of the radius. Given these attachments, the brachioradialis only crosses the elbow joint. The muscle has a fusiform shape where it widens at the muscle belly and narrows distally to become a thin tendon at its distal insertion point. The brachioradialis contributes to the muscle mass overlying the anterolateral forearm. [1][2]

The brachioradialis muscle contributes to the boundaries of the cubital fossa, forming the lateral boundary along with the wrist extensors. The cubital fossa is on the anterior surface of the elbow and contains important structures such as the brachial artery, median nerve, and biceps tendon. [3]

There are five compartments of the forearm which include the volar superficial, volar deep, dorsal superficial, dorsal deep, and mobile wad. The brachioradialis is located in the mobile wad compartment along with the extensor carpi radialis longus and extensor carpi radialis brevis muscles. [4]

The brachioradialis primarily acts as a flexor of the elbow. The muscle has been shown to be active during elbow flexion whether the forearm is supinated, neutral, or pronated. The brachioradialis functions to stabilize the forearm during elbow flexion. [5] When the forearm is supinated the brachioradialis acts as a pronator and when the forearm is pronated the brachioradialis acts as a supinator. [2] [6]

Embryology

The upper limb originates from the lateral plate of the mesoderm and somatic mesoderm and emerges as a limb bud around 26 days. [7] The somatic mesoderm contributes to the muscle and the lateral plate mesoderm contributes to the tendon and other connective tissue. The superficial muscles, such as the brachioradialis develop before the deeper muscles and can be identified by the seventh week. As the limb buds lengthen, the muscle divides into extensor and flexor components defined by connective tissue derived from the lateral plate mesoderm. The zone of polarizing activity, located at the posterior border of the upper limb bud, secretes sonic hedgehog protein to control anterior-posterior patterning. [8]

Blood Supply and Lymphatics

The brachioradialis muscle is supplied by the radial recurrent artery. The radial recurrent artery is a branch of the radial artery below the elbow joint that runs distal to proximal to contribute to the collateral circulation elbow joint. [9][10][11][10] The superficial venous drainage of the forearm and elbow has many variations. The cephalic vein and the basilic vein ascend the lateral and medial forearm, respectively. In the most commonly seen venous pattern the median cubital vein arising from the cephalic vein which then crosses the antecubital fossa to combine proximally with the basilic vein. The deep veins form paired vessels around the named arteries of the forearm and arm. [12][13][14]

The brachioradialis lymphatic drainage is part of the upper limb lymphatic system, consisting of superficial and deep lymphatic vessels. The superficial lymphatic vessels closely follow the superficial venous vasculature. Part of the superficial lymphatic system follows the basilic vein to drain into the cubital lymph nodes. Lymphatic vessels around the cephalic vein drain to the axillary lymph nodes. The deep lymphatic vessels also drain lymph from the brachioradialis and travel with the deep veins, eventually draining to the axillary lymph nodes. [15][16][17]

Nerves

The brachioradialis muscle is innervated by the radial nerve. [6][18] The innervation to the brachioradilis muscle includes contributions from spinal nerve roots C5, C6, and C7, although the brachioradialis muscle is primarily innervated by C5 and C6 nerve roots. [19] The distal portion of the radial nerve lies between the brachioradialis and the brachialis muscles anteriorly. [20] Distal to the elbow joint the radial nerve splits into superficial and deep branches. The superficial branch of the radial nerve runs distally in the forearm under the brachioradialis and lateral to the radial artery. Distally the superficial radial nerve emerges superficially between the brachioradialis and extensor carpi radialis longus tendons. [21] In a radial nerve palsy the brachioradialis is one of the first muscles to recover after nerve injury. [22]

Muscles

The brachioradialis arises with the extensor carpi radialis longus muscle from the lateral supracondylar ridge of the humerus. The brachioradialis is one of seven muscles in the superficial layer of the forearm which attach to the lateral epicondyle and supracondylar ridge of the humerus. The other six muscles originating at these attachments include the extensor carpi radialis longus, extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi, extensor carpi ulnaris, and anconeus muscle. [23]

Physiologic Variants

There are several variations of brachioradialis anatomy that have been reported. One variant described has two superficial radial nerves and two muscle bellies of the brachioradialis. One of the superficial radial nerves runs between the two muscle bellies of the brachioradialis putting the nerve at risk for entrapment. The unique double muscle belly variant of the brachioradialis formed superficial and deep layers which both shared a common origin and insertion tendon. [24] Similar variants have been reported without the duplicated superficial branch of the radial nerve. These reports include a bifid brachioradialis where the superficial branch of the radial nerve passing through it. [25] These variants both can potentially lead to Wartenberg’s syndrome, which is entrapment of the superficial branch of the radial nerve. [26]

Another variant of the brachioradialis muscle features additional muscle fibers contributing to the brachioradialis from a more proximal origin. These additional muscle fibers originate from the shaft of the humerus near the deltoid insertion and blend distally with the brachialis muscle. Some fibers have been reported to originate as proximal as the acromion. These variant muscle fibers then fuse distally with the brachioradialis. [27][28] This variant is described as the brachioradialis accessory muscle and primarily acts as a supinator. [29] This accessory muscle is clinically significant due to its potential to entrap the radial nerve leading to symptomatic nerve compression. [20]

A variant insertion of the brachioradialis has been reported where the tendon distally inserts to the third metacarpal. [30]

Surgical Considerations

The brachioradialis is an important structure in the volar approach to the radius (Henry approach), which is an exposure used for distal radius volar plating in the treatment of distal radial fractures. [31] There are two internervous planes when using the volar approach which includes the distal and proximal planes. The distal internervous plane is between the brachioradialis muscle and the flexor carpi radialis muscle, which are innervated by the radial nerve and median nerve respectively. [32][33] The more proximal internervous plane is between the brachioradialis muscle and the pronator teres, which is innervated by the radial nerve and median nerve respectively.

During superficial surgical dissection in the volar approach to the radius, the surgeon must take care to not damage the superficial radial nerve that runs along the undersurface of the brachioradialis or the palmar cutaneous branch of the median nerve. [34] Care must be taken to not apply excessive force when retracting the superficial radial nerve within the mobile wad. Excessive retraction can lead to a painful neuroma. [35] The palmar cutaneous branch of the median nerve is located at the wrist just medially to the flexor carpi radalis tendon and is also at risk to be damaged during surgery. [36] The brachioradialis acts as a deforming force on the distal radius and can be released to facilitate reduction before applying volar plating. [32] Studies have shown that releasing the brachioradialis during a distal radial fracture repair does not result in any clinically significant loss of elbow flexion and wrist function. [33][1][2][36]

Wartenberg syndrome can be caused by compression of the superficial radial nerve resulting in pain in the dorsoradial wrist and hand. [37] Wartenberg syndrome is treated by surgical decompression, which is accomplished by releasing the fascia between the brachioradialis and extensor carpi radialis longus. [38][39]

Clinical Significance

Wartenberg syndrome occurs when the superficial radial nerve is compressed by the brachioradialis and extensor carpi radialis longus tendon and by fascial bands in the subcutaneous plane [38][39] This compression is typically more symptomatic during forearm pronation. Patients may have a history of forearm fracture or wearing handcuffs, tight wrist bands, or casts. [40][41]. This type of radial neuropathy only causes deficits in sensory functioning. [42] The nerve compression causes burning pain and paresthesia over the dorsum of the hand, wrist, thumb, index, and middle fingers. [37] Patients will have no complaints of motor weakness. Physical exam maneuvers to elicit Wartenberg syndrome include Tinel sign over the superficial radial nerve, wrist flexion with ulnar deviation, and Finkelstein test. [43][44] Wartenberg syndrome can be treated using surgical decompression. [38][39]

The brachioradialis tendon is used clinically to test the C6 spinal nerve. Striking the brachioradialis tendon causes elbow flexion with forearm pronation or supination. [45] A C5-C6 herniated disc causes cervical radiculopathy and affects the C6 nerve root, resulting in sensory and reflex loss over the brachioradialis muscle. There may also be paresthesia of the thumb and index finger. In cases of C6 spinal nerve compression, tapping of the distal brachioradialis tendon may produce ipsilateral finger flexion, known as an inverted radial reflex or inverted supinator sign. [46]

A midshaft fracture of the humerus can damage the radial nerve within the radial groove of the humerus. The brachioradialis is the first muscle to be innervated by the radial nerve distal to the fracture and is an important muscle to check during the recovery process of the nerve. The brachioradialis and extensor carpi radialis longus are the two muscles that first recover innervation following an injury to the radial nerve at the radial groove. The radial nerve recovers about 1 mm per day. [47] The brachioradialis is typically reinnervated within 3 or 4 months. Surgical exploration of the radial nerve may be necessary if the brachioradialis has not recovered innervation after 6 months. [48]