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Anatomy, Shoulder and Upper Limb, Forearm Brachioradialis Muscle
Introduction
The brachioradialis is considered a posterior or extensor-compartment muscle, though it functions as a flexor (see Image. Forearm Muscles). This unique muscle is 1 of 2 forearm extensor-compartment muscles that do not cross the wrist, the other being the supinator. The brachioradialis flexes, pronates, and supinates the forearm but is innervated by the radial nerve. Therefore, weakness of this muscle or its tendon reflex during neurologic examination may indicate a radial rather than a median or ulnar nerve lesion.
Trauma and chronic overuse are the usual causes of brachioradialis impairment. In surgery, the brachioradialis is vital to hand defect reconstruction, forearm tendon transfers, and carpal tunnel release. Understanding this muscle's anatomic and clinical significance is vital to properly evaluating and managing various musculoskeletal conditions.
Structure and Function
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Structure and Function
Structure
The brachioradialis muscle can be palpated in the anterolateral forearm. The muscle has a fusiform shape, widening at the muscle belly and narrowing distally to insert via a thin tendon at its attachment site. The brachioradialis originates proximally from the proximal two-thirds of the lateral humeral supracondylar ridge and inserts distally on the base of the lateral radial styloid process. Given these attachments, the muscle crosses only the elbow joint. The brachioradialis contributes to the muscle mass overlying the anterolateral forearm.[1][2]
The brachioradialis muscle forms the lateral cubital fossa boundary along with the wrist extensors. The brachial artery, median nerve, and biceps tendon all pass this important region on the elbow's flexor surface.[3]
The 5 forearm compartments include the volar superficial, volar deep, dorsal superficial, dorsal deep, and mobile wad. The brachioradialis is located in the mobile wad compartment together with the extensor carpi radialis longus and extensor carpi radialis brevis muscles.[4]
Function
The brachioradialis primarily acts as an elbow flexor. The muscle is active during elbow flexion, whether the forearm is supinated, neutral, or pronated. The brachioradialis stabilizes the forearm during elbow flexion.[5] The muscle acts as a pronator when the forearm is supinated and a supinator when the forearm is pronated.[6]
Embryology
The upper limb originates from the lateral plate of the mesoderm and somatic mesoderm, emerging as a limb bud around 26 days.[7] The somatic mesoderm forms the muscle, while the lateral plate mesoderm gives rise to the tendon and other connective tissue. Superficial muscles like the brachioradialis develop before the deeper muscles and can be identified by the 7th week. As the limb buds lengthen, the newly formed connective tissue divides the limb muscles into extensor and flexor components. 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, a radial artery branch that supplies the forearm and contributes to the elbow joint's anastomotic network.[9][10][11]
The cephalic and basilic veins—the upper limb's main superficial veins—ascend the lateral and medial forearm, respectively. In most people, the median cubital vein arises from the cephalic vein, which then crosses the antecubital fossa to merge proximally with the basilic vein. However, the forearm and elbow's superficial venous drainage has many variations. Meanwhile, 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 and consists 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 empty into the axillary lymph nodes. The deep lymphatic vessels also drain lymph from the brachioradialis and travel with the deep veins, eventually emptying into the axillary lymph nodes.[15][16][17]
Nerves
The radial nerve innervates the brachioradialis muscle.[18] This nerve receives contributions from the C5 to C7 spinal roots, although most of the neural input to the brachioradialis comes from C5 and C6.[19] The radial nerve's distal portion lies between the brachioradialis and brachialis muscles anteriorly.[20] Distal to the elbow joint, the radial nerve splits into superficial and deep branches. The radial nerve's superficial branch 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] The brachioradialis is one of the first muscles to recover following radial nerve injury.[22]
Muscles
The brachioradialis arises with the extensor carpi radialis longus muscle from the lateral humeral supracondylar ridge. The brachioradialis is 1 of 7 muscles in the forearm's superficial layer that attach to the lateral epicondyle and supracondylar ridge of the humerus. The other 6 muscles originating from these regions include the extensor carpi radialis longus, extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi, extensor carpi ulnaris, and anconeus.[23]
Physiologic Variants
Several brachioradialis anatomic variations have been reported in the literature. One variant has 2 superficial radial nerves and 2 brachioradialis muscle bellies with a common origin and insertion point. The 2 muscle bellies form superficial and deep layers. One of the superficial radial nerves runs between the muscle bellies and is thus at increased risk for entrapment.[24] Another variant has a bifid brachioradialis, with the radial nerve's superficial branch passing through the 2 muscle slips.[25] Wartenberg’s syndrome, a condition arising from entrapment of the superficial branch of the radial nerve, can develop from these variants.[26]
Muscle fibers from a more proximal origin have also been found merging with the brachioradialis. In one variant, the proximal muscle fibers originate from the humeral midshaft near the distal deltoid insertion. The accessory muscle fibers may even arise as high as the acromial level—a variant that has been reported to enhance the muscle's supinating ability.[27][28][29] Accessory brachioradialis muscles also increase the risk of symptomatic radial nerve compression.
Brachioradialis insertion onto the 3rd metacarpal has also been documented. This variant allows the muscle to move the wrist joint.[30]
Surgical Considerations
The brachioradialis is an important landmark during a volar-approach (Henry-approach) radial surgery. The technique maximizes radial bone exposure during distal-radius volar plating.[31] The volar approach divides the surgical area into distal and proximal internervous planes. The distal internervous plane lies between the brachioradialis and flexor carpi radialis. The proximal internervous plane occupies the area between the brachioradialis and pronator teres.[32][33] The radial nerve supplies the brachioradialis, while the flexor carpi radialis and pronator teres are supplied by the median nerve.
In the wrist region, the superficial radial nerve runs deep to the brachioradialis or palmar cutaneous branch of the median nerve. The palmar cutaneous branch of the median nerve lies medial to the flexor carpi radalis tendon. Careful retraction or dissection in this area helps avoid distal radial and median nerve injuries.[34][35]
The brachioradialis acts as a molding force on the distal radius and can be released to facilitate reduction before volar plating. Studies show that releasing the brachioradialis during a distal radial fracture repair does not produce clinically significant elbow flexion and wrist function loss.[36]
Wartenberg syndrome may be caused by compression of the superficial radial nerve, resulting in pain in the dorsoradial wrist and hand.[37] The condition is treated by surgical decompression, accomplished by releasing the fascia between the brachioradialis and extensor carpi radialis longus.[38][39]
Clinical Significance
Wartenberg syndrome occurs when the brachioradialis and extensor carpi radialis longus tendons or fascial bands in the subcutaneous plane compress the superficial radial nerve. Forearm pronation precipitates the symptoms, usually burning pain and paresthesia over the dorsum of the hand, wrist, thumb, index, and middle fingers. Patients may have a history of forearm fractures or wearing handcuffs, tight wrist bands, or casts.[40][41] This type of radial neuropathy only causes sensory deficits.[42] Physical exam maneuvers that can elicit Wartenberg syndrome include the Tinel and Finkelstein tests and wrist flexion with ulnar deviation.[43][44] Surgical decompression can alleviate the condition.
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 to C6 herniated disc causes C6 radiculopathy, resulting in sensory and reflex loss over the brachioradialis muscle. Thumb and index finger paresthesia may also manifest. Tapping the distal brachioradialis tendon may produce ipsilateral finger flexion, known as an inverted radial reflex or inverted supinator sign.[46]
A humeral midshaft fracture can damage the radial nerve, which spirals around the radial groove of the humerus. The brachioradialis is the first muscle the radial nerve innervates distal to the fracture. Thus, examining brachioradialis function helps monitor the recovery of this nerve.
The brachioradialis and extensor carpi radialis longus are the first 2 muscles to regain motor strength following a humeral midshaft-level injury to the radial nerve. A lesioned radial nerve recovers function at a rate of about 1 mm per day.[47] The brachioradialis is typically reinnervated within 3 or 4 months, even with conservative treatment. Surgical arm exploration may be necessary if the brachioradialis has not recovered strength after 6 months.[48]
Media
(Click Image to Enlarge)
Forearm Muscles. This anterior view shows the biceps brachii, brachioradialis, pronator teres, flexor carpi radialis and ulnaris, palmaris longus, and flexor digitorum sublimis. Other structures in this illustration include the flexor retinaculum (not labeled) and medial epicondyle.
Henry Vandyke Carter, Public domain, via Wikimedia Commons
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