Anatomy, Shoulder and Upper Limb, Forearm Brachioradialis Muscle

Article Author:
Brandon Lung
Article Editor:
Mike Bisogno
Updated:
10/27/2018 12:31:26 PM
PubMed Link:
Anatomy, Shoulder and Upper Limb, Forearm Brachioradialis Muscle

Introduction

The brachioradialis is a superficial fusiform forearm muscle that flexes the forearm at the elbow. It originates along the proximal two-thirds of the humerus lateral supracondylar ridge and inserts into the lateral surface of the styloid process of the radius.[1] The brachioradialis is the only muscle in the body that originates from the distal end of one bone and inserts onto the distal end of another bone. In the anatomical position, the brachioradialis forms the lateral boundary of the cubital fossa and is part of the muscle mass that overlies the anterolateral forearm. During flexion of a semipronated forearm, the brachioradialis muscle bulge can be found.

Structure and Function

The brachioradialis initiates forearm pronation, forearm supination, and flexion of the elbow at the mid-prone position. Since the brachioradialis lies anterior to the elbow joint, it is an accessory flexor despite being a posterior compartment muscle. During pronation, the brachioradialis is an active elbow flexor since the biceps brachii is in a mechanical disadvantage.

Embryology

The upper limb musculature originates from the dorsolateral somite cells that move into the limb around the fourth week to form muscles. With lengthening of the buds, the muscle divides into extensor and flexor components determined by connective tissue derived from the lateral plate mesoderm. The zone of polarizing activity at the posterior border of the upper limb secretes sonic hedgehog to control anterior-posterior patterning.[2] Once the limb bud forms, the primary rami along the ventral aspect migrates into the mesenchyme, and the radial nerve arises by both of the dorsal segmental branches to innervate the brachioradialis.

Blood Supply and Lymphatics

The brachioradialis muscle is supplied by the radial recurrent artery, which contributes to an anastomotic network around the elbow joint and lateral forearm muscles. The brachioradialis lymphatic drainage is part of the upper limb lymph system, consisting of superficial and deep lymphatic vessels. The superficial vessels around the basilic vein go to the cubital lymph nodes, which are proximal to the medial epicondyle of the humerus. Vessels around the cephalic vein go to the axillary lymph nodes. The deep lymphatic vessels also drain lymph from the brachioradialis and follow the major deep veins, eventually terminating in the humeral axillary lymph nodes.

Nerves

The brachioradialis muscle is a posterior compartment muscle and is innervated by the radial nerve. The radial nerve lies between the brachioradialis and the brachialis muscles anteriorly. The first muscle to be innervated by the nerve is the brachioradialis, and it is also the first muscle to recover after nerve injury. The superficial radial nerve runs distally in the forearm under the brachioradialis and lateral to the radial artery.

Muscles

The brachioradialis is palpable as part of the thick mobile wad of muscle on the anterolateral part of the forearm. The mobile wad consists of the brachioradialis, extensor carpi radialis longus, and extensor carpi radialis brevis, with the brachioradialis forming the medial border. The brachioradialis arises with the extensor carpi radialis longus muscle from the lateral supracondylar ridge of the humerus.

As a posterior compartment muscle of the forearm, the brachioradialis is one of seven muscles in the superficial layer, along with the extensor carpi radialis longus, extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi, extensor carpi ulnaris, and anconeus. All seven superficial muscles have a common origin at the supraepicondylar ridge and lateral epicondyle of the humerus.

Physiologic Variants

Case reports have described coincidental variations of the superficial radial nerve and brachioradialis with one duplicated superficial nerve branch traveling between the two bellies of the variant brachioradialis muscle.[3] This variant brachioradialis was noted to have a superficial and deep muscle belly with one common origin and insertional tendon. This variant is significant when considering the presentation of pain and paresthesia along the dorsolateral hand due to Wartenberg syndrome.

Another variant of the brachioradialis muscle described in the literature features a separate supernumerary muscle arising from the brachioradialis and inserting into either the radius, pronator teres, or supinator muscle.[4] This variant is described as the brachioradialis accessory muscle and functions to reinforce the action of the biceps brachii, brachioradialis, and supinator muscles. This accessory muscle is clinically significant because in cases of radial nerve compression the radial nerve has been reported to pass deep to the brachioradialis accessory muscle.

Surgical Considerations

The brachioradialis is important in the volar approach to the radius, which is an exposure used for distal radius volar plating in the treatment of wrist fractures.[5] The internervous plane along the distal radius lies between the brachioradialis muscle, which is innervated by the radial nerve, and the flexor carpi radialis muscle, which is innervated by the median nerve.  The internervous plane along the proximal radius lies between the brachioradialis muscle and the pronator teres, which is innervated by the median nerve.

During superficial surgical dissection in the volar approach to the radius, the surgeon must identify the superficial radial nerve that runs along the undersurface of the brachioradialis. Careful preservation of the superficial radial nerve will prevent the formation of a painful neuroma at the operative site. The nerve may also become vulnerable to neurapraxia when the mobile wad is mobilized and vigorously retracted laterally. The brachioradialis receives arterial branches from the recurrent radial artery below the elbow. These arterial branches must be ligated and divided to allow lateral mobilization of the brachioradialis and prevention of hematoma formation. During surgical decompression for Wartenberg syndrome, neurolysis and release of fascia are done between the brachioradialis and extensor carpi radialis longus. [6]

Clinical Significance

Wartenberg syndrome occurs when the superficial radial nerve is compressed by the brachioradialis and extensor carpi radialis longus tendon during forearm pronation.[7] This type of radial neuropathy occurs in sports that involve repeated supination and pronation, and deficits are seen in only sensory functioning. Patients may have a history of forearm fracture or wearing handcuffs, tight wrist bands, or plaster casts. The superficial radial nerve is compressed by the scissoring action of the brachioradialis and extensor carpi radialis longus during forearm pronation and by fascial bands at the subcutaneous plane exit site. Unlike de Quervain tendinopathy, the radial nerve compression causes burning pain and paresthesias over the dorsum of the hand, wrist, thumb, index, and middle fingers. Patients may complain of worsening symptoms during repetitive wrist flexion and ulnar deviation, but there are 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 for one minute, and Finkelstein test to cause traction of the nerve.

Because the brachioradialis pronates the forearm when supinated and supinates the forearm when pronated, the brachioradialis acts as a deforming force in distal radius fractures when the forearm is immobilized in full pronation or supination after reduction of the fracture. The deforming force of the brachioradialis is one reason for neutral immobilization of distal radius fractures.

Other Issues

A C5-C6 herniated disc causes cervical radiculopathy and affects the C6 nerve root, resulting in sensory and reflex loss over the brachioradialis muscle. Patients with C6 radial nerve pathology will not only have loss of wrist extension but also the loss of the brachioradialis reflex. There may also be paresthesia of the thumb and index finger. In cases of spinal cord compression, tapping of the distal brachioradialis tendon may produce ipsilateral finger flexion, known as an inverted radial reflex.

When there is an injury to the humerus and radial nerve at the spiral groove, the distance between the spiral groove and the brachioradialis is about 10 to 12 cm. The brachioradialis is the first muscle to be innervated by the radial nerve and is an important muscle to check for the recovery process of the nerve. Since the radial nerve recovers about 1 mm per day, regain of the brachioradialis function can occur within 120 days of initial radial nerve injury. If the brachioradialis fails to show activity within 4 months of nerve injury, radial nerve exploration is warranted.