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Anatomy, Shoulder and Upper Limb, Brachialis Muscle
Introduction
The brachialis is an important flexor muscle of the forearm at the elbow.[1] The brachialis provides elbow flexion at all physiologic positions and is considered a "pure flexor" of the forearm at the elbow.[2] It lies in the anteroinferior area of the arm and is deeper than the biceps brachialis muscle. Brachialis contributes to the upper part of the anatomically important antecubital fossa of the elbow joint.
Structure and Function
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Structure and Function
The brachialis is an elbow flexor that originates from the distal anterior humerus and inserts onto the ulnar tuberosity. The brachialis is 1 of the largest elbow flexors and provides pure forearm flexion at the elbow.[2] It does not provide any supination or pronation of the forearm. See Image. Internal Muscles of the Chest and Shoulder. Within the literature, there are conflicting reports of the detailed anatomy of the brachialis. Traditionally, the brachialis has been described as a single-head muscle, although cadaver assessment has demonstrated that the brachialis muscle may have 2 heads, 1 superficial and 1 deep.[3] The superficial head forms the major head, which originates from the anterior mid-shaft humerus and the lateral intermuscular septum and inserts onto the ulnar tuberosity.[3] In detail, the insertion of the deep head has 3 portions: medial and lateral aponeurosis and muscular contractile fibers that attach directly to the ulna.[3] The deep head forms a smaller muscle that originates from the anterior humerus and the medial intermuscular septum and inserts mainly into an aponeurosis that branches to the ulna. Furthermore, different anatomic variants have been described in the literature, with the potential for real clinical implications. The brachialis muscle is the strongest flexor of the elbow in the absence of supination, as with supination and flexion, its mechanical momentum becomes more disadvantaged than the biceps brachialis muscle.
Embryology
As a skeletal muscle, the brachialis ultimately develops from the mesoderm layer.[4] The upper extremity musculature develops from a common muscle of origin, muscle primordia, that later develops into specific muscle heads (see Image. Right Upper Extremity Surface Anatomy).[5][6] Specifically, the muscle primordia develops from dorsolateral somite cells that migrate into limb buds around day 28 of development.[5] The muscle primordia later split into separate flexor and extensor components. This division is controlled by signaling from connective tissue deriving from the lateral plate mesoderm.[5][6] Notably, anterior-posterior development is under the control of downstream signaling of sonic hedgehog protein, secreted at the zone of polarizing activity in the posterior limb bud.[5][6] Dorsoventral differentiation is under the control of WNT7A and downstream signaling pathways thereof.[5][6] Differences in these complex developmental pathways can ultimately lead to different anatomic variants. These variants can also significantly influence surrounding neural and vascular structures.
Blood Supply and Lymphatics
The blood supply of the brachialis ultimately comes from the tributaries of the brachial artery and the radial recurrent artery. Occasionally, other arteries may supply the brachialis, including branches of the ulnar collateral arteries. The venous drainage from the brachialis muscle is ultimately the brachial vein, which later joins with the basilic vein tributary and forms the axillary vein. The upper limb contains both superficial and deep lymphatic channels. The superficial channels generally follow superficial vasculature and perforate into the deep lymphatics at various points, particularly near the cubital fossa. The deep lymphatic channels generally follow the main vessels and ultimately drain into the axillary lymph nodes.[7] It bears mentioning that research has documented various anatomic variants of this relevant vasculature.[8][9][10]
Nerves
The brachialis often has dual innervation, innervated medially by the musculocutaneous nerve and laterally by the radial nerve.[11][12] However, the musculocutaneous nerve provides most of the motor supply to the muscle. Other anatomic variants have also been described, including individual innervation by the musculocutaneous and median nerve.[13] The musculocutaneous nerve passes between the biceps and brachialis muscles, wherein the terminal branch of the lateral antebrachial cutaneous nerve emerges. The radial nerve passes between the brachialis and brachioradialis muscles in the lateral arm after spiraling and emerging from the spiral groove of the humerus.[14] However, there is documentation of anatomic variants of the brachialis muscle, which have influenced nerve location and innervation patterns.[15]
Muscles
The brachialis muscle relates anteriorly to the biceps muscle, the brachioradialis muscle, the pronator teres muscle, and the vascular-nerve bundle of the arm. See Image. Superficial Muscles of the Chest and Shoulder.
Physiologic Variants
There have been some reports of anatomic variants of the brachialis muscle.[16] One case report discussed an accessory brachialis muscle found during cadaver dissection at Harvard Medical School in 2003.[17] The accessory brachialis muscle originated from the mid-shaft of the humerus and the medial intermuscular septum. It crossed both the median nerve and brachial artery before inserting into the common tendon of the elbow flexor muscles.[17] Another report described an accessory brachialis muscle that included a fibrous/muscular tunnel containing the brachial artery and median nerve, suggesting that this anatomic variant could yield nerve compression symptomatology with muscle contraction.[18]
Surgical Considerations
The brachialis typically gets split during anterior and anterolateral surgical approaches to the humerus.[11] Such methods are common for humerus fractures, particularly supracondylar humerus fractures.[19] Of note, a supracondylar fracture of the humerus is 1 of the most common pediatric injuries of the elbow region. Estimates show that it accounts for between 15 and 17% of childhood extremity fractures.[20] Given the importance of the brachialis muscle and the proximity to various neurovascular elements, this approach requires extreme care. Notably, cadaver studies have demonstrated that anterolateral splitting of the brachialis yields a significant probability of damaging lateral branch nerve supply, even when the brachialis receives dual innervation (eg, from both the musculocutaneous and radial nerves).[11] Given anatomic variability, it may be difficult to predict the exact location of essential nerves within these muscle compartments, further complicating the surgical approach.[11] Some have described utilizing a lateral approach that does not split the brachialis muscle. However, there is a concern that the required manipulation and dissection may increase the risk of post-operative nerve palsy.[21]
Clinical Significance
The brachialis muscle is an important elbow flexor, so it is clinically relevant. Impaired elbow flexion can result from several etiologies, including neurologic, neurovascular, muscle rupture, or traumatic causes. Generally, injury to the biceps brachii is more commonly the cause of elbow flexor trauma, although there is also documentation of isolated injuries to the brachialis muscle.[22][23][24][25] The majority of these cases were attributable to either overuse injuries or strenuous weight-loading injuries. Magnetic resonance imaging is generally the most accurate means of diagnosing these isolated muscle ruptures. However, there are also suggestions regarding using ultrasound for diagnosis as a low-risk and cost-effective alternative.[24] Although the literature is limited, some have suggested that conservative, non-surgical management is sufficient for uncomplicated, isolated brachialis injury.[24]
Additionally, there has been documented use of brachialis tendon transfer, specifically to reconstruct the flexor digitorum profundus and the flexor pollicis longus after brachial plexus injuries.[26] The brachialis is an alternative donor for such applications since forearm muscles are not always available for use in these procedures. This study demonstrated that this donor strategy achieved "key pinch" and "hook grasp" in these patients.[26] For a manual evaluation of the strength of the brachialis muscle, the operator puts his resistance on the patient's wrist. At the same time, the latter holds the elbow extended and with the palm forward. The patient has to flex the elbow without supination; in this way, we are evaluating the flexor muscle strongest at the elbow.
Other Issues
Brachialis syndrome results from a permanent injury to the median nerve following poor patient positioning during surgery, particularly due to a decompression of the median nerve in the antecubital fossa.[27] According to the literature, the brachialis muscle could be 1 of the causes of the alteration of the arthrokinematics of the shoulder (thanks to the anatomic-myofascial continuum), causing pain to the movement and disturbance to the rotator cuff of the shoulder. The presence of trigger points in the brachialis muscle could be a clue. In some patients, injecting drugs in the brachialis muscle can solve the shoulder problem. Myositis ossificans in the sporting field generally involve specific muscle areas, including the brachialis muscle (see Image. Myositis Ossificans of the Brachialis Muscle).[28] The approach is often conservative, and the athlete has excellent chances of returning to full function. In severe cases where surgery is indicated, ensuring the process fully develops before intervening is essential.[29] Furthermore, radiotherapy may have a role in improving post-operative pain and functional outcomes.[30] Angiosarcoma formation can affect the brachialis muscle, particularly in deep tissues.[31] Angiosarcoma may present as a deep swelling or hematoma; a bioptic test is required to understand the nature of the swelling. Tendinopathy of the brachialis muscle at its insertion is a rare event. Generally, the symptom is pain in the antecubital area, and conservative therapy (steroid injection) is the recommended approach to management.[32]
Media
(Click Image to Enlarge)
Superficial Muscles of the Chest and Shoulder. This illustration shows the pectoralis major, deltoid, coracobrachialis biceps brachii, brachialis, and brachioradialis. Other structures included in this image are the clavicle, sternum, and lacertus fibrosus.
Henry Vandyke Carter, Public Domain, via Wikimedia Commons
(Click Image to Enlarge)
Internal Muscles of the Chest and Shoulder. The internal muscles of the chest and shoulder include the pectoralis, deltoid, subclavius, costal cartilages, ribs, pectoralis minor, serratus anterior, biceps brachii, coracobrachialis, and brachialis.
Henry Vandyke Carter, Public Domain, via Wikimedia Commons
(Click Image to Enlarge)
Right Upper Extremity Surface Anatomy. This anterior view shows the surface markings of the flexor carpi radialis, abductor and exterior pollicis longus and brevis, palmaris longus, medial antebrachial muscles, antecubital fossa, lateral antebrachial muscles, brachialis, biceps brachii, triceps brachii, and medial epicondyle.
Henry Vandyke Carter, Public Domain, via Wikimedia Commons
(Click Image to Enlarge)
Myositis Ossificans of the Brachialis Muscle
Orthobullets
References
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