Anatomy, Shoulder and Upper Limb, Brachialis Muscle

The brachialis muscle relates anteriorly to the biceps muscle, the brachioradialis muscle, the pronator teres muscle, and the vascular-nerve bundle of the arm.

There have been some reports of anatomic variants of the brachialis muscle. One case report discussed an accessory brachialis muscle found during cadaver dissection at Harvard Medical School in the year 2003.[12] The accessory brachialis muscle was found to originate from the mid-shaft of the humerus and the medial intermuscular septum and crossed both the median nerve and brachial artery before inserting into the common tendon of the elbow flexor muscles.[12] 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.[13]

The brachialis typically gets split during anterior and anterolateral surgical approaches of the humerus.[11] Such methods are common for humerus fractures, particularly supracondylar humerus fractures.[14] Of note, supracondylar fracture of the humerus is one of the most common pediatric injuries of the elbow region. Estimates are that it accounts for between 15 to 17% of childhood extremity fractures.[15] 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 branches of nerve supply, even when the brachialis receives dual innervation (e.g., 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, although there is a concern that the required manipulation and dissection may increase the risk of post-operative nerve palsy.[16] 

Given that the brachialis muscle is an important elbow flexor, it is of clinical relevance. 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.[17][18][19][20] The majority of these cases were attributable to either overuse injuries or strenuous weight loading injuries. Magnetic resonance imaging (MRI) is generally the most accurate means of diagnosing these isolated muscle ruptures, although there are also suggestions regarding the use of ultrasound for diagnosis as a low-risk and cost-effective alternative.[19] Although the literature is limited, some have suggested that conservative, non-surgical management is sufficient for uncomplicated, isolated brachialis injury.[19]

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.[21] Since forearm muscles are not always available for use, the brachialis is an alternative donor for such applications. This study demonstrated that this donor strategy yielded excellent results in achieving "key pinch" and "hook grasp" in these patients.[21] 

For a manual evaluation of the strength of the brachialis muscle, the operator puts his resistance on the patient's wrist, while the latter holds the elbow extended and with the palm of the hand forward. The patient will have to flex the elbow without supination; in this way, we are evaluating the flexor muscle stronger than the elbow.

Brachialis syndrome is the result of a permanent injury to the median nerve, following a poor patient positioning during surgery, in particular, due to a decompression of the median nerve in the antecubital fossa.

According to the literature, the brachialis muscle could be one 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, the injection of drugs in the brachialis muscle can solve the problem of the shoulder.

Myositis ossificans in the sporting field, generally, involves specific muscle areas, including the brachialis muscle. The approach is often conservative, and the athlete has excellent chances of returning to his sport.

Angiosarcoma formation can affect the brachialis muscle, in particular, in deep tissues. 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 a pain in the antecubital area, and a conservative therapy (steroid injection) is the recommended approach to management.