As stated before, the musculocutaneous nerve innervates the three muscles of the arm’s anterior compartment. The coracobrachialis, the first muscle to receive innervation, originates on the coracoid process and inserts on the middle third of the medial aspect of the humerus. It flexes and adducts the shoulder at the glenohumeral joint. It is important to note that unlike the other two muscles of the anterior compartment, the coracobrachialis does not cross the elbow joint thus has no action on the elbow.

The biceps brachii muscle has a short and long head. The short head originates on the coracoid process of the scapula, while the long head originates on the supraglenoid tubercle of the scapula. These two heads come together to form a single tendon that inserts on the radial tuberosity and the fascia of the forearm via the bicipital aponeurosis. The biceps brachii acts to flex the elbow, as well as supinate the forearm. The biceps brachii muscle receives its innervation from the C5 and C6 fibers of the musculocutaneous nerve. 

The brachialis muscle originates on the distal portion of the anterior humerus, and inserts on both the coronoid process and tuberosity of the ulna. Many will think of the biceps brachii when thinking of elbow flexion, but it is actually the brachialis that is considered the main flexor of the elbow. The brachialis is a versatile flexor in that it is able to flex the elbow from either a pronated or supinated forearm position. 

Surgeons always remain astute to the peripheral nerve anatomy due to relative susceptibility that many nerves have for intraoperative damage. Zlotolow, et al describe the many surgical exposures of the humerus and the specific nerves that may be injured with each approach. The deltopectoral approach, specifically for repairs of subscapularis tears, places the musculocutaneous nerve at risk. Anterolateral approaches for reduction of humeral fractures place the lateral antebrachial cutaneous nerve at risk.[3] Additionally, it is noted that surgeons should avoid dissecting medial to the conjoined tendon (short head of the biceps and coracobrachialis attachment on the coracoid process) due to risk of lesioning the musculocutaneous nerve.[3]

Musculocutaneous Nerve Palsy

As with all nerves, direct trauma to the musculocutaneous nerve in the form of lacerations, gunshot wounds, and nearby bone fractures has been reported.[4] While isolated musculocutaneous nerve syndromes are relatively uncommon, a few specific clinical situations have been described in the literature. Most significant is entrapment of the musculocutaneous nerve within the coracobrachialis muscle, leading to biceps brachii and brachialis weakness and atrophy with accompanying loss of sensation in the lateral forearm. It has been found that patients most apt to develop this condition are active young individuals that frequently engage in shoulder flexion and elbow flexion with the forearm in a pronated position.[5] This syndrome often occurs secondary to hypertrophy of the coracobrachialis and thus is a result of chronic overuse. It is important to note that the compressed nerve within the coracobrachialis has already given off its motor branch to the coracobrachialis, thus will not present with defects of coracobrachialis muscle function.

Another important clinical scenario is shoulder dislocation. While the most frequently injured nerve in this scenario is the axillary nerve, several cases of musculocutaneous nerve damage have been reported secondary to anterior humeral dislocations. The musculocutaneous nerve branches from the lateral cord just anterior to the glenohumeral joint, explaining its susceptibility to damage with anterior dislocations. Additionally, it has been found that downward traction and external rotation place significant tension on the nerve, and anterior humeral dislocations may place the nerve in this position.[6][7]

Lateral Antebrachial Cutaneous Nerve Palsy

In addition to the musculocutaneous nerve lesions described, specific damage to the lateral antebrachial cutaneous nerve has also been examined. While this sensory-only lesion often goes unnoticed clinically, there are a few clinical situations worth noting. Just prior to exiting the deep fascia to supply cutaneous innervation, the lateral antebrachial cutaneous nerve passes between what has been described as a tunnel created by tough fascial layers of the brachialis and bicipital aponeurosis. The nerve can be compressed by the bicipital aponeurosis in cases of chronic elbow extension with the forearm in a pronated position. [4] This can lead to hypesthesia in the lateral forearm region. 

The lateral antebrachial cutaneous nerve travels in close proximity to the cephalic vein, thus is at risk for injury during venipuncture procedures. General practice is to avoid the medial aspect of the cubital fossa during venipuncture due to cases of median antebrachial cutaneous nerve damage. However, cases of lateral antebrachial cutaneous nerve damage similar to that described by Rayegani and Azadi suggest that caution should be taken when using the lateral aspect of the cubital fossa as well. Using as superficial an approach as possible has been suggested to reduce the overall risk of peripheral nerve damage. [8]