Innervation of biceps brachii and brachialis: Anatomical and surgical approach

This study describes the anatomy of the musculocutaneous nerve (MCN) in the middle and lower thirds of the arm, with special reference to the motor branches to the biceps and brachialis muscles, given their importance in certain clinical, particularly surgical, procedures. In each dissection (46 upper limbs) we recorded the course of the MCN and its variations, and the number, type of distribution pattern and length of the motor branches to the biceps brachii and brachialis muscles. We also recorded the position at which those branches arose from the MCN trunk. We found three branching patterns for biceps brachii: 1) one branch (60.5%); 2) two branches, one for each biceps head (27.9%); and 3) two branches, one for the two biceps heads and one for the common belly (11.6%). The mean distance between the acromion and the motor branches to biceps brachii was 133.8 mm, 45.3% of the acromion-lateral epicondyle distance. The mean length of those branches was 31.2 mm. We found two branching patterns for brachialis: 1) one branch (72.1%); and 2) two branches (27.9%). These motor nerves to brachialis arose from the MCN at a mean distance of 185.3 mm, 61.5% of the acromion-lateral epicondyle distance, and their average length was 33.0 mm. The data were expressed as the percentage of the distance between the acromion and the lateral epicondyle of the humerus, to make their clinical use easy and to avoid errors caused by anthropometric differences.