Innervation pattern, localization of primary motor branch and motor points of biceps brachii muscle, and its clinical application: A study from Myanmar

Background: The biceps brachii muscle is known for its significant variability in the human body. This study aims to detail the distal branching patterns of the motor branches of the musculocutaneous nerve, with the goal of developing surgical guidelines tailored to the Myanmar population. Materials and Methods: A total of 62 arms from 31 adult cadavers (26 males, 41.9%, and 36 females, 58.1%) were dissected. A reference line was drawn from the tip of the coracoid process to the humerus's lateral epicondyle. The primary motor nerve to the biceps originated from the musculocutaneous nerve, and the gross motor entry point (GMEP) for each muscle head was measured from the tip of the coracoid process, expressed as a percentage of the reference line. Results: Innervation pattern Type I, characterized by a single primary motor branch (PMB), was present in 48 arms (77.4%), while Type II, with two separate PMBs, was identified in 11 arms (17.4%). Type III, where a common PMB bifurcated to supply both heads, with an additional motor branch innervating the common muscle belly, was seen in three arms (4.8%). The mean distance of the PMB was 12.23 cm in males and 11.49 cm in females. The mean percentage of PMB relative to the reference line was 44.78% in males and 44.04% in females. The mean distance from the GMEP to the long head was 13.48 cm in males and 12.5 cm in females. The mean distance from the GMEP to the short head was 12.115 cm in males and 11.514 cm in females. Conclusion: An anterior surgical approach is recommended for identifying the motor branches and locating the motor points, which facilitates clinical application and reduces errors due to anthropometric differences.