Morphometric study of the nerves entering into the coracobrachialis muscle.

The nerves entering into the coracobrachialis muscle are the musculocutaneous nerve (MC) and the nerve (usually consists of several thin branches) branches to the coracobrachialis. These thin branches enter the coracobrachialis proximal to the MC. The thin branches and the MC are susceptible to injury during coracoid process transfer. The purpose of this study is (1) to reveal the number and origin of the thin branches and (2) especially to report the morphometric information about the two distances between the coracoid process and the points where the first thin branch and the MC enter the coracobrachialis. These distances were named as the "distance T1" and the "distance D," respectively. Forty-two cadaver upper extremities were used and the distance between the coracoid process and the medial epicondyle of the humerus as the "arm length" was measured. The "ratio T1" was calculated by dividing the distance T1 by the arm length. The "ratio D" was calculated by dividing the distance D by the arm length. The number of the thin branches varied between one and four. In the most common type, there were two thin branches (45%). All of the thin branches originated from the MC. The mean distance T1, distance D and arm length were found as 41.5, 62 and 304.5 mm, respectively. The mean ratio T1 and ratio D were determined as 0.13 (approximately 1/8) and 0.20 (=1/5), respectively. The findings about the number and origin of the thin branches may contribute to the anatomy of the nerve to the coracobrachialis. The shoulder surgeon may calculate the predicted distance T1 and distance D of any upper extremity, dividing its arm length by eight and five, respectively.