Coracobrachialis muscle variants in human fetuses

Purpose: The current cadaveric study on human fetuses demonstrates the coracobrachialis muscle variability. It further analyzes the embryological and phylogenetic background as well as the coracobrachialis muscle variability in adults.

Materials: Classical (gross anatomy) dissection was performed on 140 (34 male and 36 female) upper limbs of human fetuses, 10 % formalin-fixed. The donated fetuses were obtained after spontaneous abortion and informed consent of both parents. The dissection was performed at the Anatomical Dissection and Donation Department of the Medical University of Lodz. Upon dissection, the following features were assessed: 1. the coracobrachialis muscle's anatomy, 2. the musculocutaneous nerve origin and course in relation to the coracobrachialis muscle, 3. the coexisting neural and muscular variants (co-variants), and 4. The morphometric details of the coracobrachialis heads and the nerve's length.

Results: Four muscle variants (based on the number of heads) were identified in 140 specimens. A two-headed muscle (typical anatomy) was identified in 60.7 %, with origins from the coracoid process, and the biceps brachii short head or the coracoid process exclusively. A one-headed muscle with origin from the coracoid process was identified in 18.6 %. Three- and four-headed muscles were identified in 10 % and 10.7 %, having origins from the coracoid process, and the biceps brachii short head. In all cases, the coracobrachialis was inserted into the distal humeral third. In cases of a muscle with two or more heads, the musculocutaneous nerve had a course between superficial and deep heads, while in cases of a single muscle head, the nerve followed a medial course to the muscle. Thirteen cases of musculocutaneous absence were identified (9.29 %), in which the median nerve supplied the coracobrachialis.

Conclusion: Four variants of the coracobrachialis muscle were identified according to the number of heads, compared to the three variants recorded in the human adult study. Coracobrachialis muscle and musculocutaneous nerve anatomy (typical and variant) are of clinical importance.

Level of evidence: II Basic Science Research.