A proposal for a new classification of coracobrachialis muscle morphology.

The coracobrachialis muscle (CRM) originates from the apex of the coracoid process, in common with the short head of the biceps brachii muscle, and from the intermuscular septum. It inserts to the medial part of the humerus between the attachment of the medial head of the triceps brachii and the brachial muscle. Both the proximal and distal attachments of the CRM, as well as its relationship with the musculocutaneus nerve, demonstrate morphological variability. One hundred and one upper limbs (52 left, and 49 right) fixed in 10% formalin solution were examined. Three main types, with subtypes, were identified. The most common was Type I (49.5), characterized by a single muscle belly with a classical origin from the coracoid process, medially and posteriorly to the tendon of the biceps brachii. Type II (42.6%), characterized by two heads, was divided into two subtypes (A-B) depending on its origin: Type IIA, where one head originated from the coracoid process posteriorly to the tendon of the biceps brachii and the second head from the short head of the biceps brachii, and Type IIB, in which both heads originated from the coracoid process; however, the superficial head fused with the insertion of a short head of the biceps brachii, while the deep head was directly originating. Finally, Type III (7.9%) was characterized by three heads: two originated from the coracoid process (superficial and deep), and the third from a short head of the biceps brachii. Two types of insertion and two types of musculocutaneous nerve (MCN) relative to CRM could be distinguished. An adapted classification is needed for all clinicians working in this area, as well as for anatomists. The CRM demonstrates morphological variability in both its proximal and distal attachments, as well as the variable course of the MCN relative to the CRM. Not much is known about the variability of coracobrachialis muscle. The present paper introduces a completely new classification, both clinical and anatomical.