A RARE CASE OF TENDINOUS INSERTION OF CORACOBRACHIALIS ASSOCIATED WITH VARIANT MUSCULOCUTANEOUS NERVE

We report a rare case of the tendinous insertion of coracobrachialis muscle which has not been reported in the literature. The insertion of the novel coracobrachialis muscle is usually into the medial border of the humerus in a 3-5cm impression at the mid-shaft level. Contrary to this, in the present case, it was seen getting inserted as a sharp slender tendon in the middle of the medial border of the humerus. Variable insertion of coracobrachialis muscle may be responsible for the causation of compression of surrounding structures like median nerve, musculocutaneous nerve, and brachial artery. This article aims to point out a rare case of the unusual tendinous insertion of coracobrachialis and its association with musculocutaneous nerve, providing necessary information to surgeons performing surgical reconstruction using coracobrachialis.


INTRODUCTION
Coracobrachialis muscle (CB) is chiefly a flexor muscle, although in the arm it corresponds to the adductor mass seen in the thigh which has become insubstantial in human beings during the process of evolution. It develops from a tissue mass forming the biceps and brachialis and is supplied by musculocutaneous nerve (MCN) (Bergman, 1988). Usually a novel CB originates from the tip of the coracoid process of the scapula in common with the short head of the biceps brachii muscle. The insertion of the muscle is usually into medial border of the humerus in a 3-5cm impression at mid-shaft level between the attachments of triceps and brachialis. CB forms an inconspicuous rounded ridge on the upper medial side of the arm (Williams et al., 1995 www.anatclinar.com.ar These variations include as accessory slip of the CB inserting 1) to medial epicondyle 2) to medial supracondylar ridge of the humerus 3) to medial intermuscular septum of the arm and 4) to lesser tubercle (Ilayperuma et al., 2016;Bohnel, 1979;El-Naggar, 2001;Guha et al., 2010;Kopuz et al., 2002). This article aims to point out a rare case of unusual tendinous insertion of CB and its association with MCN, providing necessary information to surgeons performing reconstruction, both post mastectomy and infraclavicular defects, muscle transfer in cases of facial paralysis, and procedures mandating axillary block.

CASE REPORT
During the regular dissection of 62 years old formalin embalmed male cadavers of south Indian origin in the Department of Anatomy at MVJ Medical College and Research Hospital, we came across a rare anatomical variation in the flexor compartment of the left arm. When anterior compartment muscles of arm were dissected from its origin to the insertion following the steps in Cunnigham's dissection manual, we observed a variation in the insertion of CB. Usually the CB ends on an impression, 3-5 cm in length and is inserted into the middle of the medial border of the humerus, but in the present case, CB was observed getting inserted as sharp slender tendon in the middle of the medial border of the humerus. In addition to this variation, we also observed the musculocutaneous nerve passing underneath this slender tendon. The MCN was seen to be arising normally from the lateral cord of brachial plexus running downwards and laterally to pass beneath the slender tendon of CB. The formation of median nerve (MN) was seen to be normal. Communicating branch between the MN and MCN was encountered after latter passed underneath this slender tendon (Figure 1).

DISCUSSION
An anomaly in the CB is not uncommon but such anomalies have not been well documented. Anomalies are encountered more in distal attachment (insertion) than proximal attachment (origin) of CB. Variations in the insertion of CB have been reported by various authors in the literature. These variations include as accessory slip of the CB inserting 1) to medial epicondyle 2) to medial supracondylar ridges of the humerus 3) to medial intermuscular septum of the arm and 4) to lesser tubercle (Ilayperuma et al., 2016). Kopuz C et al. (2002) stated that Wood (1867) was the first to propose three parts of CB: the upper part inserted to the capsule of the shoulder joint (CB superior or brevis or rotator humeri); the middle part to the mid-part of the humerus (CB proprius or medius); the lower part to the internal condyloid ridge, the internal intermuscular septum, or the trochlea (CB longus). We can enlighten these variations by means of comparative anatomy.
Study conducted in amphibians, reptiles, and monotremes showed that the coracobrachialis muscle has three heads each of which inserted into 1) the humerus superior to tendon of latissimus dorsicoracobrachialis brevis (profundus), 2) the humerus inferior to tendon of latissimus dorsicoracobrachialis medius (proprius), 3) inferiorly on the shaft of humerus bridging the median nerve and brachial artery -coracobrachialis longus (superficialis) (Howell and Straus, 1932;Sonntag, 1923). Similar classification was put forth by Kyou-Jouffroy et al. (as citied by Georgiev et al., 2018). In contrast, Mori (1964) conducted the study on Japanese population and concluded that the CBM was divided into superficial and deep layers. The intrinsic muscles of the upper limb develop from the limb bud mesenchyme of lateral plate mesoderm. The muscle mass may eventually differentiate into different layers of muscles which may fuse to form a single muscle mass followed by apoptosis of some muscle mass (El-Naggar, 2001). This has resulted in fusion of coracobrachialis medius and longus which gets inserted in a diffuse manner to the medial border of humerus at mid shaft level. Persistence of coracobrachialis brevis may present as accessory slip of CB in humans which gets inserted to lesser tubercle or capsule of shoulder joint. From the above explanation, it is evident that slender CB observed in our case was remnant of coracobrachialis medius, longus being absent. Our second observation was MCN passing underneath the slender tendon of CB. Wide variation in MCN (0-22%) have been reported in the previous literature. Loukas and Aqueelah (2005) (Ilayperuma et al., 2016). In conclusion, an anatomical variation of CB has become significant with the widespread use of Computed Tomogram (CT) and Magnetic Resonance Imaging (MRI) wherein any variation in CB can be misinterpreted for any other muscle. Knowledge of these kinds of variations are important as CB has also been used as a graft for chronic acromiocalvicular dislocations, breast reconstructive surgeries Long standing facial www.anatclinar.com.ar paralysis has also been treated effectively with CB flap . The MCN can get trapped underneath CB as in our case, causing paresthesia in the cutaneous area of distribution of the nerve associated with poor flexion of the elbow.

Conflict of interest Nil
Funding Nil

Ethical approval
It was obtained from the ethical committee of the institution

Contributions
The authors in this article have made the following contribution -MKHA: Study design, dissection of specimen, manuscript editing, figure editing; SM: manuscript editing, literature research.