Anat Cell Biol.  2024 Jun;57(2):194-203. 10.5115/acb.24.032.

Morphological classification, anatomical variations, innervation patterns, musculocutaneous nerve relation of the coracobrachialis muscle: anatomical study and clinical significance

• Nasr AY
• Youssef RA

• KMID: 2556563
• DOI: http://doi.org/10.5115/acb.24.032

Abstract

The anatomical variations of coracobrachialis muscle (CBM) are of great clinical importance. This study aimed to elucidate the morphological variations, innervation patterns and musculocutaneous nerve (MCN) relation to CBM. Upper limbs of fifty cadavers (30 males and 20 females) were examined for proximal and distal attachments, innervation pattern of CBM and its relation to MCN. Four morphological types of CBM were identified according to number of its heads. The commonest type was the two-headed (63.0%) followed by the single belly (22.0%), three-headed (12.0%) and lastly fourheaded (3.0%) type. Moreover, an abnormal insertion of CBM was observed in four left limbs (4.0%); one inserting into the medial humeral epicondyle, the second into the upper third of humeral shaft, the third one in the common tendon of biceps, and the fourth one showing a bifurcated insertion. Also, four different innervation patterns of CBM were identified including MCN (80.0%), lateral cord (14.0%), lateral root of median nerve (4.0%), and median nerve itself (2.0%). The course of MCN was superficial to the single belly CBM (19.0%) and in-between the heads in the other types (71.0%). Measurements of the length and original distance of CBM muscular branches originating from MCN revealed no sex or side significant difference. Awareness of the anatomic variations, innervation patterns, and MCN relation of CBM is imperative in recent diagnostic and surgical procedures to obtain definite diagnosis, effective management and good outcome.

Keyword

Coracobrachialis muscle; Variations; Musculocutaneous nerve

Figure

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  Fig. 1 The morphological types of the CBM. (A) Single belly CBM (type I) originating from the CP of scapula in common with the SH with superficial course of MCN. (B) Two-headed CBM (type II) shows superficial head (1) originating from CP and deep head (2) originating from SH with passage of MCN between them. (C) The three-headed CBM (type III) exhibits two superficial heads (1, 2) originating from the SH directly or through a common tendon and a deep head (3) originating from CP and inserts into the middle of anteromedial surface of humerus with passage of MCN between its superficial and deep heads. (D) The four-headed CBM (type IV) has two superficial heads (1, 4) originating from CP and SH respectively and two deep heads (2, 3) originating from CP directly and in common with SH with passage of MCN between the deep and superficial heads. The 4th head (4) fuses with the common tendon of biceps brachii muscle. CBM, coracobrachialis muscle; MCN, musculocutaneous nerve; LH, long head of biceps; LCF, lateral cutaneous nerve of forearm; MN, median nerve; LC, lateral cord of brachial plexus; LR, lateral root of MN; MR, medial root of MN; AX, axillary artery; BA, brachial artery; SH, short head of biceps; UN, ulnar nerve; CP, coracoid process; MC, medial cord; Cb, communicating branch; 3rd, third head of biceps; Lat. Dorsi, latissimus dorsi muscle.

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  Fig. 2 The variants of the CBM insertion. (A) The insertion of CBM bifurcates into large muscular part attaching into the anteromedial surface of humerus and small medial fibromuscular sling extending downward and medially to insert into the medial border of humerus (***) crossing the MN, AX, UN, and MCF superficially. The MCN pierces CBM and gives a Cb to MN. (B) The second variant of CBM insertion showing a long tendinous expansion (arrowheads) extending from the fleshy insertion of the muscle downward to insert into the ME of humerus close to the BA and MN. MCN passes between the superficial and deep heads of CBM. (C) The third variant of CBM insertion showing the extension of CBM insertion (****) downward into the deep surface of the common tendon of biceps brachii. (D) The fourth variant of CBM insertion showing the single belly muscle inserting mostly into the middle of anteromedial surface of humerus and small short fibrous sling extending from the medial aspect of the muscle to insert into the medial border of humerus (arrowheads) just lower to the latissimus dorsi muscle (Lat. Dorsi) tendon. MCN passes superficial to CBM without piercing. MCN, musculocutaneous nerve; CBM, coracobrachialis muscle; SH, short head of biceps; LH, long head of biceps; MN, median nerve; AX, axillary artery; BA, brachial artery; UN, ulnar nerve; MCF, medial cutaneous nerve of forearm; Cb, communicating branch; LC, lateral cord; CP, coracoid process; MC, medial cord; ME, medial epicondyle; 3rd, third head of biceps; MR, medial root.

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  Fig. 3 The different innervation patterns of the CBM. (A) In pattern (I): The muscular branch of CBM (arrow) originates from the MCN before its piercing. (B) In pattern (II): CBM innervates from a muscular branch (arrow) originating from the LC. (C) IN pattern (III): CBM innervates from a muscular branch (arrow) originating from the LR of MN. (D) In pattern (IV): CBM innervates from a muscular branch (arrow) originating from the MN itself. CBM, coracobrachialis muscle; MCN, musculocutaneous nerve; SH, short head of biceps; LH, long head of biceps; AX, axillary artery; BA, brachial artery; UN, ulnar nerve; MCF, medial cutaneous of forearm; Cb, communicating branch; CP, coracoid process; 3rd, third head of biceps; LC, lateral cord; LR, lateral root; MN, median nerve; MC, medial cord; Lat. Dorsi, latissimus dorsi muscle. *****Abnormal insertion of brachialis.

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  Fig. 4 Formation, relation to CBM, and branching patterns of the MCN. (A) The MCN is the continuation of the LC of brachial plexus. It pierces CBM and descends between biceps and brachialis muscle. MCN gives branches to biceps (1), brachialis (2) and continues as LCF (3). The MN has three roots, two lateral (R1, R2) originating from its corresponding cord the LC and one MR originating from the MC of brachial plexus. Muscular branch to CBM originating from LC (arrow). (B) The MCN has two roots; one originating from the LC (R1) and the other (R2) originating from MC. MCN pierces CBM and gives branches to biceps (1), brachialis (2) and LCF (3). Also, the MN of this limb has one root only originating from the medial cord and crosses the BA deeply from lateral to medial aspect. (C) The MCN is normally formed as a continuation of the LC and descends superficial to CBM without piercing to pass between biceps and brachialis muscles. MCN gives branches to biceps (1), brachialis (2), and LCF (3). The MN is formed on the medial side of the BA from long LR of LC and MR from MC of brachial plexus. (D) The abnormal formation of MN from MC and union of the anterior division of both UT and MT that form the LC. MN gives branches to CBM (arrow), biceps (1), brachialis (2) and LCF (3). The MN crosses the BA superficially from lateral to medial side to pass deep to the abnormal insertion (****) of the brachialis muscle. MCN is absent. CBM, coracobrachialis muscle; MCN, musculocutaneous nerve; SH, short head of biceps; LH, long head of biceps; BA, brachial artery; AX, axillary artery; LC, lateral cord; MN, median nerve; MR, medial root; MC, medial cord; LCF, lateral cutaneous nerve of forearm; UN, ulnar nerve; LR, lateral root; MC, medial cord; UT, upper trunk; MT, middle trunk; Lat. Dorsi, latissimus dorsi muscle; MCF, medial cutaneous of forearm.

Reference

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