Abstract
Purpose
The current cadaveric case series evaluates the coracobrachialis muscle morphology, the related musculocutaneous nerve origin, course, and branching pattern, as well as associated adjacent neuromuscular variants.
Materials and methods
Twenty-seven (24 paired and 3 unpaired) cadaveric arms were dissected to identify the coracobrachialis possible variants with emphasis on the musculocutaneous nerve course and coexisted neural variants.
Results
Four morphological types of the coracobrachialis were identified: a two-headed muscle in 62.96% (17/27 arms), a three-headed in 22.2% (6/27), a one-headed in 11.1% (3/27), and a four-headed in 3.7% (1 arm). A coracobrachialis variant morphology was identified in 37.04% (10/27). A three-headed biceps brachii muscle coexisted in 23.53% (4/17). Two different courses of the musculocutaneous nerve were recorded: 1. a course between coracobrachialis superficial and deep heads (in cases of two or more heads) (100%, 24/24), and 2. a medial course in case of one-headed coracobrachialis (100%, 3/3). Three neural interconnections were found: 1. the lateral cord of the brachial plexus with the medial root of the median nerve in 18.52%, 2. the musculocutaneous with the median nerve in 7.41% and 3. the radial with the ulnar nerve in 3.71%. Duplication of the lateral root of the median nerve was identified in 11.1%.
Conclusions
The knowledge of the morphology of the muscles of the anterior arm compartment, especially the coracobrachialis variant morphology and the related musculocutaneous nerve variable course, is of paramount importance for surgeons. Careful dissection and knowledge of relatively common variants play a significant role in reducing iatrogenic injury.
Similar content being viewed by others
Avoid common mistakes on your manuscript.
Introduction
The coracobrachialis muscle (CB), located in the anterior arm compartment, originates from the apex of the coracoid process (CP) of the scapula and the proximal portion of the origin of the tendon of the biceps brachii muscle (BB). The CB is perforated and innervated by the musculocutaneous nerve (MCN) that originates from the lateral cord of the brachial plexus [29]. CB has many functions mainly at the shoulder joint. It both moves the humerus forward, causing the arm’s flexion, and toward the torso, causing the arm’s adduction. To a lesser degree, it turns the humerus inwards, causing internal rotation. In addition, CB adds to shoulder joint stabilization [2].
The CB superficial head(s) may originate from the CP, and/or the medial border of the tendon of the BB short head. CB deep head(s) may arise from the lateral border of the BB short head (CP base) [7]. Superficial and deep heads after a small course, fuse, and may entrap the MCN coursing between them. The MCN before piercing CB, may give proximal branches, the so-called accessory muscle’s innervation. In cases of an MCN non-piercing CB, it looks like the CB deep head is missing [7].
Several procedures include the muscles’ mobilization attached into the CP, mainly for treating the anterior shoulder instability [6, 12, 27], and the CB transfer in reconstructive surgery [13, 16]. The MCN may be injured also in the commonly used shoulder surgery anterior deltopectoral approach [19, 30]. Hence, MCN protection is of utmost importance in these operations [10, 18, 20], while CB typical and variant morphology, as well as associated neural variants may play a significant role.
The current cadaveric case series evaluates CB typical and variant morphology, and the related MCN origin, course, and branching pattern, as well as associated neuromuscular variants of the area. A comparison of the current findings with those of other studies in different populations is performed in an effort to identify differences.
Materials and methods
A dissection of 15 formalin-embalmed donated cadavers of Greek origin (12 bilateral and 3 unilateral) was performed in the axilla and arm to identify CB possible morphologic variants with emphasis on the MCN course and coexisted neuromuscular variants. Skin, subcutaneous fat and the upper limb superficial fascia were dissected, and the CB was exposed from its proximal to distal attachment, to be carefully examined for its morphological variants, variant innervation, and its relationship with the MCN. The cadavers were donated to the Anatomy Department of the Medical School of the National and Kapodistrian University of Athens and the Anatomy and Surgical Anatomy Department of the Medical School of the Aristotle University of Thessaloniki, through the “Anatomical Gift Program” after written informed consent.
Results
Four morphological types of the CB were summarized in Table 1 with their co-variants: 1. A two-headed CB (Figs. 1L, 2A, B) in 62.96% (i.e., CB typical morphology, identified in 17/27 arms, in 5 cadavers bilaterally and in 7 arms unilaterally), 2. A three-headed CB (Figs. 3B, 4R) in 22.2% (6/27, in 4 cases unilaterally and in 1 case bilaterally), 3. A one-headed CB (Fig. 3A) in 11.1% (3/27), and 4. A four-headed CB in 3.7% (1 case). A variant CB morphology was identified in 37.04% (10/27 cases). A unilateral three-headed BB coexisted in 23.53% (4/17 cases).
Musculocutaneous nerve (MCN) variant course
Two different courses of the MCN were recorded: 1. a course between CB superficial and deep heads (in cases of two or more CB heads) (100%, 24/24), and 2. a medial course in a one-headed CB (100%, 3/3).
Muscles’ fusion via bundles
Muscle and myofascial bundles were identified in 2 cases: 1. between CB and the triceps brachii medial head, and 2. between the BB short head and the upper arm fascia.
Neural interconnections
Three neural interconnections (ICs) were identified: 1. IC of the lateral cord of the brachial plexus with the medial root of the median nerve (MN) (Fig. 5) in 18.52% (5/27 cases, 3 unilaterally and 1 bilaterally), 2. IC of the MCN with the MN in 7.41% (2/27 cases), and 3. IC of the radial nerve with the ulnar nerve in 3.71% (1/27).
Other neural co-variants
A duplication of the MN lateral root was identified in 11.1% (3/27) (Fig. 6A, B).
Discussion
The current case series identified the typical and variant CB morphology, by recording four types: type 1-a CB with two heads (one superficial and deep head, typical morphology), type 2-a CB with three heads (two superficial and one deep head), type 3-a CB with a single head, and type 4-a CB with four heads (3 superficial and one deep). Symmetrical CBs (two or three heads) were identified in 50% (6/12) and asymmetrical CBs in 50% (6/12). In the current series, the most common type was the two-headed CB in 62.96%, similar to El-Naggar [7] who recorded in almost all cases a two-headed CB. El-Naggar [7] and El Naggar and Zahir [8] were the first to suggest the CB typical two-headed nature, contrariwise to Szewczyk et al. [26] who identified a CB single belly as the most common type (49.5%), a double-headed CB in 42.6% and a three-headed CB in 7.9%. A remarkable difference concerning the most identified morphological CB variant can be pointed out between the current study (the two-headed CB, the commonest variant) and that by Szewczyk et al. (the one-headed CB, the commonest variant) [26]. The differences were explained by the different examined populations, in Szewczyk et al. [26] study, the cadavers belonged to a Central European population, while the cadavers of the current study had a Greek origin. Different populations may be correlated with differences in the topographical anatomy, as it can be confirmed by the study by Ilayperuma et al. [14] who included cadavers of Sri Lankan individuals and by the study by Larrotta et al. [15], who comprised cadavers of Colombian origin. The current series recorded two cases of muscle and myofascial bundles joining: 1. CB and triceps brachii medial head, and 2. the BB short head and the upper arm fascia. These formations narrowed the available surface area and possibly entrapped the adjacent nerves (MCN, and MN) and the brachial artery. In addition, the current series highlights the coexistence of the CB variant morphology with the BB morphologic variants in 14.8% similar to other reports [5, 8, 21, 23]. Although, CB and BB variants seem to coexist, no systematic study until now has identified this association that possibly is explained by the muscles’ developmental background. During the 4th–7th week of the embryonic development, mesenchymal cells migrate from the dorsolateral part of somites to the limb bud to create muscles [3]. The anterior arm muscles (CB, BB, and brachialis muscle) probably derive from a single mass in the embryo of 11-mm long and they can be recognized in the embryo of 14–16 mm in length, while the formation of additional heads takes place at the separation stage in an embryonal length 11–19 mm [1]. The muscles’ formation is completed before nerves’ formation. Thus, “the nerve follows the muscle”, and as a result, a developmental problem in muscle differentiation leads to the atypical innervation [4, 15, 28]. After the formation of the muscles and nerves supplying the entire limb, a plexus begins to create in the form of loops between nerve fibers, and in this way the MCN originates from the MN [3], late during arm development. Disruption in the brachial plexus differentiation may lead to the MCN absence [4, 15, 28].
In the current case series, an MCN perforating CB was observed in 88.9%, while a medial course was identified in 11.1%. Guerri-Guttenberg and Ingolotti [11], recorded a MCN not piercing CB in 11.1%, closest to the current results, while in Remerand et al. [24] ultrasound study on 388 axillae, a MCN of medial course was identified in 22%. In addition, Ilayperuma et al. [14] identified the MCN perforating CB in 83.38% and a no perforation in 16.67%. The MCN perforation easily distinguished CB superficial from deep heads. However, the relationship between the MCN course and the number of CB heads has not been systematically studied. In the current series, in 100% of the double-headed (or more) CB, the MCN coursed between the heads and in 100% of one-headed CB, the MCN coursed medially to the muscle.
In the current case series, CB variants coexisted with neural aberrations in 59.26% (16/27 arms). The IC of the MCN with the MN was identified in 11.1% (3/27 cases) of the present series, while this variant has been reported in up to 43% of cases by Guerri-Guttenberg and Ingolotti [11]. Other neural co-variants, found in the current case series, were the IC between the lateral cord of the brachial plexus and the medial root of the MN (4/27; 14.8%) and the IC of the lateral cord of the brachial plexus with the UN (2/27; 7.4%). Those variations’ prevalence has not been studied in the literature. In addition, the lateral root duplication was observed in 11.1% (3/27, all cases unilaterally) by the current case series, while Pandey and Shukla [22] recorded a lower prevalence of 4.65% (8/172 cadavers). Some studies recorded the presence of accessory branches originating at a high-level position from the MCN main trunk [10, 17, 25]. Although this research point needs further investigation from larger clinical studies, surgeons need to know this correlation when operating, as the MCN lesions result in reduced elbow flexion strength and sensory impairment of the forearm radial aspect.
Variants of the anterior arm compartment, such as the CB accessory heads or the aberrant MCN course, have clinical complications due to high number of surgical procedures performed in the area. The Latarjet procedure is frequently used to treat recurrent anterior shoulder instability with more than 20% bone loss of the humeral glenoid. Latarjet, before using this technique, studied the MCN anatomy in relation to CP and CB [20] and identified the MCN proximal branches, before the CB piercing by the MCN, and identified the cause of neurovascular injury after the procedure. El-Naggar’s study [7], several years later, claimed the separate innervation for the two-headed CB (isolated for each head), so the presence of accessory branches is clinically important. Also, orthopedic surgeons should take into consideration the bilateral existence of CB variants in 50% in the current study. It is of note that during the deltopectoral approach which is utilized for most of the open shoulder surgery, the surgeon should not extend the approach medially of the CP. There are also other procedures involving the area of the CP, such as acromioclavicular dislocations and fixations of CP fracture. Careful surgical technique and dissection of the area are of utmost importance adequate surgical visualization and injury avoidance. Moreover, in complex cases, preoperative imaging may be considered to avoid iatrogenic intraoperative lesions [9].
The presence of variant muscles of the anterior arm compartment may cause neuropathy, such as MCN neuropathy due to muscular compression. The most common site of MCN entrapment is the CB and the presence of additional heads [21]. In 88.9% of arms in the current series, the MCN coursed through the muscle, so there was a great chance of compression leading to numbness or deficit on the glenohumeral joint [21]. Also in two cases, muscle and myofascial fibers were identified, passing anterior to the brachial artery and the MN, thus probably compressing on them, and resulting in symptoms.
Limitations
As the cadaveric case series were used for the students’ educational purposes, we could not perform a detailed morphological, as well as a morphometric study. Thus, we didn’t obtain data about the vascular variants of the area. The cadavers were initially dissected by students and further by the senior authors, that’s why this case series does not conclude the co-variants regarding the area vascularization, as well as data concerning the MCN proximal branching pattern before the entrance to CB. In addition, although the gender and side of investigation were known, their impact on muscle’s morphology was not recorded due to the low number of dissected parts. Finally, the subjects’ medical history was unknown.
Conclusions
Muscle variants of the anterior arm compartment are not rare, as well as coexisted muscles and neurovascular aberrations. A relation exists between the CB typical and variant morphology and the MCN course. The knowledge of accessory CB heads and/or muscle bundles joining the muscles of the anterior arm compartment is clinically important because they may compress on the MCN, and/or the MN during their course between them, leading mainly to numbness. Also, cases of atypical MCN course, medial to CB, may complicate imaging leading to false interpretation. Alterations of the brachial plexus arrangement and occurrence of ICs between the nerves, or branches duplication are variants, essential to know in order to avoid iatrogenic injury, intraoperatively.
Data availability
Data and material related to the report will be available with the corresponding author for further reference.
Abbreviations
- CB:
-
Coracobrachialis muscle
- BB:
-
Biceps brachii muscle
- CP:
-
Coracoid process
- MCN:
-
Musculocutaneous nerve
- MN:
-
Median nerve
- IC:
-
Interconnection
References
Bardeen C (1905) Studies of the development of the human skeleton. Am J Anat 4:265–302
Bassett RW, Browne AO, Morrey BF (1990) Glenohumeral muscle force and moment mechanics in a position of shoulder instability. J Biomech 23:405–415
Benes M, Kachlik D (2021) Atypical branching of the musculocutaneous and median nerves with associated unusual innervation of muscles in the anterior compartment of the arm: case report and plea for extension of the current classification system. Surg Radiol Anat 43:671–678
Bulla A, Farace F, Steinmann G, Montella A, Uzel AP (2012) About the absence of musculocutaneous nerve. Ital J Anat Embryol 117(2):29
Catli MM, Ozsoy U, Kaya Y, Hizay A, Yildirim FB, Sarikcioglu L (2012) Four-headed biceps brachii, three-headed coracobrachialis muscles associated with arterial and nervous anomalies in the upper limb. Anat Cell Biol 45(2):136–139
Clavert P, Lutz J-C, Wolfram-Gabel R, Kempf JF, Kahn JL (2009) Relationships of the musculocutaneous nerve and the coracobrachialis during coracoid abutment procedure (Latarjet procedure). Surg Radiol Anat 31(1):49–53
El-Naggar MM (2001) A study on the morphology of the coracobrachialis muscle and its relationship with the musculocutaneous nerve. Folia Morphol 60(3):217–224
El-Naggar MM, Zahir FI (2001) Two bellies of the coracobrachialis muscle associated with a third head of the biceps brachii muscle. Clin Anat 14(5):379–382
Filippou D, Piagkou M, Natsis K, Chytas D, Kostare G, Triantafyllou G, Kostares E, Koutserimpas C, Totlis T, Salmas M, Karampelias V, Tsakotos G (2023) A rare bilateral variant of the coracobrachialis muscle with supernumerary heads and coexisted variant branching patterns of the brachial plexus and the axillary artery. Surg Radiol Anat 45:277–282
Flatow EL, Bigliani LU, April EW (1989) An anatomic study of the musculocutaneous nerve and its relationship to the coracoid process. Clin Orthop Relat Res 244(244):166–171
Guerri-Guttenberg RA, Ingolotti M (2009) Classifying musculocutaneous nerve variations. Clin Anat 22(6):671–683
Hawi N, Reinhold A, Suero EM, Liodakis E, Przyklenk S, Brandes J et al (2016) The anatomic basis for the arthroscopic Latarjet procedure: a cadaveric study. Am J Sports Med 44(2):497–503
Hobar PC, Rohrich RJ, Mickel TJ (1990) The coracobrachialis muscle flap for coverage of exposed axillary vessels: a salvage procedure. Plast Reconstr Surg 85(5):801–804
Ilayperuma I, Nanayakkara BG, Hasan R, Uluwitiya SM, Palahepitiya KN (2016) Coracobrachialis muscle: morphology, morphometry and gender differences. Surg Radiol Anat 38(3):335–340
Larrotta DRB, Porras PLF, Acuña LEB, Ballesteros D, Forero P, Ballesteros L (2018) Anatomic variations in relation to the origin of the musculocutaneous nerve: absence and non-perforation of the coracobrachialis muscle. Anatomical study and clinical significance. Int J Morphol 36(2):425–429
Lo IKY, Burkhart SS, Parten PM (2004) Surgery about the coracoid: neurovascular structures at risk. Arthrosc - J Arthrosc Relat Surg 20(6):591–595
Macchi V, Tiengo C, Porzionato A, Parenti A, Stecco C, Bassetto F et al (2007) Musculocutaneous nerve: histotopographic study and clinical implications. Clin Anat 20(4):400–406
May VJ (1970) A modified Bristow operation for anterior recurrent dislocation of the shoulder. J Bone Jt Surg 52(5):1010–1016
McFarland EG, Caicedo JC, Guitterez MI, Sherbondy PS, Kim TK (2001) The anatomic relationship of the brachial plexus and axillary artery to the glenoid: implications for anterior shoulder surgery. Am J Sports Med 29(6):729–733
Neidhardt JP, Morin A, Autissier JM, Latarjet M (1968) Entrance of the musculo-cutaneous nerve into the coraco-branchial: surgical applications. Lyon Chir 64(2):268–276
Olewnik Ł, Paulsen F, Tubbs RS, Zielińska N, Szewczyk B, Karauda P, Polguj M (2020) Potential compression of the musculocutaneous, median, and ulnar nerves by a very rare variant of the coracobrachialis longus muscle. Folia Morphol 80(3):707–713
Pandey SK, Shukla VK (2007) Anatomical variations of the cords of brachial plexus and the median nerve. Clin Anat 20:150–156
Piagkou M, Totlis T, Anastasopoulos N, Lazaridis N, Natsis K (2019) An atypical biceps brachii and coracobrachialis muscles associated with multiple neurovascular aberrations: a case report with clinical significance. Folia Morphol 78(2):444–449
Remerand F, Laulan J, Couvret C, Palud M, Baud A, Velut S, Laffon M, Fusciardi J (2010) Is the musculocutaneous nerve really in the coracobrachialis muscle when performing an axillary block? An ultrasound study. Anesth Analg 110(6):1729–1734
Singh H, Yang JS, Wiley T, Judson C, Arciero RA, Mazzocca AD et al (2020) Relationship of the musculocutaneous nerve and its twigs to the coracoid process: an operative exposure. Orthop J Sport Med 8(10):232596712095441
Szewczyk B, Polguj M, Paulsen F, Podgórski M, Duparc F, Karauda P, Olewnik Ł (2021) A proposal for a new classification of coracobrachialis muscle morphology. Surg Radiol Anat 43(5):679–688
Tom JA, Cerynik DL, Lee CM, Lewullis GE, Kumar NS (2010) Anatomical considerations of subcoracoid neurovascular structures in anterior shoulder reconstruction. Clin Anat 23(7):815–820
Uzel AP, Bulla A, Steinmann G, LaurentJoye M, Caix P (2011) Absence du nerf musculocutané et distribution à partir du nerf médian: à propos de deux cas et revue de la littérature. Morphologie 95(311):146–150
Williams PL, Bannister LH, Berry MM, Collins P, Dyson M, Dussek JE (2000) Gray’s anatomy: the anatomical basis of medicine and surgery, 38th edn. Churchill Livingstone, New York
Zlotolow DA, Catalano LW, Barron OA, Glickel SZ (2006) Surgical exposures of the humerus. J Am Acad Orthop Surg 14(13):754–765
Acknowledgements
The authors would like to express their gratitude to body donors and their families for their highest contribution to the medical education and anatomy research.
Funding
Open access funding provided by HEAL-Link Greece. None.
Author information
Authors and Affiliations
Contributions
Conceptualization: MP, GTr; methodology: MP, GTs; data collection: VK, IP, AT; analysis and interpretation of results: MP, GTs; writing—original draft: MP, GTr; writing—review and editing: MP, GTs, DC, IP, CK; critical revision of the manuscript: CK, KN; approval of the final version of the manuscript: all authors.
Corresponding author
Ethics declarations
Conflict of interest
The authors declare that they have no conflict of interest.
Ethical approval
As this is a case series that is completely on cadavers, no ethical clearance was required as cadavers are used for teaching and research purpose.
Consent for publication
All the authors gave consent for the publication of the report.
Additional information
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Rights and permissions
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
About this article
Cite this article
Piagkou, M., Tsakotos, G., Triantafyllou, G. et al. Coracobrachialis muscle morphology and coexisted neural variants: a cadaveric case series. Surg Radiol Anat 45, 1117–1124 (2023). https://doi.org/10.1007/s00276-023-03207-7
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1007/s00276-023-03207-7