Sternalis Featuring Tendomuscular Fusion to the Contralateral Sternocleidomastoid: A Cadaveric Case Report

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Methods Routine cadaveric dissection in an undergraduate anatomy laboratory revealed two sternalis muscles parasternal to the sternal body. Subsequent fine prosection of the anterior thoracic wall and neck was carried out to uncover the soft tissue attachments of both sternalis muscles. Positional relationship to neighboring anterior thoracic and neck structures, and geometric dimensions of the muscle bellies and tendons, were documented. Results Both sternalis muscles were imbedded in the pectoral fascia, with their muscle fibers running obliquely to the midsternal line. The right sternalis muscle was notably larger in length, width, and thickness compared to the sternalis muscle on the left. The right sternalis muscle featured a distinct superior cord-like intermediate tendon that traversed superolateral and fused directly with the contralateral sternomastoid and sterno-occipital portions of the left sternocleidomastoid muscle. The superior tendon of the left sternalis muscle was aponeurotic and affixed to the cord-tendon of the right sternalis muscle. A distinct tendinous entheses for the sternal head for both sternocleidomastoid muscles were noted at the manubrial body. Conclusions This case report describes a unique bilateral sternalis muscle variant with conjoined sternocleidomastoid muscle integration. This anatomical description of a sternalis-sternocleidomastoid muscle morphology may supplement radiographic interpretations and support diagnostic accuracy. anatomy anterior thoracic wall cadaveric sternalis sternocleidomastoid variant Figures Figure 1 Figure 2 Introduction The sternalis muscle (sternalis m.), also referred to as muscularis sternalis and rectus sternalis, is an aberrant muscular structure of the anterior thoracic wall that is typically situated in the subcutaneous tissue superficial to the pectoralis major muscle [ 1 ]. Since its initial reporting by Barthelemy Cabrol in 1604, comprehensive anatomical and imaging studies have described an eclectic array in geometric dimensions, fiber directions, soft tissue adherence, and distinct points of attachment, as well as proposed embryological origin and function [ 2 ]. Classification of sternalis m. variant type is predicated upon its inherent topographic positional reference and attachment to the ribs, sternum, and costal margin, and whose form contains either a single head or is composed of multiple differentiated muscular subvolumes [ 1 ]. However, the morphologic presentations of the sternalis m. are vastly broad and its definition cannot be limited in scope. The importance for augmenting clinician awareness of other variant styles, particularly the muscles’ cervical anatomical relationships with the sternocleidomastoid (SCM) muscle, warrants further report. Cadaveric studies reporting sternalis-SCM muscle conjunction highlight variable presentations, including a unilaterally distinct muscular fasciculus running alongside the contralateral SCM muscle [ 3 ] and bilateral direct continuation with the SCM muscle [ 4 , 5 ]. The embryological origin for the gross myogenesis of the sternalis m. is still debated. Proposed theories suggest that the sternalis m. is either a true homological derivative of the SCM muscle or a consequence of incidental fiber misalignment due to deficiencies or rotations in pectoralis major muscle segment development [ 1 , 6 ]. Clinically, the anatomical relationship of the sternalis m. to thoracic and cervical structures are important as they may implicate possible differential diagnoses as an incidental abnormal mass or neoplasm if a clinician is not aware of its presence [ 7 – 9 ]. Further characterization of sternalis-SCM complex variations can provide clinicians with valuable information of these unusual and uncommon anatomical features, aiding in more precise diagnostic approach and treatment planning. In the current case report, we document and morphometrically describe a unique asymmetrical bilateral sternalis muscle pair exhibiting a direct tendomuscular fusion to the contralateral SCM muscle. Grounding of this sternalis m. variant against prior findings, developmental origin, and merit in visualization & surgical applications are discussed towards clinical and scholarly reference. Materials and Methods Cadaveric Materials & Description Dissection of an 88-year-old male formalin-fixed cadaver in an undergraduate anatomy teaching laboratory was carried out. The reported cause of death was high-grade urothelial carcinoma. Further demographic and medical information of the donor was not provided. Upon dissection of the anterior thoracic wall, two bilateral heteromorphic sternalis muscles were identified parasternal and superficial to the pectroalis major muscles. Further detailed sharp and blunt prosection was carried out to further distinguish the muscles’ attachments. All cadaveric material were obtained and ethically consented for permitted education and research purposes through the Willed Body Program in the Division of Anatomy at University of Toronto. The University of Toronto Research Ethics Board deemed that this clinical case report was exempt from ethics review. Cadaveric Dissection The skin and subcutaneous tissue were excised from the anterior thoracic wall to uncover the pectoral fascia investing the whole of both the right and left sternalis muscles, being found superficial to the pectoralis major muscles. The superior tendon of the right sternalis appeared to course superolateral towards the contralateral (left) SCM muscle. To allow for measurement of dimensions and documentation of the positional relationship of the sternalis muscles to adjacent sternocostal structures and SCM muscle heads, the skin; superficial cervical fascia; and platysma muscle were cut and reflected to uncover both SCM muscles. The entirety of both SCM muscles were further exposed to their superior and inferior points of attachment. Due to the nature of dissection for teaching, the superior sternocostal factions and anterior abdominal wall (rectus sheath and external oblique) were incised and dissected such that the tendinous points of attachment for the left sternalis, and inferior tendon of the right sternalis, were inadvertently lost. Data Collection Projection and directional paths of tendinous fibers were visually confirmed as they aligned with their points of attachment. Following the complete exposure of the sternalis muscles, muscle variant type was cross-referenced against two proposed classification systems [ 1 , 10 ]. Muscle orientation, morphology, and geometric measures were documented for both sternalis muscles. Fascicular arrangement and positional relationship to regional bony and muscular structures were also recorded. Dimensions (length, width, and thickness) for muscular and tendinous components were measured using a sliding digital caliper (resolution ± 0.02 mm; Kynup, Shenzhen, GD, China). The thickness of the muscle was averaged (mean ± standard deviation) from superior, intermediate, and inferior equidistant points from the centroid of the muscle belly. The angular deviation from the sagittal plane for both sternalis muscles was measured along the superior tendomuscular fibers, and referenced to the midsagittal line, using a digital angle finder (resolution ± 0.15°; Trend, Greensboro, NC, United States). Results Right sternalis muscle The orientation and positional relationship of the right sternalis m. to the sternum and pectoralis major muscles is shown in Fig. 1 a. The right sternalis m. was monocephalic, presenting a longitudinal pennation architecture and appeared as a well-developed flat muscle, situated obliquely (deviation = 26.8°) about ribs 1 to 5 and running perpendicular to the fascicles of the sternocostal head of the right pectoralis major muscle. The entirety of the right sternalis m. belly was embedded in the pectoral fascia and did not showcase any osteological attachments. Superiorly, the right sternalis tendon was organized and cord-like, coursing superolateral about the sternal angle and decussating with the distinct sternal origin of the right SCM muscle, anteriorly. The tendon further transversed the contralateral (left) clavicular notch of the manubrium and attached directly with the sterno-occipital portion of the left SCM muscle as an intermediate tendon in a fusiform style. The sternomastoid portion of the left SCM muscle affixed to the medial margin of the right sternalis cord-tendon (Fig. 1 b). A distinct tendinous enthesis for the sterno-occipital portion (sternal origin) of the left SCM muscle was located on the body of the manubrium inferior to the left sternoclavicular joint and medial to the first costochondral joint, separate from the superior sternalis tendon (Fig. 2 a). The superior sternalis tendon had a thickness of 0.4 cm at its broadest point and measured a length of 5.7 cm from the superior myotendinous intersection to the point of attachment to the left SCM muscle (Fig. 2 b). About the anterior thoracic wall, the sternalis m. belly flattened and broadened, such that its lateral border intersected the right midclavicular line. The inferior tendon was aponeurotic and attached to the anterior rectus sheath lateral to the xiphoid process and costal margin of the sixth rib. The length of the right sternalis m. belly, measured from the superior and inferior myotendinous junctions, was 13.5 cm, and 4.8 cm wide at its broadest point. The mean muscle thickness was 1.7 ± 0.5 mm and was thickest most superiorly (Fig. 2 b). Left sternalis muscle The orientation and positional relationship of the left sternalis m. to the sternum and left pectoralis major muscle is shown in Fig. 1 a. The left sternalis m. was monocephalic, presenting a longitudinal pennation architecture and appeared as a well-developed flat muscle situated obliquely (deviation = 23.7°) about ribs 2 to 4, running slightly perpendicular to the fascicles of the sternocostal head of the left pectoralis major muscle. The entirety of the left sternalis m. belly was imbedded in the pectoral fascia of the left pectoralis major muscle. The superior tendon appeared diffuse and aponeurotic, fanning and affixing into the superior cord-tendon of the right sternalis m. superiorly, inserting along its lateral border inferior to the left jugular notch of the manubrium. The inferior tendon was also aponeurotic and attached into the anterior rectus sheath left of the xiphoid process and subcostal angle. From the superior to inferior myotendinous junctions, the sternalis m. belly was 9.5 cm long and had a width of 1.9 cm at its broadest point. The mean muscle thickness was 0.9 ± 0.6 mm and was thickest most proximally (Fig. 2 b). Discussion The present case study has described a sternalis m. variant that featured a direct tendinous attachment to the contralateral sternomastoid and sterno-occipital portions of the SCM muscle without featuring an osseous attachment to the sternum. The continuity of the right sternalis with the SCM muscle on the left side did not exclude a sternal origin for the SCM; both sternal heads featured an anatomically consistent point of origin at the manubrial body. The superior aspect of the left sternalis m. showcased a broad aponeurotic tendon that converged into the right sternalis cord-tendon, forming a tendofibrous complex anterior to the manubrial body. To the authors’ knowledge, this is the first case report that describes a converged structural arrangement stemming from musculotendinous features of individual sternalis and sternal constituents of the SCM muscle. The left aponeurotic tendon appeared to take a similar superolateral trajectory towards the right SCM counterpart but terminated pre-emptively. This may suggest a hypoplastic disposition of an arrested organized tendinous development, as the left sternalis m. belly may have initially projected towards the right SCM, or as a true and delineated connection to the cord-tendon acting as a possible structural support for the right sternalis. Based on their positional and physical characteristics, the right and left sternalis muscles do not entirely fit within established contextual groupings due to their unique superior attachment patterns, although they do share certain aligning features. Crossed against the suggested schema by Snosek et al. [ 1 ], both sternalis muscle bellies fit a “simple” type I definition. Classification proposed by Jelev et al. [ 10 ] includes variants with sternalis-SCM conjunction, where both sternalis muscles of the present study fit the type II2 composition, however the left sternalis superior attachment did not precisely categorize. Sternalis is often situated in the subcutaneous plane of the pectoral fascia, with associated soft tissue and hypodermal adhesions [ 2 ]. The nature of this superficial anatomical positioning has suggested sternalis to be developmentally derived from a truncated, atrophic vestigial panniculus carnosus muscle, which is commonly found in vertebral organisms, including other higher order primates [ 1 ]. However, though this theory cannot be ruled out, it seems underwhelming to explain the organized, converged, and defined superior and inferior attachment for sternalis in the current report, as panniculus carnosus features morphologically diffuse and irregular muscular striations and is entirely positioned in the underlying subcutaneous layer. In contrast, our findings suggest sternalis may be derived from a dysplastic differentiation from adjacent neighboring musculature, which may include the pectoralis major and SCM muscles [ 6 ], as there was direct fusion to the sternal head of SCM on the left side with aligned fascicle orientation, with the sternalis m. fiber’s being well concatenated; organized; and delimiting clear medial and lateral borders. Further, since sternalis has shown to receive neural innervation from perforating pectoral, spinal accessory, and anterior intercostal nerves [ 3 , 5 ] the early differentiation of myotomes (skeletal muscle) and syndetomes (tendons), along with axonal guidance towards their innervating targets, may further support the idea of synergistic alignment during sternalis development from localized musculature [ 11 ]. In the current report, the right (SCM conjunction) and left (no SCM conjunction) sternalis muscles may have had initially similar conditions for origin (derived from either defective prepectoral or SCM masses), but diverged upon their tendinous insertions with oblique alignments stemming from varying mechanical strains [ 4 ], resulting in differing superior tendinous configurations. Though specific incidence for sternalis-SCM conjunction is unknown, it’s current and prior reporting reflects the need bring awareness to a potential clinical setting, which may pertain to imaging and surgical modalities. Imaging of sternalis produces clear differentiation of its contours, showcasing an isoechoic (ultrasound), isointense (magnetic resonance), or irregularly dense (mammogram) form with an organized superior-inferior (craniocaudal) fiber direction [ 8 ]. However, with the diversity of form and dimensions, sternalis may be interpreted as incidental, prompting consideration in differential medical diagnosis of idiopathic thoracic, supraclavicular, or anterior cervical masses, or misdiagnosis of root cause acute and chronic chest pain [ 12 , 13 ]. For instance, a clinical sonographic report has correlated the presence of sternalis, contributing to increased parasternal volume and swelling, to symptomatic pain upon palpation and respiration excursion [ 13 ]. The sparce report of sternalis being adopted in surgical settings makes it difficult to make inference on the efficacious use of a sternalis-SCM conjoined structure in reconstructive interventions, though may be a reasonable choice as a graft or pedicle flap as its presumed function appears insignificant. However, precise anatomical interpretations of radiographic images are required to validate the exact sternalis form, as various sternalis-SCM morphologies are possible [ 3 – 5 ], to facilitate appropriate interventional course. Conclusion The current anatomical case report describes a bilateral sternalis muscle that presented distinct superior attachments, including a distinct fusion to the contralateral sternal head of the SCM muscle. Clinical awareness of the broad array of sternalis-SCM complexes may serve to improve clinical diagnostic accuracy of irregularly presenting anterior thoracic or cervical masses. Further reporting on this aberrant sternalis-SCM configuration is necessary to determine the incidence rate and functional capacity. Statements and Declarations Acknowledgments The authors sincerely thank those who donated their bodies to science so that anatomical research could be performed. Results from such research can potentially increase the collective knowledge of medical practitioners that can then improve patient care. Therefore, these donors and their families deserve our highest gratitude. The authors would like to also thank Harun Bola for his technical support. Competing Interests The authors have no relevant financial or non-financial interests to disclose. Funding No funding was ascertained for this study. Author Contributions All authors contributed to the study conception and design. Material preparation, data collection and analysis, figure creation, and manuscript drafting were performed by Benjamin Kozlowski. Data analysis, manuscript drafting, and study supervision were performed by Kristina Lisk. All authors read and approved the final manuscript. References Snosek M, Tubbs RS, Loukas M (2014) Sternalis muscle, what every anatomist and clinician should know. Clinical Anatomy 27:866–884. https://doi.org/10.1002/ca.22361 Asghar A, Naaz S, Narayan RK, Patra A (2022) The prevalence and distribution of sternalis muscle: a meta-analysis of published literature of the last two hundred years. Anat Sci Int 97:110–123. https://doi.org/10.1007/s12565-021-00632-9 Wang X, Zou H, Chen J, et al (2022) An unusual sternalis with variation of the contralateral sternocleidomastoid muscle: a case report. Surg Radiol Anat 44:987–990. https://doi.org/10.1007/s00276-022-02980-1 Raikos A, Paraskevas GK, Tzika M, et al (2011) Sternalis muscle: an underestimated anterior chest wall anatomical variant. Journal of Cardiothoracic Surgery 6:73. https://doi.org/10.1186/1749-8090-6-73 Ramakrishnan S, Mahajan I, Nikitha T, et al (2023) An X across the Chest: A Rare Case of a Criss-crossed Sternalis Muscle. Acta Med Acad. https://doi.org/10.5644/ama2006-124.420 Huntington GS (1904) The Derivation and Significance of certain Supernumerary Muscles of the Pectoral Region. J Anat Physiol 39:1-54.27 Harish K, Gopinath KS (2003) Sternalis muscle: importance in surgery of the breast. Surg Radiol Anat 25:311–314. https://doi.org/10.1007/s00276-003-0119-9 Demirpolat G, Oktay A, Bilgen I, Isayev H (2010) Mammographic features of the sternalis muscle. Diagn Interv Radiol 16:276–278. https://doi.org/10.4261/1305-3825.DIR.2733-09.2 Young Lee B, Young Byun J, Hee Kim H, et al (2006) The sternalis muscles: incidence and imaging findings on MDCT. J Thorac Imaging 21:179–183. https://doi.org/10.1097/01.rti.0000208287.04490.db Jelev L, Georgiev G, Surchev L (2001) The sternalis muscle in the Bulgarian population: classification of sternales. J Anat 199:359–363. https://doi.org/10.1046/j.1469-7580.2001.19930359.x Deries M, Thorsteinsdóttir S (2016) Axial and limb muscle development: dialogue with the neighbourhood. Cell Mol Life Sci 73:4415–4431. https://doi.org/10.1007/s00018-016-2298-7 Georgiyeva K, Kumar H, Fernandez VE (2023) Sternalis Syndrome Misidentified by Multiple Specialties Responding to Botox Treatment: A Case Report and Literature Review. Cureus 15:e42236. https://doi.org/10.7759/cureus.42236 Gruber L, Martinoli C, Tagliafico AS, et al (2016) A Rare Case of a Symptomatic Sternalis Muscle: Ultrasonograpy And MRI Correlation. Ultrasound Int Open 2:E140–E141. https://doi.org/10.1055/s-0042-113607 Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 27 Sep, 2024 Read the published version in Surgical and Radiologic Anatomy → Version 1 posted Editorial decision: Revision requested 06 Sep, 2024 Reviews received at journal 03 Sep, 2024 Reviewers agreed at journal 14 Aug, 2024 Reviewers invited by journal 14 Aug, 2024 Editor assigned by journal 14 Aug, 2024 Submission checks completed at journal 14 Aug, 2024 First submitted to journal 10 Aug, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4892812","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":350545961,"identity":"e4090543-c7d0-46ce-b75d-5ced12bf66bd","order_by":0,"name":"Benjamin Kozlowski","email":"","orcid":"","institution":"University of Toronto","correspondingAuthor":false,"prefix":"","firstName":"Benjamin","middleName":"","lastName":"Kozlowski","suffix":""},{"id":350545962,"identity":"f15c9089-e83f-418a-88f4-75fb8302ea5b","order_by":1,"name":"Kristina Lisk","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABAElEQVRIie3Qv0vEMBTA8fcIXJdi1xcK138hXVxU/Fc8hLvVsYNIpBCXousdiv9DF+dKoFP+BJfjwLnF1cGXQTnkct7okO8UQj75BRCL/cMIUAMoHiWoUVeHks4T4YnjCV+3l3wvEAB4aw4g8tHWH8PVWwFC1OPq2d5kue03QwVFpneT/GhmqFPvpRZoZPtiSd7PF6pzUC4DR01TNPwWi57gmoly6TG9GrgI3Y5JPTA5Z1KP66ctUgRInqLmi9kZEy1bvUVUgMgGDTllL41/y7JfyFUzmZNzVLYBQi7h7/m0Zw/J3WZsrk+yLBU9VdVpMQ2Qnya/9/pjfSwWi8X29QXdKVnQtsSVvgAAAABJRU5ErkJggg==","orcid":"","institution":"University of Toronto","correspondingAuthor":true,"prefix":"","firstName":"Kristina","middleName":"","lastName":"Lisk","suffix":""}],"badges":[],"createdAt":"2024-08-10 18:26:42","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4892812/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4892812/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s00276-024-03485-9","type":"published","date":"2024-09-27T15:57:45+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":64217454,"identity":"a0af0b92-8d5d-4ae9-86a8-292e6d33d737","added_by":"auto","created_at":"2024-09-10 10:08:27","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":985422,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eAnterior view of the anterior thoracic wall, picture enhancing the bilateral sternalis muscles in relation to the midsternal line (white dashed line) (a), and color-labelled portions of the sternocleidomastoid muscle in relation to the clavicle and intermediate tendon of the right sternalis (b) \u003c/strong\u003e\u003cem\u003eAbbreviations\u003c/em\u003e: \u003cem\u003eLP, \u003c/em\u003elaryngeal prominence; \u003cem\u003ePG (cut)\u003c/em\u003e, parotid gland (cut); \u003cem\u003ePM\u003c/em\u003e, pectoralis major; \u003cem\u003eSCM, \u003c/em\u003esternocleidomastoid; \u003cem\u003eSM, \u003c/em\u003esternalis muscle;\u003cem\u003e SMG\u003c/em\u003e, submandibular gland\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-4892812/v1/dfb782510f19012b23b1af86.png"},{"id":64217455,"identity":"6770ce1f-78da-4fae-a192-9e9fca7736f4","added_by":"auto","created_at":"2024-09-10 10:08:27","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":758134,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eTendinous arrangements for the sternalis and sternocleidomastoid muscles in relation to the manubrium (a) and morphometric dimensions of the right \u0026amp; left sternalis muscles (b) \u003c/strong\u003eThe angle of deviation for both sternalis muscles was taken along the medial borders of the projected superior tendons, in relation to the midsternal line (white dashed line). The right sternalis superior tendon coursed supralateral, creating a tendinous chiasm with the right sternal origin of sternocleidomastoid at the body of the manubrium (black dashed oval). A segment of the left sterno-occipital portion (sternal origin) was attached left-lateral to the midsternal line and inferior to the left sternoclavicular joint. \u003cem\u003eAbbreviations: SCM\u003c/em\u003e, sternocleidomastoid; \u003cem\u003eSM, \u003c/em\u003esternalis muscle\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-4892812/v1/7a43618714ce092681509f21.png"},{"id":65628050,"identity":"096c14a7-da29-4cf2-b7cc-0f5a271964b0","added_by":"auto","created_at":"2024-09-30 16:17:20","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2559641,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4892812/v1/e4328a84-4f99-4eb7-ae5e-fe733bfca187.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Sternalis Featuring Tendomuscular Fusion to the Contralateral Sternocleidomastoid: A Cadaveric Case Report","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe sternalis muscle (sternalis m.), also referred to as muscularis sternalis and rectus sternalis, is an aberrant muscular structure of the anterior thoracic wall that is typically situated in the subcutaneous tissue superficial to the pectoralis major muscle [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Since its initial reporting by Barthelemy Cabrol in 1604, comprehensive anatomical and imaging studies have described an eclectic array in geometric dimensions, fiber directions, soft tissue adherence, and distinct points of attachment, as well as proposed embryological origin and function [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Classification of sternalis m. variant type is predicated upon its inherent topographic positional reference and attachment to the ribs, sternum, and costal margin, and whose form contains either a single head or is composed of multiple differentiated muscular subvolumes [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. However, the morphologic presentations of the sternalis m. are vastly broad and its definition cannot be limited in scope. The importance for augmenting clinician awareness of other variant styles, particularly the muscles\u0026rsquo; cervical anatomical relationships with the sternocleidomastoid (SCM) muscle, warrants further report.\u003c/p\u003e \u003cp\u003eCadaveric studies reporting sternalis-SCM muscle conjunction highlight variable presentations, including a unilaterally distinct muscular fasciculus running alongside the contralateral SCM muscle [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e] and bilateral direct continuation with the SCM muscle [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. The embryological origin for the gross myogenesis of the sternalis m. is still debated. Proposed theories suggest that the sternalis m. is either a true homological derivative of the SCM muscle or a consequence of incidental fiber misalignment due to deficiencies or rotations in pectoralis major muscle segment development [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Clinically, the anatomical relationship of the sternalis m. to thoracic and cervical structures are important as they may implicate possible differential diagnoses as an incidental abnormal mass or neoplasm if a clinician is not aware of its presence [\u003cspan additionalcitationids=\"CR8\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. Further characterization of sternalis-SCM complex variations can provide clinicians with valuable information of these unusual and uncommon anatomical features, aiding in more precise diagnostic approach and treatment planning. In the current case report, we document and morphometrically describe a unique asymmetrical bilateral sternalis muscle pair exhibiting a direct tendomuscular fusion to the contralateral SCM muscle. Grounding of this sternalis m. variant against prior findings, developmental origin, and merit in visualization \u0026amp; surgical applications are discussed towards clinical and scholarly reference.\u003c/p\u003e"},{"header":"Materials and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eCadaveric Materials \u0026amp; Description\u003c/h2\u003e \u003cp\u003eDissection of an 88-year-old male formalin-fixed cadaver in an undergraduate anatomy teaching laboratory was carried out. The reported cause of death was high-grade urothelial carcinoma. Further demographic and medical information of the donor was not provided. Upon dissection of the anterior thoracic wall, two bilateral heteromorphic sternalis muscles were identified parasternal and superficial to the pectroalis major muscles. Further detailed sharp and blunt prosection was carried out to further distinguish the muscles\u0026rsquo; attachments. All cadaveric material were obtained and ethically consented for permitted education and research purposes through the Willed Body Program in the Division of Anatomy at University of Toronto. The University of Toronto Research Ethics Board deemed that this clinical case report was exempt from ethics review.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003eCadaveric Dissection\u003c/h2\u003e \u003cp\u003eThe skin and subcutaneous tissue were excised from the anterior thoracic wall to uncover the pectoral fascia investing the whole of both the right and left sternalis muscles, being found superficial to the pectoralis major muscles. The superior tendon of the right sternalis appeared to course superolateral towards the contralateral (left) SCM muscle. To allow for measurement of dimensions and documentation of the positional relationship of the sternalis muscles to adjacent sternocostal structures and SCM muscle heads, the skin; superficial cervical fascia; and platysma muscle were cut and reflected to uncover both SCM muscles. The entirety of both SCM muscles were further exposed to their superior and inferior points of attachment. Due to the nature of dissection for teaching, the superior sternocostal factions and anterior abdominal wall (rectus sheath and external oblique) were incised and dissected such that the tendinous points of attachment for the left sternalis, and inferior tendon of the right sternalis, were inadvertently lost.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003eData Collection\u003c/h2\u003e \u003cp\u003eProjection and directional paths of tendinous fibers were visually confirmed as they aligned with their points of attachment. Following the complete exposure of the sternalis muscles, muscle variant type was cross-referenced against two proposed classification systems [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. Muscle orientation, morphology, and geometric measures were documented for both sternalis muscles. Fascicular arrangement and positional relationship to regional bony and muscular structures were also recorded. Dimensions (length, width, and thickness) for muscular and tendinous components were measured using a sliding digital caliper (resolution\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02 mm; Kynup, Shenzhen, GD, China). The thickness of the muscle was averaged (mean\u0026thinsp;\u0026plusmn;\u0026thinsp;standard deviation) from superior, intermediate, and inferior equidistant points from the centroid of the muscle belly. The angular deviation from the sagittal plane for both sternalis muscles was measured along the superior tendomuscular fibers, and referenced to the midsagittal line, using a digital angle finder (resolution\u0026thinsp;\u0026plusmn;\u0026thinsp;0.15\u0026deg;; Trend, Greensboro, NC, United States).\u003c/p\u003e \u003c/div\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003eRight sternalis muscle\u003c/h2\u003e \u003cp\u003eThe orientation and positional relationship of the right sternalis m. to the sternum and pectoralis major muscles is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea. The right sternalis m. was monocephalic, presenting a longitudinal pennation architecture and appeared as a well-developed flat muscle, situated obliquely (deviation\u0026thinsp;=\u0026thinsp;26.8\u0026deg;) about ribs 1 to 5 and running perpendicular to the fascicles of the sternocostal head of the right pectoralis major muscle. The entirety of the right sternalis m. belly was embedded in the pectoral fascia and did not showcase any osteological attachments. Superiorly, the right sternalis tendon was organized and cord-like, coursing superolateral about the sternal angle and decussating with the distinct sternal origin of the right SCM muscle, anteriorly. The tendon further transversed the contralateral (left) clavicular notch of the manubrium and attached directly with the sterno-occipital portion of the left SCM muscle as an intermediate tendon in a fusiform style. The sternomastoid portion of the left SCM muscle affixed to the medial margin of the right sternalis cord-tendon (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). A distinct tendinous enthesis for the sterno-occipital portion (sternal origin) of the left SCM muscle was located on the body of the manubrium inferior to the left sternoclavicular joint and medial to the first costochondral joint, separate from the superior sternalis tendon (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea). The superior sternalis tendon had a thickness of 0.4 cm at its broadest point and measured a length of 5.7 cm from the superior myotendinous intersection to the point of attachment to the left SCM muscle (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb). About the anterior thoracic wall, the sternalis m. belly flattened and broadened, such that its lateral border intersected the right midclavicular line. The inferior tendon was aponeurotic and attached to the anterior rectus sheath lateral to the xiphoid process and costal margin of the sixth rib. The length of the right sternalis m. belly, measured from the superior and inferior myotendinous junctions, was 13.5 cm, and 4.8 cm wide at its broadest point. The mean muscle thickness was 1.7\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5 mm and was thickest most superiorly (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eLeft sternalis muscle\u003c/h2\u003e \u003cp\u003eThe orientation and positional relationship of the left sternalis m. to the sternum and left pectoralis major muscle is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea. The left sternalis m. was monocephalic, presenting a longitudinal pennation architecture and appeared as a well-developed flat muscle situated obliquely (deviation\u0026thinsp;=\u0026thinsp;23.7\u0026deg;) about ribs 2 to 4, running slightly perpendicular to the fascicles of the sternocostal head of the left pectoralis major muscle. The entirety of the left sternalis m. belly was imbedded in the pectoral fascia of the left pectoralis major muscle. The superior tendon appeared diffuse and aponeurotic, fanning and affixing into the superior cord-tendon of the right sternalis m. superiorly, inserting along its lateral border inferior to the left jugular notch of the manubrium. The inferior tendon was also aponeurotic and attached into the anterior rectus sheath left of the xiphoid process and subcostal angle. From the superior to inferior myotendinous junctions, the sternalis m. belly was 9.5 cm long and had a width of 1.9 cm at its broadest point. The mean muscle thickness was 0.9\u0026thinsp;\u0026plusmn;\u0026thinsp;0.6 mm and was thickest most proximally (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb).\u003c/p\u003e \u003c/div\u003e"},{"header":"Discussion","content":"\u003cp\u003eThe present case study has described a sternalis m. variant that featured a direct tendinous attachment to the contralateral sternomastoid and sterno-occipital portions of the SCM muscle without featuring an osseous attachment to the sternum. The continuity of the right sternalis with the SCM muscle on the left side did not exclude a sternal origin for the SCM; both sternal heads featured an anatomically consistent point of origin at the manubrial body. The superior aspect of the left sternalis m. showcased a broad aponeurotic tendon that converged into the right sternalis cord-tendon, forming a tendofibrous complex anterior to the manubrial body. To the authors\u0026rsquo; knowledge, this is the first case report that describes a converged structural arrangement stemming from musculotendinous features of individual sternalis and sternal constituents of the SCM muscle. The left aponeurotic tendon appeared to take a similar superolateral trajectory towards the right SCM counterpart but terminated pre-emptively. This may suggest a hypoplastic disposition of an arrested organized tendinous development, as the left sternalis m. belly may have initially projected towards the right SCM, or as a true and delineated connection to the cord-tendon acting as a possible structural support for the right sternalis. Based on their positional and physical characteristics, the right and left sternalis muscles do not entirely fit within established contextual groupings due to their unique superior attachment patterns, although they do share certain aligning features. Crossed against the suggested schema by Snosek et al. [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e], both sternalis muscle bellies fit a \u0026ldquo;simple\u0026rdquo; type I definition. Classification proposed by Jelev et al. [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] includes variants with sternalis-SCM conjunction, where both sternalis muscles of the present study fit the type II2 composition, however the left sternalis superior attachment did not precisely categorize.\u003c/p\u003e \u003cp\u003eSternalis is often situated in the subcutaneous plane of the pectoral fascia, with associated soft tissue and hypodermal adhesions [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. The nature of this superficial anatomical positioning has suggested sternalis to be developmentally derived from a truncated, atrophic vestigial panniculus carnosus muscle, which is commonly found in vertebral organisms, including other higher order primates [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. However, though this theory cannot be ruled out, it seems underwhelming to explain the organized, converged, and defined superior and inferior attachment for sternalis in the current report, as panniculus carnosus features morphologically diffuse and irregular muscular striations and is entirely positioned in the underlying subcutaneous layer. In contrast, our findings suggest sternalis may be derived from a dysplastic differentiation from adjacent neighboring musculature, which may include the pectoralis major and SCM muscles [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e], as there was direct fusion to the sternal head of SCM on the left side with aligned fascicle orientation, with the sternalis m. fiber\u0026rsquo;s being well concatenated; organized; and delimiting clear medial and lateral borders. Further, since sternalis has shown to receive neural innervation from perforating pectoral, spinal accessory, and anterior intercostal nerves [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e] the early differentiation of myotomes (skeletal muscle) and syndetomes (tendons), along with axonal guidance towards their innervating targets, may further support the idea of synergistic alignment during sternalis development from localized musculature [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. In the current report, the right (SCM conjunction) and left (no SCM conjunction) sternalis muscles may have had initially similar conditions for origin (derived from either defective prepectoral or SCM masses), but diverged upon their tendinous insertions with oblique alignments stemming from varying mechanical strains [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e], resulting in differing superior tendinous configurations.\u003c/p\u003e \u003cp\u003eThough specific incidence for sternalis-SCM conjunction is unknown, it\u0026rsquo;s current and prior reporting reflects the need bring awareness to a potential clinical setting, which may pertain to imaging and surgical modalities. Imaging of sternalis produces clear differentiation of its contours, showcasing an isoechoic (ultrasound), isointense (magnetic resonance), or irregularly dense (mammogram) form with an organized superior-inferior (craniocaudal) fiber direction [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. However, with the diversity of form and dimensions, sternalis may be interpreted as incidental, prompting consideration in differential medical diagnosis of idiopathic thoracic, supraclavicular, or anterior cervical masses, or misdiagnosis of root cause acute and chronic chest pain [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. For instance, a clinical sonographic report has correlated the presence of sternalis, contributing to increased parasternal volume and swelling, to symptomatic pain upon palpation and respiration excursion [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. The sparce report of sternalis being adopted in surgical settings makes it difficult to make inference on the efficacious use of a sternalis-SCM conjoined structure in reconstructive interventions, though may be a reasonable choice as a graft or pedicle flap as its presumed function appears insignificant. However, precise anatomical interpretations of radiographic images are required to validate the exact sternalis form, as various sternalis-SCM morphologies are possible [\u003cspan additionalcitationids=\"CR4\" citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e], to facilitate appropriate interventional course.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThe current anatomical case report describes a bilateral sternalis muscle that presented distinct superior attachments, including a distinct fusion to the contralateral sternal head of the SCM muscle. Clinical awareness of the broad array of sternalis-SCM complexes may serve to improve clinical diagnostic accuracy of irregularly presenting anterior thoracic or cervical masses. Further reporting on this aberrant sternalis-SCM configuration is necessary to determine the incidence rate and functional capacity.\u003c/p\u003e"},{"header":"Statements and Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors sincerely thank those who donated their bodies to science so that anatomical research could be performed. Results from such research can potentially increase the collective knowledge of medical practitioners that can then improve patient care. Therefore, these donors and their families deserve our highest gratitude. The authors would like to also thank Harun Bola for his technical support.\u003c/p\u003e\n\u003cp\u003e\u003cem\u003e\u003cstrong\u003eCompeting Interests\u003c/strong\u003e\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eThe authors have no relevant financial or non-financial interests to disclose.\u003c/p\u003e\n\u003cp\u003e\u003cem\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eNo funding was ascertained for this study.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll authors contributed to the study conception and design. Material preparation, data collection and analysis, figure creation, and manuscript drafting were performed by Benjamin Kozlowski. Data analysis, manuscript drafting, and study supervision were performed by Kristina Lisk. All authors read and approved the final manuscript.\u003cbr\u003e\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eSnosek M, Tubbs RS, Loukas M (2014) Sternalis muscle, what every anatomist and clinician should know. Clinical Anatomy 27:866\u0026ndash;884. https://doi.org/10.1002/ca.22361\u003c/li\u003e\n \u003cli\u003eAsghar A, Naaz S, Narayan RK, Patra A (2022) The prevalence and distribution of sternalis muscle: a meta-analysis of published literature of the last two hundred years. Anat Sci Int 97:110\u0026ndash;123. https://doi.org/10.1007/s12565-021-00632-9\u003c/li\u003e\n \u003cli\u003eWang X, Zou H, Chen J, et al (2022) An unusual sternalis with variation of the contralateral sternocleidomastoid muscle: a case report. Surg Radiol Anat 44:987\u0026ndash;990. https://doi.org/10.1007/s00276-022-02980-1\u003c/li\u003e\n \u003cli\u003eRaikos A, Paraskevas GK, Tzika M, et al (2011) Sternalis muscle: an underestimated anterior chest wall anatomical variant. Journal of Cardiothoracic Surgery 6:73. https://doi.org/10.1186/1749-8090-6-73\u003c/li\u003e\n \u003cli\u003eRamakrishnan S, Mahajan I, Nikitha T, et al (2023) An X across the Chest: A Rare Case of a Criss-crossed Sternalis Muscle. Acta Med Acad. https://doi.org/10.5644/ama2006-124.420\u003c/li\u003e\n \u003cli\u003eHuntington GS (1904) The Derivation and Significance of certain Supernumerary Muscles of the Pectoral Region. J Anat Physiol 39:1-54.27\u003c/li\u003e\n \u003cli\u003eHarish K, Gopinath KS (2003) Sternalis muscle: importance in surgery of the breast. Surg Radiol Anat 25:311\u0026ndash;314. https://doi.org/10.1007/s00276-003-0119-9\u003c/li\u003e\n \u003cli\u003eDemirpolat G, Oktay A, Bilgen I, Isayev H (2010) Mammographic features of the sternalis muscle. Diagn Interv Radiol 16:276\u0026ndash;278. https://doi.org/10.4261/1305-3825.DIR.2733-09.2\u003c/li\u003e\n \u003cli\u003eYoung Lee B, Young Byun J, Hee Kim H, et al (2006) The sternalis muscles: incidence and imaging findings on MDCT. J Thorac Imaging 21:179\u0026ndash;183. https://doi.org/10.1097/01.rti.0000208287.04490.db\u003c/li\u003e\n \u003cli\u003eJelev L, Georgiev G, Surchev L (2001) The sternalis muscle in the Bulgarian population: classification of sternales. J Anat 199:359\u0026ndash;363. https://doi.org/10.1046/j.1469-7580.2001.19930359.x\u003c/li\u003e\n \u003cli\u003eDeries M, Thorsteinsd\u0026oacute;ttir S (2016) Axial and limb muscle development: dialogue with the neighbourhood. Cell Mol Life Sci 73:4415\u0026ndash;4431. https://doi.org/10.1007/s00018-016-2298-7\u003c/li\u003e\n \u003cli\u003eGeorgiyeva K, Kumar H, Fernandez VE (2023) Sternalis Syndrome Misidentified by Multiple Specialties Responding to Botox Treatment: A Case Report and Literature Review. Cureus 15:e42236. https://doi.org/10.7759/cureus.42236\u003c/li\u003e\n \u003cli\u003eGruber L, Martinoli C, Tagliafico AS, et al (2016) A Rare Case of a Symptomatic Sternalis Muscle: Ultrasonograpy And MRI Correlation. Ultrasound Int Open 2:E140\u0026ndash;E141. https://doi.org/10.1055/s-0042-113607\u003cstrong\u003e\u003cbr\u003e\u003c/strong\u003e\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"surgical-and-radiologic-anatomy","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"sara","sideBox":"Learn more about [Surgical and Radiologic Anatomy](http://link.springer.com/journal/276)","snPcode":"276","submissionUrl":"https://submission.nature.com/new-submission/276/3","title":"Surgical and Radiologic Anatomy","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"anatomy, anterior thoracic wall, cadaveric, sternalis, sternocleidomastoid, variant","lastPublishedDoi":"10.21203/rs.3.rs-4892812/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4892812/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003e\u003cb\u003ePurpose\u003c/b\u003e\u003c/p\u003e \u003cp\u003eTo report the morphologic and spatial relationships of a bilateral sternalis muscle variant.\u003c/p\u003e\u003cp\u003e\u003cb\u003eMethods\u003c/b\u003e\u003c/p\u003e \u003cp\u003eRoutine cadaveric dissection in an undergraduate anatomy laboratory revealed two sternalis muscles parasternal to the sternal body. Subsequent fine prosection of the anterior thoracic wall and neck was carried out to uncover the soft tissue attachments of both sternalis muscles. Positional relationship to neighboring anterior thoracic and neck structures, and geometric dimensions of the muscle bellies and tendons, were documented.\u003c/p\u003e\u003cp\u003e\u003cb\u003eResults\u003c/b\u003e\u003c/p\u003e \u003cp\u003eBoth sternalis muscles were imbedded in the pectoral fascia, with their muscle fibers running obliquely to the midsternal line. The right sternalis muscle was notably larger in length, width, and thickness compared to the sternalis muscle on the left. The right sternalis muscle featured a distinct superior cord-like intermediate tendon that traversed superolateral and fused directly with the contralateral sternomastoid and sterno-occipital portions of the left sternocleidomastoid muscle. The superior tendon of the left sternalis muscle was aponeurotic and affixed to the cord-tendon of the right sternalis muscle. A distinct tendinous entheses for the sternal head for both sternocleidomastoid muscles were noted at the manubrial body.\u003c/p\u003e\u003cp\u003e\u003cb\u003eConclusions\u003c/b\u003e\u003c/p\u003e \u003cp\u003eThis case report describes a unique bilateral sternalis muscle variant with conjoined sternocleidomastoid muscle integration. This anatomical description of a sternalis-sternocleidomastoid muscle morphology may supplement radiographic interpretations and support diagnostic accuracy.\u003c/p\u003e","manuscriptTitle":"Sternalis Featuring Tendomuscular Fusion to the Contralateral Sternocleidomastoid: A Cadaveric Case Report","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-09-10 10:08:23","doi":"10.21203/rs.3.rs-4892812/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-09-06T13:27:49+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-09-03T07:12:39+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"204155839343308531887077496047112531848","date":"2024-08-14T23:59:56+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-08-14T11:09:54+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-08-14T10:56:21+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-08-14T05:33:36+00:00","index":"","fulltext":""},{"type":"submitted","content":"Surgical and Radiologic Anatomy","date":"2024-08-10T18:25:16+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"surgical-and-radiologic-anatomy","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"sara","sideBox":"Learn more about [Surgical and Radiologic Anatomy](http://link.springer.com/journal/276)","snPcode":"276","submissionUrl":"https://submission.nature.com/new-submission/276/3","title":"Surgical and Radiologic Anatomy","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"d0457b17-1cf4-43a3-9b31-7dfb213fd02e","owner":[],"postedDate":"September 10th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2024-09-30T16:10:39+00:00","versionOfRecord":{"articleIdentity":"rs-4892812","link":"https://doi.org/10.1007/s00276-024-03485-9","journal":{"identity":"surgical-and-radiologic-anatomy","isVorOnly":false,"title":"Surgical and Radiologic Anatomy"},"publishedOn":"2024-09-27 15:57:45","publishedOnDateReadable":"September 27th, 2024"},"versionCreatedAt":"2024-09-10 10:08:23","video":"","vorDoi":"10.1007/s00276-024-03485-9","vorDoiUrl":"https://doi.org/10.1007/s00276-024-03485-9","workflowStages":[]},"version":"v1","identity":"rs-4892812","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4892812","identity":"rs-4892812","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}