Abstract
Knowledge about variations in mylohyoid muscle and submental artery is essential for maxillofacial surgeons, as these structures are commonly encountered in maxillofacial ablative and reconstructive surgery. While cadaveric and radiologic studies on mylohyoid variations have been documented in the literature, we report an intraoperative variation observed in relation to mylohyoid muscle and submental artery.
Keywords: Anatomy, Anatomic variation, Mylohyoid muscle, Submental artery, Submental flap, Submental island flap, Local flap, Oral reconstruction, Reconstructive surgery
Introduction
The submandibular triangle is one of the most important anatomic regions to any maxillofacial surgeon. With the popularity of submental artery flaps for oral reconstruction, the anatomic variations encountered in this region are of utmost importance. The mylohyoid muscle, one of the supra-hyoid muscles of the head and neck region, is a flat and triangular muscle. Arising from the mylohyoid line on the mandible bilaterally, it extends from the symphysis backward till anterior to the last molar, existing as a paired muscle belly [1]. The anterior and middle fibers of the mylohyoid muscle insert into the median raphe, extending from the symphysis downward to the hyoid bone and joining the muscle fibers of the opposite side, while the posterior fibers pass inferio-medially to insert into the anterior surface of the body of hyoid [1]. In cases in which the raphe is absent, the muscle presents as a continuous diaphragm [2]. While the occurrence of mylohyoid deficiencies, also known as boutonnière, is a commonly documented finding, separate bellies of the mylohyoid muscle are considered rare [3]. The available literature provides evidence of mylohyoid variations in cadaveric and radiologic findings with no clinical documentation of the same. The dominant blood supply to the supra-hyoid muscles and floor of the mouth is known to be through the branches of the external carotid artery as well as perforating and terminal branches of the submental and sublingual artery along with their anastomosis. We report a unique case of distinct mylohyoid bellies with a variation in the submental artery course encountered during neck dissection.
Case Report
A 65-year-old female, biopsy-proven carcinoma left maxillary alveolus, underwent left hemi-maxillectomy with unilateral supra-omohyoid neck dissection. Intraoperative on table two distinct mylohyoid bellies were observed with herniated fat and submental artery traversing through the cleft (Fig. 1). While the anterior belly of the mylohyoid had its insertion into the fibers of the contralateral mylohyoid, the posterior belly insertion was into the intermediate tendon of the digastric muscle (Fig. 2). The hypoglossal nerve, lingual nerve, and submandibular duct were visualized in their normal anatomic position deep to the posterior belly of the mylohyoid muscle. Coronal cuts of the contrast-enhanced CT revealed a unilateral mylohyoid defect with adipose tissue and submental artery within it (Fig. 3).
Fig. 1.
Submental artery pedicle traversing between the anterior and posterior bellies of the mylohyoid
Fig. 2.
Anterior and posterior bellies of the mylohyoid post-ligation of the submental artery. aDG anterior belly of digastric muscle, DT digastric tendon, MB mandibular body, MD mylohyoid muscle defect, MH mylohyoid muscle, P platysma, SA submental artery pedicle, SMG submandibular gland
Fig. 3.
Contrast-enhanced coronal CT representation of dehiscence of left mylohyoid muscle with submental artery located within it. White arrow—gap in the mylohyoid muscle. Black arrow—submental artery. White star—submandibular gland. Asterisk—mylohyoid muscle. Inset image—axial scan with black arrows depicting the two separate bellies of the mylohyoid muscle on the left side
Discussion
Embryologically, the mylohyoid muscle in primates is derived from cranial somitomere 4 and develops as anterior and posterior halves. The anterior half exists superficial to the posterior half with some degree of overlap [4]. A mylohyoid defect or boutonnière occurs when these two parts fail to overlap or when the underlying developing sublingual gland interferes with the differentiation of the adjacent mylohyoid muscle [5]. The above-reported case may have been the result of the perpetuation of the gap between the anterior and posterior parts of the muscle or interference from the underlying sublingual salivary gland during development; however, the muscle bellies were found to have separate insertions. The findings of our case were similar to a cadaveric study which reported of a distinct anterior belly, the mylo-geniohyoid, and posterior belly, the mylo-digastric, of the mylohyoid muscle [2]. While reported cases of mylohyoid variations in the past have been documented based on cadaveric findings alone, the clinical finding of such a defect in conjunction with a varied course of the main submental artery branch is rare. Mylohyoid deficiencies were referred to as boutonnières, and the salivary tissue projection through these defects as sublingual boutons, with the majority of the defects being less than 5 mm [6]. Mylohyoid boutonnière represents muscle defects that may contain submandibular and sublingual salivary tissue, fat, blood vessels, or combinations of all three of these components with fat being the most common tissue [2]. These mylohyoid defects were reported to occur in 77% of individuals, found to be bilateral in 67% and unilateral in 33% of cases [2].
Other mylohyoid muscle variations documented in the literature include:
Subtotal absence of the mylohyoid [7] with an exchange of fibers with the stylohyoid, sternohyoid, omohyoid, and geniohyoid muscles which may proceed to complete fusion [8].
The presence of an accessory mylohyoid [9].
An abbreviated mylohyoid with insertion at both the anterior belly and intermediate tendon of the digastric muscle [10].
An abbreviated mylohyoid with insertion via a fibrous pseudohyoid, into the geniohyoid muscle midway between the chin and hyoid [11].
The mylohyoid muscle is innervated by the mylohyoid branch of the inferior alveolar nerve and receives its arterial supply from the sublingual branch of the lingual artery, the maxillary artery, the mylohyoid branch of the inferior alveolar artery, and the submental branch of the facial artery [1]. The submental artery, which emerges from the superior edge of the submandibular gland, runs superficial to the mylohyoid muscle, giving off several perforating branches, and ends terminally either superficial, deep or through the anterior belly of digastric muscle to end in the subcutaneous plane in the midline. It was noted that 60% had a large perforating branch through the mylohyoid with 93.2% having a larger diameter than the submental artery [12]. Variations were reported in the course of the submental artery in the blood supply to the floor of the mouth wherein it perforates the mylohyoid muscle or takes a roundabout route above the mylohyoid muscle to travel near the surface of the mandible [13]. In the case discussed, the main branch of the submental artery was found to be passing through the space between the two muscle bellies, surrounded by fatty tissue. Ligation of this branch would have proven to be detrimental to the survival of the submental island flap, if planned, in an alternative case scenario. White et al. [2] described the importance of proper imaging in the form of contrast-enhanced CT or MRI scans to rule out clinical or radiologic misdiagnosis of accessory salivary tissue within these defects as pathologic abnormality. The anterior mylohyoid defects were found to be more difficult to detect, due to overlapping by the anterior belly of the digastric muscle and CT beam hardening through the mandibular symphysis [2]. While fat-containing defects appear on CT scans as areas of discontinuity in the mylohyoid muscle, defects containing accessory salivary tissue have a similar radiologic appearance to orthotopic salivary tissue, with branches of the submental artery and vein traversing through these mylohyoid defects in 42 percent of the cases [2].
Conclusion
Studies in the past have highlighted mylohyoid muscle variations with an increased predilection to plunging ranula, the spread of sublingual space infections, and iatrogenic-related risk of injury to the hypoglossal nerve, submandibular duct, and lingual nerve [3]. This case expresses how a detailed preoperative radiologic evaluation is important particularly where it may impact local flap reconstruction (submental or mylohyoid muscle flap) with the requirement of cautious tracing of the submental artery and guarded submandibular triangle dissection. This will help surgeons, enabling a clearer understanding of variations to the mylohyoid anatomy and associated submental artery course.
Compliance with Ethical Standards
Conflict of interest
The authors declare that they have no conflict of interest.
Ethical Approval
All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 declaration of Helsinki and its later amendments or comparable ethical standards.
Informed Consent
Informed consent was obtained from the participant included in the study.
Footnotes
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
References
- 1.Standring S (ed) (2015) Gray’s anatomy e-book: the anatomical basis of clinical practice. Elsevier Health Sciences
- 2.White DK, Davidson HC, Harnsberger HR, Haller J, Kamya A. Accessory salivary tissue in the mylohyoid boutonniere: a clinical and radiologic pseudolesion of the oral cavity. Am J Neuroradiol. 2001;22(2):406–412. [PMC free article] [PubMed] [Google Scholar]
- 3.Bender-Heine A, Zdilla MJ. Variation of the mylohyoid: implications for Ludwig angina, cervical contouring, and digastric muscle transfer. Surg Radiol Anat. 2018;40(2):221–225. doi: 10.1007/s00276-017-1868-1. [DOI] [PubMed] [Google Scholar]
- 4.Moss-Salentijn L, Hendricks-Klyvert M. A bilateral, superficial location of human sublingual glands: report of a case. J Oral Maxillofac Surg. 1987;45(11):983–986. doi: 10.1016/0278-2391(87)90456-3. [DOI] [PubMed] [Google Scholar]
- 5.Schneider W, Fulgenzi A, Lupovitch A. Submental herniation of sublingual gland: report of case. J Oral Surg. 1972;30(11):824–826. [PubMed] [Google Scholar]
- 6.Gaughran GRL. Mylohyoid boutonniere and sublingual bouton. J Anat. 1963;97(Pt 4):565. [PMC free article] [PubMed] [Google Scholar]
- 7.Bergman RA, Thompson SA, Afifi AK, Saadeh FA (1988) Compendium of human anatomic variation: text, atlas, and world literature. Urban & Schwarzenberg, p 29
- 8.Von Lanz T, Wachsmuth W (2013) Praktische Anatomie: ein Lehr-und Hilfsbuch der anatomischen Grundlagen ärztlichen Handelns. Springer, Berlin, 64
- 9.Sehirli Ü, Cavdar S. An accessory mylohyoid muscle. Surg Radiol Anat. 1996;18(1):57–59. doi: 10.1007/BF03207764. [DOI] [PubMed] [Google Scholar]
- 10.Saadeh FA, El-Sabban M, Hawi JS. Rare variations of the mylohyoid muscle: case study. Clin Anat: Off J Am Assoc Clin Anat Br Assoc Clin Anat. 2001;14(4):285–287. doi: 10.1002/ca.1049. [DOI] [PubMed] [Google Scholar]
- 11.Zdilla MJ, Lambert HW. Mylohyoid insertion at the mid-belly of a combined geniohyoid via a fibrous pseudo-hyoid: an observation with implications for submental flap surgery. JPRAS Open. 2015;5:1–3. doi: 10.1016/j.jpra.2015.03.007. [DOI] [Google Scholar]
- 12.Bavitz JB, Harn SD, Homze EJ. Arterial supply to the floor of the mouth and lingual gingiva. Oral Surg Oral Med Oral Pathol. 1994;77(3):232–235. doi: 10.1016/0030-4220(94)90290-9. [DOI] [PubMed] [Google Scholar]
- 13.Katsumi Y, Tanaka R, Hayashi T, Koga T, Takagi R, Ohshima H. Variation in arterial supply to the floor of the mouth and assessment of relative hemorrhage risk in implant surgery. Clin Oral Implant Res. 2013;24(4):434–440. doi: 10.1111/j.1600-0501.2011.02348.x. [DOI] [PubMed] [Google Scholar]