The Variational Types of Anterior Belly of the Digastric Muscle and Clinical Importance: A Case Report

Abstract

In this study, we examined anatomical variations in the anterior belly of the digastric muscle (ABDM) in two cadavers: a 68-year-old male and a 54-year-old female. Variations in the length, diameter, and origin-insertion relationships were measured and categorized. The results showed unilateral insertion and origin types, emphasizing their clinical relevance in head and neck surgery and radiological assessments.

Introduction

The digastric muscle is situated bilaterally in the suprahyoid region and characteristically exhibits two muscular bellies (anterior and posterior) linked by an intermediate tendon [1, 2]. The paired digastric muscles depress the mandible or elevate the hyoid bone, functioning as single muscles with significant physiological roles [1]. ABDM attaches medially to the digastric fossa of the mandible and descends inferiorly towards the intermediate tendon. Variations in ABDM within the suprahyoid region may occur because the two bellies differ in their embryological origins and neural development [3]. During the fourth week of gestation, ABDM originates from the first pharyngeal arch, whereas posterior belly of the digastric muscle (PBDM) develops from the second pharyngeal arch. ABDM arises from the base of the mandible and is innervated by the trigeminal nerve, whereas PBDM originates from the mastoid notch and receives innervation from the facial nerve. The mylohyoid and ABDM share a common embryological origin, innervation (mylohyoid nerve), and vascular supply (submental artery) [4]. Anatomical variations in digastric muscles, such as differences in origin, insertion sites, morphology, location, number, and connections, impact diagnostic and therapeutic procedures, particularly in the submental triangle. Understanding these variations is essential for precise imaging of soft tissue masses and planning surgical interventions in the head and neck. This study explores the embryological origins and clinical implications of ABDM variations through analysis of two cases.

Case Report

In the present study, anatomical variations in the digastric muscle were observed in two cadavers: a 68-year-old male and a 54-year-old female. To visualize these variations, dissection of the skin and the superficial cervical fascia was performed in the submandibular and suprahyoid regions. The dissection procedure followed a methodology previously described in the literature [5–8]. The lengths and diameters were measured using an electronic digital caliper (Mitutoyo, Japan), with each measurement taken twice to ensure an accuracy within 0.1 mm. Their types and locations were also recorded. This study was conducted in the Cadaver Unit of the Bahcesehir University Faculty of Medicine, Department of Anatomy. We categorized the variations according to the suggestions provided by Yamada (1935) [9] and Zlabek (1933) [3]. The study identified two types of ABDM variations, unilateral insertion (left) and unilateral origin (right), in a female cadaver. The first accessory ABDM measured 5.6 cm, while the second measured 3.1 cm. Additionally, the extra ABDM measured 4.3 cm in length. For muscle dimensions, the left ABDM measured 6.9 cm, left PBDM 4.9 cm, right ABDM 6.1 cm, and right PBDM 5.4 cm. In a male cadaver, measurements were as follows: left ABDM 4.7 cm, left PBDM 3.8 cm, right ABDM 5.2 cm, and right PBDM 4.4 cm. Intermediate tendons measured 3.2 cm (left) and 3.6 cm (right) in length, with widths of 1.1 cm and 1 cm, respectively (Fig. 1).

Fig. 1.

The figure shows the anterior belly of the digastric muscle (ABDM) and accessory muscle variants (inferior view of the fixed cadaveric heads). The unilateral (left) insertion type and unilateral (right) origin type ABDM variations were found in 2 cadavers. (A) The length of the first accessory ABDM (right to left) was 5.6 cm, and the length of the second accessory muscle (left to right) was 3.1 cm in the female cadaver. (B) Length of the second accessory muscle (left) was 4.3 cm, and length of the second accessory muscle (left to right) was 3.1 cm in the male cadaver. A: Anterior, AB: Anterior Belly, ABDM: Anterior belly of the digastric muscle, HB: Hyoid bone, IT: Intermediate Tendon, L: Left, LBM: Lower border of the mandible, MHM: Mylohyoid Muscle, P: Posterior, PBDM: Posterior belly of the digastric muscle, R: Right, SHM: Suprahyoid Muscle, SMG: Submandibular Gland, THM: Thyrohyoid Muscle

Discussion

During the early stages of muscle development, the orientation of muscle fibers suggests the possibility of various types of digastric muscle variations. This study aimed to contribute to the understanding of variations in the origin and insertion sites of ABDM through anatomical dissection and analysis of their relationships. The results suggest that the origin type of the ABDM may have originated from the primordium during early developmental stages, providing insights into the evolutionary trajectory of this muscle type [2]. Various ABDM types may influence mandibular mobility and cause radiological changes due to asymmetrical temporomandibular joint (TMJ) function. These variations can affect TMJ movements, tongue elevation during swallowing, and head flexion. Understanding their origin and insertion dynamics is critical for assessing their musculoskeletal implications [4]. Fujimura et al. classified ABDM variations by their attachment sites. Studies have shown that the left ABDM usually inserts into the left digastric fossa, while an abnormal bundle inserts on the right side; the right lateral bundle may not enter the fossa. Besides the nerve supply, the insertion site plays a crucial role in muscle categorization [10]. Surgeons should be aware that variations in size and position could impact the submandibular gland, floor of the mouth, or other nearby structures such as the larynx and the TMJ. For example, the left-sided ABDM variation (6.9 cm) in the female cadaver could cause asymmetry or affect surrounding tissues more significantly than the right-sided counterpart. These asymmetries might mimic lymph nodes or benign masses (e.g., thyroglossal cysts), leading to diagnostic challenges in clinical and imaging assessments [11]. Such variations should be carefully considered during surgical procedures in the neck region, particularly during submandibulectomy, as the digastric muscle and its tendon serve as crucial anatomical landmarks [12]. Mori’s classification categorizes the seven ABDM variations. The ape variant originates from the intermediate tendon of the digastric muscle and intertendinous aponeurosis. In the anterior type, ABDM arises from both the intertendinous aponeurosis and the ABDM itself, either fused or non-fused. The posterior type features muscle fibers originating from the intertendinous aponeurosis, forming a thin muscle plate extending to the inner surface of the mandible or the mylohyoid muscle. The biceps variant bifurcates ABDM into two heads, with the medial head crossing to the contralateral side and insertion into the digastric fossa. The accessory type involves an abnormal fasciculus arising from the lateral margin of the ABDM that is inserted into the mandible. The combined form results from the coexistence of anterior and posterior types, or a complex form [13]. De-ary-pires et al. categorized ABDM variations into five types: absence (lack of ABDM), type 1 (inferior mandibular margin attachment), type 2 (two bellies attached to the mandible or mylohyoid muscle on the same or opposite sides), type 3 (three bellies with additional slips to the mandible or mylohyoid), type 4 (four bellies to the mandible or mylohyoid), and type 5 (involving the mentohyoid muscle and Macalister’s muscle) [1]. ABDM exhibits significant morphological variability, with six variations observed among 15 cadavers (40%) in the submental region, according to Mangalagiri [14]. In a Northern California study, ABDM variants were found in 31.2% of 48 cadavers, with three novel variants identified: the union of bilateral ABDMs and an ectopic tubercle under the mandible with two accessory muscles. ABDM’s wide attachment of ABDM at both the origin and insertion sites typifies its atavistic nature, with symmetric midline convergence of the right and left medial borders [15]. ABDM can be categorized into four types based on variations in origin and insertion. The origin type involves an accessory muscle originating from the primary muscle and insertion into the fibrous tendinous arch related to the hyoid bone. The insertion type features an accessory muscle arising from the intermediate tendon of the digastric muscle, medial to the ABDM. The mixed type includes an accessory muscle crossing the midline in the submental region, while the complex type combines features from the origin, insertion, and mixed types [3, 9]. Other variations in ABDM, including arrowhead, double-headed, and asymmetrical fan-shaped varieties, have been reported in recent studies [16] (Table 1). In the current study, the presence of accessory ABDM muscles, with lengths ranging from 3.1 cm to 5.6 cm, could complicate surgical approaches, especially when these variations are unilateral. These accessory muscles could be mistaken for pathological growths, such as benign cysts or lymph nodes, if not identified properly during clinical imaging or physical examination. In surgeries, particularly submandibulectomy, the left-sided ABDM in females (up to 6.9 cm) may require more careful dissection to avoid damage to surrounding structures. The intermediate tendon size remains consistent across genders and is an important landmark during neck surgeries. Understanding its length (3.4 cm) is crucial for safely navigating the area to avoid nerve or vascular damage. Given the observed variations in both the muscle and tendon dimensions, especially the unilateral insertion and origin types of the ABDM, careful attention should be paid during surgeries that may involve manipulation or resection of these muscles.

Table 1.

Previous studies on variations of ABDM

Studies	Study Design	Type	Significance
Kim et al. (2019,Grenada)	Cadaver, MRI, CT	Unilateral right: 20.6% Unilateral left: 20.2% Bilateral: 53.7% Multiple: 326 (%100) Acessory ABDM	The submental flap proved beneficial for facial reconstruction surgery, and inclusion of the ABDM was imperative in the flap design, as the submental vessels traverse deep to it
Liquidato et al. (2007, Brazil)	Cadaver Study (10, male, adult)	Unilateral right/bilateral	Consider variations in the ABDM when distinguishing cervical masses and performing surgeries in the anterior neck, particularly in the submental and submandibular triangles
Arayapisit et al. (2022, Thailand)	Cadaver Study (91, 10male,6 female had ABDM variations)	The arrowhead 6/91 (6.6) The double-headed 9/91 (9.9) The asymmetrical fanshaped 1/91 (1.1)	It facilitates surgical planning to minimize iatrogenic injuries
Mori (1964, Japan)	Cadaver Study (262)	The normal (46.1%) The ape form (1.9%,5/262) The anterior type (10.3% bilateral; 6.8% unilateral): dis/continuous forms The posterior type (0.7%): continuous/myloid forms The biceps form (medial and lateral heads) Accessory muscle fascicles The combination form	More than half of the Japanese population exhibited an abnormal ABDM (54.1%)
Mangalagiri (2009, India)	Cadaver Study (15,40%)	Unilateral left (n = 3)/bilateral (n = 3)/ Accessory slips formed a Y shaped configuration/ accessory anterior bellies arising from IT	Understanding such variations was significantly important when planning surgeries in the submental region and staging tumors
Sevinc et al. (2008, Türkiye)	Cadaver (Case, Male,55y.o)	Bilateral/The accessory bundle originated from the anterior part of the IT, subsequently bifurcated into two parts, and fused with the mylohyoid muscles	An accessory ABDM, traversing the muscles of the submental triangle, could be mistaken for a pathological lesions, such as a tumor or enlarged lymph nodes on CT/MRI
De-ary-pires (2003,Brazil)	Cadaver Study (74, adult)	Five types (I-V) for ABDM, three types (I-III) for the IT, and two types (I-II) for PBDM	This classification offered clear anatomical parameters for interpreting morphological variants of the ABDM, along with pertinent clinical and surgical correlations
Fujimura (2003, Japan)	Cadaver (Case, Male,83y.o)	On the left side, there were three muscular bundles, while on the right side, there were four. They were anchored to the digastric fossa on their respective side, the raphe of the mylohyoid muscle, and the base of the mandible	The raphe of the mylohyoid muscle exhibited a significant curvature towards the right, and the four anomalous bundles identified on the ABDM on the right side were affixed to this curved juncture
Anderson (2021,USA)	Cadaver Study (48)	Fifteen cadavers (31.2%) exhibited variations in the ABDM. And 3 unreported types were identified	Understanding the prevalence and typical patterns of ABDM variations would provide valuable insights during surgical procedures conducted in the submental region
Natsis (2018, Greece)	Cadaver (Case, Male,54 y.o)	A three-headed ABDM	A thorough understanding of anatomical variations within the submental region was crucial for surgeons, imaging specialists, and clinicians engaged in interpreting imaging studies or formulating differential diagnoses for neck masses.
Bakirci (2007,Türkiye)	Cadaver (Case 1,Male,84 y.o; Case 2,Male,60 y.o)	Case 1: bilateral ABDM (Type 2) Case 2: unilateral	Because of its location and tissue density, an accessory digastric muscle could have been misinterpreted as a pseudo-mass or a normal or metastatic submandibular or submental lymph node
Hsiao (2018, Taiwan)	Cadaver Study (15)	Unilateral type Crossover type Mixed type	Pre-existing awareness of potential anomalies is crucial for minimizing confusion during the elevation of myocutaneous flaps on the ABDM
Sargon (1999, Türkiye)	Cadaver Study (99, adult)58 male and 41 female,	Unilateral/bilateral and additional types	Various types of complex anomalies should be considered in functional studies concerning the floor of the mouth and in the evaluation of these structures in CT/MRI. These anomalies are significant in the context of embryological understanding
Unsal (2023, Cyprus)	Ultrasonographic imaging (151)	Types 1 (n = 145),2 (n = 3),7 (n = 3), (according to Kim et al.)	Given its potential clinical significance, maxillofacial surgeons and radiologists should possess a comprehensive understanding of both the normal anatomy and variations of this muscle
Present study (2024,Türkiye)	Cadaver (Case 1,Male,68 y.o; Case 2,Female,54 y.o)	Case 1: Unilateral origin and insertion types Case 2: Unilateral (right) insertion type	This study’s inclusion of both accessory ABDM muscles and specific tendon measurements (e.g., 3.4 cm intermediate tendon length) offers more granular data on ABDM’s anatomical diversity, which is crucial for surgical interventions in the neck and submandibular regions

Abbreviations: ABDM: Anterior Belly of Digastric Muscle, CT: Computed Tomography, IT: Intermediate Tendon, PBDM: Posterior Belly of Digastric Muscle, MRI: Magnetic Resonance Imaging

Conclusion

The unique presence of accessory ABDM and the extra ABDM are noteworthy. Our study’s inclusion of specific dimensions for the accessory and primary ABDM muscles (both left and right sides) adds precision and can serve as a valuable reference for future anatomical studies, particularly in surgical or clinical settings where such variations may impact procedures involving the submandibular or suprahyoid regions.

Author Contributions

EO and CB contributed to the study conception and design. EO and CB were involved in data collection and acquisition, contributing to data management and interpretation. EO and CB participated in drafting the article and revising it critically for the valuable intellectual content and writing of the manuscript. All authors have read the final approval of the submitted version.

Funding

No funding or grants were obtained.

Data Availability

Not applicable.

Declarations

Ethics Approval and Consent to Participate

All procedures performed in conformity with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.

Research Involving Human Participants and/or Animals

All procedures performed in studies involving human participants were in accordance with the ethical standards of the 1964 Helsinki Declaration and its later amendments.

Informed Consent

Not applicable.

Competing Interests

The authors declare that they have no conflict of interest.