Morphometric Analysis of the Submandibular Fossa

INTRODUCTION

The submandibular fossa or fovea is a concave region that is present in the medial side of the mandible below the mylohyoid line.^[1]

These concavities on either side of the posterior part of the mandible enclose the submandibular gland and submandibular lymph nodes.^[2] During the process of development, the submandibular salivary gland exerts compressive forces along the lingual surface of the mandible, which later on is denoted as the submandibular fossae. The vital structures associated with the fossa are the neurovascular bundle of the inferior alveolar canal, the lingual nerve, and the sublingual artery.^[3]

It is a high-risk zone during implant placement since accidental perforation of the lingual plate above the mylohyoid muscle can cause lingual nerve damage, whereas perforation of the submylohyoid space can cause injury to the sublingual arteries.^[4]

Overextension of the pilot drill into the inferior alveolar canal causes injury to the inferior alveolar nerve.^[5]

There are different methods to assess anatomical areas by touching ridges, osteometry, and diagnostic casts. But due to the presence of the mylohyoid muscle, it is not possible in the posterior part of the mandible.^[4]

According to the American Association of Maxillofacial Radiology, CBCT of the maxillofacial region in cross sections is considered an efficient imaging modality for placing dental implants.^[6]

The cross-sectional (oblique coronal) imaging of the tooth alveolar complex is helpful, as multidirectional imaging and reconstruction of cone beam computed tomography (CBCT) paves the way to determine the accurate dimensions of the alveolar bone for implant placement.^[7,8]Therefore, our study is aimed at determining the depth of the submandibular fossa, the type of submandibular fossa based on its depth, the angle of concavity, and the relationship between the depth of the submandibular fossa and the angle of concavity using CBCT.

MATERIAL AND METHODS

15 CBCT images were retrieved from our database. They were divided into two groups: group I (25-50 years) and group II (50-70 years). The exclusion criteria for our groups were the presence of any bone deformities, the presence of intraosseous/extraosseous lesions, the presence of reconstruction plates, and the presence of infections in the posterior mandible. The images were viewed using CS 9500 3D imaging software, the inferior alveolar nerve canal was traced, and the submandibular fossa region was identified. The anterior border was marked distal to the mental foramen, and the posterior border was marked distal to the third molar region. The images were then viewed in cross-section with a slice thickness of 2 mm and a slice interval of 1 mm.

For the measurement of depth, in each slice, the most prominent part of lingual cortical bone superiorly is identified and marked as S, and in the same way, the most prominent inferior part is marked as I. A line segment (SI) connecting two points is drawn. The deepest point of the lingual cortex is marked as D. A line segment (d) is drawn from point D perpendicular to SI. The meeting point is marked as T. The measurement of DT gives the depth (d) of the submandibular fossa.

On each side, the value of ‘d’ was recorded in all slices between the anterior and posterior borders; the highest distance denoted the maximum depth of the submandibular fossa [Figures 1 and 2]. A line tangential to the superior border of the inferior alveolar nerve canal is drawn and marked as AO. Another line is drawn tangential to the inferior border of the inferior alveolar nerve canal and marked as BO. The concavity angle (C) is determined by measuring the angle when the line DC meets the line CO. The type of the submandibular fossa was categorized into any one of the following types as proposed by Parnia et al.(2009), [Figure 3].

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Figure 1: DT: The Maximum depth of the submandibular fossa (d), S: Superior lingual cortical bone, I: Inferior lingual cortical bone, SI: line segment, D: Deepest point of lingual cortex, Aline segemnt (d) is drawn from point D perpendicular to SI, T: Meeting point.

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Figure 2: The maximum depth of the submandibular fossa, -3.9 mm, angle of concavity -53 AO: Superior border of inferior alveolar nerve canal, BO: Inferior border of inferior alveolar nerve canal, C and D is depth of mandibular fossa Concavity angle (C), measuring the angle when the line DC meets th line CO.

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Figure 3: Types of submandibular fossa

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TYPE I - a lingual concavity <2 mm

TYPE II - a 2-3 mm concavity

TYPE III - a concavity >3 mm

The depth of the fossa, the angle of concavity, and the type of fossa were determined, and the values were recorded. The relationship between the angle of concavity and the depth of the submandibular fossa was also recorded. The study proposal was submitted to the ethical committee of our institution and carried out after their approval.

RESULTS

Descriptive statistical analysis was done using SPSS version 20.0 (Chicago, IL) software. In addition to this, the prevalence of deeper fossa among various age groups, genders, and sides was estimated using one-way ANOVA test, Chi-square test, and independent t-test, respectively. A p <0.05 was chosen as the significance level. A total of 12.5% of the younger adults and 50.0% of the older adults had a deeper submandibular fossa (Type III). Equal distribution of deeper fossa (Type III) in the right (50%) and left sides (50%). There was no sexual dimorphism in the depth of the fossa. The angle of concavity increased as the depth of the submandibular fossa increased.

DISCUSSION

The submandibular fossa is a concavity located along the lingual surface of the mandibular body. It is situated inferior to the mylohyoid line, in the mandibular molar region.^[9]

It is formed by compressive forces exerted by the developing submandibular salivary gland on the lingual surface of the mandibular cortex. The lingual area is highly vascularized. with two arteries, the sublingual artery(branch of the lingual artery) and the submental artery(cervical branch of the facial artery). In the posterior lingual surface of the mandible, there is also the presence of the mylohyoid artery, which anastomoses with branches of the submental artery.^[10]Assessment of the submandibular fossa was done to prevent postoperative complications like lingual cortical plate perforation or injury to the lingual nerve. When the perforation occurs above the mylohyoid ridge, the lingual nerve is damaged. If the perforation occurs below the ridge, then it results in hemorrhage or infection, which may spread to deeper anatomical spaces and can also cause upper airway obstruction.^[11]

The study was done to measure the submandibular fossa depth, type of fossa based on depth and angle of lingual concavity, and the relationship between depth and angle of concavity in order to prepare the implant bed.

Our present study revealed that the maximum depth of the submandibular fossa was 4 mm and the minimum depth was 1.4 mm. The maximum angle of concavity was 67⁰, and the minimum angle of concavity was 35⁰. Descriptive statistics revealed that the mean depth in type I cases was 1.67±0.29 mm, in type II cases it was 2.16±0.47, and in type III cases the mean depth was 3.9±0.14 mm. The average depth of the fossa was 2.03±0.67. There was a statistically significant difference in the depth of the fossa among groups as determined by a one-way ANOVA test [Table 1]. The average depth in younger adults is 2.01±0.82 mm, and for older adults it is 2.06±0.47 mm. Thus, there was a significant difference between younger and older adults (p <0.05). On performing an independent t-test, the average depth in male patients was 2.15±0.80, and in females it was 1.86±0.36. There was no significant sexual dimorphism (p <0.05).

According to Parnia et al (2010), the average depth was 2.6±0.85 mm. The minimum depth was 0.4 mm, and the maximum depth was 6.6 mm. The mean fossa depth was 2.7±0.91 mm in males and 2.5±0.7 mm in females, and the differences were insignificant (p <0.05). An independent t-test revealed no sexual dimorphism, which was similar to our study. The age differences in different groups were not statistically significant (p <0.05). This fact was not in line with our study, probably due to the difference in age groups of patients involved in the study.^[9]

Kamburolu et al (2015),^[11] in their study, observed that the mean depth of the submandibular fossa was found to be 2.4 mm. The concavity depth for females on the right side was 2.26±2.05 mm, and on the left side it was 2.24±2.03 mm. The mean concavity depth for males on the right side was 2.57±2.20 mm, whereas on the left side it was 2.68±2.34 mm. However, there was no statistically significant difference in the depth of the fossa between males and females (p >0.05). The difference between the depth in the right and left submandibular fossa was also statistically insignificant (p >0.05). These results were supportive of our study.^[12]

Sina Haghanifar et al (2018),^[4] in their study, concluded that the mean depth in male patients was 2.02±0.83 mm and 1.73±0.57 mm in female patients. The average submandibular gland fossa depth on the right and left sides was 1.87±0.72 mm and 1.87±0.73 mm, respectively. On statistical analysis, there was no significant difference in depth between the right and left sides and between males and females (p >0.05). These results support our study. In contrast to our study, there was no significant difference in SM fossa depth related to age groups. It can be substantiated by the fact that in their study, the age group of the patients involved in the study was only the older population, and no comparison was established between the young and older population, like in our study.^[4] In the present study, we also determined the angle of the submandibular fossa. The descriptive statistical analysis revealed that in type I cases, the mean concavity angle was 42.80±5.71⁰. In type II cases, the mean concavity angle was 49.54±11.46⁰, and in type III cases, it was 48.0±7.07⁰. The average angle of concavity was 46.07±9.11⁰. On performing a one-way ANOVA test between groups, there was no statistical significance. Multiple comparisons using post-hoc tests also showed there was no statistical significance between the groups (p >0.05). An independent t-test was performed, which revealed that the average angle of concavity in the right side is 45.67±10.70⁰ and in the left side it is 46.47±8.63⁰, whereas for females it is 44.75±10.02⁰. The average angle of concavity in the right side is 45.67±10.7⁰, and on the left side it is 46.47±7.54⁰. In young patients, the average lingual concavity angle is 46.19±9.98⁰, and in older patients, the average lingual concavity is 45.93±8.37⁰. There was no statistically significant difference among gender, age, sex, and sides of the mandible.

According to Panjnoush et al (2016), the average concavity angle in male patients was 15.10 ±17.00⁰ and for females it was 15.70±15.45⁰. The average concavity depth of the submandibular fossa was found to be 15.45±16.17⁰. On performing statistical analysis, no significant difference was observed between males and females (p >0.05). This data supported our study.^[13]

Yoon et al (2016) put forth that the average concavity angle on the right side was 75.87⁰ for females and 75.45⁰ for males. On the left side, the average concavity angle was 75.19⁰ for males, and for females it was 75.39⁰. On performing statistical analysis, there was no significant difference between sex and age, which was again in line with our study.^[14]

Herranz-Aparicio et al (2016), in their study, stated that the average concavity angle was 66.6±8.9⁰ for males and 71.6±8.4⁰ for females. There was a statistically significant difference in concavity angle between males and females (p <0.05). This was contradictory to our study, the difference in the method used for determining the concavity angle, and the use of computed tomography instead of cone beam computed tomography was suggested as a valuable reason for variation.^[15]

Sina Hagnifar et al (2018), in their study, evaluated the concavity angle and concluded that the average concavity angle on the right side and left side were 42.63±9.59⁰ and 43.09±9.42⁰. The average concavity angle for females on the right side is 42.4±9.00, for males it is 42.8±10.1⁰. The average concavity angle for females on the left side was 41.6±8.0⁰ and for males on the right side was 44.6±10.4⁰. There was no statistical significance between the angle of concavity on the right and left side and between males and females (p >0.05). On Pearson’s correlation coefficient to determine the depth/angle of concavity ratio, he found that there was a linear relationship between angle of concavity and depth. The result was similar to our study in which a positive linear relationship was observed between the fossa depth and angle of concavity, which means that as the depth of the fossa increased, the angle of concavity also increased.^[4] Thus, from the current study, we can elicit that deeper submandibular fossae and increased angle of concavity possess a high risk of perforation of the lingual cortical plate during posterior mandibular implant surgeries.

CONCLUSION

Due to bone remodeling with age, older adults have a deeper fossa, possessing an increased risk for perforation when compared to younger adults. It is important to assess the morphology of the submandibular fossa using CBCT before implant placement. The cross-sectional image analysis can be used as an adjunct during pre-implant imaging to prevent intraoperative and postoperative complications.