Retrospective Evaluation of Position of Mental Foramen and Its Relationship with Gonial Angle Using Cone Beam Computed Tomography

Introduction

Mental foramen (MF) is an important anatomical landmark on the outer surface of the mandible in the premolar area. Physicians should be aware of its exact location when performing local anesthesia, incision, flap lift and osteotomy in the vicinity of the MF. While performing surgical procedures, particularly, implant placement, endodontic surgery, root resection, extraction of tooth, cyst, or tumor, orthognathic surgery, and fixation of facial fractures, attention must be paid to avoid damaging the neurovascular bundle exiting in the MF (1).

Radiographically, MF can be seen as a lucent and round region on both sides of the mandible. Conventional radiography techniques widely used in dentistry, including periapical and panoramic, provide a 2-dimensional image of a 3-D structure (2,3). Thus, due to the superimposition of adjacent anatomical landmarks (2,4-5), trabecular bone pattern (6), thinness of mandible (4-6), and being positioned below the end of the periapical film (4), MF may not be visible in certain cases.

Moreover, the opening of MF leans upward and toward  back, consequently, it is not in the usual view of the premolars in the longitudinal axis of the canal opening. On the other hand, due to its position on the surface of the mandible, its location in relation to the root of the teeth is affected by the change of the angle of radiation. When the image of mental foramen falls on the apex of one of the premolars, it may mimic a periapical lesion. In such cases, the presence of a dental canal, which extends to the suspicious radiolucency or a detectable lamina dura in the mentioned area, could reveal the true nature of this overcast image. The relative thinness of the lamina dura superimposed on this lucent foramen results in a significant burn out of the former thus making the diagnosis further complicated. However, a second radiography from a different angle clearly shows lamina dura as well as the change in location of MF radiolucency in comparison to the tooth apex (7).

Nowadays, Cone beam computed tomography (CBCT) has brought advancements in dentistry. This imaging modality has enabled an accurate three-dimensional evaluation of the bony structures in the maxillofacial region (1-5), and as a result, enabling the evaluation of MF in sagittal, axial, and coronal cross sections (8).

Gonial angle (GA) is an important indicator for determining growth pattern, evaluating mandibular rotation, and deciding on extraction pattern in class II patients and an influential factor in deciding on surgery in skeletal class III patients and in forensic age estimation (9). GA is formed when Ramus line (tangent to the posterior border of ramus) and mandibular line (the line in contact with the inferior border of mandible passing gnathion) intersect each other (10).

Some reports suggest that occlusal forces are related to the shape of mandibular angle (9). A high gonial angle indicates the tendency of the mandible to rotate backward with the growth of the condyles toward the back, which is reflected by a steep mandibular plane and, as a result, the posterior orientation of growth. On the contrary, a smaller gonial angle represents the vertical growth of the condyle (11).

German and French researchers made measurements of GA in the late 19th century for the first time. Their queries showed that the average mandibular angle did not alter from adulthood until at least the age of 70, except in cases where there were extensive tooth loss (12).

There is disagreement regarding the exact position of the MF and adjacent anatomical landmarks due to their various differences (13-15). Since no study has been published to investigate the morphometric relationship of gonial angle with position of mental foramen in CBCT images, the present study aimed to investigate the location of MF and its relation with gonial angle in a group of Iranian population by using CBCT imaging.

Materials and Methods

In this analytical, cross-sectional study, in order to examine the position of MF with respect to gonial angle, 64 samples of CBCT images were collected from the archives of radiology department of dental school of Guilan University of Medical Sciences (GUMS). The total of 128 gonial angles had been collected for various reasons including orthodontic treatment, surgery, implant placement, among others. The CBCT images had been prepared using Pax-i 3D unit (Vatech, Hwaseong, Korea). Exposure parameters for each sample were as follows: 95 kVp, 5.2 mAs, 9s scan time, 0.25 voxel size, and 15×15cm FOV for mandible Mode.

Inclusion criteria were full field CBCT images that covered the entire mandible, ramus, and condyle area of ​​both sides; patients aged 18 to 50 years, with their both premolars (number 4 and 5) and first and second molars in place. Exclusion criteria were images with involving lesions in mandibular canal or mental foramen; edentulous patients; and images of primary and mixed dentition. The present study was confirmed and approved by the ethics committee of GUMS and was assigned code of ethics 1397422.

All panoramic images were reconstructed by the same device software.  All CBCT images were evaluated by student under the supervision of an oral and maxillofacial radiologist.

Using Pax-i 3D unit, reconstructed panoramic and obtained cross-sectional images, the following parameters were examined:

1) Measurement of left and right gonial angles in reconstructed panoramic images: Line “a” was drawn tangentially with the most prominent point of posterior border of ramus and condyle, and line “b” was tangent with the inferior border of mandible. The angle created between lines “a” and “b” was measured as gonial angle (GA) (Figure 1). Then, scans were divided into two groups according to the size of the gonial angle: Low angle (≤125°) and high angle (>125°) (16). (Figure 1)

2) Analysis and measurement of mental foramen (MF) indices: In cross-sectional profiles, the section in which MF had the clearest view, the indices associated with this foramen were examined. The position of mental foramen was determined as follows: Type A = below the apex of first premolar; B = below the apex of second premolar; C = between first and second premolars; D = anteriorly from the first premolar; E = posteriorly from second premolar.

In addition, the frequency of various types of mental foramen in the studied images, were evaluated based on sex and age.

Then, the length of mental canal was measured based on the distance from the lingual end of canal to its opening. Moreover, the vertical distance from the lowest point on the inferior border, buccal cortex, and alveolar crest, plus the distance from buccal end of mental canal to lingual cortex of mandible, were calculated (17) (Figures 2&3).

The statistical analysis was performed using Independent t-test and Chi-square test. The relationship between frequency of position of the MF on the right and left side of mandible, as well as the relationship between sex and position of MF and measurements of mental canal were examined using Chi-square test. Independent t-test was also used to analyze the relationship between age and position of MF and mental canal measurements, the relationship between MF position and gonial angle and the relationship between mental canal and gonial angle measurements. P-values under 0.05 were considered as statistically significant values.

Results

In this study, 64 CBCT images (a total of 128 gonial angles and mental foramen) were examined for determining the relation between gonial angle, position of mental foramen, and mental canal. The mean age of the subjects was 29.13±8.36 (minimum age 18 years and maximum 49), with a median age of 27 years old.

Based on Table I, the most and least frequent types of MF position on the right belonged to type C and E, respectively. Regarding the left side, the highest occurrence was found as typeC and the least as D. There was no significant relationship between frequency of position of MF on right and left side of mandible (P=0.217).

According to Table I, there was no relation between sex, age, and position of MF, respectively. (Sex(R/L): P=0.458 / P=0.687, Age(R/L): P=0.190 / P=0.239). (Table I)

Table I: Distribution of various types of MF positions

^* A:below the apex of first premolar; B: below the apex of second premolar; C: between first and second premolars; D: anteriorly from the first premolar; E: posteriorly from second premolar

According to the information in Table II, mean and standard deviation of gonial angle measured on the right side was 121.73°±5.38° and on the left was 124.4°±5.00°. The mean gonial angle on the right was about 3 degrees smaller than the left, and Pearson correlation test showed a significant relation between the two sides (r=0.8, P<0.001). (Table II)

Table II: Gonial angle measurements in Right and left sides of mandible

According to Table I, the highest frequency of MF types in high and low gonial angles was C and the lowest in the mentioned categories was type E and D, respectively. Statistical analysis showed no relation between the position of MF and gonial angle (P=0.423).

Among the measurements taken in the high angle group, only the distance from the MF to the lingual plate on the right side followed a normal distribution, and in the low angle group, the length of the mental canal and the distance from the MF to the buccal plate on the left and the distance from the MF to the inferior border of mandible on the right side followed a normal distribution.

Based on Table III, no significant relationship was found between the measured variables in relation to the mental canal and the gonial angle on both sides (P>0.05), but between the measurements on the right and left sides of mandible – in general and not considering gonial angle – a meaningful relation was observed (P<0.001).

(Table III)

Table III: Mental canal measurements in association with gonial angle

 ^a: Distance from the lingual end of mental canal to inferior border of mandible; ^b: Distance from the lingual end of mental canal to alveolar crest; ^c: Distance from the lingual end of mental canal to buccal plate; ^d: Distance from buccal end of mental canal to lingual plate.

No relation was revealed between measurements of mental canal with sex and age, table IV and table V (P>0.05). Table (IV, V)

Table IV: Mental canal measurements in association with sex

Table V: Mental canal measurements in association with age