From our experience, the demand for seeking dental implants has grown so much that there has been a substantial increase in the surgical procedures being performed in the mandible. This requires the dentist to visualize the anatomic structure with precision to avoid any complications. As the popularity of implants increases, more challenges and complexity will need to be accepted by the implantologist and oral and periodontal surgeons. Modern imaging techniques are available, with their maximum utilization to assess and explore in-depth these difficult areas. ¹

Taking precautions before attempting the surgical procedures by clarifying and categorizing the anatomy appropriately will prepare the clinicians better to handle any complications which may arise during and after the procedure as the mandible and maxilla house many anatomical structures. The posterior maxilla often presents issues with a poor quality of bone. In addition, pneumatization of the maxillary sinus further complicates the implant placement, and restoring this area becomes a difficult task. In the anterior maxilla, a lack of facial bone creates esthetic concern, and it is difficult to achieve an emergence profile. If the nasopalatine canal is sufficiently large, it may pose difficulties in placing the implant in this region, with potential complications which may arise later. ²

Compared to the maxilla, the mandible is further complicated. The posterior mandible poses a unique challenge for many surgical procedures and implant placements, because of the location of many important anatomical structures which need to be kept in mind. Most examples of the posterior region of the mandible present with an external oblique ridge, lingual concavity, and close association of lingual nerve in the third and second molar region, making the surgical procedure relatively cumbersome. Failure to detect the lingual concavity may force the implant placement outside the border of the mandible with possible fracture of the mandibular margin. ³

Besides, negligence in not considering the anatomical landmarks that are most closely associated with dental implant surgery like the mandibular foramen, the inferior alveolar nerve, the mental foramina, the mental nerve, the lingual nerve, and the incisive canal and its associated neurovascular bundle may end up with the patient exposed to possible nerve injury and complications associated with the neurovascular bundles, including hemorrhage. ⁴

The inferior alveolar nerve and artery in the mandibular canal begins at the mandible foramen and divides into two branches. The mental foramen is typically located on the lingual side between the premolars and the bottom border of the jaw. ³ The portion of the inferior alveolar nerve which presents anteriorly to the mental foramen, before exiting the canal, is referred to as the anterior loop of the inferior alveolar nerve. ⁵

The anterior canal and the submandibular fossa convexity are important anatomic factors that can limit useful bone in the posterior region, necessitating surgical augmentation or other critical surgical treatments. Instrumentation through the lingual cortical plate which is inadvertent or careless can bring about vascular harm that can cause the formation of a sublingual or submandibular hematoma, excessive bleeding, or infection. ⁶ In addition, information about the location of the mental foramen can lead to more accurate local anesthesia techniques and can avoid damage to the mental nerve during implant placement. ³

The lower jaw remodels throughout life with growth with recognized modifications in prominent places such as the mental foramen, mandibular foramen, and mandibular canal. The shifting of the position of the mental foramen with age is a well-established fact that has been confirmed by the majority of researchers on the subject. ⁷ Further, after the extraction and subsequent resorption of the local alveolar ridge, the mental foramen location may change move closer to the alveolar bone ridge and is thus more prone to damage during surgical procedures. ^{8 , 9} However, the typical location of the mental foramen is reported to be either between the apices of the first and second premolars or below the apex of the second premolar. ³ This position of the mental foramen demonstrates anatomical variations, which can be found as far anterior as the canine ¹⁰ and as far posterior as the first molar. ¹¹

The mental foramen is an important anatomical structure representing the termination of the mandibular canal. ¹² Its accurate identification depends on knowledge of its location; this is strategically important for diagnostic and clinical procedures. ¹³ As a result, it appears that no single and consistent pattern of mental foramen placement exists across diverse groups. Hence, in clinical dental treatment, a detailed assessment of the mental location typical of each group is extremely useful. The shape, size, and position of the mental foramen must be evaluated, especially when considering various dental treatments performed in the mandible.

Currently, high resolution 3D cone-beam computed tomography (CBCT) is the most promising and accurate technique available for quantitatively determining the location of mental foramen and the presence of anterior loops. ¹⁴

The high image quality of bone tissue and anatomical structure features provided by CBCT analysis reduces the risk of lesions in the lower alveolar vascular nerve bundle. Paralysis and bleeding may result in the front area of the mandible and nearby tissues. ¹⁵

Many authors and researchers in their retrospective studies assessed the details of the mental foramen (MF). ^{16 – 19} They have found that in very few cases, the exact position of the mental foramen can be observed in coronal, axial, sagittal, cross-sectional, and three-dimensional reconstructed images using CBCT.

Moreover, the CBCT easily overpowers drawbacks of panoramic radiograph like inaccurate patient placement due to a lack of standardization in placement throughout radiography, fluctuation in anatomic structures, and situations such as interruption and magnification in the radiographs, as well as some discrepancies in measuring the length.

It has been reported in the literature that race and ethnic variation are also visible in the mental foramen, accessory mental foramen, and anterior loop. ¹⁷ Considering this, there are variations expected in the Saudi population, yet few studies have reported variations. ^{18 , 19} There are reported studies using panoramic radiographs, however, such studies may not be useful in the digital era with 3D imaging providing clearer details in comparison. Thus, this retrospective study was conducted to evaluate the shape, size, and location of mental foramen in dentulous and edentulous among the Saudi population using 3D CBCT. This study aimed to compare the positions and dimensions of MF openings between edentulous and dentate subjects matched by gender and nationality through CBCT.

From the total 500 scans, 280 were excluded as they were not good quality images and 40 as they did not fulfill other inclusion criteria such as Saudi nationals (n=22), free from systemic disease/condition (n=12), and permanent dentition (n=6). Hence, 180 were included in the analysis ( Figure 1).

The descriptive characteristics of the study variables are shown in Table 1. A total of 180 CBCT were analyzed, of which 96 were male and 84 were female. ³⁴ Among 180 CBCT, it was ensured that there was equal distribution between dentate, edentulous, and sides (left and right) and vertical position among the CBCT is LC, horizontal position shown P4 having the highest percentage 67.8%.

59 males and 63 females showed P4 horizontal position. A statistically significant association was found between the horizontal position of the mental foramen and gender (p<0.05) ( Table 2). A statistically significant association was also found between the horizontal position of the mental foramen and dentate status (p<0.05) ( Table 3). On the other hand, no statistically significant association was found between the horizontal position of the mental foramen and sides (p>0.05) ( Table 4).

The mean and standard deviation (SD) mental foramen angle was highest on the left side in the 50-59-year-old group in the edentulous group (53.95±13.97) and least on the right side in the 16-19-year-old group in the dentulous group (38.00±8.26) ( Table 5). The mean distance of the mental foramen to the alveolar crest (MF_MSB) was statistically significantly higher (t=6.235, p<0.001) in males (14.52±3.29 mm) than females (11.22±3.80 mm). In addition, the mean distance of the mental foramen to the mandibular inferior border of the mandible (MF_MIB) was statistically significantly higher (t=2.875, p=0.005) in males (11.59±2.10 mm) than females (10.25±3.95 mm). Moreover, the mean width of the mental foramen in the transverse section (MFW) was statistically significantly higher (t=3.622, p<0.001) in males (3.43±0.82 mm) than females (2.99±0.83 mm). However, the mean M angle between males (44.21±9.06) and females (46.11±12.03) was not found to be statistically significant (t=-1.205, p=0.239). The mean MF_MSB, MF_MIB, and MFW were statistically significantly higher in males (p<0.05) ( Table 6).

The mean distance of MF_MSB was found to be statistically significant (t=8.763, p<0.001) higher in dentate (15.11±2.72 mm) than edentulous (10.85±3.73 mm). On the other hand, the mean distance of MF_MIB between dentate (11.36±2.16 mm) and edentulous (10.57±3.90 mm) was not found to be statistically significant (t=1.688, p=0.093). The mean MFW between dentate (3.46±0.74 mm) and edentulous (2.99±0.89 mm) was found to be statistically significantly higher in dentate (t=3.815, p<0.001). Furthermore, the mean M angle between dentate (41.92±8.33) and edentulous (48.27±11.61) was found to be statistically significant (t=-4.217, p<0.001) ( Table 7). There was no statistically significant difference in the mean of all study variables by sides (p>0.05) ( Table 8).

A detailed understanding of the anatomical structures of the mandible and maxilla are important during periodontal and implant surgical procedure. A good surgeon always plans the surgical procedure using required diagnostic aids and gives attention to each small anatomical structure in the surgical field, including anatomical variation (if any) to avoid surgical complications. ³ The mandible, with its associated anatomical structures in the posterior region, always requires additional attention to avoid injury to the neurovascular bundles and any complications related to them. The mental foramen and associated alterations should have been given attention before surgery. Anatomically, mental foramen shows a lot of variation with the number of foramina and accessory foramina and the possibility of the presence of an anterior loop of the mental nerve. ³

In the present study, we have utilized CBCT to assess the position of the mental foramen. Many previous studies evaluating the mental foramen and its associated structure were done in cadavers, and human studies using panoramic radiographs, ultrasonography, and CBCT. ^{13 – 19} Panoramic radiographs have their disadvantages with a lack of clarity, thus smaller mental foramen may go unnoticed. In the present study, When compared to multi-detector computed tomography (MDCT) imaging, CBCT was the radiographic method of choice for visualizing the hard tissues of the mandible because it reveals anatomical structures without superimpositions and deformation, which are observed in traditional imaging techniques such as panoramic image analysis and at low levels of radiation. ²²

Although several studies are available regarding the mental foramen and its associated anatomical variation which are carried out within the Saudi population, there is still scope for more information to gather and add to the existing literature. ^{16 – 19} In addition, all the earlier studies done in Saudi Arabia were only among dentate subjects. This is the first study exclusively comparing the dentate and edentulous. Furthermore, initial studies were done using panoramic views. ^{16 – 19} However, there are drawbacks of using a panoramic view as previously outlined and the result may vary when they are compared with that of the CBCT. ¹⁰

The prevalence of horizontal position of mental foramen in the present study predominated by P4 (35% in males and 32.8% in females), followed by P3 (8.3% each in males and females). P4 appears to be the most common position, or more prevalent position, seen in almost all the studies which are done among Saudi, Jordanian, and Egyptian population. ^{18 , 23} However, variation in the position was seen in studies among the Indian (P5), European (P3), and Iranian population (P3). In the present study, the prevalence was similar to the study of Alam et al. (2018). ¹⁸ Many other studies have shown higher prevalence. The similarity and difference in the results are related to race and potentially due to change in the sample size of the age group. It may be possible that the sample size chosen is also responsible for the percentage of prevalence seen in different studies. The sample size of our study is less than the few studies mentioned above and more in a few other studies. Hence, the percentage of prevalence seen in this study may be different than the previous studies.

In the present study, the variation in the mental foramen compared between males and females was found to be statistically significant. The horizontal position of the mental foramen and gender showed a statistical significance difference, especially at the P3B, P5, and P4 positions. There was a statistically significant difference seen in MF_MSB and MFW. However, there is no significant difference in MF_MIB and MF angles. Differences in the position of mental foramen across gender and racial groups have also been reported in the literature. Present study results are similar to some parameters which were presented in Alam et al. (2018). ¹⁸ In their study, the P4 position is more prevalent in males which is similar to our study, but they have found P3 to be more prevalent in females. However, in our study, we have found P4 more commonly prevalent, followed by P3. There was a also statistical difference in P3B and P5 prevalence among males and females. A study done in the Indian population has shown slightly different results, where they have found P4 followed by P3 being common in males on the right side of the mandible and P4 for the females on the left side of the mandible. ²⁴

Li et al. (2018) and Al-Mahalawy et al. (2017) found similar results to the present study with the difference in the prevalence among gender. ^{17 , 25} In a study of the Polish population, for the right or left side in males, the average values of a horizontal and vertical diameter were significantly higher on the right side than in the female subgroup. Whereas, on the left side the average value of only the vertical diameter was significantly higher in males compared to women. ²⁶ Thus, various studies have pointed out the different or similar prevalences among males and females. Variability in the prevalence among males and females was not consistent, with few studies mentioning no difference and few other studies mentioning differences between the gender. The difference in the result could be related to the age group of the sample, sample size, and racial predilection.

The present study is unique compared to many other studies because we have taken both the dentate and edentulous patients. It is known that there is variation in the location of the mental foramen as the age advances due to resorption of the crestal bone loss or change in the position of the mandible due to differences in the growth of the mandible. In the present study, although we have found the prevalent position is P4 in both the dentate and edentulous groups, there was a statistically significant difference between these two groups. With regards to the age effect on the morphology of the foramina, studies have shown different results, although many studies did not specify the group as dentate or edentulous, the age group variation has been shown. ^{16 – 19} Differences in the division of the age group could lead to different conclusions ^{27 , 28} on variation in the mental foramen location, number of mental foramina, and anterior loop of the mental nerve.

One of the studies revealed that age-related differences of the accessory mental foramen size varied between pediatric and adult populations due to the growth process of the mandible ²⁹ ; variation in the edentulous and dentate subject across age variation is an important consideration during implant placement. In the present study, there was no statistical significance in the MF position between the left and right sides in the horizontal position. However, we have seen the difference in the results for MF_ MSB, M angle, and MFW. The present study results are similar to the studies reported among the Jordanian, Saudi, and Iranian population, ^{18 , 30} but a few other studies reported the prevalence of asymmetric mental foramen in Saudi, Egyptian, and Jordanian populations too. ^{10 , 20} The variation in the position of mental foramen in different populations may be due to racial variations and factors which may have influenced the growth of either side in a different manner. ¹⁸

The variation in the distance between the mental foramen to the alveolar crest, relation to the lower border of the mandible, and corresponding changes in the width of mental foramen in the transverse section is commonly increased by age. The mean distance from the upper border of the mental foramen to the alveolar crest in our study is 15.11 mm in dentate and 10.85 mm in edentulous, which is almost in the range of the previous study done among the Saudi population ¹⁷ where the range is 9.1–19.2 mm (Mean: 14.3 mm). Similar reports have been published by Haktanir et al. (2010), which showed a mean distance of 14.2 mm (Range: 10.7–29.8 mm), ³¹ where the lower range is close to our study, but there was a much larger difference observed in the upper range.

Changes in the vertical distance seen in this study, especially in the upper range, could be due to the resorption of the alveolar crest, age of the patient, growth pattern changes in the mandible as the age advances, and genetic changes. ¹⁷ To overcome the shadow of the alveolar crest resorption having an impact on this parameter, one author suggested the use of cementoenamel junction (CEJ) of adjacent teeth as a guide. ⁶ Similar to the results of the relation between the mental foramen and alveolar crest, many studies established and presented the distance between the mental foramen and inferior border of the mandible. In our study, we have seen a mean distance of 10.57 mm in the edentulous and 11.36 mm in the dentate subjects. These study results are again similar to the study reports of Al-Mahalawy et al. (2017), where they reported the mean distance between the inferior margin of mental foramen and lower mandibular border was 13.8 mm (Range: 8.7–16.6 mm). ¹⁷ Similarly in the reports by Von Arx et al. (2013) and Kalender et al. (2012), the average distance was found to be 13.2 mm and 12.4 mm, respectively. ^{32 , 33}

There is some limitation of the study. The sample size of the study though drawn after considering the value of previous studies, increasing the sample size may be helpful to draw a better conclusion. Further, we have considered dentate ane edentulous age group to a limited range, since the anatomy of mental foramen may vary from age to age. A further age range inclusion may broaden the result with a better conclusion. Considering the similarity in some of the parameters in the same ethnic population, differences across ethnic populations, changes in the age group, gender, and state of dentate and edentulous, a recent systematic review on the anterior loop and mental foramen rightly pointed that there is no fixed parameter to be relied on for the presence and distribution of anterior loop. ¹³ Though it appears that in a given ethnic population and age group some parameters appear to be similar, it is highly recommended not to rely on any average values available for the anterior loop. The clinician is advised to use imaging modalities available in every case wherever surgical procedure is to be performed near the mental foramen region for identification and accurate measurements of the anterior loop length to avoid any injury. ¹³ If clinicians want to avoid complications and extend comfort to the patient, it is prudent to be cautious during the operation. Clinicians should be on the lookout for unanticipated deviations, especially when doing dental operations that entail periosteal detachment and implant placement in the mental area.

Within the limitation of the study, it can be concluded that the horizontal position of the mental foramen is predominated by P4 followed by P3 in this sample of the Saudi population. There is some variation in the mental foramen location across gender and different age groups, and also variation in the position of the mental foramen among dentate and edentulous subjects. Considering the variations in the age group, gender, and between dentate and edentulous subjects, the clinician needs to be careful about the position of the mental foramen and the use of CBCT as a diagnostic aid before the surgical procedure needs to be considered.