Accessory Mental Foramen in Dry Mandibles: An Observational Study Along with Systematic Review and Meta-Analysis

1. Introduction

Mental foramen (MF) is an oval or circular opening located on the body of the mandible, typically equidistant from its superior and inferior borders. It serves as a passage for the mental nerve, artery and vein, playing a vital role in innervating the lower face, including the lower teeth, lip, labial mucosa and gingiva.

The accessory mental foramen (AMF), when present, is identified as a smaller opening located alongside the MF, typically positioned posterior to it/. The AMF plays a critical role, particularly during operating procedures, making it essential for surgeons to recognize its presence and morphology. Precise knowledge of the variations in the anatomy, shape, size, and position of the MF, along with the presence of the AMF, is invaluable for performing mandibular surgical procedures. These include filling treatments, premolar curettage, root canal therapy, dental implant placement, orthognathic and periapical surgeries, jaw surgeries, and the enucleation of pathologies in the mental region. Given the frequency of dental and maxillofacial procedures involving the mental region, it is imperative for dental practitioners to be well-informed about these anatomical details to prevent postoperative complications such as lip numbness, a significant consequence of improper handling or violation of the AMF [1].

Saywer et al. demonstrated that the morphology, position, and incidence of AMFs vary among individuals and across ethnic groups [2]. Additionally, the position of the MF differs between edentulous and dentate subjects [3]. While many studies focus on dry mandibles, others have utilized data from Cone-Beam Computed Tomography (CBCT) and panoramic radiographs. Notably, linear measurements from CBCT have been shown to align closely with those obtained from dry mandibles using a digital caliper, the gold-standard method [4]. CBCT measurements of MF size have also been validated for accuracy [5]. However, comprehensive data on AMFs remain limited.

Although numerous observational studies, case series, and case reports have been published, a systematic review and meta-analysis focusing on dry mandibles has yet to be conducted. This study aims to assess the incidence and perform a morphological and morphometric analysis of AMFs in human dry mandibles from the Greek population, complemented by a systematic review and meta-analysis of available global data.

2. Materials and Methods

The present observational study was conducted on 114 dry adult human mandibles of unknown gender and age, derived from the osteological collection of the Anatomy Department, Democritus University of Thrace, Greece.

The study objectives were to measure the location, shape, incidence, diameter, as well as the distance between AMFs and MF, AMFs to the alveolar crest and AMFs to the inferior border in each mandible. For that purpose, we used a digimatic caliper (Mitutoyo Co., Japan) of 0.1 mm accuracy. Each distance was measured from the center of each foramen, either MF or AMF. The location of each AMF was determined using two imaginary lines (horizontal and vertical) intersecting at the MF, dividing the area into four quadrants: anterior superior, posterior superior, anterior inferior, and posterior inferior. The diameter of the AMF was measured using a single wire with a diameter ranging from 0.2 mm to 1.4 mm. For oval-shaped foramina, the horizontal distance was measured using up to two wires with diameters ranging from 0.02 mm to 3mm (Figure 1).

All measurements were performed independently by two investigators. Inter-rater reliability assessment for categorical variables was assessed using the Cohen’s kappa estimates; in case of continuous variables, the intraclass correlation coefficients (ICC) estimates and their 95% confidence intervals (CI) based on a mean rating (n=2), absolute-agreement, two-way random model were alternatively applied. Means are accompanied by the relevant standard deviation (SD). Inter-rater reliability assessment for categorical variables was assessed using the Cohen’s kappa estimates; in case of continuous variables, the intraclass correlation coefficients (ICC) estimates and their 95% confidence intervals (CI) based on a mean rating (n=2), absolute-agreement, two-way random model were alternatively applied [6]. Based on the 95% confident interval of the ICC estimate, values less than 0.5, between 0.5 and 0.75, between 0.75 and 0.9, and greater than 0.90 are indicative of poor, moderate, good, and excellent reliability, respectively [7]. The level of statistical significance was set to p=0.05. The SPSS software (version 26.0.0.0) was used for statistical analysis

For the purpose of meta-analysis, we implemented the PRISMA 2020 statement guidelines [8]. We used the search term “accessory mental foramen” in PubMed, Scopus, and Google Scholar databases to detect all publications reporting prevalence and morphology of AMF in dry mandibles. Inclusion and exclusion criteria followed the PICOTS framework (Table 1).

Quality assessment was performed with the Johanna Briggs Institute (JBI) Critical Appraisal Checklist for analytical cross-sectional studies tool [9].

All data regarding author, year of publication, origin of population studied, sample size, number of cases presenting AMF, number of cases presenting unilateral AMF, number of cases presenting bilateral AMF, number of total AMFs detected, number of AMFs detected on the right side, number of AMFs detected on the left side, number of single AMFs, number of multiple AMFs, number of round-shaped AMFs, number of oval-shaped AMFs, diameter in mm (if round) or vertical axis in mm (if oval), distance from MF in mm, distance from alveolar ridge in mm, and distance from mandible lower border were recorded. Quantitative synthesis were performed in case that data were available from ≥5 sources.

All steps including literature search, records screening, and data extraction were performed independently by two authors (Z-M.T., and C.T.). In case of dispute, a third author (V.P.) was responsible for the final judgment.

Data were synthesized using STATA 18 Statistical Software version. Prevalence data were combined using the random-effects REML model after Freeman-Tuckey double arcsine transformation. Combination of means and standard deviations (SDs) were performed using the freely available online tool located at https://www.statstodo.com/CombineMeansSDs.php.

Means and estimates based on sample size, median, range, and interquartile range were calculated using the freely available online tool located at https://www.math.hkbu.edu.hk/~tongt/papers/median2mean.html [10-12].

Heterogeneity was approached using the Q test and I² statistic; Q test P value < 0.10 was indicative of a statistically significant result. Furthermore, a value of I²≤ 25% was indicative of insignificant heterogeneity, 26%-50% of low heterogeneity, 51%-75% of moderate heterogeneity, and > 75% of high heterogeneity [13,14]. Sensitivity analysis was performed using the leave-one-out procedure to investigate the effect of each included study on heterogeneity.

3. Results

3.1. Our data

According to our data, the MF was present in all observed mandibles and it was bilateral. Nine AMFs were found in 7 mandibles (5 single, 2 double), all unilateral; 5 AMFs were located at the right hemimandible and 4 at the left (Figure 2).

As far as the shape of AMFs is concerned, 5 were round and 4 oval (Figure 3).

As far as the location of the AMFs is concerned, 6 AMFs were located posterior and 3 anterior to MF (Figure 4).

Additionally, four AMFs were located inferior to the MF, three superior to it, and two at the same horizontal level as the MF (Figure 5).

AMFs presented 0.96±0.43 mm mean diameter, ranging from 0.40 to 1.70 mm (Figure 6).

Additionally, AMFs presented 4.12±2.15 mm mean distance from MF, ranging from 3.20 to 17.72 mm (Figure 7).

Of note, AMFs were measured to be located 12.68 ± 4.10 mm from alveolar ridge, and 11.92 ± 1.57 mm from mandible lower border (Table 2).

Lastly, in one mandible, the AMF was located at the same horizontal level as the MF but did not fall within any quadrant (Table 3).

Considerable interrater agreement was observed for all measurements (Table 4).

3.2. Meta-analysis

For the purpose of the meta-analysis, 252 publications were identified from search in PubMed and Scopus databases; ten additional publications were retrieved through citation searching (Figure 8).

After screening, 27 publications were included in qualitative review and in quantitative synthesis, along with data obtained from the present observational study (Table 5; Table 6).

The quality assessment is presented in detail as depicted in Table 7. In detail, 8 domains were assessed using respective questions (Q1-Q8), as follows: Q1: “Were the criteria for inclusion in the sample clearly defined?”; Q2: “Were the study subjects and the setting described in detail?”; Q3: “Was the exposure measured in a valid and reliable way?”; Q4: “Were objective, standard criteria used for measurement of the condition?”; Q5: “Were confounding factors identified?”; Q6: “Were strategies to deal with confounding factors stated?”; Q7: “Were the outcomes measured in a valid and reliable way?”; Q8: “Was appropriate statistical analysis used?”. The response options were recorded as “Yes” (Y), “No” (N), “Unclear (U)”, and “Not Applicable” (NA).

The combined AMF prevalence was 6.1% (95% CI: 4.8% - 7.6%; I² 60%) (Figure 9).

AMF were detected in 48.2% (95% CI: 35.2% - 61.2%; I² 50%) on the right side (Figure 10), and in 51.8% (95% CI: 38.8% - 64.8%; I² 50%) on the left side (Figure 11); no difference was documented as CIs referring to both sides overlapped.

The combined mean vertical axis was 1.18 ± 0.61 mm, and the combined mean distance from MF was 3.64 ± 2.29 mm. Bilateral AMFs were detected in 2.1% of AMF cases (95% CI: 0.0% - 7.3%; I² 18%) (Figure 12).

Oval shape was described in 37.0% of AMFs (95% CI: 5.1% - 75.8%; I² 83%) (Figure 13).

All but one study were close to the 95% confidence intervals of the Galbraith plot (Figure 14).

No significant publication bias was detected; the funnel plot was symmetrical by visual inspection. Moreover, no additional study was imputed using the trim-and-fill method on either side (Figure 15); both Egger’s and Begg’s test yielded a non-significant result (p=0.914, and 0.234, respectively).

The leave-one-out sensitivity analysis revealed no critical effect on combined mean prevalence due to single publications (Figure 16).

4. Discussion

This observational study investigates the location, shape, and dimensions of AMFs in dry adult human mandibles and, apart from the presentation of our own data, includes a systematic review and meta-analysis of worldwide data. Notably, this is the first meta-analysis focusing on AMF data obtained exclusively from dry mandibles. In contrast, three previously published meta-analyses primarily rely on CBCT data [41-43].

Our data is in keeping with the results of the consequent meta-analysis, thus rendering them credible and representative. Of note, we reported that the combined AMF prevalence is 6.1% (95% CI: 4.8% - 7.6%), and that there is no difference between right and left side. Moreover, AMFs were mostly rounded, while oval shape was described in 37.0% of cases; the combined mean AMF vertical axis was 1.18 ± 0.61 mm, and the combined mean distance from MF was 3.64 ± 2.29 mm. Lastly, AMFs were detected bilaterally in 2.1% of cases. The heterogeneity of our results, measured as I², ranged from unimportant to considerable, reaching 83% in case of AMF shape determination.

In this study, we assessed AMF locations by measuring distances from the AMFs to both the alveolar crest and the inferior border of the mandible. The latter measurement is considered more reliable, as it remains consistent regardless of whether the mandible is dentate or edentulous. In contrast, the distance between the AMF and the alveolar ridge can change due to resorption of the alveolar ridge with age [3].

The presence of AMF is a critical consideration in surgical procedures involving the mental region. These additional foramina house nerves and blood vessels that, if damaged during surgery, can result in complications such as lower lip numbness or bleeding. Consequently, understanding the location and content of AMFs is essential for planning and executing safe and effective surgical interventions. In their hallmark paper, Rahpeyma and Khajehahmadi have highlighted the impact of AMFs on the outcomes of various surgical procedures, including orthognathic, periapical, implant, inferior alveolar nerve repositioning, and trauma surgeries. This underscores the importance of incorporating AMF considerations into surgical planning to minimize complications and optimize patient outcomes [44].

The influence of AMFs on local anesthesia is another important aspect to consider. Their presence can affect the success of mental nerve blocks. If an AMF is located near the main mental foramen, a single anesthetic injection into the main foramen may suffice, as the anesthetic can diffuse into the AMF. However, if an AMF is located farther from the main foramen, an additional injection may be necessary to ensure effective anesthesia. Recognizing these anatomical variations allows dental professionals and anesthesiologists to tailor their techniques, ensuring patient comfort and procedural success.

The major strength of this study is its integration of primary data from the well-established osteological collection of the Anatomy Department at Democritus University of Thrace, Greece, with data from a systematic review and meta-analysis. To our knowledge, this is the first study of its kind. However, a certain limitation of the study is that the total heterogeneity, mostly attributable to ethnic-related variation, could not be further assessed, due to the lack of gender-specific information. Future research incorporating subgroup analyses or meta-regression could explore potential gender-related differences in AMF anatomy.

5. Conclusions

Dental surgeons must have a comprehensive understanding of the anatomical variations in the mental foramen (MF) and the potential presence of accessory mental foramina (AMFs). This knowledge is essential to prevent or mitigate the risk of postoperative complications such as pain, anesthesia, injury, and other adverse outcomes. By incorporating this anatomical insight into surgical and anesthetic planning, clinicians can enhance patient safety and procedural success.