Anthropometrics Analysis of Mental Foramen and Accessory Mental Foramen in Zambian Adult Human Mandibles

The mental foramen (MF) and accessory mental foramen (AMF) are the strategically important landmarks during surgical interventions and anaesthetic nerve blocks procedures involving the mental nerve. The study aimed at evaluating anthropometrics of MF and AMF in Zambian adult human mandibles and it was cleared for ethics from TDRC Ethics Review Committee (Reg. No.: 00002911; FWA: 00003729). A total of 33 Zambian adult human mandibles were evaluated for shape, position, and direction of opening of foramen. All measurements were performed using a Digital Vernier Calliper and statistically analysed for per cent frequency and mean and standard deviations, and we performed the one sample t-test for comparative analysis. Data were considered significant at p<0.05. All mandibles that were examined had bilateral MF while unilateral AMF was found in two mandibles (6%). The foramens were mostly oval in shape and their most common position was between the second premolar and first molar and the most common orientation was posterior-superior. The comparative analysis of mandibular anthropometrics showed significant variations (p<0.05) with different ethnic groups. The findings emphasize the ethnic variations and edify that the foramen position is not always as stated in reference textbooks. The clinical creditability of the study is cautioning the surgeons on possible variations of the MF and AMF anthropometrics compared to existing literature in order to avoid any unforeseen injury related to anaesthesia or dental surgeries. Further studies with large sample sizes representing whole country are recommended to establish the standard MF and AMF anthropometrics of Zambian population.


Introduction
The mental foramen (MF) is an oval or circular opening on the body of the mandible where the mandibular canal terminates. It is an exit for the mental nerve and blood vessels, which are terminal branches of inferior alveolar nerve, artery, and vein. The mental nerve provides innervations of the lower teeth, lip, gingival, and lower face [1,2]. The MF is an important anatomical landmark during osteotomy procedures, anesthetic nerve blocks, and prevention of neurovascular complications after invasive procedures on the lower jaw. Its anatomy is also useful in evaluating the morphometric symmetry of the mental triangle, microscopic and macroscopic morphology, bone remodelling activity, and paleoanthropologic features of the facial skeleton in different populations [3].
The MF is usually located in the body of the mandible at an equal distance from the superior and inferior border below or between the apex of the first and second premolar [2,4,5]. The direction of opening of the foramen from the inferior alveolar has been shown to be pointing posteriorly outward and upward [4,6]. Variability in the location of MF has been documented in different literature with the tendency of being more posterior in blacks than in whites and between the 2 The Scientific World Journal second premolar and first molar [7]. A study on Tanzanian population revealed that the most frequent locations for MF were below the apex of the second premolar and between the 2 nd premolar and 1 st molar. The MF was asymmetrically located between the right and left sides and predominantly oval. The direction of opening was mostly superior and posterosuperior and rarely labial, mesial, or posterior [4]. Another study on Zimbabwean population found that the MF was mostly oval shaped and the frequency of occurrence was highest below the lower 2 nd premolar on the right side and between 2 nd premolar and 1 st molar on the left side [8]. In a study done on Malawian population, the MF was found to be oval in shape, oriented posterosuperiorly, and located inferior to the 2 nd premolar tooth and bilaterally symmetrical in a majority of cases. Its vertical position was slightly below the midpoint of the distance between the lower border of the mandible and the alveolar margin [9].
Any foramen in addition to MF in the body of mandible is known as accessory mental foramen (AMF) and it tends to exist in the apical area of the first molar and posterior or inferior area of the mental foramen. As AMF is due to branching of mental nerve before passing through MF, its shape, size, and verification of its existence would prevent accessory nerve injury during periapical surgery [2,10]. The potential severe complication of injury of the accessory mental foramen (AMF) is sensory disturbance of the lower lip [11]. Studies [12,13] have reported AMF incidence to range from 1.4% to 9.7% with an exception of one on Japanese population, which reported very high incidence of 12.5% [14]. The distances between MF and AMF were reported to range between 0.67mm and 5.74mm [14]. Ethnic variations in relation to AMF have also been reported [12]. Absent AMF are a more common variation than MF absence in humans and the frequent reasons for absence may range from atrophy, posttraumatic fibrosis, osteoblastic hyperplasia, geriatric bony resorption, or congenital agenesis [3].
Hence location, size, shape, position, and incidence of MF and AMF would facilitate the dental surgeon to apply nerve block in different surgical procedures involving lower jaw. The anatomical research reports on variations in anthropometrics of MF and AMF between race and geographical location signifying the need to establish local values. Currently there are no established values on MF and AMF for Zambian population. Therefore, this study is aimed at evaluating anthropometrics of MF and AMF in Zambian adult human mandibles to establish specific MF and AMF anthropometrics of Zambian population in order to make a gateway to add them to the medical literature.

Study Samples.
A total of 33 Zambian adult cadavers out of 35 availed in the Anatomy Laboratory of Michael Chilufya Sata School of Medicine (MCS SoM), which met the inclusion criteria, that is, dentate adult mandibles, were considered for the study. The cadavers used in the study were legally permitted for the use of education and research purposed at the MCS SoM and the appropriate permission was obtained from the Dean, MCS SoM for their use in the study.

Ethical Compliance.
Ethical clearance for this anatomical study (Approval Reference No. TRC/C4/04/2017) was sought from TDRC Ethics Review Committee (Reg. No.: 00002911; FWA: 00003729), Ndola, Zambia. Handling of cadavers was done only in Anatomy laboratory dissecting hall and no body parts were moved out of the dissecting hall. The cadaver was treated with respect as to the respect given to human life. The information about the cadaver in terms of names or names of the relatives was not sought or collected and only information about age, sex, and race was used for the study.

Preparation of Mandibles.
The mandibles were disarticulated, cleaned, and then rinsed in 70% ethanol solution and allowed to dry for 24hrs. The mandibles were chosen according to the following criteria: (1) As a minimum, all mandibular teeth from right first molar to left first molar were present (2) All mandibular teeth from right first molar to left first molar were in a reasonably normal position and alignment  The relative position of MF/AMF was found by subtracting the average distance from symphysis menti to medial margin of MF/AMF (WY) with length of the mandibles, i.e., ratio of WY/WX.

Statistical Analysis.
Statistical analysis was done using IBM-SPSS software version 20. The most prevalent shape, position, and orientations were found by simple frequency analysis. The MF and AMF diameters for size and distances from defined positions were measured using a Digital Vernier Calliper (in mm) and statistically analysed for per cent frequency and mean and standard deviations (SD). The comparative analyses of mandibular anthropometrics from defined positions were performed using a one-sample t-test. Data were considered statistically significant at p<0.05.

Results
A total of 33 Zambian adult human mandibles of known sex that included 31 male mandibles and 2 female mandibles were evaluated for MF and AMF anthropometrics measurements.
Since the proportion of female to male samples was too low (6%), the data were not analysed and stratified by sex. The frequency distribution of MF and AMF and their shapes in Zambian adult human mandibles are shown in Table 1. The study found one mandible (n=33, 3%) with AMF on the right side (Figure 3(a)) and another (n=33, 3%) with AMF on the left side ( Figure 3(b)) along with bilateral MF, and the rest were only with bilateral MF. The AMF/MF ratio was 3% (n=33) on both sides of mandibles. The shapes of the MF and AMF were mainly oval on both sides of mandibles. The dimensions of the MF and AMF (Table 2) on the mandibles were determined by measuring their horizontal diameters (HD) and vertical diameters (VD). The average HD of the MF was 3.6±0.9mm on the right side and 3.8±0.9mm on the left side. The average VD of the MF was 2.8±0.7mm on the right side and 3.2±1.1mm on the left side. The HD and VD of AMF found on the left side were 1.9mm and 0.8mm, respectively, while for another one found on the right side they were 1.3mm and 2.1mm, respectively.
The mandibular anthropometric measurements (the distance of MF and AMF from defined landmarks) on Zambian adult human mandibles (n=33) are presented in Figure 5. The MF were positioned at mean AC of 15.5±2.9mm and mean BD of 13.7±1.5mm, likewise the AMF at AC and BD of 16.1mm and 13.5mm, respectively, on the left side. The mean AC and BD were 15.9±2.9mm and 13.9±1.7mm, respectively, for MF
The study also found that the overall mean dimension of MF was 3.35mm, while it was 3.18mm and 3.79mm in the Zimbabwean [8] and Malawian [9] adult populations, respectively. The observed average HD of the MF was 3.7mm, while it was ranging from 3.26mm to 3.41mm in the Asian populations [19,24,25]. It is also important to note that a wider HD of 5.16mm [26] and a lower HD of 2.69mm [10] were also reported. The average VD of the MF observed in the study was 3mm, while it was ranging from 2.13mm to 2.61mm in the Asian populations [19,[24][25][26]. The mean dimension of AMF found in the study was 1.5mm, while it was 0.5 -1mm in Indian adult population [10,23].
The position of the MF varies considerably by geographic location and nationality [9,10,15,17,19]. The knowledge on the position of MF to specific ethnic group is very important especially during implantology so that one can avoid putting implant in the MF. The most common position of MF was position 5 that is between the second premolar and first molar and the next common position was position 4 that is in line with second premolar, while the position of AMF on both cases were position 5. Positions 1 and 2 were not observed in this study and this is in line with the report that the position of the MF was more posterior in blacks than whites [22]. The comparative study of position of MF in relation to mandibular teeth with selected reports of different ethnic groups revealed significant differences among ethnic groups on different geographic locations [9, 10, 15-17, 19, 21]. Therefore, the variability of the position of the MF should be considered when undertaking periodontal or endodontic surgery in the area from the canine to the mesial root of the first molar. Though caution has to be taken, it does not exclude the need of radiographs prior to any periodontal or endodontic surgery.
Previous studies on mandibles of different ethnic groups [4,9,16,26] revealed that the direction of opening (orientation) of the MF was mainly posterior-superior. Similar result was observed in the present study where the most common direction of opening (orientation) of MF was posteriorsuperior (46%) and the next common was labial (31.8%), while it was posterior and posterior-superior, respectively, for the AMF found on the left and right sides of mandibles.
According to the anthropometric measurements in the study, the MF was found to be at an equal distance on both sides of the mandible but it was not true in case of AMF. The mean anterior chord (WY) and posterior chord (XZ) obtained were 28.5mm and 73.4mm, respectively, for MF on both sides of mandibles, while they were 32.6mm and 71.3mm on the left side and 27.9mm and 75.5mm on the right side for AMF. The mean length of mandibles was 101.9mm. It was also noted that the mean position of MF was found 13.86mm above the lower border of the mandible and 15.66mm below the alveolar ridge on both sides of mandibles but the AMF was found 10.9mm and 13.5mm above the lower border of the mandible and 22.5mm and 16.1mm below the alveolar ridge, respectively, on the left and right sides. The relative position of the MF and AMF in the mandible was obtained by calculating the ratio of WY/WX and it was 0.28 for MF on both sides of the mandible but it was 0.31 on the left side and 0.27 on the right side for AMF. Previous studies [9,18] reported that the MF lies approximately below one-fourth of the distance from the symphysis menti to the posterior border of the mandible on the left side, which is slightly smaller than the calculated value of the present study.
A comparative study on mandibular anthropometrics of selected reports of different ethnic groups [9, 10, 15-18, 20, 21] revealed significant variation among different ethnic groups on different geographic locations. In most cases, the mean distances of the MF from defined landmarks on the mandibles, length of mandibles, and relative position of MF on mandibles were significantly differed among different ethnic groups from different geographic locations. The relative position of the MF was ranging from 0.26 to 0.29 on the right side of mandibles and from 0.23 to 0.29 on the left side of mandibles. It was slightly higher on the right of mandibles and slightly lower on the left of mandibles for adult Asians while it was slightly lower on the right of mandibles for adult Europeans when compared to adult Africans. Though the lengths of mandibles on right side are similar among Malawians (also on left side), Zambians, and Indians, the MF lies further forward in adult Malawians and backward in adult Indians when compared to adult Zambians. In contrast, the lengths of mandibles on right side are significantly different among Turkish, Chinese, and Zambians but the relative position of the MF was in agreement. Hence, this study evidences for wide variation of MF and AMF anthropometrics of adult Zambians compared to other geographic locations and ethnicities.

Conclusion
As a preliminary attempt, the study has established specific MF and AMF anthropometrics for adult humans in Copperbelt province of Zambia. There are variations in the MF characteristics of studied Zambian mandibles compared to established values worldwide and within the region. The results of the present study highlight the racial differences in the most common position and relative position of the MF observed among different populations in the different geographic locations. The clinical creditability of the study is cautioning the surgeons on possible variations of the MF and AMF anthropometrics compared to existing literatures in order to avoid any unforeseen injury related to anaesthesia or dental surgeries. Although the study has the limitations that the findings are based on a small sample size, which is also having too low female to male proportion which was due to challenges in acquiring cadavers, the data was normally distributed and showed normal variations so the information on anthropometrics of MF and AMF presented in this article can help anatomists, prosthodontists, orthodontists, surgeons, forensic odontologists, and paleoanthropologists to predict the position of the MF and AMF and perform both diagnostics and safer clinical procedures led on the mandibles of Zambian population. However, it is recommended for further studies with large sample sizes representing whole country to establish the standard MF and AMF anthropometrics of Zambians.

Data Availability
The data used to support the findings of this study are included within the article.

Conflicts of Interest
The authors have no conflicts of interest to declare.

Authors' Contributions
Severine N. Anthony, Lumamba Mubbunu, and Majuto S. Mlawa have contributed during proposal writing. All authors contributed during anthropometric measurements, analysis of result, and preparation of manuscript.