Gr os s A na to m y Position and number of mental foramen in dry human mandibles : Comparison with respect to sides and sexes

Abstract Introduction Mental foramen of mandible has been found to vary in its position, number and distance from the surrounding bony landmarks. This variation occurs with populations as well as with factors like ethnicity, age, sex and sides. Knowledge of this variation is important for localisation of the mental nerve during surgeries in this region. This study was undertaken to document the variations in number and location of mental foramen with sexes and sides in dry mandibles of Indian origin. Materials and methods The sample consisted of 83 dried mandibles of known sex, 53 males and 30 females. The position of mental foramens was measured in relation to mandibular teeth and its distance from three bony landmarks—mental symphysis, alveolar border and lower border of mandible. Incidence of accessory mental foramen was noted. Results The most common position of mental foramen was at the apex of the second premolar (48.8%) and the next was between the apices of the first and second premolars (30.1%), this pattern being the same for both sides of both sexes with no statistical significance. Accessory mental foramen was found in 7.22% sides, bilaterally in 4.81% and was more common in males and in the right side. Mean lower border, alveolar border and mental symphysis were 12.60 ± 1.63, 12.39 ± 2.32 and 26.47 ± 2.54 mm respectively, with all values being significantly higher for males; differences between the sides were statistically insignificant. Conclusion The current study provides insight into the location and number of mental foramen in Indians and explains the discrepancy seen among findings of earlier studies.


Introduction
The mental foramen (MF), from which the mental neurovascular bundle emerges, lies below either the interval between the premolar teeth or the second premolar tooth, midway between the upper and lower borders of the body.The posterior border of the foramen is smooth and accommodates the nerve as it emerges posterolaterally 1 .
The orientation and position of the MF is important for performing anaesthetic block prior to clinical procedures in lower anterior teeth and also for preserving the integrity of the mental nerve trunk in surgical interventions 2,3 .The mental nerve injury can cause transitory or permanent sensitive, thermal and tactile changes 4,5 .Accessory mental foramen (AMF) containing a branch of mental nerve has been reported to cause mandibular contour asymmetry 6 .The situation of MF is an important factor in enabling the surgeon to plan the extent of bone removal in cancer of the floor of the mouth encroaching on the jaw 7 .
The MF morphology, in terms of modal position and number, varies not only according to age, sex and ethnicity but even within the same race, in different geographic regions and within the inhabitants of the same geographic area 8 .Standard anatomy and radiology textbooks contain conflicting statements regarding the position of MF.There is also similar disagreement among authors of various published investigations 9 .Few studies that are available on documentation of MF in the Indian population make comparisons among sides [10][11][12][13][14][15][16] , but studies concerning difference in characteristics of MF with respect to sex for this particular population are indeed scarce in literature; the only one that is available is a study done on radiographs 17 .Hence, the current study was undertaken to document the position and number of MF in dry mandibles belonging to Indian population and to study its variations between sexes and with sides.

Materials and methods
A total of 166 sides of 83 mandibles were studied from collections of the Department of Anatomy, Government Medical College, Aurangabad, India.These mandibles were derived from cadavers of Indian origin from the dissection hall of the same department.All mandibles were intact, of known sex, with sex and serial number marked on them; 53 mandibles were of males and 30 of females.For statistical analysis, 166 sides of mandibles were divided into four subsets-right male, left male, right female and left female.Age of individual before death and cause of death were unknown, but completely edentulous mandibles and mandibles with absence of the third molar or socket for third molar were excluded from the study because they were considered to be of extreme ages.

Results
For the entire sample, MFL was found to be 12.60 ± 1.6324 mm with 48 (28.9%) sides, with values between 12 and 13 mm; mean MFU was 12.39 ± 2.3251 mm with 59 (35.5%) sides between 12 and 14 mm; and mean MFM 27.47 ± 2.5405 mm with 116 (69.8%) sides between 25 and 30 mm.Mean values and values in 95% confidence interval for individual subsets are presented in Table 1.
The most frequent location of MF was at position PM2 (47.8%), followed by PM1PM2 (30.12%); this was true for all four subsets.None of the MF was found at position CPM1 and PM1PM2.Side-and sexwise position distribution of MF are presented in Table 2.
AMFs were present in a total of 12 sides (7.22%) and bilaterally in four mandibles (4.81%); see Figure 2 for representative AMF.AMFs were present most frequently on the right sides of males (six sides or 11.3%) followed by left side of males (five sides or 9.4%) and right side of females (one side or 3.3%) with no AMF on left sides of females.None of the mandibles were found with more than one AMF or absent MF.

Discussion
Comparing males with females, Students' t test revealed that the difference in the values was statistically significant for all parameters except the number of MF.However, the values were statistically insignificant when the left and right sides were compared (Table 3).

Position of mental foramen
The most frequent position of MF in relation to mandibular teeth was PM2, which is also true for all four individual subsets for left sides of female in whom 43% each of MF were present at PM2 and PM1PM2.In fact, PM2 has been shown to be the most common position of MF in almost all earlier studies done on various populations with few exceptions; see Table 4 for earlier a straight wire vertically, adjacent to the MF.It was documented as being in line with between the canine and first premolar (CPM1), below the first premolar (PM1), between the first and second premolars (PM1PM2), below the second premolar and the last one between the second premolar and first molar (PM2M1); numbering was not used to avoid confusion arising by the use of different numbering systems by various authors.
Data were analysed for central tendency calculating mean, standard deviation, range and values in 95% confidence interval.Students' t test was applied to find the significance of statistical difference between sides and sexes; p values less than 0.05 were considered statistically significant.
Distance of margins of MF was measured bilaterally from three landmarks, using digital calipers with an accuracy of 0.01 mm.The bony landmarks used were mental symphysis, alveolar margin and lower border of the body of the mandible; the distance of these landmarks from MF was designated as MFM, MFU and MFL, respectively (Figure 1).In case of presence of AMF, measurements were taken from the larger or principal mental foramen.The incidence and number of AMF were documented with respect to sides and sex.
Location of MF in relation to mandibular teeth was noted by placing the mandible in a firm horizontal surface with maximum contact between its lower border and surface and holding Asian Indians 19 , 5% in Chinese 20 and 6.5% in Turks with 0.5% bilateral 21 .In the Indian population, it has been reported to be variable as 2.6% 11 -8% of left side, 5% in right side and nil bilaterally 10 .In Brazilians, AMFs were present in 7.5% of right and 3.8% of left sides 22 .In a radiological study of the Malaya population, the incidence of unilaterally absent mental foramen was reported to be 4.7% 23 , but this could be fallacious as the MF may not appear in conventional radiographs 24 .In an American population, 1% of each of the sides of the mandibles had double and triple mental foramina 25 and in a Sri Lankan sample two and three foramina were found in 1.96% of each side 26 ; no more than two foramina were found in our sample.In our population, no mandibles were found with absent mental foramen.In a study with a large sample of 1435 dry mandibles, MF was absent in two mandibles in the right side and one in the left side, with no bilaterally absent MF.Bilateral absence of MF is indeed very rare, reported only as case reports 27 .

Distance of mental foramen from bony landmarks
Values for all measurements were higher for males than for females; similarly values were marginally high on the right side when compared with left sides (see Table 1).
Mean MFM value was higher for males than females (p = 0.0012) and on the right side than the left side (p = 0.4021).This is in accordance with a study in a Brazilian sample 5 .In contrast, in Sri Lankans, MFM was higher for females and on the right side 26 ; in both these studies, the differences were statistically insignificant.MFM was found to be greater in males for both American Whites and blacks with p less than 0.05 3 .Three Indian studies each show values of MFM to be higher in the right side 11,13,15 and left side 10,12,16 ; all six studies prove no statistically

Incidence of accessory mental foramina
In the present study, AMFs were more common on males and on the right side.Incidence of AMF was 8.33% in Sri Lankans, 6.62% in studies in the Indian population and ref. [ 18 ] for studies in other populations.No statistically significant difference was observed in position of MF with sides or sexes, again in accordance with all studies conducted so far.and higher in the left side by one 12 .Mean MFL values of other studies are summarised in Table 5.
Mean MFU values were higher in the right side for males and higher in the left side for females; difference in sides was statistically insignificant and in sexes significant.In earlier population, this has been reported to be 14.20 mm for both sides of both sexes with no statistical difference between sexes or sides; the widest variation was in the right side of female mandibles 5 .In an Indian population, it has been found to be higher in the right side by some authors 10,11,13,15,16 significant difference.Mean MFM values of other studies are summarised in Table 5.
Mean MFL value was slightly higher on the left side for males and right side for females, the difference being statistically significant for sexes but not for sides (see Table 2).In a Brazilian Earlier studies done on an Indian population compare the position and number of MF with respect to sides alone, and their results tend to contrast each other.This discrepancy in the results can be explained partially on the basis that they do not take into account the sexual difference in position of MF.The current study tries to fulfil this gap by providing the evidence that there exists a statistically significant difference between sexes in number and positions of MF in mandibles belonging to Indian origin; differences in sides for the same are statistically insignificant.

Conclusion
Incidence of AMF and the position of mental foramina in relation to mandibular teeth and other bony landmarks are crucial for a number of surgical and dental procedures.They vary in different populations of the world as well as with age, sex, sides studies on an Indian population, it has been shown to be higher on the right side by some authors 11,15 and higher in the left side by others 10,12,16 .No study could be found comparing between sexes.Mean MFU values of other studies are summarised in Table 5.

Figure 1 :
Figure 1: Distance of mental foramen from bony landmarks, MFM from mental symphysis, MFU from alveolar border and MFL from lower border of mandible.

Figure 2 :
Figure 2: Figure 2a to 2d represent unilateral AMF in M 32 and bilateral AMF in M 57.Arrows at AMF

Table 2 Position of MF with sides and sexes MF location Total sides Male left Male right Female left Female right
PM1, below the first premolar; PM1PM2, between the first and second premolars; PM2M1, below the second premolar and the last one between the second premolar and first molar

Table 4 Position of mental foramina in relation to mandibular teeth in Indian populations Author and population Sample size Position of mental foramen in percentage
Study on radiographs.CPM1, between the canine and first premolar; PM1, below the first premolar; PM1PM2, between the first and second premolars; PM2M1, below the second premolar and the last one between the second premolar and first molar. *

Table 5 Mean MFM, MFL and MFU in millimetres as seen in various populations Author and population Sample size MFM MFL MFU
**Study in CT scans.MF, mental foramen.