Medical literature is not unified in the description of the morphology of the mental foramen. Different anatomy and radiology text books give contradicting statements regarding the morphometric characteristics of the mental foramen; thereby depicting variable racial trends. ⁶

Africans: The mental foramen was observed to exhibit dimorphism; it was 14.89 mm above the lower border of the mandible in males and 14.21 mm in females. Also, it was 16.16 mm below the alveolar ridge in males and 15.66 mm in females. The average size of the long and the short axis of the foramen were 5.66mm and 3.97mm respectively in the male and 4.99 mm and 3.87 mm respectively in the female mandibles.]12These measurements were statistically similar to another study on mandibles from Alagoas state.¹⁸

Tanzanians: The mental foramen was frequently located below the apex of the second premolar and between the 2 ^nd premolar and 1st molar. A significantly less common location was between 1 ^st and 2 ^nd premolars and below 1 ^st molar. The mental foramen was a-symmetrically located between the right and left sides and predominantly oval. The direction of opening was mostly superior and postero-superior and rarely labial, mesial or posterior. ¹⁹

Mongoloids: The mental foramen was observed to be located in line with the longitudinal axis of the lower second premolar.⁵

Caucasoids: It was more medially located, between first and second premolars.

Blacks: It was found to be placed more posterior in blacks than in whites; between the second premolar and first molar.^5,20

Byzantiums: The most common position of MF was between the first and second premolars on the right side, whereas on the left side it was in line with the lower second premolar. ²¹

Japanese: The MF was observed to lie at an average height of 12.96 mm from the inferior edge of mandible. The largest horizontal diameter ranged between 3.25-3.32mm whereas the vertical diameter ranged from 2.38-2.39mm between the right and left sides. The MF was located in similar statistic proportions between the 1st and 2nd premolars and below the 2nd premolar on the right side. On the left side, it was mostly between the 1st and 2nd premolars. The MF was predominantly single and oval with larger horizontal diameter. ¹⁸

Sri Lankans: The most common MF position was in line with the longitudinal axis of the lower second premolar followed by a position between first and second premolar .The mean transverse and vertical diameters of the foramen were 3.31 and 2.5mm, respectively. The mental foramen was located 24.87 mm (right side) and 24.77 mm (left side) lateral to the symphysis menti. In a majority of cases, the mental foramen was oval in shape and its usual direction of opening was postero-superior. The incidence of multiple mental foramina was 3.92%. ²²

Israelis: The mental foramen was found to be predominantly single and oval with an average long diameter of 2.37 mm. The common location was between the roots of 1 ^st and 2 ^nd premolar.¹¹

Zimbabweans: The mental foramen was mostly oval shaped. The frequency of occurrence of the mental foramen was highest below the lower second premolar on the right side and between 2 ^nd premolar and 1 ^st molar on the left side. In a vertical plane, the mental foramen was predominantly located under the midpoint of the line joining the lower border of the mandible to alveolar ridge. ²³

Malawians: The mental foramen was seen to be oval in shape, oriented postero superiorly, located inferior to the second 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. ²⁴

Turkish: The distance of the mental foramen from the lower border of the mandible was noted to be 14.61mm and 14.29 mm on the right and the left sides, respectively. ²⁵The distance from the upper border was 13.6 mm and 14.62 mm on the right and left sides. The horizontal diameter of the mental foramen was 2.93 mm on the right side and 3.14 mm on the left side. The vertical diameter was 2.38 mm and 2.64 mm on the right and left sides respectively. The mental foramen was found under the root of the second premolar in most mandibles. The average inclination was 96.82 degrees which was in accordance with the findings of another study done on dry mandibles from Alagoas State.¹⁸

Jordanians: The mental foramen was seen most commonly below and between the mandibular premolar teeth, and the most frequent appearance was the continuous type; showing similarity to Caucasian populations. ²⁶

The location of the mental foramen in relation to chronological age

A lot of research done in the past highlights the genetic and racial influence on mental foramen variations. In 1970; Kadanoff et al demonstrated the variations in the number of mental foramina ranging from being absent to double or triple; equal or unequal sized; close or widely separate or even multiple; in the form of a cluster.²⁷ Identifying these variations by a prior digital scan may reduce the incidence of post procedural paralytic and hemorrhagic complications in the mental region, thereby significantly reducing morbidity.

Unlike non human primates, the mental foramen is usually single in humans, but accessory foramina have been recorded. It has been suggested that separation of the mental nerve into several fasciculi earlier than the formation of the mental foramen until the 12th gestational week could be a reason for the formation of accessory mental foramen.²⁸

The incidence of accessory foramina varies between ethnic groups, while non-Caucasians may have a higher incidence than Caucasians; there are yet no reported gender differences.^28,29 The incidence of accessory mental foramen among various ethnic groups, is reported as follows: 2.6% in French; 1.4% in American Whites; 5.7% in American Blacks; 3.3% in Greeks; 1.5% in Russians; 3.0% in Hungarians; 9.7% in Melanesians; and 3.6% in Egyptians .⁷ In the Japanese population, the incidence was found to be 6.7-12.5%, which is rather high as compared to other ethnic groups.²⁹ The multiple foramina are usually unequal in size: a single large foramen with smaller (satellite) foramina.³⁰ Danny.R. , in 1998, demonstrated AMF to be more frequent in Nazca, African Americans and less common in Asian Indians and American Whites. The incidence occurrence was equal for right- and left sides. However, in certain ethnic groups like African Americans, AMF were reported to be more frequent in males, though this observation may not be a representative of all populations. ³¹

The presence of nerve fibers in accessory foramina may be significant in the effectiveness of local anesthesia following a routine inferior alveolar nerve block.^32,33 Blood vessels present in accessory foramina can be a cause of intraosseous hemorrhages during implant procedures. ^34,35 The possible role of accessory foramina and their neurovascular contents in the spread of tumors of this region has also been recognized.^36-39

Absent mental foramen is an extremely rare anatomic variation. Infact, man is the only primate known to have agenesis of the mental foramen. Accessory mental foramina are a more common variation than MF absence in humans.⁴⁰ The frequent reasons for absence may range from atrophy, post traumatic fibrosis, osteoblastic hyperplasia, geriatric bony resorption or congenital agenesis.⁴⁰

Medical literature is very scant when it comes to reports of absent mental foramen. Till now, published records state only ‘four cases’ ^8,41,42,43 of unilateral absence and a ‘single’ case of bilateral absence⁴⁰. Unilateral absence of the mental foramen is very rare; the frequency ranges from less than 0.02% ⁹ to 0.47% ⁸ ; while the frequency of bilateral absence is statistically negligible.⁴⁰ The incidence of absence has been equally reported between Caucasians and Blacks; and no particular genetic or racial trend has been observed regarding this phenomenon.⁴⁰

Since the mental foramen marks the exit of mental nerves and vessels (the sensory supply of the lower lip-chin area), absence of MF would mean no outlet for this neurovascular bundle and probable sensory alterations in that area. Clinical implications would include ineffective mental block in anesthesia. Moreover, the subjects could possibly present with some neurosensory disturbance in the mental region or around lips due to this rare condition.⁴⁰

Detection is possible by macroscopic inspection of dry mandibles, plane or periapical radiography or computed tomography scans. However, radiographic localization is difficult due to lack of consistent anatomical landmarks for reference, and the foramen cannot be clinically visualized or manually palpated. The foramen may not appear on conventional radiographs, and linear measurements need to be adjusted to account for radiographic distortion and discrepancies in the findings. Computerized tomography (CT) scans are usually more accurate.

Sometimes, an anterior loop of the mental nerve may be present mesial to the mental foramen and may cause mental nerve injury during dental implants.⁴⁴ Guidelines to verify the position of the mental foramen and validating the presence of an anterior loop of the mental nerve while establishing a zone of safety (in millimeters) for implant placement can avoid such nerve injury. Furthermore, radiographic assessments result in a high percentage of false negative findings. These false findings can cause sensory dysfunction due to inferior alveolar nerve damage in the foraminal area. The foramen may occasionally misdiagnose with a radiolucent lesion in the apical area of the mandibular premolar teeth. Additionally, local anesthesia of the terminal incisive branches of the inferior alveolar and mental nerves can be obtained effectively if the mental foramen is correctly identified. Therefore, computerized tomography (CT) scans are more accurate for detecting the mental foramen than conventional radiographs.

Histological and embryological studies on human fetus sections characterize the mental foramen of the mandible as a highly suitable model to study the spatial relationship of nerves, vessels and dental primordia during bone remodeling activity. The MF exhibits changing remodeling activity at different gestational ages- apposition, inactivity, and resorption. ⁴⁵A study on human fetuses revealed the earliest position of the MF to be in the interstitial bone between primary canine and first molar. A positional change in the dorsal direction was described during the first half of the prenatal period. ⁴⁶

Paleoanthropologists believe that the mental foramen is the landmark for determining the maturity of the human mandible. Modern human adults are truncated or paedomorphic, in mental foramen position with respect to Neanderthals. However, adults of the two taxa have similar bi-mental foramen breadth with respect to mandibular size.⁴⁷