Position , shape and direction of the mental foramen in mandibles in South-Eastern Nigeria

Objectives: The mental foramen is an important anatomical and clinical landmark for undertaking a successful mental nerve anesthetic block during clinical and surgical procedures in the mandibular and mental region. This study was aimed at studying and documenting the position, shape and direction of opening of the mental foramen in dry mandibles in South-Eastern Nigeria. Methods: 66 dry adult human mandibles obtained from the region of study were used and the parameters visually examined. The position was determined in relation to the mandibular teeth, the shape was determined with the aid of a hand lens and the direction with the aid of a thin office pin inserted into the foramen. Results: The most prevalent position of the mental foramen was shown to be in line with the longitudinal axis of the second premolar teeth (58.33%) followed by the position between the second premolar and the first molar (25.76%). Round shaped foramen was more prevalent (75.76%) than the oval shaped foramen (25.24%) and the foramen pointed posterosuperiorly in 73.48% and anterosuperiorly in 21.21% of the mandibles. It was also observed that there were symmetrical occurrences in majority of the mandibles. Conclusion: The present study showed the average morphology of the mental foramen in the population studied. It may be useful in population diagnosis, clinical practices and in forensic medicine.


Materials and Method
66 dry adult human mandibles obtained from the region of study were used for the study. Fishel et al 21 proposed the most popular method for identification of mental foramen. The horizontal position of the mental foramen was recorded as either in line with the longitudinal axis of a tooth or as lying between two teeth. In this study,the position of the mental foramen was measured using the Vernier caliper or ruler to trace, with reference to the 1 st and 2 nd premolar teeth and first molar teeth. The different positions observed were classified as follows: · Position 1-Between the first and second premolar (P1) · Position 2 -In line with the long axis of the second premolar (P2) · Position 3-Between the second premolar and the first molar (P3) The shape of the mental foramen was observed and determined with the aid of a hand lens. The two shapes observed were oval and round.
The direction of the mental foramen was measured by inserting anoffice pin into the foramen from the lateral part of the mandible. The direction to which the office pin pointed was visually inspected. The results of the different directions or courses of the foramina were then grouped into 5 as follows:

Position of mental foramen
The most prevalent position of the mental foramen was found to be in line with the long axis of the second premolar teeth(58.33%) and 33 pairs were found to be bilateral out of a total 55 bilateral pairs. The position between the premolars was least prevalent with 15.91% and had 10 bilateral pairs. Total bilateral occurrence was 83.33% showing that the position of the mental foramen is often symmetric.

Shape of mental foramen
The results for the round-shaped and oval-shaped foramina are shown in Table 2. Total bilateral occurrence was at 84.85% showing that same shape mostly occurs on both sides of the mandible.

Direction of mental foramen
Mental foramen pointing postero-superiorly (D4) was found to be more prevalent than those of all other directions while D1 and D3 had the least prevalence with less than 1% each. The results in detail are found in Table 3. Most of the occurrences were symmetric since the data showed 83.33% bilateral occurrence.

Discussion
Significant differences have been reported in the position of the mental foramen among various ethnic groups and populations. In the present study in South-Eastern Nigerians, it was showed that the most prevalent position of the mental foramen was found to be in line with the long axis of the second premolar (58.33%) followed by the position between the second premolar and the first molar (25.76%).This agreed with studies by Adejuwon et al 12 16 in Zimbabweans,and Chkoura and El Wady 17 in Moroccan population,as they all showed the commonest position of the mental foramen to be in line with the long axis of the second premolar followed by theposition between the second premolar and the first molar. It indicates that most African populations have the same mental foramen position. In Northern Nigerians, however, it was reported that the most common position was in line with the interdental space between the first and second premolars 7 . This report shows that the position of the mental foramen varies in Northern and Southern Nigerians. Similar results to this present study were also obtained by Budhiraja 1 in North Indian subjects, Neo 6 in Singaporean Malays and Indians, Al Jasser and Nwoku 18 in Saudi population, Ngeow and Yuzawati 19 in Malay population, and Agarwal and Gupta 20 in South Gujarat (Indian) subjects. Koppe 11 in comparison of three population groups found the modal position to be in line with the long axis of the second premolar in the Chinese and between the first and second premolar in the European and Indian. This was similar to the comparison of Santini and Land 10 . Gungor et al 5 reported that in the Turkish population, the most common position of the mental foramen was between the first and second premolars. It can therefore be deduced from studies 5,10,11 that the most common position of the mental foramen in the European population is between the first and second premolars just like that of North Americans 9 . This agrees with statement in some Anatomy textbooks 22  are positioned similarly. It also agreed with the notion that the mental foramen was positioned more anteriorly in whites than in blacks 23 . The essence of these studies is to get the clinicians informed on what to expect when attending to patients from any particular population group and to ensure that the integrity of the mental vessels is preserved and a successful mental nerve block achieved.   26 all reported that the oval-shaped mental foramen was more prevalent than the round-shaped MF in their subjects. Comparison between the results of the present study and previous ones is presented in Table 5 below.  26 .In this study, it was posterosuperiorly in majority of the subjects (73.48%). This was in agreement with previous studies by Boonpiruk 27

Conclusion
It is importantto be aware of the variations that occur in the mental foramen among different population groups. This will help to avoid injury or paralysis of the mental nerve which is one of the principal complications of surgery in the mandibular or mental regions. The notion that the mental foramen lies between the premolars as stated in old and some new textbooks can no longer be generally accepted. In the present study, the position of the mental foramen has been shown to be in line with the long axis of the second premolar and the shape and direction showed to be oval and posterosuperior respectively in majority of the subjects. This study has provided information on the morphology of the mental foramen in mandibles of South-eastern Nigerians and we hope it will be useful in achieving a successful mental nerve block while avoiding injury to the nerve. It is therefore suggested that clinicians should carefully identify this anatomical landmark using radiographs prior to the surgical procedures.