The mandibular foramen (MF) is an irregular foramen located a little above the centre of the medial surface of the mandibular ramus. The inferior alveolar nerve and vessels pass through the MF and traverse the mandibular canal and divides into mental and incisive branches to supply the mandibular teeth and participates in the formation of the anterior loop [12]. Inferior alveolar nerve block is a common local anaesthetic technique used in dental practice. But the failure rate of this technique is reported to be as high as 20%–25% [3]. The commonest cause for inferior alveolar nerve block failure is inaccurate localization of MF [4]. The main complications during this technique are haemorrhage, injury to the neurovascular bundle, fractures, and necrosis of mandibular ramus [5]. Hence, thorough knowledge of the mandibular ramus is very essential.

Clarke and Holmes [6], have reported that the position of the MF is 1cm above the occlusal plane of the lower molars, and is also at the same height of the gingival papillae of the upper teeth when the individual is with his mouth closed. But, Nicholson [7], has said that, there is variability of the two mandibular rami in the same person, and it is not possible to standardize the foramen identification. Studies have conclusively proved that there are significant morphological differences in the mandibular anatomy among the three major racial groups—Caucasoid, Mongoloid, and Negroid [89].

Accessory MF is any opening in the mandible other than the MF, mental foramen, lingual foramen, and sockets of teeth [10]. The presence of accessory MF and additional branches of inferior alveolar nerve may lead to increased rates of failure of inferior alveolar nerve blocks as all the branches may not be anaesthetized [11]. The accessory MF has also been reported to be the site for the spread of tumours following radiotherapy in the lateral surface of mandible [12]. So the knowledge of accessory MF is imperative to radiotherapists when planning for radiation therapy in the lateral mandibular region.

Therefore, the present study aims to determine the precise location of the MF from various anatomical landmarks such as anterior and posterior borders of the mandibular ramus, mandibular notch, base of the ramus, third molar tooth and retromolar trigone from mandibles of South Indian population. This study aims to identify the MF location in relation to the limits of mandibular ramus and to the quadrant of the ramus, taking horizontal and vertical directions. The presence of accessory MF was also noted.

The study was conducted in 204 adult dry human mandibles of unknown sex and age collected from the bone bank of Anatomy and Forensic Medicine departments and students of Dhanalakshmi Srinivasan Medical College and Hospital, Siruvachur, Perambalur. Mandibles with sockets for third molar teeth, those in regular shape, and devoid of deformities were selected. The damaged bones and those having pathological abnormalities were excluded.

To precisely locate the mandibular foramen, the following parameters were measured on both sides of the mandible with a sliding vernier callipers of 0.1 mm accuracy (Figs. 1, 2).

(1) AB-MF: distance from the midpoint of anterior margin of Mandibular foramen to the nearest point on the anterior border of the ramus of mandible (Fig. 1), (2) PB-MF: distance from the midpoint of posterior margin of mandibular foramen to the nearest point on the posterior border of the ramus of mandible (Fig. 1), (3) AB-PB: breadth of the ramus from anterior to posterior border, (4) MF-MN: distance from the lowest point of mandibular notch to the inferior limit of mandibular foramen (Fig. 1), (5) MF-MB: distance from inferior limit of Mandibular foramen to the base of the mandible (Fig. 1), (6) III Molar-MF: distance from the midpoint of third molar tooth or socket to anterior margin of Mandibular foramen (Fig. 2), and (7) RT-MF: distance between the apex of the retromolar trigone and mandibular foramen (Fig. 2).

The angle of the mandible was measured with a goniometer at the junction of inferior and posterior borders of the ramus of the mandible (Fig. 1).

The mandibles were further observed for the presence of accessory mandibular foramen in and around mandibular foramen on the medial surface of mandibular ramus by means of a simple visual observation with the help of a magnifying lens and their prevalence rate was noted and analyzed.

The quadrant where mandibular foramen was located horizontally, was obtained by identifying the distance between the anterior border of the mandibular ramus and the midpoint of the mandibular foramen opening hole, calculated by subtracting, from AB-PB, the sum of the distances AB-MF, and PB-MF. The result yields the width of mandibular foramen. This distance was divided into halves to determine the midpoint of the mandibular foramen opening hole, and added to distance AB-MF. It was then calculated, in percentage, how much the distance between anterior border of the ramus and the midpoint of mandibular foramen represented in relation to the total width of the ramus (AB-PB). This value indicated in which quadrant mandibular foramen was in an anteroposterior location (Fig. 3). Values from 0% to 25% comprised the first quadrant; from 26% to 50%, the second; from 51% to 75%, the third and from 76% to 100%, the fourth. In the vertical direction, the quadrant on which Mandibular foramen was located was identified by calculating how much MF-MN represented, in percentage of the addition of MF-MN and MF-MB distance (Fig. 3).

All the above parameters were carefully tabulated and statistically analysed. SPSS version 16.0 (SPSS Inc., Chicago, IL, USA) was used for the statistical analysis of this study. Student's t test was used as test of significance to compare the mean values of right and left sides and a P-value less than 0.05 was taken to be statistically significant. The results of the present study were compared with the results of previous studies done on various ethnic groups.

The location of the mandibular foramen is essential for mandibular surgeries like vertical ramus osteotomy, inverted L osteotomy and also esthetic surgeries for dentofacial deformities. The inferior alveolar nerve is at a greater risk during these surgical procedures. Daw et al. [5] have reported great variability in the position of mandibular foramen from Non-Asian hemi mandibles. They have also emphasised that the knowledge of the location of the mandibular foramen would assist in performing a proper sagittal split of the mandibular ramus.

During pterygomandibular technique of inferior alveolar nerve blockage long needles of size 33 mm and short needles of size 21.5 mm are used. If a long needle is used in a patient with small mandible, there is a risk of perforating the parotid gland capsule and injuring the branches of facial nerve. If a short needle is used in a patient with big sized mandible, there may be a fracture of the needle when it is completely introduced in the oral tissues [13].

There are significant differences in the localization of mandibular foramen in different ethnic groups. Mbajiorgu in his study [14] on adult black Zimbabweans has reported that the mandibular foramen lies 2.56 mm behind the midpoint of the ramus width on the right side and 2.08 mm behind the midpoint of the ramus width on the left side. The mean distance from anterior border of mandibular ramus to anterior margin of mandibular foramen (AB-MF) was 18.95±0.41 mm, the mean distance from posterior border of ramus to posterior margin of mandibular foramen (PB-MF) was 14.30±0.35 mm, the mean distance from mandibular notch to inferior end of the mandibular foramen (MF-MN) was 22.5±0.5 mm and the mean distance from the inferior end of mandibular foramen to base of the ramus (MF-MB) was 28.44±0.65 mm in his study.

Oguz and Bozkir [4] have tried to localize the mandibular foramen in Turkish mandibles. Ennes and Medeiros [13] and Prado et al. [15] have studied the location of mandibular foramen in Brazilian population. Samanta and Kharb [11] and Raghavendra and Benjamin [16] have tried to localize the mandibular foramen in Indian mandibles. There are variations in the values obtained in each of the studies and the values are compared with the results of the present study in Table 5.

Kilarkaje et al. [17] have reported that mandibular foramen was within 25 mm from the third molar tooth. Varma et al. [18] have reported that the mean distance of mandibular foramen from the third molar tooth was 15 mm on the right side and 18 mm on the left side. Ghorai et al. [19] have reported the distance to be 22.8±4.9 mm on the right side and 21.7±4.7 mm on the left side. The results of the present study are similar to Kilarkaje et al. [17] and Ghorai et al. [19], as the measurements were 22.84±2.13 mm on the right side and 23.23±4.21 mm on the left side.

Valente et al. [20] have measured the distance of mandibular foramen from the apex of retromolar trigone and reported it to be 14.23±2.57 mm on the right side and 14.40±2.48 mm on the left side. In the present study, the measurements were 12.27±2.13 mm on the right side and 12.13±2.35 mm on the left side.

Ennes and Medeiros [13] have reported that the average gonial angle to be 125.6° with a standard deviation between 6.2° and 9.2°. Oguz and Bozkir [4] reported the angle of mandible to be 120.2°±4.7°. In the present study, it was 117.47°±4.95° on the right side and 117.47°±5.88° on the left side. The gonial angle is inversely proportional to the anteroposterior width of the mandibular ramus and the distance between mandibular foramen and base of the mandible (MF-MB). So, in individuals with wide gonial angle, inferior alveolar nerve blockage has to be performed at a site lower than the conventional site and with a short needle and in individuals with small gonial angle, the inferior alveolar nerve block has to be performed at a site higher than the conventional site and with a long needle [13].

Kilarkaje et al. [17] from their study have reported that the mandibular foramen maintains bilateral symmetry in dry mandibles in all ages and the foramen was found to be within 25 mm from the third molar, anterior border of ramus (AB) and mandibular notch. In the present study bilateral symmetry of distance of mandibular foramen from various landmarks of the mandibular ramus ranged between 13% to 20% only.

Ennes and Medeiros [13] have localized the mandibular foramen in the third quadrant in the anteroposterior and superoinferior axis of the mandibular ramus. Padmavathi et al. [21] have localized the mandibular foramen in the third quadrant in the anteroposterior axis and in the junction between second and third quadrant in the superoinferior axis. In the present study also the mandibular foramen was localized in the third quadrant in the anteroposterior axis and junction between second and third quadrants in the superoinferior axis of the mandibular ramus.

The embryological basis for the occurrence of accessory mandibular foramen has been explained in literatures. Chávez-Lomeli et al. [22] have reported that, initially, 3 inferior alveolar nerves, innervating each of the 3 groups of mandibular teeth are formed in the embryo. Later, all the 3 nerves fuse and a single inferior alveolar nerve is formed. The incomplete fusion of the nerves leads to the formation of double mandibular canals. In 60% of the cases, the mandibular canal was found to have the entire inferior alveolar nerve passing through it and in 40% the nerves were found to be scattered. Pancer et al. [23] has reported the incidence of accessory mandibular foramen to vary from 0.88% to 10.66%. In the study by Padmavathi et al. [21] the accessory mandibular foramen was present in 41.5% of the cases and it was present unilaterally in 29.2% of the cases and bilaterally in 12.3% of the cases. Gopalakrishna et al. [24] have reported the incidence of single accessory mandibular foramina to be 18%. Samanta and Kharb [11] have found accessory mandibular foramen in 16.66% of the mandibles of which 10% of the mandibles had single accessory mandibular foramen and 6.66% of the mandibles had double accessory mandibular foramens. Freire et al. [25] have observed a single accessory mandibular foramen in 27.93% of the Brazilian mandibles located below the mandibular foramen and in 43.24% of the Brazilian mandibles, the accessory foramen was located above the mandibular foramen. In the present study, single accessory mandibular foramen was found unilaterally in 13.72% of the mandibles and bilaterally in 8.8% of the mandibles. Double accessory mandibular foramen was found unilaterally in 8.33% of the mandibles and bilaterally in 1.5% of the mandibles (Table 4). The statistically significant predominance of unilateral single accessory mandibular foramen on the right side may be due to chance and there is no literature explanation for it. Hanihara and Ishida [26] have reported that, there is a higher prevalence of accessory mandibular foramen in Asian males, hence there is a higher incidence of accessory mandibular foramen in our study and also in the study by Padmavathi et al. [21] on Indians.

The smaller size of the accessory mandibular foramen limits its visualisation in traditional radiographs of the mandible. Panoramic radiographs of the mandible have limitations like distortion, overlap and magnification, which may lead to false interpretation of important anatomical structures [23]. Haas et al. [27] and Pancer et al. [23] have suggested that cone beam computed tomography (CBCT) is a reliable and accurate three dimensional radiographic technique to visualise the accessory mandibular foramen. The higher cost and level of radiation associated with CBCT limit its usage in dental patients.

The range of distance between the accessory mandibular foramen and mandibular foramen is clinically important, because the spread of local anaesthetic affects the efficiency of the inferior alveolar nerve block. The spread of local anaesthetic in inferior alveolar nerve blocks, depends on the drug used, its concentration and the volume of drug injected [28]. In the present study, the accessory foramens were located at a distance of less than 5 mm from the mandibular foramen, hence if a higher concentration and larger volume of local anaesthetic is used, the accessory nerves can also be anaesthetised.

The precise localization of mandibular foramen is very important to achieve a successful inferior alveolar nerve block, prior to dental surgeries in the lower jaw like osteotomy, orthognathic reconstruction surgeries of the mandible and dental implant procedures, and also to avoid injury to the neurovascular contents passing through it. Accessory mandibular foramina will serve as a route for spread of infection and tumour cells. The present study on the morphometry of the mandibular foramen and the incidence of accessory mandibular foramen will provide useful information to dental surgeons for planning and conducting dental and maxillofacial surgeries in south Indian population. This study will also help radiologist and oncologist in localizing the mandibular foramen in south Indian population.