Topographical Landmarks for Identication of Inferior Alveolar Nerve and its Surgical Implications - a Cadaveric Study

Background: Inferior alveolar nerve, which arises from the posterior division of the mandibular nerve, contains both sensory and motor bres. The intraosseous course of the nerve is variable. Mandibular foramen is situated on the medial surface of the mandibular ramus. It is an important anatomical landmark for procedures like sagittal split osteotomies and inferior alveolar nerve block. Methods: The precise location of mandibular foramen was studied by the dissection of formalin xed cadavers available for undergraduate dissection for a period of two years fron 2014-2016. Results: The mandibular foramen was 21±3.33 mm superior to the gonion at an angle of 98 0 ±5 0 with the base of the mandible. The mandibular foramen was 20.13±3.1 mm inferior to the lowest point of the mandibular notch. Accessory mandibular foramen was observed in 9.37% of the samples dissected. Conclusion: The present study explains the position of mandibular foramen in relation to prominent bony landmarks and the knowledge about the mandibular foramen, helps dental surgeons during inferior alveolar nerve blocks and split osteotomies. Practical implications: Failure rate of the alveolar has been reported to be approximately A thorough anatomical knowledge of the mandibular is essential for inferior alveolar nerve blocks and split since they are technically procedures and as


Background:
The mandibular nerve is the largest division of the trigeminal nerve containing both afferent and efferent bers. It enters the infratemporal region via the foramen ovale and divides into anterior division and posterior division. The trunk gives off two branches namely nervus spinosus and nerve to medial pterygoid. The anterior division gives four branches namely masseteric nerve, deep temporal nerve, buccal nerve and nerve to lateral pterygoid. Masseteric nerve, deep temporal nerve and the nerve to lateral pterygoid innervate the muscles of mastication whereas the buccal nerve provides sensory innervation to the skin of the cheek, buccal mucosa and buccal gingivae of mandibular molars. The posterior division gives three branches namely auriculotemporal nerve, lingual nerve and inferior alveolar nerve. [1] Inferior alveolar nerve, which arises from the posterior division of the mandibular nerve, contains both sensory and motor bres. It enters the mandibular canal, via the mandibular foramen. It then leaves the mandibular canal through the mental foramen as the mental nerve. The intraosseous course of the nerve is variable. It may have a gentle curve towards the mental foramen or an ascending or descending pathway. [1,2,3,4] Mandibular foramen is situated on the medial surface of the mandibular ramus. It is an important anatomical landmark for procedures like sagittal split osteotomies and inferior alveolar nerve block. [5,6] Inferior alveolar nerve block is a commonly performed procedure in dentistry. It involves the precise localization of the mandibular foramen and deposition of local anesthesia. [7] Failure rate of the inferior alveolar nerve block has been reported to be approximately 20-25%. [8,9] Inadequate knowledge regarding the position of the mandibular foramen and the presence of accessory mandibular foramen are the main factors responsible for the failure of this technique. [10][11][12][13][14] Bilateral sagittal split osteotomy (BSSO) and vertical ramus osteotomy (VRO) are the common procedures which are done for the correction of the mandibular prognathism, to reposition the mandible surgically. A thorough anatomical knowledge of the mandibular ramus is essential for these procedures, since they are technically di cult procedures and as they are also associated with a higher incidence of complications. [6,15] Methodology: The study was conducted on sixteen formalin xed cadavers, available for the undergraduate dissection in the Department of Anatomy. Ethical clearance was obtained from the Institute's Ethics Committee. The mandibles taken out from the cadavers were cleared of the muscle bres attached to them. The position of the mandibular foramen was examined (Fig. 1). The distance of the mandibular foramen from the angle of the mandible was measured using the vernier calliper (C). The distance of the mandibular foramen from the lowest point on the mandibular notch was additionally quanti ed (D). Using a protractor, the angle made by the mandibular foramen at the gonion with the base of the mandible was measured (E) (Fig. 2, Fig. 3). The mandibles were then examined for the presence of any accessory mandibular foramen. (Fig. 4).
Statistical analysis was performed using GraphPad Instat 3 software.
Descriptive statistics were determined for each variable.
Comparisons were made for each variable on the right and the left side using the student t test. Results:

Mandibular foramen topography
The parameters considered were: C -Distance from the angle of the mandible to the mandibular foramen D -Distance from the lowest point of the mandibular notch to the mandibular foramen E -Angle formed by the mandibular foramen at the gonion with the base of the mandible The mandibular foramen was 21 ± 3.33 mm superior to the gonion at an angle of 980 ± 50 with the base of the mandible The mandibular foramen was 20.13 ± 3.1 mm inferior to the mandibular notch In the sixteen formalin xed cadavers dissected, no statistically signi cant difference was observed in the measurements related to the topography of the mandibular foramen between the right and the left side (p > 0.05) Accessory mandibular foramen was observed in 9.37% of the specimens dissected Discussion: A precise knowledge about the position of the mandibular foramen is indispensable to attain a successful inferior alveolar nerve block. The data obtained from dry mandibles and radiographs have shown that the position of mandibular foramen is highly variable. [10,13,16] In the current study, each half of the mandibles collected from sixteen cadavers were examined for the position of mandibular foramen in relation to the gonion, base of the mandible and mandibular notch.
The position of the mandibular foramen in relation to the occlusal plane was also noted.
In the present study, in 87.5% of the samples, the mandibular foramen was at the level of the occlusal surface of the third molar. In 6.25% of the samples, it was approximately 9.51 mm above the occlusal surface of the third molar. In 6.25% of the samples, it was 8.19 mm below the occlusal surface of the third molar. Afadhali et al., advocated that the mandibular foramen could be located immediately above the occlusal plane of mandibular rst molar and second premolar. [17] Augier M et al., and Basmajian et al., described the mandibular foramen slightly above the level of the occlusal plane [18,19] whereas Thangavelu et al., demonstrated it at or below the level of the occlusal plane. [20] In the previous studies, the mandibular foramen was described in relation to the ramus of the mandible and gonion angle. In a study conducted by Nicholson et al., the mandibular foramen was located predominantly at the centre of the ramus of the mandible. [11] In another study, the mandibular foramen was located slightly above the middle of the mandibular ramus. [21] Ennes JP et al., tried to explore the relation between the gonion angle and the distance between the mandibular foramen and the base of the mandible and concluded that the distance from the mandibular foramen to the base of the mandible was inversely related to the gonion angle. [12] (Table 3) In the current study, the mandibular foramen was described in relation to the angle of the mandible. In addition, the angle formed by the mandibular foramen with the base of the mandible has been explored.
The mandibular foramen was located 21 ± 3.33 mm superior to the gonion at an angle of 98 0 ±5 0 with the base of the mandible (Table 1).
In this study, we also studied the relationship between the mandibular foramen and mandibular notch. Mandibular foramen was located 20.13 ± 3.1 mm inferior to the lowest point of the mandibular notch (Table 1). This is consistent with the ndings of Oguz et al., who observed that the shortest distance between the mandibular foramen and the mandibular notch was 22.27 mm. [22] In the present study, no signi cant statistical difference was observed in the position of the mandibular foramen between the right and left sides ( Table 2). This is consistent with the ndings of Tsai HH [23] and Braga et al., [24] indicating bilateral symmetry of the mandibular foramen.
Presence of accessory mandibular foramen could lead to ineffective inferior alveolar nerve block. [21] In the present study, accessory mandibular foramen was observed in 9.37% of the samples (Fig. 4). In all 9.37% of the samples, they are seen inferior to the mandibular foramen. In a study done in Brazilian population, accessory mandibular foramina were observed below the mandibular foramen in 22.07% of the mandibles and above the mandibular foramen in 25.22% of the population. [25] Figure 1 Topography of mandibular foramen on a cadaveric specimen.