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Inferior alveolar nerve canal position among South Indians: A cone beam computed tomographic pilot study

Purpose: To document a clinically relevant position of the inferior alveolar nerve (IAN) in complete dentate south Indian patients in the age group of 20–29 years using cone beam computerized tomograms. Materials and Methods: The investigators used a cross-sectional study design and a study sample of subjects who had a radiographically identi ﬁ able IAN canal with complete set of 28 permanent teeth excluding 3 rd molars. Predictor variables were age, tooth position, and side. Outcome variables were the linear distances between the buccal and lingual aspect of the IAN canal, buccal and lingual cortical thickness, IAN canal diameter, and the superior aspect of the IAN canal from the periapex of ﬁ rst and second mandibular molar. Descriptive statistics and Mann–Whitney U test were performed. P value of ≤ 0.05 was taken as signi ﬁ cant. Results: The study sample was composed of 10 male and 10 female patients with a mean age of 24.2 ± 3.00 years. On average, the lingual cortical thickness was 1.68 mm at 1 st molar and 1.44 at 2 nd molar level. Gender and side in ﬂ uenced the outcome with varying statistical signi ﬁ cance. Conclusions: The range of linear dimension of mandibular canal, cortical bone thickness, and distance between tooth apex and IAN canal have been presented for the South Indian population in the age group of 20–29 years. The implications of the ﬁ ndings will in ﬂ uence on the course of surgery. Further large-scale studies are needed to validate the ﬁ ndings of this study.


INTRODCUTION
Iatrogenic injuries to inferior alveolar nerve (IAN) are a welldocumented complication of third molar surgeries, implant placement, osteotomies, or fracture repair. The considerable variation in the course, the shape, curve, and direction of the nerve as well as the terminal segment of IAN complicates the regional anatomy. Hence, it is often diffi cult to predict the exact position of the nerve, thus impeding a proper preoperative planning. [1] A better understanding of the intrabony anatomy of the IAN and its relationship to mandibular molar (MM) anatomical landmarks, particularly with emphasis on the tooth may aid to decrease the risk of inadvertent IAN injury associated with various surgical interventions in the area including sagittal split osteotomies or placement of fi xation screws. Several studies have underlined the unwanted and often avoidable iatrogenic damage to the IAN. It has been reported that in harvesting procedures involving the mandibular ramus, IAN damage may cause sensory defi cits in up to 8.3%. [2] In orthognathic surgeries, this sensory change is reported in up to 65.1% of the cases and after placement of dental implants has been up to 77.8%. [2] Previous attempts to describe the IAN-MM anatomy have signifi cant limitations. Cadaver fi ndings cannot be translated to patient population owing to differences in age or disease. Skull-based studies lack demographic data or use clinically inconsistent and anatomically irrelevant landmarks or include edentulous mandibles. [1] Use of two-dimensional imaging modalities and computed tomograms has their own limitations. [2] Owing to absence of such IAN studies from this part of the world, this preliminary study was performed as a pilot effort to identify appropriate sample size. The aim of this study was to identify the linear relationship of the canal to the buccal and lingual cortex (inner and outer) and to the periapex of the molar teeth as well as to compare the difference in parameters in the apical region of the fi rst and second molar in the ipsilateral and contralateral side of the jaw.

MATERIALS AND METHODS
Randomly chosen routine cone beam computed tomography (CBCT)s of 10 males and 10 females in the age group of 20-29 years were considered for this study. The other criterias were as follows: (1) absence of any developmental disturbance or pathology or previous treatment that could infl uence the IAN or canal or position of tooth including impactions; (2) complete set of 28 teeth excluding 3 rd molars; (3) radiographically completely corticized IAN canal bilaterally; and (4) absence of radiological evidence of skeletal/dental malocclusion that could have altered the position of 1 st and 2 nd MM or IAN.
All the CBCTs were taken by the same trained personnel between November 2010 and February 2011 in the same machine (Kodak Cone Beam Computerized Tomography, 9600) at the same settings. Manufacturer's instructions regarding the positioning and placement were followed. Calibration of the linear measurements had been performed using known dimensions in millimeters. The linear measurements were performed using the software and guides with the same machine.
Using the axial, coronal, and sagittal sections, the exact location of the IAN and tooth was identifi ed for the study. Linear measurements were made in cross sections. Using 200-μm thick sections, sections at the level of the opening of periapical foramen of the mesial root of the mandibular 1 st and 2 nd molar were measured by the authors, individually. Mean of the values was considered as the measurement of the particular patient. Kappa value between observers ranged between 0.93 and 0.82 for all values.
All measurements were measured along the center of the IAN canal unless mentioned specifi cally. The measurements were the shortest distance between the lingual inner and outer cortex (A), inner cortex to outer surface of the IAN canal along lingual side (B), inner cortex to outer IAN canal along the buccal side (C), shortest distance between buccal inner to outer (D), inner cortical width of the IAN canal (E), outer cortex to outer cortex width along the centre of the IAN canal (F), and the shortest distance between the periapex to the superior surface of IAN canal (G). These measurements were modifi ed approach of previously published literature [3] [Figures 1 and 2].
All the data were entered and analyzed using SPSS, version 16 (SPSS Inc., Chicago, IL). Descriptive statistics of the variables and measurements are presented. Mann-Whitney statistics were presented for comparing the linear measurements between the 1 st and 2 nd molars as well as between both sides. P value of ≤0.05 was taken as signifi cant.

RESULTS
The mean age of the study population (n = 20) was 24.2 ± 3.00 years. The mean age of males (n = 10) was 25.2 ± 3.33 years (20-29 years). The mean age of the females (n = 10) was 23.2 ± 2.39 years (20-26 years). Table 1 depicts the mean value of the linear measurements of parameters in the study population and Table 2 depicts the various sites were at the level of the foramen opening of the mesial root of the right and left mandibular 1 st and 2 nd molar. Table 3 differentiates the difference in terms of gender among the various parameters. It is observed that the thickness of buccal and lingual cortex signifi cantly varied between genders. The other parameters had no signifi cant role in terms of these linear measurements. Table 4 depicts the results of Mann-Whitney test on comparing the tooth with respect to its position (1 st and 2 nd Molar) and side (right and left). The difference of lingual cortex thickness (A) with respect to 1 st and 2 nd molar in the right quadrant was signifi cant as well as the distance from the lingual cortex to the IAN (B) and the inner cortex to outer surface of the IAN canal along buccal side. The other dimensions were not statistically signifi cant.

DISCUSSION
The various imaging modalities including the periapical, panoramic, occlusal radiography, as well as conventional and computerized tomography have been used to preassess the site of surgeries such as implants. None of these modalities are perfect. Although periapical radiographs provide suffi cient diagnostic information in anterior mandible, surgeries of posterior mandible may require more accurate vital diagnostic information to avoid vital structures such as IAN-MM complex. Panoramic views have inherent defect of overlap in anterior region. Twodimensional radiographs do not provide information such as thickness or location of vital structures in a bucccolingual direction. Cross-sectional imaging using conventional spiral, linear, hypocycloidal computed tomogram, and magnetic resonance imaging have been used for cross-sectional imaging. [4] There have been issues with the amount of radiation exposure and the high costs associated with these modalities. [1] CBCT has been reported as a well-suited imaging modality for the craniofacial area. It provides clear and accurate images of structures, and therefore is extremely useful for assessing the bone component. As the resultant images displayed are often corrected for magnifi cation, and as a result, accurate measurements can be derived from the reformatted three-dimensional data. [5] The CBCT appearance of the mandibular canal usually is a welldefi ned radiolucent zone, lined by radiopaque borders. The radiographic density of this lucent structure is variable, with the presence of a radiopaque outline being dependent on the canal's cortication. As a result, in few young patients, the mandibular canal is not well visualized. Carter had earlier reported that radiologically invisible IAN canal may occur because the IAN bundles are not always surrounded by an ossified canal. [6] Stella had previously reported of the reliability and accuracy of conventional tomography as part of the preimplant evaluation of posterior mandibular sites to the visibility of the IAN canal. [7] Although this is a pilot and preliminary study, the values identifi ed in the study are comparable with the values mentioned in the global literature. [8] The lingual cortex is thicker at the fi rst molar level, while the buccal cortex is much thicker at the second molar level. This probably could be due to consistent remodeling owing to the oral musculature attachments in the region. The mylohyoid line that serves to attach the mylohyoid musculature is oriented at higher position in 2 nd molar region that 1 st molar region, as this line runs obliquely. Hence, the thickness of lingual cortex is more at fi rst molar level than the 2 nd molar level. In the buccal surface, masseter attachment at the posterior region (2 nd molar) plays a dominant role. As in the literature, root tip of 2 nd molar     is closely associated with the IAN than the 1 st molar root tip. [7] This is because the IAN canal runs obliquely. The IAN canal diameter begins to narrow down at the fi rst molar region than in second molar region, as the thickness of vessels greatly decreases. This is consistent as reported by Humphries SM. Concisely, the parameters studied vary between the 1 st molar and 2 nd molar.
As identifi ed in the study, females had a mildly thicker buccal and lingual cortical thickness than males. The fi ndings are not in agreement with earlier reports. [8] Although the difference is less than 0.1 mm, the phenomenon needs to be checked using a larger sample in a wider age group. However, the overall thickness of mandible is thicker in males than females, which is in agreement with earlier reports. [8] The thickness of IAN canal increases from 1 st molar to 2 nd molar. This is also in agreement with published reports. [8] The difference in between the right side and left side was remarkable, indicating that the right and left halves are not entirely symmetrical. The right halves value is noted to be always higher than the left values which is consistent with reports from the literature. [9] This pilot study has identifi ed the thickness of the buccal cortical bone in the mandible in the regions of the fi rst and second molars. This will be useful guide when monocortical plating of mandibular fractures with plates placed in the neutral zone of the mandible that is often placed in close proximity to the IAN canal. Furthermore, the values mentioned would also be helpful to relate the position of the IAN canal to the buccal bone at its medial and inferior positions from the root apex.
As this kind of study is performed for the fi rst time in this population, the comparison of the values cannot be done or our claims refuted. The aim of this study was to collect CBCT details of normal patients from archival records, as per the strict inclusion and exclusion criteria's. As CBCTs are reserved only for gross and extensive pathologies or for extensive treatment planning, most of the retrospective CBCTs do not meet the criterias and liberal selection criteria would induce selection bias in the study. Hence, only very small sample size was included for this pilot study.
In future, similar studies which compare IAN anatomy of cases with iatrogenic complication could be compared against regular cases; also CBCTs diagnostic effi cacy against other modalities could be compared.

CONCLUSION
The range of linear dimension of mandibular canal, cortical bone thickness, and distance between tooth apex and IAN canal have been presented for the South Indian population in the age group of 21-30 years. With the increasing demand for accurate preoperative assessment and planning prior to oral implant surgery or other surgeries of this region, cross-sectional images such as from CBCT may be utilized for obtaining more information on the appearance, location, and course of the canals, and their relation to other anatomical structures in the jaw bone including the apex of the tooth.  *P ≤ 0.05, significant; **P ≤ 0.001, highly significant. Distance between the lingual inner and outer cortex (A), inner cortex to outer surface of the IAN canal along lingual side (B), inner cortex to outer IAN canal along the buccal side (C), inner cortex to outer surface of the IAN canal along buccal side (D), innercortical width of the IAN canal (E), outer cortex to outer cortex width along the centre of the IAN canal (F). and the shortest distance between the periapex to the superior surface of IAN canal (G). All values are in millimeters.

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