EVALUATING THE THREE DIMENSIONAL SPREAD OF ANTERIOR LOOP OF MANDIBULAR NERVE FOR PREDICTABLE IMPLANT INSERTION IN MANDIBLE-A CBCT ANALYSIS ON NORTH-INDIAN POPULATION

1. Post-Graduate Student, Department of Prosthodontics, Govt. Dental College & Hospital, Srinagar, Jammu & Kashmir, India. 2. Professor and Head, Department of Prosthodontics, Govt. Dental College & Hospital, Srinagar, Jammu & Kashmir, India. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History Received: 30 June 2020 Final Accepted: 31 July 2020 Published: August 2020 Introduction: Implant placement requires an understanding of regional anatomy and a precise knowledge of the location of important anatomic structures such as the inferior alveolar nerve and maxillary sinus. At its anterior exit the inferior nerve creates a loop before exiting through the mental foramen. The dimensions of the anterior loop of inferior alveolar nerve are variable among individuals. This study was undertaken to assess the variability in the dimensions of this loop, so as to achieve predictable implant placement in the area of mental foramen. Materials and Methods: Eighty one CBCT scans of patients after exclusion from a sample of hundred CBCTs were used in the study. Individual sections were evaluated for the greatest dimension of the loop in the antero-posterior and supero-inferior direction. The greatest dimension was transferred to the sagittal plane, and measured perpendicular to the inferior border of mandible. T-test was used for statistical analysis of the data. Results: 68 of the 81 cases studied had an anterior loop in the mandibular nerve. The mean value obtained in males for the anteroposterior dimension was 3.65 mm on the right side, and 3.56 on the left side. For the females the value obtained was 3.02, and 2.91 mm for right and left side. In the supero-inferior dimension the corresponding values for males for the right and left side were 4.28mm and 4.18 mm. For the females group the values obtained were 3.24 mm and 3.17 on the right and left side. The results were statistically significant with p value <0.05. Conclusion: A minimum distance of 6 mm is recommended to prevent damage to the anterior loop. Using the CBCT data for treatment planning is however the gold standard and should be prioritized over using average values for implant insertion.

The inferior alveolar nerve bifurcates in the area of the mandibular premolars into two branches, the incisive nerve that runs anteriorly through the bone and the mental nerve that exits through a small opening called mental foramen apical to the roots of mandibular premolar teeth 1 . The mental nerve exits the foramen after moving buccally, anteriorly or superiorly or a combination of the above. This gives rise to the anterior loop of mental nerve, which is a structure of vital importance during treatment planning for implant placement. A safe distance must certainly be maintained between the mental nerve loop and the apical aspect of the implant osteotomy to avoid any injury to the mental nerve that can result in disruption of sensory stimuli from the region of chin, lip, labial mucosa and mucosa of anterior gingiva.
The mental portion of inferior alveolar nerve is classified as straight, vertical and anterior loop 2,3 . Among these anatomic variants, the anterior loop has been the focus of much research, owing to its variable positioning in the area. Researchers have studied the anterior loop using different modalities including direct measurement on cadavers, OPGs, CT and CBCT. Values ranging from 0.11 to 11 mm were reported along-with a prevalence of 7-88% 4 . More research is required in identifying the presence and postion of the anterior loop that can help guide the practitioners even in the absence of CBCT for safe preparation of implant osteotomies.
Thus this study was undertaken to identify the prevalence and dimensions of the anterior loop of mental nerve in a north Indian population, and to determine a safe distance for implant placement in the area of mental foramen.

Materials and Methods:-
This cross sectional study was carried out on 100 CBCT scans that had been obtained for other purposes, like diagnosis, orthodontics, oral surgical requirements, implant planning, etc. The scans were taken by the same radiology technician. Images of inadequate quality were discarded, as were scans with unerupted teeth or any pathology in the region of mandibular premolars, as well as long edentulous areas in the area of premolars. Cases with implants placed in the area of premolars were also discarded. Thus we were left with 81 cases that were included in the study. The sagittal, cross-sectional, and multiplanar reformatted images of the patients were carefully evaluated. The ALL was evaluated in the respective sections of each CBCT image and the dimension was transferred to the buccal surface of mandible, perpendicular to the sagittal plane, using the software. This was necessary owing to three dimensional curvature of the foramen that limited the ability to visualize the entire length of the loop in a single section. The distance between the lines projected onto the sagittal plane was measured to identify the antero-posterior and supero-inferior position of the foramen, in relation to the mandibular canal.

Results:-
According to the study 68 of the 81 cases studied had an anterior loop in the mandibular nerve. The mean value obtained in males for the antero-posterior dimension was 3.65 mm with a standard deviation of 1.02 on the right side, and 3.56 with standard deviation of 0.99 on the left side. For the females the value obtained was 3.02 mm with standard deviation of 0.87, and 2.91 mm with standard deviation of 0.84 respectively for the right and left side. In the supero-inferior dimension the corresponding values for males for the right and left side were 4.28mm (SD 1.16) and 4.18 mm (SD 1.15). For the females group the values obtained were 3.24 mm (SD 0.86) and 3.17 (SD 0.87) on the right and left side. Appropriate t-test (paired t-test for intra-group and independent samples t-test for inter-group variables) was used for intra-and inter-group comparison. The differences were found to be statistically significant for both intra-and inter-group comparison, with p value <0.05.

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Anteriormost point identified (a) and transferred (red line) to the buccal surface (b), posteriormost point identified (c) and transferred (red line) to the buccal surface (d) the distance between the two points measured in the sagittal plane, perpendicular to a tangent to the inferior border of mandible(e).
Superiormost point identified (f) and transferred (red line) to the buccal surface (g), inferiormost point identified (h) and transferred (red line) to the buccal surface (i), distance between the two points measured in the sagittal plane parallel to a tangent to the inferior border of mandible (j).

Discussion:-
The aim of this study was to evaluate whether an average safe distance between the mental

Discussion:-
The aim of this study was to evaluate whether an average safe distance between the mental foramen and apical extent of an implant osteotomy can be established based on CBCT study, and whether the results of this study correlate with the previously established values in other studies.
Although panoramic radiography is a commonly used radiographic diagnostic tool, its reliability is subject to question due to its two dimensional nature, and superimposition of anatomic structures 5,6 . The incidence of anterior loop reported from OPG based studies ranges from 12-40%, which is much lesser than the numbers reported from CBCT studies and study of human cadavers. Values obtained from cadaver studies include 59% 7 , 61.5% 8 , 62.7% 9 , 88% 10 . Similarly CBCT analysis has revealed a wide range of 7% by Jacobs et al 11  In the present study the values reported for the right and left side were different with statistical significance (p<0.05). This finding is not in agreement with reports from studies done previously. The difference in values of male and female population is also statistically significant (p<0.05), which is in agreement with the results from previous studies. The study done by Wong and Patil, however, reported no significant difference due to sex or ethinicity 19 .

Conclusion:-
A mean distance of 6 mm is recommended as per this study to prevent damage to the inferior alveolar nerve branches during implant osteotomy preparations in more than 95% of the population. Using a CBCT to treatment plan is the best option, as arbitrary distance of 6 mm can result in significant compromise of vertical bone available for placing an implant in the premolar area.