Journal of Indian Academy of Oral Medicine and Radiology

ORIGINAL ARTICLE
Year
: 2019  |  Volume : 31  |  Issue : 2  |  Page : 100--106

Assessing the dimensions of mandibular incisive canal and its relationship to adjacent anatomical landmarks using cone beam computed tomography in Indian sub-population – A retrospective study


Pratik Chandrakant Malusare, Amita Navalkar, Deepa Das, Bhakti Patil 
 Department of Oral Medicine and Radiology, G.D Pol Foundation Y.M.T Dental College, Kharghar, Navi Mumbai, Maharashtra, India

Correspondence Address:
Dr. Pratik Chandrakant Malusare
Department of Oral Medicine and Radiology, G.D Pol Foundation Y.M.T Dental College, Kharghar, Navi Mumbai - 410 210, Maharashtra, 604, Glen Classic, Hiranandani Gardens, Powai, Mumbai - 76, Maharashtra
India

Abstract

Objective: The purpose of the study is to explore the capability of the CBCT to provide information concerning the position and dimensions of the mandibular incisive canal in relation to adjacent anatomical landmarks. Methodology: CBCT scans of 150 subjects were retrospectively studied to analyze the dimensions of the mandibular incisive canal and to assess the relationship of the mandibular incisive canal to the adjacent anatomical structures of the mandible. Results: 150 CBCT scans were evaluated for the position and dimensions of mandibular incisive canal to adjacent structures. In 87% of cases, the MIC was identified and had a mean length of 13.4mm (range, 5.6 to 24.7 mm) on the right side and 12.4 mm (range, 4 to 22.5 mm) on the left side. From the mental foramen, the incisive canal runs forward toward the incisors, in a slight downward direction, with its diameter decreasing as it progresses towards the midline. Discussion: The data from this study indicates surgical anatomic relationships should be considered in pre-surgical planning to avoid neurosensory disturbances and other potential complications. With the increased interest in performing a thorough pre-surgical examination in the inter-foraminal region, crosssectional images should be utilized to obtain information on the appearance, location, and course of the foramina and canals and their relation to other anatomical structures of the jaw.



How to cite this article:
Malusare PC, Navalkar A, Das D, Patil B. Assessing the dimensions of mandibular incisive canal and its relationship to adjacent anatomical landmarks using cone beam computed tomography in Indian sub-population – A retrospective study.J Indian Acad Oral Med Radiol 2019;31:100-106


How to cite this URL:
Malusare PC, Navalkar A, Das D, Patil B. Assessing the dimensions of mandibular incisive canal and its relationship to adjacent anatomical landmarks using cone beam computed tomography in Indian sub-population – A retrospective study. J Indian Acad Oral Med Radiol [serial online] 2019 [cited 2019 Oct 17 ];31:100-106
Available from: http://www.jiaomr.in/text.asp?2019/31/2/100/261095


Full Text



 Introduction



One of the most frequent accidental complications that may occur during surgical procedures in the mandibular interforaminal region is a neurosensory disturbance in the chin and lower lip. This complication occurs when important structures such as the mental foramen (MF), the anterior loop of the inferior alveolar nerve and mandibular incisive canal (MIC) are not properly identified and protected.[1] Knowledge of the anatomy in the region between the mental foramens is still poorly documented.[2] Although correct identification of the anatomical structures in this region is important for the success of surgical procedures.[3] Osteotomies frequently penetrate into the incisive canal without significant risks to damage vital anatomical structures. Several case reports describe neurosensory disturbances, oedema, hemorrhage, haematoma and the failure of osseo-integration of implants during or after surgical procedures in the interforaminal region of the mandible.[4] Due to an increasing rate of surgical intervention in the region of chin such as oral implant placement, bone grafting, thus emphasizes the practical significance of the mandibular incisive canal, lingual vasculature and lingual concavity.[5] Valuing this information, great attention must be paid to the spatial relationship of the incisive canal to other mandibular structures. Locating the mandibular incisive canal, measuring its length and describing its course and relationship to other anatomical structures should become mandatory for surgeons who mainly rely on clinical and radiographic sign for pre-operative information concerning the vital structures in the area of the symphysis. Based on the interpretation of panoramic images, the anterior mandible has often been considered safe by surgeons for surgical procedures. However, the topographic anatomy may show anatomical variations which can go undetected on the 2D images. With the help of Cone Beam Computed Tomography (CBCT), the incidence, location, course, and type of the incisive canal in the anterior mandible can be mapped easily and thus caution the surgeon to its presence during surgery.

 Materials and Methodology



The study involved 150 archived CBCT scans at Oral Medicine and Radiology Department, G.D Pol Foundation's Y.M.T Dental College, Navi Mumbai. CBCT scans obtained for various reasons (implant planning, assessment of impacted teeth, trauma, etc.) were selected for the study.

Inclusion criteria

CBCT images of the entire mandible from the anterior two-third of ramus on right side to anterior two-third of ramus on left side.Region of mandibular body anterior to the mental foramen bilaterally showing the presence of one or more teeth.

Exclusion criteria

Images of poor diagnostic value.Scans showing complete edentulous regions anterior to the mental foramen bilaterally.

Scanning of the patients was accomplished by using a Kodak CS 9300 imaging machine using the following imaging protocols—field of view, 5 × 10 cm; voxel size, 0.1 8 mm; kilovoltage, 80–90 kv; and milliampere, 5–10 mA. All reconstructions and measurements were accomplished using the CS imaging software program A single observer viewed the images using a Sony Viao laptop (Intel Core i3 processor, 1.90 GHz, 4GB Ram memory, Intel HD Graphics; 15.6 HD LED LCD monitor, Windows 10 operating system). The images were analyzed and the measurements were done using the tools given in the proprietary software. On each scan, axial slices were reconstructed parallel to the inferior border of the mandible. A planning line, along the centerline of the mandibular jaw arch, was drawn on the axial cut slice at the level of mental foramen.

A Panorex “panoramic reconstruction” was automatically generated by the software and cross-sectional images were also reconstructed [Figure 1].{Figure 1}

Incidence of MIC

The incidence of the MIC was noted on the axial images of each scan on the right and left side at the level of the mental foramen. The mental foramen on one or both sides was noted. The nerve tracking tool was used to trace the canal on the reconstructed panoramic image. This was reconfirmed using the cross-sectional plane. If this structure was visible on the panoramic and cross-sectional view, it was included in the study [Figure 2].{Figure 2}

Initial part of MIC with respect to mental foramen

On the panoramic image the beginning of the incisive canal was assessed in relation to the mental foramen and was allocated to one of the group.[5]

Type 1- Mandibular incisive canal starting before the opening of the mental foramen [Figure 3]Type 2- Mandibular incisive canal starting at the level of mental foramen [Figure 4].{Figure 3}{Figure 4}

Length of MIC

After confirming the presence of the MIC, the visible length of the incisive canal, defined as the intrabony continuation of the mandibular canal was measured on the panoramic image. The visible length of the canal was measured from the mesial aspect of the mental foramen (starting point) to the most mesial location that was definitely visible (end point) on the panoramic reconstruction view of CBCT scans [Figure 5].{Figure 5}

In Type 1 MIC the length was calculated from the starting point defined as 6 mm mesial to the mental foramen. In Type 2 MIC the length was calculated from the starting point defined as the most mesial location of mental foramen.[6]

Spatial relationship of adjacent structures to MIC

To determine the position of the incisive canal, five measurements were obtained. These measurements were done on sagittal section of each tooth anterior to the mental foramen bilaterally.[7]

From the inner side of upper cortical border of incisive canal to the root tip [Figure 6]From the inner side of the inferior cortical border of the incisive canal to the outerside of inferior cortical border of mandible [Figure 7]From the inner side of buccal part of the cortical border of incisive canal to the outer side of buccal plate [Figure 8]From the inner side of lingual cortical border of incisive canal to the outer edge of lingual border [Figure 9]The longest distance between the inner cortical borders of canal, i.e. diameter of the canal [Figure 10].{Figure 6}{Figure 7}{Figure 8}{Figure 9}{Figure 10}

 Results



CBCT Scans of 150 patients were studied to assess the dimensions of the mandibular incisive canal to adjacent anatomic structures. The mandibular incisive canal was studied on 150 mandibular right and 150 mandibular left sides.

Identification of MIC

The mandibular incisive canal was identified in 262 sides out of total 300 sides examined in the present study. The visibility of the canal using CBCT in the present study was 87%.

Opening of MIC in relation to mental foramen

The relationship between the opening of incisive canal and mental foramen was categorized as Type 1 and Type 2 in our study [Table 1]. The majority of the MIC opening 54.2% (n = 142) were found at the level of the MF (Type 2), near to the vestibular surface and the rest 45.8% (n = 120), to begin before the MF. Out of total 300 sides (150 right and 150 left sides) examined, 62 Type 1 (47%), and 70 Type 2 (53%) were present on the right side and 58 Type 1 (44.6%) and 72 Type 2 (55.4%) were present on left side.{Table 1}

Length of incisive canal

The incisive canal length was evaluated in 300 sides. The canal length was 13.4 mm (range, 5.6 to 24.7 mm) on the right side and 12.4 mm (range, 4 to 22.5 mm) on the left side.

Spatial relationship of MIC to adjacent anatomical structures

In the present study, a total of 1112 teeth were examined which were present anterior to mental foramen. They consisted of 328 premolars, 282 canines and 502 incisors. To determine the position of the incisive canal, five measurements were obtained in sagittal section at each tooth anterior to the mental foramen bilaterally [Table 2].{Table 2}

 Discussion



Often during implant surgery in the mandibular symphysis area, little attention is given to the mandibular incisive canal. The region between the mental foramen is considered as a zone of choice for the placement of implants. However, complications may arise due to an extension anterior to the mental foramen that forms the mandibular incisive canal. (MIC). The section of the nerve just before the ramification of the incisive nerve is defined as the anterior loop of the inferior alveolar nerve. Therefore, the goal of our study was to prove the efficacy of CBCT in identifying the position and dimensions of the mandibular incisive canal in relation to adjacent landmarks. In the light of the increased use of dental implants, the present study aimed at gaining a better understanding of the radiographic courses of the mandibular incisive canal. The present study defined the appearance, location, and course of the MIC as compared to other anatomical landmarks on the CBCT scans of the mandible, and also evaluate the clinical significance of such analysis.

Identification of mandibular incisive canal (MIC)

The incisive canal has been described as appearing as a round radiolucent area surrounded by a radiopaque border in the cross-sectional images as seen on CBCT. Recent studies have reported that the mandibular incisive nerve has been found to be present in normal and atrophic mandibles[8],[9],[10] which justifies the inclusion of scans taken of dentate and partial edentulous patients. In our study MIC was identified in 87% of scans in an Indian subpopulation. Ramesh et al.(2015) did a similar study on Indian sub population. The incisive canal was identified in 71.66% in their study.[11] Pires et al. compared the appearance of MIC in panoramic and CBCT images. They found that 88% incidence of MIC on CBCT images versus 11.2% on panoramic radiographs. This was attributed to the superimposition of anatomical structures like cervical vertebra in the anterior mandibular region on panoramic images.[12]

The high prevalence of MIC found by means of the CT scan is comparable with direct measurement of cadaveric specimens, which can be considered a trusted method for the detection of this canal.[13] In addition, Santos et al. recently evaluated the reliability and reproducibility of measurements with CBCT, and demonstrated strong agreement between examiners.[14] This could indicate that the methodology can serve as a standard for linear measurement analysis of the mandibular canal topography and adjacent osseous structures, with high accuracy and potential of providing unambiguous information for correct diagnosis.

Differences in the prevalence of this canal have been observed[10],[15],[16],[17] when the canal is too small to be visualized on the CBCT[12] and when different systems have been used for obtaining tomographic images. These differ in sensitivity and slice thickness, because the voxel size used was different. The MIC becomes smaller as it progresses in a direction mesial to the mental foramen, towards the most anterior part of the mandible. Depending upon its size, it may or may not be visualized on CBCT. In our study, the course of MIC could be traced in the anterior region in 27% of scans.

In the present study, a total of 1112 teeth, anterior to mental foramen were examined. They consisted of 328 premolars, 282 canines, and 502 incisors. The incisive canal was located at the level of mental foramen. At this level, MIC was located more in the cross-sectional images of the premolars and canines than those of the central incisors. This can be explained by the fact that the incisive nerve loses its morphology as a true neurovascular bundle near the midline to form a neurovascular plexus narrowing its diameter, thus making its visualization on the CBCT less common.

Length of incisive canal

In our study, the mean length of the incisive canal on the right side was 13.49 ± 4.6 mm and 12.4 ± 4.3 mm on the left side. However, it was shorter than the one found by Makris et al.[18] who verified, also by using CBCT, a MIC average length of 15.1 mm in Greeks. Similarly, shorter lengths were obtained by Rosa et al.[19] and Apostolakis and Brown[20] measuring 9.11 ± 3.00 mm and 8.9 mm, respectively as compared to our study. Pires et al.[12] verified MIC lengths of 7.1 ± 4 mm and 6.6 ± 3.7 mm for the right and left side, respectively. The difference in length of MIC in various studies can be due to different methodology of measuring the length of MIC. It can also be attributed to the size of mandible, ethnic race, environment, stress, nutrition, and assessing technique can largely influence the variations in length of MIC.

Initial part of MIC with respect to Mental foramen (MF)

Yovchev et al.[5] studied on 140 CBCT scans and found the majority of the MICs 86.3%, were found to begin before the MF opening (Type 1); and the rest 13.7% at the level of the MF (Type 2), near to the vestibular surface. Using the same methodology in the present study, we found that in 54.2% of scans the MIC was found to be of Type 2, i.e. in start in close proximity to the MF and near to the vestibular surface and 45.8% Type 1 begin before the MF opening. In cases when the MIC begins close to the mental foramen (Type 2) the lingual positioning of the implants could save the neurovascular bundle in the canal. All this would lead to better conditions for osteointegration and prevent eventual neurosensory disturbances. Thus, the ability to categorize the relation of MIC to MF can provide a lesser damage to neurovascular bundle in the canal and achieve a minimally invasive yet optimally positioned implant.

Spatial relationship of MIC to adjacent anatomical structures

It is recognized that the mandibular incisive neurovascular bundle may run in the medullary spaces in a number of cases, i.e. without the formation of a distinct bony canal. Thus, it is important to provide information concerning the dimensions and position of MIC in relation to various anatomical landmarks. This information may be used by implant surgeons to reduce the number of postoperative complications after selective procedures in the symphysis area of the mandible.

The distance was measured from the root tip to the superior border of the mandibular incisive canal was measured in the premolar, canine, and incisor regions. The mean distance from root tip to the superior border of MIC was 6.4mm (range, -0.9-13.5 mm) for the premolars, 5.6 mm (range, 0.8-15.7 mm) for canines, and 8.2 mm (range, 1.8-16 mm) for incisors. The distance from the apex of the tooth to the superior border of the incisive canal decreased from first premolars to canines as a result of longer canine roots. The canine teeth are longer than incisor teeth resulting in a shorter distance to the canal without changing its superior-inferior position within the mandible. It should be noted that although the distance from the incisors to the incisive canal was greater than that of the canines, it does not indicate an inferior movement in the course of the nerve. At some locations the incisive canal could be found in close proximity to the root apex. Hence, by calculating the distance of MIC from the root tips, the proximity of the MIC to the root tip can be determined. If the implant is placed in close proximity to the nerve, it could potentially exhibit continued pressure on the nerve resulting neurosensory changes experienced by the patient. Hence, the close proximity of these two structures should be assessed prior to implant placement and surgical procedures in the mandibular anterior region to avoid iatrogenic neurosensory disturbances. The distance was measured from the inner side of the inferior cortical border of the incisive canal to the outerside of the inferior cortical border of the mandible was measured in the premolar, canine and incisor regions. In the present study, the distance between the inferior border of mandible to the incisive canal ranges between 8.6 and 9.3 mm. The distance between the inferior border to MIC decreases from premolars to incisors ranging from 9.3 mm at the premolars to 9.1 mm at the incisors.

The distance was measured from the inner side of buccal part of the cortical border of the incisive canal to the outer side of buccal plate of the mandible was measured in the premolar, canine, and incisor regions. In the present study, the distance between the buccal border to the incisive canal ranges between 3.2 mm and 3.7 mm. The distance between the buccal border to the incisive canal slightly increased from the premolars to the incisors ranging from 3.2 mm at the premolars to 3.6 mm at the central incisors with a mean buccal thickness of 3.5 mm in our study. The present study provides valuable data on bone thickness of the mandibular anterior region which can enable a surgeon to ensure proper soft-tissue support, avoid resorption of the facial bone wall following restoration and by this minimize the risk for peri-implant soft-tissue recessions.

The distance was measured from the inner side of the lingual cortical border of the incisive canal to the outer edge of the lingual border was measured in the premolar, canine, and incisor regions. The distance between the lingual border to the canal ranged from 4.6 mm at the premolars to 5.7 mm at the central incisors with a mean lingual thickness of 4.8 mm. The bone lingual to the canal decreased from the premolars to the canines and the canal approaches midline the lingual bone thickness increased in the incisor region. As per the measurements of this study, it is thus evident that due to increased thickness of lingual bone in the incisor region, as the canal approaches midline, it lies in close proximity with buccal plate. This study is in accordance with Pires et al. who stated that the course of the incisive canal is in close proximity to the buccal plate independent of its location within the mandible.[12]

The longest distance between the inner cortical borders of canal, i.e. diameter of canal was measured in the premolar, canine, and incisor region. The mean inner diameter of the incisive canal in the present study was 1.4 mm in the premolar region, 1.1 mm in the canine region, and 1 mm in the incisor region. In our study, the smallest diameter measured was 0.4 mm in the incisor region and largest diameter measured was 5.5 mm in the premolar region. We found a trend of decrease in diameter as the canal runs medially. This can be attributed that the incisive nerve loses its morphology as a true neurovascular bundle near the midline to form a neurovascular plexus narrowing its diameter. Many studies have been conducted regarding the size of the incisive canal and the thickness of the surrounding bone. Some studies have reported on the inner diameter, not including the surrounding cortication, with results ranging from a mean of 1.1 mm[16] to 1.8 mm.[21] Furthermore, Huang et al. observed that the mean diameter of MIC was 1.21 mm +/- 0.29 mm.[22] Incisive canals with large diameters may result in the failed osseointegration of implants because of decreased bone-implant contact. An implant can fail to integrate with a gap of 2 mm, which is smaller than the mean diameter of the incisive canal at every point measured. Rosenquist found that the incisive nerve causes implant failure by migration of soft tissue around the implant thereby preventing its osseotintegraion.[23] CBCT scan was capable of detecting fine MIC of diameter as small as 0.4 mm. Hence, CBCT are capable of detecting various anatomic despite their small size.

 Clinical Significance



Although the distances of the MIC from the bone plate seem to obey a pattern, the surgeon must be aware of the variable range of distribution of the MIC, so that previously established default values may pose the risk of injury. The clinical significance of this study lies in the mapping of the incisive canal and its anatomical proximity during surgical procedures in order to avoid potential injury to the incisive mandibular nerve, a purpose safely achieved with the use of CBCT. Therefore, in order to determine the appropriate location of the MIC for each individual, this should be investigated on a case-by-case basis.

 Conclusion



The data from this study indicates surgical anatomic relationships should be considered in pre-surgical planning to avoid neurosensory disturbances and other potential complications. With the increased interest in performing a thorough pre-surgical examination in the inter-foraminal region, cross-sectional images should be utilized to obtain information on the appearance, location, and course of the foramina and canals and their relation to other anatomical structures of the jaw. Large variations in the dimensions and topography of the MIC have been identified in the present study, which indicates a valuable role for this multiplanar imaging modality in assessment prior to surgical procedures. Panoramic radiographs have the least accuracy in identifying this structure. Hence, a better image such as conventional tomographs or CBCT should be used especially in the intermental foramen area. Analyzing CBCT scans using the method described in this study can be a useful tool in avoiding implant surgical complications in the anterior mandible.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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