2024 Volume 66 Issue 3 Pages 169-175
Purpose: To identify and measure the distance from the dental apices to the mandibular (MC) and mandibular incisive (MIC) canals, the diameter of the MC and the distances of the mental foramen (MF).
Methods: In this retrospective study, cone-beam computed tomography scans of 144 adult patients (males and females) from a dental school in South Brazil were evaluated. Cross-sections were selected on the MC and the MIC paths, perpendicular to the mandibular base, and measurements were taken from the dental apices to the mandibular cortices. The measurement and location of the mandibular and mental foramen on both sides were compared. Paired t-tests compared sides, while Student’s t-tests compared sexes (P < 0.05).
Results: The distance from the dental apices to the upper wall of the MC was closest in the third molar and farthest in the central incisor region. In both sexes and sides, the path of the MC is in most cases lingually in the molar regions and moves to the buccal region from the second premolar. The MF emerges in the regions between the premolars or near the second premolar.
Conclusion: The results of this study highlight the importance of evaluating specific individual characteristics of a given population.
The mandibular canal (MC) is an intraosseous anatomical feature located inside the mandible, extending from an entrance through the mandibular foramen to its exit through the mental foramen [1]. Important anatomical features found inside the canal are the inferior alveolar nerve (IAN), inferior alveolar vein, and inferior alveolar artery. The teeth and surrounding tissues receive nerve and vascular supply from the branches that arise from these structures. The canal is usually a single structure, but anatomic variations can occur [2]. The IAN is more vulnerable to damage during dental treatment, including surgery, implant dentistry, endodontics, and related specialties, because of the intimate contact between this nerve and the root apices of the posterior teeth [3].
Anteriorly to the mental foramen (MF), as an extension of the MC, the mandibular incisive canal (MIC) provides innervation to the lower anterior teeth and the surrounding mucosa. The MIC is poorly visible as the diameter [4], and bone corticalization [5] of the MC decreases from lateral to medial. Although the mandibular incisive canal (MIC) is not as closely associated with the root apices as the inferior alveolar nerve (IAN) it can also be injured, a fact that should be taken into consideration during dental procedures.
The anatomical complexity of these structures requires an exact delineation of anatomical variations of their paths [6]. Accurate detection of MC is critical to prevent nerve damage during dental procedures. The most common complications are sensory impairments and trans- or postoperative bleeding. Lesions can remain for a transitory period, or they can become permanent. Such complications can be caused by direct trauma to the IAN, or by indirect damage due to a hematoma of the inferior alveolar artery [7]. Paresthesia typically improves over time, but it could become irreversible in some cases.
Dentists must be familiar with the anatomical variations of the MC, and know how to make a correct diagnosis, to avoid complications during procedures as well as to correctly block the IAN during anesthesia. Although the importance of correct diagnosis and identification of the MC has been well described, further studies and information about the path and anatomical variations of this structure are still needed, due to individual anatomical variations related to ethnic factors. It is important to emphasize that the South Brazilian population is diverse, and comprised of individuals with European, African, and Indigenous ancestries. Due to this mixture of races, analysis of this population is particularly insightful. It is worth noting that it is through results obtained in different populations around the world that a global picture of such measures can be built.
The knowledge of the precise location of anatomical landmarks, such as the mandibular foramen, the MC and the MF and the course of the mandibular neurovascular bundle, are essential to obtain the desired results for procedures performed at different levels of the mandible body. The mastery of these important anatomical structures, especially knowing their exact location in relation to others nearby can prevent various complications [8].
Cone-beam computed tomography (CBCT) images are essential in assisting professionals to identify anatomical variations and the MC path, which aids correct planning and execution of the treatment proposed for the patient [9]. Performing an accurate measurement of the IAN on a CBCT scan is a challenge, since quantitative measurement of the MC volume with a highly sensitive technique can be beneficial in determining the probability of injury to areas along the course of the IAN [10,11]. The position and path of the MC are important parameters that should be examined with preoperative three-dimensional imaging techniques. Assessing the anatomical location of the whole path of the MC may be helpful in increasing the success of procedures and reducing the risk of neurosensory disturbances [11].
This study aimed to employ CBCT images to identify and bilaterally measure the distance from the dental apices to the upper wall of the MC, including its continuation to the region of the anterior teeth (i.e., the MIC), the distances of the mandibular external cortices (buccal and lingual), and the mandibular base to the MC walls. Additionally, the distance from the upper portion of the mental foramen to the mandibular base, and height of the mental foramen, as well as the diameter of the MC were measured.
The present study was characterized as a retrospective observational study, with quantitative and comparative analyses of the mandibular landmarks that were visualized on cone-beam computed tomography (CBCT) images. The observational stages were developed in the Radiology Service of the Dentistry Course of the School of Health and Life Sciences (DC-SHLS) of the Pontifical Catholic University of Rio Grande do Sul (PUCRS). This study did not require an Informed Consent Form because it was a retrospective study of CBCT scans of patients who were seen as part of the Dentistry Course, and who signed a consent form when they commenced treatment.
CBCT images and study sampleFor this study, the CBCT scans were selected from the database of the Radiology Service of the DC-SHLS of the PUCRS. The CBCT images were selected according to the following eligibility criteria: 1) inclusion criteria: CBCT scans of the mandible of individuals over 18 years of age who underwent examinations for diagnosis and/or treatment at the DC-SHLS of the PUCRS; 2) exclusion criteria: images of totally edentulous mandibles; or those with missing lower posterior teeth (Kennedy’s Class I); or with dental implants in the lower arch; or with the presence of plates or screws in the mandibular region; or with specific congenital deformities, such as active bone pathologies, or mandibular syndromes, which could hinder the measurements.
For the sample calculation, a confidence level of 95% and a standard deviation of 1.62 were considered for the distance from the root apex to the MC, according to the study by Nagadia et al. [12], with a margin of error of 0.2 mm. This subsequently resulted in a sample of 252 hemiarches. Since the evaluations were performed per tooth, and some patients had missing teeth, 10% was added to this value.
The tomographic images used were acquired with the i-CAT Cone Beam 3D Dental Imaging System (Imaging Sciences International LLC, Hatfield, PA, USA), serial number ICU 082130, under an exposure protocol of 120 kV power, 5 mA, and 10 mm aluminum filter protection. These tomographic examinations were performed with a voxel of 0.2 mm and with different fields of view that were determined individually for each patient, according to the area to be examined. All the images followed the same acquisition protocol.
The analyses and measurements of the tomographic images were performed on the same LG monitor (model Flatron E2250V 21.5 inches), with appropriate features for viewing, and in a room with decreased brightness for proper viewing. The software used for the analyses and measurements of the pre-established points was OnDemand software (CyberMed Inc., Irvine, CA, USA).
MeasurementsMandibular CBCT images from both right and left sides were subjected to individual analyses in accordance with the methodology described by Muñoz et al. [1]. Transverse sections, oriented perpendicular to the mandibular base, were chosen where the apex of each tooth was closest to the MC, as shown in Fig. 1A. The following distances were measured on the transverse images, as shown in the schematic illustrations in Fig. 1B: the distance from the upper border of the MC to the apex of the root closest to the canal (A); the distance from the inferior border of the MC to the external surface of the mandibular base (B); the distance from the outer/vestibular border of the MC to the buccal cortical (C); the distance from the inner/lingual border of the mandibular canal to the lingual cortical. The measurements related to the height (E) and width (F) of the MC were also included, which represented the diameter of the MC and MIC. Areas where a tooth was absent were excluded from the measurements.
Measurements were also taken in the region of the mental foramen (Fig. 1C), being the distance that was traced perpendicularly from the uppermost part of the mental foramen to the base of the mandible (oriented by the line drawn parallel to the base of the mandible) (G); the distance traced perpendicularly from the lowermost part of the mental foramen to the base of the mandible (oriented by the line drawn parallel to the base of the mandible) (H); and the distance traced at the level of the opening of the mental foramen (I).
All measurements on sampled CBCT images were performed by a single examiner (previously calibrated, trained, and proficient in manipulating the software and taking morphometric measurements by an oral and maxillofacial radiologist). Measurements were taken only after examiner calibration using 16 scans, in which points A to I were measured. Two measurement sessions were performed for these examinations, and the second session was performed after a 14-day interval.
1A) Mandible illustrating the points for analysis and measurements (3M, third molar; 2M, second molar; 1M, first molar; 2PM, second premolar; 1PM, first premolar; C, canine; LI, lateral incisor; CI, central incisor; MF, mental foramen). 1B) cross-sectional measurements: A measurement from the upper border of the mandibular canal to the apex of the root closest to the canal. B measurement from the lower border of the mandibular canal to the external surface of the mandibular base. C measurement from the outer/buccal border of the mandibular canal to the buccal cortex. D measurement from the inner/lingual border of the mandibular canal to the lingual cortex. Measurements of mandibular canal diameter: E mandibular canal width. F mandibular canal height. 1C) G distance from the uppermost part of the mental foramen to the base of the mandible; H distance traced from the lowermost part of the mental foramen to the base of the mandible (both measurements oriented by the line drawn parallel to the base of the mandible), and I mental foramen vertical diameter.
The data was statistically analyzed using the Intraclass Correlation Coefficient (ICC). The values obtained ranged from 0.801 to 0.994. To compare the differences between the right and left sides of the same individual, a paired t-test was performed for all measurements (A to I). Meanwhile, to investigate differences across sexes (male and female), Student’s t-tests for independent samples were utilized. Descriptive statistics were employed in this study, encompassing the calculation of means and standard deviations to delineate the quantitative variables. Statistical analyses were executed using GraphPad Prism 8.0 (GraphPad Software, San Diego, CA, USA) with a significance threshold of 5%.
In this investigation, the CBCT scans of 144 adult patients aged between 18 and 79 years at the time of the examination were evaluated, including 50 males and 94 females. While viewing these scans the right and left sides were treated as two distinct sites, thus totaling 288 lower hemiarches, which were analyzed and measured.
To assess discrepancies between the left and right sides across all measurements a paired t-test was employed. The outcomes revealed statistically significant differences exclusively in measurement B within the second molar region among females (P = 0.008), as well as measurement E in the first molar region, and D in the canine region among males (P = 0.036 and 0.048, respectively). Consequently, mean values of each measurement for each individual were used to analyze the differences between sexes.
When analyzing the distance from dental apices to the upper wall of the mandibular canal (A), it becomes evident that the closest proximity is observed in the third molar region (1.51 in female and 2.46 in male), while the farthest distance is noted in the central incisor region (9.56 in female and 11.16 in male). Statistically significant differences between sexes are apparent in all areas, except for the canine region. Additionally, males had greater distances in all regions than female.
Measurements taken from the lower border of the mandibular canal to the external surface of the mandibular base (designated as B) exhibited sex-based disparities across various regions, except for the canine and lateral incisor areas. In contrast to the A measurements, the distances corresponding to B were consistently shorter in all regions, except for the central incisor areas.
Measurements C and D represent the distances between the mandibular canal (MC) and the buccal and lingual surfaces of the mandible, respectively. Statistically significant differences between sexes were observed in the molar regions for both measurements. Additionally, the lingual distances were greater than the buccal ones in the more anterior areas, starting from the second premolar region.
The width and height of the mandibular canal (MC) exhibited statistically significant variations between sexes, with males generally displaying greater measurements in both dimensions, particularly in molar regions (Fig. 2).
No sex-based differences were observed in the following measurements: the distance from the uppermost point of the mental foramen to the base of the mandible (G), the distance from the lowermost point of the mental foramen to the base of the mandible (H), and the vertical diameter of the mental foramen (I) (Fig. 3).
Examination indicated that in both sexes and on both sides, the path of the MC is, in most cases, located lingually in the molar regions and moves to the buccal side from the second premolar region. This can be observed in Fig. 4.
Regarding the positioning of the mental foramen, it was observed that in both sexes the MF consistently emerges on the mandible in regions between the premolars or in close proximity to the second premolar (approximately 90% of cases) (Fig. 5).
2A) distances from dental apices to the upper wall of the MC. 2B) distances from the lower border of the MC to the external surface of the mandibular base. 2C) distances between the MC and the buccal and lingual cortical plate of the mandible. 2D) distances between the MC and the lingual cortical plates of the mandible. 2E-F) measurements of the width and height of the MC, respectively. *Significant difference (P ≦ 0.05)
Values in darker shades of gray represent a higher prevalence of MC location in relation to the dental apices in the buccolingual direction
Values in darker shades of gray represent a higher prevalence of the mental foramen exit position
To expand knowledge of anatomy, this study analyzed, through the evaluation of computed tomography scans, the path of the MC from the apexes of the molars to the region of the anterior teeth, including its exit of the MF where it continues into the body of the mandible and is identified as the MIC. Computed tomography images of 144 patients were evaluated, with the sample made up mainly of females (94), as well as 50 males, aged between 18 and 79 years old from a population located in a city in South Brazil.
CBCT in dentistry is considered an examination method with greater precision and accuracy in clinical situations in which the dentist needs information to identify the exact measurements, the precise location, the ideal surgical approach, and the preservation of the structures, utilizing three-dimensional analyses. The affordability and convenience of use are the primary factors influencing the use of this equipment [13]. Moreover, it should be indicated only when conventional radiographic techniques do not improve the clinical solutions for the patient. CBCT scanning should not be practiced routinely, but only when needed to add potential information to assist in the treatment plan.
When the right and left sides were observed and compared, only a small number of differences in measurement between the two sides were significant. However, this did not occur when the males and females were compared, where a considerable number of measurements showed statistically significant differences between sexes.
Notably, the distance between the mandibular and incisive nerves and the apices of the roots generally follows a pattern of increase up to a certain age, followed by a subsequent decrease. This apex is reached in males up to 50 years of age, and in females up to 40 years of age. This phenomenon is attributed to the continuous growth observed in females up to 40 years and in males up to 50 years [14]. However, the relative positioning of the canal and the foramen tends to maintain a relatively constant pattern across sexes and with advancing age [15]. Due to this, given the broad range of ages within the sample, specific evaluation of each age group was not undertaken.
The extrusion of endodontic material beyond the apex can cause damage to the mandibular nerve due to the proximity of the mandibular premolar and molar apexes when reaching the MC. In this study, the smallest distance of the MC to the dental apex was observed in the third molar region, being 1.51 ± 1.88 mm for females, and 2.46 ± 3.02 mm in males. This could be explained by the fact that females generally have a smaller body size, so they could be at greater risk of suffering damage to the MC. Other investigations found that also the MC was closer to the roots of the third molar [1,7,16,17]. The measurements obtained for the smallest distances from the apexes in relation to the MIC were evident in the first premolar region (5.34 ± 2.63 mm) in females and the canine region in males (6.07 ± 2.74 mm). This finding justifies why longer dental implants can be placed in the mandibular body region compared to the areas of the symphysis and close to the mandibular angle [18].
In this study, the results showed that the MC presented closer to the mandibular base in the second molar region on both sexes, being 6.38 ± 1.60 mm in females and 6.96 ± 2.08 mm in males. Nair et al. [17] also found similar results for the second molar region (6.30 mm). Additionally, other authors reported consistent findings across sexes, indicating that the distance from the lower limit of the MC to the mandibular base increases gradually from the posterior to the anterior [18,19]. Practitioners must be aware of the position of the MC in relation to the mandibular lower limit during surgical planning, as inadequate positioning of the plate at the base of the mandible can lead to invasion of the MC and neurovascular damage [18]. The MIC nearest to the mandibular base was in the canine region for both sexes, being 8.53 ± 2.00 mm in females and 8.99 ± 2.08 mm in males. The findings are in agreement with those of Pires et al. [20], who reported similar values for MIC measurements.
This study showed that the greatest distance to the buccal cortical plate was observed in the second molar region for both males (6.14 ± 1.60 mm) and females (5.61 ± 1.47 mm), although no statistically significant differences were observed among these distances. Similarly, the MC was further from the buccal cortical plate in the second molar region, with a 6.79 mm distance [12]. It was also evidenced that the distance from the MC to the buccal cortical plate in the males was greater compared to females [21]. The shortest distance from the lingual cortical plate was reported in the regions of first and second molars, both with the same distance (2.0 mm) [17]. Whereas in this research, the smallest distance of the MC to the lingual cortical plate was observed in the first molar region (2.19 mm). Clinically, these data are important to avoid neurosensory sequelae in jaw surgeries and to prove that although there is a general pattern in the spatial relationships of MC, the safety zones do not present a universal pattern [22]. Likewise, the decision to guide a dental implant buccally or lingually to prevent canal invasion is largely dependent on knowledge of the buccolingual thickness of the jaw and the position of the MC [18].
In the present study, the MIC distance was also measured from the buccal and lingual cortical plates. In relation to the vestibular cortical plate, the smallest distance was observed in the first premolar region in both sexes (3.67 ± 3.37 mm for females and 3.81 ± 1.60 mm for males) and at a distance of 4.93 ± 1.90 mm in females and 5.55 ± 2.07 mm for males in the lingual cortical plate. In both sexes these results were found in the canine region. However, in previous studies, the authors reported greater proximity to the lingual cortical plate [23,24]. This may be because the MIC becomes smaller as it progresses mesially from the MF to the anterior region of the mandible, where it may become difficult to visualize on CBCT. The risk of hemorrhage in the lower canine region is higher when angled implants are inserted in this region. To minimize the risk of laceration or perforation, it is important to consider the drill direction during the surgical procedure because the sublingual artery follows a horizontal path [25].
Considering the average measurements in both vertical and horizontal directions of the MC diameter, the molar region was the only area where the difference in dimensions between males and females was statistically significant. In this study, the mean vertical diameter of the MC was 3.79 ± 0.72 mm in males and 3.42 ± 0.68 mm in females; in the horizontal direction, males obtained 2.98 ± 0.57 mm while females obtained 2.7 ± 0.52 mm. The results found in the present research are consistent with those reported by other authors [18,26,27].
The average diameter of the MF was greater in males (10.53 mm) than in females (9.77 mm), with statistically significant differences (P = 0.007). Preoperative imaging is crucial for any surgical procedure, as well as understanding and mastering anatomy for anesthetic infiltrations and endodontic biomechanics in the mental region. Negligence can cause damage to the neuro-vascular bundle during surgeries.
The distance from the mental foramen to the mandibular base was greater for males compared to females (P = 0.0001). The finding of sexual dimorphism in this measurement agrees with findings from several other studies [18,22,28]. This sex-related statistical difference may be due to the lower rate of bone development and growth of the craniofacial skeleton in females compared to males, which is regulated by sex hormones and other local factors, such as chewing forces and muscles, resulting in less bone deposition throughout time in the bony portion of the mandibular base [28].
The most prevalent location for MF was below the apex of the second premolar on the right side, with 58.95% for females and 57.14% for males, on the left side, 47.37% in females and 51.02% in males. The results of this investigation corroborate studies previously published [22,28,29]. However, other authors obtained the MF most located between the lower premolars in samples from different populations [30,31]. It is believed that variability in MF position can be attributed to genetic factors.
The Brazilian population is characterized by a great mix of ethnic groups, the result of the immigration of populations from various countries which colonized Brazil for five centuries, resulting in an interracial mix of Europeans, Africans, Amerindians and Asians [32]. The population sample studied in the present study, belonging to a state in the south region of Brazil, is made up of a mixture of people (European and African descendants), a fact that would justify the results presented herein, similar to those of studies of populations from different locations [6,11,18,22,27]. Thus, knowledge of anatomy involving specific measurements of certain structures provides important information on the ethnic influence and ancestry of the populations in a study, which can help prevent risks and complications during surgical and endodontic procedures.
In this paper, it was observed that, particularly when males and females were compared, a considerable number of measurements showed a significant difference in the distances to the MC within the evaluated reference points. The majority of CBCT scans evaluated revealed that the patients had a substantial number of dental elements. The lack of information about systemic medical conditions that could have affected the quality and quantity of the jawbone of the patients is a limitation of this study. Another limitation refers to the observational nature of the study, which would not allow an objective assessment of changes in the mandibular canal related to the patients’ age. This fact also may occur due to the small sample of the population evaluated, even though it is bigger than other studies [33,34]. It is important for the oral and maxillofacial surgeon to be aware of cases where the patient is edentulous or has multiple missing teeth. This is because the distance to the alveolar ridge in relation to the MC and the MF will be modified, resulting in a decrease in the distance between them. If this is not considered, there could be an increased number of complications.
Based on the findings of this study, one can conclude that the females showed the shortest distances. The shortest distance from the root apexes to the upper wall of the MC was observed in the third molar region. The sample population predominantly exhibited MF below the apex of the second premolar. The shortest distance from the buccal cortical of the mandible to the MC wall was in the premolar region; the females had the shortest distances from the MC to the buccal cortical plate. When evaluating the lingual cortical of the mandible, the shortest MC distance was observed in the third molar region, with the males on the right side, and with the females on the left side. Males had the smallest distances, with the MC being nearest to the lingual cortical in the first molar region. The shortest distance from the root apexes to the upper wall of the MIC was observed in the canine region. Neurovascular complications in the region of the path of the MC and the MIC during surgical and endodontic procedures can be avoided by highlighting the importance of studies that evaluate specific individual characteristics for a given population.
CBCT: cone-beam computed tomography; IAN: inferior alveolar nerve; ICC: intraclass correlation coefficient; MC: mandibular canal; MF: mental foramen; MIC: mandibular incisive canal
The procedures were approved by the Research Ethics Committee of the Pontifical Catholic University of Rio Grande do Sul (PUCRS), Porto Alegre, Brazil (Protocol # CAAE 11130219.8.0000.5336), following Ethical Determinations according to the guidelines outlined in the 1964 Declaration of Helsinki.
All authors declare no potential conflict of interest with respect to the research, authorship, and publication of this article.
No funding
GCD: conceptualization, samples preparation, data collection, analyses, and writing; ARCM: conceptualization, data curation, formal analysis, and writing; RCH: conceptualization, data collection, statistical analyses, and writing; MIBR: conceptualization, writing, review, editing, and supervision; All authors read and approved the final version of the manuscript.
1)GCD: gabriel.castrodallazen@hotmail.com, https://orcid.org/0000-0001-8359-3603
2)ARCM: aline.morosolli@pucrs.br, https://orcid.org/0000-0001-6622-9594
2)RCH: rafael.hartmann@pucrs.br, https://orcid.org/0000-0002-3087-6922
2)MIBR*: ivete.rockenbach@pucrs.br, https://orcid.org/0000-0002-9001-0020
The authors thank the participants for all their support and contributions. Gabriel Castro Dall’Azen would like to thank the National Council for Scientific and Technological Development (CNPq) – Brazil for the financial support.
Data generated during the current study are available from the corresponding author on reasonable request. All data generated or analyzed during this study are included in this published article.
