Cone-Beam Computed Tomography Analysis on the Relationship between the Mandibular Third Molar and the Position of the Mandibular Canal in Koreans from the Yanbian Area and the Han People

Objective To analyze differences in the positional relationships between the mandibular third molar (MTM) and the mandibular canal in Korean and Han patients using cone-beam computed tomography (CBCT) and to provide a basis for preoperative risk assessments. Materials and Methods The CBCT imaging data of 260 Korean and Han patients were collected. The patients' genders, ages, impaction types and depths, relative positions between the MTMs and the mandibular nerve canals, and the shortest distances and shapes at the root tips and cortical bones were all recorded and analyzed. All data were compared using the nonparametric test, ordered logistic regression analysis, a chi-square test, and Fisher's exact test. Results The relationship between the mandibular canal and the relative position of the MTM differed between Korean and Han patients, mainly in the different types of impactions, and the difference was statistically significant (P < 0.05). The shortest distance between the mesioangular and horizontally impacted mandibular canals and the buccal side of the MTM in Korean patients was less than in Han patients, and the difference was statistically significant (P < 0.05). For horizontal impactions, the probability of cortical bone interruption was 1.980 times greater in Korean patients than in Han patients, and the difference was statistically significant (P < 0.05). The significance threshold was set at 0.05. Conclusion There are some differences in the positional relationship between the mandibular canal in the MTM region and the rate of cortical bone disruption between Koreans from the Yanbian area and the Hans. This should gain clinical attention.


Introduction
Impacted mandibular third molars (MTMs) may lead to caries, root resorptions, periodontal bone loss, cysts, or tumor formations [1]. Extracting the impacted MTM is a common surgical treatment in clinics, but one of its most serious complications is inferior alveolar nerve (IAN) injury. Te incidence of IAN injury in MTM extractions is 0.4%-8% [2]. Te MTM root, being adjacent to the mandibular canal, is the most prominent risk factor for IAN injury [3].
Bell et al. [4] proved that the diagnostic accuracy of panoramic radiography is relatively poor, so they recommended cone-beam computed tomography (CBCT) because it can provide accurate morphological images [5].
Tis includes the buccolingual structure, root number and curvature, shape of the mandibular canal, cortical disruption, and the distance between the mandibular canal and the MTM [6][7][8]. According to a recent study [9], the incidence of IAN injury is 28.6% when the mandibular nerve canal is located on the lingual side of the MTM, 28% when the mandibular nerve canal is in contact with the MTM, and 11.7% when the mandibular nerve canal is in contact with the MTM. Tis is signifcantly higher than the incidence of IAN injury during MTM extraction. Terefore, the use of CBCT can clarify the position of the mandibular canal in relation to the MTM preoperatively, predict the possibility of IAN injury, and facilitate communication between the surgeon and the patient.
It is important to be familiar with the situation of the mandibular canal in the MTM area in various demographics. Some scholars have confrmed diferences in the dental, maxillofacial, and craniofacial areas of diferent nationalities and in diferent regions [10,11]. Te mandibular plane of Korean people is fatter than that of Han people, and the mandible of Korean people shows a counterclockwise rotation and a low-angle trend [11]. Te diferent growth and rotation modes of the mandible are related to the position and curvature of the mandibular canal [12]. Tere are differences in the position of the mandibular canal among diferent ethnic groups [13], and the rate of cortical bone disruptions is also diferent under diferent positional relationships of the mandibular canal in the MTM [7,14]. Tis study aims to analyze diferences in the positional relationship between the MTM and the mandibular canal between Korean people from the Yanbian area and Han people. Tis study also aims to fll diferent nationality databases and supply an imaging basis to provide a reference for future clinical preoperative risk assessment.

Cone-Beam Computed Tomography Image Reading Analysis
(1) Te Winter classifcation was used to classify the impaction angles of the MTM. Te classes of impaction were vertical, mesial, horizontal, buccal, lingual impaction, distal, and inverted [15]. (2) Te Pell classifcation was used to record and divide the MTM impaction depths into high, middle, and low levels [16]. (3) Te Ueda classifcation was used to identify the cross-sectional shape of the mandibular canal when the distance between it and the MTM was at its shortest in the coronal section. Te shapes were round/oval, dumbbell, or teardrop and are shown in Figure 1 [7]. (4) Maegawa's classifcation was used as a reference to classify the positional relationship between the mandibular canal and the MTM at their shortest recorded distance in the coronal section. Te MTM and mandibular canal were located below the MTM root tip, the buccal and lingual positions, and between the tooth roots and are shown in Figure 2 [17]. (5) According to Park's classifcation [6], the cortical bone with the shortest distance between the mandibular canal and the MTM was categorized as either intact noncontact cortical bone, intact contact cortical bone, or disrupted cortical bone, as shown in Figure 3. (6) Te defning principle of root shape and number was used to divide the MTM root number into single, double, three, or four roots [8]. (7) Te shortest distance between the mandibular canal on the coronal plane and the root tip of the MTM was measured (accurate to 0.01 mm), and the average was recorded after three measurements, as is shown in Figure 4.

Statistical Methods.
All data were analyzed using IBM SPSS Statistics26.0 (Manufacturer International Business Machines Corporation, head ofce in Armonk, New York, USA) statistical software. Subject to skewed distribution, the measurement data were expressed using the median and interquartile range (P 25 -P 75 ) and analyzed using a nonparametric test. Te enumeration data were expressed as the sample size and percentage and compared using ordered logistic regression analysis, a chi-square test, and Fisher's exact test. Te diference was considered statistically signifcant at P < 0.05. Han patients by gender, location, and type of impaction (P > 0.05), as is shown in Table 2.

Comparative Analysis of the Diferent Positional
Relationships. Te relative positional relationship between diferent mandibular canals and MTMs in the Korean and Han patients was compared, and the results are shown in Table 3. Te percentage of mandibular canals located on the buccal side of the MTM in the Korean patients was signifcantly lower than that of the Han patients, and the diference was statistically signifcant (P < 0.05). Te percentage of mandibular canals located between the MTM roots in the Korean patients was signifcantly higher than in the Han patients, and the diference was statistically signifcant (P < 0.05).

Analysis of the Positional Relationship and the Shortest Distance between the Mandibular Canals and Mandibular
Tird Molars in Diferent Impaction Types. Te inverted, lingual, distal, and buccal impaction samples accounted for too small a sample size for each, so the vertical, mesial, and horizontal impaction samples were selected for statistical analysis. Te positional relationship and the shortest distance between the root and buccal side for the vertical, mesial, and horizontal impactions of the Korean and Han patients were compared. When the mandibular canal was located between the lingual side and the root, the shortest distance sampled between the Korean and Han patients was 0, which could not be statistically analyzed. For mesial impactions, the percentage of the mandibular canals located on the lingual side of the MTMs in the Korean patients was signifcantly lower than that in the Han patients, which was statistically signifcant (P < 0.05). However, the proportion of mandibular canals located between the roots of the MTMs in the Korean patients was signifcantly higher than in the Han patients, which was statistically signifcant    International Journal of Clinical Practice (P < 0.05). In horizontal impactions, the percentage of mandibular canals located between the roots of the MTM in the Korean patients was signifcantly higher than that in the Han patients, which was statistically signifcant (P < 0.05), and the results are shown in Table 4.
Tere was a signifcant diference in the median buccal shortest distances between the mesial and horizontal impaction (P < 0.05). According to the P 25 -P 75 comparison, the shortest distance samples of the Han and Korean patients were greater for the mesial and horizontal impactions. Te comparison of the shortest distances is shown in Table 5. Table 6, the results found that the shape distribution of the mandibular canal difered depending on the positional relationship (χ 2 � 169.349, P < 0.05).

Comparative Analysis of the Relationship between the Shape and Position of Diferent Mandibular Canals and the Cortical Bone. As is shown in
When the mandibular canal was located below the MTM root, on the buccal side, its shape in the Korean and Han patients was mainly round/oval. When the mandibular canal was located on the lingual side, between the roots of the MTM, its shape in the Korean and Han patients was mainly dumbbell-like. However, there was no signifcant diference in the shape of the mandibular canal in the Korean and Han patients in the same positional relationship (P > 0.05).

Comparative Analysis of the Risk of Cortical Bone Disruption with Diferent Impaction Types, Depths, and Root
Numbers. Taking the cortical bone status as the dependent   International Journal of Clinical Practice variable, the ordered logistic regression analysis method was used to study the nationality, impaction type, depth, and root number. Taking the Han patients as the reference level, the odds ratio value and its 95% confdence interval were obtained to represent the relative risk of cortical bone disruption in the Korean patients under the impaction type, depth, and root number of roots, as is shown in Table 7. Te results showed that the incidence of cortical bone disruption in the mandibular canal in the MTM area of the Korean patients was higher than in the Han patients. Te results also demonstrated that there was no signifcant diference in the risk of cortical bone disruption in all patients (P > 0.05). Furthermore, there was no signifcant diference in the risk of cortical bone disruption between the Korean and Han patients regarding diferent impaction depths and root numbers (P > 0.05). When the impaction type was horizontal, the risk of cortical bone disruption in the Korean patients was 1.980 times higher than in the Han patients, which was statistically signifcant (P < 0.05).

Discussion
Mandibular third molar extraction is one of the most common surgical procedures in oral and maxillofacial surgery. When the mandibular canal is close to the MTM, an IAN injury is easily caused during MTM extraction, resulting in numbness and pain in the lower lip [18]. A safer approach is to use three-dimensional CBCT to obtain more information about the position of the neural tube, which can be used to determine the most appropriate therapeutic regimen.

General Clinical Data and Distribution of Impaction Types of Korean People from the Yanbian Area and the Han
People. In this study, there were no signifcant diferences in gender, age distribution, the number of MTMs in diferent genders, and the number of diferent sides between the Korean and Han people. Tis means that further statistical analysis could be performed. Chen et al. [19] found in a CBCT study of 1,304 patients that vertical impactions were the most common type of impaction, followed by mesial and horizontal impactions, and that all other types were rare. When the MTM intersected with the mandibular canal, Xu et al. [3] found in a CBCT study of 537 patients that mesial and vertical impactions were the most common type of impaction and that horizontal impactions were the least common. Tis study screened 260 Korean and Han patients whose MTM root tips were in contact with or overlapped the mandibular canal. It was found that mesial impactions were the most common, followed by horizontal impactions, vertical impactions, and fnally, all other impaction types. Te reason for this situation may be that the difculty of vertical impaction extractions is low, so CBCTs are not taken. Te patients selected in this study were those who had high-difculty extractions for mesial and horizontal impactions.

Comparison of the Positional Relationship, Shape, and Cortical Bones of the Mandibular Canals and Mandibular Tird Molars in Korean People from the Yanbian Area and the Han People.
Tere are four positional relationships between the mandibular canal and MTM: subroot, buccal, lingual, and interroot. While it is difcult to show the relationship between the mandibular canal and the MTM in panoramic    International Journal of Clinical Practice radiography, some studies believe that the mandibular canal is mainly located on the buccal side of the MTM [20]. However, this study found that the mandibular canal in both Korean and Han patients was mainly located below the root of the MTM, which is consistent with studies by Xu et al. [3] and Quirino de Almeida Barros et al. [21] Diferent regions, nationalities, and dietary habits lead to diferences in craniomaxillofacial structures and tooth development [11]. Korean people are one of the main ethnic minorities in China, yet there are few studies comparing their MTMmandibular canal positional relationships to those of the Han people. Terefore, in this study, the MTM-mandibular canal positional relationships between the Korean and Han people were studied. It was found that there were obvious nationality-based diferences and that the diferences were statistically signifcant (P < 0.05). Except for the position below the root, the buccal distribution rate of the Han patients was higher, which is consistent with the results of the studies by Xu et al. [3], and it was signifcantly larger than that of the Korean patients. Te root-to-root incidence in Korean patients was higher than in the Han patients. Te diference in the positional relationship afects the shape of the mandibular canal and the rate of cortical bone disruption, thus afecting preoperative assessments of IAN injuries and the selected surgical treatments. Shiratori et al. [20] proposed that the cortical bone and the shape of the mandibular canal are reliable predictors of IAN injuries during MTM extractions. Te dumbbell shape has a larger area of cortical bone disruption than the teardrop oval shape, and patients showing cortical bone loss and dumbbell shapes during MTM extraction should be considered high-risk patients for mandibular canal injury [7,9,20]. In terms of the rate of cortical bone disruption of the mandibular canal, the root and buccal side of the Korean and Han people is lower compared with the rate of cortical bone disruption between the root and the lingual side, which is consistent with the results of Chen et al. [19] and Maegawa et al. [17] Tere was no signifcant diference between the Korean and Han people for the disruption rates in either location.
Diferent positional relationships afect how force is applied during tooth extraction [22]. For example, when the mandibular canal is located on the lingual side, the tooth root presses the lingual bone plate to diferent degrees when the dental elevator is used for dislocation, causing IAN injury. Shujaat et al. [23] believed that there was a potential relationship between the impaction type and the positional relationship. It is worth noting that in the Saudi population, 71.6%-78.6% of the vertical, mesial, and horizontal impacted mandibular canals are located on the lingual side of the root. Here, there is a signifcant correlation between the impaction type and the positional relationship. However, Quirino de Almeida Barros et al. [21] did not fnd a correlation between the impaction type and the positional relationship in the Saudi population. Te relationship between the impaction type and the positional relationship may be related to the ethnic group of the patient. In this study, the most commonly used clinical impaction types (vertical, mesial, and horizontal) were screened [3,5,19]. Tere was no correlation found between the impaction types and their locations. However, there was a diference in the positional relationship between the Korean and Han patients in the mesial and horizontal impactions. Te proportion of mandibular canals between the roots for the Korean patients was signifcantly higher than that of the Han patients in mesial and horizontal impactions. In mesial impactions, the proportion occurring on the lingual side of the Han patients was higher than that of the Korean patients, which indicated that there were ethnic diferences in the MTMmandibular canal positional relationship under diferent impaction types.
Tis study explored the shortest distances in diferent positional relationships and cortical bone disruptions under diferent impaction types. It was found that the shortest distance measured in the Korean patients was smaller than that of the Han patients on the buccal side of the mesial and horizontal impactions. Tis means that Korean people are more likely to have cortical bone disruptions on the buccal side, which may be related to the thicker buccal bone plate of the Han people or that their nerve tubes are closer to the buccal cortical bone. Although the shortening of the distance does not directly become a risk factor for an IAN injury in clinical surgery, the disruption of the cortical bone usually exposes the mandibular canal during operation, thus becoming a risk factor for IAN injuries [6,7,24].

Study on the Rate of Cortical Bone Disruption in Korean
People from the Yanbian Area and the Han People. In the MTM area, the mandibular canal has a complete bone tube composed of cancellous bone [25]. Tere is a continuous white line around the maxillary and mandibular canal on imaging [26], so it is called the cortical bone, with properties of cortical bone integrity and disruption. Cortical integrity is positively correlated with IAN injuries [6,7], and studies have confrmed that exposed mandibular canals have a greater rate of IAN injury than unexposed mandibular canals [27]. Te risk of cortical bone disruption was further analyzed using CBCT, which was compared with twodimensional panoramic radiography [28].
Wang et al. [29] examined 136 patients using CBCT and found that the cortical bone interruption rate of the mandibular canal was 46.3%. Chen et al. [19] found that for 1,304 MTMs, the cortical bone interruption rate of the mandibular canal was 20.2%. Park et al. [6] found that in 259 MTMs, the cortical bone interruption rate of the mandibular canal was 36.7%. Tis study found that in 213 MTMs, the cortical bone interruption rate of the mandibular canal of the Han patients was 48.4%, which was similar to the results of Wang et al. [29] Jung et al. [30] found that in 175 MTMs in South Korea, the cortical bone interruption rate of the mandibular canal was 56%. In this study, it was found that in 208 MTMs of Korean patients, the cortical bone interruption rate of the mandibular canal was 55.8% higher than that of the Han patients. However, the ordered logistic regression analysis results found no signifcant diference in the risk of cortical bone disruption between the Korean and Han patients (P > 0.05). 8 International Journal of Clinical Practice After further classifcation of the impaction types, there was a signifcant diference in the risk of cortical bone disruption between the Korean and Han people. Te risk of cortical bone disruption in the Korean patients was 1.980 times higher than that of the Han patients in horizontal impactions. Tis risk covered the calculation under the condition of intact noncontact cortical and intact cortical bones. It was more suitable for assessing the risk of injury to the IAN in clinical extractions of MTMs. Te results showed that in the horizontal impactions, in terms of the mandibular canal interruptions in the interroot, the rate for the Korean patients was signifcantly higher than for the Han patients. Additionally, the mandibular canal interruption rate was the highest in the interroot for both the Korean and Han patients.
Te deeper the impaction depth, the higher the risk of injury to the IAN of the MTM [31], as the deeper the teeth are, the more parts and opportunities there are to access the neural tube. Although the panoramic radiography screenings in this study have shown that the MTM root tip can come into contact with or overlap the mandibular canal, it was found that with the increase in depth, the cortical bone disruption rate also increases [19]. Te average cortical bone disruption rate was as follows: high (47.4%), medium (52.5%), and low (70.8%) in the Korean and Han groups, but the increase in impaction depth was not signifcantly correlated with nationality.
Previous studies on MTMs rarely considered the root number [19] because the root numbers and morphologies of MTMs varied greatly. However, in clinical practice, compared with single-root MTMs, extracting MTMs with multiple roots has a greater probability of damage to the mandibular canal [32]. In this study, the number of MTM roots in the Korean patients was the largest in double-root teeth, followed by single-root teeth, which was consistent with the number of roots in Korean people [33]. In comparing the number of roots and the risk of cortical bone in the Han patients, no diference was found between the number of roots and the risk of cortical bone disruption between the Korean and Han patients. Tis also suggests that although an increase in the number of roots increases the risk of cortical bone disruption, it is not refected in racial diferences at the same number of roots.
Tis study also has some shortcomings; as it is a retrospective study, the statistical data and analysis results are afected by the population size, distribution, ethnic afliation, and information errors in the region. Clarissa has reported the correlation of dental anomalies with certain patterns of malocclusion, an aspect that was not focused on in this study [34]. Additionally, future studies should be performed in order to evaluate also other variables and their possible relationship with third molar position, such as sella turcica bridging [35].

Conclusion
Tere are some diferences in the positional relationship between the MTM and mandibular canal and the rate of cortical bone disruption between Korean people from the Yanbian area and the Han people, all of which should be paid attention to clinically.

Data Availability
Te datasets used and/or analysed during the current study are available from the corresponding author on reasonable request. Te authors declared that materials described in the manuscript, including all relevant raw data, will be freely available to any scientist wishing to use them for noncommercial purposes, without breaching participant confdentiality.

Ethical Approval
Tis study was conducted with approval from the Ethics Committee of Afliated Hospital of Yanbian University. Tis study was conducted in accordance with the Declaration of Helsinki.

Consent
Written informed consent was obtained from all participants.