CBCT analysis of apical distance between second lower premolars, first and second lower molars and mandibular canal

The aim of this study was to determine the average vertical distances of the root apices of second premolars, first molars and second molars mesially and distally from the upper projection of the mandibular canal on the sagittal section of CBCT images, and determine if there were statistically significant differences between the age and gender groups. Material and methods The research was conducted at the Faculty of Medicine of the University of Banja Luka, and the sample consisted of 146 CBCT images of patients. CBCT images were obtained using Planmeca ProMax 3D Mid instrument (Planmeca, Helsinki, Finland) and analyzed using Planmeca Romexis Viewer software. In the sagittal section, the vertical distance from the root apex to the upper projection of the mandibular canal was measured for each examined tooth. Results The distal root of the first molar (4.88 mm) had the greatest average vertical distance of the root apex from the mandibular canal, and the distal root of the second molar had the smallest average vertical distance (2.76 mm). There was statistically significant difference between certain age groups in the values of individual roots, for the second molar mesially and distally (p < 0.05), while for the first molar mesially the value of p was at the limit of significance (p = 0.05). Conclusion The results of this study showed that distal root of the mandibular second molar had the smallest vertical distance from mandibular canal, therefore an extra caution during a root canal treatment and careful planning of oral surgery in this region is recommended.


INTRODUCTION
Cone-beam computerized tomography (CBCT) is a modern radiological imaging system, designed specifically for use in the maxillofacial region. The system overcomes many limitations of conventional radiography, creating a non-distorted, three-dimensional image of the examined area. It is used in endodontics to determine the morphology and dimensions of root canals, periapical lesions, detection and localization of resorptions, postoperative control and monitoring of treatment outcomes. It is also used in orthodontic treatment, as well as implant prosthodontics, facilitating prosthetic planning, selection of implants and the place of its installation [1, 2,3]. Also, successful endodontic treatment largely depends on an adequate radiographic method, which should provide critical information about examined teeth and their surrounding anatomy. Since its beginning, conventional radiography has remained the mainstay of auxiliary diagnostic methods in endodontics.
Data from the literature indicate that there are differences when comparing the distances of the tips of mandibular teeth from mandibular canal in relation to gender and age. However, numerous individual variations of the position of the canal in the mandible can occur, as well as the position of teeth and their mutual relationship [4]. Mandibular canal extends through the lower jaw from the mandibular opening (foramen mandibulae). In most cases, it is bilaterally symmetrical and in the form of one main canal on each side of the mandible, but variations are also possible. The contents of the mandibular canal are inferior alveolar nerve (nervus alveolaris inferior) and blood vessels of the same name. Inferior alveolar nerve is mixed and its terminal branch with its sensitive part innervates teeth and gums of the lower jaw, while motor part controls mylohyoid muscle and anterior belly of digastric muscle [5,6].
Many authors have confirmed that iatrogenic injuries of the inferior alveolar nerve are common (64.4%). Therefore, it is necessary to know the exact location of the mandibular canal and its contents for adequate endodontic treatment of lower posterior teeth as well as adequate resection of their root tips. Endodontic treatment includes mechanical use of instruments combined with chemical irrigation, medication agents and materials for final obturation of the canal system. During all these phases, the occurrence of unwanted complications is possible -mechanical, chemical or thermal injury to the nerve that can cause neuropathic pain or anesthesia in its innervation zone. During endodontic treatment of 1% of lower premolars and even 10% of the lower second molars there is possibility of an injury to lower alveolar nerve. Injury to the content of the mandibular canal is also possible during oral surgery. Such injuries sometimes require treatment in the form of microsurgical decompression of the inferior alveolar nerve [7][8][9][10].
The aim of this retrospective study was to determine the average vertical distances of the root apices of second premolars, first molars and second molars mesially and distally from the upper projection of the mandibular canal on the sagittal section of the CBCT, and determine whether there were statistically significant differences between the age and gender groups.

MATERIAL AND METHODS
The research was approved by the Ethics Committee of the Faculty of Medicine at the University of Banja Luka (18/4.141/21). The sample included 146 CBCT images of patients where the vertical canal distance of the second premolar, first molar and second molar was measured distally and mesially and statistical significance of the difference in relation to gender and age was examined. The initial database included 174 images, of which 146 met the criteria for sample selection: the presence of at least one tooth of importance for research (second mandibular premolar, first mandibular molar and second mandibular molar) and visibility of the mandibular canal on the image. Teeth with internal and external root resorption and endodontically treated teeth were not included in the study. The research included images made in the period from January 1 st 2018 until December 31 st 2018.
The imaging process was performed using Planmeca ProMax 3D Mid camera (Planmeca, Helsinki, Finland), and CBCT images were analyzed using Planmeca Romexis Viewer software (Figure 1). On the sagittal section, for each tooth (second mandibular premolar, first mandibular molar, second mandibular molar), vertical distance of the root apex to the upper projection of the mandibular canal was measured ( Figure 2). For teeth with two roots, the distance was measured for each root separately ( Figure 3).
Mandibular canal on sagittal section shows variability in appearance and usually appears as a radiolucent circle, which can be up to 4 mm in diameter. To facilitate the identification of mandibular canal, mental opening was identified on sagittal section, and the canal was followed to the level of the apex of the corresponding tooth [11].
A total of 406 measurements were performed, of which 146 for the second premolar, 42 for the first molar mesially, 42 for the first molar distally, 88 for the second molar mesially, and 88 for the second molar distally (Table 1). After the measurements, obtained values were divided according to the gender and age of the patients ( Table 2). Based on age, the sample was divided into the three groups: group  A, which included CBCT images of patients younger than 17, group B, which included CBCT images of patients aged 18 to 49, and group C, which included CBCT images of patients older than 50.

STATISTICAL ANALYSIS
All data were presented in tables and figures. R Studio Version 3.6.2 was used to analyze the obtained data. Data were processed with a 95% significance level using The Kruskal-Wallis and The Mann-Whitney U test. Based on the measured values, the average vertical distances for each root were determined, as well as average vertical distances for each root by age groups. It was also examined what this statistical significance is reflected in, by comparing individual groups separated by gender. In groups of teeth where significant statistical deviation was observed, CBCT images of persons of one gender, from one age group, were compared with CBCT images of persons of the same gender from another age group.

RESULTS
Analysis of the results of the average vertical distance for each root from the upper projection of mandibular canal showed that distal root of first molar (4.88 mm) had the highest average vertical distance and distal root of second molar (2.76 mm) the smallest. The average distance for first molar mesially was 4.86 mm, for second molar mesially 3.01 mm, and for second premolar 4.23 mm ( Figure 4).
Observed by age groups, in the group A, second molar had the greatest distance from the mandibular canal mesially (4.69 mm), and second premolar had the smallest distance (3.2 mm).
In the group B, the greatest average vertical distance of the root apex from mandibular canal was shown in first molar mesially (4.4 mm) and the smallest in second molar distally (2.2 mm). In the group C, first molar distally had the highest average vertical distance (6.4 mm), and second molar distally had the smallest distance (3.44 mm). With statistical significance of 95% and using The Kruskal-Wallis test, the average vertical distance by types of premolars and molars in relation to age groups was observed ( Table 2) but there was no statistically significant difference, p> 0.05. With statistical significance of 95% and using The Mann-Whitney U test, it was observed whether there was a statistically significant difference between age groups for each individual root and found that in the first molar mesially there was a statistically significant difference between groups B and C, with note that the value of p was at the significance limit (p = 0.05). In the second molar distally, statistically significant difference was observed between groups A and B, as well as between B and C (p < 0.05), and in second molar mesially between groups B and C (p < 0.05). No statistically significant difference was observed in other groups.
Previously mentioned groups, where statistically significant difference was found were further divided by gender in the analysis, after which persons of one gender from one age group were compared with persons of the same gender from another age group.
Comparing the vertical distance between CBCT images of females by age groups B and C for the first molar mesially, it was found that the difference was not statistically significant (p>0.05), while in males there was significant difference (p<0.05) ( Figure 5).
Comparing the vertical distance between females by age groups B and C for the second molar distally, it was found that the difference was not statistically significant   (p>0.05), while for the same groups in males statistically significant difference was found (p<0.05) ( Figure 6). Although there is a statistically significant difference for the whole age groups B and C in the second molar mesially, observing the same by gender, and from different age groups no statistically significant difference was found (p > 0.05) (Figure 7).
No statistically significant difference between different age groups in women was found, while in men it was observed only in the first molar mesially and the second molar distally, between groups A and B.
Comparing the average vertical distances for each root by gender, it was observed that all average values were higher in males. For men, the greatest average vertical distance was present in the root of first molar distally (5.57 mm) and the smallest in second molar distally (3.52 mm). In women, also, first molar distally had the greatest average vertical distance (3.50 mm), and second molar mesially had the smallest (1.64 mm) ( Figure 8).

DISCUSSION
Development of modern radiological imaging system and CBCT, designed specifically for use in maxillofacial region, allowed obtaining timely information relevant to endodontic, surgical or endodontic-surgical treatment [12,13,14]. CBCT overcomes many of the limitations of conventional radiography, creating a non-distorted, three-dimensional image of the examined area and allowing visualization of the images by layers and sections in all three dimensions [15].
The results of our study showed that distal root of the first molar (4.88 mm) had the greatest average vertical distance of the apex from mandibular canal. Distal root of second molar (2.76 mm) had the smallest average vertical distance, it is more gracile than the mesial and slightly distally oriented, and its close relationship with the mandibular canal can be attributed to the trajectory and its position in the mandible. Uğur Aydın et al. obtained partially similar results in Turkish population, where they found, based on CBCT images, that distal root of second molar (2.75 mm) had the smallest average distance from mandibular canal, while the greatest average distance was registered in the mesial root of the first molar (4.98 mm) [16].
In their study on CBCT images, Lvovsky et al. concluded that mesial root of the first molar (6.18 mm) had the greatest average distance, and distal root of the second molar (3.42 mm) had the smallest distance to mandibular canal [17]. The mean value for the mesial root of the first molar in our study was 4.86 mm.
Aksoy et al., by measuring the shortest distance from the mandibular canal, also found that the roots of the   second molar were significantly closer to mandibular canal than the roots of the first molars, first, and second premolars [18]. In the study of Denio et al. that analyzed anatomical relationships of the mandibular canal and lateral teeth by dissecting 22 mandibles, it was concluded that second mandibular premolar and second mandibular molar had the smallest distance from mandibular canal [19].
Using CBCT images, Kosumarl et al. found that mesial root of the first molar had the greatest average distance from mandibular canal, while the shortest was in distal root of second molar, both in persons with normal skeletal jaw ratio or skeletal open bite [20]. With the exception of the third molar, Pucilo et al. concluded in a systematic review that distal root of the second molar was closest to the mandibular canal, which coincided with the results of our study [21]. Littner et al. measured the average values of the distance from the apex of first and second molars from the canal on radiographic images of the cadaver and they were between 3.50 and 5.40 mm, and according to a study conducted by Kovisto et al. on CBCT images, these average values were between 1.51 and 3.43 mm [22,23]. In our study, the average values of the distance of the apices in lower lateral teeth from the mandibular canal ranged between 2.76 and 4.88 mm.
Observing the average vertical distances for each root by gender, it was noticed that all average values were higher in males, which could be related to more gracile constitution of women and smaller dimensions of the lower jaw. Similar findings were reported by other researchers (Aksoy et al. 2017; Simonton et al. 2009), who found that distances between the apices of the lower premolars and lower molars and mandibular canal were smaller in female population [18,24].
Sato et al. performed research on cadavers, and with the help of CT images and panoramic radiography measured the distances from the root apex to the upper projection of the mandibular canal. Data were grouped by gender and side of mouth. They recorded slightly lower values of the distance in first and second molars from the mandibular canal in women, compared to men [25].
It has been shown that there is four times higher possibility of developing chronic pain after endodontic treatment, if the patient is a female person, as the incidence of postoperative pain in women is also higher [26].
For a definitive conclusion on the reliability of such measurements of the distance of the tooth apices from the mandibular canal, verification in a larger group of teeth is necessary. The deviations in the measurements can be explained by the fact that they were obtained through different sagittal sections of the CBCT, which could affect the repeatability of these measurements.

CONCLUSION
Measurements obtained in our study and statistical analyses showed that distal root of mandibular second molars had the smallest vertical distance from the mandibular canal, so careful canal instrumentation and careful planning of oral surgery in this region is recommended.