Background
In the clinical environment of oral medicine, extraction of the lower third molar is a common surgical procedure. However, various postsurgery complications can occur [
1‐
5]. Among such complications, injury to the inferior alveolar nerve (IAN) is the most severe. Because the IAN occasionally contacts or is near the third molar/root, or molars with a “crooked root,” the IAN can be easily damaged during this procedure. Temporary and permanent IAN injury comprises approximately 5.0–7.0 % and 0.5–1.0 % of such incidents, respectively [
2,
6‐
12]. Therefore, information regarding the spatial relationship between the third molars and IAN is critical for preoperative procedures.
Clinically, taking panoramic film is essential for evaluation before extraction. However, because panoramic film is a two-dimensional imaging tool, the image can be distorted or overlapped [
13]. Nakagawa et al. [
14] stated that this may lead clinicians to misinterpret the results or make incorrect judgments. Therefore, to minimize postoperative complications and improve judgments, computed tomography (CT) is introduced to evaluate the spatial relationship between the third molar and IAN canal.
Based on the Cartesian coordinate system concept, Maegawa et al. [
15] reported that the probability of the IAN canals being positioned at the buccal, lingual, and inferior sides of the lower third molar and between the roots was 51, 26, 19, and 4 %, respectively. Ghaeminia et al. [
16] employed a similar method on Westerners and reported observations on the buccal side (17 %), lingual side (49 %), inferior side (19 %), and between the roots (15 %). In addition, various researchers have reported inconsistent data including the trends of position probability. This is primarily because of measurement position discrepancies among various races; it may also have resulted from differences in the tooth morphology of the mandible structure and lower third molar root, which can cause approximate and imprecise classification of the teeth (such as between the inferior and buccal and between the inferior and lingual), leading to judgment errors and varying results.
Accordingly, this study was conducted to analyze the spatial relationship between impacted third molars and the IAN, the results of which were compared with those of previous studies. Specifically, this study was aimed at (1) establishing the distribution between impacted third molars and the IAN, and (2) investigating the relative position between the lower third molars and IAN by using a cylindrical coordinate system.
Results
Based on the selection criteria, mandibular bone CT images of 75 patients (age
mean 37.0 + 18.9 years; age
range 18–77 years) were used in this study. From the 75 mandibular bones, 137 lower third molars (100 noncontact cases; 37 contact cases) were used for the measurement. For the Cartesian coordinate system (Table
1), the IAN distribution relative to the third molar was 78.8 % (108/137) in the inferior position, 11.7 % (16/137) in the lingual site, 8.8 % (12/137) in the buccal site, and 0.7 % (1/137) between the roots (Table
1).
Table 1
Relationship between the IAN and third molar using the Cartesian coordinate system
Buccal | 3 | 9 | 12 | 8.8 |
Lingual | 8 | 8 | 16 | 11.7 |
Between roots | 1 | 0 | 1 | 0.7 |
Inferior | 25 | 83 | 108 | 78.8 |
Total | 37 | 100 | 137 | 100 |
Regarding the relationship between the IAN and third molar in an angular distribution, no case between 60° and 270° was observed using the cylindrical coordinate system (Table
2). The highest probability region with 43.8 % was between 330° and 360°, followed by 32.1 % between 0° and 30°, and 21.2 % between 300° and 330°. The lowest probability with 0.8 % was between 270° and 300°. In addition, for the noncontact cases, Table
3 lists the shortest distances between the mandible canal and third molar. Most of them occurred above the 3-mm group, 11 % were in the 1–2-mm group, 11 % were at 1–2 mm, and only 3 % were in the 0–1-mm group.
Table 2
Orientation of the lower third molar in relation to the IAN using the cylindrical coordinate system
0 ~ 30° | 23 | 21 | 44 | 32.1 |
30° ~ 60° | 1 | 2 | 3 | 2.1 |
60° ~270° | 0 | 0 | 0 | 0 |
270° ~ 300° | 1 | 0 | 1 | 0.8 |
300° ~ 330° | 6 | 23 | 29 | 21.2 |
330° ~ 360° | 6 | 54 | 60 | 43.8 |
Total | 37 | 100 | 137 | 100 |
Table 3
The shortest distance between the mandible canal and the lower third molar for non-contact cases
0 ~ 1 mm | 3 | 3 |
1 ~ 2 mm | 11 | 11 |
2 ~ 3 mm | 11 | 11 |
>3 mm | 75 | 75 |
Total | 100 | 100 |
Discussion
To reduce the risk of IAN injury when extracting the mandibular third molars, the appropriate tools must be used. Therefore, using CT images to accurately identify the relationship between the molars and IAN at the buccolingual section is crucial. In previous studies, the reported probability of various IAN positions relative to the lower third molars has been inconsistent. This is possibly due to the human discrimination affects derived from the subjective judgments of different dentists. The morphologies of the alveolar bone and lower third molar are highly divergent, which easily leads to approximate and imprecise distinguishing and classification (such as between the inferior and buccal sides and between the inferior and lingual sides). This eventually results in judgment errors and inconsistent results. Compared with the traditional Cartesian coordinate system, the anatomical structure can be categorized more objectively and accurately by using the cylindrical coordinate system. This study developed a method that involves using a cylindrical coordinate system to assess the relationship between the IAN and lower third molar for Asian populations.
Previous studies have compared the ability of CT and panoramic filming for evaluating the relative position of the IAN and third molar [
15,
17]. These studies have indicated that CT is an efficient tool for determining the relative position between the IAN and lower third molar. Although CT scans were employed in determining the relative position, we varied from other studies [
15,
17,
18] in that the Down’s mandibular plane was employed because resectioning methods cause the buccolingual section slices to become perpendicular to the mandibular canal.
In recent years, dental cone-beam CT (CBCT) has been prevalently employed in dental surgery [
19‐
21] and basic research [
22‐
25]. In addition to the presurgical orthodontic, orthognathic, and endodontic treatment evaluations, CBCT images also frequently serve as a reference when extracting impacted lower third molars [
16,
26]. Therefore, although a CT image database was employed in the present study, the proposed cylindrical coordinate system proposed can also be applied to CBCT images.
The spatial relationships between the IAN and lower third molar identified in this study were compared with those reported in literature (Table
4). When the Cartesian coordinate system was used, the IAN distribution relative to the third molars was found to be consistent with that reported by Tantanapornkul et al. [
26] (Table
4). The highest distribution was on the inferior side, and the probability distribution on the lingual side was slightly higher than that on the buccal side; the lowest distribution was between the roots. Similarly, Monoco et al. [
9] also indicated that the probability that the IAN would be located on the inferior side was the highest (Table
4). However, Ueda et al. [
27] and Maegawa et al. [
15] reported that the probability of the IAN distribution on the buccal side was higher than that on the lingual side (Table
4). Ghaeminia et al. [
16], de Melo Albert et al. [
28], and Ohman et al. [
17] have reported that the probability of the distribution on the lingual side was the highest (Table
4). Variations in the results of previous studies and those of the present study may be due to the differing patient ethnicities of the patients and the various orientations of the mandibular bone in the buccolingual slices.
Table 4
The buccolingual distribution of the IAN in relation to the lower third molar, as reported in previous studies
Buccal | 8.9 % | 45.5 % | 17 % | 25 % | 45 % | 31 % | 25 % | 51 % |
Lingual | 11.8 % | 32.4 % | 49 % | 26 % | 48 % | 33 % | 19 % | 26 % |
Between roots | 0.7 % | 0.7 % | 15 % | 4 % | | 10 % | 5 % | 4 % |
Inferior | 78.6 % | 21.4 % | 19 % | 45 % | 7 % | 26 % | 51 % | 19 % |
Number | 136 | 145 | 53 | 142 | 31 | 90 | 73 | 47 |
Regarding the experimental results of previous studies, Maegawa et al. [
15], Tantanapornkul et al. [
26], and Ueda et al. [
27] have investigated conditions in Japan, which is near Taiwan. However, based on the Cartesian coordinate system, the spatial distributions in the experimental results of these studies are inconsistent. Maegawa et al. [
15] and Ueda et al. [
27] have reported that the highest distribution was on the buccal side, followed by the lingual side, inferior side, and then between the roots. The experimental results of the current study were similar to those reported by Tantanapornkul et al. [
26], which indicated that the highest distribution was on the inferior side, followed the lingual side, buccal side, and then between the roots. This difference may have resulted from the ambiguous classification derived from the Cartesian coordinate system, particularly in the buccoinferior and linguoinferior regions.
Although the cylindrical coordinate system is used in orthodontics [
29], it is not widely employed for oral surgery. During extraction of the lower third molar, protecting the IAN is critical. This raises questions as to why observations are made from the perspective of the molar to be extracted and why the position of the nerve is categorized relative to the molar. Instead, we propose setting the IAN requiring protection as the origin and then observing the relative position of the nearby molar. Therefore, in the present study, we used a cylindrical coordinate system to categorize the relationship between the IAN and lower third molar. The advantage of using the cylindrical coordinate system is that it simplifies classifying the relationship between the third molars and IAN. The two parameters identified by this system—namely, the relative angle and shortest distance between the mandible canal and third molars—provide quantitative data and a detailed understanding for further investigation. The angle data not only reveal the distribution between the IAN and lower third molar, but may also prevent misunderstanding among clinical practitioners and researchers. Furthermore, the shortest distance between the lower third molar and mandible canal provides additional information regarding the distance between the mandible canal and third molar root, which is not available when using the Cartesian coordinate system. The shorter the distance is between these two structures, the greater the possibility for IAN injury during extraction of the lower third molar. Therefore, determining this distance is crucial. As shown in Table
3, an increment of 1 mm was adopted as the scale. Our observations indicated that for 75 % of all cases (75 cases), the distance between the IAN and lower third molar exceeded 3 mm, and for 3 % of cases (3 cases) the distance was shorter than 1 mm. In two of these three cases, removing the impacted third molars presented greater difficulty (the IAN was located on the lingual side relative to the lower third molar); thus, removal of impacted lower third molars necessitates additional caution.
To prevent situations where whether the IAN is located in the buccal or inferior region (or in the lingual or inferior region) cannot be determined, the position (
r,
θ) of the IAN relative to the lower third molar can be determined using the cylindrical coordinate system. Although Miller et al. [
30] proposed inferior–lingual and inferior–buccal classifications regarding the relationship between the IAN and lower third molars, communication (particularly verbal) between clinical practitioners remain vague; by comparison, the cylindrical coordinate system (
r,
θ) provides superior clarity.
This study was subject to several limitations. First, all human samples were of Asian ethnicity, and whether the differences would occur in other ethnic groups was not explored. Second, the effects of gender and age were not investigated because of the small sample. Third, given the current status of the cylindrical coordinate system, the segmentation of impacted lower third molars was based on a manual approach. In future, an automated approach should be developed to enhance the clinical application of this system. Finally, unlike the prevalent use of the Cartesian coordinate system, the cylindrical coordinate system remains a new method for analyzing the relationship between the IAN and lower third molar. Comprehensive investigations are necessary to further verify the academic results and clinical applicability of this approach.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
MC, HH, and JH participated in the design of the study. WW, LF, and MT performed the measurement. MT and HH conducted the statistical analysis. WW and JH conceived the study, participated in its design and coordination, and drafted the manuscript. All authors read and approved the final manuscript.