Quantitative analysis of the relationship between maxillary incisors and the incisive canal by cone-beam computed tomography in an adult Japanese population
Matsumura et al. Progress in Orthodontics
Quantitative analysis of the relationship between maxillary incisors and the incisive canal by cone-beam computed tomography in an adult Japanese population
Tomonari Matsumura 0
Yuji Ishida 0
Ayako Kawabe 0
Takashi Ono 0
0 Orthodontic Science, Department of Oral Health Sciences, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University , 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549 , Japan
Background: In setting goals for orthodontic treatment, determining the morphologies of the alveolar bone and maxillary incisor root is important for avoiding root resorption, dehiscence, and fenestration. This study aimed to analyze the configurational relationships among maxillary incisors, the alveolar border, and the incisive canal by cone-beam computed tomography (CBCT). Methods: Cone-beam CT images of 93 orthodontic patients were evaluated for length of the incisive canal (L); angles between the palatal plane and the maxillary alveolar border (θ1), the incisive canal (θ2), and maxillary incisor (θ3); distance from the left maxillary incisor to the incisive canal (D); and cross-sectional areas of the incisive canal (CSAs) at three vertical levels. Comparison of variables between male and female patients was performed with the two-sample t test. Correlations between parameters were examined by Pearson's correlation analysis and Bonferroni correction for multiple comparisons. Results: Male patients exhibited significantly greater values of L than female patients. There were significant positive correlations between θ1 and θ2, θ2 and θ3, and θ3 and θ1. While the value of D was the lowest at the oral opening, that of the cross-sectional area of the incisive canal (CSA) was the greatest at the incisal root apex. Conclusions: This study demonstrated that the incisive canal had large inter-individual variability, and the proximity between the incisive canal and the incisal root could not be precisely predicted by the conventional cephalograms. Therefore, pre-treatment CBCT examination should be recommended when a large amount of maxillary anterior retraction and/or intrusion is planned in orthodontic diagnosis.
Orthodontic patients commonly require improvement of
facial esthetics. Spatial position of the maxillary incisor
is a critical factor in both facial esthetics and
maxillofacial function [
]. Therefore, three-dimensional (3D)
position and inclination of the maxillary incisor is
regarded as a dominant factor when setting goals for
orthodontic treatment [
]. With regard to maxillary
incisor movement, Ackerman et al. presented the
concept of “envelope of discrepancy” [
], which describes
the limitations of the range of orthodontic movement of
the maxillary incisor [
]. Contact with hard tissue
structures, such as the labial, palatal, or incisive canal cortical
plates, is a risk factor for apical root resorption in the
maxillary incisor [
], and it is one of the iatrogenic
complications of orthodontic treatment. In addition,
excessive tooth movement during orthodontic treatment
can induce root deviation from the alveolar housing of
dentition, leading to dehiscence and fenestration [
Limitation of maxillary incisor movement in conjunction
with cephalometric analysis has been proposed in order
to avoid these potential complications; however, this
approach remains controversial [
In orthodontics, diagnosis and treatment planning
have generally been performed by two-dimensional (2D)
analysis of lateral and anteroposterior cephalometric
measurements. However, the incisive canal and cortical
plate on the sagittal plane of maxillary incisors cannot
be precisely evaluated in conventional cephalometric
radiographs. Recent developments in 3D analysis of dental
cone-beam computed tomography (CBCT) images could
help obtain more detailed information. Several studies
using dental CBCT have suggested that determining the
morphologies of maxillary incisor roots, the incisive canal,
and the maxillary alveolar bone is important when setting
goals for orthodontic treatment [
8–10, 14, 15
While previous CT and CBCT studies have clarified
the anatomical relationships between maxillary incisors
and the incisive canal [
], they suffered from
inadequate image resolution for precise evaluation of alveolar
bone shape and thickness. The aim of this study was to
analyze the configurational relationships between
maxillary incisors and the incisive canal in the anterior region
of the maxillary alveolar bone using CBCT images.
From among patients between the ages of 18 and
39 years who sought orthodontic treatment at the Tokyo
Medical and Dental University Dental Hospital between
2012 and 2015, only those who required CBCT for
diagnosis and treatment planning were selected. Written
consent was obtained from all subjects after explanation
of the research aims and goals.
The exclusion criteria were (1) history of orthodontic
treatment, (2) missing or supernumerary maxillary
incisors, 3) midline deviation of maxillary incisors ≥2 mm
from the facial midline, (4) prosthodontic treatment of
maxillary incisors, 5) evident nasopalatine pathology
(e.g., nasopalatine duct cysts), (6) history of trauma to
maxillary incisors, and 7) congenital anomalies (e.g., cleft
lip and palate). Based on the inclusion and exclusion
criteria, 93 subjects (male, 31; female, 62; mean age, 24.3 ±
5.6 years) were finally selected. Their skeletal pattern
was Class I, Class II, and Class III and their mean ANB
was 3.1 ± 3.5 (range −4.6–9.0).
As a part of pre-treatment examination of each patient,
CBCT (Finecube; Yoshida Dental MFG. Co., Tokyo,
Japan) images of the maxillary and mandibular
dentoalveolar regions were acquired for diagnosis and treatment
planning, using the following settings: normal mode
(16.8 s, 4.10 mGy, 90 kV, and 4 mA); slice thickness,
0.147 mm; field of view (FOV), 81 × 74 mm; and voxel
size, 0.146 mm. All images were acquired with the head
positioned along the Frankfort horizontal plane, running
parallel to the floor. Images were saved as digital
imaging and communication in medicine (DICOM) files,
and sagittal and horizontal views of those were extracted
and evaluated using an image analysis software (ImageJ
version 1.48; National Institute of Mental Health, MD,
USA). Prior to measurement, the three dimensions were
calibrated and the three planes (i.e., sagittal, horizontal,
and coronal) defined in each image (Fig. 1).
In the midsagittal plane, linear and angular
measurements were defined as follows (Fig. 2):
P: palatal plane
L: length of the incisive canal
θ1, θ2, and θ3: angles between the palatal plane and
axes of the maxillary alveolar border, the incisive canal,
and maxillary left central incisor, respectively.
Linear and area measurements were acquired in the
horizontal plane at three vertical levels: n, r, and o (levels
of the nasal opening of the incisive canal, root apex of
the maxillary incisor, and oral opening of the incisive
canal, respectively; Fig. 3), and the distance from maxillary
incisors to the incisive canal (D) and the cross-sectional
area of the incisive canal (CSA) were measured at each
level (Dn, Dr, and Do, respectively; CSAn, CSAr, and
CSAo, respectively; Fig. 4). All measurements were
performed by a single examiner, who repeated each
measurement after a 2-month interval. The Dahlberg formula was
used to calculate method error, as follows:
where d = difference between two measurements and
n = number of measurement pairs [
All statistical analyses were performed using the
computer software package SPSS 22.0 (IBM, Armonk, NY,
USA). Mean values and standard deviations (SDs) were
calculated for all measurements. Comparison of
variables between male and female patients was performed
with the two-sample t test. Correlations between
parameters were examined by Pearson’s correlation analysis
and Bonferroni correction for multiple comparisons.
The significance level for all analyses was set at p < 0.05.
Method errors for linear measurements L, Dr, and Do
ranged between −0.2 and 0.2 mm, −0.4 and 0.5 mm, and
−0.3 and 0.5 mm, respectively, while those for angular
measurements θ1, θ2, and θ3 ranged between −1.5° and
2.5°, −1.4° and 1.3°, and −1.3° and 1.1°, respectively.
Method errors for area measurements CSAn, CSAr, and
CSAo ranged between −5.9 and 6.0 mm , −5.5 and
6.2 mm , and −4.9 and 5.5 mm2, respectively. There
were no significant differences between the original and
repeat measurements of any of the parameters.
The descriptive statistics of all measurements in the
sagittal plane are presented in Table 1. Only L and θ2
exhibited significant sex-specific differences. The value
of L in male patients (13.8 ± 2.2 mm) was significantly
greater compared to that in female patients (12.2 ±
2.3 mm; p < 0.05), while θ2 in male patients (105.5° ± 8.6°)
was significantly smaller compared to that in female
patients (109.6° ± 7.9°; p < 0.05). There were significant
positive correlations between θ1 and θ2 (p < 0.01; r =
0.719), θ2 and θ3 (p < 0.01; r = 0.488), and θ3 and θ1
(p < 0.01; r = 0.628; Table 2). While Dr (4.1 ± 1.8 mm)
was significantly greater than Do (3.2 ± 1.3 mm; p < 0.05),
CSAr (81.0 ± 17.3 mm ) was significantly greater than
Data are presented as mean ± standard deviation
*p < 0.05
NS not significant
CSAn (74.0 ± 13.5 mm2) and CSAo (74.5 ± 13.3 mm ;
both p < 0.05; Fig. 5).
To our knowledge, this is the first study involving a large
population to investigate the anatomical characteristics
of maxillary incisors, the incisive canal, and the
maxillary alveolar border using CBCT images. First, in the
present study, the length of the incisive canal in male
patients was significantly greater compared to that in
female patients, which is concordant with the findings of
a previous study [
]. Second, inclination of maxillary
incisors was significantly correlated with those of the
maxillary alveolar border and axis of the incisive canal.
Moreover, maxillary incisors were located closer to the
incisive canal at the level of the root apex than at the
level of oral opening of the incisive canal, and the CSA
of the incisive canal at the level of the maxillary incisor
root apex was significantly greater compared to those at
the levels of oral and nasal openings.
Since DICOM data are divided into 512 × 512
matrices, a large FOV corresponds to large voxel size and
reduction in image resolution. In contrast, a limited
FOV provides high visibility, but leads to an increase in
radiation dose per unit of tissue. Thus, FOV should be
set with consideration of object size for evaluation
]. In orthodontics, accurate diagnosis requires
careful observation of not only of the alveolar bone
but also dental roots. The FOV of CBCT images in
the present study (i.e., 81 × 74 mm) was smaller in
comparison with those reported in previous studies
]. This limited FOV, set in order to achieve an
adequate voxel size (0.146 mm) for accurate
diagnosis, provided more accurate information.
The incisive canal, an anatomical structure located on
the midsagittal plane of the maxillary bone running
parallel and posterior to maxillary incisors, involves the
nasopalatine vessels and nerves, branches of the
trigeminal nerve, and the maxillary artery and is surrounded by
a thick layer of cortical bone [
]. Because of its
proximity to maxillary incisors, the possibility of sensory
dysfunction in the anterior region and failure of
osseointegration has been reported in cases of contact of the
incisive canal through surgical interventions such as dental
implant placement [
]. Recently, Chung et al.
reported that proximity of maxillary incisal roots to the
incisive canal might influence the degree of root resorption
after large incisal retraction [
]. Therefore, when
planning orthodontic treatment, it is critical to confirm the
exact location of maxillary incisors and the incisive canal
and determine the morphology of the alveolar bone.
Imaging of anatomical structures at the maxillary anterior
region by CBCT with a limited FOV has not been
documented in literature. In the present study, the
morphologies of and positional relationships between the incisive
canal and maxillary incisors were evaluated by CBCT
images acquired with a limited FOV.
For calculating the degree of maxillary incisor
movement, 2D analysis can provide only limited information
regarding the 3D maxillofacial structures. Apical root
resorption has been frequently reported in the maxillary
incisor region [
]. In some cases, unexpected apical
root resorption in maxillary incisors after anterior
retraction has been reported to have occurred because of
proximity or contact of the roots with the labial, palatal,
or incisive canal cortical plates [
]. In clinical
settings, several orthodontic treatments till date have
been successfully performed solely with conventional
cephalometric analysis. However, recent temporary
anchorage devices (i.e., miniscrew implants) have expanded
the range of orthodontic treatment and made it possible
to achieve a large degree of maxillary incisor movement
6, 27, 31
]. To manage post-orthodontic treatment
complications such as root resorption, gingival recession,
dehiscence, and fenestration following root deviation
from the alveolar bone housing, anatomical features of
the maxillofacial area should be carefully examined in
each patient, and diagnosis should be formulated based
on 3D information. The present findings suggest that
FOV-limited CBCT is a useful modality for orthodontic
diagnosis of maxillary protrusion.
Loss of a maxillary incisor affects the morphology of
the maxillary alveolar border and, consequently, alters
the morphology of the anterior wall of the incisive canal
]. Moreover, changes in location and inclination of
the maxillary incisors lead to morphological changes in
the maxillary alveolar border [
]. Therefore, pre
and post-orthodontic treatment FOV-limited CBCT
analyses for assessment of morphological changes in the
maxillary anterior region and the incisive canal are required to
ensure precise evaluation of tooth movement-induced
anatomical changes in the surrounding tissues [
Anatomic variations that are present in the anterior
region of the maxillary alveolar bone yielded
morphometric data that might be useful for orthodontic
treatment planning in patients requiring significant
correction of maxillary incisal inclination or root
position or in patients requiring implant placement in
the anterior region.
2D: Two-dimensional; 3D: Three-dimensional; CBCT: Cone-beam computed
tomography; CSA: Cross-sectional area of the incisive canal; D: The distance
from the left maxillary incisor to the incisive canal; DICOM: Digital imaging
and communication in medicine; FOV: Field of view; L: The length of the
incisive canal; n: The level of the nasal opening of the incisive canal; o: The
level of the oral opening of the incisive canal; P: Palatal plane; r: The level of
the root apex of the maxillary incisor; θ1: The angles between the palatal
plane and the maxillary alveolar border; θ2: The angles between the palatal
plane and the incisive canal; θ3: The angles between the palatal plane and
This research did not receive any specific grant from funding agencies in the
public, commercial, or not-for-profit sectors.
YI participated in the study conception, data collection, and manuscript
formatting. AK participated in the study conception, data collection,
manuscript formatting, and statistical analysis. TO participated in the study
conception, and manuscript formatting. All authors read and approved the
Ethics approval and consent to participate
This study was approved by the Institutional Ethical Committee of the Tokyo
Medical and Dental University (approval numbers: 846 and 1254) and
conducted in accordance with the Declaration of Helsinki.
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in
published maps and institutional affiliations.
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