Remsh K. Al-Rokhami,Karim A. Sakran,Maged S. Alhammadi,Barakat Al-Tayar,Waseem S. Al-Gumaei,Enas S. Al-Yafrusee,Lina H. Al-Shoaibi,Baocheng Cao
Tridimensional Analysis of Incisive Canal and Upper Central Incisor Approximation
Envelope of discrepancy,Facial divergence,Incisive canal,Tooth movement,Upper central incisors
Objective The present study aimed to estimate the location of the incisive canal (IC) concerning the upper central incisor roots (U1) in order to explore the amount of incisor retraction as per the envelope of discrepancy amongst Chinese patients with different vertical facial growth patterns. Methods This is a cross-sectional study that used a total of 207 pretreatment cone-beam computed tomography (CBCT) scans of adults with a skeletal class I relationship. Sixty-nine cases were included in each of the normodivergent, hypodivergent, and hyperdivergent facial groups. The IC volume was measured using Mimics 21 software. The IC width and IC-U1 proximity were measured using in vivo 6 software. Linear measurements were conducted at 3 vertical levels. Results The IC has shown a larger volume in the hyperdivergent group and male patients. Overall, the IC has recorded linear width greater than the inter-root distance of U1 in 59.1%, 66%, and 68.8% amongst the normodivergent, hypodivergent, and hyperdivergent facial groups, respectively, and in 58.3% of males and 70.8% of females. The overall sagittal distances between the U1 and IC were 4.00 ± 0.82 mm, 4.60 ± 0.83 mm, and 3.60 ± 0.80 mm amongst the normodivergent, hypodivergent, and hyperdivergent facial groups, respectively. Conclusions The maximum sagittal distances between U1 and IC were 4.8 mm, 5.4 mm, and 4.4 mm amongst the normodivergent, hypodivergent, and hyperdivergent facial groups, respectively. Thereby, our findings have revised the retraction aspect of the envelope of discrepancy as per the different vertical facial growth patterns, which could serve a reference for the clinical practice involved considerable incisors movement, especially among Chinese patients.
Tridimensional Analysis of Incisive Canal and Upper Central Incisor Approximation

The present study aimed to estimate the location of the incisive canal (IC) concerning the upper central incisor roots (U1) in order to explore the amount of incisor retraction as per the envelope of discrepancy amongst Chinese patients with different vertical facial growth patterns.

This is a cross-sectional study that used a total of 207 pretreatment cone-beam computed tomography (CBCT) scans of adults with a skeletal class I relationship. Sixty-nine cases were included in each of the normodivergent, hypodivergent, and hyperdivergent facial groups. The IC volume was measured using Mimics 21 software. The IC width and IC-U1 proximity were measured using in vivo 6 software. Linear measurements were conducted at 3 vertical levels.

The IC has shown a larger volume in the hyperdivergent group and male patients. Overall, the IC has recorded linear width greater than the inter-root distance of U1 in 59.1%, 66%, and 68.8% amongst the normodivergent, hypodivergent, and hyperdivergent facial groups, respectively, and in 58.3% of males and 70.8% of females. The overall sagittal distances between the U1 and IC were 4.00 ± 0.82 mm, 4.60 ± 0.83 mm, and 3.60 ± 0.80 mm amongst the normodivergent, hypodivergent, and hyperdivergent facial groups, respectively.

The maximum sagittal distances between U1 and IC were 4.8 mm, 5.4 mm, and 4.4 mm amongst the normodivergent, hypodivergent, and hyperdivergent facial groups, respectively. Thereby, our findings have revised the retraction aspect of the envelope of discrepancy as per the different vertical facial growth patterns, which could serve a reference for the clinical practice involved considerable incisors movement, especially among Chinese patients.

The localisation of the upper incisors might be considered an important point of interest in orthodontic diagnosis and treatment planning. The posterior anatomical limit for retracting upper incisors has generally been viewed as the palatal cortical plate., Nevertheless, recent craniofacial anatomical investigations have demonstrated that the cortical bone surrounding the incisive canal (IC) is dense and has a closer relationship with roots of upper central incisors (U1) than does the palatal cortical plate. Due to IC closeness to the U1, the potential of IC surgical invasion during dental procedures has been recorded in previous studies., The idea of the “envelope of discrepancy” presented by Proffit and Ackerman clarifies the sequences conceivable with various sorts of orthodontic treatments, including camouflage orthodontics, growth modification, and surgical orthodontics., It has been imagined that the measure of changes workable for the upper incisors with orthodontic treatment alone are more in retraction and extrusion than protraction and intrusion (7, 4, 2, and 2 mm, respectively). Despite that the morphologic parameters of IC are common, its exact position corresponding to the U1 is not well reported in the 3-dimensional (3D) era. Nakada et al found that posterior movement of the root peak during orthodontic retraction may prompt its contact with the IC cortical plate and cause apical root resorption. Chung et al found the root of one of the U1 was in contact with the cortical plate of the IC, and broad root resorption was noticed after en-masse retraction. Cho et al found that more than 60% of studied cases have recorded greater linear width of IC as compared to the inter-root distance of the U1. A recent study found 53% of studied clinical cases that required U1 retraction greater than 4 mm have shown invasion of IC by the U1 roots following the retraction. Moreover, this process resulted in about a 2.39-mm root resorption, and the severe invasion revealed a 6.2-mm root resorption. Vertical facial morphology is significant for orthodontists because it influences the objectives and the manner of orthodontic treatment by affecting the growth forecast, the anchorage framework, bite power, and other functions. The aforementioned findings considering the U1 and IC relationship raised the importance of reevaluating the broadened dynamics of the envelope of discrepancy using modern 3-D technologies. Further, reevaluation is improved by including segregation of this envelope according to facial type, as it is one of the determinants of orthodontic diagnosis and treatment planning as well as orthodontic biomechanics. Therefore, the present study sought to estimate the morphologic parameters and the approximate location of the incisive canal regarding the upper central incisors and, hence, evaluate the safety zone of orthodontic retraction mentioned in the envelope of discrepancy as per the different vertical facial growth patterns of Chinese people.

Cone-beam computed tomography (CBCT) scans analysed in this cross-sectional study were derived consecutively from the archive of the Hospital of Stomatology, Lanzhou University, between 2014 and 2020, after receiving ethical committee approval from the clinical scientific research of the School of Stomatology, Lanzhou University (No. LZUKQ-2020-20). The data that support the findings of this study are available from the corresponding author upon reasonable request. The sample size was determined at alpha 5% and power of 90% utilising G power software (University of Dusseldorf) and one previous study. As a result, 207 cases were required to be included in the present study. Patient selection was established according to the following inclusion criteria: (1) 18–30 years old, (2) availability of high-quality lateral cephalograms and CBCT images, and (3) skeletal class I relationship (Point A to Nasion to Point B angle from 0° to 4°). Patients who had any of the following criteria were excluded: (1) large maxillary midline diastema, (2) maxillary midline shift ≥2 mm, (3) facial asymmetries (deviation of the mandible), (4) respiratory problems, (5) previous orthodontic/prosthetic treatment, (6) evidence of root resorption, (7) missing teeth (except third molars), and (8) tooth/bone anomalies in the upper jaw midline area. All patients signed consent forms after being given written information. The facial groups were identified according to the parameters of Fields et al. Patients were included in the normodivergent group when the mandibular plane to SN plane angle (GoMe-SN) is 27° to 37°, palatal plane to mandibular plane angle (PP-MP) is 19° to 31°, and lower facial height (ANS-Me) is between 64 and 72 mm. For patients to be included in the hypodivergent group, GoMe-SN is ˂27°, PP-MP is ˂19°, and ANS-Me is ˂64 mm. The hyperdivergent group was identified based on the following: GoMe-SN >37°, PP-MP >31°, and ANS-Me >72 mm.

The CBCT images were obtained using I-CAT Imaging System (Imaging Sciences International Inc.) under standard protocols by a specialist radiographer; field of view was 17 cm; exposure parameter setting was 120 kVp, 20.27 MAs, and 14.9 s; and voxel size was 0.4 mm. All the images were realigned corresponding to Frankfort-horizontal (FH) plane in the sagittal plane. The internal volume of the IC was segmented using 3D segmentation and image analysis software (Mimics 21, Materialise) as described elsewhere, and 3D volumetric models of the IC were generated from these segmentations (Figure, part A). The other 3D parameters and linear measurements were measured utilising Invivo Dental imaging software (version 6, Anatomage) as described by Al-Rokhami et al. The linear measurements were taken at 3 levels situated on the sagittal plane: (1) the lowest point of the incisive canal buccal wall (opening level, H1), (2) midlevel between the opening level and the root apex of the upper central incisors (midlevel, H2), and (3) the root apex of the upper central incisors (root apex level, H3) (Figure, part B). These landmarks and measurements are described in the Figure (partsC and D).

The statistical analysis was calculated utilising IBM SPSS Statistics V. 25.0 (IBM Corp.). Descriptive data were presented as mean and standard deviation (SD) or as frequency and percentage where appropriate. Since the data were normally distributed, one-way analysis of variance followed by Tukey test were applied to estimate the differences amongst the facial groups. Independent t test was used to compare variances between sexes. A P value <.05 was considered to represent statistical significance. All measures were conducted by 2 trained investigators. The intra-class correlation coefficient by Cronbach's alpha test has ensured interobserver reliability, which ranged from 0.980 to 0.999.

A total of 207 adults were enrolled in the present study, with a mean age of 23.41 ± 3.51 years, comprising 104 (50.2%) male and 103 (49.8%) female patients. An equal number of cases were included in each facial group (n = 69, 33.3%). The distribution of male and female patients amongst the normodivergent, hypodivergent, and hyperdivergent groups were 36 (52.5%) and 33 (47.8%), 37 (53.6%) and 32 (46.4%), and 31 (44.9%) and 38 (55.1), respectively. The sagittal skeletal relationship, U1-SN angle, and vertical facial measures per each facial group are reported in Table 1.

The IC has recorded volumetric values around 117.9 ± 25.97, 117.7 ± 29.68, and 130.6 ± 50.74 mm3 amongst the normodivergent, hypodivergent, and hyperdivergent groups, respectively. The IC width (Cl-Cl) showed nonsignificant differences amongst the normodivergent, hypodivergent, and hyperdivergent groups (Table 2). The sexes also showed nonsignificant differences in terms of IC volume and width (Table 3). The IC width was decreased from H1 to H3 (P ˂ .001) similarly in the facial groups and sexes (Table 4). No significant differences were found in either the inter-root distance (Rm-Rm) or posterior inter-root distance (Rp-Rp) amongst the facial groups, except at Rp-Rp-H1 (Table 2). However, the male patients in the hypodivergent group showed significantly higher Rm-Rm and Rp-Rp (P ˂ .05) than female patients at both the H1 and H2 levels (Table 3). On the other hand, the Rm-Rm was increased from H1 to H3 (P ˂ .001), whereas the Rp-Rp was decreased (P ˂ .001) (Table 4). At H1, the rate of cases with IC width greater than the inter-root distance (Rm-Rm) was comparable in the hyperdivergent and hypodivergent groups (94.4%), whilst the normodivergent group showed a slightly lower rate (86.1%). On the other hand, at H2, the hyperdivergent group recorded a higher rate of IC width greater than Rm-Rm (43.1%), whilst the normodivergent and hypodivergent groups have recorded rates around 32% and 37.5%, respectively. At both H1 and H2 levels, male patients showed almost lower rate of IC width greater than Rm-Rm in all the facial groups (Supplemental Material 1).

The Cl-Root, Rm-Canal, and Rm-Cat measures were considerably higher in the hypodivergent group (4.60 ± 0.83 mm) than in the normodivergent and hyperdivergent groups (4.00 ± 0.82 and 3.60 ± 0.80 mm, respectively) at all the vertical levels (P ˂ .001) (Table 2). Besides, the male patients showed significantly higher Cl-Root, Rm-Canal, and Rm-Cat distances compared with female patients only within the hypodivergent group (Table 3). Amongst all the facial groups and both sexes, the Cl-Root was significantly higher at H2 than at H1, whilst the Rm-Cat was significantly higher at H1 and H2 than at H3 (Table 4).

There is always a need for continuous updating of the main parameters that limit orthodontic tooth movement, especially with the vast development of 3D imaging. One of these anatomical parameters, which was ignored during the 2D era, is the IC proximity to the upper incisors. The possible effect of the IC and U1 approximation was at the assumption level a few years ago. However, with emergence of 3D analysis and the surprising results found in the literature concerning the U1-IC relationship, it is time to step back and reevaluate the limits of maxillary dentoalveolar retraction mentioned in the envelope of discrepancy. Overall, from H1 to H3, the IC width was considerably decreased, whereas the inter-root distance was significantly increased, similarly amongst the 3 facial groups and sexes. Similar findings were reported in other studies., The IC 3D volume and linear width showed nonsignificant differences between the facial groups and sexes. However, the rates of cases with IC width larger than U1 inter-root distance were slightly higher in the hyperdivergent group and female patients than in the other facial groups and male patients, which increases the risk of U1 and IC approximation or invasion. A previous study reported that in cases with larger pretreatment IC volume, there has been demonstrated IC invasion after retraction of upper incisors. Additionally, during treatment, changes in root parallelism due to poor bracket positioning or addition of root torque during retraction may likewise lead to root convergence of the U1 and then reducing the inter-root distances. The maximum amount of upper central incisors retraction is 7 mm, as reported in the “envelope of discrepancy,” and the modern improvement of skeletal anchorage has also extended the limits of orthodontic tooth movement. However, a recent study had contrary and shocking findings: 53% of cases of maximum retraction (>4 mm) showed IC invasion by the incisor roots. As a result, a different degree of root resorption was observed with an average of 2.39 mm, and the largest degree of invasion demonstrated 6.2 mm of root resorption. Further, the amount of root resorption was significantly higher in the IC invasion group than non-invasion group (2.39 mm vs 0.82 mm, P < .0001). Nonetheless, the facial type of the included patients was not reported in their study. The present findings revealed that the maximum biological (safe) sagittal distances between the U1 and IC were 4.8, 5.4, and 4.4 mm amongst the normodivergent, hypodivergent, and hyperdivergent groups, respectively: somewhat less than the traditional guideline of envelope of discrepancy (7 mm)., Besides, the normodivergent, hypodivergent, and hyperdivergent groups have recorded various overall mean sagittal distances between the U1 and IC: 4.00 ± 0.82, 4.60 ± 0.83, and 3.60 ± 0.80 mm, respectively. Hence, the hypodivergent group reported greater sagittal distance between the IC and upper centrals. This finding could be illustrated as the hypodivergent facial growth pattern having thicker alveolar bone than the other facial patterns, as reported in previous studies.16, 17, 18 Although the sagittal distance between the U1 roots and the IC has yet to be identified, 3D assessments during orthodontic diagnosis and close observation of the incisor roots throughout treatment would be beneficial in preventing possible complications, particularly in patients requiring maximum retraction. Among all the facial groups, the male patients have shown almost greater sagittal distances between the ICs and upper centrals as compared to females, which appeared signifcant only in the hypodivergent facial group. Hence, the female patients, even those with a hypodivergent facial pattern, were more likely to have UI and IC approximation with subsequent sequelae than were male patients. Overall, the present results were consistent with the findings of Klinge et al, who reported that the male patients had wider alveolar bone cross-sections than female patients. External apical root resorption is one of the most common potential issues provoked during orthodontic therapy, and it has challenged orthodontists for a long time. The radiographic estimation has revealed the incidence of root resorption in a range from 48% to 66%., Interestingly, the incidence of orthodontically induced root resorption is far greater in the upper centrals, even with their larger tooth dimensions. Multiple factors have been implicated in the incidence of orthodontically induced root resorption, including hereditary impact, age, sex, root morphology, root vicinity to the cortical bone, alveolar bone density, type of malocclusion, treatment length, level of applied force, and extent of apical root movement. However, no clear reasons for resorption were truly identified. Even though remodelling of IC was reported in some previous studies, rates of contact or invasion of the U1 roots to IC were fairly high after maximum anterior retraction. Further, contact of upper central roots and IC was also found after minor incisor movements. Chung et al reported that 24% of participants showed IC remodelling after maximum retraction, and a more recent study by Yu et al reported that 11.4% of participants showed some signs of IC remodelling. Moreover, the remodelling group still demonstrated apical root resorption, even if it was less than in the nonremodelling group. One of the limitations of this study is its cross-sectional nature and the associated limitations. Thereby, further interventional prospective clinical studies are recommended to 3-dimensionally evaluate the possible side effects of IC invasion and maximum incisor retraction to comprehensively determine the limits in the envelope of discrepancy in the 3D era. Another limitation was the small range of malocclusion types evaluated. Hence further studies that encompass different variants of malocclusion, particularly those needing camouflaged treatment, is needed to shed further light on the remdelling capacity of the IC wall after orthodontic tooth movement.

The maximum sagittal distances between the U1 and IC were 4.8, 5.4, and 4.4 mm amongst the normodivergent, hypodivergent, and hyperdivergent groups, respectively. The sagittal distances between U1 and IC were associated with the facial growth pattern, which was significantly higher in the hypodivergent group than in the normodivergent and hyperdivergent groups. Besides, there were relative differences in the sagittal distances between sexes, particularly in the hypodivergent group. The present CBCT-based findings have revised the traditional envelope of discrepancy and provided some insights for clinical practice, particularly amongst Chinese populations.

This research was supported by The Natural Foundation (1208RJZA236), The Key Technology Plan Program (20YF8FA071), and The Key Technology Support Program (1604FKCA089), of Gansu Province, China. The aforementioned funds have no any role in any aspect of the present study. All authors declare that they have no conflict of interest or any financial relationship related to this study.

RKA and KAS contributed equally to this work.

None disclosed.