Introduction
This study analyzed the influence of the 3-dimensional (3D) root apex position on the traction duration of unilateral impacted maxillary canines, compared with the cusp tip position.
Methods
Thirty-one cone-beam computed tomography scans were analyzed. Each normally erupted canine was mirrored across the midsagittal plane to create an image of its enantiomorph. The distances were measured between the root apices of the impacted canine and the enantiomorphic contralateral normally erupted canine, as well as between their cusp tips and tooth angulations. Each distance variable was further subdivided into vertical displacement, horizontal displacement, mesiodistal (MD) displacement, and labiopalatal displacement, whereas the angulation variable was divided into MD tip difference and torque difference. The correlation between each measurement and the traction duration was analyzed.
Results
The position of the root apex showed no significant correlation with traction duration. However, the 3D displacement, horizontal displacement, and labiopalatal displacement of the cusp tip ( P <0.001), vertical displacement of the cusp tip ( P <0.01), and the 3D angulation difference and MD tip difference between tooth axes ( P <0.05) showed a significantly positive correlation with traction duration. Multiple regression analysis showed that the 3D displacement of the cusp tip explains approximately 55.4% of the variance in traction duration, increasing by 1.2 months per 1 mm.
Conclusions
Although root apex position does not affect traction duration in unilateral impacted maxillary canines, the 3D displacement of the cusp tip is a key determinant, with more significant palatal displacement and a higher vertical position associated with longer traction duration.
Highlights
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Apex displacement did not affect the traction duration in unilateral impacted canines.
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Longer traction time correlated with greater 3D cusp tip displacement (∼1.2 mo/mm).
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More palatal and and higher vertical cusp tips required longer traction time.
Because of its eruption sequence, the maxillary canine is the most frequently impacted after the third molars. Leaving the impacted canine untreated at the appropriate time can lead to various dental complications, such as root resorption and cystic change. , Therefore, timely and proper intervention is crucial, with forced eruption being a commonly used treatment approach. Forced eruption of an impacted maxillary canine often accounts for a significant portion of the total treatment time. Therefore, predicting the traction duration is essential for effective treatment planning and patient communication, as the extraction of the impacted tooth and space closure or prosthetic restoration can also be considered. In addition, providing realistic expectations about treatment duration plays a critical role in maintaining the patient’s long-term compliance throughout the orthodontic treatment.
To establish reliable predictors of the traction duration, previous studies have focused mostly on the position, angulation, and overlap of the crown in relation to adjacent teeth. ,,, However, there is relatively less interest in the position of the apex, even though the apex of an impacted canine also deviates from the normal position. ,, In addition, during traction, the root apex of the impacted canine moves alongside its crown. Thus, traction of an impacted canine should be considered as a correction of both the crown and root apex, implying that an abnormal root apex position could significantly influence treatment duration. Therefore, to accurately evaluate the 3-dimensional (3D) position of an impacted canine, it is essential to assess both the crown and the root apex to gain a more precise understanding of the overall displacement pattern of the impacted maxillary canine.
At present, cone-beam computed tomography (CBCT) has been introduced as a more precise tool to evaluate impacted canines. Shin et al, found the pretreatment inclination of the canine toward the midsagittal plane to be the sole factor influencing traction duration. Another study by Goh et al, found rotation to be the only factor influencing traction duration, whereas the vertical and horizontal displacement and angulation were not associated with traction duration. Nevertheless, rather than using a full 3D assessment, most previous studies have typically evaluated the position of impacted canines using reference planes such as the occlusal plane or the midsagittal plane, measuring distances or angulations relative to these planes. As a result, the true complexity of the impacted tooth’s spatial position may not be fully understood. To overcome these limitations, this study introduces a more refined method by mirroring the normally erupted contralateral canine across the midsagittal plane to predict the ideal position of the impacted canine. This method allows for the quantitative assessment of the displacement of the impacted canine by using its ideal position as a reference, in which it would have erupted under normal conditions, rather than relying on conventional reference planes. The resulting measurements enable clinicians to intuitively understand the pattern and extent of the displacement based on numerical values alone.
This study aims to evaluate the influence of root apex position on the traction duration of unilateral impacted maxillary canines using CBCT by incorporating a novel 3D analysis method. Furthermore, this study reassesses the role of crown position, a well-established factor that affects traction duration, to provide a more comprehensive understanding of the variables that influence traction duration.
Material and methods
This retrospective study was based on data from patients with a unilateral impacted maxillary canine who visited the Department of Orthodontics, Yonsei University Dental Hospital, between January 2018 and December 2023. In total, 273 patients were initially screened using the keyword phrase “unilateral impacted maxillary canine.” Of these 273 initially screened patients, those for the final study were selected based on the following criteria. The inclusion criteria were selected to ensure that the contralateral maxillary canine erupted in its normal position: (1) an impacted canine in sectors II-V by Ericson and Kurol classification ( Fig 1 ) to standardize the direction of displacement between the crown and root apex, (2) the presence of a normally erupted contralateral maxillary canine, and (3) the maxillary arch showing a round shape and <2 mm of crowding. The exclusion criteria were selected to eliminate environmental factors, such as abnormality and treatment method: (1) systemic disease, maxillofacial deformity, and history of trauma; (2) obstacles to the eruption, such as odontomas or supernumerary teeth; (3) a missing tooth or other impacted teeth, except for the canine; (4) additional rotation control after eruption ; (5) root resorption or dilacerations of maxillary dentition; and (6) orthodontic treatment with extraction or transposition. Finally, of the 273 patients with unilateral canine impaction, 31 patients (21 females and 10 males) were selected according to these criteria. The average age of the selected patients was 14.8 years, ranging 10-38 years. On the basis of a previous study, the Pearson correlation coefficient for the alternative hypothesis was set at 0.5, with a significance level of 0.05 and power of 80%, resulting in a minimum required sample size of 23 (G∗Power, version 3.1.9.4; Franz Faul, Universität Kiel, Germany). All research procedures in this study complied with the guidelines of the Declaration of Helsinki and were reviewed and approved by the Institutional Review Board of Yonsei University Dental Hospital (2-2024-0035).
The classification of the anteroposterior position of the cusp tip about adjacent teeth was used by Ericson and Kurol. Sector I , distal part of a crown and root of the lateral incisors; Sector II , distal area of the lateral incisor divided by the tooth axis; Sector III , mesial area of the lateral incisor divided by the tooth axis; Sector IV , distal area of the central incisor divided by the tooth axis; Sector V , mesial area of the central incisor divided by the tooth axis.
All CBCT scans were performed using uniform parametric settings at the standard operational settings of the hospital (80 kVp; 10 mA) with Alphard 3030 (Alphard Roentgen Ind, Ltd, Kyoto, Japan). CBCT data were converted and assessed using Invivo 3D Imaging software (Anatomage, San Jose, Calif). The software was run on a computer equipped with an Intel Core i7-7700 CPU @ 3.60 GHz, integrated Intel HD Graphics 630, and 8.25 GB of RAM. For the reference plane, the occlusal plane was determined by 3 points: the midpoint between the incisal edges of the maxillary central incisors, and mesiobuccal cusp tips of the maxillary first molars on both sides. The midsagittal plane was perpendicular to the occlusal plane, passing ANS and PNS.
Analyses were performed by comparing the impacted canines with the normally erupted contralateral maxillary canines by symmetrical mirroring of the normally erupted contralateral maxillary canines about the midsagittal plane ( Fig 2 , A ). The cusp tip position, root apex position, and tooth axis (the line through the cusp tip and root apex) were analyzed by linear and angular measurements, including the vertical projection of the cusp tips and root apex on the occlusal plane ( Fig 2 , B ). The position of each point on the occlusal plane by the vertical projection can be understood simply as the views from the occlusal of the CBCT 3D reconstruction. The following measurements were analyzed.
Mirroring process and illustrations of measurements used in the analysis: A, Mirroring process. The normally erupted contralateral canine is reflected along the midsagittal plane to compare with the impacted canine; B, Landmarks of the impacted canine and the enantiomorphic normally erupted canine. Cn , the cusp tip of the enantiomorphic normally erupted contralateral canine; An , apex of the enantiomorphic normally erupted contralateral canine; Ci , the cusp tip of the impacted canine; Ai , the apex of the impacted canine. Points marked with (’) indicate the corresponding landmarks projected perpendicularly onto the occlusal plane; C, Illustration of the 3D displacement, vertical displacement, and horizontal displacement; D, Illustration of the horizontal displacement, LP displacement, and MD displacement on the occlusal plane; E, Illustration of the torque difference and MD tip difference (see Table I ).
First, the 3D distance between each cusp tip and the root apex of 2 canines (3D displacement) was measured. The 3D displacement was divided into distance from the occlusal plane (vertical displacement) and distance between each point of the cusp tip and root apex projected onto the occlusal plane (horizontal displacement) ( Fig 2 , D ; Table I ). Using reference lines, the horizontal displacement was further divided into the mesiodistal (MD) displacement and the labiopalatal (LP) displacement. These reference lines passed through each cusp tip and the root apex of the normally erupted contralateral canine that were parallel to the line passing the average point of the maxillary lateral incisor and the first premolar buccal cusp tip and root apex in the normally erupted canine quadrant (n = 31) ( Fig 2 , E ; Table I ). By this classification, unlike previous studies that usually used x-y (occlusal), y-z (midsagittal), and z-x (coronal) planes as axes, this study was conducted using 3 axes, the MD axis, the LP axis, and the vertical axis, to allow more clinically intuitive application.
Table I
Definitions of impacted canine measurements
| Measurements | Definition |
|---|---|
| Cusp tip displacement | |
| 3D displacement | Distance from Ci to Cn |
| Vertical displacement | Difference between the distance of Ci and Ci’ and that of Cn and Cn’ |
| Horizontal displacement | Distance from Ci’ to Cn’ |
| MD displacement | MD distance from Ci’ to Cn’ using reference line passing the average point of the maxillary left lateral incisor incisal edge and first premolar buccal cusp tip |
| LP displacement | LP distance from Ci’ to Cn’ using reference line passing the average point of the maxillary left lateral incisor incisal edge and first premolar buccal cusp tip |
| Apex displacement | |
| 3D displacement | Distance from Ai to An |
| Vertical displacement | Difference between the distance of Ai and Ai’ and that of An and An’ |
| Horizontal displacement | Distance from Ai’ to An’ |
| MD displacement | MD distance from Ai’ to An’ using reference line passing the average point of the maxillary left lateral incisor and first premolar apex |
| LP displacement | LP distance from Ai’ to An’ using reference line passing the average point of the maxillary left lateral incisor and first premolar apex |
| Canine angulation difference | |
| 3D angulation difference | Angulation from an axis passing Ci and Ai to an axis passing Cn and An |
| MD tip difference | Angulation from an axis passing Ci’ and Ai’ to an axis passing Cn’ and An’ |
| Torque difference | Difference between the angulation of the axis passing Ci and Ai and the occlusal plane, and that of the axis passing Cn and An and the occlusal plane |
Cn , cusp tip of the enantiomorphic normally erupted contralateral canine; An , apex of the enantiomorphic normally erupted contralateral canine; Ci , cusp tip of the impacted canine; Ai , apex of the impacted canine; points marked with (’) indicate the corresponding landmarks projected perpendicularly onto the occlusal plane.
Second, the 3D angulation between the 2 canines was measured (3D angulation difference). The 3D angulation difference was divided into the difference of the canine angulation to the occlusal plane (torque difference), and the canine axis difference projected onto the occlusal plane (MD tip difference), which could be considered the MD angular displacement, known as MD tip ( Fig 2 , C ; Table I ).
Medical records and clinical data were used to calculate treatment duration by applying orthodontic force to the impacted canine after the surgical opening to engage a 0.016 × 0.022-in stainless steel wire in a 0.018-in slot bracket on the maxillary dental arch. This meant that the impacted canine was sufficiently aligned. If the stainless steel wire was not used as the main archwire for the treatment, the debonding period was used as the endpoint of the traction.
Statistical analysis
All statistical analyses were conducted using SPSS software (version 27.0; IBM, Armonk, NY). Intraexaminer reliability was assessed using intraclass correlation coefficients based on 2 sets of measurements performed by a single researcher (Y.S.P) with a 2-week interval. The researcher was blinded to the traction duration of each case during the measurement process to minimize potential measurement bias. All intraclass correlation coefficient values were >0.9 ( P <0.001), indicating high consistency of the measurements. Pearson correlation coefficient and stepwise multiple linear regression analysis were used to determine correlations between the measurements of cusp tip displacement, apex displacement, canine angulation difference, and traction duration.
Results
Table II summarizes the distribution of impacted canines according to sector classification based on panoramic radiographs. Using CBCT images, impacted canines were categorized relative to the lateral incisor root, with 12 patients classified as labial impaction and 19 as palatal impaction ( Table II ). Figure 3 illustrates the vertical projection of cusp tips and apices on the occlusal plane ( Fig 3 , A and B , show cases of labially impacted and palatally impacted canines, respectively). The cusp tips of impacted canines were palatally located near the lateral incisor ( Fig 3 , C ), whereas their apices were palatally positioned near the first and second premolars ( Fig 3 , D ).
Table II
Classification of cusp tips by sectors
| Sector | Cusp tip | |||
|---|---|---|---|---|
| n | Percentage of total (%) | Labial | Palatal | |
| II | 7 | 22.6 | 2 | 5 |
| III | 11 | 35.5 | 6 | 5 |
| IV | 9 | 29.0 | 4 | 5 |
| V | 4 | 12.9 | 0 | 4 |
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