Introduction
The purpose of this study was to analyze the treatment effects after molar distalization using modified C-palatal plates with and without second molar eruption and to evaluate the three-dimensional position of the molars during long-term retention using cone-beam computed tomography.
Methods
The study sample comprised 74 third molars in 42 patients. Twenty-seven adolescent patients (mean age, 12.6 years) having 48 maxillary third molars were divided into 2 groups according to the eruption of their second molars: 15 patients with second molar eruption (group 1) and 12 patients without second molar eruption (group 2). Pretreatment, posttreatment, and long-term data (mean, 5.2 years) from cone-beam computed tomography were scanned and compared with control groups.
Results
There was less tipping movement of the first and second molars (0.94° and 3.22°) and distal tipping movement of the third molars (8.91°) in group 1 than in group 2 (4.36°, 7.39°, and 3.08°, respectively), but the treatment time was shorter and the positional change of the third molars was insignificant in group 2. In the long-term, the second molars fully erupted after distalization in group 2, and there was no difference in the third molar position between group 1, group 2, and the control group, except for the vertical position of the third molars in group 1.
Conclusions
In the long-term, the second molars fully erupted after distalization, and the third molars were in a favorable position. Therefore, these findings suggest that clinicians do not need to extract developing third molars before distalization in adolescents.
Highlights
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Molars were distalized with modified C-palatal plates with and without second molar eruption.
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Unerupted second molars erupted fully after molar distalization.
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No need to extract third molars before distalization in adolescents.
Nonextraction treatment with molar distalization is now feasible using temporary skeletal anchorage devices in patients with Class II malocclusion. To overcome tipping and extrusion problems common with conventional distalization methods, temporary skeletal anchorage devices allow for greater distalization with less distal tipping of the first molars.
However, distalization before the eruption of second molars is a challenge considering the potential for a delayed eruption of the second and third molars. Regarding the probability of eruption disturbances of the molars associated with distalization in the absence or presence of the maxillary second molars, Rubin et al suggested patients be carefully monitored to prevent impaction of the second molars when orthodontic appliances are used to maintain the mandibular arch perimeter in the mixed dentition.
For efficient molar distalization, Shpack et al have recommended headgear treatment be started before the eruption of the maxillary second molars. In growing patients, a germectomy of the third molars has been suggested before the application of a pendulum, and some researchers have found that nonextraction therapy is associated with a significant increase in the frequency of third molar impaction. , However, recently, Kang et al found no significant effect on the molar distalization of a third molar tooth follicle when temporary anchorage devices were used. Considering the possible difficulties and trauma relative to surgical extraction of impacted third molars in adolescent patients, the relation between the eruption stage and molar distalization needs to be clarified.
Treatment timing remains controversial. Previous studies assessed the distal movement of maxillary molars relative to the eruption state. Several of them suggested that the optimal treatment timing for maxillary arch distalization is before the eruption of the maxillary second molars, , , but other studies have reported that the eruption stage has only limited or negligible impact on distalization.
Clinicians have expressed concern about the delayed eruption of second molars, which can result in insufficient space for the third molars and can lengthen treatment time. Maxillary third molar impaction can be predicted in adolescents on the basis of the size of the retromolar space measured as the distance from the first molar to the pterygoid vertical along the occlusal plane. The high impaction rate of maxillary third molars may be explained by insufficient periosteal apposition at the posterior outline of the maxillary tuberosity. , Lee et al observed an insignificant difference in the posttreatment volume of maxillary tuberosity after maxillary distalization. Some studies have evaluated the skeletal effect of molar distalization in adolescents and adults, but few studies have evaluated the maxillary molar position after distalization with vs without the second molar eruption.
Kinzinger et al assessed the effectiveness of molar distalization with a modified pendulum appliance relative to the second and third molar eruption stage. However, side effects of the pendulum treatment, such as anchorage loss which resulted in increased overjet and molar tipping by a mean change in angulation of 15.7°. Modified C-palatal plates (MCPPs) used in this study showed greater distalization and intrusion with less distal tipping of the first molar and more extrusion of the incisor than the buccal miniscrews. Unfortunately, until now, there has not been any analysis or long-term evaluation of the maxillary molar distalization related to the second and third molar eruption stage. The null hypothesis in this study was that maxillary molar distalization using MCPPs induces a significant positional change of third molars in a group without the eruption of the second molar, compared with the group with erupted second molar.
Therefore, the purpose of this study was to analyze the treatment effects after molar distalization with and without the second molar eruption and to evaluate the three-dimensional (3D) position of the molars during long-term retention.
Material and methods
This study was based on Strengthening the Reporting of Observational Studies in Epidemiology reporting guidelines. This sample comprised 74 third molars in 42 patients. Twenty-seven adolescent patients (mean age, 12.6 years) having 48 maxillary third molars underwent bilateral distalization of maxillary molars with MCPPs at the Department of Orthodontics at Seoul St. Mary’s Hospital, The Catholic University of Korea from January 2009 to December 2013 and were divided into 2 groups according to the eruption of their second molars; 15 patients with second molar eruption (group 1), 12 patients without second molar eruption (group 2). In group 1, the bracket positioning was possible on second molars that had fully erupted as far as the occlusal plane. In group 2, second molars were not yet erupted under the gingival coverage ( Fig 1 ). In addition, to evaluate the eruption level of the second molar, the perpendicular distance from the occlusal line (the line connected between the mesiobuccal cusp tip and the distobuccal cusp tip of the first molar) to the midpoint of the mesiobuccal and distobuccal cusp tip of the second molar was measured ( Table I ).
| Characteristics | Group 1 | Group 2 | Control group 1 | Control group 2 | ||||
|---|---|---|---|---|---|---|---|---|
| Mean | SD | Mean | SD | Mean | SD | Mean | SD | |
| Patients (n) | 15 | 12 | 20 | 15 | ||||
| Third molars (n) | 25 | 23 | 35 | 26 | ||||
| Age, y | 13.2 | 1.32 | 12.0 | 1.24 | 12.65 | 1.14 | 19.4 | 1.69 |
| Sex (male/female), n | 5/10 | 5/7 | 12/8 | 4/11 | ||||
| Eruption level of second molar, mm ∗ | 2.12 | 0.52 | 10.35 | 1.35 | ||||
Cone-beam computed tomography (CBCT) images were scanned before and after molar distalization (group 1, 18.1 ± 13.12 months; group 2: 13.1 ± 7.04 months) for pretreatment and posttreatment evaluation. Long-term data (mean, 5.2 years) was derived from CBCT images from January 2017 to May 2019.
Control groups that have not received orthodontic treatment were used to compare skeletal growth during distalization. Control group 1 (age, 12.7 ± 1.1 years) was used as a cross-sectional sample used as a short-term study (12.9 ± 4.04 months) and was applied to compare the MCPPs treatment effect and amount of growth. The control group 2 was used for long-term evaluation (age, 19.3 ± 1.6 years) included 15 dental patients with Class II malocclusion with 26 maxillary third molars who had taken 1 CBCT image for other reasons such as an impacted tooth or pathology ( Table I ).
Institutional Review Board approval of this study was obtained from the Catholic University of Korea (KC19RESI0172). The sample size necessary for a significance level of P < 0.05, a β = 0.2, and an effect size of 1.5 was determined to be at least 23 third molars ( www.clincalc.com ). The inclusion criteria for groups 1 and 2 in this retrospective study were as follows: (1) age range from 11 to 14 years, (2) dental Class II molar relationship more than 1/4 cusp, (3) unilaterally or bilaterally developing maxillary third molars, (4) mild to moderate crowding <5 mm in the maxilla, and (5) high-quality CBCT images. For the long-term evaluation of the maxillary molar position, the control 2 group was selected on the basis of the same criteria as groups 1 and 2. An I-CAT computed tomography scanner (Imaging Science International, Hatfield, Pa) was used for all patients, with 120 kVp, 47.7 mA, a standard voxel size of 0.4 mm, 200 × 400 mm field of view. The MCPP and 3 miniscrews (2.0 mm in diameter, 8 mm in length; Jeil Corporation, Seoul, South Korea) were inserted by the same operator (Y.A.K.) in the paramedian region of the palate ( Fig 2 ). A palatal arch with 2 hooks in the anterior part was placed with the elastomeric chain applying approximately 300 g of force on each side. Class I molar relationship was achieved in all patients with normal overjet and overbite. Hawley retainers in combination with a fixed retainer were used for long-term retention.
For 3D volume rendering, the resulting data were exported in a Digital Imaging and Communications in Medicine multifile format and imported into Invivo software (version 5.3; Anatomage, San Jose, Calif).
One examiner (J.H.P.) did the orientation and took measurements of oriented landmarks ( Fig 3 , A ). The repeated measurements done by these observers were used to calculate the intraclass correlation coefficient, which ranged from 0.92 to 0.95 for intraobserver reliability.
Additional landmarks were digitized; the mesiobuccal and distobuccal cusps of the maxillary molars and the palatal root tip of the maxillary molars. For assessment of the linear and angular dimensions, the distance from the mesiobuccal cusp of the maxillary molar to the horizontal, frontal, and sagittal planes was measured. The horizontal plane (x-axis) was set as the plane passing through the right orbitale and porions. The sagittal plane (z-axis) was set perpendicular to the horizontal plane passing through the anterior nasal spine and posterior nasal spine. The frontal plane (y-axis) was set perpendicular to the x-axis and z-axis, passing through nasion. The changes in sagittal, vertical, and transverse positions were analyzed ( Fig 3 , B ).
The angles between the Frankfort horizontal line projected on the sagittal plane and the long axis of the first and second molars, defined as the line from mesiobuccal cusp to the tip of the palatal root, were measured. The angulation change of the first and second molars was analyzed ( Fig 3 , B and C ).
The angles were measured between the sagittal plane projected on the horizontal plane and the crown axis of the molars, which is defined as a line tangent to the mesiobuccal and distobuccal cusps. The mean change in rotation of the maxillary molars was analyzed ( Fig 3 , C ).
Statistical analysis
SPSS software (version 16.0; SPSS, Chicago, Ill) was used for the statistical analysis. All data were confirmed to be normally distributed using the Shapiro-Wilk test. The Wilcoxon rank-sum test was performed when the data did not follow a normal distribution. Independent and paired t tests were used to compare variables of the 2 groups and the control group for analysis of changes between pretreatment, posttreatment, and long-term effects. Age and gender were used as covariates. Differences with probabilities of less than 5% ( P < 0.05) were considered statistically significant. The Kruskal-Wallis and Mann-Whitney tests were used to evaluate the difference in skeletal variables before and after the treatment of group 1, group 2, and control group 1.
Results
Group 1, group 2, and control group 1 showed skeletal Class II malocclusion in the predistalization stage. Group 2 had a larger ANB than control group 1. In addition, group 1 had a smaller SNB than control group 1. Group 2 had a smaller mandibular plane angle than group 1. After distalization, groups 1 and 2 showed a decreased ANB compared with control group 1 ( Table II ).
| Characteristics | Predistalization (T1) | Changes between predistalization and postdistalization (T2 − T1) | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Control 1 | Group 1 | Group 2 | Control 1 | Group 1 | Group 2 | |||||||||
| Mean | SD | Mean | SD | Mean | SD | P ∗ | Mean | SD | Mean | SD | Mean | SD | P ∗ | |
| SNA | 82.0 | 3.5 | 80.1 | 2.7 | 81.1 | 1.2 | 0.197 | 0.28 | 1.63 | −0.18 | 0.47 | 0.04 | 0.39 | 0.337 |
| SNB | 78.0 | 3.5 | 75.5 | 3.1 | 76.0 | 1.7 | 0.062 (b < a) † | 0.63 | 1.58 | 0.52 | 0.39 | 1.08 | 0.75 | 0.137 |
| ANB | 4.0 | 0.4 | 4.6 | 1.3 | 5.2 | 1.2 | 0.032 (c > a) † | −0.35 | 0.22 | −0.69 | 0.50 | −1.04 | 0.60 | 0.001 (b, c < a) † |
| Facial angle | 88.7 | 3.2 | 87.3 | 3.1 | 88.4 | 3.1 | 0.565 | 0.58 | 1.43 | −1.05 | 1.47 | 1.34 | 2.09 | 0.001(b < a, c) † |
| Palatal plane angle | −2.8 | 4.2 | −4.2 | 3.0 | −3.1 | 2.9 | 0.432 | −0.21 | 1.84 | 0.23 | 0.63 | −1.62 | 3.47 | 0.290 |
| Mandibular plane angle | 25.7 | 4.3 | 28.1 | 3.7 | 23.1 | 4.3 | 0.015 (b > c) † | 0.77 | 3.29 | 2.43 | 2.51 | −0.02 | 3.76 | 0.074 |
| A point-N perpendicular | 2.0 | 3.3 | 1.8 | 3.6 | 3.3 | 2.9 | 0.363 | 0.37 | 1.53 | −0.44 | 0.75 | 0.26 | 1.27 | 0.204 |
| B point-N perpendicular | −3.4 | 5.5 | −5.1 | 5.7 | −3.0 | 4.6 | 0.779 | 1.07 | 2.34 | 0.35 | 1.49 | 2.08 | 2.22 | 0.172 |
| Maxillary length | 82.6 | 3.8 | 83.7 | 6.0 | 83.8 | 3.7 | 0.760 | 0.96 | 3.84 | 1.56 | 1.08 | 0.24 | 1.56 | 0.100 |
| Mandibular length | 109.6 | 6.3 | 109.7 | 6.1 | 107.1 | 5.9 | 0.636 | 3.27 | 6.51 | 3.06 | 1.43 | 3.13 | 2.27 | 0.551 |
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