Introduction
In this study, we compared the dentitional changes after Invisalign and conventional orthodontic treatment with 4 first premolar extractions.
Methods
This retrospective study included 57 patients whose orthodontic treatment involved the extraction of 4 first premolars because of bialveolar protrusion. A total of 27 patients were treated with Invisalign (mean age, 25.5 ± 5.2 years) and 30 patients with the fixed appliance (mean age, 24.4 ± 5.8 years). The angular and linear changes of the maxillary and mandibular central incisors, second premolars, first molars, and second molars were measured from the recordings on the basis of the lateral cephalograms taken before and after treatment. The angular changes of the canines and second premolars were measured using panoramic radiographs.
Results
The overbite and interincisal angle increased significantly in the Invisalign group compared with in the conventional fixed appliance group ( P <0.05). The maxillary central incisors showed increased lingual tipping in the Invisalign group ( P <0.05), whereas there was no statistically significant difference in the angular change of the mandibular incisors between groups ( P >0.05). The maxillary first and second molars showed mesial tipping in the Invisalign group ( P <0.05). The maxillary second premolars, first and second molars, and the mandibular second molars showed mesial movement in the Invisalign group ( P <0.05).
Conclusions
The Invisalign group showed more statistically significant lingual tipping of the maxillary central incisors, distal tipping of the maxillary canines, and mesial tipping of the maxillary first and second molars after maximum retraction of the anterior teeth compared with the fixed appliance group.
Highlights
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Invisalign causes more lingual tipping of the maxillary incisors than do fixed appliances.
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Invisalign and fixed appliance treatment resulted in similar amounts of anterior retraction.
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Invisalign brings about more mesial tipping of the maxillary molars than do fixed appliances.
As the esthetic value in dental treatment has become more critical in modern society, invisible orthodontic appliances are gaining more attention in the field of esthetic orthodontic treatment. Among many invisible orthodontic appliances, a clear aligner has been brought to the spotlight. It is a completely different force system in that it does not require conventional brackets and wires. Several companies worldwide have developed clear aligner trays and produced aligners according to a doctor’s prescription. Among them, Invisalign (Align Technology, San Jose, Calif) is the world’s first mass-produced customized transparent aligner system. It features various forms of attachments that enhance tooth movements such as extrusion, intrusion, and rotation. Power ridges, indentations to control the torque of teeth, and bite ramps are used to correct deep overbites (OBs).
Since the launch of the new system and the clinical use of Invisalign became frequent, various studies have been conducted to prove the effectiveness of orthodontic treatment using this device. Khosravi et al investigated the clinical effectiveness of OB management on the basis of cephalometric analyses in patients with deepbite, open bite, and normal bite and reported that Invisalign controlled the vertical dimension relatively well, mostly achieved by incisor movement with minimal changes of the mandibular plane angle. According to a recent study investigating improvements in deep OB using ClinCheck software (Align Technology), it was less likely to achieve an ideal bite pattern in patients having a deeper OB, even with Invisalign devices. When Invisalign with composite resin attachments and Class II elastics were used for maxillary molar distalization, the maxillary first molars (U6s) showed the distal movement of 2.25 mm without significant tipping or vertical movement of the crown based on the lateral cephalogram analysis, and no change in the vertical dimension of the facial profile was shown.
Recently, several studies have evaluated the predicted and actual outcome of anterior tooth movement with Invisalign performed on patients with severe crowding. The intrusion movements of the maxillary and mandibular incisors assessed with cone-beam computed tomography (CBCT) were not significantly shown as predicted. Jiang et al reported that labial root movement was much more predictable than lingual root movement with the evaluation using CBCT. In addition, another report showed that the mean predictability of buccal expansion in the maxillary arch (76.4%) was lower than in the mandibular arch (86.9%) based on a digital model analysis when a planned arch expansion was at least 3 mm using Invisalign. The highest predictability was 81.1% in the maxillary second premolars (U5s) and 93.0% in the mandibular first premolars, respectively. In comparison, the second molars showed the lowest predictability in both the maxilla and mandible, with 41.5% and 42.9%, respectively.
One study compared the clinical results of 2 groups after performing premolar extraction orthodontic treatment with Invisalign and fixed appliance. According to the American Board of Orthodontics Objective Grading System, there were no significant differences in alignment, marginal ridge, occlusal relationship, overjet, interproximal contacts, and root angulation. However, the Invisalign group had significantly lower scores in correcting buccolingual inclination and occlusal contacts than those of the fixed appliance group. Recently, another study suggested no significant difference in total Objective Grading System scores between groups on the basis of cast models and radiographic analysis after premolar extraction treatment. The subjects in the above 2 studies included those with severe crowding.
Most importantly, few studies have been performed to assess the clinical outcome of tooth movements on patients experiencing Invisalign treatment involving premolar extraction because of bialveolar protrusion. Selecting patients with minor crowding is crucial to identify clinical side effects, followed by 4 premolar extraction strategies. This study aimed to compare the results of tooth movements after maximum retraction was performed with Invisalign and fixed appliances on the patients with the 4 first premolars extracted. The null hypothesis of this study was that there was no significant difference in the angular and linear change of the teeth between groups after premolar extraction treatment.
Material and methods
This retrospective study was conducted on patients who visited a private dental clinic to treat lip protrusion using Invisalign (Align Technology) between January 2014 and December 2016. A single orthodontist, who had 25 years of experience in conventional fixed appliances and 10 years of experience in Invisalign, treated the experimental group in this study. All treatment plans, the so-called ClinCheck, have been reviewed and approved by 1 orthodontist. Invisalign G5 (generation 5) was used for every patient, characterized by SmartTrack material and SmartForce features (such as power ridges and optimized attachments). Attachments used for tooth axis control were bonded to the canines, second premolars, and first and second molars; dual attachment was bonded to the canines, and a single attachment was placed in the posterior teeth. Sequential movements were prescribed in the treatment plan for maximum retraction after premolar extraction. The tooth movement strategy used in this study was to retract the canine solely until a space of about 1 mm was created between the lateral incisor and the canine. Then, the movement of the canine was stopped, and the 4 incisors were retracted. When the 4 incisors met the canine, the canine moved backward again. This movement was repeated until the extraction space is closed. Class II elastics (0.25-in, 4 oz) were used during the space closure period, and vertical elastics were used for bite seating during the finishing stage. Patients were encouraged to wear the aligner for >20 hours daily and replace it every 2 weeks. An average of 2 refinements were performed to solve problems such as bite opening of posterior teeth, unclosed tooth extraction spaces, and collapsed tooth axes, and the total number of aligners ranged from 68 to 87 (lower bound, 34.0 months; upper bound, 43.4 months with 95% confidence intervals).
This study design was approved by the Institutional Review Board of Gangnam Severance Hospital (Institutional Review Board no. 3-2019-0171). Of the 185 patients received at the clinic, 27 (6 males, 21 females; mean age, 25.5 ± 5.2 years) who satisfied the inclusion criteria were selected as the experimental group.
The inclusion criteria were as follows: (1) adult patients aged ≥18 years, (2) 0°< ANB <6°, (3) Angle Class I molar relationship, (4) arch length discrepancy ≤4 mm, (5) 4 first premolar extractions, (6) patients requiring maximum anterior teeth retraction, (7) no significant periodontal disease, (8) no significant medical history, (9) no history of trauma or significant facial asymmetry, and (10) no previous orthodontic treatment or orthognathic surgery.
The control group was 30 patients (9 males, 21 females; mean age, 24.4 ± 5.8 years) of 251 patients who visited the Department of Orthodontics at Gangnam Severance Dental Hospital between January 2015 and January 2016 and were treated with 0.022-in Roth prescription brackets (Clippy-C; Tomy, Tokyo, Japan) to meet the same inclusion criteria of the experimental group. Patients in the control group were subjected to en masse retraction using temporary skeletal anchorage devices (TSADs) (Tapered type, 1.6 mm × 7 mm length; Orlus [Ortholution, Seoul, South Korea]) for maximum retraction after premolar extraction. TSADs were inserted in the interradicular bone between the second premolar and the first molar, and elastic chains were connected to the long hooks on the anterior segment. The failure rate of TSADs was 13.3%, and when they became loose during treatment, they were reinserted into the same or adjacent site. Intermaxillary elastics were used for settling in the finishing stage.
Pretreatment (T1) and posttreatment (T2) lateral cephalometric and panoramic x-rays were taken for both groups. Because of the different x-ray machines used for each group (Pax-Uni 3D, Vatech Korea, Hwaseong-si, South Korea for the experimental group; PMPROMAX Planmeca, Helsinki, Finland for the control group), the lateral cephalograms for each patient at T1 and T2 were standardized and calibrated using the ruler in each x-ray examination. The following are intraoral and radiographic images of 1 patient, each treated by Invisalign and fixed appliance ( Fig 1 ).
The demographic characteristics of the groups are shown in Tables I and II . There were no statistically significant differences between groups in the distribution of age or sex ( P >0.05). In addition, pretreatment skeletal and dental characteristics showed no statistically significant differences ( P >0.05).
| Variables | Invisalign (n = 27) | Fixed appliance (n =30) | P value |
|---|---|---|---|
| Age (y) | 25.5 ± 5.2 (23.5-27.5) | 24.4 ± 5.8 (22.3-26.5) | 0.312 |
| Sex | 0.506 | ||
| Female | 21 (77.8) | 21 (70.0) | |
| Male | 6 (22.2) | 9 (30.0) | |
| Treatment duration (mo) | 38.7 ± 12.0 (34.0-43.4) | 32.1 ± 9.1 (28.8-35.4) | 0.021 ∗ |
∗ Statistically significant differences between groups (unpaired t test) (P <0.05).
| Variables | Invisalign (n = 27) | Fixed appliance (n = 30) | P value |
|---|---|---|---|
| SNA (°) | 81.45 ± 3.39 | 81.61 ± 3.67 | 0.871 |
| SNB (°) | 77.31 ± 3.09 | 78.14 ± 3.02 | 0.179 |
| ANB (°) | 4.14 ± 1.93 | 3.46 ± 1.63 | 0.062 |
| Wits (mm) | −0.34 ± 2.77 | −1.59 ± 3.14 | 0.118 |
| Mp-SN (°) | 37.86 ± 6.20 | 35.62 ± 6.30 | 0.182 |
| U1-SN (°) | 109.96 ± 5.48 | 111.25 ± 6.65 | 0.431 |
| IMPA (°) | 100.46 ± 7.22 | 98.07 ± 6.76 | 0.202 |
| Overjet (mm) | 2.86 ± 2.34 | 2.71 ± 2.09 | 0.805 |
| OB (mm) | 1.70 ± 1.41 | 1.50 ± 1.40 | 0.587 |
| IIA (°) | 111.45 ± 12.63 | 113.48 ± 8.44 | 0.476 |
To evaluate skeletal and dental changes, the following measurements were made on the lateral cephalograms at T1 and T2: SNA (angle formed by the sella-nasion plane and nasion-A point line), SNB (angle formed by the sella-nasion plane and nasion-B point line), ANB (angle formed by the nasion-A point line and nasion-B point line), Wits appraisal, Mp-SN (angle formed by the sella-nasion plane and mandibular plane), overjet, OB, and interincisal angle (IIA).
The horizontal reference line (HRL) was defined as the line obtained by 7° clockwise rotation around the nasion of the line connecting the sella and nasion. The vertical reference line (VRL) was defined as the line orthogonal to the HRL and passing through the sella. The mandibular plane (Mp) is the plane passing through the menton and the most posteroinferior point of the mandibular angle ( Fig 2 , A ).
The tooth axes of the maxillary teeth (central incisor [U1], U5, U6, and second molar [U7]) were measured relative to the HRL, and the tooth axes of the mandibular teeth (central incisor [L1], second premolar [L5], first molar [L6], and second molar [L7]) were measured relative to the MP. The tooth axis was defined as the line passing through the incisal edge and the root tip for the central incisors, the line passing through the centroid and the root tip for the second premolars, and the line passing through the centroid and the root furcation for the first and second molars ( Fig 2 , B ). In patients in which a double image was present, the line passing the middle of the 2 images was chosen. Herein, the centroid was defined as the midpoint of the line joining the mesial and distal parts of the crown with maximal convexity ( Fig 2 , A ). ,
To assess the horizontal movement after treatment (T2 − T1), the distance from each tooth to the VRL was measured ( Fig 2 , C ). To assess the vertical movement after treatment, we measured the vertical distance of each tooth from the HRL for the maxillary dentition and from the MP for the mandibular dentition ( Fig 2 , D ). When measuring the horizontal and vertical distances, we measured the distance from the incisal edge to the reference line for the central incisors and the distance from the centroid to the reference line for the second premolars, first and second molars.
To assess the angulations between the maxillary canines (U3s) and U5s and between the mandibular canines (L3s) and L5s after treatment using panoramic radiographs, the bicondylar line was used. This was suggested by Parenti et al as the most reliable reference line for assessing the axial angulation of the U3. The bicondylar line is the line passing through the most superior points of the 2 mandibular condyles. Using this as the reference line, the interior angulation of the canines and second premolars ( Fig 3 , A ) and the exterior angulations of the mandibular dentition were measured with the reference line ( Fig 3 , B ). The angulations between the canines and the second premolars were calculated by subtracting the angulation of the second premolar from the angulation of the canine. This value was measured left and right, increasing when the crowns tilted toward the extraction site after treatment (convergent) and decreasing when the crowns were away from the extraction site (divergent).
The primary outcomes of this study were angular and linear measurement changes of incisors, second premolars, first molars, and second molars in lateral cephalograms and angulation changes between canines and second premolars in panoramic radiographs. Before treatment, we confirmed whether there was no significant difference in the skeletal pattern (SNA, SNB, ANB, Wits, and Mp-SN) or anterior teeth relationship (SN-U1, IMPA, overjet, OB, and IIA) between groups in the lateral cephalograms. After treatment, we assessed these measurements again and classified these changes as secondary outcomes. In addition, the duration of the treatment was also included as the secondary outcome. All of the tipping mentioned in this study referred to the direction of the crown.
Statistical analysis
SPSS software (version 21.0; IBM, Armonk, NY) was used for all statistical analyses in this study. To test the intraexaminer reliability, 20% of the sample was randomly chosen to be measured again 2 weeks after the first assessment. The reliability of the measures was assessed by means of an intraclass correlation coefficient with Shrout–Fleiss derivation. The intraclass correlation coefficient showed excellent reproducibility for the linear measurements (0.94), the angular measurements (0.96) on lateral cephalograms, and the angular measurements on panoramic radiographs (0.97). The value for the interrater reliability was 0.93. Paired t tests were performed to test statistically significant differences between T1 and T2 in each treatment group. After verifying normality using a Shapiro–Wilk test to compare differences between groups, we used P values from independent t tests for measurements that followed a normal distribution and Mann–Whitney U tests for measurements that did not follow a normal distribution. P <0.05 was regarded as statistically significant. The sample size was calculated according to data on changes in dental measurements between groups in the previous study. A power analysis confirmed the sample size should be >22 per group with 80% power at a significance level of 0.05.
Results
Compared with the fixed appliance group, the treatment duration of the Invisalign group was approximately 6 months longer ( P <0.05) ( Table I ). Posttreatment skeletal and dental changes in the lateral cephalogram are shown in Table III . There were no statistically significant differences in skeletal changes before and after treatment within each group and between groups. In both groups, there were significant increases in OB and IIA after treatment ( P <0.05). The Invisalign group had an increase of 1.70 ± 1.73 mm in OB ( P <0.001), whereas the fixed appliance group had an increase of 0.71 ± 1.48 mm ( P <0.05). The Invisalign group had an increase of 33.25° ± 13.20° in IIA ( P <0.001), whereas the fixed appliance group had an increase of 25.88° ± 12.61° ( P <0.001). Compared with the fixed appliance group, the Invisalign group showed a 1 mm greater increase in OB and a 7° greater increase in IIA ( P <0.05).
| Variables | Invisalign (n = 27) | Fixed appliance (n = 30) | Invisalign (n = 27) | Fixed appliance (n = 30) | P value | |||||
|---|---|---|---|---|---|---|---|---|---|---|
| T1 | T2 | P value | T1 | T2 | P value | ΔT | ΔT | t | U | |
| SNA (°) | 81.45 ± 3.39 (80.17-82.73) | 81.41 ± 3.39 (80.13-82.69) | 0.718 | 81.61 ± 3.67 (80.30-82.92) | 81.23 ± 4.64 (79.57-82.89) | 0.062 | −0.04 ± 0.63 (−0.28 to 0.20) | −0.37 ± 0.76 (−0.64 to −0.10) | 0.061 | |
| SNB (°) | 77.31 ± 3.09 (76.14-78.48) | 77.15 ± 3.10 (75.98-78.32) | 0.662 | 78.14 ± 3.02 (77.06-79.22) | 77.73 ± 3.70 (76.41-79.05) | 0.054 | −0.17 ± 0.83 (−0.48 to 0.14) | −0.45 ± 0.71 (−0.70 to −0.20) | 0.055 | |
| ANB (°) | 4.14 ± 1.93 (3.41-4.87) | 4.27 ± 1.70 (3.63-4.91) | 0.311 | 3.46 ± 1.63 (2.88-4.04) | 3.54 ± 1.66 (2.95-4.13) | 0.466 | 0.13 ± 0.82 (−0.18 to 0.44) | 0.08 ± 0.59 (−0.13 to 0.29) | 0.316 | |
| Wits (mm) | −0.34 ± 2.77 (−1.38 to 0.70) | −1.07 ± 2.49 (−2.01 to −0.13) | 0.054 | −1.59 ± 3.14 (−2.71 to −0.47) | −2.04 ± 2.07 (−2.78 to−1.30) | 0.260 | −0.74 ± 1.45 (−1.29 to −0.19) | −0.45 ± 2.15 (−1.22 to 0.32) | 0.544 | |
| Mp-SN (°) | 37.86 ± 6.20 (35.52-40.20) | 38.03 ± 5.73 (35.87-40.19) | 0.621 | 35.62 ± 6.30 (33.37-37.87) | 36.15 ± 6.60 (33.79-38.51) | 0.189 | 0.17 ± 1.69 (−0.47 to 0.81) | 0.53 ± 2.14 (−0.24 to 1.30) | 0.701 | |
| Overjet (mm) | 2.86 ± 2.34 (1.98-3.74) | 3.82 ± 0.95 (3.46-4.18) | 0.050 | 2.71 ± 2.09 (1.96-3.46) | 2.89 ± 0.78 (2.61-3.17) | 0.650 | 0.96 ± 2.44 (0.04 to 1.88) | 0.18 ± 2.11 (−0.58 to 0.94) | 0.195 | |
| OB (mm) | 1.70 ± 1.41 (1.17-2.23) | 3.41 ± 1.26 (2.93-3.89) | <0.001 ††† | 1.50 ± 1.40 (1.00-2.00) | 2.21 ± 0.91 (1.88-2.54) | 0.013 † | 1.70 ± 1.73 (1.05-2.35) | 0.71 ± 1.48 (0.18-1.24) | 0.023 ∗ | |
| IIA (°) | 111.45 ± 12.63 (106.69-116.21) | 144.70 ± 8.91 (141.34-148.06) | <0.001 ††† | 113.48 ± 8.44 (110.46-116.50) | 139.35 ± 9.90 (135.81-142.89) | <0.001 ††† | 33.25 ± 13.20 (28.27-38.23) | 25.88 ± 12.61 (21.37-30.39) | 0.035 ∗ | |
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