Introduction
This research aimed to determine whether Class II malocclusion can be treated with clear aligners after completing treatment with the initial set of aligners.
Methods
A sample of 80 adult patients were divided into Group 1 with Class I molar malocclusions ( n = 40 [11 men and 29 women]; 38.70 ± 15.90 years) and Group 2 with Class II molar malocclusions ( n = 40 [11 men and 29 women]; 35.25 ± 15.21 years). All patients had finished treatment with the initial set of Invisalign aligners (Align Technology, Santa Jose, Calif) without known centric occlusion–centric relation discrepancies, issues of compliance, or overcorrection. The 7 measurements using the American Board of Orthodontics (ABO) Model Grading System and millimetric measurements for anteroposterior (AP) and vertical dimensions were assessed and compared between the 2 groups at pretreatment, posttreatment ClinCheck (Align Technology) prediction, and posttreatment.
Results
No improvements were observed in the AP correction. The amount of AP correction in patients with Class II malocclusion was 6.8% of the predicted amount. The amount of overbite correction achieved was 28.8% and 38.9% of the predicted amounts in patients with Class I and Class II malocclusion, respectively. Significant improvements in alignment and interproximal contact scores were observed, with only slight improvements in total ABO scores. An increase in mean occlusal contacts score was observed after treatment. No patient with Class II malocclusions would meet the ABO standards after Invisalign treatment.
Conclusions
The Invisalign system successfully achieves certain tooth movements but fails to achieve other movements predictably. No significant Class II correction or overjet reduction was observed with elastics for an average of 7-month duration in the adult population. Additional refinements may be necessary to address problems created during treatment, as evidenced by a posterior open bite incidence.
Highlights
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Significant improvements in the ABO score with Invisalign treatment.
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No improvements were seen in anteroposterior correction in Class II group.
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No posttreatment occlusions met ABO standards in Class II malocclusion group.
As early as 1945, orthodontists used aligners to correct minor tooth movements; however, comprehensive orthodontic treatment was deemed impractical because of the number of impressions and laboratory time required to fabricate each aligner. Invisalign (Align Technology, San Jose, Calif) clear aligners originated in 1997, when Stanford student Zia Chishti, an orthodontic patient turned entrepreneur, received a clear retainer from his orthodontist for retention. Using the design software in a computer laboratory at the university, Chishti and his partners learned how to simulate a solid object with a computer-aided design model and then recreate that object using 3-dimensional printing technology. Through this technology and sequential staging of tooth movements capable of generating orthodontic forces, the Invisalign system was developed under the company name Align Technology. , Currently, more than 7.5 million Invisalign cases have shipped worldwide with yearly net revenues exceeding $2.3 billion. Most recently, Align has released the first U.S. Food and Drug Administration–approved clear aligner treatment for Class II malocclusion, termed Invisalign with mandibular advancement.
With the improvements in aligner materials, attachment design, and 3-dimensional software, the scientific community has responded with research measuring the success or failure of these technological progressions by measuring achieved individual tooth movements and comparing them with their predicted models. Some of the most notable conclusions in comparison with fixed appliances include the following: aligner cases demonstrated significantly poorer root control during extraction space closure, shorter treatment duration by 5.7 months on average, and higher Peer Assessment Rating scores. Studies comparing achieved vs predicted tooth movements demonstrated a total tooth movement accuracy of 41%, with extrusion, mandibular canine rotation, and labial crown movement measured at 29.6%, 29.1%, and 37.6%, respectively. Average vertical changes observed were between 0.9 mm and 1.5 mm, , whereas anteroposterior (AP) movement via maxillary molar distalization was highly predictable (88% accuracy) when at least 1.5 mm of translation was prescribed.
Some recent studies have evaluated posttreatment results using the American Board of Orthodontics (ABO) Model Grading System (MGS). Findings indicated that Invisalign-treated patients lost 13 MGS points more than patients treated with conventional braces. These scores were consistently lower for buccolingual inclination and overjet criteria, with occlusal contacts and occlusal relationship scores worsening with treatment. , Extraction cases with Invisalign have demonstrated increased treatment times compared with braces and poorer MGS scores. Retention studies have shown significantly more relapse among patients treated with Invisalign.
The present study serves to expand our knowledge of aligner treatment for patients with Class II malocclusions. Previous research has objectively evaluated posttreatment models, but with an unspecified number of refinements over an unspecified amount of time; this method skews the predicted and achieved accuracy for tooth movements. After completing treatment with the initial set of aligners, the evaluation of patients can thoroughly compare the initial, predicted, and achieved results over a specific period. Thus, the purpose of the study was to determine whether Class II malocclusion can be treated with Invisalign after completion of the treatment with the initial set of clear aligners.
Material and methods
A total of 80 Invisalign-treated patients were selected for inclusion in this retrospective study, enough for a sampling error of 10% of the mean with a critical confidence value (Z α/2 ) equal to 1.645 (α = 0.10) and 90% confidence level. The sample was divided into group 1 with Class I malocclusions ( n = 40 [11 men and 29 women]; age, 38.70 ± 15.90 years) and group 2 with Class II malocclusions ( n = 40 [11 men and 29 women]; age, 35.25 ± 15.21 years). Classifications were determined from molar relationships, as defined by the ABO, and canine relationships.
For Class II malocclusions, both end-on and full-step molars were included in the study; unilateral Class II relationships were also included. Both age and gender—29 women and 11 men per group—were matched before analysis of records to ensure even distribution. Treatments were completed by an orthodontic specialist with extensive experience in treating with clear aligners.
A Patient Data Checklist was created with the following inclusion criteria: no known reported issues of compliance, no known centric occlusion–centric relation discrepancy, no overcorrection planned, no interproximal reduction performed immediately before refinement scan, completion of all active aligners in ClinCheck (Align Technology), patients whose treatment finished after 2014 with SmartTrack (Align Technology) material, and adult patients aged 18 years and older. Overcorrection was defined as values outside of the normal range for that particular dental measurement, as defined by the ABO Discrepancy Index. For example, the range for normal overbite is >1 to ≤3 mm, yielding 0 points to the overall score. There were no limitations on missing teeth (except canines) or history of orthodontic treatment. Patients with dental bridges and those having planned extractions were not included in the study.
The protocol for evaluating digital model files at pretreatment (T1), posttreatment ClinCheck prediction (T2A), and posttreatment (T2B) were obtained from the Invisalign Web site. For this research, posttreatment was defined as the end of the first set of active aligners. Next, ClinCheck Pro (version 5.4; Align Technology) software was used to view the files and perform the digital measurements.
The measurements of the ABO MGS, including alignment, marginal ridges, buccolingual inclinations, occlusal contacts, occlusal relationships, overjet, and interproximal contacts, were assessed at T1, T2A, and T2B ( Table I ). Root parallelism was omitted from the analysis, similar to a previous study. Both overbite and AP relationships were also recorded in millimeters on the Patient Data Checklist. The amount needed was calculated using an ideal overbite and AP relationship. Measurements for each component were made from the digital models according to the protocol established in the ABO Grading System for Dental Casts and Panoramic Radiographs. All measurements were made by investigator (B.D.P.) who had successfully completed the ABO Calibration Kit administered by a current ABO Director. All scores were recorded on the Cast-Radiograph Worksheet. The ClinCheck Pro software allowed the use of a digital ruler instead of a standard ruler ( Fig 1 ). Because no measurement required precision finer than 0.5 mm, the proposed method for digital measurements provided the optimal accuracy required. According to ABO guidelines, a case that scores more than 30 points would likely fail, less than 20 points would likely pass, and between 20 and 30 points would be considered borderline.
| ABO measurement | Description | Cronbach α |
|---|---|---|
| Alignment | In the anterior region, the incisal edges with the lingual surfaces of the maxillary teeth and the labial surfaces of the mandibular teeth. In the posterior region, the mesiodistal central groove of the maxillary teeth and the buccal cusps of the mandibular teeth. | 0.903 |
| Marginal ridges | These are used to assess the proper vertical positioning of the posterior teeth, which should be at the same level. | 0.945 |
| Buccolingual inclination | This is used to assess the angulation of the posterior teeth to establish maximum intercuspation and avoid balancing interferences. | 0.938 |
| Occlusal contacts | These are used to measure the adequacy of the posterior occlusion and maximum intercuspation of the opposing teeth. | 0.973 |
| Occlusal relationship | This is used to assess the AP position of the maxillary and mandibular posterior teeth. | 0.984 |
| Overjet | This is used to assess the AP relationship of anterior teeth and transverse relationship of posterior teeth. | 0.831 |
| Interproximal contacts | This is used to assess spacing within the dental arch. | 0.923 |
The percentage of treatment accuracy for the ABO MGS measurements was calculated by 2 methods, depending on which model was used as the gold standard. If the predicted ClinCheck is the standard, then the formula is defined as percentage accuracy = (initial − achieved)/(initial − predicted) × 100. The percentage of treatment accuracy for AP movement and overbite achieved was calculated by the following equation: percentage accuracy = amount achieved/amount predicted × 100.
Statistical analysis
Descriptive statistics were calculated for all variables. Because the data distribution was not normal, the nonparametric equivalents were used for statistical analysis. Wilcoxon signed rank (paired t ) tests were used to identify intragroup differences. Mann-Whitney U (independent t ) tests were used to evaluate intergroup differences. Intraexaminer reliability was evaluated after remeasuring 10% of the sample and reported with Cronbach alpha; α = 0.05 was used as the level of statistical significance for all analyses.
Results
Descriptive statistics for the ABO MGS measurements at T1, T2A, and T2B, and the comparisons between the time points were shown in Tables II and III for patients with Class I and Class II malocclusion, respectively. Intrarater reliability ranged from 0.831 to 0.984 ( Table I ). Assuming a passing threshold of 27 or lower, all of the predicted ClinCheck occlusions (T2A) would meet ABO standards, but only 47.5% and 0% of posttreatment occlusions (T2B) would pass in the Class I and Class II malocclusion groups, respectively. After treatment, improvements were noted in the total ABO scores, alignment, and interproximal contacts, but significant differences remained compared with the predicted scores. No improvements were observed in marginal ridges, overjet, or occlusal relationships at posttreatment. Occlusal contacts were the only category that scored worse with treatment. The average number of aligners was higher in the Class II malocclusion group and the refinement percentage, as shown in Table IV . Patients with Class II malocclusion required significantly more initial aligners as well as a greater percentage at refinement than those with Class I malocclusion, indicating longer treatment time.
| Variables | Descriptive | Paired t test | ||||||
|---|---|---|---|---|---|---|---|---|
| T1 | T2A | T2B | T1-T2B | T2A-T2B | ||||
| Mean | SD | Mean | SD | Mean | SD | Sig | Sig | |
| Total score | 35.25 | 8.84 | 12.00 | 5.35 | 27.53 | 8.42 | 0.000 ∗ | 0.000 ∗ |
| AR | 20.85 | 6.27 | 2.75 | 1.63 | 7.30 | 3.74 | 0.000 ∗ | 0.000 ∗ |
| MR | 3.10 | 2.10 | 2.10 | 1.50 | 3.10 | 2.24 | 1.000 | 0.001 ∗ |
| BI | 2.30 | 1.62 | 1.95 | 1.62 | 2.40 | 1.77 | 0.440 | 0.002 ∗ |
| OJ | 3.35 | 2.68 | 2.40 | 2.69 | 3.13 | 2.78 | 0.540 | 0.032 ∗ |
| OC | 2.90 | 2.64 | 2.68 | 2.46 | 10.60 | 4.11 | 0.000 ∗ | 0.000 ∗ |
| OR | 0.65 | 1.05 | 0.13 | 0.34 | 0.73 | 1.01 | 0.645 | 0.000 ∗ |
| IC | 2.13 | 4.92 | 0.15 | 0.95 | 0.30 | 0.91 | 0.015 ∗ | 0.486 |
∗ Paired t test was significant at the 0.05 level (2-tailed).
| Descriptive | Paired t test | |||||||
|---|---|---|---|---|---|---|---|---|
| T1 | T2A | T2B | T1-T2B | T2A-T2B | ||||
| Mean | SD | Mean | SD | Mean | SD | Sig | Sig | |
| Total score | 55.98 | 10.63 | 15.38 | 5.34 | 48.78 | 10.54 | 0.000 ∗ | 0.000 ∗ |
| AR | 21.35 | 5.56 | 2.55 | 2.09 | 7.13 | 3.62 | 0.000 ∗ | 0.000 ∗ |
| MR | 4.65 | 2.73 | 3.25 | 1.66 | 5.15 | 7.25 | 0.648 | 0.094 |
| BI | 2.93 | 1.93 | 1.80 | 1.57 | 2.50 | 2.06 | 0.025 ∗ | 0.003 ∗ |
| OJ | 8.40 | 2.93 | 2.48 | 2.21 | 9.35 | 3.35 | 0.072 | 0.000 ∗ |
| OC | 2.58 | 2.34 | 3.93 | 2.56 | 12.03 | 3.99 | 0.000 ∗ | 0.000 ∗ |
| OR | 14.28 | 4.77 | 1.33 | 1.79 | 13.35 | 5.04 | 0.067 | 0.000 ∗ |
| IC | 1.55 | 3.51 | 0.03 | 0.16 | 0.38 | 0.98 | 0.037 ∗ | 0.033 ∗ |
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