Introduction
This study aimed to establish if there is a significant difference in effectiveness between 2 generations of Invisalign trays in terms of Peer Assessment Rating (PAR) score reduction for finished patients from a graduate orthodontic clinic.
Methods
Forty-five pretreatment and posttreatment patients treated with the previous Invisalign material and 49 pretreatment and posttreatment patients treated with SmartTrack material were scored using the Peer Assessment Rating (PAR) index. Both groups were controlled for initial weighted PAR score, age, gender, and treatment time. The 2 generations were compared in regard to absolute reduction, percent reduction, and great improvement in PAR score.
Results
The mean absolute reduction in weighted PAR score between the groups was not statistically significant ( P = 0.526). The mean percent reduction in weighted PAR score between the groups was not statistically significant ( P = 0.210). The proportion of great improvement between the groups was not significant ( P = 0.526). Only 6 of the 8 components of occlusion had enough variation to be modeled. An absolute reduction in unweighted PAR score was not significantly different between the groups for maxillary anterior alignment, overjet, or mandibular anterior alignment ( P = 0.996, 1.000, and 0.114, respectively). Percent reduction in unweighted PAR score was not significantly different between the 2 groups for an anteroposterior, overbite, or transverse ( P = 1.000, 1.000, and 1.000, respectively) relationships.
Conclusions
Our study indicates that both generations of Invisalign aligners improved the malocclusion to a similar degree according to the PAR index. Patient-centric benefits of SmartTrack aligner should also be considered by the provider.
Highlights
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There were no significant differences between the two groups for any of the 8 components using PAR.
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Both generations of Invisalign were able to improve the malocclusion to a similar degree.
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Patient-centric benefits of SmartTrack aligner should also be considered by the provider.
In 1997, Align Technology (Santa Clara, Calif) introduced the Invisalign system, which integrates 3-dimensional computer-based technology with virtual treatment planning to create a series of clear, polyurethane aligners to incrementally move teeth into an ideal position. Studies have shown that Invisalign is more comfortable than braces, provides preferred esthetics for some patients and allows for greater ability to maintain good oral hygiene. Invisalign claims to have treated over 7 million patients.
The original Invisalign tray material was made of 0.030-in thick plastic material. The company used variously shaped composite attachments to aid tooth movements of up to 0.25-0.30 mm per aligner and 2-3° rotational changes per aligner. These products were followed by the introduction of SmartTrack aligner material in 2013, which they claimed to be more gentle, comfortable, and predictable. ,
A recent systematic review evaluated the clinical effectiveness of Invisalign and found it was comparable to traditional brackets for orthodontic treatment in nongrowing, nonextraction patients with mild-to-moderate crowding. The article reported that Invisalign was predictable in its ability to level, tip, and derotate teeth (except premolars and canines). However, there was limited evidence that Invisalign could predictably expand arches with bodily movement, close extraction spaces, fully correct occlusal contacts, or resolve significant vertical and anteroposterior discrepancies. This systematic review included articles dating from 2003-2017, meaning several generations of aligner material were included in the studies.
There is little evidence that compares the previous aligner material used by Invisalign to their SmartTrack aligner material. Condo et al compared the 2 aligners in terms of mechanical properties, whereas Bräscher et al compared the 2 generations in terms of patient-centric values of comfort.
However, there are little, if any, quantified treatment outcomes data comparing the previous aligner to the new SmartTrack material. This is an important point because Align indicated on their Web site that, “A study Align conducted with more than 1000 patients treated with SmartTrack showed the material offered statistically significant improvement in the control of tooth movement (50% faster movement with 75% more predictability) compared with patients treated with the current Invisalign material.” This statement has not been verified by any published data in the refereed literature.
Clinical outcomes have been evaluated by both the American Board of Orthodontics (ABO) Objective Grading System (OGS) and the Peer Assessment Rating (PAR) index. They both have well-known strengths and weaknesses. This study focused on treatment effectiveness using 2 different aligner materials: the previous generation of Invisalign aligners and the SmartTrack aligner material released by Invisalign in 2013. The specific aims of this study were to establish if there were significant differences between the 2 generations of Invisalign trays in terms of overall improvements for finished patients using the PAR index, as alluded to in the manufacturer’s statements. In addition, this study evaluated PAR’s 8 occlusal components for both generations of material. Finally, this study compared treatment time, age, initial PAR, and gender between patients treated with SmartTrack and its precursor.
Material and methods
The protocol for this study was reviewed and approved by an institutional review board.
This retrospective case-control study selected Invisalign patients from a university-based clinic archive. This study compared 2 groups: the first group included patients treated with a previous Invisalign material between 2008 and 2012, whereas the second group included patients treated with SmartTrack Invisalign material treated between 2014 and 2016. All patients were treated under the same faculty between 2008 and 2016. Patients were selected on the basis of the following criteria: (1) available pretreatment and posttreatment models, (2) no auxiliary appliances other than interarch elastics, (3) no history of orthognathic surgery or craniofacial anomalies, (4) patients aged ≥16 years at treatment start, (5) no missing teeth first molar to first molar; and (6) nonextraction treatment and treated with a 2 week per tray protocol.
The sample size was determined on the basis of the availability of archived complete records gathered during the designated treatment periods (ie, 2009-2012 and 2014-2016) that met all inclusion criteria. After reviewing all available patients who met the criteria, 45 consecutive finished patients (mean age, 26.0; standard deviation [SD], 9.1; female, n = 31; male, n = 14) were selected for the previous Invisalign material group (group 1) and 49 consecutive finished patients (mean age, 28.2; SD, 11.1; female, n = 37; male, n = 12) were selected for the SmartTrack material group (group 2).
All casts were evaluated using the PAR index, which was first introduced in 1992 by Richmond et al, and it evaluates 8 components of occlusion; maxillary anterior segment alignment, mandibular anterior segment alignment, anteroposterior buccal occlusion, transverse buccal occlusion, vertical buccal occlusion, overjet, overbite, and centerline. The PAR index uses a weighted scoring system that multiplies individual scores of some components of occlusion by a unique factor before summing them to determine an overall weighted PAR score.
For this study, the PAR index was preferable for numerous reasons, including its international recognition, association with stability, prediction of orthodontic need, and superior reliability and validity. All patients used digital casts from OrthoCAD (Cadent, Fairview, NJ) or 3Shape software (OrthoAnalyzer; 3Shape, Copenhagen, Denmark) to determine the pretreatment and posttreatment PAR scores. Mayers et al showed that determining PAR score using digital models was both valid and reliable. Two investigators (Jack Tang and Mitchell Bobby) were independently calibrated for the PAR index, and both investigators were blinded to the material group they were scoring. The intrarater and interrater reliability were assessed by the intraclass correlation coefficient (ICC) determined from 17 randomly selected patients rescored more than 6 months after initial data collection.
The PAR index evaluates not only overall pretreatment and posttreatment occlusion, but it also assesses 8 individual components of malocclusion. Richmond et al originally described improvement as a reduction in PAR score of ≥30%, and great improvement as a reduction in PAR score by ≥22 points. Because most Invisalign patients treated in this study were limited to mild or moderate malocclusions, a good number of patients did not begin with a PAR score of ≥22. In addition, the lowest posttreatment PAR score shared by both groups was 2 points. Therefore, the definition of great improvement changed to be either a weighted PAR score with a reduction of ≥22 points or a weighted posttreatment PAR score of ≤2 for those patients who began with a pretreatment PAR score <22. This is a scoring modification previously introduced by Gu et al For both overall PAR scores and component PAR score analysis, both the absolute reduction in weighted PAR score and the percentage reduction in weighted PAR score [(baseline-endpoint)/baseline] were calculated.
Statistical analysis
All statistical analyses were performed with R software (R Core Team, Vienna, Austria). ICC values were calculated using a linear mixed-effect model with a random effect for subjects. Multiple linear regression with adjustment for baseline PAR score was used to evaluate differences in posttreatment PAR scores, an absolute reduction in PAR scores, and a percent reduction in PAR scores between treatment groups. Multiple comparisons were adjusted using the Bonferonni method. Multiple logistic regression with adjustment for baseline PAR score was used to evaluate the difference in probability of achieving great improvement in PAR score. The level of statistical significance for all analyses was set at α = 0.05.
Results
The 2 calibrated examiners demonstrated excellent intrarater reliability of 0.997 (95% confidence interval [CI], 0.992-1.000) and 0.999 (95% CI, 0.998-1.000), respectively. Interrater reliability was good with an ICC score of 0.885 (95% CI, 0.718-0.956).
A comparison of baseline characteristics between the 2 groups assessed age ( P = 0.292), gender ( P = 0.627), and the number of months in treatment for group 1 (mean, 13.18; SD, 8.70) and group 2 (mean, 13.86; SD, 5.82) ( P = 0.656). Only the initial PAR score was significantly different between the groups. The mean pretreatment unweighted PAR scores for groups 1 and 2 were 13.29 (SD, 4.62) and 9.53 (SD, 3.67), respectively ( P <0.01); whereas, the weighted PAR scores for groups 1 and 2 were 19.00 (SD, 8.10) and 14.90 (SD, 7.38), respectively ( P = 0.012). Because of the lack of significant difference among the groups, only initial PAR was included as a covariate in the regression analysis to adjust the pretreatment weighted PAR scores for baseline.
The mean posttreatment weighted PAR scores were not significantly different ( P = 0.526) ( Table I ), with the previous generation having a mean score of 4.333 and SmartTrack with 4.102. After adjusting for baseline weighted PAR, the mean absolute reduction in weighted PAR score for the previous generation of aligners was 14.667 and 10.769 for the SmartTrack aligner, which was not statistically significant ( P = 0.526) ( Table I ). The mean percent reduction in weighted PAR score for the previous generation of aligners was 76.931% and 68.401% for the SmartTrack aligner, which was not statistically significant ( P = 0.210) ( Table I ). The proportion of great improvement for the previous generation of aligners was 13.333% and 10.204% for the SmartTrack aligner group, which was not statistically significant ( P = 0.526) ( Tables I and II ).
| Dependent variable | |||
|---|---|---|---|
| Pretreatment | Absolute reduction | Percent reduction | |
| Group 1 vs 2 | |||
| Score | −0.53 | 0.53 | 0.061 |
| P | P = 0.526 | P = 0.526 | P = 0.210 |
| Pretreatment weighted PAR | |||
| Score | 0.186 | 0.814 | 0.006 |
| P | P = 0.001∗∗∗ | P = 0.001∗∗∗ | P = 0.063∗ |
| Constant | |||
| Score | 1.337 | −1.337 | 0.598 |
| P | P = 0.167 | P = 0.167 | P = 0.001∗∗∗ |
| Observations | 94 | 94 | 94 |
| R 2 | 0.122 | 0.744 | 0.07 |
| Adjusted R 2 | 0.102 | 0.738 | 0.05 |
| Residual standard error | 3.890 (df = 91) | 3.890 (df = 91) | 0.228 (df = 91) |
| F statistic | 6.300∗∗∗ (df = 2; 91) | 132.108∗∗∗ (df = 2; 91) | 3.428∗∗ (df = 2; 91) |
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