Introduction
The purpose of this research was to provide an update on the accuracy of tooth movement with Invisalign (Align Technology, Santa Clara, Calif).
Methods
This prospective clinical study included 38 patients treated with Invisalign Full or Invisalign Teen. All teeth, from the central incisor to the second molar, were measured on digital models created from intraoral scans. Predicted values were determined by superimposing the initial and final ClinCheck models, and achieved values were determined by superimposing the initial ClinCheck models and the digital models from the posttreatment scans. Individual teeth were superimposed with a best-fit analysis and measured using Compare software (version 8.1; GeoDigm, Falcon Heights, Minn). The types of tooth movements studied were a mesial-distal crown tip, buccal-lingual crown tip, extrusion, intrusion, and mesial-distal rotation.
Results
The mean accuracy of Invisalign for all tooth movements was 50%. The highest overall accuracy was achieved with a buccal-lingual crown tip (56%), whereas the lowest overall accuracy occurred with rotation (46%). The accuracies for mesial rotation of the mandibular first molar (28%), distal rotation of the maxillary canine (37%), and intrusion of the mandibular incisors (35%) were particularly low.
Conclusions
There was a marked improvement in the overall accuracy; however, the strengths and weaknesses of tooth movement with Invisalign remained relatively the same.
Highlights
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The mean accuracy of Invisalign for all tooth movements was 50%.
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The highest overall accuracy occurred with a buccal-lingual crown tip (56%).
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The lowest overall accuracy occurred with rotation (46%).
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Despite the improvement, the weaknesses of tooth movement with Invisalign remained the same.
In 2009, Kravitz et al conducted the first prospective clinical study on Invisalign (Align Technology, Santa Clara, Calif) to evaluate its efficacy. Prior published data included case reports, material studies, technical articles, editorials, surveys, studies comparing Invisalign to conventional fixed appliances, and a systematic review, none of which provided scientific evidence regarding the efficacy or limitations of Invisalign. Ten years after Invisalign was introduced, orthodontists were just beginning to quantify how well it moved teeth.
The landmark study by Kravitz et al evaluated the accuracy of anterior tooth movements with Invisalign. Measurements were made by superimposing the predicted and achieved ClinCheck digital models over the stationary premolars and molars, using ToothMeasure, Align’s tooth measurement software. The most accurate movement was lingual constriction (47%), and the least accurate movements were incisor extrusion (18%) and mandibular canine rotation (28%). The overall mean accuracy of Invisalign was 41%.
In a second study, using the same sample and methodology, Kravitz et al specifically evaluated the influence of interproximal reduction (IPR) and ellipsoid attachments on canine rotation. The mean accuracy of this rotation with Invisalign was 36%. The authors reported that canines which received IPR achieved the highest accuracy (43%). Most importantly, the accuracy of canine rotation significantly dropped with rotational movements greater than 15°.
Since these 2 studies were published, significant contributions have been made, further evaluating the efficacy of tooth movement with Invisalign.
In 2012, Krieger et al also evaluated anterior tooth position with Invisalign, but they studied different parameters. Rather than assessing individual tooth movements, the authors evaluated arch length, intercanine distance, overbite, overjet, and midlines by comparing initial and final plaster casts, which were measured with digital calipers. They provided a general conclusion that Invisalign effectively resolved anterior crowding by incisor proclination, but overbite correction was difficult to achieve.
In 2014, Simon et al evaluated the influence of attachments and power ridges with Invisalign for 3 specific movements: incisor torque, premolar rotation, and maxillary molar distalization. Predicted digital models were superimposed over achieved digitized plaster models, using Surfacer software. The least accurate movement was premolar rotation (40%). Similar to the findings by Kravitz et al, this accuracy significantly decreased with rotational movements greater than 15°.
In 2017, Grünheid et al evaluated the efficacy of tooth movement with Invisalign for all teeth. The predicted and achieved digital models were superimposed with a best-fit registration, using Compare software (version 8.1; GeoDigm, Falcon Heights, Minn). Although the percent accuracy was not calculated, the movements that had the greatest difference between predicted and achieved outcomes were molar torque, mandibular incisor intrusion, and mandibular lateral, canine, and first premolar rotation.
In 2018, Charalampakis et al evaluated the efficacy of incisor, canine, and premolar movements with Invisalign. The predicted and achieved ClinCheck models were superimposed over stationary first and second molars, using SlicerCFM software. Similar to the findings by Grünheid et al, the least accurate movements were mandibular incisor intrusion, followed by a rotation of the maxillary canines, mandibular premolars, and mandibular canines.
Since the publication of the 2009 studies, , the Invisalign system has undergone significant changes. The 2 most notable are the introduction of SmartForce features (2008), such as optimized attachments, pressure zones, and customized staging, and the SmartTrack aligner material (2011), which allows for a better range of force delivery and fit. In addition, physical impressions have been largely replaced by digital scans. The purpose of this prospective clinical study is to provide an update on the accuracy of Invisalign with newer technology.
Material and methods
The study group comprised 38 patients (13 males, 25 females) with a mean age of 36 years. Twenty-nine patients received Invisalign Full and 9 received Invisalign Teen. The mean number of aligners per arch was 21 maxillary and 20 mandibular. Both arches each averaged 6 attachments and less than 1 mm of IPR. The breakdown of malocclusions was as follows: 22 Class I, 13 Class II, and 3 Class III. The average time between the initial and final scans was 8.5 months ( Table I ).
| Category | n |
|---|---|
| Patients | |
| Male | 13 |
| Female | 25 |
| Malocclusion | |
| Class I | 22 |
| Class II | 13 |
| Class III | 3 |
| Type of Invisalign | |
| Invisalign Full | 29 |
| Invisalign Teen | 9 |
| Average number of aligners | |
| Maxillary | 21 |
| Mandibular | 20 |
| Frequency of attachments | |
| Maxillary | 6 |
| Mandibular | 6 |
| Average amount of IPR, mm | |
| Maxillary | <1 |
| Mandibular | <1 |
The research protocol was approved by the Institutional Review Board of European University College (no. EUC-IRB-17.2.11). Invisalign treatment was provided at a single orthodontic practice in South Riding, Virginia, and the orthodontist (N.D.K.), who prescribed all ClinCheck treatment plans, was highly experienced (Tier-Level Diamond Plus Provider [formerly Top 1% Elite] with over 2500 Invisalign cases treated). Unlike the 2009 study, overengineering of tooth moments was prescribed when deemed necessary to achieve the best result clinically.
The patients were instructed to wear their aligners for 22 hours per day and change their aligners every 10 days. At the delivery appointment, the patients understood that they were part of a research study, and honest reporting of their compliance was critical. Compliance was also verbally confirmed at each appointment. The last data collection was in November 2017.
Inclusion criteria were as follows: (1) treated with either Invisalign Full or Invisalign Teen, (2) underwent treatment in both arches, (3) completed an initial and final intraoral digital scan, and (4) confirmed good compliance throughout treatment. Exclusion criteria were as follows: (1) noncompletion in time for the study, (2) poor compliance with the aligners, and (3) oral surgery or dental restorations before the final scan. A total of 44 patients were enrolled in the study but 6 were excluded; 3 patients did not complete their treatment in time for data collection, and 3 patients had errors in their final scans.
The digital models were deidentified and imported into Compare, a tooth measurement software program. All teeth in the arch were evaluated. The total number of teeth measured was 899 (450 maxillary and 449 mandibular), which was more than twice as many as the 2009 study. The digital models were evaluated following the protocol established by Grünheid et al.
The initial ClinCheck model was segmented into individual teeth. To provide the predicted values, we globally aligned the initial ClinCheck model over the final ClinCheck model. Then the individual teeth from the initial model were superimposed over the equivalent teeth of the final model, using a best-fit algorithm. To provide the achieved values, we superimposed the individual teeth from the initial ClinCheck model on the digital model from the posttreatment scan ( Figs , A and B ).
The tooth movements measured were mesial-distal crown tip, buccal-lingual crown tip, intrusion, extrusion, and rotation. Although the software measured “torque,” in the absence of radiographs, this movement could not be confirmed and was excluded from this study. The percent accuracy was determined by the following equation: percentage of accuracy = 100% − ([(predicted – achieved)/predicted] × 100%). The equation accounted for directionality and ensured that the percentage of accuracy never exceeded 100% for teeth that achieved movements beyond their predicted value.
To evaluate the clinical relevance of our results, we printed and assessed the posttreatment scans of half the sample (19 patients), according to the American Board of Orthodontics (ABO) cast evaluation system. Patients were randomly chosen using research randomizer software. Following the protocol of the 2009 study,1 a modified-Discrepancy Index of the pretreatment malocclusion was calculated, excluding for cephalometrics and skeletal asymmetry scores. The posttreatment ABO scores were calculated by 2 operators; subsequently, 10 models were remeasured by the same examiner to assess intraoperator reliability.
Statistical analysis
The statistical analysis was performed with SPSS software (version 15; IBM, Armonk, NY). Each tooth movement was measured separately. Clinical significance was set for linear movements at <0.25 mm and angular movements at <2°, which is approximately the amount of maximum movement on a tooth per aligner. Paired t tests ( P < 0.05) compared the intraarch accuracy of tooth movement by direction (ie, buccal versus lingual), and independent t tests compared the accuracy of tooth movement by arch (ie, maxillary vs mandibular).
Results
All predicted linear and angular movements less than 0.1 mm and 1.0° were eliminated from analysis to account for error in model superimposition. Acceptable sample sizes were attained for all tooth movements, except for the lingual crown tip of the maxillary first molar (n = 6), first premolar (n = 3) and second premolar (n = 7), as well as the intrusion of mandibular second premolar (n = 8).
The mean accuracy of Invisalign for all tooth movements was 50%. The highest overall accuracy was achieved with a buccal-lingual crown tip (56%). The lowest overall accuracy occurred with rotation (46%). Specifically, the most accurate movement was the labial crown tip of the maxillary lateral incisor (70%), and the least accurate movements were the mesial rotation of the mandibular first molar (28%), followed by intrusion of the maxillary (33%) and mandibular central incisors (34%) ( Table II ).
