Efficacy of Anterior Tooth Simulations with Clear Aligner Therapy - A Retrospective Cohort study of Invisalign and Flash Aligner Systems

1. INTRODUCTION

Clear aligner therapy (CAT) is an increasingly popular treatment modality in contemporary orthodontics [1]. Studies have evaluated the biological [2-4], esthetic [5, 6], and psychological [7-10] advantages that CAT promises over conventional pre-adjusted edgewise appliances. Although CAT is fast gaining popularity amongst care seekers, the orthodontic profession still has unanswered questions regarding the efficacy and efficiency of these appliances. Introduced into the market in 1999, Invisalign® (Align Technology, Santa Clara, CA, USA) has been the forerunner and mainstay in the aligner industry, having had the first-mover advantage. The marketing and advertising campaign of the company was aggressive [11]; at the same time, however, orthodontic literature regarding the efficacy and efficiency of CAT was scarce.

One of the first research articles comparing virtual treatment outcomes with actual achieved treatment outcomes appeared in 2009 by Kravitz et al. [12]. This prospective study found that the mean accuracy of anterior tooth movement with Invisalign® was 41%. The most accurate tooth movement was lingual constriction (47.1%), and the least accurate was extrusion (29.6%), particularly for the maxillary and mandibular incisors. Rotation of the maxillary and mandibular canines and the maxillary lateral incisor was significantly less accurate compared to the other teeth. In 2014, Simon et al. [13] reported that the overall mean accuracy of tooth movement was 59%. The mean accuracy for maxillary incisor torque was 42%. Premolar derotation showed the lowest accuracy with approximately 40%, and distalization of maxillary molars was the most accurate movement with 87%. In a systematic review by Rossini et al. [14], the authors concluded that CAT was an effective treatment modality and was able to align and level the arches in non-growing subjects. However, they warned that CAT was neither effective in anterior extrusion movements nor in rotation, especially for rounded teeth such as mandibular premolars. The authors suggested the use of auxiliaries (attachments, inter-arch elastics, IPR, or altered aligner geometry) to improve the predictability of orthodontic movement with CAT.

In 2013, Align technology® launched a new aligner material and improved tooth movement algorithms based on data obtained from treated cases since their founding in 1999. Since then, the orthodontic literature has reported more encouraging and improved outcomes [15-17]. In the quest for high-level evidence, a number of systematic reviews published on the Invisalign® system have appeared in the past few years [10, 18-20]. These reviews have all reported encouraging outcomes, and some have even compared their efficacy to fixed orthodontic appliances. A large issue is that the efficacy of one system cannot be a “state of the art” reflection on aligner science!

While Invisalign® is still considered the gold standard amongst CAT globally, numerous other CAT products have also been providing services in different regions of the world, usually at a lower price-point compared to Invisalign. However, the efficacy of other systems has not been evaluated by independent researchers. It is imperative to know the accuracy of achieving simulated tooth movements using different CAT products to understand the science of aligner treatment from a holistic perspective, rather than data generated from a single CAT brand.

Flash Orthodontics® [21] (Mumbai, India) is a relatively new aligner company established in 2016. Claiming better aligner quality (with 16-micron build quality and no striations) at a lower price point, they provide the intraoral scanning for their users, aiding in a complete digital workflow for aligner planning and manufacture. The tooth movements are formulated by their in-house orthodontic team with the treating doctor’s final approval. Notably, they claim to deliver the aligners within five days of accepting the plan to offices in India.

The purpose of the present study was to compare the efficacy of tooth movement between Invisalign® aligners and Flash® aligners by comparing the predicted treatment outcomes versus actual outcomes achieved using a 3D best-fit algorithm. The null hypothesis was no significant difference in the efficacy of orthodontic tooth movement between Invisalign® and Flash® software predictions to achieve actual outcomes.

2. MATERIALS & METHODS

2.2. Procedure for Attaining Differences between Predicated and Achieved Tooth Positions

To obtain digital models of the predicted outcome, the digital models of the initial and final stage of each patient’s virtual treatment plan were exported through Align Technology’s ClinCheckPro® and Flash® plan programs. Pretreatment, posttreatment, and predicted digital models were imported into eModel Compare 8.1 software (GeoDigm Corporation, Falcon Heights, Minn), a 3-dimensional software independent of either aligner company. This software used a best-fit 3-dimension (3D) algorithm to compare virtual treatment outcomes to achieve treatment outcomes. The automation of the process precludes the measurement from any operator bias. This enabled the calculation of differences in both linear and angular dimensions for individual tooth positions between the two digital models [22]. The simulated virtual treatment plan models were segmented and compared with the unsegmented models of the achieved tooth positions. The dental arches were first aligned globally, and then individual teeth from the segmented model were superimposed on the analogous teeth of the unsegmented model using a best fit algorithm so that differences between tooth positions could be computed. The differences between tooth positions were computed in linear (mesial-distal, buccal-lingual, and occlusal gingival) and angular (tip, torque and rotation) dimensions. (Fig. 1) Data from the patient’s dentition was organized and compared in two categories, i.e., maxillary and mandibular anterior dentitions. The automation of the process precludes the measurement from any operator bias.

Table 1.

-	N	Male: Female Ratio	Mean Age	No. of Teeth Measured	Angle Class (I:II:III)	Mean tx Duration (months)	Mean no. Maxillary Aligners	Mean No. Mandibular Aligners	Mean No. Attachments both Arches
Invisalign®	38	13:25	35.5	1031	25:13:6	8.4	21.4	19.7	6.2
Flash®	24	6:18	29.2	607	14:10:0	6.9	20.7	21.4	11.3

3. RESULTS

Independent t-tests comparing the pretreatment mandibular incisor irregularity index demonstrated that the two samples were homogeneous (P>.05) for mandibular anterior crowding. The left and right dentitions were tested for differences using independent t-tests and no differences (P>.05) were detected, thus, right-left data were pooled per tooth type.

4. DISCUSSION

The aim of the current study was to evaluate the efficacy of predicted simulations for three-dimensional movements of anterior teeth with two aligner systems. The samples were consecutively treated cases by experienced clinicians in each respective group. Only adult patients were included in the study for the following reasons: 1) adults represented the majority of patients who requested CAT, 2) adults showed better compliance than adolescents [23, 24], and 3) confounding factors were minimized in adults. The present study was retrospective in nature, which presented a noteworthy limitation.

In the present study, the efficacy of Invisalign and Flash aligner systems has been compared by measuring the discrepancies between virtual treatment plans and actual treatment outcomes using a mathematical superimposition of digital models. In previous studies that assessed the accuracy of clear aligner therapy, the ABO model grading system [25-27], ToothMeasure® (Align Technology) [12, 28] or Surfacer® (Imageware, Plano, Tex) software [13] were used. While these tools were able to give a general evaluation of accuracy, the software used in the current study is uniquely able to quantify differences between objects with respect to six degrees of freedom [15, 22]. The eModel Compare software calculates the differences automatically and is not influenced by potential operator bias. It has been used previously by Grunheid et al. [15], Awad et al. [29], and Haoulli et al. [30]. In a 2017 retrospective study on the accuracy of Invisalign® in nonextraction cases, Grunheid et al. [15] found statistically significant differences between predicted and achieved movements for most tooth movements, however none were clinically relevant. These results are consistent with the results of the present study, which also showed that while all differences between predicted and achieved tooth movements with Invisalign® were statistically significant, all angular values exceeded the threshold for clinical relevance by greater than 2°.

Kravitz et al. [12] indicated that mandibular canine mesio-distal tipping was one of the least accurate tooth movements for Invisalign®. In addition, the systematic review by Rossini et al. [14] also concluded that teeth inclinations seem to be among the limitations of Invisalign® regarding accuracy. These findings were reflected in the Invisalign® results of the present study, with the discrepancies for the maxillary and mandibular tip for all anterior teeth being clinically and statistically significant. Both Kravitz et al. [12] and Charamlampakis et al. [16] indicated that mandibular canine rotations were also problematic for Invisalign®. These outcomes were reproduced in the present study, with the mandibular rotational discrepancy for all anterior teeth exceeding 2°. Castroflorio et al. [31] concluded that maxillary incisor torque could be accurately expressed using Invisalign® Power Ridges. On the contrary, however, Simon et al. [13] indicated that incisor torque accuracy was low. Results of the present study agreed with the latter, statistically and clinically significant discrepancies were observed for all maxillary and mandibular anterior teeth with regard to torque with Invisalign®.

Current scholarly literature states that there will always be a certain amount of discrepancy between the digital setup and the achieved clinical outcome [18-20, 32]. This is due to the inevitable deformation and loss of elasticity of the aligner when used for extended periods of time, reducing its efficacy in producing tooth movement [33]. According to our results, there were no clinically significant differences between the Invisalign® and Flash® groups, with Flash® being more (statistically) significantly accurate than Invisalign® in achieving predicted tipping movements for maxillary central incisors (1.3°), BL movements for maxillary canines (0.1mm), and mandibular central incisor rotations (1°).

On a detailed analysis of individual tooth movement accuracy, a trend of linear movements (MD, BL, OG) tracking better than angular movements (tip, torque, rotations) was observed (Table 2). All Invisalign® angular discrepancies and most Flash® angular discrepancies exceeded the clinically acceptable threshold of 2 degrees. Between the three anterior teeth, the maxillary central incisors were statistically (not clinically) less accurate than lateral incisors and/or canines for BL, OG, and torque movements.

Our results suggest that the accuracy of virtual simulations with Flash® is comparable with Invisalign® despite the cost of Flash being lower than Invisalign, and hence the null hypothesis can be accepted. The factors that could influence the differences between the groups, however insignificant in the present study, are 1) aligner material, 2) staging parameters and algorithms, 3) demographics of patient selection between groups, 4) quantum of tooth movement in individual teeth in a given patient or group, 5) attachment geometry, size, etc., and 6) accuracy and staging of IPR [18-20, 32, 34].

The number of CAT service providers is increasing globally [35]. Newer CAT products, where orthodontists make simulations based on commercially available software, such as OrthoAnalyzer, uLab, Archform, Blue Sky Plan and Orchestrate, etc., are gaining immense popularity as well [36]. Flash® is one such provider in emerging markets. All scholarly literature evaluating the efficacy and other clinical outcomes are based on data derived from Invisalign® treated cases only. Independent peer-reviewed independent evaluation of data from other systems is imperative for quantification of the efficacy and development of aligner science in totality. The results of the present study indicate that the digital simulations used in both systems were not accurate, with clinically significant differences present for most angular movements. These results, however, should be interpreted with caution as these aligner systems use the treating orthodontist’s input and technician’s expertise for preparing digital simulations. These evaluations do not take into account clinical scenarios like refinement protocols [16] which are a given in contemporary aligner therapy, as well as do not account for the fact that over-corrections may be built into virtual simulations.

CONCLUSION

There were no clinically important differences in clinical accuracy and efficacy between Invisalign® and Flash® aligners in achieving predicted tooth positions.

REFERENCES