Movement of anterior teeth using clear aligners: a three-dimensional, retrospective evaluation
Tepedino et al. Progress in Orthodontics
Movement of anterior teeth using clear aligners: a three-dimensional, retrospective evaluation
Michele Tepedino 0 2
Valeria Paoloni 1
Paola Cozza 1
Claudio Chimenti 0 2
0 Department of Biotechnological and Applied Clinical Sciences, University of L'Aquila , Viale S.Salvatore, Edificio Delta 6, 67100 L'Aquila , Italy
1 Department of Clinical Sciences and Translational Medicine, University of Rome Tor Vergata , Viale Oxford 81, 00133 Rome , Italy
2 Department of Biotechnological and Applied Clinical Sciences, University of L'Aquila , Viale S.Salvatore, Edificio Delta 6, 67100 L'Aquila , Italy
Background: Clear aligner treatment offers several advantages, but the available literature shows that some kind of tooth movements are unpredictable. In addition, the majority of the studies are focused on one clear aligner system, while different characteristics of various systems can provide different treatment outcomes. The aim of the present retrospective cohort study was to evaluate the predictability of Nuvola® aligner system in achieving torque movements of anterior teeth. Methods: Thirty-nine adult patients, who were consecutively treated with clear aligners, were retrospectively selected, and digital models pre-treatment (T0), post-treatment (T1) and the digital setup models (TS) were collected. Only the first phase of treatment made of 12 aligners was considered for the present study. Torque of anterior teeth was measured as labiolingual inclination on digital models at T0, T1, and TS using VAM software. Any difference between the predicted and achieved torque movements was evaluated using Wilcoxon signed-rank test or paired sample t test. First-type error was set as p < 0.008. Results: No statistically significant difference was found for all the anterior teeth between predicted and achieved torque movements. Conclusions: The studied clear aligner system was able to produce clinical outcomes comparable to the planning of the digital setup relative to torque movements of the anterior teeth.
Clear aligners; Orthodontic tooth movement; Torque
Since Kesling [
] first proposed the use of sequential
thermoformed aligners for orthodontic tooth movement
and Align Technology (Santa Clara, CA, USA) in 1997
developed this idea into a feasible treatment modality
with the introduction of CAD/CAM technologies [
clear aligner therapy has experienced increasing
diffusion. The success of clear aligner therapy probably relies
on some advantages like aesthetics, greater comfort for
the patient [
], and improved oral hygiene and
periodontal health [
] compared to fixed appliances. Growing
demand led to improvement of the technique, which is
no longer limited to simple crowding cases but extended
also to complex malocclusions [
The first study on the efficacy of Invisalign® (Align
Technology, Santa Clara, CA, USA) clear aligners
reported a mean accuracy—defined as the overlap
between the predicted and achieved movements—of 41%
], while subsequent study reported more
promising results [
]. However, there is still an open debate
about the predictability of clear aligner therapy,
especially regarding complex tooth movements. While
levelling and aligning, intrusion, and bodily distalisation of
upper molars of no more than 1.5 mm were
demonstrated to be predictable movements, rotations,
extrusion, and torque movements are difficult to achieve with
clear aligners [
]. In addition, most of the available
studies are relative to the Invisalign system, but
differences in the aligner’s material properties and thickness,
the production process, the model’s precision, and the
position of the aligner’s margin all have an effect on the
final performance of the appliance [
different results can be expected from different clear
aligner systems [
]. Recently, Lombardo et al. [
comparing planned and achieved tipping and rotation
tooth movements in patients using another clear aligners
system, found that orthodontic aligners are unable to
achieve programmed movement with 100%
predictability. In particular, although tipping movements were
efficaciously achieved, especially at the molars and
premolars, rotation of the lower canines was an
To the best of our knowledge, there is a lack of reliable
literature on this subject, especially on the effective
clinical predictability of torque movement of the anterior
teeth using different clear aligners. Therefore, the aim of
the present study was to evaluate the efficiency of a clear
aligner system (Nuvola®, GEO S.r.l., Rome, Italy) [
controlling the torque movement of upper and lower
anterior teeth. The null hypothesis was that no difference
exists between planned and achieved torque movements.
The records of patients referred to the Dental Clinic of
the Department of Biotechnological and Applied Clinical
Sciences, University of L'Aquila, from September 2013
to September 2017 for orthodontic treatment with clear
aligners, were screened for the following inclusion criteria:
– Caucasian adult patients (> 18 years) with full
– Up to 6 mm of crowding in the anterior segment of
the arch (from the distal right canine to distal left
canine) evaluated on dental casts according to
– Non-extractive orthodontic treatment with Nuvola®
– Presence of retention attachments for the buccal
surfaces of the first and second premolars;
– Treatment plan that required interproximal
– Absence of local and systemic conditions that can
alter bone metabolism.
Sample size calculation (G*Power version 18.104.22.168,
Universitat Dusseldorf, Germany) [
], according to data
on torque measurements retrieved from a previous study
on the Invisalign® system [
], revealed that having a
difference in the response of matched pairs of 0.26 and a
standard deviation of the difference of 0.49; fifty-eight
pairs would be needed to be able to reject the null
hypothesis with a power of 0.90 and a type I error of 0.008.
Thirty-nine patients (14 males, 25 females) with a mean
age of 30.7 years were retrospectively enrolled in the study
group (Table 1), for a total of 63 treated dental arches
(36 upper and 27 lower dental arches). Informed consent
was obtained from every patient before inclusion.
Orthodontic treatment protocol
All subjects underwent a non-extractive orthodontic
treatment with Nuvola® aligners. The Nuvola® system is
designed to proceed through consecutive steps of a
maximum of 12 aligners. After each step, new impressions
should be acquired to design a new setup and to move
forward to the next treatment phase.
The treatment plan required the presence of retention
attachments for the buccal surfaces of the first and
second premolars and the IPR of anterior teeth in order to
achieve the correct dental alignment. Patients were
instructed to wear their aligners for 22 h per day, except
during meal times and oral hygiene procedures. Aligners
were replaced every 14 days.
Measurement of digital models
For each treated dental arch, pre-treatment (T0), real
post-treatment (T1: at the end of the first phase of
treatment after 12 aligners), and ideal post-treatment
according to setup (TS) digital casts were available.
Pretreatment and post-treatment models were acquired
using a 3Shape E1 scanner (3Shape, Copenhagen,
Denmark), and setups were constructed using Maestro
3D Ortho Studio software (AGE Solutions S.r.l., Pisa,
Italy). The digital models of upper and lower arches at
T0, T1, and TS were acquired as .STL files [
analysed by a single operator (M.T.) using VAM software
(Vectra, Canfield Scientific, Fairfield, NJ, USA).
Fifteen landmarks were localised by the same
welltrained operator on each digital model: the gingival limit
of the lingual facial axis of clinical crown (FACC) of the
right and left first molars, the incisal spot/the papilla
between the central incisors (for upper and lower arch,
respectively), and the gingival and occlusal limit of the
buccal FACC of the central incisors, lateral incisors, and
canines (Fig. 1). The landmarks’ coordinates were
exported as a .txt file and then imported into an Excel
spreadsheet (Microsoft Excel, Microsoft, Redmond, WA,
USA). The gingival points of the right and left molars
and the point on the incisal spot/the papilla between the
central incisors were used to define a reference plane.
Then, the coordinates of the gingival and occlusal points
of the buccal FACC of each tooth were transformed
according to this reference plane. Torque was measured
as the labiolingual inclination of the FACCs relative to
the reference plane [
]. The torque angles were then
calculated using trigonometry.
Error of the method
Thirty dental arches were randomly selected using
online software (https://www.randomizer.org/), and
landmark selection and torque measurements were repeated
by the same operator after 2 weeks. For all
measurements, Dahlberg’s formula (s = √ (Sd ^ 2 )/2n, where d =
difference between the first and second measurements)
was used to calculate the standard error on the repeated
sets of measurements. Bland–Altman plots were used to
check for the intra-observer reliability between the two
sets of measurements [
A Shapiro–Wilk normality test was used to analyse the
type of data distribution for all the variables. A paired
sample t test or a Wilcoxon signed-rank test was used to
evaluate if any statistically significant difference was
present between the predicted movements (TS) and the
clinically achieved torque (T1). First-type error was set as
0.008 after applying Bonferroni correction for multiple
testing. Descriptive statistics including mean and standard
deviation were also computed for all the variables.
Regarding the error of the method, the standard error was
1.2° for central incisor’s torque, 1.3° for lateral incisor’s
torque, and 1.3° for canine’s torque. Bland–Altman plots
revealed no systematic errors, confirming the
intraobserver reliability of the measurements (Figs. 2, 3, and 4).
Descriptive statistics are reported in Table 2. The
mean predicted torque movement (predicted = TS–T0)
was 2.3° ± 2.5 for left central incisor, 2.3° ± 2.4 for left
lateral incisor, 2.6° ± 2.6 for left canine, 2.4° ± 2.4 for right
central incisor, 3.1° ± 2.9 for right lateral incisor, and 2.8°
± 2.8 for right canine in the upper arch; in the lower
arch, the mean predicted torque movement was 2.3° ±
1.7 for left central incisor, 3.1° ± 2.7 for left lateral
incisor, 3.2° ± 3.1 for left canine, 1.7° ± 1.7 for right central
incisor, 3.2° ± 3.6 for right lateral incisor, and 2.2° ± 4.0
for right canine. Increasing angular values means that
the tooth’s crown was moved labially, while decreasing
values were measured when the crown moved lingually.
The paired sample t test and Wilcoxon signed-rank
test revealed that no statistically significant difference
was present between the torque measurements at TS
and T1; therefore, the null hypothesis was accepted,
confirming that the predicted movements were generally
achieved (Table 3). The dataset containing all the
collected measurements is attached as Additional file 1.
The aim of the present study was to evaluate the
predictability of the Nuvola® aligner system in performing
torque movements on the anterior teeth, and the results
showed that the movements predicted from the digital
setup were generally achieved.
It was decided to include only adult patients in the
study group because they represent most of the patients
who require orthodontic treatment with invisible
techniques, and because those patients generally show a
better compliance, compared to adolescents [
reducing a possible source of bias. In addition, only
patients with mild crowding were included because clear
aligner treatment is not yet recommended for complex
Regarding overall treatment efficiency, some studies
evaluated the outcomes of clear aligner treatment
versus fixed appliance treatment using the discrepancy
index of the American Board of Orthodontics or the
Peer Assessment Rating (PAR) index [
authors showed that Invisalign® produced significantly
lower scores than fixed appliances and was unable to
correct buccolingual inclination, occlusal contacts,
occlusal relationships, and overjet [
], while other
authors concluded that there were no statistically
significant differences in the achieved results [
different conclusions of these two cited articles, which
were published 12 years apart, probably reflect the
development and improvements in materials,
technologies, and treatment protocols.
Regarding the efficiency of the single type of tooth
movements, a systematic review of the literature
demonstrated that rotations, especially of teeth with a rounded
shape like canines and premolars; extrusions; and bodily
translations are less predictably achieved with clear
]. On the other hand, it was proved that
Fig. 3 Bland–Altman plot for repeated measurements of lateral incisors, confirming the absence of systematic errors
intrusions and molar distalisations of up to 1.5 mm can
be efficiently achieved [
Few articles are available in the literature regarding
torque movements with clear aligners [
7, 10, 11, 27, 28
Torque movement requires the presence of a force couple
acting on the tooth, and the biomechanics of this type of
tooth movement was described by Hahn et al. [
movement inside each aligner determines that initially the
aligner does not fit the tooth crown perfectly, and a
certain amount of reversible deformation is present at the
gingival margins of the aligner. Such deformation prevents
the formation of an effective couple [
] and is the reason
why torque is difficult to achieve with clear aligners. This
was confirmed by Elkholy et al. [
] measuring the
moment/force (M/F) ratio produced during the bodily
movement of an upper central incisor with a different
aligner thickness and amount of movement for each
aligner; the authors found that the M/F ratio was not large
enough to produce an effective counter-movement in any
of the situations studied. Zhang et al. [
information from digital models and cone-beam CT images to
evaluate both crown and root position during anterior
teeth movements. Interestingly, the authors found a large
amount of crown movement and a small amount of root
movement, suggesting that clear aligners are capable of
mainly crown tilting movements.
Torque is expressed in degrees (°)
When evaluating the outcomes of clear aligner
treatment, it should be considered that several factors play a
role in determining successful tooth movement: the
attachment’s shape and position [
], the aligner’s material
and thickness [
], the amount of activation present
in each aligner , and the techniques used for the
production of the aligners [
]. To be thorough, treatment
outcomes depend also on the patient’s characteristics,
bone density and morphology [
], and crown and root
morphology of the teeth [
], as well as on factors
related to the clinician, like the accuracy in performing the
requested amount of IPR, which is usually
With these considerations in mind, it can be argued
that not all clear aligner systems could provide the same
efficiency and predictability. This is of great importance
because many systems are available on the market, but
the majority of the available studies are focused on
Invisalign® aligners. Therefore, the generalisability of the
results of those studies can be questioned.
The results of the present study showed that with
Nuvola® aligners, the torque movements for central and
lateral incisors and canines of both arches predicted in
the digital setup were, in general, clinically achieved; the
mean difference between predicted (TS) and achieved
(T1) torque was between 0.03° and 1.86°and was not
statistically significant (Table 3). This finding is in contrast
to the results presented by Simon et al. [
], who found
a significant difference between the planned and
achieved torque for an upper incisor with Invisalign® for
a movement greater than 10°, measuring a movement
accuracy of 51.5% using Power Ridges and of 41.9%
using only ellipsoid attachments. On the other hand,
Lombardo et al. [
] found results comparable to those
of the present study with another clear aligner system,
showing a mean accuracy of 72.9% and no statistically
significant difference between planned and achieved
torque for some teeth, but not for all. It must be
underlined that the results of the present study refer to the
end of a phase of 12 aligners, and not to the treatment’s
end. This is because the studied system proposes a clear
aligner treatment that progresses through subsequent
steps, each one comprising a maximum of 12 aligners; at
the end of each phase, new impressions are taken, and a
new digital setup is prepared. The maximum torque
movement planned was 12.7° in the upper arch (mean
2.6°) and 20.8° in the lower arch (mean 2.7°) for cases
presenting mild to moderate crowding, and those
amounts of torque were easily achieved clinically.
All the available studies agree that there is always a
certain amount of discrepancy between the digital setup
and the real clinical outcome: the deformation of the
aligner when accommodated in its position produces
areas of contact with the teeth and gaps where the
aligner is not touching the teeth surfaces, and this
reduces the efficacy in producing tooth movements [
As the treatment progresses and aligners are changed,
the fit between the aligner and the teeth becomes
progressively looser because tooth movements are not
properly achieved. On the other hand, the present study
demonstrated that small amounts of movement for a
short series of aligners can be predictably achieved.
Torque measurements were performed following a
protocol that was validated previously [
]: the use of a
constructed reference plane that can be only minimally
altered during orthodontic treatment allows for precise
measurements. The authors who proposed this method
reported that the average method errors of the torque
values for all teeth were 1.2° and 1.5° for the mandible
and the maxilla, respectively [
]. In the present study,
the measured error of the method was between 1.2° and
1.3°, corresponding to the lower bound of the range
reported by Huanca Ghislanzoni et al. [
The main limitation of the present study is its
retrospective design; however, care was taken to minimise
any selection bias, since all the included subjects were
treated consecutively during the considered timespan
and were chosen only according to the predetermined
inclusion criteria. It was not possible to blind the
operator who made the measurements given the nature of
the study because it would have been easy to recognise
and understand the sequence of the digital models. The
results of the present study suggest that an aligner
system that prescribes subsequent steps of treatment, with
a re-evaluation of the case and a new digital setup at
each step, can offer a clinical advantage in terms of
predictability of tooth movement, especially considering
that the new setups do not have additional costs for the
clinician, except for the impression material and the
chairside time needed to make new impressions. Further
studies are needed to evaluate if a step-by-step
progression is effective in producing a final treatment outcome
that better corresponds to the digital setup with respect
to other aligner systems.
For patients with moderate crowding up to 6 mm, the
Nuvola® clear aligner system was able to produce
clinical outcomes comparable to the planning of the digital
setup relative to torque movements of the anterior
teeth. Considering subsequent treatment steps of 12
aligners each, this allowed achievement of predictable
Additional file 1: The dataset containing all the collected
measurements. (XLSX 27 kb)
FACC: Facial axis of clinical crown; RME: Rapid maxillary expansion
The authors declare that no funding was given for the realisation of the
Availability of data and materials
The dataset supporting the conclusions of this article is included within the
article and its Additional file 1.
MT performed the measurements and the statistical analysis. VP drafted the
manuscript. PC revised the manuscript. CC conceived the study design and
supervised the work. All authors read and approved the final version of this
MT, DDS, PhD is a research assistant. VP, DDS is a PhD student. PC, MD, DDS,
MS is a professor and department chair. CC, MD, DDS, MS is a professor.
Ethics approval and consent to participate
The study was performed in accordance with the Declaration of Helsinki. It is
a retrospective analysis, and the protocol was approved by the Chairman of
the Section of Orthodontics, Department of Biotechnological and Applied
Clinical Sciences, University of L'Aquila.
Consent for publication
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in
published maps and institutional affiliations.
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