Accuracy of Tooth Movement with In-House Clear Aligners

Prévia do material em texto

Journal of the World Federation of Orthodontists 10 (2021) 177–182 
Contents lists available at ScienceDirect 
Journal of the World Federation of Orthodontists 
journal homepage: www.ejwf.org 
Research Article 
Accuracy of Tooth Movement with In-House Clear Aligners 
Sivaporn Sachdev 
a , Syrina Tantidhnazet b , Nuntinee Nanthavanich Saengfai b , ∗
a Resident, Department of Orthodontics, Faculty of Dentistry, Mahidol University, Bangkok, Thailand 
b Department of Orthodontics, Faculty of Dentisitry, Mahidol University, Bangkok, Thailand 
a r t i c l e i n f o 
Article history: 
Received 31 May 2021 
Revised 6 August 2021 
Accepted 18 August 2021 
Available online 5 October 2021 
Keyword: 
Clear aligner /in-house 
Tooth movement 
Accuracy 
a b s t r a c t 
Aim: To evaluate accuracy of tooth movements with in-house clear aligners. 
Methods: This prospective clinical study included 30 participants for anterior clear aligners with crowding 
not exceeding 4 mm. per arch, non-extraction cases with no changes in posterior relationship. Aligners 
were created with OrthoAnalyzer TM software, attachments were placed as needed and IPR was performed 
in required areas. The final stage of tooth movement was compared with predicted tooth movement 
by superimposition of the two STL models. Maxillary arch was superimposed on stable posterior teeth 
and best fit on palatal rugae while mandibular arch was superimposed on unmoved posterior teeth. The 
amount of differences in predicted and achieved tooth movements were compared. Six types of tooth 
movement were included in the comparison which were labial, lingual, mesiodistal, intrusion, extrusion 
and rotation. 
Results: Total sample consisted of 259 anterior teeth (126 maxillary, 133 mandibular). The achieved 
tooth movements were significantly lesser than the predicted tooth movements in all the six types of 
tooth movement. Overall accuracy of tooth movement with clear aligner was 56.18%. The most accurate 
tooth movement was mesiodistal (72.33%). The least accurate movement was intrusion (43.28%). While 
mesiodistal, labial, rotation and lingual tooth movements were more predictable than intrusion and ex- 
trusion. 
Conclusion: Understanding the accuracy of different tooth movements might help in case selection, treat- 
ment plan and development or increase accuracy and predictability of in-house clear aligners. 
© 2021 World Federation of Orthodontists. Published by Elsevier Inc. All rights reserved. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
1. Introduction 
For many years, fixed appliances have been used for orthodontic
treatment. Nonetheless, the aesthetic demand during orthodontic
treatment has led to the development of clear aligners [1] . Clear
aligners are series of removable plastic appliances that are used
for moving teeth. The concept of using clear aligners to straighten
teeth was first introduced by Kesling in 1946, where a piece of
flexible rubber was produced from laboratory wax up of teeth.
This method allowed for minor movements while maintaining
the alignment of remaining teeth in the arch and was initially
Competing interest: Authors have completed and submitted the ICMJE Form for 
Disclosure of potential conflicts of interest. None declared. 
Funding: This project was partly funded by Faculty of Dentistry, Mahidol Uni- 
versity. 
Provinence and peer review: Not commisioned and externally peer reviewed. 
∗ Correspondence to: Department of Orthodontics, Faculty of Dentistry, Mahidol 
University, 6 Yothi Road, Phayathai, Ratchathewi, Bangkok, 10400, Thailand. 
E-mail address: nuntinee.nan@mahidol.ac.th (N.N. Saengfai). 
 
 
 
 
 
 
 
 
2212-4438/$ – see front matter © 2021 World Federation of Orthodontists. Published by E
https://doi.org/10.1016/j.ejwf.2021.08.003 
known as positioner. However, due to limitations of technology at
that time, the only possible tooth movement with positioner was
crown tipping [2] . 
The technique of using clear aligner with interproximal tooth
reduction (IPR) was introduced by Sheridan in the early 90s. A new
set-up was necessary for every tooth movement. Therefore, a new
impression had to be taken every visit for a new aligner produc-
tion, making the process time consuming and requiring heavy lab-
oratory workload. 
In 1999, align technology commercialized ‘The Invisalign Sys-
tem’, which was the first orthodontic appliance to use computer-
aided design (CAD) and computer-aided manufacturing (CAM). This
was a practical approach as multiple tooth set-ups could be cre-
ated on the computer from a single impression. This noticeably
reduced the clinical and laboratory time. Later, other aligner sys-
tems were released and used similar principles to achieve their
results [3] . 
Although studies have been done to evaluate the efficacy of
clear aligners, many of the studies published were based highly
on expert opinions, case reports and case series [4 , 5] . While few
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178 S. Sachdev et al / Journal of the World Federation of Orthodontists 10 (2021) 177–182 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
studies were clinical studies, they were mainly focused on major
manufacturing company, Invisalign [6-8] . 
Additionally, in-house clear aligner and other aligner systems
have also been developed. Unlike other aligners system, in-house
clear aligners eliminate the requirement of an outside service,
while integration of the software to 3D scanners and printers al-
low orthodontist to gain full control over aligners’ workflow. This
eliminates the drawbacks from outsourcing in terms of additional
fees and additional time required for aligners’ production. More-
over, in house clear aligners differ from other aligners in term of
software use, production process, material used, model precision,
attachments designs and aligner margins. These differences may
result in various efficacy of clear aligners [9-11] . Despite the devel-
opment of in-house clear aligners and other aligners system, there
exist only insubstantial studies regarding the subject, especially the
ones concerning accuracy [12 , 13] . 
Considering the fact that there are still very limited reports on
accuracy of tooth movements with in-house clear aligners, the ob-
jective of this prospective study was to investigate the accuracy of
tooth movements with in-house clear aligners. 
2. Materials and methods 
Patients were consecutively recruited at Department of Or-
thodontics, Mahidol University, Thailand. A total of 59 patients
were recruited and 32 participants were selected according to
the inclusion criteria. Participants were treated by orthodon-
tic residents under one supervisor. The treatment plans, attach-
ment placements and digital simulation of every treatment was
evaluated by a professional practitioner who has more than 10
years of experience in aligner treatment. All 32 participants re-
quired anterior aligner on at least one arch without extraction for
their orthodontic treatment. Ethics approval was carried out from
the Mahidol university research ethics committee (MU-DT/PY-IRB
2017/062.2411). 
The inclusion criteria were as follows (I) Crowding on anterior
teeth not exceeding 4 mm. per arch (II) No extraction required (III)
No changes in posterior relationship (IV) Adult patients aged be-
tween 18-45 years (V) Overjet and overbite of 0-5 mm (VI) Vertical
correction notmore than 3 mm (VII) Rotation corrections not more
than 30 degrees. 
Pre-treatment intraoral scans of participants were taken with
3Shape Trios intraoral scanner (Copenhagen, Denmark). Scanned
files were then transferred to OrthoAnalyzer TM software (Copen-
hagen, Denmark) for virtual model set up. Teeth were segmented
and anterior teeth were aligned into desired position with the help
of the software. Attachments were placed on teeth according to the
attachments protocol. Threshold for attachments placement were
(I) 5 degrees of rotation (II) 0.2 mm. of extrusion (III) 0.5 mm.
of intrusion (IV) 0.75 mm. of mesiodistal movement. IPR was set
in required areas as planned. Total number of aligners were deter-
mined by maximum tooth movement, not exceeding 2 mm and 2
degrees in each aligner. 
After total number of clear aligners were obtained, models in
each set up along with model template for attachment place-
ment were printed using Formlabs photopolymer resin for den-
tal model (Somerville, Massachusetts, United States) from Formlabs
3D printing machine with printing resolution of 50 μm. Thermo-
plastic sheet of 0.5 mm thickness (DURAN; Scheu Dental, Iserlohn,
Germany) was thermoformed on printed 3D model with pressure
molding device (Biostar®; Scheu Dental, Iserlohn, Germany). Af-
ter thermoforming, clear aligners were cut and finished at cervical
margin of each tooth. 
Patients were instructed to wear each set of aligners at least 22
hours/dayfor 2 weeks, where removal was only permitted during
food consumption and brushing. Patients were seen once a month
to check aligner fitness and attachments intactness, as well as to
dispense new aligners. After completing all aligners in the given
set, achieved tooth movement was scanned with 3Shape Trios in-
traoral scanner for superimposition. In the case that treatment out-
come was achieved, clear retainers were delivered. On the other
hand, when treatment outcome was not accomplished, additional
aligners were used to correct remaining malocclusion. 
3. Outcome Measurement 
OrthoAnalyzer TM software was used for model superimposi-
tion and outcome measurement. The final stage of achieved tooth
movement was compared with predicted tooth movement. Max-
illary arch was superimposed on stable posterior teeth and best
fit on palatal rugae while mandibular arch was superimposed on
unmoved posterior teeth. Superimposition of 3D digital model on
maxillary arch by using best fit method on palatal surfaces revealed
accurate and reliable measurements [14] . In mandibular arch, sur-
face superimposition on stable structures was found to be simple,
reproducible and reliable method [15] . Accuracy from superimpo-
sition on posterior segment was determined by color-coding scale;
white to yellow color, suggesting that posterior teeth were properly
superimposed allowing discrepancy from 0 to 0.2 mm ( Figure 1 ).
All predicted translational movements less than or equal to 0.2 mm
and rotational movements less than or equal to 1 degree were ex-
cluded from data analysis to elucidate error from model superim-
position [8] . This sensitivity threshold was determined by our al-
lowance of superimposition error appointed in the program. A total
of six tooth movements including labial, lingual, mesiodistal, intru-
sion, extrusion and rotation were measured between predicted and
achieved tooth movements. 
4. Statistical analysis 
Statistical analysis was performed with SPSS software (version
25.0; IBM, Armonk, NY, USA). To assess intra examiner reliability,
intraclass correlation coefficients (ICC) estimates at 95% intervals
was used. Two weeks after initial measurements, 10% of the sub-
jects were remeasured by the same examiner. Kolmogorov-Smirnov
test was used for normality test and the result showed non-normal
distribution of the data. In order to compare the differences in
achieved tooth movement to predicted tooth movement, Wilcoxon
signed rank test ( P( Plateral incisor could have
influenced the poor results observed. Likewise, other studies have
also found intrusion of anterior teeth to be problematic. One retro-
spective study evaluated cases that needed refinement found that
intrusion was the most inaccurate of all linear movements and had
median differences ranging from 0.8 to 1.5 mm [21] . Other studies
found that overbite had highest mean difference between predicted
and achieved tooth movements [ 20 , 26 ]. Anterior teeth were also
found to be located more occlusally than predicted [27] . Kravitz
et al., reported intrusion accuracy of 41.3% with lateral incisor hav-
ing the lowest accuracy [8] . A possible commentary for low efficacy
in intrusion may have resulted from relative extrusion, as more
than half of the teeth that were intruded were also retracted at
the same time. This could create antagonistic effects making the
teeth less intruded. Another reason could be from bite blocks ef-
fects on posterior teeth. Considering the biomechanical aspects of
bite block, slight molar intrusion could be effectively accomplished
if used at least 12 to 16 hours per day [28] . Bite-blocks effects of
clear aligner may result in intrusion of the molars and cause an-
terior teeth to appear less intruded on achieved tooth movement
model after superimposition [21] . 
Clear aligners move teeth by pushing forces; therefore, they
need to push on the attachment surfaces to be able to extrude a
tooth. Most studies have addressed low efficacy of extrusion with
clear aligners. The efficacy of extrusion in our study was 48.26%
which was second least accurate but this number was largely high
when compared to the other studies. According to Kravitz et al.,
extrusive movements with aligners are considered to be the least
accurate movement with accuracy of only 29.6% [8] . Other stud-
ies have also reported tooth movement in vertical planes to be
more difficult to achieve than horizontal planes [ 20 , 21 , 26 ]. A po-
tential explanation for improved efficacy in extrusion is that at-
tachments were always used even for the smallest extrusion of 0.2
mm. Another factor could be the molar intrusion from the thick-
ness of aligner materials on posterior teeth, making anterior teeth
appear more extruded. A study regarding treatment of open bite
cases with clear aligners stated that aligners allowed vertical con-
trol and molar intrusion by bite-block effect while incisors were
extruded when appropriate attachments were used [29] . 
In addition, higher accuracy was found in lingual and labial
tooth movements of 63.07% and 60.10% and mean differences of
0.24 mm and 0.23 mm respectively compared to pre-treatment
tooth positions. This was in agreement with another study that
found horizontal movements of incisors to be accurate with small
differences of 0.20-0.25 mm [21] . Kravitz et al., reported lingual
constriction to be more accurate than labial movement with ac-
curacy of 47.1% and the accuracy of labial expansion to be 40.5%
[8] . In our study the number of teeth that were moved lingually
were almost 4.7 times higher than those that were moved labially,
as most of the cases needed reduction in protrusion. No significant
differences were found between labial and lingual movements, yet
a larger sample size for labial movement would be required for fur-
ther validation. IPR was performed whenever space was required
for lingual movement. 
Considering rotational movement, the mean accuracy was
50.06%. Interestingly, our study found lowest accuracy with signif-
icant differences in maxillary central incisor of 35.01%. This could
be attributed to the presence of rotation in both maxillary central
182 S. Sachdev et al / Journal of the World Federation of Orthodontists 10 (2021) 177–182 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
incisors in majority of recruited patients leading to poor fitting of
aligners to tooth surfaces and thus central incisors harder to be ro-
tated. This result was contradictory to the previous studies which
found rounded teeth like canines to be the least accurate, as far
as rotatory movements are concerned. Kravitz et al., found mean
accuracy of rotation to be 32.2% in maxillary canine and 29.1% in
mandibular canine and when rotation was more than 15 degrees,
accuracy significantly decreased [8] . Another study by Kravitz et al,
reported a 35.8% [7] accuracy in rotation of canines. 
There are three main limitations in this study. First, the efficacy
of tooth movements on posterior teeth were not evaluated as they
were needed as reference for superimposition. Second, modifica-
tion of the treatment to obtain better treatment results such as us-
ing intermaxillary elastics was not feasible as we wanted to evalu-
ate the efficacy solely of clear aligners. Lastly, the root movement
could not be evaluated as cone beam CT was not taken and super-
imposed with digital model. Therefore, it was not possible to evalu-
ate if horizontal movements such as labial, lingual and mesiodistal
movements occurred as translation or tipping. 
This study reiterated that in-house clear aligners are beneficial
for non-complex cases and the treatment goals can be achieved at
lower cost. The incorporation of CAD-CAM in in-house clear aligner
treatment has improved the effectiveness of aligners’ production,
as series of tooth movements were feasibly created from final out-
come. Nonetheless, a limitation of in-house clear aligner is its de-
pendency on the software. The software we used only offered at-
tachments design that are preformed but different shapes of at-
tachments could play an important role in accuracy of tooth move-
ment as well. 
Most studies on accuracy of clear aligners in literature uti-
lized commercial aligners but this study focused on the accu-
racy of aligners produced through in-house technique. Knowledge
from this study allowed us to understand which tooth movements
could be achieved with clear aligners and which tooth movements
needed more attention to attain the treatment goal. In the matter
of tooth movements that are more challenging, modalities such as
modification of attachments and overcorrection on predicted plan
should be undertaken to improve the treatment efficacy. 
7. Conclusion 
Through the analysis regarding the accuracy of tooth move-
ments with in-house clear aligners with the use of attachments,
the following conclusions were drawn: 
1. Achieved tooth movements were significantly lesser than pre-
dicted tooth movements in all six types of tooth movements
evaluated including labial, lingual, mesiodistal, intrusion, extru-
sion and rotational movements. 
2. Mesiodistal, labial, rotation and lingual tooth movements were
more predictable, while intrusion and extrusion were less pre-
dictable. 
3. Overall accuracy of tooth movement with anterior clear align-
ers was 56.18%. The most accurate tooth movement was
mesiodistal movement (72.33%) and the least accurate move-
ment was intrusion (43.28%). 
CRediT authorship contribution statement 
Sivaporn Sachdev: Formal analysis, Methodology, Project ad-
ministration, Writing – original draft. Syrina Tantidhnazet:
Conceptualization, Methodology, Writing – review & editing.
Nuntinee Nanthavanich Saengfai: Conceptualization, Methodol-
ogy, Writing – review & editing, Supervision. 
Acknowledgments 
Special thanks to Dr. Somsak Mitrirattanakul for his help in sta-
tistical analysis; Yasumin Khianwong and Rapirat Jiratrapi for their
help in aligners production. 
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	Accuracy of Tooth Movement with In-House Clear Aligners
	1 Introduction
	2 Materials and methods
	3 Outcome Measurement
	4 Statistical analysis
	5 Results
	5.1 Comparing mean of achieved tooth movement to predicted tooth movement
	5.2 Comparing accuracy between different types of tooth movement
	5.3 Comparing accuracy of each tooth movement among individual tooth
	6 Discussion
	7 Conclusion
	CRediT authorship contribution statement
	Acknowledgments
	References