Thickness Variations of Thermoformed and 3D-Printed Clear Aligners

Abstract

Objective
To assess thickness variations of thermoformed and 3D-printed clear aligners.

Materials and Methods
Six different thermoplastic materials with different initial thicknesses were used for aligner thermoforming using Biostar® device (Biostar®, SCHEU-DENTAL GmbH, Iserlohn, Germany). Also, two different dental resins were used to create the printed aligners in three digitally designed thicknesses using IZZI Direct printer (3Dtech, Zagreb, Croatia). The aligners were measured using an electronic micrometer (ELECTRONIC UNIVERSAL MICROMETER, Schut Geometrical Metrology, Groningen, The Netherlands, accuracy: 0.001 mm) on a total of 20 points per aligner. Statistical analysis was performed using the JASP program (JASP, University of Amsterdam, Amsterdam, The Netherlands).

Results
The difference between the thermoformed and printed groups was statistically significant. Significant differences between different thermoformed materials and between 3D-printed materials were found. The thickness of thermoformed aligners deviated more in the upper jaw, whereas the thickness of printed aligners deviated more in the lower jaw. Both differences were statistically significant. The greatest average deviation from the initial thickness was found in Duran 0.75; Erkodur 0.6; Erkoloc-Pro 1.0; IZZI 0.5; NextDent 0.6 and NextDent A 0.6. NextDent group had the lowest deviations for all teeth of both jaws, except for upper and lower first molar where NextDent A group was more accurate.

Conclusions
Thermoformed aligners showed decreased values, while printed ones showed mostly increased values compared to the original material thickness. The highest mean deviation belonged to IZZI group, and the NextDent group had the lowest mean deviation. The thickness of both aligners was thinner at the edges compared to the thickness at cusps and fissures.

Full text

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145
ActastomatolCroat.2024;58(2):145-155.
DOI:10.15644/asc58/2/4
ORIGINAL SCIENTIFIC PAPER
IZVORNI ZNANSTVENI RAD
RužicaBandić1*,KatarinaVodanović1*, IvnaVukovićKekez1,IvanaMedvedecMikić2,3,IvanGalić3,4,DanijelaKalibovićGovorko1,4
Thickness Variations of Thermoformed and 3D-Printed Clear
Aligners
Varijacije u debljini termoformiranih i 3D-printanih prozirnih
ortodontskih udlaga
ACTA
STOMATOLOGICA
CROATIC A
www.ascro.hr
1 DepartmentofOrthodontics,UniversityofSplitSchoolofMedicine
Katedra za ortodonciju Medicinskog fakulteta Sveučilišta u Splitu
2 DepartmentofEndodonticsandRestorativeDentalMedicine,UniversityofSplitSchoolofMedicine
Katedra za endodonciju i restaurativnu dentalnu medicinu Medicinskog fakulteta Sveučilišta u Splitu
3 DepartmentofOralSurgery,UniversityofSplitSchoolofMedicine
Katedra za oralnu kirurgiju Medicinskog fakulteta Sveučilišta u Splitu
4 DepartmentofMaxillofacialSurgery,UniversityHospitalofSplit
Zavod za maksilofacijalnu kirurgiju KBC-a Split
*Authorscontributedequallytothiswork•Autori su jednako pridonijeli ovom radu
Abstract
Objective: Toassessthicknessvariationsofthermoformedand3D-printedclearaligners.Materials
and Methods: Sixdifferent thermoplasticmaterialswithdifferent initial thicknesseswereusedfor
alignerthermoformingusingBiostar®device(Biostar®,SCHEU-DENTALGmbH,Iserlohn,Germany).
Also,twodifferentdentalresinswereusedtocreatetheprintedalignersinthreedigitallydesigned
thicknessesusingIZZIDirectprinter(3Dtech,Zagreb,Croatia).Thealignersweremeasuredusingan
electronicmicrometer(ELECTRONICUNIVERSALMICROMETER,SchutGeometricalMetrology,Gronin-
gen,TheNetherlands,accuracy:0.001mm)onatotalof20pointsperaligner.Statisticalanalysiswas
performedusing theJASPprogram (JASP,Universityof Amsterdam,Amsterdam,The Netherlands).
Results: Thedifferencebetweenthethermoformedandprintedgroupswasstatisticallysignicant.
Signicantdifferences between different thermoformed materialsandbetween3D-printedmateri-
alswerefound.Thethicknessofthermoformedalignersdeviatedmoreintheupperjaw,whereasthe
thicknessofprintedalignersdeviatedmoreinthelowerjaw.Bothdifferenceswerestatisticallysig-
nicant.ThegreatestaveragedeviationfromtheinitialthicknesswasfoundinDuran0.75;Erkodur
0.6;Erkoloc-Pro1.0;IZZI0.5;NextDent0.6andNextDentA0.6.NextDentgrouphadthelowestdevi-
ationsforallteethofbothjaws,exceptforupperandlowerrstmolarwhereNextDentAgroupwas
moreaccurate.Conclusions: Thermoformed alignersshoweddecreasedvalues,while printedones
showedmostlyincreasedvaluescomparedtotheoriginalmaterialthickness.Thehighestmeandevi-
ationbelongedtoIZZIgroup,andtheNextDentgrouphadthelowestmeandeviation.Thethickness
ofbothalignerswaslowerattheedgescomparedtothethicknessatcuspsandssures.
Received:January5,2024
Accepted:April30,2024
Address for correspondence
DanijelaKalibovićGovorko
UniversityofSplitSchoolofMedicine
DepartmentofOrthodontics
danijela.kalibovic.govorko@mefst.hr
MeSH Terms:OrthodonticAppliances
Removable;Three-Dimensional
Printing;SyntheticResins
Author Keywords:Orthodontics;
Printing;Three-dimensional;
OrthodonticAppliances;Removable
RužicaBandić:ORCID0000-0003-0811-6983:
KatarinaVodanović:ORCID0000-0001-6985-584
IvnaVukovićKekez:ORCID0000-0001-9616-420X
IvanaMedvedecMikić,ORCID0000-0003-0202-3399
IvanGalić:ORCID0000-0003-0387-9535
DanijelaKalibovićGovorko:ORCID0000-0002-2598-9009
Introduction
Clear aligners are transparent, removable orthodon-
tic appliances that represent an alternative to conventional
orthodontic treatment (1). eir advantages over xed orth-
odontic appliances are as follows: improved aesthetics and
comfort, reduced in-chair time and emergency visits, less
pain and better quality of life for the patients (2-5). Howev-
er, clear aligners still have several limitations, such as the in-
ability to treat certain types of malocclusions, dependence on
the patient’s motivation and production costs (3, 4).
e standard clear aligners fabrication workow includes
digital image acquisition via intraoral scanning, virtual treat-
Uvod
Ortodontske udlage su prozirni, mobilni ortodontski
aparati i alternativa su konvencionalnom ortodontskom lije-
čenju (1). Njihove prednosti u odnosu na ksne ortodontske
naprave jesu poboljšana estetika i udobnost, manje vremena
provedenog na kontrolama i u hitnim posjetima, manje bo-
li i bolja kvaliteta života pacijenata (2 – 5). No te naprave još
uvijek imaju nekoliko ograničenja, kao što su nemogućnost
liječenja određenih vrsta malokluzija, ovisnost o motivaciji
pacijenta i troškovi proizvodnje (3, 4).
Standardni postupak izrade prozirnih ortodontskih ud-
laga uključuje intraoralno skeniranje zubnih lukova, virtu-

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Thermoformed and 3D-Printed Clear AlignersBandić et al.
146
ment planning and 3D-printing of a series of orthodontic
models. Subsequently, thin thermoplastic sheets are heated
and pressed over physical models. Finally, after the thermo-
forming process, aligners are trimmed and polished and are
ready for clinical use (6).
Apart from being time-consuming, thermoforming can
aect the material properties and subsequently clinical per-
formance of the appliance. Signicant changes in exural
and elastic modulus, hardness, Young’s modulus, and trans-
parency of clear aligners after thermoforming were observed
(7-10).
e development of digital technologies and 3D printing
oers new possibilities such as 3D printing of clear aligners
directly from a digital le, thus eliminating the cumulative
errors that can occur during thermoplastic fabrication (11).
3D printing generates signicantly less waste than the con-
ventional thermoforming method, thus increasing eciency
and reducing the adverse impact on the environment (1, 12,
13). Other benets are good accuracy and t, load- and de-
formation resistance and shorter production time (1, 12). In
comparison with thermoformed aligners, 3D printed align-
ers have superior mechanical and geometrical properties and
are more suitable for intraoral use due to their resistance to
mastication and biting forces (11, 14). ey can also deliver
optimal forces for orthodontic tooth movement (15).
e inuence of changes in aligner thickness on the
forces required for tooth movement needs to be considered
when planning orthodontic treatment with clear aligners (2,
16, 17). According to the study by Gao et al., aligners with
higher thicknesses and gingival edge exerted higher forces
than thinner ones without an edge (18). Mechanical prop-
erties of clear aligners depend on material type, thickness,
and amount of activation. Appliances fabricated from thin-
ner thermoplastic materials and those with greater amount
of activation deliver lower orthodontic forces (19). Elkholy et
al. recommended a novel sequence in clear aligner treatment
with new thinner aligners for signicant amount of force re-
duction. is approach may reduce the risk of root resorp-
tion by tooth overloading (20, 21). However, another study
reported sucient tooth movement and similar eect on the
principal stresses in periodontal ligament using clear align-
ers with dierent thicknesses, with thicker aligner exerted
only slightly higher load on the tooth compared to thinner
one (22).
Mantovani et al. analyzed the thickness of thermoformed
aligners and their research showed that it was not homog-
enous, especially in the molar regions. Consistent aligner
thickness is clinically important; otherwise some orthodon-
tic tooth movements can be reduced (23). Although ther-
moforming is a reproducible process, thickness of thermo-
formed clear aligners is decreased compared with the original
thickness of the thermoplastic sheet (24, 25). Regarding dif-
ferences across the arch, thickness and gap width of thermo-
formed aligners are smaller at anterior teeth and gingival re-
gions than that at posterior teeth and occlusal surfaces (26).
On the other hand, the results of a previous study have re-
vealed that 3D printed clear aligners have higher thickness
compared to digital designed dimensions (27). Post-printing
alno planiranje liječenja i 3D printanje niza ortodontskih
modela. Tanke termoplastične folije zatim se zagrijavaju i
prešaju preko zičkih modela. Na kraju, poslije procesa ter-
moformiranja, ortodontske udlage se obrezuju i poliraju te su
spremni za kliničku uporabu (6).
Uz to što oduzima mnogo vremena, termoformiranje
može utjecati na svojstva materijala i posljedično na klinič-
ka svojstva aparata. Nakon toga postupka uočene su značajne
promjene u modulu elastičnosti, tvrdoći, Youngovu modulu
i prozirnosti ortodontskih udlaga (7 – 10).
Razvoj digitalnih tehnologija i trodimenzionalnog ispi-
sa nudi nove mogućnosti kao što je 3D printanje ortodont-
skih udlaga izravno iz digitalne datoteke čime se eliminiraju
kumulativne pogreške koje se mogu pojaviti tijekom termo-
formiranja (11). 3D printanje generira znatno manje otpada
od konvencionalne metode termoformiranja, čime se poveća-
va učinkovitost i smanjuje negativni utjecaj na okoliš (1, 12,
13). Ostale su prednosti dobra točnost i prilagodba, otpor-
nost na opterećenje i deformaciju te kraće vrijeme proizvod-
nje (1, 12). U usporedbi s termoformiranim ortodontskim
udlagama, 3D printane ortodontske udlage imaju superiorna
mehanička i geometrijska svojstva te su prikladnije za intrao-
ralnu upotrebu zbog otpornosti na žvačne sile (11, 14). Tako-
đer otpuštaju optimalne sile za ortodontski pomak zuba (15).
Pri planiranju ortodontskog liječenja ortodontskim ud-
lagama potrebno je uzeti u obzir utjecaj promjene debljine
ortodontskih udlaga na sile potrebne za pomak zuba (2, 16,
17). Prema istraživanju Gaoa i suradnika, ortodontske udla-
ge s većom debljinom i gingivnim rubom producirale su ve-
će sile od onih tanjih bez ruba (18). Mehanička svojstva or-
todontskih udlaga ovise o vrsti materijala, debljini i iznosu
aktivacije. Aparati izrađeni od tanjih termoplastičnih materi-
jala i oni s većim iznosom aktivacije, daju manje ortodontske
sile (19). Elkholy i suradnici preporučili su novi slijed u tera-
piji ortodontskim udlagama, s novim tanjim ortodontskim
udlagama za značajno smanjenje sile. Takav pristup može
smanjiti rizik od resorpcije korijena zbog preopterećenja zu-
ba (20, 21).
Međutim, autori jedne druge studije izvijestili su o do-
voljnom pomicanju zuba i sličnom učinku na parodontni li-
gament u korištenju ortodontskih udlaga različitih debljina,
pri čemu je deblja ortodontska udlaga samo malo više opte-
rećivala zub od tanje (22).
Mantovani i suradnici analizirali su debljinu termofor-
mirane ortodontske udlage i njihovo je istraživanje pokaza-
lo da ona nije homogena, osobito u području molara. Kon-
zistentna debljina ortodontske udlage klinički je važna jer se
inače neki ortodontski pomaci zuba mogu smanjiti (23). Ia-
ko je termoformiranje ponovljiv proces, debljina termoformi-
ranih ortodontskih udlaga smanjena je u usporedbi s izvor-
nom debljinom termoplastične folije (24, 25). Kad je riječ o
razlici duž zubnoga luka, debljina i širina razmaka između
zuba i aparata termoformiranih ortodontskih udlaga manja
je na prednjim zubima i gingivnim dijelovima nego na stra-
žnjim zubima i okluzalnim površinama (26). S druge strane,
u jednoj studiji autori ističu da su 3D printane ortodontske
udlage deblje u usporedbi s digitalno dizajniranim dimenzi-
jama (27). Uvjeti nakon printanja također su vrlo važni pri

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Termoformirani i 3D ispisani prozirni poravnačiBandić i sur. 147
conditions are also very important when printing clear align-
ers. Dierent temperatures and curing time durations may
inuence possible clinical eciency of appliances (28).
Material and methods
In this study the thickness of thermoformed and 3D
printed clear aligners was examined. Intraoral scan of a eug-
nathic patient was taken with Trios scanner (3Shape, Copen-
hagen, Denmark) and STL le was created using Maestro
3D Ortho Studio, version 5 (AGE Solutions®, Pontedera, Ita-
ly). 3D models of patient’s dentition were printed from STL
le using IZZI Ortho printer (3Dtech, Zagreb, Croatia).
ermoplastic sheets were thermoformed on printed models
using Biostar® device (Biostar®, SCHEU-DENTAL GmbH,
Iserlohn, Germany). ree samples of six dierent materials
with dierent initial thickness were used: Duran+ (0.5 mm,
0.625 mm, 0.75 mm; Duran+, SCHEU-DENTAL GmbH);
Erkodur (0.5 mm, 0.6 mm, 0.8 mm; Erkodur, Erkodent Er-
ich Kopp GmbH, Pfalzgrafenweiler, Germany); Zendura A
(0.76 mm; Zendura A, Bay Materials LLC, Fremont, Cali-
fornia, USA), Erkoloc-Pro (1.0 mm, 1.3 mm; Erkoloc-Pro,
Erkodent Erich Kopp GmbH); Zendura FLX (0.76 mm; Ze-
ndura FLX, Bay Materials LLC); CA PRO+ (0.75 mm; CA
PRO+, SCHEU-DENTAL GmbH).
Printed aligners were printed directly from the patient’s
STL le using two dierent dental resins. IZZI standard
(3DTech, Zagreb, Croatia) is an opaque resin that is nor-
mally used to produce dental models via 3D printing (group
IZZI). NextDent Ortho Flex resin (NextDent B.V., Soes-
terberg, the Netherlands) is used for 3D printing of dental
splints and retainers. We used NextDent resin to produce
two groups of aligners: one group was produced according to
the manufacturer’s instructions (group NextDent) and an-
other, with modied post-printing protocol, without drain-
ing of the residual resin (group NextDent A). All 3D printed
aligners were produced using IZZI Direct printer (3Dtech,
Zagreb, Croatia) in three digitally designed thicknesses (0.5
mm, 0.6 mm i 0.7 mm). Two samples of each material were
printed. e study’s sample size was computed using the re-
source equation method (29).
Left sides of the upper and lower aligners were used for
the analysis. All samples were measured by three measur-
ers using electronic micrometer (ELECTRONIC UNI-
VERSAL MICROMETER, Schut Geometrical Metrology,
Groningen, the Netherlands, accuracy: 0.001 mm) on a to-
tal of 20 points per aligner. e thickness of aligner on cen-
tral incisor was measured at: vestibular edge, incisal edge,
cingulum and palatal/lingual edge; on canine at: vestibular
edge, cusp tip, cingulum and palatal/lingual edge; on rst
premolar at: vestibular edge, buccal cusp tip, central ssure,
palatal/lingual cusp tip and palatal/lingual cusp tip; on rst
molar at: vestibular edge, mesiobuccal cusp tip, distobuccal
cusp tip, central ssure, mesiopalatal/mesiolingual cusp tip,
distopalatal/distolingual cusp tip and palatal/lingual edge.
e data were checked for normality by using the Kol-
mogorov-Smirnov test, and they showed non-parametric
distribution. Basic statistical parameters were calculated for
ispisu prozirnih ortodontskih udlaga. Različite temperature
i trajanje polimerizacije mogu utjecati na kliničku učinkovi-
tost aparata (28).
Materijal i metode
U ovoj studiji ispitivana je debljina termoformiranih i
3D printanih ortodontskih udlaga. Intraoralni sken eugna-
tog pacijenta snimljen je skenerom Trios (3Shape, Kopen-
hagen, Danska), a STL datoteka napravljena je u programu
Maestro 3D Ortho Studio, verzija 5 (AGE Solutions®, Pon-
tedera, Italija). 3D modeli pacijentove denticije isprintani su
iz STL datoteke korištenjem printera IZZI Ortho (3Dtech,
Zagreb, Hrvatska). Termoplastične ploče termoformirane su
na printanim modelima s pomoću uređaja Biostar® (Biostar®,
SCHEU-DENTAL GmbH, Iserlohn, Njemačka). Upotrije-
bljena su po tri uzorka svakoga od šest različitih materija-
la drukčije početne debljine: Duran+ (0,5 mm, 0,625 mm,
0,75 mm; Duran+, SCHEU-DENTAL GmbH); Erkodur
(0,5 mm, 0,6 mm, 0,8 mm; Erkodur, Erkodent Erich Kopp
GmbH, Pfalzgrafenweiler, Njemačka); Zendura A (0,76 mm;
Zendura A, Bay Materials LLC, Fremont, Kalifornija, SAD),
Erkoloc-Pro (1,0 mm, 1,3 mm; Erkoloc-Pro, Erkodent Erich
Kopp GmbH); Zendura FLX (0,76 mm; Zendura FLX, Bay
Materials LLC); CA PRO+ (0,75 mm; CA PRO+, SCHEU-
DENTA L GmbH).
Printane ortodontske udlage rađene su izravno iz paci-
jentove STL datoteke od dviju različitih dentalnih smola.
IZZI standard (3DTech, Zagreb, Hrvatska) opakna je smola
koja se inače koristi za izradu dentalnih modela 3D printa-
njem (grupa IZZI). NextDent Ortho Flex smola (NextDent
B.V., Soesterberg, Nizozemska) upotrebljava se za 3D printa-
nje zubnih udlaga i retencijskih naprava. Od NextDent smo-
le proizveli smo dvije skupine ortodontskih udlaga: jednu
prema uputama proizvođača (skupina NextDent) i drugu,
s modiciranim protokolom nakon printanja, bez cijeđenja
zaostale smole (skupina NextDent A). Svi 3D printane orto-
dontske udlage proizvedene su s pomoću printera IZZI Di-
rect (3Dtech, Zagreb, Hrvatska) u trima digitalno dizajnira-
nim debljinama (0,5 mm, 0,6 mm i 0,7 mm). Printana su po
dva uzorka svakog materijala. Veličina uzorka studije izraču-
nata je metodom jednadžbe resursa (29).
Za analizu je korištena lijeva strana svake gornje i do-
nje ortodontske udlage. Tri mjeritelja mjerila su sve uzorke
elektroničkim mikrometrom (ELECTRONIC UNIVER-
SAL MICROMETER, Schut Geometrical Metrology, Gro-
ningen, Nizozemska, točnost: 0,001 mm) na ukupno 20 to-
čaka po ortodontskoj udlazi. Debljina ortodontske udlage na
središnjem sjekutiću mjerena je na vestibularnom rubu, inci-
zalnom rubu, cingulumu i palatinalnom/lingvalnom rubu;
na očnjaku na vestibularnom rubu, vrhu kvržice, cingulumu
i palatinalnom/lingvalnom rubu; na prvom pretkutnjaku na
vestibularnom rubu, vrhu bukalne kvržice, središnjoj suri,
vrhu palatinalne/lingvalne kvržice; na prvom kutnjaku na
vestibularnom rubu, vrhu meziobukalne kvržice, vrhu dis-
tobukalne kvržice, središnjoj suri, vrhu meziopalatinalne/
meziolingvalne kvržice, vrhu distopalatinalne/distolingvalne
kvržice i palatinalnom/lingvalnom rubu.

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Thermoformed and 3D-Printed Clear AlignersBandić et al.
148
each variable. e Kruskal-Wallis non-parametric test and
Dunn Post-hoc test with Bonferroni correction were used to
determine statistically signicant dierences between sam-
ples. e signicance level was set at p<0.05. Statistical anal-
ysis was performed in JASP program (JASP, University of
Amsterdam, Amsterdam, the Netherlands).
Results
Differentmeasurers
e mean value of the measured deviation percentage
from the declared thickness for three measurers was: mea-
surer 1 (-22.115%), measurer 2 (-21.675%) and measurer 3
(-21.731%) but those dierences between measurers weren’t
statistically signicant (Kruskal-Wallis test, p=0,878).
Thermoformedvs.3Dprintedaligners
Descriptive statistics of the deviation percentage from
the declared thickness for samples produced by conventional
thermoforming method and samples produced by 3D print-
ing is presented in Table 1. e dierence between the ther-
moformed and printed groups was statistically signicant
(Kruskal-Wallis test, p<0.001).
Normalnost podataka provjerena je Kolmogorov-Smir-
novljevim testom i pokazali su neparametrijsku distribuci-
ju. Za svaku varijablu izračunati su osnovni statistički pa-
rametri. Za utvrđivanje statistički značajnih razlika između
uzoraka korišteni su Kruskal-Wallisov neparametrijski test i
Dunnov post-hoc test s Bonferronijevom korekcijom. Razi-
na značajnosti postavljena je na p < 0,05. Statistička analiza
obavljena je u programu JASP (JASP, Sveučilište u Amster-
damu, Nizozemska).
Rezultati
Različitimjeritelji
Srednja vrijednost izmjerenog postotka odstupanja od de-
klarirane debljine za tri mjeritelja bila je: mjeritelj 1 (-22,115
%), mjeritelj 2 (-21,675 %) i mjeritelj 3 (-21,731 %), ali te ra-
zlike među mjeriteljima nisu bile statistički značajne (Kru-
skal-Wallisov test, p = 0,878).
Termoformiranevs.3Dprintaneortodontskeudlage
Deskriptivna statistika postotka odstupanja od deklari-
rane debljine za uzorke proizvedene konvencionalnom me-
todom termoformiranja i uzorke proizvedene 3D printanjem
nalazi se u tablici 1. Razlika između termoformiranih i prin-
tanih skupina bila je statistički značajna (Kruskal-Wallisov
test, p < 0,001) .
Table 1 Percentagesofdeviationsfromdeclaredthicknessfordifferentmanufacturingprocess
Tablica 1. Postotakodstupanjaoddeklariranedebljinezarazličiteproceseproizvodnje
Manufacturing process •
Proizvodni proces NMean SD SE Coecient of variation •
Koecijent varijacije
ermoforming 3960 -39.764 14.096 0.224 -0.354
3D printing 2160 11.020 21.434 0.461 1.945
Thermoformedmaterials
e mean values of thickness deviation percentage of dif-
ferent thermoformed aligners, from lowest to highest were:
Zendura A (-32.408%), Zendura FLX (-35.455%), Duran
(-35.993%), Erkodur (-37.446%), CA PRO+ (-42.604%) and
Erkoloc-Pro (-53.31%). Signicant dierences between dif-
ferent thermoformed materials were found (Kruskal-Wallis
test, p<0.001). Dunn’s Post Hoc Comparisons with Bonfer-
roni correction for thermoformed materials are presented in
Table 2 .
3Dprintedmaterials
e mean thickness deviation percentages of printed
aligners from lowest to highest were: NextDent (+4.04%),
NextDent A (+5.883%) and IZZI (+23.137%). According to
the Kruskal-Wallis test, the dierence between printed ma-
terials was found (Kruskal-Wallis test, p<0.001) and Dunn’s
post-hoc analysis with Bonferroni correction showed signif-
icant dierence between IZZI group and both NextDent
groups, while between NextDent and NextDent A no statis-
tically signicant dierence was found (Table 3).
Termoformiranimaterijali
Srednje vrijednosti postotka odstupanja u debljini ra-
zličitih termoformiranih ortodontskih udlaga, od najmanje
do najveće, bile su: Zendura A (-32,408 %), Zendura FLX
(-35,455 %), Duran (-35,993 %), Erkodur (-37,446 %), CA
PRO+ (-42,604 %) i Erkoloc-Pro (-53,31 %). Utvrđene su
značajne razlike između različitih termoformiranih materi-
jala (Kruskal-Wallisov test, p < 0,001). Dunnova post-hoc
usporedba s Bonferronijevom korekcijom za termoformirane
materijale prikazana je u tablici 2.
3Dprintanimaterijali
Prosječni postotak odstupanja u debljini printanih or-
todontskih udlaga od najmanjega do najvećega bio je:
NextDent (+4,04 %), NextDent A (+5,883 %) i IZZI
(+23,137 %). Prema Kruskal-Wallisovu testu utvrđena je ra-
zlika između printanih materijala (Kruskal-Wallisov test, p
< 0,001), a Dunnova post-hoc analiza s Bonferronijevom ko-
rekcijom pokazala je značajnu razliku između IZZI skupine
i obje NextDent skupine, a između NextDenta i NextDen-
ta A nije ustanovljena statistički značajna razlika (tablica 3.).

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Termoformirani i 3D ispisani prozirni poravnačiBandić i sur. 149
Uppervs.lowerjaw
e dierence between thickness deviations in upper and
lower jaw, for both thermoformed and printed aligners was
analyzed. e thickness of thermoformed aligners deviated
more in the upper jaw and the dierence was statistically sig-
nicant (Kruskal-Wallis test, p<0.001) (Supplement Table 1).
e dierence between the upper and lower jaw for printed
aligners was also signicant with more thickness deviations
in the lower jaw (Kruskal-Wallis test, p<0.001) (Supplement
Table 2).
Gornjačeljustvs.donjačeljust
Analizirana je razlika između odstupanja debljine u gor-
njoj i donjoj čeljusti za termoformirane i za printane orto-
dontske udlage. Debljina termoformiranih ortodontskih
udlaga više je odstupala u gornjoj čeljusti i razlika je bila
statistički značajna (Kruskal-Wallisov test, p < 0,001) (do-
punska tablica 1.). Razlika između gornje i donje čeljusti za
printane ortodontske udlage također je bila značajna s većim
odstupanjima u debljini u donjoj čeljusti (Kruskal-Wallisov
test, p < 0,001) (dopunska tablica 2.).
Comparison z Wi Wj p pbonf
CA PRO + - Duran -7. 3 0 9 1.773.688 2.282.246 <.001*** <.001***
CA PRO + - Erkodur -5.413 1.773.688 2.150.341 <.001*** <.001***
CA PRO + - Erkoloc-Pro 11.738 1.773.688 90 7.4 01 <.001*** <.001***
CA PRO + - Zendura A -9.330 1.773.688 2.568.778 <.001*** <.001***
CA PRO + - Zendura FLX -6.534 1.773.688 2.330.469 <.001*** <.001***
Duran – Erkodur 2.681 2.282.246 2.150.341 0.007** 0.110
Duran – Erkoloc-Pro 24.994 2.282.246 9 0 7. 4 01 <.001*** <.001***
Duran – Zendura A -4.118 2.282.246 2.568.778 <.001*** <.001***
Duran – Zendura FLX -0.693 2.282.246 2.330.469 0.488 1.000
Erkodur – Erkoloc-Pro 22.596 2.150.341 90 7.4 01 <.001*** <.001***
Erkodur – Zendura A -6.014 2.150.341 2.568.778 <.001*** <.001***
Erkodur – Zendura FLX -2.589 2.150.341 2.330.469 0.010** 0.144
Erkoloc-Pro – Zendura A -22.512 9 0 7.401 2.568.778 <.001*** <.001***
Erkoloc-Pro – Zendura FLX -19.283 9 0 7.4 01 2.330.469 <.001*** <.001***
Zendura A – Zendura FLX 2.797 2 .568.778 2.330.469 0.005** 0.077
* p <05, ** p <.01, *** p <001
Table 2 Dunn’sPostHocComparisonswithBonferronicorrectionforthermoformedmaterials
Tablica 2. Dunnovapost-hocusporedbasBonferronijevomkorekcijomzatermoformiranematerijale
Comparison z Wi Wj p Pbonf
IZZI – NextDent 17. 2 8 8 1.445.626 8 7 7.3 4 9 <.001*** <.001***
IZZI – NextDent A 16.035 1.445.626 918.525 <.001*** <.001***
NextDent – NextDent A -1.253 8 7 7. 349 918.525 0.210 0.631
* p < 05, ** p < 01, *** p < 001
Table 3 Dunn’sPostHocComparisonswithBonferronicorrectionforprintedmaterials.
Tablica 3. Dunnovapost-hocusporedbasBonferronijevomkorekcijomzaprintanematerijale
Samematerialwithdifferentinitialthicknesses
Deviations from initial thicknesses of aligners pro-
duced from the same material but with several dierent ini-
tial thicknesses (Duran 0.5 mm, 0.625 mm and 0.75mm;
Erkodur 0.5 mm, 0.6 mm and 0.8 mm; Erkoloc-Pro 1.0 mm
and 1.3 mm; IZZI 0.5 mm, 0.6 mm, 0.7 mm; NextDent
0.5 mm, 0.6 mm and 0.7 mm; NextDent A 0.5, 0.6 and
0.7 mm) were compared. e greatest average deviation from
the initial thickness was found in Duran 0.75 (-37.149%);
Erkodur 0.6 (-37.897%); Erkoloc-Pro 1.0 (-58.331%); IZZI
0.5 (+27.997%); NextDent 0.6 (+5.954%) and NextDent A
0.6 (+10.337%) (Table 4).
e analysis of thickness deviations from three dierent
initial thicknesses of Duran material showed a signicant
dierence between Duran 0.625 mm and Duran 0.75 mm
(Kruskal-Wallis test, p=0.007; Dunn’s Post Hoc, p=0.008,)
(Supplement Table 3). Erkodur material showed no signi-
Istimaterijalsrazličitimpočetnimdebljinama
Uspoređena su odstupanja od početnih debljina orto-
dontskih udlaga proizvedenih od istog materijala, ali s ne-
koliko različitih početnih debljina (Duran 0,5 mm, 0,625
mm i 0,75 mm; Erkodur 0,5 mm, 0,6 mm i 0,8 mm; Erko-
loc-Pro 1,0 mm i 1,3 mm; IZZI 0,5 mm , 0,6 mm, 0,7 mm;
NextDent 0,5 mm, 0,6 mm i 0,7 mm; NextDent A 0,5, 0,6
i 0,7 mm). Najveće prosječno odstupanje od početne deblji-
ne ustanovljeno je kod sljedećih ortodontskih udlaga: Duran
0,75 (-37,149 %); Erkodur 0,6 (-37,897 %); Erkoloc-Pro 1,0
(-58,331 %); IZZI 0,5 (+27,997 %); NextDent 0,6 (+5,954
%) i NextDent A 0,6 (+10,337%) (tablica 4.).
Analiza odstupanja u debljini triju različitih početnih de-
bljina materijala Duran pokazala je značajnu razliku izme-
đu Durana 0,625 mm i Durana 0,75 mm (Kruskal-Wallisov
test, p = 0,007; Dunnov post-hoc test, p = 0,008) (dopunska
tablica 3.). Materijal Erkodur nije pokazao značajnu razli-

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Thermoformed and 3D-Printed Clear AlignersBandić et al.
150
Table 4 Percentagesofdeviationsfromthedeclaredthicknessformaterialswithseveraldifferentinitialthicknesses–descriptivestatistics
Tablica 4. Postotakodstupanjaoddeklariranedebljinezamaterijalesvišerazličitihpočetnihdebljina–deskriptivnastatistika
Initial thickness •
Početna debljina NMean SD SE Coecient of variation •
Koecijent varijacije
Duran
0.5 360 -36.584 12.928 0.681 - 0.353
0.625 360 -34.246 13.022 0.686 -0.380
0.75 360 -37.149 13.168 0.694 -0.354
Erkodur
0.5 360 -37.29 7 11.8 07 0.622 - 0.317
0.6 360 -37.897 11.859 0.625 -0.313
0.8 360 -37.145 12.353 0.651 -0.333
Erkoloc-Pro 1360 -58.331 7. 459 0.393 -0.128
1.3 360 -48.289 9.414 0.496 -0.195
IZZI
0.5 240 27.997 18.413 1.189 0.658
0.6 240 25.427 15. 2 35 0.983 0.599
0.7 240 15.988 13.237 0.854 0.828
Nextdent
0.5 240 0.354 19.077 1.231 53.865
0.6 240 5.954 22.906 1.479 3.847
0.7 240 5.813 20.962 1.353 3.606
Nextdent A
0.5 240 8.532 24.823 1.602 2.910
0.6 240 10.337 18.4 61 1.192 1.786
0.7 240 -1.220 16.483 1.064 -13.511
cant dierence in thickness changes between dierent initial
thicknesses (Kruskal-Wallis test, p=0.565), while the dier-
ence in thickness deviations between two initial thicknesses
of Erkoloc-Pro material was statistically signicant (Kruskal-
Wallis test, p<0.001). ickness deviations of IZZI materi-
al with 0.7 mm initial thickness were signicantly dierent
from thickness deviations of both IZZI 0.5 mm and IZZI
0.6 mm (Kruskal-Wallis test, p<0.001; Dunn’s Post Hoc,
p<0.001) (Supplement Table 4). e analysis of thickness de-
viations from three dierent initial thicknesses of NextDent
material showed that NextDent 0.5 mm was signicantly
dierent from NextDent 0.6 mm and NextDent 0.7 mm
(Kruskal-Wallist test, p=0.003; Dunn’s Post Hoc, p=0.011,
p=0.006) (Supplement Table 5). In the case of NextDent
material with a modied post-printing protocol (NextDent
A group), signicant dierences in thickness deviations be-
tween NextDent A 0.7 mm and both other groups NextDent
A 0.5 mm and NextDent A 0.6 mm were found (Kruskal-
Wallis test, p<0.001; Dunn’s Post Hoc, p<0.001) (Supple-
ment Table 6).
Variationsduetotoothtype
e percentage of thickness changes for each tooth of
the upper and lower jaw was analyzed. If the results of both
thermoformed and printed aligners in the upper and lower
jaw are considered together, the Ercoloc-Pro material had the
highest number of deviations (Supplement Tables 7 and 8).
On each tooth, the Erkoloc-Pro was thinned by more than
50% after the thermoforming process and the greatest thin-
ning was found at the upper rst molar (-58.146%). Next-
Dent group had the lowest deviations for all teeth of both
jaws, except for the upper and lower rst molar where Next-
Dent A group was more accurate. Signicant dierences be-
tween dierent materials for teeth in both the upper and
lower jaw were found (Kruskal-Wallis test, p<0.001) (Sup-
plement Tables 9-16).
ku u promjenama debljine između različitih početnih deblji-
na (Kruskal-Wallisov test, p = 0,565), a razlika u odstupanju
u debljini između dviju početnih debljina materijala Erko-
loc-Pro bila je statistički značajna (Kruskal-Wallisov test, p
< 0,001). Odstupanja u debljini materijala IZZI s početnom
debljinom od 0,7 mm, značajno su se razlikovala od odstu-
panja u debljini i IZZI 0,5 mm i IZZI 0,6 mm (Kruskal-
Wallisov test, p < 0,001; Dunnov post-hoc test, p < 0,001)
(dopunska tablica 4.). Analiza odstupanja u debljini od tri-
ju različitih početnih debljina materijala NextDent pokazala
je da se NextDent 0,5 mm značajno razlikuje od NextDen-
ta 0,6 mm i NextDenta 0,7 mm (Kruskal-Wallisov test, p
= 0,003; Dunnov post. hoc test, p = 0,011, p = 0,006) (do-
punska tablica 5.). U slučaju materijala NextDent s modi-
ciranim poslijeprintanim protokolom (skupina NextDent A)
utvrđene su značajne razlike u odstupanjima u debljini iz-
među NextDenta A 0,7 mm i obje druge skupine NextDen-
ta A 0,5 mm i NextDenta A 0,6 mm (Kruskal-Wallisov test,
p < 0,001; Dunnov post-hoc test, p < 0,001) (dopunska ta-
blica 6.).
Varijacijeovisnoovrstizuba
Analiziran je postotak promjene u debljini za svaki zub
gornje i donje čeljusti. Ako se rezultati termoformiranih
i printanih ortodontskih udlaga u gornjoj i donjoj čeljusti
promatraju zajedno, materijal Ercoloc-Pro imao je najveći
broj odstupanja (dopunske tablice 7. i 8.). Na svakom zu-
bu Erkoloc-Pro je stanjen za više od 50 % poslije procesa
termoformiranja, a najveće stanjenje zabilježeno je na prvo-
me gornjem molaru (-58,146 %). Skupina NextDent imala je
najmanja odstupanja za sve zube obiju čeljusti, osim za gornji
i donji prvi molar gdje je skupina NextDent A bila preciznija.
Utvrđene su značajne razlike za vrstu zuba između različitih
materijala u gornjoj i donjoj čeljusti (Kruskal-Wallisov test, p
< 0,001) (dopunske tablice 9. – 16.).

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Termoformirani i 3D ispisani prozirni poravnačiBandić i sur. 151
Variationsduetothepositionofmeasuringpoints
To determine changes in thickness according to tooth
morphology, measured points were grouped in three groups:
1-protrusive forms (cusps, incisal edge and cingulum), 2-all
aligner edges (vestibular, palatal, lingual), and nally 3-all
ssures on rst premolars and rst molars.
e percentage of deviation for thermoformed aligners
was the highest on the edges (Table 5). e dierences be-
tween the three morphological groups for thermoformed
aligners were signicant (Kruskal-Wallis test, p<0.001;
Dunn’s Post Hoc, p<0.001) (Supplement Table 17).
In the printed aligner group, the highest deviation from
the declared thickness was found at ssures (+28.567%), and
the lowest at the edges (+6.906%) (Table 6). Although all de-
viations were positive, the printed aligner group also followed
the trend of thinner edges and thicker ssures. e dierence
between dierent measuring points of the teeth for print-
ed aligners was statistically signicant (Kruskal-Wallis test,
p<0.001; Dunn’s Post Hoc, p<0.001) (Supplement Table 18).
Discussion
Light continuous forces are required to achieve ide-
al orthodontic tooth movement. Otherwise, the rate of the
tooth movement will be slower or root resorption may oc-
cur (30). Aligner thickness is one of the factors that can af-
fect the biomechanical characteristics of aligners and must
be considered when planning orthodontic treatment (2, 16,
17). From the previous studies, it is known that appliance
thickness has an impact on forces and moments during treat-
ment with aligners (18, 31-33).
Clear aligners for clinical use are produced by the ther-
moforming process; however, the development of 3D print-
ing makes it possible to print aligners directly from a digital
le (1, 34).
In this study, the variations of aligner thickness after
thermoforming and 3D printing were analyzed and com-
pared. Measurements were made with a digital micrometer
with the accuracy of 0.001 mm. All samples were measured
Odstupanjazbogpoložajamjernihtočaka
Kako bi se odredile promjene u debljini prema morfolo-
giji zuba, izmjerene točke grupirane su u tri skupine: 1. izbo-
čeni dijelovi (kvržice, incizalni rub i cingulum), 2. svi rubo-
vi ortodontske udlage (vestibularni, palatinalni, lingvalni) i
3. sve sure na prvim pretkutnjacima i prvim kutnjacima.
Postotak odstupanja za termoformirane ortodontske ud-
lage bio je najveći na rubovima (tablica 5.). Razlike izme-
đu triju morfoloških skupina za termoformirane ortodont-
ske udlage bile su značajne (Kruskal-Wallisov test, p < 0.001;
Dunnov post-hoc test, p < 0,001) (dopunska tablica 17.).
U skupini printanih ortodontskih udlaga najveće od-
stupanje od deklarirane debljine utvrđeno je u surama
(+28,567 %), a najmanje na rubovima (+6,906 %) (tablica
6.). Iako su sva odstupanja bila pozitivna, skupina printanih
ortodontskih udlaga također je pratila trend tanjih rubova i
debljih sura. Razlika između različitih mjernih točaka zu-
ba za printane ortodontske udlage bila je statistički značajna
(Kruskal-Wallisov test, p < 0,001; Dunnov post-hoc test, p <
0,001) (dopunska tablica 18.).
Rasprava
Za postizanje idealnoga ortodontskog pomaka zuba po-
trebne su lagane kontinuirane sile. U suprotnomu bit će sporija
brzina pomicanja zuba ili se može dogoditi resorpcija korijena
(30). Debljina ortodontske udlage jedan je od čimbenika ko-
ji može utjecati na njezine biomehaničke karakteristike i mora
se uzeti u obzir pri planiranju ortodontske terapije (2, 16, 17).
Iz dosadašnjih istraživanja poznato je da debljina aparata utje-
če na sile i momente tijekom terapije ortodontskim udlagama
(18, 31 – 33).
Prozirne ortodontske udlage za kliničku upotrebu proi-
zvode se postupkom termoformiranja, no razvoj 3D printa-
nja omogućio je njihovu izradu izravno iz digitalne datoteke
(1, 34).
U ovom istraživanju analizirane su i uspoređene varijaci-
je u debljini ortodontskih udlaga poslije termoformiranja i 3D
printanja. Mjerenja su obavljena digitalnim mikrometrom toč-
nosti 0,001 mm. Sve su uzorke tri puta mjerila tri neovisna
Measuring points •
Mjerna mjesta NMean SD SE Coecient of variation •
Koecijent varijacije
11998 -39.649 13.511 0.302 - 0.341
2156 6 -42.147 14.407 0.364 -0.342
3396 -30.922 12.055 0.606 -0.390
Table 5 Percentageofdeviationfromdeclaredthicknessfordifferentmeasuringpointsonthetoothforthermoformedaligners–descriptive
statistics
Tablica 5. Postotakodstupanjaoddeklariranedebljinezarazličitemjernetočkenazubuzatermoformiranealignere–deskriptivnastatistika
Table 6 Percentageofdeviationfromdeclaredthicknessfordifferentmeasuringpointsonthetoothforprintedaligners–descriptive
statistics
Tablica 6. Postotakodstupanjaoddeklariranedebljinezarazličitemjernetočkenazubuzaprintanealignere–deskriptivnastatistika
Measuring points •
Mjerna mjesta NMean SD SE Coecient of variation •
Koecijent varijacije
11080 11.002 21.394 0.651 1.944
2864 6.906 19.285 0.656 2.793
3216 27.5 67 21.917 1.491 0.795

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Thermoformed and 3D-Printed Clear AlignersBandić et al.
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three times by three independent measurers. According to
statistical analysis, there was no signicant dierence be-
tween the measurers (Kruskal-Wallis test, p=0.878), which
means that the measurement method is repeatable.
e results of this study showed that the printed aligners
had signicantly lower deviations from the planned thickness
than thermoformed ones (Kruskal-Wallis test, p<.001) and po-
tentially more predictive treatment outcomes. ermoformed
aligners were thinner than the original thermoplastic foil in all
measured samples, while thickness of printed aligners tended
to increase in comparison to the thickness of digital les.
Our results were in concordance with the study of Park
et al. whose results also showed dierences between ther-
moformed and 3D-printed groups. ermoformed aligners
had reduced thickness in comparison to the original foil and
3D-printed aligners were thicker than digitally designed les
(35). Another study also examined thickness changes after
thermoforming and 3D printing of clear aligners (36). A dig-
ital calliper was used for measuring the thickness in the mid-
dle and at both ends of rectangular specimens. e results
showed that the average thickness of PET-G (Polyethylene
terephthalate glycol) specimens was only 54,7% of the initial
thickness, while the one of 3D printed specimens was 12%
higher than the thickness designed in the digital le (36).
From the previous research, it is known that thermo-
forming is a reproducible process but can reduce aligner
thickness (25). e study by Palone et al. showed that both
gap width and aligner thickness after thermoforming are
dierent across the arch and between aligner manufacturers
(26). In another study, the thickness of four types of aligners
was analyzed before and after thermoforming, and the lat-
ter was reduced by 0.017-0.022 (37). Ryokawa et al. reported
that thermoformed material thicknesses ranged from 74.9 to
92.6% of the nominal sheet thicknesses (38).
e heating of thermoplastic foil and the use of pressure
during the thermoforming process as well as material com-
position can aect both, the mechanical and physical prop-
erties of aligners (10, 24). Dierent thermoplastic polymers
are used for clear aligner production via thermoforming. e
most commonly used polymers are PET-G and TPU (er-
moplastic polyurethane) (6). ey are transparent and have
good mechanical properties (39, 40). To achieve better prop-
erties, polymer blends have also been developed (6).
Our analysis of the thermoformed aligners showed nega-
tive thickness deviations at all measuring points. Similar to
other studies that found dierences in biomechanical proper-
ties between single- and multi-layered thermoplastic materi-
als (41, 43), the results of our study showed that Ercoloc-Pro,
multi-layered material had the highest mean thickness devia-
tion (-53.31%), while single-layered Zendura A had the lowest
(-32.408%). When several thicknesses of the same thermo-
formed material were compared, signicant dierences were
observed between Duran 0.625 and Duran 0.75 (Kruskal-
Wallis test, p=0.007; Dunn’s Post Hoc, p=0.008), as well as
between Erkoloc-Pro 1.0 and Erkoloc-Pro 1.3 (Kruskal-Wal-
lis test, p<0.001).
In the group of 3D-printed aligners, the greatest mean
deviation from digitally designed thickness was found in
mjeritelja. Prema statističkoj analizi nije bilo značajne razlike
između mjeritelja (Kruskal-Wallisov test, p = 0,878), što znači
da je metoda mjerenja ponovljiva.
Rezultati ove studije pokazali su da printane ortodontske
udlage znatno manje odstupaju od planirane debljine nego ter-
moformirani (Kruskal-Wallisov test, p < 0,001) i imaju poten-
cijalno predvidljivije ishode liječenja. Termoformirane orto-
dontske udlage bile su tanje od originalne termoplastične folije
u svim mjerenim uzorcima, a debljina printanih ortodontskih
udlaga imala je tendenciju povećanja u usporedbi s planira-
nom debljinom iz digitalnih datoteka.
Naši rezultati bili su u skladu sa studijom Parka i suradni-
ka čiji su rezultati također pokazali razlike između termofor-
miranih i 3D printanih skupina. Termoformirane ortodontske
udlage imale su smanjenu debljinu u usporedbi s izvornom fo-
lijom, a 3D printane ortodontske udlage bile su deblje od digi-
talno dizajniranih datoteka (35). U drugom istraživanju autori
su također ispitivali promjene u debljini poslije termoformira-
nja i 3D printanja prozirnih ortodontskih udlaga (36). Za mje-
renje debljine u sredini pravokutnih uzoraka i na oba kraja ko-
rišten je digitalni kaliper. Rezultati su pokazali da je prosječna
debljina PET-G (polietilen tereftalat glikola) uzoraka bila sa-
mo 54,7 % početne debljine, a kod 3D printanih uzoraka bila
je 12 % veća od debljine planirane u digitalnoj datoteci (36).
Iz dosadašnjih istraživanja poznato je da je termoformira-
nje ponovljiv proces, ali može smanjiti debljinu ortodontske
udlage (25). Studija Palonea i suradnika pokazala je da se širi-
na razmaka i debljina ortodontske udlage poslije termoformi-
ranja razlikuju s obzirom na promatrani dio zubnoga luka i iz-
među proizvođača ortodontskih udlaga (26). U drugoj studiji
analizirana je debljina četiriju vrsta ortodontskih udlaga prije
termoformiranja i poslije toga postupka, a potonja je smanjena
za 0,017 do 0,022 (37). Ryokawa i suradnici izvijestili su da se
debljina termoformiranog materijala kreće od 74,9 do 92,6 %
nominalne debljine folije (38).
Zagrijavanje termoplastične folije i korištenje pritiska tije-
kom procesa termoformiranja, kao i sastav materijala, mogu
utjecati na mehanička i zikalna svojstva ortodontskih udlaga
(10, 24). Različiti termoplastični polimeri koriste se za proizvod-
nju prozirnih ortodontskih udlaga termoformiranjem. Najčešće
korišteni su PET-G i TPU (termoplastični poliuretan) (6). Pro-
zirni su i imaju dobra mehanička svojstva (39, 40). Za postizanje
boljih svojstava proizvode se i mješavine polimera (6).
Naša analiza termoformiranih ortodontskih udlaga poka-
zala je negativna odstupanja u debljini na svim mjernim točka-
ma. Slično rezultatima drugih istraživanja, prema kojima po-
stoje razlike u biomehaničkim svojstvima između jednoslojnih
i višeslojnih termoplastičnih materijala (41, 43), rezultati na-
šeg istraživanja pokazali su da Ercoloc-Pro, višeslojni materi-
jal, ima najveće (-53,31 %), a jednoslojna Zendura A najmanje
srednje odstupanje debljine (-32,408 %). Usporedbom nekoli-
ko debljina istoga termoformiranoga materijala uočene su zna-
čajne razlike između Durana 0,625 i Durana 0,75 (Kruskal-
Wallisov test, p = 0,007; Dunnov post-hoc test, p = 0,008) i
između Erkoloc-Proa 1,0 i Erkoloc-Proa 1,3 (Kruskal-Walli-
sov test, p < 0,001).
U skupini 3D printanih ortodontskih udlaga najveće
srednje odstupanje od digitalno planirane debljine utvrđeno

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Termoformirani i 3D ispisani prozirni poravnačiBandić i sur. 153
IZZI group (+23.137%). e mean deviation value for Next-
Dent group was only +4.04% and it was closest to the initial
thickness. IZZI group was signicantly dierent from both
NextDent groups (Kruskal-Wallis test, p<0.001; Dunn’s Post
Hoc, p<0.001). When several thicknesses of the same printed
material were compared, IZZI 0.7 was signicantly dierent
from both IZZI 0.5 and 0.6 (Kruskal-Wallis test, p<0.001;
Dunn’s Post Hoc, p<0.001), NextDent 0.5 from both Next-
Dent 0.6 and 0.7 (Kruskal-Wallis test, p=0.003; Dunn’s
Post Hoc, p=0.011, p=0.006) and NextDent A 0.7 from
both NextDent A 0.5 and 0.6 (Kruskal-Wallis test, p<0.001;
Dunn’s Post Hoc, p<0.001).
Since IZZI resin was originally used for model rather than
aligner printing, its subpar performance was somewhat ex-
pected. e results of our study are in accordance with anoth-
er study investigating the eect of digitally designed thickness
on 3D-printed clear aligners, in which Edelmann et al. report-
ed that 3D-printing could increase aligner thickness by more
than 0.2 mm (27). Lee et al. have explained that a possible rea-
son for aligner thickness increase after 3D printing could be
the usage of high denition projector as a light source in DLP
printers (36). Other possible causes of aligner overbuilding are
over-penetrance of light in transparent materials during 3D
printing, print orientation, as well as polymerization of residu-
al resin during the post-curing process (27, 36, 44).
When comparing thickness deviations between the up-
per and lower jaw, signicant dierences were found in both
groups (Kruskal-Wallis tests, p<0.001). e thickness of
thermoformed aligners deviated more in the upper jaw, while
in the case of printed aligners it deviated more in the lower
jaw. If the thickness on each tooth was analyzed separately,
aligners produced from Erkoloc-Pro material were thinned
by more than 50% on all teeth of the upper and lower jaw,
whereas both NextDent groups had the lowest deviations.
For both, the thickness of thermoformed and printed
aligners was signicantly lower at edges than at cusps and
ssures (p<0.001). It is important to note that in the case of
printed aligners, thickness deviations of all three groups (edg-
es, cusps and ssures) were positive and that the thickness of
edges was closest to digitally designed (+6.906%). ese re-
sults are in line with the results from other studies (23, 24,
26, 35). Mantovani et al. reported that Invisalign thickness
was not consistent across the arch, but a signicant dierence
was found only between gingivolingual and occlusal sites in
the molar region (23). Another research reported that aligner
thickness was generally smaller at anterior teeth and gingival
sites, unlike the thickness at the posterior teeth and occlusal
surfaces, and only one of the six tested commercial materi-
als had homogeneous thickness across the arch (26). Similar
results were obtained in another study where thermoplastic
materials, both single- and multi-layered, had reduced thick-
ness at buccal and buccogingival areas (35). is can be due
to dierent tooth anatomy and less stretching of thermoplas-
tic foil in the occlusal and posterior regions during the ther-
moforming process (24). As for printed aligners, Park et al.
reported that they also were thicker at incisal and occlusal
areas. is can happen due to errors in layering during 3D
printing of complex tooth morphology (35).
je u skupini IZZI (+23,137 %). Srednja vrijednost odstupa-
nja za grupu NextDent bila je samo +4,04 % i bila je naj-
bliža početnoj debljini. Skupina IZZI značajno se razlikova-
la od obiju skupina NextDenta (Kruskal-Wallisov test, p <
0,001; Dunnov post-hoc test, p < 0,001). Kada se uspoređu-
je nekoliko debljina istoga printanog materijala, IZZI 0,7 zna-
čajno se razlikuje od IZZI-ja 0,5 i 0,6 (Kruskal-Wallisov test,
p < 0,001; Dunnov post-hoc test p < 0,001), NextDent 0,5
od NextDenta 0,6 i 0,7 ( Kruskal-Wallisov test, p = 0,003;
Dunnov post-hoc test, p = 0,011, p = 0,006) i NextDent A 0,7
od NextDenta A 0,5 i 0,6 (Kruskal-Wallisov test, p < 0,001;
Dunnov post-hoc test, p < 0,001) .
Budući da je smola IZZI izvorno korištena za ispis mode-
la, a ne ortodontske udlage, njezina je loša izvedba donekle bila
očekivana. Rezultati naše studije u skladu su sa studijom koja
je istraživala utjecaj digitalno dizajnirane debljine na 3D prin-
tane ortodontske udlage, u kojoj su Edelmann i suradnici izvi-
jestili da 3D printanje može povećati debljinu ortodontske ud-
lage za više od 0,2 mm (27). Lee i suradnici objasnili su da bi
mogući razlog za povećanje debljine ortodontske udlage nakon
3D printanja mogao biti korištenje projektora visoke razluči-
vosti kao izvora svjetla u DLP printerima (36). Drugi mogući
uzroci za povećano zadebljavanje ortodontske udlage jesu pre-
komjerno prodiranje svjetla u prozirne materijale tijekom 3D
printanja, orijentacija printa i polimerizacija zaostale smole ti-
jekom procesa naknadnog stvrdnjavanja (27, 36, 44).
Usporedbom odstupanja u debljini između gornje i do-
nje čeljusti utvrđene su značajne razlike u objema skupinama
(Kruskal-Wallisov test, p < 0,001). Debljina termoformiranih
ortodontskih udlaga više je odstupala u gornjoj čeljusti, a kod
printanih ortodontskih udlaga više je odstupala u donjoj čelju-
sti. Ako se posebno analizira debljina na svakome zubu, orto-
dontske udlage proizvedene od materijala Erkoloc-Pro istanjile
su se za više od 50 % na svim zubima gornje i donje čeljusti, a za
obje skupine NextDenta zabilježena su najmanja odstupanja.
I za termoformirane i za printane ortodontske udlage de-
bljina je bila značajno manja na rubovima nego na kvržicama i
surama (p < 0,001). Važno je napomenuti da su kod printanih
ortodontskih udlaga odstupanja od debljine u svim trima sku-
pinama (rubovi, kvržice i sure) bila pozitivna te da je debljina
rubova bila najbliža digitalno dizajniranoj (+6,906 %). Ti su re-
zultati u skladu s rezultatima u drugim studijama (23, 24, 26,
35). Mantovani i suradnici izvijestili su da debljina ortodontske
udlage Invisalign nije bila konzistetna u cijelome zubnome lu-
ku, ali je značajna razlika pronađena samo između gingivolin-
gvalnih i okluzalnih mjesta u molarnoj regiji (23). I u dugom
istraživanju istaknuto je da je debljina ortodontskih udlaga op-
ćenito manja na prednjim zubima i mjestima uz gingivu, za ra-
zliku od debljine na stražnjim zubima i okluzalnim površina-
ma, a samo je jedan od šest testiranih komercijalnih materijala
imao homogenu debljinu duž cijeloga luka (26). Slični rezul-
tati dobiveni su u studiji u kojoj su termoplastični materijali,
jednoslojni i višeslojni, imali smanjenu debljinu na bukalnom
i bukogingivnom području (35). To može biti posljedica druk-
čije anatomije zuba i manjeg istezanja termoplastične folije na
okluzalnim i posteriornim regijama u procesu termoformiranja
(24). Kad je riječ o printanim ortodontskim udlagama, Park i
suradnici izvijestili su da su i oni deblji u incizalnim i okluzal-

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Thermoformed and 3D-Printed Clear AlignersBandić et al.
154
e success of orthodontic treatment depends, among
other things, on the quality of the appliances that are being
used. e quality of the appliance is reected in the ability
of appliance to move the tooth to the planned position in the
planned time without causing any damage. Material manu-
facturers often fail to mention the potential problems that
end users may encounter. erefore, an independent research
like ours is very important, especially when new things are
introduced such as the current transition from thermoform-
ing to 3D printing. Direct 3D printing of orthodontic align-
ers brings numerous advantages compared to previous pro-
duction technology; it saves time, and human resources,
reduces waste, and it is clear that this is the future of orth-
odontic aligner production. However, as our research has
shown, despite great progress, it is still necessary to work on
the development of materials and technology that will justify
the abandonment of thermoforming, which, despite its ma-
ny shortcomings, still works quite well in everyday practice.
is research was co-nanced by the European Union
from the European Regional Development Fund under the
Operational Program Competitiveness and Cohesion 2014-
2020
Conclusions
ermoformed aligners had signicantly more thick-
ness deviations than printed ones. All thickness deviations of
thermoformed aligners were negative. Erkoloc-Pro (PET-G/
TPU) diered the most and Zendura A (TPU) the least from
the initial thickness of the thermoplastic sheet; ickness de-
viations of printed aligners were mostly positive. e high-
est mean deviation had IZZI group, while NextDent group
had the lowest one. For both, the nal thickness of thermo-
formed and direct-printed aligners was signicantly smaller
at edges than at cusps and ssures.
Conict of interest: Nonedeclared
Author’s contribution: R. B., K. V., I. V. K. - data collection, analysis
andinterpretationofresults,originaldraftpreparation;I. M. M., I. G.
-reviewingandediting;D. K. G.-studyconceptionanddesign,analy-
sisandinterpretationofresults,reviewingandediting
nim područjima. To se može dogoditi zbog pogrešaka u sloje-
vanju tijekom 3D printanja složene morfologije zuba (35).
Uspjeh ortodontskog liječenja ovisi, između ostaloga, i o
kvaliteti aparata koji se koriste. Kakvoća se ogleda u njiho-
voj mogućnosti da pomaknu zub u planirani položaj u pla-
niranom vremenu bez ikakva oštećenja. Proizvođači materija-
la često propuštaju spomenuti potencijalne probleme s kojima
se krajnji korisnici mogu susresti, a ovakva neovisna istraživa-
nja vrlo su važna, posebno kada se uvode nove stvari, kao što je
trenutačni prijelaz s termoformiranja na 3D printanje. Izravni
3D ispis ortodontskih ortodontskih udlaga ima brojne pred-
nosti u usporedbi s dosadašnjom proizvodnom tehnologijom
– štedi vrijeme i ljudske resurse, smanjuje otpad i jasno je da
je to budućnost u proizvodnji ortodontskih udlaga. No, kako
je naše istraživanje pokazalo, unatoč velikom napretku, po-
trebno je još raditi na razvoju materijala i tehnologije koji će
opravdati odustajanje od termoformiranja koje, unatoč mno-
gobrojnim nedostatcima, još uvijek dosta dobro funkcionira u
svakodnevnoj praksi.
Ovo istraživanje sunancirala je Europska unija iz svoje-
ga Fonda za regionalni razvoj u sklopu operativnog programa
Konkurentnost i kohezija 2014. – 2020.
Zaključci
Termoformirane ortodontske udlage znatno su više od-
stupale u debljini od printanih. Sva odstupanja u debljini ter-
moformiranih ortodontskih udlaga bila su negativna. Erko-
loc-Pro (PET-G/TPU) razlikovao se najviše, a Zendura A
(TPU) najmanje od početne debljine termoplastične folije.
Odstupanja u debljini printanih ortodontskih udlaga bila su
uglavnom pozitivna. Najveću srednju devijaciju imala je sku-
pina IZZI, a najmanju skupina NextDent. I kod termofor-
miranih i printanih ortodontskih udlaga debljina je bila zna-
čajno manja na rubovima nego na kvržicama i surama.
Sukob interesa:Autorinisubiliusukobuinteresa.
Doprinos autora: R. B., K. V., I. V. K.–prikupljanjepodataka,analizai
interpretacijarezultata,pripremaizvornognacrta,I. M. M., I. G.–re-
cenziranjeipriređivanje;D. K. G.–koncepcijastudijeidizajn,analiza
iinterpretacijarezultata,recenziranjeiuređivanje
Sažetak
Cilj:Procijenitivarijacijedebljinetermoformiranihi3Dprintanihprozirnihortodontskihudlaga.Ma-
terijali i metode:Šestrazličitihtermoplastičnihmaterijalasrazličitimpočetnimdebljinamakorište-
nojezatermoformiranjeortodontskihudlagapomoćuBiostar®uređaja(Biostar®,SCHEU-DENTAL
GmbH,Iserlohn,Njemačka).Također,dvijerazličitedentalnesmolekorištenesuza izradu3D-prin-
tanihortodontskihudlagautridigitalnodizajniranedebljinepomoćuIZZIDirectpisača(3Dtech,Za-
greb,Hrvatska).Ortodontskeudlagesuizmjereneelektroničkimmikrometrom(ELECTRONICUNIVER-
SALMICROMETER, Schut Geometrical Metrology, Groningen,Nizozemska, točnost: 0,001mm) na
ukupno20točakapoortodontskojudlazi.StatističkaanalizaprovedenajespomoćuprogramaJASP
(JASP,SveučilišteuAmsterdamu,Amsterdam,Nizozemska).Rezultati:Razlikaizmeđutermoformira-
nihiprintanihskupinabilajestatističkiznačajna.Pronađenesuznačajnerazlikeizmeđurazličitihter-
moformiranihmaterijalaiizmeđu3Dprintanihmaterijala.Debljinatermoformiranihortodontskihud-
lagavišejeodstupalaugornjojčeljusti,dokjedebljinaprintanihortodontskihudlagavišeodstupala
udonjojčeljusti.Objesurazlikebilestatističkiznačajne.Najvećeprosječnoodstupanjeodpočetne
debljineutvrđenojekodDurana0,75;Erkodur0,6;Erkoloc-Pro1.0;IZZI0,5;NextDent0.6iNextDent
A0.6.NextDentskupinaimalajenajmanjaodstupanjazasvezubeobijučeljusti,osimzagornjiidonji
prvikutnjakgdjejeNextDentAskupinabilapreciznija.Zaključci:Termoformiraneortodontskeudla-
gepokazalesusmanjenevrijednosti,doksuprintanepokazaleuglavnompovećanevrijednostiuus-
poredbisizvornomdebljinommaterijala.NajvećusrednjudevijacijuimalajeskupinaIZZI,anajma-
njusrednjudevijacijuskupinaNextDent.Debljinaobjeortodontskeudlagebilajemanjanarubovima
uusporedbisdebljinomnakvržicamausurama.
Zaprimljen:5.siječnja2024.
Prihvaćen:30.travnja2024.
Adresa za dopisivanje
DanijelaKalibovićGovorko
MedicinskifakultetSveučilištauSplitu
Katedrazaortodonciju
danijela.kalibovic.govorko@mefst.hr
MeSH pojmovi:mobilneortodontske
naprave;trodimenzionalniotisak
Autorske ključne riječi:Ortodonci-
ja;3D-printanje;Ortodontskiapara-
ti;Mobilni

www.ascro.hr
Termoformirani i 3D ispisani prozirni poravnačiBandić i sur. 155
References
1. TartagliaGM,MapelliA,MasperoC,SantanielloT,SeranM,Far-
ronatoM,etal.Direct3DPrintingofClearOrthodonticAligners:
Current State and Future Possibilities. Materials (Basel). 2021
Apr5;14(7):1799.
2. WeirT.Clearalignersinorthodontictreatment.AustDentJ.2017
Mar:62Suppl1:58-62.
3. ZhengM,LiuR,NiZ,YuZ.Efciency,effectivenessandtreatment
stabilityofclearaligners:Asystematicreviewandmeta-analysis.
OrthodCraniofacRes.2017Aug;20(3):127-133.
4. Tamer I, Oztas E, Marsan G. Orthodontic Treatment with Clear
AlignersandTheScienticRealityBehindTheirMarketing:ALit-
eratureReview.TurkJOrthod.2019Dec1;32(4):241-246.
5. MillerKB,McGorraySP,WomackR,QuinteroJC,PerelmuterM,
GibsonJ,etal.AcomparisonoftreatmentimpactsbetweenInvis-
alignalignerandxedappliancetherapyduringtherstweekof
treatment.American journal of orthodontics anddentofacialor-
thopedics:ofcialpublicationoftheAmJOrthodDentofacialOr-
thop.2007Mar;131(3):302.e1-9.
6. BichuYM,AlwaA,LiuX,AndrewsJ,LudwigB,BichuAY,etal.Ad-
vancesinorthodonticclearalignermaterials.BioactMater.2022
Oct20:22:384-403.
7. DalaieK,FatemiSM,GhaffariS.Dynamicmechanicalandthermal
propertiesofclearalignersafterthermoformingandaging.Prog-
ressinorthodontics.2021;22(1):15.Epub2021/06/29.
8. RyuJH,KwonJS,JiangHB,ChaJY,KimKM.Effectsofthermoform-
ingonthephysicalandmechanicalpropertiesofthermoplastic
materialsfor transparent orthodontic aligners.KoreanJOrthod.
2018Sep;48(5):316-325.
9. GolkhaniB, WeberA,Keilig L,ReimannS, BourauelC.Variation
ofthemodulusofelasticityofalignerfoilsheetmaterialsdueto
thermoforming..JOrofacOrthop.2022Jul;83(4):233-243.
10. Tamburrino F, D'Anto V, Bucci R, Alessandri-Bonetti G, Barone
S, Razionale AV. Mechanical Properties of Thermoplastic Poly-
mersforAlignerManufacturing:InVitroStudy.Dentistryjournal.
2020;8(2).
11.
JindalP,JunejaM,SienaFL,BajajD,BreedonP.Mechanicalandgeo-
metricpropertiesofthermoformedand3Dprintedcleardentalalign-
ers.AmJOrthodDentofacialOrthop.2019Nov;156(5):694-701.
12. PeetersB,Kiratli N,Semeijn J.A barrieranalysisfordistributed
recyclingof3Dprintingwaste:Takingthemakermovementper-
spective.JCleanProd.2019;241:118313.
13. Bayirli B, Kim-Berman H, Puntillo A, editors. Embracing Novel
TechnologiesinDentistryandOrthodontics.2020.
14. Jindal P,Worcester F,Siena FL,Forbes C, JunejaM, BreedonP.
Mechanicalbehaviourof3Dprintedvsthermoformedclearden-
talalignermaterialsundernon-linearcompressiveloadingusing
FEM.JMechBehavBiomedMater.2020Dec:112:104045.
15. HertanE,McCrayJ,BankheadB,KimKB.Forceproleassessment
ofdirect-printed alignersversusthermoformedaligners andthe
effectsofnon-engagedsurfacepatterns.ProgOrthod.2022Nov
29;23(1):49.
16. WheelerTT.Orthodonticclearalignertreatment.SeminarsinOr-
thodontics.2017;23:83-9.
17. PhanX, LingPH.Clinical limitationsofInvisalign.JCan DentAs-
soc.2007;73(3):263-6.
18. GaoL,WichelhausA.ForcesandmomentsdeliveredbythePET-G
alignertoamaxillarycentralincisorforpalataltippingandintru-
sion.TheAngleorthodontist.2017;87(4):534-41.
19. KohdaN,IijimaM,MugurumaT,BrantleyWA,AhluwaliaKS,Mizo-
guchiI.Effectsofmechanicalpropertiesofthermoplasticmateri-
alsontheinitialforceofthermoplasticappliances.AngleOrthod.
2013May;83(3):476-83.
20. ElkholyF,SchmidtF,JagerR,LapatkiBG.Forcesandmomentsde-
liveredbynovel,thinnerPET-Galignersduringlabiopalatalbodily
movementofamaxillarycentralincisor:Aninvitrostudy.TheAn-
gleorthodontist.2016;86(6):883-90.
21. ElkholyF,SchmidtF,JagerR,LapatkiBG.Forcesandmomentsap-
pliedduringderotationofamaxillarycentralincisorwiththinner
aligners:Anin-vitrostudy.AmJOrthodDentofacialOrthop.2017
Feb;151(2):407-415.
22. SeoJH, Eghan-AcquahE, KimMS,LeeJH, JeongYH, JungTG,et
al. Comparative Analysis of Stress in the Periodontal Ligament
andCenterofRotationintheToothafterOrthodonticTreatment
Depending on Clear Aligner Thickness-Finite Element Analysis
Study.Materials(Basel).2021;14(2).
23. MantovaniE,ParriniS,CodaE,CugliariG,ScottiN,PasqualiniD,
etal.MicrocomputedtomographyevaluationofInvisalignalign-
erthicknesshomogeneity.AngleOrthod.2021May1;91(3):343-
348.
24. LombardoL,Palone M,LongoM,ArvedaN,NacucchiM,De Pas-
calisF,etal.MicroCTX-raycomparisonofalignergapandthick-
nessofsixbrandsofaligners:anin-vitrostudy.Progressinortho-
dontics.2020;21(1):12.
25. BucciR,RongoR,LevateC,MichelottiA,BaroneS,RazionaleAV,
etal.Thickness of orthodontic clear alignersafter thermoform-
ingandafter10daysofintraoralexposure:aprospectiveclinical
study.Progressinorthodontics.2019;20(1):36.
26. PaloneM, LongoM,ArvedaN, NacucchiM,PascalisF, Spedica-
toGA, et al. Micro-computed tomography evaluationofgeneral
trends in aligner thickness and gap width after thermoforming
procedures involving six commercial clear aligners: An in vitro
study.KoreanJOrthod.2021Mar25;51(2):135-141.
27. EdelmannA,EnglishJD,ChenSJ,KasperFK.Analysisofthethick-
nessof3-dimensional-printedorthodonticaligners.AmJOrthod
DentofacialOrthop.2020Nov;158(5):e91-e98.
28. JindalP,JunejaM,BajajD,SienaF,BreedonP.Effectsofpost-cur-
ingconditionsonmechanicalpropertiesof3Dprintedclearden-
talaligners.RapidPrototypingJournal.2020;ahead-of-print.
29. MeadR.TheDesignofExperiments.NewYork.CambridgeUniver-
sityPress;1988:634.p.
30. ProftWR.Contemporaryorthodontics.WilliamR.Proft,editor.
5thed.ed.St.Louis,Mo:Elsevier/Mosby;2013.
31. HahnW, DatheH, Fialka-FrickeJ,Fricke-ZechS, ZapfA, Kubein-
MeesenburgD,etal.Inuenceofthermoplastic appliancethick-
nessonthemagnitudeofforcedeliveredtoamaxillary central
incisor during tipping. Am J Orthod Dentofacial Orthop. 2009
Jul;136(1):12.e1-7;discussion12-3.
32. Alhasyimi AA,Ayub A,Farmasyanti CA.Effectiveness of theAt-
tachment Design and Thickness of Clear Aligners during Orth-
odontic Anterior Retraction: Finite Element Analysis. European
journalofdentistry.2023.
33. GrantJ,Foley P,BankheadB,MirandaG, AdelSM,KimKB.Forc-
esandmomentsgeneratedby3Ddirectprintedclearalignersof
varyinglabialandlingualthicknessesduringlingualmovementof
maxillarycentralincisor:aninvitrostudy.ProgOrthod.2023Jul
10;24(1):23.
34. MasperoC,Tartaglia GM.3DPrintingofClearOrthodonticAlign-
ers:Where We Are and WhereWeAreGoing.Materials (Basel).
2020;13(22).
35. ParkSY,ChoiSH,YuHS,KimSJ,KimH,KimKB,etal.Comparison
oftranslucency, thickness, andgapwidthofthermoformed and
3D-printedclearalignersusingmicro-CTandspectrophotometer.
SciRep.2023Jul5;13(1):10921.
36. LeeSY, KimH,KimHJ, ChungCJ, ChoiYJ,Kim SJ,etal.Thermo-
mechanicalpropertiesof3Dprintedphotocurableshapememory
resinforclearaligners.SciRep.2022Apr15;12(1):6246.
37. DasyHDA,AsatrianG,RózsaN,LeeHF,KwakJH..Effectsofvari-
ableattachmentshapesandalignermaterialonalignerretention.
AngleOrthod.2015Nov;85(6)):934-40.
38. RyokawaHMY,FujishimaA,MiyazakiT,MakiK. Themechanical
propertiesofdentalthermoplasticmaterialsinasimulatedintra-
oralenvironment.OrthodonticWaves.2006(65(2)):64-72.
39. DupaixRBBM.Finitestrainbehaviorofpoly(ethyleneterephthal-
ate)(PET)andpoly(ethyleneterephthalate)-glycol(PETG). Poly-
mer.2005;46(13):4827-38.
40. FrickRA.CharacterizationofTPU-elastomers bythermalanalysis
(DSC).PolymerTesting.2004(23(4)):413-7.
41. Jia L,Wang C,Wang C,Song J, FanY. Efcacyof variousmulti-
layersoforthodonticclearaligners:asimulatedstudy.Comput
MethodsBiomechBiomedEngin.2022Nov;25(15):1710-1721.
42. LombardoL,MartinesE,MazzantiV,Arreghini A,MollicaF,Sicil-
ianiG.Stressrelaxationpropertiesoffourorthodonticalignerma-
terials:A24-hourinvitrostudy.AngleOrthod.2017Jan;87(1):11-
18.
43. Simunovic L,Jurela A,Sudarevic K, BacicI, MestrovicS. Differ-
entialStabilityofOne-layerandThree-layerOrthodonticAligner
BlendsunderThermocycling:ImplicationsforClinicalDurability.
ActaStomatolCroat.2023Dec;57(4):286-299.
44. McCartyMC,ChenSJ,EnglishJD,KasperF.Effectofprintorienta-
tionanddurationofultravioletcuringonthedimensionalaccura-
cyofa3-dimensionallyprintedorthodonticclearalignerdesign.
AmJOrthodDentofacialOrthop.2020Dec;158(6):889-897.