Japanese Dental Science Review 59 (2023) 403–411

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Japanese Dental Science Review

journal homepage: www.elsevier.com/locate/jdsr

From biomimetics to smart materials and 3D technology: Applications in

orthodontic bonding, debonding, and appliance design or fabrication
Theodore Eliades a, *, Nearchos Panayi a, b, Spyridon N. Papageorgiou a
a
Clinic of Orthodontics and Pediatric Dentistry, Center of Dental Medicine, University of Zurich, Zurich, Switzerland
b
European University Cyprus, School of Dentistry, Nicosia, Cyprus

A R T I C L E I N F O A B S T R A C T

Keywords: This review covers aspects of orthodontic materials, appliance fabrication and bonding, crossing scientific fields
Orthodontic and presenting recent advances in science and technology. Its purpose is to familiarize the reader with de­
Biomaterials velopments on these issues, indicate possible future applications of such pioneering approaches, and report the
Biomimetic
current status in orthodontics. The first section of this review covers shape-memory polymer wires, several
Self-healing
Shape-memory
misconceptions arising from the recent introduction of novel three-dimensional (3D)-printed aligners (mistak­
3D printed enly termed shape-memory polymers only because they present a certain degree of rebound capacity, as most
non-stiff alloys or polymers do), frictionless surfaces enabling resistance-less sliding, self-healing materials for
effective handling of fractured plastic/ceramic brackets, self-cleaning materials to minimize microbial attach­
ment or plaque build-up on orthodontic appliances, elastomers with reduced force relaxation and extended
stretching capacity to address the problem of inadequate force application during wire-engagement in the
bracket slot, biomimetic (non-etching mediated) adhesive attachment to surfaces based on the model of the
gecko and the mussel, and command-debond adhesives as options for an atraumatic debonding. This review’s
second section deals with the recent and largely unsubstantiated application of 3D-printed alloys and polymers in
orthodontics and aspects of planning, material fabrication, and appliance design.

1. Introduction non-stiff alloys (multistrand wires) or polymers do.

b.frictionless surfaces enabling resistance-less sliding.
Advances in the field of biomimetics, termed from the greek βιο-bio c.self-healing materials for effective handling of fractured plastic/
meaning living and μίμησις-mimetic (adjective, meaning imitating/ ceramic brackets.
copying), have brought a significant growth in material development d.self-cleaning materials to minimize microbial attachment and
and applications. Despite numerous patents and substantial break­ plaque build-up on bracket, aligner, wire, and elastomers.
throughs in biomedical or engineering materials, the orthodontic pro­ e.elastomers with reduced force relaxation and extended stretching
fession has yet to explore the feasibility of introducing such biomimetic capacity to address the problem of inadequate force application during
materials in clinical practice. wire-engagement in the bracket slot.
The purpose of this comprehensive review is to familiarize the reader f.biomimetic, non-etching mediated, adhesion based on the gecko or
with these developments, indicate possible applications of such pio­ mussel model.
neering approaches in the introduction of materials/processes in the g.command-debond adhesives as options for an atraumatic
broader field of biomedical or engineering/industrial materials, and debonding.
report the status of applications in orthodontics. The clinical challenges for each issue will be discussed followed by
The first section of this review covers: corresponding advances in material manufacturing and the introduction
a.shape-memory polymer wires, aiming to clarifying several mis­ of a relevant application in technology and industry.
conceptions arising from the recent introduction of three-dimensional The second section of this review deals with the recent and largely
(3D)-printed aligners, mistakenly termed shape-memory polymers unsubstantiated application of 3D-printed alloys and polymers in or­
only because they present a certain degree of rebound capacity, as most thodontics and aspects of planning, material fabrication, appliance

* Correspondence to: Clinic of Orthodontics and Pediatric Dentistry, Center of Dental Medicine, University of Zurich, Plattenstrasse 11, 8032 Zurich, Switzerland.
E-mail address: theodore.eliades@zzm.uzh.ch (T. Eliades).

https://doi.org/10.1016/j.jdsr.2023.10.005
Received 6 September 2023; Received in revised form 24 October 2023; Accepted 26 October 2023
1882-7616/© 2023 Published by Elsevier Ltd on behalf of The Japanese Association for Dental Science. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
T. Eliades et al. Japanese Dental Science Review 59 (2023) 403–411

design. ligature) that greatly affect exerted forces.

2. Biomimetics and smart materials 2.2.2. Available materials

A possible solution to the abovementioned problems might come
2.1. Shape-memory polymers from broader industrial metallurgical applications mimicking the flaked
surface texture of snakes. These reptiles show remarkably smooth
2.1.1. Clinical challenge movement across various terrains, owing not to the smooth surface of
The clinical problem that led to the development of polymeric or their skin, but to the textured pattern and the specific arrangement of
surface-altered wires is the aesthetic problems from the use of metallic their external flake layers [4].
wires. Relevant research has demonstrated that the friction coefficient of
Earlier attempts to introduce solid plastic polymeric wires such as textured surfaces of snakes with a characteristic orientation of the
Optiflex (Ormco Co., Glendora, CA) for the initial treatment stages superimposed skin flakes was almost 50% reduced compared to the
resulted in the application of polymers with reduced modulus of elas­ modeled untextured one.
ticity, stiffness, but also low resilience and thus low elastic energy stored Steel surfaces used in industry or manufacturing made to resemble
in the wire (which in the stress-strain curve is provided by the area under the snakes’ external flake layer were built at the end of the first decade of
the curve until the yield point, or the so called ‘range’)—precluding their 2000 and were met with success but have yet to be introduced in
use in most cases with average crowding. Moreover, their low bending orthodontics.
stiffness adversely affected their levelling capacity [1]. This is different
from the aligning properties of the wire as the bent wire in the buccal 2.3. Self-healing materials
segment of deep curve of Spee cases needs have high bending stiffness
and cross sections filling the edgewise size of the bracket, i.e., 0.018- or 2.3.1. Clinical challenge
0.022-inch. This contradicts the false notion that Nickel-Titanium (NiTi) Ceramic, and to less extent plastic, brackets were introduced as an
wires were introduced exactly because with their low modulus can appliance with superior aesthetics, but several issues arose from their
effectively level the curve of Spee. A round 0.016-inch stainless steel large-scale clinical use. Most research on this topic is focused on their
wire has much higher stiffness than all NiTi or fracture strength and debonding characteristics, due to their unfavor­
Titanium-Molybdenum-Alloy (TMA) wires of any cross-section or shape. able debonding pattern that was reported in the early 1990 s [5]. The
Similarly, rectangular 0.016 × 0.022-inch NiTi and TMA wires to level observation that ceramic brackets fracture frequently, usually at the
the occlusion in the straightwire technique also face the problem of wings and most often during debonding, is universal knowledge for most
reduced bending stiffness owing to the reduced contribution to this ef­ readers. These materials are composed of atoms bound together with
fect of the flatwise dimension of the wire, in addition to the fact that they such strong forces that their flexibility is notably impaired. As a result,
show play in the edgewise dimension (in the 0.018-inch system). force application on them leads to a minimum elastic deformation and
Other plastic wires were developed in the late 1990′s-early 2000’s by no permanent deformation. It follows that these materials maintain their
BioMers Products, LLC (Jacksonville, Fla) to address these concerns and dimensions / shape after fracture, as no deformation has set in, due to
efficiently align / derotate teeth, but their hydrolytic degradation after their bonding energy and strong directional characteristics. Also, the
extended periods of time precluded their widespread use. atomic packing factor of such materials is high, implying that their dense
atomic distribution in 3D arrays results in high crystal density. There­
2.1.2. Available materials fore, no plastic deformation of the wings is possible and, when the force
Developed in the mid 2000′s, shape-memory polymers—which have exceeds a certain value, the wing fractures.
nothing to do with the claimed memory properties of recently intro­
duced 3D-printed aligners—were introduced initially for biomedical 2.3.2. Available materials
applications. Exposure of initially developed polymer threads to heat or These issues led to interest being attracted to the production of self-
to a specific wavelength of light [2,3] resulted in modification of their healing polymeric materials, owing to the potential flexibility in syn­
shape to their pre-engineered shape. Therefore, thrombectomies and thesizing polymers with desirable properties. One of the first such at­
removals of vessel clots were facilitated through the catheterization of tempts involved including monomer-filled spheres, which upon crack
the vessel with a shape-memory thread, which after being navigated propagation would empty their content to the fracture vicinity to
through the thrombus, had its external to the patient’s body part facilitate filling of the fracture path with an auto-polymerized material.
exposed to a specific stimulus that reverted the thread to its initial shape Later applications, which have already been adopted by the industry,
(in that case of a cork), thereby facilitating clot removal. include the development of self-healing of oxetane-chitosan compounds
However, this application has since 2005 not yet received any in the form of coatings that, upon exposure to ultraviolet sunlight,
consideration for use in orthodontic wires or orthodontic aligner initiate a cascade of events bringing these two molecules together to
applications. form a coating that closes potential damage of the initially formed
coating. This found applications in the automobile industry, with pro­
2.2. Frictionless movement tective car coatings already being marketed.
In the area of ceramics, development of such materials is more
2.2.1. Clinical challenge challenging as the necessity of altering ceramic structural defects usually
Friction, although often overemphasized, plays a significant role in requires high temperatures, which exceed the intraoral spectrum. In as
determining the magnitude of force applied to the tooth crown through much, additional processes such as application of combustion chambers
an activated wire or a prescription bracket. It has also been shown to be [6] or electric field-induced colloidal aggregation [7] to fill cracks and
an impeding factor against effective tooth movement and efforts to inhibit propagation, thereby avoiding a catastrophic failure, have no
overcome this included the use of wires with smoother surfaces or feasible application mode in orthodontics. Therefore, despite the ad­
alternative ligation modes such as self-ligation, which however have vances in that field, the potential application to orthodontic materials
never associated with improved tooth movement rates in vivo. The involves only polymeric products (brackets and aligners), which on the
problem lies with factors traditionally ignored in oversimplified in vitro other hand, are ductile and present a much lower clinical failure
studies, such as the specific tooth movement pattern (which deviates incidence.
from the straight line path and includes several tilting spatial alter­
ations) and the in vivo ageing of the involved materials (bracket, wire,

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2.4. Self-cleaning materials involved the development of Titanium photocatalysis [12], which in­
volves the coating of the bracket with a layer of Ti oxide that upon
2.4.1. Clinical challenge sunlight exposure releases reactive oxygen species or hydroxyl radicals,
Plaque accumulation on a biomaterial exposed to a biological system which have antimicrobial, odor-removing effects. However, such ap­
is accompanied by the organization of a non-cellular biofilm by spon­ plications have yet to be widely employed in orthodontics.
taneous adsorption of extracellular macromolecules, composed mostly
of glycoproteins and proteoglycans. These films induce a conditioning
2.5. New elastomers
effect that modifies the biomaterial’s surface properties and alters both
the response of the subsequently attached cells and the interactions
2.5.1. Clinical challenge
occurring at the biomaterial-host interface. This conditioning effect is
The use of polyurethane-based elastomeric ligatures and modules in
based on the differing capacities of artificial surfaces to fractionate
engaging archwires and closing spaces in orthodontics is accompanied
proteins from biological fluids, such as saliva or blood, and the ability to
by a notable force relaxation that accounts for up to 40% of the initially
induce conformation and orientation changes of adsorbed proteins.
applied load. This effect is accentuated by the laborious and multi­
The outcome of biofilm adsorption is dependent on the biological
perspective intraoral ageing pattern of these polymers that comprises of
fluid flow rate at the site of contact, the type of interfacial interactions
hydrolytic degradation, swelling, and softening, which further degrade
involved, and the attachment strength with the substrate. Under static
the mechanical properties and decrease the exerted forces from the
conditions or low flow rates, the biomaterial surface chemistry is the
material. At the same time, they favor plaque build-up and contribute to
fundamental factor affecting the composition and organization of ac­
the microbial colonization of the bracket-wire-elastomer complex.
quired biofilms, whereas in environments with high flow rates, substrate
Elastomeric force relaxation derives from the material’s macroscopic
surface molecular motion and roughness are also important factors. In
degradation in the form of tearing of the structural surface and bulk
addition, with long exposure periods, material properties such as
structure, presenting discontinuities because of sustained load. Micro­
porosity, sorption, corrosion, and biodegradation further modify
scopically, the extension of the molecular chains that, in some cases
biomaterial-host interactions. Finally, the material’s wettability is
fracture, leads the load to be exerted by fewer number of bearing units,
modulated by its critical surface tension and plays a significant role in
and as a result presenting higher deformation.
the development of adverse effects either on the tooth’s enamel (in the
Efforts to address this problem in the broader biomedical literature
form of white spot lesions) or periodontal inflammation.
have focused on increasing the crosslinking of chains or their length to
provide more area for load-distribution and longer chains. The problems
2.4.2. Available materials
arising from these approaches have to do with the increased initial
Most efforts in this field have concentrated around the alteration of
stiffness in the cases of fortifying the crosslinking (which would result in
the wetting characteristics of orthodontic materials such as brackets
increased exerted forces) and the entanglement in the scenario of
adhesives and elastomers. The problem with adhesives lies with the fact
incorporating much longer chains which cause the noodle effect–i.e. the
that a reduced wettability to disrupt the developed biofilm on the
increase in stiffness due to the perplexed structure and the entanglement
microbia-attracting adhesive margins, would also result in reduced
of chains.
wetting of the enamel etching-induced tags and projection of the resin in
Thus, ideally elastomers should maintain the applied load for at least
the enamel, thus adversely affecting the bond integrity of the bracket to
4–6 weeks, while they should be hydrophobic to reduce the attraction of
enamel.
species and be water tolerant.
Potential solutions for brackets or elastomers could be their
manufacturing with reduced wettability modelled by the lotus effect [8],
2.5.2. Available materials
which is based on the example of the synonymous plant that, through a
Elastomers used in industrial applications are structures with chains
dense network of hairs developed on its surface, allows the formation of
forming multi-dimensional networks [13,14]. As a result, these elasto­
a large contact angle, which is indicative of reduced wetting (Fig. 1).
mers have much higher toughness and strength, producing extension
The formation of superhydrophobic surfaces has been adopted in
ranges in the order of 50 times the original length (in general rubber
medicine [9], dentistry [10], and everyday applications ranging from
elasticity refers to a property of a material to be capable of being
utility material surfaces, dishes, handles, car surface coatings of cars to
extended 10 times its size).
decrease dirt, and fabric repellant surfaces, among others.
An alternative approach could be engaging the wire tying a silk fiber
In orthodontics, the fabrication of self-healing brackets [11] has
around the bracket wings, as silk has remarkable properties [15] and has
recently been produced synthetically [16], leading to a product stronger
than Kevlar and more elastic than nylon.

2.6. Biomimetic adhesives

2.6.1. Clinical challenge

Etching-mediated orthodontic bonding and consequently debonding
is basically an interventional procedure that irreversibly alters enamel
structure, roughness, and composition by introducing resin tags into the
enamel structure. These formations remain after debonding and
constitute a substrate where a series of effects such as decalcification and
color alteration take place. The latter might also be attributed to the
polymerization shrinkage that leads to detachment of resin-tags from
the enamel walls, thereby allowing the penetration of colorants from the
Fig. 1. A sign of reduced wettability. Note the high contact angle arising from
the formation of a high meniscus of liquid on the substrate. This wetting re­ oral environment or corrosion products of the welded mesh of the
duces the formation of biofilm, which is the first step for the adherence of bracket base, leaving characteristic black spots on the adhesive.
plaque and colonization of the surface by microbia. This model is pursued to be Along with glass-ionomer bonding that is associated with reduced
applied to brackets and elastomers to reduce plaque accumulation adhesive penetration depths, alternative methods have been developed
on appliances. in the biomedical literature for bonding of tissues and materials.

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2.6.2. Available materials

In the mid-2000′s, efforts to replicate the ability of some animals to
sustain their bodies against gravity were developed, using the lizard
gecko as a model (Fig. 2). Analysis of the gecko’s feet identified a dense
hair-network, which increased substantially the effective contact area of
the foot to the underlying surface providing a mechanism such as Velcro
materials [17]. These surfaces were used as a model to fabricate carbon
nanotube networks embedded into a polymer surface [18]. In addition
to this mechanism, and with the objective of facilitating bonding to wet
surfaces, the mussel model was adopted [19], as the latter have the
ability to bond chemically to a number of dissimilar surfaces ranging
from rock to wood or metal. The resulting adhesive synthesized was
Fig. 3. The size of the cutting section of the bur would not be significantly
patented as geckel [20], a biomedical product which can be applied as
larger than the vertical dimensions of the adhesive to prevent from adverse
substitute of deep layer stitching instead of sutural materials. effects on the free enamel surface during grinding.
These biomimetic adhesives could become a standard, introducing a
bonding method without irreversible effects on enamel and, ideally,
and therefore, the deleterious effect of large index differences is well
without the necessity of applying rotary instruments for the removal of
substantiated and should be avoided.
adhesive remnant.
Iron fillers have the advantage of being of the preferred size and,
most importantly, can be distributed within the bulk adhesive with a
2.7. Command-debond adhesives preferred orientation, which would vary among different layers and thus
provide optimum mechanical properties to specific loading directions
2.7.1. Clinical challenge [21]. Moreover, after completion of the adhesive’s intended service,
Debonding of orthodontic appliances and attachments involves alternating-polarity magnets can be used to force the iron fillers to move
several traumatic procedures, which include the use of rotary in­ inside the polymer structure and induce internal cracks. This internal
struments to remove the polymerized adhesive that was transformed induction of catastrophic failure as a method of facilitating composite
into a crystalline structure from its initially viscous pre-polymerized debonding can be also achieved with the use of thermally expandable
form. This exposes enamel to alteration risks in the form of groove fillers [22]. In this instance, heat can induce an increase in the volume of
formation from the rotating bur (Fig. 3), potentially affecting the fillers by 50–100 times, thus causing cracks and fragmentation of the
enamel’s color parameters and gloss. The aforementioned process in­ polymerized resin composite.
duces irreversible damage to enamel regardless of the etching-mediated In addition, research in this field has been directed towards using
process, i.e., the presence or resin tags after debonding and resin photooxidation-initiated, hydrolytic, or other forms of degradation to
removal. Therefore, introducing a method that enables atraumatic induce a reversal of the adhesive from the crystalline status to the
appliance removal constitutes an independent variable offering the viscous state [23]. However, this method is only available for industrial
possibility to minimize some of these enamel adverse effects. applications of adhesives, as this mode of debonding is non-compatible
with biomedical applications.
2.7.2. Available materials The use of thermal treatment on the adhesive could also be applied to
Polymeric adhesives have been experimentally synthesized for in­ achieve a reversal of the condition of the adhesive phase. Glass transi­
dustrial applications by adding iron fillers. Orthodontic adhesives usu­ tion temperature (Tg) in polymers indicates the temperature above
ally contain fillers in a ratio of 0.6–0.7 (% wt) in the form of silica which the polymer structure is transformed from the crystalline to the
particles or barium glasses, which possess a refractive index of 1.55 at viscous state. In relevant research, 80% of the Tg initiates this trans­
the wavelength of the photoinitiator. Evidence available on the field of formation to a viscous state. Thus, heating the adhesive could transform
resin composites suggests that maximum light scattering occurs at par­ it to a paste-like phase which could be removed without any rotary in­
ticle sizes equivalent to the half of the wavelength of the polymerization struments. The problem with this approach lies in the relatively high Tg
photoinitiator, which for camphorquinone systems is 468 nm– hence a of currently available adhesives, which range above 100–110 degrees
favorable filler size would be about 230 nm. However, the typical mi­ Celsius. Therefore, an 80%-Tg temperature could be well over 80 de­
croscopy picture of an adhesive includes a large size variation of filler grees, which could have adverse effects in the physiology of the pulp.
particles. The reflectance (r) of a composite material consisting of two From the opposite side, a recent approach with conventional adhe­
different phases of indices n1 and n2 is given by the equation: sives has demonstrated that freezing the bracket-adhesive complex with
the use of freezing probe, resulted in an alteration of the remnant ad­
r= (n1-n2)2/(n1+n2)2 hesive index showing reduced adhesive remaining onto enamel after
debonding (Fig. 4).

2.8. Concluding remarks on the application of already existing

technological advances on orthodontic materials and processes

The application of many advantageous material applications seems

to be impeded by:

a. lack of an overall knowledge on the level of scientific advances and

related armamentaria of materials / tools that could potentially be
used to optimize orthodontic materials / processes—as in the case of
self-healing polymeric brackets or tribological considerations of
metals;
Fig. 2. A photograph of the edge of the feet of the lizard gecko, which is able to
sustain its weight vertically . Note the dense network of hair, which increase the
contact area with the substrate and provide mechanical retention.

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used in orthodontics for manufacturing dental models, occlusal splints,

metallic appliances, direct printed aligners, brackets etc. Finally,
following 3D printing, other machine units are used to clean and
completely polymerize the 3D object.
In essence, a fully digital orthodontic office consists of software that
gathers and analyses all digital input taken by cone beam computed
tomography, an intraoral scanner, if possible, a face scanner, CAD
software, and 3D printers. Intraoral and face records are also input into
the software to create a “virtual patient” in contrast to the traditional
way, where all the records (study models, cephalograms, panoramic
radiographs, etc.) cannot be gathered and evaluated as a single patient
file [24].

2.10. Orthodontic appliance designing and fabrication

In order to be able to design and manufacture customized ortho­

dontic appliances we need to be able to 3D scan the oral cavity, design
Fig. 4. The post-deboding appearance of enamel with a remnant adhesive of the appliance in special CAD software, use a material (resin, slurry,
full thickness is an indication of a failed debonding. Ideally a portion of the metallic powder, etc.), use a 3D printer to fabricate the object, and
adhesive, both area- and thickness-wise) should be removed along with the finally proceed with the final steps of cleaning, curing, and debonding-
bracket on debonding with a preferred failure pattern being the adhesive sintering. Most of these procedures can be performed in the orthodontic
cohesive fracture of the composite. office, while printing metallic appliances (made by Cobalt-Chromium
[CoCr], stainless steel, or titanium) can only be performed in special
b. lack of interest, perhaps due to the projected reduced market share of laboratories.
orthodontic materials, as in the case of self-cleaning brackets, elas­
tomers, or aligners; and 2.10.1. Dental models and aligners
c. limitations arising from the biomedical character and necessity to Printing of dental models using special resins is the first digital
apply these materials in the human body, as in the case of photo­ procedure that was performed in orthodontics by Invisalign (Align
oxidation of composites to facilitate debonding. Technology, Santa Clara, Calif) at the beginning of our century, while up
to that point dental models were only made by pouring plaster into
2.9. 3D technology and orthodontics alginate or silicon impressions. The intraoral cavity was digitized using a
scanner or by scanning dental impressions and subsequently the 3D
During the last years, digital technology changed the way dentistry dental arches were imported in a CAD software where a setup was
and orthodontics is practiced, transforming many of its aspects from performed resulting in exporting multiple dental models. Those models
analogue to digital. It is not an exaggeration to say that digitization of were used to create aligners using the thermoforming procedure, which
the oral cavity using 3D scanners is the most important part of this is not a new technique, but was developed by Sheridan back in the early
evolution and is responsible for all digital changes that we are observing 90′s [26]. Nowadays, the same workflow can be performed in the or­
in the profession. The ability to digitally capture in detail the structures thodontic office where thermoformed aligners are manufactured on
of the oral cavity is the trigger that allows designing and manufacturing dental models that are printed following a setup in commercially
of customized appliances. On the other hand, a 3D object without the available CAD software. CAD software can be found online with the
proper tools to design, edit, and modify the 3D object would be useless. option of buying orthodontic cases (according to the number of dental
For this reason, computer-aided-designing (CAD) software is an essential models that are exported) or can be bought and installed in the ortho­
tool that can be used to virtually design the orthodontic appliances in a dontic office.
“tailor-made” manner. CAD software has existed for more than 40 years Aligners that would be directly printed without the intermediate step
for engineering, aerospace and other fields. In orthodontics, CAD soft­ of model-printing and thermoforming have always been in the mind of
ware started to appear at the beginning of our century mainly for clinicians and companies (Fig. 5). Nevertheless, the obstacle of creating
handling and creating dental models, or more recently, for the purpose a proper resin that would adequately create an active appliance could
of manufacturing thermoformed clear aligners from manipulated dental not be easily overcome. Essentially, an aligner is the only customized
models. In the course of time, and as technology was advancing, other orthodontic appliance that is printed and directly exerts force to move
software was developed for the designing of more complex appliances
such as maxillary expanders, lingual arches, molar-distalizers, etc. The
last years, and as new materials appeared in the market, CAD software
has been used to design directly printed aligners or even customized
brackets manufactured in the orthodontic office, creating a digital self-
sufficient environment.
The stage of bringing virtual appliances to real life is called un-
digitization and refers to the process of transforming a virtual 3D ob­
ject into a real world object [24]. 3D manufacturing is the actual term
pertaining to machines being used to perform this process [25] and is
divided into subtractive and additive manufacturing. Subtractive
manufacturing is the procedure where material is removed from a mass
(disc) to create 3D objects. This procedure is rarely used in orthodontics.
On the other hand, additive manufacturing is the process, where an Fig. 5. Aligners printed in a vertical orientation. Note the supports that are
object is developed by laying down successive layers of a material to needed for a successful printing which will be removed in the post print­
build up an object. This procedure is commonly called 3D printing and is ing procedure.

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teeth. Anything else is not an active, but rather a passive appliance, concluded that the ability to change the thickness of printed aligners and
which is not meant to move teeth of its own (occlusal splints, metallic the change in force and moments, could be used to optimize the pre­
appliances, customized brackets). For this reason, a directly printed scribed orthodontic movement while minimizing unwanted tooth
aligner is an appliance that needs to possess properties, which will movements. This opens new possibilities in the provision completely
enable adequate and efficient tooth movement in the desired direction. individualized mechanotherapy for each patient, according to the spe­
On one side, the material properties are limiting the efficacy of the cific needs of each treatment phase.
movement, if inadequate, and on the other side the multistep workflow, As can be shown from the abovementioned studies, each step of the
which is not consistent, creates a non-stable and repeatable environment 3D printed aligners workflow, if not performed correctly, could alter the
that might pose threats in the demand to have every time the same final result. On the other hand, different printers, software, or curing
aligner quality. Briefly, the workflow of printed aligner manufacturing units currently available in the market could have a different effect on
includes 3D scanning, importing of the scans in a CAD software, where the finally printed aligner outcome in terms of their mechanical prop­
setup and designing of the virtual aligners is performed, 3D printing erties, transparency, leaching, roughness, etc. For this reason, it is
using aligner resin, removing of the excess resin, and finally post essential to investigate what is the effect of using different printers on
printing curing of the aligners in order to give the aligner its final the final outcome. A recent study comparing the mechanical properties
properties. of directly printed aligners using five different 3D printers showed that
The last two years studies have been conducted to investigate many mechanical properties of 3D-printed orthodontic aligners are directly
aspects of directly printed aligners. One of the first available resins for dependent on the 3D printer used [36].
printed aligners that appeared in the market was made by Graphy As is well known, uncured resin (monomer/ oligomer) might exert
(Seoul, Korea), followed by 3Dresyns (Barcelona, Spain), Luxcreo possibly toxic and allergic side effects on human cells. It is possible that
(Luxmark, Belmont, USA) and Clear A (Senertek, Ismir, Turkey) . The incomplete post-printing cure can increase the chances of toxic or
first study concerning Graphy’s aligner resin investigated the properties allergic reactions to the patient and therefore, relevant studies on this
of printed aligners aged for one week of use in the mouth of patients issue should be carried out before such appliances see widespread use.
[27]. The results showed a non-significant decrease in all the mechanical Software for direct aligner printing resembles the ones for
properties of the aligners at the end of the week. Cytotoxicity and manufacturing of thermoformed aligners. The only difference is that the
estrogenicity are terms that are discussed in the community when it operator must design the virtual aligner on the 3D model that will be
comes to dental materials and units. In another study, no signs of later printed (Fig. 6). In addition, the operator must define the proper
cytotoxicity and estrogenicity were found in specimens of directly thickness and offset of the aligner, while in Deltaface (Coruo, Limoges,
printed aligners [28]. Leaching is similarly an important factor that can France) software there is an option, where the software detects the tooth
lead to problems with the appiance’s integrity and also to patient health movement prescribed in setup and adds more material on the opposite
hazards. In a recent study, urethane was detected in sets of printed side of the direction of tooth movement (from 0.1 mm to 0.9 mm ac­
aligners [29]—even though the impact of urethane to humans is not cording to the operator’s wish) (Fig. 6). However, the usefulness of this
precisely known. Nevertheless, since aligners are the only appliances feature in achieving more predictable tooth movement needs to be
that are renewed every week, possible leaching phenomena of any clinically proven. Artificial intelligence (AI) is another feature that can
substance will be kept in constantly high levels in the patient’s mouth, be incorporated in such software to facilitate faster designing workflow
thereby creating potential health hazards to the patient. Roughness is at the steps of teeth segmentation and setup. In addition, a central
another property that should not be overlooked, especially in appliances server, where all the data will be gathered from multiple offices, can be
that are made of polymers. A study comparing Invisalign aligners and the center where AI analyzes and gives feedback to the orthodontist for
directly printed aligners revealed higher roughness values after one future aligner orthodontic treatment. Thus, more accurate and predict­
week of wearing for the latter [30]. This could lead to easier aligner able results might be obtained with the help of big data analysis through
microfractures, loss of transparency, material leaching, and overall the use of AI.
deterioration of their mechanical properties. Nevertheless, the study
was done using the initial printed aligner manufacturing workflow and 2.10.2. Customized metallic appliances
this could have affected the final outcome. A new study with optimized CoCr is an alloy used for decades in dentistry for casting removable
protocols might have shown more favorable results for printed aligners. or fixed partial dentures. During the last decades, a novel 3D printing
Comparison of thermoformed and in-office directly printed aligners technology, called selective laser sintering, was introduced, allowing the
in terms of dimensional accuracy revealed higher accuracy for the latter, 3D printing of various metallic appliances in orthodontics. CoCr alloys
while there was also a 12% increase of the thickness and significant are used most of the time, while stainless steel, and titanium ones are
thickness decrease of thermoformed aligners. Due to the inconsistent
multistep procedure, which is prone to errors, a different workflow
configuration could present different results [31]. However, in another
study, printed aligners were found to apply a constant light force to the
teeth owing to their flexibility and viscoelastic properties [32]. The force
profile of printed aligners versus thermoformed was also studied from
another research team concluding that the forces delivered by printed
aligners in the vertical dimension were more consistent and of lower
magnitude compared to forces exerted from thermoformed ones [33].
Furthermore, a comparison of the mechanical properties between
printed and thermoformed aligners revealed a significant difference in
elastic modulus, ultimate tensile strength, and stress relaxation. In
addition, moisture of the simulated oral environment showed to have a
greater effect on the mechanical properties of the printed aligners
compared to the thermoformed ones, and this might affect printed
aligners’ ability to generate and maintain force levels appropriate for Fig. 6. Printed aligners are easily designed on the virtual model. Deltaface
tooth movement throughout their use [34]. software (Coruo, Limoges, France) offers the ability to increase the aligner
Increases in thickness of the directly printed aligners and their effect thickness on specific areas. This thickness increase is automatically added by
on tooth movement was investigated in a recent vitro study [35], which the software in places where movement of teeth is detected.

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also available but not used that often. Several companies have released customized orthodontic brackets. In both systems a 3D scanning is
in the market machines for metallic printing with different names and performed, which is then sent to the company to produce the customized
technology variations to manufacture metallic objects. However, their brackets. Such online CAD software enables the orthodontist to create a
big plethora and potentially hazardous materials used in the printing virtual setup, where customized brackets are designed, approved by the
procedure do not allow their installation in the average orthodontic orthodontist, and then printed by the company.
office. The development of this technology allowed the shift from the The evolution of 3D-technologies and competition between manu­
manual designing and manufacturing of orthodontic appliances to a facturers enabled the development of faster, more accurate, and cheaper
digital one. Both techniques, analogue and digital, possess advantages 3D printers. New materials were invented and introduced to the market,
and disadvantages which should be taken into consideration when with CAD software being an integral part in the appliances’ design. The
manufacturing orthodontic appliances. Designing orthodontic appli­ last years, many orthodontic offices have installed all the necessary units
ances can be performed using dedicated CAD software such as 3Shape for designing and 3D printing, thus creating small digital laboratories of
ortho system software (3Shape, Copenhagen, Denmark), OnyxCeph their own and thermoformed or directly printed aligners, occlusal
(Chemnitz, Germany), Deltaface (Coruo, Limoges, France), etc. For the splints, indirect bonding trays, and dental models are nowadays often
skilled designing personnel, professional engineering CAD software manufactured within the orthodontic office.
(Meshmixer, Blender, etc) can also be used to design appliances. How­ Nevertheless, orthodontic treatment is mainly based on fixed appli­
ever, the bigger disadvantage of CoCr alloys is the almost complete lack ances, the orthodontic brackets, which up to now could not be
of flexibility of the appliances. 3D-printed bands are rigid compared to adequately manufactured in the orthodontic office. 3D technology ad­
commercially-available bands and cannot pass the maximum circum­ vancements enable the orthodontic office to become a small lab that can
ference of the molars, thereby creating retention problems. Several or­ print customized orthodontic brackets. Novel software called Ubrackets
thodontic appliances can be designed and printed such as maxillary- (Coruo, Limoges, France) enables the orthodontist to perform a digital
expanders, lingual arches, and distalizers (Fig. 7). Nevertheless, many setup of imported dental scans and automatically design customized
times an orthodontic technician must manually add pre-fabricated parts orthodontic brackets together with their customized archwires [24]. The
such as expander-screws, springs, or other parts for molar distalization, workflow of manufacturing customized brackets can be divided into the
because these cannot be printed. The behavior of CoCr alloy was eval­ designing and the printing part. At the designing part the operator
uated in a study examining CoCr-based orthodontic appliances placed separates the teeth from the gingiva in a stage called segmentation to
for 6 months in the oral environment. The results showed that intraoral perform the setup of the dental arches. At the next step the orthodontist
ageing did not influence the mechanical properties of the appliances, but chooses to design labial or lingual customized brackets which will be
the appliance showed degradation in the breakdown potential of the later printed. Following that, the brackets are automatically positioned
protective oxide layer, which results in pitting corrosion. Thus, it is on a flat rectangular archwire opposite the teeth’s surfaces and with
possible that Cobalt may be released in the patient’s mouth [37] and this special manipulators the operator positions the brackets on the desired
might be potentially detrimental to the patient’s health. place creating customized brackets where their bases are adapted to the
tooth surfaces (Fig. 8). The next step is to design indirect bonding trays
2.10.3. Customized orthodontic brackets or positioning keys for each bracket which will help the orthodontist
After introduction of the original Edgewise appliance by E.H. Angle bond them in an accurate way, and which should be easily removed after
[38], the development of the straight-wire appliance [39] was in fact the bonding. Customized archwires can also be exported as 3D files for an
first attempt to create customized orthodontic appliance (even if it was archwire bending robot or in a electronic drawing for manual plier
tailored simply to the average patient). A real need for completely bending. Maybe the most important issue to be solved is the material
customized brackets appeared when lingual orthodontic appliances that will be used to create the customized brackets. In the orthodontist’s
were introduced, due to the unique nature of the lingual surfaces of the armamentarium several printers can be found together with specialized
teeth. The following years, customized lingual appliances played a big software to solve the problem of designing and printing. Nevertheless,
role in orthodontic treatment, offering adequately predictable results, the key to creating good-quality brackets for orthodontic treatment is
even for difficult cases. the material. Attempts have been made to print customized brackets
It wasn’t until a few years ago that Ormco (Orange, Calif, USA) using hybrid ceramic permanent crown resins (Fig. 9), while the first
created its own series of labial customized brackets, while LightForce study to compare two resins, normally used for temporary and perma­
(Burlington, Massach, USA) introduced its polycrystalline 3D-printed nent crown resins, to print brackets was published a few years ago [40].
The study concluded that there was no significant difference between

Fig. 7. 3D technology enables the orthodontist to design customized ortho­

dontic appliances such as the rapid palatal expansion with anterior hooks for
face mask treatment seen in the picture. The material used for 3D printing is Fig. 8. Ubrackets software (Coruo, Limoges, France) enable the designing and
mostly CoCr alloy. Nevertheless, not all parts can be printed (i.e. such as the printing customized brackets. The brackets are automatically placed on a flat
screw) needing the involvement of a dental technician to combine the screw rectangular virtual archwire opposite their corresponding teeth and can be
with the printed parts. moved in all directions using a digital manipulator.

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T. Eliades et al. Japanese Dental Science Review 59 (2023) 403–411

Fig. 9. The picture presents the first attempt to print customized brackets using
permanent hybrid ceramic crown resin. Positioning keys placed on the teeth
cusps and incisal edges were designed in Ubrackets in order to facilitate ac­
curate bonding.

the two resins in terms of mechanical properties. In addition, hardness, a

property that is very important for brackets, as it presents the resistance
to wear off, was relatively low (though almost double compared to
Fig. 10. Zirconia is a material used for dental crown manufacturing using
commercial plastic brackets available in the market). In another study,
milling technology. A zirconia 3D printer was used for the first time to print
3D printed zirconia brackets (Fig. 10) made in a ZiproD (AON, Seoul,
customized orthodontic brackets (with a 0.018 × 0.025–inch slot). The disad­
Korea) zirconia printer were compared to commonly-available Clarity
vantage of zirconia is its white color. Nevertheless, new translucent zirconia
(3 M, Monrovia, USA) and LightForce (Burlington, Massach, USA) material has been launched to counteract the current problem.
brackets [41]. The study revealed high hardness values for Clarity
brackets followed by LightForce and zirconia brackets. Despite that,
assistant in our efforts to create better smiles in a more efficient and
hardness was above the ideal value for brackets making them all suitable
predictable matter.
for orthodontic treatment. In the same study, zirconia exhibited the
higher fracture toughness values followed by Clarity and LightForce. A
disadvantage of zirconia brackets was the color, which is white, while a Funding
solution to that could be the use of special zirconia paintings that are
used to color the brackets the same color with the patient’s teeth. Lately, None.
AON (Seoul, Korea) released a translucent zirconia that can be used for
brackets printing. Declaration of Generative AI and AI-assisted technologies in the
Designing and printing brackets in the orthodontic office is definitely writing process
a big step towards creating digital, self-sufficient orthodontic offices. In
addition, it seems that the cost of customized brackets is much lower None.
compared to the commercial pre-fabricated ones, while the ability to
reprint brackets, in cases of accidental debonds, must not be under­
estimated. Nevertheless, the advantages of customized brackets are not Conflict of Interest
limited only to the above. Brackets can be designed in bigger mesiodistal
widths when great rotations must be corrected or can be smaller when TE and SNP have no conflict of interest to declare. NP discloses a
severe crowding does not allow the use of regular sized brackets. In financial interest with the company Coruo (Limoges, France) concerning
addition, in cases where increased torque must be used when retracting the orthodontic computer-aided design software UBrackets.
upper incisors, additional torque can be incorporated in the prescrip­
tion. Overcorrection can be included to the canine brackets to coun­ References
teract the tipping and rotational side effects movements when they are
[1] Lim KF, Lew KK, Toh SL. Bending stiffness of two aesthetic orthodontic archwires:
moved distally into extraction spaces. In the case of customized an in vitro comparative study. Clin Mater 1994;16(2):63–71. https://doi.org/
brackets, no single one prescription exists that will optimally move the 10.1016/0267-6605(94)90099-x.
teeth in a specific way. On the contrary, the operator must create a [2] Lendlein A, Jiang H, Jünger O, Langer R. Light-induced shape-memory polymers.
Nature 2005;434(7035):879–82. https://doi.org/10.1038/nature03496.
unique prescription for each specific patient following existing di­
[3] Small Iv W, Wilson T, Benett W, Loge J, Maitland D. Laser-activated shape memory
agnostics and treatment-plan in a tailor-made fashion. The orthodontist polymer intravascular thrombectomy device. Opt Express 2005;13(20):8204–13.
can choose from a big amount of brackets variation to fulfill the or­ https://doi.org/10.1364/opex.13.008204.
[4] Greiner C, Schäfer M. Bio-inspired scale-like surface textures and their tribological
thodontic treatment in an efficient and predictable way.
properties. Bioinspir Biomim 2015;10(4):044001. https://doi.org/10.1088/1748-
Lastly, AI is already a reality in our lives and penetrates more and 3190/10/4/044001.
more medicine, dentistry, and orthodontics. In the case of customized [5] Eliades T, Viazis AD, Lekka M. Failure mode analysis of ceramic brackets bonded to
brackets, AI can play the role of decreasing the designing time through enamel. Am J Orthod Dentofac Orthop 1993;104(1):21–6. https://doi.org/
10.1016/S0889-5406(08)80120-5.
automation of steps such as teeth segmentation, setup and most [6] Farle A, Boatemaa L, Shen L, Gövert S, Kok JBW, Bosch M, Yoshioka S, van der
important customized brackets designing and positioning [42,43]. A Zwaag S, Sloof WG. Demonstrating the self-healing behaviour of some selected
centralized server gathering all data from different offices could use AI ceramics under combustion chamber conditions. Smart Mater Struct 2016;25:
084019. https://doi.org/10.1088/0964-1726/25/8/084019.
analysis to provide feedback for the creation of optimized appliances. AI [7] Punckt C, Jan L, Jiang P, Frewen TA, Saville DA, Kevrekidis IG, Aksay IA.
can be a reliable assistant to our orthodontic treatment, assisting in Autonomous colloidal crystallization in a galvanic microreactor. J Appl Phys 2012;
digital setups, tooth segmentation, predicting teeth movement and 112:074905. https://doi.org/10.1063/1.4755807.
[8] Marmur A. The lotus effect: superhydrophobicity and metastability. Langmuir
growth, tracing cephalograms, etc. Nevertheless, 3D technologies and AI 2004;20(9):3517–9. https://doi.org/10.1021/la036369u.
will never substitute the orthodontist but might prove a valuable [9] Ciasca G, Papi M, Businaro L, Campi G, Ortolani M, Palmieri V, Cedola A, De
Ninno A, Gerardino A, Maulucci G. Recent advances in superhydrophobic surfaces

410
T. Eliades et al. Japanese Dental Science Review 59 (2023) 403–411

and their relevance to biology and medicine. Bioinspir Biomim 2016;11:011001. [28] Pratsinis H, Papageorgiou SN, Panayi N, Iliadi A, Eliades T, Kletsas D. Cytotoxicity
https://doi.org/10.1088/1748-3190/11/1/011001. and estrogenicity of a novel 3-dimensional printed orthodontic aligner. Am J
[10] Yin J, Mei ML, Li Q, Xia R, Zhang Z, Chu CH. Self-cleaning and antibiofouling Orthod Dentofac Orthop 2022;162(3):e116–22. https://doi.org/10.1186/10.1016/
enamel surface by slippery liquid-infused technique. Sci Rep 2016;6:25924. j.ajodo.2022.06.014.
https://doi.org/10.1038/srep25924. [29] Willi A, Patcas R, Zervou SK, Panayi N, Schätzle M, Eliades G, Hiskia A, Eliades T.
[11] Sato Y, Miyazawa K, Sato N, Nakano K, Takei Y, Kawai T, Goto S. Study on Leaching from a 3D-printed aligner resin. Eur J Orthod 2023;45(3):244–9. https://
fabrication of orthodontic brackets with the photocatalytic function of titanium doi.org/10.1093/ejo/cjac056.
dioxide. Dent Mater J 2009;28(4):388–95. https://doi.org/10.4012/dmj.28.388. [30] Koletsi D, Panayi N, Laspos C, Athanasiou AE, Zinelis S, Eliades T. In vivo aging-
[12] Baransi K, Dubowski Y, Sabbah I. Synergetic effect between photocatalytic induced surface roughness alterations of Invisalign® and 3D-printed aligners.
degradation and adsorption processes on the removal of phenolic compounds from 14653125221145948 J Orthod 2022. https://doi.org/10.1177/
olive mill wastewater. Water Res 2012;46(3):789–98. https://doi.org/10.1016/j. 14653125221145948.
watres.2011.11.049. [31] Koenig N, Choi JY, McCray J, Hayes A, Schneider P, Kim KB. Comparison of
[13] Ducrot E, Chen Y, Bulters M, Sijbesma RP, Creton C. Toughening elastomers with dimensional accuracy between direct-printed and thermoformed aligners. Korean J
sacrificial bonds and watching them break. Science 2014;344(6180):186–9. Orthod 2022;52(4):249–57. https://doi.org/10.4041/kjod21.269.
https://doi.org/10.1126/science.1248494. [32] Lee SY, Kim H, Kim HJ, Chung CJ, Choi YJ, Kim SJ, Cha JY. Thermo-mechanical
[14] Goff J, Sulaiman S, Arkles B, Lewicki JP. Soft materials with recoverable shape properties of 3D printed photocurable shape memory resin for clear aligners. Sci
factors from extreme distortion states. Adv Mater 2016;28(12):2393–8. https:// Rep 2022;12(1):6246. https://doi.org/10.1038/s41598-022-09831-4.
doi.org/10.1002/adma.201503320. [33] Hertan E, McCray J, Bankhead B, Kim KB. Force profile assessment of direct-
[15] Griffiths JR, Salanitri VR. The strength of spider silk. J Mater Sci 1980;15(2): printed aligners versus thermoformed aligners and the effects of non-engaged
491–6. https://doi.org/10.1007/BF02396800. surface patterns. Prog Orthod 2022;23(1):49. https://doi.org/10.1186/s40510-
[16] Copeland CG, Bell BE, Christensen CD, Lewis RV. Development of a process for the 022-00443-2.
spinning of synthetic spider silk. ACS Biomater Sci Eng 2015;1(7):577–84. https:// [34] Shirey N, Mendonca G, Groth C, Kim-Berman H. Comparison of mechanical
doi.org/10.1021/acsbiomaterials.5b00092. properties of 3-dimensional printed and thermoformed orthodontic aligners. Am J
[17] Berengueres J, Saito S, Tadakuma K. Structural properties of a scaled gecko foot- Orthod Dentofac Orthop 2023;163(5):720–8. https://doi.org/10.1016/j.
hair. Bioinspir Biomim 2007;2(1):1–8. https://doi.org/10.1088/1748-3182/2/1/ ajodo.2022.12.008.
001. [35] Grant J, Foley P, Bankhead B, Miranda G, Adel SM, Kim KB. Forces and moments
[18] Yurdumakan B, Raravikar NR, Ajayan PM, Dhinojwala A. Synthetic gecko foot- generated by 3D direct printed clear aligners of varying labial and lingual
hairs from multiwalled carbon nanotubes. Chem Commun (Camb) 2005;30: thicknesses during lingual movement of maxillary central incisor: an in vitro study.
3799–801. https://doi.org/10.1039/b506047h. Prog Orthod 2023;24(1):23. https://doi.org/10.1186/s40510-023-00475-2.
[19] Cristescu R, Mihailescu IN, Stamatin I, Doraiswamy A, Narayan RJ, Westwood G, [36] Zinelis S, Panayi N, Polychronis G, Papageorgiou SN, Eliades T. Comparative
Wilker JJ, Stafslien S, Chisholm B, Chrisey DB. Thin films of polymer mimics of analysis of mechanical properties of orthodontic aligners produced by different
cross-linking mussel adhesive proteins deposited by matrix assisted pulsed laser contemporary 3D printers. Orthod Craniofac Res 2022;25(3):336–41. https://doi.
evaporation. Appl Surf Sci 2009;255(10):5496–8. https://doi.org/10.1016/j. org/10.1111/ocr.12537.
apsusc.2008.11.012. [37] Zinelis S, Polychronis G, Papadopoulos F, Kokkinos C, Economou A, Panayi N,
[20] Lee H, Lee BP, Messersmith PB. A reversible wet/dry adhesive inspired by mussels Papageorgiou SN, Eliades T. Mechanical and electrochemical characterization of
and geckos. Nature 2007;448(7151):338–41. https://doi.org/10.1038/ 3D printed orthodontic metallic appliances after in vivo ageing. Dent Mater 2022;
nature05968. 38(11):1721–7. https://doi.org/10.1016/j.dental.2022.09.002.
[21] Erb RM, Libanori R, Rothfuchs N, Studart AR. Composites reinforced in three [38] Angle EH. The latest and best in orthodontic mechanism. Dent Cosm 1929;71:
dimensions by using low magnetic fields. Science 2012;335(6065):199–204. 164–74.
https://doi.org/10.1126/science.1210822. [39] Andrews LF. The Straight-Wire Appliance: Syllabus of Philosophy and Techniques.
[22] Banea MD, da Silva LFM, Campilho RDSG, Sato C. Smart adhesive joints: an Rev. ed. San Diego: University of Michigan publications,; 1975.
overview of recent developments. J Adhes 2014;90(1):16–40. https://doi.org/ [40] Papageorgiou SN, Polychronis G, Panayi N, Zinelis S, Eliades T. New aesthetic in-
10.1080/00218464.2013.785916. house 3D-printed brackets: proof of concept and fundamental mechanical
[23] Celina M. Review of polymer oxidation and its relationship with materials properties. Prog Orthod 2022;23(1):6. https://doi.org/10.1186/s40510-022-
performance and lifetime prediction. Polym Degrad Stabil 2013;98(12):2419–29. 00400-z.
https://doi.org/10.1016/j.polymdegradstab.2013.06.024. [41] Polychronis G, Papageorgiou SN, Riollo CS, Panayi N, Zinelis S, Eliades T. Fracture
[24] Panayi NC. DIY Orthodontics: Design it Yourself. Chicago: Quintessence toughness and hardness of in-office, 3D-printed ceramic brackets. Orthod Craniofac
Publishing; 2021. Res 2023;26(3):476–80. https://doi.org/10.1111/ocr.12632.
[25] Shahrubudin N, Lee TC, Ramlan R. An overview on 3D printing technology: [42] Hao J, Liao W, Zhang YL, Peng J, Zhao Z, Chen Z, Zhou BW, Feng Y, Fang B, Liu ZZ,
technological, materials, and applications. Procedia Manuf 2019;35:1286–96. Zhao ZH. Toward clinically applicable 3-dimensional tooth segmentation via deep
https://doi.org/10.1016/j.promfg.2019.06.089. learning. J Dent Res 2022;101(3):304–11. https://doi.org/10.1177/
[26] Sheridan JJ, LeDoux W, McMinn R. Essix retainers: fabrication and supervision for 00220345211040459.
permanent retention. J Clin Orthod 1993;27(1):37–45. [43] Schneider L, Arsiwala-Scheppach L, Krois J, Meyer-Lueckel H, Bressem KK,
[27] Can E, Panayi N, Polychronis G, Papageorgiou SN, Zinelis S, Eliades G, Eliades T. Niehues SM, Schwendicke F. Benchmarking deep learning models for tooth
In-house 3D-printed aligners: effect of in vivo ageing on mechanical properties. Eur structure segmentation. J Dent Res 2022;101(11):1343–9. https://doi.org/
J Orthod 2022;44(1):51–5. https://doi.org/10.1093/ejo/cjab022. 10.1177/00220345221100169.

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