Content uploaded by Ioannis A. Tsolakis
Author content
All content in this area was uploaded by Ioannis A. Tsolakis on Apr 03, 2023
Content may be subject to copyright.
Citation: Tsoukala, E.; Lyros, I.;
Tsolakis, A.I.; Maroulakos, M.P.;
Tsolakis, I.A. Direct 3D-Printed
Orthodontic Retainers. A Systematic
Review. Children 2023,10, 676.
https://doi.org/10.3390/
children10040676
Academic Editor: Maria Grazia
Cagetti
Received: 1 March 2023
Revised: 30 March 2023
Accepted: 31 March 2023
Published: 3 April 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
children
Review
Direct 3D-Printed Orthodontic Retainers. A Systematic Review
Efthimia Tsoukala 1,*, Ioannis Lyros 1, Apostolos I. Tsolakis 1,2 , Michael P. Maroulakos 1
and Ioannis A. Tsolakis 2,3
1Department of Orthodontics, National and Kapodistrian University of Athens, School of Dentistry,
11527 Athens, Greece
2Department of Orthodontics, School of Dentistry, Case Western Reserve University, Cleveland, OH 44106, USA
3
Department of Orthodontics, School of Dentistry, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
*Correspondence: efi.ts.78@gmail.com
Abstract:
Three-dimensional (3D) printing technology has shed light on many fields in medicine
and dentistry, including orthodontics. Direct 3D-printed prosthetics, implants or surgical devices are
well-documented. The fabrication of orthodontic retainers using CAD technology and additive manu-
facturing is an emerging trend but the available data are scarce. The research approach of the present
review included keywords in Medline, Scopus, Cochrane Library and Google Scholar up to December
2022. The searching process concluded with five studies eligible for our project. Three of them
investigated directly 3D-printed clear retainers
in vitro
. The other two studies investigated directly
3D-printed fixed retainers. Among them, one study was
in vitro
and the second was a prospective
clinical trial. Directly 3D-printed retainers can be evolved over time as a good alternative to all the
conventional materials for retention. Devices that are 3D-printed are more time and cost efficient, offer
more comfortable procedures for both practitioners and patients and the materials used in additive
manufacturing can solve aesthetic problems, periodontal issues or problems with the interference of
these materials with magnetic resonance imaging (MRI). More well-designed prospective clinical
trials are necessary for more evaluable results.
Keywords:
additive manufacturing; 3D direct-printed retainers; 3D orthodontic retainers;
3D-printed retainers
1. Introduction
The term orthodontic retention refers to the means used by the clinician to prevent
tooth relapse from the optimal aesthetic and functional condition after the end of the
orthodontic treatment. In fact, retention is part of the orthodontic treatment [
1
–
3
]. The teeth
have a tendency to return to their initial situation. Age changes in soft or skeletal tissues,
occlusal limitations, pressure from cheeks, lips or tongue or tension from the periodontal
fibers can lead to some degree of relapse [
1
,
3
–
6
]. For this reason, a proper retention protocol
should be applied to every patient. In particular, the retention phase is more imperative
for young patients, who have remaining growth potential even after the end of the active
orthodontic treatment.
The most frequently used devices for retention can be divided in two categories. The
first category includes removable retainers. These can be made from acrylic plate and
stainless-steel wire work such as the broadly known Hawley-type appliance (HR) or Begg
or Barrer-type, which is more common among children, or from transparent thermoplastic
material, the vacuum-formed retainers (VFR) [
7
]. The second category are the fixed retainers
bonded at the lingual surfaces of the upper or lower anterior teeth [
1
,
3
,
7
,
8
]. These are made
up of plain or multistranded stainless-steel wires, nickelium/titanium wires or fiberglass
in different thicknesses and designs. Although aesthetic and compatible with allergies or
MRI artifacts, fiberglass splints have deteriorated mechanical properties in contrast to their
metal analogs [8–16].
Children 2023,10, 676. https://doi.org/10.3390/children10040676 https://www.mdpi.com/journal/children
Children 2023,10, 676 2 of 13
However, as dentistry is undergoing a technological revolution and is moving to
a digital era, orthodontic retention should also be considered in this way [17,18].
In 1980, Hideo Kodama of Nagoya Municipal Industrial Research Institute in Japan
invented 3D printing. Three years later, Charles (Chuck) Hull invented stereolithography
(SLA), and in 1987 he also introduced the first 3D printer, the SLA-1
®
, to the market through
his own company “3D Systems, Inc. (Rock Hill, SC, USA)” [
17
,
18
]. Nowadays, the first
step into this digital workflow is the acquisition of data from intraoral scanners or cone
beam computed tomography (CBCT). These data are then turned to standard tessellation
language (STL) files and uploaded in a software program for 3D processing and designing of
the device of choice. The next step is the uploading of the digital file on the 3D-printer digital
platform. The most common types of printers used in dentistry are stereolithography (SLA),
which utilize beams of UV light for the curing of photopolymers, direct light processing
(DLP), which uses a light projector that projects slides of the 3D object to cure the resin,
and material jetting (MJ), a new promising printing method. Printed materials vary among
ceramic, metal or thermoplastic resin. The last step is post-manufacturing modification to
ensure the proper construction of the product [17,19].
Three dimensional (3D)-printed dental appliances directly from a digital file, such as
3D-printed surgical splints, implant guides and indirect bonding transfer trays or biomedi-
cal and pedodontics appliances have been reported [
17
,
20
–
23
]. CAD technology and 3D
printing have also introduced new tools and materials for the direct 3D-printing fabri-
cation of either of the two types of retainers [
24
]. Intraoral scanners produce 3D digital
models that so far have many diagnostic and treatment planning applications [
21
,
22
,
25
,
26
].
The 3D-printed models can also serve as plaster analogues for the thermoforming proce-
dure of plastic retainers; however, directly 3D-printed clear or fixed retainers are not well
established in clinical practice.
Some clinical reports have been made [27–29]. The first attempt for direct fabrication
of a 3D-printed clear retainer was mentioned in 2014 by Nasef et al. from a computed
tomography (CBCT) scan with satisfying results [
30
]. Moreover, a milling method from
PEEK material was used by Zachrisson in 2018 to digitally fabricate a new type of fixed
retainer [
24
]. Nowadays, retainers made of PEEK can be fabricated either with milling or
printing technology [
24
,
29
]. PEEK material (poly-ether-ether-ketone) seems promising due
to its biocompatibility, which could address issues such as allergies and MRI artifacts or
nickel–titanium (NiTi) archwires covered in PEEK could also address esthetic concerns.
However, more research is necessary to investigate other aspects such as the mechanical
properties or the cytotoxicity of this polymer [31].
The interest towards direct 3D-printed fixed or clear retainers can be explained, as the
new technologies in dentistry allow in-house fabrication capabilities. In-house scanners,
3D printers at low cost and free software that designs different kinds of orthodontic
appliances are now available. This technological evolution has given the capability to the
clinician of delivering to the patient their personalized retainer on the same day as the
removal of the orthodontic appliances [
25
,
32
]. Marsh et al. have reported the use of the
in-house virtual bracket removal (VBR) technique, a software that digitally removes the
braces from the intraoral scan to produce a 3D-printed model for fabrication of the clear
thermoformed retainers that facilitates same-day delivery [
32
]. It is important to mention
that children often lose their retainers or they destroy them. As a result, they have to
revisit the orthodontist to create new retainers. Three-dimensional technology allows the
dentist to have an electronic storage of the final casts and to easily print the retainers for the
patients. This way patients do not need multiple appointments to receive their retainers.
Therefore, the step forward in this technological breakthrough is to virtually design
the retainers, fixed or clear, on the 3D digital models and directly print the retainers from
the 3D printer, in this way eliminating the extra step of printing the 3D dental models and
fabricating the retainers on them secondarily.
In this systematic review, the aim was to gather all the available data in the litera-
ture concerning directly 3D-printed retainers. However, a meta-analysis was not feasible
Children 2023,10, 676 3 of 13
because the clinical and experimental available data were not comparable due to the
heterogeneous study design.
2. Materials and Methods
Protocol and registration
This study is designed and conducted according to the Preferred Reporting Items
for Systematic Reviews and Meta-Analysis (PRISMA) Statement [
33
]. The protocol for
this systematic review was registered on the Open Science Forum Database (Protocol:
10.17605/OSF.IO/6W2KU).
Search Strategy
Four Databases (Medline (via Pubmed), Scopus, Cochrane Library and Google Scholar)
were thoroughly searched by two authors (ET and IT) for studies up to December 2022.
There was a language limitation for English studies only. The key words used in the
search strategy were «3D-printed retainers», «three-dimensional orthodontic retainers»,
«3D direct-printed retainers» and «additive manufacturing» (Table 1).
Table 1. Electronic databases searched, the search terms used and the results for each database.
Database Searched Search Terms Results
Medline (via Pubmed)
3D-printed OR three-dimensional-printed orthodontic retainers OR direct-printed retainers.
13
Scopus 3D-printed orthodontic retainers. 126
Cochrane Library 3D-printed orthodontic retainers. 3
Google scholar 3D-printed orthodontic retainers. 2490
Eligibility Criteria
The eligibility criteria were applied in accordance with the PICO research approach,
as follows:
•Population/Patients: post-treatment human dental arches or their digital models.
•Intervention: 3D-printed orthodontic removable or fixed retainers.
•Comparison: conventional removable or fixed Retainers or no comparison.
•
Outcome: successful fitting and efficiency (retention, oral health and longevity) of
the device.
Study Selection and Data Extraction.
Inclusion criteria were prospective or retrospective clinical trials, observational studies
and
in vitro
studies. Exclusion criteria were reviews, authors’ opinions, thesis articles, case
reports and case series. Duplicate studies were eliminated after all the retrieved studies
were incorporated into the Mendeley reference management system (Elsevier, 2021). Two
authors chose the studies separately and twice (ET, IL). Any discrepancies were clarified by
conversation with a third author (IT). The authors, institutions or research conclusions of
the studies were not kept a secret from the two authors. Following the reading of abstracts
and the elimination of non-eligible papers, possibly pertinent studies were identified by
title. Following this step, a manual search of the eligible studies’ references was conducted
to uncover any undiscovered new papers. After thoroughly reading the articles, a decision
was taken in accordance with the prerequisites.
Initially, 2632 articles were retrieved. After duplicate removal and title and abstract
screening by the two authors, 14 articles remained for full text evaluation. Finally, 5 articles
were included in the analysis (Figure 1).
Children 2023,10, 676 4 of 13
Children 2023, 10, x FOR PEER REVIEW 4 of 13
Figure 1. Prisma flow chart selection of records.
3. Results
Out of the five studies included, three evaluated the characteristics of direct 3D-
printed clear retainers and all three were in vitro studies [21,22,34]. The other two evalu-
ated the characteristics of direct 3D-printed fixed retainers. The study of Firlej et al. [24]
was an in vitro study and the one performed by Shah et al. [35] was an in vivo randomized
prospective clinical trial.
Risk of bias assessment
To assess the risk of bias, the Quin tool for the in vitro studies [36] and the RoB 2.0
tool [37] for the prospective randomized clinical trial according to the Cochrane guide-
lines, were applied. The risk of bias assessment was independently performed by two re-
view authors (ET and IL). Any disagreement was resolved by a third author (IAT).
Figure 1. Prisma flow chart selection of records.
3. Results
Out of the five studies included, three evaluated the characteristics of direct 3D-printed
clear retainers and all three were
in vitro
studies [
21
,
22
,
34
]. The other two evaluated the
characteristics of direct 3D-printed fixed retainers. The study of Firlej et al. [
24
] was
an
in vitro
study and the one performed by Shah et al. [
35
] was an
in vivo
randomized
prospective clinical trial.
Risk of bias assessment
To assess the risk of bias, the Quin tool for the
in vitro
studies [
36
] and the RoB 2.0
tool [
37
] for the prospective randomized clinical trial according to the Cochrane guidelines,
Children 2023,10, 676 5 of 13
were applied. The risk of bias assessment was independently performed by two review
authors (ET and IL). Any disagreement was resolved by a third author (IAT).
Table 2displays the analytical results of the quality assessment of the risk of bias
with the Quin tool and Table 3shows the results of the quality assessment with the RoB
2.0 tool [
37
]. All of the studies were characterized by a high risk of bias. The
in vitro
studies exhibited a high risk of bias mainly considering the detailed explanation of sample
size calculation, detailed explanation of sampling technique, details of the comparison
group, operator details, randomization, blinding and presentation of the results. The
in vivo
clinical trial was also characterized by a high risk of bias, considering mainly the
randomization process, the missing data and the selection of the reported result.
Table 2.
Risk of bias assessment for
in vitro
studies using the Quin tool. 1: Clearly stated aims/objects;
2: Detailed explanation of sample size calculation; 3: Detailed explanation of sampling technique;
4: Details of comparison group; 5: Detailed explanation of the methodology; 6: Operator details;
7: Randomization; 8: Method of measurement outcome; 9: Outcome assessor Details; 10: Blinding;
11: Statistical analysis; and 12: Presentation of results; H: High; L: Low; U: Unclear.
Signaling Questions
Study 1 2 3 4 5 6 7 8 9 10 11 12 Summary
Cole et al., 2019 [38] L H H H L L U L L H L L H
Naeem et al., 2022 [39] L L H H L H L U L H L L H
Williams et al., 2022 [40] L L L H L H L U L H L U H
Firlej et al., 2022 [24] L H H H H H H U U H H H H
Table 3. Risk of bias assessment for the in vivo study using the RoB-2.0 tool.
Study
Bias Arising
from the Ran-
domization
Process.
Bias Due to
Deviations from
the Intended
Interventions.
Bias Due to
Missing
Data.
Bias in
Measurement
of the
Outcome.
Bias in
Selection of
the Reported
Result.
Overall
Risk
Shah et al., 2022 [35] High. Some concerns. High. Low. Some concerns. High.
Study Characteristics
3D-Printed Clear Retainers
The 3D-printed clear retainers were assessed regarding fitting or accuracy, trueness
and precision. Before the review of the studies, it is good to clarify the terms accuracy,
trueness and precision [41].
Accuracy: The closeness of agreement between a quantity value obtained by measure-
ment and the true value of the measurement.
Precision: The closeness of agreement between the independent test results obtained
under stipulated conditions.
Trueness: The closeness of agreement between the average value obtained from a large
series of test results and an accepted reference value.
In other words, high accuracy is a combination of high precision and high trueness.
Cole et al. [
21
] compared the accuracies of nine 3D-printed retainers with equal number
of both traditional vacuum-formed retainers (TVF) and commercially obtainable vacuum-
formed retainers (CVF) fabricated by Invisalign (Align Technology, San Jose, CA, USA) (29
in total). To determine the fit of the retainers, reference points were set at the 3D models.
With the help of engineering software, superimposing of the digital images of the printed
retainers and their original models was used to calculate the differences in these marked
points. The results showed that TVF retainers had the most accurate fit, with the least
difference for almost all the reference points measured, followed by CVF retainers. The
largest variations from the reference models were measured for the 3D-printed retainer
Children 2023,10, 676 6 of 13
group. However, this last group showed the least difference between the reference points
and 3D models at the incisal edges and cusp tips. The «weak points» of the 3D-printed
retainers were the smooth surfaces of all teeth. However, all of the measurements for all
three groups were below 0.50 mm, which is an «accuracy threshold» of a digital articulation,
according to the study.
Naeem et al. [
22
] compared the accuracies of fifteen 3D-printed clear retainers fabri-
cated by four different commonly used 3D printers: SLA (stereolithography), digital light
processing (DLP), continuous light processing (cDLP) and polyjet photopolymer (PPP)
printers. Six reference points and intercanine (ICW) and intermolar width (IMW) were
compared after digital superimposing of the 3D digital models and the digital data of the
printed retainers. The «accuracy threshold» according to this study was set at 0.25 mm.
According to the results, all four kinds of 3D-printed retainers were below this tolerance
level of accuracy. More precisely, the PPP printed retainer had the least deviations from the
reference models at the region of the incisors, the DLP group at the region of the canines
and the cDLP and SLA groups at the region of the molars. Regarding the ICW and IMW,
the PPP printers resulted in the most accurate replication of width, followed by the SLA
printers. A greater width discrepancy was observed for the DLP and cDLP printer groups.
The lower inter-arch distortion of the PPP printers may be attributed to the horizontal
orientation of printing in this group, as opposed to the other three groups of printers that
printed with an angulation, or the smallest printing height of the PPP printers. Estimated
from all aspects, his study concluded that PPP and SLA printers were the most accurate
and DLP and cDLP were the most precise for printing retainers.
Williams et al. [
34
] studied the accuracies of sixty different 3D-printed clear retainers,
printed at various angulations of 15
◦
, 30
◦
, 45
◦
, 60
◦
and 90
◦
. All were printed from an SLA
printer and used clear resin for the fabrication (Clear, Formlabs Inc., Somerville, MA,
USA). Again, like Naeem et al., eight specific landmarks were set as reference points
for digital superimposing and a 0.25 mm accuracy threshold was set as a measure of
clinically acceptable accuracy. Their results showed that there is not a unique optimal
print angulation. Overall, all print angulations were found to fabricate retainers within
an accuracy threshold (from 0.074 to 0.225 mm) at the cusp tips and the incisal edges.
However, smooth facial surfaces exhibited differences up to 0.480 mm and were not
appraised as clinically agreeable. The most time-effective printing (1 h and 30 min) was
observed when fabricating one retainer at 15◦angulation. Printing at 60◦and 90◦took up
to 2 h and 15 min. The most cost-effective printing was that at 45
◦
angulation with 5.20 mL
of resin required per retainer, whereas printing at 30
◦
resulted in the most amount of resin
required overall.
3D-Printed Fixed Retainers
Firlej et al. [
24
] examined the mechanical properties of 3D-printed fixed retainers.
The retainers were digitally designed and printed in predetermined dimensions of 3 mm
width, 30 mm length and in three different thicknesses of 0.8 mm, 1 mm and 1.2 mm. The
resin used was Nextdent MFH C&B N1, a commonly used resin for printing restorations
and bridges. The printer was a Phrozen MINI4k printer. The subject of the study was
to examine whether the thickness of the retainers affected their strength under loaded
conditions. Aging in artificial saliva at 37
±◦
C was performed too. Flexural strength, elastic
properties, deflection and creep were evaluated. It was found that the thickness of the
material plays a crucial role in the mechanical properties of the retainer. All of the properties
examined (flexural modulus, deflection, strength and creep) were superior for the 1.2 mm
retainer sample. Surprisingly, the 1.0 mm sample had the inferior properties. This indicates
that the mechanical properties and the width of the 3D printing resin are not directly
proportional. After the aging process, it was also assumed that under simulated loading
and humidity oral environment, the properties of resin-based materials are deteriorated
and only the 1.2 mm sample retains its properties for a longer period of time. These first
results concerning the mechanical characteristics of 3D-printed fixed retainers, revealed
that they can satisfyingly replace metal retainers. It is also easy, cheap and fast to reproduce
Children 2023,10, 676 7 of 13
new ones in case of the breakage of the previous ones. However, this was an
in vitro
study
and additional long-term clinical trials should be conducted for further evaluation.
Shah et al. [35] compared usual and customized 3D-printed fixed lingual retainers in
terms of periodontal health, amount of relapse and failure. His study was a prospective
randomized clinical trial. He randomly divided a group of 34 patients into two. The
first group received a conventional multistranded braided stainless-steel fixed orthodontic
lingual retainer; however, there was no reference to the company name of the product. The
second group received a customized 3D-printed lingual retainer, with no reference to the
3D printer or resin used. The retainers were bonded at time T0, and the patients of each
group were recalled after 3 months (time T1) and 6 months (time T2) of bonding for the
evaluation of plaque index (PI), gingival index (GI), bleeding on probing (BoP), microbial
load and P. Gingivalis estimation. The estimation of the amount of relapse was recorded via
Little’s irregularity index (LII) using a digital Vernier caliper. The estimation of incidence
of failure was recorded for wire breakage or bond failure at time T1 and T2 or whenever
they occurred. The results showed that there were significant differences between the two
types of retainers in plaque index, gingival index and bleeding on probing. The 3D-printed
retainer was superior to the conventional one relating to periodontal health. However,
neither of the two cultures collected from the conventional retainer group or the 3D-printed
group were detected to contain P. gingivalis, indicating no chronic periodontal disease.
A greater irregularity index (LII) was found for the group of conventional fixed retainers
at time interval of 6 months. No wire breakage was reported for any of the two groups
during the study. Bond failures occurred for both types of retainers, and between the two,
more failures were reported for the conventional fixed retainer group. Therefore, for all
aspects studied, the 3D-printed fixed retainers proved to be better.
4. Discussion
Removable retainers, either VFRs or HRs, provide better oral hygiene opportunities
to the patient [
42
–
44
]; however, the literature so far contends that the difference between
fixed and removable retainers in influencing the long-term development of periodontitis
is insignificant [
45
–
54
]. In contrast with fixed retainers, removable retainers need lifelong
compliance and cooperation from the patient [2,12,46,47,55].
Comparing the removable appliances, HRs provide better settling of the occlusion com-
pared with VFRs, and they also seem to be more durable in terms of breakage [
7
,
48
,
56
,
57
].
Bowen Li et al. concluded in their systematic review that patients using HRs showed
healthier periodontal condition than those using VFRs [
58
]. In contrast to this, VFRs
are more aesthetic, do not interfere with speech and are therefore more acceptable to
the patient [
55
,
59
,
60
]. Additionally, VFRs are more cost effective and easier to manufac-
ture [
7
,
48
,
56
,
61
]. Moreover, they seem to provide better retention of anterior teeth irregular-
ities [
38
,
39
,
62
]. However, both VFRs and HRs seem to maintain sufficiently orthodontically
corrected intercanine and intermolar widths [57,63–65].
Other adjunctive procedures have also been reported that enhance the stability of the
orthodontic result [
66
]. Among them, circumferential supracrestal fiberotomy and inter-
proximal reduction are the more prevalent for the moment [
5
]. Moreover, pharmacological
agents have been mentioned in the literature, but they are still in the experimental stage.
Raloxifene has been proven to decrease post-orthodontic treatment relapse
in vitro
[
67
]. Re-
sults with the injection of bisphosphonates
in vitro
also suggested enhanced post-treatment
stability [
68
]. Similar results have been suggested by Li et al. [
69
] with intermittent parathy-
roid hormone administration, by Duliamy et al. [
70
] with local injection with strodium,
by Al-Fakhry et al. [
71
] with injectable platelet rich fibrin (i-PRF) administration and
by
Zhao et al. [40]
with local OPG (osteoprotegerin) gene transfer to periodontal tissues.
Ozturk et al. [
72
] investigated the effect of photobiomodulation and high frequency me-
chanical vibration three-dimensionally and molecularly and found that both can support
post-treatment retention. The supportive effects of vibration for orthodontic retention were
Children 2023,10, 676 8 of 13
also highlighted in the study of Yadav et al. [
73
]. Though promising, all these procedures
need further clinical investigation in order to be implemented.
It seems that orthodontists have plenty of options so far for the retention phase of the
treatment. Although efforts have been made through the years for the standardization of
the retention protocol, this is impracticable. Every patient needs tailormade approaches
according to their age, pre-treatment characteristics, periodontal and dental situation,
compromises during treatment, compliance, etc., [
12
,
74
–
76
]. Knaup et al. suggested the
application of both removable and fixed appliances to enhance anterior teeth alignment
stability over time [77].
Now that CAD-CAM and 3D-printing technology have found their application in
orthodontics, it is just a matter of time for commercially available direct 3D-printed aligners
and retainers to be released.
However, this review of the available literature proved that directly 3D-printed retain-
ers are still at their infancy. Either fixed or removable clear, the retainers from 3D-printing
technology so far are mainly resin based. So, a possible reason for this retardation is
that there is no affirmed biocompatible resin that is commercially obtainable for this pur-
pose [63,73].
EnvisionTEC (EnvisionTEC, Inc.; Dearborn, MI, USA) and Formlabs have announced
the release of novel resins for the fabrication of clear aligners and occlusal splints, respec-
tively, with appropriate biocompatibility and physical properties [
21
,
34
]. PEEK polymer is
a material that has been used in some clinical case reports for fixed retention [
29
,
78
]. It is
already known for its application in dentistry for implants or prosthetic restorations, but it
also seems to be a promising material for orthodontic purposes due to its superior physical,
mechanical and aesthetic properties, biocompatibility, low plaque affinity and flexural
modulus close to enamel and dentin. However, more clinical research is necessary [
31
,
79
].
A case report also referred to an experimental resin (Genial Printing Resin, GC) from GC
Orthodontics (GC America Inc., Alsip, IL, USA) for 3D printing of fixed retention [28].
The studies so far are mostly
in vitro
experiments or case reports of «laboratory-stage»
products. Further well-designed clinical trials examining the properties and the behavior
of 3D-printed retainers in long time periods are necessary.
One aspect to be examined in long-term clinical studies is the behavior of resin over
time. It is known that resin-based materials become more fragile over time. The humidity
of the oral environment, the chemical and temperature imbalances from food or beverage
consumption, the stress loading during mastication or other factors also affect the properties
and the behavior of the resin [24].
Leaching from 3D-printed resin is also one more aspect that should be examined.
There are a number of biocompatible resins available for use in many fields such as in
medicine and in dentistry [
80
]. However, the cytotoxic potential of 3D resin products is
not yet well researched as the information available is scarce on this subject. The protocols
used differ from study to study, making them not comparable and studies do not tend
to reach consensual outcomes. A systematic review investigating the biological effects of
3D-printed resins used in orthodontics concludes that 3D-printed aligners exhibit higher
levels of cytotoxicity and genotoxicity than thermoplastic resins, especially if they have not
been subjected to a final surface treatment. This prompts the clinician to act with caution,
especially when treating young children and growing patients in order to protect them
from fertility issues [81].
Another conclusion that we can reach from the studies included in our research for
the 3D-printed clear retainers was that the anterior regions of the 3D-printed clear retainers
are the most problematic, regardless of print angulation [
22
]. Williams et al. in particular,
concluded that the smooth facial surfaces of central incisors provided greater differences
up to 0.480 mm [34].
Deviations of fitting of a 3D-printed clear retainer were attributed to various causes.
Cole et al. attributed the greatest deviations of the 3D-printed retainer group to the PMMA
resin used, which is very rigid, and assumed that this kind of resin is not the material of
Children 2023,10, 676 9 of 13
choice for this purpose [
21
]. Naeem also concluded that the print orientation, location of
the model on the baseplate, post-processing procedures of the retainers, overexposures of
some layers as the build develops because of a clear resin and errors from the CBCT scan
affected the accuracy of the aligners [22].
Cole et al. set the accuracy threshold at 0.50 mm somewhat arbitrarily, as it is not
clarified by the literature. He relied on studies that have indicated that measurements up to
0.50 mm are clinically agreeable for the appraisal of digital articulation. Naeem set 0.25 mm
of maximum deviation as the accuracy threshold according to a study by Johal et al. [
82
],
who tested the fit of thermoformed retainers
in vitro
and concluded that this threshold was
the maximum millimeter deviation. The same clinically acceptable maximum distance was
adopted by Williams et al., who relied on a study of Boyd and Vlaskalic [
83
]. Boyd et al.
concluded that a distance of 0.15–0.25 mm is necessary between the aligner and its model
cast in order to deliver its forces appropriately. Consequently, Williams et al. considered
that deviations up to 0.25 mm were considered clinically acceptable for a retention device.
Therefore, an acceptable accuracy threshold is also something that needs to be further
examined to be used universally in future studies.
The application of direct 3D-printed retainers in 3D printing pharmaceuticals is an-
other revolutionary technology. Three-dimensional printing has given the capability of
manufacturing drug delivery devices such as occlusal splints or retainers with customized
design and drug delivery rates. Reports have been made of customized 3D-printed mouth-
guards loaded with vanillic acid in humans [
84
]. Another study delivered personalized
orthodontic retainers loaded with clonidine hydrochloride for local sustained release of the
drug with satisfying results [
84
,
85
]. This promising technology could have applications
for the treatment of oral diseases as well, such as periodontitis, oral candidiasis, herpetic
gingivostomatitis or oral carcinoma. Personalized 3D-printed drug-loaded orthodontic
retainers seem to be an emerging technology for the sustained release of drugs [85].
Moreover, 3D-printed fixed retainers seem very promising. The resin-based type of
retention is an alternative for patients with metal allergies and aesthetic problems and the
material used does not interfere with magnetic resonance imaging (MRI). However, even
the metal-type 3D-printed retainers are better than the conventional fixed retainers [
86
].
The amount of adhesive used can be eliminated because of the shape and size individuality
of these devices [
24
]. Moreover, they have smooth surfaces that accumulate less plaque [
35
]
and their fabrication is time and cost effective [24].
Strengths and limitations
This review was constructed on well-established guidelines as outlined in the Materials
section. The searching procedure was extensive up to December 2022 and was detailed
including every potentially eligible report.
The limitations of the present review might be associated with the nature of the in-
cluded research and the data characteristics (four
in vitro
studies and only one prospective
clinical trial with a short follow-up of six months). Additionally, the English language
restriction can be considered as a weakness. The heterogeneity of protocols among the pre-
sented studies and the high risk of bias in the majority of them discouraged the performance
of further meta-analysis.
5. Conclusions
The transition from conventional methods to a fully digital workflow for the produc-
tion of 3D-printed retainers is the future trend. Affordable intraoral scanners, 3D printers
and free available software make 3D-printing technology more time and cost effective
than the conventional techniques. It is also an easier procedure for the practitioner and
a more convenient experience, especially for uncooperative or younger patients. However,
more
in vitro
and
in vivo
clinical trials should be conducted in a well-organized manner in
order to clarify some «grey» points concerning the most suitable materials in 3D printing
technology: the print angulation and the post-processing procedures, especially for direct-
printed clear retainers, as well as the longevity of these appliances in intraoral conditions.
Children 2023,10, 676 10 of 13
The safety of these products should be verified because the majority of the patients in-
volved in orthodontic treatment are growing patients who will keep these retention devices
intraorally for the rest of their lives.
Author Contributions:
Conceptualization, I.A.T.; methodology, E.T. and I.A.T.; validation, ET. and
I.A.T. formal analysis, E.T.; investigation, I.A.T. and E.T.; resources, E.T., I.L., A.I.T., M.P.M. and
I.A.T.; data curation, I.A.T.; writing—original draft preparation, E.T., I.L., A.I.T., M.P.M. and I.A.T.;
writing—review and editing, E.T., A.I.T. and I.A.T.; visualization, I.A.T.; supervision, I.A.T.; project
administration, I.A.T. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Data is contained within the article.
Conflicts of Interest: The authors declare no conflict of interest.
References
1. Johnston, C.D.; Littlewood, S.J. Retention in orthodontics. Br. Dent. J. 2015,218, 119–122. [CrossRef]
2. Fleming, P.S.; Littlewood, S.J. Orthodontic retention—A final frontier? Br. Dent. J. 2021,230, 41415. [CrossRef] [PubMed]
3. Littlewood, S.J.; Kandasamy, S.; Huang, G. Retention and relapse in clinical practice. Aust. Dent. J. 2017,62, 51–57. [CrossRef]
4. Fudalej, P.S.; Renkema, A.M. A brief history of orthodontic retention. Br. Dent. J. 2021,230, 777–780. [CrossRef] [PubMed]
5.
Millett, D. The rationale for orthodontic retention: Piecing together the jigsaw. Br. Dent. J.
2021
,230, 739–749. [CrossRef]
[PubMed]
6.
Steinnes, J.; Johnsen, G.; Kerosuo, H. Stability of orthodontic treatment outcome in relation to retention status: An 8-year
follow-up. Am. J. Orthod. Dentofac. Orthop. 2017,151, 1027–1033. [CrossRef]
7.
McNally, M.; Mullin, M.; Dhopatkar, A.; Rock, W.P. Orthodontic Retention: Why When and How? Dent. Update
2003
,30, 446–452.
[CrossRef]
8.
Littlewood, S.J.; Millett, D.T.; Doubleday, B.; Bearn, D.R.; Worthington, H.V. Retention procedures for stabilising tooth position
after treatment with orthodontic braces. Cochrane Database Syst. Rev. 2016,2016, CD002283. [CrossRef]
9. Kartal, Y.; Kaya, B. Fixed orthodontic retainers: A review. Turk. J. Orthod. 2019,32, 110–114. [CrossRef]
10.
Littlewood, S.J.; Dalci, O.; Dolce, C.; Holliday, L.S.; Naraghi, S. Orthodontic retention: What’s on the horizon? Br. Dent. J.
2021
,
230, 760–764. [CrossRef]
11.
Kirschen, R.; Littlewood, S.J.; Blazewska-Amin, A.; Fleming, P.S. Bonded orthodontic retention: A practical guide. Br. Dent. J.
2021,230, 709–716. [CrossRef] [PubMed]
12.
Al-Moghrabi, D.; Littlewood, S.J.; Fleming, P.S. Orthodontic retention protocols: An evidence-based overview. Br. Dent. J.
2021
,
230, 770–776. [CrossRef] [PubMed]
13.
Tacken, M.P.E.; Cosyn, J.; De Wilde, P.; Aerts, J.; Govaerts, E.; Vannet, B.V. Glass fibre reinforced versus multistranded bonded
orthodontic retainers: A 2 year prospective multi-centre study. Eur. J. Orthod. 2010,32, 117–123. [CrossRef] [PubMed]
14.
Foek, D.L.S.; Yetkiner, E.; Özcan, M. Fatigue resistance, debonding force, and failure type of fiber-reinforced composite, polyethy-
lene ribbon-reinforced, and braided stainless steel wire lingual retainers
in vitro
.Korean J. Orthod.
2013
,43, 186–192. [CrossRef]
[PubMed]
15.
Mummolo, S.; Quinzi, V.; Nota, A.; Marino, C.; Pittari, L.; Manenti, R.J.; Tecco, S. Metal versus Fiberglass Post-Orthodontic
Retainers Short-Term Effects on Plaque Index and Microbial Colonization: An Observational Study. Life
2022
,12, 331. [CrossRef]
[PubMed]
16.
Foek, D.L.S.; Ozcan, M.; Krebs, E.; Sandham, A. Adhesive properties of bonded orthodontic retainers to enamel: Stainless steel
wire vs. fiber-reinforced composites. J. Adhes. Dent.
2009
,11, 381–390. Available online: http://www.ncbi.nlm.nih.gov/pubmed/
19841765 (accessed on 1 May 2009). [PubMed]
17.
Tian, Y.; Chen, C.X.; Xu, X.; Wang, J.; Hou, X.; Li, K.; Lu, X.; Shi, H.; Lee, E.S.; Jiang, H.B. A Review of 3D Printing in Dentistry:
Technologies, Affecting Factors, and Applications. Scanning 2021,2021, 9950131. [CrossRef] [PubMed]
18.
Tsolakis, I.A.; Gizani, S.; Panayi, N.; Antonopoulos, G.; Tsolakis, A.I. Three-Dimensional Printing Technology in Orthodontics for
Dental Models: A Systematic Review. Children 2022,9, 1106. [CrossRef]
19.
Turkyilmaz, I.; Wilkins, G.N. 3D printing in dentistry—Exploring the new horizons. J. Dent. Sci.
2021
,16, 1037–1038. [CrossRef]
20.
Tsolakis, I.A.; Gizani, S.; Tsolakis, A.I.; Panayi, N. Three-Dimensional-Printed Customized Orthodontic and Pedodontic Appli-
ances: A Critical Review of a New Era for Treatment. Children 2022,9, 1107. [CrossRef]
21.
Cole, D.; Bencharit, S.; Carrico, C.K.; Arias, A.; Tüfekçi, E. Evaluation of fit for 3D-printed retainers compared with thermoform
retainers. Am. J. Orthod. Dentofac. Orthop. 2019,155, 592–599. [CrossRef] [PubMed]
Children 2023,10, 676 11 of 13
22.
Naeem, O.A.; Bencharit, S.; Yang, I.H.; Stilianoudakis, S.C.; Carrico, C.; Tüfekçi, E. Comparison of 3-dimensional printing
technologies on the precision, trueness, and accuracy of printed retainers. Am. J. Orthod. Dentofac. Orthop.
2022
,161, 582–591.
[CrossRef] [PubMed]
23.
Thurzo, A.; Šufliarsky, B.; Urbanová, W.; ˇ
Cverha, M.; Strunga, M.; Varga, I. Pierre Robin Sequence and 3D Printed Personalized
Composite Appliances in Interdisciplinary Approach. Polymers 2022,14, 3858. [CrossRef]
24.
Firlej, M.; Zaborowicz, K.; Zaborowicz, M.; Firlej, E.; Domagała, I.; Pieniak, D.; Igielska-Kalwat, J.; Dmowski, A.; Biedziak, B.
Mechanical Properties of 3D Printed Orthodontic Retainers. Int. J. Environ. Res. Public Health
2022
,19, 5775. [CrossRef] [PubMed]
25. Cousley, R.R. Introducing 3D printing in your orthodontic practice. J. Orthod. 2020,47, 265–272. [CrossRef]
26.
Tartaglia, G.M.; Mapelli, A.; Maspero, C.; Santaniello, T.; Serafin, M.; Farronato, M.; Caprioglio, A. Direct 3D printing of clear
orthodontic aligners: Current state and future possibilities. Materials 2021,14, 1799. [CrossRef]
27.
Koizumi, S.; Seimiya, K.; Park, H.; Nakashizu, T.; Suzuki, K.; Otsuka, T.; Kobayashi, M.; Hoshi, N.; Kimoto, K.; Yamaguchi, T.
A metal retainer manufactured by 3D printing. Orthod. Waves 2020,79, 95–98. [CrossRef]
28.
Doldo, T.; Di Vece, L.; Cagidiaco, E.F.; Nuti, N.; Parrini, S.; Ferrari, M.; Carboncini, F. A new generation of orthodontic retainer
using 3D printing technology: Report of two cases. J. Osseointegration 2018,10, 142–148.
29.
Beretta, M.; Mangano, A.; Gianolio, A.; Negrini, S.; Canova, F.F.; Cirulli, N. A Fully Digital Workflow for PEEK Fixed Retainers.
J. Clin. Orthod. 2021,55, 249–253.
30.
Nasef, A.A.; El-Beialy, A.R.; Mostafa, Y.A. Virtual techniques for designing and fabricating a retainer. Am. J. Orthod. Dentofac.
Orthop. 2014,146, 394–398. [CrossRef]
31.
Nai, T.A.P.; Aydin, B.; Brand, H.S.; Jonkman, R.E.G. Present and Theoretical Applications of Poly-Ether-Ether-Ketone (PEEK) in
Orthodontics: A Scoping Review. Materials 2022,15, 7414. [CrossRef] [PubMed]
32.
Marsh, K.; Weissheimer, A.; Yin, K.; Chamberlain-Umanoff, A.; Tong, H.; Sameshima, G.T. Three-dimensional assessment of
virtual bracket removal for orthodontic retainers: A prospective clinical study. Am. J. Orthod. Dentofac. Orthop.
2021
,160, 302–311.
[CrossRef] [PubMed]
33.
Page, M.J.; McKenzie, J.E.; Bossuyt, P.M.; Boutron, I.; Hoffmann, T.C.; Mulrow, C.D.; Shamseer, L.; Tetzlaff, J.M.; Akl, E.A.;
Brennan, S.E.; et al. The PRISMA 2020 statement: An updated guideline for reporting systematic reviews. BMJ
2021
,372,
2020–2021. [CrossRef]
34.
Williams, A.; Bencharit, S.; Yang, I.H.; Stilianoudakis, S.C.; Carrico, C.K.; Tüfekçi, E. Effect of print angulation on the accuracy
and precision of 3D-printed orthodontic retainers. Am. J. Orthod. Dentofac. Orthop. 2022,161, 133–139. [CrossRef] [PubMed]
35.
Shah, R.; Nair, R.R. Comparative Evaluation of Periodontal Health, Amount of Relapse and Incidence of Failure between
Conventional and Customized 3D Printed Orthodontic Fixed Lingual Retainers: An In-Vivo Study. JPSP 2022,6, 6960–6968.
36.
Sheth, V.H.; Shah, N.P.; Jain, R.; Bhanushali, N.; Bhatnagar, V. Development and validation of risk of bias tool for assessing
in vitro studies conducted in dentistry: The QUIN. J. Prosthet. Dent. 2022,in press. [CrossRef]
37.
Sterne, J.A.C.; Savovi´c, J.; Page, M.J.; Elbers, R.G.; Blencowe, N.S.; Boutron, I.; Cates, C.J.; Cheng, H.Y.; Corbett, M.S.;
Eldridge, S.M.; et al. RoB 2: A revised tool for assessing risk of bias in randomised trials. BMJ 2019,366, l4898. [CrossRef]
38.
Outhaisavanh, S.; Liu, Y.; Song, J. The origin and evolution of the Hawley retainer for the effectiveness to maintain tooth position
after fixed orthodontic treatment compare to vacuum-formed retainer: A systematic review of RCTs. Int. Orthod.
2020
,18,
225–236. [CrossRef]
39.
Demir, A.; Babacan, H.; Nalcaci, R.; Topcuoglu, T. Comparison of retention characteristics of Essix and Hawley retainers. Korean J.
Orthod. 2012,42, 255–262. [CrossRef]
40.
Zhao, N.; Lin, J.; Kanzaki, H.; Ni, J.; Chen, Z.; Liang, W.; Liu, Y. Local osteoprotegerin gene transfer inhibits relapse of orthodontic
tooth movement. Am. J. Orthod. Dentofac. Orthop. 2012,141, 30–40. [CrossRef]
41.
Menditto, A.; Patriarca, M.; Magnusson, B. Understanding the meaning of accuracy, trueness and precision. Accredit. Qual. Assur.
2007,12, 45–47. [CrossRef]
42.
Gökçe, B.; Kaya, B. Periodontal effects and survival rates of different mandibular retainers: Comparison of bonding technique
and wire thickness. Eur. J. Orthod. 2019,41, 591–600. [CrossRef] [PubMed]
43.
Rody, W.J.; Elmaraghy, S.; McNeight, A.M.; Chamberlain, C.A.; Antal, D.; Dolce, C.; Wheeler, T.T.; McGorray, S.P.; Shaddox, L.M.
Effects of different orthodontic retention protocols on the periodontal health of mandibular incisors. Orthod. Craniofacial Res.
2016,19, 198–208. [CrossRef] [PubMed]
44.
Storey, M.; Forde, K.; Littlewood, S.J.; Scott, P.; Luther, F.; Kang, J. Bonded versus vacuum-formed retainers: A randomized
controlled trial. Part 2: Periodontal health outcomes after 12 months. Eur. J. Orthod. 2018,40, 399–408. [PubMed]
45.
Salvesen, B.F.; Grytten, J.; Rongen, G.; Vandevska-radunovic, V. Patient-Reported Outcome Measures on Oral Hygiene, Periodon-
tal Health, and Treatment Satisfaction of Orthodontic Retention Patients up to Ten Years after Treatment—A Cross-Sectional
Study. Int. J. Environ. Res. Public Health 2022,19, 4843. [CrossRef]
46.
Al-Moghrabi, D.; Pandis, N.; Fleming, P.S. The effects of fixed and removable orthodontic retainers: A systematic review. Prog.
Orthod. 2016,17, 1–22. [CrossRef]
47.
Al-Moghrabi, D.; Johal, A.; O’Rourke, N.; Donos, N.; Pandis, N.; Gonzales-Marin, C.; Fleming, P.S. Effects of fixed vs. removable
orthodontic retainers on stability and periodontal health: 4-year follow-up of a randomized controlled trial. Am. J. Orthod.
Dentofac. Orthop. 2018,154, 167–174e1. [CrossRef]
Children 2023,10, 676 12 of 13
48.
Madurantakam, P.; Kumar, S. Fixed and removable orthodontic retainers and periodontal health. Evid. Based Dent.
2017
,18,
103–104. [CrossRef]
49.
Arn, M.L.; Dritsas, K.; Pandis, N.; Kloukos, D. The effects of fixed orthodontic retainers on periodontal health: A systematic
review. Am. J. Orthod. Dentofac. Orthop. 2020,157, 156–164.e17. [CrossRef]
50.
Steegmans, P.A.J.; Meursinge Reynders, R.A. Fixed orthodontic retainers and periodontal health. Evid. Based Dent.
2020
,21,
146–149. [CrossRef]
51.
Adanur-Atmaca, R.; Çokakoglu, S.; Öztürk, F. Effects of different lingual retainers on periodontal health and stability. Angle
Orthod. 2021,91, 468–476. [CrossRef]
52.
Levin, L.; Samorodnitzky-Naveh, G.R.; Machtei, E.E. The Association of Orthodontic Treatment and Fixed Retainers with Gingival
Health. J. Periodontol. 2008,79, 2087–2092. [CrossRef] [PubMed]
53.
Eroglu, A.K.; Baka, Z.M.; Arslan, U. Comparative evaluation of salivary microbial levels and periodontal status of patients
wearing fixed and removable orthodontic retainers. Am. J. Orthod. Dentofac. Orthop. 2019,156, 186–192. [CrossRef] [PubMed]
54.
Han, J.Y.; Park, S.H.; Kim, J.; Hwang, K.G.; Park, C.J. Clinical factors affecting the longevity of fixed retainers and the influence of
fixed retainers on periodontal health in periodontitis patients: A retrospective study. J. Periodontal Implant Sci.
2021
,51, 1–16.
[CrossRef] [PubMed]
55.
Saleh, M.; Hajeer, M.Y.; Muessig, D. Acceptability comparison between Hawley retainers and vacuum-formed retainers in
orthodontic adult patients: A single-centre, randomized controlled trial. Eur. J. Orthod. 2017,39, 453–461. [CrossRef] [PubMed]
56. Meade, M.J.; Millet, D.C. Vacuum-Formed Retainers: An Overview. Dent. Update 2015,42, 24–34. [CrossRef]
57.
Ashari, A.; Xian, L.; Mohamed, A.M.F.S.; Wahab, R.M.A.; Kit, Y.C.; Tata, M.D.; Sinnasamy, S.; Kuppusamy, E. One-year compara-
tive assessment of retention of arch width increases between modified vacuum-formed and Hawley retainers: A multicenter
randomized clinical trial. Angle Orthod. 2022,92, 197–203. [CrossRef] [PubMed]
58.
Li, B.; Xu, Y.; Lu, C.; Wei, Z.; Li, Y.; Zhang, J. Assessment of the effect of vacuum-formed retainers and Hawley retainers on
periodontal health: A systematic review and meta-analysis. PLoS ONE 2021,16, e0253968. [CrossRef]
59.
Dogramaci, E.J.; Littlewood, S.J. Removable orthodontic retainers: Practical considerations. Br. Dent. J.
2021
,230, 723–730.
[CrossRef]
60.
Wan, J.; Wang, T.; Pei, X.; Wan, Q.; Feng, W.; Chen, J. Speech effects of Hawley and vacuum-formed retainers by acoustic analysis:
A single-center randomized controlled trial. Angle Orthod. 2017,87, 286–292. [CrossRef] [PubMed]
61.
Hichens, L.; Rowland, H.; Williams, A.; Hollinghurst, S.; Ewings, P.; Clark, S.; Ireland, A.; Sandy, J. Cost-effectiveness and patient
satisfaction: Hawley and vacuum-formed retainers. Eur. J. Orthod. 2007,29, 372–378. [CrossRef] [PubMed]
62.
Rowland, H.; Hichens, L.; Williams, A.; Hills, D.; Killingback, N.; Ewings, P.; Clark, S.; Ireland, A.J.; Sandy, J.R. The effectiveness
of Hawley and vacuum-formed retainers: A single-center randomized controlled trial. Am. J. Orthod. Dentofac. Orthop.
2007
,132,
730–737. [CrossRef]
63. Mai, W.; He, J.; Meng, H.; Jiang, Y.; Huang, C.; Li, M.; Yuan, K.; Kang, N. Comparison of vacuum-formed and Hawley retainers:
A systematic review. Am. J. Orthod. Dentofac. Orthop. 2014,145, 720–727. [CrossRef] [PubMed]
64.
Ashari, A.; Nik Mustapha, N.M.; Yuen, J.J.X.; Saw, Z.K.; Lau, M.N.; Xian, L.; Syed Mohamed, A.M.F.; Megat Abdul Wahab, R.;
Yeoh, C.K.; Deva Tata, M.; et al. A two-year comparative assessment of retention of arch width increases between modified
vacuum-formed and Hawley retainers: A multi-center randomized clinical trial. Prog. Orthod.
2022
,23, 1–11. [CrossRef]
[PubMed]
65.
Ramazanzadeh, B.; Ahrari, F.; Hosseini, Z.S. The retention characteristics of Hawley and vacuum-formed retainers with different
retention protocols. J. Clin. Exp. Dent. 2018,10, e224. [CrossRef]
66.
Kaklamanos, E.G.; Makrygiannakis, M.A.; Athanasiou, A.E. Could medications and biologic factors affect post-orthodontic tooth
movement changes? A systematic review of animal studies. Orthod. Craniofacial Res. 2021,24, 39–51. [CrossRef] [PubMed]
67.
Azami, N.; Chen, P.J.; Mehta, S.; Kalajzic, Z.; Dutra, E.H.; Nanda, R.; Yadav, S. Raloxifene administration enhances retention in an
orthodontic relapse model. Eur. J. Orthod. 2020,42, 371–377. [CrossRef]
68.
Tokhtah, H.A.; Alhadlaq, A.M. Utility of injectable bisphosphonates in enhancing orthodontic retention in a goat model:
A split-mouth study. Saudi Dent. J. 2022,34, 282–287. [CrossRef]
69.
Li, T.; Yan, Z.; He, S.; Zhou, C.; Wang, H.; Yin, X.; Zou, S.; Duan, P. Intermittent parathyroid hormone improves orthodontic
retention via insulin-like growth factor-1. Oral Dis. 2021,27, 290–300. [CrossRef]
70.
Al-Duliamy, M.J.; Ghaib, N.H.; Kader, O.A.; Abdullah, B.H. Enhancement of orthodontic anchorage and retention by the local
injection of strontium: An experimental study in rats. Saudi Dent. J. 2015,27, 22–29. [CrossRef]
71.
Al-Fakhry, H.; Al-Sayagh, N. Effects of Injectable platelet rich fibrin (i-PRF) on reduction of relapse after orthodontic tooth
movement: Rabbits model study. J. Orthod. Sci. 2022,11, 10. [CrossRef] [PubMed]
72.
Öztürk, T.; Gül Amuk, N. Three-dimensional imaging and molecular analysis of the effects of photobiomodulation and mechanical
vibration on orthodontic retention treatment in rats: Effects of photobiomodulation and mechanical vibration on orthodontic
retention treatment. J. Orofac. Orthop. 2022,83, 24–41. [CrossRef] [PubMed]
73.
Yadav, S.; Assefnia, A.; Gupta, H.; Vishwanath, M.; Kalajzic, Z.; Allareddy, V.; Nanda, R. The effect of low-frequency mechanical
vibration on retention in an orthodontic relapse model. Eur. J. Orthod. 2016,38, 44–50. [CrossRef] [PubMed]
74.
Andriekute, A.; Vasiliauskas, A.; Sidlauskas, A. A survey of protocols and trends in orthodontic retention. Prog. Orthod.
2017
,
18, 31. [CrossRef] [PubMed]
Children 2023,10, 676 13 of 13
75.
Bahije, L.; Ennaji, A.; Benyahia, H.; Zaoui, F. A systematic review of orthodontic retention systems: The verdict. Int. Orthod.
2018
,
16, 409–424. [CrossRef] [PubMed]
76.
Lyros, I.; Tsolakis, I.A.; Maroulakos, M.P.; Fora, E.; Lykogeorgos, T.; Dalampira, M.; Tsolakis, A.I. Orthodontic Retainers-A Critical
Review. Children 2023,10, 230. [CrossRef]
77.
Knaup, I.; Schulte, U.; Bartz, J.R.; Niederau, C.; Craveiro, R.B.; Jäger, A.; Wolf, M. Post-treatment Stability in Orthodontic Retention
with Twistflex Retainers—Do Patients Benefit from Additional Removable Retainers? Clin. Oral Investig.
2022
,26, 5215–5222.
[CrossRef]
78.
Aboulazm, K.; von See, C.; Othman, A. Fixed lingual orthodontic retainer with bilateral missing lateral incisors produced in
PEEK material using CAD/CAM technology. J. Clin. Exp. Dent. 2021,13, 549–551. [CrossRef]
79.
Paglia, M.; Beretta, M.; Quinzi, V.; Colombo, S. PEEK polymer in orthodontics: A scoping review. Eur. J. Paediatr. Dent.
2022
,23,
137–139.
80.
Guttridge, C.; Shannon, A.; O’Sullivan, A.; O’Sullivan, K.J.; O’Sullivan, L.W. Biocompatible 3D printing resins for medical
applications: A review of marketed intended use, biocompatibility certification, and post-processing guidance. Ann. 3D Print
Med. 2022,5, 100044. [CrossRef]
81.
Francisco, I.; Paula, A.B.; Ribeiro, M.; Marques, F.; Travassos, R.; Nunes, C.; Pereira, F.; Marto, C.M.; Carrilho, E.; Vale, F. The
Biological Effects of 3D Resins Used in Orthodontics: A Systematic Review. Bioengineering 2022,9, 15. [CrossRef] [PubMed]
82.
Johal, A.; Sharma, N.R.; McLaughlin, K.; Zou, L.F. The reliability of thermoform retainers: A laboratory-based comparative study.
Eur. J. Orthod. 2015,37, 503–507. [CrossRef] [PubMed]
83.
Boyd, R.L.; Waskalic, V. Three-dimensional diagnosis and orthodontic treatment of complex malocclusions with the invisalign
appliance. Semin. Orthod. 2001,7, 274–293. [CrossRef]
84.
Seoane-Viaño, I.; Trenfield, S.J.; Basit, A.W.; Goyanes, A. Translating 3D printed pharmaceuticals: From hype to real-world clinical
applications. Adv. Drug Deliv. Rev. 2021,174, 553–575. [CrossRef] [PubMed]
85.
Jiang, H.; Fu, J.; Li, M.; Wang, S.; Zhuang, B.; Sun, H.; Ge, C.; Feng, B.; Jin, Y. 3D-Printed Wearable Personalized Orthodontic
Retainers for Sustained Release of Clonidine Hydrochloride. AAPS PharmSciTech. 2019,20, 260. [CrossRef]
86.
Roser, C.; Hilgenfeld, T.; Sen, S.; Badrow, T.; Zingler, S.; Heiland, S.; Bendszus, M.; Lux, C.J.; Juerchott, A. Evaluation of magnetic
resonance imaging artifacts caused by fixed orthodontic CAD/CAM retainers—An
in vitro
study. Clin. Oral Investig.
2021
,25,
1423–1431. [CrossRef]
Disclaimer/Publisher’s Note:
The statements, opinions and data contained in all publications are solely those of the individual
author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to
people or property resulting from any ideas, methods, instructions or products referred to in the content.
