medicina

Review
Orthodontic Aligners: Current Perspectives for the Modern
Orthodontic Office
Chung How Kau 1,2, * , Jen Soh 3 , Teti Christou 1 and Akanksha Mangal 4

1 Department of Orthodontics, University of Alabama at Birmingham, Birmingham, AL 35233, USA;

tetich@uab.edu
2 Department of Surgery, Heersink School of Medicine, University of Alabama at Birmingham,
Birmingham, AL 35233, USA
3 Private Orthodontic Practice, Braces & Aesthetics Dental Clinic Pte Ltd., Singapore 307506, Singapore;
sohjen69@yahoo.com
4 Orthodontic Clinical Fellowship, School of Orthodontics, Jacksonville University, Jacksonville, FL 32211, USA;
akanksha.mngl@gmail.com
* Correspondence: ckau@uab.edu

Abstract: Orthodontic aligners are changing the practice of orthodontics. This system of orthodontic
appliances is becoming the mainstay appliance of choice for orthodontic offices in many countries.
Patient preferences and lifestyle needs have made this appliance the primary choice when seeking
care. In the early days, appliances lacked the efficiency and effectiveness of traditional bracket-wire
systems, but modern systems are now able to handle a more comprehensive orthodontic caseload.
Current systems provide newer biomechanical strategies and artificial intelligence-driven tooth
movements for better outcomes. These improvements now mean that an orthodontist can be better
prepared to manage a larger number of orthodontic malocclusions. This paper aims to discuss some
of the evolution of orthodontic aligners and to describe to orthodontists the fundamentals of aligner
therapy. In addition, it will provide an evidence-based outcome to the existing treatment outcomes in
the current literature.

Keywords: orthodontic aligners; aligner biomechanics; innovations

Citation: Kau, C.H.; Soh, J.; Christou, T.;

Mangal, A. Orthodontic Aligners: 1. Introduction
Current Perspectives for the Modern
Orthodontic aligner therapy (OAT) is changing the dental and orthodontic land-
Orthodontic Office. Medicina 2023, 59,
scape [1]. Recent reports suggest that OAT now represents 30–45% of an orthodontic prac-
1773. https://doi.org/10.3390/
tice caseload, and there are shifting demographics on patient preferences and awareness of
medicina59101773
OAT [2–4]. Early orthodontic appliances certainly had many reported shortcomings [5,6],
Academic Editor: Bruno Chrcanovic but the modern versions of OAT have had many improvements and better outcomes [7–9].
As a result of this success, many orthodontic practices have had to re-invent their practices
Received: 6 September 2023
Revised: 23 September 2023
to meet patient needs and alter clinical workflows [10].
Accepted: 29 September 2023
In order for the modern appliances to be understood, a short history of appliance
Published: 5 October 2023 inception must be understood. The first minor tooth movement appliance was introduced
by Remensnyder in 1925. This appliance, the Flex O-Tite, was used primarily to treat
periodontitis. This was followed by Kesling, who introduced the “tooth positioning appli-
ance” in 1945 [11]. In 1959, Nahoum built on Kesling’s idea of using a series of positioners
Copyright: © 2023 by the authors. to achieve tooth movements and introduced the first documented clear thermoplastic
Licensee MDPI, Basel, Switzerland. appliance, which was vacuum-based and showed a firm adaptation to the patient’s cast.
This article is an open access article This appliance could also be used as a retainer, splint, surgical pack holder, medicament
distributed under the terms and
carrier, etc. He also used auxiliary elements, like acrylic buttons for interarch elastics, a
conditions of the Creative Commons
concept that is similar to the use of attachments and auxiliaries in the current practice of
Attribution (CC BY) license (https://
clear aligner therapy, suggesting that it may have originated from Nahoum’s treatment
creativecommons.org/licenses/by/
mechanics [12]. Later, in the 1970s, Ponitz proposed an “Invisible retainer” that was initially
4.0/).

Medicina 2023, 59, 1773. https://doi.org/10.3390/medicina59101773 https://www.mdpi.com/journal/medicina

Medicina 2023, 59, 1773 2 of 14

fabricated for finishing and retaining orthodontic cases [13]. In 1985, McNamara modified
the vacuum form technique developed by Ponitz and used a Biostar machine that applied
positive air pressure to fabricate an invisible retainer using a 1 mm thick Biocryl sheet,
which could also be used for final detailing and retention [14]. In 1993, Sheridan further
modified it by reducing the thickness to 0.030 and introduced the “Essix appliance” [12].
The core principle in OAT is the determination of the final position of all of the teeth.
This philosophy of beginning with the “end in mind” was tedious and time-consuming
when the concept was first proposed. The anticipated final position of the teeth was
determined by sectioning the teeth on a dental cast and manipulating their positions to
create tooth movement. Through trial and error, an adequate amount of tooth movement
in six degrees of freedom was prescribed and achieved. However, in 1997, two Stanford
graduates, Zia Chishti and Kelsey Wirth, along with two orthodontists, founded Align
Technology (Santa Clara, CA, USA) [15]. Essentially, the first patents that were written
described a computerized method to move teeth, through a series of polymeric shells, from
a start point to a final result or end point. This revolutionary method produced the first
orthodontic appliance fabricated using transparent and thermoplastic polymeric materials,
with the aid of modern CAD/CAM technology. This manufacturing workflow meant that
the original methods involving the manual fabrication of clear aligners could be discarded
and that mass production workflows could be created. Today, the early manual methods
are no longer used for commercial aligner fabrication, and the current digital fabrication
method employs CAD/CAM technology and a digital workflow protocol [16–20]. In many
ways, Align Technology must be credited with the continuous innovation and research
in enhancing tooth movement and biomechanics in orthodontic aligner therapy (OAT)
for both the clinician and patient [21]. Since its inception, Invisalign appliances have
evolved drastically from the first-generation aligners to the present-day eight-generation
aligners. During this evolutionary process, the main progress has been seen in the types
of material from Proceed 30 (a polymer mixture) to Smart Track material (multi-layer
aromatic thermoplastic polyurethane/co-polyester material). In addition, “SmartForce”
features allow for the addition of different types of auxiliaries and attachments to the
clinical software simulation. Furthermore, significant improvements to the software now
allow for the incorporation of CBCT scans into the ClinCheck software, “SmartStage”,
for extraction cases [22], “Invisalign first” for mixed dentition malocclusions [23], and
mandibular advancement for Skeletal Class II correction, which all aim to provide greater
control for tooth movements [24]. These tools also help clinicians communicate effectively
to patients and also visualize the clinical treatment plans. At present, Invisalign has been
the most popular to date and continues to be a leader in the market in a large pool of
companies all across the world.

2. Beginning with the End in Mind

Many OAT methods allow for the visualization of the patient end result. This key
treatment planning feature now allows for many clinicians to enhance their treatment plans
and optimize smile aesthetics [25]. Recent publications showed that finished treatments
presented to the American Board of Orthodontics still required significant improvements
when comparing Invisalign and traditional fixed appliance therapy despite the advantage of
having the ClinCheck visualization software tool that predicts the simulated final outcome
of the treated occlusion [26,27]. The ability to visualize the final result in conjunction with
the use of cone beam computed tomography has expanded the diagnostic ability to study
the position of roots, apical root resorption, and bony changes associated with the planned
tooth movements (Figures 1–3) [28,29].
 Medicina 2023, 59, x FOR PEER REVIEW 3 of 14
Medicina 2023, 59, 1773 3 of 14
Medicina 2023, 59, x FOR PEER REVIEW 3 of 14

Figure 1. Standard array of pictures for the capture of malocclusion in an 11-year-old patient.
Figure 1. Standard Figure 1. Standard
array of picturesarray of pictures
for the capturefor
of the capture of malocclusion
malocclusion in an 11-year-old
in an 11-year-old patient. patient.

Figure 2. CBCT data show short roots in the anterior zone of the maxillary teeth. Careful treat-
ment planning with the Smartee check clinical simulation software reduced the overall rate of
tooth movement in the patient’s treatment plan and also allowed for visualization of the roots.
Total number of trays was 30 in this treatment sequence.
 Medicina 2023, 59, x FOR PEER REVIEW 4 of 14

Figure 2. CBCT data show short roots in the anterior zone of the maxillary teeth. Careful treatment
planning with the Smartee check clinical simulation software reduced the overall rate of tooth move-
Medicina 2023, 59, 1773 4 of 14
ment in the patient’s treatment plan and also allowed for visualization of the roots. Total number of
trays was 30 in this treatment sequence.

Figure 3. (a,b) Completion of 30 sets of trays in under 24 weeks. No further root resorption to the
Figure 3. (a,b)
anterior teethCompletion
and a highly of 30 setsesthetic
desirable of trays in under
result 24 weeks.
for maximum Noprojection.
smile further root resorption to the
anterior teeth and a highly desirable esthetic result for maximum smile projection.
3. Clinical Effectiveness of Orthodontic Aligner Therapy
3. Clinical Effectiveness of Orthodontic Aligner Therapy
Although the range of treatment indications for OAT has broadened over time, the
Although the
predictability range
of all of treatment
movements plannedindications
in the mainfor OATofhas
phase broadened
treatment over
does not time, the
reach
predictability
anywhere nearof all100%.
movements
The averageplanned in the
accuracy of main phase of
OAT ranged treatment
between 41%does not reach
and 73%, de- any-
pending
where near mainly
100%. Theon the type of accuracy
average movement, ofits magnitude,
OAT ranged and the tooth
between 41%type
andinvestigated
73%, depending
[30,31].
mainly onSimple studies
the type to understand
of movement, itsthe “tracking” and
magnitude, outcomes of clinically
the tooth planned simu-
type investigated [30,31].
lations
Simple and actual
studies tooth movement
to understand will help inoutcomes
the “tracking” understanding the effectiveness
of clinically plannedof OAT
simulations
and(Figure
actual 4). A number
tooth movement of studies havein
will help also described problematic
understanding movements.
the effectiveness These(Figure
of OAT in- 4).
clude root torque, bodily translation [31], and the derotation of round-shaped teeth [32],
A number of studies have also described problematic movements. These include root
which often entail mid-course corrections and/or additional finishing phases. Extrusion
torque, bodily translation [31], and the derotation of round-shaped teeth [32], which often
entail mid-course corrections and/or additional finishing phases. Extrusion and intrusion
mechanics have also been challenges, but clinicians have overcome some of these short-
falls by changing the pathways in which these teeth move on the computer simulations.
Table 1 describes some of the movements reported in the literature. Many clinicians have
also described the importance of the type of aligner and the material the aligner is made
from, not to mention the planning accuracy, the manual skill of the orthodontist, and patient
compliance [32].
 and intrusion mechanics have also been challenges, but clinicians have overcome some of
these shortfalls by changing the pathways in which these teeth move on the computer
simulations. Table 1 describes some of the movements reported in the literature. Many
Medicina 2023, 59, 1773 clinicians have also described the importance of the type of aligner and the material the 5 of 14
aligner is made from, not to mention the planning accuracy, the manual skill of the ortho-
dontist, and patient compliance [32].

Figure
Figure 4. 4.Superimposition
Superimposition ofofSmartee
Smarteecheck toothtooth
check simulated results results
simulated with realwith
intra-oral patient scanpatient
real intra-oral
data. The results of the patient at T30 were less than 0.25 mm across all tooth movements. This
scan data. The results of the patient at T30 were less than 0.25 mm across all tooth movements.
showed that the planned clinical result is producing the desired tooth movement and is one way to
Thisensure
showed thattracking
proper the planned
of theclinical
alignersresult
duringisthe
producing
patient’s the desired tooth movement and is one way
treatment.
to ensure proper tracking of the aligners during the patient’s treatment.

4. Materials
Orthodontic aligners have undergone multiple transformations over the years. The need
for improved clinical efficiency to treat various malocclusions has increased, so there has
been an evolution in clinical materials. Every new aligner manufacturer claims to be better
than the previous one. Many manufacturers claim that various factors like the material type,
the thickness of the plastic, and its mechanical properties influence the effectiveness and
efficiency of the ultimate aligner produced. The first generation of aligners used a polymer
mixture called Proceed 30. Its main aim was to treat mild crowding and close spaces in simple
cases [34]. However, it did not meet all the physical, chemical, and clinical requirements for
orthodontic tooth movement [35,36]. These early issues led to modifications and advances
in aligner systems. These changes included the addition of attachments for better control of
the tooth movement, and changes in the material that was used to produce aligners, aiming
to incorporate the properties that are essential to achieve the predicted orthodontic tooth
movement. “Exceed 30” was a single-layer polymer material that provided 1.5 times greater
elasticity and 4 times more adaptability than Proceed 30. It was also more patient-friendly
in terms of the insertion and removal of the aligners. The pursuit to improve the aligner
material by improving the aligner material properties continues to this day. In 2013, the
Exceed 30 material was replaced by a new multi-layer material called the SmartTrack TM
(LD30). This material was very superior to its precursors as it performed better. It delivered
consistent forces that were gentle and had long-term action, and it had improved properties
like greater elasticity, chemical stability, and a precise fit [37–40]. It was also more comfortable
to patients [41]. Since then, developments in the aligner systems have been involved with
increasing the ability to treat more complex and severe malocclusions compared to the ones
that can be treated successfully with fixed orthodontic treatments.
Medicina 2023, 59, 1773 6 of 14

Table 1. Table showing the range of tooth movements for various aligners and their effectiveness as reported in the literature.

Year Author Study Design Malocclusion Patient Group Extrusion Intrusion Rotation Bodily Movement Tipping
Retrospective
need for
Greatest correction necessary 20.5% inclination
Palone et al. [30] overcorrection
(buccal–lingual crown
aligner system: F22 (measured the 150 patients - 40.2% upper incisors
Class I malocclusion, tipping)
Aligners amount of correction (80 women and 70 - 39.7% lower incisors
2022 minimal crowding 11.7% 22% 14.5%
Overall: required in the men; mean age: 33.7 - 28.8% upper canines
(≤3 mm) angulation
approximately 20% finishing phase to years) - 28.7% lower canines (mesial–distal crown
(19.42%) achieve the
tipping)
prescribed
outcomes)
Lowest overall accuracy (46%)
Extrusion of the
maxillary incisors - Difficult for canines,
premolars, and molars Buccal–lingual crown
(55%) had the Incisor intrusion remained
Haouili et al. [33] - Distal rotation (37%) tip was overall most
highest accuracy, a challenge
aligner system: 22 Class I, was significantly less accurate (56%) tooth
Prospective 38 patients whereas extrusion And did not improve (35%),
2019 Invisalign 13 Class II, and accurate than mesial movement
follow-up study Mean age: 36 years of the maxillary whereas second molar
Mean accuracy for 3 Class III rotation (52%) Labial crown tip max
and mandibular intrusion had high
all TMs: 50% - Mesial rotation of max LI (70%)—most
molars accuracy (51%)
1st molars (28%—least accurate
(40%) had the
lowest accuracy accurate)

- Highest ac-
curacy: pure
tipping (77.48%)
Translation: 49.50% - Controlled
Retrospective tipping: 65.24%
measurement of 69 patients - Less effective
Jiang et al. [31] on maxillary in- - Torque: 35.21%
different types of Crowding Age ≥ 20 years (lingual root
2019 aligner system: cisors compared
incisor Non-extraction Overall efficacy: movement was
Invisalign to mandibular
movements in 55.58% significantly
sagittal plane incisors
more difficult
to accomplish
than labial root
movement)
Medicina 2023, 59, 1773 7 of 14

Table 1. Cont.

Year Author Study Design Malocclusion Patient Group Extrusion Intrusion Rotation Bodily Movement Tipping

- Most inac-
curate of
all linear
movements - Most inaccurate Horizontal movements
Class I (mild, mod, - Accurate of all incisors
- Differences - Discrepancy of 0.9
Charalampakis severe) - No sta- (buccal-lingual crown
20 adult patients from 0.8 to 1.5 to 3.06 degrees
et al. [34] - Crowding tistically tip):
2018 Retrospective Avg age: 37 years mm - Highest inac-
aligner system: - Deep bite significant accurate, with either
and 6 months - The maxil- curacy in the
Invisalign - Open bite differences small (0.20–0.25 mm)
lary central maxillary canine
observed or insignificant
incisors had region
the greatest differences
difference of
1.5 mm

- Not effective
in controlling
anterior extru-
sion Anterior intrusion is Orthodontic Aligner
- Contrasting achievable with Therapy (OAT) is
OAT is not effective in
Rossini et al. [32] results for OAT and is effective in controlling
controlling rotations,
2015 aligner system: Systematic review posterior ver- comparable to that upper molar bodily
especially
Invisalign tical control, reported for the movement when a
for rounded teeth
so a definite straight distalization of 1.5 mm is
conclusion wire technique prescribed
cannot be
drawn
Medicina 2023, 59, 1773 8 of 14

The latest trend in the development of OAT is shape-memory polymers (SMPs) and
direct 3D printing of the aligners [42–44]. SMPs are a type of smart material or stimuli-
responsive polymer material. These materials are able to react suitably with external
stimuli, such as thermal, electrical, or magnetic input, producing a predictable repeatable
output. SMPs have the ability to change their macroscopic shape under a proper stimulus.
Some preliminary in vitro investigations using a thermal responsive SMP (shape-memory
sheet (ClearX)) by Kline Europe GmbH, Düsseldorf, Germany and Graphy material have
shown tremendous promise [44]. Unlike shape-changing materials (conventional materials),
shape-memory materials have the capacity to maintain a stable temporary shape until
they are appropriately activated to recover their original shape. Shape recovery forces
generated upon appropriate thermal stimuli were used to move a tooth on a typodont.
The aim was to overcome the rate-limiting staging of conventional aligner materials and
show the possibility of using one shape-memory aligner instead of three subsequent
conventional aligners in order to decrease the number of aligners used per treatment,
saving money and time, reducing plastic consumption, and consequently decreasing the
total cost. One added advantage was that thermo-responsive SMPs were transparent and
aesthetically pleasing. This, along with their intrinsic shape recovery property, made them
a novel orthodontic material. Elshazly and co-authors concluded that experimentally,
tooth movement could be conducted on a typodont model by using clear aligners made
of shape-memory polymers (SMPs) [45]. The aligner, however, should undergo different
steps of special heat treatment above its transition temperature in order to initiate its shape-
memory recovery. Aligners made of SMPs could be a promising future choice for aesthetic
orthodontic treatment [46].
Just as digital impressions and 3D-printed models proved superior to manual impres-
sions and plaster models, clear aligners that are 3D printed directly may eliminate the errors
resulting from the thermoplastic workflow, apart from the errors that result from analog
impressions [14]. In September 2021, the South Korean manufacturer, Graphy, showcased
the world’s first direct 3D-printed aligner, which was produced from the company’s own
3D printing resin. This resin is a patented technology called Tera Harz TC-35, which is a
clear biocompatible photocurable resin that claims to be equipped with a shape-memory
function that, according to Graphy, is the only one available in the current market [43].
Graphy also claimed that these aligners could be produced from any 3D printer. Another
feature includes the ability of the aligner to rotate teeth by up to 35◦ , which other aligners
may struggle to achieve. Compared to the manufacturing method of existing transparent
aligners, model output is not required, and manual work such as thermoforming, cutting,
and finishing is not required, so there are few errors and less waste during the manufac-
turing process, and the manufacturing time and cost are also reduced. Moreover, heat
sterilization is possible with this material. In the case of thermoforming materials, they are
completely deformed in hot water, whereas in the case of the material used by Graphy, their
shape and physical properties are restored in hot water, so if the device is used for a long
time or if the device is contaminated by food or foreign substances, it can be heat-sterilized
by putting it in hot water and can always be kept clean (Figure 5). An in vitro study found
that changing the thickness of a direct 3D-printed aligner (TC-35) resulted in changes to
the magnitude of forces and moments generated that were more optimized with minimal
unwanted side effects, thereby producing more predictable tooth movement [42]. Yet
another study found that a direct 3D-printed aligner (TC-35) was able to achieve higher
extrusive forces on the maxillary central incisor without attachments by using pressure
columns in the aligner itself when compared with thermoformed aligners [47]. Finally, a
direct 3D-printed aligner was found to deliver more consistent forces with fewer unwanted
side effects over a 14-day period in an in vitro study [43]. The development of this material
is revolutionary as it incorporates the benefits of shape memory and 3D printing in one
aligner system.
 to achieve higher extrusive forces on the maxillary central incisor without attachments by
using pressure columns in the aligner itself when compared with thermoformed aligners
[47]. Finally, a direct 3D-printed aligner was found to deliver more consistent forces with
fewer unwanted side effects over a 14-day period in an in vitro study [43]. The develop-
Medicina 2023, 59, 1773 ment of this material is revolutionary as it incorporates the benefits of shape memory
9 of 14 and
3D printing in one aligner system.

Figure
Figure A direct
5. direct
5. A 3D-printedaligner
3D-printed aligner being
beingplaced
placedinin a water bath
a water of 35
bath ofdegrees Celsius.
35 degrees Notice
Celsius. that that
Notice
the aligner is now able to deform under pressure but recovers its shape memory
the aligner is now able to deform under pressure but recovers its shape memory when placed backwhen placed back
intointo
thethe water
water bath.This
bath. Thisisisan
an important
important first
firststep
stepinindirect 3D-printed
direct 3D-printed aligners.
aligners.
5. Commercial Aligner Companies
5. Commercial AlignerisCompanies
Align Technology the pioneering parent company of Invisalign that offers commercially
Align Technology
produced clear aligners.is The
the interactive
pioneering parentfor
software company of Invisalign
tooth movement that
planning offers com-
is known
as ClinCheck. Invisalign provides a clinical guide on the complexity of malocclusion
mercially produced clear aligners. The interactive software for tooth movement planning that
governs the choice of the different delivery packages based on the number of
is known as ClinCheck. Invisalign provides a clinical guide on the complexity of maloc-aligners that
are predicted
clusion by thethe
that governs ClinCheck
choice of software that simulates
the different thepackages
delivery OAT outcomes.
based The packages
on the number of
comprise Express, Lite, Moderate, and Comprehensive, with an increasing number of aligners
for each respective package to treat the increasing complexity of the malocclusion.
Besides Align Technology, other recent commercial companies that produce clear
aligners include Henry Schein (Reveal), Straumann (Clear Correct), Angel Align, Ormco
(Spark), and Smartee, to name a few. Each company has its proprietary interactive tooth
movement planning software for communication between the clinician and the technician,
assisting with the tooth movement planning process.
Many companies incorporate clinical teeth that allow for the integration of 3D CBCT
data into the crowns of the teeth. These revolutionary changes now mean that a true
3D virtual patient may be acquired for diagnosis, treatment planning, and appliance
fabrication [28,29].

6. OAT Differentiations
As clinicians make choices for their dental offices, how would one differentiate the vast
number of OAT appliance manufacturers? This paper proposes a simple method to evaluate
the various OAT appliance manufacturers. Table 2 describes the various decisions that a
clinician can make when investing in a particular system. Not all practices are ready to
embrace a fully integrated orthodontic office, and enhanced decisions have to be made based
on patient population, practice need, investment in the practice, and brand positioning.
Medicina 2023, 59, 1773 10 of 14

Table 2. A simple table that any clinician can use when evaluating the aligner manufacturer of choice
for the dental office.

Comprehensive Normal Basic Limited

Patient Records Clinical Pictures and Radiographs X X X
Intra-Oral Scan Uploads /
Dedicated X X
PVS Impressions
3rd Party X X
Software Interface Interactive X X
Static X X
Tooth Movement All Teeth X X
Most Teeth X
Some Teeth X
Attachments All Teeth X X X
3D Controls Real-Time Movements X X X
Real-Time Attachments X X
Real-Time IPR X
Real-Time Simulation of Doctor Changes X
Tray Cut-Outs X
Complex Tooth Movements Tooth Expansion X
Sagittal Correctors X
Interactive Patient Tools Predictive Simulation X X
Compliance Indicators Built Into
X
Aligners
Software Subscription Fees X X X X
Clinical Research X X
Post Treatment Provision of Invisible Retainers X X
Duration of Coverage of Aligner
X X
Production Fees (Clearly Stated Fees)
X—needed requirement.

7. Direct-to-Consumer (DTC) Aligners

Direct-to-Consumer (DTC) aligners pose a challenge to the modern orthodontic office
that offers CAT in that the delivery of aligners is directly from the commercial companies
to the patients. A recent modification to the original modus operandi of some of these DTC
companies was the enlistment of clinicians from participating dental clinics to conduct a
general clinical and radiographic dental examination for caries and periodontal problems,
place attachments, and issue aligners and to perform an interproximal reduction based on
the instructions dictated by the treatment plan developed by the companies. The drawback
of DTC aligners is the lack of clinical ownership, as the participating clinician is not the one
who assesses and plans the tooth movement; it is performed remotely by the clinical team
that is employed by the DTC companies. The modern orthodontic office must be prepared
to manage and treat failures of DTC aligners. Thus, a modern orthodontic office must be
equipped not only to deliver in-office OAT, but also to educate patients on the differences
between in-office OAT and DTC aligners [48–50].

8. Clinical Performance of Orthodontic Clear Aligners

A prospective study conducted in 2009 found that the mean accuracy of tooth move-
ment with Invisalign aligners was at best 41%. Lingual constriction was found to be the
Medicina 2023, 59, 1773 11 of 14

most accurate at 47.1%. The least accurate movements were the extrusion of maxillary and
mandibular incisors, followed by the mesiodistal tipping of canines. A rotational movement
greater than 15 degrees resulted in a decrease in the accuracy of the tooth movement that
was simulated [51]. A similar prospective study conducted in 2020 found that the mean
accuracy had improved to 50% for all tooth movements. Although the overall accuracy had
improved, the study concluded that the strengths and weaknesses of Invisalign aligners
did not change much over the years of its research and development [33].
A systematic review concluded that Invisalign was a viable tool to treat mild to moder-
ate malocclusions in non-growing patients with a non-extraction approach.
Clear recommendations for other treatment strategies with the use of aligners could not
be ascertained due to a lack of standardized protocols and high variations of clinical and
research methods. Limited efficacy was found for arch expansion with bodily tooth move-
ment, the closure of extraction spaces, corrections of greater vertical and anteroposterior
discrepancies, and establishing occlusal contacts [52]. Another systematic review found
a low-to-moderate level of evidence for tooth movement efficacy in orthodontic aligner
therapy. Acceptable outcomes were comparable with fixed appliance therapy for mild to
moderate malocclusions. The review also concluded that predictable tooth movements
could not be achieved with a single series of aligners [53].
A retrospective clinical study found that one in every six patients who started with
OAT was converted to fixed appliance therapy to complete the treatment. Patients with
an average number of 81 aligners were more likely to switch and complete the treatment
with a fixed appliance. A longer overall treatment time was demonstrated in patients who
converted to a fixed appliance. Only 6% of OAT patients completed their treatment without
additional refinement aligners. The study also concluded that two to three additional series
of refinement aligners could be expected, and the average treatment time was 2 years [54].
An unwanted effect associated with OAT is unplanned molar intrusion due to the
interface between the upper and lower aligner materials contacting each other. A study
found that 74% of patients demonstrated unplanned molar intrusion after treatment. Max-
illary molar intrusion accounted for 15.5%, and mandibular molar intrusion accounted for
32.8%, while maxillary and mandibular molar intrusions accounted for 25.9%. A vertical
facial type and muscular stimulus while wearing aligners were proposed as possible factors
associated with this unplanned effect. Although the investigators were of the opinion that
unplanned molar intrusion was self-limiting, vertical occlusal settling and spontaneous
re-equilibration over a longer period of time were not evaluated and would require further
investigation and validation [55].
In summary, the recent research studies suggest that the efficacy of aligners has yet to
achieve a high level of predictability for the full range of tooth movements. This would
impact the overall treatment duration when the predicted number of aligners needed to
deliver the outcome is not matched clinically. Greater amounts of clinical time and effort
are needed with either an additional series of refinement aligners or a conversion to a
fixed appliance in order to finish and detail the occlusion to achieve the required treatment
outcome.

9. Future Research
The recent research on orthodontic aligner therapy has provided a better understand-
ing of plastic material behavior and tooth movement responses. However, much research
is needed to provide evidence-based clinical data for the following:
1. Factors that determine the choice of 7-, 10-, or 14-day protocols for an aligner change.
2. The in vivo performance of optimized versus conventional attachments for all types
of tooth movements besides rotation and extrusion.
3. The accuracy of the amount of interproximal reduction in the enamel determined by
the software to facilitate the required tooth movement that will be clinically achieved.
Medicina 2023, 59, 1773 12 of 14

4. The magnitude and degree of overcorrection for the various types of tooth movements
in order to compensate for the inherent limitation of the plastic material to fully express
the desired tooth movement or to counteract unwanted tooth movement.
5. The mid- and long-term outcomes of unplanned molar intrusion on the vertical
settling of functional occlusion during retention.
6. An improvement in the predictability of the software simulations of treatment out-
comes that are realistically achievable clinically.
7. The accuracy of the prediction of gingival margin height changes with tooth move-
ment such as extrusion and intrusion.

10. Conclusions and Final Thoughts

The future brings tremendous promise for clinicians. The ability to customize and
individualize a patient’s treatment journey is exciting yet daunting. The authors have
found that the early adoption of technology can sometimes be exciting, but beta versions
of new technology often do not live up to their expectations [56]. The fundamentals of
orthodontic examination and diagnosis, treatment planning, and biomechanics combined
with clinical competency, experience, and acumen remain the pillars of sound orthodontic
treatment. As a result, a careful understanding of technology coupled with clinical-based
evidence is the key to success.

Author Contributions: Conceptualization: C.H.K. and A.M.; methodology, C.H.K., A.M. and J.S.;
writing-originial draft preparation, C.H.K. and A.M.; writing-review and editing, C.H.K., A.M., J.S.
and T.C. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Informed Consent Statement: Written informed consent has been obtained from the patient(s) to
publish this paper.
Conflicts of Interest: The authors declared no conflict of interest.

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