Aligners: The Science of Clear Orthodontics

Abstract

Since their inception, clear aligners have undergone a multitude of advancements that aimed to make orthodontic treatment less conspicuous. Used for moving teeth for alignment and creating healthy and beautiful smiles, like many other orthodontic appliances, clear aligners have proven to be a much more comfortable and esthetic alternative. The treatment success relies on clinicians’ knowledge and experience with the aligners, proper case selection, and the patients’ adherence to the treatment protocols. The area of exploration of this review is limited to providing an overview of the clear aligner treatment including benefits and limitations, principles and biomechanics of clear aligner treatment, materials used,
methods of fabrication, and wear time. Umpteen manufacturers have emerged accounting for the increased demand by patients seeking orthodontic treatment, with the number increasing every year. It is outside the purview of this article to elucidate and compare the array of clear aligner systems available today.

Full text

38 International Journal of Dental and Medical Specialty Vol 7 ● Issue 1 ● Jan-Jun 2020
Aligners: e Science of Clear Orthodontics
Sunegha Kundal1, Tulika Shokeen2
1Specialist Orthodontist, Private Practitioner, Jammu and Kashmir, India, 2Specialist Orthodontist, Private
Practitioner, Delhi, India
ABSTRACT
Since their inception, clear aligners have undergone multitude of advancements that aimed to make the orthodontic
treatment less conspicuous. Used for moving teeth for alignment and creating healthy and beautiful smiles, like many
other orthodontic appliances, clear aligners have proven to be a much more comfortable and esthetic alternative. The
treatment success relies on clinicians’ knowledge and experience with the aligners, proper case selection, and the patients’
adherence to the treatment protocols. The area of exploration of this review is limited to providing an overview of the clear
aligner treatment including benets and limitations, principles and biomechanics of clear aligner treatment, materials used,
methods of fabrication, and wear time. Umpteen manufacturers have emerged accounting for the increased demand by
patients seeking orthodontic treatment, with the number increasing every year. It is outside the purview of this article to
elucidate and compare the array of clear aligner systems available today.
Key words: Esthetics, clear aligners, orthodontics, tooth movement
INTRODUCTION
Orthodontic treatment has long been synonymous to a
mouth full of “unattractive” wires. The stigma extended
beyond outer appearance, with people perceiving those with
metal braces to have a lower intellect.[1] Introduction of clear
aligners thus came as a respite to the orthodontic patient, for
their esthetic superiority made them more attractive to metal
and ceramic brackets.[2] These are the active clear plastic
trays fitting snugly onto the teeth, each worn for 2 weeks
on an average, and changed sequentially to accomplish the
incorporated tooth movements. From being used for mild
malocclusion cases at the outset to treating vast variety of
cases contemporarily, from minimal crowding to bicuspid
extractions, clear aligners have come a long way and still
continue to evolve.
THE ORIGIN STORY
Contrary to popular belief, clear aligners are not a new
find. The chronology leading up to their present-day use
dates to 1945, when Dr. Harold D. Kesling first advocated
a rubber appliance for moving teeth.[3] In 1964, Nahoum
fabricated the dental contour appliance in what was
to be the first thermoformed plastic sheet for moving
teeth.[4] Sheridan modified the same in 1993, calling it
the Essix Appliance.[5] However, fabricating the appliance
was a laborious process requiring impression-making at
every successive appointment. The solution arrived in
the form of a clear aligner system created digitally by two
Stanford graduates – Zia Chishti and Kelsey Wirth. Called
Invisalign®, it was launched in 1997 by Align Technology©
(Santa Clara, CA) and is credited with bringing a comfortable
alternative to braces to mainstream orthodontics. In 2005,
a competitor emerged in the form of OrthoClear® – a
clear aligner system developed independently by one of the
original founders Zia Chishti. However, it was withdrawn a
year later due to lawsuits alleging patent infringement. Many
practicing clear aligner users suffered as a result, one of whom
was Dr. Willis Pumphrey whose 400 patients were stranded
mid-treatment. He responded by developing ClearCorrect
Address for Correspondence:
Dr. Sunegha Kundal, Specialist Orthodontist, Private Practitioner, Jammu - 180011, Jammu and Kashmir, India. Phone: +91-
9086505505. E-mail:suneghakundal@gmail.com
Submission: 27 May 2020; Revision: 16 June 2020; Acceptance: 28 June 2020
Review Article
Access this article online
Publisher
Website:
www.ijdms.in
DOI: 10.30954/IJDMS.1.2020.9

Kundal and Shokeen: Clear aligners
International Journal of Dental and Medical Specialty Vol 7 ● Issue 1 ● Jan-Jun 2020 39
(ClearCorrect, Round Rock, TX, USA), an Invisalign®
alternative that quickly gained popularity. An onslaught of
clear aligner systems has since followed that are being used
to correct minor to complex malocclusions.[6]
BENEFITS AND LIMITATIONS
Benets
i. Esthetics: A transparent, clear design that is discreet.
ii. Comfort: Free from brackets or wires and resultant cuts
and ulcerations.
iii. Improved oral hygiene and periodontal health:
Removable nature allows proper brushing and flossing.
iv. No food restrictions: Patients are free to eat or drink
anything during clear aligner therapy (CAT).
v. Predictable treatment time: Digital planning ensures a
precise estimation of treatment time.
vi. Decreased dental office visits: Patients themselves change
their subsequent aligners, requiring fewer office visits.
vii. Minimal emergencies: Significantly reduced emergency
appointments with CAT as compared to fixed orthodontic
treatment.
viii. Possible to include teeth with structural anomalies and
difficult-to-bond surfaces.
Limitations
i. Dependency on patient compliance: A wear time of 22
h/day is mandatory for therapy to be effective.
ii. Limited extent of tooth movements by aligners alone:
For complex movements, auxiliaries are required as an
adjunct to aligners.
iii. Higher cost.
iv. Initial slurring of speech: Subsides subsequently after
2–3 days of wear.
v. Breakages: While wearing or removing if the patient
applies excessive force.
vi. Chances of losing the aligners: Removable nature makes
aligners prone to being misplaced.
vii. Inconvenience: Removing aligners every time one eats
or drinks can be a burden.
viii. Manufacturing defects: Results in ill-fitting aligners.
PRINCIPLES OF CAT
CAT works on two basic principles:[7]
1. Creating space: Through arch expansion, tooth
extraction, or interproximal reduction (IPR).
2. Force/pressure application: Through small incremental
movements incorporated in aligner itself and with
auxiliaries (attachments, dimples, elastics, or digital
power chains). The amount of pressure required for
moving a tooth depends on factors such as tooth shape,
tooth size, type of movement, and periodontal condition.
BIOMECHANICS OF CAT
The mechanism of tooth movement with clear aligners is
centered around two systems:[8,9]
1. Displacement-driven system: Controls tipping and minor
rotations. Aligners are fabricated as per next staged
position; the tooth continues to move till the aligner
becomes passive. No root movement elicited.
2. Force-driven system: The software determines the type
of movement required for an individual tooth, the
mechanical principles needed to achieve that movement,
and aligner shape. Pressure points and attachments are
incorporated into aligners that apply the forces required
for planned movements.
TYPE OF MATERIALS USED
Clear aligner materials have evolved from a single layered
or monophasic plastic to the 2nd generation polyurethane
material, to the currently used 3rd generation multilayered
polyurethane-like material that comprises of hard and
soft layers. While the soft layer imparts the property of
elastic deformation allowing smooth seating of the
aligner, the hard layer ensures strength and durability.[10]
Polyethylene terephthalate glycol modified (PET-G) remains
the commonly used material. Other materials include
polypropylene, polycarbonate, thermoplastic polyurethanes,
and ethylene vinyl acetate.[11] Table 1 enlists popular clear
aligner brands and the material used for aligner fabrication.[12]
Table 1: Clear aligner brands and the material used by them for
aligner fabrication
Code Thickness Product
name
Manufacture Component
(MSDS)
EVA 1.0 mm Bioplast Scheu-Dental,
GmbH
Ethylene-
vinyl acetate
copolymer
PE 1.0 mm Copyplast Scheu-Dental,
GmbH
Polyethylene
PETG 1.0 mm Duran Scheu-Dental,
GmbH
Polyethylene
terephthalate
glycol
PP 0.8 mm Hardcast Scheu-Dental,
GmbH
Polypropylene
PC 0.75 mm Imprelon
“S”
Scheu-Dental,
GmbH
Polycarbonate
A+ 0.040 in. Essix A+ Raintree
Essix, Inc.
Copolyester
C+ 0.040 in. Essix C+ Raintree
Essix, Inc.
Polypropylene/
ethylene
copolymer
(>95%),
stabilizers (<5%)
PUR 0.030 in. Invisalign Align
Technology,
Inc.
Polyurethane
from methylene
diphenyl
diisocyanate and
1,6-hexanedial,
Additives
Component as obtained from the manufactures’ material safety data sheet

Kundal and Shokeen: Clear aligners
40 International Journal of Dental and Medical Specialty Vol 7 ● Issue 1 ● Jan-Jun 2020
PROPERTIES OF CLEAR ALIGNER
MATERIAL
Mechanical Properties
Force delivery of clear aligners depends on:
a. Amount of activation: The extent of movement
prescribed per aligner differs for every clear aligner
system. ClearSmile® system allows 0.5 mm, ClearCorrect
(ClearCorrect, Round Rock, TX, USA) prescribes
0.3 mm, whereas Invisalign (Align Technology, Inc.,
Santa Clara, CA, USA) incorporates 0.25 mm of tooth
movement per aligner.[13,14]
b. Type of material: Pressure formed appliance exerts greater
force systems at higher rates of activation as compared
to vacuum formed ones.[15,16]
c. Thickness of material: Ranges from 0.5 to 1.5 mm; thicker
the material, higher the force.[17,18]
d. Size and shape of adjacent teeth.[16]
Wear resistance of an aligner is indicated by the hardness
of its material; higher the hardness value, better the wear
resistance. Similarly, higher elastic index indicates brittleness
of the material; a high modulus of elasticity increases the
force delivery capacity under constant strain, which is a
desirable property for an aligner material.[19]
Aging
Mechanical properties of the aligner material deteriorate
with time. Microcracks, delaminated areas, calcified
biofilm deposits, and loss of transparency were reported
in Invisalign (Align Technology, Inc., Santa Clara, CA,
USA) aligners worn for 2 weeks.[20,21] The residual stresses
decrease in the thermoplastic materials with time.[22] A
change in fit of the appliance and resultant orthodontic
forces can be seen as a result of intraoral hygroscopic
expansion. Water absorption of thermoplastic materials
increases with time, with Invisalign (Align Technology,
Inc., Santa Clara, CA, USA) showing the highest
absorption followed by PET-G.[12]
Thermal Properties
All polymers have a glass-transition temperature (Tg) at
which the rigid state converts to a rubbery state. When
thermal properties of three clear aligners (Invisalign
[Align Technology, Inc., Santa Clara, CA, USA), Simpli5
[Allesee Orthodontic Appliances, Sturtevant, WI, USA],
and ClearCorrect [ClearCorrect, Round Rock, TX, USA])
were compared, all three aligner systems were found to
have a Tg above the accepted oral maximum temperature,
with no significant difference seen before and after
clinical use.[23]
Stainability
The aligner material should be able to maintain its color
stability and transparency. To avoid pigment adsorption, it
is recommended to remove aligners during eating and while
drinking colored drinks. Color changes in aligners have been
reported on exposure to staining solutions such as coffee,
black tea, and red wine.[24] Invisalign (Align Technology, Inc.,
Santa Clara, CA, USA) was more prone to pigmentation
when exposed to coffee or red wine.[25]
FABRICATION OF CLEAR ALIGNERS
Manual
The individual teeth are sectioned and repositioned
sequentially using a wax setup. Once a working cast is
obtained, teeth are removed from cast and moved into
the planned position, with aligners sheets molded onto
the realigned models using pressure molding or vacuum
machine.[26]
Digital (CAD-CAM)[27]
• Patients’ impressions obtained digitally by intraoral
scanner and uploaded for virtual treatment planning.
• In case of traditional impressions (polyvinyl siloxane
putty), casts are obtained, scanned, and uploaded.
• Digital models are then analyzed for IPR and expansion
requirements.
• Teeth are sectioned digitally.
• IPR prescription is reviewed for validity and if acceptable,
IPR is performed digitally and plan is communicated to
the doctor.
• Teeth are then moved into correct alignment digitally
and a digital overlay model is created by superimposing
final model onto the original model.
• Sequence of aligners fabricated using the overlay model.
• The force bumps, attachments, or auxiliaries are then
planned for aiding tooth movements.
GINGIVAL MARGIN DESIGN OF
ALIGNERS
Clear aligner should seat completely on insertion and
be properly anchored to the dental arch without any
dislodgement or vertical lifting. A decline in aligner forces
at gum line is seen due to flexibility of gingival margins, thus
impeding the ability to produce certain tooth movements.
The possible margin designs are scalloped, straight cut at the
gingival zenith, and straight cut 2 mm above the gingival
zenith (most retentive) [Figure 1].[28]

Kundal and Shokeen: Clear aligners
International Journal of Dental and Medical Specialty Vol 7 ● Issue 1 ● Jan-Jun 2020 41
WEAR TIME OF ALIGNERS
Wearing an aligner for 22 h/day for 2 weeks is mandated to
ensure maximum expression of aligner prescription.[29] A
decrease in daily wear time likely decreases the efficacy of
certain incisor movements, reportedly labial inclination and
buccolingual translation of mandibular incisors, rotation of
maxillary central incisors, as well as buccolingual translation
and intrusion of maxillary lateral incisors.[30] However, most
incorporated movements occur within the 1st week, thus
casting doubt over usefulness of the additional 1 week of
aligner wear.[9,31]
TOOTH MOVEMENTS WITH CLEAR
ALIGNERS
Replacing braces with clear polyurethane trays for
orthodontic treatment have raised questions regarding its
efficiency in moving the teeth. Tooth movement obtained
most accurately with clear aligners is retrusion, followed by
rotation, fan-type expansion, and protrusion, respectively.[32]
Acceptable results can be achieved for the buccolingual
inclination of maxillary and mandibular incisors in mild-to-
moderate malocclusions.[33]
CAT is recommended in non-extraction cases with mild-
to-moderate malocclusions in non-growing patients.[34] In
extraction cases, proper root angulations can be attained
with use of suitable attachments and adequate knowledge
of the system.[35] Maxillary molar distalization (2.5 mm)
and premolar extraction space closure (7 mm) are the
most predictable and controlled movements that can be
obtained.[36] To its demerit, as occlusal contacts are lost due
to thickness of aligners, problems with the final settling of
the occlusal plane can arise.[37]
ROOT RESORPTION WITH CLEAR
ALIGNERS
While fixed orthodontic appliances cause root resorption
as a side effect of heavy orthodontic forces, there is no clear
consensus on root resorption with CAT. A lower incidence
and severity of root resorption has been reported, with
incisors being the most affected teeth.[38] The incidence of
root resorption was seen to be comparable between aligners
and light orthodontic forces.[39] Another study revealed
a lower severity and prevalence of root resorption with
aligners.[40]
CONCLUSION
Clear aligner treatment has flourished since its introduction
into orthodontics. Continued developments on the
technological front are being adapted to improve its
efficiency, especially in complex cases. Being a removable
appliance, patient compliance is the foremost criteria for
success of treatment with aligners and patient motivation
is indispensable to avail the planned treatment outcomes.
Arrays of companies are manufacturing aligners today,
providing them directly to consumers, or routing through
the dentists; it falls beyond the scope of this article to study
the individual characteristics of each system.
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How to cite this article: Kundal S, Shokeen T. Aligners: The Science of
Clear Orthodontics. Int J Dent Med Spec 2020;7(1):38-42.
Source of Support: None; Conicts of Interest: None