Effect of Clear Aligner Therapy on Jaw Motor Function

by

Tiantong Lou

A thesis submitted in conformity with the requirements for the degree of Master of Science (Orthodontics)

Faculty of Dentistry University of Toronto

© Copyright by Tiantong Lou 2019

ii

Effect of Clear Aligner Therapy on Jaw Motor Function

Tiantong Lou

Master of Science (Orthodontics)

Faculty of Dentistry University of

Toronto

2019

Recent evidence indicates that clear aligner therapy (CAT) induces mild to moderate tooth and

jaw muscle pain. This study aimed to measure the effect of CAT on the electromyographic (EMG)

activity of the masseter. It was hypothesized that CAT induces an increase in the EMG activity of

the masseter during day-time.

The EMG activity of the masseter was recorded in 17 adults over 4 weeks (baseline – no aligners;

a passive aligner not moving teeth; first active aligner; and second active aligner) using portable

devices.

CAT induced a significant increase in masseter activity (all p<0.001). Compared to baseline, the

passive and the first active aligner induced +152.4% and +155.1% relative increase in EMG

activity, while the second active aligner induced a +61.7% EMG increase (all p<0.001).

CAT induces an increase in the EMG activity of the masseter. Caution should be taken while using

CAT in patients at risk for temporomandibular disorders.

iii

Acknowledgments

To my research supervisor, Dr. Iacopo Cioffi, I will always be thankful for your continued and

endless guidance, support and advice throughout the past three years. This research would not

have been possible without your knowledge and expertise. I will always appreciate your

exceptional leadership, mentorship and supervision with this project.

I also like to express my sincerest gratitude to my research committee members, Dr. Eszter

Somogyi-Ganss and Dr. Angelos Metaxas, for taking time out of your busy schedules to help me

with my research and to provide me with your advice and guidance throughout my thesis. I

would like to acknowledge Align Technology for their research grant in support of this project.

To my co-residents, thank you for the past three years of wonderful experiences and memories.

To the entire Faculty and Staff of the Orthodontics Department, I would like to thank everyone

for all that you have taught me over the years.

I would like to thank my family and friends, especially my mother Xiao Lou and my father

Jianzhong Huang, who have supported and believed in me throughout this journey. Lastly, I

would like to dedicate this thesis to the loving memory of my late grandfather, Xia Songde, who

encouraged me to always strive to improve and be better every day.

iv

Table of Contents

Acknowledgments.......................................................................................................................... iii

Table of Contents .......................................................................................................................... iv

List of Figures ............................................................................................................................... vi

List of Appendices ........................................................................................................................ vii

List of Abbreviations ................................................................................................................... viii

Chapter 1 Introduction .................................................................................................................. 1

Clear Aligner Therapy ........................................................................................................... 1

1.1 Introduction to Clear Aligner Therapy ...................................................................... 1

1.2 Historical Precursors to Clear Aligners ...................................................................... 2

1.2.1 Contemporary Clear Aligners ..................................................................................... 4

1.3 The Concept of Clear Aligner Therapy ...................................................................... 5

1.4 Benefits, Limitations and Controversies of Clear Aligner Therapy ........................ 6

1.5 Patient Perception of Clear Aligner Therapy ............................................................ 6

1.5.1 Attractiveness and Acceptability ................................................................................ 6

1.5.2 Social Perceptions ....................................................................................................... 7

1.6 Categories of Aligners................................................................................................... 8

1.7 The Invisalign® System ................................................................................................ 9

Jaw Motor Responses during Orthodontic Treatment ....................................................... 11

2.1 Role of Occlusion in Jaw Muscle Activity................................................................. 11

2.1.1 Influence of Occlusal Relationship in Jaw Muscle Activity ..................................... 11

2.1.2 Effect of Occlusal Interferences in Jaw Muscle Activity ......................................... 12

2.2 Psychosocial Factors in the Modulation of Jaw Muscle Activity ........................... 13

2.2.1 Effect of Psychological factors on Jaw Muscle Activity .......................................... 13

2.2.2 Psychological Profile of the Clear Aligner Patient ................................................... 14

Pain in Orthodontics ............................................................................................................ 14

3.1.1 Progression of Orthodontic Pain ............................................................................... 14

3.1.2 Physiological Responses to Orthodontic Pain .......................................................... 15

3.1.3 Pain in Clear Aligner Therapy .................................................................................. 16

Surface Electromyography .................................................................................................. 20

4.1 Assumptions in the use of EMG for Assessment of Muscle Activity ...................... 20

4.2 Applications of sEMG in Clinical Research ............................................................. 20

4.3 Factors and Limitations Affecting the Electrical Activity of Jaw Muscles ........... 21

4.4 Reproducibility ............................................................................................................ 22

v

Significance of Problem....................................................................................................... 22

Objectives of Study ............................................................................................................... 23

Hypothesis ............................................................................................................................ 23

Chapter 2 Materials and Methods ............................................................................................... 24

Research Ethics .................................................................................................................... 24

Patient Recruitment ............................................................................................................. 24

Experimental Setup .............................................................................................................. 25

3.1 Surface Electromyography Recordings .................................................................... 26

Sample Size ........................................................................................................................... 28

Data Processing and Statistical Analyses ........................................................................... 28

Consent to Participate .......................................................................................................... 28

Chapter 3 Results ......................................................................................................................... 30

Effect of Aligner Condition on sEMG Activity ................................................................... 30

Effect of Individual Daily Recording Session on sEMG Activity ...................................... 31

Effect of Psychological factors and Oral behaviors on sEMG response to CAT .............. 32

Chapter 4 Discussion ................................................................................................................... 33

Summary of effects of Clear Aligner Therapy on Somatosensory Function .................... 33

Effect of Clear Aligner Therapy on Jaw Motor Function ................................................. 33

2.1 The Adaptation of Jaw Muscles to Clear Aligner Therapy .................................... 36

2.1.1 Occlusal Hypervigilance ........................................................................................... 36

2.1.2 Occlusal Interferences ............................................................................................... 36

2.1.3 Possible Alternative Pathway ................................................................................... 37

Individual daily recordings and reproducibility of data ..................................................... 37

Analysis of psychological traits and oral behaviors ........................................................... 38

Conclusions .......................................................................................................................... 39

Clinical Significance ............................................................................................................ 39

Limitations ............................................................................................................................ 39

Future Studies ...................................................................................................................... 40

Appendices .................................................................................................................................... 41

References .................................................................................................................................... 70

vi

List of Figures

Figure 1-1. Contemporary clear aligners systems used for orthodontic treatment.

Figure 1-2. Horizontal rectangular attachment with gingival bevel used for the Invisalign®

system in the ClinCheck® software.

Figure 1-3. Effect of aligner condition on orthodontic tooth pain (VAS 0-100mm). Recreated

with permission. All pairwise comparisons were statistically significant (all p<0.001).

Figure 1-4. Effect of aligner condition on jaw muscle tenderness (VAS 0-100mm). Recreated

with permission. All pairwise comparisons were statistically significant (all p<0.001) except for

baseline with first active aligner and passive aligner with second active aligner (p>0.05 for

both).

Figure 2-1. Schematic Diagram Illustrating Flow of Patient Recruitment.

Figure 2-2. Schematic Illustration of Experimental Design.

Figure 3-3. Effect of aligner condition on the masseter muscle activity. Mean Z-Scores (SEM)

of standardized Root Mean Squared sEMG activity for each aligner condition. All pairwise

comparisons, except for passive-first active aligner, were statistically significant (p<0.001).

Figure 3-2. sEMG activity recorded during each day of the four conditions. Mean Z-Scores

(SEM) of standardized Root Mean Squared sEMG activity for each day, separated by each

aligner condition. All pairwise comparisons within each week, except for day 1-day 3 and day 3-

day5 of the passive aligner, were statistically significant (all p<0.001).

vii

List of Appendices

Appendix 1 - Health Sciences Research Ethics Board (REB) Approval

Appendix 2 - Research Diagnostic Criteria for Temporomandibular Disorders (RDC/TMD)

Examination Form

Appendix 3a – Diagnostic Criteria for TMD demographics

Appendix 3b - TMD-Pain Screener

Appendix 4a – Oral Behavior Checklist

Appendix 4b – Somatosensory Amplification Scale

Appendix 4c – Beck Depression Inventory

Appendix 4d – Pain Catastrophizing Scale

Appendix 5 – Information and Consent for Research Study

Appendix 6 – Research Flyer for Recruitment of Participants

Appendix 7 – Customized Pain Diary

Appendix 8 – Sample Customized Calendar for the Experimental Period

Appendix 9 – Written Instructions for Operating the sEMG Device

Appendix 10 – Video Instructions for Operating the sEMG Device

Appendix 11 – Tabulated data of the mean muscle activity (Z-score), standard error and the 95%

confidence interval for effect of aligner condition on the masseter muscle activity.

Appendix 12 – Tabulated data of the daily mean muscle activity (Z-score), standard error and

the 95% confidence interval for effect of aligner condition on the masseter muscle activity.

viii

List of Abbreviations

ANCOVA Analysis of covariance

ANOVA Analysis of variance

BDI Beck depression inventory

CAD/CAM Computer-aided design and computer-aided

manufacturing

CAT Clear aligner therapy

CPM Conditioned pain modulation

CT Computed tomography

EMG Electromyography

IL-1β Interleukin-1β

MVC Maximum voluntary contraction

OBC Oral behaviour checklist

OTM Orthodontic Tooth Movement

PCS Pain catastrophizing scale

PgE Prostaglandin E

PPT Pressure pain threshold

ix

RDC/TMD Research diagnostic criteria for

temporomandibular disorders

SEM Standard error of the mean

sEMG Surface electromyography

SPSS Statistical package for the social sciences

SSAS Somatosensory amplification scale

STAI State-trait anxiety inventory

TMD Temporomandibular joint disorder

TMJ Temporomandibular joint

VAS Visual analogue scale

1

Chapter 1 Introduction

Clear Aligner Therapy

1.1 Introduction to Clear Aligner Therapy

Although the specialty of orthodontics may seem like a contemporary branch of dentistry, the art

of perfecting one’s smile has been long desired. It is the advent of new appliances and

techniques, developed over the past centuries that has gradually evolved orthodontics into the

sophisticated specialty it is today.

The concept of using aligners to treat malocclusions dates as far back as the early 20th century

with the “Flex-O-Tite” appliance by Remensnyder1. From this, Kesling2 in 1945 created the

rubber-based tooth positioner appliance and proposed the concept of using them in successive

series for incremental tooth movements. It was not until the 1960s that Nahoum3 would

introduce the first clear thermoplastic appliance capable of orthodontic tooth movement. Based

on his idea, Ponitz developed the first “invisible retainer”4 in the 1970s, which was then later

refined by McNamara in the 1980s. A similar appliance known as the Essix retainer was

developed by Sheridan in 1993.5

With the rise of the digital age of the 21st century, we have since been able to integrate modern

technology with these earlier fundamental principles to create a variety of contemporary clear

aligner systems that allow for a more comprehensive approach to orthodontic treatment (Figure

1-1).6, 7

Figure 1-1. Contemporary clear aligners systems used for orthodontic treatment.8

2

1.2 Historical Precursors to Clear Aligners

In 1925, Orrin Remensnyder developed the dental massage device, with the intended use of

exercising and stimulating the gingiva, such as in treatment of periodontitis.1 It was a device

made out of soft rubber, covering the clinical crowns, the marginal gingiva and marketed as the

“Flex-O-Tite” gum-massaging appliance. In the subsequent years, he reported observations of

minor tooth movement occurring with the use of this appliance.9

The fundamental concepts of modern clear aligner therapy can be traced back to Herald Dean

Kesling in 1945.2 The desire for this type of treatment was driven by Kesling’s vision of a simple

appliance that would guide the movement of all teeth into their ideal positions with relation to

one another, without the interferences from any traditional bands or wires. This led to the

conception of a device known as the “Tooth Positioning Appliance”. It is an active orthodontic

appliance used for final artistic positioning of teeth as well as serving as an effective retainer. As

a finishing appliance, the positioner took advantage of the fact that most teeth are still unstable

and mobile from the on-going treatment and should respond readily to its influence. A modern-

day version of the Tooth Positioner Appliance is still available from TP Orthodontics, Inc., an

orthodontic supply company founded by Kesling. The positioner appliance is originally

fabricated from a one-piece pliable rubber material from a wax set up for which it can be

patterned over. It is designed to completely fill the freeway space, as well as covering the labial

and lingual surfaces of the maxillary and mandibular dentition. The Positioner appliance was

intended for the correction of mild dental discrepancies, such as spacing, residual overbites, and

mesio-distal or bucco-lingual relationships. However, as with all new appliances, the tooth

positioner also had its share of drawbacks.10 This included reliance on patient compliance, foul

of the taste of the rubber material, deepening of the overbite, lack of proper inter-digitation and

poor settling of the occlusion.11-15

The first documented clear thermoplastic appliance for the use in dentistry was developed by

Henry Isaac Nahoum in 19593, fabricated using an industrial-grade vacuum former16. It was

known as the dental contour appliance as it was originally designed to maintain or change

contours17. The process could accommodate various materials, including acetates, vinyl, styrene,

polyethylene, and butyrate with translucent, clear or colored sheets.18 Nahoum postulated that

this appliance could be used in orthodontics both as a retainer and for achieving minor

3

orthodontic tooth movements, such as minor rotations and space closure. For its fabrication, an

altered cast is formed by using a jeweler’s saw to section the teeth and baseplate wax is used to

hold them in their new position. From here, the contour appliance is formed over the model. The

resiliency of the appliance material will exert pressure until the teeth have attained their

predetermined positions. In addition to its function as an orthodontic positioner and retainer,

Nahoum also proposed the use of the contour appliance for various other aspects of dentistry,19

such as splints, bite plates, surgical pack holders, medicament carriers and provisional crowns.20-

22 Nahoum built on Kesling’s idea of using a series of appliances in an incremental fashion for

progressively achieving a desired tooth movement. This concept was developed with the

realization that some tooth movements were too great to be corrected in one step. He made

progressive adjustments to the teeth on the altered cast by gradually moving them through the

wax and fabricated a new vacuum-formed retainer for each step. This method was recommended

for usage predominantly in the anterior dentition. The auxiliary elements used in today’s clear

aligner therapy also had origins in Nahoum’s methodology. For example, when both arches are

treated, he suggested the use of bonded acrylic buttons for the attachment of interarch elastics.

Builting on the knowledge of his predecessors, Robert John Ponitz in 1971 provided a more

streamlined fabrication process for vacuum-formed clear plastic appliance.4 The material for

these appliances was proposed to be made out of cellulose acetate butyrate, polyurethane,

polyvinylacetate-polyethylene polymer, polycarbonate-cycolac, and latex. The fabrication

procedure involved preheating a clear plastic material in an oven and using a vacuum unit to

form the material to the shape of the dental arch from a cast. Ponitz proposed that teeth can be

moved and repositioned on the cast using wax prior to the formation of the retainer, thus

allowing for the patient’s teeth to be moved to new positions by the means of the appliance.

Moreover, acrylic bite planes can be formed over or under the appliance and secured with self-

curing acrylic liquid. In cases involving edentulous regions, denture teeth can also be attached to

the retainer by the same method. The main advantages of these clear invisible retainers included:

ease of fabrication; speed of insertion; minimal chair side adjustment; as well as reparability via

heat guns. These appliances, at the time, were also used as holders for periodontal dressing,

surgical splints, temporary partial dentures, as well as splints for occlusal trauma and bruxism.23,

24

4

Ponitz’s technique for fabricating invisible retainers for retention and final detailing were later

refined by James A. McNamara in 1985.25 He reported the fabrication of these appliances using

1 mm thick BiocrylTM polymers with a Biostar forming machine. Rather than the vacuum

pressure technique described by Ponitz, the Biostar machine used positive air pressure to adapt

the thermoplastic BiocrylTM to the surface of the cast. This appliance was reported to be used in

80% of his private practice cases.21 McNamara ultimately concluded that although clear

removable retainers had their advantages, they did not have the same long-term durability of

traditional acrylic or bonded retainers.

In 1993, John J. Sheridan introduced his variation to the family of thermoplastic appliances,

known as the Essix retainer, designed to function both as a retainer and positioner.5 It was

fabricated using a 0.030” sheet of thermoplastic copolyester from Raintree Products. He

advocated the use of a positive air pressure method for the thermoforming process, which would

reduce the thickness of the sheet to 0.015” after completion. In contrast to Nahoum’s idea of

using serial appliances for successive movements, the fundamental principle of the Essix system

is based on the use of a single appliance for in-course adjustments to achieve treatment goals.

The two primary methods of creating tooth movement in the Essix system are via alterations in

the aligner or the tooth surface. The first method involves spot-thermoforming the aligners via

Hilliard thermopliers. The second method, known as mounding, involves alterations to create

projections on the tooth surface, such that a force will be exerted as the resiliency of the aligner

material presses against it. This is usually achieved by bonding composite materials, in the shape

of a mound.

1.2.1 Contemporary Clear Aligners

The concept of introducing a mainstream computer-aided design & computer-aided

manufacturing (CAD/CAM) based clear aligner technology to the mass market was first

conceived by Zia Chishti, a business student from Stanford University.26 After receiving his

orthodontic treatment, he did not consistently wear his clear retainer as prescribed by his

orthodontist and not surprisingly, he experienced relapse of crowding of his lower anterior teeth.

Chishti attempted to use his current retainer to realign his teeth but was frustrated with the

progress.27 This inspired him to develop a computer-aided system that designed a series of these

clear appliances to incrementally move teeth. From this concept, Chishti and Kelsey Wirth,

5

another Stanford MBA student, along with two other orthodontists founded Align Technology in

1997 in a garage in Palo Alto.27

1.3 The Concept of Clear Aligner Therapy

Although the concept of using aligners in orthodontics has existed for many decades, the

planning and fabrication processes were done manually, through tedious and laborious

procedures such as sequential wax set-ups.7 The major drawback of the manual fabrication

process was limited to only a small subset of aligners, and thus cannot be used for

comprehensive orthodontic treatments. The recent advancements in CAD/CAM and rapid

prototyping techniques has allowed for an industrial approach7 to the treatment planning and

manufacturing of clear thermoplastic aligners.28, 29

Contemporary aligners of the 21st century combine the principles pioneered by Remensnyder9,

Kesling2, Nahoum3 and others4, 5, 25 and integrate them with modern CAD/CAM technology.

Today’s aligners are made using transparent and thermoplastic polymeric materials, custom

fabricated to the patients’ individual dental arches.30 This approach achieve orthodontic tooth

movement through the usage of a plurality of successive aligners, where each aligner

incrementally moves teeth by a predetermined amount. The force system of aligners is generated

when there is a pre-established geometric mismatch between the shape of the aligner tray and the

dental arch.7 The force system of aligners can vary by the mechanical properties of the

thermoplastic material, thickness of the aligners, amount of activation as well as the addition of

auxiliary elements.

The CAD-based process involves multiple steps, beginning from the 3-dimensional

reconstruction of the patients’ oral anatomy to the manufacturing of the aligners. The digital

reconstructions are performed through either intra-oral scanning or digital scanning of a study

model.31 The computer algorithm will then segment the individual clinical crowns from the rest

of the digitized 3-dimensional model. The orthodontic treatment plan is then developed and

partitioned into a sequence of smaller movements by the CAD software.29 The manufacturing of

the physical molds of the dentition at each stage of treatment is performed using the rapid

prototyping technique.28, 29 The customized aligners are then produced using a thermoforming

process and trimmed to the final configurations.7

6

1.4 Benefits, Limitations and Controversies of Clear Aligner Therapy

One of the main primary advantages of clear aligners is its perception by patients to be an

esthetically superior alternative to the fixed labial appliances.32-35 Aligners also allow for ease of

removal during eating and cleaning.36 Removable appliances in general, including clear aligners,

have been shown to have less functional impediments than fixed appliances.32, 37 Not

surprisingly, this leads to decreased speech impairment when compared to fixed lingual

appliances38, making clear aligners more suitable for professionals that require a great deal of

talking and function in a representative capacity. Lastly, clear aligners have been thought to

reduce the number of emergency appointments and chair time in the orthodontic office.39, 40

The primary disadvantage of clear aligner therapy, as noted by many studies and reviews40-44, is

the low-level of available evidence in the published literature regarding this modality of

treatment and its biomechanics. Most of the articles are expert opinions or retrospective studies

with design flaws and biases.39 Moreover, the modern technology that allowed clear aligner

therapy to become available to the masses comes at higher laboratory fees that may affect the

patient.45

Although the clear aligners have been shown to reduce chair time and emergency appointments,

it comes at the cost of additional material expenses to the office and additional time required of

the clinician to perform the virtual treatment plan.39, 40 Despite earlier claims by some that

periodontal health is better during clear aligners therapy than fixed lingual appliances46-51, the

latest available research has suggested that there may not actually be any differences between

these two treatment modalities.52 Lastly, the level of pain associated with clear aligner therapy is

still controversial,45, 53-55 and its effect on the masticatory muscle activity is presently unknown.

1.5 Patient Perception of Clear Aligner Therapy

1.5.1 Attractiveness and Acceptability

The specialty of orthodontics has entered an era where optimal dental and facial esthetics has

become an integral part of diagnosis and planning for orthodontic treatment.34 The past several

decades has seen a trend progressing from decrease in bracket size56, transition of metallic to

7

clear brackets57, lingual appliances58-60, clear thermoplastic aligners36, 61, other adjuncts including

clear elastic ties and clear wires.62, 63

From a study on the patients’ perspective of attractiveness of orthodontic appliances by

Ziuchkovski et al.,33 it was found that the amount of visible metal in the appliance was

negatively correlated to its attractiveness. Lingual appliances and clear aligners, with no visible

metallic components, were rated the highest in attractiveness. The next highest rated appliance

were the ceramic appliances, while hybrid or stainless-steel self-ligating and twin brackets were

rated the lowest. This trend was thought to parallel the patients’ ever increasing desire for whiter

teeth.64 This hierarchy was verified in a follow-up study by Rosvall et al.,65 in which the

attractiveness of the appliance decreased as the amount of metal display increased.

In terms of patients’ acceptability of orthodontic treatment, surveys have shown that despite the

desire for correction of their malocclusion, between 33% to 62% would reject treatment if a

visible appliance is involved.32, 66, 67 Subsequent studies on patients’ acceptability of appliances

revealed that over 90% of adults find clear aligners, lingual braces68, 69 and ceramic brackets to

be acceptable, while traditional metallic fixed appliances only received a 55% acceptance rate.65

Patients surveyed also indicated that they were willing to pay additional fees for more esthetic

treatment options, such as clear aligners or lingual appliances.

1.5.2 Social Perceptions

Profiling studies have revealed that among female patients between ages of 20-30 years old,

clear aligners are preferred over fixed buccal appliances for esthetics and over fixed lingual

appliances for functional reasons.37, 38 A cross-sectional study70 on the perceptions of adults

wearing orthodontic appliances revealed that the orthodontic appearance can exert an influence

on how the wearer is being perceived in social situations. The data revealed that a greater

perceived intellectual ability was associated with no appliance, lingual appliance and clear

aligners. Other social perceptions, such as psychological adjustment and social competence was

not correlated with any type of orthodontic appliance. These results signify that social

interactions and social well-being of the orthodontic patient may be influenced by the visibility

of their appliance, however, the extent to which this could impact their psychological well-being

is unknown.70

8

The rise of the digital age during the 21st century has led to new methods of communication.71

Social media outlets such as Twitter has grown exponentially and become a primary method of

multipurpose communication throughout the world.72, 73 Free from recall bias of traditional

surveys74, real time data from Twitter has been utilized75, 76 to study many fields, such as product

feedback77, political elections78, 79 and the stock market80. A Twitter analysis by Noll et al.71 in

2017 demonstrated no significant differences in patients’ sentiments regarding traditional fixed

appliances or clear aligners. Majority of the tweets were positive (61%), expressing gratitude for

their new smile. Among the negative tweets (39%), pain was the most frequent complaint, with

lisps from aligners being another objection. Other studies using Twitter data have shown that

traditional orthodontic treatment may either attract bullying or help stop bullying.81 Therefore, it

is possible that the esthetic advantages from clear aligner therapy may potentially contribute to

reducing bullying by eliminating the stereotypical appearances associated with traditional

orthodontics.81

1.6 Categories of Aligners

There are several major categories of clear aligner products, classified based on their clinical

applicability and method of delivery to the patient. These aligner systems range from those

available direct to the consumer to comprehensive systems designed to treat more complex

malocclusions.

Direct to consumer aligner systems are marketed directly to patients, designed for patients to

treat themselves “at home”. These systems require the patient to take photographic records and

make their own impressions of their dental arches. The resulting aligners are subsequently

fabricated and delivered to the patient, without the direct supervision of a dental professional.

Much of the marketing behind these products include convenience and reduced cost. The biggest

provider of this system is Smile Direct Club82, which was recently approved for use in Canada.

Other companies include Smilelove, SnapCorrect Orthly. As more of these “do-it-yourself” or

over-the-counter aligner companies arise, some researchers have expressed concern over the

patients’ safety without a health care provider supervising their treatment.83 According a recent

consumer alert by the American Association of Orthodontists (AAO), patients may be subjecting

themselves to increased risk of damage to their teeth, periodontium, and even adverse events,

such as allergenic reactions, some of which could be life-threatening.

9

Minor Tooth Movement (MTM) aligners are designed to provide limited clinical treatment, such

as single arch or anterior alignment only. These systems have been marketed as a cheaper and

faster alternative to comprehensive clear aligner therapy. Products in this category include

Originator, Simpli 5, MTM Clear Aligner, and Clearguide System.

In-house fabricated aligners are created by companies that provide the 3D treatment planning

software to the orthodontist’s office. This software can be integrated with 3D scanners and

printers to allow the orthodontist to fabricate their own aligners directly. Products in this

category include Arcad, Elemetrix, OrthoAnalyzer, Orchestrate 3D and Archform.

Comprehensive aligner systems allow for 3D interactive treatment planning via incorporation of

3D CAD/CAM tooth movements. They include usage of various auxiliary elements, such as

bonded resin attachments, and slits for elastics. These systems provide various additional

features that allow for more complex tooth movements in all planes of space and more

comprehensive treatment than the previous options. The majority of this literature review will

focus on the capabilities of comprehensive aligner systems. Current providers of these products

include Invisalign®, ClearCorrect, ClearPath, eAligner, K-Line and Orthocaps.

1.7 The Invisalign® System

Since the initial development of Invisalign® by Align Technology in 1997,84 this product

received initial clearance by FDA in 1998 for use in patients with permanent teeth and

contraindicated the device for patients presenting with mixed dentition, severe overbite, severe

overjet, tooth malocclusion requiring surgical correction, adolescent patients with a skeletally

narrow jaw, and adult patients with dental prosthetics or implants.36 When the Invisalign®

system first came to the market in 1999, its initial availability was limited to orthodontists and

later expanded to general practitioners as well.84 This appliance consists of a series of clear

thermoplastic aligners that are worn for 2-week periods each. Each aligner was staged to achieve

0.25-0.30 mm of orthodontic tooth movement per tray. Later in 2009, it was announced that the

FDA had removed permanent dentition limitation as indication for use, and previous

contraindications had been changed to precautions.85The first iteration of aligners worked behind

a displacement-driven system86, where they were solely dependent on its shape to achieve

results.87 No auxiliary elements were incorporated at that time. Limited research is available on

efficacy of tooth movement by first generation aligners, with the only study done by Djeu et al.

10

in 2005.88 The second generation of aligners began the use of various auxiliary elements for the

purposes of enhancing the efficacy of orthodontic tooth movement.87 These included the use of

attachments, incorporation of composite buttons and the use of inter-maxillary elastics (Figure 1-

2). The major novelty in the third generation of aligners was the introduction of optimized

attachments that can be placed automatically by manufacturer’s software.87 Clinician can still

request non-precision attachments in cases where they felt necessary. The three main geometries

of attachments are ellipsoid, beveled and rectangular.

Figure 1-2. Horizontal rectangular attachment with gingival bevel used for the Invisalign®

system in the ClinCheck® software.

The size of the SmartForce optimized attachments are created by the software with the shape of

the tooth. SmartForce attachments can be placed by the software automatically without input

from the operator. These attachments are claimed by Align Tech to result in improve control of

tooth movements, especially root control.86 Examples of SmartForce attachments include:

optimized extrusion attachments, multi-tooth anterior extrusion attachment, optimized rotation

attachment, multi-plane attachment, and root control attachment.

SmartTrack is a proprietary multi-layered polyurethane based polymer, designed to be highly

elastic.89 It was first introduced in 2013, designed to replace the previous Exceed-30 and Exceed-

40 aligner material. It has since been incorporated as the new standard aligner material of

Invisalign®. According to Align Technology Inc.’s internal studies27, they were designed to

maintain a more constant and gentler force over time, without losing force expression due to

stress-relaxation cycles over 2 weeks wear. They were also aimed to conform to tooth

morphology, attachments and interproximal spaces. Theoretically, it should result in improved

tracking, as well as improved control of tooth movement through stabilizing the contacts

between the teeth and aligner. It is important to note that there is a lack current evidence in the

11

scientific literature to back up majority of the claims made for the SmartTrack material. The

current available evidence shows that thermoplastic aligners do not maintain a constant force

over the 2-weeks of wear.90-93 Only one recent study mentioned that SmartTrack appears to be

more comfortable than the previous aligner materials used, however, the authors noted that more

studies are needed to further investigate these claims.55

Jaw Motor Responses during Orthodontic Treatment

Jaw muscles are a group of units that are among the most powerful skeletal muscles in the

human body.94 They are capable of producing forceful contractions for mastication, as well as

providing assistance in various other functional demands, such as speech and deglutition.95 Jaw

muscle activity controls the movement and positioning of the mandible, and consequently, they

impart forces onto the dentition and the temporomandibular joints.96 The consequences of their

actions are relevant to many aspects of dentistry, including orthodontic treatment of

malocclusions, prosthetic modifications of the occlusion, as well as surgical corrections of

various craniofacial deformities.97-99

Changes to the functional demands of jaw muscles will lead to modifications of their activity,

such as contractile velocity, frequency, force generation and other pertinent properties.94 This

process occurs due to our body’s ability to adapt to new changes and may take place under

various different mechanisms.100, 101 The main factors affecting jaw muscle activity during

orthodontic treatment are tooth pain and occlusal changes.102 Psychosocial factors can also

influence and modulate the jaw muscle activity.103-105

2.1 Role of Occlusion in Jaw Muscle Activity

2.1.1 Influence of Occlusal Relationship in Jaw Muscle Activity

The occlusal relationships in all three planes of space have been shown to affect jaw muscle

activity.106-108 Individuals with a mesial jaw relationship demonstrates a tendency to posture

anteriorly, leading to an increased temporalis and masseter muscle activity over those with neural

or distal jaw relationships.106 Further studies have shown that the more severe the skeletal

discrepancy is, the more the muscle activity increases.109 Interestingly, the sagittal relationship

does not affect the maximum voluntary contraction nor the jaw muscle fiber composition.106, 109-

111

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In the vertical dimension, those with deep overbites are associated with increases in the masseter

and temporalis muscle activities when compared to those with open bites.107 This finding

correlate well with the established literature data, as those with deep overbites are often

brachyfacial in craniofacial morphology, whereas the open bite individuals are often

dolichofacial in morphology.107 The fiber composition of the masseter muscle also varies, with

deep bite individuals having more type II fibers and open bite individuals having more type I

fibers.112

Transverse discrepancies, such as mandibular asymmetries, can also affect the activity and

morphology of the jaw muscles. The masseter muscle on the deviated side of the mandible has

been shown to have significantly smaller length, volume and are orientated more vertically.108

Additionally, the smaller volume of the masseter muscle is also associated with a decrease in its

muscle activity during functional use.113 Individuals with unilateral crossbites have demonstrated

lower muscle activity on the affected side.114 Following successful treatment of the unilateral

crossbites, no bilateral differences in the muscle activities can be detected.115

2.1.2 Effect of Occlusal Interferences on Jaw Muscle Activity

Our understanding of the role of occlusal interferences in jaw muscle activities has gradually

changed over time. Earlier studies suggested that the introduction of occlusal interferences may

trigger a cascade of events that lead to muscle hyperactivity, possibly increasing the risk for

bruxism and TMD.116 However, more recent studies using double-blinded randomized crossover

designs have shown that occlusal interferences lead to a decrease in jaw muscle activity within

the first two days and gradually increase towards baseline thereafter.117 This change has been

hypothesized to be an avoidance behavior developed by the participants in response to the

perceived occlusal discomfort.101 Indivduals may develop adaptation to the occlusal disturbance,

thus returning towards baseline levels of muscle activity. It is possible that the reaction to

occlusal interferences may be different in patients with a history of TMD or occlusal

hypervigilance.118

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2.2 Psychosocial Factors in the Modulation of Jaw Muscle Activity

2.2.1 Effect of Psychological factors on Jaw Muscle Activity

Psychological factors not only play a major role in the patients’ perception of pain, but has also

been linked to various parafunctional behaviors, such as bruxism.104, 119-121 Changes in the jaw

motor activity may encompass a variety of functional disturbances, including oral parafunction

and bruxism, with a potential increase in the risk for temporomandibular disease.122 The etiology

of these behaviors is often multifactorial, possibly involving both peripheral factors, such as

occlusal interferences, and central factors, such as stress, anxiety and depression.123

The theory of parafunction being related to the central effect, such as stress, was first postulated

by Laskin in 1969.124 Later studies reinforced this idea by showing a positive correlation

between life events, jaw muscle EMG activity and muscle pain.125 A review showed that awake

bruxism, which occurs semi-voluntary during daytime126, may be related to state anxiety, such as

a transitory anxious reaction to stressful daily events.127 Additionally, awake bruxism was also

associated with depression and trait anxiety.128, 129 The frequency of tooth clenching has also

been positively correlated with the patients’ psychological distress.120, 121 Other studies have also

shown that highly anxious individuals have a heightened pain experience during orthodontic

treatment.130

Another factor which is thought to affect jaw muscle activity is occlusal hypervigilance, which

is related to somatosensory amplification – the tendency to amplify the experience of a minor or

mild somatic sensation as a more severe or intense sensation.131 Somatosensory amplification

involves bodily hypervigilance, focusing on weak sensations and the disposition of reacting with

cognitions that intensify them.132 Occlusal hypervigilance is analogous to bodily hypervigilance,

describing the heightened attention and selective focus on occlusal changes.118 This may be a

protective behavior that result in the continuous checking of the occlusion for any changes.

Minor alterations of the occlusion may be perceived as more intense changes. This monitoring

process may lead to repeated tooth-to-tooth contacts or possible clenches, resulting in increased

jaw muscle activity.105 As a matter of fact, individuals with a high frequency of self-reported

awake oral parafunction present higher occlusal sensitivity.133

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2.2.2 Psychological Profile of the Clear Aligner Patient

The patient’s preference and perception of the orthodontic appliance may reflect their personality

traits or psychosocial status.134 This knowledge could play an important role for the clinician in

managing the orthodontic patient’s adaptation and adjustability to their appliance.134 A study

regarding the association between personality traits and satisfaction of orthodontic treatment

revealed that neurotic personality traits were associated with greater dissatisfaction of

treatment.135 When looking at personality traits and psychological features, patients who

preferred clear aligners and lingual appliances had greater levels of somatization.134 Patients with

various degrees of anxiety, especially bodily concerns, also tend to shy away from traditional

buccal fixed appliances, preferring concealed appliances such as clear aligners and lingual

braces.134 The narcissistic trait demonstrated no relationship with appliance selection. In terms of

pain perception, Abu Alhaija et al.136 reported no relationship between personality traits and

perception of pain during orthodontic treatment. However, in a 2013 study by Cooper-Kazaz et

al.,134 the narcissistic personality trait showed correlation with higher reported levels of pain,

especially in cases of lowered self-esteem, among clear aligners, fixed lingual appliances as well

as fixed buccal appliances. In terms of pain adaptation, psychological features were thought to

play minimal effect, as the majority of patients’ response and recovery seemed to be relatively

easy and uneventful.134

Pain in Orthodontics

Pain as defined by the International Association for the Study of Pain is “an unpleasant and

emotional experience associated with actual or potential tissue damage or described in terms of

such damage”.137 Fear of pain is a major reason for patients to forego orthodontic treatment.138-

140 In one particular survey, patients rated pain as the highest area of dislike in regards to

orthodontic treatment, and ranked fourth among major fears and apprehensions.141

3.1.1 Progression of Orthodontic Pain

The vast majority of patients will experience pain at one point or another during orthodontic

treatment.142 The initial pattern of pain experienced by patients with traditional fixed appliance

therapy has been well studied,142-147 however, knowledge concerning the pain and discomfort

that may be experienced by patients further into treatment is limited. In the initial stages, patients

15

experience peak levels of pain within approximately the first 24 hours of wire placement,

followed by a gradual decrease towards baseline levels within 7 days.45, 53-55, 142, 148 Similar

studies found that the first 4 to 7 days were most critical for the patient in terms of general

discomfort.146 These results are in agreement with studies that found patients are generally able

to adapt to new appliances within a week after placement.147

The initial increase in pain and discomfort following the first 24 hours of appliance activation

has been correlated with an acute inflammatory response.149 The compression of the periodontal

ligament (PDL) due to the applied initial orthodontic force has been thought to be the cause of

the pain and discomfort. During this initial period of 24 to 48 hours, the local periodontium

experiences ischemia, edema and release of proinflammatory mediators.150, 151 A similar pattern

is observed with the levels of PgE and IL-1β found in gingival crevicular fluid, which reaches a

peak level within the first 24 hours of initial appliance activation, and gradually reduces to

baseline levels after one week.152 Therefore, the clinically observed pattern of pain progression

during the first week may be attributed to the changes at the molecular levels involving these

inflammatory mediators of the local periodontium.55

Numerous studies have shown that patients’ perception of pain, discomfort and quality of life

varied between fixed appliances and removable appliances.53 Fixed appliances generally produce

more discomfort, tension, pressure, tightness, pain and sensitivity than removable or functional

appliances.146, 153-155 Removable appliances can provide intermittent levels of force application,

which has been thought to allow the dentoalveolar tissues time to repair and organize before the

compressive forces are reapplied.156 However, patients receiving functional or removable

appliances experienced problems relating to speech and swallowing more frequently than fixed

appliance patients, likely due to impingement of tongue space.71, 146, 147, 155, 157

3.1.2 Physiological Responses to Orthodontic Pain

The Pain-Adaptation model has been used to explain the observation that the pain associated

with the initial tooth movement can cause patients to express an avoidance in chewing and a

suppression of jaw muscle activity.158 Indeed, pain is associated with a decrease in

electromyographic activity of agonist muscles and an increase in the activity of antagonist

muscles.101 In this model, the feedback of pain to the motor command lowers the agonist muscle

output via excitation of the inhibitory motor neurons and inhibits the excitatory motor neurons

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supplying the agonist group. Conversely, the same pain stimulus also increases the activity of the

antagonist muscles by suppressing the activity of the inhibitory motor neurons and by

stimulating the excitatory motor neurons innervating the antagonist group. These two pathways,

controlling the agonist and antagonist muscle groups, are reciprocal.101

Clinical findings reveal that some patients wearing clear aligners report jaw muscle tenderness159

and present wear facets on aligner trays, thus suggesting that the aligners may have acted as

occlusal splints.160 It is possible that a different adaptation mechanism involving repetitive tooth

clenching may have occurred in these patients. This has also been suggested in a study in which

it was found that the frequency of daytime tooth clenching increases while wearing

Invisalign®.161 It is likely that patients are triggered to clench on the trays to alleviate

orthodontic pain. Indeed, clenching on the trays, similarly to clenching on plastic wafers162, may

promote blood flow through the periodontal ligament, thus preventing the accumulation of pro-

algesic mediators in the periodontal ligament space, and promoting pain relief.163 In addition,

clenching on the aligner trays can act as a conditioning stimulus to reduce the perception of the

orthodontic noxious stimuli in a conditioned pain modulation paradigm.100

3.1.3 Pain in Clear Aligner Therapy

Miller et al.53 conducted the first study evaluating the differences in pain and impact on quality

of life experienced by patients undergoing aligners and fixed appliance therapy. This was a

prospective longitudinal cohort study with 33 aligner patients and 27 fixed appliance patients.

The participants were asked to use a daily diary for 7 days, measuring functional, psychosocial

and pain-related impacts.164 The diary consisted of questions adapted from the Geriatric Oral

Health Assessment Index165, a 5-point Likert scale, a visual analog scale for pain as well as

demographics information. The results showed that the progression of pain in aligner treatment

followed a similar pattern to fixed appliances, in which pain peaked after 24 hours and gradually

returned to normal. Additionally, although the initial levels of pain were higher for the fixed

appliance group along with higher levels of analgesic consumption, both groups recovered to

baseline within 7 days.

In a subsequent study by Shalish et al.45, 68 patients being treated by either buccal fixed

appliance, lingual fixed appliance or clear aligners were recruited to complete a health-related

quality of life questionnaire166-170 with previous validation171-174 and a five-point scale for

17

dysfunction during the first week and the 14th day. Their results showed that the average initial

pain levels were consistently higher in the lingual fixed appliance and clear aligner groups, with

analgesic consumption paralleling the dynamics of the pain levels, although the difference did

not reach statistical significance. In all groups, the pain levels subsided within one week. These

results contradict the findings by Miller et al.53, which the authors have attributed to a greater

mechanical force being applied in the aligner group compared to the buccal fixed appliance

group.

To further elucidate and compare the pain levels between these orthodontic treatment modalities,

Fujiyama et al.54 conducted a prospective clinical trial with 145 patients receiving either aligner

therapy, fixed appliance therapy or a combination of both. Using a visual analogue scale, the

participants were asked to record their pain levels at time points of 60 s, 6 h, 12 h, 1 to 7 days

post-appliance insertion. This was repeated at the 3rd and the 5th week after appliance delivery.

Their results illustrated a similar pattern of pain progression during the first week of appliance

delivery for all groups studied. However, the overall pain levels were significantly more intense

and longer lasting for fixed appliance group than either aligner or the combined group.

In a recent study by White et al.,55 patients were randomly allocated to either clear aligner or

fixed appliance treatment group to study the differences in their pain levels. The participants

were asked to complete a daily diary with pain measured on a visual analogue scale. The diary

was completed at initial appliance delivery, daily for the first week, as well as the first 4 days

after their next two follow-up appointments. The pattern of pain progression during the first

week following initial appliance activation was in good agreement with previous studies.45, 53, 54,

142-144, 175 The clear aligner group experienced consistently lower discomfort than the fixed

appliance group during most the first week, with statistically significant differences observed

after 2-3 days. Similarly, over a relatively longer term of 2 month, the level of pain was less in

the aligner group than the fixed appliance group.

In a recent parallel study, 26 healthy adult patients undergoing Invisalign® therapy were

recruited to study the somatosensory changes during treatment.176 The participants reported tooth

pain and jaw muscle tenderness on 100mm visual analogue scales (VAS) in pain dairies over a 4

week period. Each of the 4 weeks represent a different experimental condition: 1) baseline; 2)

passive aligner; 3) first active aligner and 4) second active aligner. Data from the self-reported

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VAS showed that CAT resulted in an increase in orthodontic tooth pain (Figure 1-3). All three

aligners produced significant increases in tooth pain when compared to the baseline levels

(p<0.001 for all), but of limited clinicl significance. The passive aligner demonstrated the highest

increase in tooth pain and the difference was significantly higher than both active aligners

(p<0.001 for both). From here, tooth pain levels decreased from the first active aligner to the

second active aligner (p<0.001).

Figure 1-3. Effect of aligner condition on orthodontic tooth pain (VAS 0-100mm). Recreated

with permission.176 Differing asterisks indicate statistical significance between the pairwise

comparisons (p<0.001).

The self-reported VAS (0-100mm) for jaw muscle tenderness showed that both passive aligners

and the second active aligners lead to an increase when compared to the baseline levels (all

p<0.001) (Figure 1-4). The first active aligner resulted in less mean muscle tenderness than both

passive and second active aligners (p<0.001 for both) and no statistical difference when

compared to the baseline (p>0.05). The second active aligner was not statistically different than

the passive aligner (p>0.05).

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Figure 1-4. Effect of aligner condition on jaw muscle tenderness (VAS 0-100mm). Recreated

with permission.176 Differing asterisks indicate statistical significance between the pairwise

comparisons (p<0.001).

The results of pain and discomfort comparison studies between clear aligners and fixed

appliances by White et al.55, Fujiyama et al.54 and Miller et al.53 are in general agreement with

one another, and as well as past studies that demonstrated fixed appliances cause more pain than

removable appliances.153, 155, 177, 178 These results were in contrast to the findings from Shalish et

al.45, which reported that the pain was greater in patients treated with aligners than fixed

appliances. One possible explanation for this discrepancy could be the variations in the initial

arch wires used between the studies. For example, the classic nitinol wires used in Shalish’s

study has been shown to display higher peak discomfort than the superelastic copper nitinol

wires used in White’s study.179, 180 Furthermore, White’s study was the only one to utilize

SmartTrack, a new aligner material by Align Technology in 2013,89, 181 whereas the previous

studies used the Exceed-30 aligner material. The limited evidence suggest SmartTrack may be

more comfortable than older materials182, although further studies are needed to verify this.55

Lastly, Shalish’s article mentioned that the differences in pain levels observed may have been

due to a higher level of mechanical force being applied early in treatment for the aligner group.

20

Although orthodontic pain does exist with clear aligner therapy, the available evidence seems to

suggest that it is of a lesser degree than fixed appliances, especially during the first week.

Deformation of the aligner tray has been reported as the most frequent cause of the pain and

discomfort.54 Other issues leading to pain associated with the aligner trays included non-smooth

edges, missing tray materials as well as attachment deformation.54

Surface Electromyography

Surface Electromyography (sEMG) is an objective method of measuring information of the

muscle of interest through placement of electrodes over the skin.183, 184 Its simplicity and non-

invasive nature have brought wide-spread use to researchers in the field of dentistry for both

basic science and clinical studies.185 Studies have shown that sEMG is a powerful method for

physiological investigations of jaw elevator muscles.186, 187 Additional studies have used it for

studying temporomandibular disorders (TMD)188, 189, as well as muscle hyper- and hypo-

activity190, 191, muscle imbalance192 and fatigue.193-195

4.1 Assumptions in the use of EMG for Assessment of Muscle Activity

Surface EMG detects signals from numerous muscle fibers within the vicinity of the electrode

and are not selective towards any one group of fibers.184 The measured signals are weighted

summation of the spatial and temporal activity of many motor units combined. These signals are

restricted to the analysis of general muscle activity, cooperation of various muscle groups and

temporal changes of their activity. Surface EMG is also limited towards detecting superficial

muscles located close under the skin. The masseter and temporalis are the only masticatory

muscles suitable for the evaluation of activity via sEMG. One limitation of the technique is that

the hair overlying the skin can interfere with the signal detection and needs to be removed in

cases of posterior and medial temporalis or facial hair overlying the masseter.

4.2 Applications of sEMG in Clinical Research

The use of sEMG recordings for various basic science and clinical applications has been widely

studied.185 The topic in using sEMG for jaw elevator muscles at rest is extensively researched.

The physiological rest position of the jaw muscles is generally thought to range between

complete absence to only minimal muscular activity.196, 197 Further studies have also

21

demonstrated some degree of spontaneous activity of the elevator muscles at the rest position.198,

199 During static contraction, such as clenching, studies have shown that a balanced EMG activity

is an indicator of good neuromuscular adaptation to the occlusal condition.200, 201 However, static

contractions by itself is not sufficient for diagnosis of neuromuscular conditions.185 During

dynamic contraction, such as chewing, sEMG has also been used to study the energy expenditure

used using mastication.202

Jaw muscle adaptation is a physiological process that may be assessed using various parameters,

including muscle activity, muscle force output, as well as cross-sectional area of each individual

fibers.94, 203 The most commonly employed technique to physiologically study the changes jaw

muscle activity is via sEMG.94 Its non-invasive nature makes it feasible for clinical trials and can

record the amplified motor unit action potentials of the jaw muscles.204

4.3 Factors and Limitations Affecting the Electrical Activity of Jaw Muscles

Age is a critical socio-medical factor when considering any assessment of the masticatory

muscle activity.184 The development of the orofacial musculature as well as the

temporomandibular joints are immature in children when compared to adults.205 Previous sEMG

studies confirm that the muscle activity of the masseter and temporalis is different between

children and adults.206 The role of sex in the influence of masticatory muscle activity is presently

unclear. While Ferrario et al.207 reported no differences in the rest activity of temporalis and

masseter muscle between males and females, Pinho et al.208 reported higher resting masseter and

temporalis activity for women than men. The electrical activity of masticatory muscles can also

be influenced by the day and night cycle. Several studies have demonstrated that the resting

activities of the masseter and temporalis muscles decrease when recorded during the night.209-211

Variations in craniofacial morphology also has considerable influence on the electrical activities

of the jaw muscles.184 Malocclusions occurring in all 3 dimensions of space can have an impact

of the jaw muscle activities. For example, anterior open bites can exhibit decreased jaw muscle

activity during clenching, Class II malocclusions show higher temporalis activity during chewing

and posterior crossbites result in decrease of the ipsilateral masseter muscle activity.212, 213

The primary limitation of this technique is that it only describes the general gross activity of the

muscle as a whole and is limited to only the specified motor task.214-216 In order to study the

22

changes in myoelectric activities of the various parts of a muscle, the electrodes would have to

be placed intramuscularly.217 This technique involves the percutaneous insertion of needle

electrodes into the muscle to transmit signals.218 However, this is more invasive and its use is

limited by the patients willingness to participate and research ethics, but may find use in various

animal experiments.219, 220

4.4 Reproducibility

Various studies have shown that the reliability of accuracy of sEMG is dependent on the

underlying physiological structures and methodological factors of the study.221-223 Reliability

studies have shown that sEMG recordings can have large variabilities, depending on the day of

recording at the same site.185, 188 This finding was in agreement with subsequent studies, which

also showed that recordings made within the same day had smaller variations.224 This had led to

some controversies regarding the reproducibility of sEMG measurements225, while others have

argued that reliable measurements can be obtained from carefully controlled experimental

setup.216, 226 These variations in experimental setup that could improve reproducibility of sEMG

recordings include method of electrode placement227-229, shape and location of electrode over

muscle230, 231, operator training189, 232, body posture233 and psychological factors of patient.196, 234,

235 Therefore, it has been proposed by many to use templates and tattoos to assist in positioning

of electrodes236, 237, increasing electrode distances238 and normalization of signals.200

Significance of Problem

Jaw muscles are versatile structures with the capability to adapt their biological characteristics to

the various functional demands imposed on them.94 These adaptive changes include altering their

physical size, fiber properties, muscle activity and force of contraction.204, 239, 240 Clear aligners

have seen a rapid rise in their popularity and use, however, little is known about their effect on

the jaw motor function.241 It is presently unknown what effects, if any, CAT will have on the jaw

muscle activity and whether these changes are transient or persistent in nature. Understanding

these effects may lead to a significant impact in the orthodontic management of patients with

masticatory muscle pain and temporomandibular joint disorders.

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Objectives of Study

The objective of this study is to evaluate the changes in daytime activity of the masseter muscles

by means of ambulatory surface electromyography in patients subjected to orthodontic treatment

with clear aligner therapy with Invisalign®.

Hypothesis

We hypothesize that patients subjected to CAT during the first few weeks will have a transient

increase in masseter muscle activity from baseline levels.

24

Chapter 2 Materials and Methods

Research Ethics

Ethics approval was obtained from the Human Research Ethics Program (HREP) from the

University of Toronto Research Ethics Board (Approval # 34215, Appendix 1).

Patient Recruitment

Patients treated using CAT with Invisalign® (Align Technology, Santa Clara, California, USA)

at the graduate orthodontic clinics of University of Toronto and University of Western Ontario

were recruited to participate in the study. The research protocol was uniform and standardized

for both sites. A single examiner at each site conducted all research-related matter with

calibration. Treatment at all clinics was provided by graduate orthodontic residents under the

supervision of an instructor. Eligible patients for the study were 18 years or older with no prior

history of clear aligner use. Exclusion criteria consisted of: (1) current TMD pain; (2) current use

of muscle relaxants and other related medications affecting jaw muscle activity; (3) neurological

disorders; (4) craniofacial syndromes; (5) presence of any systemic disorders; (6) presence of

orofacial pain; (7) current use of analgesics; (8) unwillingness to shave prior to sEMG

recordings.

In order to assess for current TMD or orofacial pain, each patient was subject to a preliminary

TMD examination according to the Diagnostic Criteria for temporomandibular disorders

(DC/TMD) prior to the start of the study242 (Appendix 2). Prior to the TMD clinical assessment,

a preliminary screening questionnaire based off of a modified version of the TMD-Pain screener

questionnaire243 was also completed by each potential patient (Appendix 3). The TMD-Pain

screener questionnaire is used to detect facial TMD pain in individuals and has a high specificity

and sensitivity, 99.1% and 96.9% respectively.243 This questionnaire has proven to be a valid tool

to identify patients with symptoms of TMD.

In addition to TMD screening, each patient also answered the Oral behavior checklist244, the

State Trait anxiety Inventory245, the Somatosensory Amplification scale131, the Beck depression

inventory246, 247, and the Pain catastrophizing scale248 (Appendix 4a-d). These questionnaires will

allow for controlling for the association with certain psychological traits, as well as pre-existing

25

parafunctional oral behaviors, on differences in muscle activity. Each patient was provided a

verbal explanation and given an informed consent package regarding the study (Appendix 5). All

questions and concerns by the patients were answered. The patients provided written and verbal

consent acknowledging the receipt of the information package and willingness to participate in

the study.

The initial pool of potential participating patients consisted of 5 males and 19 females, giving a

total of 24 eligible patients. A research flyer for the invitation to participate in the research

project was also posted with ethics approval (Appendix 6). After discussion of study with the

patients, 7 declined to participate for various reasons, such as time commitment and

compensation (Fig. 2-1). The final sample size was a total of 17 participants for the sEMG

portion of the study, with 1 male and 16 females (mean age ± SD = 35.3±17.6 years). The was

no dropout throughout the entirety of the study and all patients fully completed the EMG

recordings and longitudinal monitoring of pain and jaw muscle tenderness aspect of the study.

Figure 2-1. Schematic Diagram Illustrating Flow of Patient Recruitment. The main reason some

patients declined to participate was related to the time commitment required.

Experimental Setup

All patients were treated with the latest generation of Invisalign® clear aligners. The main

aligner material was SmartTrack©, which is a multi-layer thermoplastic polyurethane-based

26

material with an elastomeric component.157, 158 Utilizing the ClinCheck Pro software, the first set

of maxillary and mandibular aligners for all patients were designed to have no active tooth

movements (passive or “dummy” trays). Active tooth movements from the aligners were only

incorporated at the subsequent stages. To negate any potential effect on the results from the

auxiliary bonded attachments on the teeth, all attachments were placed either at the beginning of

baseline measurements or after the experimental period.

Figure 2-2. Schematic Illustration of Experimental Design.

3.1 Surface Electromyography Recordings

Surface electromyography (sEMG) was used to evaluate the daytime activity of the masseter

musle in the patients undergoing CAT. Data from this analysis provide assessment of the

adaptive changes of the masseter to CAT. Previous data from literature has shown that sEMG

can be an objective, reliable and non-invasive tool for evaluation of the masticatory muscles.249

Participants were given a take-home kit containing a portable sEMG device (MicroEMG, OT

Bioelettronica, Turin, Italy) to self-record electrical signals of the masseter muscles for 4 hours

per session starting any time after 12:00 noon (Fig. 2-2). Each sEMG kit contained numerous

disposable bipolar self-adhesive concentric electrodes, which were used for recording the EMG

signals. In order to diminish impedance, prior to electrode placement, patients were instructed to

clean the surface of skin with a disposable alcohol swab.250 The kits also contained a customized

calendar to help remind the patient of which day they need to be performing the sEMG readings

(Appendix 8). Patients were instructed to place the electrodes at the right masseter muscle, along

a line projecting from the mandibular angle to the lateral canthus of the eye, approximately 20

mm above the mandibular angle.251 Instructions were provided to the patient via verbal

instructions, written instructions (Appendix 9) and video instructions as well (Appendix 10)252.

27

The electrodes were centered on a landmark over the external cheek that closely approximates

the largest muscle “bulge” when clenched. In order to reduce artifacts during sEMG recordings,

all participants were instructed to avoid exercising, chewing and eating during recording

sessions. In the event that these activities did occur during recording, participants were instructed

to record the time in the pain diary of the corresponding day.

Data from the sEMG recordings were collected 3 times per week for 4 weeks in total (Fig. 2-2).

These 4-hour recording sessions occurred during the 1st, 3rd and 5th day of each week. Each of the

4 weeks represent a different condition and timepoint for which the sEMG is recorded under: 1)

baseline recording, done prior to the patient receiving their first Invisalign® tray; 2) passive

aligner recording, consist of the patient using a dummy Invisalign® tray that provides no active

orthodontic forces; 3) first active aligner recording, consist of the patient receiving their actual

Invisalign® tray that is intended for their treatment; 4) second active aligner recording, consist

of the patient receiving their actual Invisalign® tray that is intended for their treatment.

The purpose of performing the first week of sEMG recording without any aligners is to act as a

baseline measurement of masseter activity, for which the subsequent weeks can be compared

against. This allows each patient to serve as their own controls, account for factors such as

crowding, craniofacial morphology, and their malocclusion. The passive aligner tray allowed the

determination of whether or not the presence of clear aligners by itself (without active tooth

movement forces) could elicit a change in masseter activity. Patients were informed of the

passive aligner during the instruction period. During the third and fourth weeks, one active

aligner was worn per week to study the effect of orthodontic tooth movement sEMG activity.

The decision was made to solely use one brand of CAT for all patients in the experiment to

eliminate any potential confounding factors associated with using a variety of clear aligner

manufacturers. These confounding factors would include differences in plastic aligner material

composition, thickness, flexibility, force activation, stress-relaxation differences, etc. It has been

shown that the quality of orthodontic force exerted by a thermoplastic clear aligner appliance is

highly dependent on the mechanical properties of its fabrication material.253

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Sample Size

Based on a previous study254 17 participants were required to detect a 10% change in EMG

amplitude with the Invisalign® appliance, with effect size at 0.86, alpha error set at 0.05, beta error

at 0.1. Assuming a 20% dropout rate, a minimum of 21 could have been required. However, no

dropouts were recorded.

Data Processing and Statistical Analyses

The raw sEMG signals were downloaded from the device memory cards. Root Mean

Square (RMS) values were computed. The sEMG data for all patients were manually analyzed

for artifact identification, based on information provided by the participants and careful

examination of the sEMG signals. The identified artifacts were removed from the dataset. The

sEMG data was standardized (Z scores) using 1-minute averages. The sEMG data were exported

and compiled into a tabulated format suitable for statistical analysis using a custom-made

computer algorithm. This was written using macros for Microsoft Excel using Visual Basic

programming language. (Microsoft Corp. Released 2019. Microsoft Excel, Version 16.0,

Redmond, WA: Microsoft Corp)

A generalized linear mixed effect model with Bonferroni corrections was used to test differences

between conditions (baseline, dummy, aligner 1, aligner 2) and between the recording days of

each condition. Interactions between days of recording and experimental conditions were

tested. The fixed factors in the model were the recording day and the experimental condition.

Patient ID was included as a random factor. A sensitivity analysis was also performed using

linear regression to test the effect of various behavioral and psychological factors (OBC, trait

anxiety, SSAS, BDI) on the sEMG recording relative changes from baseline EMG

measurements. The statistical software used for all data in this study was SPSS (IBM Corp.

Released 2016. IBM SPSS Statistics for Windows, Version 24.0. Armonk, NY: IBM Corp.). The

statistical significance was set at p<0.05.

Consent to Participate

Each patient was given an information and consent package regarding the current study

(Appendix 5). Each participant was asked to sign the consent acknowledging the receipt of the

29

information package and willingness to participate in the study. Each recruit would have

previously consented to be treated at the Faculty of Dentistry in the Graduate Orthodontic clinic.

The compensation provided for each participant was $350 CAD, with appropriate proration

based on full participation or early drop out. This consent included both consent for

care/treatment as well as consent for collection, use and disclosure of personal information

including “for research and publication purposes on an anonymous basis.”

30

Chapter 3 Results

Effect of Aligner Condition on sEMG Activity

The expected wear time of the sEMG device was 48 hours over a period of 4 weeks for each

participant. The average EMG wear time of the participants was (meanSD) 45.9314.60 hours,

demonstrating good compliance with the protocol. The sEMG activity trajectories is plotted in

terms of Z-scores, with each week representing a different aligner condition, as previously

described (Figure 3-1). All clear aligners, passive, first and second active trays, showed varying

degrees of significant increases in sEMG activity from the baseline measurement (p<0.001 for all

three conditions). Relative to the baseline, the largest increase in sEMG activity seen with the

passive tray and the first active tray, while the least increase was with the second active tray

(Appendix 11).

Figure 3-1. Effect of aligner condition on the masseter muscle activity. Differing asterisks

indicate statistical significance between the pairwise comparisons. Mean Z-Scores (SEM) of

31

standardized Root Mean Squared sEMG activity for each aligner condition. All pairwise

comparisons, except for passive-first active aligner, were statistically significant (p<0.001).

When going from the passive aligner to the first active aligner, the sEMG activity did not change

significantly during the two weeks (p=0.751). Conversely, the second active aligner produced a

significant decrease in sEMG activity when compared to the passive aligner (p<0.001).

Likewise, there was also a significant decrease in sEMG activity between the first active aligner

and the second active aligner (p<0.001).

Effect of Individual Daily Recording Session on sEMG Activity

Figure 3-2. Effect of individual daily recording sessions on the sEMG activity. Differing

asterisks indicate statistical significance between the pairwise comparisons. Mean Z-Scores

(SEM) of standardized Root Mean Squared sEMG activity for each day of research, separated

by each aligner condition. All pairwise comparisons within each week, except for day 1-day 3

and day 3-day5 of the passive aligner, were statistically significant (p<0.001).

The daily changes in sEMG activity for each of the aligner conditions is reported in figure 3-2

(Appendix 12). During the baseline sEMG recording, there was a gradual decrease in sEMG

activity from day 1 to day 3 to day 5, as well as day 1 to day 5 (all p<0.001). The week of

passive aligners showed an increase in sEMG activity, from day 1 to day 5 (p<0.001), while the

32

differences between day 1 or day 5 with day 3 was not significant (p<0.001). However, the data

trend still shows a general increase in sEMG throughout the week. For the week with the first

passive aligner, there was a remarkable decrease in sEMG activity from day 1 to day 3

(p<0.001). The sEMG activity then slightly returned from day 3 to day 5 (p<0.001), but overall,

still lower than day 1 (p<0.001). During the week of the second active aligner, all three days

followed the similar trend as the week of the first active aligner. There was also a drop in sEMG

activity from day 1 to day 3, although smaller in magnitude but still statistically significant

(p<0.001). The sEMG activity then increased again during day 5 when compared to day 1 and

day 3 (p<0.001 for both).

Effect of Psychological factors and Oral behaviors on sEMG response to CAT

Various psychological factors, such as stress and anxiety, has been previously shown to

influence jaw muscle activity and pain.122, 255 To account for these factors, a regression analysis

was used to test the effects of some psychological variable on standardized EGM outcomes. The

psychological traits used were the Trait Anxiety Inventory245 (second portion of STAI), OBC244,

the SSAS131, the BDI.246, 247 The predicting variables showing high inter-correlations were

excluded from the model.

Table 3-1. Regression analysis – Effect of psychological traits and oral behaviors on the

standardized masseter muscle EMG activity.

Predictor B Std. Error Sig.

Trait Anxiety 75.04 27.70 0.027

OBC -34.12 10.34 0.011

SSAS 21.01 27.78 0.471

BDI -62.44 44.91 0.202

The results of the analysis showed that both trait anxiety and OBC were predictors for sEMG

activity (Table 3-3). The trait anxiety score was positively associated with an increase in sEMG

activity (B = 75.04, p = 0.027) while the OBC was associated with a decrease in sEMG activity

(B = -34.12, p = 0.011).

33

Chapter 4 Discussion

Summary of effects of Clear Aligner Therapy on Somatosensory Function

This research is part of a larger multi-centered study aiming at investigating the effects of clear

aligner therapy on both somatosensory function and jaw motor function. In order to provide a

comprehensive discussion of all the findings, a brief summary of the concurrent study done at

Western University will be provided here regarding somatosensory function.176 To complete the

findings, the same pool of participants was also given a pain diary with 100 mm VAS to record

tooth pain, jaw muscle soreness, occlusal discomfort and stress at four time points per day

throughout the aligner wear. The pressure pain threshold was also taken at baseline, 4-weeks and

4-month time points to assess somatosensory changes associated with CAT.

The results from that study revealed that CAT produced mild tooth pain and jaw muscle

tenderness of limited clinical significance (Figure 1-3 and 1-4). The level of tooth pain

experienced reached the highest point during the week of passive aligners and decreased in the

following weeks where active aligners were worn. Likewise, the level of jaw muscle tenderness

reached the highest point during the week of passive aligners but persisted in the subsequent

weeks. However, the magnitude of the increase in jaw muscle tenderness was small and only of

limited clinical relevance. The amount of stress experienced by the participants was shown to

have a substantial modulating effect in the perception of pain and jaw muscle soreness and

played a key role in the adaptation process. The use of aligners for a period of 4 month produced

somatosensory changes that were not statistically significant in the trigeminal and extra-

trigeminal muscle regions.

Effect of Clear Aligner Therapy on Jaw Motor Function

The current understanding of the effect of CAT on daytime jaw muscle activity is extremely

limited. Our results from sEMG data showed that CAT is associated with a transient increase in

jaw muscle activity. In a recent study by Brien using self-reported questionnaires, patients

undergoing CAT showed a significant increase in oral parafunctional behaviors during the first 2

weeks of treatment.161 These symptoms were found to have returned to baseline measurements

overtime, demonstrating the jaw muscle’s adaptive mechanism to the appliance gradually taking

34

effect.101, 161 The limitation of this study is mainly the use of self-reports, which may suffer from

recall bias and objectivity. However, their result is still consistent with our findings, where the

sEMG activity increased during the first and second weeks of aligner wear and trended down

towards baseline after the fourth week.

Increases in jaw muscle activity may be due to a variety of factors, including function,

parafunction or muscle tone changes.249 Our data showed a significant increase in sEMG activity

from baseline week to the passive aligner week (152.4% relative increase, p<0.001). It is

assumed that the participants were not increasing their functional behaviors, such as eating,

during the sEMG recording, as they were manually filtered from the data according their diaries.

Although an increase in muscle tone can occur, it is unlikely to contribute to the significant

relative increase seen in our data. Although not directly measured, it is possible that the increase

in jaw muscle activity observed may be due to parafunction, such as tooth clenching.

Furthermore, since there is no deception involved in this study, the patients were fully aware that

their first aligner was solely passive and for habituation purposes only. This may be explained by

the introduction of a foreign object into the oral cavity, such as the thermoplastic aligner

material, causing an initial disturbance in the balance of the orofacial musculatures possibly

leading to occlusal hypervigilance.118 Consequently, we see an initial increase in sEMG and

muscle activity.

Interestingly, upon insertion of the first active aligner, there was no significant difference in the

sEMG activity measured when compared to the previous week with the passive aligner (5.0%

relative increase, p = 0.751). Likewise, when looking at the sEMG activity from baseline to the

first active aligner, it was nearly identical to that of the passive aligner (155.1% relative increase,

p<0.001). The main difference, in theory, between the passive aligner and the first aligner is the

introduction of orthodontic tooth pain, via orthodontic tooth movement (OTM). The results from

this study suggest that the mere presence of an aligner itself is the major contributing factor to

the increase in muscle activity, rather than the actual tooth pain from OTM.

These results also directly parallel the coincident study done at Western University176 using self-

reported daily diaries, where tooth pain and jaw muscle tenderness has been recorded on a VAS

throughout the same time period. (Figure 1-3) Following a similar trend as the sEMG data,

muscle tenderness also had the greatest increase when changing from baseline to the passive

35

aligner (mean VAS increase = 3 mm, p<0.001, Figure 1-4). However, the increase is relatively

mild in magnitude and persisted throughout the active aligner trays as well (maximum VAS =

8mm).

Likewise, their results also showed that the most significant increase in tooth pain was from

baseline (mean VAS = 4mm) to the introduction of the passive aligner (mean VAS = 8mm), and

the pain report slightly decrease upon insertion of the first active aligner (mean VAS = 6mm,

Figure 1-3). These observed changes were all statistically significant and in agreement with the

observation that the mere insertion of the aligner played a more critical role in the patients’ pain

and muscle adaptation, rather than the actual tooth pain.

During the second and third week of the sEMG study, it has been shown that the muscle activity

increased by the presence of the aligner inserted between teeth. When the second active aligner is

placed during the fourth week, the muscle activity increased when compared to the baseline

(61.7% relative increase, p<0.001) but decreased relative to the passive or first active aligner

(173.1% relative decrease, p<0.001 and 169.6% relative decrease, p<0.001 respectively). This

result is indicative of the adaptive mechanisms gradually taking place overtime since the first

insertion of the aligner.

Likewise, the concurrent study using pain diaries also reported a similar trend in the progression

of tooth pain. (Figure 1-4) The relative increase in orthodontic tooth pain from baseline (mean

VAS = 4mm) was less for the second active aligner (mean VAS = 6mm), and also decreased

relative to the passive or first active aligner (mean VAS = 11mm and 8mm, respectively).

Further supporting the hypothesis of the habituation process is from Brien’s study, where the

patients self-reported questionnaires showed an initial increase in oral parafunctional-like

behaviors and symptoms during the first 2 weeks of CAT, but gradually reduced towards

baseline overtime.161

In other words, data from these studies all support our initial hypothesis that CAT is associated

with a transient increase in muscle activity and tooth pain, however, after a few weeks, the

adaptive mechanism of the orofacial musculatures take over and gradually returns towards

normal.

36

2.1 The Adaptation of Jaw Muscles to Clear Aligner Therapy

In the Pain-Adaptation model, orthodontic tooth pain from classical fixed appliances leads to a

fear of avoidance behavior, in which patients avoid tooth contact, such as clenching or chewing,

in order to reduce the perception of pain.158 This has been well demonstrated by previous sEMG

studies.101, 158 If this behavior is indeed taking place with CAT as well, it would lead to an

expected decrease in muscle activity, due to the avoidance behavior during treatment. However,

our data shows the exact opposite, in which the sEMG activity increased in response to the

aligner insertion. This result suggests that the pain-adaptation model is not consistent with CAT.

2.1.1 Occlusal Hypervigilance

One possible explanation for the observed increase in jaw muscle activity is due to occlusal

hypervigilance.105, 118 Occlusal hypervigilance is a term that describes the tendency to increase

occlusal perceptions and heighten attention on changes to the dentition.118 Patients with occlusal

hypervigilance may continuously check their occlusion and selectively focus on detecting

changes to their occlusal sensation.118 This continuous monitoring results in repetitive tooth-to-

tooth contacts and clenches as a way to detect and search for possible threats in the oral cavity.105

Since the increase in pain that was reported in the sister study is only of limited clinical

significance176, it may not have been harmful enough to trigger the avoidance behavior seen in

the pain-adaptation model. However, the presence of the foreign objects in the oral cavity, such

as the clear aligners, may be sufficient to trigger occlusal hypervigilance. The clear aligner trays

used in this study were made from SmartTrack®, proprietary multi-layered polyurethane based

polymer, designed by Align Technology to be highly elastic and maintain a constant force over-

time.89, 181 The current research on this material is limited182, but further research into its

properties, such as thickness and hardness may provide insight into the results observed here.55

2.1.2 Occlusal Interferences

Related to the idea of occlusal hypervigilance is the concept of occlusal interferences. It is

possible that the aligner itself may be a source for the introduction of occlusal interferences.121,

256 Clear aligners are appliances that provide full occlusal coverage of the entire dentition in both

arches.257 Due to the thickness of the thermoplastic material, they are clinically assumed to have

the propensity to introduce occlusal interferences. These occlusal interferences may lead to the

37

observed muscle hyperactivity. Furthermore, since the pain level associated with clear aligners is

very low (Figure 1-3), the fear of avoidance behavior is not seen. Related studies in this area

have demonstrated that the introduction of occlusal interferences may lead to varying degrees of

oral parafunction, with a minor impact in those with lower frequency of parafunction and an

aggravation in those with high frequencies of parafunction.121

Another explanation for the observed result may be properties inherent to the aligner itself, such

as its fit and material composition. The aligners fabricated in this study first came from

polyvinylsiloxane (PVS) impressions that were then poured and digitally scanned. The digitized

models were then used for rapid prototyping,28, 29 and the aligners were finally made via the

thermoforming process.7 All of the steps involved may be subject to distortion and could have

compounded beyond a critical threshold such that the dummy aligner to be not have been truly

passive.

2.1.3 Possible Alternative Pathway

Although the data from this study suggest that the Pain-Adaptation model should be ruled out,

there is insufficient evidence to definitively prove another mechanism taking place under CAT.

However, we hypothesize that Conditioned Pain Modulation (CPM) may act as an alternative

mechanism in response to the stimulus from CAT.100 In this paradigm, a secondary stimulus can

act as a conditioning stimulus to relieve the perception of pain from the original stimulus, similar

to the bite wafer effect.163 In simpler terms, CPM describes the phenomenon in which “pain

inhibits pain”. This theory correlates well with our experimental data, in which a transient

increase in masseter muscle activity and tooth pain was observed, with associated jaw muscle

tenderness as a consequence of the increased jaw muscle hyperactivity (Figure 3-1). It would be

prudent for future studies to investigate if Conditioned Pain Modulation (CPM) may act as an

alternative mechanism in response to the stimulus from CAT.

Individual daily recordings and reproducibility of data

It is well established in the existing literature that sEMG signals may have large variabilities

when recorded at the same site on different days258 and sometimes even within different

recording sessions.188, 224, 259 These variations may be due to a multitude of factors, including

38

method of electrode placement227-229, location of electrode over muscle230, 231, operator

training189, 232, body posture233 and psychological factors of patient.196, 234, 235

In order to improve reproducibility of the results, many of these factors have been controlled for

in our study, and as a result, the relative differences between the three days of recording within

each week is quite low (Figure 3-2). These methods included using concentric electrodes to have

a consistent distance, providing patients with an instructional video on the sEMG device,

marking the location of the masseter muscle for consistent electrode placement, requesting the

participants to record time of activities such as drinking and coughing and manually filtering

those results from the data.

These standardized protocols among the research units has improved reproducibility and allowed

the variance of the data within each day of the week to be very low. However, it is still very hard

to analyze the effect, if any, of each recording day within a given week. For example.

psychological factors, such as stress, has been shown to be a modulating factor in sEMG

activity.196, 234, 235 One limitation of the study is that patients were not restricted to start recording

at any particular day of the week. This means that patients starting the sEMG recording on a

weekend may be at a less stressful mood than patients who started their recording on a weekday,

thus making analyzing the effect of each individual day unreliable.260 The impact of this

limitation is lessened by averaging the three days into a weekly analysis, as this accounts for all

the days within the given week.

Analysis of psychological traits and oral behaviors

The results of the sensitivity analysis showed that both trait anxiety and OBC were predictors for

sEMG activity. The trait anxiety score was positively associated with an increase in sEMG

activity (B = 75.04, p = 0.027) while the OBC was negatively associated with a decrease in

sEMG activity (B = -34.12, p = 0.011). The exact changes associated with these parameters

could not be estimated, due to the standardization using Z-scores. These results suggest that in

highly anxious individuals, orthodontic treatment with CAT may lead to a further increase in the

jaw muscle activity. This data is consistent with recent studies demonstrating that individuals

with increased trait anxiety and concurrent facial pain report more frequent oral parafunctions

than those without pain.105 Indeed, occlusal sensations has been associated with psychosocially

loaded situations, such as distress, anxiety, negative emotions and depression, which may

39

increase the abnormal occlusal sensation, leading to the patient to focus more on their occlusal

changes.118

Conclusions

1. Clear aligner therapy produces a transient increase in masseter muscle activity within the

first two weeks of treatment and decreases towards baseline thereafter.

2. Presence of the aligner tray is the main determinant of masseter muscle activity.

3. Psychological factors have a modulating effect on masseter muscle activity during CAT.

Clinical Significance

Jaw muscle hyperactivity may encompass various functional disturbances, such as oral

parafunction, which is a risk factor for temporomandibular disease.261 Our data shows that clear

aligners are associated with jaw muscle hyperactivity. While these changes in jaw motor

response are adaptive and transient in healthy patients, caution might be taken in treatment

involving patients with current or history of TMD, as well as patients with high trait anxiety.

However, further studies are needed to address the effect of clear aligner therapy on patients with

orofacial pain and TMD.

Limitations

The main limitations of this study are related to the sEMG device, various patient factors and

aligner fabrication. The use of ambulatory sEMG has technological limitations, such as the lack

of real time feedback. Since the data is stored locally on the device itself, it remains with the

participant until they return to the clinic. This makes it difficult to check for patient compliance

or technique errors in using the device during the experimental time period. Attempts to

overcome this limitation during this study included providing written instructions, in-person

demonstration of the device, as well as custom-filmed instructional videos and calendars

(Appendices 8-10).252 Surface electromyography is also limited to the study of the general gross

activity of the muscle as a whole,214-216 to study the changes in myoelectric activities of the

various parts of a muscle, intramuscularly electromyography would have to be used.217 However,

this technique is more invasive and would be more challenging to implement.

40

There is also a significant burden of participation to the potential patient interested in the study.

This study required the patient to record sEMG signals for 48 hours over a period of 4 weeks,

limitations to their daily activities, such as the avoidance of exercising, eating and drinking. The

large time commitment detracted some patients from participating in the study. Despite our best

efforts to instruct the participants through written, verbal and video guides, not all participants

followed the protocol correctly (Appendix 9 and 10).252 Although participants were asked to

perform three clenches at maximum voluntary contraction, many of them did not clench at the

beginning of the recording session, thus eliminating the ability to analyze via maximum

voluntary contractions. If the data could be analyzed based on maximum voluntary contractions,

this would have allowed for the direct assessment of oral parafunction, rather than indirectly

through the analysis of standardized estimates of muscle activity (Z scores). Lastly, the

participants were all female expect for a single male, which may limit the external validity and

generalizability of the study.

In regard to the actual aligners used in this study, it is possible that the dummy aligners may not

have been truly passive. This is because of the possibility for distortion to take place in several

steps in the aligner manufacturing process, starting from the PVS impression to the 3D

digitization process, to the rapid prototyping technique,28, 29 and the thermoforming process.7

Future Studies

With new advancements in digital technology, future studies with more sophisticated hardware

and software could improve subsequent studies using ambulatory sEMG measurements.262 This

includes new generations of sEMG devices that are equipped with wireless communication

protocols, allowing connection with smartphones or computers to give real-time feedback of

sEMG signals.262 This in turn, may facilitate easier and simpler ambulatory sEMG recordings for

the patient and reduce their burden of participation, and improving the quality of data

Future research may benefit from data analysis using maximum voluntary contractions to allow

for measurement of parafunctional behaviors rather than just muscle activity. It would also be

prudent to study the effect of clear aligner therapy on TMD patients, children and adolescents, as

the morphological and structural differences in these groups of patients may lead to different

responses.263, 264

41

Appendices

Appendix 1 - Health Sciences Research Ethics Board (REB) Approval

42

Appendix 2 - Research Diagnostic Criteria for Temporomandibular Disorders (RDC/TMD)

Examination Form (page 1/2)

43

Appendix 2 - Research Diagnostic Criteria for Temporomandibular Disorders (RDC/TMD)

Examination Form (page 2/2)

44

Appendix 3a – Diagnostic Criteria for TMD demographics (page 1/3)

45

Appendix 3a – Diagnostic Criteria for TMD demographics (page 2/3)

46

Appendix 3a – Diagnostic Criteria for TMD demographics (page 3/3)

47

Appendix 3b - TMD-Pain Screener

48

Appendix 4a – Oral Behavior Checklist

49

Appendix 4b – Somatosensory Amplification Scale

50

Appendix 4c – Beck Depression Inventory (page 1/3)

51

Appendix 4c – Beck Depression Inventory (page 2/3)

52

Appendix 4c – Beck Depression Inventory (page 3/3)

53

Appendix 4d – Pain Catastrophizing Scale

54

Appendix 5 – Information and Consent for Research Study (page 1/5)

55

Appendix 5 – Information and Consent for Research Study (page 2/5)

56

Appendix 5 – Information and Consent for Research Study (page 3/5)

57

Appendix 5 – Information and Consent for Research Study (page 4/5)

58

Appendix 5 – Information and Consent for Research Study (page 5/5)

59

Appendix 6 – Research Flyer for Recruitment of Participants

60

Appendix 7 – Customized Pain Diary (1 page used per day)

61

Appendix 8 – Sample Customized Calendar for the Experimental Period

62

Appendix 9 – Written Instructions for Operating the sEMG Device (page 1/5)

63

Appendix 9 – Written Instructions for Operating the sEMG Device (page 2/5)

64

Appendix 9 – Written Instructions for Operating the sEMG Device (page 3/5)

65

Appendix 9 – Written Instructions for Operating the sEMG Device (page 4/5)

66

Appendix 9 – Written Instructions for Operating the sEMG Device (page 5/5)

67

Appendix 10 – Video Instructions for Operating the sEMG Device (Screen captures of the video

provided)

https://www.youtube.com/watch?v=M8ZqYdN32iE&feature=youtu.be

68

Appendix 11 – Tabulated data of the mean muscle activity (Z-score), standard error and the 95%

confidence interval for effect of aligner condition on the masseter muscle activity.

Week Mean (Z-Score) Std. Error

95% Confidence Interval

Lower Upper

Baseline -0.227 0.036 -0.297 -0.158

Passive Aligner 0.119 0.035 0.050 0.188

First Active

Aligner 0.125 0.036 0.056 0.195

Second Active

Aligner -0.087 0.035 -0.156 -0.017

69

Appendix 12 – Tabulated data of the daily mean muscle activity (Z-score), standard error and

the 95% confidence interval for effect of aligner condition on the masseter muscle activity.

Differing asterisks for the individual days within each week indicate statistical significance

between the pairwise comparisons.

Week Day Mean (Z-

Score)

Std.

Error

95% Confidence interval

Lower Upper

1

1* -0.180 0.042 -0.261 -0.098

3** -0.197 0.042 -0.280 -0.115

5*** -0.305 0.041 -0.385 -0.226

2

1* 0.071 0.040 -0.009 0.150

3*, ** 0.128 0.041 0.047 0.208

5** 0.158 0.041 0.079 0.238

3

1* 0.267 0.042 0.185 0.349

3** -0.004 0.041 -0.084 0.077

5*** 0.113 0.042 0.031 0.195

4

1* -0.115 0.041 -0.195 -0.035

3** -0.149 0.041 -0.229 -0.068

5** 0.003 0.042 -0.078 0.085

70

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