A COMPARISON OF ROOT RESORPTION BETWEEN INVISALIGN TREATMENT
AND CONTEMPORARY ORTHODONTIC TREATMENT
by
Brandon Fowler
________________________________________________________________
A Thesis Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
MASTER OF SCIENCE
(CRANIOFACIAL BIOLOGY)
May 2010
Copyright 2010 Brandon Fowler
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DEDICATION
I dedicate this thesis to my beautiful wife, Melissa. Without her patience, love, support,
and laughter, I would not be who I am today. I could not have accomplished what I have
so far without you and I look forward to the future because I have my best friend on my
team.
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ACKNOWLEDGEMENTS
Dr. Kurt Stormberg: Thank you for allowing me to come down to your office numerous
times in order to gather all the needed records for my data collection. Thanks also to you
and your staff for being so welcoming to me. This research would have not been possible
without your generosity and help.
Dr. Glenn Sameshima: Thank you to my research advisor for all your help with this
thesis. Thank you for your amazing statistical crunching powers and for your extensive
knowledge on root resorption.
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TABLE OF CONTENTS
Dedication ii
Acknowledgements iii
List of Tables vi
List of Figures vii
Abstract viii
Chapter One: Introduction 1
I. History of Removable Appliances 1
a. Functional Appliances For Growth Modification 1
b. Removable Appliances for Tooth Movement 4
II. Contemporary Removable Appliances 6
a. Tooth Movement Utilizing Removable Appliances 6
1. Arch Expansion 6
2. Spring Retainers 7
Chapter Two: Literature Review 10
I. Advantages of Removable Appliances 10
II. Introduction of Clear Removable Appliances 10
III. Align Technology Inc. Overview 12
a. How The Invisalign System Works 13
b. Case Selection for Invisalign 15
c. New Improvements of Invisalign 16
1. Optimized Extrusion Attachments 16
2. Optimized Rotation Attachments 17
3. Power Ridges 17
4. Velocity Optimization 18
5. IPR Improvements 18
d. Invisalign Popularity 18
IV. Competitors of Invisalign 20
1. Bioliner Plus 21
2. ClearCorrect Aligners 22
3. NuBrace 23
V. Success of Invisalign 24
VI. Failures of Invisalign 26
VII. Forces 28
1. Pressure Tension Theory 28
2. Types of Forces 30
3. Types of Movements Accomplished 31
VIII. Forces of Invisalign 32
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IX. Root Resorption 34
X. Biological Factors 37
1. Root Morphology 37
2. Gender 38
3. Age 38
4. Genetics 39
5. Systemic Factors 40
6. Trauma Before Treatment 40
XI. Mechanical Factors 41
1. Treatment Times 41
2. Types of Forces 41
3. Types of Appliances 42
4. Extraction Pattern 42
5. Orthodontic Movement Type 43
6. Orthodontic Force 43
XII. Clinical Diagnostic Aids To Identify Root Resorption 44
XIII. Cascade of Affected Teeth 45
XIV. Treatment of Root Resorption 46
XV. Apical Root Resorption with Removable Appliances 47
XVI. Root Resorption with Invisalign 48
Chapter Three: Hypothesis 49
Chapter Four: Materials and Methods 50
Chapter Five: Results 60
Chapter Six: Discussion 73
Chapter Seven: Conclusion 83
References 84
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LIST OF TABLES
Table 1: Descriptive statistics for Conventional Fixed Appliances 60
(Group C)
Table 2: Descriptive statistics for Invisalign Removable Aligners 60
(Group I)
Table 3: Group Statistics for Group C and Group I 61
Table 4: Levene’s Test for Equality of Variances 62
Table 5: t-test for Equality of Means 63
Table 6: Frequency tables for root resorption seen on all 64
maxillary teeth for Group C and Group I
Table 7: Frequency tables for root resorption seen on all 66
mandibular teeth for Group C and Group I
Table 8: Root shape for all teeth displayed by each individual 68
tooth type and location.
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LIST OF FIGURES
Figure 1: The Monobloc Appliance 2
Figure 2: The Activator Appliance 2
Figure 3: Maxillary Schwartz Appliance 4
Figure 4: Crozat Appliance 5
Figure 5: Split Plate and Rapid Palatal Expander 7
Figure 6: Spring Retainers 8
Figure 7: Clear removable “Suck down” retainer 11
Figure 8: Invisalign Aligners 12
Figure 9: Example of ClinCheck Screen; Before and After 14
Figure 10: Optimized Extrusion Attachment 17
Figure 11: Optimized Rotation Attachment 17
Figure 12: Power Ridges 18
Figure 13: Visual difference between Conventional Fixed and Invisalign 20
Figure 14: Levander and Malmgren root resorption visual classification 55
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ABSTRACT
The purpose of the study was to compare the incidence and severity of External
Apical Root Resorption (EARR) in patients treated with different orthodontic appliances.
The definition of EARR can be that of blunting or shortening of the root apex, a
condition often associated with orthodontic treatment. In previous studies it have been
found that the teeth most susceptible to root resorption are the maxillary and mandibular
incisors, with the maxillary lateral incisors have the greatest incidence and degree of
resorption. A number of studies have evaluated the role of variables such as age, sex,
malocclusion, dental root anatomy, appliance type, type of orthodontic force, and
treatment time in the development of external apical root resorption. Few studies have
dealt with the effects of different orthodontic bracketing systems on EARR. Standard
edgewise, straight-wire, and Begg appliances, as well as the newer self-ligating bracket
systems, have began to be investigated. The introduction and popularity of Invisalign in
treating more and more complex orthodontic malocclusions provoked the investigation of
its effect on external apical root resorption. Only a few clinical studies have been
published with regards to Invisalign and very few studies have been completed that
analyzed the effects of the Invisalign aligners. A further aim was to analyze the degree of
EARR in different tooth groups in patients presenting with root resorption. The sample
consisted of 90 female patients aged 10 – 58 years at the beginning of orthodontic
treatment. A conventional edgewise appliance (MBT prescription, 0.22-inch slot) and the
Invisalign appliance were used. Root resorption in all anterior tooth groups, (canines,
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lateral incisors, central incisors), of both maxilla and the mandible were evaluated from
pre- and post-treatment panoramic radiographs. Of the tooth groups, maxillary lateral
incisors showed EARR most frequently, followed by the maxillary central incisors and
then the mandibular incisors. Root resorption was significantly correlated with the
conventional edgewise appliance treatment and not with the Invisalign treatment. The
most severe resorption was seen in the maxillary lateral incisors. It is concluded that with
the fixed conventional edgewise appliance, the risk of EARR is higher than when
malocclusions are treated with the Invisalign appliance. In this study there was not any
recordable external apical root resorption present when the malocclusions were treated
with the Invisalign clear removable aligners.
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CHAPTER ONE: INTRODUCTION
I. The History of Removable Appliances
Most of the early removable appliances in orthodontics were functional
appliances which were used to change the posture of the mandible in order to promote
growth. This can be done by holding the mandible open, or open and forward. This
action in turn creates pressure on the muscles due to the constant stretching of the
muscles and soft tissues which is transmitted to the skeletal and dental structures and
moves the teeth and modifies growth.
A. Functional Appliances For Growth Modification
In 1879, Norman W. Kingsley was the first to use the forward positioning of the
mandible in orthodontic treatment. Kingsley’s removable plate with molar clasps might
be considered the prototype of all functional appliances, having a continuous labial wire
and a bite plane extending posteriorly. (Wahl, 2006)
The Monobloc, developed by Pierre Robin in 1902, is considered the forerunner
of the functional appliances. His appliance influenced muscular activity by changing the
spatial relationship of the jaws. The Monobloc extended all along the lingual surfaces of
the mandibular teeth, but it had sharp lingual imprints of the crown surfaces of both the
maxillary and mandibular teeth. It also incorporated an expansion screw in the palate to
expand the dental arches. (Wahl, 2006)
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Figure 1: The Monobloc Appliance. Courtesy of nimrodental.co.uk.
In the 1920s, the Activator became the first widely used functional appliance. The
Activator was developed by Viggo Anderson in Norway and became known as the
“Norwegian system” of treatment. The philosophy behind the Activator was first
discovered by Anderson in 1909 when he removed his daughter’s fixed appliances before
she left for her summer vacation, as was customary at the time, and placed a Hawley-type
maxillary retainer. On the mandibular teeth, Anderson placed a lingual horseshoe flange
that guided the mandible forward about 3 to 4 mm in occlusion. When his daughter
returned from her summer vacation, he noticed that her nighttime wear of the appliances
had eliminated her Class II malocclusion, and it was found to be stable. (Wahl, 2006)
Figure 2: The Activator Appliance. Courtesy fo the Joseph Burke Lab
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The Activator appliance was continually adjusted and improved over the years
with most of the improvements developed by Karl Haupl, a fellow German dental school
faculty member with Anderson at the University of Oslo. Haupl believed that the only
stable tooth movement was that produced by natural forces from the body and the
alterations in function produced by these removable appliances would give stable
corrections to the malocclusions they were treating. (Proffit, 2007) This orthodontic
philosophy was distinctly different from the teaching and philosophy of Edward Angle in
the United States. Angle and his followers emphasized the achievement of precise
positions of teeth and this was believed to be accomplished only through the use of fixed
appliances.
During 1925 to 1965, the orthodontics that was performed in the United States
was based primarily on exclusive use of fixed appliances. This was not the case in
Europe during the same time period. In Europe, the orthodontics was completed primarily
with removable appliances. This was because in Europe there were much more rapidly
forming social welfare systems which placed more emphasis on limited orthodontic
treatment for a majority of the population and the treatment itself was often performed
and completed by general practitioners rather than orthodontic specialists. Also, the
precious metal that was needed for the fixed appliances was less available in Europe due
to these social systems, as well as, it being banned in Nazi Germany. (Proffit, 2007) This
therefore forced the German orthodontists and general dentists to use removable
appliances to complete the orthodontic treatment of their patients.
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B. Removable Appliances for Tooth Movement
The second group of removable appliances were those of the “active plates”
group, which were used more for pure tooth movement. (Proffit, 2007) Martin Schwartz
developed his “Split plate” in Vienna which could produce different types of tooth
movements as well as expand the maxilla or mandible.
Figure 3: Maxillary Schwartz Appliance. Courtesy of the Clinical Foundation of
Orthopedics and Orthodontics.
In the early 1900’s, George Crozat developed one of the first removable
appliances in the United States used for tooth movement. His appliance consisted of
clasps around the first molars with a heavy gold wire as the framework, and lighter gold
wires that acted like finger springs, which would produce the desired tooth movement.
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The drawback of this appliance, like all removable appliances, was that the forces it
produced were mainly a tipping of the tooth and not bodily movement.
Figure 4: Crozat Appliance. Courtesy of the Clinical Foundation of Orthopedics
and Orthodontics.
Tooth movement without bands, brackets, or wires was described as early as 1944
by Harold Kesling who reported a flexible tooth-positioning appliance he called the
“tooth positioner”. (Kesling, 1945) His new technique involved taking impressions of a
patient’s dentition who was nearing completion of their orthodontic treatment and
resetting the teeth into ideal position which helped close the spaces left after debanding.
From the new models, a rubber positioner was made that, if worn enough hours, acted as
a finishing appliance. It could also be used as a retainer and later versions of the tooth
positioner were made of other materials, including clear plastic. (Wahl, 2006) In 1971,
Ponitz introduced his so called “invisible retainers.” He claimed that by using base plate
wax to reposition the teeth on the master model before the retainer was made, he could
achieve limited tooth movement with the appliance. (Ponitz, 1971)
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In the 1960s there was a blending of the European and United States orthodontic
philosophies. Fixed appliances became more popular in Europe, and removable
functional appliances began to catch on in the United States. Since the 1960s there has
not been a distinct difference between orthodontics performed in Europe or the United
States as both have accepted and adapted to include each other’s philosophy into their
own.
II. Contemporary Removable Appliances
In today’s orthodontic treatment protocols, removable appliances that are not
being used for growth modification, but solely used for moving teeth can be categorized
into two classes. The first classification includes removable appliances being used for
tooth movements in pre-adolescents, and the second classification being that of the use of
clear plastic aligners which are becoming more popular for tooth movement in adults.
A. Tooth movement utilizing Removable appliances.
The removable appliances used for moving teeth in children can further be
classified into 1) Removable appliances used for arch expansion and 2) Removable
appliances used to reposition individual teeth within the arch.
1. Arch Expansion Appliances
Active plates or palatal expanders are used for arch expansion primarily in the
maxilla. Palatal expanders consist of a base plate that has clasps attached to it and
usually has a jackscrew embedded in it which holds the two pieces of the plate together.
The jackscrew is activated and opened by turning a key, which separates the two pieces
of the base plate. The screw controls the amount of movement of the teeth, but the force
that is delivered each time the screw is turned is very different from the ideal force for
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moving teeth. The jackscrew produces a heavy force with each activation and this heavy
force decays rapidly rather than the ideal light continuous force, which has been found to
be congruent with moving teeth. When the screw is reactivated, multiple heavy forces
are placed on the teeth, which allows for potential damage to the roots of the teeth. Also,
if the jackscrew is activated to often without allowing for the expansion of the bone and
teeth, the appliance will be displaced and no longer fit, which makes the appliance
ineffective.
Figure 5. Split Plate (left) and Rapid Palatal Expander (right) (Courtesy
thewirebenders.com)
2. Spring Retainers
Removable appliances with springs for tooth movements do not produce the
heavy forces like that of a jackscrew in a palatal expander, but instead produce the nearly
ideal light continuous force needed to move teeth. Like the edges of the active
plates/palatal expander, the removable appliances with the springs have a disadvantage
because the springs only contact the tooth at one point and therefore can only produce a
tipping tooth movement. The guidelines for using a removable appliance with a spring is
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therefore only acceptable if the tooth movement desired is a few millimeters of tipping.
For movements of 3 to 4 mm of crown movement, control of the roots of the teeth needs
to be accounted for.
According to Proffit, in designing springs for tooth movement, there are two
principles that need to be accounted for: 1) the design of the appliance must provide
adequate springiness and range while also possessing acceptable strength, and 2) the
spring must be positioned so that its force is exerted only in the desired direction. To
account for these two principles, the springs on the removable appliances are usually
made out of larger diameter wires (greater than 0.5mm) for strength. By increasing the
length of the spring through the use of loops and curves in the spring, the needed amount
of springiness can be achieved. (Proffit, 2007)
Figure 6: Spring Retainers (Courtesy of ParOrthoLab.com)
In Belfast, England, Philip Adams modified the arrowhead clasp into what he
called the Adams clasp. The Adams clasp has become the basis for the English
removable appliances and to this day is still considered the most effective clasp for
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removable orthodontic purposes. The Adams clasp is a very important part of the
removable appliance because if the appliance is not effectively retained in the mouth then
the appliance becomes ineffective. Adams clasps are usually made of 0.7mm wire and
the retentive points of the clasp must fit well into the undercuts of the teeth for the needed
retention.
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CHAPTER TWO: LITERATURE REVIEW
I. Advantages of Removable Appliances
Removable orthodontic appliances have three distinct advantages when compared
with conventional orthodontic bracketing systems: 1. They are fabricated in a dental
laboratory rather than placed directly in the patient’s mouth, which reduces the chair time
of the practicing orthodontist, 2. They can be removed in social situations when having
braces may be seen as undesirable, and 3. Removable orthodontic appliances can be
made of clear plastics materials that make them almost invisible to others. All of these
benefits make orthodontic treatment more acceptable to adult patients, which is why we
see more adults receiving orthodontic treatment. The thought of straightening an adult’s
teeth with removable appliances can now be seen as more esthetic than conventional
brackets and wires. Because of these advantages, there is more and more interest in the
continuing expansion of clear removable orthodontic appliances.
II. Introduction of Clear Removable Appliances
The introduction of vacuum-formed clear thermoplastic sheets in the early 1980s
allowed orthodontists to change the way they looked at moving teeth. With this new
technology, the thermoplastic sheets could be heated in order to soften the material and
then could be vacuumed down onto a model of the patient’s teeth to have the sheets fit
very tightly over the teeth. These “suck down” materials were originally used as another
form of retention, but it became clear that these new materials could also be useful in
another way. If the teeth in the arch were reset on the model in a slightly different
position before making the “suck down retainer” there would in effect have been created
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a tooth-moving device rather then just a retainer. This device became known as an
“aligner” because it could align mildly displaced or crowded teeth back into proper
occlusion. Originally this was thought of as a solution to the slight amount of relapse seen
in patients who had previous orthodontic treatment and had stopped wearing their
retainers.
Figure 7: Clear removable ‘Suck down” retainer. (Courtesy of Invisalign.com)
At first, only small degrees of tooth movements were possible with a single
aligner due to the stiffness of the material. In order to generate more movement from the
aligner, the aligner would need to be modified. The material’s ability to be heated up and
then remolded led to the use of a dimple pliers to alter the aligner shape slightly in order
to generate more movement to move the teeth to a new position. A major limitation in
this protocol of modifying the aligners was that the plastic could only be stretched a
maximum of about 3mm, in 1mm increments, before it became too thin and lost its
ability to deliver the needed force. (Proffit, 2007) A newer technique was developed
which incorporated hard plastic bumps that snap into a hole in the aligner, therefore
modifying the aligner and creating more tooth movement. This had the advantage of not
12
needing to stretch the aligner plastic, but it was still not a practical way to manage more
difficult orthodontic problems or solve the problem of having to modify the aligners.
It became very evident that a sequence of several aligners made from a series of
dental casts with teeth positioned in slightly different locations was the best way of
creating an orthodontic moving system. This however, was seen to be extremely time
consuming in the dental laboratory and not many orthodontists had the time or the desire
to attempt this new technique.
III. Align Technology Inc, Overview
In the late 1990s, Align Technology Inc., whose headquarters are located in Santa
Clara, California, decided to try to computerize the process of producing a sequence of
casts with incremental changes on which aligners could be fabricated. They first
introduced the Invisalign system in 1999 and are considered the pioneers in “invisible
orthodontics,” which is perceived as being the ultimate esthetic form of orthodontics. The
Invisalign treatment system combines a three-dimensional modeling software with
custom manufacturing technology to produce a series of clear, removable aligners in
order to straighten teeth.
Figure 8: Invisalign aligners. (Courtesy of Invisalign.com)
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The aligners are made from a clear, thin plastic, which fits over the buccal,
lingual, and occlusal surfaces of the teeth. The Invisalign aligners are composed of
polyurethane with added methylene diphenyl diisocyanate and 1,6hexanediol. The
diphenyl structure provides the stability needed to be used as an orthodontic appliance.
(Schuster, 2004) The aligners are to be worn by the patient for 22 hours per day to
achieve gradual tooth movement and are then changed and advanced to the next aligner
in the series every two weeks. (Schuster, 2004) Each aligner is designed to move a tooth
about 0.25-0.33mm. (Joffe, 2003 and Boyd, 2000)
A. How The Invisalign System Works
The entire process begins with the practitioner taking a Vinyl Polysiloxane
impression of the patient’s maxillary and mandibular teeth which are then shipped
directly to Align Technology. Their approach is to scan the original pre-treatment dental
casts in order to create a digital model of the casts. From these digital casts Align could
then make small changes in the position of the teeth. Once the resetting of the teeth is
complete, Align produces a stereolithographic cast upon which the aligners will be made.
Additional incremental changes can be made on the digital model and Align will produce
a matching series of modified casts. From this series of modified casts, a series of
custom-made clear “aligners” are produced. Each aligner moves the teeth in small
incremental steps and is then replaced by the next aligner in the series until the final
desired tooth position is achieved. Once the impressions of the patient’s maxillary and
mandibular arches, along with all radiographic records, and the completed comprehensive
treatment planning form have been shipped to Align Technology, they create a three-dimensional
treatment plan customized to each patient. The orthodontist can log onto the
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Invisalign website using their own password and in their own personal domain about two
weeks later, to view the patient’s virtual models in 3-D which is referred to as the
ClinCheck. The treatment plan submitted has now been translated into tooth movements
and is able to be viewed in this virtual correction stage by stage and from any angle. The
Invisalign technique uses this 3-D computer imaging technology to depict the complete
treatment plan from the initial position of the teeth to the final desired position. Using the
ClinCheck software, Align Technology is able to demonstrate the planned treatment
along with the number of aligners in the series that will be used to achieve the final
desired result. Most of the time there is a need for Inter-proximal reduction (IPR) when
there is not enough space for the teeth to move past each other in crowded cases. The
amount of IPR needed, as well as the locations between which teeth, is also included in
the ClinCheck for review by the treating orthodontist
Figure 9: Example of ClinCheck Screen; Before and After. Courtesy of
enhancedental.com
If there is anything that is not satisfactory to the treating orthodontist, alterations to
the treatment plan can be submitted. These changes in the virtual treatment plan are
unlimited and can be made until the orthodontist is completely satisfied. Once satisfied
with the virtual treatment plan, the process is completed by confirming that Align
Technology can go ahead and manufacture the aligners. The entire set of aligners will
15
then be fabricated and shipped to the office of the orthodontist within four weeks and are
ready for delivery to the patient.
Once treatment is complete with the series of aligners, if the treatment outcome is
not exactly what the orthodontist wanted, refinement aligners can be made. Refinement
aligners are a series of aligners that can be added at the end of treatment with the original
series of aligners in order to complete any tooth movement not fully expressed by the
original series of aligners. A new set of impressions is needed and the previous steps up
to confirming the ClinCheck are again completed in order to confirm fabrication of the
refinement aligner series.
B. Case Selection for Invisalign
With careful attention to detail and planning Align has created an orthodontic
appliance that can handle treating mild to moderate cases of crowding. Align technology
has made it clear that Invisalign is indicated for patients with mild to moderate crowding
(1-6mm), mild to moderate spacing (1-6mm), and non-skeletal constricted arches, and
those who have experienced relapse after fixed appliance therapy. (Turpin, 2005). It was
also declared that since growth changes could not be accounted for with the protocol of
fabricating the aligners, this treatment option would be limited to non-growing adult
patients. In the initial developmental stages of this new technique of moving teeth there
were definitely some kinks that needed to be worked out. Therefore, the overall
acceptance of Invisalign was not very significant with many orthodontists to begin with.
Invisalign has been indicated by its manufacturer to be used in adults and
adolescents who have fully erupted permanent dentitions. There is still much debate as to
which categories of malocclusions are good cases to be treated with Invisalign. Of
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course, Align Technology claims that basically any case, from mild crowding to
moderately complex cases, can be treated successfully with the Invisalign system, but
Joffe has defined some specific criteria as to who should use Invisalign. He warned that
caution should be taken with malocclusions with more than 5mm of spacing or crowding,
skeletal anteroposterior discrepancies greater than 2mm, teeth rotations greater then 20
degrees, anterior or posterior open bites, teeth extrusion, teeth tipping of greater than 45
degrees, teeth with short clinical crowns and arches with multiple missing teeth. (Joffe,
2003)
C. New Improvements of Invisalign
Since these early stages of development there have been many new improvements
to the technique and the popularity of Invisalign in the orthodontic community is
beginning to significantly increase. Align Technology is constantly trying to improve
their product and to expand its parameters of treatment to include treatment of more
complex cases. There have been advances in the attachments to the teeth, which allow
more complex tooth movements. The attachments are tooth colored composite buttons
that allow the aligners to de-rotate, intrude, or extrude a tooth.
The newest improvements to the Invisalign technique are discussed briefly:
1.Optimized Extrusion Attachments.
These attachments are used for extrusion of maxillary and mandibular teeth which
are customized for each patient and based on the width, long axis, and contour of each
tooth. They include an active surface area with a tapered gingival area designed to
deliver more active forces with a more esthetic design.
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Figure 10: Optimized Extrusion Attachment. Courtesy of Aligntechinstitute.com
2. Optimized Rotation Attachments.
These attachments are used to help improve rotations of upper and lower canines.
The attachment is on placed on the buccal side of the tooth and as the aligner engages the
attachment, it is also designed to engage the lingual side of the tooth which would
provide a rotational moment along the long axis of the canine.
Figure 11: Optimized Rotation Attachement. Courtesy of Aligntechinstitute.com
3. Power Ridges.
Designed to optimized forces on maxillary incisors to deliver lingual root torque
without having to bond attachments. Can be used to upright Class II Division II
retroinclined incisors. Designed to produce equal moment to force to ratios to produce
lingual root torque.
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Figure 12: Power ridges. Courtesy of Alignetechinstitute.com
4. Velocity Optimization.
Designed to provide more controlled movement of the tooth’s root and crown.
Improved ClinCheck protocols are designed to limit the speed of crown and root
movements, including rotations, to optimal ranges. ClinCheck is now designed to factor
in crown and root movements at every aligner stage.
5. IPR Improvements.
Improved ClinCheck protocols are set up to be designed for the IPR to be
performed when the teeth are more aligned. In crowded cases, the prescribed IPR is now
setup in later stages of the aligners so that tooth contacts needing IPR may be easier to
access. (www.aligntechinstitute.com, 2010)
D. Invisalign Popularity
Align Technology serves doctors around the world, and since its inception has
claimed to have trained more than 58,000 doctors, started more than 1,000,000 patients in
treatment, and manufactured more than 46 million unique Invisalign aligners. (Invisalign
website). Invisalign was originally only marketed to orthodontists and the general public.
After a class action lawsuit was brought upon them by a group of general dentists, Align
Technology began offering its services to general dentists as well. Through extensive
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marketing, Invisalign has become a treatment modality that most orthodontists and
general dentists are now familiar with and using, all within a little more than a decade
after its development. (Vicens, 2010) Align Technology’s marketing campaign to the
general public has also sparked a huge interest and demand for the Invisalign treatment
and at the time was thought of as a revolutionary way to market the product not only to
the potential treating orthodontists, but also directly to the consumers.
A study in 2009 was conducted which polled current orthodontic residents in the 65
US orthodontic programs to determine their future clinical practice plans. From this
study, 84.06% of the orthodontic residents stated they plan to use Invisalign and 63.04%
plan to use self-ligating brackets. (Noble, 2009) Noble et al, also completed another
study in 2009 regarding the motivations and future plans of Canadian orthodontic
residents and it was determined that again, almost 80% of the residents will be utilizing
Invisalign in their practices once they graduate. (Noble, 2009) The popularity of
Invisalign is therefore expected to grow in both in the United States and Canada because
of the projected future use by current orthodontic residents.
Today’s orthodontists are beginning to focus on the patient’s perspective of
treatment and how much value they place on quality of care and the esthetics of the
braces they will have in their mouth. A study done by Rosvall et al looked at the
attractiveness, acceptability, and value of orthodontic appliances. Fifty adults were asked
to rank by appearance three categories of appliances that are routinely used: 1) Clear
aligners and Lingual appliances, 2) Ceramic appliances and 3) Stainless steel appliances.
Clear aligners and Lingual appliances were preferred over all fixed appliances, both
ceramic and stainless steel. In addition, the fifty adults polled in this survey were much
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more willing to pay more for the appliances which they considered more esthetic.
(Rosvall, 2009)
Figure 13: Visual difference between Conventional Fixed and Invisalign.
Courtesy of inviaslign.com.
Besides esthetics, Invisalign could offer other advantages to its users such as, not
impeding oral hygiene procedures, reducing eating restrictions, and possibly reducing the
risk of decalcification, caries, gingivitis, and periodontal disease. (Bollen, 2003)
IV. COMPETITORS OF INVISALIGN
With the popularity of the Invisalign system being evident in today’s orthodontic
climate, there have been numerous competing companies that have started up and have
developed similar treating techniques as Invisalign. Align Technology Inc has previously
filed a complaint with the U.S. International Trade Commission against a competing
company, OrthoClear, Inc., in 2006. OrthoClear was a clear plastic aligner manufacturer
that had been started by a former Align Technology employee. Align Technology
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eventually acquired OrthoClear’s disputed intellectual property and shut down production
of OrthoClear’s aligner system, however other competing companies remain.
1. Bioliner Plus
The Bioliner Plus System is fabricated by Five Star Orthodontic Laboratory &
Supply, whose headquarters are located in Denton, Texas. The Bioliner Plus system is a
competitor of Invisalign and is used to correct cases with rotated teeth, tipped teeth, and
minor cases. The Bioliner system does not make any claims about the ability to correct
Class II or Class III malocclusions. They also state that their system is not to be used to
correct open bites, closed bites, excessive crowding or cases that require sliding
mechanics.
The protocol of the Bioliner Plus system begins the same as Invisalign’s with the
practitioner taking a Vinyl Polysiloxane impression of the patient’s maxillary and
mandibular teeth. This is then shipped to Texas, where a 0.85mm ultra clear plastic
aligner is fabricated with a soft thermal plastic insert. The Bioliner has a soft insert
producing gentle, continuous realignment on all teeth which have been reset. The soft
insert allows individual tooth movement up to 1.5mm per tray, depending on space which
is a lot more movement compared to Invisalign’s 0.33mm. The Bioliner is worn for 2 to 3
weeks. The Plus, an aligner without a soft insert, is then worn to finish for 1 to 2 weeks.
If needed after this series of aligners, a new impression can be taken for a new Bioliner.
Up to three Bioliners can be made from the original models. Taking a new impression
after three Bioliners gives control back to the orthodontist, and eliminates improper
fitting trays. The Bioliner can also be used for bleaching trays in conjunction with
22
realignment. Upon finishing the case, the Plus aligner may then be used as a retainer,
however a Hawley retainer is recommended for final retention. (fivestarortho.com, 2010)
2. ClearCorrect Aligners
Another strong competitor of Invisalign is the ClearCorrect Aligner system.
ClearCorrect, Inc., is headquartered in Houston, Texas and was founded and owned by
cosmetic dentist, Dr. Willis Pumphrey. The ClearCorrect system works the exact same
way as the Invisalign treatment in that clear plastic aligners are manufactured by taking a
mold of the patient’s teeth, then cast into a plaster model, and scanned by ClearCorrect
into a three-dimensional model. A computer program then designs a series of plastic
dental inserts, which are replaced every two weeks. The inserts apply pressure to the
teeth, slowly moving them into the desired position.
One of the benefits of ClearCorrect over Invisalign is that there are no midcourse
correction or refinement fees. They also claim their lab fees are lower and all products
include initial retention at no extra charge. ClearCorrect’s exponential growth seems to
be a direct correlation of its focus on educating the professionals using the system, rather
than the consumer, in an effort to build relationships with orthodontists and general
dentists, who will in turn recommend the advantages of ClearCorrect to their patients.
This is directly in opposition to how Invisalign attracts its potential consumers, with a
consumer based advertisement campaign as their main marketing technique.
The similarity between ClearCorrect’s product and Invisalign prompted the
company to be proactive in addressing patent issues between itself and Align
Technology. ClearCorrect did not want to end up in the same legal predicament as
23
OrthoClear did in 2006, so in February of 2009, the company filed a declaratory
judgment against Align Technology in court. In a declaratory judgment the plaintiff asks
the court to resolve a potential dispute before it proceeds to a lawsuit. ClearCorrect
claimed that some of Align Technology’s patents were invalid, and therefore
ClearCorrect’s product did not infringe on them. ClearCorrect has since voluntarily
dismissed the suit after Align Technology represented to the court that it did not plan on
pursuing a patent infringement lawsuit against them. Therefore, ClearCorrect can
proceed with manufacturing a very similar product without the fear of a lawsuit in the
future. (Clearcorrect.com, 2010)
3. NuBrace
Dr. Tom Kalili is the CEO and founder of this removable orthodontic treatment
system based out of Beverly Hills, California. Nubrace has developed the “BAR”
(Bracket-less Anti Resorption) system which is incorporated in this removable
orthodontic appliance. They claim this also allows for ease of hygiene, less irritation to
soft tissue, and eliminating the need for brackets and other attachments on the teeth.
BAR is achieved by placing grooves in the patient’s stone model, which translates into a
BAR on the aligner for greater orthodontic control. They claim this technique produces
the same force with less stress. Like the Bioliner Plus system, NuBrace has soft inner
linings that give the patient greater comfort and ease of use. It also claims to allow for
longer duration of tooth movement per aligner and less localized stress to minimize root
resorption and bone damage.
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NuBrace is custom fabricated each and every time the practitioner sees the patient
for exact fit. This is different from Invisalign, which may take one impression and rely on
computer projections for fabrication of aligners based on how the teeth may move. Each
patient’s dentition may vary in degree of movement, rate of movement, and extent of
movement. To pre-fabricate a series of aligners does not take into consideration if the
impression is faulty, if a patient's teeth move faster than others, or even if the patient is
wearing the aligners for the appropriate amount of time each day. With NuBrace, the
practitioner is able to monitor the patient prior to each and every aligner being fabricated
for exact fit. If there is a problem the practitioner can request for a new NuBrace aligner
based on a new impression and cuts the loss of treatment time immediately.
The treatment protocol is similar to Invisalign, but does have some differences.
The first step in the protocol is for detailed impressions of the patient’s teeth. NuBrace,
like Invisalign, uspes 3-D computer technology to determine the desired tooth
movements and how the aligners will move the teeth. A NuBrace aligner will be
fabricated at each stage of the orthodontic treatment based on the practitioner's
recommendation for most accurate results. The aligners are worn for four-week intervals
instead of the two-week interval of Invisalign. Nubrace claims that its treatment is ideal
for minor tooth movement of the maxillary and mandibular anterior teeth. (Nubrace.com,
2010)
V. SUCCESS OF INVISALIGN
With all the advantages of the Invisalign treatment system it has become a very
popular choice of orthodontic patients. Some of these advantages of Invisalign Aligners
25
include the ability to remove aligners to eat, brush and floss, and the superior comfort and
ease of use. Align has claimed that 90% of orthodontic patients are candidates for
Invisalign. (Boyd, 2001)
With this increased popularity of Invisalign, numerous studies have been completed
looking at the effectiveness of the Invisalign treatment system. Many of these studies
have compared the treatment outcomes of Invisalign with the treatment outcomes of
conventional fixed appliances. Clements et al measured the PAR scores before and after
Invisalign treatment and he concluded that the aligners were most successful in
improving anterior alignment and least successful in improving buccal occlusion,
transverse relationships, and overbite. Invisalign was also found to be moderately
successful at improving midline and overjet. (Clements, 2003)
One study compared the treatment impacts of Invisalign versus fixed appliances
within the first week of treatment, usually the most difficult week for new orthodontic
patients. It was found that the Invisalign group experienced less pain the first week and
the fixed appliance group took more pain medications on the second and third day of
treatment. Overall, the Invisalign patients experienced less pain and fewer negative
impacts on their lives during the first week of orthodontic treatment than did those treated
with fixed appliances. (Miller, 2007)
A recent study looked at whether or not Invisalign aligners are superior to fixed
appliances as far as periodontal health is concerned. The modified plaque index was
significantly lower in the Invisalign group but the periodontal condition of both the fixed
appliance group and the Invisalign group were nearly identical. Periodontal health is not
26
jeopardized with Invisalign even though the aligners cover the entire teeth and part of the
keratinized gingival. (Miethke, 2005)
VI. FAILURES OF INVISALIGN
The clear removable aligners also have some significant disadvantages to them as
well. These include the fact that the effectiveness of the appliance is placed into the
hands of the patient and depends on their compliance with wearing the appliance the
needed amount of time to accomplish the desired movement of the teeth. Another
disadvantage is that it is hard to obtain the needed two-point contact on a tooth in order to
move it in more complex ways than just tipping the tooth.
One systematic review of Invisalign research found that there were no strong
conclusions that could be made regarding the treatment effects of the Invisalign
appliances. They concluded that randomized clinical trails are the only way to address
the concerns that orthodontists have with the Invisalign system. (Lagravere, 2005). In
one case controlled cohort study comparing the treatment results of Invisalign patients to
conventional fixed appliance patients it was found that the Invisalign group had a lower
score when graded using the American Board of Orthodontics objective grading system
(OGS). It was implied that treatment with fixed braces was superior to the Invisalign
treatment. (Djeu, 2005)
Clements found that the aligners are the most successful in improving anterior
alignment and least successful at improving buccal occlusion. He stated that this lack in
buccal occlusion correction is most likely due to the lack of inter-arch mechanics or
27
because the arches are prevented from occluding with each other by means of the
appliances which overlay on the occlusal surfaces of the teeth. (Clements, 2003)
A study conducted by Sheridan reported that uncorrected rotations were the most
prevalent problem encountered after Invisalign treatment. These uncorrected rotations
were usually corrected by placing conventionally fixed appliances. (Sheridan, 2004) The
derotation of cylindrically shaped teeth presents a problem as the aligners do not have an
undercut to grasp onto and therefore the aligner slips off the tooth and does not derotate
it. Invisalign is now utilizing bondable attachments to the teeth, which are tooth colored
composite attachments, in order to help with rotating, intruding, and extruding teeth.
Align technology recommends the use of these attachments, as well as interproximal
reduction and overcorrection of the rotations to help aid in the rotational movement.
(Kravitz, 2008). Kravitz, however found that the suggested vertical-ellipsoid attachments
and interproximal reduction does not significantly improve the accuracy of canine
rotation with the Invisalign system. (Kravitz, 2008) It has been suggested by Boyd that
the use of labial and lingual attachments will aid in the desired rotational movement.
Boyd also suggests a 10% overcorrection, which he refers to as the “11/10 rule,” in order
to fully correct rotations with the Invisalign system. (Boyd, 2001)
Looking at post-retention of Invisalign cases, one study found that patients treated
with Invisalign relapsed more than those treated with conventional fixed appliances, with
the maxillary anterior alignment worsening more in the Invisalign group. (Kuncio, 2007)
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VII. FORCES
1. Pressure Tension Theory
It has been found that the act of tooth movement relies on chemical signals as the
stimulus for a cellular differentiation process, which ultimately leads to tooth movement.
The Pressure –Tension theory states that an alteration in blood flow within the
periodontal ligament (PDL) is produced by a sustained pressure that causes the tooth to
shift position within its PDL space. This pressure compresses the ligament in some areas
and stretches the PDL in other areas. The blood flow is decreased in the areas where the
PDL is compressed, while the blood flow is either maintained or increased where the
PDL is under tension (Proffit, 2007).
Orthodontic tooth movement is dependent on the ability of the periodontal cells to
react to the mechanical stimuli. For tooth movement to occur, osteoclast recruitment and
activation must be induced to remove bone from the area adjacent to the pressure side of
the periodontal tissue. Hyalinization in periodontal tissue, on the other hand, will limit
the tooth movement. This hyalinization inhibits the osteoclastic recruitment in the
compressed area for frontal resorption, but also induces the undermining resorption. This
degenerative change is caused by excessive force or the periodontal tissue.
In orthodontics many different forces are used and generated throughout
treatment. With Invisalign and other removable appliances there is a light continuous
force produced. When light, but prolonged, forces are applied to a tooth, blood flow
through the partially compressed PDL decreases as soon as fluids are expressed from the
PDL space, which allows the tooth to move in its socket. After only a few hours of this
29
continuous force, a change in the chemical environment occurs, which produces a
different pattern of cellular activity in the tooth socket. Animal experiments have shown
that increased levels of cyclic adenosine monophosphate (AMP), which is known as the
second messenger for many important cellular functions, appear after around four hours
of sustained pressure (Proffit, 2007).
This four-hour threshold period correlates well with the human response to
removable appliances. If a removable appliance is worn for less than four hours per day
there will be no orthodontic effects or movements. However, above this 4-6 hour
threshold, tooth movement will be seen to occur. (Proffit, 2007)
The key to producing tooth movement is that a sustained force be applied to the
tooth for a certain period of time. Increasingly effective tooth movement is produced if
force is maintained for longer durations. Continuous forces that are produced by fixed
appliances do not require patient compliance and will produce more tooth movement than
removable appliances unless the removable appliance is worn almost full-time.
Removable appliances worn for decreasing amounts of time produce decreasing amounts
of tooth movement. With removable appliances, not only are some patients
noncompliant in wearing the appliance, but also the two-point contacts on teeth needed
for tooth movement are very difficult to produce.
Duration of force is also a concern with removable appliances, in that the force
magnitude delivered to the tooth changes as the tooth moves positions. Profitt states that,
“Only in theory is it possible to make a perfect spring, one that would deliver the same
force day after day, no matter how much or how little the tooth moved in response to that
30
force.” (Proffit, 2007) The same can be stated about aligners. In fact, some decline of
force magnitude will be seen in all removable appliances after the tooth with the force
applied to it has moved in position even the slightest amount.
Tomizuka et al determined that initially light and gradually increasing force
induced tooth movement without the lag phase and showed smooth recruitment of
osteoclasts and inhibition of hyalinization. (Tomizuka, 2007). He also stated that the
conventional orthodontic appliances are not really suitable for generating this light force
because of their material properties and the force does not increase over time, but actually
decreases as the tooth moves. (Tomizuka, 2007)
2. Types of Forces
Orthodontic force duration is classified into three categories by the rate of force decay.
1. Continuous – force is maintained at a good proportion of the original force
2. Interrupted – force levels decline down to zero before reactivation
3. Intermittent – force levels decline abruptly to zero when the orthodontic
appliance is removed, and returns to the original level at a later point when
the appliance is replaced.
Intermittent forces are produced by all removable appliances which are usually
not maintained enough hours of the day to have significant effects on tooth movement.
There is a very important interaction between force magnitude and how quickly the force
declines as the tooth responds. For example, with a nearly continuous light force, a
relatively smooth progression of tooth movement will result from frontal resorption of the
31
bone surrounding the tooth. If the continuous force is heavy, tooth movement will be
delayed until the undermining resorption can remove the bone necessary to allow tooth
movement. This heavy force cuts off the blood supply to the PDL and can be destructive
to both the PDL structures as well as the root of the tooth. Therefore, a light continuous
force produces the most efficient tooth movement. This force is what conventional fixed
appliances can provide and what the Invisalign aligners aim to produce.
3. Types of Movements accomplished
During orthodontic treatment there are numerous types of movements needed to
occur and each has its own optimal force level needed in order to complete the
movements. Ideal orthodontic treatment requires optimal force delivery that leads to a
maximum rate of tooth movement with minimal irreversible damage to the tissues. If
insufficient force is delivered to the tooth, tooth movement might not be obtained. The
simplest form of orthodontic movement is tipping of the tooth. This movement requires
only a single force applied against the crown of the tooth and the tooth will then rotate
around its center of resistance, which is located about half-way down its root. The
optimal force for tipping movement of one tooth is 50-75g (Proffit, 2007)
If two forces are applied to the tooth at the same time, the tooth can be moved
bodily. During this movement the tooth’s crown and root apex both move in the same
direction. The forces for translation, or bodily movement, should stay around 70-120gm.
Forces needed for rotation and extrusion of a tooth are also light and should be in the
range of 35-60gm. An extremely light force is needed in order to intrude a tooth. The
light force seems counter-intuitive, but it is needed because the force for intrusion will be
32
concentrated in a very small area at the apex of the tooth. If the force is kept extremely
light, around 10-20gm, the tooth can be intruded successfully. (Proffit, 2007)
VIII. FORCES OF INVISALIGN
The exact force level that Invisalign produces has not been published, but it can
be assumed that since each aligner is designed to move the teeth up to 0.2mm, the force
levels the teeth experience are in the lower range of the orthodontic force levels.
(Brezniak, 2008) In another study, however, the measured forces delivered from the
clear plastic aligners for tipping was found to be approximately 3 to 11 times higher than
the ideal forces (0.35-0.60N) irrespective of the thickness of the material. (Hahn, 2009)
Thermoplastic orthodontic aligners are generally recommended to be used for two
weeks in each stage. During this period, temperature fluctuations in the oral cavity can
change the properties of these materials. These appliances are also subject to deflection
changes during placement or removal from the mouth. Thermoplastic orthodontic
aligners are mainly composed of polyethylene or polypropylene. Although many
successful clinical results with aligners have been reported, the force and energy delivery
properties of these materials when the appliance is returning to its resting position after
resetting are still not fully understood or investigated. (Kwon, 2008)
There is the concern that the thermal changes in the mouth might influence the
mechanical properties of the aligners. It was reported in a study that tested the aligners
for 36 days in thermal conditions that mimicked those found in the mouth, that there was
no significant change in the delivered force in the deflection range of the optimal tipping
33
movement of the tooth. (Kwon, 2008). This tested time period far exceeds the two-week
intervals that Invisalign recommends use of its aligners. However, the same study found
that repeat “load cycling”, removing and placing of the aligners, did influence and
decrease the force delivery properties. (Kwon, 2008).
Brezniak researched the forces needed to be generated with the Invisalign clear
aligners in order to move teeth. He had found previously that both bodily movements
and extrusions were not accomplished the way Invisalign had claimed. (Brezniak, 2008)
Invisalign has developed multiple different shapes and sizes of tooth colored composite
attachments in order to create the force and moments needed in order to accomplish these
more difficult tooth movements.
Brezniak also describes what has been termed the “watermelon seed effect.”
When an aligner is placed on the teeth it is expected that the desired tooth movement will
occur. If this does not happen, the aligner will give way to the stiffer teeth in the arch
and ultimately become distorted. The aligner’s gingival edges move away from the teeth
and no force can be exerted in the gingival area while the force is concentrated only in
the occlusal part. This distortion prevents the possibility of a couple to be developed and
no bodily movement of the tooth is therefore possible. However, this occlusal force does
encourage intrusion and it is not uncommon to see teeth that are undesirably intruded
using the aligners. (Brezniak, 2008) The action is mimicked when you squeeze a
watermelon seed between your finger and thumb and it shoots out of your grasp. Hahn et
al also confirmed this, when he stated that removable thermoplastic aligners deliver
complex force systems and despite their tipping force it is possible to show and quantify
the intrusive component. (Hahn, 2009) Another recent study by Hahn stated that due to
34
the complex shape of the crown of a maxillary incisor, no pure moments or continuously
acting and changing horizontal forces can be measured when assessing the forces of clear
thermoplastic aligners. (Hahn, 2010)
The forces of the Invisalign system seem to be a lot lighter than those of the
conventional fixed appliances, but as has been recorded in the literature, all forces do
cause some amount of root resorption. There have been many proven studies showing
that one of the acceptable outcomes with conventional orthodontic treatment is the
occurrence of root resorption. Hahn et al also went on to say that “It was previously
demonstrated that the amount of root resorption is directly proportional to the magnitude
of force applied, but this is not the case when removable thermoplastic appliances, in
particular, are used.” (Hahn, 2009) There is the continuous struggle to limit the actual
amount of external root resorption with the different bracket system. In contrast, the
claims made by the manufacturers of Invisalign that their treatment does not produce any
resorption still needs to be proven.
IX. ROOT RESORPTION
There are many types of root resorption that can occur at different stages of a
person’s life. Physiological root resorption is part of the normal process that occurs with
the loss of deciduous teeth as the permanent teeth begin to erupt. Root resorption may be
simply defined as the loss of apical root tissue, but this overly simplifies a complex
multifactor phenomenon. Root resorption is one of the few deleterious consequences of
tooth movement and most commonly suffered by patients being treated with
35
orthodontics. Even though root resorption seems almost unavoidable while in orthodontic
treatment, the actual amount of resorption from orthodontic treatment has not been seen
to produce a decrease in the longevity or the function of a tooth. Premature tooth loss due
to root resorption has not been reported in the literature. (Killiany, 1999).
During orthodontic tooth movement, forces are transmitted through the tooth to
the periodontal ligament. Areas of compression of the ligament result in the generation
and influx of osteoclasts resulting in bone resorption. Areas of tension result in osteoblast
differentiation and the deposition of bone. When an imbalance between resorption and
deposition occurs and is combined with the loss of some of the protective characteristics
of cementum, this imbalance contributes to the cementoclasts and osteoclasts resorbing
areas of the root. (Ngan, 2004) It is thought that root resorption occurs during periods of
active tooth movement when the balance is tipped in its favor. The opposite deposition or
cemental repair occurs during periods of quiescence. If these two mechanisms are
disrupted or an imbalance towards resorption is maintained then a permanent loss of root
tissue occurs. (Brezniak and Wasserstein, 1993) Both bone and cementum respond
similarly when an orthodontic force is applied to a tooth. Usually cementum is more
resistant to resorption and the bone surrounding the tooth is resorbed instead of the tooth
surface. The exact mechanism of root resorption is still unknown, but some studies have
reported that resorption lacunae can appear as early as 10 days after the application of an
orthodontic force. (Reitan, 1974).
The form of resorption that is commonly associated with orthodontic tooth
movement is External Apical Root Resorption (EARR), which is a form of surface
36
resorption. EARR refers only to the loss of apical root tissue, whereas other forms of
resorption can be observed on other areas of the root. For example, lateral resorption on
the buccal surface of molar roots during rapid maxillary expansion treatment has been
recorded in the literature. (Heithersay 1999).
EARR is thought to occur in about 3-5% of orthodontic patients with average
resorption amounts being between 1 to 2mm. (Kennedy, 1983). In the majority of patients
root loss is limited to 2mm or less. (Sameshima and Sinclair, 2001) Another study found
that 4% of orthodontic patients experience “generalized” resorption of more than 3mm,
and about 5% of adults, and 2% of adolescents, are likely to have at least one tooth that
resorbs more than 5mm during treatment. (Mirabella, 1995) Severe resorption (over 1/4
of the root length) may have adverse effects on the longevity of the tooth. (Kaley and
Phillips, 1991)
The mesial and distal surfaces of orthodontically treated teeth have considerably
more resorption lacunae than other surfaces. (Wierzbicki, 2009). Using Micro-Computed
Tomography (Micro-CT), Wierzbicki et al, were able to quantify the amount of root
resorption that occurs after one year of orthodontically treated teeth. They found that the
average amount of resorption was 0.9% of the tooth root volume. (Wierzbicki, 2009)
These small resorption lacunae, usually on the lateral aspects of roots, are thought to be
harmless and are seen as a normal occurrence during the on-going balance between
resorption and deposition.
There are numerous factors that have been linked to causing root resorption of the
permanent teeth; physiologic tooth movement, adjacent impacted tooth pressure,
37
periapical or periodontal inflammation, continuous occlusal trauma, tumors or cysts,
orthodontic treatment and idiopathic factors. (Brezniak, 1993). Physiologic root
shortening has also been reported in individuals who have never had orthodontic
treatment (Harris, 1993). Although it is an undesirable occurrence, root resorption is ever
present, and therefore the aim is to minimize the amount and effects on the teeth during
orthodontic treatment. We will now look into some of the more recently studied
biological and mechanical factors of root resorption.
X. BIOLOGICAL FACTORS
1. Root Morphology
It is difficult to determine if a patient will be susceptible to EARR, but studies
have shown that the shapes of the roots prior to orthodontic treatment may give a clue as
to the amount of root resorption expected. Levander and Malmgren stated that blunted
roots and pipette shaped roots are more often affected by EARR than normal shaped
roots. (Levander, 1994) Dilacerated roots are also seen to be more prone to root
resorption. Sameshima also confirmed that teeth with abnormal root shape, whether that
be pipette, pointed, or dilacerated, appear to be more susceptible to EARR. (Sameshima,
2001). Long, narrow, and deviated roots also are seen to have more root resorption
occurring when compared to normal shaped teeth. (Smale, 2005) Another identified risk
factor is if the patient already has shortened roots or has had recorded root resorption in
the past (Linge and Linge, 1983) Invagination and taurodontism have been found to be a
risk factor for root resorption when a force is applied to these teeth. (Kjar, 1995) Normal
38
as well as small and peg-shaped laterals have been reported not to be associated with a
higher risk than normal for root resorption. (Sameshima, 2001)
2. Gender
The literature has conflicting results when root resorption is related to gender.
Many studies have shown that there is no significant difference between males and
females. Linge and Kjar found females to be more susceptible to EARR than males.
(Linge and Linge, 1983, Kjar, 1995) Newman found the idiopathic root resorption ratio to
be 3.7:1 females to males, respectively. (Newman, 1975) Preoteasa found that males
were more susceptible to root resorption. (Preoteasa, 2009) Males were also seen to
suffer from EARR more often in a study by Baumrind looking at patients over the age of
20 years old. (Baumrind, 1996)
3. Age
Age and root resorption is one category that has been researched extensively. The
majority of studies confirms a positive relationship of age and root resorption and has
showed that adults have significantly more root resorption when compared to children.
(Sameshima, 2001). Even without orthodontic treatment, the incidence of root resorption
has been seen to increase with age. (Massler, 1954.) Linge reported that the risks of
resorption seem to increase in individuals after 11 years of age. (Linge and Linge, 1983).
Preoteasa found that individuals older than 12 years old had root resorption with all
degrees of severity whereas patients younger than 12 did not have root resorption.
(Preoteasa, 2009)
39
4. Genetics
Research has discovered that there may be a familial and genetic basis for root
resorption. The importance of genetic factors in bone metabolism has been confirmed,
and a study looking at twins has indicated a strong genetic component to EARR (Ngan,
2003). Sameshima and Sinclair have reported that Caucasian and Hispanic patients are
more vulnerable to root resorption than Asian patients. (Sameshima, 2001).
Several genes located on the Interleukin-1 gene cluster have been confirmed to
contribute to root resorption. Within this gene cluster, the genes IL-1A and IL-1B encode
for two pro-inflammatory cytokines, IL-1 (alpha) and IL-1 (beta), two very potent stimuli
for bone resorption. (Al-Qawasmi, 2003) A genetic susceptibility to EARR has been
supported, with 70% genetic contribution being proposed by one researcher (Harris,
1997). Another candidate gene for EARR in orthodontic treatment is tissue non-specific
alkaline phosphatase (TNSALP), the product of which plays an important role in
mineralization and cementum formation. Mice lacking a functional TNSALP gene were
found to have defective acellular cementum formation along the molar roots. (Beertsen,
1999) This is important because the cementum is usually harder than the bone
surrounding it and therefore when teeth are moved through the bone, the bone resorbs
before the cementum. However, if poorly formed cementum present then the root will
resorb more so than the surrounding bone during tooth movement.
40
5. Systemic Factors
Some hormonal imbalances have been implicated to exacerbate root resorption.
Researchers have examined the connections with other conditions such as chronic asthma
(McNab, 1999), and its relationship to endocrine disturbances such as hyperthyroidism
(Levander and Malmgren, 1988) High bone turnover (i.e. hyperthyroidism) can increase
the amount of tooth movement compared with a normal or low bone turnover state in
adult patients. Low bone turnover (i.e. hypothyroidism) can result in more root
resorption, suggesting that patients with a decreased bone turnover rate, the risk of root
resorption could be increased. (Verna, 2003) Owman-Moll found that patients with
allergies have an increased risk for root resorption but the findings were not statistically
significant. (Owman-Moll, 2000) All of these associations have only proven to be weak
and further research is needed to confirm these findings.
6. Trauma Before Treatment
Root resorption has been reported to occur after trauma and in response to
inflammation by a number of authors (Andreasen, 1985 and Tronstad, 1988). The
average amount of root resorption after orthodontic therapy for traumatitized teeth was
1.07mm compared to 0.64mm for non-traumatized teeth. (Linge, 1983). Previously
traumatized teeth showing root resorption that are then moved orthodontically are known
to be more susceptible to further loss of root structure. (Linge, 1983 and Brin, 1991). One
study found that if a patient has detectable root resorption on periapical radiographs
during the first 6 months of orthodontic treatment, they are very likely to continue to
experience more root resorption throughout the rest of the orthodontic treatment. (Artun,
41
2005) Traumatized teeth have also been found to exhibit external root resorption without
any orthodontic treatment. (Brin, 1991). Endodontically treated incisors are less affected
by root resorption it seems than normal teeth. (Owman-Moll, 1995)
XI. MECHANICAL FACTORS
1. Treatment Times
Most of the literature has reported that the severity and frequency of root
resorption is directly correlated to the extent of the treatment time. (Sengal, 2004,
Apajalahti, 2007, Baumrind, 1996, Snellgrove, 1995). One study found that the duration
of the treatment time was the highest correlated factor for root resorption in the maxillary
incisors. (Taithongchai, 1996) Levander and Malmgren found in their study that 34% of
the examined teeth showed some amount of root resorption after six to nine months of
treatment, whereas at 19 months of treatment, root resorption increased to 56%.
(Levander and Malmgren, 1988)
2. Types of Forces
There is conflicting evidence that exists as to whether an increase in force levels
during orthodontic treatment will increase the severity of root resorption (Owman-Moll,
1995; Owman-Moll, 1996). However, heavier forces do seem to lead to greater amounts
of resorption (Chan, 2004), but there is a great amount of individual variation (Acar,
1999). Owman-Moll’s study looked at the effects of doubling and quadrupling the
amount of orthodontic force placed on a teeth. They found that when the orthodontic
42
force was doubled there was not significant change in tooth movement or root resorption.
When the force was multiplied by four times, the root resorption did significantly
increase, but the amount of tooth movement did. (Owman-Moll, 1996)
3.Types of Appliances
Numerous studies regarding root resorption and the differences in types and
manufacturers of appliances used have been reported in the literature. It has been
concluded that removable appliances cause less root resorption than fixed appliances
(Linge, 1983) The Tweed technique and appliance has been shown to create the same
amount of root resorption as the Begg appliance. (Beck, 1994). The edgewise appliance
has been reported to affect the teeth with significantly less resorption than that of the
Begg appliance. (McNab, 2000). It has recently been recorded that patients treated with
self-ligating brackets experience significantly less root resorption in their maxillary
anterior teeth than with treatment completed using conventional fixed appliances.
(Wong, 2009). However, another study found that there was no difference in the amount
of EARR between conventional and passive self-ligating appliances. (Pandis, 2008)
4.Extraction Pattern
McFadden stated that there is no difference in the amount of root resorption in
cases where teeth were extracted when compared to non-extraction cases. (McFadden,
1989). McNab found that the incidence of EARR was 3.72 times higher for patients who
had extractions performed, compared with those patients that did not have extractions.
(McNab, 2000) Taner et al. found that in cases with extraction, the patients with Angle
Class II have twice as much root resorption when compared with patients with Class I
43
malocclusions. This can be explained by the greater difficulty in treatment presented
with a Class II occlusion. (Taner, 1999)
5. Orthodontic Movement Type
The intrusion of teeth causes about four times more root resorption than extrusion.
(Han, 2005). The use of Class II elastics was a found to have an increased amount of
EARR associated with it. (Mirabella, 1995). In one study, root resorption was seen in all
three on Angle’s molar classifications of occlusion. The most severe root resorption was
seen in patients who had either Class II or Class III malocclusions. All of the patients
with Class III malocclusions were found to have a very high degree of root resorption.
(Preoteasa, 2009)
6.Orthodontic Force
In the literature there are numerous studies regarding the optimal type and
magnitude of force for moving teeth. The current concept of optimal force is that a force
of a certain magnitude and duration is capable of producing a maximum rate of tooth
movement without tissue damage and with maximum patient comfort. The optimal force
for tooth movement therefore may differ for each tooth and for each patient. (Ren, 2003).
Acar completed a study on the difference between continuous and discontinuous
forces and recorded the amount of root resorption present. They found that with
discontinuous force delivery there was a smaller mean percentage of resorption-affected
areas, less severe apical blunting of the teeth, and less over-all root resorption when
compared to teeth where a continuous force was applied. (Acar, 1999)
44
Another study analyzing the difference between continuous and interrupted forces
found that horizontal tooth movement with continuous force was more effective than with
Interrupted force. They also determined that there was no difference between the amount
or severity of root resorption between the two forces. (Owman-Moll, 1995)
XII. CLINICAL DIAGNOSTIC AIDS TO IDENTIFY ROOT RESORPTION
External Apical Root Resorption is most commonly detected towards the end of
treatment when a progress, or end of treatment, panoramic radiograph is taken. At the
beginning of treatment most orthodontists take panoramic and lateral cephalometric
radiographs, but these may not be enough to accurately see the amount of root resorption.
The numerous advantages of panoramic radiographs to a full set of periapical radiographs
are less radiation exposure, less patient chair time, less operator time, and better patient
cooperation. (Sameshima, 2001). However, there are also limitations to panoramic
radiographs, which include the quality of the radiograph being dependent on the
operator’s correct positioning of the patient and the closeness of the desired structures to
the focal trough of the radiograph. Periapical films have been found to show greater
detail and less distortion than panoramic radiographs. Panoramic radiographs have been
shown to overestimate the amount of root resorption by 20% when used to measure pre-and
post-treatment root resorption. (Sameshima, 2001) Sameshima also found that root
dilacerations and other abnormal root morphologies appeared normal on panoramic
radiographs when they were clearly seen as abnormal on periapical radiographs. Ericson
explained the limitation of conventional diagnostic tools used to analyze root resorption
45
and stated that periapical and panoramic radiographs will demonstrate normal root
morphology, but CT scans can reveal the real damage that may exist. (Ericson, 2000)
XIII. CASADE OF AFFECTED TEETH
There have been numerous studies in the literature which have attempted to
determine the cascade of most commonly affected teeth to the least commonly affected
teeth. There is still no all-inclusive consensus about the cascade but many authors have
attempted to determine one. Brezniak has completed many studies on root resorption and
has determined that the cascade of affected teeth begins with the maxillary laterals being
the most commonly affected tooth, followed by the maxillary centrals, mandibular
incisors, the distal root of mandibular first molars, mandibular second premolars and
maxillary second premolars. (Brezniak and Wasserstein, 1993). Sameshima also
confirmed that the laterals incisors had the most root resorption, (Sameshima, 2001.)
while Sjolien, stated that the mandibular incisors were the most affected teeth. Most
researchers can agree that the molars and the bicuspids, either maxillary or mandibular,
experience the smallest amount of root resorption. Janson found that the most effected
teeth were the maxillary centrals, followed by the maxillary laterals, mandibular centrals,
and lastly the mandibular lateral incisors. (Janson, 2000) Preoteasa found that the
mandibular anterior teeth had a higher percentage of root resorption when compared to
the maxillary teeth. The mandibular central incisor was found to have the most root
46
resorption and the maxillary canine with the least amount of root resorption. (Presoteasa,
2009)
XIV. TREATMENT OF ROOT RESORPTION
If EARR is found to be occurring in a large amount, the correct way of handling
this would be to have an inactive phase of 4-6 months (Sameshima and Sinclair, 2001).
The orthodontic braces can be left attached to the teeth with the wires removed, or all
appliances can be removed during this inactive phase. If the root has experienced a small
amount of resorption, the root can heal itself. Once the orthodontic force falls below a
certain threshold, the root resorption ceases and the roots can begin the healing process.
With more severe levels of resorption, however, the occurrence is permanent.
Repair begins two weeks after force removal with the placement of acellular
cementum succeeded by cellular cementum. This process is evident in 38% of human
premolar lacunae after two weeks and in 82% of the lacunae after five weeks. (Owman-
Moll, 1998) Another study looked at the timeframes for the repair process. The
percentage of areas that had begun to repair themselves ranged from 28% after one week
of retention to 75% after eight weeks of retention. The healing cementum was found to
be mostly of the cellular type. There was also found to be no major differences in the
healing potential in the cervical, middle and apical thirds of the root. (Owman-Moll,
1995)
Copeland found that resorption occurring during active treatment was 2.93mm
and the mean root resorption during the post-treatment period was 0.1mm. (Copeland,
47
1986) This is in agreement with many previous studies that have stated that root
resorption halts after active treatment is complete, but it also shed light on the fact that
root resorption can still occur in the retention phase. This resorption is nowhere near the
amount seen in active treatment and is probably attributed to other factors such as
traumatic occlusion and active forces delivered by the retainers. Although the resorption
process stops once the active appliances are removed, severely resorbed teeth may be lost
prematurely in patients who are also susceptible to marginal periodontal breakdown.
(Artun, 2005).
XV. APICAL ROOT RESORPTION WITH REMOVABLE APPLIANCES
The use of removable appliance in orthodontics has been around for a long time,
but the investigations into the amount of root resorption that occurs with their use is just
beginning to be investigated.
Janson compared patients that used a Frankel removable appliance to an untreated
control group. The study used the Levander and Malmgren scoring system to quantify the
root resorption present and found that there was significantly greater resorption in the
Frankel group when compared to the untreated control group. The amounts of resorption
were predominantly small and the prevalence of resorption for the incisors was greatest
in the maxillary centrals, followed by the maxillary laterals, mandibular centrals and
mandibular laterals. (Janson, 2007) It was found in another study that when removable
orthodontic appliances were used there was no root resorption present and when fixed
48
appliances were used, root resorption was found and generally present in high prevalence
and low severity. (Preoteasa, 2009)
XVI. ROOT RESORPTION WITH INVISALIGN
Since EARR was first described in the literature, orthodontists have been looking
for a treatment protocol where root resorption does not occur. Within the last ten years,
Invisalign has become more and more popular and so far there are not many studies
completed regarding EARR and the Invisalign treatment.
The Invisalign treatment technique belongs in the removable appliance treatment
category of orthodontic modalities. The aligners apply intermittent forces to the teeth in
the same manner as most active removable appliances. There have been several articles
that state that this pause in treatment with intermittent force allows the resorbed
cementum to heal and prevent further resorption. (Reitan, 1964 and Dougherty, 1968)
In the current literature, there have been very few cases of EARR occurring after
the Invisalign treatment. Brezniak reported one case in which a patient was seen to have
root shortening of the four maxillary incisors. This involved root resorption apically,
from 2mm to one third of the original root length (Brezniak, 2008). There has been a
recently completed longitudinal study of 100 consecutive Invisalign patients that showed
no measureable root resorption, whereas with conventional fixed appliances an average
of 10% of the patients have shown clinically significant root resorption of at least 3mm.
(Boyd, 2007). Needless to say the topic of root resorption with the Invisalign treatment
needs to be further investigated and that was what this study aimed at doing.
49
CHAPTER THREE: HYPOTHESIS
Research Hypothesis, Ha:
External Apical Root Resorption is significantly lower for cases treated with the
Invisalign clear removable aligners than for cases treated with conventional fixed
orthodontic appliances.
Null Hypothesis, Ho:
There is no significant difference in External Apical Root Resorption between cases
treated with the Invisalign clear removable aligners and those treated with conventional
fixed orthodontic appliances.
50
CHAPTER FOUR: MATERIALS AND METHODS
The records of the patients used in this study were collected from a private
practice office in San Diego, California where one solo practitioner, who has been trained
to use both appliances, started and completed all stages of treatment for both groups. All
patients used in this study were both female and of Caucasian ethnic backgrounds. The
patients were classified into two equal groups: Group 1 (n=45) was bonded and treated
with an 0.022-inch slot appliance, MBT prescription, conventional edgewise appliance,
and Group 2 (n=45) received Invisalign orthodontic treatment. Forty-five female patients,
(age 10 to 47 years old), treated with the conventionally bonded brackets were included
in this cohort investigation. Their selection was based on a list of recently completed
orthodontic cases in the office and was based on the inclusion and exclusion criteria listed
below. Forty-five female patients, (age 12 to 58 years old), treated with the Invisalign
technique were also included in this cohort investigation. Their selection from a larger
pool of 200 patients successfully treated with Invisalign from this private practice office
was based on the inclusion and exclusion criteria listed below. The Invisalign cases
were selected randomly according to the inclusion and exclusion criteria, and the
conventionally treated cases were selected to match the number of Invisalign cases.
Patient records:
Complete patient records were obtained including pre-treatment and post-treatment
intra-oral photographs, panoramic and cephalometric radiographs, traced
cephalometric radiographs in Dolphin Imaging, as well as progress photos. Intra-oral
51
pictures were used to classify pre-treatment malocclusion. All panoramic radiographs
and lateral cephalometric radiographs before and after treatment were taken from the
same radiographic machine, (Planmeca XC, Proline with Dimax 2 Ceph) and by the
same operator. The same treating orthodontist also traced all lateral cephalometric
radiographs.
To quantify resorption, post-treatment panoramic radiographs were used and the
teeth analyzed included the maxillary canines, laterals, and centrals, as well as the
mandibular canines, laterals and centrals.
Inclusion Criteria:
The criteria for inclusion were the following:
1. Same Ligation method: Patient treatment that consisted of using the same
ligation method for the entire treatment and by the same orthodontist. Patients
that had transferred into the practice during their active treatment or were started
by another orthodontist were not considered for this study.
2. Molar Classification: Angle Class I for all of the conventional MBT treated
cases. For the Invisalign cases molar classification was not taken into account
since there was no attempt at trying to correct molar classification if they were
not already Angle Class I.
3. Extraction Pattern: Non- extraction only
4. Gender: Female only
5. Age: Between 10-60 years of age
6. Ethnicity: Caucasian
52
7. Root formation: All roots must be completely formed at the start of treatment
8. Radiographs: Must have complete set of radiographs, including pre-treatment
and post-treatment panoramic and lateral cephalometric radiographs.
Radiographical records were only deemed complete if they included a traced pre-and
post-treatment lateral cephalometric radiographs. The treating orthodontist
completed all tracings.
9. Intra-oral photographs: Must have complete series of pre-treatment and post-treatment
intra-oral photographs
Exclusion Criteria:
In addition, the exclusion criteria for case selection were the following:
1. Teeth present: Must have all anterior teeth of the maxilla and mandible.
(Canines, lateral incisors, central incisors)
2. Previously restored teeth: Patients with a history of restorations on the
anterior teeth (e.g. PFM, large amalgam or composite restorations, fixed partial
bridges, implants) were not used in this study.
3. Endodontically treated teeth: Endodontically treated teeth were excluded.
4. Expansion: Patients whose treatment required the use of any maxillary or
mandibular expansion device (e.g. rapid palatal expander, quad helix,
Schwartz appliance) were excluded from the study.
5. Extra-oral Appliances: Cases that utilized extra-oral traction appliances
(e.g. headgear, reverse-pull headgear, J-hook headgear) were excluded.
6. Functional appliances: Patients whose treatment included the use of
53
functional appliances (e.g. Herbst, MARA, Twinblock, Forsus, Bionator) were
excluded.
7. Impactions: Cases with impacted teeth were excluded from the study
8. Surgical Treatment: Any patients whose orthodontic treatment was
combined with surgical treatment were excluded
9. Asymmetries: Cases with skeletal asymmetries were excluded from the
study
10. TMJ dysfunction: Patients who had symptomatic temporamandibular joints
prior to treatment or who developed TMJ symptoms during treatment were
excluded from the study.
11. Parafunctional habits: Patients with any parafunctional oral habits (e.g.
tongue thrust, thumb sucking, lip sucking) were excluded from the study.
12. Trauma: Patients with a history of trauma to their teeth were excluded
13. Developmental problems: Patients with a history of any developmental
problems (e.g. cleft palate and cleft lip) were excluded
14. Abnormal root morphology: Patients with extreme cases of abnormal root
morphology (e.g. large dilacerations greater than 45 degrees, fused roots,
extremely short roots), were excluded from this study.
15. Radiographs: Any patient without complete pre- and post-treatment
panoramic and lateral cephalometric radiographic, as well as traced lateral
cephalometric radiograph records was excluded from the study.
54
Data Collection:
The following information was obtained and calculated using patient’s treatment
records and personal history forms.
1. Age at start of orthodontic treatment
2. Treatment time in months
3. Beginning Overjet and Ending Overjet - Measured from the most facial
aspect of the most anterior mandibular incisor to the middle of the maxillary
incisor’s incisal edge
4. Shape of each tooth – Shape assessment was performed for each tooth
on the pretreatment panoramic radiographs. A 5-parameter ordinal scale (1.
Normal, 2. Blunt, 3. Dilacerated (up to 45 degrees), 4. Pointed, 5. Pipette) was
used.
5. Amount of root movement. This measurement was based on the
superimposition of the pre- and post-treatment lateral cephalometric radiographs
that were superimposed using Ricket’s criteria of using the palatal plane.
Horizontal and vertical root movement was measured. For horizontal root
movement, the tip of the root was measured in millimeters and given a positive
number if the root tip moved anteriorly and a negative number if the root tip
moved posteriorly. For the vertical root movement, a positive number correlated
with extrusion of the root tip where a negative number was given to a root tip that
had been intruded.
6. Amount of root resorption seen on each tooth - Used the Levander and
55
Malmgren scoring system of categorizing the amount of root resorption seen to
have occurred on the pre-treatment panoramic radiographs to the post-treatment
panoramic radiographs. The scoring system is gradated as follows: Grade 0 –
Absence of root resorption; Grade 1 – mild resorption, root with its normal length
and only an irregular contour; Grade 2 – moderate resorption, small area of root
loss with the apex exhibiting an almost straight contour; Grade 3 – accentuated
resorption, loss of almost one third of root length; Grade 4 – extreme resorption,
loss of more than one third of the root length.
Figure 14: Levander and Malmgren root resorption visual classification classes.
Measurements:
To quantify resorption the difference in root length was obtained by visual
comparison of digital pre-treatment panoramic radiographs and post-treatment panoramic
radiographs. These images were enlarged to three times the original size on 24-inch
56
high-resolution computer monitors. Two monitors placed side by side were used in the
visual comparison, one with the pre-treatment enlarged panoramic radiograph and the
other with the enlarged post-treatment panoramic radiograph. A visual comparison of the
root length was then collected and categorized using the above Levander and Malmgren
scoring system.
The maxillary and mandibular canines, lateral incisors, and central incisors,
totaling in 1080 teeth, were examined. The decision to work with only the canines and
the incisors was reached because they are the teeth subjected to greater movement during
treatment with both orthodontic treatment techniques, and because most authors agree
that they are more frequently and intensely absorbed during treatment. (Janson, 2000).
Case matching Criteria:
Cases in the Invisalign treated group were matched to cases in the conventionally
ligated group according to the following criteria:
1. Ethnicity: Caucasian
2. Gender: Female
3. Extraction Pattern: Non-extraction
4. Radiographs: Complete radiographical records
Archwire Sequence for Conventional Ligated MBT Brackets:
The orthodontist who treated these cases used the same wire sequence for all the
patients.
57
1. 0.14 NiTi or 0.16 NiTi (depending on initial crowding)
2. 0.18 NiTi Wire
3. 0.19X0.25 CuNiTi
4. 0.19X0.25 SS
Assumptions:
There are numerous factors that could have potentially influenced the rate of
orthodontic tooth movement, which would influence the amount of external apical root
resorption. This study was therefore based on the following assumptions:
1. Patients do not have any known predisposition to root resorption.
2. The biological differences in the rate of root resorption due to orthodontic forces
among patients are controlled and accounted for by the large sample size.
3. The treatment mechanics and archwire sequence for the conventional group is
assumed to be consistent and be similar for all cases completed with the MBT
bracket system.
4. The Invisalign group has the same treatment protocol and the amount of aligners
used is consistent with the amount of initial crowding that existed.
5. The cases analyzed in the two groups are matched close enough to be relevant.
6. All of the cases used in this study were completed similarly with respect to the
quality of finish.
7. The protocol for this study is appropriate and reproducible since it has been used in
numerous previous studies just never with the Invisalign treatment protocol.
58
Data analysis:
Descriptive statistics were computed for all variables. Continuous variables were
graphed and the distributions examined and tested for normality and homogeneity.
Equality of variances were tested with Levene's test. Parametric or non-parametric
independent t-tests were used. Significance was established at alpha = 0.05. The
variable Root Resorption was recorded as an integer, converted to a string variable,
and compared between groups using Mantel Hanzeal statistics; correlations with apical
distance (two separate directions, both ratio variables) were tested with Friedman's
test.
Method Error:
A random sample of 15 pre-treatment and post-treatment records were re-analyzed
and re-measured one month after the original data was collected for both the
Invisalign group and the conventional group. There were no significant differences
found between the original measurements and the re-measured panoramic records. The
same researcher analyzed and measured all of the root lengths at both time periods. All of
the cases selected for this study were chosen based on the criteria that the initial and final
panoramic and lateral cephalometric radiographs were all taken on the same x-ray
machine and by the same operator. Because of the retrospective nature of this study the
error from the radiographic procedure could not be studied. Also, when any operator
takes panoramic radiographs, there is the chance of error in the angulation of the patient’s
head. That could mean that the same angle of the pre-treatment radiographs will not
59
exactly mimic the angulation of the post-treatment panoramic radiographs and thus there
could be some error incorporated. Some other variables that could not be controlled
include: film/patient position, accuracy of the x-ray unit used, and the ability of the x-ray
technician to follow the instructions of the x-ray unit manufacturer. However, these
sources of errors were assumed to be insignificant in the accuracy of the results.
There also is the difficulty in using panoramic radiographs to look at root
resorption when compared to periapical radiographs. The periapical radiographs are the
best radiographic medium to visualize root resorption, however, the researcher could not
find any orthodontic offices that had both pre-treatment full mouth periapical radiographs
and post-treatment periapical radiographs for cases treated with Invisalign. It is a rare
occurrence to even find orthodontic offices that take post-treatment panoramic
radiographs and lateral cephalometric radiographs of their Invisalign treated cases.
Since the treating orthodontist traced all of the lateral cephalometric radiographs,
we are assuming his accuracy of the pre- and post-treatment points placed was accurate to
a non-significant degree. If teeth in the pre- and post-treatment radiographs were slightly
different sizes, the pre-treatment incisor length was measured from the incisal edge to the
root tip and this length was then used for the post-treatment incisor. A small dot was
placed where the correct root tip should be based upon the pre-treatment incisor length
and the post-treatment angluation of the incisor. The difference was then measured in the
vertical and horizontal directions in millimeters.
60
CHAPTER FIVE: RESULTS
The average age of patients treated with the conventional fixed appliances was
15.82 years and ranged from 10 to 47 years old. The Invisalign group’s mean age was
more than double the conventional group at 38.24 years with a range of 12 to 58 years of
age. The actual months in treatment of the conventional fixed appliance group was 19.69
months and the Invisalign group was found to be slightly higher at 20.36 months.
Table 1: Descriptive statistics for Conventional Fixed Appliances (Group C)
Table 2: Descriptive statistics for Invisalign Removable Aligners (Group I)
61
When looking at the root movement produced by both appliances it was found
that the conventional fixed appliances moved the root on average 0.80 mm in an
extrusive direction. This was a lot larger than the Invisalign group which recorded only
0.07 mm of movement in an extrusive pattern. The horizontal root movement of the
conventional group was 0.91mm in a retractive direction whereas the root movement of
the Invisalign group was 0.25mm in a protrusive pattern. Both appliances were found to
have decreased the amount of overjet from the beginning of treatment to the completion
of treatment. The conventional fixed appliances reduced the overjet by an average of
0.55 mm while the Invisalign group reduced the overjet by an average of 0.42 mm.
Table 3: Group Statistics for Group C and Group I.
62
Table 4: Independent Sample Tests. Levene’s Test for Equality of Variances
63
Table 5: t-test for Equality of Means
Root resorption was found to have occurred in the conventional fixed appliances
whereas there was no root resorption recorded in the Invisalign group. The most affected
maxillary teeth were the maxillary laterals. 51.1% of the lateral teeth showed some sign
of root resorption. The degrees of severity of root resorption was also found to be the
most severe in the maxillary laterals with a few teeth experiencing a level 3 on the
Levander and Malmgren visual test. The next most affected maxillary teeth were the
central incisors. The maxillary right incisor was found to be affected in 35.6% of the
cases and the maxillary left incisor in 31.1% of the cases. The maxillary canines were the
least affected teeth in the maxilla in this study. The maxillary right canine experience
64
some amount of root resorption in 6.7% of the teeth analyzed and the maxillary left
canine in 8.9% of the teeth.
Table 6: Frequency tables for root resorption seen on all maxillary teeth for
Group C and Group I.
65
Table 6 (cont): Frequency tables for root resorption seen on all maxillary
teeth for Group C and Group I.
Similar to the maxilla, the mandibular lateral incisors were the most commonly
affected teeth with the most severe amounts of root resorption. The most affected
mandibular tooth was the lower right lateral incisor. It was seen to have root resorption
in 20% of the teeth analyzed. The mandibular left lateral incisor as well as the
mandibular left central incisor experience root resorption in 17.8% of the teeth. The
mandibular right central incisor had 13.3% of its teeth experience some amount of root
66
resorption. The mandibular canines, like in the maxilla, were the least affected teeth in
the arch at 8.9% for the left canine and 2.2% for the right canine.
Table 7: Frequency tables for root resorption seen on all mandibular teeth for
Group C and Group I.
67
Table 7 (cont): Frequency tables for root resorption seen on all mandibular teeth
for Group C and Group I.
The cascade of affected teeth found in this study was the most affected teeth being
the maxillary lateral incisors, followed by the maxillary central incisors, the mandibular
lateral incisors, mandibular central incisors, maxillary canines, and the mandibular
canines being the least affected tooth. The least affected tooth in this entire study was the
mandibular right canine. Only one tooth in this group was seen to have a slight amount
of root resorption present.
68
The shapes of the roots were also analyzed and it was found that maxillary lateral
incisors had the most frequent occurrences of dilacerated roots when compared to all
other teeth. The maxillary left lateral incisor was found to be dilacerated in 14.4% of the
cases and the maxillary right lateral incisor in 11.1% of the cases. When analyzing all
1080 teeth, it was found that the most common root morphology was normal root shape
at 91.3% of the time, blunted roots at 3.4%, dilacerated roots at 3.1%, pointed roots 1.9%
and pipette roots at 0.3%.
Table 8: Root shape for all teeth displayed by each individual tooth type and
location. (N=Normal, D= Dilacerated, B= Blunted, O=Pointed, P=Pipette)
69
Table 8 (cont): Root shape for all teeth displayed by each individual tooth type
and location. (N=Normal, D= Dilacerated, B= Blunted, O=Pointed, P=Pipette)
70
Table 8 (cont): Root shape for all teeth displayed by each individual tooth type
and location. (N=Normal, D= Dilacerated, B= Blunted, O=Pointed, P=Pipette)
71
Table 8 (cont): Root shape for all teeth displayed by each individual tooth type
and location. (N=Normal, D= Dilacerated, B= Blunted, O=Pointed, P=Pipette)
72
Table 8 (cont): Root shape for all teeth displayed by each individual tooth type
and location. (N=Normal, D= Dilacerated, B= Blunted, O=Pointed, P=Pipette)
73
CHAPTER SIX: DISCUSSION
The goal of this research study was to compare the amount of external apical root
resorption (EARR) that was found in two groups of patients treated with two different
types of treatment appliances, but all being completed in the same private orthodontic
office by the same orthodontist. The two appliances used in this study were the fixed
conventional 0.22 MBT prescription brackets and the Invisalign treatment system using
removable clear aligners. It was hypothesized that there would be much less root
resorption found on the roots of the patients treated with Invisalign when compared to the
conventionally fixed orthodontic brackets.
Sample Analysis:
The sample of patients used for this study was collected from a private practice in
San Diego County according to a predetermined, very specific inclusion and exclusion
criteria which is found in the materials and methods section. All the patients used for this
study were Caucasian females.
Meier surveyed patients who were interested in Invisalign treatment and reported
that the characteristic profile was women (72%) between the ages of 20-29 years old.
(Meier, 2003). This was part of our decision to use only females in this study as well as
the fact that in the literature some authors had determined that females were more
susceptible to EARR. (Linge, 1983, Kjar, 1995, and Newman, 1975) At the office where
the records were collected there seemed to be more female patients who had recently
finished with conventional fixed braces compared to males in the practice.
74
Caucasian females were chosen for this study based on the previous studies by
Sameshima and Sinclair who reported that Caucasian and Hispanic patients are more
vulnerable to root resorption than Asian patients. (Sameshima, 2001).
There was a statistically significant difference between the ages of the Invisalign
and conventional braces groups. The mean age of the Invisalign group (15.82 years old)
was greater than that of the conventional braces (38.24 years old) by almost 22 years.
This large discrepancy was expected due to the fact that adults are much more likely to be
interested in treatment with Invisalign which offers greater esthetics and comfort than
conventional braces do. Invisalign is also limited to patients whose complete permanent
dentition has already erupted whereas the conventional braces groups average age was
lower because it included pre-adolescent children. This age difference is not a huge factor
since it has been recorded in many studies in the literature that there is actually more root
resorption seen in older adult patients compared to young children. In this study we did
not find any EARR in the Invisalign patients and found EARR in our younger patients
treated with fixed conventional brackets. However, the tooth movement in both groups
should be similar regardless of age if all other things, such as periodontal condition and
compliance, being equal. (Sameshima 2001, Massler 1954, Linge 1983, Preoteasa 2009)
The decision to work with only the incisors and canines was reached because they
are the teeth subjected to greater movement during treatment and because most authors
agree that they are the most frequently and intensely resorbed during treatment. External
apical root resorption has been found to occur mainly in the anterior teeth. Most studies
in the literature looked primarily at the anterior teeth when analyzing the amount of root
resorption present after orthodontic treatment. This study collected data from canine to
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canine in both the maxillary and mandibular arches. Also with our patients, we used
Angle Class I molar cases with mild to moderate crowding. We looked at these anterior
teeth because they would be the teeth with the most force on them and the most
movement. Most frequently the resorption is found at the apical area of the roots because
the orthodontic forces are concentrated there. Also the apical tooth structure is usually
cellular cement which is less mineralized and more friable and easy to injure. (Preoteasa,
2009)
There has not been any set criteria as to determine who will and who won’t be a
patient that suffers from root resorption. In this study, we tried to limit the number of
variables that could affect the amount of resorption. We chose Angle Class I molar
malocclusions that were treated with non-extraction treatment. We chose this group of
patients because malocclusions treated with extraction treatments usually extends the
treatment time as well as the actual distance that the teeth are moved in the dental arch.
Both of these factors have been shown to have increased the amount of root resorption
seen after treatment. (Sengal 2004, Apajalahti 2007, Baumrid 1996, Snellgrove 1995,
McFadden 1989, McNab 2000)
Subjective methods, such as the Levander and Malmgren method used in this
study are predominantly used in root resorption studies following tooth movement, in
contrast to other methods that quantify resorption by comparing measurements obtained
in radiographs before and after treatment, and therefore they seem to be very reliable.
Janson et al, is one of many recent studies that have used the same visual test for their
studies looking at apical root resorption. (Janson, 2007)
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The decision to use panoramic radiographs for all the analysis of the root
resorption is simply due to the fact that an orthodontic offices could not be found that had
both pre-treatment full mouth periapical radiographs and post-treatment periapical
radiographs for cases treated with Invisalign. It is a rare occurrence to even find
orthodontic offices that take post-treatment panoramic radiographs and lateral
cephalometric radiographs of their Invisalign treated cases. The periapical radiographs
are the best radiographic medium to visualize root resorption, because they can show
more detail of the roots, as well as, they do not have as much distortion in them as
panoramic radiographs do. Sameshima found that panoramic films showed significantly
greater average apical root resorption than periapical films. It was also found that root
dilacerations and other abnormal shapes, clearly visible on periapical films, often appear
normal of periapical films. The use of panoramic films to measure pre- and post-treatment
root resorption has also be seen to overestimate the amount of root loss by
20%. (Sameshima, 2001). With all the overestimations of root resorption with
panoramic radiographs, there was still no sign of apical root resorption with the
Invisalign treatment. Our results should therefore be seen as being reliable. Our root
resorption values for the fixed conventional braces, which did show apical root
resorption, may therefore be an overestimation due to the use of panoramic radiographs.
Significance of the Findings:
After analyzing all the data from this research study it was determined that the
Invisalign group did not have any signs of External Apical Root Resorption. Not a single
tooth, of the 540 teeth analyzed, in the Invisalign group showed any signs of losing any
77
apical root structure. This is significant and is in agreement with Boyd an his previous
study. (Boyd, 2001).
When analyzing the conventionally fixed orthodontic appliances, there was found
to be significant root resorption found in these patients. Root resorption of some level
was found in 33 of the 45 patients analyzed (73.3% of the patients). Different degrees of
root resorption were also found to be present, but the majority of the root resorption seen
was of the Levander and Malmgren classification level 1. When looking at all the teeth
of the study in their own classification of type of tooth (i.e. central incisor, lateral incisor
and canines), all of the types of teeth showed at least the classification level 1 of root
resorption. Seven of the twelve types of teeth analyzed showed the root resorption
classification level 2, and only the maxillary laterals showed a tooth with root resorption
classification level of 3.
The teeth that were most affected by root resorption in this study were the
maxillary lateral incisors. This confirms the findings of other studies that also found the
maxillary lateral incisors to be the most affected tooth. (Brezniak, 1993 and Sameshima,
2001) The lateral incisors in this study had more than 50% of the teeth affected with
some amount of root resorption. It was also found that they had the most severe amount
of root resorption found, with classification levels of 2 and 3 more often than any other
tooth. When looking at the root shapes of the maxillary incisors there was also a
significant amount of dilacerated roots compared to any other tooth in the mouth. The
maxillary left lateral incisor was found to have dilacerated roots (less than 45 degrees), in
14.4% of the teeth and the maxillary right lateral incisor with 11.1% of the teeth. This
can be one explanation as to why this type of tooth was seen to have the most root
78
resorption present. Lateral incisors which have curved or dilacerated roots have been
found to have a greater degree and frequency of root resorption. (Sameshima, 2001 and
Smale, 2005). Another reason for the increased frequency of root resorption in the lateral
incisors is that maxillary lateral incisors are often displaced mesially and need to be
moved a significant distance distally to orient them correctly. (Wong, 2009)
The next most affected teeth were the maxillary central incisors with the
maxillary right central incisor affected 35.6% of the time and the maxillary left central
incisor 31.1% of the time. This is also confirmed in the literature and there is even
literature that would suggest the central incisor is the most affected tooth. (Janson,
2000). The fact that the maxillary central incisors were the second most affected tooth in
the mouth could be explained by the fact that the cases we looked at were all Angle Class
I molar with mild to moderate overjet. The cases where therefore corrected to a smaller
amount of overjet by using the orthodontic appliance. This action would therefore have
had the most impact on the central incisors which are usually the tooth that is the furthest
protracted in crowded Angle class I cases. Therefore, these teeth will be moved the most
in a retractive movement which could lead to more root resorption.
The mandibular incisors were all found to be in the same range for the frequency
of root resorption seen. The mandibular right lateral was the most affected mandibular
incisor at 20% of the time, followed by both the mandibular left lateral incisor and central
incisor at 17.8%, and the mandibular right central at 13.3% of the time. Again with
crowded teeth of an Angle class I case, the incisors will be moved the most. The lower
incisors are usually flared a little bit when fixed orthodontic appliances are placed. The
mandibular anterior teeth did not show as much root resorption as the maxillary teeth and
79
this can be due to the fact that the cases used in this study all started with a Class I
occlusion with mild to moderate crowding and relatively level Curves of Spee.
Therefore, there was not the need to intrude any of the incisors which, as previous studies
have shown, cause an increase in the amount of root resorption seen on the incisors
because the force is directly focused on the apex of the incisor roots. (Beck, 1994 and
Parker, 1998)
Confirming the finding of Preoteasa, the canines in both the maxillary and
mandibular arches showed the least amount of root resorption. (Proeteasa, 2009). The
least affected canine was the mandibular right at a frequency of 2.2%. Again in Angle
Class I cases, the canine will not be moved great distances and will not have as much
force applied to move it compared to correction of a Class II or Class III case with
extractions where a canine will be moved a significant distance to close space. This was
confirmed by Taner et al, when his study showed that patients with Angle Class II had
twice as much root resorption when compared with Class I patients. (Taner, 1999)
When looking at the amount of movement of the root apex following the
treatment with the two different orthodontic appliances, it was seen that the root moved
significantly more with the conventional brackets when compared to the Invisalign group.
The mean vertical movement of the root was found to be significantly greater in the
conventional fixed group compared to the almost lack of movement in the Invisalign
group. The horizontal movement of the root was also seen to be moved further in the
conventional group when compared to the Invisalign group. The conventional fixed
group actually had a mean movement of -0.91mm, which correlates to retracting the
maxillary incisors whereas the Invisalign group’s mean horizontal movement was
80
0.25mm. These differences can be explained by the fact that with the fixed conventional
appliances with the wires and brackets you are able to control the teeth and the roots
more precisely than with the clear Invisalign aligners. With the aligners, the teeth are
protracted or maintained in position through the use of interproximal reduction to the
teeth. In Invisalign this allows for space to be made in order to round out the arch and
accommodate all the teeth into the arch. If there is initial spacing present, the Invisalign
aligners help to close the space by tipping the teeth together without real regard to the
root position. It is much harder to control root position with the Invisalign aligners
compared to fixed brackets. With the conventional fixed appliances, the orthodontist has
more control of the movement of the teeth, as well as the roots, through the use of his
mechanics with the brackets and wires, for example tie-backs of the archwire do not
allow the teeth to flare forward.
The analysis of the beginning and ending overjets confirm this fact in that the
average correction in overjet with the Invisalign group was found to be 0.42mm whereas
the conventional fixed appliances corrected the overjet with an average of 0.55mm. This
again can be attributed to the fact that conventional fixed appliances allow for better
control of the teeth and roots than aligners.
The force delivered to a tooth has been proven to directly correspond to the
amount of root resorption. (Owman-Moll, 1995) The difference between the two groups
analyzed is the amount of force delivered to the tooth as well as the type of force. With
conventional fixed braces the ideal force is a light continuous force applied to the teeth in
order to create the desired movement of the teeth. With both the fixed appliances and the
Invisalign removable appliance, these light forces can be achieved. The main difference
81
is the fact that the Invisalign aligners are removable and can therefore be taken out of the
mouth, which ultimately removes the force placed on the teeth. With fixed appliances the
force is continuous and always present. With removable aligners, the force is
discontinuous. Previous studies have shown that with a discontinuous force delivery a
smaller mean percentage of resorption-affected areas, less severe apical blunting of teeth,
and less overall root resorption was found compared to when continuous forces were
delivered. (Acar, 1999) This can also be attributed to the fact that the teeth may have a
little more time to heal and repair the apical root structure as suggested in previous
studies. (Owman-Moll, 1998).
The reasons as to why there was no external apical root resorption seen on any of
the analyzed teeth which treated with the Invisalign aligners is multidimensional. First,
the fact that the forces the Invisalign aligners place on the teeth are very light as well as
being discontinuous. This in itself leads to less root resorption as the heavier the force
delivered to the tooth, the greater chance for root resorption. Also with discontinuous
forces, the teeth have a period of quiescence where the process of healing and laying
down repairative cementum can occur if there was a loss of apical root structure. There
is also the fact that with the Invisalign aligners there is not as much movement of the root
as with conventional fixed appliances. In this study, it was found that the root apex of the
teeth with the Invisalign aligners barely moved while there was recordable movement
with the fixed appliances. The less distance that a root has to move, the less chance there
is of apical root resorption occurring. The other factor that comes into play is that the
tooth movement with the Invisalign system is primarily tipping rather than true bodily
movement. Again, the less movement the root undergoes, the less chance of root
82
resorption. The actual amount of treatment time to complete the cases in this study was
not a factor as both groups, on average, finished within a month of each other.
83
CHAPTER SEVEN: CONCLUSION
This study is the beginning of the much needed research aspect of the effects of
the Invisalign treatment system. There has not been many studies completed that have
looked at the effects of Invisalign on root resorption or forces attributed to Invisalign
aligners. Most of the published reports about Invisalign are case reports, which are of
much interest to orthodontists, but do not present significant evidence to support
treatment based on evidence. With this study it was attempted to quantify the level of
root resorption present after the use of the Invisalign treatment with removable aligners.
We compared and analyzed the results for the Invisalign system with conventional fixed
orthodontic appliances and confirmed our hypothesis that with the Invisalign treatment
there would be less external apical root resorption present. Our study found that there
were not any amounts of root resorption present with the Invisalign aligners while finding
an expected amount of root resorption with the conventionally fixed orthodontic brackets
and wires.
84
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