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EFFECT OF ALIGNER MATERIAL, DURATION, AND FORCE LEVEL ON TOOTH MOVEMENT
By
NEHA D. PATEL
A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE
UNIVERSITY OF FLORIDA
2014
© 2014 Neha D. Patel
To my husband and family for helping me reach for the stars and to God for giving me the strength to reach that high
4
ACKNOWLEDGMENTS
Thank you to my mentors, Dr. Timothy Wheeler, Dr. Susan P. McGorray, and Dr.
Madhu Nair, for your support and guidance through this process. Thank you to my
husband and family for their support throughout my entire residency. Thank you to my
instructors, Dr. Wellington Rody and Dr. Calogero Dolce for their pearls of wisdom and
patience these past three years.
5
TABLE OF CONTENTS page
ACKNOWLEDGMENTS.............................................................................................................. 4
LIST OF TABLES ......................................................................................................................... 6
LIST OF FIGURES....................................................................................................................... 7
ABSTRACT ................................................................................................................................... 8
CHAPTER
1 INTRODUCTION................................................................................................................... 9
2 MATERIALS AND METHODS ......................................................................................... 14
Study Design........................................................................................................................ 14 Enrollment and Study Participation .................................................................................. 14 Collection of Data ................................................................................................................ 16
Clinical Tooth Movement ............................................................................................ 16 Radiographic Measures .............................................................................................. 16
Statistical Considerations .................................................................................................. 17
3 RESULTS ............................................................................................................................. 22
4 DISCUSSION ...................................................................................................................... 28
5 CONCLUSIONS .................................................................................................................. 34
LIST OF REFERENCES ........................................................................................................... 35
BIOGRAPHICAL SKETCH ....................................................................................................... 40
6
LIST OF TABLES
Table page 2-1 Outline of inclusion and exclusion criteria. ................................................................. 19
2-2 Schedule of Study Events ............................................................................................. 20
3-1 Demographics of subjects............................................................................................. 24
3-2 Age of subjects ............................................................................................................... 24
3-3 Material differences in average percent of tooth movement at Day 14 (Pct14), Day 25 (Pct25), and time points in between (Diff3514). .......................................... 24
3-4 Material differences in average compliance on Day 14 and Day 25...................... 24
3-5 Average percent of tooth movement achieved between the materials and subject composition of movement groups. ................................................................. 25
3-6 Average percent of tooth movement achieved and average compliance scores for accompanying percentage groups............................................................ 25
3-7 Differences in average tooth movement achieved between genders.................... 25
3-8 Comparison of CBCT Measures and Tooth Movement at Day 14 (Pct 14) and Day 25 (Pct 25)............................................................................................................... 26
7
LIST OF FIGURES
Figure page 2-1 Overview of radiographic measurements of the target tooth. ................................. 21
3-1 Tooth movement (in mm) by material over the 25-day study period. .................... 26
3-2 Tooth movement by gender and material over 25 days. ......................................... 27
3-3 Overall tooth movement between previous UF studies and the current study by material. ...................................................................................................................... 27
8
Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science
EFFECT OF ALIGNER MATERIAL, DURATION, AND FORCE LEVEL ON TOOTH
MOVEMENT
By
Neha D. Patel
May 2014
Chair: Timothy T. Wheeler Major: Dental Sciences - Orthodontics
SmartTrackTM, created by Align Technologies®, has a lower initial insertion force
and a longer working range compared to the older EX30 material to aid orthodontic
tooth movement (OTM). To investigate the effect of SmartTrackTM on OTM in-vivo over
a 25-day period compared to the EX30 material. Aligners made of one of the two
materials and programmed for 0.25mm of buccal movement of a maxillary incisor were
used in 33 subjects (17 females and 16 males), 18-40 years old, for 22 hours/day for 25
days, in a randomized, blinded manner. Cone beam computed tomography (CBCT)
imaging was conducted prior to tooth movement. SmartTrackTM achieved significantly
higher mean OTM (73.1%) compared to EX30 (42.8%) by Day 14. No difference in
OTM occurred from Day 14-25. Mixed modeling revealed SmartTrackTM and compliance
separately provided 16% and 9% higher OTM. No difference between CBCT
measurements and tooth movement was noted. SmartTrackTM achieved a higher mean
OTM with a seemingly shorter lag phase compared to the EX30 material over a 25-day
period. A combination of SmartTrackTM effects and compliance play a collective role in
achievement optimal OTM.
9
CHAPTER 1 INTRODUCTION
Clear orthodontic aligners are actively sought by the public through its thin and
easily adaptable nature 1, ideal aesthetic optical properties, ease of proper hygiene, and
comfort of wear2-4. Kusy defined ideal archwires as those with properties of high
strength, resilience, formability, are springy, biocompatible, have good springback, good
range, and are highly esthetic5. Similar ideals are held for orthodontic treatment utilizing
aligners. Although the advent of clear aligner therapy sparked interest within the public
to seek orthodontic treatment, initial case studies showed that treatment is successfully
achieved in minor to moderate crowding. It has been reported that those patients
exhibiting crossbites, a deep bite6, 7, an anterior open bite or shallow over bite, and
those with periodontal problems may all be treated with clear aligner therapy8.
Extraction spaces have been shown to be difficult to close with bodily tooth movement
since tipping occurs readily as orthodontic force is placed9-11. Newer treatment
strategies have been able to overcome the drawbacks of aligner therapy by being able
to treat complex maloccusions7, 12, 13, and integration of aligner therapy with fixed
treatment in orthognathic surgery cases and with functional appliances has been able to
achieve orthodontic success14-17.
The aligners tend to function optimally by producing tooth movements in an
anterior-posterior (AP) direction and improving transverse relationships generally by
tipping movements11, 18. More difficult movements, such as controlling root position and
tooth rotations have been aided with the advent of Power Ridges and attachments to
facilitate movement 2, 7, 19. Polyurethane is the main element in fabrication of the
aligners and can be affected by several variables, including heat and moisture. The
10
prescribed wear to allow for proper aligner prescription expression is at least a twenty
hour interval per day for two weeks2. The majority of tooth movement has been proven
to occur within the first week of aligner wear, but controversy still remains as to whether
the extra week of aligner usage will aid in fulfilling the prescription of the tray 10 20.
Vardimon et al found that the distortion energy, or von Mises strain, during maxillary
incisor retraction decreased over a fifteen day period, with the highest at day 1,
decreasing at day 2, and then maintaining levels from day 2 to day 15 with no complete
relapse to the original level21. Due to the decaying forces, it has been suggested to have
patients wear aligners for 24 hours for the first two days per aligner and then
intermittently up until day 1521. Baldwin et al. had reported of a clear aligner company
suggesting a wear pattern of three weeks for full expression of aligner prescription9.
Clinical studies previously conducted have shown labial expansion of incisors to
achieve only 38%-61.6% of the full aligner prescription expression20, 22-24. According to
Kravitz et al, retroclining maxillary incisors is two times more predictable in lingual
tipping as opposed to than proclination24.
Orthodontic tooth movement tends to occur in three general phases of initial, lag,
and postlag phases25. In rat models, Arias and Marquez-Orozco 26 had reported that the
first four days of OTM tended to be in the initial and lag phases. The transition to the
postlag phase may take as long as 20 days to complete27, which is after the two week
wear pattern for aligners. To facilitate OTM, the coupling of bone resorption and bone
regeneration are necessary in response to the high orthodontic force placed on teeth27-
29. Accomplishing tooth movement usually requires 20-150g of orthodontic force per
tooth 30. Light, continuous forces are generally preferred over heavy forces to prevent
11
hyalinization of the PDL and unstable bone resorption that may occur with heavy forces,
which tends to surpass a capillary blood pressure of 20-25 gm/cm2 of root surface.
Light forces allow for a proper biologic response and allow for stable bone resorption 27.
Alveolar bone height concurrent with root length also greatly influence tooth movement
and the position of the tooth center of resistance (CRes). Over time, alveolar bone
levels tend to decrease, thus displacing the CRes to a more apical level 31. Since CRes
affects how force applied at the coronal level, moments may be greater if alveolar bone
levels are decreased.
In 2005, a randomized and controlled clinical trial was conducted at UF to assess
the effects of recombinant human relaxin during OTM with clear aligners of a single
anterior incisor over an eight week period 23. Although the results of the study found no
significant variance between treatment and control groups, other effects were noted.
These effects include a higher rate of OTM during the first week compared to the
second week of continuous aligner wear during the two-week prescription cycle, the
absolute expression of the aligners was not achieved, the age of subjects had a positive
correlation with OTM, and OTM differed greatly between subjects, which also was
echoed in previous studies20.
Performance of aligner material influences tooth movement. Studies have shown
that a hard versus soft aligner material had no statistically significant difference in
outcome of extraction space closure9-11, but it has been shown that a combination of a
hard aligner material and a two-week interval allows for optimal tooth movement10. As
with any aligner, working range and initial insertion force are two of the vital traits in
performance of the aligner material. Initial insertion force is defined as the force
12
influencing the movement of the teeth according to the prescription expressed in the
aligner upon initial insertion. Patients tend to feel more elevated levels of discomfort
within the initial days after insertion of the appliance 3, 32. Working range of an appliance
is the difference between the full amount of force imparted by the appliance at initial
activation until deactivation. Larger working ranges are favorable due to its decreased
discomfort perceived by patients and the force influencing tooth movement is able to
work over a larger range of distance. An improved material, called SmartTrackTM,
developed for Invisalign® treatment compared to the existing EX30 material was shown
to be superior in both traits. MSDS information procured from Align Technologies
reports the material is made from multilayer aromatic thermoplastic polyurethane, and
suggests the material may show improved flexibility and a longer working range similar
to the properties reported of a multistranded archwire5. Multistranded stainless steel
wires tend to have high springback, moderately low stiffness33, high range, and
administer a lower force34, which are properties desired in an aligner35. The improved
material imparts a force of 4N, compared to the old material force of 4.5N, and also has
a working range of 0.20mm, compared to 0.15mm of the old material.
The purpose of this study is to investigate the effect of appliance activation force
over time on total tooth movement utilizing a human tooth movement model using the
standard material (Material S) and an experimental material (Material E). The benefit of
utilizing CBCT imaging will also aid in evaluating the alveolar bone relationship to tooth
movement. Since the lag phase may last longer than the 2 week prescribed wear
period, a longer investigational period from a two-week wear pattern to a four-week
wear pattern will be evaluated to determine if tray prescription expression can be
13
enhanced. Tooth movement curvatures will also be evaluated to investigate whether
Material E may allow for a more linear phase of tooth movement as opposed to the
three defined phases25.
14
CHAPTER 2 MATERIALS AND METHODS
Study Design
The basis of the study design is echoed off of previous aligner research20, 23. IRB
approval had been obtained to conduct a single center clinical trial at the University of
Florida Graduate Orthodontic Clinic. Participating subjects were between and including
the ages of 18-40, had minor incisor misalignment, and would undergo orthodontic
treatment. Five groups of subjects with five to seven participants in each group were
randomized to a material (S or E) with a standard activation schedule of 0.250 mm over
a twenty-five day study period. Study clinicians were blinded to the aligner material and
an equal number of subjects were randomized to each material group.
Enrollment and Study Participation
Each subject was initially phone screened and then scheduled for the first of two
preliminary visits. The inclusion and exclusion criteria are outlined in Table 2-1. The first
visit is the Eligibility Visit, which is to distinguish potential study subjects with minor
maxillary incisor malalignment and to exclude those falling under the exclusion criteria.
The following procedures will be performed. The informed consent form, completed
medical history, and intraoral clinical examination were completed. Those who are
deemed eligible based on this preliminary visit were then allowed to proceed to the
Screening Visit. The goal of this visit was to determine if the subject is fully eligible for
completion of the study according to the Cadent iTero™ digital impression scans, which
were sent to Align Technology for fabrication of Invisalign® appliances, intraoral and
extraoral photographs, and a cone-beam computed tomography radiograph (CBCT). A
CBCT is standard protocol before orthodontic treatment at the University of Florida.
15
Women were required to undergo a urine pregnancy test to ensure a negative reading
before any imaging procedures. Those who are deemed eligible for completion of the
study had digital impression scans sent to Align Technology for Invisalign® appliance
fabrication, given a study number, and enrolled in the study. An outline of data collected
at each study visit is depicted in Table 2-2.
Once a participant was accepted into the study, one central incisor, the right or
left, was chosen for activation through the aligners and termed as the “target tooth”. The
target tooth had to follow proper criteria, such as no adjacent teeth to block tooth
movement and must have been able to move a 0.50mm in an AP direction. The maxilla
was the only arch receiving an aligner, which was fabricated using either Material S or
Material E and had an activation prescription in the antero-posterior (AP) direction of
0.250mm. Instead of utilizing polyvinyl siloxane (PVS) impressions as in previous
studies20, 23, digital scans were taken with an iTero™ optical scanner for each subject.
The iTero™ optical scanner is reliable in replication of the dentition and an alternative to
the conventional PVS impressions36. The single maxillary aligner was delivered on a
Monday, and scans were taken on days 1, 2, 3, 4, 7, 9, 11, 14, 16, 18, 21, 23, and 25.
The study terminated at day 25, and final data collection was taken.
A total of 33 subjects were enrolled in the study after initial screening. If a subject
missed more than 2 study visits, they were involuntarily dropped from the study. One
subject was dismissed midway through treatment due to the lack of follow-up with study
visits. Data was utilized from the terminated subject since the last study visit. Subjects
were required to wear the aligners full time except for brushing their teeth, drinking, and
eating. Compliance with aligner wear was documented through a daily diary distributed
16
to study subjects at the initial delivery of aligners. A completed medical and dental
history was also taken at the initial visit and routinely checked along with the daily diary
at every visit to monitor any excessive medication intake.
At the end of data collection, study subjects were given the opportunity to be
treated at the University of Florida Graduate Orthodontic Clinic for routine orthodontic
treatment using the Invisalign® appliances.
Collection of Data
Clinical Tooth Movement
The measurement of the AP movement of the target tooth was done using
iTero™ digital impression scans and a Bite Matching algorithm, a proprietary software
developed by Align Technologies® to superimpose scans more efficiently and
effectively.
Radiographic Measures
Each cone beam radiograph was sliced with a 0.4mm and 3.6mm thickness to
include the full length of the target tooth for each subject with accompanying apical
bone, making sure to account for the incisive canal. Sliced images were then flattened
to an 8bit TIF image, minimizing any compression. ImageJ® software was used to
magnify the image without changing the innate properties of the initial file. A fractal
analysis (FA) was conducted to quantify bone by evaluating the organization and
complexity of bone structure using the TACT® workbench. This analysis was used to
qualitatively and quantitatively assess bone in the apical region of the tooth prescribed
for movement. A FA is advantageous compared to gray level analysis in that it is
independent of projection geometry or exposure parameters. Digital subtraction
techniques require multiple images with reproducible projection geometry to yield
17
meaningful results and is, thus, impractical in the clinical scenario. Independent of other
extraneous variables, FA utilizes the range of signal intensities, converts the intensities
into a frequency plot in order to plot the intensities to a 2D regression plot. All images
were normalized by utilizing a reference image and a 32 by 32 region of interest (ROI)
within the apical bone of the target tooth. The FA was conducted for each ROI using a
power spectrum method within the Tuned Aperture Computed Tomography (TACT)®
workbench. Following a 2D fast Fourier transformation (FFT), the log of the magnitude
was plotted against the log of frequencies created by the FFT. The slope of the
regression line generated the fractal dimension for each specific ROI 37.
Measurements of the target tooth mimic that of previous studies20 and are
summarized in Figure 2-1. All measurements were done using InVivo Anatomage®
Software, version 5, and included measurements of tooth length, root length, crown
length, crown to root ratio, the distance of the root apex to the buccal plate, and the
widest buccolingual dimension of the target tooth at the crest of the alveolus was done
at three different timepoints by one researcher (NDP) and averaged.
Statistical Considerations
Statistical assessments were able to utilize a parametric analysis due to the normal
distribution of the data. The outcomes of interest were to observe tooth movement from
baseline to Day 14 and to Day 25. Primary comparisons were done between treatment
groups and the outcomes as well as correlations of CBCT measures and the outcomes.
Additional variables such as age, gender, and compliance were also taken into
consideration. Analyses conducted included two-sample t-tests and Pearson correlation
coefficients with a p value less than 0.05 considered statistically significant. With the
addition of multiple data points in this study, we had the advantage of using mixed
18
modeling to test interactions between radiographic measures, material, compliance,
gender, age, and tooth movement.
19
Table 2-1. Outline of inclusion and exclusion criteria.
Inclusion criteria
1. Males or females between and including the ages of 18 and 40 years old, desiring correction of minor incisor malalignment using the Invisalign® appliance. Subjects may have had previous orthodontic treatment.
2. Adult dentition with all upper anterior teeth present.
3. At least one maxillary central incisor that has sufficient space between it and adjacent teeth to allow AP movement of 0.5mm (crown tipping only).
4. Normal pulp vitality and healthy periodontal tissues as determined by intraoral exam.
5. Good health as determined by medical history.
6. Willingness and ability to comply with study procedures, attend study visits, and complete the study.
7. The ability to understand and sign a written informed consent form, which must be signed prior to initiation of the study procedures.
Exclusion Criteria
1. Significant periodontal disease (>3mm pocket depth or >1mm of recession on maxillary anterior teeth).
2. Active dental disease not under care of either a dentist or periodontist.
3. Chronic daily use of any nonsteroidal anti-inflammatory medication, estrogen, calcitonin, or corticosteroids.
4. History of use or current use of any bisphosphonate medication or other medication for treatment of osteoporosis.
5. Current smoker (must not have smoked in the last six months).
6. Fibroblast function is necessary for proper conservation of periodontal structures. Nicotine has been shown to be destructive to periodontal elements such as periodontal ligament fibroblast growth in a dose dependent manner and can inhibit cell proliferation and diminish protein synthesis58. Tobacco products may also be a major factor in the development of periodontal disease.
7. Women must not be pregnant. Negative urine pregnancy tests prior to exposure to cone beam computed tomography (CBCT) imaging is require to verify pregnancy status. Dental x-ray exposure with pregnant women has been questioned in potential association to pituitary and thyroid disorders and low birth weight in infants59.
8. Any condition or use of medication which in the opinion of the investigator interferes with the biology of tooth movement.
9. Any condition which in opinion of the investigator results in increased risk to the subject.
20
Table 2-2. Schedule of Study Events
Event Prelim
1 Prelim
2 Day
0 Days
1, 2, 3, 4, 7, 9, 11, 14, 16, 18, 21, 23
Day 25
Informed Consent1 X Inclusion/Exclusion X X Medical History X Pregnancy Test X Intraoral Exam X X X X Maxillary occlusal and frontal photos X X X
Cadent iTero™ digital impression scan X X X
Intraoral photos (complete set) X Extraoral photos X Cone beam imaging2,3 X Dispense aligners and daily diary X
Collect aligners and daily diary X Begin planned orthodontic treatment X
1Informed consent must be given before any subject can progress further within the study. 2Cone beam imaging is a usual protocol for orthodontic treatment at the University of Florida. 3Women will be required to take urine pregnancy tests immediately prior to exposure to verify they are not pregnant.
21
Tooth Length -Measured from incisal tip to apex of the target tooth
Crown Length -Measured from incisal tip perpendicular to a line connecting the buccal and lingual cortical plates. Root Length -Measured from a line connecting the buccal and lingual cortical plates to the root apex. Crown to root ratio -The division of the crown length into the root length. Distance from apex to buccal cortical plate -The measurement of the perpendicular distance from the apex to the buccal cortical plate Widest buccolingual width of tooth at crest of alveolus -The measurement of the buccolingual thickness of the tooth at the alveolar crest.
Figure 2-1. Overview of radiographic measurements of the target tooth.
22
CHAPTER 3 RESULTS
An outline of the subject demographic information is displayed in Table 3-1 and
Table 3-2. An equal number of males (48.48%) and females (51.52%) participated in
this study with an average age of 26.95 (± 5.31) years. Orthodontic tooth movement
between materials was statistically significant at Day 14 (p=0.02) and Day 25 (p=0.02),
and values are depicted in Table 3-3. Material E was able to achieve a higher average
percentage of tooth movement at Day 14 (73.06%) and Day 25 (77.38%) compared to
Material S (42.75% and 50.07%, respectively). A diagrammatic representation of the
average amount of tooth movement achieved at all time points between the materials is
displayed in Figure 3-1. The term Diff2514 is defined as the percentage of tooth
movement achieved at Day 14 subtracted from the percentage of tooth movement
achieved at Day 25 to evaluate tooth movement between the time points. No significant
difference was noted in tooth movement between the materials and over time from Day
14 to Day 25 (p=0.18).
In comparing a two-week aligner protocol versus a four-week aligner protocol, no
difference was noted (p=0.23) in tooth movement achieved between Day 14 and Day
25. Material S noted 9.73% of tooth movement from Day 14 to Day 25, as defined by
the Diff3514 variable, while Material E was -0.47%, thus showing that the majority of
tooth movement was achieved by Day 14 with Material E while Material S still allowed
for a residual amount tooth movement to occur past Day 14. Average percentage of
tooth movement in relation to age was not significant at Day 14 (r= -0.05, p=0.78), Day
25 (r= -0.03, p=0.86), or time points in between (r= -0.08, p=0.67).
23
Compliance between the materials at Day 14 and Day 25 showed no significant
differences in the average number of hours of reported aligner wear (p=0.65; p=0.61) as
shown in Table 3-4. Thus, compliance was similar between the materials with
accompanying similar maximum and minimum hours of wear. Table 3-5 shows 10
subjects had ≥80% of average tooth movement achieved at Day 14, and 9 out of those
10 subjects were assigned to Material E. Table 3-6 shows that of those 10 subjects, the
hours of compliance were slightly higher than the other percentage groups. Thus,
slightly higher compliance may point towards higher amounts of tooth movement.
Gender and tooth movement is depicted in Table 3-7. No significant relationship
can be seen in tooth movement between the genders at Day 14 (p=0.39), Day 25
(p=0.65), and time points in between (p=0.69). Females and males had higher average
percentages of tooth movement with Material E than Material S, as seen in Figure 3-2.
Males tended to have higher percentages of tooth movement than women at all time
points of average percentages of tooth movement.
Radiographic measures had no significant relationships to tooth movement as
displayed in Table 1-8. Marginal significance was noted with tooth length in relation to
tooth movement achieved at Day 14 (r=0.33, p=0.07) and Day 25 (r=0.31, p=0.09), and
root length had marginal significance at Day 14 (r=0.33, p=0.07). Mixed model analyses
between all variables noted significant interactions between tooth movement with
compliance (p=0.0001) and Material E (p=0.0017).
Overall average tooth movement in millimeters between the materials is
displayed in Figure 3-3. The average tooth movement achieved at Day 14 and Day 25
24
with Material S is similar to the previous studies done at the University of Florida20, 22, 23.
Material E outperformed the previous studies at each time point.
Table 3-1. Demographics of subjects
Variable N Material E Material S P-Value Total 33 17 16 p=0.87
(NS) Females 17 9 8 Males 16 8 8
Table 3-2. Age of subjects
N Mean (yrs) SD Min
(yrs) Max (yrs) P-Value
Total 33 26.95 5.31 20.06 40.07 p=0.74 (NS) Material E 17 26.65 5.44 22.05 40.07
Material S 16 27.72 5.32 20.06 38.08
Table 3-3. Material differences in average percent of tooth movement at Day 14
(Pct14), Day 25 (Pct25), and time points in between (Diff3514).
Variable N Mean SD Min Max P Value
Material E Pct 14 16 73.06% 37.98% -18.00% 133.00% Pct 14
p=0.02**
Pct 25 p=0.02**
Diff2514 p=0.18 (NS)
Pct 25 16 77.38% 30.36% 17.00% 130.00% Diff2514 15 -0.47% 19.96% -35.00% 39.00%
Material S Pct 14 16 42.75% 30.93% -31.00% 103.00% Pct 25 15 50.07% 31.06% 4.00% 120.00%
Diff2514 15 9.73% 20.62% -27.00% 42.00%
Table 3-4. Material differences in average compliance on Day 14 and Day 25. Variable Mean (hrs) SD (hrs) Min (hrs) Max (hrs) P-Value
Material E Meanwear 14 21.40 1.73 15.89 22.81 Meanwear
14 p=0.65 (NS)
Meanwear
25 p=0.61 (NS)
Meanwear 25 21.44 1.36 17.69 22.84
Material S Meanwear 14 21.17 0.87 19.39 22.42
Meanwear 25 21.21 0.99 18.74 22.63
25
Table 3-5. Average percent of tooth movement achieved between the materials and subject composition of movement groups.
Percentage of Tooth Movement Achieved at Day 14 Material E (N) Material S (N) Total (N)
≤35% 2 7 9 36-79% 5 8 13 ≥80% 9 1 10
Table 3-6. Average percent of tooth movement achieved and average compliance
scores for accompanying percentage groups.
Percentage of Tooth Movement Achieved at
Day 14
Mean (hrs) SD Min
(hrs) Max (hrs)
Mean Wear 14
≤35% 20.76 0.84 19.39 22.10 36-79% 20.90 1.68 15.89 22.42 ≥80% 22.21 0.63 20.89 22.81
Table 3-7. Differences in average tooth movement achieved between genders.
Variable Mean SD Min Max P Value
Females
Pct 14 52.1% 42.5% -31.0 119.0
Pct 14 p=0.39 (NS)
Pct 25
p=0.65 (NS)
Diff2514 p=0.69 (NS)
Pct 25 61.3% 36.7% 4.0% 108.0%
Diff2514 6.3% 21.6% -35.0% 39.0%
Males
Pct 14 63.7% 31.8% 20.0% 133.0%
Pct 25 66.9% 30.6% 9.0% 130.0%
Diff2514 3.2% 20.3% -35.0% 42.0%
26
Table 3-8. Comparison of CBCT Measures and Tooth Movement at Day 14 (Pct 14) and Day 25 (Pct 25).
Variable Mean SD Pct 14 P-Value
Pct 25 P-Value
Tooth Length 23.66mm 1.45mm 0.07 0.09
Crown Length 11.93mm 0.81mm 0.66 0.49
Root Length 11.35mm 2.38mm 0.07 0.12
Crown to Root Ratio 1.03 0.13 0.20 0.24
Fractal Analysis 1.83 0.32 0.78 0.80
Distance from apex to buccal cortical plate 2.40mm 1.19mm 0.21 0.16
Widest Buccolingual Width of Tooth at Crest of Alveolus 6.63mm 1.57mm 0.89 0.62
Figure 3-1. Tooth movement (in mm) by material over the 25-day study period.
Toot
h M
ovem
ent A
chie
ved
(mm
)
27
Figure 3-2. Tooth movement by gender and material over 25 days.
Figure 3-3. Overall tooth movement between previous UF studies and the current study
by material.
Toot
h M
ovem
ent A
chie
ved
(%)
Toot
h M
ovem
ent A
chie
ved
(%)
28
CHAPTER 4 DISCUSSION
Orthodontic tooth movement (OTM) is the product of intricate cellular
mechanisms. These delicate mechanisms are the foundation of which exogenous
pharmacologic influences, endogenous influences, and vitamin supplements tend to
affect the rate of OTM. Studies have shown that the exogenous use of estrogen,
bisphosphonates, corticosteroids, and nonsteroidal anti-inflammatory drugs can affect
the rate of OTM26, 38-40. As orthodontic treatment begins, discomfort may ensue,
resulting in patients reaching for pain relief medications. Miller et al.3 compared a fixed
appliance treatment group to an Invisalign treatment group and has shown that at every
time point over a 7-day period, Invisalign treatment was perceived as more comfortable
and manageable than fixed treatment. Although both treatment groups experienced a
decrease in quality of life at the beginning of treatment, the quality of life scores for
those undergoing fixed appliance treatment decreased as treatment progressed and
experienced decreases in various aspects of their daily lives. Analgesic usage was
highest among the fixed appliance subjects, but became insignificant after the third
day3. Shalish et al. compared a lingual fixed treatment group, buccal fixed treatment
group, and an Invisalign treatment group and reported 0% of analgesic usage among
Invisalign subjects by the 6th day of treatment32. The lingual and buccal fixed treatment
groups on average were not able to completely recover from initial declines in food
consumption until the 5th-10th day of treatment, while the Invisalign treatment subjects
had recovered by the 4th day at most32. Although this study did not specifically track
analgesic usage over the 25-day experimental period, the effects of an analgesic may
29
not have had a significant effect on tooth movement since the amount of tooth
movement achieved is similar to previous studies20, 22, 23.
Tooth movement with clear aligners over a 14-day period has been shown to
produce less than 100% of prescription expression20, 22-24. The results found in this
study tend to echo similar sentiments. As the data shows, tooth movement over a 14-
day period provides a similar amount of tooth movement as a 25-day period. This is the
first study to take such minute time points, which proved that tooth movement with clear
aligners still follows the three phases previously described25, but the SmartTrackTM
material possibly exhibits a short lag phase of 2-3 days and completes the majority of
tooth movement by the 14th-day of wear. The SmartTrackTM material also proved to
achieve a higher mean percentage of tooth movement at Day 14 and Day 25, which
was significantly higher than the standard EX30 material.
When aligners are worn, there are short-term and long-term forces involved.
Short-term forces are when an aligner is fitted initially in the mouth and long-term forces
are after initial fitting of aligner, forces are placed on teeth through the prescription in the
tray and occlusal loading35, 41. Modes of deformation pertinent to aligners are tension,
bending, and torsion. The elastic property of aligners allow the aligners to adapt to the
prescribed movement when strain occurs in the aligners, but the viscoelastic property of
aligners causes the fatigue of force over time. Long-term moisture exposure of 48 hours
may affect aligner performance after strain placement and tensile properties tend to
decrease35. Stress relaxation of the aligner decreases the ability of the material to
produce the appropriate amount of force for an appropriate amount of tooth
movement42. Mimicking of an intraoral environment in a water bath of 37.0oC and
30
placement of 5% strain upon thermoplastic material has shown to quicken the stress
relaxation of the material. Furthermore, only about 42.3-66.6% of the initial forces was
delivered after the immersion in the water bath42. As thermoplastic polyurethane
materials age, a process called plasticization occurs in which water is absorbed as the
material relaxes. The absorption of water may play a role in the breakup of Hydrogen
bonds present within the composition of the material, and the hygroscopic expansion of
the material may possibly accelerate further degradation43. A study by Ryokawa et al.
compared the Invisalign EX30 material to various other thermoplastic materials by
submerging the material in a distilled water bath at 37.0oC and reported the highest
water absorption by the Invisalign material after 24 hours and 336 hours of immersion44.
This may be due to the amorphous polymer composition, allowing for more free space
to be utilized by the water absorption. It is interesting to note that the Invisalign material
did not have the highest hygroscopic expansion rate when compared to other
thermoplastic materials after immersion in the water bath, possibly due to previous
water absorption and hygroscopic expansion at room temperature44. This combination
of water absorption and the temperature of the oral cavity may play a role in the
continued degradation of the aligner material43. The oral cavity exhibits a more
complicated nature and temperature differences and the intermittent pressures placed
on the teeth with aligner removal and reinsertion may also play an additive role in the
fatigue of force levels. As temperatures rise, the amount of water absorption also
increases43 and oxidation of methylene diphenylene units within the composition of the
thermoplastic polyurethane material can lead to degradation of the elastic properties45.
With a plastic device in the oral cavity, the use of Invisalign appliances has been shown
31
to be chemically stable46 and not induce any cytotoxic effects on human gingival
fibroblasts and lack any estrogenic effects of the material eluents47.
A noticeable amount of wear has been proven evident in the aligners after the
recommended two week interval. Calcification, distortion, cracking, and development of
biofilms within the aligner material have been shown46, 48, 49. Plaque accumulation may
interfere with the surface contour of the aligner50. Buccal aspects of worn appliances
have also shown an increase in hardness from mastication-induced cold working 49. A
randomized and controlled clinical trial performed at UF in 200920 studied the effects
and efficiency of OTM by inserting a fresh aligner after one week of wear, replacing the
worn aligner. The results showed that there is no added benefit of placing a fresh
aligner with the same prescription after one week of wear instead of utilizing the same
aligner for the two week period. This finding suggests that material fatigue and
deactivation is not the rate-limiting factor in the event of tooth movement.
In terms of force application, the SmartTrackTM material imparts an overall force
of 4.0N to the maxillary incisor, but studies have shown that even a 1.80N (183.5g)
force with a SentalloyTM wire for intrusion or extrusion of a single tooth may be too high
and may contribute to root resorption over a 4 week period51. Although there is no
scientific evidence of specific numeric force values necessary for tooth movement52, the
force levels of thermoplastic appliances with an activation of 0.50mm may range from
2.91N to 5.10N24, 53. A recent study showed that a lower aligner activation of 0.50mm
tended to impart a higher level of force than a 1.0mm activation, and thinner aligner
materials delivered lower forces overall than thicker materials53. The higher force level
of the lower activation may be due to possible slippage of the material from the tooth
32
due to a high activation distance. Invisalign utilizes a thin material of 0.30 inches, which
is easily adaptable in the mouth and cause minimal posterior intrusion1. The high forces
imparted upon the teeth from clear aligners may dissipate through stretching of the
aligner material from repeated removal and insertion thus reducing the high impact of
the overall force. Although Invisalign may use a higher force range than usual fixed
appliance therapy, root resorption is usually not a cause for concern for treatment with
Invisalign appliances54. Recent data shows that 100 consecutively treated Invisalign
patients had little to no root resorption. On the other hand, 10% of patients treated with
fixed appliances had root resorption of at least 3mm7.
Our findings have shown that a lower activation of 0.25mm with a low initial
insertion force and longer working range with the new SmartTrackTM material were able
to achieve a higher average percentage of tooth movement achieved than previous
studies with 0.50mm of activation per aligner after 14 days20, 23. Tipping within the
periodontal ligament (PDL) space was achieved during the initial phase since the PDL
space is generally 0.15-0.20mm55 from Day 0-Day 2, and the lag phase of tooth
movement seemed to be limited to Day 2 to Day 4. This finding suggests that the lower
millimeter activation coupled with a more adaptable material may have imparted a much
more operational force on the incisor compared to higher millimeter activation with the
standard material. Since proclination of maxillary incisors is a difficult movement
compared to retroclination24, overcorrection of maxillary buccal tipping movements may
aid in achieving the desired measurement1.
As previous studies have shown3, 20, 22, 23, 32, a larger percentage of female
subjects partook in Invisalign studies, but this study had an approximately equal
33
distribution of males and females. No difference in the average percent of tooth
movement achieved was noted between age groups or genders, compared to
In evaluating the data, some subjects were able to experience more than 100%
of tooth movement prescribed. Occlusal factors and lack of intra-arch contacts were
analyzed and found that even those patients with a proximal contact in place, tooth
movement may be affects by the opposing occlusion. Although it has been found that
intra- and inter-arch contacts may negatively affect tooth movement56, the opposite
situation may have been shown in this study with the added effect of the lower anterior
occlusion affecting the excessive positive outcome of the maxillary incisor AP
movement.
Future studies investigating tooth movement with clear aligners may include the
use of cyclical force devices, local delivery of exogenous molecules such as
inflammatory cytokine IL-1 for stimulation of osteoclasts, and low-level laser therapy57.
Animal studies conducted so far show promising results, but human clinical trials with
defined methods and durations are necessary to help implement a proper treatment
protocol to achieve exceptional tooth movement in an efficient and effective manner.
34
CHAPTER 5 CONCLUSIONS
The SmartTrackTM material with a lower initial insertion force and a longer working
range had an increased mean percentage of tooth movement compared to the standard
EX30 Invisalign® material. The SmartTrackTM material was more effective at tooth
movement over a 14-day span than the EX30 material, but the amount of average
residual tooth movement from Day 14 to Day 25 between the materials is not
statistically significant. A higher percentage of tooth movement was achieved over a 25-
day period compared to a 14-day period, but this difference is not statically significant.
Initial decrease of strain in the SmartTrackTM aligner may deliver a more physiologic
force on the maxillary incisor to allow for more optimal tooth movement. No statistical
difference was noted between tooth movement and gender, age, or CBCT
measurements. Our study echoed previous studies, showing that 100% of aligner
prescription was not achieved. The SmartTrackTM material had a higher mean
percentage of tooth movement at Day 14 and Day 25 compared to previous studies
done at UF. Orthodontic tooth movement still follows the three phases of tooth
movement, even with a decrease in initial insertion force and an increase in working
range.
35
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BIOGRAPHICAL SKETCH
Neha D. Patel was born in Chicago, Illinois to Doctors Dipak and Neela Shah.
Her father is an interventional radiologist and her mother is a retired pediatric dentist.
Neha has one sibling, Akash Shah, who is a dual fellowship trained radiologist. Neha’s
family moved from Chicago to Galesburg and finally to Moline, Illinois, where the rest of
her childhood was spent.
Upon graduation from Moline Senior High School, Neha attended the University
of Illinois at Urbana-Champaign and received a BS in psychology. After her
undergraduate training, Neha was awarded her DDS at the University of Michigan in
Ann Arbor, Michigan. Fresh from dental school, Neha applied for residency in
orthodontics and matched at the University of Florida. After her first year of residency,
Neha was married in Moline, Illinois to Pratik S. Patel. In May of 2014, Neha will
graduate from the University of Florida with a certificate in orthodontics and a Master of
Science. Neha and her husband will move to Indianapolis, Indiana in June of 2014 to
begin their lives together.
Neha is a member of the Omicron Kappa Upsilon dental honor fraternity, Delta
Sigma Delta dental fraternity, The National Scholars Honor Society, and Phi Kappa Phi.
During her dental training, she was given the International College of Dentists Award,
Comprehensive Care Award, and Russell W. Bunting Award in Periodontics. Neha has
published one article in the Journal of Public Health Dentistry, titled Pediatric patients'
orthodontic treatment need, quality of life, and smiling patterns–an analysis of patient,
parent, and provider responses.
Neha has many hobbies, including travelling, rock climbing, running, baking, and
Indian classical dancing.