Download LOWER INCISOR STABILITY COMPARING TRADITIONAL BONDED RETAINERS TO THE MAGNETAINER®™

LOWER INCISOR STABILITY COMPARING TRADITIONAL BONDED
RETAINERS TO THE MAGNETAINER®™
Adam W. Armstrong, D.M.D.
A Thesis Presented to the Graduate Faculty of
Saint Louis University in Partial Fulfillment
of the Requirements for the Degree of
Master of Science in Dentistry
2014
® Copyright by
Adam Williams Armstrong
ALL RIGHTS RESERVED
2014
i
COMMITTEE IN CHARGE OF CANDIDACY:
Associate Clinical Professor, Donald R. Oliver
Chairperson and Advisor
Professor Eustaquio Araujo
Associate Professor Ki Beom Kim
ii
DEDICATION
This work is dedicated to my wonderful wife Megan. Your
belief in me and constant encouragement has allowed me to
accomplish my goals. I love you and will attempt to support
you as much as you have supported me.
To the many people who have taken a chance on me and
allowed me to prove myself, Drs. Richard Simonsen, Russell
Gilpatrick, and Rolf Behrents. Thank you for the
opportunity to further my education and enter into the
fields of dentistry and orthodontics. I will strive to make
you proud.
Lastly to the faculty of Saint Louis University, thank
you for teaching me the skills and knowledge that I will
build upon and will allow me to have a wonderful career and
bright future.
iii
ACKNOWLEDGEMENTS
This project would not have been possible without the
help from the following individuals:
Dr. Oliver. Thank you for your guidance and feedback
during the preparation of this project and for all you have
taught me both clinically and in the classroom.
Dr. Rolf Behrents. Thank you for allowing me the
opportunity to come to Saint Louis University and further
my education.
Dr. Ki Beom Kim. Thank you for your input and advice on
this project and for all that you have taught me in both in
the classroom and in the clinic.
Dr. Eustaquio Araujo. Thank you for all of your
contributions to my thesis and to my clinical education
Drs. Gene and Aron Dellinger. Thank you for entrusting me
with the data and study models relating to the
MagneTainer®™, this project would not exist without your
contributions.
Dr. Heidi Israel. Thank you for your assistance with
understanding the statistical analysis for this thesis.
iv
TABLE OF CONTENTS
List of Tables. . . . . . . . . . . . . . . . . . . . . vii
List of Figures. . . . . . . . . . . . . . . . . . . . viii
CHAPTER 1: INTRODUCTION. . . . . . . . . . . . . . . . . .1
CHAPTER 2: REVIEW OF THE LITERATURE
Magnetism and magnetic properties. . . . . . . . . ..4
Magnetic field and its properties. . . . . . . .4
Hard vs. soft magnets. . . . . . . . . . . . . .6
Magnetic properties of matter. . . . . . . . . . . .6
Types of magnets. . . . . . . . . . . . . . . . . . .7
Aluminum-nickel-cobalt magnets. . . . . . . . ..8
Ferrite magnets. . . . . . . . . . . . . . . . .8
Rare earth magnets. . . . . . . . . . . . . . ..9
Biological considerations. . . . . . . . . . . . . .11
Corrosion of rare earth magnets. . . . . . . ..11
Biological effects of magnetic fields. . . . . 14
Magnets in orthodontics. . . . . . . . . . . . . . .21
Magnetic brackets. . . . . . . . . . . . . . ..22
Space closure. . . . . . . . . . . . . . . . ..22
Molar distalization. . . . . . . . . . . . . ..23
Extrusion of teeth. . . . . . . . . . . . . . .24
Intrusion of teeth. . . . . . . . . . . . . . .24
Impacted teeth. . . . . . . . . . . . . . . . .25
Maxillary expansion. . . . . . . . . . . . . ..25
Open bite correction. . . . . . . . . . . . . .25
Magnetic functional appliances for Cl II. . . .27
Magnetic functional appliances for CL III. . . 29
Rare earth magnets for retention. . . . . . . .29
Retention. . . . . . . . . . . . . . . . . . . . . .30
Alteration of arch form. . . . . . . . . . . ..31
Periodontium and stability. . . . . . . . . .. . . 33
The role of lower incisor shape. . . . . . . ..33
The impact of pre-treatment malocclusion. . . .34
The effect of extractions on stability. . . . .35
Retainers. . . . . . . . . . . . . . . . . . . . . .36
Hawley retainers. . . . . . . . . . . . . . . .36
Vacuum-formed retainers. . . . . . . . . . . ..38
Bonded retainers. . . . . . . . . . . . . . . .39
Little’s Irregularity Index. . . . . . . . . . . . .43
Statement of thesis. . . . . . . . . . . . . . . . .44
Literature cited. . . . . . . . . . . . . . . . . . 46
v
CHAPTER 3: JOURNAL ARTICLE
Abstract. . . . . . . . . . . . . . . . . . . . . ..59
Introduction. . . . . . . . . . . . . . . . . . . ..61
Materials and methods. . . . . . . . . . . . . . . .63
Sample. . . . . . . . . . . . . . . . . . . . . . . 63
Technique for measurement of digital models. . . . .64
Error study. . . . . . . . . . . . . . . . . . . . .66
Statistical analysis. . . . . . . . . . . . . . . . 66
Results. . . . . . . . . . . . . . . . . . . . . . .67
Bonded retainer sample. . . . . . . . . . . . .67
MagneTainer®™ sample. . . . . . . . . . . . . .70
Findings between the samples. . . . . . . . . .74
Discussion. . . . . . . . . . . . . . . . . . . . ..75
Conclusions. . . . . . . . . . . . . . . . . . . . .76
Literature cited. . . . . . . . . . . . . . . . . ..78
Vita Auctoris. . . . . . . . . . . . . . . . . . . . . ..83
vi
LIST OF TABLES
Table 2.1 Development of different types of magnets . . .87
Table 3.1 Mean, median, standard deviation, and variance
for the bonded retainer sample. . . . . . . . .69
Table 3.2 Paired t-tests for significance of variables of
bonded retainer group. . . . . . . . . . . . ..70
Table 3.3 Pearson’s correlation for variables of bonded
retainer group. . . . . . . . . . . . . . . . .71
Table 3.4 Mean, median, standard deviation, and variance
for the MagneTainer®™ sample. . . . . . . . . .73
Table 3.5 Paired t-tests for significance of variables of
the MagneTainer®™ sample. . . . . . . . . . . .74
Table 3.6 Pearson’s correlation for variables of the
MagneTainer®™ sample. . . . . . . . . . . . . .75
Table 3.7 Paired t-tests for significant differences of
variables between bonded retainer and
MagneTainer®™ sample. . . . . . . . . . . . . .76
vii
LIST OF FIGURES
Figure 2.1
The MagneTainer®™ in place on a patient. .15
Figure 3.1
Little’s Irregularity Index and intercanine
width measured using OrthoAnalyzer®
software. . . . . . . . . . . . . . . . . 65
Figure 3.2
Little’s Irregularity Index and intercanine
width measured using OrthoCAD® software. .65
viii
Chapter 1: Introduction
One of the most difficult challenges in orthodontics
is retaining the teeth after orthodontic treatment has
ended. This problem is not new, indeed back in 1919 C.A.
Hawley, quoting a friend stated “If anyone would take my
cases when they are finished, retain them and be
responsible for them afterward, I would gladly give him
half the fee.1” No matter how good the finished result of
orthodontic treatment, the teeth tend to return to their
pre-treatment positions.
Many theories and ideas have been postulated as to why
this is the case. And while it still cannot be said with
certainty why teeth won’t remain in their post-treatment
location, we do know there are several factors that
contribute to non-stability of teeth. It is well known that
arch perimeter or length and intercanine width decrease as
a person ages.2,
3
This occurs whether or not orthodontic
treatment is initiated.4-6 In fact, some of these changes are
known to worsen in those who have undergone orthodontic
treatment versus untreated controls.6
1
Blake and Bibby pointed out many of the changes that
occur to the dental arches are part of normal human
development. As they discuss, arch width increases until
the permanent cuspids erupt, this is followed by a decrease
in intercanine width and arch length.7 The decrease in arch
length continues into the 20s, and for some individuals
into their 30s at which point it slows down.6 Intermolar
width is relatively stable from 13 to 20 years with a
reduction of the antero-posterior dimension of the mandible
over time. Incisor irregularity tends to increase during
the teenage years with a more pronounced degree of
irregularity seen in females.7
Nowhere in the dentition is instability more
pronounced than the mandibular incisors. Studies have shown
that 40% to 90% of individuals have unacceptable dental
alignment of the lower incisors ten years after orthodontic
treatment.6,
8-10
These findings led Little to conclude that
relapse of the mandibular incisors is almost inevitable,
regardless of when orthodontic treatment is initiated and
the technique employed.6,
8, 9
Little also stated that the
only predictable way to ensure stability of orthodontic
treatment would be lifetime retainer wear.11
2
Recent trends in orthodontics are towards more bonded
retainers and fewer Hawley retainers, especially in the
mandibular arch.12 This allows for the orthodontist to
eliminate patient compliance and to increase the length of
the retention phase of treatment.
A new retentive appliance known as the MagneTainer®™
is being developed by Gene and Aron Dellinger of Fort Wayne
Indiana. It is a fixed, magnetic retainer made from several
(10 total) neodymium iron boron magnets bonded from the
mesial of the lower canine to the mesial of the
contralateral canine (see figure 1). This unique design
allows the retainer to function as a bonded retainer with
the added ability to floss the teeth between the magnets.
The purpose of this study is to determine the efficacy of
the MagneTainer®™ magnetic retainer as compared to other
canine-to-canine bonded retainers that are bonded to each
tooth.
Figure 1.1 The MagneTainer®™ in place on a
patient. OSS Orthodontics, Fort Wayne Indiana
3
Chapter 2: Review of the Literature
Magnetism and Magnetic Properties
While it is uncertain as to exactly when and where
magnetism was discovered, we do know that the ancient
Romans, Greeks, and Chinese were aware of magnetism.13
Magnetism is a fundamental force of nature and is closely
related to electricity. Magnetism is due to magnetic fields
produced either by an electric current, or due to the
natural alignment of charged particles within an element or
substance.14
The effects of magnetic fields have been
studied and utilized for centuries. Recently, magnets have
been incorporated into orthodontic appliances.
Magnetic Field and its Properties
The area around a magnet where its magnetic force can
be felt and measured is known as the magnetic field.
Magnetic fields are concentrated on the ends or poles of
the magnet, with each pole exhibiting an opposite direction
of magnetic force. Like poles will repel one another and
opposite poles will attract one another. The stronger a
magnet is, the larger its magnetic field will be.15
4
The intensity of a magnet’s magnetic field is called
its flux density or field density. Flux density is measured
in units of Gauss (G), Tesla (T) and Millitesla (mT).15
Magnets are susceptible to being demagnetized if a
strong enough magnetic field is applied to them. The
strength of an external magnetic field needed to
demagnetize a magnet is known as coercivity. Permanent
magnets have a high coercivity.15
For all magnets, the amount of force between two poles
is proportional to their magnitudes and inversely
proportional to the square of the distance between the
poles. This is known as Coulomb’s Law. Coulomb’s law must
be considered when selecting a magnet to be used in an
orthodontic appliance so that sufficient force is
maintained through all distances that the appliance will
function under.15
All magnets are susceptible to losing their magnetism
if exposed to a certain temperature that varies depending
on the chemical composition and fabrication method of the
magnet. This temperature is known as the Curie point.15
Curie point is a critical consideration for a magnet to be
used intraorally as some magnets have Curie points below
body temperature. Any appliance to be used intraorally
5
should also have the ability to be sterilized so a magnet
with a high Curie point will be required.
Hard vs. Soft magnets
Hard magnets are resistant to demagnetization in the
presence of a magnetic field that is stronger than its own
magnetic field. Hard magnets also stay magnetic at a size
of 1mm or less. Soft magnets possess properties that are
opposite to hard magnets.15
Magnetic Properties of Matter
Magnetic materials fall into three broad categories
based on their response to an externally applied magnetic
field. They are termed diamagnetic, paramagnetic, and
ferromagnetic.13,
14, 16
In diamagnetic materials, the magnetic moment that is
induced when a magnetic field is applied is in the opposite
direction of the applied magnetic field. This occurs as the
atoms in the material respond to the applied magnetic
field. Diamagnetic materials have a repulsive effect that
is weak in strength and demonstrate no permanent magnetism.
The repulsive moment induced by an applied magnetic field
disappears when the applied magnetic field is removed.
Common diamagnetic materials are metallic bismuth and many
6
organic molecules that have a cyclic structure, enabling
electric currents to flow through them.13,
14, 16
Paramagnetic materials are weakly attracted to an
applied magnetic field. The molecules in a paramagnetic
material all line up in the same direction as the applied
magnetic field and a small attractive force results.
Substances that contain an unpaired electron will
demonstrate paramagnetism. Paramagnetism is temporary and
exists only as long as an external magnetic field is
applied. Iron salts and many rare-earth salts demonstrate
paramagnetism.
13, 14, 16
Ferromagnetic materials are strongly attracted to an
applied magnetic field. This is due to the atoms in the
material lining up in a parallel fashion when a magnetic
field is applied. This alignment will persist after the
magnetic field is removed. Common ferromagnetic materials
include iron, nickel, cobalt, chromium dioxide and alloys
containing these elements.
13, 14, 16
Types of Magnets
Permanent magnets are all made from ferromagnetic
materials and all have a persistent magnetic field. The
properties of individual magnets are related to their
7
composition and fabrication.16 Table 1 shows the development
of several different types of magnets.
Table 2.1 development of
modified from Ravindran15
different
Year of development
1910
1925
1935
1945
1950
1955
1965
1985
1993
types
of
magnets,
Tungsten-steel
Cobalt-steel
Aluminum-nickel
Aluminum-nickel-cobalt
Aluminum-cobalt
Ferrite
Samarium-cobalt
Neodymium-iron-boron
Samarium-iron-nitride
Aluminum-Nickel-Cobalt Magnets (Alnico)
Alnico magnets were the first type of magnets used in
biomedicine. Alnico magnets are made from cobalt, aluminum,
nickel, and iron.16,
17
These magnets are available in many
shapes and sizes and are widely available. Alnico magnets
have several drawbacks that prevent them from being used in
orthodontics. Alnico magnets are expensive, prone to
demagnetization, and are not very strong for their size.15,
16
Ferrite Magnets
Ferrite magnets are magnets containing salts of iron.
Common ferrite magnets are iron-barium and iron-strontium.
8
Ferrite magnets are highly resistant to demagnetization and
corrosion and are very inexpensive to produce. Ferrite
magnets are among the most common magnets produced and have
been used in many industries but are not used in
orthodontics due to size constraints.15,
16
Rare Earth Magnets
Rare earth magnets, so named because they contain
elements from the lanthanide series of the periodic table
are the most recently developed magnets. Rare earth magnets
exhibit a property known as magnetocrystalline anisotropy.18
Magnetocrystalline anisotropy means that as the magnet is
produced, it is made up of a single crystal that is aligned
in the same direction.15,
16
This allows for much stronger
magnetism. Due to magnetocrystalline anisotropy, rare earth
magnets are up 20 times stronger than previously developed
magnets.15 This increase is magnetic strength allows for
much smaller magnets to produce higher levels of force than
was previously possible. Incorporation of rare earth metals
also increases the Curie temperature of the magnet.15
Clinically, this allows for rare earth magnets to be
sterilized and still retain their magnetic properties.
Development of rare earth magnets has allowed for many
orthodontic appliances that incorporate these magnets to be
9
produced. Rare earth magnets currently exist in three main
forms: Samarium-cobalt, Neodymium-iron-boron, and most
recently, Samarium-iron-nitride.16,
19
Samarium-cobalt(Sm-Co) magnets were first developed in
the 1960s. These magnets are very strong and have a Curie
point of 600˚C. These magnets are costly to produce and are
very brittle.16,
17
Neodymium-iron-boron (Nd-Fe-B) magnets are stronger,
much less expensive to produce, and less brittle than
Samarium-Cobalt magnets. The main disadvantages of
neodymium-iron-boron magnets are a lower Curie point (from
100˚C to 200˚C depending on the grade of the magnet) than
Sm-Co magnets and a very low resistance to corrosion.15,
20, 21
16,
For intraoral use, Nd-Fe-B magnets must be coated. Even
with these drawbacks, Nd-Fe-B magnets are still the most
widely used magnet in orthodontics.
Samarium-iron-nitride (Sm-Fe-N) magnets are the most
recently developed rare earth magnet and offer many
qualities that make them attractive for use in
orthodontics. Sm-Fe-N magnets are very strong, have higher
resistance to corrosion than other rare earth magnets, and
have a higher Curie point (700˚C to 800˚C depending on
grade) than other rare earth magnets.16
10
Biological Considerations
Prior to any material being introduced into clinical
practice, it must be studied for effectiveness, and more
importantly for safety. The side effects of the material
and its components must be evaluated to determine if they
can be safely used intra-orally. All dental materials
should be studied to determine if they will produce side
effects at either local or systemic levels. Testing should
also be done to determine if any toxic, allergic, or
carcinogenic side effects exist.22
Corrosion of Rare Earth Magnets
One of the main drawbacks to the use of rare earth
magnets, particularly those containing neodymium is their
tendency to corrode when placed in aqueous environments.22,
23
Various studies have found that while Ne-Fe-B magnets are
more prone to corrosion, their corrosion byproducts are
less cytotoxic than other rare earth magnets, particularly
those containing cobalt.22-25 Neodymium-containing magnets
corrode via oxidation of neodymium rich grains resulting in
the release of oxides of neodymium.21
In their study, Bondemark, Kurol, and Wennberg used
two different methods to study cytotoxicity of new,
11
clinically used, and recycled magnets Sm-Co magnets. The
methods used were a Millipore filter method and an
extraction method. The Millipore filter method found that
the corrosion products of Sm-Co magnets were mildly
cytotoxic with only one magnet showing any cytotoxicity.23
The extraction experiment yielded similar results,
showing mild cytotoxicity.23 The authors also found that
cytotoxicity reduced upon retesting of new magnets and
after the magnets had been used clinically.
These findings
suggest that most corrosion occurs during the initial
placement of the magnets intra-orally.23
In another study using the same testing techniques, it
was shown that different types of rare earth magnets
demonstrated different levels of toxicity. They found that
SmCo5 magnets were highly cytotoxic while Sm2Co17 magnets
were only moderately cytotoxic. Ne2Fe14B magnets were also
tested and found to have negligible cytotoxicity.25
To overcome the corrosive nature of rare earth
magnets, several authors have recommended that the magnets
either be coated, or hermetically sealed in a protective
jacket to isolate the magnets from the intra-oral
environment.16,
20, 22-26
12
Various coating materials have been compared. Coatings
made from nickel and from chromium have been tested and
while both reduced corrosion of rare earth magnets, the
nickel itself was found be by cytotoxic.26 Tsutsui et al.
concluded that a chromium coating was far superior as it is
biologically compatible.26
Acrylic has also been studied as a coating for rare
earth magnets. When corrosion of magnets coated with
acrylic were compared to uncoated magnets it was found that
although an acrylic coating served as a good barrier to
corrosion, corrosion was still seen and a better coating is
needed.24
Parylene coating has been shown to reduce the
cytotoxicity of rare earth magnets to negligible levels.25
as mentioned previously, most cytotoxic corrosion occurs
very quickly after being exposed to an aqueous environment.
It has been suggested that recycling previously used
magnets, or soaking new magnets in water for twenty-four
hours prior to use may be a good way to reduce
cytotoxicity.23
When magnets coated with parylene were compared to
magnets coated with polytetrafluoroethylene (PTFE) in a
simulated intra-oral environment, it was found that both
13
parylene and PTFE provided protection against corrosion
when compared to controls with no significant difference
between the coatings.27
It two separate studies, Wilson et al. found that
biofilm accumulation played a role in the corrosion of
magnets. They demonstrated that the both the biofilms
themselves, and the metabolic byproducts of the bacteria in
the biofilms increased corrosion of rare earth magnets.20,
21
Biological Effects of Magnetic Fields
A large number of studies have been conducted to
determine the effects of magnetic fields on living tissue
with results ranging from little to no effect28-37, to having
negative effects.38-40
In a split mouth study, Bondemark, Kurol, and Larsson
examined the effects of static magnetic fields on human
pulp and gingival tissue. They found no change in the
dental pulp or in the gingival tissue adjacent to magnets.28
In another split-mouth study, tissue response to space
closure was examined comparing coil springs to magnets. No
significant differences in the rate of space closure, bone
formation, or epithelial thickness were shown.29 This study
did show significant differences in bony resorption and
14
tetracycline uptake when compared to the control side.
These findings suggest increased metabolic activity. While
these findings were statistically significant, the authors
suggest they were transitory and had negligible clinical
side effects.29
Sato et al. studied human cultured cells (HeLa cells
and human fibroblast cells) to determine the effects of
static magnetic fields on these cells. They grew these
cultured cells in the presence of a non-homogenous static
magnetic field and observed their growth, morphology, and
later DNA synthesis and content. They demonstrated that
static magnetic fields had no effect on cell morphology,
cell growth, DNA content, or DNA synthesis.31
In another study of the effect of magnetic field, five
titanium caps were implanted into the mandibles of five
dogs. Three of the caps contained rare earth magnets and
two were empty. The caps were left in place for six months
and then the animals were sacrificed and the mandibles
sectioned and examined microscopically for any changes. No
pathological changes were reported when the sites with
magnets compared to control sites.32
In another study performed on dogs, magnetic
appliances were left intraorally for six months. After six
15
months, no evidence of tissue damage was found either
clinically or upon microscopic examination.33
Esformes, Kummer, and Livelli grew tissue cultures of
several cells lines and also did an in-vivo study of wound
and bone healing in rats. They found that magnetic fields
from rare earth magnets had no effects on cell growth rate,
morphology, or their ability to grow. They also found no
negative effects on bone or wound healing.35 The authors
were careful to state that this was a short-term study and
that long-term effects of magnetic fields still need to be
investigated.35
Darendeliler, Sinclair, and Kusy performed a study on
tooth movement comparing both pulsed and static magnetic
fields to move incisors of guinea pigs. They found that
both the pulsed and static magnetic fields led to an
increase in the rate of tooth movement.34 They also found
bony wound healing and repair increased in the presence of
both pulsed and static magnetic fields, this led the
investigators to conclude that the rate of tooth movement
could be increased by magnetic fields by altering bone
metabolism.34
Effects magnetic fields generated by rare earth
magnets on cultured osteoblasts have also been studied. The
16
osteoblasts were exposed to magnetic fields of attracting
magnets as well as magnetic fields from repelling magnets.
After exposing the cells to the magnetic fields for 21
days, the cells were microscopically examined. No
difference in osteoblast activity was noted for any of the
groups.36
Camilleri and McDonald studied the effects of intense
magnetic fields (4,100G) produced by Ne-Fe-B magnets on the
sagittal suture of rats. This study demonstrated that
mitotic activity in the sagittal suture decreased for a
brief period with a tendency to return back to normal.41
An in vitro study of human fibroblasts found that
fibroblasts grown in the presence of magnetic fields showed
significantly impaired attachment and growth when compared
to controls.38 The authors of the study did note that the
observed deleterious effects could have been due to
corrosion byproducts of the magnets as no steps were taken
to limit corrosion.38
Linder-Aronson, Lindskog, and Rygh designed a study
done to determine the effects of static magnetic fields on
gingival epithelium and alveolar bone in monkeys.39 In their
study, splints were fabricated that contained SmCo rare
earth magnets, for this study, the magnets were completely
17
imbedded in acrylic. The splints were left in place for
fourteen weeks. Tetracycline was injected at time of splint
placement, and at four weeks. At the end of their
experiment, epithelium adjacent to the magnets was examined
microscopically. Alveolar bone adjacent to the magnets was
also examined microscopically. The authors found that the
epithelium was morphologically normal but was on average
two cell layers thinner when compared to controls.39 Patchy
tetracycline uptake was also noted, suggesting either a
decrease in osteoblast activity or an increase in
osteoclast activity. As with previous studies, the authors
did note that they could not rule out the possibility of
corrosion byproducts being responsible for the changes seen
in their experiment.39 It is likely that corrosion did occur
due to the choice of acrylic to limit corrosion. As
Vardimon and Mueller showed, acrylic is an inadequate
barrier to corrosion of SmCo magnets.24
The same authors performed another study, this time
looking at bone surface and skin reactions to magnetic
fields produced by rare earth magnets. For this study,
rings containing rare earth magnets were fabricated and
placed externally around the tibias of rats. The rings were
left in place for 2, 3, and 4 weeks after which the animals
18
were sacrificed and the tissue and bone adjacent to the
magnets were examined. In this study, a reduction in the
number of epithelial cells was seen as was a stimulation of
bone-lining osteoblasts.40 This study was highly criticized
by Blechman for several reasons.37 Blechman pointed out that
in the study, the magnetic device they used did not
simulate any orthodontic appliance or application. The
device was constructed in a way that magnets were straddled
both sides of the bone, this situation would not be
encountered in an orthodontic setting. The device was also
placed in a way that could have led to pressure ischemia
which could explain the observed negative effects.37
DeVincenzo agreed that pressure to the underlying
epithelium may have been responsible for the findings of
the study.42 DeVincenzo also criticized the conclusions of
Linder-Aronson and Lindskog stating that skin and bone
changes in rat tibias due not necessarily apply to oral
mucosa.42
Linder-Aronson, Rygh, and Lindskog did do a follow-up
study with that same parameters but adding a group were the
magnets were removed after eight weeks and the sacrificed
eleven weeks after the rings were removed.30 This study was
done to confirm the results of the first study and to
19
determine if the observed negative effects were reversible.
The same changes to epithelium and surface bone were noted
as in the first study. It was also determined that these
effects were reversible. They concluded that side effects
from rare earth magnets were negligible and reversible.30
After review of the literature regarding the safety of
magnetic fields for orthodontic use, the majority of
researchers have concluded that magnetic fields either
produce negligible side effects or no side effects at all.
28-37
In the studies where negative side effects were noted,
other possible reasons for these negative effects could not
be ruled out. Cytotoxicity due to corrosion of the magnets
was noted as a possible reason for the negative effects in
several studies.38,
39
Poor design of the testing apparatus
was also cited as a reason for observed negative effects.37,
42
After many studies, and many years of experience
working with magnetic fields, Blechman concluded:
“to my knowledge there has not been a single
reported and documented adverse clinical effect
because of these specifically therapeutic fields.”37
20
A search of the literature agrees with Blechman’s assertion
that magnetic fields are safe and can be utilized for
clinical benefit.
Magnets in Orthodontics
Magnets were first use in dentistry in 1952. These
magnets were implanted in the jaws to aid in retention of
full dentures.43 Magnets were also used to aid in retention
of maxillofacial prosthesis.44 Magnets were first used in
orthodontics by Blechman and Smiley who attached magnets to
the teeth of cats and proved that teeth could be moved via
magnetic forces.19
Darendeliler et al. found that pulsed electromagnetic
fields could enhance the effects of both mechanical and
magnetic forces on tooth movement.45 In another study,
Darendeliler, Sinclair, and Kusy found that pulsed
electromagnetic fields led to increased boney healing and
the elimination of the lag phase of tooth movement.34 This
led them to suggest that under ideal conditions, the
maximum rate of tooth movement could be increased from 1mm
per month to up to 3mm per month.34
Since the introduction of magnets into the field of
orthodontics, magnets have been used in a wide variety of
21
appliances. Magnets have been used both to orthodontically
move teeth,46-49 and for orthopedic correction.50-52
Magnetic orthodontic appliances have, in theory,
several advantages over traditional appliances. Magnetic
appliances reduce patient cooperation, exert continuous
forces that do not decay over time, have reduced friction,
exert predictable force levels, and can exert force through
bone and mucosa.15,
16, 22
Magnetic Brackets
Magnetic brackets were first designed by Kawata,
Okada, and Horisaka.53 This design first used iron-cobalt
magnets which were later replaced by chromium coated rare
earth magnets.54 Kawata et al. found that these magnets
could be used for mesial and distal movement of teeth as
long as the interbracket distance was 3mm or less.54 While
magnetic brackets were shown to effectively move teeth,
this method of tooth movement has not gained wide
acceptance.
Space Closure
Blechman and Abraham demonstrated that rare earth
magnets could be used to successfully achieve both
intermaxillary and intramaxillary mechanics.47 Muller
22
reported success with using magnets to close a midline
diastema without using archwires.55 Muller also suggested
that magnets could be used for more complicated mechanics
such as rotational correction, tooth uprighting, and bodily
movement.55
Molar Distalization
Gianelly et al. were the first to use repelling
magnets to successfully distalize first molars while the
second molars were unerupted. Their appliance was a
modified Nance appliance that incorporated magnets.49 They
were able to distalize the first molars 3mm in seven weeks
with 1mm of anterior anchorage loss.49 This design has been
duplicated with similar results.48
Attempts have also been made to distalize both first
and second molars using rare earth magnets. The appliance
was successful but not without drawbacks. Tipping and
rotation of the molars was noted as was proclination of the
upper incisors.56
Bondemark, Kurol, and Bernhold compared nickel
titanium springs to magnets for distalization of first and
second molars.57 It was found that while the magnets were
able to distalize molars, the coil springs achieved greater
23
amount of distalization with more consistent force levels
and less loss of force. They concluded that nickel-titanium
springs were a more efficient means of distalizing
maxillary first and second molars.57
Extrusion of Teeth
McCord and Harvie used attractive magnets to extrude a
damaged incisor to aid in its eruption.58 Bondemark et al.
also used rare earth magnets to extrude teeth where the
crowns had fractured.59 They reported success in extrusion
of the remaining portion of the tooth with no soft tissue
dehiscence, root mobility, or root resorption. They also
found no coronal bone loss or need to reshape the bone
prior to a final restoration being placed.59
Intrusion of Teeth
Magnetic appliances have also been combined with a
corticotomy to intrude posterior teeth that had
supererupted.60 The supererupted teeth were able to be
intruded without extruding the adjacent teeth. Similarly,
rare earth magnets encased in acrylic bite blocks have been
used to intrude posterior teeth.61
24
Impacted Teeth
Magnetic traction of impacted teeth was first reported
by Sandler, et al. An impacted canine was surgically
exposed and a magnet was bonded to the tooth surface. A
second magnet was embedded in a removable acrylic
appliance.62 Darendeliler, and Friedli used a similarly
designed appliance to erupt an impacted canine as well.63
Magnetic traction of impacted canines offers many
advantages over traditional mechanics. No hooks or elastics
need to be used, fewer adjustments are needed, and reactivation is not necessary.16
Maxillary Expansion
Vardimon et al. were the first researchers to attempt
using magnets to expand the maxilla in monkeys.64
Darendeliler, Strahm, and Joho also used magnetic
appliances for maxillary expansion. They found that
magnetic appliances could produce the required 250-500
grams of force needed to produce both dental and skeletal
expansion.65
Open Bite Correction
Dellinger was the first to introduce a magnetic
appliance to correction anterior open bites via posterior
25
intrusion. Dellinger termed his appliance the Active
Vertical Corrector (AVC).66 For this appliance, Dellinger
used four pairs of Sm-Co repelling magnets imbedded in a
removable acrylic plate to intrude upper and lower molars.66
The design of the appliance was updated and now uses Ne-FeB magnets to deliver less force using much smaller
magnets.67 This device is appealing as it allows for a nonsurgical approach to treat anterior open bites. As the
molars are intruded, the mandible is allowed to rotate
forward and lower anterior facial height is reduced.66,
67
Darendeliler, Yuksel, and Meral developed an appliance
they named the magnetic activator device IV (MAD IV).68 This
appliance uses Ne-Fe-B magnets. Attractive magnets are
placed at the anterior of the appliance to guide and
position the mandible forward. At the same time, repulsive
magnets are used to intrude the posterior teeth. The MAD IV
appliance is available in three configurations, each
modified to treat a different type of malocclusion.68
When the dental and skeletal effects of treatment with
the MAD IV appliance were examined, the upper incisors
became more upright, the lower incisors become more
proclined, and the posterior teeth intruded.55 Skeletally,
forward rotation of the mandible which led to a decreased
26
mandibular plane angle, decreased lower anterior facial
height, and a decrease in the ANB angle were seen.55
Woods and Nanda have also shown that posterior teeth
can be intruded utilizing magnetic appliances.69 Similar to
previous studies, their appliance used magnets embedded in
acrylic bite plates.69
Magnetic Functional Appliances for Class II Correction
Magnetic Class II functional appliances are designed
to work in the same manner as traditional functional
appliances, by positioning the mandible to a more
favorable, forward position. Magnetic appliances differ in
that they use magnetic force to reposition the mandible to
a new rest position.51,
70
Vardimon, et al. developed an appliance called the
Functional Orthopedic Magnetic Appliance (FOMA) II. This
appliance uses attractive magnets positioned anteriorly to
position the mandible more forward.52
Another functional magnetic appliance was developed by
Darendeliler, Chiarini, and Joho. This appliance is known
as the magnetic activator device (MAD).51 The MAD is
available is several varieties, the MAD I for lateral
displacement51, the MAD II for Class II correction51,
27
71, 72
,
the MAD III for Class III correction73, and as previously
discussed, the MAD IV for open bite correction68.
A retrospective study on an appliance known as the
Functional Magnetic System (FMS) was done by Vardimon et
al.74 They found that the Class II correction achieved by
the appliance varied from region to region with more
correction found in the incisors versus the molars.74 Half
of the molar relationship was corrected skeletally while
the other half was corrected dentally, whereas in the
incisor region one third of the correction was skeletal
with the remaining two thirds dental.74 An increase in lower
facial height was seen and was attributed mainly to the
eruption of the lower molars. The authors also found that
the net increase in Ar-Gn was larger for the FMS than the
other groups, this was postulated to be due to the magnets
providing a longer time in which the mandible was postured
forward as compared to other functional appliances.74
When examining the effects that magnetic functional
appliances had on the dentofacial complex it was found that
on average, mandibular length increased 3.2mm, the angle of
facial convexity decreased 2.8°, the upper and lower teeth
intruded an average of 1.5 mm each, and the mandibular
plane angle decreased 1.3°. During a follow-up period, the
28
authors found some relapse due to eruption, but all other
changes were found to be stable.75
Magnetic Functional Appliances for Class III
Correction
Darendeliler, Chiarini, and Joho developed the
Mandibular Activator Device (MAD) III to achieve functional
orthopedic correction of Class III malocclusion.73 This
device can be used with or without a reverse-pull facemask
and maxillary expander.
Vardimon et al. created the Functional Orthopedic
Magnetic Appliance (FOMA) III to treat patients with Class
III malocclusion and a midface deficiency.50 This appliance
was shown to achieve midface protraction as well as
advancement of the upper incisors and molars.50
Rare Earth Magnets Used for Retention
The first example of a magnetic retainer is from a
case report from 1999 when Springate and Sandler used
neodymium-iron-boron magnets bonded to a patient’s teeth to
hold a midline diastema closed.22 Others have used rare
earth magnets not as retainers but to aid in the placement
of bonded retainers.76-78
29
Retention
Retention is the final phase of orthodontic treatment.
Retention has been defined by Moyers as “the holding of the
teeth following orthodontic treatment in the treated
position for the period of time necessary for the
maintenance of the result.79” Reidel gave a more brief
definition of retention: “the holding of teeth in ideal
esthetic and functional position.80” No matter the
definition, retention is a difficult and often trying
aspect of orthodontics, indeed, Oppenheim referred to
retention not just as a difficult problem in orthodontics
but as the problem in orthodontics.81
Pratt et al. stated that there are two phases to
orthodontic retention which they termed the retention
phase, and the post-retention phase.12
The first phase of retention, the “retention phase” is
the time in which the teeth are held in their posttreatment position during which the periodontium remodels.10
On average, the periodontal ligament takes three to four
months to remodel, the collagen-fiber network takes four to
six months to remodel, and the supracrestal fibers take
about 12 months to completely remodel.82,
83
Thus, this first
stage of retention should last at least one year.12
30
The second phase of retention, the “post-retention
phase” starts once the remodeling of the periodontium is
complete and, according to Pratt et al. lasts the rest of
the patient’s life.12
Alteration of Arch Form
Most studies of archform and arch perimeter show that
both should be maintained rather than altered as both have
a tendency to return toward pre-treatment dimensions.84,
85
This is especially true of mandibular intermolar and
intercanine widths.8,
86-88
There are some studies that have shown stability
following modification of arch form and width. Mills showed
that alteration of the lower archform via incisor
proclination
was stable in patients whose incisors were
initially retroclined, had deep bites, and who also had a
digit or lip entrapment habit.89 Similarly, Artun said that
proclination could be stable in cases where the incisors
were initially retroclined and the etiology of the
retroclination could be corrected.90 Studies have also been
done that suggest that arch expansion may be more stable in
patients with Class II div. 2 malocclusion than Class I and
Class II div. 1 malocclusions.91,
92
Blake and Bibby point
out that the sample size of these studies was very small
31
and the results may not be reliable.7 This is supported by
Little who showed that relapse of both intermolar and
intercanine width were independent of initial malocclusion.8
Haas, and later, Sandstrom and Klapper demonstrated
that alteration of arch width via expansion may be stable
in some instances. They both had similar result results
which showed maintenance of three to four millimeters of
mandibular intercanine width and up to six millimeters of
mandibular intermolar width when this change in mandibular
arch width was accompanied by rapid palatal expansion.93,
94
As Blake and Bibby pointed out however, both of these
studies had flaws. Both were short-term studies with small
sample sizes.7
Moussa, O’Reilly, and Close who had a larger sample
size, and an 8 year post-retention follow-up showed good
stability of upper intercanine and upper and lower
intermolar width expansion when this expansion was done in
conjunction with rapid palatal expansion. Their study also
showed that stability of mandibular intercanine expansion
was poor and tended to relapse.95
32
Periodontium and Stability
As was mentioned previously, the periodontium plays a
significant role in relapse and stability. This is
especially true rotational relapse.96,
97
Because the
periodontium can take up to a year to completely remodel,
many have suggested a surgical procedure to sever the
gingival fibers immediately following orthodontic
treatment. This procedure is known as circumferential
supracrestal fiberotomy.98,
99
Studies done on the effects of circumferential
supracrestal fiberotomy have shown a significant decrease
in incisal irregularity at six, nine, and fourteen years
post-treatment with no significant loss of attachment or
other periodontal issues.100-103
The Role of Lower Incisor Shape
Peck and Peck suggested that relapse of the lower
incisors was due, at least in part to the ratio between the
mesiodistal and faciolingual height and width of the lower
incisors and suggested altering the dimensions of the lower
incisors to correct this ratio.104
Peck and Peck’s work has been criticized for using
untreated rather than treated cases. It has also been noted
33
that the cases in Peck and Peck’s study were not followed
long-term these cases would likely have shown crowding if
followed for a longer period of time.7 A long term study
comparing treated to untreated cases demonstrated a very
weak association between irregularity and
faciolingual/mesiodistal ratios.105 Other studies have not
only confirmed this, but have shown that less than six
percent of crowding can be explained by this ratio.106-109
The Impact of Pre-treatment Occlusion
Although some studies have shown that Class II div. 2
cases have shown to be more stable,91,
92
most studies do not
support this finding and show that relapse is largely
independent of Angle’s classification.
110-112
Deep bite relapse has been shown to be dependent on
the amount of bite opening achieved during treatment. On
average 30% to 50% of deep bite correction is stable.8,
114
Open bites are difficult to retain with successful
stability being shown only 60% of the time.115
Uhde, Sadowsky, and Begole found that 41% of late
lower incisor crowding could be explained by changes to
overjet, overbite, intercanine width, and intermolar
width.113 These same authors also found that the highest
34
113,
single factor related to late mandibular crowding was
change to intermolar width which was shown to contribute to
12.5% of post-treatment relapse.113
The Effect of Extractions on Stability
Little et al. studied cases in which patients had
first premolars extracted as part of their treatment plan.
These cases were examined at least ten years posttreatment. Changes in arch length, intercanine width or
length of treatment were found to have no association to
prediction of post-treatment crowding.6,
8
In this same
study, Little et al. defined the overall success of
stability of treatment as having an Irregularity Index of
3.5mm or less and found a success rate less than 30% with
20% showing severe relapse.6 When this same sample was
studied to attempt to find an association or prediction
between dental cast measures and cephalometric measures, no
association or prediction could be found.116
Studies of serial extractions in which no other
treatment was done have shown varied results. In one study,
patients who had serial extractions and no other treatment
showed in increase in post-treatment irregularity at 10
years.9 In another study which looked at patients 20 years
35
post-treatment, relapse of crowding was seen but to a
lesser extent than untreated controls.117
Similar results have also been shown when serial
extractions are combined with later orthodontic treatment.
Anterior relapse as high as 73% has been reported.9 Relapse
is seen regardless of the serial extraction pattern. No
difference in anterior relapse was seen whether the first
or second premolars were extracted.118
Retainers
Orthodontists today have many choices when it comes to
choosing retainers for their patients. The main types of
retainers are Hawley retainers, vacuum-formed retainers,
and a variety of bonded retainers. According to a recent
survey of members of the American Association of
Orthodontics, Hawley retainers are the most commonly used,
the next most common retainers are vacuum-formed retainers,
and the least common retainers are bonded retainers.12 This
survey also noted a trend towards less Hawley retainers and
more vacuum-formed and bonded retainer use.12
Hawley Retainers
Hawley retainers are available in a wide variety of
designs but generally consist of an acrylic plate that
36
rests behind the teeth and contacts the lingual surface of
the teeth and the gingiva and a labial wire that contacts
the facial surface from canine to canine. Hawley retainers
are a popular choice among orthodontists due to their
adjustability and durability. Patients have shown to be
more compliant to wearing Hawley retainers long-term
compared to other kinds of retainers.119 The main drawbacks
to Hawley retainers are fabrication time, cost and
compliance in wearing.
Hawley retainers are often chosen for their
durability, with one author stating that Hawley retainers
have lasted up to fifteen years in his practice.2
Many studies have demonstrated that Hawley retainers
also offer the advantage of allowing the posterior teeth to
settle which often results in more favorable
intercuspation, better occlusal contacts, and improved
marginal ridge alignment.12,
120, 121
It is important to note
that this settling will not occur where the wire of the
retainer crosses over the occlusion. If settling of the
posterior teeth after treatment is desired, and a Hawley
retainer is chosen, the retainer must be designed in a way
to allow settling to occur.
37
Many studies have shown that Hawley retainers are not
as effective in holding the anterior teeth in their posttreatment position as vacuum-formed retainers, especially
the lower anterior teeth.122-124 Other studies have shown no
statistically significant differences between effectiveness
of Hawley retainers compared to vacuum-formed retainers.122,
125
These findings have led many clinicians to alter their
retention protocols in favor of less Hawley use, especially
for the mandibular teeth.12
Vacuum-formed Retainers
Vacuum-formed retainers are made from clear plastic
that is warmed and vacuum-formed onto a dental cast. These
retainers are often chosen because they are less noticeable
and cover the entire surface of the teeth. Vacuum formed
retainers can be fabricated quickly and inexpensively but
these retainers are not adjustable and are not as durable
as other retainers.
As previously mentioned, vacuum-formed retainers have
been found by many studies to be more effective at
retaining the lower teeth.122-124 Many clinicians now use
either vacuum-formed retainers on both arches or in the
lower arch with a Hawley retainer on the upper arch.
Because vacuum-formed retainers cover all surfaces of the
38
teeth, settling of the occlusion is prevented. To overcome
this, Lindauer and Shoff suggested modifying the lower
vacuum-formed retainer to cover only canines and
incisors.122 This design maximizes the strengths of the
vacuum-formed retainer in holding the anterior teeth, but
still allows for posterior settling to occur.
Bonded Retainers
Bonded retainers are made from either one wire or
several wires braided or twisted together. In a study done
by Baysal et al., three different wires were compared (an
0.0215” five-stranded wire, 0.016 x 0.022” eight-stranded
dead-soft braided wire, and 0.0195” dead-soft coaxial wire)
to determine the performance of each of the wires. They
concluded that while detachment force was similar for all
wires, the dead soft wires showed more deformations. They
also found that pull out force was significantly higher for
the five-stranded coaxial wires than for the other wires.
They concluded that of the wires tested, the five-stranded
coaxial wire was the best choice for bonded lingual
retainers.126 Renkema et al. studied the effectiveness of a
0.0195,” 3 stranded wire (Wildcat wire, GAC International,
Bohemia, NY) and found it to be an effective retainer.127
39
For bonded retainers, once the wire is chosen, the
wire is bonded to the lingual surface of the teeth holding
them in place. Special attention must be taken during the
bonding procedure as this has shown to be very technique
sensitive and the long-term survival of the retainer is
dependent on good initial placement and bond strength.
Studies have shown that most failures occur in the first
six months after retainer placement.128 The overall failure
rate varies depending on how many of the teeth the retainer
is bonded to and the experience of the practitioner placing
the retainer. Overall failure rates range from around 35%128,
129
to as high as 46%.130 Scheibe and Ruf found that while the
failure rate varies with time, overall orthodontists had a
failure rate of 40.3% while graduate students had a failure
rate of 40.3%129 Their results were corroborated by Segner
and Heinrici131
Bonded retainers are most commonly used on the lower
arch and can be bonded to just the lower canines or to all
six anterior teeth. Several considerations must be taken
into account when making the decision of how many teeth the
retainer should be bonded to. Bond failure and
effectiveness of the retainer are the two most important
factors to consider.
40
Bond failure of lower bonded retainers has shown to be
higher when the retainer is bonded only to the canines.129,
132, 133
Although bond failure is higher when the retainer is
bonded only to the cuspids, this may be beneficial because
the patient will be more sensitive to bond failures. If the
retainer is bonded to all anterior teeth, a bond failure at
just one tooth may go unnoticed by the patient leading to a
delay in repair an increase in the potential for relapse134.
While retainers bonded only to the canines are
effective at retaining the lower anterior teeth, some
studies have shown an increase in incisor regularity that
is higher than if the retainer is bonded to all anterior
teeth.135,
136
A particular weakness of retainers bonded only
to the canines is that they may allow for labial movement
and rotational relapse of the anterior teeth.12,
133, 135
Many studies have been done to examine the effects
that bonded retainers have on the gingiva and teeth. In
theory, a wire bonded to the teeth provides a source for
accumulation of biofilm which in turn, could lead to
increased gingival inflammation, periodontal problems, and
caries. Johnsson, Tofelt, and Kjellberg found that while
plaque accumulation was increased in patients with lower
bonded retainers, only 2% of their sample showed any new
41
gingival recession.137 Other studies have shown that bonded
retainers have no detrimental effects to the gingiva.3,
138
Johnsson, Tofelt, and Kjellberg also noted that no
demineralization or carious lesions were seen in teeth
contacting the bonded retainer.137 Similar results have been
found in several other studies.133,
139-141
In studying different types of wires for bonded
retainers Jongsma et al. found that single-stranded wires
showed less biofilm accumulation than multi-stranded
wires.142 The also showed that biofilm accumulation on multistranded wires were more resistant to antimicrobials and
are harder to remove via toothbrushing.142 This led them to
recommend single-stranded wires for fabrication of bonded
retainers. This conclusion was supported by Pazera,
Fudalej, and Katsaros.134
Bonded retainers are chosen because they eliminate
compliance to wear. Compliance is one of the greatest
difficulties of orthodontic retention. Heyman, Grauer, and
Swift found that fewer than half of their patients wore
their retainers as instructed two years post-treatment.3 The
disadvantages of bonded retainers are debonding, technique
sensitivity of placement procedure, and that patients with
bonded retainers must be continuously monitored.127
42
Little’s Irregularity Index
Little’s Index was introduced by Dr. Robert Little in
1975 as a means to quantify lower incisor irregularity. Dr.
Little proposed measuring the linear distance between the
anatomical contact points of the lower anterior teeth (from
mesial of canine to mesial of contralateral canine) to the
nearest tenth of a millimeter. These differences were added
together and the sum was termed the Irregularity Index.143
Since the Irregularity Index was introduced, it has
gained wide acceptance and has been used in multiple
studies. Little’s Irregularity Index has been used to
measure initial crowding, effectiveness of orthodontic
appliances at resolving incisor crowding, and a wide
variety studies to determine effectiveness of different
orthodontic retainers and retention protocols.
Little’s Index is not free of flaws. The Index is
measured by calipers on dental models that remain flat on a
table. Due to this, the Index does not take into account
the vertical displacement of the teeth and its impact on
crowding.144 Intra-examiner variability of Little’s Index has
also been shown to be high.144 Another drawback to Little’s
Index is subjectivity when locating the contact points to
be measured.145 Utilizing three-dimensional digital scanning
43
for intraoral scans and to create digital models has been
suggested as a way to improve both the accuracy and
precision of Little’s Index.144,
145
Despite the limitations
of Little’s Index, it is still one of the most widely used
methods to give a quantitative measure of effectiveness of
orthodontic treatment, particularly effectiveness of
retentive devices and retention protocols.
Statement of Thesis
This study will be a retrospective study to examine
whether the MagneTainer®™ is as effective as canine-tocanine bonded retainers that are bonded to each tooth. Two
samples will be gathered. The first sample contains 39
consecutively treated individuals treated by Dr. Aron
Dellinger of Fort Wayne Indiana. This sample had the
MagneTainer placed at the conclusion of full orthodontic
treatment. The second sample, consisting of 41 patients
will be gathered from records of patients treated at Saint
Louis University’s Center for Advanced Dental Education.
This sample will have had canine-to-canine retainers bonded
to each tooth placed at the conclusion of full orthodontic
treatment. For both samples, any individuals who had
extractions, missing teeth, or spacing prior to treatment
will be excluded.
44
Dental casts from pre-treatment, debond, and two
years post-treatment will be digitized and a Little’s Index
will be taken on these digital models at each time point.
The dependent variable will be the change in Little’s
Irregularity Index from debond to two years post-treatment.
The alternate hypothesis is that there will be a
difference between the MagneTainer®™ group when compared to
the bonded canine-to-canine group in terms of Little’s
Index. The null hypothesis is that there will be a no
difference between these two groups.
45
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55
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56
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58
Chapter 3: Journal Article
Abstract
Introduction: Retention has long been one of the most
challenging aspects of orthodontic treatment. Difficulties
with retention have led many to conclude that the only way
to retain the teeth in their post-treatment positions is
long-term retention. The MagneTainer®™ is a newly developed
magnetic retainer bonded to the lower anterior teeth for
long-term retention. Purpose: This study examines the
efficacy of the MagneTainer®™ as compared to canine-tocanine retainers bonded to each tooth. Materials and
Methods: Using digital models of 39 patients who had the
MagneTainer®™ placed at debond and 41 patients who had
canine-to-canine retainers placed at debond, Little’s
Irregularity Index was calculated prior to treatment, at
placement of retainers, and two years post-placement. The
groups were compared to see if any significant differences
exist. Results: Significant differences within each group
were seen for changes in Little’s Index and space
discrepancy at two years compared to debond. The only
significant differences between the groups were related to
intercanine width. Correlations were found between initial
intercanine width, initial space deficiency, and initial
Little’s Index. Little’s Index and a modified tooth mass
59
arch length discrepancy were shown to have a strong,
positive correlation.
Conclusions: The differences found within each group were
all less than 0.5mm. There are no significant differences
in the effectiveness of bonded canine-to-canine retainers
bonded to each tooth and the MagneTainer®™. Little’s Index
and a modified tooth mass arch length discrepancy are both
good measures of initial crowding for this sample
60
Introduction
Retention is considered by many practitioners to be
one of the most difficult aspects of orthodontic treatment.
Oppenheim actually referred to retention not just as a
problem in orthodontics but as the problem in orthodontics.1
Natural changes to the dental arches that occur during
ageing and maturation are known to contribute to the
problem of post-treatment relapse, particularly decreases
to arch length and intercanine width.2,
3
These changes have
been shown to occur in both treated and untreated
individuals4-6, some studies have even shown that decreases
in arch length and intercanine width are worse for those
who have undergone orthodontic treatment.6
The lower anterior teeth have been shown to be the
most prone to relapse with many studies showing 40% to 90%
of patients treated orthodontically to have unacceptable
incisor alignment 10 years after treatment.6,
8, 11-13
These
studies led Little to conclude that the only way to ensure
long-term stability would be lifetime retainer wear.11
In a recent survey of retention methods of members of
the American Association of Orthodontists, a trend towards
more bonded, and fewer Hawley retainers was seen.7 One of
61
the main advantages of bonded retainers is that patient
compliance is reduced.
Blechman and Smiley were the first to use magnets to
move teeth.14 Beginning in the late 1980s and continuing
into the early 1990s, many magnetic appliances with a wide
variety of applications have been developed. Magnets have
been used to distalize, intrude, extrude, align, and even
retrieve impacted teeth.15-28 Magnets have also been used for
orthopedic correction of both Class II and Class III
malocclusion.29-34 Magnetic appliances exert continuous
forces that can be measured and do not decay over time,
have reduced friction, and can exert their force through
bone and mucosa.35-37
Studies have shown that both magnets themselves and
the magnetic fields they produce are effective and safe for
use intra-orally as long as the magnets are isolated from
the intra-oral environment by either being coated or
encased in a protective covering.38-47
A new retentive appliance known as the MagneTainer®™
is being developed by Gene and Aron Dellinger of Fort Wayne
Indiana. It is a fixed, magnetic retainer made from 10
neodymium iron boron magnets bonded from the mesial of the
lower canine to the mesial of the contralateral canine.
62
This unique design allows the retainer to function as a
bonded retainer with the added ability to floss the teeth
between the magnets. The purpose of this study is to
determine the efficacy of the MagneTainer®™ magnetic
retainer as compared to other canine-to-canine bonded
retainers that are bonded to each tooth.
Materials and Methods
Sample
For this study, two samples were gathered. The first
sample was acquired from patients consecutively treated by
Dr. Aron Dellinger of Fort Wayne Indiana. These patients
had the MagneTainer®™ placed at the time of orthodontic
appliance removal. Digital study models acquired using an
iTero® (Align Technology, Inc., San Jose California) intraoral scanner were taken at initiation of treatment,
appliance removal, and two years post-retainer placement
were gathered. This sample consisted of 44 patients.
The second sample was acquired from patients treated
at the Saint Louis University Center for Advanced Dental
Education (Saint Louis, Missouri). These patients all had
bonded canine-to-canine retainers where each tooth was
bonded placed at the time of orthodontic appliance removal.
63
Study models from initiation of orthodontic treatment,
appliance removal, and two years post-retainer placement
were digitized using the 3Shape® R700 desktop scanner
(3Shape A/S, Copenhagen, Denmark). This sample consisted of
44 patients.
Exclusion criteria included spacing at initiation of
treatment, extractions, and incomplete or inadequate
records. After applying exclusion criteria, six patients
were excluded from the MagneTainer®™ group, and three were
excluded from the bonded retainer group.
Technique for Measurement of Digital Models
Because the samples were acquired in different ways,
different software was required to analyze the digital
models. The MagneTainer®™ sample was analyzed using
OrthoCAD® software (Align Technology, Inc., San Jose
California)and the bonded retainer sample was analyzed
using OrthoAnalyzer® software from 3Shape® (3Shape A/S,
Copenhagen, Denmark). Although the software differed, the
same measurements were made for each individual in the
sample. For each time point (start, debond, 2 years postretainer placement) Little’s Irregularity Index and
intercanine width were measured (see figures 3.1 and 3.2).
64
Figure 3.1 Little’s Irregularity Index and Intercanine width
measured using OrthoAnalyzer® software from 3Shape® (3Shape
A/S, Copenhagen, Denmark)
Figure 3.2 Little’s Irregularity Index and Intercanine Width
measured using OrthoCAD® (Align Technology, Inc., San Jose
California) software.
65
A modified tooth mass arch length discrepancy was also
measured at each time point. The widths of the anterior
teeth were measured and summed yielding the tooth mass of
the anterior teeth. The arch length was measured from the
distal one canine to the distal of the contralateral
canine. The difference between the anterior tooth mass and
anterior arch length was then calculated.
Error Study
Five patients were randomly selected from each sample
to be re-measured. All variables in both samples had intraclass correlation values of 0.85 or above.
Statistical Analysis
Paired t-tests were run using SPSS® software (IBM
Corporation, Armonk, New York). Tests were run to determine
any differences between and within groups initially, at
appliance placement and two-years post appliance placement.
Pearson’s correlation coefficients were also measured to
determine the relationship, if any between Little’s
Irregularity Index, intercanine width, and the modified
tooth mass arch length discrepancy.
66
Results
Bonded Retainer Sample
Table 3.1 shows descriptive statistics for the bonded
retainer sample. Statistical analysis via paired t-tests
within the bonded retainer group showed significant
differences for Little’s Irregularity Index at debond
compared to Little’s Irregularity Index at two years.
Significant differences were also found between the
modified tooth mass arch length discrepancy at debond and
two years post-retainer placement (see table 3.2).
Table 3.1 Mean, median, standard deviation, and variance for the bonded retainer
sample. (LI=Little’s Index, ICW=Intercanine width, Space=Modified tooth mass arch
length discrepancy)
Mean
Initial LI
Debond LI
LI Difference from Initial
2 Year LI
LI Difference from Debond
LI 2Yr difference from Initial
Initial ICW
Debond ICW
ICW Difference from Initial
ICW 2Yr
ICW 2Yr Difference From Initial
ICW 2Yr Difference from Debond
Initial Space
Debond Space
Debond Space Diff. From Initial
2Yr Space
2Yr Space Difference from Initial
2Yr Space Difference from Debond
Median
6.15
0.56
5.61
0.90
0.41
5.24
25.45
26.96
1.95
26.70
1.71
0.40
-2.69
-0.45
3.08
-0.67
2.78
0.34
67
5.00
0.46
4.75
0.63
0.29
4.48
25.35
27.08
1.79
26.97
1.5
0.29
-2.5
-0.36
2.77
-0.70
2.62
0.31
Standard
Deviation
4.16
0.36
4.09
0.56
0.38
4.11
1.64
1.51
1.26
1.42
1.05
0.40
2.87
0.43
2.04
0.45
2.04
0.25
Variance
17.29
0.13
16.79
0.31
0.14
16.86
2.68
2.29
1.58
2.01
1.09
0.16
8.23
0.19
4.17
0.21
4.16
0.06
The mean initial Little’s Irregularity Index for the
bonded retainer sample was 6.15mm with mean initial
intercanine width of 25.45mm. The modified tooth mass arch
length discrepancy showed an average -2.69mm of space
deficiency prior to treatment.
After orthodontic treatment the Little’s Index was
reduced to an average of 0.56mm, the intercanine width was
expanded an average of 1.51mm. The modified tooth mass arch
length discrepancy showed an average of -0.45mm of space at
debond.
Two years post-retainer placement, the Little’s
Irregularity to changed 0.9mm, this was a change of 0.34mm.
The intercanine width relapsed an average of 0.4mm and the
modified tooth mass arch length discrepancy showed an
average 0.34mm of space loss.
Table 3.2 Paired t-tests for Little’s Irregularity Index at debond and two years postretainer placement as well as modified tooth mass arch length discrepancy at debond and 2
years post-retainer placement. Both tests were significant with a p-value of 0.05
Mean
LI Debond
LI 2Yr
Space Debond
Space 2Yr
0.56
0.90
-0.45
-0.67
Standard
Deviation
0.36
0.56
0.43
0.45
Mean
Difference
0.56
0.90
-0.45
-0.67
t-value
10.02
10.41
-6.57
-9.49
Significance
0.00
0.00
0.00
0.00
The changes to both Little’s Irregularity Index space
measured via the modified tooth mass arch length were shown
to be significant.
68
The mean change in Little’s Index was 0.34mm. It must
be remembered that the Little’s Index measures discrepancy
between all contact points from the mesial of one canine to
the contralateral canine. Thus, this change of 0.34mm is
distributed among the contact points of all six anterior
teeth.
The mean change in space was 0.23mm. This means that
on average, 0.23mm of space loss was seen at two years
post-retainer placement. As with the Little’s Index, this
space loss is distributed among all six anterior teeth.
Pearson’s correlation coefficients were calculated for
Little’s Index, intercanine width, and the modified tooth
mass arch length discrepancy to see if any relationship
existed between these variables. The results of these tests
can be found in table 3.3
Table 3.3 Pearson’s correlation for initial Little’s Index to initial intercanine width,
intercanine width change to Little’s Index at two years, and initial Little’s Index to
initial space measured via a modified tooth mass arch length discrepancy
Correlation Coefficient
Significance (2-tailed)
-0.317
0.044
0.451
0.003
-0.831
0.000
LI initial to ICW initial
ICW change from 2yrs to
debond to LI at 2yrs
Initial LI to initial space
69
A statistically significant correlation was found
between initial Little’s Index and initial intercanine
width. These variables show a negative, moderate
correlation. As intercanine width decreases, there is a
moderate correlation to a larger Irregularity Index.
Change in intercanine width was shown to correlate
moderately to Little’s Index at two years post-retainer
placement. As intercanine width changes, the Irregularity
Index gets larger.
A strong, negative correlation was shown between
initial space as measured by the modified tooth mass arch
length discrepancy and initial Little’s Index. This is
expected, as the amount of available space in the arch
decreases, more crowding is expected and a larger
Irregularity Index should be expected as well. The degree
of this correlation also shows that the modified tooth mass
arch length discrepancy and Little’s Irregularity Index
agree as to the amount of crowding of the anterior teeth.
MagneTainer®™ Sample
Table 3.4 shows descriptive statistics for the bonded
MagneTainer®™ sample. Statistical analysis via paired ttests within the MagneTainer®™ group showed significant
70
differences for Little’s Irregularity Index at debond
compared to Little’s Irregularity Index at two years.
Significant differences were also found between the
modified tooth mass arch length discrepancy at debond and
two years post-retainer placement (see table 3.5).
Table 3.4 Mean, median, standard deviation, and variance for the MagneTainer®™
sample. (LI=Little’s Index, ICW=Intercanine width, Space=Modified tooth mass arch
length discrepancy)
Initial LI
Debond LI
LI Difference from Initial
2 Year LI
LI Difference from Debond
LI 2Yr difference from Initial
Initial ICW
Debond ICW
ICW Difference from Initial
ICW 2Yr
ICW 2Yr Difference From Initial
ICW 2Yr Difference from Debond
Initial Space
Debond Space
Debond Space Diff. From Initial
2Yr Space
2Yr Space Difference from Initial
2Yr Space Difference from Debond
Mean
Median
6.62
0.71
5.98
0.87
0.28
5.76
26.55
28.72
2.31
28.25
1.97
0.62
-1.63
-0.45
2.07
-0.97
2.14
0.59
6.05
0.55
5.25
0.80
0.20
5.05
26.40
28.75
1.95
28.00
1.65
0.50
-1.30
-0.40
1.40
-0.80
1.30
0.40
Standard
Deviation
Variance
4.33
0.65
4.21
0.49
0.27
4.27
2.39
1.63
1.73
1.66
1.64
0.60
2.64
0.41
1.89
0.96
1.93
0.90
18.70
0.42
17.69
0.245
0.08
18.22
5.76
2.64
2.98
2.81
2.69
0.36
6.99
0.17
3.60
0.92
3.71
0.81
The mean initial Little’s Irregularity Index for the
MagneTainer®™ sample was 6.62mm with mean initial
intercanine width of 26.55mm. The modified tooth mass arch
length discrepancy showed an average -1.63mm of space
deficiency prior to treatment.
After orthodontic treatment the Little’s Index was
reduced to an average of 0.71mm, the intercanine width was
71
expanded an average of 2.31mm. The modified tooth mass arch
length discrepancy showed an average of -0.45mm of space at
debond.
Two years post-retainer placement, the Little’s
Irregularity changed to 0.87mm, this was a change of
0.16mm. The intercanine width relapsed an average of 0.62mm
and the modified tooth mass arch length discrepancy showed
an average 0.59mm of space loss.
Table 3.5 Paired T-tests for Little’s Irregularity Index at debond and two years postretainer placement as well as modified tooth mass arch length discrepancy and debond and
2 years post-retainer placement for the MagneTainer®™ group. Both tests were significant
with a p-value of 0.05
Mean
Standard
Mean
t-value
Significance
Deviation
Difference
LI Debond
LI 2Yr
Space
Debond
Space 2Yr
0.71
0.87
-0.44
0.64
0.49
0.41
0.71
0.87
-0.44
6.77
10.85
-6.66
0.00
0.00
0.00
-0.92
1.02
-0.92
-5.47
0.00
The changes to both Little’s Irregularity Index space
measured via the modified tooth mass arch length were shown
to be significant.
The mean change in Little’s Index was 0.16mm. It must
be remembered that the Little’s Index measures discrepancy
between all contact points from the mesial of one canine to
the contralateral canine. Thus, this change of 0.16mm is
distributed among the contact points off all six anterior
teeth.
72
On average, 0.48mm of space loss was seen at two years
post-retainer placement. As with the Little’s Index, this
space loss is distributed among all six anterior teeth.
Pearson’s correlation coefficients were calculated for
Little’s Index, intercanine width, and the modified tooth
mass arch length discrepancy to see if any relationship
existed between these variables. The results of these tests
can be found in table 3.6
Table 3.6 Pearson’s correlation for initial Little’s Index to initial intercanine width
and initial Little’s Index to initial space measured via a modified tooth mass arch
length discrepancy
Correlation Coefficient
Significance
(2-tailed)
LI initial to ICW initial
Initial LI to initial space
-0.37
0.023
-0.480
0.002
A significant correlation was found between initial
Little’s Index and initial intercanine width. These
variables show a negative, moderate correlation. As
intercanine width decreases, there is a moderate
correlation to a larger Irregularity Index.
A moderate, negative correlation was shown between
initial space and initial Little’s Index. The degree of
this correlation was not as large as for the bonded
retainer group.
73
Findings Between the Two Samples
Paired t-tests were run comparing each variable from
the bonded retainer sample to the MagneTainer®™ sample to
see if any statistical differences existed. Significant
differences were found for initial intercanine width,
intercanine width at debond, and change in intercanine
width at two years vs. debond (see table 3.7)
Table 3.7 paired t-tests for the significant differences between groups.
ICW=intercanine width.
Mean
Standard
Deviation
t-value
Significance
Bonded Retainer Initial ICW
25.30
1.46
-2.60
0.01
MagneTainer®™ Initial ICW
26.55
2.39
-2.60
0.01
Bonded Retainer Debond ICW
26.94
1.56
-4.21
0.00
MagneTainer®™ Debond ICW
27.72
1.63
-4.21
0.00
Bonded Retainer ICW change
0.39
0.40
-2.59
0.014
0.62
0.60
-2.59
0.014
From Debond to 2yrs
MagneTainer®™ ICW change From
Debond to 2yrs
The MagneTainer®™ group initial intercanine width was,
on average 1.25mm larger than the bonded retainer group.
The average expansion of the mandibular canines was 0.64mm
for the bonded retainer group and 1.17mm for the
MagneTainer®™ group. The average intercanine relapse at two
years post-retainer placement was 0.39mm for the bonded
retainer group and 0.62mm for the MagneTainer®™ group. No
significant differences were found for Little’s Index or
74
modified tooth mass arch length discrepancy at any time
point.
Discussion
While significant differences for Little’s
Irregularity Index and a modified tooth mass arch length
discrepancy were seen within the groups, no such
differences were found between the groups. The differences
that were found within the groups were all <0.5mm.
The significant differences between the groups that
were found were all related to intercanine width. The
MagneTainer®™ sample had, on average, a larger intercanine
width than the bonded retainer sample. The MagneTainer®™
sample’s intercanine width was also expanded more than the
bonded retainer group. This did not translate into
significant differences in Little’s Index or space
discrepancy at debond or at two years. As has been
mentioned previously, both intercanine width and arch
length are known to decrease with time. Following both
samples for a longer period of time may yield different
results than are presently available from this relatively
short-term study.
Correlation coefficients among the variables within
each group show expected results in terms of intercanine
width and available space. As the intercanine width
75
decreased, less space was available and the Little’s
Irregularity Index was higher. A correlation was also seen
between Little’s Irregularity Index and the modified tooth
mass arch length discrepancy, this was strongly correlated
for the bonded retainer group and moderately correlated for
the MagneTainer®™ group. This shows that these two measures
agree in showing the amount of crowding present initially.
For the bonded retainer sample, a correlation was
also seen for the change in intercanine width at two years
post-retainer placement compared to intercanine width at
debond and the Little’s Irregularity Index.
The purpose of this study was to determine if the
MagneTainer®™ was as effective as a canine-to-canine bonded
retainer that is bonded to each tooth. In terms of Little’s
Irregularity Index and a modified tooth mass arch length
discrepancy, we must fail to reject the null-hypothesis
that there is a difference in effectiveness between the
MagneTainer®™ and canine-to-canine retainers that are
bonded to each tooth.
Conclusions
1.
There is no difference in effectiveness of a canineto-canine retainer bonded to each tooth and the
MagneTainer®™ at two years post-retainer placement,
76
2.
Little’s Irregularity Index and a modified tooth mass
arch length discrepancy agree in terms of measuring
initial crowding, and
3.
Statistically significant relapse was seen within both
the bonded retainer sample and the MagneTainer®™
sample in terms of Little’s Index and a modified tooth
mass arch length discrepancy.
77
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82
VITA AUCTORIS
Adam Armstrong was born in Bountiful, Utah on April
4th, 1980 and has an older brother and sister as well as a
younger brother and sister. Adam attended Brigham Young
University and graduated with a Bachelor of Science in
Nutritional Science in 2006.
Adam continued his education as a member of the
inaugural class of Midwestern University’s College of
Dental Medicine earning his DMD in 2012. He began his
orthodontic training in the spring of 2012 at Saint Louis
University.
Adam has been married to his wife Megan since 2003 and
they have four children together. Adam will graduate from
Saint Louis University in December 2014 completing his
Masters of Science in Dental Medicine. Upon graduation,
Adam will return to Arizona with his family and join a
private orthodontic practice in Phoenix, Arizona.
83