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UNLOADING BEHAVIOR AND POTENTIAL BINDING OF
SUPERELASTIC ORTHODONTIC LEVELING WIRES:
A GINGIVALLY MALPOSED CUSPID MODEL
Trenton D. Thalman, D.D.S.
An Abstract Presented to the Faculty of the Graduate School
of Saint Louis University in Partial Fulfillment
of the Requirements for the Degree of
Master of Science in Dentistry
2008
ABSTRACT
Objective:
To examine unloading behavior of superelastic
(SE), nickel-titanium-alloy (NiTi) leveling wires within a
gingivally malposed cuspid model.
Materials and Methods:
A universal testing machine deflected continuous, 0.014inch SE NiTi wires gingivally at the right-cuspid position
of an orthodontic, dental-arch model.
Binding points
(smallest deflections at which frictional forces stop
leveling wires from sliding through supporting bracketslots) and unloading plots (from beneath binding points)
were obtained with self-ligation (SL) and new (unrelaxed)
elastomeric ligation (EL) at the support sites.
Unloading
data were collected with SL from 2.5-, 3.5-, 4.5-, and 5.5mm deflections; and from 1.5- and 2.5-mm deflections with
EL.
Force-loss was quantified as the ordinate difference
between the peak and the trough of a plot.
Descriptive and
inferential statistics, the latter Kruskal-Wallis with
Mann-Whitney U-tests, were run to analyze the force-loss
data. Results:
Binding occurred with SL at a deflection of
7.5 mm, and at 3.5 mm with EL.
Mean force-loss ranged from
87.0 ± 10.1 grams to 0.0 ± 0.0 grams with SL and EL during
unloading from 5.5 and 1.5 mm, respectively.
Significant
differences (p < .01) in force-losses were obtained across
1
all SL groups, but not between the EL groups.
Conclusions:
1. Binding of 0.014-inch SE NiTi leveling wires occurs at
smaller deflection-amplitudes with EL than with SL.
2. Relatively constant aligning forces from 0.014-inch SE
NiTi leveling wires should not be expected in most clinical
situations.
3. Binding of 0.014-inch SE NiTi leveling
wires occurs at a deflection-amplitude threshold rather
than at a deflection-amplitude.
2
UNLOADING BEHAVIOR AND POTENTIAL BINDING OF
SUPERELASTIC ORTHODONTIC LEVELING WIRES:
A GINGIVALLY MALPOSED CUSPID MODEL
Trenton D. Thalman, D.D.S.
A Thesis Presented to the Faculty of the Graduate School
of Saint Louis University in Partial Fulfillment
of the Requirements for the Degree of
Master of Science in Dentistry
2008
COMMITTEE IN CHARGE OF CANDIDACY:
Assistant Professor Ki Beom Kim
Chairperson and Advisor
Professor Emeritus Robert J. Nikolai
Adjunct Associate Professor Kirk D. Satrom
i
DEDICATION
This thesis is dedicated to my family: my wonderful
wife Debbie and my four amazing daughters Audrey, Lauryn,
Tricia, and Julia, and to my parents Jeff, Ilene, David,
and Patricia who have provided support and encouragement
throughout my higher education.
ii
ACKNOWLEDEGMENTS
The author would like to express appreciation to his
committee members: Dr. Ki Beom Kim, Dr. Robert Nikolai, and
Dr. Kirk Satrom.
Each has contributed much time and effort
on the author’s behalf.
A special thanks to Mr. Joe
Tricamo and his associates at the Saint Louis University
machine shop for the many hours spent in manufacturing the
experimental equipment needed to complete this study.
The author would also like to thank GAC International
for donating brackets and tubes, Ormco Corp. for donating
archwires, and TP Orthodontics Inc. for donating ligatures
and a Straight Shooter.
iii
TABLE OF CONTENTS
LIST OF TABLES........................................... vi
LIST OF FIGURES......................................... vii
CHAPTER 1:
INTRODUCTION.................................. 1
CHAPTER 2:
REVIEW OF THE LITERATURE...................... 4
INTRODUCTION............................... 4
OCCLUSOGINGIVAL WIRE ACTIVATION ......... 4
DEFLECTION OF THE SE NITI WIRE........ 5
WIRE INDUCED NORMAL FORCES............ 7
DEFLECTED WIRE-LENGTH................. 8
HIGH-CUSPID LEVELING .................... 8
NORMAL FORCES AND TOOTH MOVEMENT..... 11
ARCHWIRE SLIDING DURING LEVELING..... 11
COULOMB FRICTION.......................... 12
SLIDING FRICTION THEORY ................ 12
STATIC VS. DYNAMIC FRICTION ............ 13
FRICTION IN ORTHODONTICS ............... 13
NORMAL FORCES IN ORTHDONTICS......... 14
Potential Normal Forces ........... 14
Ligation Method ................... 16
APPLIANCE STIFFNESS.................. 19
Design Stiffness .................. 19
Wire Stiffness .................... 20
SUMMARY................................... 21
REFERENCES................................ 23
CHAPTER 3:
JOURNAL ARTICLE.............................. 26
ABSTRACT.................................. 26
INTRODUCTION.............................. 27
MATERIALS AND METHODS..................... 32
MECHANICAL TESTS ....................... 32
STATISTICS ............................. 36
RESULTS................................... 37
DISCUSSION................................ 39
BINDING ................................ 39
UNLOADING PLOTS ........................ 42
LIGATION EFFECTS ....................... 45
FACTORS NOT EXAMINED ...................46
CLINICAL RECOMMENDATIONS ............... 48
CONCLUSIONS............................... 48
REFERENCES................................ 50
iv
VITA AUCTORIS............................................ 53
v
LIST OF TABLES
Table 1.
Descriptive Statistics: Force Losses (grams)... 38
Table 2.
Mann-Whitney U-Tests of Force Losses (p < 0.01) 39
vi
LIST OF FIGURES
Figure 1.1: Force-diagram illustrating active and
responsive occlusogingival forces and couples during
continuous-wire cuspid-leveling........................... 9
Figure 1.2:
An illustration of couples remaining when
binding occurs........................................... 10
Figure 1.3: Arrows representing potential normal forces
exerted in bracket-slots by ligation or archwires. A, B,
C, Facial views of both common edgewise and self-ligating
brackets. D, A gingival view of a sectioned common
edgewise bracket. E, A gingival view of a sectioned
“active” self-ligating bracket........................... 16
Figure 2.1: Geometry and formulas to estimate the length
(L) of wire between lateral-incisor and first-bicuspid
brackets with that wire sling-tied to the cuspid bracket. 29
Figure 2.2: Diagram illustrating occlusogingival forces
and couples accompanying continuous-wire cuspid leveling. 31
Figure 2.3: The model positioned in its fixture. The
fixture was bolted to the base of the testing machine.... 33
Figure 2.4: A representative unloading plot from the 5.5mm SL group. The dashed curve includes the estimated
plateau. The dimension-symbol denotes the quantified
force-loss............................................... 36
Figure 2.5: Representative unloading plots from each group
......................................................... 37
Figure 2.6: An illustration of couples present when
binding occurs........................................... 41
Figure 2.7: Representative unloading plot from the SL 5.5mm group. The solid curve is the recorded plot. The
dashed curve represents an assumed estimate of wirebehavior without friction. The cross-hatched area
represents the energy-loss occurring as a result of
friction present......................................... 44
vii
Figure 2.8: Comparison of representative unloading plots
from the EL and SL 2.5-mm groups. The solid line is the EL
2.5-mm plot. The dashed line is the SL 2.5-mm plot. The
cross-hatched area represents the difference in energy
transfer from the two groups............................. 45
viii
CHAPTER 1:
INTRODUCTION
Superelastic (SE), nickel-titanium-alloy (NiTi) wires
have become the wires of choice for orthodontic leveling
and aligning mechanics.
These wires readily sustain large
deflections without exceeding the elastic limit of the
alloy.
The large elastic range of SE NiTi is largely due
to its pseudoelastic characteristic.
For example, Burstone
et al.1 found that an 80˚ activation of SE NiTi wire
produced a 91% recovery compared to 20% for stainless steel
wire and 65% for martensitic NiTi wire.
Some orthodontists
may believe that any large activation with SE NiTi wire
will result in the desired tooth movement; however, four
recent investigations report that friction can stop SENiTi-wire displacements through ligated bracket-slots.2-5
When a wire does not slide through supporting bracketslots, desired tooth movement is unlikely to occur.
Binding is defined herein as a condition in which an
orthodontic leveling wire is prevented from sliding through
ligated bracket-slots.
Note that “binding” in this context
differs from the binding defined by Kusy and Whitley.6
Binding can alter the desired tooth-moving forces.7
Factors leading to SE-NiTi-wire binding in leveling
mechanics apparently have not been studied.
Binding of
wires in bracket-slots during sliding mechanics has been
studied extensively;8-11
however, contributing factors may
not play the same roles during leveling mechanics.
Larger
wires are more likely to bind during sliding mechanics;
smaller wires may more readily bind during leveling
mechanics.
Ward3 and Franchi and Baccetti4 found that
0.014-inch SE NiTi wire bound, and 0.016-inch SE NiTi wire
did not bind, under the same leveling conditions.
These
findings were incidental and not intended results of the
investigations.
Not surprisingly, binding in leveling mechanics is
somewhat unlike binding in sliding mechanics.
During
sliding mechanics, when binding causes resistance to
sliding, active retraction forces can be increased in size
to overcome the friction; however, in leveling mechanics
the only forces present to overcome friction are those
generated by the springback potential of the deflected
wire.
If the archwire does not produce sufficient force to
overcome the friction present, binding will occur.
Understanding the factors that lead to SE NiTi wire-binding
in leveling mechanics are important in establishing
clinical guidelines for the efficient use of this wire.
Binding of SE NiTi wire in clinical practice could
lead to no tooth movement or undesired tooth movement.
2
The
desired tooth moving force will be completely negated if
the wire binds.
Burstone12 has pointed out that leveling of
a gingivally positioned cuspid with a continuous SE NiTi
wire leads to responsive forces with undesired displacement
tendencies.
The aim of this investigation was to determine
the potential for binding and to describe the unloading
behavior of continuous 0.014-inch round SE NiTi wire in the
leveling of a gingivally malposed cupsid.
3
CHAPTER 2:
REVIEW OF THE LITERATURE
INTRODUCTION
This review of the literature begins with a discussion
of the dynamics of leveling a gingivally positioned cuspid
with a continuous SE NiTi archwire.
The frictional forces
that arise in orthodontic leveling are then examined.
Understanding these concepts leads to the rationale for
this study of unloading behavior and potential binding in
bracket-slots supporting SE NiTi leveling wires.
OCCLUSOGINGIVAL WIRE ACTIVATION
When engaging an SE NiTi wire to level a gingivally
malposed cuspid, several mechanics-entities occur
simultaneously.
First, the wire is deflected and is
attached to or engaged in the cuspid bracket.
Second, in
response to the springback potential of the wire at the
cuspid, normal forces are induced at the adjacent
(supporting) brackets.
Third, the effective length of wire
between the adjacent brackets increases as wire is drawn
through those adjacent brackets to reach the crown of the
4
gingivally positioned cuspid.
Each of these aspects is
examined individually below.
DEFLECTION OF THE SE NITI WIRE
As an SE NiTi wire is deflected, it first deforms in
an elastic manner in the austenitic state.
As stress
induced in the wire increases, a phase-transformation
begins (from austenitic toward martensitic metallurgy).
This occurring stress-induced phase-transformation is
accompanied by a lessening in stress-increase needed to
continue the deflection.13
In practice, the transformation
likely is incomplete at wire-engagement.
When the wire is
then allowed to unload, hysteresis occurs and is followed
by a relatively level unloading plot as the (partial)
transformation reverses, and the alloy returns to its
austenite phase.14
When the wire reaches the austenitic
state, unloading is completed via Hookean elastic
deactivation.
The wire, then, may undergo large
deflections and completely return to its initial shape.13
Throughout activation, the wire will conform to the lowest
energy-state that can be achieved as constrained, resulting
in specific curvatures within the interbracket spaces.
5
The flexural stiffness of SE NiTi wire is dependent
upon its size and shape, the temperature-transition range
(TTR), and the amount of wire-deflection.1,12,14
Flexural-
stiffness measurements for SE NiTi wires are based upon
linear regressions of the unloading curves.1
These factors
are discussed below.
The cross-sectional size and shape of NiTi wire have
the same influences on its flexural stiffness(es) as they
have on the stiffness(es) of other orthodontic wires.13
This topic is discussed later.
Stiffnesses of SE NiTi wire are influenced by the TTR;
as the TTR increases, stiffnesses decreases.
The TTR can
be altered 1) by adding trace-elements to the alloy or
2) by heat-treating the wire.14
Copper is added to some
NiTi alloys to lower the TTR, thus lessening wires’ loading
and unloading stiffnesses.14
The unloading stiffness of SE NiTi wire is also
dependent upon the extent of activation.
As the activating
deflection is increased, the unloading stiffness decreases.1
Wilkinson et al.2 found that a wire deflected to 1 mm
generated an initial unloading force of 247 grams.
When
the same wires were deflected to 4 mm, they unloaded from
only 74 grams of force.
This phenomenon is the rationale
6
for Profitt’s statement, “the force delivered by an A-NiTi
wire can be changed by releasing and retying it.”13
Because of the many factors that influence SE-NiTi-wire
stiffnesses, large variations in structural properties
exist between manufacturers2,15 and within manufacturers.
Only two of these factors are under the direct control of
the orthodontist.
First, the wire selected with its size,
shape, and TTR influences the loading and unloading
stiffnesses.
Second, the extent of activation/deflection
may be varied, affecting the unloading stiffness.
WIRE INDUCED NORMAL FORCES
When a continuous wire is deflected and attached to a
gingivally malposed, right-cuspid bracket, the wire
contacts the distogingival and mesio-occlusal edges of the
lateral-incisor bracket-slot and the mesiogingival and
disto-occlusal edges of the first-bicuspid bracket-slot.
These contacts create normal forces between the slots and
wire.
The magnitudes of these normal forces are partially
dependent upon the loading stiffness as the wire is engaged
and the unloading stiffness as the wire unloads.8
Wire-
curvatures in relation to the bracket-slots dictate which
normal force will be the greatest.
7
The normal force
adjacent to the greatest wire-curvature will be the largest
normal force in the system.16
DEFLECTED WIRE-LENGTH
When an archwire is deflected to reach a gingivally
malpositioned cuspid, the wire is deflected gingivally,
drawing wire through the adjacent brackets toward the
cuspid.
The result is a greater length of wire between the
lateral-incisor and first-bicuspid brackets than the
mesiodistal distance between them.
To illustrate this
inequality, the geometry can be simplified with two right
triangles, and the Pythagorean Theorem is invoked.
If a
wire is deflected 4 mm and sling-tied to a high cuspid with
a 13-mm distance between the lateral-incisor and firstbicuspid brackets, there is approximately 15.3-mm of wire
between the brackets.
If the wire is to level the cuspid,
2.3 mm of wire must slide through the adjacent brackets so
that only 13 mm of wire remains between those brackets.
HIGH-CUSPID LEVELING
With a continuous wire secured to a high cuspid, in
addition to the action against the cuspid, potentially
8
counterproductive forces are created, posing potential
problems for the clinician using a straight-wire technique.
Burstone wrote: “With a high canine, a [passively] straight
wire tends to tip the buccal segment toward the canine.”9
Creating this undesired displacement potential are
responsive intrusive forces and couples exerted on the
adjacent teeth through their brackets.
See Figure 1.1.
Figure 1.1: Force-diagram illustrating active and responsive
occlusogingival forces and couples during continuous-wire cuspidleveling.
Forces present in the system originate from the elastic
deformation of the wire.
As the wire attempts to spring
back toward its original shape, an occlusal force is
created at the cuspid.
In response to the occlusal force,
9
gingival forces are applied to the anchorage sites.
Force-
magnitude at each anchorage site is approximately one-half
of the occlusal force at the cuspid.
Additionally, couples
are applied within the first-bicuspid and lateral-incisor
bracket-slots, creating tipping/torquing moments.
Couples
are created by the normal forces between bracket-slots and
the archwire.16 Excessive frictional forces can effectively
cancel the springback potential at the cuspid, and the net
extrusive action disappears, leaving only the couples and
friction until the resistances(s) can be overcome.
See
Figure 1.2.
Figure 1.2:
An illustration of couples remaining when binding occurs.
10
NORMAL FORCES AND TOOTH MOVEMENT
An archwire having been deflected gingivally at the
cuspid, the net normal forces on the adjacent brackets are
individually nearly equally as approximately one-half of
the cuspid force mesially and distally.
The normal forces,
local wire-curvatures, and the need for the wire to slide
through the adjacent bracket-slots to progress toward a
correction, create the friction leading to potential
binding of the leveling wire.
Potential for binding may
particularly be raised with SE NiTi wires because a large
deflection decreases the unloading stiffness.1
ARCHWIRE SLIDING DURING LEVELING
As discussed previously, a deflection to level a
gingivally malposed cuspid will produce an “excess” length
of wire between the lateral-incisor and first-bicuspid
brackets.
In order for the desired leveling to occur, this
“excess” wire must slide through the supporting bracketslots.
In the presence of wire-slot contact, frictional
forces oppose this sliding motion.
The potential for
binding of leveling wires has not been examined, although
11
some incidental binding has been reported in the process of
other investigations.2-5
COULOMB FRICTION
Friction is the force that opposes the tangential
movement of two bodies in contact.
The Coulomb model of
friction is characterized by the following equation: F =
N*µ where F is the frictional force, µ is the coefficient
of friction, and N is the normal force.9
Coulomb friction
is explored in Sliding-Friction Theory, Static and Dynamic
Friction, and Friction in Orthodontics to follow.
SLIDING FRICTION THEORY
Sliding friction is defined as friction between two
solid objects in relative translational motion.
This type
of friction is associated with orthodontic mechanics.
Two
major influences on maximum static and kinetic sliding
friction are the magnitudes of the normal forces and the
relative surface-roughness of the objects in contact.16
12
STATIC VS. DYNAMIC FRICTION
Three categories of Coulomb friction are defined:
static friction, maximum static friction, and dynamic
friction.
Static friction is present when relative motion
is attempted, but none occurs.16
This frictional force
serves to maintain equilibrium when active forces are
insufficient to produce movement.
Maximum static friction
is the tangential force present just before motion starts.
This force is ordinarily the greatest resistance before the
object begins to slide.17
Dynamic friction is usually less
than maximum static friction, and it is the frictional
force present when objects are sliding across one another.16
Separate coefficients of friction may be determined
experimentally, for a pair of contacting surfaces, for both
maximum static and dynamic friction.17
FRICTION IN ORTHODONTICS
Coulomb’s explanation of friction becomes more
complicated in orthodontics.
First, the normal forces
present in orthodontics are very situation-dependent and
are rarely constant.
Second, the velocity at which
movement occurs is very slow, leading to inconsistency in
13
the type of frictional forces present.
Third, the surface-
roughnesses of the materials depend upon more than just the
chemical compositions.
Each of these aspects is discussed
further within the following subsections.
NORMAL FORCES IN ORTHDONTICS
Two categories of normal force are present in
orthodontic leveling.
First, slot-wire contact(s) exist,
due to wire-curvatures at/through the slot.
Second, the
ligation securing the wire in the slot can create normal
force.
These normal forces for a gingivally displaced and
ligated wire are described below.
Potential Normal Forces
When the wire is activated gingivally, but only
contacts the edge of one wing of the bracket, a normal
force exists at that point only.
Depending upon the wire-
curvature through the slot, normal forces may be present at
both ends of the bracket.
From a facial perspective, then,
normal forces are possible at opposite slot-edges or in the
middle of the slot as well.
Potential normal forces can
also be viewed from the gingival perspective of the
14
bracket.8
When the wire is ligated, contact may be present
along the entire bracket-width to produce a normal force
between the bracket and archwire.
Normal forces from
ligation may differ between self-ligated and conventional
edgewise brackets.
A potential contact exists at the
lingual surface of the slot of both self-ligated and common
edgewise brackets.
Potential contacts may be made between
the ligature and the wire at both the mesial and distal
aspects of a common edgewise bracket.
The self-ligated
bracket, at its clip, may receive a facially directed
normal force from the wire.
The various normal forces are
illustrated in Figure 1.3.
15
A
B
D
C
E
Figure 1.3: Arrows representing potential normal forces exerted in
bracket-slots by ligation or archwires. A, B, C, Facial views of both
common edgewise and self-ligating brackets. D, A gingival view of a
sectioned common edgewise bracket. E, A gingival view of a sectioned
“active” self-ligating bracket.
Ligation Method
Conventional ligations are of two material categories:
stainless steel and (polymeric) elastomers.
Other
ligations have been introduced in attempts to reduce
friction.
These include self-ligating brackets, non-
conventional elastomeric ligatures,4,5 and brackets with six
tie-wings.18
Self-ligating brackets can be subdivided as
having integral “active” or “passive” slot-closures.
16
Steel ties are often used to ligate SE NiTi wires.
The
normal force created by this ligature may be very small.19
Bednar et al.9 found that steel ligatures produced the least
friction when sliding a bracket along a wire when compared
to elastomeric and self-ligations; however, Iwasaski et
al.20 found significant variances across clinicians in
placing steel ties.
This variation ranged between 150 and
1470 grams with steel ties compared to 1600 grams with
elastomers.
Elastomeric ligations initially produce relatively
large normal forces with sizable friction potential, as
confirmed in many investigations;2,4,5,9,19-21 however, none of
the friction-literature reviewed accounts for sizable
(perhaps 50% or greater) force-losses as the elastomers
relax.22
Significant variations in initial force magnitudes
for a constant stretch have also been obtained within
samples of alike elastomeric ligatures.22-24
The normal
force may also vary in magnitude with wire and bracket
sizes.
It has been found that an increase in wire- and/or
bracket-dimensions results in more stretch of the
elastomeric ligatures and thus greater normal forces.25
Self-ligating brackets have been introduced into the
orthodontic marketplace with claims of less friction and
lighter forces.
Laboratory testing has confirmed less
17
friction with self-ligating brackets;2,3,19,21,24,26 however,
during straight-wire leveling, larger active forces are
generally produced with these brackets because there is
less friction.2,3
The slot-closure device varies within the family of
self-ligating brackets; two groups of brackets are termed
“active” and “passive.”
Within the “active” brackets the
ligating mechanism presses against the archwire (if it
exceeds a minimum faciolingual dimension, i.e., 0.018
inches), seating it in the bracket.
The ligation of a
“passive” bracket will not press against an archwire.
The
stated, seemingly simple difference, “active” vs. “passive”
brackets, is flawed, however, because activated archwires
are often (first-order) angulated in the slot, and such
angulations often result in the creation of normal forces.
Differences in normal forces, generated in “active” vs.
“passive” self-ligating brackets, have been reported.
Hain
et al.24 found that, with a 0.019- x 0.025-inch stainless
steel archwire, 1.61 Newtons were produced (faciolingually)
in an “active” bracket, and no normal force existed in a
“passive” bracket.
This difference in normal forces has
been shown to change with wire-size.
Shivapuja and Berger27
found no difference between “active” and “passive” self-
18
ligating brackets in faciolingual normal forces exerted
against an 0.018-inch stainless-steel wire.
APPLIANCE STIFFNESS
Normal forces can be directly related to the stiffness
of the activated archwire.8
As the flexural stiffness(es)
of a wire increase, the magnitude(s) of normal forces
produced by the wire increase.
Archwire-stiffness depends
upon both the wire itself and the local design of the
appliance.
The “unit” wire-stiffness is dependent upon the
wire-material stiffness and the cross-sectional shape and
size.28
The unit stiffness of a SE NiTi wire is further
complicated by its deflection-dependence.1
Design Stiffness
The design-stiffness is dependent upon two variables:
the length(s) of wire and the support(s) of the wire.
The
wire-stiffness varies inversely with its length from one
support or between two supports.13,28
supported in three ways.
The archwire may be
First, the wire can be a
cantilever, and the stiffness is dependent upon the length
19
of the cantilever.
As the length of cantilever increases,
the stiffness decreases.
Second (and third), wires can be
supported at both ends either loosely or tightly.
When
comparing wires of the same size and shape, with equal
length, stiffness increases as wire-support clearance
decreases.
Loosely supported wires are stiffer than
cantilevered wires and tightly supported wires are stiffer
than loosely supported wires.13
The orthodontist can vary the wire-length between
crown-attachments.
A loop can effectively increase wire-
length, and bracket-width alters interbracket distance and
effectively changes wire-length.
Frank and Nikolai8 found
that an increase in bracket-width increased friction during
cuspid-retraction.
Wire Stiffness
Unit wire-stiffness is dependent upon wire materialstiffness and cross-sectional shape and size.
Material-
stiffnesses have been determined for stainless steel, βtitanium (β-Ti) alloys, and stabilized martensitic NiTi
alloys.29
The material-stiffness of SE NiTi wire is notably
dependent upon the extent of deflection and can vary from
20
7% to 41% of the stiffness of orthodontic stainless
steel.1,13
As wire-size increases, the stiffness of the wire
increases to the fourth power for round wires and generally
as a cubic function of the dimension in the direction of
flexure; for example, if a round wire is doubled in its
dimension in the flexure direction, the stiffness increases
sixteen times (24 = 16).
For rectangular wires the
dimension in the direction of deflection has the greater
influence on stiffness.13,28
When an archwire is brought into contact with mesial
and/or distal corners of the slot, due to the local wirecurvature as activated, wire-stiffness becomes a
predominant factor in friction potential because of the
normal force(s) generated.8
SUMMARY
It is easily seen that friction in orthodontics is
complex.
This complexity is further increased with SE NiTi
archwires.
The information that is available currently
concerning friction and SE NiTi wires has been derived
primarily from displacing a bracket along a guiding wire or
21
pulling a wire through a bracket-slot or series of
slots.21,25,26,30-32
More recently, leveling with SE NiTi wires
has been examined from the perspective of friction
affecting forces delivered to the malposed tooth/teeth.2-5,33
Apparently, though, all of these inquires involved
arbitrarily selected deflections, some of which
incidentally produced binding.2-4
No attempts to determine
the binding points for SE NiTi leveling wires have been
found in the published literature.
Due to the complex nature of the factors involved,
indirect measures for potential binding of SE NiTi
archwires in leveling are impossible.
A model simulating a
gingivally malposed cuspid has been developed to examine
the binding of 0.014-inch-diameter SE NiTi archwires.
22
REFERENCES
1. Burstone CJ, Qin B, Morton JY. Chinese NiTi wire--a new orthodontic
alloy. Am J Orthod 1985;87:445-452.
2. Wilkinson PD, Dysart PS, Hood JA, Herbison GP. Load-deflection
characteristics of superelastic nickel-titanium orthodontic wires. Am J
Orthod Dentofacial Orthop 2002;121:483-495.
3. Ward BL. Friction in Alignment Mechanics: The effects of ligation,
perturbation, and wire size on orhtodontic aliging forces Center for
Advanced Dental Education. Saint Louis, MO: Saint Louis University;
2007.
4. Franchi L, Baccetti T. Forces released during alignment with a
preadjusted appliance with different types of elastomeric ligatures. Am
J Orthod Dentofacial Orthop 2006;129:687-690.
5. Camporesi M, Baccetti T, Franchi L. Forces released by esthetic
preadjusted appliances with low-friction and conventional elastomeric
ligatures. Am J Orthod Dentofacial Orthop 2007;131:772-775.
6. Kusy RP, Whitley JQ. Influence of archwire and bracket dimensions on
sliding mechanics: derivations and determinations of the critical
contact angles for binding. Eur J Orthod 1999;21:199-208.
7. Rossouw PE. Friction: An Overview. Sem Orthod 2003;9:218-222.
8. Frank CA, Nikolai RJ. A comparative study of frictional resistances
between orthodontic bracket and arch wire. Am J Orthod 1980;78:593-609.
9. Bednar JR, Gruendeman GW, Sandrik JL. A comparative study of
frictional forces between orthodontic brackets and arch wires. Am J
Orthod Dentofacial Orthop 1991;100:513-522.
10. Andreasen GF, Quevedo FR. Evaluation of friction forces in the
0.022 x 0.028 edgewise bracket in vitro. J Biomech 1970;3:151-160.
11. Tidy DC. Frictional forces in fixed appliances. Am J Orthod
Dentofacial Orthop 1989;96:249-254.
12. Burstone CJ. Application of Bioengineering to Clinical
Orthodontics. In: Graber TM, Vanarsdall RL, Vig KWL, editors.
Orthodontics: Current Principles and Techniques. St. Louis, MO:
Elsivier, Mosby; 2005. p. 293-330.
13. Proffit WR. Contemporary Orthodontics. 3rd Ed. St. Louis, MO:
Mosby; 2000.
14. Matasa CG. Biomaterials in Orthodontics. In: Graber TM, Vanarsdall
RL, Vig KWL, editors. Orthodontics: Current Principles and Techniques.
St. Louis, MO: Elsevier, Mosby; 2005. p. 345-390.
23
15. Thayer TA, Bagby MD, Moore RN, DeAngelis RJ. X-ray diffraction of
nitinol orthodontic arch wires. Am J Orthod Dentofacial Orthop
1995;107:604-612.
16. Nikolai RJ. Bioengineering Analysis of Orthodontic Mechanics.
Philadelphia, PA: Lea & Febiger; 1985.
17. Rossouw PE, Kamelchuk LS, Kusy RP. A Fundamental Review of
Variables Associated with Low Velocity Frictional Dynamics. Sem Orthod
2003;9:223-235.
18. Thorstenson GA, Kusy RP. Effects of ligation type and method on the
resistance to sliding of novel orthodontic brackets with second-order
angulation in the dry and wet states. Angle Orthod 2003;73:418-430.
19. Hain M, Dhopatkar A, Rock P. The effect of ligation method on
friction in sliding mechanics. Am J Orthod Dentofacial Orthop
2003;123:416-422.
20. Iwasaki LR, Beatty MW, Randall CJ, Nickel JC. Clinical ligation
forces and intraoral friction during sliding on a stainless steel
archwire. AmJ Orthod Dentofacial Orthop 2003;123:408-415.
21. Henao SP, Kusy RP. Evaluation of the frictional resistance of
conventional and self-ligating bracket designs using standardized
archwires and dental typodonts. Angle Orthod 2004;74:202-211.
22. Taloumis LJ, Smith TM, Hondrum SO, Lorton L. Force decay and
deformation of orthodontic elastomeric ligatures. Am J Orthod
Dentofacial Orthop 1997;111:1-11.
23. Khambay B, Millett D, McHugh S. Archwire seating forces produced by
different ligation methods and their effect on frictional resistance.
Eur J Orthod 2005;27:302-308.
24. Hain M, Dhopatkar A, Rock P. A comparison of different ligation
methods on friction. Am J Orthod Dentofacial Orthop 2006;130:666-670.
25. Kapila S, Angolkar PV, Duncanson MG, Jr., Nanda RS. Evaluation of
friction between edgewise stainless steel brackets and orthodontic
wires of four alloys. Am J Orthod Dentofacial Orthop 1990;98:117-126.
26. Henao SP, Kusy RP. Frictional evaluations of dental typodont models
using four self-ligating designs and a conventional design. Angle
Orthod 2005;75:75-85.
27. Shivapuja PK, Berger J. A comparative study of conventional
ligation and self-ligation bracket systems. Am J Orthod Dentofacial
Orthop 1994;106:472-480.
28. Burstone CJ. Application of Bioengineering to Clinical
Orthodontics. In: Thomas M. Graber RLV, Katherine W.L. Vig, editor.
Orthodontics: Current Pricniples and Techniques. St. Louis, MO:
Elsivier, Mosby; 2005. p. 293-330.
24
29. Burstone CJ. Variable-modulus orthodontics. Am J Orthod 1981;80:116.
30. Baccetti T, Franchi L. Friction produced by types of elastomeric
ligatures in treatment mechanics with the preadjusted appliance. Angle
Orthod 2006;76:211-216.
31. Tecco S, Festa F, Caputi S, Traini T, Di Iorio D, D'Attilio M.
Friction of conventional and self-ligating brackets using a 10 bracket
model. Angle Orthod 2005;75:1041-1045.
32. Kusy RP, Whitley JQ. Effects of surface roughness on the
coefficients of friction in model orthodontic systems. J Biomech
1990;23:913-925.
33. Fuck L-M, Drescher D. Force systems in the initial phase of
orthodontic treatment -- a comparison of different leveling arch wires.
J Orofacial Orthop 2006;67:6-18.
25
CHAPTER 3:
JOURNAL ARTICLE
ABSTRACT
Objective:
To examine unloading behavior of superelastic
(SE), nickel-titanium-alloy (NiTi) leveling wires within a
gingivally malposed cuspid model.
Materials and Methods:
A universal testing machine deflected continuous, 0.014inch SE NiTi wires gingivally at the right-cuspid position
of an orthodontic, dental-arch model.
Binding points
(smallest deflections at which frictional forces stop
leveling wires from sliding through supporting bracketslots) and unloading plots (from beneath binding points)
were obtained with self-ligation (SL) and new (unrelaxed)
elastomeric ligation (EL) at the support sites.
Unloading
data were collected with SL from 2.5-, 3.5-, 4.5-, and 5.5mm deflections; and from 1.5- and 2.5-mm deflections with
EL.
Force-loss was quantified as the ordinate difference
between the peak and the trough of a plot.
Descriptive and
inferential statistics, the latter Kruskal-Wallis with
Mann-Whitney U-tests, were run to analyze the force-loss
data. Results:
Binding occurred with SL at a deflection of
7.5 mm, and at 3.5 mm with EL.
Mean force-loss ranged from
87.0 ± 10.1 grams to 0.0 ± 0.0 grams with SL and EL during
26
unloading from 5.5 and 1.5 mm, respectively.
Significant
differences (p < .01) in force-losses were obtained across
all SL groups, but not between the EL groups.
Conclusions:
1. Binding of 0.014-inch SE NiTi leveling wires occurs at
smaller deflection-amplitudes with EL than with SL.
2. Relatively constant aligning forces from 0.014-inch SE
NiTi leveling wires should not be expected in most clinical
situations.
3. Binding of 0.014-inch SE NiTi leveling
wires occurs at a deflection-amplitude threshold rather
than at a deflection-amplitude.
INTRODUCTION
Unloading behavior of superelastic (SE), nickeltitanium-alloy (NiTi) orthodontic wires has been well
documented from cantilever1 and three-point2,3 bending tests.
These tests have shown that SE NiTi wires have large
elastic ranges and can exert relatively constant forces
over sizable portions of those ranges.1-3
SE NiTi wires
have also displayed deflection-dependent unloading
stiffnesses; larger deflections resulted in smaller
unloading stiffnesses.1-4
Properties of SE NiTi wires have
led to recommendations from Burstone et al.1 and Proffit4
27
that they be the wires of choice when large deflections and
relatively constant tooth-moving forces are required.
The unloading behavior of SE NiTi wires has been
tested within rather complex models.3,5-8
These models all
involved multiple brackets and various ligation methods.
Under some of these conditions Wilkinson et al.,3 Camporesi
et al.,5 Franchi and Baccetti,6 and Ward8 found that SE NiTi
wires can cease to slide through supporting bracket-slots.
The archwire failing to slide through supporting bracketslots is termed “binding” herein.
Note that this
definition differs from the wire-slot binding described by
Kusy and Whitley.9
Discovery of binding in leveling
mechanics raises questions concerning the behavior of SE
NiTi wire.
Some pertinent questions are as follows:
What
activating deflection will cause the SE NiTi wire to bind?
What factors lead to SE NiTi wire-slot binding?
How do SE
NiTi wires respond as they begin to bind?
Wire-length between brackets is a source of potential
binding of a continuous leveling wire.8
As a leveling wire
is deflected to engage a malposed tooth, to obtain the
necessary added length locally, the wire slides through the
adjacent ligated brackets and tubes.
For the malposed
tooth/teeth to align, the wire must slide back through the
supporting brackets and tubes.
28
“Free” wire-length between
two brackets adjacent to a malposed cuspid can be estimated
by superimposing two right triangles on the curved wire as
illustrated in Figure 2.1.
Assuming a 13-mm lateral-
incisor-to-first-bicuspid interbracket distance and a 4-mm
deflection to the cuspid bracket, 15.3 mm is calculated as
the wire-length between the lateral-incisor and firstbicuspid brackets.
An estimated 15.3 mm of wire-length
demonstrates that, for the wire to (deactivate and) become
straight, approximately 2.3 mm of wire must slide through
the slots of the adjacent ligated brackets and tubes.
c
b
a
L≈2c
c2=a2+b2
Figure 2.1: Geometry and formulas to estimate the length (L) of wire
between lateral-incisor and first-bicuspid brackets with that wire
sling-tied to the cuspid bracket.
29
Sliding of a leveling wire may be opposed by frictional
forces.6
Excessive frictional forces will stop the wire
from sliding, resulting in a bound wire.3,6,8
Information
currently available concerning friction and SE NiTi wires
has been derived primarily from displacing a bracket along
a guiding wire or pulling a wire through a bracket-slot or
series of slots.10-15
More recently, leveling with SE NiTi
wires has been examined from the perspective of friction
affecting forces delivered to the malposed tooth/teeth.3,5-8
Apparently, though, all of these inquiries involved
arbitrarily selected deflections, some of which
incidentally produced binding.3,6,8
No attempts to determine
the binding points for SE NiTi leveling wires have been
found in the published literature.
Self-ligating brackets have been introduced into the
orthodontic marketplace with claims of less friction and
lighter forces.
Laboratory testing has confirmed that
often there is less friction with self-ligating
brackets;3,8,11,12,16,17 notably, during straight-wire leveling,
larger active forces are generally produced with these
brackets because there is less friction.3,8
The purpose of this study was to examine the binding
and unloading behavior of a SE NiTi wire within a
gingivally-malposed-cuspid model.
30
Ligation was varied:
self-ligation or new (unrelaxed) elastomerics.
Burstone18
has pointed out that leveling of a gingivally positioned
cuspid with a continuous wire tends to tip adjacent teeth
towards the cuspid.
Forces responsible for these tooth
movements are created by the wire-curvatures at and through
the first-bicuspid and lateral-incisor bracket-slots.
These curvatures induce pairs of normal forces at the
supporting bracket-slots.
Wire-curvatures indicate that
the normal forces closest to the cuspid are greater than
the normal forces farther from the cuspid.19
These
unbalanced pairs of normal forces result in the couples and
the intrusive forces illustrated in Figure 2.2.
Figure 2.2: Diagram illustrating occlusogingival forces and couples
accompanying continuous-wire cuspid leveling.
31
When a leveling wire binds, occlusal springback at the
cuspid bracket temporarily ceases, but the couples remain,
frictional forces disallow wire-sliding, and the curved
shape of the wire is maintained.
The responsive couples
are seen to potentially tip/torque the supporting teeth
such that the crowns could move toward the cuspid.
Such
responsive displacements would reduce wire-curvatures,
reducing friction, and tend toward unbinding the wire.
MATERIALS AND METHODS
MECHANICAL TESTS
A model was constructed of ¼-inch-thick tool-steel
plate, machined to match the Tru-Arch, maxillary, small
archform (Ormco Corp., Glendora, CA).
Attached to the
machined edge were two first-molar tubes with zero firstorder rotation and nine In-Ovation-R brackets (GAC
International, Bohemia, NY) from second bicuspid to second
bicuspid (0.022-inch slots/tubes); material was removed at
the right-cuspid site to permit gingival deflections there.
Brackets and tubes were spaced according to the typical
maxillary tooth size of the adult-male dentition as
32
modified by Wilkinson et al.3
The set of brackets and tubes
was aligned with a 0.022-inch-diameter stainless-steel
archwire, shaped to match the model; the attachments were
direct-bonded with Pad Lock® (Reliance Orthodontic
Products, Itasca, IL).
See Figure 2.3.
Figure 2.3: The model positioned in its fixture.
bolted to the base of the testing machine.
The fixture was
All trials were conducted with a universal testing
machine (Model 1011, Instron Corp., Canton, MA)
Tests were
initiated by deflecting small, maxillary, 27°C Copper NITI®, 0.014-inch, Tru-Arch archblanks gingivally at the
right-cuspid position.
Wires were deflected with a 0.01033
inch-wide-by-0.31-inch-long blade at a crosshead speed of 8
mm per minute and deactivated at a crosshead speed of 2 mm
per minute.
A chart-recorder (Model 2310-069, Instron
Corp.) plotted force vs. deflection during unloading.
Tests were carried out in a dry field within a Plexiglas®
case where the temperature was maintained at 36 ± 5°
Celcius.
Archwires were ligated in the brackets with either
In-Ovation-R clips or new (unrelaxed) elastomerics (#361080, TP Orthodontics, Inc., La Porte, IN); the elastomerics
were placed with a Straight Shooter® (TP Orthodontics,
Inc.).
Tests were run first on all self-ligation (SL)
specimens; then the clips were removed from the brackets to
enable solely new elastic ligation (EL) placements.
Initial binding-point estimates, separately with SL
and EL strap-ups, were obtained by repeatedly cycling a
single wire with deflection-amplitudes increasing by 0.5
mm.
When a force of zero grams was observed throughout
attempted deactivation, the test was terminated, and the
last deflection-amplitude became the initial binding-point
estimate.
Five wires were deflected to each initial bindingpoint estimate.
If all five wires did not bind during
deactivation, the deflection-amplitude was increased in
34
increments of 1 mm until five consecutive wires bound.
The
smallest amplitude at which five consecutive wires bound
was defined as the estimated binding point.
Unloading data were collected from ten wires from each
group. (A “group” is defined by ligation and deflectionamplitude.)
Group tests began at a deflection-amplitude 1
mm below the estimated binding point, and the amplitude was
decreased by 1 mm for each additional group.
New groups
were added until a relatively level, unloading plateau was
observed.
Groups containing wires that bound were excluded
from the unloading data-set.
Initial unloading plots revealed a trough deviating
from the plateau that is typically obtained in three-point
bending tests of SE NiTi wires.
The ordinate difference
between the trough and estimated plateau was defined as
force-loss.
See Figure 2.4.
Force-loss measurements were
taken from all unloading plots.
35
Unloading Force (g)
250
200
150
100
50
0
0
1
3
2
5
4
6
Deflection Distance (mm)
Figure 2.4: A representative unloading plot from the 5.5-mm SL group.
The dashed curve includes the estimated plateau. The dimension-symbol
denotes the quantified force-loss.
STATISTICS
Force-loss measurements were analyzed with SPSS 14.0
for Windows (SPSS Inc., Chicago, IL).
Descriptive
statistics were compiled for each group.
A Kruskal-Wallis
test was run to study the effects of deflection-amplitude
and ligation method on a parameter defined as “force loss.”
Post-hoc Man-Whitney U-tests were conducted.
differences were sought at p < 0.01.
36
Significant
RESULTS
The estimated binding points were at deflectionamplitudes of 3.5 mm with EL and 7.5 mm with SL.
Unloading
data were collected at deflection-amplitudes of 1.5 and 2.5
mm with EL; and 2.5, 3.5, 4.5, and 5.5 mm with SL.
The
6.5-mm SL group was excluded because some wires unloaded
completely and other specimens bound.
A large dip from the
SE-characteristic plateau was noted from each of the SL
groups deflected to 4.5 and 5.5 mm; see Figure 2.5.
Unloading Force (g)
250
200
Legend
SL 5.5
150
SL 4.5
SL 3.5
100
SL 2.5
EL 2.5
50
EL 1.5
0
0
1
2
3
4
5
6
Deflection Distance (mm)
Figure 2.5:
Representative unloading plots from each group
Descriptive statistics are presented in Table 1.
Force-loss increased from 3.5 grams to 87.0 grams as the
37
SL-group deflection-amplitudes approached the estimated
binding point.
Mean force-losses from the EL groups were
0.0 grams and 0.5 grams from 1.5- and 2.5-mm deflectionamplitudes, respectively.
Table 1.
Descriptive Statistics: Force Losses (grams)
Group
Mean
SD
Minimum
Maximum
SL 5.5-mm
87.0
±10.1
75
105
SL 4.5-mm
34.5
±6.0
30
50
SL 3.5-mm
15.0
±4.7
10
20
SL 2.5-mm
3.5
±2.4
0
5
EL 2.5-mm
0.5
±1.6
0
5
EL 1.5-mm
0.0
±0.0
0
0
The Kruskal-Wallis analysis revealed statistical
differences across the mean force-loss data.
Mann-Whitney
U-Tests determined statistically significant differences
within all pairs of SL force-loss groups and with the EL
groups; the force-losses between the EL groups were not
statistically different.
See Table 2.
38
Table 2.
Mann-Whitney U-Tests of Force Losses (p < 0.01)
Groups
(mm)
SL 5.5
SL 5.5
n/a
SL 4.5
Sig
n/a
SL 3.5
Sig
Sig
n/a
SL 2.5
Sig
Sig
Sig
n/a
EL 2.5
Sig
Sig
Sig
Sig
n/a
EL 1.5
Sig
Sig
Sig
Sig
Non Sig
SL 4.5
SL 3.5
SL 2.5
EL 2.5
EL 1.5
n/a
DISCUSSION
BINDING
SE NiTi wires respond mechanically very differently
from Hookean orthodontic-alloy wires.
model, the SE NiTi wire may bind.
Within the present
After the wire is
deflected and deactivation begins, wire-slot friction at
the supports has the effect of decreasing the net unloading
force at the cuspid bracket.
Changes in frictional and
springback forces occur for two reasons.
First, frictional
forces result in particular from wire-and-bracket-slot
contacts at the lateral-incisor and first-bicuspid
39
brackets.
As deflection-amplitude is increased, wire-
curvatures increase between the cuspid and the supporting
bracket-slots, and greater wire-curvatures result in larger
normal and frictional forces at the supporting bracketslots.
Second, increases in deflection-amplitude, with SE
NiTi wires, decrease unloading stiffnesses and springback
forces.1
A deflected state may be reached where the
springback potential of the wire cannot overcome the
frictional forces, and the wire binds in those slots.
Bound wires may “free-up” and unload because of the
couples exerted that could torque the lateral-incisor and
first-bicuspid toward the malposed cuspid.
See Figure 2.6.
The frictional forces are only fractions of the sizes of
the normal forces; thus, the couples dominate in
displacement potential.
The torquing could reduce wire-
curvatures, lessen frictional forces, and occlusal
unloading at and movement of the cuspid could resume.
Application of the estimated binding points herein to
clinical practice becomes somewhat challenging.
In this
research, binding-point estimates were specified at
deflection-amplitudes where binding occurred consistently.
The SL estimate discounted the binding of several specimens
within the SL 6.5-mm group.
In future research, binding
40
thresholds, where binding begins to occur, might be more
useful for clinical application.
Figure 2.6:
An illustration of couples present when binding occurs.
Understanding that SE NiTi wires may bind is important
clinically.
A bound wire will produce either no tooth
movement or unwanted tooth movement.20
It should be noted
that binding occurred here within both SL and EL groups.
Some researchers have (erroneously) claimed that selfligating brackets result in a “virtually friction-free”
appliance where binding cannot occur.21
41
UNLOADING PLOTS
Unloading plots in Figure 2.5 reveal several notable
findings.
First, SE NiTi wires activated between two
bracket-slots generally did not produce relatively constant
forces.
Plots from the 2.5- and 3.5-mm SL groups exhibited
relatively constant force during 1.5 and 2.5 mm of
unloading, respectively.
All other unloading plots
displayed substantially varying forces.
Varying unloading
forces resulted from frictional forces impairing levelingwire sliding.
Attaining constant tooth/teeth moving forces
may not be important for orthodontic displacement, as
optimal tooth/teeth moving forces are unknown.22
Second,
frictional forces altered unloading plots from the SL
groups, often creating troughs deviating from the
anticipated plateaus.
Small troughs occurred in EL 2.5-mm
curves and no troughs occurred within the EL 1.5-mm group.
Third, a “step” was observed at 200 grams in each unloading
plot.
The source of this “step” is unclear.
Fourth, a
“spike” in unloading force occurs in all SL plots at about
0.5 mm of deflection.
This spike, a sudden increase in
force, may reflect the wire undergoing completion of the
reverse transformation back to the austenitic metallurgic
phase.
42
FORCE-LOSSES
Force-losses quantified herein, partially evaluating
the SE NiTi wire behavior, demonstrated the increasing
nemesis of friction within the SL groups as deflectionamplitude was increased.
Wire-curvatures between the
cuspid and supporting bracket-slots are responsible for
much of the friction.
As wire-curvatures increase, larger
normal forces are induced in the supporting bracket-slots.
The unloading plot from the largest deflection-amplitude is
illustrated in Figure 2.7.
This figure compares the
expected behavior of a SE NiTi wire with friction absent to
that with friction impeding wire displacements through the
ligated slots.
The energy-loss depicted is a direct result
of frictional forces.
43
Unloading Force (g)
250
200
150
100
50
0
0
1
3
2
5
4
6
Deflection Distance (mm)
Figure 2.7: Representative unloading plot from the SL 5.5-mm group.
The solid curve is the recorded plot. The dashed curve represents an
assumed estimate of wire-behavior without friction. The cross-hatched
area represents the energy-loss occurring as a result of friction
present.
The EL groups also experienced energy-loss as a result
of friction.
The pattern of energy-loss was quite
different from that of the SL groups.
Figure 2.8 is a
comparison of unloading plots from the EL and SL 2.5-mm
groups.
These plots are compared because the deflection-
amplitudes were equal; therefore, the “friction-free”
deactivation-plot is the same for both groups.
Very little
force was lost (3.5 grams) to friction from the SL 2.5-mm
group.
The EL plot did not display the trough or much of
the plateau typically exhibited by the SL groups.
44
Unloading Force (g)
250
200
150
100
50
0
0
1
3
2
4
5
6
Deflection Distance (mm)
Figure 2.8: Comparison of representative unloading plots from the EL
and SL 2.5-mm groups. The solid line is the EL 2.5-mm plot. The
dashed line is the SL 2.5-mm plot. The cross-hatched area represents
the difference in energy transfer from the two groups.
Although, there is greater energy-loss within the EL
2.5-mm group, both groups displayed adequate tooth/teeth
moving forces.
Also, the force-loss difference would be
expected to decrease as elastomerics relax.23
LIGATION EFFECTS
The self-ligating clips of the In-Ovation brackets
were not expected to contribute normal forces with 0.014inch wire engaged.
Faciolingual space between bracket-slot
base and clip accepts wires (passive from an occlusal
45
perspective) less than 0.018 inches in diameter without
contact between the archwire and clip.24
Beyond the influence of wire-curvatures, frictional
forces also arise from elastomeric ligation stretched
around leveling wires, sliding through support-slots.
Effects of EL in this model resulted in energy-losses that
were greater from the EL 2.5-mm group than from the SL 2.5mm group.
The greater energy-loss from the EL 2.5-mm group
is displayed as the cross-hatched area in Figure 2.8.
FACTORS NOT EXAMINED
Other factors not studied within this research could
affect binding points and the unloading behavior of SE NiTi
leveling wires.
These factors include relaxation of
elastomeric ligatures, simulating extraneous intraoral
forces, and varying interbracket distances, wire diameter,
and wire temperature-transition range (TTR).
Stretched elastomeric ligatures have been shown to
relax, losing one-half or more of their initial force
magnitudes over time.25
This relaxation results in reduced
normal forces between archwires and bracket-slots and
ligatures.
Recently, Petersen23 found average SE-NiTi-wire
unloading forces of 71 grams with new (unrelaxed)
46
elastomerics, 112 grams with relaxed elastomerics, and 128
grams with self-ligation.
Binding deflection-amplitudes
and unloading plots from SE NiTi wires may change as
elastomerics relax.
As previously mentioned, leveling-wire curvatures are
responsible for the normal forces between the wire and the
bracket-slots.
Changing the local wire-curvature will
alter the associated normal forces and, therefore, the
leveling-wire behavior.
Increasing (Decreasing)
interbracket distance(s), particularly from the cuspid,
will decrease (increase) wire-curvatures.
Engaging the
archwire in the cuspid-bracket-slot vs. sling-tying the
wire to the bracket will affect curvatures as well.
Perturbations, simulations of extraneous intraoral
forces, are another factor that may alter binding points
and the unloading behavior of SE NiTi wires.
Braun et al.27
found that substantial perturbations momentarily reduced
frictional forces to zero, allegedly sufficient for wireslips (as in “stick-slip”).
Ward8 found that, with small
perturbations, SE NiTi leveling wires can still bind.
Wire-diameter may also play a role in unloading
behavior of SE NiTi wires.
Franchi and Baccetti6 and Ward8
found that 0.014-inch SE NiTi wire bound, but 0.016-inch SE
NiTi wire did not bind under the same conditions.
47
The TTR is another factor that may influence binding
points.
Wilkinson et al.3 found that 0.016-inch 35˚C Copper
NI-TI® (Ormco Corp.) bound, but other 0.016-inch SE NiTi
wires, with lower TTRs, did not bind.
CLINICAL RECOMMENDATIONS
A recommendation of 0.014-inch 27˚C Copper NI-TI® wire
(Ormco Corp.) to level a gingivally malposed cuspid can be
made from the present data.
Conservative suggestions would
be to sling-tie activations with 3-mm maximum deflections
with conventional edgewise brackets and elastomeric
ligatures and sling-tied activations to 5-mm deflections
with self-ligating brackets.
CONCLUSIONS
1. Binding of 0.014-inch SE NiTi leveling wires occurs at
smaller deflection-amplitudes with EL than with SL.
2. Relatively constant aligning forces from 0.014-inch SE
NiTi leveling wires should not be expected in most clinical
situations.
48
3. Binding of 0.014-inch SE NiTi leveling wires occurs at a
deflection-amplitude threshold rather than at a deflectionamplitude.
49
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VITA AUCTORIS
Trenton Dayle Thalman was born on February 15, 1976,
in Salt Lake City, Utah.
He is the second child of seven:
five boys and two girls.
Upon graduation from high school
Trent attended Snow College in Ephraim, Utah, for one
quarter.
He then served a two-year voluntary mission for
his church in British Columbia.
Trent returned to his
studies at Snow College and graduated with an Associate’s
degree.
He further studied at Southern Utah University in
Cedar City, Utah, and at Utah Valley State College in Orem,
Utah.
In March of 1998, Trent married his sweet wife Debbie.
They are the parents of four daughters: Audrey, Lauryn,
Tricia, and Julia.
From July 2001 to May 2005 he attended the Indiana
University School of Dentistry.
From IU he received the
degree of Doctor of Dental Surgery.
In 2005 he began graduate studies in orthodontics at
Saint Louis University in St. Louis, Missouri, where he is
currently a Candidate for the degree of Master of Science
in Dentistry.
After graduation he and his family will
return home to Richfield, Utah, to begin private practice
and enjoy time with friends and family.
53