effect of force directions in maxillary anterior teeth
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EFFECT OF FORCE DIRECTIONS IN MAXILLARY
ANTERIOR TEETH RETRACTION WITH SKELETAL
ANCHORAGE: FINITE ELEMENT ANALYSIS
BY
MISS NANTAPORN RUENPOL
A DISSERTATION SUBMITTED IN PARTIAL FULFILLMENT OF
THE REQUIREMENTS FOR THE DEGREE OF
THE DOCTOR OF PHILOSOPHY (ORAL HEALTH SCIENCE)
FACULTY OF DENTISTRY
THAMMASAT UNIVERSITY
ACADEMIC YEAR 2015
COPYRIGHT OF THAMMASAT UNIVERSITY
Ref. code: 25585013320022TJR
EFFECT OF FORCE DIRECTION IN MAXILLARY
ANTERIOR TEETH RETRACTIOIN WITH SKELETAL
ANCHORAGE: FINITE ELEMENT ANALYSIS
BY
MISS NANTAPORN RUENPOL
A DISSERTATION SUBMITTED IN PARTIAL FULFILLMENT OF
THE REQUIREMENTS FOR THE DEGREE OF
THE DOCTOR OF PHILOSOPHY (ORAL HEALTH SCIENCE)
FACULTY OF DENTISTRY
THAMMASAT UNIVERSITY
ACADEMIC YEAR 2015
COPYRIGHT OF THAMMASAT UNIVERSITY
Ref. code: 25585013320022TJR
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Dissertation Title Effect of Force Direction in Maxillary Anterior
Teeth Retraction with Skeletal Anchorage: Finite
Element Analysis
Author Miss Nantaporn Ruenpol
Degree Doctor of Philosophy (Oral Health Science)
Department/Faculty/University Faculty of Dentistry
Thammasat University
Dissertation Advisor
Dissertation Co-Advisor
Asst. Prof. Dr.Nongluck Chareonworaluck,
D.D.S., Grad.Dip.Clin.Sc.(Prosthodotnics),
Dr.med.dent.(Orthodontics)
Asst. Prof. Dr.Vitoon Uthaisangsuk,
Dr.-Ing.
Academic Year 2015
ABSTRACT
The aim of the study was to compare the force directions to allow bodily-
like parallel retraction of maxillary anterior teeth. To calculate the tooth elements,
surface model of the tooth was made based on a dental study model (i21D-400C;
Nissin Dental Products, Kyoto, Japan). This procedure consisted of 3 steps: Firstly,
sectional images of the dental study model were taken by using dental cone-beam
computed tomography (i-CAT, U.S.A.). Secondly, the finite element model was
developed using MSC Patran (MSC Software, Inc.,USA.). The thickness of
periodontal ligament (PDL) was considered to be uniform (0.25 millimeters). The
alveolar bone was constructed to follow the curve of cementoenamel junction. The
thickness of cortical bone was 0.5 millimeters. The bracket slot was 0.018x0.025 inch
(Gemini, 3M Unitek, U.S.A.), in which the 0.016x0.022 inch stainless steel archwire
was inserted. Brackets of the anterior teeth were ligated firmly to the archwire. On
these teeth, the archwire and the brackets were moved as an united body.
Alternatively, buccal tubes of the posterior teeth were loosely engaged to the archwire
so that they could be slided along the archwire, but sagittal rotation of the archwire
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was restricted by bracket slots. The frictional coefficient was ignored. Mini-screws or
fixed points were placed in alveolar bone between the second premolars and the first
molars in position of 3.0, 5.0, 7.0 and 9.0 millimeters from the archwire bilaterally to
simulate the en masse retraction of the anterior teeth. Power arms which were made
from stainless steel wire were bonded to the archwire between the lateral incisors and
the canines. Thirdly, orthodontic forces were applied from the mini-screws to the
power arms. A line joining the mini-screw with the power arm was the line of action
of the force. According to clinical cases, the magnitude of the orthodontic force was
assumed to be 2.0 N (200 grams). In order to change the force direction, the length of
the power arms was varied from 3.0, 5.0, 7.0 and 9.0 millimeters. The incisal edge
and apex changes of maxillary central incisor were calculated and compared to each
force direction.
The results were divided into two parts: non-adjusted base of brackets
model or non-straight wire model and adjusted base of brackets model which caused
archwire be straightened. In non-adjusted base of brackets model or non-straight wire
model, wire was not straight and did not afford for teeth movement. High stress
occurred in cervical of maxillary lateral incisor, maxillary first molar and buccal of
maxillary of second molar. Crown of maxillary lateral incisor displaced more than the
other teeth. In periodontal ligament, the highest strain occurred nearly the cervical of
maxillary lateral incisor. In adjusted base of brackets model or straight wire model,
wire was straight and afforded for teeth movement like the teeth which have been
leveled and aligned already. High stress occurred in cervical of maxillary lateral
incisor, maxillary first molar, maxillary canine, maxillary second premolar and buccal
of maxillary of second molar. Crown of maxillary lateral incisor displaced more than
the other teeth. In periodontal ligament, the highest strain occurred nearly the cervical
of maxillary lateral incisor.
The incisal edge and apex changes of maxillary central incisor were
calculated and compared for each force direction. In two simulated models, 3.0
millimeters of mini-screws and 9.0 millimeters of power arm showed the least
differential change or nearly zero of the incisal edge and apex. However, the
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differential change of the incisal edge and apex in adjusted base of brackets model or
straight wire model was only minus value. That means while incisal edge of maxillary
central incisor moved linqually in upper anterior teeth retraction, apex of maxillary
central incisor moved more labially. The next order which had the least change or
nearly zero in adjusted base of bracket model was 9.0 millimeters of mini-screws and
9.0 millimeters of lever arm in adjusted base of bracket model or straight wire model.
So during the simulation, 9.0 millimeters mini-screw and 9.0 millimeters lever arm
was the force direction which bodily movement nearly to be occurred. The maxillary
lateral incisor was protruded and more protruded in adjusted base of brackets model
or straight wire model than non-adjusted base of brackets model or non-straight wire
model. It might be hardly movement of adjacent teeth that limited maxillary lateral
incisor movement of non-adjusted base of brackets model or non-straight wire model.
In clinical application, less crowding and fair aligned teeth can be applied anterior
retraction force, and there is less protrude of maxillary lateral incisor than well
aligned teeth. However, periodontal strain in adjusted base of brackets model or
straight wire model is greater than non-adjusted base of brackets model or non-
straight wire model, which risk or induce to occur root resorbed in these area.
Keywords: Skeletal anchorage, Finite element analysis, Retraction
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ACKNOWLEDEMENT
The author would like to express deep appreciation and gratitude to
advisor, Asst. Prof. Dr.Nongluck Chareonworaluck, D.D.S., Grad.Dip.Clin.Sc.
(Prosthodotnics), Dr.med.dent., Asst. Prof. Dr.Vitoon Uthaisangsuk, Dr.-Ing. for their
kind support, valuable guideance and dedication in this study.
Grateful acknowledgement is also extended to Mr. Sedthawat
Sucharitpwatskul and Mr. Prasit Wattanawongsakun from National Metal and
Materials Technology Center (MTEC) for technical support, general helps during the
long-lasting analysis and occasionally encouragement during the study.
Miss Nantaporn Ruenpol
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TABLE OF CONTENTS
Page
ABSTRACT (1)
ACKNOWLEDGEMENTS (4)
LIST OF TABLES (8)
LIST OF FIGURES (9)
LIST OF ABBREVIATIONS (13)
CHAPTER 1 INTRODUCTION 1
1.1 Biomechanics of maxillary anterior teeth retraction 3
1.1.1 Sequential retraction 3
1.1.2 En Masse retraction 4
1.2 Anchorage in orthodontic treatment 7
1.3 Skeletal anchorage 7
1.4 En masse retraction with skeletal anchorage 12
CHAPTER 2 REVIEW OF LITERATURE 14
2.1 Center of resistance 14
2.2 Length of power arm 15
2.3 Skeletal anchorage position 15
2.4 Finite element analysis 19
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Page
CHAPTER 3 RESEARCH METHODOLOGY 21
3.1 Research problem 21
3.2 Hypothesis 21
3.3 Objectives of the study 21
3.4 Scope and limitation of the study 22
3.5 Conceptual framework 22
3.6 Materials and Methods 23
CHAPTER 4 RESULTS AND DISCUSSION 26
4.1 Results 26
4.1.1 Model for simulation 26
4.1.1.1 Non-adjusted base of brackets or non-straight wire model 26
4.1.1.2 Adjusted base of brackets or straight wire model 30
4.1.2 Compare the incisal edge and apex changes of maxillary 34
central incisor in Y axis
4.1.3 Compare the incisal edge and apex changes of maxillary 39
central incisor in Z axis
4.2 Discussion 43
4.2.1 Model for simulation 43
4.2.2 Compare the incisal edge and apex changes of maxillary 44
central incisor
CHAPTER 5 CONCLUSIONS AND RECOMMENDATIONS 55
REFERENCES 56
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APPENDICES 61
APPENDIX A: Non-straight wire model 62
APPENDIX B: Staight wire model 63
BIOGRAPHY 89
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LIST OF TABLES
Tables Page
1.1 The number of mini-screws in the delivery program of different companies. 10
3.1 Material properties. 25
4.1 The change of the incisal edge and apex in original and new position in 36
Y axis of non-straight model.
4.2 The change of the incisal edge and apex in original and new position in 37
Y axis of straight wire model.
4.3 The differential change of the incisal edge and apex in original and new 38
position in Y axis of non-straight wire model.
4.4 The differential change of the incisal edge and apex in original and new 39
position in Y axis of straight wire model.
4.5 The incisal edge change in original and new position in Z axis of 40
non-straight wire model.
4.6 The apex change in original and new position in Z axis of 41
non-straight wire model.
4.7 The incisal edge change in original and new position in Z axis of 42
straight wire model.
4.8 The apex change in original and new position in Z axis of 43
straight wire model.
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LIST OF FIGURES
Figures Page
1.1 Anchorage loss during canine retraction: a) initially - class I on both side b) 2
finally - cusp to cusp relationship due to mesial displacement of upper molars.
1.2 Force vectors in a case of translation movement of the anterior teeth. 6
1.3 Graphic presentations of various types of orthodontic cortical anchorage. 9
1.4 Different types of AbsoAnchor mini-screws. 11
1.5 Diagonal (oblique) and perpendicular insertion of mini-screws. 11
2.1 Clinical setups for en masse retraction. 16
2.2 Force system involved:total force (F), intrusive force (i), retractive force (r) 16
(r is much greater than i).
2.3 Step-by-step construction of mini-screw system. Bodily retraction of upper 17
incisors is induced by appropriate line of force. A, Step 1: type of tooth
movement required is determined. B, Step 2: desirable line of force is
determined from a diagram or lateral cephalogram. C, Step 3: mini-screws
are placed exactly on the planned lines of force. To complete the line of
force, the power arms are extended so that the position of elastics
coincides with the planned lines of force.
2.4 Limitations in construction of line of force. Both the insertion site and the 18
length of the power arm are limited on labial/buccal side of the arch of the
soft tissue impingement.
2.5 In case of bodily retraction of incisors, power arms are extended from the 18
main arch, and the line of force becomes closer to the center of resistance.
Additional torque on the archwire is required for the maintenance of proper
moment/ force (/F) ratio in force system.
3.1 Model for tooth movement simulation 21
3.2 Dental study model (i21D-400C; Nissin Dental Products, Kyoto, Japan). 24
4.1 Non-adjusted base of brackets or non-straight wire model. 26
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4.2 Bracket, archwire and power arm in non-adjusted base of brackets or 27
non-straight wire model
4.3 Teeth in simulated model. 27
4.4 Stress in non-adjusted base of brackets or non-straight wire model. 28
4.5 Displacement in non-adjusted base of brackets or non-straight wire model. 29
4.6 Periodontal ligament strain in non-adjusted base of brackets or non-straight 30
wire.
4.7 Adjusted base of brackets or straight wire model. 31
4.8 Bracket, archwire and lever arm in adjusted base of brackets or straight 31
wire model.
4.9 Stress in adjusted base of brackets or straight wire model. 32
4.10 Displacement in adjusted base of brackets or straight wire model 33
4.11 Periodontal ligament strain in adjusted base of brackets or straight wire 34
Model.
4.12 Incisal edge (upper arrow) and apex (lower arrow) of maxillary central 35
Incisor.
4.13 3D finite element model of maxillary dentition, including PDL, alveolar 45
bone, brackets, and archwire.
4.14 Movement patterns with a low-position miniscrew placed 4.0 millimeters 47
gingivally to the archwire. In all cases, the entire dentitions rotate,
because the lines of action of the force pass below both centers of
resistance (CR1 and CR2). Rotation and intrusion of the entire dentition
decreases with an increase in the length of the power arm. A, Power arm
1.0 millimeter in length; B, power arm 4.0 millimeters in length; C,
power arm 8.0 millimeters in length.
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Page
4.15 Movement patterns with a high-position mini-screw placed 8.0 millimeters 48
gingivally to the archwire. In all cases, rotations of the entire dentition are
smaller than those in the case of low-position mini-screws;the anterior move
almost bodily, because the lines of action of the force shift closer to the
centerof resistance of the anterior\teeth (CR1) and pass above the center
of resistance of the posterior teeth (CR2). Rotation and intrusion of the
entiredentition decrease with an increase in the length of the power arm.
A, Power arm 1.0 millimeter in length; B, power arm 4.0 millimeters in
length; C, power arm 8.0 millimeters in length.
4.16 Intraoral picture and the illustration of the retraction of the anterior tooth 50
using various lengths of power arm and implant anchorage in sliding
mechanics.
4.17 Illustration of the cross section of a play constructed in the model. 50
4.18 Loading conditions when controlled anterior tooth movements are 51
performed (a) in case of using 0.018 × 0.025 inch stainless steel archwire
(without play); (b) in case of using 0.016 × 0.022 inch archwire (with play).
CRe, center of resistance; CRo, center of rotation.
4.19 Sagittal cross section at the mesial surface of the maxillary central incisor 52
bracket in the model which has a play. (a) before orthodontic force
applied; (b) after the application of the force at the level of 12.0
millimeters
6.1 Non-straight wire model in frontal view. 62
6.2 Non-straight wire model after separate teeth, brackets, power arms and wire 63
in occlusal view.
6.3 Non-straight wire model after separate teeth, brackets, power arms and wire 63
in frontal view.
6.4 Non-straight wire model after separateteeth, brackets, power arms and wire 63
in superior view view.
6.5 Non-straight wire model after separate teeth, brackets, power arms and wire 64
in palatal view.
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6.6 Non-straight wire model after separate teeth, brackets, power arms and wire 65
in occlusal view.
6.7 Non-straight wire model after separate teeth, brackets, power arms and wire 66
in occlusal view.
6.8 Non-straight wire model after separate teeth, brackets, power arm and wire 66
in lateral view (mesh structure).
6.9 Non-straight wire model after separate teeth, brackets, power arm and wire 67
in frontal view (mesh structure).
6.10 Non-straight wire model after separate teeth, brackets, power arm and wire 67
in occlusal view (mesh structure).
6.11 Non-straight wire model after separate teeth, brackets, power arm and wire 68
in occlusal view (mesh structure).
6.12 Non-straight wire model after separate teeth, brackets, power arm and wire 68
in palatal view (mesh structure).
6.13 Non-straight wire model after separate brackets, power arm and wire in 69
lateral view.
6.14 Non-straight wire model after separate brackets, power arm and wire in 69
occlusal view.
6.15 Non-straight wire model after separate brackets, power arm and wire in 70
palatal view.
6.16 Stress of teeth in non-straight wire model in lateral view. 71
6.17 Stress of teeth in non-straight wire model in frontal view. 71
6.18 Stress of periodontal ligament in non-straight wire model. 72
6.19 Play of archwire in bracket slot in non-straight wire model(pink is original 72
position).
6.20 Displacement of teeth in non-straight wire model in occlusal view 73
(mesh structure) .
6.21 Displacement of teeth in non-straight wire model in lateral view 73
(mesh structure).
6.22 Displacement of teeth in non-straight wire model in frontal view. 74
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6.23 Displacement of teeth in non-straight wire model in palatal view. 74
6.24 Strian in periodontal space in non-straight wire model. 75
6.25 Straight wire model in frontal view. 76
6.26 Straight wire model in frontal view (mesh structure). 76
6.27 Straight wire model after separate teeth, brackets, power arms and wire 77
in occlusal view.
6.28 Straight wire model after separate teeth, brackets, power arms and wire in 78
superior view view.
6.29 Straight wire model after separate teeth, brackets, power arms and wire in 78
frontal view.
6.30 Straight wire model after separate teeth, brackets, power arms and wire in 79
palatal view.
6.31 Straight wire model after separate teeth, brackets, power arms and wire in 79
lateral view (mesh structure).
6.32 Straight wire model after separate teeth, brackets, power arms and wire in 80
occlusal view.
6.33 Straight wire model after separate teeth, brackets, power arms and wire in 80
occlusal view.
6.34 Straight wire model after separate teeth, brackets, power arm and wire in 81
lateral view (mesh structure).
6.35 Straight wire model after separate teeth, brackets, power arm and wire in 81
occlusal view (mesh structure).
6.36 Straight wire model after separate teeth, brackets, power arm and wire in 82
frontal view (mesh structure).
6.37 Straight wire model after separate teeth, brackets, power arm and wire in 82
occlusal view (mesh structure).
6.38 Straight wire model after separate teeth, brackets, power arm and wire in 83
palatal view (mesh structure).
6.39 Straight wire model after separate brackets, power arm and wire in 83
lateral view.
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Page
6.40 Straight wire model after separate brackets, power arm and wire in 84
occlusal view.
6.41 Straight wire model after separate brackets, power arm and wire in 84
palatal view.
6.42 Stress of teeth in straight wire model in lateral view. 85
6.43 Stress of teeth in straight wire model in frontal view. 85
6.44 Stress of periodontal ligament in straight wire model. 86
6.45 Play of archwire in bracket slot in non-straight wire model (pink is original 86
position).
6.46 Displacement of teeth in straight wire model in occlusal view (mesh structure).87
6.47 Displacement of teeth in straight wire model in lateral view (mesh structure). 87
6.48 Displacement of teeth in non-straight wire model in frontal view. 88
6.49 Displacement of teeth in non-straight wire model in palatal view. 88
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LIST OF ABBREVIATIONS
Symbols/Abbreviations Terms
PDL
CEJ
mm
3D
FE
FEM
Pa
CR, CRe
CRo
M/F
N
MI
SAS
TAD
TSR
ER
Periodontal ligament
Cementoenamel junction
millimeter
3 dimension
Finite element
Finite element method
Pascal
Center of resistance
Center of rotation
moment/force
Newton
Mini implant, Micro-implant
Skeletal anchorage system
Temporary anchorage device
Two-step retraction
En masse retraction
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CHAPTER 1
INTRODUCTION
Orthodontics is one of the area of dentistry concerned with the
supervision, guidance and correction of growing and mature dento-facial structures,
including those conditions that require movement of the teeth or correction of mal-
relationships and malformations of related structures, by the adjustment of
relationships between and among teeth and facial bones by the application of forces
and/or the simulation and redirection of the function forces within the craniofacial
complex.1
In corrective orthodontic treatment, the objectives are to correct the
abnormality of teeth, occlusion and facial skeletal relations and to correct overall
abnormal problems of orthodontic patients. Corrective orthodontic treatment with
fixed appliances consists of five phases as follows:
1. Leveling phase: the objectives are to level the tooth positions and to
correct tooth relations.
2. Movement phase: the objectives are to move the canine, premolar and
molar teeth in three planes in order to establish the
posterior occlusion as determined.
3. Contraction phase: the objectives are to sagitally move the maxillary and
mandibular anterior teeth and to adjust the maxillary
and mandibular tooth inclination in order to establish
normal overjet and overbite.
4. Adjustment phase: the objectives are to precisely adjust the occlusion.
5. Retention phase: the objectives are to control tooth position and
occlusion change after removal of fixed appliances.1
Orthodontic movements have been described in the literature for over
100 years, since Edward Hartley Angle had introduced foundations of malocclusion
treatment. Numerous appliances and techniques have been designed to accomplish
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treatment goals. Independently on the treatment plan calling either for reduction of
teeth number or dental arch expansion and despite modern and sophisticated
orthodontic appliance or technique. The most currently performed dental movements
base on Newton’s 3rd law established already in 1687:to every action there is always
opposed an equal reaction or the mutual actions of two bodies upon each other are
always equal, and directed to contrary parts.2
Figure 1.1 Anchorage loss during canine retraction: a) initially - class I on both side
b) finally - cusp to cusp relationship due to mesial displacement of upper
molars.3
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1.1 Biomechanics of maxillary anterior teeth retraction13
Several techniques of space closure are used in orthodontics. Especially in
case of maxillary teeth protrusion or crowding, maxillary first premolars are often to
be removed. Maxillary anterior teeth retraction is identified by retraction of the
anterior teeth as one group. The most frequently used techniques are: Two-step
retraction (TSR) or Sequential retraction (retraction of canine teeth followed by
retraction of all four incisors) and En masse retraction (ER) (retraction of all six
anterior teeth). The two-step retraction approach allows retraction of canine teeth
independently, followed by retraction of incisors in a second step, this helps to obtain
greater retraction of the anterior teeth by reducing the tendency of anchorage loss
through incorporating more teeth in the anchorage unit (Figure 1.1). However,
closing spaces in two-steps might take a longer treatment time. In addition, when
canines are retracted individually they tend to tip and rotate more than when the six
anterior teeth are retracted as a single unit. That means en masse retraction is good in
keeping the alignment of the anterior teeth during treatment.
1.1.1 Sequential retraction
Two-step retraction is indispensable sometimes, especially when teeth
are misaligned, severely protruded, and severely crowded. Open bites also may
support the sequential retraction approach; not only because of dental protrusion, but
also because of tongue thrusting that usually accompanies such cases, which makes
optimal forces adjustment mostly impossible. Ectopic eruption of canines may lead
the orthodontist to follow sequential retraction, as it is impossible to retract the
anterior teeth as a group when canines are highly leveled.
Sequential retraction has two phases. Firstly, canines are moved
posteriorly, then canines are congregated with the posterior units of second premolars
and first molars (in addition to second molars if they are banded) to form one group.
Secondly, the anterior four incisors are retracted. Sequential retraction may cause
temporary spaces, which are often unwelcome, especially between lateral incisors and
canines. In the first phase of sequential retraction, it is recommended that a stop on
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the mesial of the molar tube be placed14, to maintain the anchorage by preventing its
“burning” (by a potential movement of the first molar mesial).4 This stop has its own
reaction on the pertinent incisors; consequently, the incisors tend to move anteriorly
during the canine retraction phase, which increases the burden on the anchorage units
during “phase two” or incisors retraction. In other words, the concept that sequential
retraction reduces anchorage requirements is gradually becoming a debatable issue.
Orthodontists have utilized sequential retraction mechanics for
decades, based on a hypothesis that presumes two-phased retraction protects
anchorage better than one-phased retraction. The sequential retraction technique has
been mistakenly related to anchorage fortification. Recently, there has been a
revitalization of the en masse retraction technique.
1.1.2 En Masse retraction
As “anchorage preserving” has been the sought-after notion that often
preoccupies orthodontists, it is recommended that another “out of squad” notion be
mentioned—that is, “anchorage burn” as a part of treatment plan. It is worthwhile to
discuss cases that either need minimum anchorage or require burning of anchorage.
As such cases are seen in a clinician’s daily practice. Examples include cases where
posterior teeth have interdental spaces or when a treatment plan requires mesial
translation to one or more posterior molars. In such cases, en masse retraction helps as
a facilitation factor, if forces are correctly adjusted to fulfill such intentions. It is
logical that sufficient force be adjusted to retract the anterior teeth and “protract” the
targeted posterior tooth/teeth. As a result, the clinician is encouraged to analyze the
requirements of forces and moments of each case independently, whether to retract
the anterior tooth/teeth, protract the posterior tooth/teeth, or retract and protract teeth
simultaneously.
An advantage of en masse retraction in maintaining the “Leveling of
Alignment” of anterior teeth should be taken into account by the clinician.
Application of the “Theory of Optimal Force Values”, which depends on using
continuous low force, as minimal as available, and simultaneously over the due
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threshold that is sufficient to cause tooth movement, it is possible to retract canines
and incisors “en masse” without causing excessive anchorage loss in the posterior
segments. It can be suggested that forces necessary to retract the anterior segment
dissipate and are below the biological threshold to cause substantial movement of the
posterior anchorage units. The reasons for the revitalization of the notion of en masse
retraction are:
1. The advent of mini-screws and relevant temporary anchorage devices.
2. The perception of “Optimal Forces Application”
3. The application of contemporary biomechanic principle that could support
the “one-phased retraction” approach, as translation movement helps (in
the case of en masse retraction) in conserving the leveling and alignment
of anterior teeth and in avoiding the high “moment-values” effects. (High
moments may affect the anchorage units, as a reaction, to counteract the
moments of the anterior teeth). Nonetheless, en masse retraction is not
indicated in all cases and is not a panacea. In other words, there is no
magic potion in orthodontics; thus, contraindication of en masse retraction
should also be respected.
There are examples of the contraindication of en-masse retraction and
the indication of sequential retraction in extraction cases:
1. Severe crowding, especially when anterior teeth are severely misaligned
and badly leveled.
2. The cases where sectional archwires are indicated.
3. Ectopic eruption of the canine, which may be a contraindication to one-
phased retraction because of difficulty to perform such multiple
simultaneously objectives.
4. Cases of severe protrusion when optimal forces are difficult to attain.
Especially, when crowns of anterior teeth should be tipped posteriorly and
retracted.
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The clinician is encouraged to discern between cases; consequently, en
masse retraction may be applied when:
1. Anterior teeth are in order with good alignment.
2. Crowding is mild.
3. Protrusion is either moderate or mild.
4. Continuous archwire mechanics are used.
5. The least friction and absence of notching is preferable with the notion of
optimal force.
6. Cases where burning of anchorage forms a part of treatment plan.
7. Cases when mini-screws are used.
The optimal force notion in en masse retraction depends on the ability
to apply low force slightly above the thresholds to retract the anterior teeth in the
expected direction such as translation movement (Figure 1.2), while simultaneously
such forces are counteracted and dissipated in the posterior teeth without remarkable
anchorage loss. The “optimal force” here is the “minor force” which is slightly above
the “threshold force” and the threshold force is the least available force to move a
tooth.
Figure 1.2 Force vectors in a case of translation movement of the anterior teeth.13
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Clinically, en masse retraction is an easy approach, as it is available to
be applied when the anchor units are the posterior teeth such as second premolars,
first molars, and second molars if included. In spite of the belief to which many
orthodontists subscribe regarding sequential retraction (as they think that it conserves
more of the anchorage in extraction cases), the clinical views show sometimes
contradictory paradigms to such a notion.
1.2 Anchorage in orthodontic treatment
Anchorage control plays a role in the effective management of orthodontic
patients for obtaining both structural and facial esthetics. Anchorage is defined as the
resistance to unwanted tooth movement or as the desired reaction of posterior teeth to
spaced closure biomechanic therapy.5-6 Depending on the requirement, it can be
classified as minimum, medium, or maximum anchorage.7 Maximum anchorage is
needed when the treatment objectives require that no or very little anchorage can be
lost.8 Obtaining maximum or absolute anchorage always has been a goal for the
orthodontist, often resulting in a condition, called anchorage loss. Anchorage loss is
the reciprocal reaction of the anchor unit that can obstruct the success of orthodontic
treatment by complicating anteroposterior correction.15 To address this problem,
many appliances and techniques have been devised; Nance holding arch, transpalatal
arch, extraoral traction. Multiple teeth at the anchorage segment and differential
moments are some commonly used ones.16-18 However, all these methods have a few
inherent disadvantages: complicated designs, need for exceptional patient
cooperation, elaborate wire bending, and so on.
1.3 Skeletal anchorage
In recent years, titanium screws have gained enormous popularity in the
orthodontic community and are being considered as absolute sources of orthodontic
anchorage.9,19-20 Their primary advantages are easy placement and removal,
immediate loading, placement at various anatomic locations including the alveolar
bone between the roots of teeth, and low cost. There have been much research to
support it.
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In 1945, Gainsforth and Higley introduced concept of skeletal anchorage,
conductive to the later discovery of osseointegration.21 In 1969, Brånemark et al.
developed the concept of osseointegration, using pure titanium implants.22 Later, the
discoveries were going on. In 1980, several animal studies on the use of titanium
implant in orthodontics reported successful results. Kokich et al. also reported a novel
source of absolute anchorage, ankylosis of deciduous tooth was used to protract the
maxilla in 1985.23 Subsequently, mid-palatal implants and onplants are introduced to
use as skeletal anchorage. In 1997, Kanomi reported using a mini-implant for
orthodontic anchorage.24 Moreover, Umemori et al., used the titanium mini-plates for
anchorage to intrude the lower posterior teeth anchorage.25 Afterward, many books
and many type of mini-screws implant anchorage were published (Figure 1.3, 1.4,
Table 1.1).9-12 Orthodontists enable to move patient's teeth in a variety of ways and
mini-screws implants can be placed in the jaw bone, as well as between the root of
teeth. The only disadvantage that has been linked to the use of mini-screw is the pain
caused during the creation of the starter hole drilled before driving a mini-screw into
the bone. These devices have been called by various names some of the popular ones
are52
• Mini-screw
• Mini implant- MI
• Micro-implant- MI
• Skeletal anchorage system- SAS
• Temporary anchorage device- TAD
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Figure 1.3 Graphic presentations of various types of skeletal anchorage.11
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Table 1.1 The number of mini-screws in the delivery program of different companies11
Number of mini-screws
Mini-screw name Company Diameter Lenght Design
variants Total
Aarhus-Mini-implant Medicon, Germany 3 11 4 11
AbsoAnchor Dentos, Korea 7 6 7 154
Anchor Plus /
NeoAnchor Plus Myungsung, Korea 2 5 2 40
Ancotek Tekka, France 3 4 22 70
Dual-top Anchor
Screw
Jeil Medical Corp,
Korea 3 3 5 40
LOMA Mondeal, Germany 3 5 3 44
MTAC American
Orthodontics, USA 2 2 1 4
O.A.S.I. Lacer Orthodontics,
USA 1 3 2 6
Orlus Ortholution, Korea 2 8 7 11
Ortho Anchor Screw KLS Martin, Germany 1 2 1 2
Ortho Easy* Forestadent, Germany 1 2 1 2
Ortho implant IMTEC Corp, USA 1 3 1 3
Orthoanchor Dentsply-Sankin,
Japan 1 3 1 3
Orthodontic Mini
Implant Leone, Italy 2 4 5 18
Orthodontic Mini
Implants
Bio Materials
Korea,Korea 9 9 4 19
Spider Screw HDC, Italy 2 7 6 21
tomas*-pin Dentaurum, Germany 1 3 2 6
*Screws that are available as sterile and non-sterile were count only once
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Figure 1.4 Different types of AbsoAnchor mini-screws.54
Figure 1.5 Diagonal (oblique) and perpendicular insertion of mini-screws.53
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These screws have spawned many clinical applications, such as en masse
retraction of anterior teeth. Buccal interdental gingival are the areas which mini-
screws or small size of skeletal anchorage are placed.9-11 Also in choosing the proper
length of a mini-screw, the path of insertion of the mini-screw must be considered. A
mini-screw can be placed either in a diagonal direction or a perpendicular direction
relative to the cortical bone surface. It is better and easier to place mini-screw in a
perpendicular direction, but, there are many situations in which the mini-screw should
be placed in a diagonal direction so as to avoid injury to an adjacent tooth root. When
the mini-screw is placed in a diagonal direction rather than perpendicular direction, it
is better to use a slightly longer mini-screw (Figure 1.5). Mini-screws made by
titanium alloys of this thickness can be inserted safely without pre-drilling on
maxillary buccal areas. However, the treatment effects of skeletal anchorage for teeth
movement are largely unsubstantiated.26
1.4 En masse retraction with skeletal anchorage
When the maxillary anterior teeth are retracted in premolar extraction
cases, the control of force vectors and moments is important to achieve the desired
tooth movement. The applied moment-to-force ratio on the upper anterior teeth
determines the type of tooth movement, such as uncontrolled tipping, controlled
tipping, bodily or root movement. In addition, the direction and the application point
of retraction force in relation to the location of the center of resistance (CR) are
critical factors in predicting and planning the esthetic tooth movement of anterior
teeth. The direction of force application has been controlled mainly by changing the
vertical vector of the force, influenced by the length and the position of retraction
hooks on working wire and position of mini-screws placement. In 2011, Tominaga et
al. studied placement of the power arm of an archwire between the lateral incisor and
canine enables orthodontists to maintain better control of the anterior teeth in sliding
mechanics. Both the biomechanical principles associated with the tooth’s center of
resistance and the deformation of the archwire should be taken into consideration for
predicting and planning orthodontic tooth movement.27 However, anatomical
limitations and individual patient variations have made this control difficult.
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Moreover, the combination of appliance with the proper position of mini-screw is
expected to allow bodily-like parallel and the efficiency retraction of anterior teeth.
Therefore, the direction of forces which come from temporary anchorage devices and
length of power arm in en masse retraction are effect to create the pattern of maxillary
anterior teeth movement such as bodily movement or tipping movement.
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CHAPTER 2
REVIEW OF LITERATURE
The development of mini-screw or the temporary skeletal anchorage
device has contributed to various applications. In case of maxillary anterior teeth
retraction, the retraction force on maxillary anterior teeth as well as to the
establishment of absolute anchorage such as buccal mini-screw anchorage doesn’t
need more surgery. It’s easy to orthodontist and patient to achieve the goal of
treatment. That means, the total of retraction force effected directly to maxillary
anterior teeth while anchorages did not movement which are the purpose of treatment.
There are many studies reported the success of the treatment.
In buccal mini-screws anchorage appliance, Park et al. reported the
efficacy of sliding mechanics with mini-screw implant anchorage on the treatment of
skeletal Class II malocclusion.28 In 2008, Upadhyay et al. studied buccal mini-
implants were efficient for intraoral anchorage reinforcement for en masse retraction
and intrusion of maxillary anterior teeth (Figure 2.1, 2.2).29 Maximum anchorage was
achieved without appliances in the posterior dentition. En masse retraction of the six
anterior teeth can be accomplished by using buccal temporary skeletal implant as the
only source of anchorage.30
2.1 Center of resistance
The appropriate center of resistance point (CR) is effect to bodily
movement of the target teeth. Melson et al. and Sung et al. had estimated the CR of
six anterior teeth to be located 13.5 millimeters posteriorly and 9.0 millimeters
superiorly from the center of the archwire.31-32 In 2007, Sia et al. concluded that the
location of the center of resistance of the maxillary central incisor was approximately
0.77 of the root length from the apex.33 Where in a single force passing through the
CR causes bodily tooth movement. Since the location of the CR of the incisor was
determined to be at the level of 7.2 millimeters apically from the bracket slot from FE
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analysis, bodily movement was expected to be produced at 7.2 millimeters of the
height.34
2.2 Length of power arm
During anterior tooth retraction with sliding mechanics, controlled crown-
lingual tipping, bodily translation movement, and controlled crown-labial movement
could be achieved by attaching a power arm length that was lower, equivalent, or
higher than the level of the center of resistance, respectively. The power arm length
could be the most easily modifiable clinical factor in determining the direction of
anterior tooth movement during retraction with sliding mechanics. Sia et al. reported
6.8 millimeters, 6.5 millimeters, and 7.5 millimeters power arm length which are
center of resistance of maxillary central incisor in 0.018x0.025 inch slot brackets with
0.016x0.022 inch Elgiloy archwire, three maxillary central incisor in three subjects
have translation movement.33 In 2010, Kim et al. studied the length of the power arm
increased as its position was moved from the lateral incisor to the premolar in
0.022x0.025 inch slot brackets with 0.021x0.025 inch stainless steel archwire. This
was because the length of the power arm must be increased to be in equilibrium
mechanically. The application of a 150 grams retraction force at this position
indicates more stable movement of the anterior teeth when approximate 5 millimeters
length of power arm was positioned between the canine and the lateral incisor.
Moreover, the parallel translation of the maxillary anterior teeth could be generated
more effectively.35 In the treatment of Angle Class II division 1 malocclusions, the
use of a power arm height of 4.0 millimeters to 5.0 millimeters for 0.018x0.025 inch
slot brackets with 0.018x0.025 inch stainless steel archwire is recommended to obtain
controlled lingual crown tipping of the maxillary central incisor.27
2.3 Skeletal anchorage position
In 2010, Sung et al. studied position of mini-screws placement 10.0 and
12.0 millimeters from the 0.019x0.025 inch stainless steel archwire in 0.022x0.025
inch slot brackets and 0.016x0.022 inch stainless steel archwire in 0.018x0.025 inch
slot brackets. They reported the position of mini-screws placement 12.0 millimeters
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and 8.0 millimeters anterior retraction hook condition, the force vector was applied
just above the CR for the six anterior teeth.32 In 2012, Kojima Y et al. stated that
movement pattern with a high position mini-screw placed 8.0 millimeters gingivally
to the 0.018x0.025 inch stainless steel archwire in 0.018x0.025 inch slot brackets,
rotations of the six anterior teeth in en masse retraction were smaller than in the case
of low position mini-screws. The anterior teeth moved almost bodily because the
action force line shifts closer the CR of the anterior teeth.36
Figure 2.1 Clinical setups for en masse retraction.29
Figure 2.2 Force system involved:total force (F), intrusive force (i),retractive force (r)
(r is much greater than i).29
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Figure 2.3 Step-by-step construction of mini-screw system. Bodily retraction of upper
incisors is induced by appropriate line of force. A, Step 1: type of tooth
movement required is determined. B, Step 2: desirable line of force is
determined from a diagram or lateral cephalogram. C, Step 3: mini-screws
are placed exactly on the planned lines of force. To complete the line of
force, the power arms are extended so that the position of elastics coincides
with the planned lines of force.12
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Figure 2.4 Limitations in construction of line of force. Both the insertion site and the
length of the power arm are limited on labial/buccal side of the arch of the
soft tissue impingement.12
Figure 2.5 In case of bodily retraction of incisors, power arms are extended from the
main arch, and the line of force becomes closer to the center of resistance.
Additional torque on the archwire is required for the maintenance of
proper moment/ force (M/F) ratio in force system.12
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2.4 Finite element analysis
New advances in 3-dimensional (3D) technology, such as computer-aided
design and computerized to motion imaging, allow for a more accurate description of
dental anatomy. Although the associated force transfer through the dentition during
orthodontic treatment frequently is statically indeterminate, these systems can be
solved by incorporating the principles of solid mechanics. However, current finite
element analysis that could predict applied forces with a continuous archwire is rarely
combined with 3D multiple tooth systems.
The finite element (FE) analysis has been proved to be a useful tool to
study orthodontic tooth treatments. Orthodontic tooth movement is achieved by
remodeling processes of the alveolar bone, which are triggered by changes in the
stress/strain distribution in the periodontium. In the past, the finite element (FE)
method has been used to describe the stressed situation within the periodontal
ligament (PDL) and surrounding alveolar bone. The present study sought to determine
the impact of the modeling process on the outcome from FE analyses and to relate
these findings to the current theories on orthodontic tooth movement. In a series of
FE analyses simulating teeth subjected to orthodontic loading, the influence of
geometry/morphology, material properties, and boundary conditions was evaluated.37
As a pioneer, Bourauel et al. executed the simulation of orthodontic tooth movement
in comparison with experimental investigations, and suggested that the movement is
controlled predominantly by mechanical deformations of the periodontal ligament
(PDL) rather than strains in alveolar bone.38-39 Subsequently, the FE simulation done
by Schneider et al., revealed that it possible to integrate a mechanical bone
remodeling algorithm into a realistic 3D tooth and jawbone model.40 Then, in order to
consider the characteristic nonlinear behavior of PDL, a hyperelastic approach was
developed by Natali et al., to analyze the mobility of human dentition under the action
of short lasting loads.41 Recently, Marangalou et al. constructed a computational
model to calculate the rate of orthodontic tooth bodily movement.42 Additionally,
based on the external bone remodeling mechanism, some of researcher has developed
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a numerical model to reproduce an orthodontic treatment of mandibular canine
tipping movement.43
Later study, finite element analysis can clarify the tooth movement and the
force system in en-masse sliding mechanics.36 Three-dimensional en-masse retraction
of the anterior teeth as an independent segment can be accomplished by using
partially osseointegrated buccal implants as the only source of anchorage, an intrusion
overlay archwire, and a retraction hook.44 De Lima Araújo et al. reported the buccal
mini-implants provided an adequate anchorage for the retraction of the anterior teeth,
and there was no loss in the anchorage of the posterior teeth.45
In 2014, Deepak et al. also showed the clinical study which the implant
group is better in three dimensional controlling compared to the non-implant group
during retraction. Therefore, the implant group definitely was superior to conventional
method.46 However, there are not enough retraction studies which clearly reported and
clarified about force direction that produced from the different length of powerarms
and mini-screws positions in maxillary arch (Figure 2.3-2.5).
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CHAPTER 3
RESEARCH METHODOLOGY
3.1 Research question?
Research question: Which force direction is expected to allow bodily-like
parallel retraction of maxillary anterior teeth?
3.2 Hypothesis
The changes of maxillary central incisor position in retraction with skeletal
anchorage by different force directions are not different.
3.3 Objective of the study
The aim of the study was to compare the force directions to allow bodily-
like parallel retraction of maxillary anterior teeth.
Figure 3.1 Model for tooth movement simulation.
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3.4 Conceptual framework
3.5 Scope and limitation of the study
The scope of the study is defined by the following conditions:
1. The geometrical assessment results were based on the computer model,
which may be different from living tissue.
2. The magnitude and direction of forces were taken from the literature to
evaluate mechanical performance by means of a finite element method.
3. The mechanical testing of human subjects is beyond the scoped of the
study.
Mini-screws
position
-3.0 millimeters
-5.0 millimeters
-7.0 millimeters
-9.0 millimeters
Length of power
arm
Force
Directions
-3.0 millimeters
-5.0 millimeters
-7.0 millimeters
-9.0 millimeters
Maxillary central
incisor position
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3.6 Material and Method
To calculate the tooth elements, surface models of the tooth were made
based on a dental study model (i21D-400C; Nissin Dental Products, Kyoto, Japan)
(Figure 3.2). This procedure consisted of 3 steps:
Firstly, sectional images of the dental study model were taken by using
dental cone-beam computed tomography (i-CAT, U.S.A.).
Secondly, the finite element model was developed using MSC Patran.
(MSC Software, Inc.,USA.) The model was meshed with 4-node-tetrehedron
elements. (Figure 3.1) Model was composed of elements varying from 113,341 to
118,728, and nodes ranging from 27,500 to 28,330. The thickness of PDL was
considered to be uniform (0.25 millimeters). The alveolar bone was constructed to
follow the curve of cementoenamel junction(CEJ) with cortical bone 0.5 millimeters.
The bracket slot was 0.018 inch (Gemini, 3M Unitek, U.S.A.), which the 0.016x0.022
inch archwire was inserted. Brackets of the anterior teeth were ligated firmly to the
archwire. On these teeth, the archwire and the brackets moved as an unit body.
Alternatively, buccal tubes of the posterior teeth were loosely engaged to the archwire
so that they could slide along the archwire, but sagittal rotation of the archwire was
restricted by bracket slots. The frictional coefficient was ignored. Proper material
properties were assigned for teeth, PDL, cortical bone, trabecular bone and stainless
steel (Table 3.1)47, with the assumption that all materials were isotropic and linearly
elastic. The Mini-screws or fixed points were placed in alveolar bone between the
second premolars and the first molars in a position of 3.0, 5.0, 7.0 and 9.0 millimeters
from the archwire bilaterally to simulate the en masse retraction of the anterior teeth.
Power arms were bonded to the archwire between the lateral incisors and the canines.
The power arms were made from stainless steel wire.
Thirdly, orthodontic forces were applied to the power arms from the mini-
screws. A line joining the mini-screw with the power arm was the line of action of the
force. According to clinical cases, the magnitude of the orthodontic force was
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assumed to be 2.0 N (200 grams). To change the force direction, the length of the
power arms was varied from 3.0, 5.0, 7.0 and 9.0 millimeters.
Maxillary anterior teeth movements were achieved by recorded the incisal
edge and apex of maxillary central incisor in original and new position. Then
calculated the incisal edge and apex changes of maxillary central incisor. And finally,
compared these changes of each force direction (Figure 3.1).
Figure 3.2 Dental study model (i21D-400C; Nissin Dental Products, Kyoto, Japan)
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Table 3.1 Material properties52
Poisson's ratio Young's modulus (Pa)
Teeth 0.31 1.80E+10
PDL 0.3 1.75E+09
Cortical bone 0.31 1.37E+10
Trabecular bone 0.3 1.37E+09
Stainless steel 0.3 2.00E+11
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CHAPTER 4
RESULTS AND DISCUSSION
4.1 Results
4.1.1 Model for simulation
4.1.1.1 Non-adjusted base of brackets or non-straight wire model
A dental study model (i21D-400C; Nissin Dental Products,
Kyoto, Japan) was used in this study. The bracket slot was 0.018x0.025 inch (Gemini,
3M Unitek, U.S.A.), which the 0.016x0.022 inch archwire was inserted (Figure 4.1).
When separated the materials, wire was not straight and did not afford for teeth
movement (Figure 4.2). Located X, Y and Z axis were mesio-distal, labio-lingual and
inciso-gingival consequently (Figure 4.3). Stress, displacement and strain was showed
in figure 4.4-4.6. High stress occurred in cervical of maxillary lateral incisor,
maxillary first molar and buccal of maxillary of second molar (Figure 4.4). Crown of
maxillary lateral incisor displaced more than other teeth (Figure 4.5). In periodontal
ligament, the highest strain occurred nearly the cervical of maxillary lateral incisor
(Figure 4.6).
Figure 4.1 Non-adjusted base of brackets or non-straight wire model.
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x
y
z
Figure 4.2 Bracket, archwire and power arm in non-adjusted base of brackets or non-
straight wire model.
z
x
y
Figure 4.3 Teeth in simulated model.
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Equivalent stress (MPa)
Figure 4.4 Stress in non-adjusted base of brackets or non-straight wire model.
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Displacement (millimeter)
z
x
y
Figure 4.5 Displacement in non-adjusted base of brackets or non-straight wire model.
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Strain (-)
Figure 4.6 Periodontal ligament strain in non-adjusted base of brackets or non-straight
wire.
4.1.1.2 Adjusted base of brackets or straight wire model
A dental study model (i21D-400C; Nissin Dental Products,
Kyoto, Japan) was used in this study. The bracket slot was 0.018x0.025 inch (Gemini,
3M Unitek, U.S.A.), which the 0.016x0.022 inch archwire was inserted and adjusted
base of bracket to achieve wire straighted (Figure 4.7). When separated the materials,
wire was straight and afforded for teeth movement (Figure 4.8). Located X, Y and Z
axis were mesio-distal, labio-lingual and inciso-gingival consequently (Figure 4.3).
Stress, displacement and strain was showed in figure 4.9-4.11. High stress occurred in
cervical of maxillary lateral incisor, maxillary first molar, maxillary canine, maxillary
second premolar and buccal of maxillary of second molar (Figure 4.9). Crown of
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maxillary lateral incisor displaced more than other teeth (Figure 4.10). In periodontal
ligament, the highest strain occurred nearly the cervical of maxillary lateral incisor
(Figure 4.11).
Figure 4.7 Adjusted base of brackets or straight wire model.
Figure 4.8 Bracket, archwire and lever arm in adjusted base of brackets or straight
wire model.
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Equivalent stress (MPa)
Figure 4.9 Stress in adjusted base of brackets or straight wire model.
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Displacement (millimeter)
z
x
y
Figure 4.10 Displacement in adjusted base of brackets or straight wire model.
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Strain (-)
Figure 4.11 Periodontal ligament strain in adjusted base of brackets or straight wire
model.
4.1.2 Comparison of the incisal edge and apex changes of maxillary
central incisor in Y axis.
The incisal edge and apex changes of maxillary central incisor in non-
straight wire model and straight-wire model in Y axis were calculated and compared
the changes of each force direction (Table 4.1-4.2). In two simulated model, 3.0
millimeters of mini-screws and 9.0 millimeters of power arm showed the least
differential change or nearly zero of the incisal edge and apex in original and new
position (Table 4.3-4.4). However, the differential changes of the incisal edge and
apex in original and new position in adjusted base of bracket model or straight wire
model were only minus value. That means while incisal edge of maxillary central
incisor moved linqually in upper anterior teeth retraction, apex of maxillary central
incisor moved more labially. The next order which had the least change or nearly zero
in adjusted base of bracket model or straight wire model was 9.0 millimeters of mini-
screws and 9.0 millimeters of power arm (Table 4.4).
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Figure 4.12 Incisal edge (upper arrow) and apex (lower arrow) of maxillary central
incisor.
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Table 4.1 The change of the incisal edge and apex in original and new position in
Y axis of non-straight wire model
Mini-screw
(mm)
Lever arm
(mm) Position
The change
(mm)
9 9 Incisal edge 4.605E-04
Apex 6.543E-05
9 7 Incisal edge 5.905E-04
Apex 2.375E-05
9 5 Incisal edge 5.641E-04
Apex 5.002E-05
9 3 Incisal edge 4.722E-04
Apex 5.233E-05
7 9 Incisal edge 6.756E-04
Apex -1.971E-05
7 7 Incisal edge 5.320E-04
Apex 6.979E-05
7 5 Incisal edge 6.143E-04
Apex 4.982E-05
7 3 Incisal edge 6.031E-04
Apex 6.031E-04
5 9 Incisal edge 4.565E-04
Apex 5.834E-05
5 7 Incisal edge 4.956E-04
Apex 1.389E-04
5 5 Incisal edge 5.304E-04
Apex 4.879E-05
5 3 Incisal edge 6.463E-04
Apex 4.887E-05
3 9 Incisal edge 4.144E-04
Apex 9.545E-05
3 7 Incisal edge 4.956E-04
Apex 1.389E-04
3 5 Incisal edge 5.155E-04
Apex 6.633E-05
3 3 Incisal edge 6.883E-04
Apex 6.986E-05
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Table 4.2 The change of the incisal edge and apex in original and new position
in Y axis of straight wire model
Mini-screw
(mm)
Lever arm
(mm) Position
The change
(mm)
9 9 Incisal edge 3.568E-04
Apex -2.845E-05
9 7 Incisal edge 5.376E-04
Apex -1.448E-04
9 5 Incisal edge 6.030E-04
Apex -1.681E-04
9 3 Incisal edge 5.452E-04
Apex -1.049E-04
7 9 Incisal edge 4.792E-04
Apex -1.238E-04
7 7 Incisal edge 5.634E-04
Apex -1.488E-04
7 5 Incisal edge 6.204E-04
Apex -1.675E-04
7 3 Incisal edge 5.651E-04
Apex -1.036E-04
5 9 Incisal edge 4.802E-04
Apex -1.178E-04
5 7 Incisal edge 5.356E-04
Apex -1.423E-04
5 5 Incisal edge 6.096E-04
Apex -1.545E-04
5 3 Incisal edge 5.630E-04
Apex -8.963E-05
3 9 Incisal edge 4.846E-04
Apex -1.178E-04
3 7 Incisal edge 5.776E-04
Apex -1.486E-04
3 5 Incisal edge 6.094E-04
Apex -1.423E-04
3 3 Incisal edge 5.510E-04
Apex -7.444E-05
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Table 4.3 The differential change of the incisal edge and apex in original and
new position in Y axis of non-straight wire model
Mini-screw (mm) Lever arm (mm) The differential change(x10-4)
(mm)
3 9 3.19
5 7 3.57
3 7 3.57
9 9 3.95
5 9 3.98
9 3 4.20
3 5 4.49
7 7 4.62
5 5 4.82
9 5 5.14
7 3 5.44
7 5 5.65
9 7 5.67
5 3 5.97
3 3 6.18
7 9 6.95
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Table 4.4 The differential change of the incisal edge and apex in original and
new position in Y axis of straight wire model
Mini-screw (mm) Lever arm (mm) The differential change(x10-4)
(mm)
3 9 -3.67
9 9 3.85
5 9 5.98
7 9 6.03
3 3 6.25
9 3 6.50
5 3 6.53
7 3 6.69
5 7 6.78
9 7 6.82
7 7 7.12
3 7 7.26
3 5 7.52
5 5 7.64
9 5 7.71
7 5 7.88
4.1.3 Comparison of the incisal edge and apex changes of maxillary
central incisors in Z axis.
The incisal edge and apex changes of maxillary central incisor in
non-straight wire model and straight wire model in Z axis were calculated and
compared the changes of each force direction (Table 4.5-4.8). In all force directions ,
the incisor edge changes in these models showed plus value but minus value in the
apex changes. Therefore, in two simulated models, incisal edge of maxillary central
incisors were extruded and apex of maxillary central incisors were intruded during
retraction. In straight wire model 9.0 millimeters of mini-screws and 9.0 millimeters
of power arm showed the least change of the incisal edge and apex in original and
new position (Table 4.7-4.8). So, there was least movement in Z axis in this force
direction.
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Table 4.5 The incisal edge change in original and new position in Z axis of non-
Straight wire model
Mini-screw (mm) Lever arm (mm) The change(x10-4)
(mm)
3 9 1.28
9 9 1.50
7 9 1.56
9 3 1.58
5 7 1.59
3 7 1.59
3 5 1.86
5 5 1.98
7 7 1.99
9 5 2.19
9 7 2.29
7 3 2.39
7 5 2.41
5 3 2.65
3 3 2.84
7 9 3.01
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Table 4.6 The apex change in original and new position in Z axis of non-straight
wire model
Mini-screw (mm) Lever arm (mm) The change(x10-5)
(mm)
5 7 -2.8
3 7 -2.8
3 9 -4.3
7 7 -6.1
3 5 -6.6
9 9 -6.7
9 3 -6.7
5 9 -7.0
7 3 -7.2
9 5 -7.5
3 3 -7.5
5 5 -7.6
5 3 -8.2
7 5 -8.6
9 7 -9.8
7 9 -11.6
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Table 4.7 The incisal edge change in original and new position in Z axis of
straight wire model
Mini-screw (mm) Lever arm (mm) The change(x10-4)
(mm)
9 9 1.25
7 9 2.29
5 9 2.31
3 9 2.42
3 3 2.52
9 3 2.56
5 3 2.59
9 7 2.62
7 3 2.65
5 7 2.73
7 7 2.79
3 7 2.79
9 5 3.03
3 5 3.09
5 5 3.09
7 5 3.16
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Table 4.8 The apex change in original and new position in Z axis of
straight wire model
Mini-screw (mm) Lever arm (mm) The change(x10-4)
(mm)
9 9 -0.9
3 9 -1.1
3 3 -1.2
5 9 -1.2
7 9 -1.3
5 3 -1.3
5 7 -1.3
7 3 -1.3
9 3 -1.3
3 7 -1.4
3 5 -1.4
9 7 -1.4
7 7 -1.4
5 5 -1.5
7 5 -1.6
9 5 -1.6
4.2 Discussion
4.2.1 Model for simulation
Non-adjusted base of brackets model or non-straight wire model and
adjusted base of brackets model or straight wire model were constructed for simulated
the movement of maxillary central incisor. The results were divided into two parts:
non-adjusted base of brackets model or non-straight wire model and adjusted base of
brackets model which caused archwire be straightened. In non-adjusted base of
brackets model or non-straight wire model, wire was not straight and did not afford
for teeth movement. High stress occurred in cervical of maxillary lateral incisor,
maxillary first molar and buccal of maxillary of second molar (Figure 4.4). Crown of
maxillary lateral incisor displaced more than the other teeth (Figure 4.5). In
periodontal ligament, the highest strain occurred nearly the cervical of maxillary
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lateral incisor (Figure 4.6). In adjusted base of brackets model or straight wire model,
wire was straight and afforded for teeth movement like the teeth which have been
leveled and aligned already. High stress occurred in cervical of maxillary lateral
incisor, maxillary first molar, maxillary canine, maxillary second premolar and buccal
of maxillary of second molar (Figure 4.9). Crown of maxillary lateral incisor
displaced more than the other teeth (Figure 4.10). In periodontal ligament, the highest
strain occurred nearly the cervical of maxillary lateral incisor (Figure 4.11). It may be
becaused of the small size of maxillary lateral incisor, high stress, displacement and
high strain can be easy occurred.
4.2.2 Comparison of the incisal edge and apex changes of maxillary
central incisor
In this study, it was found that a close relationship existed between the
degree of labiolingual tipping of the maxillary central incisor and the direction of
retraction force on the power arm or length of power arm and position of mini-screw.
All of 16 movement patterns in 16 force directions in non-adjusted base of brackets
model or non-straight wire model and adjusted base of brackets model or straight
wire model which nearly passed leveling phase, the incisal edge of maxillary central
incisor moved lingually. In both models, 3.0 millimeters mini-screw and 9.0
millimeters power arm seem to be the least differential change or nearly zero of
incisal edge and apex in original and new position which effect the bodily movement.
On the other hand, apex of the maxillary central incisor moved more labially than
incisal edge in adjusted base of brackets model or straight wire. It may be caused the
prominence of root. 9.0 millimeters mini-screw and 9.0 millimeters power arm was
the second order in the least differential change or nearly zero of incisal edge and
apex in original and new position of the maxillary central incisor. It was nearly to
occur the ideal bodily movement than other force directions in adjusted base of
brackets model or straight wire. Closely to other studys which concluded the high
level of the horizontal force direction could effect to bodily movement of maxillary
central incisor than low level of the horizontal force direction because of high level of
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the horizontal force directions were near to the center of resistance of the maxillary
central incisor.27, 33-36, 47
In horizontal force, Tominaga et al. studied in finite element model
and stated that an appliance with 0.018x0.025 inch bracket slots and an 0.018x0.025
inch and 0.016x0.022 inch stainless steel archwire was generated and 150 grams
horizontal force, 5.0, 11.0 millimeters power arm produced bodily movement of the
maxillary central incisor consequenly.27
Figure 4.13 3D finite element model of maxillary dentition, including PDL, alveolar
bone, brackets, and archwire.27
Sia et al. have investigated in three patients for determine of optimal
force system required for control of anterior tooth movement in sliding mechanics
0.018x0.025 inch slot brackets and 0.016x0.022 inch cobalt-chromium alloy wires
were used.47 Retraction forces of 1.5 N were applied parallel to the archwire. The
location of the center of rotation of the target tooth varied according to the different
heights of the retraction forces. As the height of force application was shifted apically,
the center of rotation also displaced almost exponentially in the same direction. An
infinity range between hooks 4 and 5 (6.0-8.0 millimeters from the bracket position)
was noticed in all 3 subjects, the location of the center of resistance. No tooth rotation
but bodily translation movement would occur at this level.47
In horizontal and oblique forces, Yukio Kojima et al. studied in the
archwire was made from a stainless steel wire, 0.018x0.025 inch in slot 0.018 inch.
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To calculate the tooth elements, surface models of the tooth were based on a dental
study model (i21D-400C; Nissin Dental Products, Kyoto, Japan). When the power
arm was lengthened, rotation of the entire maxillary dentition decreased. The
posterior teeth were effective for preventing rotation of the anterior teeth through an
archwire. In cases of a high position of a mini-screw, bodily tooth movement was
almost achieved. The vertical component of the force produced intrusion or extrusion
of the entire dentition.36
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Fig 4.14 Movement patterns with a low-position miniscrew placed 4.0 millimeters
gingivally to the archwire. In all cases, the entire dentitions rotate, because
the lines of action of the force pass below both centers of resistance (CR1
and CR2). Rotation and intrusion of the entire dentition decreases with an
increase in the length of the power arm. A, Power arm 1.0 millimeter in
length; B, power arm 4.0 millimeters in length; C, power arm 8.0 millimeters
in length.36
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Fig 4.15 Movement patterns with a high-position miniscrew placed 8.0 millimeters
gingivally to the archwire. In all cases, rotations of the entire dentition are
smaller than those in the case of low-position miniscrews;the anterior teeth
move almost bodily, because the lines of action of the force shift closer to
the centerof resistance of the anterior teeth (CR1) and pass above the center
of resistance of the posterior teeth (CR2). Rotation and intrusion of the
entire dentition decrease with an increase in the length of the power arm. A,
Power arm 1.0 millimeters in length; B, power arm 4.0 millimeters in
length; C, power arm 8.0 millimeters in length.47
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Ashekar et al. studied in finite element model. 0.019x0.025 inch stainless
steel archwire and 150 grams of retraction force are used in this study. They reported
that 8.0 millimeters mini-screw and 8.0 millimeters power arm showed more bodily
movement during maxillary teeth retraction.48
From a biomechanical point of view, the relationship between the line of
action of a force and the location of the center of resistance (CR) of a tooth
determines the type of tooth movement, such as lingual crown tipping, bodily
movement, or lingual root tipping.34,49-51 However, tooth movements analyzed in
some studies were not in agreement with that concept based on biomechanical
principles, where a single force passing through the CR causes bodily tooth
movement. Since the location of the CR of the incisor was determined to be at the
level of 7.2 millimeters apically from the bracket slot from finite element (FE)
analysis, bodily movement was expected to be produced at the height of 7.2
millimeters. Moreover, FE analysis showed that bodily movement of the incisor
occurs at the height of 5.0 millimeters, which is 2.2 millimeters incisal to the level of
CR, with 0.018×0.025 inch archwire (Figure 4.19 (a)). On the other hand, bodily
movement occurs at the level of 11.0 mm, which is 3.8 mm apical to the CR, with
0.016 × 0.022 inch archwire under the condition that a play exists (Figure 4.19 (b)).
The concept based upon the theoretical considerations did not work with the
multibracket appliances. This may be mainly due to the existence of play between the
bracket and the archwire.34
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Figure 4.16 Intraoral picture and the illustration of the retraction of the anterior tooth
using various lengths of power arm and implant anchorage in sliding
mechanics.34
Figure 4.17 Illustration of the cross section of a play constructed in the model.34
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Figure 4.18 Loading conditions when controlled anterior tooth movements are
performed (a) in case of using 0.018 × 0.025 inch stainless steel archwire
(without play); (b) in case of using 0.016 × 0.022 inch archwire (with
play). CRe, center of resistance; CRo, center of rotation.34
Tominaga et al. also reported that when relatively long power arms
were used, a substantial amount of bending moment was generated at the base of the
power arms as a cantilever effect. It was considered that the greater play between the
archwire and the bracket, the weaker normal forces were, and thereby weaker the
movement moment transmitted to the incisor. Although one of the keys to an
estimation of how the tooth movement was an appreciation of the relationship of a
line of action of the retraction force and the CR of a tooth, the effect of the archwire
deflection within the bracket slot on force system acting on a tooth should also be
taken into consideration.34
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Figure 4.19 Sagittal cross section at the mesial surface of the maxillary central incisor
bracket in the model which has a play. (a) before orthodontic force
applied; (b) after the application of the force at the level of 12.0
millimeters.34
In case, excessively long power arms are used, an impingement may
be produced on the buccal mucosa. On the other hand, clinically appropriate length of
the power arm allows any types of anterior tooth movement when a full-size archwire
is used. This situation in which there is no play between the archwire and the bracket
has no any clinical disadvantages in sliding mechanics. The friction that produced will
prevent the tooth from sliding along an archwire. Therefore, the existence of archwire
play in the bracket slot and its dimension should be taken into account in an actual
clinical situation in order to precisely predict the tooth movement after going through
the treatment. In addition, it is necessary to give full consideration to anatomical
parameters that vary from one individual to another.
Especially, an estimation of the CR position is of utmost clinical
importance prior to the treatment. Because the length of the power arm itself does not
have any significant importance. Since the height of the retraction force producing a
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certain type of tooth movement is closely related to the location of the CR, an optimal
power arm length should be back-calculated from the location of the CR. Considering
not only the relationship between the line of action of a retraction force and the
location of the CR of a tooth but also the effect of the archwire deformation, including
the torsion within the bracket slot, will be a great help in establishing an optimal
treatment plan and achieving speedy, effective, and accurate orthodontic tooth
movement. However, this concept may not work with the multibracket appliances.
This may be mainly due to the existence of play between the bracket and the
archwire.34
By the muco-buccal limitation, the lever arm and mini-screw could
not extend in higher level. Results of this study was closely related to the results of
other studies.27, 34, 36, 48 This was one of the knowledge to select the line of force in
orthodontic treatment. However, several variables affecting biomechanical behavior
of teeth movement were not taken into consideration in this study. Further
investigation using Finite Element Method (FEM), including factors such as wire size,
bracket slot size, play between bracket slot, archwire and variable anatomical
parameters is still needed. Not only these variables, but time also effected to these
movements. In this study, this is initial movement. In the further study, long term
orthodontic movement may be used to investigated the pattern of movement together.
This study had investigated the effect of force directions in maxillary
anterior teeth retraction with skeletal anchorage. Dental model which was used in this
study, had fair teeth alignment. But after bonded brackets and inserted archwire, the
archwire was not straight for retraction phase. All of these reasons, the simulation was
presented again with straight wire by adjusted base of brackets before bonded to teeth
surface. During the simulation, 9.0 millimeters mini-screw and 9.0 millimeters power
arm were the force direction which bodily movement nearly to be occurred in the
expected movement. Maxillary lateral incisor was protruded and a little more
protruding in adjusted base of brackets model or straight wire model than in non-
adjusted base of brackets model or non-straight wire model. It may be hardly
movement of adjacent teeth that limits the movement of lateral incisor in non-straight
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wire model. In clinical application, less crowding and fair aligned teeth can be applied
anterior retraction force, and there is less protrude of lateral incisor than well aligned
teeth. However, periodontal ligament strain in adjusted base of brackets or straight
wire model is greater than non-adjusted base of brackets model or non- straight wire
model which risk to occur root resorbed in these area. Furthermore, this was only
computer simulation from dental model which closed to in human being and there are
many variations in shape and size of each teeth. These results can used to be the
guideline for maxillary anterior teeth retraction in orthodontic treatment. In the
future, clinical trial in patients is very important to confirm this results.
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CHAPTER 5
CONCLUSIONS AND RECOMMENDATIONS
9.0 millimeters mini-screw and 9.0 millimeters power arm were the force
direction which bodily movement nearly to be occurred in straight wire model or
model which nearly passed leveling phase.
Lateral incisor was protruded and more protrude in adjusted base of
brackets model or straight wire model than non-adjusted base of brackets model or
non-straight wire model. In clinical application, less crowding and fair aligned teeth
can be applied anterior retraction force, and there is less protrude of lateral incisor
than adjusted base of brackets model or straight wire model which is like well aligned
teeth before retraction.
Periodontal strain in straight wire model is greater than non-straight wire
model which risk to occur root resorption in these area.
Anterior retraction in fair aligned teeth, leveling and aligning can not be
involved in the step of orthodontic treatment. Start with anterior retraction deal to
effect of less protrusion of maxillary lateral incisor and decrease in treatment time.
By the muco-buccal limitation, the power arm and mini-screw cannot
extend in higher level for comparisons the results. However, several variables
affecting biomechanical behavior of teeth movement were not taken into
consideration in this study. Further investigation using Finte Element Method,
including factors such as wire size, bracket slot size, play between bracket slot,
archwire, and variable anatomical parameters are still needed. Not only these
variables, but time also effected these movements. In the further study, long term
orthodontic movement may be used to investigated the pattern of movement together.
However these are only computer simulations from dental model Tthere are many
variations in shape and size of each teeth, the results in human being may confirmed
these simulations in the future study.
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APPENDICES
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APPENDIX A
Non-straight wire model
Figure 6.1 Non-straight wire model in front view.
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Figure 6.2 Non-straight wire model after separate teeth, brackets, power arms and
wire in occlusal view.
Figure 6.3 Non-straight wire model after separate teeth, brackets, power arms and
wire in frontal view.
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Figure 6.4 Non-straight wire model after separate teeth, brackets, power arms and
wire in superior view.
Figure 6.5 Non-straight wire model after separate teeth, brackets, power arms and
wire in palatal view.
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Figure 6.6 Non-straight wire model after separate teeth, brackets, power arms and
wire in occlusal view.
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Figure 6.7 Non-straight wire model after separate teeth, brackets, power arms and
wire in occlusal view.
Figure 6.8 Non-straight wire model after separate teeth, brackets, power arm and
wire in lateral view (mesh structure).
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Figure 6.9 Non-straight wire model after separate teeth, brackets, power arm and
wire in frontal view (mesh structure).
Figure 6.10 Non-straight wire model after separate teeth, brackets, power arm and
wire in occlusal view (mesh structure).
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Figure 6.11 Non-straight wire model after separate teeth, brackets, power arm and
wire in occlusal view (mesh structure).
Figure 6.12 Non-straight wire model after separate teeth, brackets, power arm and
wire in palatal view (mesh structure).
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Figure 6.13 Non-straight wire model after separate brackets, power arm and wire in
lateral view.
Figure 6.14 Non-straight wire model after separate brackets, power arm and wire in
occlusal view.
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Figure 6.15 Non-straight wire model after separate brackets, power arm and wire in
palatal view.
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Equivalent stress (MPa)
Figure 6.16 Stress of teeth in non-straight wire model in lateral view.
Equivalent stress (MPa)
Figure 6.17 Stress of teeth in non-straight wire model in frontal view.
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Equivalent stress (MPa)
Figure 6.18 Stress of periodontal ligament in non-straight wire model.
Displacement (millimeter)
Figure 6.19 Play of archwire in bracket slot in non-straight wire model (pink is
original position).
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Displacement (millimeter)
Figure 6.20 Displacement of teeth in non-straight wire model in occlusal view.
Displacement (millimeter)
Figure 6.21 Displacement of teeth in non-straight wire model in lateral view.
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Displacement (millimeter)
Figure 6.22 Displacement of teeth in non-straight wire model in frontal view.
Displacement (millimeter)
Figure 6.23 Displacement of teeth in non-straight wire model in palatal view.
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Strain (-)
Figure 6.24 Strian in periodontal ligament in non-straight wire model.
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APPENDIX B
STRAIGHT WIRE MODEL
Figure 6.25 Straight wire model in front view.
Fig 6.27 Straight wire model in frontal view (mesh structure).
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Figure 6.27 Straight wire model after separate teeth, brackets, power arms and wire
in occlusal view.
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Figure 6.28 Straight wire model after separate teeth, brackets, power arms and wire
in superior view.
Figure 6.29 Straight wire model after separate teeth, brackets, power arms and wire
in frontal view.
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Figure 6.30 Straight wire model after separate teeth, brackets, power arms and wire
in palatal view.
Figure 6.31 Straight wire model after separate teeth, brackets, power arms and wire
in lateral view.
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Figure 6.32 Straight wire model after separate teeth, brackets, power arms and wire
in occlusal view.
Figure 6.33 Straight wire model after separate teeth, brackets, power arms and wire
in occlusal view.
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Figure 6.34 Straight wire model after separate teeth, brackets, power arm and wire
in lateral view (mesh structure).
Figure 6.35 Straight wire model after separate teeth, brackets, power arm and wire
in occlusal view (mesh structure).
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Figure 6.36 Straight wire model after separate teeth, brackets, power arm and wire
in frontal view (mesh structure).
Figure 6.37 Straight wire model after separate teeth, brackets, power arm and wire
in occlusal view (mesh structure).
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Figure 6.38 Straight wire model after separate teeth, brackets, power arm and wire
in palatal view (mesh structure).
Figure 6.39 Straight wire model after separate brackets, power arm and wire in
lateral views.
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Figure 6.40 Straight wire model after separate brackets, power arm and wire in
occlusal view.
Figure 6.41 Straight wire model after separate brackets, power arm and wire in
palatal view.
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Equivalent stress (MPa)
Figure 6.42 Stress of teeth in straight wire model in lateral view.
Equivalent stress (MPa)
Figure 6.43 Stress of teeth in straight wire model in frontal view.
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Equivalent stress (MPa)
Figure 6.44 Stress of periodontal ligament in straight wire model.
Displacement (millimeter)
Figure 6.45 Play of archwire in bracket slot in straight wire model
(pink is original position).
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Displacement (millimeter)
Figure 6.46 Displacement of teeth in straight wire model in palatal view (mesh
structure).
Displacement (millimeter)
Figure 6.47 Displacement of teeth in straight wire model in frontal view (mesh
structure).
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Displacement (millimeter)
Figure 6.48 Displacement of teeth in straight wire model in frontal view.
Displacement (millimeter)
Figure 6.49 Displacement of teeth in straight wire model in palatal view.
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BIOGRAPHY
Name Miss Nantaporn Ruenpol
Date of Birth May 29, 1972
Educational Attainment 1996: Doctor of Dental Surgery
Work Position
Head of Dental Public Health Department,
Ayutthaya Provincial Public Health Office
Scholarship -
Publications
-
Work Experiences
2012-Present
2010
2002
1997
1996
Head of Dental Public Health Office,
Ayutthaya Provincial Public Health
Dentist
Dental department, Bangsay Hospital, Ayutthaya
Dentist
Private practice
Dentist
Dental department, Somdejprasunkkaraj
(Wasanamahatara) Hospital, Ayutthaya
Dentist
Dental department, Bantan Hospital, Chaiyapoom
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