Effect of force directions in maxillary anterior teeth ...

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


central incisor in Z axis

4.2 Discussion 43

4.2.1 Model for simulation 43


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


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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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Page

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.


in frontal view.

6.4 Non-straight wire model after separateteeth, brackets, power arms and wire 63

in superior view view.


in palatal view.

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in occlusal view.


in occlusal view.

6.8 Non-straight wire model after separate teeth, brackets, power arm and wire 66

in lateral view (mesh structure).


in frontal view (mesh structure).


in occlusal view (mesh structure).




in palatal view (mesh structure).

6.13 Non-straight wire model after separate brackets, power arm and wire in 69

lateral view.


occlusal view.


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.


frontal view.


palatal view.


lateral view (mesh structure).


occlusal view.


occlusal view.

6.34 Straight wire model after separate teeth, brackets, power arm and wire in 81

lateral view (mesh structure).


occlusal view (mesh structure).


frontal view (mesh structure).


occlusal view (mesh structure).


palatal view (mesh structure).

6.39 Straight wire model after separate brackets, power arm and wire in 83

lateral view.

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occlusal view.


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-


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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Figure 4.9 Stress in adjusted base of brackets or straight wire model.

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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


Apex 2.375E-05


Apex 5.002E-05


Apex 5.233E-05


Apex -1.971E-05


Apex 6.979E-05


Apex 4.982E-05


Apex 6.031E-04


Apex 5.834E-05


Apex 1.389E-04


Apex 4.879E-05


Apex 4.887E-05


Apex 9.545E-05


Apex 1.389E-04


Apex 6.633E-05


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)


Apex -2.845E-05


Apex -1.448E-04


Apex -1.681E-04


Apex -1.049E-04


Apex -1.238E-04


Apex -1.488E-04


Apex -1.675E-04


Apex -1.036E-04


Apex -1.178E-04


Apex -1.423E-04


Apex -1.545E-04


Apex -8.963E-05


Apex -1.178E-04


Apex -1.486E-04


Apex -1.423E-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


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


(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


(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


(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.


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.


wire in frontal view.

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wire in superior view.


wire in palatal view.

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Figure 6.8 Non-straight wire model after separate teeth, brackets, power arm and

wire in lateral view (mesh structure).

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wire in frontal view (mesh structure).


wire in occlusal view (mesh structure).

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wire in occlusal view (mesh structure).


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.


occlusal view.

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palatal view.

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Figure 6.16 Stress of teeth in non-straight wire model in lateral view.


Figure 6.17 Stress of teeth in non-straight wire model in frontal view.

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Figure 6.18 Stress of periodontal ligament in non-straight wire model.


Figure 6.19 Play of archwire in bracket slot in non-straight wire model (pink is

original position).

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Figure 6.20 Displacement of teeth in non-straight wire model in occlusal view.


Figure 6.21 Displacement of teeth in non-straight wire model in lateral view.

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Figure 6.22 Displacement of teeth in non-straight wire model in frontal view.


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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in superior view.


in frontal view.

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in palatal view.


in lateral view.

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in occlusal view.


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).



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in frontal view (mesh structure).



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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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occlusal view.


palatal view.

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Figure 6.42 Stress of teeth in straight wire model in lateral view.


Figure 6.43 Stress of teeth in straight wire model in frontal view.

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Figure 6.44 Stress of periodontal ligament in straight wire model.


Figure 6.45 Play of archwire in bracket slot in straight wire model

(pink is original position).

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Figure 6.46 Displacement of teeth in straight wire model in palatal view (mesh

structure).


Figure 6.47 Displacement of teeth in straight wire model in frontal view (mesh

structure).

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Figure 6.48 Displacement of teeth in straight wire model in frontal view.


Figure 6.49 Displacement of teeth in straight wire model in palatal view.

Ref. code: 25585013320022TJR

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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

Ref. code: 25585013320022TJR

Effect of force directions in maxillary anterior teeth ...

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