Thesis final /certified fixed orthodontic courses by Indian dental academy

INTRODUCTION
INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy.com

COMPARISON OF STRESS DISTRIBUTION DURING
TIPPING AND TORQUING TOOTH MOVEMENTS- A
FINITE ELEMENT STUDY
BY
DR. AVINASH KUMAR
PG STUDENT
GOVERNMENT DENTAL COLLEGE &
RESEARCH INSTITUTE, BANGALORE
UNDER THE GUIDANCE OFUNDER THE GUIDANCE OF
Dr. SHASHIKALA KUMARI. VDr. SHASHIKALA KUMARI. V
PROFESSOR & HEAD
CO GUIDE
Dr. UMA. H. L
ASSISTANT PROFESSOR
www.indiandentalacademy.co
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Orthodontic tooth movement can be considered as one
of the basic pillars of treatment.
 Sound orthodontic therapy has its basis inSound orthodontic therapy has its basis in
mechanics; a through understanding of themechanics; a through understanding of the
biomechanics of tooth movement is required.biomechanics of tooth movement is required.
 The study of orthodontic biomechanics in-turnThe study of orthodontic biomechanics in-turn
requires the understanding of the nature of stressrequires the understanding of the nature of stress
and strain in the periodontium, induced byand strain in the periodontium, induced by
orthodontic forces.orthodontic forces.
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The purpose of this study was to investigate the
distribution of stress caused by tipping and torque
movements, specifically at maxillary central incisor -
using a finite element study.
This tooth was chosen because it undergoes the most
detailed tooth movement and is at higher risk for root
resorption than all other teeth except the maxillary
lateral incisor.
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 Finite element analysis: is a highly precise numericalFinite element analysis: is a highly precise numerical
method of analysis that allows the study of stressmethod of analysis that allows the study of stress
distribution in biological systems.distribution in biological systems.
 This method enables the stress-strain in the interior of theThis method enables the stress-strain in the interior of the
structures to be calculated. It also has the potential forstructures to be calculated. It also has the potential for
equivalent mathematical modelling of a real object ofequivalent mathematical modelling of a real object of
complicated tridimentional geometry.complicated tridimentional geometry.
 At the same time it permits the application of various forceAt the same time it permits the application of various force
systems at a set point, and the study of distribution of suchsystems at a set point, and the study of distribution of such
forces through the following structures: alveolar bone,forces through the following structures: alveolar bone,
tooth and periodontal ligament. It also provides qualitativetooth and periodontal ligament. It also provides qualitative
and quantitative results.and quantitative results.
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aIms & ObjeCTIves
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 To analyze distribution of stress on the tooth and
periodontal structures when simple tipping and
torquing movement is produced, using a finite
element model.
 To demonstrate the areas of highest stress
concentration on the root, periodontal ligament
and the alveolar bone
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maTeRIals
& meThOD
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 In this study a 3-dimensional finite element
model of permanent maxillary central incisor
and its supporting structures was generated
and used to analyze the stress distributed due
to tipping and torquing tooth movements.
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Oblique view of the three-dimensional finite element model of the
maxillary central incisor. Occluso-gingival levels (A through D)
where the principal stresses and Von Mises stress were
determined.
A
B
C
D
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Periodontal ligament was simulated as a 0.20 mm
thick ring around the model of the tooth. The finite
element model consisted of 1, 04,760 elements and
35,702 nodes
The software used for the geometric modeling was
ANSYS 10.
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Material Parameters used in the Finite Element Model
Material
Young’s modulus
(kg/mm2
)
Poisson’s Ratio
Tooth 2.0 × 103
0.15
Periodontal ligament 6.8 × 10-2
0.49
Alveolar bone 1.4 × 103
0.15
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A tipping force of 100grams in a labio-lingual direction was
applied Perpendicular to the axis of the tooth.
The point of application of force was established on the midpoint of
labial face of the crown, which coincides with the point where the
bracket is normally placed in clinical practice
Application of force
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A Pair of 50 grams forces were applied on both the nodal elements
found in the centre of the labial face of the crown for torquing tooth
movement
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These loading conditions were measured for the alveolar bone,
the periodontal ligament and the root surface at four different
radicular levels, and selected in an occluso-gingival direction.
These were established at: A, B, C and D.
Different nodal points were chosen at each level in order to
register stress levels. These nodes were placed in a mesial,
distal, labial and lingual position for each of these structures.
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This information was later organized graphically for each
structure (tooth, ligament and bone) and for each
radicular zone (labial, lingual, mesial and distal) relating
the amount of load suffered in g/cm2
with each of the
occlusogingival levels.
The load was described as three principal stresses (PS) that
determine the general state of a body, namely: maximum
principal stress; intermediate principal stress & minimum
principal stress.
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In some cases, the occurrence of principal stresses
could mask the tensional state of the body at a certain
point. Thus in these cases it is easier to use only a
single stress number i.e. Von Mises stress
Von Mises stress, is used to estimate yield criteria for
ductile materials. The Von Mises stress is defined by the
distortion energy theory as a combination of all principle
stresses. The Von Mises stresses by definition is always
positive and do not provide an indication of a tensile or
compressive state.
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RESULT
S
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Von Misses stress distribution on the model caused by 100
grams of tipping force
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Von Misses stress distribution on the model caused
by torquing forces
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Graphical representation of stress distribution on the
periodontal ligament in loading condition for a tipping force
of 100 g.
LABIAL SIDE LINGUAL SIDE
PERIODONTAL LIGAMENT PERIODONTAL LIGAMENTwww.indiandentalacademy.co
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MESIAL SIDE DISTAL SIDE
PERIODONTAL LIGAMENT PERIODONTAL LIGAMENT
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Pattern of stress distribution for tipping force of 100g
 The maximum and intermediate stresses of
periodontal ligament were very similar. A zero
level of stress was found between levels B and C.
 On the tooth, the greatest degree of stress appeared
on the labial side, between levels B and C.
 In the bone the maximum values of stress were
registered at level A,
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Pattern of stress distribution for crown torque of
100g/mm
 The maximum values of stress on the tooth were
found at level A,
 The periodontal ligament showed its highest level
of stress at an apical level.
 The maximum level of stress in the bone was
observed at level A,
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discussion
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In this study the distribution of three principal stresses
(maximum , intermediate & minimum) were very similar
in the periodontal ligament. The tooth apex and bony
alveolar crest are the areas that suffer the greatest stress
when tipping and torquing movements are produced.
Similar results were found in previous study done by
Puente et. al, when tooth movements were carried out on
lower canine that used FEM.
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Jeon et. al., studied the stress response in the periodontium of
the maxillary first molar to different moment to force ratios;
After various counter tipping moments were applied, the stress
concentration was observed on the root surface at the furcation
level.
In this study we used an anterior tooth, the maxillary central
incisor, which displays high stress concentration at the apical
level of periodontal ligament when torquing tooth movement is
produced.
This result may suggest that the root morphology of the
maxillary central incisor makes it more susceptible to apical
root resorption relative to posterior teeth during tooth
movement.
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LIMITATIONS OF FINITE ELEMENT METHOD [FEM]
As with any theoretical model of a biological system, there
are some limitations with FEM.
For the force values and their distribution on the structures,
it can be said, that differences exist between the FEM
models.
In the Finite element analysis, assumptions related to material
properties of simulated structures are not absolute
representation of the structure.
In reality the structures modeled are much more dynamic.
Moreover, the physical characteristics of tissue vary from site
to site and from individual to individual.www.indiandentalacademy.co
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In the light of these limitations, the quantitative results
reported in this study may be of limited significance
outside the context of the model assumptions. Still the
qualitative trends observed are meaningful.
The thickness of periodontal ligament is not uniform
throughout the root surface, also differs at different ages
and between individuals.
The forces applied are perpendicular to the long axis of
the tooth in this experiment. In mouth, actual force may
not be linear, perpendicular to the long axis of the tooth.
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conclusion
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It shows that the applied force is not equally distributed
over the tooth, periodontal ligament and bone.
In these types of tooth movement, maximum tensional
stresses are situated on the bony marginal crest and the
dental apex.
Rotation centre is not a unique point, but a radicular
area, situated between the B and C levels in the model.
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 If the clinician is concerned about placing heavy stresses on
the root apex (for example, a patient whose incisors show
previous root resorption) then torquing forces must be
applied with caution.
 However, the link between external forces and apical root
resorption is far from clear-cut.
 Because of the low incidence of severe root resorption and
the lack of a reliable animal model, we simply do not know
why similar mechanical forces affect one person so
differently from another.
 It is likely that root resorption is a complex, multifactorial
system with biochemical thresholds that vary significantly
among individuals.
What are the clinical implications of this model?
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