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5. Force
an act upon a body that changes or tends to
change the state of rest or the motion of that
body
Vectors , having both magnitude and
direction
Magnitude
light
heavy
Direction
Pull
Push
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6. Force
Point of application
Center of Mass
Center of Gravity
Center of Resistance
Each body has a point in its mass,
which behaves as if the whole mass is
concentrated at that single point
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7. Point of application of force in
relation to center of mass
Tipping
Force passing above or below the
center of mass of the pencil
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8. Point of application of force in
relation to center of mass
Translation
Force passing through the center of
mass of of the pencil
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9. Point of application of force in
relation to center of mass
Center of Resistance
Point at which the resistance to tooth movement
is concentrated
Depends on
root length
no. of roots
Alv. bone support
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10. Moments
Moment of the
force
Force applied at any
point other than
through the center of
resistance,in addition
to moving the center
of resistance would
create a moment
“Rotation”,“Tipping”or
Torquing
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11. Factors controlling the
moment
M=F x d
Either increasing the magnitude of the force
or applying the same force even further from
the center of resistance increases the
tendency for rotation
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12. Type of tooth movement
A-Uncontrolled tipping
B-Controlled tipping
C-Torque
D-translation or bodily movement
Center of resistance of a tooth does not
change ,only the center of rotation changes
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13. Couple
Moment of couple
Two forces equal in
magnitude and
opposite in direction
at a distance
Additive/Subtractive
A couple applies a
pure moment to a
tooth
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14. Couple – Clinical point
We can apply couple
only on the exposed
part of the tooth
Depending on the plane
in which the couple is
acting, this moment has
been called
“Rotation”(first order),
“Tipping”(second
order), “Torque”(third
order)
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15. Tooth movement
Equivalent force systems
Helps to predict the type of
tooth movement that will
occur with the appliance
activation by determining the
force system at the center of
resistance
forces are replaced at the center of resistance;
moment of force generated at the bracket is also
placed at the center of resistance; Third.the applied
moment is placed at the COR. Finally the applied
moment and the moment of force are added to
determine the net moment
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16. Center of resistance ¢er
of rotation
Point around which the
tooth rotates is called the
center of rotation
Can be at the center of
res.,at the root apex or at
infinity
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17. Equivalent force system
cont…
the resulting force system describes the expected
tooth movement
by determining the equivalent force systems,it
becomes apparent that achieving desired and
predictable tooth movement requires an awareness
of both applied forces and moments
The relationship between the applied force system
and the type of tooth movement can be described by
the Moment-to-Force ratio
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18. Moment-to-Force ratio
The ratio of the counter-balancing moment produced
to the net force that is applied to a tooth will
determine the type of tooth movement that will occur
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20. Tooth movement
Thus by changing the ratio of the moment from the
applied couple(counter-balancing moment) to the force
applied, the center of rotation of the tooth can be
varied to produce the type of tooth movement desired
Uncontrolled tipping
M/F RATIO=0
Center of rotation just
apical to center of rotation
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21. Tooth movement cont……
Controlled tipping
M/F RATIO =7:1
Center of rotation is at the
apex of the tooth
Translation
M/F RATIO=10:1
the center of
rotation is at infinity
because no rotation
occurs www.indiandentalacademy.com
22. Tooth movement cont……
Root movement
M/F RATIO = 13:1
Center of rotation is at the
crown of the tooth and only
root movement occurs
Pure rotation
M/F RATIO = infinity
center of rotation is
coincident with center
of resistance
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23. Predicting orthodontic force
systems
To fully understand the response to
appliance activation the displacement at one
end of the appliance has to be considered
with the effects at the other
Force systems by orthodontic appliance must
be resolved separately
To deduce how each tooth will be displaced
as a result of activation ,the equivalent force
systems acting at the centers of resistance
must be assessed subsequently
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24. Predicting orthodontic force
systems
Static equilibrium
Physical laws of statics are governed by
Newtons laws of motion
For any orthodontic appliance
The sum of all forces present must
equal zero
The sum of all moments present
must equal zero
The sum of all forces and moments
present must equal zero
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25. Predicting orthodontic force
systems
Static equilibrium cont…
Mesiodistal,buccolingual and
extrusion-intrusion forces in balance
Two equal
moments, system in
balance
Unequal moments,
unbalanced system
sum of these force
systems=0,but system
rotates clockwise
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27. Predicting orthodontic force
systemsThe application of unequal moments result in
more complex force systems,and may occur
with bracket misalignment,eccentric
“V”bends, gable bends or use of auxiliary
space closing springs
When applied moments are unequal
“additional” forces must be present to oppose
moment difference
Being aware of the force system produced by
an orthodontic appliance in equilibrium aids in
the prediction of response to the treatment
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29. Determinate Vs. Indeterminate
force systems
Determinate force systems
• moments and forces can readily be discerned
• E.g… one couple appliance system
Indeterminate force system
• Too complex for precisely measuring all forces
and moments involved in the equilibrium
• Only the direction of net moments and approx..
force levels can be determined
• E.g… two couple appliance system
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30. Biomechanical classification
of orthodontic appliances
No couple
appliance system
• e.g..elastic band
stretched b/w two
points
• Appliance is in
equilibrium as the
sum of forces and
moments is zero
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31. Biomechanical classification
of orthodontic appliances
One couple appliance system
• One end of the appliance
experiences couple(bracket or
tube) and other end is tied as a
point contact(which can not
produce a couple but only a simple force at that site)
• E.g..cantilever spring activated to extrude a high
maxillary canine and auxiliary intrusion or
extrusion arches
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33. One couple appliance system
Auxiliary intrusion/extrusion arches
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34. Two couple appliance system
Both ends of the appliance are engaged into
attachments(brackets or tubes).A couple is
thus generated by the wire at either or both
sites
Statically indeterminate force system
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35. Two couple appliance system
Force system
depends on wire-
bracket geometry
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36. Two couple appliance system
Straight segment of
wire in misaligned
brackets
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38. Two couple appliance system
2x6 appliance can be used for molar
control
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39. Two couple appliance system
Lingual arches as two couple system
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40. How to predict?
It is sometimes beneficial not to
engage a straight wire into all of
the teeth in an arch
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41. Conclus
the choice of appliances and technique
used by practitioners varies radically among
individuals, but the fundamental forces and
moments they produce are universal.
Appliance will always act according to the
laws of physics
Understanding the basic force systems helps
achieving controlled tooth movement in a
predictable manner
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This simple example shows how to determine an appliances equilibrium state
Prior knowledge of any mechanical side effects makes possible compensation before the effects occur.