Orthodontic Force Systems Explained

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

www.indiandeantalacademy.com

Orthodontic force
systems


Orthodontic force systems
BIOLOGY +MECHANICS =
BIOMECHANICS


Biomechanical concepts
Forces
Moments
Couples
Moment-to-Force ratios


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

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

Point of application of force in
relation to center of mass
Tipping

Force passing above or below the
center of mass of the pencil

Translation

Force passing through the center of
mass of of the pencil

Center of Resistance
Point at which the resistance to tooth movement
is concentrated

Depends on
root length
no. of roots
Alv. bone support


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

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

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 www.indiandeantalacademy.com
the center of rotation changes

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


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)

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

Center of resistance &center
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


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


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


Moment-to-Force ratio
Translation or bodily movement


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


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

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

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

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

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

systems
Force system from intrusion arch in
equilibrium


systems of unequal moments result in
The application
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

Biomechanical classification
of orthodontic appliances
Equal and opposite force systems
One couple force systems
Two couple appliance system


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

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

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


Cantilever spring


Auxiliary intrusion/extrusion arches


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


Force system
depends on wirebracket geometry


Straight segment of
wire in misaligned
brackets


“V” bend principle


2x6 appliance can be used for molar
control


Lingual arches as two couple system


How to predict?

It is sometimes beneficial not to
engage a straight wire into all of
the teeth in an arch


Conclusion
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

Thank you
For more details please visit
www.indiandentalacademy.com


Orthodontic Force Systems Explained

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Orthodontic Force Systems Explained

Similar to Orthodontic Force Systems Explained (20)

More from Indian dental academy

More from Indian dental academy (20)

Orthodontic Force Systems Explained

Editor's Notes