PPT_BioMechanical Principles.pptx

BIO MECHANICAL PRINCIPLES
IN
ORTHODONTICS
Seminar on,

CONTENTS
3
 Introduction
 Mechanical Concepts in Orthodontics
Scalar, Vector, Force
Resultant and Components of Orthodontic Force
Systems
Resultant of Forces
Components of a Force
Center of Mass, Gravity, Resistance
Center of Resistance During Anterior Teeth Retraction
Center of Resistance During Anterior Teeth Intrusion
Center of Rotation

INTRODUCTION
Begg PR, Keslin PC. Begg Orthodontic Theory & technique, 2nd Edition April 5, 2022 4
o The centerbiologicenterc cascade of events that ultimately results in bone remodeling
and orthodontic tooth movement begins with the mechanical activation of an
orthodontic appliance.
o The force systems produced by orthodontic appliances, consisting of both forces and
moments, displace teeth in a manner that is both predictable and controllable.
o By varying the ratio of moment to force applied to teeth, the type of tooth movement
experienced can be regulated by the orthodontist.

INTRODUCTION
Cont…
o Orthodontic appliances obey the laws of physics and can be activated to generate the
desired force systems to achieve predetermined treatment goals for individual patients.
o Likewise, any orthodontic appliance can be analyzed to define the mechanical force
systems it produces.
o Understanding the biomechanical principles underlying orthodontic appliance
activations is essential for executing efficient and successful orthodontic treatment.

INTRODUCTION
Cont…
o Mechanics is the discipline that describes the effects of forces on bodies
o Biomechanics refers to the science of mechanics in relation to biologic system

MECHANICAL CONCEPTS IN ORTHODONTICS
o An understanding of several fundamental mechanical concept is necessary to
understand clinical relevance of biomechanics in orthodontics.

SCALAR:
o When a physical property (weight, temperature, force) has
only magnitude, it is called a scalar quantity.
o Ex.: A force of different magnitude such as 20gm, 50gm
etc)
VECTOR:
o When a physical property has both magnitude and
direction it is called a vector quantity.
o Ex. A force vector characterized by magnitude, line of
action, point of origin and sense)
Cont…

FORCE:
o Force is equal to mass times acceleration
o F= ma
o Forces are actions applied to bodies
Cont…

RESULTANTS AND COMPONENTS OF ORTHODONTIC FORCE SYSTEMS:
o Teeth are often acted upon by more than one force. Since the movement of a tooth (or
any object) is determined by the net effect of all forces on it, it is necessary to combine
applied forces to determine a single net force, or resultant.
o At other times there may be a force on a tooth that we wish to break up into components.
For example, a cervical headgear to maxillary molars will move the molars in both the
occlusal and distal directions.
o It may be useful to resolve the headgear force into the components that are parallel and
perpendicular to the occlusal plane, in order to determine the magnitude of force in each
of these directions
Cont…

RESULTANTS AND COMPONENTS OF ORTHODONTIC FORCE SYSTEMS:
o Teeth are
Cont…

RESULTANT OF FORCES:
o The parallelogram methTeethod of determining the resultant of 2 forces having common
point of application.
Cont…

COMPONENTS OF A FORCE:
Cont…

COMPONENTS OF A FORCE:
o The resultant of 2 force with
different point of application can
be determined by extending the
line of action to construct a
common point of application
Cont…

CENTER OF MASS:
o Each body has a point on its mass, which behaves as if the whole mass is concentrated
at that single point. We call it the center of mass in a gravity free environment.
CENTER OF GRAVITY:
o The same is called the centre of gravity in an environment when gravity is present.
Cont…

CENTER OF RESISTANCE:
o Center of mass of a free body is the point through which an applied force must pass to
move it linearly without any rotation.
o This center of mass is the free objects “Balance Point”
o The center of resistance is the equivalent balance point of a restrained body.
o Center of resistance varies depending upon the
• Root length & morphology
• Number of roots
• Level of alveolar bone support
Cont…

Cont…

CENTER OF RESISTANCE DURING ANTERIOR TEETH RETRACTION:
o Center of resistance of 6 anterior teeth: ± 7mm apical to the interproximal bone
o Center of resistance of 4 anterior teeth: ±5mm apical to the interproximal bone
o Center of resistance of 2 anterior teeth: ±3.5mm apical to the interproximal
bone
o The location of the instantaneous center of resistance shifted apically as the number of
dental units consolidated (2, 4, and 6) increased.
o Clinical implication: They suggest that little difference in the moment/force ratio (M/F) is
required to translate a two- or four-teeth unit. However, for the retraction of a six-teeth
segment, the M/F ratio of a retraction spring should be calibrated for a higher value to
facilitate translation.
Cont…

o Clinical Implication:
o They suggest that little difference in the moment/force ratio (M/F) is required to translate
a two- or four-teeth unit.
o However, for the retraction of a six-teeth segment, the M/F ratio of a retraction spring
should be calibrated for a higher value to facilitate translation.
Cont…

Cont…

CENTER OF RESISTANCE DURING ANTERIOR TEETH INTRUSION:
o For an anterior segment comprising two central incisors, the center of resistance was
located on a projection line parallel to the midsagittal plane on a point situated at the
distal half of the canines.
o For an anterior segment that included the four incisors, the center of resistance was
situated on a projection line perpendicular to the occlusal plane between the canines and
first premolars.
o For a rigid anterior segment that included the six anterior teeth, the center of resistance
was situated on a projection line perpendicular to the occlusal plane distal to the first
premolar.
Cont…

CENTER OF RESISTANCE DURING ANTERIOR TEETH INTRUSION:
o The center of resistance was found in different occlusoapical positions, depending on the
direction of the force.
o Thus the location of the center of resistance cannot be considered to be constant,
independent of the direction of loading, for a tooth with a given support.
o A force always acts to displace the center of resistance in the direction the force is acting
(support being the same).
Cont…

Cont…

CENTER OF ROTATION:
o If a model of a tooth is attached to a piece of paper by a pin, the point with the pin in it
cannot move, and this point becomes the center of rotation about which the tooth can
spin.
o If the pin is placed at the incisal edge, only movement of the root is possible
o If it is placed at the root apex, movement is limited to crown tipping.
o In each case, the center of rotation is determined by the position of the pin.
o Thus, in two dimensional figures, the center of rotation may be defined as a point about
which a body appears to have rotated, as determined from its initial and final positions.
Cont…

CENTER OF ROTATION:
o The more nearly translational the movement, the farther apically the center of rotation
would be located.
o In the extreme case, with perfect translation, the center of rotation can be defined as
being an infinite distance away.
Cont…

CENTER OF ROTATION:
o A simple method for determining a center of
rotation is to take any two points on the tooth
and connect the before and after positions of
each point with a line.
o The intersection of the perpendicular
bisectors of these lines is the center of
rotation
Cont…

CENTER OF ROTATION:
o Burstone stated in his simple formula:
y × (M/F) = 0.068 h2
where
y = Distance from center of resistance to center of rotation
M/F = Distance from center of resistance to point of force application
h = Root length
Thus, in this special case of a two-dimensional parabolic root, 0.068 is a constant for a
given root length.
Cont…

CENTER OF ROTATION:
o Two important parameters σ2 and γ, which measure the resistance of the tooth to tipping,
were found to be constants for loading in one plane of space, independent of the position
of occlusoapical force.
o Using experimental σ2 or γ values, one can calculate the location of the center of rotation
of the tooth for a given force position or, conversely, when a center of rotation is desired,
the position of the force (or the equivalent moment/force ratio at the bracket) can be
calculated.
Cont…

CENTER OF ROTATION:
o Because the γ values differed as the load was changed from one plane to another
through the long axis of the tooth, it was shown that different centers of rotation would be
produced for a given force location if the direction of loading was changed.
o The center of rotation located more apically to the center of resistance with forces
directed labiopalatally than mesiodistally.
Cont…

CENTER OF ROTATION:
Cont…

PPT_BioMechanical Principles.pptx

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