Mech

BIOMECHANICS
www.indiandentalacademy.com

Introduction
Theories of tooth movement
Phases of tooth movement
Types of forces
Mechanical concepts in orthodontics
Mechanical aspects of extraction space closure
Mechanical aspects of anchorage control
Mechanical aspects of headgear appliance

Mechanics is the discipline
that describes the effect of
forces on bodies;
Biomechanics refers to the
science of mechanics in
relation to biologic systems.

Theories of orthodontic tooth
movement
•Pressure tension theory-Shwartz 1932
•Blood flow theory-Bein 1966
•Piezoelectric theory

Phases of Tooth Movement
•Initial phase
•Lag phase
•Post lag phase

Types of forces
•Continuous
•Interrupted
•Intermittent

• Orthodontic tooth movement results from the
application of forces to teeth. These forces are
produced by the appliances (wires, brackets,
elastics, etc.) inserted and activated by the
clinician.
• The basis of orthodontic treatment lies in the
clinical application of
biomechanical concepts. Mechanics is the
discipline that describes the effect of forces on
bodies; biomechanics refers to the science of
mechanics in relation to biologic systems.

MECHANICAL
CONCEPTS IN
ORTHODONTICS

Newtons Laws
1.Law of inertia: Every body continues ina state
of rest or uniform motion in a straight line unless
its compelled to change by the forces impressed
on it
2.Law of acceleration: The change in motion is
proportional to the motive force impressed and is
made in the direction of the straight line in which
the force is impressed
3:The law of action and reaction: To every action
thee is always opposing and equal reactionwww.indiandentalacademy.com

Force - a load applied to an object that
will tend to move it to a different position
in space. Force, though rigidly defined in
units of Newtons (mass times the
acceleration of gravity), is usually
measured in weight units of grams
or ounces.

Center of Resistance. All objects have a center of
mass. The center of mass is the point through
which an applied force must pass for a free
object to move linearly without any rotation; in
other words, the center of mass is an
object's "balance point.'
A tooth within a periodontal support system is not
a free body because it is restrained by the
periodontium. The center of resistance is analogous
to the center of mass for restrained bodies.

The center of resistance of a tooth is dependent on the
root length and morphology, the number of roots, and
the level of alveolar bone
Analytical studies have determined that the center of
resistance for single-rooted teeth with normal alveolar
bone levels is about one-fourth to one-third the
distance from the cementoenamel junction (CEJ) to
the root apex.
Thee center of resistance of maxilla is slightly inferior
to Orbitale for the maxilla and distal to the lateral
incisor roots for intrusive movements of maxillary
anterior four teeth.

A moment is defined as the product of the force
times the perpendicular distance from the
point of force application to the center of
resistance, and thus is measured in units of gm-
mm (or equivalent). If the line of action of an
applied force does not pass through the center of
resistance, a moment is necessarily created. Not
only will the force tend to translate the
object, moving it to a different position, it also
will tend to rotate the object around the center of
resistance.

Couple-two forces equal in magnitude
and opposite. in direction.
A couple will produce pure rotation,
spinning the object around its center
of resistance, while the combination of a
force and a couple can change the way an
object rotates while being moved

Center of rotation- the point around which
rotation actually occurs when an object is
being moved. If a force and a couple are
applied to an object, the center of rotation
can be controlled and made to have any
desired location. The application of a
force and couple to the crown of a tooth, in
fact, is the mechanism by which bodily
movement of a tooth or even greater
movement of the root than the crown can
be produced. www.indiandentalacademy.com

Force 50gm gives 750 gm-mm moment
0.5mm bracket with a torsional force of 1500 gm gives 750 moment

The moment of the force is
determined by the magnitude of
the force and the distance from the
point of force application to the
center of resistance. For most
teeth, this is 8to 10mm, so MF will
be 8 to lO times the force

Moment-to-force ratios of 1 to 7
would produce controlled
tipping, ratios of 8 to 10
(depending on the length of the
root) would produce bodily
movement, and ratios greater
than 10 would produce torque.

Narrow vs. Wide Brackets
The wider bracket reduces both
the force needed to generate the
moment and the contact angle and
is thus advantageous for space
closure by sliding.

Retraction F=100
Distance from Cr to bracket is 10mm
Moment has to be 1000
If bracket is 1mm wide it needs 1000 gm
If bracket is 4mm wide it needs only 250 gm ( 4x 250 = 1000)

Effect of Bracket Slot Size

MECHANICAL ASPECTS OF ANCHORAGE
CONTROL
When teeth slide along an arch wire, force is
needed for two.purposes: to overcome frictional
resistance, and to create the bone remodeling
needed for tooth movement

When one moving object contacts another,
friction at their interface produces resistance to
the direction of movement. The frictional force
is proportional to the force with which the
contacting surfaces are pressed together and is
affected by the nature of the surface at the
interface

Surface Qualities of Wires
Surface Qualities of Brackets

Force of Contact.
Flexible wires
Larger Bracket
Rigid cap instead of tight ligature

BIOMECHANICS OF
EXTRACTION SPACE
CLOSURE

Anchorage: The nature and
degree of resistance to
displacement
offered by an an anatomic unit
for the purpose of effecting
tooth movement

Depending on anchorage demand
•Maximum anchorage (Group A)
•Moderate anchorage(Group B)
•Minimum anchorage(Group C)

Single cuspid Vs en-mass retraction

Space closure : biomechanical prospective
•Requires translatory movement of anterior
and posterior teeth
•This requires application of equal and
opposite
force and moment
•M/F ratio should be 10/1 for bodily movement

Group A
•Mesial forces acting on posteriors must
be minimised or neutralized –third Law
•For this it requires a relative increase in M/F
ratio(lesser F) and a decrease in
anterior M/F ratio (increased F)
•Extra oral appliance or intermaxillary
Elastics.

Segmental en-mass extraction space closure

Off center V-bend

AT 6mm activatin M/F is6/1
At 4mm its 10/1
At 2 mm its 12/1

Symmetric space closure Group –B
Group C space closure

Biomechanics of Head gear appliance

Line of action through the center of
resistance causes bodily movement
where as any where above or below
the centre of resistance will cause
tipping
M=TxP

Cervical traction

Occipital traction

Parietal traction

THANK YOU

Mech

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