Biomechanics in Orthodontics

BDS [Orthodontics] course

1. BIOMECHANICS
IN
ORTHODONTICS
PROF (Dr.) SAIBEL FARISHTA

2. BIOMECHANICS
In orthodontics, biomechanics is commonly used
in discussion of the reaction of the dental and
facial structures to orthodontic force, whereas,
mechanics is reserved for the properties of the
strictly mechanical components of the appliance
system.
BIOLOGY +MECHANICS = BIO-MECHANICS

3. MECHANICS IS THE STUDY OF THE
EFFECT OF FORCES ON A BODY
MECHANICS

4. Orthodontic therapy depends on the reaction of
the teeth and more generally the facial structures
to gentle but persistent force.

5. COMMON TERMINOLOGIES IN BIOMECHANICS
FORCE
MASS
CENTER OF MASS (C.M.)
CENTER OF RESISTANCE (Cres)
CENTER OF ROTATION (Crot)
MOMENT
COUPLE
MOMENT OF FORCE (M
F
)
MOMENT OF COUPLE (M
C
)

6. FORCE: is defined as an act upon a body that
changes or tends to change the state of rest or the
motion of the body

7. FORCE:
A load applied to an object that will tend to move
the object to different position in space.
Units: Newton
Force equal to mass times acceleration.
F = ma
In metric system, it is usually measured in grams
or ounces. In orthodontics, forces are obtained in a
variety of ways. Deflection of wires, activation of
springs, elasticsand magnetsare the common
means of producing orthodontic force.

8. FORCE
STATIC:Which deals with bodies at rest
DYNAMIC:Which deals with moving bodies

9. MASS AND WEIGHT:
The mass of any body is the quantity of matter it
contains. The quantity of mass of a particle can be
measured by weighing the particle, since mass and
weight are proportional but not the same.

10. CENTER OF MASS (C.M.):
The point at which the mass of a body may be considered to be
concentrated is known as center of mass.
In other words the center of mass is an object’s balance point.
E.g. in a space, if one could push a box in line with its center of
mass, the box will translate away as its entire mass is
concentrated at that single point.
Practically speaking one can predict the behavior of any body in
space if one knows forces in relation to their center of mass.

11. CENTER OF RESISTANCE ( Cres):
For an object in free space, the center of resistance is the same as
the center of mass. If the object is partially restrained as in case
of tooth embedded in bone, the center of resistance is equivalent
to the balance point for restrained bodies.

12. The center of resistance depends upon,
·Root length and morphology
·Number of roots
·Level of alveolar bone support
The exact location of Cres for a tooth is not easily identified.
Although Cres for single rooted teeth with normal alveolar bone
levels is about one fourth to one third of the distance from the
cemento-enamel junction, to the root apex. Cres for multirooted
teethlies just below the furcation area, i.e. 1-2 mm apical to the
furcation.
Cres of maxilla lies in the area of postero-superior aspect of
zygomatico-maxillary suture.
IN CASE OF A TOOTH;

13. Although its precise location is typically unknown, it is
important to have a conceptual awareness of Cres in selecting
and activating an orthodontic appliance. The relationship of the
force system acting on the tooth to the Cres determines the type
of tooth movement expressed.It is the point through which pure
force will produce only translation i.e. all the points on the tooth
moving in parallel straight line.

14. CENTER OF ROTATION (Crot):
It is the point around which rotation actually occurs
when an object is being moved. Depending upon the
force system applied, the center of rotation may vary.
E.g. In case of controlled tipping center of rotation
will be at root apex while in case of perfect translation
it will be at infinity.

15. MOMENT:
Is product of force times the perpendicular distance
from the point of the force application and to the
center of the resistance.
Moment = Force xPerpendicular distance from Cres to
point of force application
Thus it is measured in the unit of gm-mm

16. Couple
Two equal and opposite, non-colinear forces are called acouple

17. COUPLE:
Two forces equal in magnitude and opposite in
direction produce couple. The result of applying two
forces in this way is a pure moment, since translatory
effect of the forces cancels out.
A couple will produce pure rotation, spinning the
object around its Cres. While combination of a force
and a couple can change the way an object rotates
while it is being moved

18. MOMENT OF FORCE (M
F
):
If the line of action of an applied force does not
pass through the center of resistance, the force will
produce some rotation.
The potential for rotation is measured as moment
and the magnitude of the moment is equal to the
magnitude of the line of force multiplied by the
perpendicular distance of the line of the action of
force to the Cres. The direction of moment can be
determined by continuing the line of action of the
force around the Cres.
Unit of M
F
is gm-mm.
Two factors determine M
F
.
1)Magnitude of force
2)Distance

19. MOMENT OF FORCE (M
F
)

20. MOMENT OF COUPLE (MC):
Another method of achieving rotational movement is
through the M
C
. A couple is two parallel forces of equal
magnitude acting in opposite direction separated by a
distance.
The magnitude of a couple is calculated by multiplying
the magnitude of force by distance between them.
Unit: gm-mm.

21. We all know and have experienced that the force
required to turn a bigger wheel is considerably lesser
than that required to turn a smaller wheel. The reason
for that being, in the bigger wheel the moment arm is
larger and hence the force to generate a moment of
particular magnitude is less.

22. In order to retract incisor we apply a force on the crown of
the tooth. This force creates a moment, as it is away from
the center of resistance and will cause tipping. To keep
tipping of the tooth to a minimum we have to create a
moment on this tooth in a direction opposite to that created
by the force. This can be done easily
by applying a couple having an anticlockwise moment.

23. Since moment is force times the distance the force is
equal to moment divided by the distance. Thus it
requires a large force to generate the counter
balancing moment.
Inherent moment
of a couple
Inherent couple acting at
a distance from the Cres producing
secondary moment of a couple

24. TYPES OF TOOTH MOVEMENT:
Basic tooth movements are categories into,
1.Tipping
2.Translation
3.Root movement
4.Rotation
Each movement is the result of variation of the
applied moment and force (either by magnitude
or point of application).

25. TIPPING:Is greater movement of the crown of the
tooth than that of the root. Crot is apical to the
Cres.
Tipping can be further classified on the basis of
the location of the center of rotation as
Uncontrolledtipping and Controlledtipping.

26. Uncontrolled tipping
A horizontal force at the level of bracket will
cause movements of the root apex and crown in
opposite directions. This is simplest type of
tooth movement. It requires single force and no
applied moment.
Uncontrolled tipping
Crot lies just below the Cres.

27. Controlled tipping
It is achieved by an application of force to
move the crown, as done in uncontrolled
tipping and application of a moment to
control or maintain the position of the root
apex.Crot lies at the root apex
M/F ratio = 7:1

28. Translation:This type of tooth movement is also
known as ‘bodily movement’.
Translation of a tooth takes place
when the root apex and crown move the same
distance and in the same direction.
A horizontal force applied at the Cres of a tooth
will result in this type of tooth movement.

29. Root movement (TORQUE):Root movement is
achieved by keeping the crown of a tooth stationary
and applying a moment and force to move only the
root.
Root movement is termed as ‘torque’.
Point of force application –a point apical to the
Cres
Center of Rotation –at the incisal edge or bracket.
M/F = 12:1

30. Pure rotation:This type of tooth movement occurs
when tooth rotates about its center of resistance.
A couple is required to produce pure rotation.

31. Intrusion and extrusion: It is tooth movement in axial
direction.
Intrusionis the bodily displacement of a tooth along its
long axis in an apical direction.
Extrusionis bodily displacement of a tooth along its
long axis in an occlusal direction.

32. Biomechanical considerations serve not only to
explain the effect of an orthodontic appliance but also
to detect side effects of therapy and to assist in
planning strategies for the avoidance or therapeutic
exploitation of these side effects.
Efficient orthodontic treatment requires that sound
treatment plans be carried with sound mechanical
plans.
Conclusion

33. Thank You