The document discusses biomechanics principles related to orthodontic tooth movement. It covers topics such as: - Forces applied to teeth can cause movement through bone remodeling. - Biomechanics refers to mechanics applied to biological systems. Knowledge of forces is needed to control orthodontic treatment. - Teeth can move through light forces during normal function but heavier sustained forces over 1 second are needed for orthodontic tooth movement. - Forces have magnitude and direction, while scalars only have magnitude. Resultant forces and moments from multiple applied forces are calculated. - Different force systems and moment-to-force ratios produce different types of tooth movement such as tipping, translation, rotation, and torque.

1. Prepared by –
SK AZIZ IKBAL
Final Prof. 2015-2016
NORTH BENGAL DENTAL COLLEGE &
HOSPITAL
DEPARTMENT OF ORTHODONTICS
Under the Guidance of -:
Dr Prakash Ch Roy
Dr Rajib Sheal
Dr Abhijit Das
Dr Amit Shaw
1

2.  Orthodontic treatment was possible due
to the fact that whenever a prolonged
force is applied on a tooth, bone
remodelling occurs around the tooth
resulting in it's movement.
2

3.  Biomechanics refers to the science of mechanics in
relation to biologic systems.
 Where as term mechanics refers to the discipline that
describes the effect of forces on bodies.
 WHY TO STUDY BIOMECHANICS?
 Knowledge of the bio mechanical principles and
governing forces is necessary for the control of
orthodontic treatment
3

4. It is the naturally occurring tooth movements
that take place during and after tooth eruption
1. Tooth eruption
2. Migration or drift of teeth
3. Changes in tooth position during
mastication
4

5.  Normal force of mastication – 1 to 50 kg
 It occurs in cycles of 1 second or less duration
 Teeth exhibit slight movement within the socket and
return to their original position on withdrawal of the
force
 Whenever the force is sustained for more than 1
second, periodontal fluid is squeezed out & pain is
felt as the tooth is displaced into the the periodontal
space
5

6.  Scalars : it do not have direction , it have only magnitude. Example: weight, temperature ,
distance , mass etc.
 Vectors: it have both magnitude and direction. Example : force etc.
6

7.  1.RESULTANT FROM FORCE WITH COMMON
POINT OF APPLICATIO N:
THE PARALLELOGRAM METHOD:
If 2 force have common point of application, than they are
considered as the side of the parallelogram
We complete the parallellogram.
And diagonal is considered as the resultant.
(length: magnitude of resultant F, arrowhead: direction )
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8. 8

9. THE LAW OF TRANSMISSIBILITY OF FORCE
 First we make artificial/ constructed point of
application in space, along the line of application of
force.
 Vectors are moved along the line of application to the
constructed point of application ,maintaining their
original length.
 Then parallelogram is constructed, and resultant F is
obtained
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10. 10

11.  We divide a single force into components at
right angle to each other.
 We consider the force vector to be the
diagonal of parallelogram,
 The component are parallel and
perpendicular to occlusal plane, dividing into
vertical and the horizontal component.
11

12. 12

13.  Each body has a point in its mass, which
behaves as if the whole mass is concentrated
at that single point, which we call CENTER OF
MASS in a gravity free environment.
13

14. 14

15. 15

16. 16

17.  It is the point on the tooth when a single force
is passed through it, would bring about its
translation along the line of action of the force

18.  The center of resistance is on the long axis of
tooth between one third and one half of the
root length apical to alveolar crest.
18

19.  For multirooted tooth, the center of
resistance is probably b/w the roots 1-2 mm
apical to furcation.
19

20. 20
• Factors affecting Centre of Resistance
1. Number of roots
2. Degree of Alveolar Bone loss
3. Degree of Root Resorption

21.  Point, around which body appears to rotate, determined from initial and final
position.
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The centre of rotation can be at the centre of resistance,apical to it,at the root
apex or at infinity
TYPES OF TOOTH MOVEMENT & POSITION OF THE CENTER OF ROTATION
Translation – lies at infinity
Uncontroued tipping – slightly apical to COR
Controlled tipping – Apex of root
Root movement of torquing - incisal or occlusal edge

22. 22
Extrusion or Intrusion- outside the body

23.  The moment of the force is the tendency for a force to produce rotation. It is
determined by multiplying the magnitude of the force by the perpendicular
distance of the line of action to the center of resistance
 It is measured in gm-mm.
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24. 24

25.  If two forces of the couple act on opposite
sides of the center of resistance, their effect
is additive.
 However, if they are on the same side of the
center of resistance, their effect is
subtractive.
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26. 26
If two forces of the couple act on opposite sides of
the center of resistance, their effect is additive.

27. 27
However, if they are on the same side of the
center of resistance, their effect is subtractive.

28. COUPLE - A couple consists of two forces of equal magnitude, with parallel but non
colinear lines of action and opposite senses.
The result is a moment with no net force.
 The object rotates about it’s centre of resistance regardless of the point of application of
the couple.

29. MOMENT FORCE RATIO
It is the relationship between the force and the counterbalancing
couple.
‘Moment-to-Force’ ratio= Counterbalancing moment/ Force
29

30. CONTROLLED
TIPPING
TIPPING –
simplest tooth movement.
Controlled tipping- Centre of rotation at its apex.
Uncontrolled tipping- Centre of rotation is very close or at centre of
resistance.
UNCONTROLLED
TIPPING

31. 31
BODILY MOVEMENT
TRANSLATION –
•All points of tooth move equally and in same
direction.
•Here line of action of applied force passes through
centre of resistance.
•To counteract the tendency for tipping, a couple
can be applied intentionally to produce a moment
of equal magnitude in the opposite direction. when
a moment –to –force ratio of 10:1 is applied to the
bracket ,the equivalent force system at the center
of resistance is a single force with no net moment.
•In the pure translation, the center of
rotation is considered to be at infinity.

32. 32
ROTATION –
If only a couple ,and no force is applied to a
tooth the tooth will rotate around it’s centre of
resistance and the tooth will not translate.
because the action of a couple does not
depend on it’s point of application,a pure
moment always acts at the center of
resistance.
ROTATION

33. 33
ROOT
UPRIGHTING
ROOT UPRIGHTING –
When the counter moment applied
intentionally at the bracket is more than the
moment of force , the root moves in the
direction of force but the crown tips in the
opposite direction.when the moment-to –
force ratio of 13:1 is applied at the
bracket,the equivalent force system at the
center of resistance is a force to move the
tooth plus a small net tendency for the root to
tip in the direction of force.
The center of rotation ,when the
moment to force ratio is 13:1,is at the
crown of the tooth and only root
movements occur.

34. 34
EXTRUSIONINTRUSION
INTRUSION & EXTRUSION- They are bodily displaced of a tooth along the
long axis of it. When the displacement is in apical direction, it is intrusive and
when towards occlusal direction, it is extrusive.

35. M/F 0 to 5 : 1 Uncontrolled tipping
M/F 7 : 1 Controlled tipping
M/F 8 to10 : 1 Translation
M/F >10 : 1 Root movement or Torque
MOMENT TO FORCE RATIO FOR VARIOUS
TOOTH MOVEMENTS
35

36. 36
TYPE OF MOVEMENT FORCE REQUIRED (gm)*
Tipping 35-60
Bodily movement (translation) 70-120
Root uprighting 50-100
Rotation 35-60
Extrusion 35-60
Intrusion 10-20
*values depend in part on the size of the tooth, smaller values appropriate for
incisors, higher values for multirooted tooth

37.  A moment can be referred as ROTATING,TIPPING,TORQUING
 When the tooth is embedded within the alveolar bone, a
couple can be applied only on the exposed part of the
tooth.Various tooth alignment procedures can be achieved by
this couple mechanism. Depending on the plane in which the
couple is acting this rotational tendency is called
a.Rotation{first order} b.Tipping{second order}
c.Torque{third order}
37

38. 38
Force applied on a tooth
Overcome the moment
Created by the force applied
to the crown
Counter moment
Crown moves more than root
To maintain the inclination
Of the tooth

39. To maintain axial inclination
Apply the force close to
the center of resistance
Create a 2nd moment
In the direction opposite
to the first
Practical difficulty
Power arm
Counter moment
Tooth remain upright
And move bodily
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42. 42

43.  Mc/Mf=0 –PURE TIPPING (Crot and Cres same, thus the tooth
rotates around the Cres)
 0‹Mc/Mf‹1-CONTROLLED TIPPING (Crot displaced away from Cres,
and the root and crown move in the same direction)
 Mc/Mf=1-BODILY MOVEMENT (equal movement of crown and root)
 Mc/Mf›1-TORQUE (root apex moves further than crown)
43
[Mf- Moment produced by force applied on tooth, Mc- Counter balancing moment]

44. 44

45. TORQUE
A rectangular wire in
a rectangular slot
Generate the moment
of a couple necessary
to control root
position
Torque acting as the
counter moment
Bracket system Clinical implication
45

46. 46

47.  Equal and opposite force system.
 One couple appliance system.
 Two couple appliance system.
47

48.  Simplest orthodontic appliance ,an elastic
band stretched between two points of
attachment is the best example. This
produces force of equal magnitude on either
end but opposite direction.
48

49.  One end of the appliance experiences couple and the other end is tied as a
point contact.
49

50.  The both the ends of the appliance are engaged
into attachments{brackets or tubes}.A couple may
be generated by the wire at either or both
attachment sites. The force systems produced by
two couple appliances cannot be measured
clinically and so they are referred as statically
indeterminate.
50

51.  Variety of combinations of two- bracket
systems and their force systems
51

52.  Continuous Force (ideal spring)
 Interrupted Force (removable active plates)
 Intermittent Force (removable appliances)
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56.  Forces that bring about orthodontic tooth
movement are continuous and should have
a minimum magnitude (threshold)
 Below this threshold limit, the PDL has the
ability to stabilize the tooth by active
metabolism
 The minimum pressure required is 5 to 10
gm/cm2 (current concept)
56

57.  Contemporary Orthodontics .
 Orthodontics – The Art and Science .
 Biomechanical and esthetic considrations NANDA
 Text book of orthodontics TOM GRABER
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