1. MECHANICS OF
TOOTH MOVEMENT::
- RICHARD J. SMITH
-CHARLES J. BURSTONE
VOLUME 85, NO 4 ,
AM. J. ORTHODONTICS , APRIL 1984
PRESENTED BY: KIRTI AGRAWAL J.R.1

2.  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.
 Force vector is represented by arrows:
 Origin of arrow: point of application of force
 Body of arrow: line of application
 Arrowhead: direction of force( sense)

3. RESULTANT AND COMPONENT OF
ORTHODONTIC FORCE SYSTEM:
 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 )

4. The parallelogram method:

5. RESULTANT WITH DIFFERENT
POINT OF APPLICATION:
 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

7. RESOLVING THE FORCE INTO
COMPONENT:
 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.

9. Force and movement:
 Center of gravity: a point at which a free
body is perfectly balanced.
 Center of resistance : a force with a line of
action passing through the center of
resistance produce translation.
Example: center of resistance of a single rooted
tooth , probably between 1/3 and ½ of root
length apical to alveolar crest.
Center of resistance of a multirooted tooth , is
probably 1-2 mm apical to the furcation.

11.  Center of resistance depends on :
 1) root length
 2) alveolar bone height

13. Types of movements:
 Translation
 Rotation
 Combination of translation and rotation

14.  Translation: if line of action of force passes
through the center of resistance of a
tooth, the tooth will respond to pure bodily
movement / translation

15. MOMENT OF A FORCE:
 If line of application of force does not pass




through the center of resistance, the force
will produce some rotation, the potential for
rotation is measured a MOMENT.
M= F × d( perpendicular distance of line
of action of force to the center of resistance)
Unit: gram milimeters
Symbol: curved arrow.
Clockwise moments defined as POSITIVE and
counterclockwise moments negative.

16.  The direction of the moment can be
determined by continuing the line of action of
force around center of resistance.

18. COMBINED TRANSLATION AND
ROTATION:
 If the line of action of force does not pass
through the center of resistance.
 It will create a moment about center of
resistance, and result is a combination of
translation and rotation.

20. Rotation :
 Pure rotation means tooth spins about center
of resistance.

22. Type of movement
Center of rotation
translation
infinity
Pure rotation
At center of resistance
Root movement
Incisal edge
Controlled tipping
Apex
Uncontrolled tipping
Intrusion
slightly apical to the center of
resistance
On a line which is approximately
perpendicular to long axis

23. COUPLE:
 A single F cannot produce pure rotation , only
a force system can produce pure rotation of
tooth is called a COUPLE.
 Example : 2 F are the same
magnitude, parallel to each other , in the
opposite direction but not coincide, form a
couple.