# force# centerof resistance # center of rotation # type of tooth movement # type of forces # tipping # torquing# controlled and uncontrolled tooth mob Cement # continuous, interrupted, intermittent forces

1. THE MECHANICS OF
TOOTH MOVEMENT
DR. SHWETA A. KOLHE
BDS. MDS. (ORTHO) PHD SCHOLAR
ASSOCIATE PROFESSOR
VYWS DENTAL COLLEGE AND HOSPITAL,
AMRAVATI

2. INTRODUCTION
• Every body continues in it’s state of rest or of uniform
motion in a straight line, unless it is compelled to
change that state by forces impressed upon it ( Sir
Isaac Newton) – and teeth are no exception.
• Mechanics – defined as that branch of engineering
science that describes the effect of force on a body.

3. FORCE
• Force can be defined as an act upon a
body that changes or tends to change
the state of rest or of uniform motion
of that body.
• Force is vector, having definite
magnitude, a specific direction and a
point of application.
• f =ma. ( unit is newton or gram
millimeter/second2)
• In clinical practice it is either a push

4. • Resultant force
• Multiple forces can be combined
to form a resultant force by
vector addition.
• Determined by the law of
parallelogram of vectors by
connecting the vectors ‘ head to
tail’ while maintaining the length
and direction of the line of

5. STRESS AND STRAIN
• Stress is the force applied per unit area / external
force acting upon a body.
• Strain is the internal distortion per unit area.
• Strain can be expressed in the form of a change in
either the external dimension or internal energy of the
body.
• Stress and strain are inter-related terms.

6. COUPLE
• Couple is a pair of concentrated forces having equal
magnitude and opposite direction with parallel but non-
collinear line of action.
Or
• Couple is two parallel forces of equal magnitude but
opposite in direction.
• Couple = force x distance. ( unit is grams milimeter)

7. • A couple when acting upon a body brings about pure
rotation around the center of resistance.
• A couple exerts no net force on the center of resistance
because the two forces that comprise it are opposite in
direction and cancel each other.
• Couple acts to rotate, tip, or torque the Tooth around the
center of resistance.
• Moment of couple is the rotational tendency of the couple.

8. Couple – two forces of
same magnitude, parallel
to each other and acting
in opposite direction.
Clinical application of a
couple in treatment of
rotations.

9. • Engaging a wire
in an angulated
bracket slot
results in a
couple that helps
in uprighting a
• Engaging A
rectangular wire
into a slot
produces a couple
that is used in
producing the
required torque.
• Couples are
applied by
engaging a
wire in an
edge wise
bracket Slot.

10. CENTER OF RESISTANCE
• Every body or free object behaves as if it’s mass is
concentrated at a single point on which it can be
perfectly balanced.
• For physical calculation, this point can be taken as the
point where the whole body weight is concentrated and
can be termed center of gravity or mass.

11. • Center of resistance of tooth can be
defined as that point on the tooth
when a single force is passed
through it, would bring about it’s
translation along the lie of action of
the force.

12. • Center of resistance of tooth is
constant.
• In a single rooted tooth it lies between
one third and of one half of the root,
apical to the alveolar crease
• While in a multi-rooted tooth the center
of resistance lies between the roots, 1-
2 mm apical to the furcation.
• The center of resistance exists for -
single tooth, units of tooth, the entire
dental Arch and the jaws.

13. FACTOR WHICH CAN CHANGE THE
POSITION OF THE CENTER OF RESISTANCE
-
• Root length- longer the root, the
center of resistance will be placed
more apically.
• Alveolar bone height – if the
alveolar crest is higher, the center
of resistance will be placed more
coronally.

14. MOMENT
• Moment can be defined as the measure of rotational
potential of a force with respect to a specific axis.
• Moment not only produce a linear motion but also
produce rotation.
• Moment = Magnitude of force x distance
• Unit is gram millimeters.

15. • There are two variables that
determine the moment of
force – the magnitude of force
and the distance from the
center of resistance.
• Either one of these two
variables can be manipulated
to produce the desired force
systems.

16. CENTER OF ROTATION
• Center of rotation is a point, about which a body
appears to have rotated, as determined from it’s initial
and final positions.
• Center of rotation is a variable point and changes
according to the type of tooth movement.
• In case of controlled crown tipping, the center of
rotation – at the root apex.
• In case of perfect translation – at infinity.

17. CENTER OF ROTATION
• At root apex –
during
controlled
tipping.
• At incisal edge
– during
torquing.
• Outside root –
during
Intrusion &
extrusion.
• At away from
root apex –
during
uncontrolled
tipping.

18. TYPE OF TOOTH MOVEMENT
• The prime motive of orthodontic treatment is to move
the teeth into more favourable and corrected
positions.
• Variety of tooth movement in all the 3 planes –
sagittal, coronal, and transverse.
• Tooth movement within oral cavity can be listed –
1. Tipping. 2. Bodily movement
3. Intrusion 4. Extrusion
5. Torquing 6. Uprighting

19. TIPPING
• Tipping is a simple type of tooth movement
• where a single force is applied to the crown
• Tipping force results in Movement of the crown in the
direction of the force and the root in the opposite
direction.
• Tipping can be of two type –
1.Controlled Tipping
2.Uncontrolled tipping

20. CONTROLLED TIPPING
• It occurs when a tooth tips about a
center of rotation at its apex.
• There is a lingual movement of the
crown with minimal movement of
the root in labial direction.

21. UNCONTROLLED TIPPING
• It describes the movement of a tooth
that occurs about a center of rotation
apical to and very close to the center
of resistance.
• It is characterized by the crown
moving in one direction while the root
moves in the opposite direction.

22. BODILY MOVEMENT
• If the line of action of an applied
force passes through the center
of resistance of a tooth.
• All the points on the tooth will
move an equal distance in the
same direction signifying a bodily
displacement or translation.

23. INTRUSION AND EXTRUSION
• Intrusion is the bodily
displacement of a tooth
along it’s long axis in an
apical direction.
• Extrusion is the bodily
displacement of a tooth
along it’s long axis in an
occlusal direction.

24. ROTATION
• Rotations are labial or lingual movement of a tooth around
it’s long axis.

25. TORQUEING
• Torqueing can be
considered as a reverse
tipping characterized by
lingual movement of the
root.

26. UPRIGHTING
• During orthodontic treatment, the
crowns of certain teeth will be
tipped in a mesio-distal direction
with the roots tipped in the opposite
way.
• Tipping these roots back to get a
parallel orientation is termed
uprightng

27. •Tooth movements can be classified basically
into three:
1. Pure translation
2. Pure rotation
3. Generalized rotation.

28. PURE TRANSLATION
• Occurs when all points on the tooth move an equal
distance in the same direction.
• This is brought about when the line of action of an
applied force passes through the center of resistance
of a tooth.
• Pure translation of three types :
1.Intrusion
2.Extrusion
3.Bodily movement ( mesio- distal, labio- lingual)

29. PURE ROTATION
• A displacement of the body, produced by a couple,
characterized by the center of rotation coinciding with
the center of resistance i.e. the movement of points of
the tooth along the area of a circle, with the center of
resistance being the centre of the circle.
• Pure rotations can be divided into 2 types:
1.Transverse rotation
2.Long axis rotation

30. 1.Transverse rotation – Those tooth
displacements during which the
long – axis orientation changes e.g.
Tipping and torquing.
2.Long – axis rotation – Here the
angulation of the long – axis is not
altered. E.g. Rotation of tooth
around it’s Long axis.
3.Generalized rotation- . Any
movement that is not pure
translation or rotation can be
described as a combination of both
translation and rotation. E.g. Seen

31. TYPES OF FORCES
• As is well stated by the pioneers in the subject, if
malocclusion is the disease in orthodontics, force is
definitely it’s medicine.
• It is therefore very important for the clinician to use
his knowledge and experience in determining the
type, amount and direction of force required to bring
about efficient treatment results.
• Based on the duration of application –
1.Continuous force
2.Intermittent force

32. Graph showing the
different types of force

33. CONTINUOUS
FORCE
• It is an active orthodontic force
that decrease little in Magnitude
between appointment periods. E.g.
Light wire appliance.
• For an appliance to deliver
continuous force, the appliance
components should be highly
flexible and the activation must be
done to a relatively low force level.
• This is because continuous forces
are expected To bring about direct

34. INTERMITTENT FORCE
• It is an active orthodontic
force that decays to zero
magnitude or nearly so prior
to the next appointment. E.g.
Removable active plates.

35. • The appliance components should have high stiffness
and the initial activation should be twice the Expected
corresponding soft – tissue deformation. Due to a
relatively high activation, a greater force is exerted on
the teeth.
• lead to undermining resorption and corresponding
tooth movement.
• Once the tooth has moved, the force will decay
considerably so that repair of the necrosed soft tissue
and resumption of blood supply occurs within the
periodontium.

36. INTERRUPTED FORCE
• It is an orthodontic or orthopedic
force that is inactive for intervals
of time between appointments.

37. For an interrupted force to be delivered, the pre-
requisites include:
1.It should deliver heavy forces.
2.There should not be any force decay.
3.There should be a specific magnitude-time pattern for
pattern for example, 200-300 gms of force 10-14 hours
a day.
4.The inactive period of each day must be sufficient to
keep the periodontal ligament healthy over the total
period of time of use of the appliance.

38. • Thinking in the normal way, it might occur to us that a
continuous force may bring about a continuous
movement or an increased force may lead to an
Increased tooth movement.
• It often exhibits, cyclic, long-term magnitude-time
pattern. E.g. Force exerted by an extra- oral appliance
worn only at night.
• But these assumption are far from reality both In the
clinical and cellular context.
• The rate of tooth movement is highly dependent upon the
complex biologic responses to the various types of forces
that are yet to be clearly understood.