This document discusses biomechanics and mechanics of tooth movement in orthodontics. It covers: 1. The basic definitions of mechanics, stress, strain, stiffness, strength and other mechanical properties relevant to orthodontic tooth movement. 2. Theories of tooth movement including biomechanical, pressure-tension, fluid dynamics and piezoelectric theories. 3. Factors that influence tooth movement including force magnitude, duration, and decay over time. Light continuous forces produce faster movement through bone remodeling compared to heavy forces which cause bone necrosis. 4. Types of tooth movement including tipping, bodily movement, intrusion, extrusion, rotation and root uprighting.

1. Ibn Sina University
Faculty of Dentistry
Department of Orthodontics
Biomechanics,
Mechanics of tooth
Movement
Mohanad Elsherif
BDS (U of K), MFD RCSI, MFDS RCPS(Glasg), MSc (Orthodontics), M.Orth. RCSEd

2. Mechanics and Biomechanics
Mechanics:
The branch of engineering that study the effect of a force
to the body.
Biomechanics:
The study of he reaction of the dental and facial structures
to orthodontic force.

3. Basic proprieties of wire material
Stress:
 Internal distribution of the load. Measured as force per unit area.
 It can be classified into compressive, tensile or shear stress.
force force
force
force
forceforce
Compressive
stress
Tensile stress Shear stress

4. Basic proprieties of wire material
Strain
 It is the internal distortion produced by the load. Measured deflection per
unit length.
 It can be classified as elastic or plastic.
Elastic strain
Plastic strain

5. Basic proprieties of wire material
Hooks law:
 Stress is proportional to the strain within the elastic limit.
Proportional limit
Stress
Strain

6. Basic proprieties of wire material
Elastic (Proportional) limit
 The greatest stress to which material can be subjected
to and still can get back to its normal shape.
Yield strength:
 The point 0.1% permanent deformation.
Ultimate tensile strength:
 The maximum stress after which the material under go
permanent deformation.
Failure point:
 The point at which material fracture
Ultimate tensile strength
Yield strength
Proportional limit
Failure point
Stress
Strain

7. Basic proprieties of wire material
Stiffness:
 The ability of the material to resist deformation.
Strength:
 The ability of the material to resist deformation without
fracture.
Elasticity
 The ability of a stressed material to return to its original
form

8. Basic proprieties of wire material
Range
 Range is defined as the distance that the
wire will bend elastically before permanent
deformation occurs.
Springback:
 Springback is the range of activation of a
wire.
Ultimate tensile strength
Yield strength
Proportional limit
Failure point
Stress
Strain
Range
Springback

9. Basic proprieties of wire material
Resiliency:
The maximum amount of energy
a material can absorb without
undergoing permanent
deformation.
Formability:
The amount of permanent
deformation that a wire can
withstand before failing.
------Yield strength ------
-Proportional limit---
Strain
Stress
FORMABILITY
RESILIENCE

10. The Biologic Basis of
Orthodontic Therapy
 Orthodontic treatment is based on the principle that if
prolonged pressure is applied to a tooth, tooth movement
will occur as the bone around the tooth remodels.
 In essence, the tooth moves through the bone carrying
its attachment apparatus with it, as the socket of the tooth
migrates.
 Because the bony response is mediated by the
periodontal ligament, tooth movement is primarily a
periodontal ligament phenomenon.

11. Theories of tooth movement
1. Biomechanical theory.
2. Fluid dynamic (Blood flow) theory.
3. Pressure tension theory.
4. Piezoelectric theory.

12. Tooth movement
Tipping movementBodily movement

13. Theories of tooth movement
Osteoclast in
the pressure
area to resorb
bone.
Osteoblast in
the tension
area to form
bone.
Chemical
agent
Electrical
signals
Pressure tension
Blood flow
Biomechanical
theory
Piezoelectrical
theory

14. Theories of tooth movements
Light force => frontal
resorption => faster tooth
movement.
Heavy force =>
undermining resorption
=> slower tooth
movement.

15. Periodontal Ligament and Bone
Response to Sustained Force
 The response to sustained force against the teeth is a function of force magnitude.
 Lighter forces are compatible with survival of cells within the PDL and a
remodeling of the tooth socket by a relatively painless “frontal resorption” of the
tooth socket.
 Heavy forces lead to rapidly developing pain, necrosis of cellular elements within
the PDL, and the phenomenon “undermining resorption” of alveolar bone near the
affected tooth.
 In orthodontic practice, the objective is to produce tooth movement as much as
possible by frontal resorption.
 However recognizing that some areas of PDL necrosis and undermining resorption
will probably occur despite efforts to prevent this.

16. Pressure–Tension in Periodontal
Ligament
 This the classic theory of tooth movement, relies on chemical rather than
electric signals as the stimulus for cellular differentiation and ultimately
tooth movement.
 There is no doubt that sustained pressure against a tooth causes the
tooth to shift position within the PDL space, compressing the ligament in
some areas while stretching it in others.
 The mechanical effects on cells within the ligament cause the release of
cytokines, prostaglandins, and other chemical messengers.
 Chemical messengers are important in the cascade of events that lead to
remodeling of alveolar bone and tooth movement, and both mechanical
compression of tissues and changes in blood flow can cause their release.

17. Pressure–Tension in Periodontal
Ligament
Step (1)
Initial compression of
tissues and alterations in
blood flow associated
with pressure within the
PDL.
Step (2)
The formation and/or
release of chemical
messengers.
Step (3)
Activation of cells.

18. Time Light force Heavy force
<1 second
1-2 seconds
PDL fluid incompressible, alveolar bone bends, piezoelectric signal generated PDL fluid expressed,
tooth moves within PDL space
3-5
seconds
Blood vessels within PDL partially compressed
on pressure side, dilated on tension side; PDL
fibers and cells mechanically distorted
Blood vessels within PDL occluded on
pressure side.
Blood flow altered, oxygen tension begins to
change; prostaglandins and cytokines released
Blood flow cut off to compressed
PDL area
Metabolic changes occurring: chemical
messengers affect cellular activity, enzyme levels
change.
Minutes
Hours Cell death in compressed area
~4 hours
Increased cAMP levels detectable, cellular
differentiation begins within PDL.
~2 days
Tooth movement by frontal resorption begin as
osteoclasts/ osteoblasts remodel bony socket
3-5 days
Cell differentiation in adjacent narrow
spaces, undermining resorption begins
7-14
days
Undermining resorption removes lamina
dura adjacent to compressed PDL, tooth
movement occurs

19. Effects of Force Distribution and
Types of Tooth Movement
1.Tipping
movement:
• Simplest type of tooth
movement.
• The force passes above
the center of
resistance.
• The crown moves in
one direction and the
root in opposite
direction.

20. Effects of Force Distribution and
Types of Tooth Movement
2. Bodily
movement:
• The force passes
through the center
of resistance.
• The crown and the
root moves together
in the same
direction.
• Optimal force 70-

21. Effects of Force Distribution and
Types of Tooth Movement
3. Intrusion:
• The tooth moves
bodily in apical
direction.
• Optimal force10- 20
gm

22. Effects of Force Distribution and
Types of Tooth Movement
4. Extrusion:
• The tooth moves bodily in
coronal direction.
• Optimal force 35- 60 gm

23. Effects of Force Distribution and
Types of Tooth Movement
5. Rotation:
• The tooth moves around
it’s long axis.
• Optimal force 35- 60 gm.

24. Effects of Force Distribution and
Types of Tooth Movement
6. Root uprighting
(torqueing):
• Movement of the root without
the movement of the crown.
• Optimal force 50- 100 gm.

25. Effects of Force Duration and
Force Decay
 The key to producing orthodontic tooth movement is the
application of sustained force, which does not mean that the
force must be absolutely continuous.
 It does mean that the force must be present for a
considerable percentage of the time, certainly hours rather
than minutes per day.
 Clinical experience suggests that there is a threshold for
force duration in humans in the 4 to 8 hour range (Mean is 6
hours) and that increasingly effective tooth movement is
produced if force is maintained for longer durations.

26. Effects of Force Duration and
Force Decay

27. Effects of Force Duration and
Force Decay
 Duration of force has another aspect, related to how force magnitude
changes as the tooth responds by moving.
 Only in theory is it possible to make a perfect spring, one that would
deliver the same force day after day, no matter how much or how little
the tooth moved in response to that force.
 In reality, some decline in force magnitude (i.e., force decay) is noted
with even the springiest device after the tooth has moved a short
distance though with the superelastic nickel–titanium materials, the
decrease is amazingly small.
 With many orthodontic devices, the force may drop all the way to zero.

28. Effects of Force Duration and
Force Decay
 From this perspective, orthodontic force duration
is classified by the rate of decay as:
 Continuous—force maintained at some appreciable
fraction of the original from one patient visit to the next
(e.g. fixed appliances).
 Interrupted—force levels decline to zero between
activations (e.g. closing loops).
 Intermittent—force levels decline abruptly to zero when
the appliance is removed (e.g. removable appliances).

30. Effects of Force Duration and
Force Decay
 Both continuous and interrupted forces can be produced by
fixed appliances that are constantly present.
 Intermittent forces are produced by all patient-activated
appliances such as removable plates, headgear, and elastics.
 Forces generated during normal function (e.g., chewing,
swallowing, speaking) can be viewed as a special case of
intermittently applied forces, most of which are not
maintained for enough hours per day to have significant
effects on the position of the teeth.

31. Effects of Force Duration and
Force Decay
 There is an important interaction between force magnitude and
how rapidly the force declines as the tooth responds.
 Light continues force is desirable, however heavy continuous
forces are to be avoided because there is no time for the PDL to
heal.
 Heavy intermittent forces, though less efficient, can be clinically
acceptable.
 To say it another way: the more perfect the spring in the sense
of its ability to provide continuous force, the more careful the
clinician must be that only light force is applied.

32. Effects of Force Duration and
Force Decay
 Experience has shown that orthodontic appliances should not be
reactivated more frequently than at 3-week intervals, and a 4- to 6-week
appointment cycle is more typical in clinical practice.
 Undermining resorption requires 7 to 14 days the tooth movement
occurs in the first 10 days or so, and there is an equal or longer period
for PDL regeneration and repair before force is applied again.
 This repair phase is highly desirable and needed with many
appliances.
 Activating an appliance too frequently, short circuiting the repair
process, can produce damage to the teeth or bone that a longer
appointment cycle would have prevented or at least minimized.

33. Declaration
 The author wish to declare that; these presentations are his original work, all
materials and pictures collection, typing and slide design has been done by the
author.
 Most of these materials has been done for undergraduate students, although
postgraduate students may find some useful basic and advanced information.
 The universities title at the front page indicate where the lecture was first
presented. The author was working as a lecturer of orthodontics at Ibn Sina
University, Sudan International University, and as a Master student in Orthodontics at
University of Khartoum.
 The author declare that all materials and photos in these presentations has been
collected from different textbooks, papers and online websites. These pictures are
presented here for education and demonstration purposes only. The author are not
attempting to plagiarize or reproduced unauthorized material, and the intellectual
properties of these photos belong to their original authors.

34. Declaration
 As the authors reviews several textbooks, papers and other references during
preparation of these materials, it was impossible to cite every textbook and journal
article, the main textbooks that has been reviewed during preparation of these
presentations were:
Contemporary Orthodontics 5th edition; Proffit, William R, Henry W. Fields, and
David M. Sarver.
Handbook of Orthodontics. 1st edition; Cobourne, Martyn T, and Andrew T. DiBiase.
Clinical cases in orthodontics. Martyn T. Cobourne, Padhraig S. Fleming, Andrew T.
DiBiase, Sofia Ahmad
Essentials of orthodontics: Diagnosis and Treatment; Robert N. Staley, Neil T. Reske
Orthodontics: Current Principles & Techniques 5th edition; Graber, Lee W, Robert L.
Vanarsdall, and Katherine W. L. Vig
Orthodontics: The Art and Science. 3rd Edition. Bhalajhi, S.I.

35. Declaration
 For the purposes of dissemination and sharing of knowledge, these
lectures were given to several colleagues and students. It were also
uploaded to SlideShare website by the author. Colleagues and students
may download, use, and modify these materials as they see fit for non-
profit purposes. The author retain the copyright of the original work.
 The author wish to thank his family, teachers, colleagues and students
for their love and support throughout his career. I also wish to express
my sincere gratitude to all orthodontic pillars for their tremendous
contribution to our specialty.
 Finally, the author welcome any advices and enquires through his
email address: Mohanad-07@hotmail.com

36. Thank You