This document discusses the biomechanics of orthodontic tooth movement. It defines key terms like biomechanical principles and analyzes force systems used for orthodontic tooth movement. Physiologic tooth movement includes eruption, migration, and movement during chewing. Histologically, tooth movement occurs through bone remodeling on pressure and tension sides in response to optimal continuous forces. Excessive forces can delay movement or cause root resorption. To control tooth movement, orthodontists calculate the moment to force ratio based on a tooth's center of resistance and applied forces.

1. Biomechanics of Orthodontic
Tooth Movement
Dr. Nabil Al-Zubair

2. Objectives
• Define the term “biomechanical principles” with
respect to various methods of orthodontic
therapy.
• Evaluate various force systems used for
orthodontic tooth movement and orthopedic
application.
• Describe the biology of orthodontic tooth
movement and discuss the histologic changes
that occur with various force systems.

3. Physiologic tooth movement
• Include :
• Definition: naturally
occurring tooth 1. Tooth eruption
movement that take 2. Migration or drift of teeth
place during & after 3. Changes in tooth position
tooth eruption. during mastication

4. Tooth eruption
• Definition: the axial movement of the tooth
from its developmental position in the jaw to
its final position.

5. Theories of tooth eruption
1 The root elongation theory
2 The hydrostatic theory (vascular theory)
3 The alveolar bone growth theory
4 The pulp theory
5 The periodontal ligament theory

6. Migration or drift of teeth
• Refers to the minor changes in tooth position observed after eruption of teeth
The human dentition
Shows a natural tendency to move in a mesial & occlusal direction
• • howevertrue lower jaw show certain variation)
largely the in case of maxillary dentition

7. Migration or drift of teeth
Tooth migration is usually as a result of
Proximal & occlusal wear of teeth
Teeth undergo proximal & occlusal wear
Move in a mesial & occlusal direction
to maintain inter-proximal & occlusal contact.

8. Tooth movement during mastication
Slight movement within its socket
Force absorbed by bone bending
Return to its original position as
soon as the load is removed
The teeth & periodontal structures
1-50 Kg based on type of food
Intermittent heavy forces
Short duration
Cycles of one second or less

9. Tooth movement during mastication
1-2 secs – PDL fluid displaced
Pain
2-3 secs – PDL tissues compressed
The teeth & periodontal structures
Long duration 1-50Kg / continuous

10. Histological basis of tooth movement
• Single point force application to crown of tooth Tilting
Center of Rotation

11. Histological basis of tooth movement
Pressure side
+ --
Tension side + -
+ -
+
+ +
+
- + Tension side
- +
- +
- +
-
-
Pressure side

12. Histological basis of tooth movement
Continuous optimal force-
Direct bone resorption
Pressure side
-
- Cell proliferation
-
-
- Osteoclasts
-
-
-
-
Pressure side -

13. Histological basis of tooth movement
Continuous optimal force-
Osteoid
mineralised
+
Tension side +
+ PL fibers arebone
Woven stretched
+ +
+ +
+ Tension side
+
Fibroblasts and osteoblasts +
+
Mature lamella bone.

14. Histological basis of tooth movement
•If force exceeds capillary blood pressure
Reduces blood flow
Pressure side
Will not produce optimal tooth movement.
+ --
Tension side + -
+ -
+
+ +
+
- + Tension side
- +
- +
- +
-
-
Pressure side
Tooth movement occurs with a force duration for only 6 hrs/day

15. Excessive force
1. No direct resorption of bone
2. Compression of blood vessels
This results in sterile necrosis of cells (known as hyalinization because
of the structureless appearance under the microscope.
No tooth movement initially. Pressure side
Delay of 2-3 weeks then there is
undermining resorption outwards
from the marrow spaces of the
adjacent alveolar bone.
Pressure side

16. • The optimum force for tooth movement is 25 g/cm2 of
root surface area.
• The size of force is determined by surface area of the
root and the type of movement i.e tipping < force than
bodily movement.

17. Force – 50-75g
Force –100-150g
Force –50-100g
Tooth movement
Force –15-25g
Force –50g

18. Excessive forces results in:-
– delay in tooth movement
– discomfort to patient
– tooth mobility
– root resorption
– excessive force dispersed over anchor teeth, threshold
exceeded

19. Timing of treatment
Usually the early permanent dentition 10-14 yrs.
Reasons
• Bone remodelling in response to orthodontic forces
are more rapid in children.
• Patient co-operation best around early teens.
• Active tooth movement cannot begin until after
eruption.
• Growth can be used for overbite reduction,
anteroposteior arch correction and spontaneous
tooth movement.

20. Biomechanics
Free bodies :
their mass is concentrated at a single point.
This point is called the mass center
• If, e.g., a free body
is found in space
and a force is
applied at a
specific point on it,
then all body
points will follow
the same course.

21. The tooth not a free body
The periodontal ligament Restrict its movement
The alveolar bone.
The point that corresponds to the mass center of a free
body is the center of resistance of the tooth.

22. The center of resistance
The point where tooth resistance to a movement may be
concentrated;
Center of Rotation (CR) M
CR
in other words, when a force is
applied through this point, tooth
D
translation ensues.
Forces applied at a distance from the CR also F
create a moment that tends to rotate the
tooth.
Moment = Force x Distance

23. Moment = Force x Distance
M
Moment magnitude depends on
1. force magnitude
2. distance of the force vector from the CR
D
F

24. Moment / force ratio
• When the orthodontist knows the location of a tooth’s CR,
Moment / force ratio for every tooth movement may be calculated;
thus, tooth movement may be controlled
M
D
Distance = Moment / Force
F

25. Moment / Force = Distance
• Translation • Torque
• Controlled tipping
Moment to force ratio is between 1 and 7
Moment to force ratio is between higher 10 10
Moment to force ratio is 8 and than

26. Moment / force ratio
• If, e.g., tooth translation is to be achieved
M
• An opposite moment
counteracting the first
one D
• a force applied
through the center
F
of resistance
• Should be incorporated into the system.

27. M
D
• If, e.g., tooth translation is to be achieved
F
• An opposite moment counteracting the first one

28. Dr. Nabil Al-Zubair