Bio-mechanical Principle in
Orthodontic Treatment
Xi'an Jiaotong University
Stomatology hospital
Orthodontic Department Zou Min
Appliance
Force
tooth、Alveolar 、bone
Displacement
Ⅰ Biological characteristics of
Periodontal tissue
Ⅰ Biological characteristics of
Periodontal tissue
（一）Stability of Periodontal
ligament Environment
Periodontal ligament （PDL）
is made up of Fibrous connective
tissue ，mostly connect teeth to
alveolar ，so that teeth fixed in
the alveolar.
Ⅰ Biological characteristics of
Periodontal tissue
（一）Stability of Periodontal ligament
Environment
Cell
Periodontal ligament has three cells
(Three components) ：
1 compound cell
2 Absorption cell
3 Undifferentiated cells
Ⅰ Biological characteristics of
Periodontal tissue
（一）Stability of Periodontal ligament
Environment
Tissue fluid : Periodontal ligament is full of tissue
fluid, which is to absorb external force. As the
liquid is incompressible ， the force gave to
teeth is quickly spread to the alveolar .
Ⅰ Biological characteristics of
Periodontal tissue
（二） Plasticity of alveolar
Bone especially alveolar has
the general robustness of bone
tissue and highly Plasticity.In
the physiological state, constantly
updated and translated，through
the conversion and absorption to
achieve a new balance .
Ⅰ Biological characteristics of
Periodontal tissue
(三) Resistance to stress of the
cementum
Root surface is covered
with a thin layer of cementum
，it fights against external
forces ，protect cementum
not to be absorbed.
Ⅱ Biological theory of teeth movement
1. Piezoelectric theory
Piezoelectric effect is the phenomenon by
crystal material deformation produced by
charge . Negatively charged concave, convex
positively charged
Ⅱ Biological theory of teeth
movement
Piezoelectric theory
Piezoelectric effect has two characteristics：
1 The loss of speed is very quickly. If the
force continues, it dose not continue to produce
electrical signals.
2 The disappearing of force brings the same
amount but the opposite direction of the signal
Ⅱ Biological theory of teeth
movement
1. Piezoelectric theory
Have a positive effect on the normal
alveolar bone metabolism.
The effect about the continuing
orthodontic force is not clear .
Results show: Electrical changes in the
environment can affect the
permeability of cell membrane, causing
Ⅱ Biological theory of teeth
movement
2. Mechanical - chemical theory of bone：
external force
teeth
blood
change of PDL
Chemical environment of
PDL（the level of oxygen and other relative levels of
metabolites ，for example Concentration of
Central - phosphate ）
Cell differentiation
Bone formation and absorption
Ⅱ Biological theory of teeth
movement
2. Mechanical - chemical theory of bone:
Tension side ：
The amount of blood increase
blood
oxygen content of the blood increase
Osteoblast differentiation
bone formation
Pressure side ：
Decreased blood flow
Decreased blood
oxygen content
Osteoclast
differentiation
Bone resorption
Ⅲ Organizational change in the
process of orthodontic treatment
（一） The response of alveolar and periodontal
ligament to orthodontic force
1. Pressure side
Periodontal ligament narrowed
Reduce blood flow
Degradation and absorbtion
of collagen fibers
Alveolar bending deformation
Osteoclast differentiation
Alveolar resorption
Movement of teeth
Multinuclear
osteoclasts
Ⅲ Organizational change in the process
of orthodontic treatment
（一） Response to orthodontic force by alveolar and
periodontal ligament
Large treatment force——Local vascular hemorrhage ，
Periodontal tissue degeneration transparent ，Transgender
organizations was absorbed by Phagocyte ，Newborn
fibroblasts 、
Vascular invasive ，repair them
——
Indirect absorption happens on the
pressure of the alveolar ，
indirect absorption also known as
“Absorption of prowling ”.
root tip is the mostly area of the root of
teeth.
Ⅲ Organizational change in the
process of orthodontic treatment
Tension side
：
Periodontal fiber elongation
stretch
Blood flow
increased
Fibroblast
proliferation ， Alveolar bone
bending deformation
Osteoblast differentiation
The surface of bone
trabeculae to form new
Trabecular bone
Osteogenic
（1）the response of alveolar and periodontal
ligament to orthodontic force
In the process of tooth movement ，Cells
generally appear within 36 hours after the burner.
Ⅲ Organizational change in the
process of orthodontic treatment
（2）Other
1. Dental tissue
Changes in cementum
u
Changes in root
u
Changes in pulp tissue
u 2. Impaction of deciduous tooth
movement to constant tooth germ
3. Changes in suture
u
4. Impact on the adjacent teeth
Ⅲ Organizational change in the process of
orthodontic treatment
（3）Periodontal tissue alterations and
recovery after teeth movement
Periodontal tissue alterations usually takes
four weeks to complete, which is one-month clinical
follow-up visits the patient basis on the biological
for the physiological movement of teeth.
Ⅲ Organizational change in the process
of orthodontic treatment
（三）Periodontal tissue alterations and
recoveries after teeth movement
Periodontal ligament fibers to keep nine months is still
anomaly.
New bone to maintain 9 months is not mature .
normal combination of periodontal ligament and alveolar
need 3 months.
Root absorption and recovery are more than 3
months.
Therefore, after treatment it needs to keep more than 1
year .
Ⅳ The effect of strength of
orthodontic force
Strength of different organizations have
different power levels of impaction. If the force is too
small, it can not cause periodontal tissue changes in
the chemical environment, the teeth will not move.
Excessive force can cause some periodontal tissue
degeneration and necrosis, degeneration and necrosis
of these organizations need to remove and repair a
longer period of time, tooth movement will be slowed
down, even not to move.
Ⅳ The effect of Strength of
orthodontic force
(一）The size of forces
Ⅰ level ：too small to cause teeth moving
Ⅱ level ：moderate and long-lasting ,and
can cause organizations response in 5~6
hours generally
Ⅲ level ：so large that local periodontal
fiber denaturize and necrosis
Ⅳ level ：Periodontal ligament compression
，and cementum and alveolar adhesion
Ⅳ The effect of Strength of
orthodontic force
（二） the best strength of treatment
In theory , it should be moderate and
long-lasting, and the strength can not
exceed the pressure of periodontal
ligament capillary, so that the capillaries is
in a semi-closed state, generally around
75-100g.
Ⅳ The effect of Strength of
orthodontic force
1. Indications of determine the appropriate
corrective force in clinic
No significant pain consciously
No significant response to percussion
Forced teeth has no obvious loosening ?(
< II )
Obvious teeth movement
There is no pathological changes of roots
and periodontal tissue on X-ray
Ⅳ The effect of Strength of
orthodontic force
2.Appropriate orthodongtic force
observed by Histological
The pressure side vessel is compressed, but
not closed
The cells have the greatest response
Organization is in a activity, but not
nonecrosis
Direct absorption of alveolar more than the
indirect absorption
Ⅳ The effect of Strength of
orthodontic force
（三）The type of tooth movement and the
distribution of power
1 Conception
Center of Resistance
It is the simplify center of the resistance
movement of objects ,also known as resistance,
the center of objects. It is inherent, and not
impacted by outside force.
Ⅳ The effect of Strength of
orthodontic force
（三）The type of tooth movement and the
distribution of power
1 Conception
Center of Rotation
Refers to objects in the external force
by the next rotation around the point center of
rotation with the external force and torque
changes.
Ⅳ The effect of Strength of
orthodontic force
（三） The type of tooth movement and
the distribution of power
1 Conception
Force: size, direction, point
moment=force×arm
(Resistance force from
the center line of the
vertical distance ) 。
Moment has two
variables ：Force and
distance. The direction
of moment consistent
with the direction and
force of resistance
centers +is Anticlockwise rotation ，-is
clockwise rotation.
couple：Two forces
with equal size but
opposite direction
operated on the same
odjects. They are
parallelled but not in a
line. Couple of teeth
can cause a simple
rotation.
Couple moment ：one
of the force×arm
u
Orthopedic Force ： Large and
discontinuous, causing the power to
change the bone morphogenetic .
u
Orthodontic force ：Smaller sustained,
causing the power to change the tooth
and dental arch .
Ⅳ The effect of Strength of
orthodontic force
2. The type of tooth movement
（2）Incline movement：
Crown and root movement are in opposite
direction with a fulcrum which is in the border
between the 1/3 of root and 1/3 of middle of root by
a single horizontal force.The periodontal tissues of
root apiex and alveolar was afforded the largest
power which is about 50g .
Ⅳ The effect of Strength of
orthodontic force
2. The type of tooth movement
（2）Bodily movement：
Crown and root movement are in
one direction at the same time. On the
one side of the periodontal tissues is
compressed, the other side is tracted.The
power distribution can support greater
forces which is generally believed that
100 - 150g or so.
Ⅳ The effect of Strength of
orthodontic force
2 The type of tooth movement
（3）Vertical movement ：
Teeth instrusion：In this movement , there is
only a small amount periodontal ligament of apical
squeezed. If the force is too large, the obstacles to the
blood circulation will be easier to caused in the root
zone, resulting in group symptoms of necrosis and
pulp texture. Therefore the power to suppress should
be very modest. generally power: 15 - 25g
Ⅳ The effect of Strength of
orthodontic force
2. The type of tooth movement
（3）Vertical movement ：
Teeth extrusion：In the
movement, the
periodontal fibers was force to extrusion,and new
alveolar bone was formated in Alveolar fossa which
make the alveolar increased with the teeth extrusion.
As Periodontal ligament supporting a large of forces
, extrusion movement can endure a larger elongation
force. Force value of 50 - 100g
rotation
Ⅳ The effect of Strength of
orthodontic force
2. The type of tooth movement
（4）Rotation movement ：
A couple of forces can made the crown and
root rotate along the tooth axis. Most of
periodontal fiber was forced by the horizontal
direction level of traction, and flat tooth root
formation will make two pressure zone and one
tension zone. The role of the force is between
extrusion and intrusion
Ⅳ The effect of Strength of
orthodontic force
2. The type of tooth movement
（5）Torque
Torque movement is also known as rootcontrolled movement, which can cause the root to
move without the moving of the crown .Alveolar
resorption occurred in the 1 / 3 of the roots first, and
apical has the largest movement. Periodontal
ligament has larger force area which is similar to the
bodily movement.
Ⅴ Factors that affect the
tooth movement
1. Force faction
bodily：150
tipping：50
large：>350g
extrusion：25
strength of force medium：60~350g
instrusion：100
weak：<60g
rotation：100
torque：150
Type of force
Mechanical force, biological force, and
orthodontic force, orthopedic force ,and so on
Forcing time
（Continuous force, intermittent force）
Ⅳ The effect of Strength of
orthodontic force
2. Body condition
u
Age
Health status
Partial anatomical factors: Root length,
alveolar height
Ⅳ The effect of Strength of
orthodontic force
3. External factors
u
Hormones: parathyroid hormone,
etc.
Drug: prostaglandins, such as
Salvia
Physical stimulation: electrical
u
stimulation, magnetic, etc.