2. Physiologic tooth movement
3. Histology of tooth movement
4. Optimum orthodontic force
6. Phases of tooth movement
7. Theories of tooth movement
8. Bone deposition
9. Bone resorption
Orthodontic treatment is made possible by the fact
that teeth can be moved through the alveolar bone by
applying appropriate forces . orthodontic tooth
movement is a unique process where a solid object
(tooth) is made to move through a solid medium
Orthodontic treatment is possible due to the fact
whenever a prolonged forces is applied on a tooth ,
bone remodeling occurs around the tooth resulting in
its movement. Bone subject to pressure as a result of
compression of periodontal ligament resorbs while
bone forms under tensile as a result of stretching of
the periodontal ligament.
Physiologic tooth movements are naturally occurring
tooth movements that take place during and after
tooth eruption .
1) Tooth eruption
2) Migration or drift of tooth
3) Changes in tooth position during mastication
It is the axial movement of the tooth from its
developmental position in the jaw to its final
position in the oral cavity.
Following are some of the theories:
A) Blood pressure theory: The tissue around the
developing end of the root is highly vascular. This
pressure is believed to cause the axial movement
of the teeth
B) Root growth: The apical growth of roots
results in an axially directed force that brings
about the eruption of the teeth.
C ) Hammock ligament: A band of fibrous tissue
exists below the root apex spanning from one side
of the alveolar wall to the other. The fibrous
tissue appears to form a network below the
developing root and is rich in fluid droplets.
D) Periodontal ligament traction: It states that
the periodontal ligament is rich in fibroblast that
contains contractile tissue. The contraction of
these periodontal fibers results in axial movement
of the tooth.
It refers to the minor changes in tooth
position observed after eruption. Human
dentition shows a natural tendency to move
in a mesial and occlusal direction.
This is largely true in case of maxillary
dentition. As the teeth undergo occlusal
and proximal wear, they move in a mesial
and occlusal direction to maintain inter-
proximal and occlusal contact.
During mastication, the teeth and periodontal
structures are subjected to intermittent heavy
forces which occur in cycles of one second or less
and may range from 1-50 kilograms based on the type
of food being mastication.
When a tooth is subjected to such heavy forces, the
tissue fluid present in the periodontal space, being
incompressalbe, prevents major displacement of the
tooth within the socket.
Whenever the forces of mastication are sustained
and are more than the usual one second cycle, the
periodontal fluid is squeezed out and a pain is felt as
the tooth is displaced into periodontal ligament
When a tooth is moved due to application of
orthodontic force , there is bone resorption on
the pressure side and new bone formation on the
tension side . the histologic changes seen during
tooth movement can be studied under 2 headings :
Changes following application of mild force
Changes following application of extreme force
When a force is applied to a tooth, areas of
pressure and tension are produced.
a. Changes on the pressure side: - the
periodontal ligament in the direction of the
tooth movement get compressed to almost
1/3rd of its original thickness and vascularity
is increased which helps in the mobilization
of cells such as fibroblast and osteoclasts
. Osteoclasts are bone resorbing cells that
line up the socket wall on the pressure side.
When continued forces are applied a change
in orientation of bony tabeculae is seen.
Trabeculae are usually parallel to the long axis
to the tooth and become horizontally
oriented i.e. parallel to the direction of
orthodontic force. The osteoclasts start
resorbing bone when the force are applied
within physiologic limits, the resorption is
seen in the alveolar plate immediately
adjacent to the ligament this is known as
b. Changes on the tension side: The area of tooth
opposite to the direction of force is called the
tension side. On application of force the
periodontal membrane on the tension side gets
stretched, the distance between alveolar
process and tooth is widened. Vascularity is
increased which causes mobilization of the cells
such as fibroblast and osteoblast in response
osteod is led down by osteoblast in the
c. Secondary modeling changes:
When force is applied the adjacent bone shows
osteoclastic and osteoblastic activity on the
pressure and tension side. Bony changes also
take place elsewhere to maintain the width or
thickness of the alveolar bone. these changes
are called as secondary remodeling.
Whenever extreme forces are applied to the
teeth , it results in crushing or total
compression of the periodontal ligament. On
the pressure side the root closely
approximates the lamina Dura, compresses the
periodontal ligament and leads to occlusion of
the blood vessels. The ligament is deprived
of nutritional supply leading to regressive
changes called hyalinization. Bone resorption
occurs in the adjacent marrow spaces and in
the alveolar plate below, behind and above
the hyalinized zones this kind of resorption
is called undermining or rearward resorption.
On the tension side, the periodontal ligament gets
over-stretched leading to tearing of the blood
vessels and ischemia.
Thus when the extreme force is applied there is
net increase in osteoclastic activity as compare
to bone formation with the result that tooth
become loosened in its socket. Pain and hyperemia
of the gingiva may occur due to application of
extreme forces during orthodontic tooth
It is one which moves teeth most rapidly in the
desired direction, with the least possible damage
to tissue and with minimum patient discomfort.
Equivalent to capillary pulse pressure which is
20-25 gm/sq. cm
It has following characteristics:
Produces rapid tooth movement
Minimal patient discomfort
The lag phase of tooth movement is minimal
No marked mobility of the teeth being moved.
From a histologic point of view the use of optimum
force has the following characteristics:
The vitality of the tooth and supporting
periodontal ligament is maintained
Initiates maximum cellular response
Produces direct or frontal resorption
It is a form of tissue degeneration
characterized by formation of a clear,
eosinophilic homogenous substance. This
hyalinization occurs in organs such as
kidneys,lungs,etc.Hyalinization of the
periodontal ligament denotes a compressed
and locally degenerated periodontal
ligament.the convention pathologic process
of hyalinization presents irreversible one,
however , hyalinization is a reversible
process in PDL.
The change observed during formation of hyalinized
bone are as follows:
There is gradual shrinkage of the periodontal fibers
The cellular structures become indistinct. Some nuclei
becomes smaller(pycnotic ) while some nuclei
The compressed collagenous fibres gradually unite
into a more or less cell free mass
Certain changes also occur in the ground substance
There is a breakdown of the blood vessel walls leading
to spilling of their contents
Osteoclasts are formed in marrow spaces and
adjacent are of the inner bony surface offer aperiod
of 20 to 30 hrs .
The presence of hyalinized zone indicate that the
ligament is non functional and therefore bone
resorption cannot occur .
The elimination of hyalinized tissue occurs by 2
Resorption of the alveolar bone by osteoclasts
differentiating in the peripheral intact
periodontal membrane and adjacent marrow
Invasion of the cells and blood vessels from the
periphery of the compressed zone by which the
necrotic tissue is removed . the invading cells
penetrate the hyalinised tissue and eliminate the
unwanted fibrous tissue by enzymatic action and
Greater the force wider is the area hyalinization.
If the lighter forces are used the hyalinised
zones are smaller and the larger area of
functioning ligament is available. Frontal
resorption predominates in case where lighter
forces are used.
The location and extent of hyalinized tissue
largely depends upon the nature of tooth
movement. In case of tipping tooth movement the
hyalinization would be closed to alveolar crest
while in case of bodily tooth movement it would
be closure to the middle portion of the root.
Whenever exxesive forces are applied during
tipping tooth ,movement , it can result In 2 areas
of hyalinization , 1 in the apical region and other
ion the marginal area.
Studies have shown that tooth movement
through 3 stages.
Burstone categorises the stages as:
Post lag phase
Lag phase Post lag phase
During this phase very rapid tooth
movement is observed over a short distance
which then stops. This movements
represents displacement of the tooth in
the periodontal membrane space and
probably behind of alveolar bone to a
certain extent. The tooth movement in the
initial phase is between 0.4 to 0.9 mm and
usually occurs in a week’s time.
little or no tooth movement occurs during this
phase. this phase is characterized by formation
of hyalinaised tissue in the periodontal ligament.
The duration of the lag phase depends on the
amount of force used to move the tooth if light
forces are used the area of hyalinization is small
and frontal resorption occurs . If heavy forces
are used the are of hyalinization is large . the la
phase usually extends for 2 to 3 weeks but at
times be as long as 10 weeks the duration of lag
phase depends on a varied no of factors including
the density of alveolar bone , age of the patient
an the extent of the hyalinized tissue.
After the lag phase tooth movement
progresses rapidly as the hyalinized zone
is removed and bone undergoes
resorption. During this post lag period
osteoclasts are found over a large surface
area resulting in direct resorption of bony
surface facing the periodontal ligament.
The mechanism of movement of a tooth by an
orthodontic force is a subject of ongoing
research for decades . numerous theoris have
been put forward to explain the same . the
theories that have been accepted and have
stood the test of time are :
PRESSURE TENSION THEORY by Schwarz:
BLOOD FLOW THEORY by bien:
BONE BENDING PIEZO ELECTRIC THEORY
Oppenheim in 1911 was the first person to study
the tissue changes in the bone incident to
orthodontic tooth movement . Schwarz (1932) is
said to be the author of this theory according
to him when a tooth is subjected to in
orthodontic force is results in areas of
pressure and tension. The area of periodontium
in the direction of tooth movement is under
pressure while the area of periodontium
opposite the tooth movement is under tension .
according to him the areas of pressure show
bone resorption while areas of tension show
This theory Is also called as the blood flow
theory as proposed by bien . According to this
theory tooth movement occurs as a result of
alteration in fluid dynamics in the periodontal
ligament . The periodontal ligament occupies the
periodontal space which is confined between two
hard tissues namely the tooth and the alveolar
socket . The periodontal space contains the fluid
system made up of interstitial fluid cellular
elements blood vessels and viscous ground
substance in addition to the periodontal fibers.
It is a confined space and the passage of
fluid in an out of the space is limited . The
contents of the periodontal ligament thus
creates a unique hydrodynamic condition
resembling a hydrolic mechanism and a
shock absorber when the force is removed
the fluid is replenished by diffusion from
capillary walls and recirculation of the
when the force applied is of short duration
such as during mastication the fluid in the
periodontal space is replenished as soon a the
force is removed. But when a force of
greater magnitude and duration is applied
such a during orthodontic tooth movement
the interstitial fluid in the periodontal
space gets squeezed out and move toward
the apex and cervical margins and results in
decreased tooth movement . this is called the
squeeze film effect by bien .
Farrar (1876) first noted deformation or
bending of interseptal alveolar walls. He was
the first to suggest that BONE BENDING
may be a possible mechanism for bringing about
Piezoelectricity is a phenomenon observed in
many crystalline materials in which a
deformation of the crystal structure produces
a flow of electric current as a result of
displacement of electrons from one part of the
crystal lattice to the other.
The possible sources of electric current are:-
Mucopolysaccharide fraction of the ground
When a crystal structure is deformed, electrons
migrate from one location to another resulting in an
electric charge. As long as the forces is
maintained, the crystal structure is stable and no
further electric effect is observed. When the
force is released the crystals return to their
original shape and a reverse flow of electrons is
observed. This rhythmic activity produces a constant
interplay of electric signals where as occasional
application and release of force produces occasional
Piezoelectric signals have 2 unusual characteristics:-
Quick decay rate: when a force is applied, a
piezoelectric signal is produced. This electric charge
quickly dies away to zero even though the force is
When the force is released, electron flow in a
opposite direction is seen.
On application of force on tooth,adjacent
Area of concavity :negative charged evoke
Area of convexity :positive charge evoke
Bone formative changes are observed on the
tension side.In the process of bone deposition
there is increase in the number of osteoblasts
which are bone forming cells. they are ovoid cells
with basophilic cytoplasm and have an oval nucleus.
They lie against the bone surface where active
bone formation is in progress i.e.
periosteum/endosteum and help in the formation
of the organic matrix and also control the
deposition of mineral salts.
The osteoblasts are derived from paravascular
connective tissues cells, closely associated with
the blood vessels. Whenever there is stress or
strain due to orthodontic force, these
osteoprogenitor cells undergo increase in nuclear
volume and form the G1 stage preosteoblasts.
With further DNA synthesis the g1 stage pre
osteobalst transform into g2 stage pre osteoblast
and these mitosis and form osteoblast.
The bone formed passes through 3 stages they
are THE OSTEOID, BUNDLE BONE and THE
The bone formed by the osteoblast is the
osteoid which is tightly calcified with further
calcification it forms bundle bone . the fibres of
the periodontal apparatus that attached to the
bundle bone. When the bundle bone reaches a
certain maturity part of it get re organized into
mature lamellated bone.
It is brought about by cells called
osteoclasts . they are multinucleated giant
cells and may have 12 or more nuclei .
they are irregularly oval or club shaped
with branching processes . they occur in a
bay like depressions in bone called
howships lacunae and have prominent
mitochondria, lisosomes and vacuoles. Each
of their nuclei has a single nucleolus. The
part of osteoclasts in contact with the
resorbing bone has a ruffeled border
The osteoclasts derived from :
Activation of previously present inactive
Migration from adjacent bone
Formation of new osteoclasts from local
macrophages of periodontal ligament
Influx of monocytes from blood vessels
During bone resorption 3 processes occur in more
or less rapid succession .
Degradation of matrix
Transport of soluble products to the extra
cellular fluid or blood vascular system.
Organic acid such as citric acid and lactic acid
are secreated by ruffeled border of the
osteoclasts which increase the solubility of
hydroxyappetite leading to decalfication .
Degradation of matrix is brought about by the
activity of cathepsin B-1 .
Bio –Physical Reactions
compression of periodontal
Production of first messengers
Production of second messengers
+VE increase in cells of
-VE increase in cells of
ORTHODONTIC TOOTH MOVEMENT
Summary of biochemical reactions