Changes in periodontal ligament during orthodontic tooth movement /certified fixed orthodontic courses by Indian dental academy

CHANGES IN PERIODONTAL LIGAMENT
DURING ORTHODONTIC TOOTH MOVEMENT

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INDIAN DENTAL ACADEMY
Leader in continuing dental education



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CONTENT
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INTRODUCTION
PERIODONTAL LIGAMENT
CELLS OF THE PERIODONTAL LIGAMENT
EXTRACELLULAR SUBSTANCE
FIBERS OF THE PERIODONTAL LIGAMENT
PERIODONTAL LIGAMENT FUNCTION
PERIODONTAL LIGAMENT RESPONSE TO NORMAL FUNCTION
THEORIES OF TOOTH MOVEMENT
ORTHODONTIC TOOTH MOVEMENT AS RELATED TO BONE DEFORMATION
ORTHODONTIC TOOTH MOVEMENT AS RELATED TO BIOCHEMICAL REACTION
BIOLOGIC MECHANISMS INVOLVED IN THE TRANSFORMATION OF EXTERNAL
STIMULI TO SPECIFIC TISSUE REACTIONS
RESPONSE OF PERIODONTAL LIGAMENT ON: PRESSUE SIDE & TENSION SIDE
TYPES OF TOOTH MOVEMENT
OPTIMUM FORCES FOR ORTHODONTIC MOVEMENT
EFFECTS OF FORCE DURARTION AND FORCE DECAY
EFFECTS OF FORCE MAGITUDE
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DRUG EFFECT ON RESPONSE TO ORTHODONTIC FORCE

INTRODUCTION
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. Bone is
selectively removed in some areas and added in others. In
essence, the tooth moves through the bone carrying its
attachment apparatus with it, as the socket of the tooth
migrates because the periodontal ligament mediates the bony
response, tooth movement is primarily a periodontal ligament
phenomenon.


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The periodontium comprises
four connective tissues :
1.two mineralized connective
tissues :
alveolar bone and cementum
2. the two fibrous connective
tissues :
periodontal ligament and
lamina propria of the gingiva.
The periodontal ligament
occupies the periodontal space,
which is located
between the cementum and the
periodontal
surface of the 5

Periodontal ligament
- approximately 0.25mm wide
- Is the soft, richly vascular and cellular connective tissue.
- It surrounds the roots of the teeth and joins the root cementum
with the lamina dura or the alveolar bone proper.
- It functions as a shock absorber during mastication
-The fibrils of the PDL are embedded in a ground substance, which is
the amorphous structure left after all the cells, capillaries and fibers have
been removed.

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-It contains connective tissue polysaccharides, salts and water.
-The connective tissue as well as the ground substance varies in
different species as well as with age.
-For instance, the tissue response to orthodontic forces, including
both cell mobilization and conversion of collagen fibers, is
considerably slower in elderly individual than in children and
adolescent.
-The ground substance has a more rapid turnover than the
collagen fibers.
- It is also essential for movement of teeth in orthodontic
treatment

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Development :
The periodontal ligament develop
from the dental follicle. Cells of the
dental follicle that differentiate into
fibroblasts synthesis the fibers and
ground substance of the periodontal
ligament. The fibers of the periodontal
ligament are embedded in newly
developed cementum and bone as
tooth erupts.


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The true periodontal ligament fibers, the principal fibers, develops
in conjunction with the eruption of the tooth.

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FIBERS OF PERIODONTAL LIGAMENT:

1 ALVEOLAR CRESTAL FIBERS
2 HORIZONTAL
3. OBLIQUE
4. APICAL
5. INTERRADICULAR


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Development of Sharpey’s fibers
Collagen fibers are embedded into cementum on one side of the
periodontal space and into alveolar bone on the other. The
embedded fibers are termed as sharpey’s fibers.

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Root cementum

fine fibrils join in
periodontal space


bone surface

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CELLS OF PERIODONTAL LIGAMENT
The principal cells of the healthy, functioning periodontal ligament
are:
1. DIFFERNTIATED CELLS for
-Synthesis and resorption
2. PROGENITOR CELLS.
-The differentiated cells are concerned with the synthesis and
resorption of alveolar bone and the fibrous connective tissue of
the ligament and cementum. Consequently the cells of the
periodontal ligament can be divided into three main categories:
SYNTHETIC CELLS
-Osteoblasts, fibroblast, cementoblast
RESOPTIVE CELLS
-Osteoclasts, fibroblasts, cemetoblast
PROGENITOR CELLS

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A) SYNTHETIC CELLS:

OSTEOBLASTS: The osteoblasts covering the periodontal
surface of the alveolar bone constitutes a modified endosteum.
Osteoblasts secrete the type I collagen as well non-collagenous
matrix of bone. Osteoblasts differentiate from progenitor cells of
the connective tissue at site of bone formation. As the osteoblasts
secrete the organic matrix of bone, it is at first devoid of mineral
salts and is called osteoid tissue and after mineraliztion of
osteoblasts, they become embedded in it and form osteocytes and
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are important in deposition of bone.

FIBROBLASTS:
Fibroblast in various stages
of differentiation, and their
progenitor, are found in the
periodontal ligament, they
are surrounded by fibers and
ground substance.
In longitudinal section, the
cells of the ligament appear
to be oriented parallel to the
oriented bundles of collagen.
They are important in
synthesis of collagen.

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CEMENTOBLASTS: Soon after Hertwig’s sheath breaks up,
undifferentiated mesenchymal cells from adjacent connective
tissue differentiate into cementoblasts. Cementoblasts synthesis
collagen and protein, which constitutes the organic matrix of
cementum. Uncalcified matrix is called cementoid. They are
unimportant in synthesis of cementum. This cementoid is lined by
cementoblasts. Connective tissue fibers from the periodontal
ligament pass between the cementoblasts into the cementum.
Numerous collagen fibrils
embedded into the cementum
are called Sharpey’s fibers.


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B) RESORPTIVE CELLS:
• OSTEOCLASTS:
osteoclasts are cells that
resorb bone and tend to be
large and multinucleated
cells. These multinucleated
osteoclasts are formed by
their precursor, the
circulating monocytes, but
they may also differentiate
from the mesenchymal cells
in situ.


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FIBROBLASTS:
Periodontal ligament can be
resorbed under physiologic
conditions by mononuclear
fibroblasts.
These cells exhibit lysosomes that
contain fragment of collagen that
appear to be undergoing digestion.
Fibroblast is capable of both
synthesis and resorption.
Collagen-resorbing fibroblasts are
inhabitants of normal functioning
periodontal ligament, and their
presence, like that of osteoclast in
relation to bone, indicates
resorption of fibers occurring
during physiologic turnover or
remodeling of periodontal
ligament.

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CEMETOCLASTS:
They resemble osteoclast and are occasionally found in
normal functioning periodontal ligament. Cementum is not
remodeled in the fashion of alveolar bone and periodontal
ligament but that it undergoes continual deposition during
life. However, resorption of cementum can occur under
certain circumstances, and in these instances mononuclear
cementoclasts or multinucleated giant cells, often located in
howship’s lacunae, are found on the surface of the cementum.
The origin of cementoclast is similar to that of osteoclasts
from ciruclating monocytes.


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C) PROGENITOR CELLS: (Undifferentiated mesenchymal cells)
-All connective tissues, including periodontal ligament, contain
progenitor cells that have the capacity to undergo mitotic division. If
they were not present, there would be no cells available to replace
differentiated cells dying at the end of their life space or as a result of
trauma.
- It is believed that generally, after division, one of the daughter cells
differentiates into functional type of connective tissue cell (i.e., any one
of cell types as described above ) while the other remains an
undifferentiated progenitor cell retaining the capacity to divide when
stimulated appropriately.
EXTRACELLULAR SUSBSTANCE:
A. FIBERS
-Collagen (predominantly type I and also type III)
-Oxytalan
B. GROUND SUBSTANCE
-Proteoglycans
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-Glycoproteins

FUNCTIONS OF MAJOR COMPONENTS OF PDL :
1.The cellular elements
2.The tissue fluids

Both play an important role in normal function and in
making orthodontic tooth movement possible.


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The collagen of the ligament is constantly being remodeled
and renewed during normal function. The same cells can serve
as both fibroblasts, producing new collagenous matrix
materials, and fibroblasts, destroying produced collagen.
Remodeling and recontouring of the bony socket and the
cementum of the root is also constantly being carried out,
though on a smaller scale, as a response to normal function.
Fibroblasts in the PDL have properties similar to osteoblasts,
and new bone probably is formed by osteoblasts that
differentiated from the local cellular population.


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Bone and cementum are removed by specialized Osteoclasts and
cementoclasts, respectively. These multinucleated giant cells are
quite different from the osteoblasts and cementoclasts that
produce bone and cementum. Despite years of investigation, their
origin remains controversial. Most are of hematogenous origin;
some may be derived from the pluripotent stem cells in the local
area .


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Finally, function of tissue fluid
PDL space is filled with fluid; this fluid is that same as that
found in all other tissues, ultimately derived from the
vascular system.
A fluid-filled chamber with retentive but porous walls could
be a description of SHOCK ABSORBER, and in normal
function, the fluid allows the PDL space to play just this
role.


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PERIODONTAL LIGAMENT RESPONSE TO NORMAL
FUNCTION:
TOOTH IS SUBJECTED TO HEAVY LOADS
PDL & INCOMPRESSIBLE TISSUE FLUID IN PDL
ALVEOLAR BONE

[ Prevents tooth displacement ]

BONE BENDS
PIEZOELECTRIC CURRENT
STIMULUS FOR SKELETAL REGENERATION & REPAIR
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PHYSIOLOGICAL RESPONSE TO HEAVY RESPONSE
TIME ( sec )

EVENTS

<1

PDL fluid incompressible , alveolar bone bends,
piezoelectric signal generated.
[ the resistance provided by tissue fluid allow normal
mastication, with its force application of 1sec or less
to occur without pain.]

1 -2

PDL fluid expressed, tooth moves within PDL space

3-5

PDL fluid squeezed out, tissue compressed;
immediate pain if pressure is heavy.

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WHAT MAKES THE TISSUE RESPOND ?
HOW APPLICATION OF SUSTAINED FORCE
TRANSFORMS INTO CELL REACTION NECESSARY FOR
REMODELLING OF TOOTH SUPPORTING STRUCTURE ?
Two possible control elements, biologic electricity and pressure
tension in the PDL that affects blood flow, are explained in two
major theories of orthodontic tooth movement


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THEORIES OF TOOTH MOVEMENT
The bioelectric theory relates to changes in bone metabolism
controlled by the electric signals that are produced when alveolar
bone flexes and bends
The pressure tension theory relates to cellular changes produced by
chemical messengers, traditionally thought to be generated by
alteration in blood flow through the PDL. Pressure and tension with
the PDL, by reducing (pressure) or increasing (tension) the diameter
of blood vessels in the ligament space, could certainly alter blood
flow.
The two theories are neither incompatible nor mutually
exclusive. From a contemporary perspective, it appears that both
mechanisms may play a part on the biologic control of tooth
movement.
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ORTHODONTIC TOOTH MOVEMENT AS RELATED TO
BONE DEFORMATION;
BIOELECTRICITY: First suggested by Farrar (1888)
Bones have a remarkable ability to remodel their structure such a way
that stress is optimally resisted (WOLFF’S LAW)
Electric signals that might initiate tooth movement initially were
thought to be piezoelectric.
Piezoelectricity is a phenomenon observed in many crystalline
materials in which deformations of the crystalline materials in flow
of electric currents are displaced from one part of the crystal
lattice to another. The piezoelectricity of inorganic crystal has been
recognized and now organic crystal structure can also have a
piezoelectric property. Not only bone mineral a crystal structure with
piezoelectric properties; collagen itself is piezoelectric, and stressgenerated potentials in dried bone specimens can be attributed to
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piezoelectricity.

TOOTH IS SUBJECTED TO SUSTAINED FORCE
PDL & INCOMPRESSIBLE TISSUE FLUID IN PDL
ALVEOLAR BONE
BONE BENDS
PIEZOELECTRIC CURRENT
TOOTH MOVEMENT STARTS

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Piezoelectricity signals have two unusual characteristics:

A quick decay rate (i.e., when a force is applied, a piezoelectric signal is
created in response that quickly dies away to zero even though the
force is maintained) and
The production of an equivalent signal, opposite in direction, when the
force is released.

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PRESSURE
CRYSTAL LATTICE E.g. Bone, Collagen
DEFORMED LATTICE
MIGRATION OF ELECTRONS
FORMATION OF ELECTRIC CURRENT
QUICKLY DECAYS
WHEN PRESSURE IS RELEASED
OPPOSITE CURRENT FLOWS

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Cells are sensitive to these strain-generated potential
(piezoelectric effect).
Zengo in his experiments on dogs alveolar bone showed that
bending of bone may create negative fields occurring in the
concave aspects of the bone surface leading to deposition and
positive fields occuring on the convex bone surface leading to
bone resorption. Zengo et al 1973- 74 )
It has been suggested that ions in the fluids surrounding living
bone interact with the electrical fields generated when the bone is
bent. These currents of small voltages are called streaming
potentials.


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Zengo et al (1973) in his vivo experiment observed the
electrical potentials highest at the enamel surface of the tooth
moved, less in the cementum and dentin and least in the
alveolar bone. At the same time a bioelectric effect could be
observed in the gingiva as well as in the proximal teeth and
their supporting tissues.
Observation has shown that not only bone deformation but also
tension may create bioelectrical signals. Thus in vivo
experiments conducted by Roberts et al (1981) have revealed
that a negative electrical field is created in areas where the
periodontal ligament is widened.


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Some investigators have reached highly diverging conclusions as
regards the stimulus required for producing bone deformation.
Baumrind et al conducting experiments on 99 rats concluded,
“bone deflection can be produced by forces lower than those
required to produce consequential changes in the periodontal
ligament width.”
Contrary to this statement, Murphy et al using the tetracycline
microfluorescent technique, observed no bending of the alveolar
bone during retraction of teeth in the monkey. Obviously deflection
of thin bone lamellae occurs quite frequently during treatment, but
in most cases deformed bone walls tend to move back to their
former position as a result of bone elasticity and fiber contraction
as soon as space has been created by resorption.

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Baumrind and Buck et al investigated the changes in rat
periodontium at varying periods up to 72 days by injecting
radioactive precursors that labeled individual cellular and
connective tissue elements to be investigated:
1. CELL STUDY :Tritiated thymidine for observation of cell
replication as indicated by the presence of DNA, tritiated
uridine for RNA assessment.
2. COLLAGEN STUDY : Tritiated proline for observation of
collagen formation.
The auto radiographic findings showed significant increase in
cell division adjacent to the roots of the experimental teeth as
compared with those of the nondisplaced control teeth.
Collagen synthesis seemed to decrease in areas adjacent to
the experimental teeth. On the pressure side there was an
increase in the number of labeled cells fairly similar to that on
the tension side. It was suggested that the major physiologic
and mechanical changes might occur not in the
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periodontal ligament but rather in the alveolar bone.

There is no longer doubt that stress-generated signals are
important in the general maintenance of the skeleton. Without
such signals, bone mineral is lost and general atrophy ensues.
Signals generated by the bending of the alveolar bone during
normal chewing almost surely are important for maintenance of
the bone around the teeth.
On the other hand, sustained force of the type used to induce
orthodontic tooth movement does not produce prominent stressgenerated signals. It appears that stress-generated signals,
important as they may be for normal skeletal function, probably
have little if anything to do with the response of orthodontic
tooth movement.

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A second type of endogenous electric signal, which is called the
“bioelectric potential” can be observed in bone that is not being
stressed.
1. Electronegative charges are observed in areas of metabolically
active bone or connective tissue where bone growth or
remodeling is occurring.
2. Inactive cells and areas are nearly electrically neutral.
Although the purpose of this bioelectric potential is not known,
adding exogenous electric signals can modify cellular activity.
The effects, presumably, are felt at cell membranes. Membrane
depolarization triggers nerve impulses and muscle contraction,
but changes in membrane potentials accompany other cellular
response as well.

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The external electric signals probably affect cell membrane
receptors, membrane permeability, or both.
Both animal and human experiment indicate that when low voltage
direct current is applied to the alveolar bone, modifying the
bioelectric potential, a tooth moves faster than its control in response
to an identical spring.(Davidovitch et al AJO 1980 )


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Electromagnetic fields also can affect cell membrane
potentials and permeability and thereby trigger changes in
cellular activity.
In animal experiments, a pulsed electromagnetic field
increased the role of tooth movement; apparently by
shortening the initial “lag phase” before tooth movement
begins. ( Sinclair at al AJO 1987 )
Electromagnetic fields can be induced within tissues by
adjacent magnets, without the contact required by electrodes,
and bone healing has been shown to be enhanced by certain
types of fields. It is possible that this effect can be utilized in
the future to enhance orthodontic tooth movement and or alter
jaw growth.

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ORTHODONTIC TOOTH MOVEMENT AS
RELATED TO BIOCHEMICAL REACTION
PRESSURE TENSION THEORY:
The pressure-tension theory, the classic theory of tooth
movement, relies on chemical rather than electric signals as the
stimulus for cellular differentiation and ultimately tooth
movement.
Chemical messengers are important in the cascade of events that
lead to remodeling of the alveolar bone and tooth movement..
In essence, this view of tooth movement shows three stages:
1. Alteration in blood flow associated with pressure with in the
PDL
2. Formation andor release of chemical messengers, and
3. Activation of cells.

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• 1. Alteration in blood
flow associated with
pressure with in the
PDL
• 2. Formation andor
release of chemical
messengers, and
• 3. Activation of cells.


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PRESSURE TENSION THEORY
SUSTAINED PRESSURE
TOOTH DISPLACEMENT WITHIN PDL SPACE
PDL COMPRESSED

PDL STRETCHED

PDL SIZE REDUCED

PDL SIZE INCREASED

BLOOD VESSEL COMPRESSED

BLOOD VESSEL DILATES

BLOOD FLOW
OXYGEN LEVEL
METABOLITES

BLOOD FLOW
OXYGEN LEVEL
METABOLITES

STIMULATES RELEASE OF CHEMICAL MESSANGER

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FORMATION AND RELEASE OF CHEMICAL
MESSENGERS:
Mechanical stresses alter the structural and functional properties of
cells at cellular, molecular, and genetic levels, leading to both
rapid responses in the neighboring tissues.
Cellular responses to mechanical stresses involve interplay
between structural elements and biochemical second messengers.


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EXTRACELLULAR SIGNALING:
Cell-surface receptor proteins water-soluble extracellular
signaling molecules (ligands) referred to as first
messenger.
-The endocrine cells secrete hormones that travel through the
bloodstream to influence target organs.
-Nerve cells from junction (synapses) with the target cells
and secrete chemical neurotransmitters, which act on the
target cells.
-Local chemical mediators are produced by many cells and
act only on cells in the local environment, since they are
rapidly destroyed (chemotatic factors, histamine,
prostaglandin’s).

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INTRACELLULAR SIGNALING:
There seems to be two ways in which the signals are generated into
intracellular signals.
-One is activation of a membrane- bound enzyme (adenylate cyclase)
that acts upon adenosine phosphate (ATP) to increase (in some cells to
decrease) the synthesis of the adenosine monophosphate (cAMP)
serves as a universal signaling molecule within cells and controls
many different intracellular signals.
-Other way in which extracellular signals are generated into
extracellular signals: that the surface receptors may open and close
gated ion channels in the plasma membrane and in turn change the
flux of ions into the cell. Particularly important in the influx of Ca+.
Both cAMP and Ca+(though not the only intracellular mediators for
extracellular signals) activate intracellular protein kinases and are
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referred to as second messengers.

The protein kinases are special enzymes that phosphorylate a
great number of inactive enzymes and thereby activate them to
become third messengers. When this cascade of enzyme is in
increased, a high cellular activity can be the result.
Among the extracellular local chemical mediators the
prostaglandins (PG) are known to bind to cell receptors. They
have different biologic effect and are very potent.
On a general basis prostaglandins are very vasoactive and
influence the Ca+ concentration in the cellular cytosol.
Another important effect of prostaglandins is regulation of the
intracellular concentration of cAMP through an effect on the
enzyme adenylate cyclase
On the other hand Ca+ liberation in cells may influence
prostaglandin synthesis directly or through cAMP

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Synthesis of prostaglandins takes place in cellular membranes
where fatty acids are cleared from membrane phospholipids by
enzymes (phospholipase) into arachidonic acids. By a change of
arachidonic acid from an extended into a folded conformation and
by further oxidation steps by different enzymes (cyclo-oxygenases)
one of the prostaglandins will be produced.
Experimental evidence proves that the open state of ion channels
seen in most cells depends on stress exerted at the membrane, a
finding that again is influenced by the maintenance and composition
of the extracellular matrix. When an orthodontic force displace the
periodontal ligament, such a movement may result in cell
perturbation, altering the influx of Ca+ and other ions, which in turn
has been believed to alter the synthesis of cAMP.

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Davidovitch and Shanfeld (1975) reported findings indicating that
the levels of cAMP and cGMP increased in PDL cells and alveolar
bone following the application of orthodontic forces to teeth.
Somjen et al (1980) demonstrated that the synthesis of cAMP
coincident with the stretching of the cells is prostaglandin dependant.
This report and others led to the assumption that what happens to the
cellular structures during transduction of orthodontic mechanical
forces, which results in physical deformation of cell membranes with
resultant prostaglandin synthesis. This would result in activation of
membrane-bound adenylate or guanylate cyclases responsible for
converting the respective substrates to cAMP and cGMP. Which
would have a effect upon the amount of collagen synthesized and
degraded intracellularly and perhaps phagocytosis of extracellular
collagens.

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Lear and Moorreess et al in their histochemical study found that the
macrophages of the new blood vessels that penetrate the tissues of a
hyalinized are could be identified by their high aryly-sulfatase and
aminopeptidase-M activity, enzymes known as markers of macrophage
activity. These cells also revealed high prostaglandin synthetase (PgS)
activity. This prostaglandin synthetase enzyme converts arachidonic acid
to prostaglandin. It has been assumed that macrophages invading and
removing hyalinized tissue, may release prostaglandins and also
stimulate the formation of osteoclast. On the other hand osteoclast-like
cells showed typical high acid phosphatase activity, which are in contact
with bone.
Davidovitch and Shanfield et al showed a rise of prostaglandin E2
(PGE2) levels in the periodontal ligament and alveolar bone, both in
sites of tension and compression in orthodontically treated animals.

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Yamasaki et al (1980) have shown that prostaglandins that
were produced during orthodontic tooth movement may
increase bone resorption activity. These author performed
series of experiments in rats, monkeys, and humans;
Prostaglandin E was injected in the gingiva of the teeth to be
moved. They reported enhanced rate of tooth movement.

Now increasing evidence proves that although Prostaglandin E
is involved in the transduction of mechanical stress on the PDL
and alveolar bone during orthodontic tooth movement, several
other inflammatory mediators are active. It appears that
orthodontic forces may activate the nervous as well as
immune systems.
.

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Results of experiments performed by Davidovitch et al (1988) and
Kvinnsland (1990) indicate the that increase in second messengers
(cAMP, cGMP) in periodontal cells do not result solely from the direct
effects of the mechanical forces but also caused by endogenous
signaling agents
Mechanical stress alter the level and distribution of the
neurotransmitter substances in PDL. Neuropeptide: Substance P (SP),
vasoactive intestinal polypeptides (VIP), calcitonin gene related
peptide (CGRP) and other neurotransmitters from sensory nerve fibers
in the PDL supply a link between physical stimulus and the
biochemical response. Pain sensitive nerve endings release stored
substance P into the PDL, leading to binding of SP to specific cellular
receptor. Through interaction with endothelial cells there will be a
rapid vasodilatation and migration of leucocytes from blood vessels.

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Also immune systems play a regulatory part in orthodontic tissue
reactions.
Pronounced vasodilatation has been reported in areas of tension and
in the periphery of compressed of the PDL in experimental tooth
movement.
Macrophages have been identified near the blood vessels. It is
generally accepted that vasodilatation leads to migration of
macrophages, lymphocytes, proteins and fluid into the extracellular
space.
These inflammatory cell as well as fibroblasts and osteoblasts,
produce signaling molecules, cytokines: interleukins 1a and 1b that
attract leucocytes, stimulate fibroblast proliferation and enhance bone
resorption and so called tumor necrosis factors A which induces
interleukin 1 production ofwww.indiandentalacademy.com
monocytes, enchances PGE2 and collagenous
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production, and increase the number of osteoclasts.

It has been reported in experimental studies that stress produced by
orthodontic forces would cause a marked increase in the staining
intensity of interleukin 1a in all cell types of the PDL, in particular
osteoblasts in tension sites and PDL cells and osteoclasts at
compression.
Similarly, orthodontic forces would produce a marked increase in
the cellular staining intensity of interleukin beta particularly in the
osteoblasts at PDL tension sites and osteoclasts at compression site.
Even TNF-alpha and gamma Interferon were observed in the PDL
in experiment tooth movement. (Davidovitch and Shanfeld et al)


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SUSTAINED PRESURE [ MECHANICAL STRESS ]
DEFORMATION OF CELLS
PEPTIDES, PG,AMINO ACID, EPINEPHRINE
(LIGANDS, act as 1st messenger )
BIND TO CELL SURFACE RECEPTOR PROTEIN
ACTIVATES ENZYMES
INCREASED OR DECREASED CONC. OF
INTRACELLULAR SIGNALING COMPOUND ( 2nd
messenger, e.g. cAMP, cGMP )

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RESPONSE OF THE PERIODONTAL LIGAMENT ON
THE PRESSURE SIDE:
- It is the side towards which a tooth is being moved.
-The periodontal space becomes narrower
-The crest of the alveolar bone is slightly deformed.
-Cellular changes occur within the PDL and on the surface of
the alveolar bone.
-Similar cellular changes and remodeling of the supporting
tissue of teeth occur with physiological migration.
-Difference being the rate of changes

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Histological studies :
Shows resorption of the alveolar bone surface on the
side towards which the tooth is moving.
In physiological tooth movement (where tartarateresistant acid phosphatase-TRAP – is used to identify
osteoclast), a few osteoclast can be observed resorbing
the alveolar bone wall. Vascular activity is low and few
leucocytes and macrophages are seen (Brudvig and
Rygh).
However, whereas with physiological migration the
number of osteoclasts is usually low (indicating slow
process).

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Orthodontics forces elicit more dramatic
changes.
Such changes can be categorized broadly into
“direct resorption” where the pressure is
relatively light, and “hyalinization”, where the
pressure is large enough to produce degenerative
changes.


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DIRECT RESORTION
Osteoclasts appear in the PDL along the alveolar bone
surface, some hours after the application of orthodontic force
In children aged between 10-13years, Reitan et al found
occasional evidence of resorption after 12 hours, and
resorption was invariably seen by 40 hours. Within optimal
force after 3-4 days, numerous osteoclasts are present along
the alveolar wall.
In light microscopical sections:
A clear zone often separate resorbing cells from the bone.
This artifact appears to be related to the destruction of both
alveolar bone and sharpey’s fibers.
Electron microscope: Shows the ruffled border of
osteoclasts in close contact with the resorbing bone surface
and both crystals and collagen fibers may be found between
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the cell processes.

• Garant et al observed Fibroblasts with increased
amounts of intracellular collagen profiles near
osteoclasts in PDL.
• These fibroblasts also play a role in bone resorption
and formation of new collagen, which becomes
attached to the alveolar bone by localized bone
deposition.
• There is extensive remodeling of collagen throughout
the PDL with possible exception of Sharpey’s fibers
at the root surface.
• Collagen detached from alveolar bone during
resorptive activity may become reattached to bone or
to pre-existing periodontal collagen fibers by local
activity of osteoblasts or fibroblasts respectively.

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The precise pathway by which degraded collaged collagen is
removed is unknown.
1. Extracellular breakdown by fibroblasts-like cells occurs.
2.Compressed necrotic tissue in hyalinized zones is removed
to a considerable extent by macrophages.

The ground substance appears to be the major water-binding
constituent of connective tissues. Even though it is presumed
that both bound and unbound water are present in the PDL, it
is not known whether there is movement of water between the
vascular and extravascular compartments during loading and
unloading of a tooth.

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The rich blood supply to the PDL may play a role
concerning the reactions of the periodontal vasculature to
moderate pressure. However, unless an adequate vascular
supply is present, the differentiation of specialized cells will
not take place.
The width of the periodontal space seems to be important
in determining the reaction of the PDL to load.
Recently erupted teeth in children have a wider PDL than
those in adults. This may help explain why children’s teeth are
more easily moved orthodontically than adult’s teeth.

62

Under non-pathological conditions, the width of the
PDL will give an indication of its capacity for
remodeling during the initial phase of increased
loading of the tooth.
With extensive direct resorption forces, within the
PDL is increased.
During the initial phase of orthodontic tooth
movement and reactivation of an applied force is
there a narrowing of the PDL on the pressure side.

63

HYALINIZATION
• Increased pressure in all localized region of the PDL can
easily exceed the optimum force and inhibit the
differentiation of osteoclasts.
• As a result, the direct resorption of alveolar bone, which
would relieve the pressure in the PDL, cannot occur.
• Instead a series of degenerative tissue reactions takes place,
commencing within a few hours. The term ‘hyalinization’ is
used to describe these tissue reactions, owing to the fact that
the degenerated tissue has a “glassy appearance”.
• The presence of hyalinization has been interpreted as
representing a change in consistency of the collagenous
matrix rather than its regeneration

64

•
•
•

•

•

With pronounced pressure tissue changes within the PDL are
characterized by :
Edema, gradual obliteration of blood vessels, and breakdown of
walls of veins.
Leakage of blood constituents into the extravascualar space
occurs.
Changes in fibroblasts are also seen. These often begin with
moderate swelling of the endoplasmic reticulum. More extensive
swelling and the formation of vacuoles occur later, followed by
rupture of cytoplasmic membrane and loss of cytoplasm. These
leaves isolated nuclei, which undergo lysis over a period of
several weeks. Mostly the collagen undergoes a longitudinal
splitting.
The degenerative processes of the different tissue components
persist as long as the pressure is maintained and, by doing so
they prevent recolonization of the damaged tissue by cells from
the adjacent, undamaged PDL.
With time accumulated erythrocyte breakdown products in the
65
pressure region may undergo crystallization

• All tooth movement stops until the adjacent bone is
resorbed by cells that differentiate on spongiosa surface of
the bone or subperiosteally. Where there is no cancellous
bone between lamina dura and the external cortical bone.
This indirect (’undermining ‘) resorption occurs at the same
time as invasion of phagocytosing cells from the peripheral
undamaged ligament and from the marrow spaces. All tissue
components damaged during compression are eventually
removed. Extracellular breakdown by fibroblast-like cells
occurs locally in some areas of PDL (i.e. removed of surface
cementum in pressure zone and compressed necrotic tissue
in hyalinized zones is removed by macrophages
• Hyalinized areas are normally removed after a 3-5 week
period, provided that, if any further force is to be applied,
should be gentle reactivation. The ‘post-hyalinized’ PDL
under pressure is markedly wider than before, perhaps in
order to withstand the greater mechanical influences 66

• RESPONSE ON THE PERIODONTAL LIGAMENT
ON TENSION SIDE:
• The tooth is drawn away from the alveolar bone on tension
side.
• When continuous force is applied to the crown of tooth, the
periodontal space will become wider on the side where the
tooth is drawn away from the alveolar bone. Bundles of
fibers are stretched and the alveolar crest is pulled in the
same direction. The blood vessels appear to be distended.
• In areas of experimental tension of the PDL, a great increase
in vascular activity could be observed. This was indicated
by an increase in the space occupied by the blood vessels
most commonly seen in the middle of the PDL and towards
the alveolar bone. www.indiandentalacademy.com
67

•

A number of cellular processes are activated within the PDL.
There is an increase in the number of connective tissue by cell
division. For young humans, incipient cell proliferation is seen
after 30-40hours, particularly near the socket wall. Osteoid
tissue will be deposited on the socket wall shortly after. Where
the fibrous bundles are thick, new bone appears to be deposited
along them. If bundles are thin, a more uniform layer is
deposited along the root surface. Calcification in deeper layers
of the osteoid starts shortly afterwards, while the superficial
parts remain uncalcified.

• In longitudinal sections of the tooth, fibroblasts in the PDL are
oriented in the same direction as the principal fibers: in the
direction of strain. The fibroblasts appear spindle shaped. The
cells adjacent to the alveolar wall often appear more spherical

68

• It ha been thought that PDL fibers at the alveolar bone surface became
entrapped passively by the advancing front of new bone formation to
form Sharpey’s fibers. However findings suggest that new Sharpey’s
fibers or new fibrils are secreted simultaneously with new bone
deposition. As the fibroblasts migrate with the bone, they may deposit
either entirely new Sharpey’s or new fibrils, which are incorporated
into existing fibers. While part of the newly synthesized collagen will
be incorporated into the new osteoid, some will be incorporated in to
the PDL, perhaps associated with increase in width on the tension
side. Lengthening of fibers seems also to occur by incorporation of
new fibrils into existing fibers.
• Observation by Ten Cate et al that fibroblast were able both to break
down and to produce collagen fibrils led to the assumption that, in the
healthy PDL, assumption that, all collagen degradation was
intracellular. Rapid remodeling of the periodontal tissue during
experimental or therapeutic tooth movement would be characterized
69
by a very high ratio of internalized collagen or fibroblasts volume.

• Areas of experimental tension of the PDL, great increase in
vascular activity could be observed by an increase in the
space occupied by the blood vessels, most commonly in the
middle of the PDL and towards the alveolar bone.
• Macrophage and other leucocytes that migrate out of the PDL
blood vessels simultaneously with proteins and fluids are
known to be capable of producing and releasing a variety of
factors.
• In fact, these cells are not only active removers of tissues that
have been altered by pathological conditions, but, mostly
importantly, they are producers of numerous signal molecules
from chemoattraction to stimulation of mitogensis and
cytodifferentiation.
• Experiments revealed that in local areas of tension, the
volume of the collagen fibers running from tooth alveolar
bone was reduced as the volume of blood vessels increased

70

TYPES OF TOOT MOVEMENT:
For the purposes of illustration, forces and
movements are often discussed in terms of tipping,
torque, bodily movement, rotation, extrusion and
intrusion.
TIPPING:
• Tipping of a tooth leads to a concentration of
pressure in limited areas of PDL. A fulcrum is
formed, which enhances root movement in the
opposite direction. If the fulcrum is located in the
coronal part, the apex o the root is torque.

71

• A tipping movement nearly
always results in the
formation of a hyalinized
zone slightly below the
alveolar crest, particularly
when the tooth has a short,
underdeveloped root. If the
root is fully developed, the
hyalinized zone is located a
short distance from the
alveolar crest. Tipping of a
tooth by light continuous
force results in a greater
movement within a shorter
time than that obtained by
any other method. www.indiandentalacademy.com

72

• In most young orthodontic patients, bone resorption
result form a moderate tipping movement is usually
followed by compensatory bone formation. The degree of
such compression varies individually and depends
primarily on the presence of bone forming osteoblasts in
the periosteum. Compensatory periosteal bone apposition
occurs in the apical region.
• Tipping of adult teeth in labial direction may result in
bone destruction of the alveolar crest, with little
compensatory bone formation. Such undesirable bone
resorption has been observed even in young patients. In
addition after a prolonged movement of the bone plate in
the apical region may occur so rapidly that the root is
finally moved through the bone.

73

TORQUE :
• During the initial movement of torque the pressure areas
is usually located close to the middle region of the root.
• This occurs because of the PDL in normally wider in the
apical third than the middle third. After resorption of
bone areas corresponding to the middle third, the apical
surface of the root gradually begins to compress adjacent
periodontal fibers and a wider pressure area in
established.
• Direct bone resorption was observed on the pressure
side. However, if more torque in incorporated in the arch
wire, the magnitude of force may be considerably
increased. A cell free area of short duration may be
created adjacent to the middle third of the root.
74

• The force exerted during light
wire torque in of the continuous
type. Reitan and Kvam found
in experimental study that the
tissue reaction caused
hyalinization and root
resorption in two areas. The
first was in the middle third of
the root and the second, which
was formed after the first
undermining resorption has
terminated, was all along the
apical third of the root.


75

BODILY MOVEMENT
• Bodily movement is obtained by
establishing a couple of force along
parallel lines and distributing the force
over the whole bone surface. This is
favorable method of displacement
provided the magnitude of force does
not exceed a certain limit.
• Hyalinization during an initial bodily
movement occurs largely as a result of
mechanical factors. Shortly after the
movement is initiated, no bodily
movement of the tooth is a mechanical
sense is observed, but instead a slightly
tipping is noticed. The result is
compression on the pressure side with
formation of a hyalinized zone between
the marginal and middle region of the
root.


76

• The short duration of the hyalinization
result from an increased bone
resorption on both sides of the
hyalinized tissue, especially in the
apical region of the pressure side.
• This leads to rapid elimination of the
hyalinized zone. The PDL on the
pressure side is usually considerably
widened by the resorption process.
• Further tooth movement, in most
cases, produces only minor hyalinized
reaction on the pressure side is partly
caused by gradually increased
stretching of fiber bundle on the tension
side, which tends to prevent the tooth
from further tipping. New bone layer
along these bundles.

77

ROTATION:
• In rotation of a tooth around its long axis the force can be
distributed over the entire PDL rather than over a narrow
vertical strip, whereas forces can be applied than in other tooth
movements.
• Histologically, however the tissue transformation that occurs
during the rotation is largely influenced by the anatomic
arrangement of the supporting structure.
• In the marginal region most of the periodontal fiber bundles
consist of free gingival and transseptal fiber group. Although the
principal fibers of the middle and apical thirds are anchored to
the root surfaces and the alveolar bone, the supralveolar fibers
are connected to the whole fiber system of the supralveolar
structure.
• This difference in the attachment of fiber bundles has proved to
be of great importance, particularly during the retention period.

78

• Most teeth to be rotated
create two pressure sides
and two tension sides.
Rotation may cause
certain variations in the
type of tissue response
observed on the pressure
side. Occasionally,
hyalinization and
undermining resorption
takes place in one
pressure zone while
direct bone resorption
occurs in the other.

79

• In the marginal region rotation usually causes
marked displacement of fibrous structure. The free
gingival fiber groups are arranged obliquely from
the root surface, because these fibers bundles
interlace with the periosteal structure and the whole
supraalveolar fibrous system, rotation also causes
displacement of the fibrous tissue located some
distance from the rotated tooth.
• On the tension side of the middle third, new bone
spicules are formed along stretched fiber bundles
arranged more or less obliquely. In addition, the
new bone on the tension sides consists partly of
uncalcified bone spicules.

80

EXTRUSION:
• Extrusive tooth movements
ideally produce no areas of
compression within the
PDL, only tension. Varying
with the individual
reaction, the periodontal
fiber bundles elongate and
new bone is deposited in
areas of alveolar crest as a
result of the tension exerted
by these stretched fiber
bundles.

81

• In young individual,
extrusion of a tooth
involves a more prolonged
stretch and displacement
of the supraalveolar fiber
bundles than of the
principal fibers of the
middle and apical thirds.
• In adult, the fibers bundles
also are stretched during
extrusion, but they are less
readily elongated and
rearranged after treatment.

82

INTRUSION:
• Intrusion requires careful control of force magnitude. Light force is
required because the force is concentrated in a small area at the
tooth apex. Primarily the anterior teeth intruded a light continuous
force, such as that obtained in the light wire technique, has proved
favorable for intrusion in young patient. In other cases the alveolar
bone may be closer to the apex, increasing the risk for apical root
resorption. If the bone of the apical region is fairly compact, as is
some adults, a light interrupted force may be preferable.
• Unlike extruded teeth, intruded teeth in young patients undergo
only minor positional changes. Stretched in exerted primarily on
the principal fibers. As intruding movement may therefore cause
formation of new spicules in the marginal region. These new bone
layers occasionally become slightly curved as a result of tension
exerted by stretched fiber bundles. Such tension is also seen in the
83
middle of the roots. www.indiandentalacademy.com

OPTIMUM FORCES FOR OTHODONTIC
TOOTH MOVEMENT
TYPE OF MOVEMENT

FORCE (gms)

•
•
•
•
•
•

•
•
•
•
•
•

TIPPING
TRANSLATION
TORQUE
ROTATION
EXTRUSION
INTRUSION

35-60
70-120
50-100
35-60
35-60
10-20


84

EFFECTS OF FORCE DURATION
AND FORCE DECAY
• Animal studies suggest that only after forces is
maintained for approximately 4 hrs, second
messengers are produced, which are required to
stimulate cellular differentiation.
• Continuous forces produced by fixed appliance
produce more tooth movement than removable
appliance unless removable appliance is present
almost all the time.

85

ORTHODONTIC FORCE
DURATION IS CLASSIFIED BY
RATE OF DECAY AS
• CONTINUOUS – Forces that are maintained
between activation of orthodontic appliance.
• INTERUPPTED – Forces level declines to zero
between activations. when a fixed appliance is
temporarily deactivated .


86

• INTERMITENT- Forces level declines abruptly to zero
intermittently.
- when orthodontic appliance is removed by the
patients.
- intermittent forces are produced by all patient
activated appliances. Such as removable appliance,
headgear, elastics.
It must be admitted that intermittent and interrupted
are more or less synonymous, but they are used for
differenent types of tooth movement, e,g interrupted
designates movement of short duration elicited by fixed
appliance, intermittent is used mainly to designate
removable appliances.

87

Effects of continuous force:
Effects of light continuous force:
- Smooth tooth movement will result from
frontal resorption.
Effects of heavy continuous force :
- Tooth movement will be delayed until
undermining resorption can remove the
bone necessary for tooth movement. Tooth
will change its position rapidly.
- Constant heavy force will prevent repair of
PDL and create the need for further
undermining resorption.
- It is destructive for both the tooth and
PDL.

88

Effects of light interrupted force:
• the tooth will move a small amount by frontal
resorption and then remain in that position until the
appliance is activated again.
Effects of heavy interrupted force :
• Heavy forces produce undermining resorption,
tooth will move when undermining resorption is
eliminated. Since the force has dropped to zero, the
tooth will remain in same position until next
activation.
• Although the original force is heavy, after the tooth
moves there is a period of regeneration and repair of
the PDL before force is applied again.

89

EFFECTS OF FORCE MAGNITUDE:
• It is generally considered lighter forces moves tooth rapidly with less
injuries to supporting tissue than heavy ones. What is considered to
light or heavy forces depends on the mode of application and the
mechanical arrangement of the recipient tooth units. In addition to any
applied force, the chewing forces are always present.
• In a study by Rygh et al 1986, reaction to heavy, continuous loads
(50cN) experimenting tipping of first molar in rats was performed.
• The result indicated a pattern in the action of PDL and bone on
tensional stress:
1. Up to certain level of stress, the reaction of PDL with increasing
vascualization,cell proliferation, fiber formation, osteoid formation
occurred.
2. Beyond certain level of stress, decreased vascular supply in the PDL
and destruction of cell stretched occurred. Removal of bone was
marked with undermining resorption of Sharpey’s fibers occurred
3. Stronger force of long duration resulted in vertical reduction of the
alveolar bone
90

• Generally, the magnitude of the force determines the
duration of the hyalinization.
• This is shorter with the light force levels, although
the tendency is towards a longer initial hyalinization
period, and also formation of secondary hyalinized
zones when excessively strong forces are applied.
• Another reason for applying light forces is that it
results in less discomfort and pain to the patient.
Unmyelinated nerve endings persist in the
hyalinized tissue, and they are more or less
compressed during the initial stage.

91

DRUG EFFECT ON THE RESPONSE TO ORTHODONTICS FORCE:

Two types of drugs are known to depress the response of
orthodontic forces
1.Prostaglandin inhibitors
2. Bisphosphonates
PROSTAGLANDIN INHIBITORS:

• If prostaglandins E plays an important role in the cascade of
signals that leads to tooth movement, so inhibitors of its
activity affects tooth movement.
• Drugs that affects prostaglandins are activity fall in two
categories:
1. Corticosteroids and
2. Non-steroidal anti-inflammatory drugs(NSAID)

92

• Corticosteroids inhibits the phospholipase activity.
• Indomethacin inhibits cyclo-oxygenase and may also
inhibit the total homeostasis in the body, provided
the dose are high.
• Aspirin and other acetylsalicylic acids inhibits the
cyclo-oxygenases irreversibly.
• Both children and adult on Steroids and NSAID may
encounter possibilities of difficulties in tooth
movement. The fact that analgesics often are
prostaglandin inhibitors raises the possibility that
the medication used for pain after orthodontic
treatment could interfere with tooth movement.

93

• BISPHONATES, used in osteoporosis bind to hydroxyapatite in
bone. They acts as a specific inhibitors of osteoclast-mediated bone
resorption, so the bone remodeling is slower in this medication. If
orthodontic treatment is necessary in older woman's taking medication
for osteoporosis it’s worthwhile to explore to her physician the
possibility of switching to estrogen as replacement for drug which
inhibits tooth movement.

•
•
•
•
•

Other drugs which inhibits prostaglandin synthesis:
TRICYCLIC ANTIDEPRESSANTS(impramine,amitryptin)
ANTI-ARRHYTHMIC AGENTS (procaine)
MALARIAL DRUGS(quinine,quinidine,chlorquine)
ANTICONVULSANT (phenytoin)
TETRACYCLINES(doxycycline) inhibits osteoclast recruitement
similar to biphosphonates.

94

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