By Dr. Arti Kripalani
Part I P.G.
K.M.Shah Dental College
Physiologic tooth movement
Optimum orthodontic force
Types of forces
Histology of tooth movement
Phases of tooth movement
Theories of tooth movement
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
Bone is selectively removed in some areas and added
The tooth moves through the bone carrying its
attachment apparatus with it, as the socket of the tooth
Because the bony response is mediated by the
periodontal ligament, tooth movement is primarily a
periodontal ligament phenomenon.
During masticatory function, the teeth
and periodontal structures are subjected
to intermittent heavy forces.
Tooth contacts last for 1 second or less.
Forces are quite heavy, ranging from 1 or 2 kg while soft
substance are chewed up to as much as 50 kg against a
more resistant object.
Naturally occurring tooth movements that take place
during and after tooth eruption.
B)Migration or drift of teeth.
C)Changes in tooth position during mastication.
Tooth eruption is the axial movement of tooth from its
developmental position in the jaw to its final position
in the oral cavity.
a)Blood pressure theory:
The tissue around the developing end of the root is highly
This vascular pressure is believed to cause the axial
movement of teeth.
The apical growth of roots result in an axially directed
force that brings about the eruption of teeth.
This theory was rejected.
c)Periodontal traction theory:
The periodontal ligament is rich in fibroblasts that contain
The contraction of these periodontal fibers (mainly the
oblique group of fibers) results in axial movement of the
d)Hammock ligament theory:
According to Sicher, a band of fibrous tissue exists below
the root apex spanning from one side of alveolar wall to
This fibrous tissue appears to form a network below the
developing root and is rich in fluid droplets.
The developing root forces itself against this band of
tissue, which in turn applies an occlusally directed force on
Refers to minor changes in tooth position observed
Human dentition shows a natural tendency to move in
a mesial & occlusal direction.
Usually a result of proximal and occlusal wear of teeth,
to maintain inter-proximal and occlusal contact.
This was pointed out for the first time by Stein and
Physiologic tooth migration usually is related to mesiodistal
movements. However, the teeth also exhibit a continued
eruption, even after full emergence, accompanying the
growth in height of the alveolar processes.
Studies of craniofacial dimensions have demonstrated that
significant changes occur in human beings even during
adulthood (Bjork A, Skieller V, 1983).
During mastication ,the teeth and PDL structures are
subjected to intermittent heavy forces which occurs in
cycles of one second or less and may range from 1-50
kg based on the type of food being masticated.
Is one which moves teeth most rapidly in the desired
direction ,with the least possible damage to tissue and
with minimum patient discomfort.
Schwarz defined it as the force leading to a change in
tissue pressure ,that approximated the capillary vessel
& blood pressure. Thus preventing their occlusion in
the compressed PDL.
Below the optimal level cause no reaction in PDL.
Forces exceeding optimal level would lead to areas of
Schwarz’s definition was slightly modified by
Oppenheim who advocated the use of lightest force
capable of bringing about tooth movement.
Oppenheim and Schwarz following extensive studies
stated that the optimum force is equivalent to the
capillary pulse pressure which is 20-25 gm/sq.cm of
root surface area.
Produce rapid tooth movement.
Minimal patient discomfort.
The lag phase of tooth movement is minimal.
No marked mobility of the teeth being moved.
The vitality of the tooth and supporting PDL is
Initiates maximum cellular response.
Produces direct or frontal resorption.
Continuous force : the force magnitude
is maintained at almost the same level
in the period between two activations.
Interrupted force : declines to zero
Intermittent force : falls to zero when
the appliance is removed and return to
original level on re insertion.
Classic histologic research about tooth movement by
Sandstedt, Oppenheim and Schwarz led to the
hypothesis that a tooth moves in the periodontal space
by generating a ‘pressure side’ and a ‘tension side.’
Later ultrastructural studies by Rygh (1972) and
Brudvik and Rygh (1994), gave a very detailed
description of events.
The findings of the above research have been
meticulously summarized by Krishnan and
Changes following application of light forces:
Changes on pressure side:
The PDL gets compressed to almost 1/3rd of it’s original
A marked increase in the vascularity of PDL on this side is
observed due to increase in capillary blood supply.
This increase in blood supply helps in mobilization of cells
such as fibroblasts and osteoclasts.
When forces applied are within physiologic limits,the
resorption is seen in alveolar plate immediately adjacent to the
ligament.This kind of resorption is called frontal resorption.
Changes on tension side:
Distance between alveolar process & tooth is widened.
Mobilization of fibroblasts & osteoblasts.
Osteoid is laid down by osteoblast in PDL immediately
adjacent to lamina dura.
Lightly calcified bone mature to form woven bone.
Secondary remodelling changes:
Bony changes also takes place elsewhere to maintain the width
or thickness of alveolar bone. These changes are called
secondary remodeling changes.
For eg:-If a tooth is being moved in a lingual direction there is
compensatory deposition of new bone on the outerside of the
lingual alveolar bony plate and also a compensatory resorption
on the labial side of the labial alveolar bone.
This is to maintain the thickness of the supporting alveolar
On the pressure side :-
Root closely approximates the lamina dura .
Compresses the PDL and leads to occlusion of blood vessels.
The PDL is hence deprived of its nutritional supply leading to
regressive changes called hyalinization .
Undermining/Rearward resorption occurs in the adjacent
marrow spaces and alveolar plate below, behind & above the
On the tension side:-
Over stretched PDL .
Tearing of blood vessels & ischaemia.
Extreme forces applied net increase in osteoclastic activity
and tooth loosened in socket.
Is orthodontic movement possible for a tooth that has
undergone endodontic treatment ?
Is it possible to move an ankylosed tooth ?
Form of tissue degeneration characterized by formation
of a clear, eosinophilic homogenous substance.
Denotes a compressed and locally degenerated PDL.
Occurs in almost all forms of orthodontic tooth
movement but the areas are wider when the force
applied is extreme.
Gradual shrinkage of PDL fibres.
Cellular structures become indistinct.
Collagenous tissues gradually unite
into a more or less cell free mass.
break down of blood vessel walls
leading to spilling of their contents.
Osteoclasts are formed after a period
of 20-30 hrs.
The presence of hyalinised zone indicates that the
ligament is non-functional and therefore bone
resorption cannot occur.
The tooth is hence not capable of further movement
until the local damaged tissue has been removed and
the adjacent alveolar bone resorbs .
1. By osteoclasts differentiating in the peripheral intact
PDL membrane and in the adjacent marrow spaces.
2. Invasion of cells and blood vessels from the
periphery of the compressed zone by which necrotic
tissue is removed by enzymatic action and
Greater the forces wider is the area of hyalinization.
Thus larger areas of the ligament becomes
functionless, thereby showing larger areas of rearward
If lighter forces are used, the hyalinised zone is
smaller and a larger area of functioning ligament is
available and frontal resorption predominates.
The location and extent of hyalinised tissue largely
depends upon nature of tooth movement.
Tipping – close to alveolar crest.
Excessive force during tipping- two areas, one on
apical region and other in marginal area.
Bodily- closer to middle portion of root.
Authors: Journal: Level of
Result and Conclusion:
E. B. W.
J. C. Maltha
EJO 2012 4 To study and assess
behaviour of the
A rapid transposition
during the first few
seconds was found.
However, it was
significantly less for
hyalinized than for non-
The hyalinized tissue
offers resistance to tooth
movement and it is
proportional to the
degree of hyalinization.
Burstone categorize the stages as:-
Post lag phase
Rapid tooth movement is observed over a short distance which
Represents displacement of tooth in PDL membrane space and
probably bending of alveolar bone .
Both light and heavy forces displace the tooth to same extent .
Between 0.4 to 0.9mm usually occurs in a weeks time.
Little or no tooth movement occurs .
Formation of hyalinized tissue .
Extent upto 2-3 weeks but may at times be as long as
Tooth movement progresses rapidly as the hyalinized
zone is removed and bone undergoes resorption .
Osteoclasts are found over a larger surface area.
If the duration of the movement is divided into an initial and a
secondary period, direct bone resorption is found in the secondary
period, when the hyalinized tissue has disappeared after
undermining bone resorption.
Study by Pilon et al performed on beagles,
divided the curve of tooth movement into 4 phases:
The first phase lasts 24 hrs to 2 days and represents the
initial movement of the tooth inside its bony socket.
Second phase when the tooth movement stops for 20 to 30
After the removal of necrotic tissue formed during the
second phase , tooth movement is accelerated in the third
phase and continues into the fourth linear phase.
1. The pressure-tension theory
2. Bone bending & piezoelectric theory
3. Blood flow/fluid dynamic theory
Classic histologic research about tooth movement by
Sandsted (1904),Oppenheim (1911) and Schwarz
(1932) led them to hypothesize that a tooth moves in
the periodontal space by generating a “pressure side”
and a “tension side.”
According to them, areas of pressure show bone
resorption while areas of tension show bone
Farrar was the first to suggest, in 1888, that alveolar
bone bending plays a pivotal role in orthodontic tooth
This hypothesis was later confirmed with the
experiments of Baumrind in rats and Grimm in
Piezo-electricity is a phenomenon observed in many
crystalline materials in which deformation of the
crystal structure produces a flow of electric current.
As a result, displacement of electrons from one part of
the crystal lattice to the other.
A small electric current is generated & bone is
mechanically regenerated and repaired.
The possible source of electric current are :-
3. Collagen hydroxyapetite interface.
As long as the force is maintained ,the crystal structure is
stable & no further electric effect is observed.
When the force is released the crystals return to their
original shape & reverse flow of electrons is observed.
This rhythmic activity produces a constant interplay of
electric signals .
Quick decay rate
When the force is released electrons flow in the opposite
On application of a force on a tooth ,
Areas of concavity negative charges bone deposition.
Areas of convexity +ve charges and bone resorption.
It was proposed by Bien in 1966.
Acc. to this theory:
“Tooth movement occurs as a result of alterations in fluid
dynamics in the PDL.”
There are three interacting fluid systems in the PDL:
1. Vascular system
2. Cellular system
3. Interstitial fluid system
These fluids in confined periodontal space create a unique
compression of PDL
Occlusion of blood vessels on pressure side
and dilation on tension side
formation of aneurysms
Fluid is squeezed out of vessel wall
O2 level falls in compressed area and rises on tension side
metabolite changes take place
change in chemical environment stimulates release of chemical
They stimulate cellular differentiation into Osteoblasts and Osteoclasts
Bone Remodeling takes place.
Cytokines (tumor necrosis factor [TNF], interleukin-1 alpha, 6-alpha
[I]), macrophage colony stimulating factor (M-CSF), granulocyte,
macrophage colony stimulating factor (GM-CSF) and prostaglandin
Osteoclast differentiation is also mediated by the interaction of
two molecules produced by osteoblasts, namely osteoprotegrin
(OPG) and RANK ligand (RANKL) Receptor activator of nuclear
factor kappa B ligand
At the “Biology of tooth movement” conference held at
Formington in Nov.,1986 it was suggested that the ans
was likely to be found in the field of CYTOKINE
According to this hypothesis,formation and resorption
1) the cytokines
2) the functional state of available target cells.
The cytokines are defined as short range soluble
mediators released from cells which modulate the
activity of other cells.
Includes interleukines, interferons, chemotactic
factors,tumor necrotic factors,colony stimulating
factors and assorted growth factors.
Q) How do cytokines mediate mechanically induced
bone remodelling ?
A) Osteoblasts have receptors for prostaglandins, PTH,
Vit D but osteoclasts don’t have.
It is cytokine that transmits signals from osteoblasts to
Cytokines either activate osteoclasts or promote
differentiation of the precursor cells.
Secondly, osteoblasts produce collagenase enzyme that
Osteoclasts cannot resorb bone untill the surface
osteoid layer is removed.
Although cytokine is a potent stimulator of bone
resorption,it can also stimulate osteoblast proliferation.
Therefore, whether resorption or formation, depends
upon the cytokines and functional state of the available
Tooth movement ultimately depends on specific
activation of bone precursor cells ( osteoprogenitors)
which form osteoblasts and osteoclasts.
Davidovitch and Shanfeld,
Davidovitch et al., Yamasaki et al. (first reported
human study, 1982-83),Lee et al., Selinkale et al. and
Mohammed et al injected PG inhibitor, leukotriene
inhibitor and both together, in rats and performed tooth
vasoactive intestinal polypeptide (VIP)
calcitonin gene related peptide (CGRP).
Davidovitch et al demonstrated that incubation of
substance P in vitro significantly increased the
concentration of cAMP in the cells and of PGE2 in the
medium within 1 minute.
Authors: Journal: Level of
Result and Conclusion:
Dental clinic of
4 To test the
tooth movement is
modulated by factors
derived from the
nervous and vascular
Administration of SP and
IL-1 beta to human PDL
fibroblasts in vitro for 1 to
60 minutes resulted in
significant increases in the
levels of the intracellular
"second messenger" cAMP,
as well as of PGE2.
This tend to support the
cytokines play a regulatory
role in orthodontic force-
induced alveolar bone
Three distinct isoforms of NOS are:
- a neuronal form (nNOS),
- an endothelial form (eNOS),
- an inducible form (iNOS).
Both nNOS and eNOS are constitutively expressed
and are collectively referred to as constitutive NOS
It was found that iNOS plays a role in the
response of periodontal tissue to orthodontic force.
1. NO activates guanylyl cyclase in periodontal
ligament fibroblasts, leading to an increased level of
cGMP. This second messenger in cell cytoplasm raises
lysosome membrane permeability, leading to
exocytosis of lysosome content resulting in resorption
of organic and mineral elements of bone.
2. Nitric oxide synthesizes prostaglandins by direct
activation of cyclo-oxygenase.
3. Nitric oxide influences the function of osteoclastic
differentiation and osteoblast function.
1) strain sensitive ion channels and shear stress receptors
in cell membrane:
calcium and potassium channels.
Channel gating may be caused by direct mechanical
perturbation or secondarily by the activation of stretch
sensitive phospholipase C or D.
They respond to mechanical strain by causing in and
out movement of ions, creating changes in the electric
These changes enable the signal to be propagated
2)Signal transduction by integrins and focal
Sutherland and Rall established the second-messenger
basis for hormone actions in 1958.
They proposed that the first messenger binds to a
specific receptor on the cell membrane and produces an
intracellular chemical second messenger.
This second messenger then interacts with cellular
enzymes which evokes a response.
The receptor activator of nuclear factor kappa B ligand
(RANKL), its decoy receptor (RANK) and
osteoprotegrin (OPG) were found to play important
roles in regulation of bone metabolism.
Evidences suggest osteoblast itself regulates the
differentiation of osteoclast.
The talk between an osteoblast and osteoclast is
accomplished through an osteoblast membrane bond
RANKL which can interact with osteoclast precursors
to cause them to differentiate into osteoclasts.
Another membrane bond molecule and its bonding
ligand OPG can develop to block RANKL and prevent
Extensive studies done by Alhashimi et al., Aihara et
al., Kanzaki et al.,and Yamaguchi et al. have
demonstrated that RANKL promotes osteoclastogenesis
while OPG inhibits this effect.
One of the most distinctive features of bone
remodeling of both cortical and trabecular bone is the
precise coupling of bone resorption and formation.
The A-R-F process is similar for all types of bone
remodeling; the multicellular unit for a trabecular bone
surface is essentially a hemisection of the
cutting/filling cone for cortical bone.
(1) bone microdamage results in release of inflammatory
cytokines (prostaglandins, interleukin 1-, and so on) and
exposure of mineralized collagen to extracellular fluid.
(2) T cells, attracted by the inflammatory cytokines and
exposed mineralized collagen, produce the ligand RANKL,
which induces osteoclastogenesis
(3) preosteoclasts from circulating blood have RANK
receptors, which are activated by the RANKL to from
(4) as bone is resorbed, growth factors are released that
stimulate preosteoclasts to produce OPG which then retract
from the bone surface.
(5) mononuclear cells move in and coat the scalloped
resorbed surface with cementing substance (green)
(6) perivascular osteogenic cells migrate through the
low cell density zone and differentiate to preosteoblasts
that then divide and form two osteoblasts each.
(7) osteoblasts form new bone, filling the resorption
cavity and completing the turnover process.
1. Effect on the pulp
Heavy intrusive force can sever the blood vessels as they
enter the root apex and that can affect the tooth vitality.
2. Effects on the root structure
Some root remodeling constantly occurs during orthodontic
But permanent root loss ( root resorption) occurs if:
there are conical roots with pointed apices,
dilacerated / distorted root form,
excessive force over prolonged period,
root apices in contact with cortical bone,
history of trauma.
PDL usually widens during orthodontic treatment
hence some mobility of teeth is unavoidable during
Heavy sustained O.F. results into greater amount of
undermining resorption and thereby greater mobility.
If tooth becomes excessively mobile during treatment,
forces should be discontinued for time being till
Pain during orthodontic treatment occurs due to development
of ischemic areas in the PDL.
Heavier forces will cause more pain.
Upon activation using light forces can also produce pain for
few hours to few days.
This also greatly depends upon individual pain threshold.
If patient is engaged in repeated chewing during initial 8 hrs.
after activation the pain can be reduced.
Excessive pain during activation is an indication of too heavy
force which should be avoided.
Orthodontic tooth movement consequent t application
of force is outcome of complex chain of events,
eventually leading to bone resorption and bone
What signals the ‘cells’ in periodontium to perceive
orthodontic force to transform into bone forming and
bone resorbing cells is not yet fully understood.
Future research in the field of molecular biology is
expected to unfol many more secretes of this