Biology Of Tooth Movement: Understanding The Cellular Responses That Facilitate Orthodontic Force

Biology Of
Tooth Movement
www.indiandentalacademy.comwww.indiandentalacademy.com

CONTENTSCONTENTS
 IntroductionIntroduction
 HistoryHistory
 Normal Structure.Normal Structure.
 Bone resorption & deposition.Bone resorption & deposition.
 Force - ClassificationForce - Classification
 Effect of forces- Heavy / Light.Effect of forces- Heavy / Light.
 Theories.Theories.
 PathwayS Of Tooth Movement.PathwayS Of Tooth Movement.
 Chemical MessengerS.Chemical MessengerS.
 Phases of tooth movement.Phases of tooth movement.
 ConclusionConclusion
 BibliographyBibliography

 What does an appliance do?
 An Orthodontic appliance transfers mechanical
stresses.
 What is the medium? How does it do it?
 Through the tooth to the periodontium where
they are translated into signals- physical,
chemical, and electrical .
 Signals sent to cells that activate tissue
remodeling to allow tooth movement.
 What does the clinician do?
 The clinician is able to control the quantity and
quality of the force system applied to the
teeth.
What does the biologic response do?
 The speed and way in which teeth move is
ultimately determined by the biological
response. www.indiandentalacademy.comwww.indiandentalacademy.com

Clinicians RoleClinicians Role
To understand the process of each
interface.
This will enable to interpret the
biological responses to activation of any
orthodontic appliance.
There may be the application of
molecular and cell biology which is
important in medical science, to the
field of orthodontics.

HistoryHistory
 18th Century18th Century  HunterHunter provided the first explanation for orthodonticprovided the first explanation for orthodontic
tooth movement.tooth movement.
 18151815  DelabbareDelabbare remarked that pain and swelling of paradental tissuesremarked that pain and swelling of paradental tissues
occur following theoccur following the application of orthodontic forcesapplication of orthodontic forces to teeth.to teeth.
Delabbare introduced the notion that inflammation is an integral part ofDelabbare introduced the notion that inflammation is an integral part of
orthodontic tooth movement.orthodontic tooth movement.
 18881888  FarrarFarrar hypothesized that tooth movement is due,hypothesized that tooth movement is due,
partly , to bending of alveolar bone by applied forces.partly , to bending of alveolar bone by applied forces.
 18921892  WolffWolff supported Farrar in that he said internal architecture ofsupported Farrar in that he said internal architecture of
bone is dictated by the mechanical forces that act upon it.bone is dictated by the mechanical forces that act upon it.www.indiandentalacademy.comwww.indiandentalacademy.com

 1904-051904-05  SandstedtSandstedt reported for the first time on thereported for the first time on the
histomorphology of tissues surrounding orthodonticallyhistomorphology of tissues surrounding orthodontically
treated teeth.treated teeth.
 That landmark experiment, which was performed in one dog,That landmark experiment, which was performed in one dog,
concluded that force induced tissue changes are limited toconcluded that force induced tissue changes are limited to
the PDL and its alveolar bone margin.the PDL and its alveolar bone margin.

 At the end of 3 weeks of treatment.At the end of 3 weeks of treatment.
 Sandstedt observedSandstedt observed::
 No bone growth in the stretched PDL,
 Bone resorption in the area of PDL
compression.
 Cell death occurred in the compressed PDL
when the applied force was excessive
 The alveolar bone resorbed as a result of
osteoclastic activity in adjacent marrow
spaces (Undermining resorption).

 OppenheimOppenheim reported the experiment on areported the experiment on a
juvenile baboon.juvenile baboon.
 In contrast toIn contrast to Sandstedt, OppenheimSandstedt, Oppenheim saw nosaw no
demarcation between the old and new bone,demarcation between the old and new bone,
There was seen a trabecular structure.There was seen a trabecular structure.
This strongly suggested aThis strongly suggested a completecomplete
transformationtransformation of the entire alveolar bone.of the entire alveolar bone.

Schwartz (1932) defined orthodontic
forces as being “not greater than the
pressure in the blood capillaries (20 to
26 g/cm2 of root surface).

 The PDL and alveolar bone, due to their fluid-The PDL and alveolar bone, due to their fluid-
fiber composition, can be deformedfiber composition, can be deformed elasticallyelastically
by external forces.by external forces.
 This also evoke cellular activities.This also evoke cellular activities.
 When the tissue elastic limit is reached, itWhen the tissue elastic limit is reached, it
starts to deformstarts to deform plasticallyplastically, with adaptive, with adaptive
proliferation and remodeling reactions.proliferation and remodeling reactions.

 Prolonged forces that exceed the bioplastic limitProlonged forces that exceed the bioplastic limit
result in biodisruptive deformation, with:result in biodisruptive deformation, with:
Ischemia,Ischemia,
Cell death,Cell death,
Inflammation,Inflammation,
Repair.Repair.
 Thus Reitan and Storey’s investigations demonstrated the complexity of the tissue reaction during tooth movement.

 It was no longer perceived as a simple
phenomenon.
 Simple phenomenon being of applied force causing the tooth to move
within the PDL, leading to tension and compression, and subsequent
bone formation and resorption.

 They are perceived as a dynamic set of events
that involved profound alterations in cellular
functions and changes in matrix composition.
 This enabled other researchers to ask “why”
and “how” PDL and Alveolar bone responds to
applied forces.

PhysicsPhysics
BiologyBiology
…the point of convergence…

The problem.
The goal.
The solution !
Tooth movement

TheThe
Biology.Biology.

PERIODONTIUM
PDL
CEMENTUM
ALVEOLAR BONE
GINGIVA

Alveolar crest group
Horizontal group
Oblique group
Apical group
Transseptal group
Interradicular group
PERIDONTAL LIGAMENT STRUCTURE
Principal fibers are divided in 6 groups

PDL - Fibers
Principal fibers & Sharpey’s fibers
Alveolar crest fibres
Horizontal fibres
Oblique fibres
Apical fibreswww.indiandentalacademy.comwww.indiandentalacademy.com

 Thickness of PDL:-Thickness of PDL:- 0.15 -0.38 mm0.15 -0.38 mm
 Principal fibers –Principal fibers –
Collagenous .Collagenous .
Arranged in bundlesArranged in bundles ..
 Follow a wavy course.Follow a wavy course.
 Terminal portionTerminal portion inserting into alveolar bone orinserting into alveolar bone or
cementum is termed ascementum is termed as SHARPEY’S FIBERS

Periodontal ligamentPeriodontal ligament
 Contents ofContents of PDLPDL
1-CELLS
 Synthetic cellsSynthetic cells
 Resorptive cellsResorptive cells
 OsteoblastsOsteoblasts
 OsteoclastsOsteoclasts
 FibroblastsFibroblasts
 CementoblastsCementoblasts
 CementoclastsCementoclasts
Cellular elements:
Connective tissue cells:
Epithelial rests of
Malassez.
Cells associated with
neurovascular elements.
Progenitor cellsProgenitor cells
OthersOthers
Mast cells, Macrophages.Mast cells, Macrophages.

2-Extracellular substance
Fibers- 1-collagen 2-oxytalin
 Ground substance:Ground substance:
ProteoglycansProteoglycans
GlycoproteinsGlycoproteins
 Blood vessels ,nerves ,lymphaticsBlood vessels ,nerves ,lymphatics

What is Collagen?
Proteins composed of amino acids-
glycine, proline, hydroxyproline &
hydroxylysine.
Collagen in a tissue can be determined
by its hydroxyproline content
 Synthesized in
fibroblasts,chondroblasts,osteoblasts.
 Transverse striations at a periodicity of 640
Angstrom(overlapping of tropocollagen molecules)

Collagen
 Tensile strength is greater than that ofTensile strength is greater than that of
steelsteel
 Principal fibers are composed of mainlyPrincipal fibers are composed of mainly
type I collagentype I collagen
 CollagenCollagen of PDL is turned over atof PDL is turned over at
fastest rate among all connectivefastest rate among all connective
tissues of the bodytissues of the body

Alveolar bone...

Tooth can be moved through the alveolar boneTooth can be moved through the alveolar bone
byby application of appropriate forces.application of appropriate forces.
This raises the possibility of anyThis raises the possibility of any
orthodontic treatment .orthodontic treatment .
The tooth (solid object) movesThe tooth (solid object) moves
through a solid medium.through a solid medium.

 Orthodontic force application leads toOrthodontic force application leads to
tooth movement.tooth movement.
 Leads to remodelling changes in dental &Leads to remodelling changes in dental &
paradental tissues:paradental tissues:
 PulpPulp
 PDLPDL
 Alveolar bone,Alveolar bone,
 Gingiva.Gingiva.

Orthodontic toothOrthodontic tooth
movementmovement
Characterized by abrupt creation ofCharacterized by abrupt creation of
Compression & Tension regions in theCompression & Tension regions in the
PDL .PDL .
 Movement can occur rapidly or slowly.Movement can occur rapidly or slowly.
Depends on-Depends on-
 The physical characteristics of theThe physical characteristics of the
applied force.applied force.
 The size & biological response of PDL.The size & biological response of PDL.

1. CONTINUOUS1. CONTINUOUS
 Force maintained at someForce maintained at some appreciable fractionappreciable fraction of theof the
original from one patient visit to the next.original from one patient visit to the next.
 Eg:In case of elastics worn continuously as prescribed.Eg:In case of elastics worn continuously as prescribed.

2.Interrupted2.Interrupted
 Force levelsForce levels decline to zerodecline to zero betweenbetween
activations.activations.
 Eg: Coil spring, e-chain if not changedEg: Coil spring, e-chain if not changed
until patient’s next visit.until patient’s next visit.

3.Intermittent3.Intermittent
 Force levels declineForce levels decline abruptly to zeroabruptly to zero whenwhen
appliance is removed.appliance is removed.
 Eg:Hawleys plate, headgear etc. whenEg:Hawleys plate, headgear etc. when
removed by the patientremoved by the patient..

Type of movementType of movement Force (gms)Force (gms)
TippingTipping 50-7550-75
Bodily movementBodily movement 100-150100-150
Root up rightingRoot up righting 75-12575-125
RotationsRotations 50-7550-75
ExtrusionExtrusion 50-7550-75
IntrusionIntrusion 15-2515-25
Optimum forces for differentOptimum forces for different
types of tooth movementstypes of tooth movements

EFFECTS OF FORCE MAGNITUDE:
Time
Event
Light Pressure Heavy pressure
< 1 sec PDL fluid incompressible, alveolar bone bends,
piezoelectric signals generated
1-2 sec PDL fluid expressed, tooth moves within PDL
space
3-5 sec Blood vessels within PDL partially compressed on
pressure side, dilated on tension side; PDL fibers
and cells mechanically distorted
Minutes Bloods flow altered, oxygen tension begins to
change; prostaglandin's and cytokines released
Hours Metabolic changes occurring: chemical messengers affect
cellular activity, enzyme levels change
~4 hours Increased cAMP levels detectable, cellular
differentiation begins within PDL
~2 days Tooth movement beginning as
osteoclasts/osteoblasts remodel bony socket
3-5 sec Blood vessels within PDL occlused on pressure
side
Minutes Blood flow cut off to compressed PDL area
Hours Cell death in compressed area
3-5 days Cell differentiation in adjacent marrow spaces,
undermining resorption begins
7-14 days Undermining resorption removes lamina dura adjacent to
compressed PDL, tooth movement occurs
Physiologic response to sustain pressure against the tooth.
www.indiandentalacademy.com

 Aim – Information of histological & chemical changesAim – Information of histological & chemical changes
of orthodontic tooth movement.of orthodontic tooth movement.1111
 Update on the recent development in cellular,Update on the recent development in cellular,
molecular ,tissue & genetic reactions in response tomolecular ,tissue & genetic reactions in response to
orthodontic force application.orthodontic force application.
 Process of remodeling in response to orthodonticProcess of remodeling in response to orthodontic
force, of –force, of –
 Bone, PDL, Gingiva.Bone, PDL, Gingiva.

The tissues are exposed toThe tissues are exposed to
MECHANICAL LOADINGMECHANICAL LOADING
They vary in degree -They vary in degree -
MAGNITUDEMAGNITUDE
FREQUENCYFREQUENCY
DURATION.DURATION.
Express extensive macro &Express extensive macro &
microscopic changes.microscopic changes.

ORTHODONTIC FORCEORTHODONTIC FORCE
 ““Force applied to teeth for the purposeForce applied to teeth for the purpose
of effecting tooth movement , generallyof effecting tooth movement , generally
having a magnitude lower than anhaving a magnitude lower than an
orthopedic force”orthopedic force”

Determinants of Skeletal homeostasis & Bone
changes in OTM13
 BMP- Bone morphogenic protein.
 Cbfa1- Transcription factor ; Earliest marker of osteogenesis.
 CGRA- Calcitonin gene related peptide.
 ClCN7- Chloride channel 7.
 CSF-1- Colony stimulating factor 1.
 CTGF- Connective tissue growth factor.
 ER- beta- Estrogen receptor beta .
 GH – Growth Hormone.
 GLAST- Glutamate /Aspartate transporter.
 Hoxa 2/Msx-2- Homeobox gene.
 IGF - Insulin like growth factor.
 LRP 5 - Low density lipoprotein receptor –related protein 5.
 NOS – Nitrous oxide synthetase.
 OPG - Osteoprotegerin.
 PGHS 2- Prostaglandin G/H Synthetase.
 PTH - Parathyroid Hormone.
 RANK/RANKL- Receptor activator of nuclear factor kappa-b and Ligand.
 S mad - Cytoplasmic signaling molecules.
 SOST - Gene for sclerostin.
 TGF beta –Transforming growth factor beta-family .
 TNF/R - Tumor necrosis factor and receptor.

Mechanical force induced 13
reciprocal communication b/w
5 environments of OTM

 Forces alter the PDL vascularity & blood flow.Forces alter the PDL vascularity & blood flow.
 Results in-Results in-
 Synthesis & Release of key molecules-Synthesis & Release of key molecules-
 Neurotransmitters, Cytokines, growthNeurotransmitters, Cytokines, growth
factors,colony stimulating factors &factors,colony stimulating factors &
arachidonic acid metabolites.arachidonic acid metabolites.
 These molecules evoke cellular responses byThese molecules evoke cellular responses by
various cell types in & around teeth.various cell types in & around teeth.
 Provides favourable microenvironment forProvides favourable microenvironment for
tissue deposition or resorption.tissue deposition or resorption.www.indiandentalacademy.comwww.indiandentalacademy.com

ORTHOPEDIC FORCEORTHOPEDIC FORCE
 Force of higher magnitude in relation toForce of higher magnitude in relation to
an orthodontic force, when deliveredan orthodontic force, when delivered
via teeth for 12 -16 hours/day, isvia teeth for 12 -16 hours/day, is
supposed to produce a skeletal effectsupposed to produce a skeletal effect
on the maxillofacial complex.on the maxillofacial complex.

Orthodontic MechanotherapyOrthodontic Mechanotherapy
 Aimed at tooth movementAimed at tooth movement
 By remodeling & adaptive changes in theBy remodeling & adaptive changes in the
paradental tissues.paradental tissues.
 For this outcome small amount of forcesFor this outcome small amount of forces
might be required- 20-150gm/tooth.might be required- 20-150gm/tooth.

Craniofacial OrthopedicsCraniofacial Orthopedics
 Aimed at delivering higher magnitudes ofAimed at delivering higher magnitudes of
mechanical forces- > 300gms.mechanical forces- > 300gms.
 This attempts to modify the form ofThis attempts to modify the form of
craniofacial bones.craniofacial bones.
 The appliances are calledThe appliances are called
CRANIOFACIAL ORTHOPEDIC DEVICSCRANIOFACIAL ORTHOPEDIC DEVICS
Delivers macroscale mechanical forcesDelivers macroscale mechanical forces
Produce microstructural sutural bone strain &Produce microstructural sutural bone strain &
Induce cellular growth response in sutures.Induce cellular growth response in sutures.

OPTIMAL ORTHODONTICOPTIMAL ORTHODONTIC
FORCEFORCE
 Mediated (settled /balanced) by coupling boneMediated (settled /balanced) by coupling bone
resorption & deposition in compressed &resorption & deposition in compressed &
stretched sides of the PDL.stretched sides of the PDL.
 Forces alter the blood flow & localizedForces alter the blood flow & localized
electrochemical environment.electrochemical environment.
 Upsets the homeostatic environment of theUpsets the homeostatic environment of the
PDL space.PDL space.
 This abrupt alteration initiates biochemical &This abrupt alteration initiates biochemical &
cellular events which reshape the bonycellular events which reshape the bony
contours of the alveolus.contours of the alveolus.

 Optimum forces moves teeth efficiently inOptimum forces moves teeth efficiently in
the desired position ,without causingthe desired position ,without causing
discomfort or tissue damage to the patient.discomfort or tissue damage to the patient.
 Basis of optimal forces –Basis of optimal forces –
PROPER MECHANICAL PRINCIPLESPROPER MECHANICAL PRINCIPLES
Enables clinician to move teethEnables clinician to move teeth
1.1. Without Traumatizing structures,Without Traumatizing structures,
2.2. Without moving dental roots redundantlyWithout moving dental roots redundantly
(Round tipping) ; or(Round tipping) ; or
3.3. Into danger zones.Into danger zones.

 Schwarz,1932, “the force leading to aSchwarz,1932, “the force leading to a
change in tissue pressure thatchange in tissue pressure that
approximated the capillary vessel’s B.P.,approximated the capillary vessel’s B.P.,
thus preventing their occlusion in thethus preventing their occlusion in the
compressed PDL.compressed PDL.
 Force below optimum produce noForce below optimum produce no
reaction .reaction .
 Force above leads to Tissue Necrosis,Force above leads to Tissue Necrosis,
preventing frontal resorption of thepreventing frontal resorption of the
alveolar bone.alveolar bone.

 Oppenheim (1942)& Reitan(1957).Oppenheim (1942)& Reitan(1957).
 Recommended applying light forces..Recommended applying light forces..
 Demonstrated cell free compressedDemonstrated cell free compressed
areas in PDL.areas in PDL.

 Storey & Smith( 1952).Storey & Smith( 1952).
 Studied distal movement of Canine.Studied distal movement of Canine.
 Recommended applying light forces..Recommended applying light forces..
 When force isWhen force is >> Optimum.---Optimum.---
 Rate-Rate-
 Tooth movement appeared zero.Tooth movement appeared zero.

Current concept ofCurrent concept of
Optimum forceOptimum force
Viewed as an extrinsic mechanicalViewed as an extrinsic mechanical
stimulus, that evokes a cellularstimulus, that evokes a cellular
response .response .
Aims to restore equilibrium byAims to restore equilibrium by
remodeling periodontal supportingremodeling periodontal supporting
tissues.tissues.

 OPTIMAL is considered hence-OPTIMAL is considered hence-
 The mechanical input that leads toThe mechanical input that leads to
max. rate of tooth movement withmax. rate of tooth movement with
minimal irreversible damage tominimal irreversible damage to
tooth, PDL & alveolar bone.tooth, PDL & alveolar bone.

Theories of OrthodonticTheories of Orthodontic
mechanismsmechanisms
 Orthodontic tooth movement has beenOrthodontic tooth movement has been
defined as the result of a biologic response todefined as the result of a biologic response to
interference in the physiologic equilibrium ofinterference in the physiologic equilibrium of
the dentofacial complex by an externallythe dentofacial complex by an externally
applied force.applied force.
 Two main mechanisms were proposed.Two main mechanisms were proposed.
 Pressure Tension theory.Pressure Tension theory.
 BONE BENDING THEORY.BONE BENDING THEORY.

Pressure Tension theory.Pressure Tension theory.
Sandstedt(1904),Oppenheim(1911),Sandstedt(1904),Oppenheim(1911),
Schwarz(1932).Schwarz(1932).
Hypothesized that a tooth moves in theHypothesized that a tooth moves in the
periodontal space by generating aperiodontal space by generating a
“pressure side” and a “tension side.”“pressure side” and a “tension side.”
On the pressure side, the PDL displaysOn the pressure side, the PDL displays
disorganization and diminution of fiberdisorganization and diminution of fiber
production.production.
Here, cell replication decreasesHere, cell replication decreases
seemingly due to vascular constriction.seemingly due to vascular constriction.

On the tension side-On the tension side-
Stimulation is produced byStimulation is produced by
stretching of PDL fiber bundles.stretching of PDL fiber bundles.
Results in an increase in cellResults in an increase in cell
replication.replication.
Due to this enhanced proliferativeDue to this enhanced proliferative
activity it eventually leads to anactivity it eventually leads to an
increase in fiber production .increase in fiber production .

Schwarz concluded-Schwarz concluded-
The forces delivered as part of orthodonticThe forces delivered as part of orthodontic
treatment should not exceed the capillary bedtreatment should not exceed the capillary bed
blood pressure -blood pressure -
20-25 g/cm20-25 g/cm22
of root surface.of root surface.
If exceeding this pressure, compressionIf exceeding this pressure, compression
could cause tissue necrosis by-could cause tissue necrosis by-
““Suffocation of the strangulatedSuffocation of the strangulated
periodontium.”periodontium.”

Early theories of tooth movementEarly theories of tooth movement
Farrar
Angle
Breitner
Sandstedt
Sicher& Wienmann
Brash

Recent theoriesRecent theories22
Pressure -Tension
Bioelectric
Chemical signals
Electric signals
Strain in bone
Strain in PDL
FLUID DYNAMIC THEORYwww.indiandentalacademy.comwww.indiandentalacademy.com

Physiologic tooth movementPhysiologic tooth movement
 Dental drift & tooth eruption.Dental drift & tooth eruption.
 Slow processSlow process
 Occurs mainly in buccal direction intoOccurs mainly in buccal direction into
cancellous bone orcancellous bone or
 Due to growth into cortical bone.Due to growth into cortical bone.

FLUID DYNAMIC THEORY:
 Proposed by Bien
This theory is also called the blood flow theory.
 Tooth movement occurs as a result of
alterations in fluid dynamics in the
periodontal ligament.
 The contents of Periodontal ligament
create unique hydrodynamic condition.

STAGES OFSTAGES OF
PRESSURE-TENSION THEORYPRESSURE-TENSION THEORY
 1.Alterations in blood-flow.1.Alterations in blood-flow.
 2.Formation or/and release of chemical2.Formation or/and release of chemical
messengers.messengers.
 3.Activation of cells.3.Activation of cells.

Pressure-Tension theoryPressure-Tension theory

THEORIES OF TOOTH
MOVEMENT:
Pressure-Tension Theory.
 Schwartz in 1932.
 Relies on Chemical rather than electric
signals for cellular differentiation and
tooth movement
TOOTH
ORTHODONTIC FORCE
AREAS OF
TENSION
AREAS OF PRESSURE
BONE
DEPOSITION
BONE
RESORPTION

Pressure-Tension theoryPressure-Tension theory
FORCE
Pressure sideTension side
Changes in fibres
Changes in vessels
Changes in fibres
Changes in vessels

Areas of
pressure
Areas of
tension

Pressure zone . . .Pressure zone . . .

Changes in pressure zone2
compression of principal fibres
Compression of blood vessels
Decreased oxygen level
Force
Cellular response
Partial ?
Complete ?
Dies?
Lives ?
PDL fliud expressed

BONE RESORPTION :BONE RESORPTION :
 FRONTALFRONTAL
 UNDERMININGUNDERMINING

Frontal resorption
Application of ideal orthodontic force.Application of ideal orthodontic force.
The resorption seen during toothThe resorption seen during tooth
movement at the pressure site.movement at the pressure site.
Here ,osteoclasts resorbs the bone.Here ,osteoclasts resorbs the bone.

Undermining resorption:
Application of heavy force (continuous).Application of heavy force (continuous).
Blood vessels occlude.Blood vessels occlude.
Results in sterile necrosis at the compressionResults in sterile necrosis at the compression
site.site.
The cells disappear.The cells disappear.
Forms anForms an avascularavascular area.area.
Termed as hyalinized zone.Termed as hyalinized zone.
It does not form hyalinized tissue.It does not form hyalinized tissue.
Devoid of cells & appears plain.Devoid of cells & appears plain.

 Remodeling of bone adjacent to theRemodeling of bone adjacent to the
necrotic area has to occur.necrotic area has to occur.
 Achieved by deriving cells from theAchieved by deriving cells from the
adjacent undamaged areas.adjacent undamaged areas.
 The cellular elements from the undamagedThe cellular elements from the undamaged
adjacent area include the necroticadjacent area include the necrotic
( Hyalinized) area.( Hyalinized) area.
 This invasion of osteoclasts from the underThis invasion of osteoclasts from the under
side of lamina- dura is termed asside of lamina- dura is termed as
undermining resorption.undermining resorption.

Blood vessels3
Light force Heavy force
Partial compression Complete occlusion
Blood flow
patent
Blood flow cut off
Chemical mediated
cell response
Necrosis
Frontal resorption Undermining resorption
O2 O2

PDL
Os.cl
Marrow
Os.cl
HFLF
Frontal resorption Undermining resorption
Cell action Cell deathwww.indiandentalacademy.comwww.indiandentalacademy.com

Tension zone . . .Tension zone . . .

Changes in tension zone
Stretching of periodontal fibres
Dilatation of blood vessels
Osteoblastic activity
Force
Bone formation

Force
Tension
zone
Bone
formation
Osteoblastic
activity

Pressure-Tension hypothesis...Pressure-Tension hypothesis...
reconsidered.reconsidered.

PDL is a continuous hydrostatic system with 3
distinct fluid compartments:
a. cells of PDL
b. vascular & lymph channels
c. interstitial fluids
Pressure – Tension hypothesis reconsideredPressure – Tension hypothesis reconsidered44
In keeping with Pascal’s Law, any force would be
distributed evenly throughout the system.
Bien

Experiments to
disprove :
Pressure – Tension hypothesis reconsidered
Systemically administered lathyritic agents to rats
By Nanda & Heller:
They disrupt collagen metabolism & function
Histological response of alveolar bone to orthodontic
force normal

Experiments to disprove
Pressure – Tension hypothesis reconsidered 5
Studied the rates of cell proloferation & collagen
metabolism
By Baumrind:
No striking difference b/w tension & pressure sites
Crown of the 1st
molar displaced 10 times more
than the reduction in PDL width.
proposed an alternate hypothesis

PDL is viscous & rubbery rather than watery.6
No objective evidence for the “squeezing out”
of tissue fluids on pressure side
PDL is a continuous system. Fluid if squeezed
out in one area will squeeze out from other
areas too.
Few highlights . . .

In accordance with universally operating physical
laws, each of the 3 structure, is deformed.
The amount of deformation produced is a
function of elastic property of the material.
The elastic property of the teeth is not been
studied.
Of the other 2 materials, bone deforms far more
readily than the PDL.7
The alternative hypothesis . . .

Bioelectric TheoryBioelectric Theory

Force
Bone bending
Piezoelectric current
Cell signal
Cell activation
Tooth movement
Bone remodeling
TheThe
concept
concept

BONE BENDING AND PEIZOELECTRIC THEORY:
* Phenomenon observed in many crystalline materials.
* Deformation of crystals produces a flow of electric
current.
* When a force is applied to a crystalline
structure (like bone or collagen), a flow of
current is produced that quickly dies away.
* When the force is released an opposite
current flow is observed.
* The piezoelectric effect results from
migration of electrons within the crystal
lattice.

PiezoelectricityPiezoelectricity 22
2 properties
Quick decay though force is maintained
Produce equal & opp. signal on force
release
Sustained force or Rhythmic force

Is pressure zone a pressure zone?

Tension zone in
alv. bone
Compression
Zone in alv. bone
Demonstrations of Epker & FrostDemonstrations of Epker & Frost 88

Convergence of the
two theories.

Biologic Pathways9
Orthodontic forces
Bone bending Tissue injury
Piezoelectricity PGs
Matrix charge
polarization
Os.clast-os.blast
cAMP
Inflammation
Hydrolytic enzyme
Collagenase
RemodellingRemodelling
Pressure-Tension

Cell

Cell . . .
Mitochondria
Nucleus & nucleolus
Granular & smooth ER
Centrioles
Microfilaments
Microtubules
Cell membrane
Cytoplasm
Lysosomes
Ribosomes

The molecularThe molecular
mechanism . . .mechanism . . .

Arachidonic acid
Prostaglandins
Leukotrienes
Cyclic nucleotides
Cytokines

• FORCE
• E C Matrix
• Cell membrane
• Cytoplasm
• Signals
NUCLEUS

ECM
Cell . . .
Changes inChanges in
ECMECM
membranemembrane
organellesorganelles
Nuclear changesNuclear changes
StimulusStimulus

What is ECM . . .

Cell membrane . . .
IntegrinsIntegrins
PhospholipidsPhospholipids
Channel proteinsChannel proteins

MEMBRANE PHOSPHOLIPID
ARACHIDONIC ACID
PHOSPHOLIPASE A
MECHANICAL
/ ELECTRIC
STIMULI
COX LOX
PROSTAGLANDINS LEUKOTRIENES
AA pathway simplified . . .AA pathway simplified . . .
12,14

PROSTAGLANDINS

Prostaglandins . . .
Discovered by Von Euler in 1934 as a product of
prostate gland
Produced by most cells including PDL cells
Vasodilatation, inflammation , metabolic reactions,
promotes Ca+ mobilization from bone . . .
Stimulate osteoclastic bone resorption

Prostaglandins 14
. . .
action is mediated by cyclic nucleotides intracellularly
acts on adenylate cyclase, a membrane bound enzyme
PGE2 participates in orthodontic tooth movement
studies reveals that they cause bone formation too.

Enter the cell . . .

2 pathways
cAMP
Phosphoinositide

Cyclic AMP. . . as 2nd
messenger
REF: U Sathyanarayana . Biochemistry ; Books & Allied (P) Ltd.
second messenger of intracellular signalling
discovered by Earl Sutherland (noble prize)
membrane bound adenylate cyclase coverts ATP
to cAMP.
consists of adenine, ribose & a phosphate

Shape change . . .
REF: J.R.Sandy et al. Recent advances in under standing mechanically induced bone remodeling & their relevance to orthodontic theory &
practice.AJO 1993;103:212-222
metabolic activity of a cell is also related to its shape
change
`phorbol esters cause rounding of fibroblasts leading
to altered gene expression, causing increased
collagenase & decreased collagen.
PGs & PTH induce changes in microfilament system
REF:J.R.Sandy.Tooth eruption & orthodontic movement. Br Dent J 1992:172;141-149
TGF-ß induces shape change in osteoblasts
(alkaline phosphatase activity)

Shape change . . .
REF: J.R.Sandy et al. Recent advances in under standing mechanically induced bone remodeling & their relevance to orthodontic theory &
practice.AJO 1993;103:212-222
mechanical force causes changes in cytoskeleton
by reducing tubulin & thus mediates mechanical stress
pressure sites has rounded cells & have catabolic
effects
tension sites has flattened cells & have synthetic effect
REF:J.R.Sandy.Tooth eruption & orthodontic movement. Br Dent J 1992:172;141-149
flattened cells synthesize more DNA than
rounded cells

Summary of events

3 messengers.3 messengers.
REF:Lee W.Graber.Orthodontics State of the Art Essence of the Science:Mosby; !986;Pg.101-102
2 steps;2 steps;
ExtracellularExtracellular
signallingsignalling
IntracellularIntracellular
signallingsignalling
Electric / chemical / bothElectric / chemical / both
cAMP & CalciumcAMP & Calcium
1
2
Enzymes activated byEnzymes activated by
Protein kinasesProtein kinases
3

Influencing factors . . .

Factors affecting
tooth movement . . .
Heat
Vitamins
Drugs
Hormones

Heat 18
helical coil springs were used to separate incisors
of white rabbits & subjected to heat
incisors were seperated a distance nearly twice
that of the controls
osteoclastic & osteoblastic activity was more in bone
disorganization of PDL fibres was more on pressure
side in heated animals
locally applied heat increases rate of tooth movement

Vitamins

Vitamin C
REF: StephenF.Litton Orthodontic tooth movement during an ascobic acid deficiency.AJO 1974 65;290-302
essential for hydroylation of proline & lysine
hydroxyproline & hydroxylysine is essential for
collagen cross linking & fiber strength
hydroxyproline & hydroxylysine is essential for
collagen cross linking & fiber strength
Deficiency reduces rates of tooth movement

Hormone
s

Calcitriol
REF: T.Takano-Yamamoto et al.Effect of rate of tooth movement with local use of 1,25(OH)2D3;J DentRes71(8);1487-1492,Aug,1992
Rapid appearance of multinucleated osteoclasts
on the compression side
no lag phase
faster tooth movement
physiologically active form of Vitamin D
stimulates Ca uptake by osteoblasts & promotes
remodeling

REF: Gianelly A.a & Schnur,R.M : Parathyroid hormone effects on Orthodontic tooth movement (Abstract).AJO 1971 50;A259
Parathormone
PTH causes demineralization of bone by
osteoclasts
function is to elevate serum Ca
stimulates the enzymes pyrophosphatase
& collagenase
rat experiments reveals that local use of PTH prior
to force application enhances tooth movement

Drugs
Biphosphonates
Corticosteroids
NSAIDs

biphosphanates inhibits osteoclast – mediated bone
formation
osteoporesis is a problem in post menopausal
females & aged persons of both genders
physician consultation to switch over to estrogen
therapy in older women is required
REF: William R. Profitt. Contemporary Orthodontics ; 3rd
ed.Pg.300-301
Biphosphonates are used in the treatment of
osteoporesis
Biphosphonates

REF: Lin Liu et al ; Effects of local administartion of clodronate on orthodontic tooth movement & root resorption in rats;EJO 26(2004)469-473
Clodronate is an anti-resorptive & anti – inflammatory
drug used in the treatment of metabolic bone disease
suppress signs of inflammation , inhibits production
& release of cytokines & PGs in osteoblasts
local use caused reduction in tooth movement
less osteoblasts in injected site
ongoing reasearch on its beneficial effects
Biphosphonates

used in treatment of asthma, arthritis,
and renal transplantations16
Corticosteroids
reduce PGs synthesis by inhibiting
production of arachidonic acid
REF: William R. Profitt. Contemporary Orthodontics ; 3rd
ed.Pg.300-301

NSAIDs
Aspirin
routinely used in cardiac patients
prophylactic drug in aged persons to prevent
cardiac arrest
aspirin inhibits the conversion of arachidonic acid
to prostaglandins
slows the rate of orthodontic tooth movement

Indomethacin19
REF:
PGs plays an important role in bone resorption
during orthodontic tooth movement
indomethacin is a PG inhibitor used in arthritis
inhibits prostaglandin synthetase, collagenase
& phopsphodiesterase
oral administration of indomethacin in experimental
animals slowed the rate of tooth movement
NSAIDs

Miscellaneous
Tricyclic anti-depressents
Anti-arrhythmic agents
Anti-malarial drugs
Methyl xanthines
Anti-convulsant drugs
Doxycycline
PGsPGs
OsteoClastsOsteoClasts

“The two theories are neither incompatible
nor
mutually exclusive. From a contemporary
perspective, it appears that both mechanisms
may
play a part in the biologic control of
tooth movement.” William R. Profitt

The application of a force to a tooth can stimulateThe application of a force to a tooth can stimulate
the process of alveolar bone resorption by creatingthe process of alveolar bone resorption by creating
areas of pressure in the attachment apparatus.areas of pressure in the attachment apparatus.
The cellular mechanisms are the same even whenThe cellular mechanisms are the same even when
there are two forms of resorption (frontal andthere are two forms of resorption (frontal and
undermining) which have been described andundermining) which have been described and
related, in part, to the magnitude of the appliedrelated, in part, to the magnitude of the applied
force.force.11

Functions of PDLFunctions of PDL
 PhysicalPhysical
 Formative & remodellingFormative & remodelling
 Nutritional & sensoryNutritional & sensory

 Hour glass appearance thinnest in midHour glass appearance thinnest in mid
root portionroot portion
 This suggests that middle portion is theThis suggests that middle portion is the
fulcrum of physiologic tooth movementfulcrum of physiologic tooth movement

Alveolar processAlveolar process
 It is the portion of maxilla & mandibleIt is the portion of maxilla & mandible
that forms and supports the tooththat forms and supports the tooth
socketssockets
 It consists of ---alveolar bone properIt consists of ---alveolar bone proper
(cribriform plate or lamina dura )(cribriform plate or lamina dura )
--external plate of cortical bone--external plate of cortical bone
--spongy /cancellous bone filled--spongy /cancellous bone filled
between cribriform &cortical platebetween cribriform &cortical plate

 CompositionComposition
Inorganic –65%Inorganic –65%
Organic -35%Organic -35%
In organicIn organic
Collagen- 90%Collagen- 90%
Non collagenous proteins likeNon collagenous proteins like
Osteocalcin, osteonectin,Osteocalcin, osteonectin,
Bone morphogenetic-protein- 10 %Bone morphogenetic-protein- 10 %
Phosphoproteins, ProteoglycansPhosphoproteins, Proteoglycans
Lipids - 0.4%Lipids - 0.4%www.indiandentalacademy.comwww.indiandentalacademy.com

 Bone marrow –Bone marrow –
Common locations - maxillary tuberosity,andCommon locations - maxillary tuberosity,and
maxillary and mandibular molar & premolar areamaxillary and mandibular molar & premolar area
(these areas may be visible radiographically as zones of radiolucency)(these areas may be visible radiographically as zones of radiolucency)
 Periosteum & Endosteum –Periosteum has aPeriosteum & Endosteum –Periosteum has a
inner layer composed of cells that haveinner layer composed of cells that have
potential to differentiate into osteoblasts &potential to differentiate into osteoblasts &
outer layer rich in blood vessels, nerves andouter layer rich in blood vessels, nerves and
fibersfibers
 Bundles of periosteal collagen fibers penetrateBundles of periosteal collagen fibers penetrate
the bone binding periosteum to bonethe bone binding periosteum to bone
 Endosteum is composed of single layer ofEndosteum is composed of single layer of
osteoprogenitor cells and connective tissueosteoprogenitor cells and connective tissuewww.indiandentalacademy.comwww.indiandentalacademy.com

CementumCementum
 It is a mineralized dental tissue.It is a mineralized dental tissue.
 Covers the anatomic roots of human teeth.Covers the anatomic roots of human teeth.
 It furnishes [provides] a medium for the
attachment of collagen fibers which binds the
tooth to surrounding structures
 It is a specialized connective tissue that shares
some physical ,chemical and structural
characteristics with compact bone ,unlike bone
,cementum is avascular

Cementum
 Cementum is thinnest at cementoenamel
junction - 20-50 mu &
 Thickest at the apex 150-200 mu.
 More resistant to resorption than bone ,
 For this reason the orthodontic tooth
movement is possible.
 The difference in resistance of bone and
cementum to pressure may be caused by the
fact that bone is richly vascularized

Physiologic Tooth MovementPhysiologic Tooth Movement
 It designates slight tipping of functioningIt designates slight tipping of functioning
teeth in their socket and also the changes inteeth in their socket and also the changes in
tooth position that occur in young personstooth position that occur in young persons
during and after tooth eruptionduring and after tooth eruption
 New tissue deposited during tooth migrationNew tissue deposited during tooth migration
represents various stages of calcificationrepresents various stages of calcification
* osteoid* osteoid
** Bundle boneBundle bone
** Lamellated boneLamellated bone

 Osteoid –Osteoid –
Appears as white line or outgrowth ,it isAppears as white line or outgrowth ,it is
uncalcified and not resorbed byuncalcified and not resorbed by
osteoclastsosteoclasts

 Bundle bone –Bundle bone –
Newly calcified tissue ,as well as ofNewly calcified tissue ,as well as of
longer existence .longer existence .
It is basophilic ,it is characterized byIt is basophilic ,it is characterized by
scarcity of fibrils in the intercellularscarcity of fibrils in the intercellular
substance therfore it appears dark insubstance therfore it appears dark in
routine hematoxylin & eosin stains .routine hematoxylin & eosin stains .

 Lamellated bone –Lamellated bone –
Cells & fiber bundles get incorporated inCells & fiber bundles get incorporated in
bundle bone during its life cycle .bundle bone during its life cycle .
When it has reached a certain thicknessWhen it has reached a certain thickness
and maturity , parts of the bundle boneand maturity , parts of the bundle bone
will be reorganised into lamellated bone.will be reorganised into lamellated bone.

Fills gap between fibers and cells
Two main components:
1 Glycoaminoglycans :
Hyaluronic acid and
proteoglycans.
2 Glycoproteins: Fibronectin and leminin
3 High water content
GROUND SUBSTANCEGROUND SUBSTANCE

On application of even greater forceOn application of even greater force
levelslevels
There is physical contact between teethThere is physical contact between teeth
and bone.and bone.
Leading to –Leading to –
1.Resorption in areas of pressure &1.Resorption in areas of pressure &
2.Undermining resorption or2.Undermining resorption or
hyalinization inhyalinization in
adjacent marrow spaces.adjacent marrow spaces.

Histologic Studies ofHistologic Studies of
Periodontium.Periodontium.
Postulates that the width changes in thePostulates that the width changes in the
PDL cause changes in cell population.PDL cause changes in cell population.
There is increases in cellular activity.There is increases in cellular activity.
There is an apparent disruption ofThere is an apparent disruption of
collagen fibers in the PDL.collagen fibers in the PDL.
Evidence of cell and tissue damage.Evidence of cell and tissue damage.

Hyalinization is seen.
There is the presence of
pyknotic nuclei in cells,
followed by areas of acellularity, or
cell-free zones.

undermining resorptionundermining resorption
The problem is resolved when cellular elements
such as macrophages, foreign body giant cells &
osteoclasts invade the necrotic tissue.
They invade from adjacent undamaged areas.
These cells resorb the underside of bone just
adjacent to the necrotic PDL area.
Remove it together with the necrotic tissue.
This process is undermining resorption

frontal resorptionfrontal resorption
Osteoclasts line up in the margin of the alveolarOsteoclasts line up in the margin of the alveolar
bone adjacent to the compressed PDL, &bone adjacent to the compressed PDL, &
produce direct bone resorption.produce direct bone resorption.
This is known asThis is known as frontal resorptionfrontal resorption

According to authors:
When an orthodontic appliance is activated, forces
delivered to the tooth are transmitted to all
tissues near force application.
These forces bend bone, tooth, and the solid
structures of the PDL.
Bone was found to be more elastic than the other
tissues .
Bends far more readily in response to force
application.

The active biologic processes thatThe active biologic processes that
follow bone bending involve:follow bone bending involve:
Bone turnover &Bone turnover &
Renewal of cellular and inorganicRenewal of cellular and inorganic
fractions.fractions.
These processes are accelerated whileThese processes are accelerated while
thethe
bone is held in the deformed position.bone is held in the deformed position.

Authors further stated that:Authors further stated that:
““reorganization proceeds not only at thereorganization proceeds not only at the
lamina dura of the alveolus, but alsolamina dura of the alveolus, but also onon
thethe
surface of every trabaculumsurface of every trabaculum within thewithin the
corpus of bone.”corpus of bone.”
The force delivered to the tooth isThe force delivered to the tooth is
dissipateddissipated
throughout the bone by development ofthroughout the bone by development ofwww.indiandentalacademy.comwww.indiandentalacademy.com

Further force application becomes aFurther force application becomes a
stimulus for altered biological responses ofstimulus for altered biological responses of
cells lying perpendicular to the stress lines.cells lying perpendicular to the stress lines.
The altered activity of cells in turn modifiesThe altered activity of cells in turn modifies
the shape and internal organization of bone,the shape and internal organization of bone,
to accommodate the exogenous forcesto accommodate the exogenous forces
acting on it.acting on it.

With this this theory, & fromWith this this theory, & from
Wolff’sWolff’s
law,these authors could explainlaw,these authors could explain
certain facts.certain facts.

1-The relative slowness of en-masse tooth1-The relative slowness of en-masse tooth
movementmovement
{Here much bone flexion is needed for the rapidity of{Here much bone flexion is needed for the rapidity of
alignment of crowded teeth, and when thinness makesalignment of crowded teeth, and when thinness makes
bone flexion easier}.bone flexion easier}.
2- The rapidity of tooth movement toward an extraction2- The rapidity of tooth movement toward an extraction
site &site &
3-The relative rapidity of tooth movement in children,3-The relative rapidity of tooth movement in children,
who have less heavily calcified and more flexiblewho have less heavily calcified and more flexible
bones than adults.bones than adults.

Zengo et al (1974),Bassett and BeckerZengo et al (1974),Bassett and Becker
(1962)(1962)
& Pollack et al (1984) demonstrated that& Pollack et al (1984) demonstrated that
orthodontic canine tipping, bends theorthodontic canine tipping, bends the
alveolaralveolar
Bone.creating on it concave and convexBone.creating on it concave and convex
surfaces identical to those generated insurfaces identical to those generated in
bentbent
long bones.long bones.

In areas of PDL tension, the interfacingIn areas of PDL tension, the interfacing
bone surface assumes a concavebone surface assumes a concave
configuration.configuration.
Here the molecules are compressed.Here the molecules are compressed.
In zones of compressed PDL, the adjacentIn zones of compressed PDL, the adjacent
alveolar bone surface becomes convex.alveolar bone surface becomes convex.
There is no contradiction between theThere is no contradiction between the
response of alveolar bone and other partsresponse of alveolar bone and other parts
ofof
the skeleton to mechanical loading.the skeleton to mechanical loading.

There lies some confusion due toThere lies some confusion due to
usage of same descriptions forusage of same descriptions for
different tissues.different tissues.
Orthodontic tension refers to the PDL,Orthodontic tension refers to the PDL,
an orthopedist might say that thean orthopedist might say that the
area is under compression, becausearea is under compression, because
the bone near the stretched PDL hasthe bone near the stretched PDL has
become concave.become concave.

Bioelectric signals inBioelectric signals in
orthodontic toothorthodontic tooth
movementmovement
Bassett and Becker (1962) proposed that, inBassett and Becker (1962) proposed that, in
response to applied mechanical forces,response to applied mechanical forces,
there is generation of electric potentials inthere is generation of electric potentials in
the stressed tissues.the stressed tissues.
These potentials might chargeThese potentials might charge
macromolecules that interact with specificmacromolecules that interact with specific
sites in cell membranes or mobilize ionssites in cell membranes or mobilize ions
across cell membranes.across cell membranes.

Zengo et al (1974) measured theZengo et al (1974) measured the
electric potential in mechanicallyelectric potential in mechanically
stressed dog alveolar bone during in-stressed dog alveolar bone during in-
vivo and in-vitro experiments.vivo and in-vitro experiments.

It has been proposed by Davidovitch etIt has been proposed by Davidovitch et
al that a physical relationship existsal that a physical relationship exists
between mechanical and electricalbetween mechanical and electrical
perturbation of bone.perturbation of bone.

Bending of bone causes 2 classes ofBending of bone causes 2 classes of
stress-generated electrical effects.stress-generated electrical effects.
Their experiments with exogenousTheir experiments with exogenous
electrical currents in conjunction withelectrical currents in conjunction with
orthodontic forces demonstrated :orthodontic forces demonstrated :
-enhanced cellular activities in the PDL-enhanced cellular activities in the PDL
-alveolar bone, as well as rapid tooth-alveolar bone, as well as rapid tooth
movement.movement.

These findings suggest that bioelectricThese findings suggest that bioelectric
responses (piezoelectricity andresponses (piezoelectricity and
streaming potentials) which arestreaming potentials) which are
propagated by bone bending in relationpropagated by bone bending in relation
to orthodontic force application mightto orthodontic force application might
function as pivotal cellular firstfunction as pivotal cellular first
messengers.messengers.

Piezoelectricity is a phenomenon observedPiezoelectricity is a phenomenon observed
in many crystalline materials, in which ain many crystalline materials, in which a
deformation of a crystal structure producesdeformation of a crystal structure produces
a flow of electric current as electrons area flow of electric current as electrons are
displaced from 1 part of the lattice todisplaced from 1 part of the lattice to
another.another.

The 2 unusual properties of piezoelectricity, whichThe 2 unusual properties of piezoelectricity, which
seem to not correlate well with orthodontic toothseem to not correlate well with orthodontic tooth
movement are:movement are:
1-A quick decay rate,1-A quick decay rate,
(where the electron transfer from 1 area to another, after force application(where the electron transfer from 1 area to another, after force application
reverts back when the force is removed This is not desirable once orthodonticreverts back when the force is removed This is not desirable once orthodontic
treatment is over.)treatment is over.)
22-Production of an equivalent signal in the opposite-Production of an equivalent signal in the opposite
direction upon force removal.direction upon force removal.

When mechanical forces are applied, theseWhen mechanical forces are applied, these
respond concomitantly, resulting in tissuerespond concomitantly, resulting in tissue
remodeling:remodeling:
Cells &Cells &
Extracellular matrix of the PDL and alveolarExtracellular matrix of the PDL and alveolar
bone.bone.
At early phases of tooth movement, PDL fluids areAt early phases of tooth movement, PDL fluids are
shifted.shifted.
Produces:Produces:
Cell and matrix distortions.Cell and matrix distortions.
Interactions between these tissueInteractions between these tissue

In response to these physicochemicalIn response to these physicochemical
events and interactions,there is release of-events and interactions,there is release of-
Cytokines,Cytokines,
Growth factors,Growth factors,
Colony-stimulating factors,&Colony-stimulating factors,&
Vasoactive neurotransmitters.Vasoactive neurotransmitters.
This initiates and sustains the remodelingThis initiates and sustains the remodeling
activity.activity.
This facilitates tooth movement.This facilitates tooth movement.

PHASES OF TOOTHPHASES OF TOOTH
MOVEMENTMOVEMENT
Burstone (1962)Burstone (1962)
If the rates of tooth movement wereIf the rates of tooth movement were
plotted against time, there wouldplotted against time, there would
be 3 phases of tooth movement—be 3 phases of tooth movement—
Initial phase,Initial phase,
Lag phase, andLag phase, and
Postlag phase.Postlag phase.

Initial phaseInitial phase
Characterized by rapid movement or toothCharacterized by rapid movement or tooth
Occurs immediately after the application of forceOccurs immediately after the application of force
This rate can be largely due to the displacement of theThis rate can be largely due to the displacement of the
tooth in the PDL space.tooth in the PDL space.

LAG PHASELAG PHASE
Immediately after the initial phase, there is a lagImmediately after the initial phase, there is a lag
period, with relatively low rates of tooth displacementperiod, with relatively low rates of tooth displacement
or no displacement. It has been suggested that the lagor no displacement. It has been suggested that the lag
isis
produced by hyalinization of the PDL in areas ofproduced by hyalinization of the PDL in areas of
compression. No further tooth movement occurs untilcompression. No further tooth movement occurs until
cells complete the removal of all necrotic tissues.cells complete the removal of all necrotic tissues.

postlag phasepostlag phase
 The third phase of tooth movementThe third phase of tooth movement
follows the lag period.follows the lag period.
 The rate of movement gradually orThe rate of movement gradually or
suddenly increases.suddenly increases.

Recent studies have proposed a newRecent studies have proposed a new
time/displacement model for toothtime/displacement model for tooth
movement.movement.
Beagles.Beagles.
Divided the curve of tooth movementDivided the curve of tooth movement
intointo 4 phases4 phases..
The first phase lasts 24 hours to 2The first phase lasts 24 hours to 2
days and represents the initialdays and represents the initial
movement of the tooth inside itsmovement of the tooth inside its
bony

Followed by a second phase:Followed by a second phase:
Tooth movement stops for 20 to 30Tooth movement stops for 20 to 30
days.days.
After theAfter the removal of necrotic tissueremoval of necrotic tissue formedformed
during the second phase, tooth movementduring the second phase, tooth movement
is accelerated in the third phase andis accelerated in the third phase and
continues into the fourth phase.continues into the fourth phase.

The third and fourth phases compriseThe third and fourth phases comprise
most of the total tooth movementmost of the total tooth movement
during orthodontic treatment.during orthodontic treatment.
Cellular and tissue reactions start inCellular and tissue reactions start in
the initial phase of tooth movement,the initial phase of tooth movement,
immediately after force application.immediately after force application.

By the compression and stretch of fibersBy the compression and stretch of fibers
and cells in PDL pressure and tensionand cells in PDL pressure and tension
areas,areas,
respectively,the complex process ofrespectively,the complex process of
recruitment of osteoclast and osteoblastrecruitment of osteoclast and osteoblast
progenitors, as well as extravasation andprogenitors, as well as extravasation and
chemoattraction of inflammatory cells,chemoattraction of inflammatory cells,
begins.begins.

In the second phase, areas of compression areIn the second phase, areas of compression are
easilyeasily
recognized by the distorted appearance of therecognized by the distorted appearance of the
normalnormal
PDL fiber arrangement. The disruption in bloodPDL fiber arrangement. The disruption in blood
flowflow
due to this distortion leads to the developmentdue to this distortion leads to the development
ofof
hyalinized areas and the arrest of toothhyalinized areas and the arrest of tooth
movement,movement,
which can last from 4 to 20 days.which can last from 4 to 20 days.

On removal of necrotic tissue andOn removal of necrotic tissue and
bone resorption from adjacentbone resorption from adjacent
marrow spaces (indirect resorption)marrow spaces (indirect resorption)
and from the direction of the viableand from the direction of the viable
PDL (undermining resorption) allowPDL (undermining resorption) allow
the resumption of tooth movement.the resumption of tooth movement.

This process requires recruitment ofThis process requires recruitment of
phagocytic cells –phagocytic cells –
Macrophages,Macrophages,
Foreign body giant cells,Foreign body giant cells,
OsteoclastsOsteoclasts
-From adjacent undamaged areas-From adjacent undamaged areas
of the PDL.of the PDL.
-Alveolar bone marrow cavities.-Alveolar bone marrow cavities.
These cells remove necrotic tissues fromThese cells remove necrotic tissues from
compressed PDL sites and adjacent alveolar bone.compressed PDL sites and adjacent alveolar bone.

areas of PDL tensionareas of PDL tension
Here quiescent osteoblasts (bone surfaceHere quiescent osteoblasts (bone surface
lininglining
cells) are enlarged.cells) are enlarged.
Start producing new bone matrix.Start producing new bone matrix.
(osteoid)(osteoid)..
New osteoblast progenitors are recruitedNew osteoblast progenitors are recruited
from the fibroblast-like cells (pericytes)from the fibroblast-like cells (pericytes)
around PDL capillaries.around PDL capillaries.

These PreosteoblastsThese Preosteoblasts
-Proliferate-Proliferate
-Migrate-Migrate
toward the alveolar bone surface, alongtoward the alveolar bone surface, along
the stretched Sharpey’s fibers.the stretched Sharpey’s fibers.
Simultaneously, PDL fibroblasts in tensionSimultaneously, PDL fibroblasts in tension
zones begin multiplying and remodeling theirzones begin multiplying and remodeling their
surrounding matrix.surrounding matrix.

acceleration and linearacceleration and linear
phasesphases
33rdrd
& 4& 4thth
Phase.Phase.
Acceleration and Linear phases .Acceleration and Linear phases .
Start about 40 days after the initialStart about 40 days after the initial
force application.force application.
The pressure sides of teeth exhibitThe pressure sides of teeth exhibit
collagen fibers without propercollagen fibers without proper
orientation.orientation.

Here, irregular bone surfaces areHere, irregular bone surfaces are
found, indicating direct or frontalfound, indicating direct or frontal
resorption.resorption.

Some report presented data onSome report presented data on
hyalinization zones at the pressure areashyalinization zones at the pressure areas
even during this stage, especially in areaseven during this stage, especially in areas
where high forces were applied.where high forces were applied.
This finding suggests that the developmentThis finding suggests that the development
and removal of necrotic tissue is aand removal of necrotic tissue is a
continuous process during toothcontinuous process during tooth
displacement, not a single event.displacement, not a single event.

Melsen’s hypothesis supports this-Melsen’s hypothesis supports this-
““indirect bone resorption at the pressure side is notindirect bone resorption at the pressure side is not
a reaction to force but an attempt to removea reaction to force but an attempt to remove
ischemic bone lying adjacent to the hyalinizedischemic bone lying adjacent to the hyalinized
tissue.tissue.
The direct bone resorption could be considered partThe direct bone resorption could be considered part
of the remodeling process.”of the remodeling process.”
The tension sides in the third and fourth phasesThe tension sides in the third and fourth phases
clearly show bone deposition.clearly show bone deposition.

SIGNALING MOLECULES AND METABOLITES INSIGNALING MOLECULES AND METABOLITES IN
ORTHODONTIC TOOTH MOVEMENTORTHODONTIC TOOTH MOVEMENT
Early phase of orthodontic tooth movementEarly phase of orthodontic tooth movement
involves an acute inflammatory response.involves an acute inflammatory response.
Characterized by:Characterized by:
Periodontal vasodilatation and ,Periodontal vasodilatation and ,
Migration of leucocytes out of the capillaries.Migration of leucocytes out of the capillaries.
These migratory cells produce various cytokines.These migratory cells produce various cytokines.
They are local biochemical signal molecules, thatThey are local biochemical signal molecules, that
interact directly or indirectly with the entireinteract directly or indirectly with the entire
population of native paradental cells.population of native paradental cells.

Cytokines evoke the synthesis andCytokines evoke the synthesis and
secretion of numerous substances bysecretion of numerous substances by
their target cells, includingtheir target cells, including
prostaglandins, growth factors,prostaglandins, growth factors,
and cytokines.and cytokines.

These cells comprise the functionalThese cells comprise the functional
units that remodel the paradentalunits that remodel the paradental
tissues and facilitate toothtissues and facilitate tooth
movement.movement.

Arachidonic acid metabolitesArachidonic acid metabolites
Arachidonic (eicosatetraenoic) acid,Arachidonic (eicosatetraenoic) acid,
the main component of phospholipidsthe main component of phospholipids
of the cell membrane, is released dueof the cell membrane, is released due
to the action of phospholipaseto the action of phospholipase
enzymes.enzymes.

Prostaglandins in toothProstaglandins in tooth
movementmovement
Von Euler introduced this term.Von Euler introduced this term.
Yamasaki et al found an increase inYamasaki et al found an increase in
osteoclast numbers after a localosteoclast numbers after a local
injection of prostaglandins intoinjection of prostaglandins into
paradental tissues.paradental tissues.

Role of prostaglandins (PGE1 andRole of prostaglandins (PGE1 and
PGE2) in stimulating bonePGE2) in stimulating bone
resorption are identified.resorption are identified.
Direct action on osteoclasts inDirect action on osteoclasts in
increasing their numbers .increasing their numbers .

The cAMP pathwayThe cAMP pathway
Internal signaling systems are thoseInternal signaling systems are those
that translate many external stimulithat translate many external stimuli
to a narrow range of internal signals orto a narrow range of internal signals or
second messengers .second messengers .
cAMP and cGMP arecAMP and cGMP are
2 second messengers associated with bone2 second messengers associated with bone
remodeling.remodeling.

This signaling molecule plays a keyThis signaling molecule plays a key
role in synthesis of nucleic acids androle in synthesis of nucleic acids and
proteins as well as secretion ofproteins as well as secretion of
cellular products.cellular products.

The Phosphoinositide[PI] dual signaling
systems
There is another Second-messenger systemThere is another Second-messenger system
reviewed extensively in relation toreviewed extensively in relation to
orthodontic tooth movement- Phosphoinositideorthodontic tooth movement- Phosphoinositide
pathway.pathway.

This reaction in turn leads to a release ofThis reaction in turn leads to a release of
calcium ions from intracellular stores.calcium ions from intracellular stores.
Phosphorylation of inositol triphosphatePhosphorylation of inositol triphosphate
yields Ins P4.yields Ins P4.
This controlsThis controls calcium entrycalcium entry at the plasmaat the plasma
membrane through calcium channels.membrane through calcium channels.
Inositol triphosphate is a mediator ofInositol triphosphate is a mediator of
mitogenesis in mechanically deformedmitogenesis in mechanically deformed
tissues through an increase in DNAtissues through an increase in DNAwww.indiandentalacademy.comwww.indiandentalacademy.com

•The importance of the second-The importance of the second-
messenger conceptmessenger concept
to orthodonticsto orthodontics
The second-messenger hypothesis postulatesThe second-messenger hypothesis postulates
thatthat
target cells respond to external stimuli,target cells respond to external stimuli,
chemical or physicalchemical or physical,,
by enzymatic transformation of certain
membrane-bound and cytoplasmic molecules to
derivatives capable of promoting the
phosphorylation of cascades of intracellular
enzymes.

Hence , temporal increases in the tissue or cellularHence , temporal increases in the tissue or cellular
concentrations of second messengers are generallyconcentrations of second messengers are generally
viewed as evidence,viewed as evidence,
that an applied extracellular first messenger,such asthat an applied extracellular first messenger,such as
an orthodontic force, has stimulated target cells.an orthodontic force, has stimulated target cells.
There are significant elevations in the concentrationsThere are significant elevations in the concentrations
of intracellular second messengers in paradentalof intracellular second messengers in paradental
cells after exposure to appliedcells after exposure to applied
mechanical forces.mechanical forces.

Vitamin D and diacylglycerolVitamin D and diacylglycerol
An important factor in orthodontic toothAn important factor in orthodontic tooth
movement ismovement is
1, 25,dehydroxychloecalciferol (1, 25, DHCC).
A biologically active form of vitamin D.A biologically active form of vitamin D.
Has a potent role in calcium homeostasis.Has a potent role in calcium homeostasis.

Potent stimulator of bone resorption .Potent stimulator of bone resorption .
Induces differentiation of osteoclasts
from their precursors.from their precursors.
Implicated inImplicated in increasing the activity ofof
existing osteoclasts.existing osteoclasts.
Has bone-resorbing activity.Has bone-resorbing activity.
Stimulates bone mineralization andand
osteoblastic cell differentiation.osteoblastic cell differentiation.
Vitamin D and diacylglycerolVitamin D and diacylglycerol

ConclusionsConclusions
1.1. The osteoblast is now perceived as the cell thatThe osteoblast is now perceived as the cell that regulatesregulates bothboth
the formative and resorptive phases of the bone remodeling cyclethe formative and resorptive phases of the bone remodeling cycle
in response to hormonal and mechanical stimuli.in response to hormonal and mechanical stimuli.
2. To date PGs have been the only chemical mediators of orthodontic
tooth movement to have been used clinically.
Lipoxygenase products may have a similar role.Lipoxygenase products may have a similar role.
3. Cytokine production by mechanically deformed tissues may account3. Cytokine production by mechanically deformed tissues may account
for many cellular effects associated with orthodontic toothfor many cellular effects associated with orthodontic tooth
movement.movement.

4. Second messenger involvement in orthodontic4. Second messenger involvement in orthodontic
tooth movement is unlikely to be restricted totooth movement is unlikely to be restricted to
cAMP.cAMP.
The phosphatidy-lino-sitol pathway is likely toThe phosphatidy-lino-sitol pathway is likely to
account for a number of cellular events seen inaccount for a number of cellular events seen in
mechanically deformed tissues.mechanically deformed tissues.
5. Cytoskeletal matrix interactions associated5. Cytoskeletal matrix interactions associated
with a change in cell shape trigger a series ofwith a change in cell shape trigger a series of
cell responses that are highly relevant tocell responses that are highly relevant to
orthodontic tooth movement.orthodontic tooth movement.

1. K Utley, Activity of alveolar bone incident to orthodontic tooth movement; AJO 1968; Mar; Pg.167-201 ..
2. William R. Profitt. Contemporary Orthodontics ; 3rd ed.Pg.298-301
3. A. Gianelly, Force induced changes in vascularity of PDL .AJODO 1969;Jan;pg.5-11
4. J.R. Sandy.Tooth eruption & orthodontic movement. Br Dent J 1992:172;141-149.
5. J.R. Sandy et al. Recent advances in understanding mechanically induced bone remodeling & their relevance to
orthodontic theory & practice ;AJO 1993;103:212-222.
6. Sheldon Baumrind. A reconsideration of the propriety of the pressure-tension hypothesis; AJO-DO;Jan;1969.
7. S. Baumrind. A reconsideration of the propriety of the pressure-tension hypothesis; AJO-;Jan;1969.
8. Fred M. Grimm. Bone bending, a feature of orthodontic tooth movement AJO-DO;vol.62;No.4;1972.
9. Yehya A.Mostafa. Orchestration of tooth movement; AJODO,1983;March:245-250.
10. Graber, T. M.: Orthodontic principles and practice, Philadelphia, 1961, W. B. Saunders Company, pp. 405-438.
11. Vinod Krishnana and Ze’ev Davidovitch ,Cellular, molecular, and tissue-level reactions to orthodontic
force,AJODO,2006;469e.1-469e.32.
12. R. Sandy. Recent adv. in understanding mechanically induced bone remodeling & their relevance to ortho theory &
practice. AJODO ,1993;103:212-22.
13. Masella ,Meister, Current concepts in the biology of orthodontic tooth movement,AJODO,2006;129:458-468.
14. K D Tripathi. Essentials of medical pharmacology.5th ed; Jaypee ..
15. U Sathyanarayana . Biochemistry ; Books & Allied (P) Ltd.
16. Kalia Melson Verna. Tissue reaction to orthodontic tooth movement in acute & chronic corticosteroid treatment;
17. Orthod Craniofacial Research 7,2004/26-34.
18. Jack A. Tweedle & Roy E. Bundy. Effect of local heat on tooth movement .AO 1965 Vol. 35 ,No.3, 218-225.
19. Brent Chumbley and Orban C.Tuncay. The effect of indomethacin(an aspirin-like drug) on the rate of orthodontic
BIBLIOGAPHY

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Biology Of Tooth Movement: Understanding The Cellular Responses That Facilitate Orthodontic Force

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