BIOLOGY OF BIOLOGY OF TOOTH MOVEMENTTOOTH MOVEMENT

By:By:

Dr shabeel pnDr shabeel pn

IntroductionIntroductionOrthodontic tooth movement is a Orthodontic tooth movement is a

uniqueunique process where a solid process where a solid object (tooth) is made to move object (tooth) is made to move

through a solid medium (bone). through a solid medium (bone).

Orthodontic treatment is possible Orthodontic treatment is possible due to the fact that due to the fact that whenever a whenever a

prolonged force is applied on a prolonged force is applied on a tooth,bone remodelling occurs tooth,bone remodelling occurs around the tooth resulting in around the tooth resulting in

its movementits movement..

Bone subject to Bone subject to pressurepressure as a as a result of result of compressioncompression of of periodontal ligament periodontal ligament resorbsresorbs. . While, bone While, bone formsforms under under tensiletensile force, as a result of force, as a result of stretchingstretching of of periodontal ligament.The bony periodontal ligament.The bony response is mediated by the response is mediated by the periodontal ligamentperiodontal ligament, Tooth , Tooth movement is primarily a movement is primarily a periodontal ligament phenomenon.periodontal ligament phenomenon.

Structure of periodontal Structure of periodontal ligament:ligament:

The The PDLPDL occupies a space approximately occupies a space approximately 0.5mm 0.5mm in width around all parts of in width around all parts of roots. roots.

Major component of the ligament are:-Major component of the ligament are:-

1.1. Network of parallel Network of parallel collagenous fiberscollagenous fibers..

2.2. Cellular elementsCellular elements along with vascular along with vascular and neural elements. and neural elements.

3.3. Tissue fluidsTissue fluids. . The cellular element and fluid play an The cellular element and fluid play an

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

I.Physiologic Tooth I.Physiologic Tooth MovementMovement

Naturally occurringNaturally occurring tooth tooth movements that take place during movements that take place during and after tooth eruptionand after tooth eruption. .

This include:This include: A)A)Tooth EruptionTooth Eruption.. B)B)MigrationMigration or drift of teeth. or drift of teeth. C)C)ChangesChanges in tooth position in tooth position during during

masticationmastication. .

TOOTH ERUPTION:TOOTH ERUPTION: Tooth eruption is the Tooth eruption is the axial movementaxial movement of of

tooth from its development position in the tooth from its development position in the jaw to its final position in the oral cavity.jaw to its final position in the oral cavity.

The following are some theories which The following are some theories which explains the eruption process.explains the eruption process.

a)a)Blood pressure theoryBlood pressure theory::

According to this theory,the According to this theory,the tissue around the developing end of the tissue around the developing end of the root is highly vascular.This vascular root is highly vascular.This vascular pressure is believed to cause the axial pressure is believed to cause the axial movement of teeth.movement of teeth.

b)Root Growthb)Root Growth:: According to this theory,the apical According to this theory,the apical

growth of roots result in an axially directed force growth of roots result in an axially directed force that brings about the eruption of teeth.that brings about the eruption of teeth.

This theory was rejected because:This theory was rejected because: The tooth moves at a greater distance than the root The tooth moves at a greater distance than the root

length.length. Onset of root growth and eruption do not coincide.Onset of root growth and eruption do not coincide. Teeth without roots also erupt.Teeth without roots also erupt.

c)Hammock ligament theory:c)Hammock ligament theory: According to Sicher, a band of fibrous According to Sicher, a band of fibrous

tissue exists below the root apex spanning from one tissue exists below the root apex spanning from one side of alveolar wall to other. This fibrous tissue side of alveolar wall to other. This fibrous tissue appears to form a network below the developing appears to form a network below the developing root and is rich in fluid droplets.the developing root root and is rich in fluid droplets.the developing root forces itself against this band of tissue, which in forces itself against this band of tissue, which in turn applies an occlusally directed force on tooth.turn applies an occlusally directed force on tooth.

d)Periodontal traction theory:d)Periodontal traction theory:

This theory states that This theory states that the periodontal ligament is rich the periodontal ligament is rich in fibroblasts that contain in fibroblasts that contain contractile tissue.The contractile tissue.The contraction of these periodontal contraction of these periodontal fibers (mainly the oblique group fibers (mainly the oblique group of fibers) results in axial of fibers) results in axial movement of the tooth.movement of the tooth.

B) B) Migration or drift of Migration or drift of teeth:teeth: Refers to the minor changes in tooth Refers to the minor changes in tooth

position observed after eruption .position observed after eruption . Human dentition shows a natural Human dentition shows a natural

tendency to move in a mesial & tendency to move in a mesial & occlusal direction.occlusal direction.

Usually a result of proximal and Usually a result of proximal and occlusal wear of teeth,occlusal wear of teeth,

They move in a mesial and occlusal They move in a mesial and occlusal direction to maintain inter-proximal direction to maintain inter-proximal and occlusal contact.and occlusal contact.

C)C)Tooth movement during Tooth movement during mastication:mastication:

During mastication ,the teeth During mastication ,the teeth and PDL structures are and PDL structures are subjected to intermittent heavy subjected to intermittent heavy forces which occurs in cycles of forces which occurs in cycles of one second or less and may one second or less and may range from 1-50 kg based on range from 1-50 kg based on the type of food being the type of food being masticated.masticated.

Physiologic response to heavy Physiologic response to heavy pressure against a toothpressure against a tooth:-:-

<1 sec – PDL fluid incompressible, <1 sec – PDL fluid incompressible, alveolar alveolar

bone bends, piezoelectric bone bends, piezoelectric effect.effect.

1-2 sec – PDL fluid compressed, tooth 1-2 sec – PDL fluid compressed, tooth moves moves

within the PDL space.within the PDL space. 3-5 sec – PDL fluid squeezed out, tissue 3-5 sec – PDL fluid squeezed out, tissue

compressed, immediate pain.compressed, immediate pain.

II.Orthodontic Tooth II.Orthodontic Tooth MovementMovement

It is a pathological process from which It is a pathological process from which the tissue recovers.the tissue recovers.

Histology of tooth movement:Histology of tooth movement: Orthodontic movement bring about Orthodontic movement bring about

areas of pressure and tension around areas of pressure and tension around the tooth. The histologic changes seen the tooth. The histologic changes seen during tooth movement vary according during tooth movement vary according to the amount and duration of force to the amount and duration of force applied. applied.

Changes following Changes following application of mild forces:application of mild forces:

Classically the movement of Classically the movement of teeth has been explained via the teeth has been explained via the pressure:tension hypothesis in pressure:tension hypothesis in which PDL tissues in pressure which PDL tissues in pressure side results in bone resurption , side results in bone resurption , while placing the PDL tissues while placing the PDL tissues under tensile force lead to bone under tensile force lead to bone deposition. deposition.

Changes on pressure side:Changes on pressure side: The PDL in direction of tooth movement The PDL in direction of tooth movement

gets compressed to almost 1/3rd of it’s gets compressed to almost 1/3rd of it’s original thickness. original thickness.

A marked increase in the vascularity of PDL A marked increase in the vascularity of PDL on this side is observed due to increase in on this side is observed due to increase in capillary blood supply. capillary blood supply.

This increase in blood supply helps in This increase in blood supply helps in mobilization of cells such as fibroblasts and mobilization of cells such as fibroblasts and osteoclasts. osteoclasts.

Osteoclasts are bone resorbing cells that lie Osteoclasts are bone resorbing cells that lie in Howship’s lacunae .in Howship’s lacunae .

when forces applied are within physiologic when forces applied are within physiologic limits,the resorption is seen in alveolar plate limits,the resorption is seen in alveolar plate immediately adjacent to the ligament.This immediately adjacent to the ligament.This kind of reorption is called frontal resorption.kind of reorption is called frontal resorption.

Changes on tension side:Changes on tension side: PDL stretchedPDL stretched Distance between alveolar process Distance between alveolar process

& tooth is widened.& tooth is widened. Increased vascularity.Increased vascularity. Mobilization of fibroblasts & Mobilization of fibroblasts &

osteoblasts.osteoblasts. Osteoid is laid down by osteoblast Osteoid is laid down by osteoblast

in PDL immediately adjacent to in PDL immediately adjacent to lamina dura.lamina dura.

Lightly calcified bone mature to Lightly calcified bone mature to form woven bone.form woven bone.

Secondary remodelling changes:Secondary remodelling changes: Bony changes also takes Bony changes also takes

place elsewhere to maintain the width place elsewhere to maintain the width or thickness of alveolar bone.These or thickness of alveolar bone.These changes are called secondary changes are called secondary remodeling changes.remodeling changes.

For egFor eg:-:-If a tooth is being moved in a If a tooth is being moved in a lingual direction there is compensatory lingual direction there is compensatory deposition of new bone on the outerside of deposition of new bone on the outerside of the lingual alveolar bony plate and also a the lingual alveolar bony plate and also a compensatory resorption on the labial side compensatory resorption on the labial side of the labial alveolar bone. of the labial alveolar bone.

This is to maintain the thickness of This is to maintain the thickness of the supporting alveolar process .the supporting alveolar process .

Change following application Change following application of extreme forces:of extreme forces:

On the pressure sideOn the pressure side :- :- Root closely approximates the lamina Root closely approximates the lamina

dura .dura . Compresses the PDL and leads to Compresses the PDL and leads to

occlusion of blood vessels. occlusion of blood vessels. The PDL is hence deprived of its The PDL is hence deprived of its

nutritional supply leading to regressive nutritional supply leading to regressive changes called hyalinization .changes called hyalinization .

Underminig/Rearward resorption occurs Underminig/Rearward resorption occurs in the adjacent marrow spaces and in the adjacent marrow spaces and alveolar plate below, behind & above alveolar plate below, behind & above the hyalinized zone.the hyalinized zone.

On the tension side:-On the tension side:- Over stretched PDL .Over stretched PDL . Tearing of blood vessels & Tearing of blood vessels &

ischaemia.ischaemia. Extreme forces applied net Extreme forces applied net

increase in osteoclastic activity increase in osteoclastic activity and tooth loosened in socket. and tooth loosened in socket.

Optimum orthodontic Optimum orthodontic force:force: Is one which moves teeth most rapidly in the Is one which moves teeth most rapidly in the

desired direction ,with the least possible desired direction ,with the least possible damage to tissue and with minimum patient damage to tissue and with minimum patient discomfort. discomfort.

SchwarzSchwarz proposed the classic concept of the proposed the classic concept of the optimal force. He defined optimal optimal force. He defined optimal continuous force as the force leading to a continuous force as the force leading to a change in tissue pressure ,that change in tissue pressure ,that approximated the capillary vessel & blood approximated the capillary vessel & blood pressure.Thus preventing their occlusion in pressure.Thus preventing their occlusion in the compressed PDL. the compressed PDL.

Below the optimal level cause no reaction in Below the optimal level cause no reaction in PDL. PDL.

Forces exceeding optimal level would lead to Forces exceeding optimal level would lead to areas of tissue necrosis ,preventing frontal areas of tissue necrosis ,preventing frontal bone resorption. bone resorption.

Schwarz’sSchwarz’s definition was slightly definition was slightly modified by modified by OppenheimOppenheim who who advocated the use of lightest force advocated the use of lightest force capable of bringing about tooth capable of bringing about tooth movement.movement.

OppenheimOppenheim and and SchwarzSchwarz following following extensive studies state that the extensive studies state that the optimum force is equivalent to the optimum force is equivalent to the capillary pulse pressure which is capillary pulse pressure which is 20-26gm/sq.cm of root surface 20-26gm/sq.cm of root surface area. area.

From a clinical point of From a clinical point of view,Optimum orthodontic view,Optimum orthodontic

force has the following force has the following characteristics:characteristics:

Produce rapid tooth movement.Produce rapid tooth movement. Minimal patient discomfort.Minimal patient discomfort. The lag phase of tooth The lag phase of tooth

movement is minimal.movement is minimal. No marked mobility of the No marked mobility of the

teeth being moved. teeth being moved.

From a histological point of From a histological point of view the use of optimum view the use of optimum force has the following force has the following

characteristics:-characteristics:-

The vitality of the tooth and The vitality of the tooth and supporting PDL is maintained.supporting PDL is maintained.

Initiates maximum cellular Initiates maximum cellular response.response.

Produces direct or frontal Produces direct or frontal resorption resorption

Hyalinization:Hyalinization: Form of tissue degeneration Form of tissue degeneration

characterized by formation of a characterized by formation of a clear, eosinophilic homogenous clear, eosinophilic homogenous substances substances

Denotes a compressed and locally Denotes a compressed and locally degenerated PDL. degenerated PDL.

Reversible process. Reversible process. Occurs in almost all forms of Occurs in almost all forms of

orthodontic tooth movement but orthodontic tooth movement but the areas are wider when the force the areas are wider when the force applied is extreme.applied is extreme.

Changes observed during Changes observed during formation of hyalinized formation of hyalinized

zone are: zone are: Gradual shrinkage of PDL fibres.Gradual shrinkage of PDL fibres. Cellular structures become indistinctCellular structures become indistinct Collagenous tissues gradually unite into Collagenous tissues gradually unite into

a more or less cell free mass.a more or less cell free mass. changes also occur in the ground changes also occur in the ground

substance.substance. break down of blood vessel walls leading break down of blood vessel walls leading

to spilling of their contents.to spilling of their contents. Osteoclasts are formed after a period of Osteoclasts are formed after a period of

20-30 hrs.20-30 hrs.

The presence of hyalinised zone The presence of hyalinised zone indicates that the ligament is indicates that the ligament is non-functional and therefore non-functional and therefore bone resorption cannot bone resorption cannot occur.The tooth is hence not occur.The tooth is hence not capable of further movement capable of further movement until the local damaged tissue until the local damaged tissue has been removed and the has been removed and the adjacent alveolar bone resorbs .adjacent alveolar bone resorbs .

Elimination of hyalinised Elimination of hyalinised tissue tissue

2 mechanism:-2 mechanism:-1.1. By osteoclasts differentiating in the By osteoclasts differentiating in the

peripheral intact PDL membrane peripheral intact PDL membrane and in the adjacent marrow spaces. and in the adjacent marrow spaces.

2.2. Invasion of cells and blood vessels Invasion of cells and blood vessels from the periphery of the from the periphery of the compressed zone by which necrotic compressed zone by which necrotic tissue is removed. The invading cells tissue is removed. The invading cells penetrate the hyalinized tissue and penetrate the hyalinized tissue and eliminate unwanted fibrous tissue by eliminate unwanted fibrous tissue by enzymatic action and phagocytosis.enzymatic action and phagocytosis.

Forces & HyalinizationForces & Hyalinization Greater the forces wider is the area of Greater the forces wider is the area of

hyalinization. Thus larger areas of the hyalinization. Thus larger areas of the ligament becomes functionless ,thereby ligament becomes functionless ,thereby showing larger areas of rearward showing larger areas of rearward resorption resorption

If lighter forces are used,the hyalinised If lighter forces are used,the hyalinised zone is smaller and a larger area of zone is smaller and a larger area of functioning ligament is available and functioning ligament is available and frontal resorption predominates. frontal resorption predominates.

The location and extend of hyalinised The location and extend of hyalinised tissue largely depends upon nature of tissue largely depends upon nature of tooth movement.tooth movement.

A-A-TippingTipping –close –close to alveolar crestto alveolar crest

B-B-ExcessiveExcessive forceforce during tipping-during tipping-two areas,one on two areas,one on apical region and apical region and other in marginal other in marginal area.area.

C-C-BodilyBodily-closer to -closer to middle portion of middle portion of rootroot

Phases of tooth Phases of tooth movementmovement

Burstone categorize the stages Burstone categorize the stages as:-as:-

Initial phaseInitial phase Lag phaseLag phase Post lag phasePost lag phase

Initial phase:Initial phase: Rapid tooth movement is observed Rapid tooth movement is observed

over a short distance which then stops. over a short distance which then stops. Represents displacement of tooth in Represents displacement of tooth in

PDL membrane space and probably PDL membrane space and probably bending of alveolar bone .bending of alveolar bone .

Both light and heavy forces displace Both light and heavy forces displace the tooth to same extent .the tooth to same extent .

Between 0.4 to 0.9mm usually occurs Between 0.4 to 0.9mm usually occurs in a weeks time.in a weeks time.

Both light & heavy forces displace the Both light & heavy forces displace the tooth to the same extent during this tooth to the same extent during this phase.phase.

Lag phase:Lag phase:

Little or no tooth movement Little or no tooth movement occurs .occurs .

Formation of hyalinized Formation of hyalinized tissue .tissue .

Extent upto 2-3 weeks . Extent upto 2-3 weeks .

Post lag phase:Post lag phase:

Tooth movement progresses rapidly Tooth movement progresses rapidly as the hyalinized zone is removed as the hyalinized zone is removed and bone undergoes resorption .and bone undergoes resorption .

Osteoclasts are found over a larger Osteoclasts are found over a larger surface area .surface area .

Theories of tooth Theories of tooth movement:movement:

1.1. Pressure tension theory by Pressure tension theory by Schwarz.(classic theory)Schwarz.(classic theory)

2.2. Fluid dynamic theory by Bien/ Fluid dynamic theory by Bien/ blood flow theory: blood flow theory:

3.3. Bone bending & piezoelectric Bone bending & piezoelectric theory: theory:

Pressure tension theory by Pressure tension theory by Schwarz.(classic theory) Schwarz.(classic theory)

Schwarz(1932) - author of this Schwarz(1932) - author of this theory .theory .

According to him ,whenever a According to him ,whenever a tooth is subjected to an tooth is subjected to an orthodontic force it results in orthodontic force it results in areas of pressure and tension .areas of pressure and tension .

Areas of pressure show bone Areas of pressure show bone resorption while areas of tension resorption while areas of tension show bone deposition. show bone deposition.

Fluid dynamic theory by Fluid dynamic theory by Bien/ Blood flow theoryBien/ Blood flow theory

According to this theory tooth According to this theory tooth movement occurs as a result of movement occurs as a result of alternations in fluid dynamics in PDL alternations in fluid dynamics in PDL located in periodontal ligament space. located in periodontal ligament space.

PDL space contains a fluid system PDL space contains a fluid system made up of interstitial fluid, cellular made up of interstitial fluid, cellular elements , blood vessels and viscous elements , blood vessels and viscous ground substances in addition to PDL ground substances in addition to PDL fibres. fibres.

It is a confined space and passage of It is a confined space and passage of fluid in & out of this space is limited. fluid in & out of this space is limited.

The contents of PDL creates a unique The contents of PDL creates a unique hydrodynamic condition resembling a hydrodynamic condition resembling a hydraulic mechanism & shock absorber .hydraulic mechanism & shock absorber .

When force is removed, the fluid is When force is removed, the fluid is replenished by diffusion from capillary replenished by diffusion from capillary walls & recirculation of interstitial fluids. walls & recirculation of interstitial fluids.

Squeeze film effect by Squeeze film effect by BienBien . . When orthodontic force applied,the When orthodontic force applied,the

compression of ligament results.Blood compression of ligament results.Blood vessels of PDL gets trapped between the vessels of PDL gets trapped between the principle fibres & this results in stenosis. principle fibres & this results in stenosis.

Vessels above the stenosis then balloons Vessels above the stenosis then balloons resulting in formation of an aneurysm resulting in formation of an aneurysm

Stenosis Stenosis ++ Aneurysm Aneurysm blood blood gases to escape into gases to escape into interstitial fluids, creating interstitial fluids, creating favourable local environment favourable local environment for resorption. for resorption.

Bone bending & Bone bending & piezoelectric theory:piezoelectric theory:

Farrar in 1876, first noted deformation or Farrar in 1876, first noted deformation or bending of interseptal alveolar bones. bending of interseptal alveolar bones.

suggest that bone bending may be a suggest that bone bending may be a possible mechanism for bringing about possible mechanism for bringing about tooth movement. tooth movement.

Piezo-electricity is a phenomenon Piezo-electricity is a phenomenon observed in many crystalline materials in observed in many crystalline materials in which deformation of the crystal structure which deformation of the crystal structure produces a flow of electric current as a produces a flow of electric current as a result displacement of electrons from one result displacement of electrons from one part of the crystal lattice to the other.A part of the crystal lattice to the other.A small electric current is generated & bone small electric current is generated & bone is mechanically deformed. is mechanically deformed.

The possible source of electric The possible source of electric current are :-current are :-

1.1. Collagen.Collagen.

2.2. Hydroxyapetite.Hydroxyapetite.

3.3. Collagen hydroxyapetite Collagen hydroxyapetite interface.interface.

4.4. Mucopolysaccharide.Mucopolysaccharide.

As long as the force is As long as the force is maintained ,The crystal structure is maintained ,The crystal structure is stable & no further electric effect is stable & no further electric effect is observed. observed.

When the force is released the When the force is released the crystals return to their original crystals return to their original shape & reverse flow of electrons is shape & reverse flow of electrons is observed. observed.

This rhythmic activity produces a This rhythmic activity produces a constant interplay of electric signals constant interplay of electric signals ..

Piezoelectric signals have Piezoelectric signals have two unusual two unusual

characteristics.characteristics. Quick decay rate: Quick decay rate: When the force is released electrons When the force is released electrons

flow in the opposite direction.flow in the opposite direction.

On application of a force on a toothOn application of a force on a tooth , , Areas of concavity Areas of concavity negative negative

charges charges bone deposition. bone deposition. Areas of convexity Areas of convexity + +veve charges and charges and

bone resorption. bone resorption.

Bone deposition:Bone deposition:

On the On the tension side.tension side. Increase in number of osteoblasts .Increase in number of osteoblasts . Osteoblasts are ovoid cells with Osteoblasts are ovoid cells with

basophilic cytoplasm and a oval basophilic cytoplasm and a oval nucleus. nucleus.

Osteoblast increase in number by Osteoblast increase in number by proliferation of their precursor cells. proliferation of their precursor cells.

The PDL fibers readapt to new The PDL fibers readapt to new position of the tooth by proliferation position of the tooth by proliferation of intermediate zone.of intermediate zone.

Bone resorption:Bone resorption: By By OSTEOCLASTSOSTEOCLASTS Multi-nucleated giant cells and may Multi-nucleated giant cells and may

have 12 or more nuclei. have 12 or more nuclei. They are irregularly and or club They are irregularly and or club

shaped with branching processes .shaped with branching processes . They lie in bay like depressions They lie in bay like depressions

called Howship’s lacunae. called Howship’s lacunae. The part of osteoclast in contact The part of osteoclast in contact

with resorbing bone has a ruffled with resorbing bone has a ruffled border. border.

Osteoclasts derived from:-Osteoclasts derived from:-

Activation of previously present Activation of previously present inactive cells.inactive cells.

Migration from adjacent bones.Migration from adjacent bones. Formation of new osteoclasts Formation of new osteoclasts

from local macrophages of PDL.from local macrophages of PDL. Influx of monocytes from blood Influx of monocytes from blood

vessels.vessels.

During bone resorption three During bone resorption three processes occur:processes occur:

1.1. Decalcification.Decalcification.

2.2. Degradation of matrix.Degradation of matrix.

3.3. Transport of soluble Transport of soluble products to the extracellular products to the extracellular fluid or blood vascular fluid or blood vascular system system

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