Biology Of tooth movement

BIOLOGY OF TOOTH

MOVEMENTBy Dr. Arti Kripalani

Part I P.G.K.M.Shah Dental College

CONTENTS 11/1/16 Introduction Periodontal ligament Physiologic tooth movement Optimum orthodontic force Types of forces Histology of tooth movement Hyalinization Phases of tooth movement Theories of tooth movement

INTRODUCTION Orthodontic treatment is based on the principle that if

prolonged pressure is applied to a tooth, tooth movement will occur as the bone around the tooth remodels.

Bone is selectively removed in some areas and added in others.

The tooth moves through the bone carrying its attachment apparatus with it, as the socket of the tooth migrates.

Because the bony response is mediated by the periodontal ligament, tooth movement is primarily a periodontal ligament phenomenon.

PERIODONTAL LIGAMENTSTRUCTURE AND FUNCTION

PERIODONTAL LIGAMENTRESPONSE TO NORMAL FUNCTION

During masticatory function, the teeth and periodontal structures are subjected to intermittent heavy forces.

Tooth contacts last for 1 second or less.

Forces are quite heavy, ranging from 1 or 2 kg while soft substance are chewed up to as much as 50 kg against a more resistant object.

PHYSIOLOGIC TOOTH MOVEMENT Naturally occurring tooth movements that take place

during and after tooth eruption.

This include:A)Tooth Eruption.B)Migration or drift of teeth.C)Changes in tooth position during mastication.

TOOTH ERUPTION Tooth eruption is the axial movement of tooth from its

developmental position in the jaw to its final position in the oral cavity.

a)Blood pressure theory: The tissue around the developing end of the root is highly

vascular. This vascular pressure is believed to cause the axial

movement of teeth.

b)Root Growth: The apical growth of roots result in an axially directed

force that brings about the eruption of teeth. This theory was rejected.

c)Periodontal traction theory: The periodontal ligament is rich in fibroblasts that contain

contractile tissue. The contraction of these periodontal fibers (mainly the

oblique group of fibers) results in axial movement of the tooth.

d)Hammock ligament theory: According to Sicher, a band of fibrous tissue exists below

the root apex spanning from one side of alveolar wall to other.

This fibrous tissue appears to form a network below the developing root and is rich in fluid droplets.

The developing root forces itself against this band of tissue, which in turn applies an occlusally directed force on tooth.

MIGRATION OR DRIFT OF TEETH Refers to minor changes in tooth position observed

after eruption.

Human dentition shows a natural tendency to move in a mesial & occlusal direction.

Usually a result of proximal and occlusal wear of teeth, to maintain inter-proximal and occlusal contact.

This was pointed out for the first time by Stein and Weinmann.

Physiologic tooth migration usually is related to mesiodistal movements. However, the teeth also exhibit a continued eruption, even after full emergence, accompanying the growth in height of the alveolar processes.

Studies of craniofacial dimensions have demonstrated that significant changes occur in human beings even during adulthood (Bjork A, Skieller V, 1983).

TOOTH MOVEMENT DURING MASTICATION During mastication ,the teeth and PDL structures are

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

OPTIMUM ORTHODONTIC FORCE Is one which moves teeth most rapidly in the desired

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

Schwarz defined it as the force leading to a change in tissue pressure ,that approximated the capillary vessel & blood pressure. Thus preventing their occlusion in the compressed PDL.

Below the optimal level cause no reaction in PDL.

Forces exceeding optimal level would lead to areas of tissue necrosis.

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

Oppenheim and Schwarz following extensive studies stated that the optimum force is equivalent to the capillary pulse pressure which is 20-25 gm/sq.cm of root surface area.

FROM A CLINICAL POINT OF VIEW, OPTIMUM ORTHODONTIC FORCE HAS THE FOLLOWING CHARACTERISTICS:

Produce rapid tooth movement. Minimal patient discomfort. The lag phase of tooth movement is minimal. No marked mobility of the teeth being moved.

FROM A HISTOLOGICAL POINT OF VIEW THE USE OF OPTIMUM FORCE HAS THE FOLLOWING CHARACTERISTICS:- The vitality of the tooth and supporting PDL is

maintained. Initiates maximum cellular response. Produces direct or frontal resorption.

TYPES OF FORCES Continuous force : the force magnitude

is maintained at almost the same level in the period between two activations.

Interrupted force : declines to zero between activations.

Intermittent force : falls to zero when the appliance is removed and return to original level on re insertion.

HISTOLOGY OF TOOTH MOVEMENT Classic histologic research about tooth movement by

Sandstedt, Oppenheim and Schwarz led to the hypothesis that a tooth moves in the periodontal space by generating a ‘pressure side’ and a ‘tension side.’

Later ultrastructural studies by Rygh (1972) and Brudvik and Rygh (1994), gave a very detailed description of events.

The findings of the above research have been meticulously summarized by Krishnan and Davidovitch (2006).

Changes following application of light forces:

Changes on pressure side:

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

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

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

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

Changes on tension side:

PDL stretched. Distance between alveolar process & tooth is widened. Increased vascularity. Mobilization of fibroblasts & osteoblasts. Osteoid is laid down by osteoblast in PDL immediately

adjacent to lamina dura. Lightly calcified bone mature to form woven bone.

Secondary remodelling changes: Bony changes also takes place elsewhere to maintain the width

or thickness of alveolar bone. These changes are called secondary remodeling changes.

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

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

CHANGE FOLLOWING APPLICATION OF EXTREME FORCES:

On the pressure side :-

Root closely approximates the lamina dura . Compresses the PDL and leads to occlusion of blood vessels. The PDL is hence deprived of its nutritional supply leading

to regressive changes called hyalinization . Undermining/Rearward resorption occurs in the adjacent

marrow spaces and alveolar plate below, behind & above the hyalinized zone.

On the tension side:-

Over stretched PDL . Tearing of blood vessels & ischaemia. Extreme forces applied net increase in osteoclastic activity

and tooth loosened in socket.

Is orthodontic movement possible for a tooth that has undergone endodontic treatment ?

Is it possible to move an ankylosed tooth ?

HYALINIZATION Form of tissue degeneration characterized by

formation of a clear, eosinophilic homogenous substance.

Denotes a compressed and locally degenerated PDL. Reversible process. Occurs in almost all forms of orthodontic tooth

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

CHANGES OBSERVED DURING FORMATION OF HYALINIZED ZONE ARE:

Gradual shrinkage of PDL fibres. Cellular structures become indistinct. Collagenous tissues gradually unite

into a more or less cell free mass. break down of blood vessel walls

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

of 20-30 hrs.

The presence of hyalinised zone indicates that the ligament is non-functional and therefore bone resorption cannot occur.

The tooth is hence not capable of further movement until the local damaged tissue has been removed and the adjacent alveolar bone resorbs .

ELIMINATION OF HYALINISED TISSUE 2 mechanism:-

1. By osteoclasts differentiating in the peripheral intact PDL membrane and in the adjacent marrow spaces.

2. Invasion of cells and blood vessels from the periphery of the compressed zone by which necrotic tissue is removed by enzymatic action and phagocytosis

FORCES & HYALINIZATION Greater the forces wider is the area of hyalinization.

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

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

The location and extent of hyalinised tissue largely

depends upon nature of tooth movement.

Tipping – close to alveolar crest.

Excessive force during tipping- two areas, one on apical region and other in marginal area.

Bodily- closer to middle portion of root.

THE BIOMECHANICAL BEHAVIOUR OF THE HYALINIZED PERIODONTAL LIGAMENT IN DOGS DURING EXPERIMENTAL ORTHODONTIC TOOTH MOVEMENT

Authors: Journal: Level of evidence:

Aim and objectives:

Result and Conclusion:

S. H. Jónsdóttir, E. B. W. Giesen and J. C. Maltha

EJO 2012 4 To study and assess the biomechanical behaviour of the hyalinized periodontal ligament in experimental orthodontic tooth movement

A rapid transposition during the first few seconds was found. However, it was significantly less for hyalinized than for non-hyalinized PDL.

The hyalinized tissue offers resistance to tooth movement and it is proportional to the degree of hyalinization.

PHASES OF TOOTH MOVEMENT Burstone categorize the stages as:- Initial phase Lag phase Post lag phase

INITIAL PHASE

Rapid tooth movement is observed over a short distance which then stops.

Represents displacement of tooth in PDL membrane space and probably bending of alveolar bone .

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

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

LAG PHASE

Little or no tooth movement occurs .

Formation of hyalinized tissue .

Extent upto 2-3 weeks but may at times be as long as 10 weeks.

POST LAG PHASE

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

Osteoclasts are found over a larger surface area.

If the duration of the movement is divided into an initial and a secondary period, direct bone resorption is found in the secondary period, when the hyalinized tissue has disappeared after undermining bone resorption.

Study by Pilon et al performed on beagles,divided the curve of tooth movement into 4 phases:

The first phase lasts 24 hrs to 2 days and represents the initial movement of the tooth inside its bony socket.

Second phase when the tooth movement stops for 20 to 30 days.

After the removal of necrotic tissue formed during the second phase , tooth movement is accelerated in the third phase and continues into the fourth linear phase.

THEORIES OF TOOTH MOVEMENT1. The pressure-tension theory2. Bone bending & piezoelectric theory3. Blood flow/fluid dynamic theory

THE PRESSURE-TENSION THEORY

Classic histologic research about tooth movement by Sandsted (1904),Oppenheim (1911) and Schwarz (1932) led them to hypothesize that a tooth moves in the periodontal space by generating a “pressure side” and a “tension side.”

According to them, areas of pressure show bone resorption while areas of tension show bone deposition.

BONE BENDING & PIEZOELECTRIC THEORY: Farrar was the first to suggest, in 1888, that alveolar

bone bending plays a pivotal role in orthodontic tooth movement.

This hypothesis was later confirmed with the experiments of Baumrind in rats and Grimm in humans.

Piezo-electricity is a phenomenon observed in many crystalline materials in which deformation of the crystal structure produces a flow of electric current.

As a result, displacement of electrons from one part of the crystal lattice to the other.

A small electric current is generated & bone is mechanically regenerated and repaired.

The possible source of electric current are :-

1. Collagen.2. Hydroxyapetite.3. Collagen hydroxyapetite interface.4. Mucopolysaccharide.

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

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

This rhythmic activity produces a constant interplay of electric signals .

PIEZOELECTRIC SIGNALS HAVE TWO UNUSUAL CHARACTERISTICS.

Quick decay rate When the force is released electrons flow in the opposite

direction.

On application of a force on a tooth , Areas of concavity negative charges bone deposition. Areas of convexity +ve charges and bone resorption.

It was proposed by Bien in 1966.

Acc. to this theory:“Tooth movement occurs as a result of alterations in fluid

dynamics in the PDL.”

There are three interacting fluid systems in the PDL:1. Vascular system2. Cellular system3. Interstitial fluid system

3) BLOOD FLOW/FLUID DYNAMIC THEORY

These fluids in confined periodontal space create a unique hydrodynamic condition.

ORTHODONTIC FORCE

compression of PDL

Occlusion of blood vessels on pressure side and dilation on tension side

formation of aneurysms

Fluid is squeezed out of vessel wall

O2 level falls in compressed area and rises on tension side

metabolite changes take place

change in chemical environment stimulates release of chemical messengers

They stimulate cellular differentiation into Osteoblasts and Osteoclasts

Bone Remodeling takes place.

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RECENT ADVANCES IN BIOLOGY OF TOOTH MOVEMENT

OSTEOBLASTS

Growth factors • Bone

morphogenetic proteins (BMP)

• Transforming growth factor (TGF β- I and II)

• Insulin-like growth factor (IGF-I and II),

• Platelet derived growth factor (PDGF)

• Fibroblast growth factor (FGF)

Genes • C-fos• C-jun• Egr-1• Cbfa1

(Runx2/OSF2)• Osterix

Proteins

• Integrin• gamma

carboxyglutamic acid (GLA) protein

OSTEOCLASTSCytokines (tumor necrosis factor [TNF], interleukin-1 alpha, 6-alpha [I]), macrophage colony stimulating factor (M-CSF), granulocyte, macrophage colony stimulating factor (GM-CSF) and prostaglandin (PGE2).

Osteoclast differentiation is also mediated by the interaction of two molecules produced by osteoblasts, namely osteoprotegrin (OPG) and RANK ligand (RANKL) Receptor activator of nuclear factor kappa B ligand

CASCADE OF EVENTS THAT FOLLOW AFTER APPLICATION OF ORTHODONTIC FORCE

PATHWAYS OF TOOTH MOVEMENT

HOW DO CELLS DISTINGUISH BETWEEN TENSION AND COMPRESSION At the “Biology of tooth movement” conference held

at Formington in Nov.,1986 it was suggested that the ans was likely to be found in the field of CYTOKINE biology.

According to this hypothesis,formation and resorption depends upon:

1) the cytokines2) the functional state of available target cells.

The cytokines are defined as short range soluble mediators released from cells which modulate the activity of other cells.

Includes interleukines, interferons, chemotactic factors,tumor necrotic factors,colony stimulating factors and assorted growth factors.

Q) How do cytokines mediate mechanically induced bone remodelling ?

A) Osteoblasts have receptors for prostaglandins, PTH, Vit D but osteoclasts don’t have.

It is cytokine that transmits signals from osteoblasts to osteoclasts.

Cytokines either activate osteoclasts or promote differentiation of the precursor cells.

Secondly, osteoblasts produce collagenase enzyme that resorbs osteoid.

Osteoclasts cannot resorb bone untill the surface osteoid layer is removed.

Although cytokine is a potent stimulator of bone resorption,it can also stimulate osteoblast proliferation.

Therefore, whether resorption or formation, depends upon the cytokines and functional state of the available target cells.

Tooth movement ultimately depends on specific activation of bone precursor cells ( osteoprogenitors) which form osteoblasts and osteoclasts.

MESSENGER SYSTEMS First messengers

Cytokines• Interleukin-1, Interleukin-6, Interleukin-11, Tumor

necrosis factor, Osteoclast differentiating factor, Bone formation Interleukin-4, Interleukin-13, Interleukin-18, Interferon, Osteoprotegrin

Growth factors• Insulin-like growth factors I & II ,

Transforming growth factor , Fibroblast growth factor, Platelet derived growth factor , Connective tissue growth factors

Hormones• Polypeptides, Parathyroid hormones ,

Calcitonin, Insulin, Growth hormone , Steroid, 1,25, dihydroxy vitamin D3, Glucocorticoids , Sex steroids ,Thyroid hormones

Colony-stimulating factors• M-CSF• G-CSF• GM-CSF

Others • Prostaglandins• Leukotriens• Nitric oxide

ROLE OF PROSTAGLANDIN AS 1ST MESSENGER Von Euler (1934) – prostate fluid. Imp role in stimulating bone resorption. One of the chief mediators of inflammation cause an

increase in intracellular cAMP and Ca++ accumulation.

Cell membrane phospholipid phospholipase- A

ARACHIDONIC ACID

Cyclo-oxygenase lipo-oxygenase pathway pathway

PG2 thromoxane leukotrienes HETE

Davidovitch and Shanfeld, Davidovitch et al., Yamasaki et al. (first reported

human study, 1982-83),Lee et al., Selinkale et al. and many others. 

Mohammed et al injected PG inhibitor, leukotriene inhibitor and both together, in rats and performed tooth movement.

ROLE OF NEUROTRANSMITTERS AS 1ST MESSENGER

substance P vasoactive intestinal polypeptide (VIP) calcitonin gene related peptide (CGRP).

Davidovitch et al demonstrated that incubation of substance P in vitro significantly increased the concentration of cAMP in the cells and of PGE2 in the medium within 1 minute.

NEUROTRANSMITTERS, CYTOKINES, AND THE CONTROL OF ALVEOLAR BONE REMODELING IN ORTHODONTICS

Authors: Journal: Level of evidence:

Aim and objectives: Result and Conclusion:

Davidovitch et al

Dental clinic of N.American1988

4 To test the hypothesis that tissue remodeling during orthodontic tooth movement is modulated by factors derived from the nervous and vascular systems.

Administration of SP and IL-1 beta to human PDL fibroblasts in vitro for 1 to 60 minutes resulted in significant increases in the levels of the intracellular "second messenger" cAMP, as well as of PGE2.

This tend to support the hypothesis that neurotransmitters and cytokines play a regulatory role in orthodontic force-induced alveolar bone remodeling.

ROLE OF NITRIC OXIDE AS 1ST MESSENGER Three distinct isoforms of NOS are:- a neuronal form (nNOS), - an endothelial form (eNOS), - an inducible form (iNOS).

Both nNOS and eNOS are constitutively expressed and are collectively referred to as constitutive NOS enzymes (eNOS).

It was found that iNOS plays a role in the response of periodontal tissue to orthodontic force.

1. NO activates guanylyl cyclase in periodontal ligament fibroblasts, leading to an increased level of cGMP. This second messenger in cell cytoplasm raises lysosome membrane permeability, leading to exocytosis of lysosome content resulting in resorption of organic and mineral elements of bone.

2. Nitric oxide synthesizes prostaglandins by direct activation of cyclo-oxygenase.

3. Nitric oxide influences the function of osteoclastic differentiation and osteoblast function.

MECHANICAL STRAIN AS 1ST MESSENGER1) strain sensitive ion channels and shear stress receptors

in cell membrane: calcium and potassium channels. Channel gating may be caused by direct mechanical

perturbation or secondarily by the activation of stretch sensitive phospholipase C or D.

They respond to mechanical strain by causing in and out movement of ions, creating changes in the electric potential.

These changes enable the signal to be propagated intracellularly.

2)Signal transduction by integrins and focal adhesions

SECOND MESSENGERS Sutherland and Rall established the second-messenger basis

for hormone actions in 1958.

They proposed that the first messenger binds to a specific receptor on the cell membrane and produces an intracellular chemical second messenger.

This second messenger then interacts with cellular enzymes which evokes a response.

- cAMP- cGMP- IP3

CAMP PATHWAY

CGMP PATHWAY

PHOSPHATE INOSITOL PATHWAY

RANK-RANKL-OPG The receptor activator of nuclear factor kappa B ligand

(RANKL), its decoy receptor (RANK) and osteoprotegrin (OPG) were found to play important roles in regulation of bone metabolism.

Evidences suggest osteoblast itself regulates the differentiation of osteoclast.

The talk between an osteoblast and osteoclast is accomplished through an osteoblast membrane bond RANKL which can interact with osteoclast precursors to cause them to differentiate into osteoclasts.

Another membrane bond molecule and its bonding ligand OPG can develop to block RANKL and prevent osteoclast formation.

Extensive studies done by Alhashimi et al., Aihara et al., Kanzaki et al.,and Yamaguchi et al. have demonstrated that RANKL promotes osteoclastogenesis while OPG inhibits this effect.

SUMMARY OF BIOLOGICAL PROCESSES OF TOOTH MOVEMENT

COUPLING OF BONE FORMATION TORESORPTION One of the most distinctive features of bone

remodeling of both cortical and trabecular bone is the precise coupling of bone resorption and formation.

The A-R-F process is similar for all types of bone remodeling; the multicellular unit for a trabecular bone surface is essentially a hemisection of the cutting/filling cone for cortical bone.

THE FOLLOWING SEQUENCEIS HYPOTHESIZED: (1) bone microdamage results in release of inflammatory

cytokines (prostaglandins, interleukin 1-, and so on) and exposure of mineralized collagen to extracellular fluid.

(2) T cells, attracted by the inflammatory cytokines and exposed mineralized collagen, produce the ligand RANKL, which induces osteoclastogenesis

(3) preosteoclasts from circulating blood have RANK receptors, which are activated by the RANKL to from osteoclasts

(4) as bone is resorbed, growth factors are released that stimulate preosteoclasts to produce OPG which then retract from the bone surface.

(5) mononuclear cells move in and coat the scalloped resorbed surface with cementing substance (green)

(6) perivascular osteogenic cells migrate through the low cell density zone and differentiate to preosteoblasts that then divide and form two osteoblasts each.

(7) osteoblasts form new bone, filling the resorption cavity and completing the turnover process.

DELETERIOUS EFFECTS OF HEAVY SUSTAINED FORCES

1. Effect on the pulp Heavy intrusive force can sever the blood vessels as

they enter the root apex and that can affect the tooth vitality.

2. Effects on the root structure Some root remodeling constantly occurs during

orthodontic tooth movement. But permanent root loss ( root resorption) occurs if: there are conical roots with pointed apices, dilacerated / distorted root form, excessive force over prolonged period, root apices in contact with cortical bone,

history of trauma.

3. Mobility

PDL usually widens during orthodontic treatment hence some mobility of teeth is unavoidable during orthodontic treatment.

Heavy sustained O.F. results into greater amount of undermining resorption and thereby greater mobility.

If tooth becomes excessively mobile during treatment, all forces should be discontinued for time being till mobility reduces.

4. Pain

Pain during orthodontic treatment occurs due to development of ischemic areas in the PDL.

Heavier forces will cause more pain.

Upon activation using light forces can also produce pain for few hours to few days.This also greatly depends upon individual pain threshold.

If patient is engaged in repeated chewing during initial 8 hrs. after activation the pain can be reduced.

Excessive pain during activation is an indication of too heavy force which should be avoided.

CONCLUSION Orthodontic tooth movement consequent t application

of force is outcome of complex chain of events, eventually leading to bone resorption and bone formation.

What signals the ‘cells’ in periodontium to perceive orthodontic force to transform into bone forming and bone resorbing cells is not yet fully understood.

Future research in the field of molecular biology is expected to unfol many more secretes of this phenomenon.

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