Biology of tooth movement /certified fixed orthodontic courses by Indian dental academy

Biology of tooth
movement

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Physiologic tooth migration
Naturally occurring tooth movements are :




Tooth eruption
Migration or drift of teeth
Changes in tooth position during tooth movement


Theories of tooth eruption
 Bone remodeling.
 Root growth.
 Vascular pressure.
 Periodontal ligament traction.


Biologic control of tooth
movement


 Two major theories have been proposed for orthodontic tooth

movement:
1. Pressure-tension theory.
2. Biologic electricity.
 The bioelectric theory relates tooth movement at least in part to

changes in bone metabolism controlled by electric signals that are
produced when alveolar bone flexes and bends.
 The pressure tension theory relates tooth movement to cellular changes

produced by chemical messengers .
 The two theories are neither compatible nor mutually exclusive.

Bioelectric theory


 There is a popularly held view that orthodontic forces in vivo produce

electric perturbations of the alveolar bone surrounding the teeth.
 These transient charges, in turn, mediate cell membrane changes which

can be physiologically expressed as bone remodeling- the application of
Wolff’s law.
 Relatively negatively charged areas are thought to promote bone

apposition.
 Positively charged areas have been depicted as sites of bone loss.


 Thus by judicious use of applied or induced currents and voltages to

discrete anatomical areas , one might expect to promote increased
cellular activity and bone turnover, the rate limiting factor of tooth
movement.
 Alternatively , one might be able to promote retention of teeth in their

new positions by an electrically induced deceleration of bone turnover.
 It follows that application of direct current or the induction of current

in bone may be an efficacious means of controlling tooth movement,
depending on the result desired.


 Two types of electric signals have been described that arise in the bone

endogenously:
1. Stress generated potential.
2. Bioelectric potential.


Stress generated potential
 This is the measurable voltage found when bone is mechanically

stressed.
 Studies on the origin of this signal clearly determined that the

potentials have their origin in the organic part of the bone, with little
or no contribution from the mineral component.
 When the organic component of bone was disrupted or enhanced by

physical and chemical means, the voltage amplitude varied with the
amount of cross linked collagen present.


 The physical origin of this stress generated potential appears to be

composed of two parts:
1. Piezoelectric potential.
2. Streaming potential.
 When a crystalline material such as dried bone is mechanically stressed

, the generated electric potential is called piezoelectricity.
 Minute voltage elicited when a moist bone is subject to mechanical

stress is called streaming potential.


Piezoelectric potential
 Was proposed by Bassett (1965)
 Piezoelectricity is a phenomenon observed in many crystalline

materials in which a deformation of crystal structure produces a flow of
electric current as electrons are displaced from one part of the crystal
lattice to another.
 It is attributed to both organic as well as inorganic materials.
 Not only is bone mineral a crystal structure with piezoelectric

properties , collagen itself is piezoelectric, and stress generated
potentials in dry bone specimens can be attributed to piezoelectricity.


Streaming potentials
 Ions in the fluids that bathe living bone interact with the complex

electric field generated when the bone bends, causing temperature
changes as well as electric signals.
 The small voltages that are observed are called “streaming potential”.
 These voltages , though different from piezoelectric signals in dry

material , have in common their rapid onset and alteration, as changing
stresses are placed on the bone.


 Studies done by Ryaby, Jones etal have shown that by adding

exogenous electric signals, similar to those generated in vivo, to various
cell culture models , it has been shown that cAMP levels and
phosphorylation of oncogene proteins can be mediated .

 One type of signal has also been shown to inhibit Parathyroid hormone

(PTH) induced coupling of adenylate cyclase system.


 Signals generated by the bending of alveolar bone during normal

chewing almost surely are important for maintainance of bone around
the teeth.

 On the other hand sustained force of the type used to induce

orthodontic tooth movement does not produce prominent stress –
generated signals.


 It appears that stress generated signals, important as they

may be for normal skeletal function, probably have little if
anything to do with the response to orthodontic tooth
movement.


Bioelectric potential
 A second type of endogenous signal, which is called bioelectric

potential can be observed in bone that is not being stressed.
 Metabolically active bone or connective tissue cells (in area of active

growth or remodeling ) produce electronegative charges that are
generally proportional to how active they are; inactive cells and areas
are nearly electrically neutral.
 The external electric signals probably effect cell membrane receptors,

membrane permeability or both.


 Both animal and human experiments indicate that when low voltage

direct current is applied to the alveolar bone, modifying the bioelectric
potential, a tooth moves faster than its control in response to an
identical spring.
 The undermining resorption, soft tissue inflammatory reaction, and

tension on the PDL complex, all produce a bioelectric response.


Effect of pulsed electromagnetic
fields on orthodontic tooth movement


 In dentistry electromagnetic fields have been used to:
1. Speed the healing of periodontal defects.
2. Reduce the amount of alveolar ridge resorption seen following

extractions.
3. Increase the rates of healing of facial fractures.
4. Stimulate the rate of mandibular condylar growth.


 Numerous laboratory studies of the effects of electrical fields on living

tissue show that electrical fields:
1. Alter the normal electric states of bone and cartilage.
2. Induce increased rates of cellular division and metabolism.
3. Thus promote increased healing of bone and cartilaginous defects.


 By combining electric stimulation and mechanical stress , Davidovitch

has shown both increased cellular activity and accelerated rates of
tooth movement.
 It was hypothesized that the application of electrical currents during

tooth movement potentiated the effect of the mechanical forces,
leading to enhancement of cell activation and tissue remodeling.
 Davidovitch has suggested that the generation of electric potentials in

mechanically stressed bone may be the signal activating the cells that
participate in the remodeling process.
 Thus it is possible that PEMFs and orthodontic mechanical forces

may be capable of functioning together in a synergistic manner.

 Numerous studies have shown increase in the rate and amount of

orthodontic tooth movement by using PEMFs.
 It can be postulated that:
1. In the presence of a mechanical deformation, the PEMF functions at

the cell membrane through an interaction with the calcium ions and
cyclic nucleotides to induce within the cells a higher degree of
receptivity and reactivity.
2. The PEMF may also be providing the signal for the recruitment of

undifferentiated stem cells into the osteoblastic and osteoclastic
processes.

3. the piezoelectric currents generated within the alveolar bone by the
pressure and tension of the orthodontic force are thought to provide the
signal for the directionality, ie deposition or resorption- of the
remodeling process.


 It is believed that the cell membrane represents the interface between

the external stimulus, be it mechanical or electrical, and the cell’s
specific response.

 The nature of effect seen clinically seems to be dependant on such

factors as :
cell type,
cellular environment,
and the electrical field parameters used.


 Bassett has suggested that specific parameters of the PEMF, such as

waveform and frequency , may be responsible for the activation of a
certain group of cells.

 It appears that electrical energy, whether applied as a direct current

or a PEMF, has the ability to effect both the depository and resorptive
activities of bone and the cartilage cells.

 Thus there is increased rate of production and action of both

osteoclasts and osteoblasts as a result of application of PEMF.


PEMF

increased localized calcium deposition

neutralizes the tissues net negative charge
( allows)
subsequent vascular invasion

initiation of osteogenesis

Advantage of PEMF over direct currents
 Although both direct current and PEMFs appear to have the ability to stimulate

increased rates of tooth movement, it has been suggested that PEMF may
eventually prove to be more clinically useful due to:
1. Their ability to alter the PEMF pulse characteristics to achieve specific cellular

effects.
2. In addition, the complete non-invasiveness of the technique,
3. the absence of faradic reactions that are potentially tissue damaging,
4. and the ability to generate a predictable current through a more efficient and

less cumbersome apparatus , should enhance its potential for clinical use.


Pressure tension theory


 The pressure tension theory , the classic theory of tooth movement ,

relies on chemical rather than electric signals as the stimulus for
cellular differentiation and ultimately tooth movement .

 In essence this view of tooth movement shows three stages:
1. Alterations in blood flow associated with pressure within PDL.
2. The formation and release of chemical messengers.
3. Activation of cells.


 The heavier the sustained pressure, the greater should be the reduction in

blood flow through compressed areas of the PDL, up to the point that vessels
are totally collapsed and no further blood flows.
 When light but prolonged forces are applied to a tooth, blood flow through

partially compressed PDL decreases as soon as fluids are expressed from the
PDL space and the tooth moves in its socket(ie in a few seconds).
 Within a few hours , the resulting chemical environment produces a different

pattern of cellular activity.
 Increased levels of cAMP the “second messenger” are produced after about 4

hours of sustained pressure.


What happens in the first few hours after sustained
force is applied against a tooth?
 Experiments have shown that prostaglandin and interleukin-1 beta levels

increase within the PDL within a short time after the application of pressure,
and it is now clear that prostaglandin E is an important mediator of the cellular
response.
 Changes in cell shape probably play a role.
 There is evidence that prostaglandins are released when cells are mechanically

deformed (ie pg release might be a primary than a secondary response to
pressure).
 Mobilization of membrane phospholipids , which leads to the formation of

inositol phosphates , is another pathway towards the eventual cellular
response.


 Other chemical messengers, particularly members of the cytokine

family, also Nitric Oxide and other regulators of cellular activity are
also involved.
 For a tooth to move osteoclasts and osteoblasts are required for bone

resorption and formation on pressure and tension side respectively.
 PG-E has the interesting property of stimulating both the osteoclastic

and osteoblastic activity, making it particularly suitable as a
mediator of tooth movement.


Frontal resorption
 When the PDL is mechanically stimulated osteoclasts appear within the

compressed PDL within 48 hours.
 Osteoclasts arrive in two waves :
1. some (the first wave), maybe derived from a local cell population

within the PDL.
2. while others (the larger second wave) are brought from distant areas
via blood flow.
 Osteoclasts attack the adjacent lamina dura, removing bone in the

process of “frontal resorption”, and tooth movement begins soon
thereafter.


 Simultaneously osteoblastic activity ensues, but it lags behind a little,

so that the PDL space becomes enlarged.
 Osteoblasts are recruited locally from progenitor cells in PDL.
 Their action is to:
1. form bone on the tension side.
2. and begin remodeling activity on the pressure side.
 In frontal resorption a steady attack on outer surface of lamina dura

results in smooth continuous tooth movement.


Undermining resorption
 The course of events is different if sustained forces against a tooth is great

enough to totally occlude blood vessels and cut off the blood supply to an area
within the PDL.
 Due to its histological appearance as the cells disappear, an avascular area in

the PDL has been referred to as hyalinized.
 Despite the name , the process has nothing to do with the formation of hyaline

connective tissue but represents the inevitable loss of all cells when the blood
supply is totally cut off.
 When this happens remodeling of bone bordering the necrotic area of the PDL

must be accomplished by cells derived from adjacent undamaged area.


 More importantly, osteoclasts appear within the adjacent bone marrow spaces

and begin an attack on the underside of the bone immediately adjacent to the
necrotic PDL area .
 This process is appropriately described as undermining resorption

since the attack is from underside of the lamina dura.
 When hyalinization and undermining resorption occur , an inevitable

delay in tooth movement results.
 This is caused by a delay in stimulating differentiation of cells within

the marrow spaces , and second because a considerable thickness of
bone must be removed from the underside before any tooth movement
can take place.

Time course of treatment with frontal vs undermining
resorption


 Not only tooth movement is more efficient when areas of PDL necrosis

are avoided but pain is also lessened.
 However, even with light forces , small avascular areas are likely to

develop in the PDL and tooth movement will be delayed until these can
be removed by undermining resorption.
 In clinical practice, tooth movement usually proceeds in a more

stepwise fashion because of the inevitable areas of undermining
resorption.


PDL and bone response to
sustained orthodontic force


• The response to sustained force against the teeth is a function of force

magnitude:
•

Heavy forces lead to a rapidly developing pain, necrosis of cellular
elements within the PDL, and the phenomenon of “undermining
resorption”, of alveolar bone near the effected tooth.

 Lighter forces are compatible with survival of cells within the PDL and

a remodeling of the tooth socket by a relatively painless “frontal
resorption” of the tooth socket.


Phases of tooth movement


 In 1962 Burstone suggested that, if the rates of tooth

movement were plotted against time, there would be 3
phases of tooth movement:

1. An initial phase.
2. A lag phase.
3. A postlag phase.


 Two recent studies done by Pilon etal , and Vas Leeuwen etal have proposed

a new time/displacement model for tooth movement.
 These studies, performed on beagles, divided the curve of tooth movement into

4 phases.
 The first phase lasts for 24 hours to 2 days and represents the initial movement
of the tooth inside its bony socket.
 It is followed by a second phase, when 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
phase.


Initial phase
 Cellular and tissue reactions start in the initial phase of tooth

movement, immediately after force application.
 Events taking place in this phase are:
 compression and tension areas develop in the PDL
 recruitment of osteoclast and osteoblast progenitors
 extravasation and chemoattraction of inflammatory cells


presence of some hyalinized areas was demonstrated even in this
early stage


Second phase
 Areas of tension are easily recognized by distorted appearance of the

normal PDL fiber arrangement.
 Disruption of the blood flow due to this distortion leads to development

of hyalinized areas and the arrest of tooth movement, which could last
from 4 to 20 days.
 Only the removal of necrotic tissue, and bone resorption from adjacent

marrow spaces, and from the direction of the viable PDL, allow the
resumption of tooth movement.


 This process requires the recruitment of phagocytic cells such as

macrophages, foreign body giant cells, and osteoclasts.

 These cells act in tandem to remove necrotic tissue from compressed

PDL sites and adjacent alveolar bone.

 In areas of PDL tension, quiescent osteoblasts are enlarged and start

producing new bone matrix (osteoid).


 New osteoblast progenitors are recruited from the population of
fibroblast-like cells(pericytes) around PDL capillaries.

 These preosteoblasts proliferate and migrate toward the alveolar bone

surface along the stretched Sharpey’s fibers.

 Simultaneously the PDL fibroblasts in tension zones begin multiplying

and remodeling their surrounding matrix.


 The third and fourth phases of orthodontic tooth movement , also

known as the acceleration and linear phases, respectively, start about
40 days after the initial force application.
 They comprise most of the total tooth movement during orthodontic

treatment.
 The pressure sides of teeth exhibit collagen fibers without proper

orientation. Here irregular bone surfaces are found, indicating direct or
frontal resorption.
 Tension sides during these phases clearly show bone deposition.


 Recent reports by Von Bohl etal demonstrated that teeth subjected to

high forces show hyalinization more often than teeth experiencing light
forces.
 Thus development of hyalinization zones has a definite relationship to

force magnitude , but it was found to have no relationship to the rate
of tooth movement.
 These investigators have concluded that , once tooth movement has

started after the second(arrest) phase, bone remodeling takes place at
a certain rate, independent of the force magnitude.


Effects of force distribution


 The optimum force levels for orthodontic tooth movement should be

just high to stimulate cellular activity without completely occluding
blood vessels in the PDL.
 Both the amount of force delivered to a tooth and also the area of the

PDL over which that force is distributed are important determinants of
the biologic effect.
 The PDL response is determined not by force alone, but by force per

unit area, or pressure.


 Since the distribution of force within the PDL , and

therefore the pressure differs with different types of tooth
movement, its necessary to specify the type of tooth
movement as well as the amount of force in discussing
optimum force levels for orthodontic purposes.


Types of tooth movement


Tipping


 Tipping of a tooth leads to concentration of pressure in limited areas of

the PDL.
 Forces used to tip a teeth must be kept quite low.
 Both experiments and experience with humans suggest that the tipping

forces should not exceed approximately 50 gm.
 Tipping of a tooth by light continuous forces results in greater

movement within a shorter time than that obtained by any other
method.
 Coronal portion of a tooth is chiefly what is moved.

 In most young patients , bone resorption resulting from a moderate

tipping movement usually is followed by compensatory bone
formation.
 The degree of such compensation varies individually and depends

primarily on the presence of osteoblasts in the periosteum.
 Compensatory periosteal bone apposition in the apical region is also

subject to variation, according to whether osteoblasts are present or
absent in the periosteum.


 Tipping of adult teeth in a labial direction may result in bone

destruction of the alveolar crest , with little compensatory bone
formation.

 In addition after a prolonged movement of the apical root portion in

the opposite direction, resorption of the bone plate in the apical region
may occur so rapidly that the root finally is moved through the bone.


Torque
 A torquing movement of tooth involves tipping of the apex.
 During the initial movement of torque the pressure area usually is

located close to the middle region of the root.
 This occurs because the pdl is normally wider in the apical third than

the middle third.
 After resorption of bone areas corresponding to the middle third , the

apical surface of bone gradually begins to compress adjacent
periodontal fibres and a wider pressure area is established.
 Tipping forces of 50-60 gm in the anterior teeth of humans are

sufficient.

Bodily movement (Translation)
 If two forces acting along parallel lines and distributed over the whole

alveolar bone surface are applied simultaneously to the crown of a
tooth, the tooth can be bodily moved.(ie the crown and the root apex
move in the same direction and the same amount.)
 Slight tipping movement also occurs during translation of a tooth.
 In translation the total PDL area is loaded uniformly.
 Forces in the range of 70-120 gm are sufficient for this purpose.


Extrusive movements during tipping and
translation.
 A tooth that is moved by tipping is also frequently somewhat extruded,

which is a direction of fiber stretching that facilitates further tipping.

 Even a tooth moved bodily may become slightly extruded unless the

archwire has been adjusted to compensate for any tendency to
extrusion.


Rotation
 In rotation of a tooth, theoretically forces to produce rotation around

its long axis could be much larger than those to produce other tooth
movements, since the force would be distributed over the entire PDL
rather than a narrow vertical strip.
 In fact, however , it is essentially impossible to apply a rotational force

so that the tooth also does not tip in its socket, and when this happens
an area of compression is created just as in any other tipping
movement.
 For this reason, appropriate forces of rotation are similar to those for

tipping, ie 50-60 gm.


Retention considerations after rotation of teeth
 The elongation and oblique arrangement of the supporting fiber

bundles necessitate a retention period after treatment has been
completed.
 In the marginal region rotation usually causes considerable

displacement of fibrous structures.
 The free gingival fiber groups arranged obliquely from the root surface,

interlace with the periosteal structures and the whole supra alveolar
fibrous system.
 Thus rotation also causes displacement of fibrous tissues located some

distance from the rotated tooth.

 The fiber bundles and the new bone layers of the middle and apical

third rearrange themselves after a fairly short retention period.
 However , the free gingival fibers remain stretched and displaced for

as long as 232 days and possibly longer.
 According to these observations, over-rotation or fiberotomy has been

recommended.


Extrusion
 Extrusive movements ideally produce no areas of compression within

the PDL, only tension.
 Even if compressed areas could be avoided, heavy forces risk extraction

of the tooth.
 Light forces , however, move the alveolar bone with the tooth.
 Varying with the individual tissue reaction, fiber bundles elongate and

new bone is deposited in areas of alveolar crest as a result of the
tension exerted by these stretched fiber bundles.


 The force exerted must not exceed 25 -30 gm, because extrusion

constitutes the type of tooth movement that requires minimal force.
 The open space in the apical region consists partly of an uncalcified

osteoid, which is not perceptible on a radiograph.
 After 4-5 weeks, calcified bone starts to become visible in the apical

area.


Intrusion
 During intrusion of teeth stretch is exerted primarily on the principal

fibers.
 Relapse usually does not occur, partly because the free gingival fiber

bundles become slightly relaxed.
 Due to the stretch of principal fibers an intrusive movement may

therefore cause formation of new bone spicules in the marginal region.
 Rearrangement of the principal fibers occurs after a retention period of

2-3 months.


 In a young patient the intruded tooth may remain fairly stable.
 In adults, however, relapse after intrusion may occur, particularly

when the retention period has been too short.
 Intrusion requires careful control of force magnitude.
 Light force is required because force is concentrated in a small area at

the tooth apex.


 Primarily the anterior teeth are intruded.
 A light continuous force has proved favourable for intrusion in young

patients.
 In other cases the alveolar bone may be closer to the apex, increasing

the risk for apical root resorption.


 If the bone of the apical region is fairly compact, as it is in some adults,
1. a light interrupted force may be preferable to provide time for cell

proliferation to start,
2. and direct bone resorption may prevail when the arch wire is

reactivated after the rest period.


Effect of force duration and
force decay


 The key to produce orthodontic tooth movement is the application of

sustained force, which does not mean that the force must be absolutely
continuous.
 It does mean that the force must be present for a considerable

percentage of time, certainly hours rather than minutes per day.
 Animal experiments suggests that only after force is maintained for

approximately 4 hours do cyclic nucleotide levels in the PDL increase,
indicating that this duration of pressure is required to produce the
“second messenger” needed to stimulate cell differentiation.


 Clinical experience suggests that there is a threshold for force duration

in humans in the 4-8 hour range, and that increasingly effective tooth
movement is produced if force is maintained for longer durations.
 Continuous forces , produced by fixed appliances that are not effected

by what the patient does , produce more tooth movement than the
removable appliances, unless removable appliance is present almost all
the time.
 Removable appliances worn for decreasing duration of time produce

decreasing amounts of tooth movements.


How force magnitude changes as tooth
responds by moving?
 Duration of force has another aspect, related to how force magnitude

changes as tooth responds by moving.
 Some decline in force magnitude (ie force decay) is noted with even

the springiest device after the tooth has moved a short distance.
 From this perspective, orthodontic force duration is classified by the

rate of decay as:
1. Continuous – force maintained at some appreciable fraction of the
original from patient’s one visit to the next.
2. Interrupted- force levels decline to zero between activations.


 There is an important interaction between force magnitude and how

rapidly the force declines as the tooth responds.
 To understand this we consider:
1. The effect of a nearly continuous force.
2. Effect of forces that decay fairly rapidly.
 Theoretically there is no doubt that light continuous forces produce the

most efficient tooth movement.
 Despite the clinicians best efforts to keep forces light enough to

produce only frontal resorption, some areas of undermining
resorption are probably produced in every clinical patient.

 The heavier forces are physiologically acceptable, only if :
1. Force levels decline so that there is a period of repair and regeneration

before the next activation.
2. Force decrease at least to the point that no second and third rounds of
undermining resorption occurs.
 Heavy continuous forces are to be avoided; heavy intermittent forces

though less efficient , can be clinically acceptable.


Reactivation schedule
 Experience has shown that orthodontic appliances should not be

reactivated more frequently than at 3 week intervals.
 A 4-6 week appointment cycle is more typical in clinical practice.
 Undermining resorption requires 7-14 days.


Diurnal variation in tooth
movement in response to
orthodontic forces


 Diurnal rhythms have been found in various parameters related to bone

formation and resorption, including plasma calcium and phosphate,
calcium regulating hormones, osteocalcin.
 Diurnal rhythms have also been found in various local events that

influence bone turnover , such as proliferation and matrix synthetic
activities of osteogenic cells, osteoclast-bone surface contact, and
enzyme activities related to bone formation and resorption.
 Kotaro Miyoshi etal conducted a study to investigate whether there is

any differnce in orthodontic tooth movement in rats when orthodontic
force is applied at different times of the day and night.


 More tooth movement was achieved in the rats that received

orthodontic force during day time, than those which received the forces
during night time.
 Results of their study showed for the first time that the response to

orthodontic force varies depending on the time of the day, the force is
applied.
 Also application of force during animal’s rest period may be more

effective than while it’s active.


 In rats both bone formation and bone resorption are most active in the

environmental light period, and are minimal in the environmental dark
period.

 This indicates that a greater bone-formative response can be obtained when

orthodontic force is applied during the light period, the period of active
bone formation, rather than during the dark period.

 Thus the magnitude of the response may depend on the underlying

physiologic activity of bone formation , which varies with the time of the
day.

 Possible explanations of this diurnal rhythm can be:
1. Hormonal variations.
2. Diurnal variation in masticatory function.

Hormonal variation
 In rats, rhythms of cell proliferation in cartilage and bone parallel the

serum corticosterone level, while the rhythm, of matrix synthesis in
bone is parathyroid dependant.
 It has been shown that diurnal rhythms in calcium metabolism in both

rats and human beings are regulated by undefined serum factors.
 Therefore it is reasonable to assume that the observed variation in

tooth movement is also caused by these hormonal rhythms.


Diurnal masticatory rhythm
 Another possible cause for this diurnal variation may be a bio-

mechanical rhythm, ie diurnal variation in masticatory function.
 The rats being more active during nocturnal period , consume more

food during the dark period than during day time, indicating that their
masticatory activities increase during night.
 Thus it is conceivable that increased masticatory activities during the

dark period might interfere with orthodontic force, and thus may
inhibit tooth movement.


 If this is applicable to diurnal human beings, more tooth movement

would be expected at night than during the day.

 Stutzman and Petrovic have shown that the rate of alveolar bone

turnover in human beings is higher in the night-time than in daytime.


Cellular response to orthodontic
force


 Tooth movement ultimately depends on specific activation

of precursor (osteoprogenitor) cells, which form osteoblasts
and osteoclasts.

 Understanding these level mechanisms and controlling the

bone remodeling response is essential to intelligently
evaluating current treatment methods and developing
innovative approaches in the future.


Mechanically induced bone remodeling
 Remodeling is divided into 3 major categories:
1. Turnover in response to micro fractures.
2. Reorientation of bone mass to optimally resist stress (Wolff’s law).
3. Net change in bone volume related to functional load.
 At the tissue level the bone turnover cycle is divided into three phases:
1. Activation – occurs in matter of hours.
2. Resorption- occurs in a month.
3. Formation- occurs in 2-3 months.


 This ARF sequence is basically a repair process, requiring about 4

months in adult human cortical bone.

 Stress patterns within the periodontium, resulting from functional

and/or orthodontic loads , dictate the form of periodontal bone.


Force system


 Application of a force system to the crown of a tooth produces a cascade

of viscoelastic, biochemical, and biophysical effects within the
periodontium and supporting bone.
 From a cellular point of view, distribution of stress, displacement of

the PDL, and bone deformation are critical factors.
 The characteristics of force system that relate to its biologic effects are:
1. Magnitude.
2. Frequency.
3. Direction and moment to force ratio.
4. Constancy.
5. Functional modification.

Transduction


 Transduction is the conversion of mechanical energy into

a biologic signal affecting a remodeling response.
 By stress and strain , the mechanical load of an orthodontic

force system is transferred to the periodontium, resulting
in:
1. Altered stress patterns.
2. Viscoelastic displacement of the PDL.
3. Bone deformation.


 These biophysical events are currently thought to be

transduced into a cellular response by one of the following
mechanisms:
1. Cell perturbation.
2. Bioelectric signals.
3. Micro environmental factors.
4. Accumulation of micro fractures.


 Because the PDL is viscoelastic it resists displacement by

short acting loads such as mastication and swallowing.
 However it is readily displaced by light continuous or even

interrupted forces such as orthodontic forces or postural
habits.
 Displacement results in PDL that is widened in osteogenic

areas or compressed in cell free or hyalinized areas.


orthodontic force
PDL displacement
influx of Na and Ca
( Ca influx)
cell perturbation
inhibits enzyme Adenylate cyclase

slows down the production of 2nd messenger cAMP

low levels of cAMP(intracellular)


initiation of proliferation in bone and
cartilage progenitor cells

 Thus stress/ strain brought about by a c AMP mediated

mechanism, involving direct perturbation and/ or a
bioelectrical signal, appears to initiate proliferation in
osseous progenitor cells, an important aspect in activation
of bone remodeling.


Micro-environmental factors

 Among the micro-environmental factors implicated in initiation of the

response of the PDL are :
1. Vascular flow (partial pressure oxygen and carbon dioxide).
2. Cell density changes (widening of PDL breaks contact inhibition, which

induces proliferation).
3. Ground substance -collagen ratio.


During PDL displacement

Ground substance (from compression areas)
(predominantly negatively charged)
(migrates with respect to fixed
positively charged collagen)

Areas of tension (widened zones)


 Thus it is evident that in the stressed PDL:
1) Osteogenic /Tension areas:

high ground substance :collagen ratio
net negative charge
decreased c AMP levels
increased osteoblastic activity

2) Resorptive / Compression areas: low ground substance:collagen

ratio
net positive charge
increased c AMP activity
increased osteoclastic activity

 As reviewed by Storrey , bone is an anisotrophic, viscoelastic material

progressing from elastic to plastic to disruptive deformation, depending
on the magnitude and frequency of the load.

 Even light loads, continuously applied, produce progressive

deformation and may eventually lead to fracture.

 At the histological level, micro fractures are any disruption from a

minute crack to a fractured trabeculus.


Remodeling response


Osteoclasts recruitment

 Appearance of increased number of osteoclasts occurs

within hours of application of orthodontic force.

 Even after force is terminated, osteoclasts persist for

several days in rats and up to 10 days in humans.




The important distinction is that active osteoclasts are recruited, and
are not dependant on local PDL cell proliferation/differentiation as
was previously thought.



A significant factor in the orthodontic response is probably
recruitment of previously present inactive osteoclasts into the active
fraction that supports tooth movement.


Osteoblast histogenesis
 Osteoblasts in response to orthodontic force are produced locally by

proliferation and differentiation of PDL fibroblast like cells.
 This remarkable potential of PDL is related to a large population of

osteoblast(Ob) precursor cells.
 More than a third of the fibroblast like population in the PDL are

preosteoblasts , the immediate proliferating predecessors of
osteoblasts.


 Following three aspects are related to osteoblast histogenesis:

1. Cell cycle analysis.
2. Nuclear morphometry.
3. Photoperiod influence.


Cell cycle analysis


Nuclear volume and photoperiod


Alveolus translocation
 Alveolus translocation is the basic bone forming and resorbing

response within and immediately adjacent to the PDL following
application of orthodontic force.
 Alveolus response is a special case of remodeling in which bone

formation and resorption occurs simultaneously on opposite sides of
the alveolus resulting in drift of the bony socket.
 Movement of one mineralized tissue(root), through another (alveolar

bone) reflects the unique transduction and activation properties of the
PDL.


 During initiation of tooth movement , the osteogenic reaction is

slower(days), than the osteoclastic response (hours), in affecting bone
remodeling changes.

 Particularly when cell free zones and undermining resorption are

involved, the resorptive aspect of alveolar translocation is the rate
limiting step in tooth movement.


Alveolar bone remodeling
 Turnover in human skeleton varies from 5% per year for long bone

cortex to 33% for trabecular bone.
 The rate of alveolar bone turnover is intermediate between the two.
 Tooth movement is not only a response of PDL but also involves

generalized remodeling of the adjacent alveolar process.
 Half or more of the entire alveolar bone is remodeled during typical

orthodontic treatment.


 Areas of hyalinization and necrosis are noted even during physiologic

drift and relapse, it is unlikely that significant orthodontic tooth
movement occurs without increased bone deformation and remodeling
of adjacent alveolar bone.


Retention period following remodeling
 Reitan observed that newly deposited bundle bone is readily resorbed

during relapse or reverse tooth movement
 Retention involves not only prevention of relapse but also stabilization

of a new functional entity.
 Considering
1. orthodontically induced remodeling transients,
2. completion of secondary remineralization,
3. and functional adaptation of the alveolar process,

four to six months is a biologically relevant , minimal retention period.

Clinical considerations in bone remodeling
 Activation of remodeling pockets in resisting bone may be an

important factor in rate of tooth movement.
• The burst of resorption (A

R) ahead of a moving tooth might be
considered “telegraphing” a resorptive signal to decrease bone mass
preparatory to complete resorption associated with alveolar
translocation.

 Typical reactivation interval for devices with relatively short ranges

of activation (such as closure loops) is about one month.


 From a biological perspective, modern orthodontics is quite primitive.
 For example under ideal conditions, bone resorbed is about 100

microns per day, which would relate to orthodontic translation at a
rate of 3 mm per month.
 This greatly exceeds the efficiency of any clinical method, indicating a

considerable gap between clinical proficiency and biologic potential.


Role of neurotransmitters in
tooth movement


 Substance -P like immunoreactive nerve fibers have been shown to be

distributed in the dental pulp, and PDL , and in the soft tissue of the
TMJ.
 A study done by Davidovitch et al revealed that in living animals,

sensory nerve fibers in the PDL may provide neurotransmitters,
specifically SP, to cells populating and bordering the periodontium.
 It suggests that in vivo the peripheral nervous system acts as a

possible supplier of a link between physical stimulus and the
biochemical response.


Pressure

Pain reflex

Substance P release from sensory nerve terminals
Antidromic vasodilation
Migration of leukocytes out of capillaries

increased supply of PG’s

Secretion of lymphokines

interaction with Responsive Paradental cells
Synthesis of 2nd messengers

First and second messenger
interactions in stressed
connective tissue


 Connective tissue cells are capable of responding to a variety of stimuli

in a mechanism which involves ligand-receptor interactions.
 These stimuli are regarded as “first messengers”, whose interaction

with specific cell surface receptors leads to enzymatic activities and
formation of “second messengers” intracellularly.
 It was postulated that cellular stimulation in mechanically stressed

bone may result from the motion of fluids and the generation of
endogenous electric potentials, leading to enhanced cellular ionic
refluxes.


 First messengers :
1. Mechanical - motion of fluids, endogenous electric potentials
2. Chemical - Substance-P, PGE
 Second messengers :

cAMP, cGMP, Ca


 Davidovitch etal suggested that connective tissue cells, particularly

bone cells, may be activated by stress generated potentials, or
streaming potentials resulting from force-induced motion of
extracellular fluids which bring a variety of ions and molecules in
contact with the plasma membrane of native cells.
 Moreover , stretching or compressing cells may alter the permeability

properties of their membranes , facilitating ion fluxes.


Reaction of connective tissue cells to mechanical forces


 Thus it appears that physical and chemical first messengers may be

operant in the initial stage of connective tissue response to mechanical
forces.
 The interaction of these signals with cells in the affected areas seems

to involve elevations in the cellular content of second messengers such
as cAMP, c GMP, and Calcium.


Role of Prostaglandins and
Interleukins in tooth movement


 In studies of vascular response done by Kenichi Yamasaki on rats, it

was demonstrated that periodontal vascular permeability increased
both on pressure and tension sides 15 minutes to 1.5 hours following
the application of orthodontic force.
 These reports suggest that initiation of inflammation was triggered

by chemical mediators such as histamine and bradykinin, and PG’s.
 Furstman and Bernick reported that pain reaction associated with

OTM was from periodontal inflammation caused by mechanical
stress, and was induced by chemical mediators such as
histamine,bradykinin, and PG’s.


 Prostaglandins are produced from arachidonic acid .
 They are released into the extracellular environment, where they

participate in cell-cell communication by interacting with
prostaglandin receptors on neighboring cells.
 Prostaglandin E is involved in :
1. Bone resorption.
2. Bone stimulation.
3. Cell proliferation.
4. Collagen synthesis.


 This dual effect of PGE on bone formation and resorption is consistent

with increased bone turnover triggered by mechanical stimulation.
 Thus PG’s may effectuate part of the coupling observed between bone

resorption and bone formation.
 Biochemical mechanism for PG mediated bone resorption is not

known.
 In many systems PGE1 and E2, the most potent bone resorption

agents , were shown to stimulate adenylate cyclase activity, which is
an enzyme catalyzing synthesis of c AMP.

 In vitro experiments carried out by Rodan etal confirmed

that mechanical perturbations , which approach a
physiologic range , augment prostaglandin production in
the osteoblast –enriched cell population.


Cell types synthesizing PG’s
 The precise identity and location of PG synthesizing cells is not

known.
 Davidovitch etal from their studies elucidated that PG’s do not seem to

be synthesized by PDL cells, native or migratory, but to be bound by
these cells.
 The paradental cells associated with the PDL and the alveolar bone,

such as fibroblasts, macrophages, cementoblasts, osteoblasts, and
osteoclasts are the potential sources of PGE during tooth movement
caused by orthodontic forces.


 William G Grieve etal in their study on PGE, and IL1b levels in GCF during

human orthodontic tooth movement found that these bone resorbing factors
produced within the PDL are detectable in GCF during early phases of tooth
movement, and return to their baseline within 7 days.
 IL b stimulates bone resorption and concomitantly inhibits bone formation.
 IL 1 b levels increased at – 1 hr, 24 hrs and 1 week.
 PGE levels increased at - 24 hrs, 48 hrs , and 1 week.

 IL b modulates PGE production.


 Kenichi Yamasaki in an vitro study on rats suggested that orthodontic

mechanical stress induces the synthesis of PG’s by localized cells,
which, in turn stimulate osteoclastic bone resorption.
 Lilja etal investigated the enzymatic profile in periodontal tissues

histochemically, and suggested that PG synthetases are found in bone
marrow in areas undergoing OTM in rats.
 Davidovitch and Shanfeld reported the involvement of PGE2 in the

bone remodeling of orthodontically treated cats, showing a rise of
PGE2 levels in the alveolar bone.


 The mechanism of synthesis and secretion of PG’s from mechanically

stressed periodontal tissues is unknown.
 Buck etal found lipids which may be correlated with PG’s in stressed

human periodontal tissues, especially in cell-free zones.
 Stress could conceivably cause a perturbation of periodontal tissue

cell membranes with a resultant increase in the synthesis and
secretion of PG’s , since the precursors of PG’s are the lipids which
constitute the cell membrane.


Action of PG’s
 c AMP and intracellular calcium have been reported to be involved in

the action of PG’s.
 Action of PG’s is known to cause an increase in concentration of both of

them.
 These second messengers are involved in the induction of osteoclasts

from monocytic cells during bone resorption induced by orthodontic
mechanical forces.


Orthodontic mechanical stress
damage/perturbation of PDL tissues

PG synthesis
Tooth movement
intracellular c AMP and calcium
accumulation by monocytic cells
Bone resorption
modulation and activation of osteoclastic activity


The second messengers


 The second -messenger hypothesis postulates that target

cells respond to external stimuli, chemical or physical, by
enzymatic transformation of certain membrane- bound
and cytoplasmic molecules to derivatives capable of
promoting phosphorylation of cascades of intracellular
enzymes.


 The first messenger binds to a specific receptor on cell membrane and

produces an intracellular second messenger.
 This second messenger then interacts with cellular enzymes, evoking a
response, such as protein synthesis or glycogen breakdown.
 Two main 2nd messenger systems are now recognized:
1. The cyclic nucleotide pathway.
2. Phosphatidyl inositol(PI) dual signaling system.
 These systems mobilize internal calcium stores and activate protein

kinase C respectively.


The c AMP pathway

 c AMP and c GMP are 2 second messengers associated with bone

remodeling.
 Bone cells, in response to hormonal and mechanical stimuli, produce

c AMP in vivo and in vitro.
 Alterations of c AMP levels have been associated with synthesis of

polyamines, nucleic acids, and proteins, and secretion of cellular
products.


 The action of c AMP is mediated through phosphorylation of specific

substrate proteins by its dependant protein kinases.
 In contrast to this role, c GMP is considered an intracellular regulator

of both endocrine and non endocrine mechanisms.
 This signaling molecule plays a key role in synthesis of nucleic acids

and proteins as well as secretion of cellular products.


The PI dual signaling systems
activation of cell surface receptors by 1st messengers

hydrolysis of PI 4,5 biphosphonate

inositol triphosphate formation

release of calcium ions from
intracellular stores


mitogenesis in mechanically
deformed tissue through an increase
in DNA synthesis

Role of calcium in tooth
movement


Tension(bone apposition) side
Ca influx

inhibits intracellular enzyme Adenylate cyclase
( which)
slows down production of 2nd messenger c AMP

initiation of proliferation among bone and cartilage
progenitor cells

stimulates bone apposition

Pressure (bone resorption) side
membrane deformation
Ca influx

increases activity of PLA2 enzyme
Increase in osteoclasts/bone resorption

stimulates release of arachidonic acid from
membrane phospholipids
synthesis of 2nd messenger c AMP
PGE2 synthesis


Role of Vitamin D in
Orthodontic tooth movement


 1,25, dehydroxycholecalciferol (1,25, DHCC) has been identified as an

important factor in tooth movement.
 This agent is a biologically active form of Vitamin D, and has a potent

role in calcium homeostasis.
 This molecule has shown to be a potent stimulator of bone resorption,

by inducing differentiation of osteoclasts from their precursors..
 It is also known to stimulate bone mineralization and osteoblastic cell

differentiation in a dose-dependant manner.


 Kale et al compared the effects of local administration of 1,25, DHCC ,

and PGE2 on orthodontic tooth movement in rats, and reported that
both molecules enhanced tooth movement significantly.
 In this study, 1,25, DHCC was found to be more effective than PGE2 in

modulating bone turnover during tooth movement, because of its well
balanced effect on bone formation and resorption.


Drug effects on the response to
orthodontic force


 Administration of certain pharmacologic agents may:
1. Stimulate tooth movement.
2. Depress tooth movement.

in response to orthodontic forces.

 It is quite possible that use of these pharmacologic agents to

manipulate tooth movement in both directions will come into common
use.


Drugs stimulating tooth movement



Vitamin D administration can enhance the response to orthodontic
force.



The E series of PG’s appears to be the most potent stimulators of
bone resorption.



A series of studies have shown that they produce a dose related
increase in calcium release from bone in vitro.



Direct injection of PG into the PDL has shown to increase the rate of
tooth movement, but this is quite painful, and not very practical.


 Based on his experiments on OTM in rats Kenichi Yamasaki

suggested that it is possible that PG’s may be useful in clinical
orthodontic movement in the form of local administration combined
with OTM.
 He suggested that local administration of PG’s combined with OTM :
1. Reduces the hyalinization areas in PDL.
2. Induces more rapid bone remodeling.
3. Provides continuous tooth movement and accelerates the rate of tooth

movement.


Drugs depressing tooth movement
 Two types of drugs are known to depress the response to orthodontic

force, and may influence current treatment:
1. The bisphosphonates used in treatment of osteoporosis. eg

alendronate.
2. Prostaglandin inhibitors. eg indomethacin.

 Bisphosphonates act as specific inhibitors of osteoclast -mediated

bone resorption


 Drugs that effect PG activity fall into two categories:
1. Corticosteroids, and NSAIDs that interfere with PG synthesis.
2. Other agents that have mixed agonistic and antagonistic effects on

various PG’s.

 Both children and adults on chronic steroid therapy may be

encountered, and the possibility of difficult tooth movement in these
patients must be kept in mind.


 Fortunately, although potent PG inhibitors like indomethacin can

inhibit tooth movement, the common analgesics (ibuprofen, aspirin)
seem to have little effect on tooth movement at the dose levels used
with orthodontic patients.
 Some other drugs which can effect PG levels, and therefore affect the

response to orthodontic force are:
Tricyclic antidepressants (amitriptyline)
antiarrythmic drugs(procaine)
antimalarial drugs (quinine,quinidine)
anticonvulsant drugs (phenytoin)
some tetracyclines( eg doxycycline)


 Recently, the possibility that locally applied PG inhibitors could be

used to decrease the response of specific teeth has been explored.
 It is now possible in periodontal therapy to place miniature spheres

that release a specific antibiotic into the gingival sulcus and into the
periodontal pockets.
 If a PG inhibitor was placed in similar mini- spheres , and could be

maintained in the sulcus around teeth that were to serve as anchors,
the improved anchorage would allow more effective tooth movement
of the teeth whose movement was desired.


Apical root resorption associated with
otm


 Apical root resorption is one of the most common complications associated

with orthodontic treatment.
 The general consensus is that apical root resorption of vital teeth occurs to

some extent in all orthodontic patients.
 Following are some important facts about apical root resorption associated

with otm:
 Sinclair and Sameshima in their study on apical root resorption reported that :
1. Apical root resorption occurs mainly in the anterior teeth , averaging over 1.4

mm.


2.

The average amount of root resorption found for molars and
premolars is very low( less than 1mm).

3. Within the anterior segment maxillary teeth are more affected than
mandibular teeth by a factor of 2.
4.

Among the maxillary anterior teeth following is the frequency of root
resorption :
laterals>centrals>canines

5.

Asian patients had significantly less root resorption than white
patients.


5.

Increased root length and overjet are correlated with greater root
resorption for the maxillary anterior dentition.

6.

Adults have significantly more resorption than children in the
mandibular anterior teeth.



Spurrier etal in their study on comparison of root resorption
during otm of vital teeth vs endodontically treated teeth found that
endodontically treated incisors resorb with less frequency and
severity than vital teeth.


Effect of extraction and orthodontic
treatment on dentoalveolar support


 In a study done by David B Kennedy etal on effect of extraction and

orthodontic treatment on dentoalveolar support reduced alveolar bone
heights were noted :
1. in the extraction sites due to bone loss.
2. in the areas of root resorption due to otm.
 The most marked alveolar bone loss was seen interproximally for

maxillary and mandibular permanent incisors , and at the pressure
side of closed first premolar extraction sites.


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