��BIOLOGY OF TOOTH MOVEMENT�� Dr. Vincy Antony

CONTENTS

• Theories of tooth movement
• Optimum orthodontic force
• Histology of tooth movement
• Phases of tooth movement
• Biological pathway of tooth movement
• Effect of drugs on the response to orthodontic force
• Deleterious effects of orthodontic force

Theories of Tooth Eruption

• Blood Pressure Theory
• Root Growth Theory
• Hammock Ligament Theory
• Periodontal Traction Theory

Components of PDL

1. Fibres

2. Cellular components, which include:

i. Undifferentiated mesenchymal cells

ii. Fibroblasts, osteoblasts, osteoclasts

iii. Blood vessels

iv. Nerve endings associated with pain and

proprioception

3. Tissue fluids act as shock absorber

The different types of periodontal fibres:

​

1. Trans-septal group

2. Alveolar crest group

3. Horizontal group

4. Oblique

5. Apical group

6. Inter-radicular fibres

​

• Thickness of normal PDL – 0.5 mm.
• Collagenous fibres of PDL connects the cementum and lamina dura.
• The fibers run at an angle attaching farther apically on the tooth than on the adjacent alveolar bone.
• PDL space is filled with fluid derived from vascular system.

RESPONSE TO NORMAL FUNCTION

​

• 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.

RESPONSE TO SUSTAINED ORTHODONTIC FORCE HISTOLOGY OF TOOTH MOVEMENT

• Changes following application of mild force

​

• Changes following application of heavy force

​

Changes following application of mild force�

• On pressure side
• Narrowing of periodontal space
• Compression of periodontal ligament
• Blood vessels are still patent and there is increased vascularity
• Cellular infiltration of tissues with proliferation of fibroblasts and osteoblasts
• Within 24-48 hours, osteoclasts appear along the bone surface and direct bone resorption occurs.

�

• On tension side:
• Widening of periodontal ligament
• Blood vessels become distended
• Cellular infiltration of tissues with proliferation of fibroblasts and osteoblasts
• Rearrangement of fibroblasts
• Deposition of osteoid tissue along the bone surface immediately adjacent to the lamina dura
• Osteoid bone is progressively replaced by bundle bone

Pressure and tension site following force application. �Bone gets deposited in tension zones and undergoes resorption in pressure zone

Changes following application of heavy force�

On pressure side

• Periodontal ligament is crushed
• Blood vessels are occluded
• Periodontal ligament becomes acellular & hyaline in appearance
• Osteocytes of the underlying bone die
• Adjacent to the hyalinised area, in the underlying bone osteoclasts appear & remove bone and hyalinised area by undermining resorption
• Tooth movement will eventually occur

​

• On tension side:
• periodontal ligament gets over stretched
• Blood vessels become ruptured and ischemia
• Thus net increase in osteoclastic activity
• The tooth becomes loosened in its socket

​

On application of light force

• On application of heavy force

HYALINISATION

• 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 is accomplished by cells derived from adjacent undamaged areas.

OPTIMAL ORTHODONTIC FORCE

Optimal force is one that produces maximum tooth movement in the desired direction with minimum damage to the supporting tissues and without any discomfort to the patient.

• Optimal orthodontic force would be equivalent to the capillary pulse pressure or 20 to 26gm.per sq.cm of root surface.

CHARACTERISTICS OF OPTIMUM ORTHODONTIC FORCE

​

A. Produce rapid tooth movement

B. Minimal patient discomfort

C. The lag phase of tooth movement is minimal.

D. No marked mobility of the teeth being moved

Optimum Orthodontic Force- �Characteristics from Histologic Point of View

1. Vitality of tooth and supporting periodontal ligament is maintained
2. Initiates maximum cellular response
3. Produces direct or frontal resorption

​

Advantages of �optimum orthodontic force�

• Tooth movement is efficient with optimum orthodontic force.
• Resorption is mainly of the periosteal type.
• Elimination of lag phase.
• Elimination of hyalinized zone with optimum force.
• Pain is lessened.
• Damage to the supporting structures is avoided.
• Chances for root resorption are minimized.

THEORIES OF ORTHODONTIC TOOTH MOVEMENT

• PRESSURE TENSION THEORY
• BLOOD FLOW THEORY
• PIEZO ELECTRIC THEORY

• PRESSURE TENSION THEORY
• Schwartz- in 1932
• Classic theory

sustained pressure

​

shift in tooth position within periodontal space

​

compression of ligament stretching of ligament

​

​

PRESSURE

TENSION

Contd……

​

blood flow decreased blood flow maintained

or increased

​

​

​

​

​

​

​

bone resorption bone deposition

​

-Release of chemical messengers

-Activation of cells

frontal / direct

• Bone resorption

undermining / rearward

• Frontal resorption-light forces
• Osteoclasts appear in 2 waves
• from local sites,
• From distant sites via blood
• Resorption of adjacent lamina dura
• Undermining resorption
• When heavy forces are used
• Hyalinization

​

BLOOD FLOW THEORY/FLUID DYNAMIC THEORY

• By Bien in 1966

​

• ‘Tooth movement occurs as a result of alterations in fluid dynamics in the periodontal ligament’.

​

• 3 distinct, interacting fluid systems
• Vascular system
• Cellular system
• Interstitial fluid system

orthodontic force

↓

compression of periodontal ligament

↓

occlusion of blood vessels

↓

formation of aneurysms

↓

alteration in the chemical environment; decreased oxygen concentration in compressed areas

↓

blood gases escape into local environment

↓

favourable environment for resorption

​

​

​

• ‘Squeeze film effect’-A force of greater magnitude and duration causes the interstitial fluid in the periodontal ligament space to get squeezed out and move towards the apex and cervical margins.

​

This results in the slowing down of the tooth movement and is called the "squeeze film" effect.

• The constriction of a blood vessel by the periodontal fibers.
• The flow of blood in the vessels is occluded by the entwining periodontal fibers.
• Below the stenosis, the pressure drop gives rise to the formation of minute gas bubbles, which can diffuse through the vessel walls.
• Above the stenosis, fluid diffuses through the walls of the cirsoid aneurysms formed by the build‐up of pressure.

BONE BENDING/ PIEZOELECTRIC/ BIOELECTRIC THEORY by Farrar

• Deformation of crystal structure produces a flow of electric current –piezoelectricity
• Possible sources of current
• Bone
• Collagen
• Hydroxyapatite
• Mucopolysaccharide
• Characteristics:
1. Quick decay rate
2. Production of equivalent signal opposite in direction when force is released

​

When a force is applied to a crystalline structure�(like bone or collagen), a flow of current is produced that quickly dies away. When the force is released, an opposite current flow is observed. The piezoelectric effect results from migration of electrons within the crystal lattice.

*

34

​

• When crystal structure is deformed , electrons migrate from one location to another result in an electric change .
• As long as force is maintained , the crystal structure is stable and no further electric effect is observed .
• When the force is released the crystals returns to their original shape and a reverse flow of electron is observed .

Bioelectric Theory includes

• Piezoelectricity
• Streaming Potential
• Bioelectric Potential

​

• ‘Streaming potential’-as bone bends, both convection and conduction currents can be detected in the extracellular fluids. The small voltages that are thus generated are called the "Streaming Potentials."
• ‘Bioelectric potential’-Endogenous electric signals can also be observed in bone that is not stressed. These are called the "Bioelectric Potentials"
• On application of force
• Areas of concavity negative charge bone deposition
• Areas of convexity positive charges bone resorption

The main drawback of bone bending theory is that this theory is based on stress generated signals. This type of signals are produced by vibratory type of orthodontic tooth movement .But for optimum tooth movement light continuous forces are applied.

​

​

Phases of tooth movement

• By Burstone-1962

​

Initial phase

Lag phase

Post lag phase

INITIAL PHASE

• Very rapid tooth movement is observed
• Represents displacement of the tooth in the periodontal ligament space & probable bending of the alveolar bone
• Both light & heavy forces displace the tooth to the same extent
• Tooth movement is about 0.4-0.9mm & occurs in a week

LAG PHASE

• Little or no tooth movement occurs
• Phase is characterised by formation of hyalinised tissue in the periodontal ligament
• If light forces- the area of hyalinisation is small- and frontal resorption occurs
• If heavy forces - the area of hyalinisation is large- and longer lag period
• Lag phase duration usually 2-3 weeks

POST LAG PHASE

• Tooth movement progresses rapidly as the hyalinised zone is removed & bone undergoes resorption

CURRENT VIEW OF ORTHODONTIC TOOTH MOVEMENT

3 PATHWAYS

• Tissue injury & inflammation
• Bone bending & bioelectric response
• Direct alteration of cell membrane through mechanotransduction

​

Orthodontic Force

​

Bone bending Tissue injury & inflammation Direct alteration of cell membrane through mechanotransduction

Inflammatory type of reaction

​

​

Release of First Messengers – activate extracellular signals

​

Release of Second Messengers

conversion of extracellular signal to intracellular signal

​

cyclic AMP second messenger pathway Phosphoinositide second messenger pathway

​

Release of Third Messengers

​

Phosphorylation of cells

​

​

Increased osteoclastic activity Increased osteoblastic activity

Bone Resorption Bone Deposition

​

• Decreased vascularity and overstretching of periodontal ligament induces inflammatory changes
• oxygen levels are altered
• because of inflammatory type of reaction, mediators like prostaglandin are released

• First Messengers
• Prostaglandin becomes the first messenger
• function of first messengers are to activate the extra cellular signals
• first messengers bind to the cell surface receptors & the extracellular signals are activated
• other first messengers are parathormone, substance P, vasoactive peptides

• Second Messengers
• next step is conversion of extra cellular signal into an intra cellular signal by -
• 2 main second messenger systems are:

Cyclic AMP pathway

Phosphoinositide pathway

• It takes nearly 6 hours of sustained pressure to produce second messenger

• Third messengers
• the cAMP & Ca ions act on the protein kinase enzymes within the cells
• Protein kinase enzymes are the third messengers
• Protein kinase causes phosphorylation of the cells. Phosphorylation results in differentiation & activation of osteoclasts and osteoblasts which ultimately produce remodelling.
• tooth movement begins as the osteoclasts and osteoblasts remodel the bony socket

Drug Effects on the Response to�Orthodontic Force

• Two types of drugs are known to depress the response to orthodontic force:
• the bisphosphonates used in treatment of osteoporosis (e.g., alendronate IFosamax] or risedronate [Actonel]), and
• prostaglandin inhibitors (especially the more potent members of this group that are used in treatment of arthritis, like indomethacin).

Prostaglandin E plays an important role in the cascade of signals that leads to tooth movement, thus inhibitors of its activity would affect tooth movement.

Drugs that affect prostaglandin activity fall into two categories:

​

1. corticosteroids and nonsteroidal anti inflammatory drugs(NSAIDs) that interfere with prostaglandins synthesis, and

​

(2) other agents that have mixed agonistic and antagonistic effects on various prostaglandins.

Phospholipids

Arachidonic acid

Prostaglandins

Corticosteroids reduce prostaglandin synthesis by inhibiting the formation of arachidonic acid

NSAIDs inhibit the conversion of arachidonic acid to prostaglandins.

Formation of Prostaglandins in body

• Difficult tooth movement may be seen in patients taking steroids and analgesics.

​

• Tricyclic antidepressants (doxepin, amitriptyline,imipramine), antiarrhythmic agents (procaine), antimalarial drugs (quinine, quinidine, chloroquine), and methyl xanthines affect prostaglandin levels.

​

• In addition, the anticonvulsant drug phenytoin has been reported to decrease tooth movement in rats, and some tetracyclines (e.g., doxycycline) inhibit osteoclast recruitment, an effect similar to bisphosphonates.

Orthodontic Force Values

​

• According to Schwarz, optimal force approximated the capillary vessels’ blood pressure, thus, preventing their occlusion in the compressed periodontal ligament.(20-26 gm/cm² of the root surface area)
• Force duration and force decay
• Threshold duration: 4-8 hours
• Light continuous force-frontal resorption
• Heavy continuous force-undermining resorption

​

​

Orthodontic force is classified based on the duration and decay rate of orthodontic force by Proffit WR

1. Continuous
2. Interrupted
3. Intermittent

Continuous : Force maintained at some appreciable fraction of the original from one patient visit to the next,for example, elastics, NiTi coil springs, etc.

​

Interrupted : Force levels decline to zero between activations. Produced by appliances such as screws, etc.

​

Intermittent : Force levels decline abruptly to zero intermittently when the orthodontic appliance is removed by the patient. Produced by all patient activated appliances, such as removable plates, headgear, etc.

​

Deleterious effects of orthodontic force

• Mobility
• detachment of fibers
• Pain
• Crushing of fibers
• Lasts for 2 to 4 days
• Decreased by repetitive chewing for first 8 hours
• Light forces are the key

​

• Effects on pulp
• Transient inflammatory response
• Results in patient discomfort
• Loss of vitality
• Effects on root structure
• Attack on cementum occurs ,but usually repair occurs
• Heavy force- severe root resorption
• Effect on alveolar bone
• Rare
• Loss of crestal bone height

• Orthodontic appliances should not be reactivated less than 3 weeks.
• The period of 3 weeks interval for the activation of the appliance is given for tissue repair in the PDL.

Possible Questions

• Discuss theories of tooth movement
• Discuss histological as well as bio chemical pathway of tooth movement
• Difference between frontal resorption and undermining resorption
• What are the stages of tooth movement according to Burstone

Acceleration of tooth movement

ROOT RESORPTION IN ORTHODONTICS

ROOT RESORPTION

• More accurately, resorption of root cementum and dentin.
• Normal ageing process in many individuals
• Root resorption induced by light orthodontic forces is reversible (by regeneration and repair of cementum and/or dentin).
• Can lead to tooth mobility in severe cases.

Factors affecting Root Resorption:

• Individual tooth vulnerability,
• Endodontically treated teeth,
• Age,
• Orthodontic appliances,
• Magnitude of Force,
• Duration of Force and
• Direction of Tooth Movement

Root Resorption

• Generalized Root Resorption

​

• Localized Root Resorption

Generalized Root Resorption

• Affects most, if not all, teeth; maxillary incisors more susceptible than other teeth.
• Could be moderate or severe but commonly in the range of up to 2.5 mm.
• Etiology largely unknown but predisposing factors include conical roots with pointed apices, distorted tooth form, or a history of trauma.

Types

• physiological - resorption of deciduous teeth
• Inflammatory
• replacement - ankylosis
• Idiopathic - no identifiable cause

Classification

Kaley & Phillips,1991

• grade 0 - no resorption
• grade 1 - slight blunting
• grade 2 - moderate resorption up to ¼ of root length
• grade 3 - severe resorption > ¼ root of length

Localized Root Resorption

• Can’t always be distinguished from generalized root resorption.
• Maxillary incisors more susceptible than other teeth.
• Only in rare cases can the causes, such as heavy orthodontic forces, be pinpointed.
• Etiology largely unknown.