controlling factors: at the time the primary second molars are lost. both the maxillary and...

Controlling Controlling factors:factors:At the time the primary second molars are lost. Both the maxillary and mandibular molars tend to shift mesially into the leeway space. but the mandibular molar normally moves mesially more than its maxillary counterpart.

a characteristic of the growth pattern at this age is more growth of the mandible than the maxilla, so that a relative deficient mandibule gradually catches up.

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Biological Basis of Orthodontic Therapy

Dr. Manar K AlhajrasiBDS,MSC,SBO,M.Orht.

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ContentsTooth supporting tissues

PDL-structure and functionRole of PDL – eruption & stabilization

Response to orthodontic forceBiologic basis of tooth movement

Biologic electricity Pressure-tension

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ContentsHow teeth move clinically-Concepts of optimal force

Effects of force distribution and types of tooth movement

Force duration and decay

Effects of drugs

Root resorption in orthodontic tooth movement

Tongue are usually not balanced. In some areas, as in the mandibular anterior, tongue pressure is greater than lip pressure. In other areas, as in the maxillary incisor region, lip pressure is greater. Active stabilization produced by metabolic effects in the PDL probably explains why teeth are stable in the presence of unbalanced pressures that would otherwise cause tooth movement.

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The PeriodontiumOrthodontic force ⇆ Changes in the supporting structure.Periodontium is a connective tissue organ covered by epithelium, that attaches the teeth to the bones of the jaws and provides a continually adapting apparatus for support of teeth during function.PDL = cells + fibers + tissue fluid4 connective tissues

Two fibrous - Lamina propria of the gingiva. - Periodontal ligament

Two mineralized -Cementum -Alveolar bone

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Gingival fibers

Circular fibersDentogingival fibersDentoperiosteal fibersTransseptal fibres (Accesory

fibres)

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PDLProgenitor cells

Synthetic cells a) Osteoblasts b) Fibroblasts c) Cementoblasts

Resorptive cells A) Osteoclasts B) Fibroclasts C) Cementoclasts

Fibres -Collagen -Oxytalan

Ground Substance -Proteoglycans -Glycoproteins

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PDLConstant remodeling- fibers, bone & cementum.Principal fibres -

1. Alveolar crest group2. Horizontal group3. Oblique group4. Apical group5. Trans-septal group

Tissue fluid-Derived from the vascular systemActs as Shock absorber-retentive chamber with porous walls.

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Normal function

Heavy forces- > 1 sec-force transmitted to boneBone bending

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PDL- Adaptive

Prolonged force

Remodeling of adjacent bone

Short duration

Pressure / Force

Role of PDL

Implications:Continued eruptionActive stabilization- threshold for orthodontic force.( 5- 10 gm/cm2 )

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Theories of tooth movement

Pressure- Tension theory

Fluid –Dynamic theory –BienSqueeze- Film effectOxygen tension

Bone bending theory

Neither incompatible nor mutually exclusive

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Bone Bending

Farrar- (1888) was the first to suggest-alveolar bone bending plays a pivotal role- tooth movement

Orthodontic appliance is activated- forces delivered to the tooth are transmitted to all tissues near force application- bend bone

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Biologic electricity

2 types of electric signalsPiezoelectricity

Electric current flows- electrons are displaced from 1 part of the crystal to the other.

Bioelectric Potential Unstressed bone

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PiezoelectricityPiezoelectricity is a phenomenon observed in many crystalline materials in which a deformation of the crystal structure produces a flow of electric current as electrons are displaced from one part of the crystal lattice to another Bone- Organic crystal Quick decay & equal & opposite signal

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Biologic electricityZeev Davidovitch – 1980

Applied electric currents to bone- 15 μ amps combined force ( 80 g)

Enhanced bone resorption near the anode & bone deposition at the cathode compared to controlsOrthodontic tooth movement accelerated

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Biologic electricity

Observations-The application of PEMF – increased both rate & final amount tooth movement

Histologic evidence- increase amount of bone deposition & more osteoclasts

Increased protein metabolism-indicated by creatinine, creatinine phosphokinase, uric acid.

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Pressure-tension

Sandstedt (1904), Oppenheim (1911),and Schwarz (1932).

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Classically:

This hypothesis explained that, on the pressure side, the PDL displays disorganization and diminution of fiber production. Here, cell replication decreases seemingly due to vascular constriction. On the tension side, stimulation produced by stretching of PDL fiber bundles results in an increase in cell replication

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Fluid Dynamic theory

Force of longer duration- interstitial fluid squeezed out

Vascular stenosis – decreased oxygen level- compressionAlteration in the chemical environmentAlterations in the blood flow- changes the chemical environment

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Cytokines

Named based on presumed targets-Leukocytes- InterleukinesMaintained original names-eg. G-CSF

IL, IFNs,TNF,CSFs,GFs & Fractalkines (chemokine family)

1980s- cytokines were produced by osteoblasts & fibroblasts- normal physiologic turnover.

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Effects of force magnitude

Frontal resorption

Hyalinization

Undermining resorption

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Effects of force magnitudeFrontal resorption- Cells attack the adjacent lamina duraThe osteoblasts lag behind in differentiation-PDL space enlarges- further initiate osteoclast remodeling

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Heavier force- The blood vessels- excessively compressed and occlude

Sterile necrosis- tissue injury = Hyalinization

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Vascular circulation impeded- dec. celluar differentiation- degradation of cellular and vascular structures

Glass like structure (1-2mm) - Hyalinization

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Undermining resorption- Hyalinization- remodeling

of bone around Necrotic connective tissue-derived from adjacent undamaged areaOsteoclasts appear – adjacent bone marrow spaces- attack on underside of bone next to necrotic area

Inevitable delay-Cell differentiationConsiderable thickness to be removed

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Force distribution & Type of tooth movement

Optimal force-The amount of force & the area of distribution The force distribution varies with the type of tooth movementTipping -

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Forces should be kept low- high concentration of forcesDestruction of the alveolar crest

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Bodily tooth movement-uniform loading of the teeth is seen.

• To produce the same pressure-same biologic response- force required is twice

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• Torque-Initially- Pressure close to middle region-PDL wider at the apex

• Later part-apical regionbegins to compress • Rotation-2 pressure &

tension sidesTipping – some

hyalinization does occur

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• Intrusion-very light forces-concentrated in a small area

• Stretch- principal fibres

• Extrusion-Only areas of tension• Light forces- could loosen teeth

considerably

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Optimum forces for various tooth movements-Proffit

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Force Duration

• Sustained force- cyclic nucleotides appear- only after 4 hours

• Longer & constant the force- faster the tooth movement

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Type force duration-force decay

• Teeth move in response to force- force changes

• May drop to zero

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• Intermittent force- abrupt decline to zero– Removable appliances( HGs), elastics– Tooth movement occurs-Forces decline (interrupted)– Force removed- tooth moves back to tension side– The PDL-improved circulation- formative changes

occur– semihyalinization

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• Continuous force-– Light- frontal resorption– Heavy- undermining resorption- constant-

further U.Resorption• Destructive to the PDL & tooth

• Force decay- – Light force-FR- no movement till activation– Heavy–UR- force drops-repair &

regeneration occurs

controlling factors: at the time the primary second molars are lost. both the maxillary and...

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