Controlling 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.

Biological Basis of Orthodontic Therapy Dr. Manar K Alhajrasi BDS,MSC,SBO,M.Orht.

Contents Tooth supporting tissues PDL-structure and function Role of PDL – eruption & stabilization Response to orthodontic force Biologic basis of tooth movement Biologic electricity Pressure-tension

Contents How 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 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.

The Periodontium Orthodontic 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 fluid 4 connective tissues Two fibrous - Lamina propria of the gingiva. - Periodontal ligament Two mineralized -Cementum -Alveolar bone

Gingival fibers Circular fibers Dentogingival fibers Dentoperiosteal fibers Transseptal fibres (Accesory fibres)

PDL Progenitor cells Fibres -Collagen Synthetic cells -Oxytalan a) Osteoblasts b) Fibroblasts c) Cementoblasts Resorptive cells A) Osteoclasts B) Fibroclasts C) Cementoclasts Fibres -Collagen -Oxytalan Ground Substance -Proteoglycans -Glycoproteins

PDL Constant remodeling- fibers, bone & cementum. Principal fibres - Alveolar crest group Horizontal group Oblique group Apical group Trans-septal group Tissue fluid- Derived from the vascular system Acts as Shock absorber-retentive chamber with porous walls.

Normal function Heavy forces- > 1 sec-force transmitted to bone Bone bending

Pressure / Force Prolonged force Short duration Remodeling of adjacent bone PDL- Adaptive

Role of PDL Implications: Continued eruption Active stabilization- threshold for orthodontic force.( 5- 10 gm/cm2 )

Theories of tooth movement Pressure- Tension theory Fluid –Dynamic theory –Bien Squeeze- Film effect Oxygen tension Bone bending theory Neither incompatible nor mutually exclusive

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

Biologic electricity 2 types of electric signals Piezoelectricity Electric current flows- electrons are displaced from 1 part of the crystal to the other. Bioelectric Potential Unstressed bone

Piezoelectricity Piezoelectricity 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

Biologic electricity Zeev 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 controls Orthodontic tooth movement accelerated

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.

Pressure-tension Sandstedt (1904), Oppenheim (1911),and Schwarz (1932).

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

Fluid Dynamic theory Force of longer duration- interstitial fluid squeezed out Vascular stenosis – decreased oxygen level- compression Alteration in the chemical environment Alterations in the blood flow- changes the chemical environment

Cytokines Named based on presumed targets- Leukocytes- Interleukines Maintained original names-eg. G-CSF IL, IFNs,TNF,CSFs,GFs & Fractalkines (chemokine family) 1980s- cytokines were produced by osteoblasts & fibroblasts- normal physiologic turnover.

Effects of force magnitude Frontal resorption Hyalinization Undermining resorption

Effects of force magnitude Frontal resorption- Cells attack the adjacent lamina dura The osteoblasts lag behind in differentiation-PDL space enlarges- further initiate osteoclast remodeling

Effects of force magnitude Heavier force- The blood vessels- excessively compressed and occlude Sterile necrosis- tissue injury = Hyalinization

Effects of force magnitude Vascular circulation impeded- dec. celluar differentiation- degradation of cellular and vascular structures Glass like structure (1-2mm) - Hyalinization

Effects of force magnitude Undermining resorption- Hyalinization- remodeling of bone around Necrotic connective tissue-derived from adjacent undamaged area Osteoclasts appear – adjacent bone marrow spaces- attack on underside of bone next to necrotic area Inevitable delay- Cell differentiation Considerable thickness to be removed

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 movement Tipping -

Force distribution & Type of tooth movement Forces should be kept low- high concentration of forces Destruction of the alveolar crest

Force distribution & Type of tooth movement Bodily tooth movement-uniform loading of the teeth is seen. To produce the same pressure-same biologic response- force required is twice

Force distribution & Type of tooth movement Torque-Initially- Pressure close to middle region-PDL wider at the apex Later part-apical region begins to compress Rotation-2 pressure & tension sides Tipping – some hyalinization does occur

Force distribution & Type of tooth movement Intrusion-very light forces-concentrated in a small area Stretch- principal fibres Extrusion-Only areas of tension Light forces- could loosen teeth considerably

Optimum forces for various tooth movements-Proffit

Force Duration Sustained force- cyclic nucleotides appear- only after 4 hours Longer & constant the force- faster the tooth movement

Type force duration-force decay Teeth move in response to force- force changes May drop to zero

Type force duration-force decay 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

Type force duration-force decay 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