1. 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.

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

3. 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

4. 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

5. 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.

6. 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

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

8. 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

10. 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.

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

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

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

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

15. 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

16. 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

17. 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

18. 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

19. 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.

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

21. 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

22. 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

24. 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.

25. Effects of force magnitude
Frontal resorption
Hyalinization
Undermining resorption

26. 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

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

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

29. 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

30. 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 -

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

32. 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

33. 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

34. 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

35. Optimum forces for various tooth movements-Proffit

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

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

38. 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

39. 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