The biological basis of the orthodontic Therapy

1. The biological basis of the orthodontic Therapy Fengshan Chen Tongji University

2. Law 1In orthodontics, tooth moves through bone and brings the periodontal ligament with it.

3. The basis of the Periodontal Ligament (PDL) • Normal width 0.25 mm or 250 micrometers. • Cells, fibers, ground substance.

4. Cells of PDL • Fibroblasts • Osteoblasts, osteoclasts • Cell rests of Malassez • Mesenchymal stem cells • They all proliferate at different stages of tooth movement. • You must know what functions each has in tooth movement.

5. Fibers of the PDL • Collagen and oxytalan • Some of them are stretched, torn and ruptured, whereas others are compressed and undergo aseptic necrosis

7. Ground substance of the PDL • Proteoglycans and other proteins • Their contents and expression are altered upon tooth movement • Water squeezed in and out during tooth movement

8. Alveolar Bone • Thin cortical bone and porous (lamina dura) • Fluid pumped in and out of the PDL • Trabecular bone underneath • Must remodel before teeth can be moved

9. C:Cell F:Fiber AB: Alveolar bone G:Gingivity CE:

10. Law 2Tooth cannot move unless bone apposition and resorption take place.

11. Susan M. Ott Univ of Washington

12. Law 3There will be no tooth movement unless there is a force.

13. The basis of Force • The force must have the right characteristics such as the magnitude and duration ---- it must meet certain threshold.

14. Force Types • Light, continuous forces • Never declines to zero. • Interrupted forces • Declines to zero • Intermittent forces • Declines to zero

15. Force Magnitude (Level) • In the range of 10 to 200 grams. • Varies with the type of tooth movement. • Light, continuous forces are currently considered to be most effective in inducing tooth movement. • Heavy forces cause damages and fail to move the teeth.

16. Force Duration • Threshold --- 6 hrs per day. • No tooth movement if forces are applied less than 6 hrs/d. • From 6 to 24 hrs/d, the longer the force is applied, the more the teeth will move.

17. Law 4Orthodontic tooth movement is not the only type of tooth movement.

18. Types of Tooth Movement • Eruption • Active • Passive • Lateral drifts • Physiological • Due to loss of adjacent teeth • Orthodontic tooth movement

19. Types of Tooth Movement • Intrusion • Extrusion • Tipping • Bodily movement • Rotation

23. Mechanisms of Tooth Movement • Piezoelectric theory.

25. Mechanisms of Tooth Movement • Pressure-tension theory.

30. The Optimal Force • “High enough to stimulate cellular activity without completely occluding blood vessels in the PDL” (Proffit et al. 2000). • Actively being investigated in a scientific field known as mechanotransduction.

32. Law 5Orthodontic tooth movement cannot occur unless cells are at work.

33. Force --- fluid flow --- cell-level strain • Deformation of cell membrane leading to cytoskeletal changes • Second messenger pathways • Gene upregulation in fibroblasts, osteoblasts and osteoclasts

34. Effect of the light force on the PDL • Light, continuous forces • Osteoclasts formed • Removing lamina dura • Tooth movement begins • This process is called “FRONTAL RESORPTION

35. “Frontal resorption” because it occurs between the root and the lamina dura.

36. Phase 3 Phase 2 Phase 1 Tooth movement (mm) Time (Arbitrary Unit) Light force leading to frontal resorption • Phase 1 – Mechanical compression and tension of the periodontium • Phase 2 --- Mechanically induced cellular and genetic responses; no tooth movement • Phase 3 --- Accelerated tooth movement due to frontal bone resorption

37. Effects of heavy force on the PDL • Heavy, continuous forces • Blood supply to PDL occluded • Aseptic necrosis • PDL becomes “hyalinized” – “HYALINIZATION” • This process is called “UNDERMINING RESORPTION”.

38. “Undermining resorption” because it occurs on the underside of lamina dura, not between lamina dura and the root.

39. Law 6Frontal resorption occurs in the PDL, whereas undermining resorption occurs underneath the lamina dura.

40. Phase 3 Phase 2 Phase 1 Tooth movement (mm) Time (Arbitrary Unit) Heavy force leading to undermining resorption • Phase 1 – Mechanical compression and tension of the periodontium • Phase 2 --- Continuing mechanical compression; little cellular and genetic responses; no tooth movement • Phase 3 --- Cells recruited from the undermining side of lamina dura, not within the PDL, to induce undermining bone resorption

41. Phase 3 Phase 3 Phase 2 Phase 2 Phase 1 Phase 1 Tooth movement (mm) Frontal resorption Time (Arbitrary Unit) Undermininging Resorption Tooth movement (mm) Time (Arbitrary Unit)

42. Law 7Frontal resorption facilitates orthodontic tooth movement, whereas undermining resorption impedes orthodontic tooth movement.

43. Anchorage • Newton’s law: for every action, there is reaction. • Defined as “resistance to unwanted tooth movement.” • The “anchorage value” of any tooth is roughly equivalent to its root surface area. Thus, molars and canines generally have higher anchorage values than incisors and bicuspids.

44. Anchorage types • Reciprocal anchorage. • Reinforced anchorage. • Stationary anchorage. • Cortical anchorage.

45. Reciprocal anchorage • Both units move roughly equal distance. • Exemplified by closing a diastema between two central incisors.

46. Reinforced anchorage • Unit A has substantially more anchorage value than Unit B. Thus, Unit A moves little but Unit B moves a lot. • Exemplified by retracting anterior teeth to close an extraction space by using posterior teeth as a reinforced anchorage unit. Unit A Unit B

47. Biomechanics of Tooth Movement • Center of Resistance --- A point on the tooth around which the tooth shall move. For most teeth, COR is 2/5 way between the apex and the crest of the alveolar bone. • Center of Rotation --- The point around which rotation occurs when an object is being moved.

49. Force and Couple • Force • Is applied by orthodontic appliances. • Induces tipping, translation, intrusion, extrusion and/or rotation. • Couple • Two forces of opposite directions and with non-overlapping points of application. • Translation of teeth occurs in response to appropriate force couples.