1. The Bio-mechanics of Orthodontic Tooth Movement D 657 Dr. Shiva Shanker Dr. Allen Firestone

2. Contemporary Orthodontics, Chapter 10, 3rd Edition: pp. 337-360 2nd Edition: pp. 302-315 Recommended: Smith and Burstone, Mechanics of tooth movement. American Journal of Orthodontics, 1984; 85:294-307 LECTURE READING

3. 1. Definition of terms2. Mechanics of tooth movement3. Anchorage in orthodontic appliances LECTURE OBJECTIVES

4. 1. Review Laboratory Procedures: Arch Length Analysis Molar Uprighting: Preparing typodont LECTURE OBJECTIVES

5. Definitions • Force • Center of Resistance (Cres) • Types of Tooth Movement

6. Force • Physical property e.g. distance, weight, temperature, force. • Physical properties are, mathematically, scalars or vectors • Scalars have a magnitude and no direction e.g. weight, temperature • Vectors have a magnitude and a direction e.g. force

7. Vectors • Line of Action • Sense • Magnitude • Point of Application

8. Resultant of Forces • Common point of application • Different points of application • Resolving force into components

9. Common point of applicationResultant of Forces

10. Different points of applicationResultant of Forces

11. Resolving Force Into ComponentsResultant of Forces

12. Force • A load applied to an object that will tend to move it in the direction of the applied force • Defined in units of Newtons • Orthodontic purposes measured as grams or ounces

13. Definitions • Force • Center of Resistance (Cres) • Types of Tooth Movement

14. Center of Resistance • Free body: center of mass/gravity = point of balance • Restrained body (tooth): center of resistance (Cres) • By definition, a force acting through Cres moves tooth with no change in orientation = translation

15. Center of Resistance • A point at which resistance to movement can be concentrated for mathematical analysis (= Cres) • The center of resistance for a tooth is 1/2 - 1/3 (40%) root length apical to alveolar crest • Cres varies with root length and alveolar crest height

16. Center of Resistance • Cres varies with root length and alveolar crest height

17. Definitions • Force • Center of Resistance (Cres) • Types of Tooth Movement

18. Types of Tooth Movement • Translation • Rotation • Combination/Tipping • Moment • Couple • Center of Rotation (Crot)

19. Translation = Bodily Tooth Movement • A force applied in line with the center of resistance; the tooth is translated with no rotation relative to the force • Orthodontically, the point of attachment is irrelevant; the line of action of force determines the effect on the tooth

20. Translation or Bodily Tooth Movement

21. Tipping Tooth Movement • A force that doesn’t pass through Cres causes translation + rotation = tipping i.e., tends to tip the tooth, movement with a rotational component.

22. Definitions • Force • Center of Resistance (Cres) • Types of Tooth Movement Moment Couple Center of Rotation (Crot)

23. Moment • If the line of action of a force is at a distance from the Cres the force will produce some rotation. The potential for rotation is measured as a moment

24. Moment - Magnitude • Magnitude = perpendicular distance from Cres to the line of action X magnitude of force (unit = gram mm) • Direction: Clockwise or Counter-cw

25. Couple • No single force can cause pure rotation • Only a couple can • Two forces: equal magnitude; parallel and non-collinear; opposite sense • Couple is a ‘free vector’

26. Couple • Two forces; equal magnitude; parallel and non-collinear; opposite sense • Translational effects cancel each other out

27. Couple • The moments are in the same direction and are additive

28. + = 1000 gm mm Couple • The Sum of the Moments = 50 g X 10 mm 50 g X 10 mm

29. Couple • The moment of a couple is equal to the magnitude of one of the forces X distance between them (50g X 20 mm) • Only a couple can cause pure rotation

30. Definitions • Force • Center of Resistance (Cres) • Types of Tooth Movement MomentCouple Center of Rotation (Crot)

31. Center of Rotation Crot • The point around which rotation occurs when an object is being moved • This point will vary depending on the force/moment/couple being applied Bodily movement or translation Tipping movement

32. Determining Crot • Connect the before and after positions of 2 points • The intersection of the perpendicular bisectors of these lines is Crot

33. Type of Movement Translation Uncontrolled tipping Controlled tipping Root movement Center of Rotation Infinity Slightly apical to Cres Apex Incisal edge Tooth Movement and Crot

34. Control of Center of Rotation Point of application of force: Closer to Cres Smaller moment Less rotation More translation

35. 1. Definition of terms2. Mechanics of tooth movement3.Anchorage in orthodontic appliances LECTURE OBJECTIVES

36. Anchorage • Resistance to unwanted tooth movement • Resistance to reaction forces • Maximizing tooth movement and minimizing unwanted “reactionary effects”

37. Strategy 1 • Reciprocal space closure • Minimum anchorage requirements

38. Strategy 2 - Differential Anchorage Values • Differential space closure • Intermediate anchorage requirements 1237 706

39. Strategy 2A - Differential Tooth Movement • Differential space closure • Intermediate anchorage requirements

40. Strategy 3 • Dissipate reactionary forces over as many teeth (or as widely) as possible • Reinforced anchorage • Maximum anchorage

41. Frictional Effects on Anchorage • Frictional resistance to sliding archwires against brackets

42. Factors Affecting Friction • Surface of wires/bracket - • 1) Stainless steel slides well on stainless steel; • 2) Nickel titanium alloy wires have greater frictional resistance • 3) Ceramic brackets also exhibit greater frictional resistance

43. 1. Definition of terms2. Types of tooth movement3. Anchorage in orthodontic appliances LECTURE OBJECTIVES

44. 1. Review Laboratory Procedures: Arch Length Analysis Molar Uprighting: Preparing typodont LECTURE OBJECTIVES

45. Arch Length Analysis = Space Analysis • Permanent Dentition • Mixed Dentition

46. Space AnalysisPermanent Dentition +Arch length available - M/D width of 12 teeth: first molar to first molar __________________________ = Arch length deficiency or excess

47. Arch Length The arch circumference: measured at the proximal contacts of posterior teeth and the incisal edges of anterior teeth i.e., the widest part of the tooth.

48. Measuring Arch Length

49. Space Analysis Measuring Tooth Width

50. Space Analysis: Mixed Dentition • The same procedure in themixed dentition but • Need to estimate the size of unerupted permanent teeth (premolars and canines) • Moyers proportionality tables, Tanaka and Johnston