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The Bio-mechanics of Orthodontic Tooth Movement

The Bio-mechanics of Orthodontic Tooth Movement. D 657 Dr. Shiva Shanker Dr. Allen Firestone. Contemporary Orthodontics , Chapter 10, 3rd Edition: pp. 337-360 2nd Edition: pp. 302-315

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The Bio-mechanics of Orthodontic Tooth Movement

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  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

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