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Ortho. Biomechanics

Mechanism of Orthodontics
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Title: Ortho. Biomechanics 1 Mechanical Principles inOrthodontic Force Control By Manar Alhajrasi BDS,MS,Ortho SBO, Morth. 2 Two Types of Orthodontic AppliancesRemovable vs. Fixed 3 Fixed appliances
  • Bands
  • Brackets
  • Wires
  • Accessory appliances
  • 4 Brackets
  • Metal bracket
  • 24K plating gold bracket
  • Clear Bracket
  • Plastic brackets
  • Ceramic brackets
  • Metal reinforced Ceramic brackets
  • 5 Plastic brackets
  • Staining and discoloration
  • Poor dimensional stability
  • Larger friction
  • 6 Ceramic brackets
  • Advantages over plastic brackets
  • Durable, resist staining
  • Dimensionally stable
  • Disadvantages over metal brackets
  • Bulkier than metal bracket
  • Fractures of brackets
  • Friction is bigger than that in metal bracket
  • Wear on teeth contacting a bracket
  • Enamel damage on debonding
  • 7 Self ligating bracket Advantage Less friction 8 Self ligating bracket Smart Clips 9 Invisible orthodontics?
  • Lingual brackets
  • Invisalign 10 Step 3 You receive your aligners in a few weeks. Step 5 You've finished treatment! Step 1 Visit your orthodontist or dentis Step 2 Invisalign makes your aligners Step 4 You wear your aligners 11 Invisalign vs. braces
  • patients treated with Invisalign relapsed
  • more than those treated with conventional
  • fixed appliances.
  • Kuncio D, et al. Angle Orthod 200777 864-9
  • 12 6 weeks later 13 Wires
  • Type
  • NiTi wire (Nickel-Titanium wire)
  • TMA wires (Titanium-Molybdenum-Alloy)
  • Stainless steel wire
  • Shape
  • Round wire
  • Rectangular wire
  • 14 (No Transcript) 15 Fixed appliance properties of arch wires related to force levels, rigidity, formability, etc. 16 General Characteristics ofOrthodontic Forces Optimal light, continuous Ideal material Maintains elasticity Maintains force over a range of tooth movement low load deflection rate 17 Materials Production of OrthodonticForce Elastic behavior Defined by stress-strain response to external load Stress internal distribution of the load force/unit area Strain internal distortion produced by the load deflection/unit length 18 Orthodontic Model Beam Force applied to a beam stress Measure deflection strain examples Bending Twisting Change in length Defined by 3 points 1. Proportional limit Point at which permanent deformation is first observed, Similar to elastic limit 2. Yield strength Point at which 0.1 deformation occurs 3. Ultimate tensile (yield) strength Maximum load wire can sustain 19
  • Ultimate tensile (yield) strength
  • Maximum load wire can sustain If the wire is deflected beyond its
  • yield strength, it will not return to its original shape, but clinically useful
  • springback will occur unless the failure point is reached.
  • Defined in force
  • deflection or stress strain diagrams
  • Useful properties
  • Stiffness
  • Range, springback
  • Strength
  • Each is proportional to the slope of
  • the elastic portion of the force-deflection
  • Curve. The more horizontal the slope,
  • the springier the wire the more
  • vertical the slope, the stiffer the wire.
  • 20 Stiffness versus Springiness Reciprocal relationship Springiness 1/stiffness Related to elastic portion of force deflection curve (slope) Range Distance wire will bend elastically before permanent deformation, This distance is measured in millimeters (or other length units) Springback Found after wire deflected beyond its yield point, it will not return to its original shape but Clinically useful Wires often deflected past yield point Strength stiffness x range 21 Resilience, Formability Resilience Area under stress strain curve to proportional limit Represents energy storage capacity Formability The amount of permanent deformation a wire can withstand before breaking 22 Ideal Orthodontic Wire Material Deflection properties High strength Low stiffness (usually) High range High formability Other properties Weldable, solderable Reasonable cost No one wire meets all criteria! Select for purpose required 23 Wire Materials Precious metal alloys Before 1950s gold alloys, corrosion resistant Stainless steel, cobalt-chromium (elgiloy) alloys Improved strength, springiness Corrosion resistant chromium Typical 18 chromium, 8 nickel Nickel-titanium (NiTi) alloys 1970s applied to orthodontics Demonstrates exceptional springiness Two special properties shape memory, super elasticity 24 Uses of Ni-Ti Arch wires
  • Good choice
  • Initial stages of Tx
  • Leveling and aligning (good stiffness, range)
  • Poor choice
  • Finishing (poor formability)
  • 25 Effects of Length (Cantilever) Strength Decreases proportionately E.g., double length half the strength Springiness Increase by cube of ratio E.g., double length 8x the springiness Range Increases by square of ratio E.g., double length 4x the range 26 Effects of Diameter Cantilever Strength Changes to third power Ratio between larger to smaller beam E.g., double diameter deliver 8x strength Springness Changes to fourth power, Ratio between smaller to larger beam E.g., double diameter Range E.g., double diameter half the range 27 Biomechanical Design Factors inOrthodontic Appliances Terms Force (F) load applied to object that will tend to move it to a different position in space Units grams, ounces Center of resistance (CR) point at which resistance to movement can be concentrated Object in free space CRcenter of mass Tooth root CR halfway between root apex and crest of alveolar bone 28 Design Factors in OrthodonticAppliances Moment product of force times the perpendicular distance from the point of force application to the center of resistance Units gm/mm Created when line of action of a force does not pass through the center of resistance Force will translate and tend to rotate object around center of resistance 29 Design Factors in OrthodonticAppliances Couple two forces equal in magnitude but opposite in direction No translation Produces pure rotation around center of resistance 30 Design Factors in OrthodonticAppliances Center of rotation point around which rotation occurs when object is being moved Can be controlled with couple and force Can be used to create bodily tooth movement 31 Friction
  • Can dramatically affect the rate of tooth movement
  • Considerations
  • Contact angle between orthodontic bracket and arch wire
  • 2. Arch wire material
  • 3. Bracket material
  • 32 Contact Angle When sliding a tooth on an arch wire Tooth tips Further tipping prevented by moment created as bracket contacts wire contact angle Increase contact angle increase resistance Greater force needed to overcome friction 33 Friction and Tooth Movement Effects of arch wire material The greater titanium content, the more friction Due to surface reactivity (chemistry) Sliding resistance titanium gt stainless steel arch wires Effects of bracket material Stainless steel least friction Titanium brackets high friction likely Ceramic Rough, hard surface Increases friction Ceramic with steel slot Reduced friction 34 Alternatives to Sliding (Friction)
  • Segmented mechanics or closing loops mechanics
  • Activate loops
  • Loops close to original shape
  • Retract teeth toward space as loops close
  • No sliding, no frictionFrictionless mechanics
  • 35 Summary Ideal orthodontic forces Wire properties Strength, stiffness, range (springback) Resilience, formability Wire materials Changes in diameter, length Design factors Force, center of resistance, moments, couples, center of rotation Use of rectangular wires couples Friction Contact angle, wires, brackets
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