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Smart And Visible Biomechanics – Optimizing Treatment Mechanics Based On Prediction Strategy For Long-Term Tooth Movement Noriaki Yoshida Episode 29 IOC 2020

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Farooq Ahmed

Bowing effect in sliding space closure

Bowing is:

  • Extrusion of upper incisors
  • Retroclination of labial segment – uncontrolled tipping
  • = Increase in overbite extend treatment time

 

Possible solution to bowing effect investigated using finite model analysis:

 

  • Using a powerarm in retraction – distal to upper lateral incisors
    1. 6mm powerarm – Good control of anterior torque but still bowing effect
    2. 10mm powerarm with 17×22 archwire in 018 slot – Good control of anterior torque, no bowing but intrusion of anterior segment
    3. 12 mm powerarch with 19×25 archwire in 022 slot – Good control of anterior torque and little change in occlusal plane

 

Difference in archwire slot size due to differences in slop, recommend the following:

  • 018 slot = 9mm powerarm with 17×22 archwire
  • 022 slot = 12mm powerarm with 19×25 archwire

 

Not possible with 022 slot to prevent any anterior loss of torque or bowing

 

Solutions to preventing bowing effects

  • Compensatory curve anteriorly 2-6mm
  • High torque prescription in the labial segment

 

Compensatory curve details

  • The greater the compensatory curve = greater friction produced
  • Friction = coef of friction x normal force = greater friction in the later stages of space closure

 

Reduction of friction

  • Archwire with rectangular anterior cross section, and round posterior cross section = reduced friction as less binding

 

Forces in space closure using power arm

  • Forces unequally distributed in conventional en-mass retraction
    • High force canine 60g, lateral and central 10-20g of force

 

Solution to unequal forces on anterior segment

  • Retract canine and lateral incisors with different space closing mechanics

 

Rowboat effect – posterior anchorage loss during space closure due to trying to maintain anterior torque

  • Use of TADs
  • Loop space closure with = loss of anterior torque
    • Incorporating a 30 degree gable bend does not significantly help with anterior torque loss
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