Biomechanics and Force Delivery Systems- Wires to Screws to Plastic Ravi Nanda 2021 Edward H. Angle Award Lecture. AAO June 2021. Episode 87

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Farooq Ahmed

Biomechanics and Force Delivery Systems: Wires to Screws to Plastic  Ravi Nanda

2021 Edward H. Angle Award Lecture


Edward Angle textbook

Some innovations today relate to Edward’s appliances

·      Expansion arch

·      Single tie bracket

·      Headgear

·      Biomechanics

o   Pushing, pulling and rotational forces

·      Bakers anchorage = class 2 elastics




·      Significant number of studies, however relate little to the outcome



·      Significant change to biomechanics with NitI

·      Properties of NiTi wire, show larger wire requires greater deflection to achieve superelastic plateau, delivers higher force  Pesce 2011

o   Ravi recommended using larger wires to give greater control

·      Location of the bend can result in variation in moment depending on location

·      Example of class 2 and moments

o   Molar correction: using a simple lever arm through the molar and premolars to distalise molar, followed by class 2 elastics to retract


Force Vs moment

·      Ravi says moment is more important than the amount of force, for example headgear, 500g is less important than the direction of moment from the outerbow


TADs and biomechanics

·      Learning from experience: Direct retraction on TADs in class 2 cases results in increase in overbite  Davoody 2012

·      Directing moment at different points of the cenre of rotation changes the type of tooth movement – Ravi 2014 JO

·      Infrazygmatic TAD can be applied to retract the upper incisors and passivng the moment through the centre of rotation of the anterior teeth = retract and no increase in overbite

·      Buccal shelf TAD: in class 3 case with AOB, powerchain results in retraction for AP as well as counterclockwise rotation which closes AOB



·      Started with clinicians depending on the company on how to move the teeth

·      Advances in aligners and engagement with biomechanics = force delivery through deformation of plastic

o   Pressure area, attachments, auxillaries

·      Force from different levels of activation of aligners Li 2016 KJO

o   Varied of activation 0.2-0.6 and force loss

§  0.2mm activation with time results in loss of force up to 40% at 7 hours

§  Initial force 8 Newtons at 7 days 2 Newtons


·      AOB cases and aligners Harris 2020

o   Most correction occurs through retraction and IPR, relative extrusion of upper and lower incisors, not molar intrusion


Aligner Attachment

·       Different attachment design for canine teeth Ojima in press paper

o   Conclusion:

§  Did not matter shape of attachment, the moment was key

§  Without attachment uncontrolled tipping


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