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