Aligner biomechanics: The hidden truths. Madhur Upadhyay

Join me for a summary of Madhur’s lecture on the biomechanics and aligners, where he describes the fundamentals biomechanics of aligners, explains the reason for the shortcomings of aligners and his tips to reduce them.


How do aligner work?

Two approaches
Shape moulding effect – shape driven
Move position of teeth to ideal
Invisalign 80% through shape moulding
Attachments
20% force delivery
Perpendicular surface with Invisalign
When shape moulding is considered 2g/mm2 of stress, with attachments 20g/mm2, higher stress to aligner, which is a downside

How to aligners treat malocclusions

Deep bite

“Water-melon seed effect” – squeeze a seed and it compression it moves left or right
Squeezing through shape moulding effect
Aligners can apply force through to the centre of resistance of the tooth


Openbite
Advantages in aligners when compared to fixed

Draw bridge effect
Relative extrusion through Retroclination 60% drawer bridge effect Harris 2020
Lever principle
Autorotation effect 28% Harris 2020


Space closure
Common lateral openbite in space closure, why this happens
Equal moment to posterior and anterior segment during space closure
Counter clockwise posterior segment, clockwise anterior segment = BOWING EFFECT
Does not commonly happen in fixed due to archwire rigidity

How good are aligners at moving teeth

Root movement
NOT OBSERVED!

Space closure
Extraction of 4 premolars - Dai 2019
Molars
2mm unplanned anchorage loss
6 degree molar tipping
Incisors
Retrocline 6 degrees more
No incisor root retraction – uncontrolled tipping
Root movement
Conclusion: Roots cannot be moved with aligners such appliances cause tooth movement by tilting motion Zhang 2015

Why can’t aligners move teeth as efficiently as braces?
First principles based understanding
Interplay of dumping and counter play of moment = bodily tooth movement
Braces – create AND CHANGE couple and counter couple through size of wire, material
Aligner – cannot change material properties

Material factors
Stress-relaxation 80% of force lost after 100 minutes Fang 2019 AJODO
Modulus of elasticity – ability to transfer energy
Higher modulus – greater tooth movement
Niti 45GPa, Aligner 1-2GPa Khoda 2013
Inferior in delivering energy
Viscoelastic material
Absorbing shock, dissipating energy in the form of heat
Ideal for retainer – force dampener

Tips
Use more trays as possible, 2 -3 trays per week
Smaller activation
More activation = less force
Average aligners 80-90 aligners = less refinement
Thicker material for heavier movement such as torque and root movement
Thick tipping movement – less root control, improves predictability
Advise reduce insert / removal Skaik 2019

Biomechanics is the law – everything else is just a recommendation


Madur’s lecture in full available on Youtube:

   • Aligner (Invisalign) Biomechanics: Th...  

EDIT - THE PAPER ON THE LECTURE TITLE BY UPADHYAY AND ARQUB IS FREELY AVAILABLE AT JWFO
Biomechanics of clear aligners: hidden truths & first principles
https://www.jwfo.org/article/S2212-44...
Thank you to the author Sarah Abu Arqub for informing me about it.