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
- Counterclockwise 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
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 counterplay 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
Biomechanics is the law – everything else is just a recommendation
Madur’s lecture in full available on youtube:
https://www.youtube.com/watch?v=ycNokW1ojIY&t=3s&ab_channel=orthobites.org
