Aligner biomechanics: The hidden truths. Madhur Upadhyay. Episode 145. February 8th 2022

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Farooq Ahmed

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

  1. Shape moulding effect – shape driven
    1. Move position of teeth to ideal
    2. Invisalign 80% through shape moulding
  2. Attachments
    1. 20% force delivery
    2. Perpendicular surface with Invisalign
    3. 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


  1. “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



Advantages in aligners when compared to fixed


  1. Draw bridge effect
    • Relative extrusion through Retroclination 60% drawer bridge effect Harris 2020
  2. 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


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


Biomechanics is the law – everything else is just a recommendation


Madur’s lecture in full available on youtube:



Biomechanics of clear aligners: hidden truths & first principles
Thank you to the author Sarah Abu Arqub for informing me about it.
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