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 www.jwfo.org/article/S2212-4438(21)00057-6/pdf Thank you to the author Sarah Abu Arqub for informing me about it.

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