Relevancy Of Biomechanics In Clinical Orthodontics In 2020 Ravindra Nanda
Time of aligners and tads biomechanics can be overlooked
Features of biomechanics
- Magnitude of the force
- Direction of the force
- Duration of the force
- Point of force application
- Centre of resistance
- Centre of rotation
- Friction
Intrusion arch – conventional
- Higher force anteriorly
- A distance from the CoR of the anterior segment = moment = mesial rotation / counter clockwise
- Equal and opposite force posterior segment, extrude the molars
- Again moment created posteriorly, distal rotation / clockwise rotation
Space closure with TADs
- Using retraction hooks
- If maintain the same line of action, Ui will retrocline / loss of torque
- Require variation of the line of action as it changes through space closure –
AOB case with TADs
- Requires anterior extrusion
- Extrusion arch – Similar to ricketts intrusion arch inverted
- Extrusion force anteriorly, generate moment to retrocline anteriors
- Posterior extrude and moment mesial rotation – prevented by TADs 5-6 region and rigid attachment with wire, preventing any movement of posterior teeth
Aligners and bodily movement
- Fixed appliances entail bodily movement due to the archwire essentially not fitting, it creates a couple / force system to resolve the ‘misfit’
- Create a force system with aligners
- Modification to aligner : grooves, material
- Modification to tooth surface: Attachments
- Auxillaries: TADs elastics
Difference aligners Vs fixed
- Plastic deformation
- Inconsistent force system in both!
- Unpredictability of aligner force system Vs full expression in fixed cases
AOB cases with aligners
- Horizontal attachments on anterior teeth – achieve extrusion
- Stripping of dentition, average 2-4mm, 35% of cases 4mm+
- Set up – shows significant posterior intrusion and anterior extrusion, not achieved clinically. Most movements with anterior extrusion upper, and some lower anterior extrusion.
- Refinement 50% of cases required 1 refinement – plastic deformation can not give full expression. K. Harris 2020
References
Harris, K., Ojima, K., Dan, C., Upadhyay, M., Alshehri, A., Kuo, C.L., Mu, J., Uribe, F. and Nanda, R., 2020. Evaluation of open bite closure using clear aligners: a retrospective study. Progress in Orthodontics, 21(1), pp.1-9.
