Join me as I summarise the 1st day of the Saudi Orthodontic Clear Aligner meeting, where Mazyar Moshiri delivered a masterclass on biomechanics with aligners.
Conclusions:
· Attachment design: perpendicular to resultant force
· Rotations: Place oblique attachment to counter intrusive force of aligner
· Overcorrect torque, add mesial crown tip due to wagon wheel effect
· Deep bite: synergistic movements: proclinatiion and extrusion
· AOB: synergistic movements: retroclination and extrusion
Concepts
Intrusion attachment
1. Intrusion on aligners
a. No intrusion attachment
o Require attachments on adjacent teeth
o If loss of tracking = occurs on adjacent teeth, with the aligner extruded off the teeth
Rotation and intrusion
· Force labial face and palatal face to create rotation moment.
· ‘Squeeze force’ occurs = vertical force = intrusion force.
· Solution
o Optimised attachment = Extrusive vector, usually gingival bevel and oblique
o Resolve resultant force, horizontal and vertical = oblique attachment
Torque
· Wagon wheel effect, for every 4 degrees of lingual crown torque there is distal crown tipping / mesial root tipping.
o Solution: OFFSET with 1 degree of mesial crown tip
· Torque lags behind, takes longer , like rectangular NiTi archwire
· Buccal root torque: Upper molar buccal attachment to provide extrusive buccal force to create moment.
Design of attachment
· KEY CONCENT: Attachment perpendicular to the force
· Perpendicular force to rotation = Long vertical attachment
· But rotational force has intrusive force the ‘squeeze force’
o Therefore horizontal and vertical force = attachment perpendicular to the force = oblique attachment
Root tip
· Open space 0.2mm interproximal = so more plastic wrap around tooth
· Then start root movement in staged way.
Deep bite
Concept:
Synergistic movements, procline = relative intrusion
Staging AP movements
o Proclination 1st to open the bite
o Then IPR
§ Not at the same time as IPR = retroclination, therefore should not procline and iPR at the same time as bite will not open
§ Does round trip – procline and intrude, then IPR and align
Staging deep bite
· Staging of OB, canines intrude 1st, then 2-2
· Likely loose tracking on lower incisors or 4s – anchorage horizontal G5/G7 attachments
Curve of Spee
· If COS not corrected, then AP also difficult to correct
· If not correct COS:
o Posterior openbite – caused by anterior interferences
§ Caused by inadequate attachment
§ Lack of over correction
OGN case deep bite
· Order 10 passive aligners near end stage pre surgery = minimum 2 months to make up for ‘lag’ or vivera retainer / essix – thicker, greater expression of torque
· TAD – intermaxillary elastics x 2
o TAD maxilla:
§ Buttons lower premolars
§ Intermaxillary elastics to guide lower premolar teeth into the tray
Openbites
Concept
Synergistic movement: Retrocline and extrude
· Aligners do not extrude along the long access
· Aligner pushes tooth lingually and extrudes in the process, retroclines
Posterior intrusion is unpredictable, anterior extrusion is more likely
Thank you to the Saudi Orthodontic Society for facilitating this podcast
