EXPERT VIEW - Sandra Tai's Q&A session

Dr. Sandra Tai is an Ambassador of the European Aligner Society, and author of a definitive clinical handbook about clear aligner therapy.  She has produced a clinical insight through a Q&A session for the first issue of EASy Update.

Q1: Deep Bite Biomechanics

Dr Tai, for the correction of deep bite malocclusions, in previous clear aligner courses, it has been suggested to always add large, rectangular attachments on 2nd premolars and molars to allow for better anchorage while levelling the curve of Spee.  What are your thoughts?


The principle for deep bite correction is the same - we need adequate anchorage to hold the aligner down posteriorly in order for the anterior intrusion to express clinically.  With the horizontal rectangular attachments, these are conventional attachments and they act as a handle to hold aligner to the teeth. These are biomechanics and attachment designs that were used BEFORE the innovation of optimized deep bite attachments with extrusion.  The difference with optimized deep bite attachments is that if you requested for posterior extrusion, there is an activation built into the aligner to extrude the premolars to help level the curve of Spee.

Q2: Class II Sequential distalization

I noticed that there were no attachments on the upper molars for a Class II case with sequential distalization. If there is no vertical attachment on the molars to control the root, do you worry that the molar will tip distally during the distalization? Does the existence of unerupted third molars prevent the teeth from distalizing? Is it necessary to remove third molars prior aligner treatment? If it is not necessary to remove it, how much can we predictably distalize the teeth?


If the ClinCheck came back with NO attachments on the molars that are being distalized, then I do not add attachments. What we really need is a very good registration of the distal surface of the second molar whether by scanner or PVS impression. If the second molar comes back on the ClinCheck with only a partial crown (maybe the distal is cut off) then I would add an attachment - usually a vertical rectangular if there is enough clinical crown height, or a horizontal rectangular if the clinical crown is too short - to increase the engagement of the aligner on the tooth.

I have completed many distalization cases without molar attachments. This is because there is aligner plastic wrapped completely around the tooth - especially once the distalization spaces are open - this is very good aligner engagement and if the ClinCheck comes back without any molar attachments, I do not usually add them on.

In younger teens where the wisdom teeth are very high up, there is no need to remove them. But in older teens 16 years or older and adult patients, it may be best to extract the third molars before distalizing. Studies have shown we can predictably distalize approx. 2.25 mm in adult patients. I will allow 2.5 mm in adults and 3.0 mm max in teens.

Q3: Unilateral expansion

I have a case where Align has returned a ClinCheck plan with unilateral expansion for a patient with a unilateral cross bite. Is that biomechanically possible? Is this patient better off with a fixed expander?


First of all, using a third party software for superimposition, we have found that where ClinCheck software shows unilateral expansion, it is expressed bilaterally - but of course, since the aligners are so flexible. Usually, a unilateral crossbite + midline deviation = functional shift. In this case, I would treat it with a Bonded rapid maxillary expander - the occlusal acrylic will disclude occlusion and eliminate the functional shift, giving the patient's muscles time to adapt to not shifting to the side. After the expansion is completed, hold patient in the expander for 3 months to allow mid-palatal suture to fill in with new bone. Then after that, you can do a clear aligner treatment.

Q4: Invisalign First

I am treating an 8-year-old girl with Invisalign First, which is going very well. We are currently at Aligner 18 of 26. Her arch development is progressing well and her overbite has improved (she had an anterior open bite). I was opening space for the upper lateral incisors, but the upper right lateral has now erupted outside of the eruption compensation space. How do I manage this case?


The lateral incisors look very large and clearly not erupting into the eruption compensation that was designed. Unfortunately for UR2, it does not look like you have enough space at this time to move it labially. If you did, you could bond a button on the lingual, trim away part of the pontic on the lingual, and make a slit on the labial, then ask the patient to wear the elastic from the lingual button to the labial slit to move the tooth labially.  For now, I would keep wearing the aligners to keep opening up that space more before you rescan to include both lateral incisors. You can start trimming the lingual of the pontic to see if the tooth will move towards the labial by itself as you open up the space.

Q5: Posterior Open Bite after MA

When finishing with Invisalign with Mandibular Advancement, if a posterior open bite is present, what is the correct way of continuing with treatment? I have my first case with MA and she finished with POB since posterior premolars are still erupting, how do I manage this case, close the posterior open bite and complete treatment?


There are several ways to manage posterior open bite after MA.

  1. Plan a 'rest' period with NO aligners to allow the occlusion to settle. In your case since you mention premolars still erupting, I think a rest period without aligners may be beneficial to allow premolars to erupt before scanning again to make additional aligners with no precision wings.
  2. Depending on the cause of the posterior open bite and whether the patient is brachyfacial or dolichofacial, the strategies for closing the posterior open bite are different. In a brachyfacial, decreased lower face height patient, it would be desirable to close the posterior open bite with posterior extrusion to help increase the lower face height. In this case, I would bond buttons and run posterior extrusive elastics to close the posterior open bite before scanning for Additional Aligners. In a dolichofacial, vertical skeletal pattern, I would make Additional aligners for more upper and lower incisor intrusion to remove anterior occlusal interference and allow the mandible to autorotate upwards an forwards to close the posterior open bite. This will be seen as a simulation jump on the ClinCheck software plan.
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