Invisalign tips teeth into extraction spaces.
We are finally seeing more research papers on aligners. One of the most prolific research teams has published several studies based on a vast database of aligner cases. I previously discussed their work when they examined incisor torque, mandibular advancement, and overbite reduction. In this latest paper, they investigated the effects of Invisalign on closing extraction spaces.
Most research indicates that aligners are effective in treating mild to moderate malocclusions. However, some orthodontists attempt to address more complex issues, and there is insufficient evidence to support their success in these cases. Highly paid clinical salespeople claim that extraction treatments are possible with aligners. Yet, when they showcase their cases on social media, which has become their preferred method of publication, the results often fall short of expectations. A recent paper examining extraction treatment has yielded some interesting but disappointing results for those practitioners who advocate for treating extraction cases with aligners. You may think I sound too cynical, but I encourage you to read on…
Abirami Thilagalavanian, Tony Weir, and Maurice J. Meade
AJO advanced access. https://doi.org/10.1016/j.ajodo.2024.06.014
This paper is open access, so everyone can read it without being a member of the AAO, which is nice.
What did they ask?
They did this study to ask the following question.
“What is the expression of root angulation in the canine premolar and 1st molar teeth adjacent to premolar extraction sites in the maxilla with an initial series of Invisalign aligners”?
What did they do?
The team used data derived from the Australian Aligner Research Database. This database contains information on 12,000 Invisalign patients treated with Invisalign at the start of the study. Seventeen specialist orthodontists treated these patients between 2013 and 2021. Each orthodontist had a minimum of 10 years experience with clear aligners.
The team employed a standardized method to measure tooth movement by comparing “actual” datasets with “predicted” ones. They focused on the canine, premolar, and molar teeth, assessing the initial, predicted, and achieved models. This approach allowed them to evaluate the accuracy of the planned changes in root angulation.
What did they find?
The study analyzed 54 patient records. Of these, 28 patients had their upper first premolars extracted, while 26 underwent treatment that involved extracting their upper second premolars. The majority of the patients were female, and the average number of aligners used in the treatment was 48.
They provided a large amount of data, and I will concentrate on what I felt were the most important values.
In upper premolar extraction patients, statistically and clinically significant differences were seen between the predicted and observed amount of canine crown tip (10.97o ±7.16). For the molar tip, this was 7.43° (plus or minus 7.4). Similar values were reported for upper second premolar teeth, with a molar tip difference of 9.12o(7.05 =/-7.05) and canines 9.62o (6.03).
They conducted a linear regression analysis to identify predictors of crown tipping. The results indicated that optimized attachments caused the tooth crown to tip away from the extraction site by 2.5° compared to conventional attachments.
Interestingly, no significant differences in accuracy were found when they compared optimised and conventional attachments on 1st premolars first molars and canines for either extraction pattern.
They wrote a clear discussion. The key takeaway was that the change in angulation achieved did not align with the predicted change. Notably, clear aligner therapy appears to lead to crown tipping into extraction spaces.
When roots were predicted to tip towards the extraction site as part of space closure, there was a greater likelihood of under-expression of this movement.
Their overall conclusion was
“The achieved root angulation in teeth adjacent to premolar extraction sites differed significantly from that predicted”.
What did I think?
This was another interesting study from this team. The strong points of this study were that they used a very large database of patient records and that experienced aligner operators treated the patients.
I understand there is controversy regarding actual versus predicted tooth movements. However, no one has provided a strong rebuttal to this methodology. I also wonder if those who criticize this methodology are in denial.
This study suggests that clear aligner therapy may not be as effective as we have been led to believe. In the absence of research that contradicts these findings, we must conclude that this treatment has significant limitations.
The results of these retrospective studies offer valuable insights. However, we should approach these findings with caution due to the potential for bias associated with retrospective methodologies. It may be beneficial for some key opinion leaders (KOLs) in the field of Invisalign to conduct a prospective study. This would be a helpful advancement in our understanding of this treatment approach.
Emeritus Professor of Orthodontics, University of Manchester, UK.
Dear Sir,
Thank you for your post on tipping in extraction-aligner cases.
The bodily movement of posterior roots is a challenge even for the most sophisticated fixed appliance +/- TAD support. However, this study again highlights the limitation of AI in predicting tooth movement. One cannot help feeling that more frequent refining might give more favourable results.
Probably the best use of aligners would be where extractions are not indicated or refused by the patient so that the suboptimal (compromised) finish is accepted by all even before starting treatment.
The fact that only 54 cases were studied out of a possible 12000, shows that there aren’t many instances of heroic treatments with aligners but we need to know about these nevertheless to improve our evidence base.
Beneslider like devices might ultimately add the control that we seek with Aligner therapy to try and make aligners an equivalent alternative to the gold standard conventional fixed appliances.
I find it interesting that ChatGPT has taken hold only recently but in Orthodontics we have been using similar software to predict tooth movement for decades. I feel that by frequent refinements we can predict better and the patient can benefit from the experience of their supervising Orthodontist rather than be treated by an algorithm.
Thank you always.
Yours sincerely,
Karun Sagar BDS
It is time to understand that the Burstonian bomechanics rules as published in 1984, are unfortunately false.
a. No single vector, what-so-ever, can develop translation, for the simple reason: nobody knows where the Cres is. Adding hooks to the system as suggested by Burstone, cannot change the above concept, in removable applinces. Therefore, do not expect to translate a tooth by ckear aligners.
b. As long as the Cres is located in the root, due to the natural restraint (bone and PDL), as while using removable applinces, including clear aligners, no translation movement can be accompished by any vector, a single one or a combination of several vectors, namely, a parallelogram.
c. The only way to develop translation is by a combination of 2 tipping movrments, when the first one, or the initial one is when the Cres is at the root, and the center of this tipping movement is at the Cres, meaning that the apex moves almost equally as the crown’s edge, in opposite direction. The second one is when the Cres moves to the mid-slot zone due to the highly restraint levels, developed by the torqued wire, as the friction between the metal rec. archwire and the inside part of the slot, thus increasing the artificial restraint of the bonded applinces, to a level that force the Cres to move to the mid-slot zone. The rotaional movement developed by the rec. wire in the slot, moves the apex much more than the crown’s edge. This movement contnues until the restraint level at the bracket/wire junction decreases, letting the free of force Cres, to move to the root to develop again the movement where the apex and the crown’s edge, move equally, while again, increaing the restraint in the slot, moving the Cres there…
Those combinational movements that mutually translate the tooth, cannot be accomplished by any removable appliance including clear aligners.
Unfortunately, no orthodontic journal, until today, is willing to brake the glass ceiling of the orthodontic biomechanics and forget Burstone’s biomechsnic rules which unfortunately overuled the basic physics law.
I know that it sounds odd, but this is the only way to understand orthodontic tooth movement, and ortodontic biomechanics. Again, it is unfortunate, but no removable appliance can develop such a movement, clear aligners are included.
Although proof is still lacking, this study implies that once again, overcorrection is necessary in aligner treatment.
Yes, just as we are forced to do when wire bending! Good thing is that this data gets us closer to the mark as defaults improve. Funny thing Morris is that we all ignore the biggest variable – the patient!! 🙂
*With all due respect, I am not sure which party is in denial.
Your Blog my have been titled “Invisalign, just as we know with Fixed Appliances, Tips Teeth into Extraction Sites”.
In the interest of brevity, a few points below prior to a blinkered acceptance of the “critical analysis” of the research mentioned above.
“Analysis of predicted and achieved root angulation changes in teeth adjacent to maxillary premolar extraction sites in patients treated with the Invisalign appliance”. I would have thought that an investigation with primary variable being “root angulation” would have, at some stage included…. a root. There is a deafening silence regarding the extrapolated measurement of root positioning and angle of change reference both within the article and the analysis above. Yes, I accept that virtual roots (actually long axis) were constructed via the 2 occlusal crown and 1 gingival landmarks, and it was the change in the long axis of the tooth, as defined via crown movement. In this situation, perhaps the title would be re-considered if we are to be rigorous. Nothing wrong with acknowledging what was measured – here was strictly extrapolated long axis change based on crown superimposition, a tooth being a solid object, albeit it small change if we accept basal EARR.
– “Invisalign tips teeth into extraction spaces” – anyone who purports to comprehend fixed appliance mechanics knows that fixed appliances move teeth with a series of tips. There is no such thing, as also communicated via described in Dr Brezniak’s comments above – of “bodily” tooth movement activated via orthodontic appliances. What is important is achieving a physiologic and functional final occlusion and alignment in the healthiest possible manner; along with achievement of other treatment goals. Unfortunately this study does not provide this information. It is not an outcome study. It assesses mid-treatment crown position in relation to a digital model used primarily to program a force system (not to represent a tooth position). It also confirms what we know about any mechanical system involving several engineering steps, particularly when translated to a biologic specimen – we can never achieve 100% of programmed activation; analogous to what we “see” with fixed appliance activation.(several thousand bracket prescriptions and designs, rebonding, wire bending…)
– “..yielded some interesting but disappointing results for those practitioners who advocate for treating extraction cases with aligners”. After reading through your blog, comments and the paper in question, and being very familiar with the M&M involved, I am not disappointed with the results; rather with the blinkered interpretation, failure to ask questions of the study and subsequent failure to place findings in the context of what we know about “gold standard” fixed appliances (our “control”).
– The second paragraph and third concision are incongruous; this is not an outcome study. These are not final records. You are extrapolating and misinterpreting outcome: there are no root positions analyzed, nor occlusion, nor alignment, nor any soft tissue parameters, nor records within this data base to evaluate outcomes. This, as the authors clearly document is a comparison of programmed movement and resulting crown position after less than 12 months of treatment (as judged via number of aligners). Would we assess outcome of treatment or final tooth position based on less than 12 months of fixed appliance treatment in patients undergoing extraction?
– the esteemed authors were obliged to list several limitations of their study, it is an “oversight” that these were not reflected in the blog introduction. “…..Moreover, as the sample size calculation referred to the primary outcome only, subgroup analyses may have lacked sufficient power for definitive conclusions.”…
– Comments regarding optimized attachment findings (one of these subgroups) are not entirely accurate – a significant, “substantial improvement” was found regarding effectiveness of optimized versus conventional attachment for upper first molars in upper first premolar extraction, as noted by the authors in both discussion and table V. The abstract an conclusion statement appeared to ignore these findings- what am I missing?.
– Most intrigued regarding type of optimized attachments (isolated or part of engineered integrated force system such as G6 that came into being in 2015 and reiterated in 2018) moving crowns in the opposite direction to the extraction site- this is a positive finding as this is exactly what the optimized attachments used in extraction space closure are engineered to achieve – crown activation in the opposite direction of aligner space closure to counter the directional force of the reducing arch length of the aligner – invariably the root will then be tending towards the site; recognizing orthodontic tooth movement is via series of tips or controlled / uncontrolled tip. Optimized attachments are additionally smaller, more comfortable, make aligners less retentive and user friendly, are cheaper to apply, faster to remove with arguably less enamel involvement than conventional attachments – especially those that clinicians had to resort to when treating patients undergoing extraction. Do they, will they ever singularly achieve 100% of programmed movement – has there been a bracket engineered in 125 years that has done so, expressed 100% of the bent wire? .. silly to even assume this. Even if there were only equivalent clinical efficacy, there are clinical and patient advantages in using optimized attachments.
– retrospective selection bias – please read the sample selection. Of 17000 (not the 12000 mentioned above), the authors selected 54 subjects of 217 treated with upper premolar extraction, between 2013 and 2019 – a 6 year period. One quarter of the rapidly evolving life of the Invisalign system.
– Age of ultimate derived sample was 27.42 +/_ 9.51. I would have thought this would be relevant for discussion – an older cohort than we may typically see in analysis of orthodontic tooth movement degree of expression
– I am still unable to determine how aligner wear protocol (another subgroup finding) was determined in this and all studies using this database of scan data (I hope no PVS!!) , when this was retrospective, “blinded” as to treating clinicians – does data base collected from clinicians include this data in any formal way? Would like to gain this information especially as we have numerous studies resulting from this sample and significant effort by the authors (authors did note this finding was unable to reach statistical significance).
– “It may be beneficial for some key opinion leaders (KOLs) in the field of Invisalign to conduct a prospective study. This would be a helpful advancement in our understanding of this treatment approach.” Totally agree; first things first; as a control and benchmark for our “gold standard”, I would like to conduct a prospective study on initial root alignment of patients being diagnosed and treatment planned for re-treatment using Invisalign (or fixed) , who have undergone orthodontic treatment including extraction as adolescents. Multi-center study in the planning.
*VV – provides sponsored lectures for Align Technology – manufacturer of the only evidence based computer programmed aligner system
Thanks for your comments on my interpretation of the paper. It is great to see that you are planning a prospective study. As I keep saying these are long overdue. When do you aim to start enrolling participants?
If you are interested please let me know. You have my contact Kevin. I would also like to run the M and M by you for your opinion.
Thanks, I cannot take a role in this study because I am retired and enjoying it! But I will happily provide you with advice.