Joseph O’Connor, Tony Weir, Elissa Freer, Brett Kerr

Clinical expression of programmed maxillary buccal expansion and buccolingual crown inclination with Invisalign EX30 and SmartTrack aligners and the effect of 1-week vs. 2-week aligner change regimes: A retrospective cohort study.

- Korean J Orthod 2024;54:142-152

I greatly appreciate the authors’ hard work, and the study is particularly interesting given the increasing variety of materials and protocols in the clear aligner field. In order to enhance readers’ comprehension of your study, I have a few questions and would appreciate further insights from the authors.

Q1. Was there a significant difference between the groups in terms of the amount of tooth movement per step, the total number of aligners used, or the presence/absence of attachments?

Q2. It is intriguing that overexpression of buccal crown inclination occurred, albeit in a small number of cases. Could you elaborate on the possible hypotheses for why this overexpression happens? For example, could it be that overexpression is influenced by greater expansion in adjacent teeth, where the inclination follows the movement of neighboring teeth with larger expansion requirements?

Q3. Based on Figures 4–6, it seems that clinically significant inaccuracies increase as the planned buccal expansion increases. Do you have any clinical guidelines or thresholds for the maximum amount of transverse expansion that can be reliably achieved with clear aligners before inaccuracies become problematic?

Q4. Buccal expansion can be achieved through either buccal crown inclination or bodily buccal movement, and the strategy for clear aligners may differ depending on the type of tooth movement. Could you provide insights on which materials, aligner change protocols, and attachment shapes/positions are most effective for achieving each type of buccal expansion?

Questioned by

Hyo-Won Ahn

Department of Orthodontics, School of Dentistry, Kyung Hee University, Seoul, Korea

Thank you for your kind words and for your interest in our research.

A1. The amounts of movement per step were all within the parameters used by Align Technology (2 degrees of rotation, 0.25 mm linear movement and 1 degree of torque per aligner), but this applies to the guide tooth—i.e., the tooth that has the furthest to move. In all the cases chosen, the guide tooth was other than a posterior tooth expanding, so movement per stage for expanding posterior teeth was universally well below the maximum threshold. Any movements per stage were thus only going to differ, across a treatment group average, by 0.1 mm or less. Total number of aligners was not statistically significant, and the attachments were identical in number for the SmartTrack groups and fewer (although not statistically) for the EX30 group. Effectively therefore the groups were similar save for material used and aligner change protocol prescribed.

A2. The role of adjacent teeth could certainly be a factor, as indeed could be the amount of vertical change planned for a case. Patients with heavy occlusions notably distort aligners transversely—the aligner widens posteriorly and the aligner fit over the posterior teeth worsens as the buccal and palatal walls of the aligner spread apart. Increasing non-fit of aligners will allow undesired movements to occur. As control of bodily movement or torque requires the formation of a couple, loss of a second point of contact due to poor aligner fit will mean no couple can be formed and thus tooth tipping will result. This is well demonstrated for anterior teeth in the article by Hahn et al.¹ Aligner fit also varies over time, not just due to movement lag or aligner cooperation—i.e., Linjawi and Abushal² that the closest adaptation of aligners to teeth occurred at 15 days, making the 7-day change protocol more possibly subject to poorer inclination control. I also refer the reader to the studies by Goh et al.³ and Lim et al.⁴ on Curve of Wilson changes in clear aligner treatment where transverse control was found to be variable in cases where the primary objective was relief of crowding and overbite control.

Other factors such as crown height and anatomy could significantly impact aligner retention on tooth crowns and thus the ability to gain and maintain sufficient contact for prescribed control.

A3. The literature is replete with reports of the lack of stability associated with long term posterior dental expansion, most notably the work of Little.⁵ The work of Goh et al.³ and Lim et al.⁴ make for some interesting findings where certain tooth types behave differently and different jaws behave differently. The question of when something becomes problematic is difficult to answer as the definition of problematic itself is imprecise, but also because the problem could be multifactorial—for example it seems that upper canines do not expand as reliably as lower canines, so a case where the upper and lower canines are planned to expand by the same amount is likely to result in an interference as the lowers respond significantly better than the uppers. Bowman et al.⁶ highlighted the effect of the buccal tip on occlusal contacts with aligner treatment.

For all the reasons given above, my personal preference is to perform as little posterior dental expansion as possible, certainly in non-growing patients, as unwanted tipping would seem a natural consequence of using aligners, particularly those made of stiffer materials. The study we reported seems to indicate, as you have highlighted, that the more you expand, the more the likelihood of clinically significant inaccuracy.

A4. This is an excellent question, and we do not yet have excellent evidence. In the light of anecdote, clinical experience and the results from this paper and some others quoted below, it would appear that 2-weekly wear protocols and stiffer modulus aligner materials would be the most beneficial in attempting bodily posterior tooth expansion with aligners. Indeed, that is what we do with fixed appliances also—we use stainless steel and take time to achieve difficult movements of large-rooted posterior teeth, not nickel titanium and rapid treatments. I see no reason why aligner treatments should not also obey similar principles. If you want to just tip teeth then rapid change and soft material applies. If you want bodily movement or some degree of root tip control, stiff aligner material and longer wear protocols should be considered. Blundell et al.⁷ reported that stiffer aligner material was superior to more flexible material when it came to achieving bite opening. Also, Blundell et al.⁸ reported that a 2-week wear protocol was the only thing that made any significant difference to the success of closing anterior openbites.

Replied by

Tony Weir

Department of Orthodontics, The University of Queensland, Brisbane, Australia