February 27, 2023

Invisalign does not move teeth as effectively as fixed appliances?

More evidence is starting to emerge on the effectiveness of clear aligner treatment. While case reports show very well-treated cases, I cannot help feeling that these are cherry-picked and wonder whether aligners are not as good as some claim. While this may be the cynic in me, there is little evidence to suggest an alternative viewpoint. This new paper in the AJO-DDO concludes that fixed appliance treatment is more effective than Invisalign. It is worth a very close look.

A team from Columbus, Ohio, did this study. The AJO-DDO published the paper.

What did they ask?

They did the study to:

‘Compare maxillary tooth movement between Invisalign and fixed appliances”.

What did they do?

A retrospective study of case records selected from the Graduate clinic archive.  The main inclusion criteria were.

  • Graduate students and Faculty treated patients older than 16 years old
  • Treatment was done on a non-extraction basis.
  • Complete records were available.

They included the records of 30 patients treated with Invisalign (mean age 34.5 +/- 14.5 years) and 30 patients (mean age 28 +/- 12.0 years).

The outcome measures were the PAR Index.  They also measured tooth movement from the digital models superimposed on the palatal rugae.  When they had superimposed the sequential models, they segmented individual teeth. Then they calculated individual tooth movement using specific landmarks within the teeth.

Finally, they compared the tooth movements between the two groups by using the relevant linear statistical models.

What did they find?

They produced a large amount of data in the text and graphs. Unfortunately, this was very detailed and needed to be more explicit. Consequently, I hope I have clearly interpreted this information through overloaded brain fog.

Firstly, there were no pre-treatment differences between the groups in PAR scores. Unfortunately, I could not find any information on the post-treatment PAR scores in the text. Nevertheless, in the abstract, the authors stated that the post-treatment results were similar.  However, we have no information on the overall quality or duration of the two treatments.

When they looked at the maxillary tooth movement, they found that there was less tooth movement with Invisalign than with fixed appliances. However, most of these differences were not clinically significant. For example, the crown translation movement of the upper central incisor with Invisalign was significantly lower than with fixed appliances, -0.53 mm.

Nevertheless, some other measurements were clinically significant. For example, the torque movement with Invisalign for the central incisor was significantly lower than for fixed appliances (mean difference -5.69 degrees p<00001).

Their overall conclusion was:

“When comparing fixed appliance to Invisalign, we found that patients with fixed appliance treatment had significantly more maxillary tooth movement in directions”.

What did I think?

It is great to see that more research is being published in peer-reviewed journals.  We are now beginning to build an evidence base for this treatment. I have thought carefully about the recent publications on aligners.

In summary, the evolution of research into this treatment follows the same path as much orthodontic research.  This is characterised by the early papers being case reports that may contain helpful information. This is then followed by retrospective studies that provide us with further knowledge. Nevertheless, we need to appreciate that this method of research is biased.  The final stages, in the progression, are randomised trials and systematic reviews.  These tend to show more minor effects than retrospective investigations. This paper represents the middle stage of the research evolution. We need to bear this in mind when we critically appraise it.


My general feeling is that this paper does provide us with some helpful information. Nevertheless, we must consider that several significant issues result in high uncertainty. Therefore, I will try to be brief:

  • This is a retrospective sample of cases treated in one dental school. Notably, the records were selected based on their availability. Unfortunately, this means the sample must have considerable selection bias.
  • The sample was tiny and there was no sample size calculation.
  • We must consider why the groups of patients were treated differently in the absence of randomisation.

These points are critical because the differences between the treatments may have occurred due to the operators using fixed appliances because they felt they needed more significant tooth movement.

The good points of the study were that the measurement method of tooth movement was novel and accurate.

It was also important that they did not measure the difference between the tooth movement and Clinchecks.  This has been done in other studies and while it provides information, it does not consider the variation in Clinchecks due to operator prescription etc.  This has been highlighted in other posts on this blog.

Final comments

We need to think about where this study leaves us.  At the most superficial level, it reinforces the general feeling that clear aligner therapy is effective for mild malocclusions. However, the other claims that are being made need further research before we can accept them.

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Have your say!

  1. Great food for thought! Over the past 40 years, I’ve often wondered about the efficacy of evolving protocols and techniques. After working with many cases in both fixed and aligner therapy, it is apparent that there are specific cases and movements that brackets are far superior and more efficient in expressing (leveling lower arches, upper anterior vertical positioning). Aligners out preform brackets in specific cases and movements as well (lower anterior alignment, excessive vertical case control, arch macro-symmetry).

    Willy Dayan says it best….you don’t use a boat to travel on land nor a car to travel by sea. It is an amazing time indeed to have the strengths of both protocols at our disposal!

  2. Congratulations! I really enjoy your blog and point of view!

  3. It is nearly 23 years since this brand was introduced (AAO, Chicago, 2000). What took us so long for this type of study to emerge? Seems to be “the cart before the horse” as is often the case. This outcome might indicate that patients and providers are willing to accept “less alignment” with “aligners”. A trade-off for interim aesthetics? Patiently waiting for the follow-up retention studies.

  4. I cannot belive the surprise. In 1933 Brodie said that removable apoliances are inferior to fixed appkiance, because they are removable. No more no less. Nothing can change it. Nothing. The inly thing that we experirnce is a change in our concsotions. G8 is not better than G1. The computer technology is much better. Not the appliance. Sorry. We sell our patiets inferior quality on overpiced cost. We sell them a brand name, like rolex or chanell. Sorry. There is nothing like fixed appliaces. Unfortunately, the last generation (25)years of new orthodontists lost the knowledge of the real orthodontics. Alignodontia instead of orthodontia. We are responsible for it. Sorry again.

  5. Hi Kevin –
    will need to read the paper as I am still confused as to what we may reasonably conclude. If the sample size was adequate and measurement protocol accurate as you say, if the initial PAR was not statistically significant the the paper suggest “outcomes were similar”. So it is not “less alignment” that they are reporting, but I believe “less movement”. This is only meaningful if we know how much movement the clinicians intended. Perhaps “less was more” in the treatment goal?
    The paper does not appear to be measuring outcome but comparing maxillary tooth movement. From the information above, I can not see any possibility of measuring the “effectiveness” of aligners v fixed, as the title of blog suggests- as you pointed out we have no data on treatment time and other such variables. So from what I understand, we have measurement of the degree of maxillary tooth movement pre and post, and have identified some differences. I am not sure of the meaning of this unless we compare it to what was programmed. Unless the researchers show that 5 degrees of torque was programmed in the aligner patients and 5 degrees of torque was activated in the bracket mechanics, the eventual movement data alone I am not sure what this means? What am I missing?
    We know that is impossible with fixed appliances (that are not digitally programmed). For most aligner systems we can measure what is programmed and compare to what is expressed. We cannot do this with non digital fixed. This data would contribute to the question of degrees of expression of movements, such as torque or rotations. We know that we never obtain 100% of activation with fixed appliances, (high torque wires, bracket slop, wire bending, manual variation, inability to measure manual activations) -we should not expect that with aligners, especially in light of the patient variable that Dr Brezniak alludes to. But we can likely come closer in the future as we do learn more about the tolerance of each movement, within each system, within each patient……
    *VV provides lectures on behalf of Align Technology

    • Hi and thanks for the comments. Yes, I agree with you as we do not really know how much movement the operators were intending to achieve. This was behind my comment on the problems with the study. They may have been trying to achieve more movement with the fixed appliances.Again, the study was limited to the movement of only a few teeth and it is not clear to me, why they did this.

      However, it is a study into aligners and we certainly need some more of these. It is a great shame and frustration that Invisalign have not funded any meaningful research into the effectiveness of their treatment.

  6. If this is the conclusion for aligner treatment delivered by orthodontists and graduate students with access to radiography, where does that leave aligner treatment delivered remotely using photographs and impressions taken by patients themselves?

    Stephen Murray
    Swords Orthodontics

    • Good point; however, I am unsure whether any of the direct-to-consumer aligner companies will get involved with research. I imagine that the outcomes will be poor. But only time will tell!…

  7. When it comes to comparing fixed and removable appliances, such as clear aligners, there is a lot that needs to be considered. One of the most important factors is the experience of the provider(s). However, it’s also crucial to be careful when reading and interpreting articles on the subject.
    Thanks to Dr. O’Brien and my friend and mentor, Dr. Gerry Samson, I have gained (I hope I did!) a better understanding of how to approach these comparisons critically. For example, the conclusion of an article published in the Angle Orthodontist in 2022: “Differences in finished case quality between Invisalign and traditional fixed appliances: A randomized controlled trial,” by Eric Lin et al., states that while patients with simple malocclusions require 4.8 months longer treatment times with aligners than with traditional braces, the post-treatment outcomes are similar for both types of appliances.
    While this may seem like good news, upon further examination of the methods used in this study, we can see that they made patients wearing aligners change them every other week – a practice that very few experienced aligner providers still use today.
    In contrast, another article published in the Angle Orthodontist in 2020 by Kravitz et al. explains why changing aligners every week is preferable: “Achieving a clinically similar accuracy between the 7-day protocol and 14-day protocol in half the treatment time suggests a 7-day protocol as an acceptable treatment protocol.” Changing every week instead of every other week can cut treatment time by half! That is a major difference.
    Returning to the article that is the subject of this blog post, there was one key detail missing from their comparison: attachments bonded on teeth during clear aligner treatment. The type, size, orientation and location where attachments are bonded on teeth will all make a difference in terms of treatment outcomes, but this study did not seem to take them into account.
    In fact, comparing clear aligners with traditional braces without considering attachments is like comparing two systems of brackets but using different wires (a round one and a rectangular one) and concluding that one system is less efficient for torque correction. For me, it’s like comparing Pink Floyd (50-year anniversary today of the release of Dark side of the moon) and the Beatles (don’t ask me which one represents brackets and which one represents clear aligners).
    It’s important to remember that our beliefs can influence how we interpret results from these kinds of studies. But at the end of the day, as an interesting article from The New Yorker points out (https://www.newyorker.com/magazine/2017/02/27/why-facts-dont-change-our-minds), beliefs will always trump facts. So let’s continue to approach these comparisons with a critical eye like Dr. O’Brien and Dr. Samson are teaching us!

  8. It’s really quite simple. After 20+ years, where are all those “amazing” aligner finishes (with full records) that would galvanize the rest of us into paroxysms of shock and awe? These results are nowhere to be found, because they don’t (and can not) exist.

    What is truly stupefying, is that so many members of a specialty (once known for it’s intellectual acumen), have all collectively suspended disbelief and relinquished any understanding of Biomechanics and Outcome assessment, when it comes to treatment with aligners.

    How can one (unless you are a paid shill for Align) compare a biomechanically inferior removable appliance that does not predictably generate couples, to a sophisticated fixed appliance?

  9. I never thought Invisalign to be appropriate for anything more then minor retreatment cases of those who eventually stopped retainer use and experienced slight relapse. The fact that they essentially need a bracket (or as they call it, “attachments”) to accomplish the majority of corrections just shows how ineffective these trays are at a biomechanical level. And still, even with attachments, they can’t seem to adequately address the issue of rotations because the company fails to acknowledge and implement designs based on the basic underlying biomechanics of tooth movement. Perhaps if they were less focused on appliance aesthetics, they might be able to come up with something that’s scientifically possible.

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