September 25, 2017

Invisalign is more comfortable than fixed appliances: A trial

Invisalign has become an established orthodontic treatment method. However, there have been few trials into its effectiveness. This new study looks at whether there is less discomfort with Invisalign compared to fixed appliances.

The Angle Orthodontist published this study. A team from Texas did the trial.


David White et al

Angle Orthodontist: On line  DOI: 10.2319/091416-687.1



They did this study to answer this primary question:

“Is there a difference in reported patient discomfort produced by Invisalign or fixed appliance treatment”?

They also looked at analgesic use and sleep disturbances.

What did they do?

They recruited a sample of patients who were in an existing study. Their sample size calculation showed that they needed 16 patients in each group.  They screened 240 patients and obtained a final sample of 41 patients.  The PICO was

Participants: Adult patients with Class I canines and molars with crowding of less than 4mm and treated without extractions.

Intervention: Invisalign

Control: Fixed appliances

Outcome: Discomfort recorded from pain diaries.

They recorded the outcome data for 7 days following the fitting of the appliances. They then repeated this for 4 days after subsequent appointments at 1 and 2 months.

Unfortunately, they did not provide any information on randomisation, sequence generation, concealment or blinding.  As I have previously explained, this is a major problem.  This means that the study is at high risk of bias. However, we do not know the direction of the bias.

What did they find?

They enrolled 18 participants (6 males and 12 females) into the fixed appliance group and 23 (11 males and 12 females) into the Invisalign group.  As a result, the groups were unbalanced (I shall discuss this later).

They produced a large amount of interesting data. I have put the most relevant into this table

Maxilla1.1 (1.56)0.76 (1.55)0.61
Mandible0.77 (1.46)1.69 (2.0)0.008

They found that the overall pattern of discomfort for the 3 data collection points was similar.  Their conclusion was;

“The patients in the fixed appliance group reported more discomfort than those in the Invisalign group”.

There were no real differences in analgesic consumption or sleep disturbances between the groups.

They put forward several good reasons for these finding.  The most logical was that the differences in pain experience could be explained by considering that the aligners produced intermittent force and could be removed. Whereas the fixed appliances provided a continuous force and could not be removed.

They also, quite rightly, pointed out that the study was supported by Align.

What did I think?

My initial feeling was that this study appeared to provide us with useful information. But when I read it carefully, I was not too sure.  This is because I found several major problems with this trial. These were:

They did not provide information on the method of concealment of the allocation of treatments. This is important because the investigators should not be able to guess or influence the treatment allocation.

They were not clear on their method of blinding.  The blinding of examiners ensures that the data cannot be interpreted or influenced by the investigators, albeit unconsciously.

The most important problem was that the groups were not of equal size. I cannot see how this happened with a random allocation. Furthermore, there were more males in the Invisalign group than the fixed appliance group.  These issues suggest that the randomisation was not effective.

I can only conclude that the study is at very high risk of bias.

Does this make a difference?

We now need to consider if these problems influence our interpretation of the findings.  My feeling is that this paper does have considerable problems. However, the data are interesting.

Anyway it is not for me to put my opinion forwards on the value of a study. This is up to you, as a reader of the paper/post, to decide if the findings are robust.  It would be great if the authors could provide further information on this trial in the comments sections of this post?

Finally, the authors should be congratulated on carrying out this study. It is surprising that Invisalign has not been evaluated in trials and it is great to see this study.

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Have your say!

  1. Kevin add some additional information about your point about the potential impact of lack of allocation concealment in this RCT I would like to direct the readership to a recent paper published in JDR establishes that in oral health trials the potential impact of lacking allocation concealment is an increase in effect size estimates of around 0.15 (Saltaji H, Arijo-Olivo S, da Costa B, Amin M, Cummings G, Flores-Mir C. The impact of bias on effect size estimates in randomized controlled trials. J Dent Res 2017 Aug 16. Doi: 10.1177/0022034517725049). Given that information it could be argued that the difference is likely to have been inflated, but at the same time not likely to deny the portrayed difference in comfort between both treatment approaches. The difference in responses between both methods is too large. Having stated this it has to be considered that other methodological characteristics may also come into play and the small potential impact of each one could be altogether clinically meaningful.

  2. I find the inclusion criteria concerning. Class I with less than 1mm crowding. Did they actually warrant treatment? Even without an NHS contract I don’t treat many like this, so is it applicable to the rest of my patients?

  3. Very interesting article, but I think the sample used was small, so there was a difficulty of the authors to justify this quantity of the presented sample.

  4. What is the pain measured? Are we measuring tooth pain or a discomfort from wearing something bulky that rubs the soft tissue? Does it matter because, discomfort is discomfort no matter what the source?

  5. This article was bad. It didn’t even discuss the force levels on the teeth (an .016 for your first wire?) or the fact that Invisalign can move/push on different teeth for each different trays. But guess what, Align will run with this and it will support their idea that Invisalign is faster and more comfortable. I’m a SLB user, and my pts truly have little to no pain. Align’s interests have shifted.

  6. Hmmm. I presume the article itself names the fixed appliances. Some of them are going to be more comfortable than others – I don’t mean from a tooth moving perspective but just from the perspective of how much they rub the mucosa.

    And 1mm or less of crowding in a class I patient? Seriously? Even if Invisalign was more painful, it would probably be more popular for this due to treatment time. Even in a balanced sample the amount of physical tooth movement seems so little I don’t think you could draw a conclusion from the results that could be extrapolated to cases where there was significant tooth movement.

  7. Hello Kevin, just wanted to tell you that I really enjoy reading your blog! I saw this article on your blog and went to read it, just wanted to mention that the crowding criteria that they had on their paper was less than 4mm, not 1mm.

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