There is no doubt that the development of aligners has changed orthodontics.  However, I often wonder if we know whether aligners “work as well” as fixed appliances.  This post is about a newly published study that provides us with some information.

While aligners have become part of the standard delivery of orthodontic treatment, I have seen few studies that have evaluated their effectiveness.  I think that this lack of research may reflect the nature of this fast moving field. This is because developments in appliances may make studies, which may take five years to complete, redundant.  Nevertheless, this does not mean that new appliances should not be studied.

vervolg: Kevin O'Brien: Which is better Invisalign v Fixed appliances: Does this study help?


I came across this new study in the AJO. It is a retrospective study. I do not usually review this type of research. However, because it may be “the best we have got”, I have decided to have a look at it.

Evaluation of Invisalign treatment effectiveness and efficiency compared with conventional fixed appliances using the Peer Assessment Rating index.

Gu J et al

Am J Orthod Dentofacial Orthop. 2017 Feb;151(2):259-266. doi: 10.1016/j.ajodo.2016.06.041.

A team from Columbus, Ohio did this study. I had close links with this department several years ago. I even went to watch the Columbus Clippers baseball team.

In their literature review they pointed out there was little evidence underpinning the use of aligners.  They wanted to ask the following question:

“Was Invisalign as effective as Fixed appliances”?

What did they do?

They carried out a retrospective study by screening the records of orthodontic patients who had been treated in the University Department at OSU.  They looked at the records of 1500 conventional and 250 Invisalign patients. The orthodontic Faculty and residents had treated the patients. They stated that they did not select the patients according to the quality of their treatment.  The main criteria for the selection were that the patients needed to be over 16 years old, treated non-extraction and had available records.

At the end of their review of records they identified 62 fixed appliance and 61 Invisalign patients. They then removed the patients who they felt had terminated their treatment early.  The final sample was 48 patients in each group.

The primary outcome was PAR scores. A secondary outcome was the duration of treatment. They carried out an appropriate statistical analysis.

What did they find?

They looked at the pre-treatment features of the two groups. They did not find any differences except the Invisalign group were older (26 years old) than the fixed appliance group (22.1 years).

At the end of treatment they found the following;

Effectiveness of Invisalign and Fixed appliances (mean and 95% CIs)

 PAR before treatmentPAR after treatmentTreatment Duration
Invisalign 20.8 (18.1-23.4) 4.08 (2.3-5.6) 13.3
Fixed 22.7 (20.4-24.9) 2.6 (1.9-3.2) 19.0

You can see that there are no real differences in the PAR scores, apart from the CI for final PAR being wider for Invisalign and this suggests there is possibly more variation in the final treatment finish with Invisalign.

When they did their statistical analysis they also showed that fixed appliances were significantly more effective at reducing PAR scores than Invisalign. Nevertheless, there was no difference in the end of treatment result.

Their overall conclusions were that:

  • Fixed appliances were more effective than Invisalign at improving malocclusion
  • Treatment with Invisalign was faster

What did I think?

Firstly, this was a retrospective study.  While this type of study may provide us with some information we should look carefully at the methods. This is because we need to evaluate any evidence of selection bias in the sample.  When I looked at this I wondered whether the method of selection of the patients records could have caused some bias.  For example, we need to consider that the final sample included 48/1500 (3.2%)  conventional and 48/250 (19.2%) Invisalign cases from the record store. We do not know much about the large number of patient records that were not included in the final sample. Most importantly, they also rejected cases that they felt terminated early. We have to consider whether the results would have been the same if these terminated cases had been included.  While, I may be being critical here, we do have to give the authors credit for their detailed reporting that allows us to consider the risk of bias.

My next step in assessing the paper was to think about whether the bias could have influence the results.  It is important to remember that we should not make assumptions on the direction of any bias. It can go either way.  As a result, I think that we can conclude that there are limited differences between the results of Invisalign and fixed appliance treatments.  However, we need to be cautious and the authors point this out.

This study also provides us with great data that can be used for a sample size calculation for a definitive trial. As a result, it is a useful study.

I have just spotted a new systematic review on the effectiveness of aligners. In my next post, I will discuss this review.  Lets see if we can find out more..


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