RME does not influence upper airway volume? A trial
One of the newer “developments” in orthodontics is the treatment of the airway. One method of increasing the airway that is being promoted is Rapid Maxillary Expansion. Let’s see if this new trial shows this effect of RME.
RME is a well-established treatment to treat maxillary transverse discrepancies. Traditionally, orthodontists anchored RME devices to the teeth. Recently, new skeletal anchorage devices have been developed. This has resulted in several claims being made for their superiority over tooth-borne methods. These include more significant expansion and the improvement of the airway. This is particularly relevant to the new speciality of “Airway Friendly Orthodontics”.
This new trial looked at the effects of three different expansion devices on the airway.
A team from Sydney did the study. The EJO published the paper.
Gordon C. Cheung et al.
European Journal of Orthodontics, 2020, 1–11. doi:10.1093/ejo/cjaa031
What did they ask?
They did the study to answer this question;
“What are the short-term changes in upper airway volume after RME with a conventional tooth-borne (Hyrax), tooth-bone-borne (Hybrid Hyrax) and the Keles keyless expander measured by CBCT”?
What did they do?
They did a randomised controlled trial with a 1:1:1 allocation ratio. The PICO was;
Participants: 66 healthy 10-16-year-old orthodontic patients with unilateral or bilateral crossbites and a maxillary transverse deficiency of more than 5mm measured from the cusp tips of the upper first molars.
Interventions: Hyrax expander
Comparators: Keles expander or Hybrid Hyrax expanders
Outcome: They chose a primary outcome of treatment-induced changes in overall airway volume. Secondary outcomes were other airway measurements, all derived from CBCT images.
They collected the data at the start of the expansion and at 6 months at appliance removal. They took the CBCT images using a standardised protocol and asked the participants to stay still during scanning. An experienced operator recorded the data from the scans. The threshold was set to between 50 and 70 for all the scans.
They did a sample size calculation based on detecting an effect size of 20% increase in airway volume. This showed that they needed a minimum of 15 patients per group to power the study. They used pre-prepared randomisation and good allocation in sealed envelopes. Finally, their statistical analysis was relevant.
What did they find?
They randomised 66 participants. Unfortunately, there were failures of the Keles device and they switched 8 participants to the Hyrax appliance. As a result, at the end of the study, they were left with data from 51 patients. These were divided into 19 Hybrid-Hyrax and only 13 Keles expanders. There were no differences between the groups at the start of treatment.
When they analysed the data they found that the Hybrid-Hyrax group had an increase in airway volume of 5903mm2 (+8.3%), the Hyrax increase was 2537 mm2 (+3.8%), and the Keles group had an increase of 3001.4 mm2(+4.5%). These differences were not statistically significant.
Their final conclusion was:
“All three RME devices produced relatively small increases in the total volume of the upper airway, and this casts doubts on their clinical relevance”.
They also stated:
“Any benefits in terms of increased airway volume obtained from RME are not necessarily translated into an improved function for breathing disordered patients”.
However, I could not find any data in this paper that underpinned this strong statement.
What did I think?
Firstly, I thought that this was a nicely set up and designed study. Nevertheless, the investigators raised some limitations. I believe that the most important of these was that there was no untreated control group. As a result, we cannot conclude that the treatment caused changes in the outcome measures. These could have arisen from average growth. However, I wonder if they could have used a delayed treatment group as a control. This has been the method used in other orthodontic studies.
It is also essential to consider that the failure rate of the Keles appliance was high. Ideally, they should have used an Intention to Treat analysis. This would have included the data from all patients, even those whose appliance failed.
We also need to consider that the high drop out rate has resulted in a lack of power, and this may explain the absence of statistical significance. Nevertheless, the effect sizes are small, and they are unlikely to be clinically significant.
Finally, I am aware that there are difficulties in the accurate measurement of the airway from CBCT images, and this relies on the phase of inspiration/expiration of the participant.
So what can we make of the results of this paper? I think that the most important fact is that the treatment changes were small. This is an important finding and suggests that RME does not result in a meaningful change in airway dimensions. It is important to consider that this study used RCT methodology. They, therefore, addressed the authors criticism of other studies that relied on lower levels of evidence and reported on minuscule levels of change.
I think that it is imperative to remember that the investigators did not measure disordered breathing neither were the participants assessed by ear, nose and throat specialists for adenoids, tonsil size or any kind of airway obstruction.
However, this study does provide us with useful information on the effect of RME on airway volume and casts some doubt on the use of RME in airway focussed orthodontics to improve childhood breathing. In many ways, this study reinforces the AAO paper on orthodontics and breathing disorders.
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Emeritus Professor of Orthodontics, University of Manchester, UK.