May 03, 2021

Micro-implant expansion again: A new trial.

It is great to see that authors are doing more studies on methods of expansion. This new trial on MARPE adds to our knowledge of this relatively new technique.

 In previous blog posts I have mentioned that there are very few trials of Maxillary Skeletal Expansion or MSE. However, since then journals have published several trials and this new study is the latest.

What did they ask?

They did this study to ask this straightforward question about micro implant assisted rapid palatal expansion:

“Are there any differences between Micro-implant RPE  and tooth-borne expanders to treat maxillary deficiency during the post-pubertal growth stage”?

What did they do?

They did a prospective randomised controlled trial with a 1:1 allocation.  The PICO was:


60 orthodontic patients with a skeletal maxillary deficiency needing more than 5mm dental expansion, a negative buccal corridor, and during the post-pubertal growth spurt.


Micro implant Rapid Palatal Expansion device


Hyrax RME device


The primary outcome was the ratio of skeletal to dental expansion at the level of the maxillary first molar. There were many secondary outcomes derived from CBCT images.

They used pre-prepared randomisation and concealed the allocation in sealed envelopes.  Patients and orthodontists were not blinded to the treatment allocation.  I could not find any information on whether they used a blinded data collection.

They carried out a sample size calculation before the study started. This showed that they needed a sample of 28 patients in each group to detect a difference of 15% in the ratio of skeletal to dental expansion.

They took CBCT scans and dental casts one week before and one week after active expansion. They evaluated the scans mainly on two coronal sections and one axial slice.

Finally, they used univariate analysis to compare the interventions.  This meant that they carried out 23 tests and ran the risk of false positives amongst the many variables that they evaluated.

What did they find?

Unfortunately, they produced a mass of data and I cannot go into all of this in the space that we have.  As a result, I am going to concentrate on their primary outcome and some of the many secondary outcomes.

These main findings were:

  • There were no differences between the two groups at the start of treatment.
  • 30 participants in each group completed the trial.
  • The ratio of skeletal to dental expansion was approximately two times higher in the Micro Implant RPE group (61.4%) compared to the Hyrax group (32.2%).
  • The success rate of opening the midpalatal suture was higher in the MARPE group (100%) than in the Hyrax group (86.7%).
  • The centre of rotation of the zygomatico-maxillary complex was located in the same position for each group.

In order to simplify my interpretation, I looked at several of the outcomes that they measured from the CBCT images more closely. These were the intermolar width at the level of the molars, the amount of sutural expansion, and the alveolar bone height at the molars.

They presented this as before and after values and changes in the variables. We need to remember that there are problems in looking at the change in orthodontic variables because this tends to mask small effect sizes. As a result, I concentrated on the post-expansion variables. I think that this is a reasonable decision. This is because if you take a sample of patients and randomly allocate a treatment. Then it is logical to assume that any post-treatment differences between the groups are due to the treatment. Importantly, taking this step clearly shows effect sizes.  This table includes this data (in mm) at the end of the expansion.


MARPE Hyrax Difference
Sutural width 3.82 2.2 1.6 (1.1 to 2.1)
Dental width 53.2 52.8 0.4 (-1.4 to 2.2)
Ratio skeletal/dental change 61.4 32.2
Alveolar bone height at molars 13.16 13.69 0.53 (-0.9 to 1.96)


Their overall conclusions were:

“MARPE was more predictable and resulted in greater skeletal expansion compared to the Hyrax expander”.

What did I think?

This was an interesting and good study of a clinically relevant problem.  Interestingly, it appears that expansion can be achieved by most methods in patients who are past the pubertal growth spurt.  The mean age of the MARPE group was 15.1 years and the Hyrax was 14.8 years.

I was a little concerned that they did not provide any information on whether the data was recorded blind. This is important and puts the trial at a reasonable risk of bias.

I think that it is also important to consider the effect size. I am always cautious when I see data presented as percentages, as this tends to “mask” the effect size.  We need to remember to look to see if the effect size is clinically significant.  As a result, I looked closely at the post-treatment values. This showed that the effect sizes were rather small with wide confidence intervals.  From this simplified evaluation of the data, I think that I can conclude.

  • Both MARPE and Hyrax expanded the maxilla.
  • There was a greater sutural expansion with the MARPE than the Hyrax with an effect size of 1.6mm (it could just as easily range between 1.1mm or 2.1mm).
  • The amount of dental expansion was similar.
  • There was little effect of either appliance on alveolar bone height.
Final comments

This was an interesting paper that gave us some useful information on the effects of the most commonly used rapid expansion appliances.  However, I had some concerns about bias with respect to the recording of the cone-beam data.  This does mean that the trial is at risk of bias.

Finally, The most important factor to review in the appraisal of this paper is to consider the effect size. What you need to do as a reader of this blog and the paper, is to decide whether gaining an additional 1.6mm sutural expansion is worth the more invasive approach of micro-implant assisted expansion.  I would stick with RME because this is simpler, but I may be old-fashioned and cautious?



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Have your say!

  1. Thanks for this great post!
    I think maybe it’s worth the skeletal involvement in these post pubertal patients. Not only because of the additional average of 1.6 mm skeletal expansion. But also the success ratio of the opening of the suture: 100% vs 86.7%? 🙂

    Never stop this great blog please! A big fan!

  2. Will the design of MARPE make a difference ?

  3. Thank you very much for this post!
    Mean total expansion and treatment duration in each group were similar I guess (?)

    The expansion protocol was 1 turn/day? How did they confirm that patients did not miss a turn? Maybe MARPE patients were more motivated due to the more complicated procedure as they could assume they had a more significant problem.

    I guess I should go and read the article!
    Thanks for all the effort!

  4. Some long-term data analyzed years ago showed that only around 25% of the expansion was skeletal using RME. There was some significant relapse both deadly and skeletal years after the expansion was completed. So that could also be another variable that may make even smaller the portrayed difference years later if the relapse potential is shown to be relatively similar. We need more data of long-term stability.
    A clinically useful approach is to prepare the expander with some wire loops to be used to “host” TADs if a conventional expansion activation gets too much resistance in more mature palates. This avoids the need to do another expander. For the patient the less aggressive approach is the first line of action.

  5. Dear Dr O’brien,
    We know that RME is more effective at school children between age of 8-14. On the other hand MARPE type expansion is a perfect alternative for young patients (15-19 years old) whose median suture is already fixed and will resist to open with RME technique.
    From this point of view, comparing these two techniques is unfair due to different indications of each techniques.
    This study shows that MARPE has greater skelatal effects compared with RME. The more skeletal anchorage, the more skeletal results.
    My opinion is similar with yours at clinical practice. It may be horroble for the patient to offer drilling her/his palatinal bone with two screws during palatal expansion. Thus classical bonded or banded RME is more accaptaple in daily use.
    After 25 years clinical experiance and a lot of studies about RME my conclusions are:
    RME is the more effective at ages between 8-14 and have many additional affects on these patients with chainging the functional matrix in upper and lower airways.
    MARPE is the best option with its skeletal effects for opening palatal suture without resistance at ages between 15-19.
    SARME is necessary at non-grower patients older than 20 years. Besides, slow Maxillary expansion or semi-Rapid expansion may be tried at these patients who dont want to be operated.

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