March 20, 2022

At last! A trial on Bone Anchored Maxillary Protraction.

Bone anchored maxillary protraction is a treatment for Class III malocclusion. It has been widely promoted and researched in retrospective studies using a small sample of cases. I have tried to look for trials into this treatment method. Unfortunately, I could not find any. I have posted about a previous study, and I was somewhat critical because I was not sure it was a trial.

This lack of high-quality research is disappointing because this treatment, in theory, may be valuable. I was, therefore, pleased to see this new study looking at the development of Bone Anchored Maxillary Protraction (BAMP). They replaced the Bollard plates used in BAMP with a hyrax appliance and mini-screws in the mandible. They called this Miniscrew anchored maxillary protraction (MAMP). Wilmes et al. first described this approach in 2011. The authors of this paper have also published a small case series on this method.

What did they ask?

They did this trial to answer this question.”

“What are the dento-skeletal effects of Miniscrew anchored maxillary protraction (MAMP) using a Hybrid Hyrax or a Convential Hyrax”?

What did they do?

They did a single centred RCT with two parallel arms with a 1:1 allocation ratio. Finally, the authors registered the trial.

The PICO was:

Participants: Orthodontic patients aged 9-13 years with Class III malocclusion. The participants were in the late mixed or early permanent dentition. They had a maxillary deficiency of a Witts of less than -1mm with an anterior crossbite or edge-to-edge incisors.

Intervention:  Hybrid Hyrax anchored MAMP expander.

Comparator: Conventional Hyrax anchored MAMP expander.

Outcomes:  The primary outcomes were overjet and Co-A point changes. Secondary outcomes were various cephalometric measurements.

The screw activation protocol was the same for both intervention groups. The patient activated the expander screw a quarter turn twice a day for 14 days. This achieved 5.6 mm expansion. The operator applied intermaxillary traction using Class III elastics from the expanders to two mandibular miniscrews. The force was 150g per side for the first month and then 250g. The clinicians continued the protraction for 11.3 months in the HH and 11.0 months for the CH.

The authors collected CBCT data at the start and end of the protraction phase of treatment.

They did a sample size calculation that showed they needed to enroll nine patients into each group to detect a clinically significant difference of 2mm in maxillary length change (Co-A).

The team used a pre-prepared randomization scheme and concealed the allocation in sealed envelopes. Notably, a different operator was responsible for randomization, allocation concealment and data processing.

They blinded their data analysis.  However, the operator and the patients were aware of the treatment allocation.

They did an Intention to Treat analysis using data imputation to deal with any dropouts. Finally, they did the relevant multivariate statistics.

I thought that this was a nicely done trail.

What did they find?

Importantly, they found undesirable dental effects in the maxillary arch in the CH group. As a result, they stopped treatment in the CH group at 11 months and before treatment started for 5 participants.

This meant that 20 patients had HH treatment. Two of these were excluded because of interruption of treatment and miniscrew loss.

They reported problems with the CH treatment. 2 patients had an extreme mesial movement of the upper molars and labial displacement of the maxillary canines. As a result, they did not start treatment on the last 5 participants. One of these patients had missing data at T2.

The data

I will concentrate on what I felt were the most important results.

Firstly, the clinicians corrected the overjet in 17 out of 18 (94.5%) patients in the HH group. On the other hand, ten out of 14 (71.4%) had an overjet correction in the CH group. However, this difference was not statistically significant.

They also reported similar skeletal effects for the 18 other cephalometric measurements. The only difference they found was a more significant mesial movement of the upper molars on the CH group with a mean difference of 1.22mm (95% CI 0.33-2.11). I did not think that this was clinically significant.

Both groups had clinically significant changes in Wits. For the HH group, this was 1.82mm, and for the CH group, this was 1.54mm.

They concluded:

“MAMP with hybrid and conventional hyrax expanders was successful at correcting reverse or edge to edge incisal relationships”.

“There were no differences in the skeletal and dental effects between the groups”.

“The HH was more effective at preventing mesial movement of the upper molars than the CH appliance”.

They did state that their null hypothesis was rejected. However, as there were no differences between the groups, I am unclear on this. Unless it was a typo?

What did I think?

This was a nicely done small trial into a really interesting clinical development. I have always thought that the original Bone Anchored Maxillary Protraction protocols were potentially excessive. This was because placing the plates was complex and traumatic. In this respect, the MAMP approach has considerable advantages.

MAMP with either the conventional or hybrid hyrax is effective at correcting reverse overjets. It was also interesting to see that there were no discernible differences in the skeletal effects of the appliances. Nevertheless, the problems that they reported with mesial molar movement with the CH approach would persuade me to use the hybrid hyrax.


We need to be somewhat cautious with other potential conclusions. For example, we cannot conclude that the effect of this treatment is similar to Bone Anchored Maxillary Protraction because they have not been compared in a trial. The current paper’s authors point out that other groups have reported a change in Wits of 5.9mm. This change is far greater than this study. However, the BAMP studies have used the same carefully selected retrospective samples in several studies. I, therefore, wonder if this significant difference is a result of selection bias.

We also cannot come to any conclusions about this treatment’s overall effectiveness because there was no untreated control. This is an area of future research that is needed.

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Have your say!

  1. In reflection to the measurement validity they used for the study. The witts of -1 mm and the presences of edge to edge Incisal relationship is not a valid diagnosis of skeletal class III MAlocclysion. moreover, there is no report of registration of CO and CR.

  2. It’s an interesting approach but the study lacks rigor IMHO. First, the concept of “protraction” needs definition. We all understand the term clinically but what is happening to the shape, size and location of the maxilla, specifically? Second, note that “movement” or “advancement” is translation thru space – so what is the mechanism, since the maxilla can’t move like the mandible can? Perhaps there is another part to this study to be published later since the authors say “Future studies should be conducted to demonstrate 3D maxillary changes of MAMP therapy with HH expanders compared with BAMP therapy”. Thus, the inappropriateness of cephalometric analyses might be addressed using 3D geometric morphometrics in a later study. Finally, I believe the null hypothesis was rejected on the basis of only one parameter compared with the other four. So what is the alternative hypothesis in future studies? The real data lies dormant in 3D statistical shape space.

  3. The one thing about this study that I would have a question about is the large change in the molar position in the subjects that had the screw anchored RPE. There was almost a 2 mm change in the U6 – A Point measurement. That seems like a lot of lost anchorage to me. I would expect to see almost no change in that measurement if it were true skeletal anchorage.

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