September 20, 2022

Is a 2×4 appliance better than aligners for mixed dentition crowding treatment?

The traditional method of providing alignment of the mixed dentition may be using a 2X4 appliance. Nevertheless, clear aligners may be an excellent alternative treatment that does not involve bonding brackets to the incisors. This new randomized trial examined the effectiveness of aligners or 2×4 appliances.

In the introduction, the authors pointed out that correction of crowded incisors may be carried out in the mixed dentition. This is a controversial area, and the clinician’s drive to provide early interceptive treatment clearly influences the need for early treatment. I understand that this type of care is popular in the USA. However, I am not sure it is carried out in Europe. This means that this paper may be somewhat controversial.

A team from Bauru, Brazil, did this study. The Angle Orthodontist published the paper (this means that it is open access, and anyone can read it).

Comparison between clear aligners and 2 3 4 mechanics in the mixed dentition: a randomized clinical trial

Vinicius Merino da Silva. Angle Orthodontist, advanced access. DOI: 10.2319/032322-237.1


What did they ask?

They did this study to

“Evaluate and compare the efficacy and efficiency between fixed appliances and clear aligners for resolving maxillary incisor irregularity in the mixed dentition”.

What did they do?

They did a parallel-sided randomized trial with a 1:1 allocation to answer this question. The PICO was


Orthodontic patients who they had treated with RME for posterior crossbites. The participants were between 7 and 11 years old, in the mixed dentition with a maxillary Little’s Index of at least 3mm.


Clear aligner treatment. The aligners were replaced every 15 days, and they asked the patients to wear them for at least 20 hours per day.


Fixed appliance treatment with the 2X4 appliance. They bonded brackets to the upper incisors and tubes to the molars. They used a standardized wire sequence, with the final wire being a 0.20 round ss wire.


The primary outcomes were maxillary incisor irregularity and treatment time. They also selected secondary outcomes, including arch widths, perimeter, plaque index, and caries.  The team collected the data six months after RME treatment (T1) and at the end of incisal alignment (T2).

They did a sample size calculation that showed they needed 16 patients in each intervention group.

Finally, they did a simple univariate statistical analysis. The study team used stratified block randomization on irregularity. Allocation was done by a coin toss. They did not provide any information on concealment.

What did they find?

Firstly, they did not find any differences between the intervention groups at the start of treatment. However, I was concerned about marked gender differences between the groups. Chi-squared tests showed this was not statistically significant, but I wonder if this could have occurred because of a lack of power?

When they looked at tooth movement. The mean reduction in the Little’s Index was 5.8 for the aligner group and 5.15 for the fixed appliance group. This was not statistically or clinically significant. The treatment duration was 8.0 (2.9) months for the aligner and 8.6 (2.65) for the FA group. Again, these were not statistically or clinically significantly different.

They did not find any other differences in the outcomes that they measured.

Their overall conclusion was

‘Clear aligners and fixed 2X4 mechanics showed similar efficacy and efficiency for correcting maxillary incisor crowding in the mixed dentition”.

“Both appliances showed similar dental plaque index and white spot lesion incidence during treatment”.

What did I think?

This was an interesting study for those who provide interceptive treatment of maxillary crowding. It is also of interest to those looking to provide aligner treatment to children. Importantly, it shows that aligners are as effective as 2×4 fixed appliances for this treatment. This means that the advantages of invisible appliances are available to our younger patients.

While the trial came up with interesting information, I was disappointed that there was limited information on randomization, allocation, and concealment.  Furthermore, the treatment endpoint was unclear, with a lack of operator blinding. This is likely to mean that the study is at risk of bias.  As a result, we must bear this in mind when interpreting the data.

In addition, while there were no differences in treatment duration between the treatments. I was disappointed that they did not provide any information on the number of attendances between the interventions.

This is important because currently, some KOLs in the pay of aligner companies are suggesting we only need to see young patients a few times during treatment. This information would have helped to inform us of these ambitious treatment protocols.

I also thought that the treatment times were rather long. I certainly would not intervene at this early stage, as this adds to the overall burden of care and is potentially unnecessary. .  I would correct this irregularity as part of a single comprehensive course of treatment.

Final comments

Finally, it is great to see trials being done on aligners. These are beginning to provide helpful information on the relative merits of aligners and fixed appliances. This research is becoming increasingly important.

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Have your say!

  1. I think a better question might have been: Is there any benefit to the patient of undertaking mixed dentition incisor alignment?

  2. From what I can tell, the programmed movement using aligner mechanics was a very conservative 0.2mm per month, (0.1mm per aligner, change each 15 days). All movements were also overcorrected by 20% .This may contribute to the extended treatment time you alluded to Kevin. I would be interested to see a follow up in retention phase.

  3. My question is if you wanted to align the teeth
    A few brackets wires cost $50 and 10 minutes of my time at most ( rest is hygienist)
    Or a few thousand dollars for aligners ( from a company).

    • Great. And nobody counts the process time for aligners, as Preparing the patient data, patient protocol, time for virtual planning, time for virtual planning modification, time for aligners manufacturing and shipping.

  4. A big advantage of aligners in mixed dentition is the ability to do dental expansion in the lower arch simultaneously, something not done routinely with 2×4 fixed appliance tx

  5. I never saw the point of aligning incisors in the early mixed dentition stage. In my day, you wouldn’t get NHS approval anyway!

  6. For clarification the study does state the aligners were designed with Maestro 3D and the models printed on a Moonray S printer (older model) and made in a PETG plastic so they are in-house designed and fabricated aligners. I think many would use a more frequent change schedule and my personal experience (sorry for anecdote) is that these early treatment cases move well with 10 day change (my routine) and often get away with 7 days. Takes very little of my time to design in the software I use and certainly less of my clinical time. So it is an option for the right case that will wear it (as some won’t). Having said that, it is usually only used for anterior crossbites, significant overjets, or where the appearance is socially affecting the child, otherwise I advise they wait for one comprehensive phase later.

  7. There are several ,valid reasons to perform “phase 1”orthodontic treatment .I have worked both sides of the Atlantic and can,maybe give a little perspective.
    1-whether or not the NHS “covers it”,or not, should never be the basis for any clinical recommendations.This may seem naive but I firmly believe this !
    2-blaming “corporate “clinics for carrying out phase one treatment ,unnecessarily ,is a distorted view of the situation .I work with a corporate group ,run by orthodontists,who give the clinicians the absolute right to practice as they wish!
    3-in my view ,N.American parents are more orthodontically attuned than UK ,EU parents in several respects.They see value in a ,albeit temporary ,aesthetic improvement provided by phase 1 ortho.We are in an age of fascination with appearance,social media and associated pathologies eg.eating disorders .N.American parents see value in providing an esthetic smile at a “ difficult “stage of adolescent developement.They are well aware of the extra costs and that a “later comprehensive “stage may well be needed.
    N.American parents see value in an initial phase in several situations eg.not allowing upper permanent cuspids to increase their impaction !
    A philosophy of wait and see or benign neglect is not appreciated.Litigation is a major issue here in N.America and I understand dental litigation has been increasing,rapidly ,in the UK. This can,unavoidably ,be a major influence on clinical decision making.

    • Thank you for your thoughtful discussion. Incisor crowding correction in the mixed dentition cause less root resorption compared to permanent dentition as showed by Sweden studies. In addition, Dugoni and collaborators showed that one third of patients that undergo phase I, will not need phase 2. Anticipating facial esthetics is an advantage if quality of life is improved. Quality of live studies in early correction of incisor crowding should be carried out.

  8. Disclosure : I speak on behalf of Align Technology
    Kevin, think we need to be careful and specific when we evaluate and discuss “aligners” and “aligner mechanics”. All aligners are not necessarily equivalent; and actually necessarily not!
    Patents prevent companies from “copying” more features and processes so that no 1 company is even able to produce equivalent systems. Even if we attempt to ignore the marketing of inherent “science”, there are now thousands of companies and hundreds of software programs, manufacturing methods and material differences ranging from manual in-house to fully digital computer programmed aligners. These aligners all have specific activations, and do not have the ability to have equivalent mechanical features.

    Are we to accept that because the appliances look “similar”, that they all have similar clinical expression? That’s just plain ignorance. Elon Musk and his Tesla would have a word to say about that…(do we care?)…

    Perhaps we are driven to overlook the differences as it suits the companies behind the “new” aligner systems? They don’t then need to produce the “evidence” that was, and still is, demanded of the pioneer computer programmed aligner system; they just show cases treated by the other companyIs it OK to be a KOL for a newer company, but show cases treated with Invisalign mechanics?- Is it OK to be a KOL for a newer company, but show cases treated with Invisalign mechanics? – and claim that because a result was achievable with this clear piece of plastic that I learned to use over many years, with billions of dollars of investment into making it work, that I can immediately obtain the same results with this new system because they make brackets that work, because they are not the #1 brand in sales, because they are not as cocky, because they give me wine?…. Or perhaps because we largely place “fixed appliance” effectiveness in the same bucket and if we can make 1 bracket system work in our hands, we have the skill to work and compensate for with another? Unfortunately its not the same comparison or consideration with “aligners”, unless you want it to conveniently be that way.

    The initial capture of anatomic data, programming (computer vs manual vs activation vs sequence vs force driven vs shape driven to name but a few differences), the material (modulus of elasticity, ability to produce a moment of force within physiologic tooth moving range- (clarity? – not unless we are now marketing and ignoring science), are not variables that may be compensated for by clinician experience nor will; as perhaps we may overcome differences with fixed appliance systems. Simply because the clinical process for clinician and staff are largely similar,: capture data, work on a computer, deliver plastic; does not make the appliance systems capability equivalent, especially when considering lumping them together for purpose of evaluation in any objective research question of effectiveness. Fixed appliances are largely clinician driven, reactive in nature. Clinician experience by default becomes one of the major variables in outcome. Computer programmed aligner systems, like it or not, are pre-programmed via patented software defaults that may – or may not be – tweaked only to limited extent by clinicians. For sure clinician experience in working with the particular software system may have an impact on outcome, however we are bound within the limitation of the particular software , mechanical features available and the rest of that products material capabilities.

    I hope that I have made my point, that aligner systems are likely not as similar with respect to clinical effectiveness as their final product – the aligners – would have us believe. Our ability as clinicians to overcome differences maybe very different with aligner mechanics than with fixed appliance systems. This is of course yet to be put to the test of rigorous research. Until that time, it seems to be most convenient for us to ignore the fact that all systems apart from Invisalign have virtually no body of evidence, even anecdotal, in sufficient completed case numbers; but because an aligner is clearer, cheaper, or they make brackets, it must be at least as good!

    The hypocrisy kills me; many of these are (my friends) and the same folks that were / are screaming for evidence from Align Technology, but it’s OK that there is far, far less, perhaps none from the others they choose to “sell”? Are they trying to punish Align, their intelligence, their patients or all? ..Bring on the naked Emperor.

  9. Dear Dr. O’Brien, please, replace the previous reply for the following one with mistypes corrections, please. We also added a new sentence in the manuscript. We are appreciated. Best regards, Daniela

    Thank you for sharing information on our RCT and for your thoughtful discussion. Both groups were followed once a month during treatment. For randomization, the envelops were concealed. The major limitation of our sample is the absence of severe crowding and severe rotation of incisors – we assume there would be difference between the appliances for severe crowding and rotation.

    Regarding early treatment, incisor crowding correction in the mixed dentition can be performed instead of should be performed. Parents and child’s expectations and complaints should be considered in the decision-making. Incisor alignment in the mixed dentition cause less root resorption compared to the correction in the permanent dentition as showed in a previous study from Norway (Mavagrani et al. 2002). In addition, Dugoni and collaborators showed that one third of patients who undergo Phase I, did not need phase 2 treatment. This is a potential advantage of early treatment. On the other hand, patients who need phase II had the complexity of malocclusion decreased by 50% (Pangrazio-Kulbersh et al 2018). Anticipating smile esthetics is another advantage of early treatment of incisor crowding what might have an important impact on quality of life and well-being among children and adolescents.

  10. It is interesting that someone discusses a possible effect produced by the clear aligner brand, but not by the brand, prescription, etc, of the fixed appliance.
    Furthermore, we should take a look at the intergroup difference for the plaque index.

  11. I love me a good 2×4 but it makes me super upset when docs just pull C’s to make space and don’t develop the arches first, thus causing a sequelae of future issues.

  12. Depois de toda a avaliação do trabalho, o comentário mais importante:
    Precisa mesmo interceptar apinhamento na dentição mista??
    Concordo com o Dr. O’Brien.
    Pra mim, fase 1 na dentição mista é para controle de hábitos ou mordida cruzada ou perda precoce.

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