February 14, 2022

A Massive Cochrane review on Orthodontic treatment for crowding.

The Cochrane team has been busy with orthodontics. Hot off the heels of the review on crossbite, here is a review on the treatment of crowding. Again, it shows how much we know and what we don’t know.

We can divide treatment for crowding into two main parts. The first is interceptive care, which is frequently carried out in Phase I. This has the “hopes and dreams” of correcting crowding in the transitional dentition. The second part is when we correct crowding as part of active treatment; when our patients are in the secondary dentition. This review addressed these two main areas.

For Phase I, they looked at the use of lip bumpers, lingual arches, and extraction of primary teeth. In adolescent treatment, they included many interventions. For example, types of brackets, vibrational devices, and archwires.

A team from Liverpool, Cork, and the great Northern city of Manchester did this review. The Cochrane Library published the paper. This is open access in several areas of the World, so I hope you can access it.

Orthodontic Treatment for Crowded Teeth In children.

Sarah Turner, Jayne E Harrison, Fyeza NJ Sharif, Darren Owens , Declan T Millett

Cochrane Database of Systematic Reviews 2021, Issue 12. Art. No.: CD003453.

DOI: 10.1002/14651858.CD003453.pub2.

What did they ask?

They did this study to:

“Assess the effects of orthodontic interventions for preventing or correcting crowding teeth in children”.

What did they do?

The authors did a standard Cochrane Review. As I have said before, Cochrane reviews follow a precise methodology. The Cochrane Oral Health Group also provides input to help the authors. They also monitor the progress and quality of the study. This means that the reviews are generally of high quality.

The authors did an electronic and hand search of the relevant literature. Then they identified pertinent papers to the review. This was then followed by data extraction, appropriate meta-analysis, and assessment of bias with the Cochrane Risk of Bias tool. In the final stage, they considered the strength of the evidence using the GRADE approach.

The PICO for the review was;

Participants: Children aged 16 years old or under who received orthodontic treatment to prevent or correct crowding.

Intervention:  These included fixed appliances, removable appliances and auxiliaries, and extractions.

Control: No treatment, delayed treatment, or other interventions.

Outcomes: Primary outcome was the amount of crowding measured in mm or any index of malocclusion. There were many other secondary outcomes.

What did they find?

The team identified a final sample of 24 studies that they could include in quantitative analysis; they used  7 in a meta-analysis.

This meant that they included 24 RCTs that provided data on 1512 participants. The studies presented 27 different comparisons.

The authors presented a large amount of data on many comparisons from single trials. They divided the studies into those that investigated the following:

Fixed appliances and auxiliaries, lip bumpers, cervical pull headgear, lower lingual arches, bracket designs, archwires, lacebacks, vibrational devices, removable appliances, eruption guidance appliances, extractions, extractions of lower deciduous canines, and extraction of wisdom teeth. I think that this just about covered every type of orthodontic treatment!

One factor that was a little problematic was that there was only one study for most of the interventions. As a result, a lot of the data was presented as a simple analysis. Therefore, I have decided to present my personal choice of data.

Lip bumper

The use of a lip bumper resulted in a decrease of lower incisor crowding by 4.39mm when compared to untreated control. High risk of bias.


There was no difference between self-ligating and conventional brackets on crowding. High risk of bias.


There were no real differences in archwires treating crowding. High risk of bias.

Eruption guidance

The use of an eruption guidance appliance reduced the number of children requiring later treatment for crowding. The chance of remaining crowded without treatment was 5.3 times more than in the treatment group. There was a high risk of bias.

Vibrational appliances

There was no evidence that these appliances were effective. High risk of bias.

Extraction of primary canines

The reduction in crowding in the extraction group was more significant than the control with a mean difference of 4.75 mm. However, there was an overall reduction of arch length in the extraction group by 2.73mm. High risk of bias.

You may have spotted that I wrote that the authors classified all the studies as high risk of bias, according to the Cochrane Risk of Bias tool. They made this classification because, in all the studies, the operators were aware of the treatment allocation. I will return to this later.

My interpretation of the authors overall conclusions was:

“The evidence from this review was very low quality. It appears that lip bumpers, Schwarz appliances, and extracting primary canines may reduce crowding. None of the other interventions had clinically significant effects”.

What did I think?

As with all Cochrane reviews, this was a large amount of work well done and written up to a high standard. Nevertheless, I must admit that I was disappointed by the conclusion that the studies provided a very low level of certainty.

Risk of  Bias

One of the main reasons for this was that the trials were at high risk of bias. The rationale for this evaluation was that the operators were aware of the treatment allocation. Unfortunately, I think that this is a problem for orthodontic trials; because it is not possible to blind the operator to the treatment.  Orthodontic treatment is not like a pack of pills or a vaccine, the identity of which can be concealed from the operator. Nevertheless, we cannot get away from the fact that there is a risk of bias if an operator is not blinded. It is, therefore, up to you to decide if the assessment is too harsh and take this into account when you interpret a Cochrane review.

What can we conclude?

When I looked at these results, I decided that I would consider any risks, bearing in mind the uncertainty in the data.  As a result, I felt that we can reduce the probability of incisor crowding by using lip bumpers. However, we need to be cautious about extracting primary canines because of the loss of the posterior space. I was interested to see that eruption guidance appliances may have potential, but I would like to see some more research. It is clear that different archwires, fancy bracket designs, and orthodontic vibrators do not have a meaningful effect on the correction of crowding.

My final feeling is that I wonder if keeping treatment simple with a lip bumper is the optimum method of addressing potential lower incisor crowding? It is low risk and appears to be effective.


I have worked with Jayne Harrison and Declan Millett on several research projects over the many years that we have known each other.

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Have your say!

  1. Interesting that virtually all treatments assessed have a high risk of bias.

    Extracting primary canines may allow the permanent incisors to align, but where did the crowding go? Neither arch circumference nor arch length increased.

    A lip bumper might allow the permanent incisors to align by effectively moving the crowding posteriorly and/ or by allowing the lower incisors to procline.

    All we can conclude is that we cannot apply the same formula to all patients and that good orthodontic treatment requires good diagnosis with our biased treatments.

  2. Fascinating that use of a lip bumper might reduce crowding. My experience that often there even more Cl II afterwards to correct is no big whoop…I like studies that ask and answer more relevant clinical questions. These are sadly rare.

  3. Seems to me…you make your choice of treatment intervention…and you take your chances! Nothing I have seen here would convince me that, based on evidence, I made the wrong treatment choices or the right treatment choices during my practicing career. My patients wanted straight teeth which is what I gave them at the time and followed up for a year or so.

  4. Two questions:

    1.What percentage of these 1,314 (7-16 yr old) ‘children’ do you think might have benefitted from earlier diagnosis of transverse deficiency and appropriately applied arch development intervention, say, before the age of 7?

    2. How likely might it have been that some of these children at T-1 and/or T-2 had possible sleep and airway hygiene compromise co-morbidities with their malocclusions?

    Thanks for considering my inquiries Kevin.

    • Thanks for the questions, this data was collected from published randomised trials that had been published. As a result, the information in the review is totally dependent on the trial publications. With respect to your question about arch development, this would have been included in the review if randomised trials had been done into this. As they did not report on this, we can be assured that there are no RCTs into “arch development”. I think that these comments also apply to your question about airway etc. It is clear that research needs to be done into these problems. I hope that I have answered your questions.

      • Thank you for your reply Kevin.

        Yes, sir, of course, arch development and airway structure/function were not evaluated in this Cochrane Review, and, to date, indeed there are no published data derived from RCT’S on either of these topics; and furthermore, it is doubtful that there ever would be an ethically-approved RCT that might propose to withhold a known therapeutic intervention (RPE for Dx:Maxillary Transverse Deficiency) from a control cohort merely for comparative purposes. So be it, please let me rephrase my questions to you:

        1. Might you agree that when maxillary transverse deficiency (MTD), with or without the presence of a posterior cross-bite, is detected before the age of 72 months old (primary/early mixed dentition), it will seldom, if ever, self-correct, will often worsen without deliberate intervention, and often become co-morbid with sagittal and/or vertical malocclusion phenotypes?

        2. As myriad recently published papers in PUB MED journals report on QOL improvements associated with RPE intervention for Dx: MTD (please see references below), might you agree that sometimes we should consider treating MTD in the primary/early mixed dentition regardless of peer pressure from those of our colleagues who’d prefer to wait until the problem worsens with age?

        ‘The role of rapid maxillary expansion in the promotion of oral and general health.’ McNamara et al. Progress in Orthodontics (2015),16:33 (https://pubmed.ncbi.nlm.nih.gov/26446931/); ‘The impact of rapid palatal expansion on children’s general health: a literature review,’ Eichenberger, M. and Baumgartner, S. Eur J Peds Dent (2014), 15(1):67-71.(https://pubmed.ncbi.nlm.nih.gov/24745597/); and, ‘Craniofacial and upper airway morphology in pediatric sleep-disordered breathing and changes in quality of life with rapid maxillary expansion.’ Katyal, V. et al. Am J Orthod Dentofacial Orthop (2013), 144(6):860-71 (https://pubmed.ncbi.nlm.nih.gov/24286909/).

        • Thanks for your comments. I disagree with your view that a trial of RPE would not be ethicak. In fact, it is unethical to promote a treatment with a low evidence base. It would be perfectly feasible and ethical to carry out a trial in which treatment is delayed for a period. We did this in the early Class II treatment studies. For another example, I am in a trial of a COVID 19 vaccine. I was randomised to get a vaccine or saline. This was at the time of the high levels of COVID infection in the UK. The risk was two sided. Firstly, the vaccine was experimental with side effects that were not confirmed from large samples and I was at risk of getting COVID if I got the placebo. If this is ethical then nearly all orthodontic trials that are at low risk of harming the participants are likely to be ethical. In answer to your question number 1, I do not know, but I would treat in the early secondary dentition. With regards to your other questions I will need to read the references and I am taking a break at the moment. I will het back to you

  5. so after over 100 years of orthodontic clinical interventions and research into a very common orthodontic feature (crowding) we do not know which treatment is best, we do not know if there are health benefits, we do not know if the health gain (if any) is lasting, we do not know who is most likely to benefit from treatment (if there is any benefit)

    lets hope governments who fund orthodontic treatment dont look too carefully at our evidence base for care

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