May 29, 2023

There is no need to treat lower incisor crowding early.

One of the first features of malocclusion that we detect is lower incisor crowding. We also know that parents and children become concerned with this feature in transitioning from primary to secondary dentition. As a result, we may carry out early correction. But is this additional treatment necessary?

This post could be one of the back-to-basics series we have been publishing. It originates in remembering and appreciating the spontaneous changes in children’s teeth. For example, we were all taught that lower incisor crowding in the transitional dentition would improve, yet there appears to be a tendency to try to correct this feature early. This interesting systematic review looked at the amount of spontaneous change that we may expect.

A team from Brazil did this study. Progress in Orthodontics published the paper.

Spontaneous changes in mandibular incisor crowding from mixed to permanent dentition: a systematic review

Cibelle Cristina Oliveira dos Santos et al

Progress in Orthodontics Online advanced.

What did they ask?

“What is the physiological behaviour of mandibular incisor crowding in transitioning from the mixed to the permanent dentition”.

What did they do?

They carried out a standard systematic review of the literature. The PECO was


Children in the mixed dentition


Mandibular incisor crowding


Before and after crowding in each participant


Change in crowding from mixed to permanent dentition.

Study design:

Observational and clinical studies.

They followed the usual systematic review methodology of electronic and hand searching, identification of papers, data extraction and assessment of bias.

The team assessed the risk of bias of the final included papers using the Risk of Bias in non-randomised studies of Exposure (ROBINS-E) tool. They also considered the certainty of evidence using GRADEsoftware.

What did they find?

At the end of the literature searches and selection of papers, the team identified a final sample of five studies.

The five studies were all cohorts. Of these, two were prospective, three were retrospective. The sample included data from 243 participants: 124 males and 119 females. The mean age at the start of the data collection ranged from 7 to 9 years. Three studies followed the sample for four years, one study lasted five years. The longest follow-up was six years.

Regarding the primary outcome of mandibular incisor crowding, two studies used Little’s Index. Two looked at the differences between the widths of the incisors and the space between the primary canines. Finally, one study used the Leighton method. The differences in measurement techniques between the studies resulted in marked heterogeneity. As a result, the team could not perform a meta-analysis, and they presented a qualitative data analysis.

I thought that the main points of their data were:

  • The retrospective studies with moderate risk of bias showed average reductions of 0.17mm after four years of follow-up.
  • The prospective study revealed a reduction in crowding from 0.3 to 0.6mm.
  • The study with a low risk of bias showed that children with 2mm initial crowding had a reduction of 4.6mm in crowding.

Their assessment of the GRADE certainty of the evidence was moderate.

Their overall conclusion was:

“There was a moderate level of uncertainty that mandibular crowding reduces in the transition from primary to secondary dentitions by up to 4.62mm. The greater the initial crowding, the greater the spontaneous correction”.

They also reminded us that this spontaneous change was due to the leeway space, increased arch width, and permanent incisor protrusion.

What did I think?

This paper was clearly written and presented a simple systematic review that answered a relevant clinical problem.

When I considered the value of this review, I felt that it reinforced what I was taught and already knew about the resolution of lower incisor crowding the transitional dentition. As a result, it reinforces our knowledge. Ironically this study also shows that interceptive treatment simply addresses a change that is likely to occur anyway.

Notably, the spontaneous change occurs without appliances, proclination of lower incisors, the burden of wearing fixed appliances and additional income to the orthodontist.

Finally, this study also reminds us that we must not forget the basics of dentofacial growth and development.

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Have your say!

  1. And there goes another income stream. At this rate, Phase 1-addicted practitioners won’t have anything to hide behind.

    • The Phase 1-addicted will find something.

      As noted in a previous issue of this blog, irrespective of the AAO white paper on airway, AAO invited several “airway speakers” at the recent Chicago meeting extoling the virtue of pre-school active airway treatment. Evidence be damned!

  2. Could this be a game changer? Let me ponder for a moment: should I trust this profound paper, or should I trust Ricketts? Hmmmm…I think I will stick with Ricketts….

  3. I totally agree with Kevin “that we must not forget the basics of dentofacial growth and development”. However, since he graduated a fair while back, there has been a tremendous change in the knowledge base but it appears that it might not have gotten into the mainstream as yet. It’s interesting that the authors use the old functional matrix hypothesis to partly explain their findings; however, the integration of temporo-spatial patterning might also further elucidate the mechanisms by which crowding is partially relieved, as follows: During growth, the spatial and functional alignment of the maxilla and mandible is maintained through remodeling of bony surfaces, including the periodontium, to permit function (through tooth eruption). However, environmentally-induced (epigenetic) changes of the early morphologic relationship (say, due to bottle-feeding) result in a ‘new solution’ (phenotypic variation), clinically recognized as lower incisor crowding here. This departure from the genetically encoded ‘developmental program’ (temporo-spatial patterning) is now part of a clinical diagnosis (malocclusion) because the system will regress to homeostasis. Thus, developmental compensation occurs to permit compromised function — and decompensation is required (perhaps through orthodontic treatment). In fact, appropriate spatial signaling can re-establish (genomic) pattern formation for optimal form and function at least to some extent (as noted in this example) since the cranio-caudal gradient of development ensures that allometry of the maxillary spatial matrix will permit concomitant mandibular development. The early orthodontists mused that wisdom tooth eruption might be associated with lower incisor crowding. Now, it appears that partial auto-resolution of lower incisor crowding might be the harbinger of impacted wisdom teeth because of temporo-spatial patterning?

  4. Other than wallet fattening (or lightening on the other side of the equation), what purpose does orthodontic alignment of mandibular incisors serve in the mixed dentition? If the e-space will eventually be utilized to align the mandibular incisors then early alignment prior to loss of the dec molars will procline the incisors and will be round tripped when space is available. Then there are the downsides of hygiene in an 8-year old, risk of decalcification, and straining of possibly thin attached gingiva. One practical reason why I avoid it is that I find that a lower 2×4 setup is particularly prone to breakage. Then there is the problem of retaining alignment while awaiting transition to complete. When I explain this to parents who are wondering why I don’t treat the lower arch along with maxillary expansion I never get any pushback. The one exception in my practice is when the lateral incisors have distally tipped over the canine eruption space. I’d be interested in hearing a good “why” from any other readers.

  5. I am having a little trouble with these conclusions for the following reasons:
    -Leeway space in the lower arch is usually lost as the first molar drifts forward into Class 1 from the end to end position of the mixed dentition and thus is not available for correction of crowding.
    -If I recall correctly, arch width/inter canine width does not change appreciably after age 8, thus no additional space.
    -Lower incisor proclination as a method of creating “spontaneous correction of lower incisor crowding” makes no sense because I am generally reducing proclination or at least holding incisors to prevent proclination so no space for me there.
    If the 3 conditions above are true, there is no space for “spontaneous correction”.
    The statement “The greater the initial crowding, the greater the spontaneous correction”. WHAT???!! They are saying that The greater the TSALD, the greater the spontaneous alignment of teeth?!?! How? something else has to change for that to occur and it is probably not something I have treatment planned for in my patients.

  6. However I think it is necessary to maintain the lee-way through an LLHA, I wonder if this can be considered phase 1

  7. Yes, Dr McDonald, I agree with you that “something else has to change for that to occur and it is probably not something I have treatment planned for in my patients”. I suggested that the crowding (developmental compensation) is pushed downstream and may later manifest as lower third molar impaction – which can be decompensated clinically. I also inadvertently omitted citations, which my suggestion is based upon, so here is a condensed list.

    Singh GD, Lee SM. Midfacial development and the wisdom of teeth: A case series. Clin Case Rep. 2021. Authorea. September 25, 2021. doi: 10.22541/au.163257033.36760750/v1
    Singh GD. Spatial matrix hypothesis. Brit. Dent. J. 202(5), 238-239, 2007.
    Singh GD. On Growth and Treatment: The spatial matrix hypothesis. In: Growth and treatment: A meeting of the minds. McNamara JA Jr (ed.) Vol 41, Craniofacial Growth Series, Ann Arbor, USA, 2004, 197-239.
    Singh GD. Report on a Symposium “Advances in the modeling of form: Clinical and computational interfaces”. Amer J Orthod Dentofac Orthop 125(1), 124-125, 2004
    Chaplain MAJ, Singh GD and McLachlan JC (Eds). On Growth and Form: Spatio-Temporal Patterning in Biology. John Wiley and Sons Publishers, England, 1999.
    Moss ML. The functional matrix hypothesis revisited. Am J Orthod Dentofacial Orthop. 1997;112(1):8-11.

  8. Unfortunately, this systematic review is one more among many, representing our inability to move forward and carry out studies that really bring some response. It is a perfect example of your August 2022 blog post. “I feel this way because, unfortunately, many reviews include low-quality papers and retrospective studies full of bias. The authors seem to include these papers because they could not find any trials. This simply dilutes the quality of the review, no matter which convenient “tool for bias” is used. “

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