Expanding is one of the key methods of resolving crowding in non-extraction treatment. This post is my take on a recent publication about expansion with aligners.
In my experience, the vast majority of GDP orthodontics appears to be non-extraction alignment. This means that crowding must be relieved using an alternative method of creating space. Expansion seems to be one popular approach.. I thought it was important to identify how closely planning software correlates with actual delivered expansion and came across this article recently.
The predictability of expansion with Invisalign: A retrospective cohort study (AJODO 2022)
What was the study about?
This retrospective study examined the predicability of expansion with aligners, specifically Invisalign. The details are listed below
Setting - 2 "highly experienced" specialist orthodontists in private practice in Australia
Notable inclusion criteria - expansion of at least 1 maxillary tooth pair >3mm
Notable exclusion criteria - IPR distal to canines, planned sequential expansion
Sample size - 57 patients after application of inclusion/exclusion criteria to 775 consecutive patients. A sample size calculation is not provided, but they did find statistically significant results.
They obtained digital models for the initial, predicted (i.e. based on the ClinCheck) and achieved tooth positions for each patient. On each model, they measured arch width at the canines, premolars and molars, using 3 different methods (which I am not going to discuss here, but all of which showed excellent reliability).
What did they find?
They calculated predictability by expressing achieved expansion as percentage of predicted expansion. The results for maxillary teeth are listed below.
Tooth | Predictability of expansion |
Canines | 72% |
FIrst premolars | 79% |
Second premolars | 81% |
First molars | 64% |
Second molars | 42% |
The table shows that at best, we can expect about 80% predictability of expansion, and that predictability is worse for the molars, which is not unexpected. They found statistically significant differences between predicted and achieved expansion for all teeth, except for second molars.
Importantly, they found 13% of measurements indicated greater expansion than was predicted. The authors propose this may be caused by planned bodily movement being expressed as tipping, causing a greater buccal movement of cusp tips (where measurements for width were made). The exact mechanism is difficult to pin down, and the authors are frank in accepting the limitations of any explanation.
My own feeling is that the cause is multi-factorial, and may be related to the aetiology of the narrow arch; an upper arch with teeth tipped palatally will be easier to expand out by buccal tipping than an arch which is already quite wide.
Does this effect clinical practice?
The results are not unexpected, but do help to reinforce some planning principles.
We often over-correct or build in additional expansion to the aligner plans to account for under-performance of the aligner. The results of this study help to determine more accurately the amount of over-correction should be requested. However, this advice must be used with caution as over-expansion is equally undesirable as under-expansion, and potentially more damaging to periodontal health.
As ever, emphasis is placed on thorough clinical assessment and sound treatment planning. Identification of cases which are more likely to be receptive to expansion (along with use of mechanics which facilitate this type of movement) will, presumably, result in higher predictability of treatment. It is advisable to undertake some orthodontic course or training to understand these principles and implement them in your clinical practice.
Orthodontic Learning Points
About the study
Expansion is most predictable around premolars, and worst around second molars
Actual expansion may be under- or over-delivered
How to learn more about this
Read around aetiology of narrow arches
Understand indications for expansion
Attend our orthodontic course to gain confidence in your planning abilities.
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