Does a 7 or 14-day aligner change influence treatment duration?
This post discusses a new trial investigating the effects of various aligner wear protocols. I have written this post with Martyn Cobourne, as he makes similar comments to mine in his excellent Evidence Based Orthodontic Group which he runs with Andrew DiBiase.
Currently, clinicians are utilising different protocols for the wear duration of clear aligners, based on 7- and 14-day aligner changes. These protocols have been established for several years. However, limited research has explored the clinical effectiveness of varying wear times. This new trial addresses this clinically relevant issue.
The development of aligner treatment has relied on interpreting operators’ clinical experiences, so its evidence base is relatively low. Recently, researchers have started publishing the results of clinical trials. This is a welcome development for orthodontists who want to practice evidence-based care. As a result, we were very interested in this new trial, which looked at the efficiency of aligner treatment for adolescent four premolars. A team from Wenzhou, China, did the study. Orthodontics and Craniofacial Research published the paper.
What did they ask?
The authors pointed out that there are few or no relevant studies evaluating the outcome of Invisalign treatment for adolescent cases. As a result, they did this study to ask the question
“Are there any differences between 7, 10 and 14 day wear protocols in the results, duration of treatment, number of aligners and restarts”?
What did they do?
They did an open randomised, controlled clinical trial with a 1:1:1 allocation. The PICO was
Participants
Aged between 12-18 years with Class I malocclusion having orthodontic Invisalign treatment involving the extraction of four first premolars.
Interventions:
Group A replaced aligners every 7 days, Group B did a 10 day aligner change and Group C changed their aligners at 14 days. The patients wore the aligners for 20-22 hours.
Outcomes:
The primary outcome was treatment duration. Other outcomes were number of aligners, refinements, and occlusal index scores (DI and CRE).
One experienced operator treated the patients. They attended every 2-3 months.
They did a sample size calculation. Unfortunately, they did not include any information on the clinically effective difference they wanted to detect. As a result, we could not repeat it .
They used a pre-determined randomisation. Unfortunately, they did not provide information on concealment or operator blinding. The data was analysed blind.
They analysed the data with ANOVA and the Tamhanes T2 test. We are not familiar with this latter test but found that it was a multiple comparison test for data with unequal variances. However, when we looked at the data, it was unclear why they chose this test.
What did they find?
They enrolled 50 participants into the trial. Five dropped out. They finally analysed 15 participants in each group.
There were no differences in age, gender or occlusal index score before treatment.
We extracted the following data from the paper (means and standard deviations).
| 7 days | 10 days | 14 days | |
| OGS | 14.13 (2.4) | 14.27 (2.27) | 12.56 (1.6) |
| Duration | 28.07 (4.7) | 28.33 (4.9) | 31.6 (3.09) |
| Max aligners | 101.3 (18.6) | 96.27 (10.04) | 95.3 (10.3) |
| Mand aligners | 98.4 (17.3) | 94.9 (8.84) | 92.2 (8.3) |
| Refinements | 6.0 (1.25) | 4.9 (0.7) | 3.9 (0.96) |
They did not detect any statistically significant differences between the groups. However, they stated that the treatment duration for the 14-day group was 3 months longer than that of the other groups. This was not statistically significant, and we will discuss it later!
When they examined the components of the occlusal index scores, they pointed out a statistically significant improvement for the 7—and 14-day protocols compared to the 10-day protocols. However, these were only a few points, and unlikely to be clinically significant when you consider them overall, and there were no differences in total index scores between the groups.
Their conclusion was:
“There were no differences in the efficacy or efficiency of tooth movement across the three treatment protocols. However, the 7-day protocol demonstrated greater efficiency, reducing treatment duration by three months”.
What did I think?
Conventional manufacturer-led protocols have suggested for many years that aligners should be changed every two weeks. However, in 2016, Align Technology introduced a one-week aligner wear protocol. This shift was primarily based on advancements in aligner material, treatment predictability, and unpublished data derived from 200 in-progress cases held on file. It also had the advantageous effect of reducing predicted treatment times by 50%. Unfortunately, a previous systematic review examined the efficacy of clear aligner wear protocols and concluded that the evidence base was minimal, with little difference observed between the wear protocols. Consequently, this trial is timely.
This was a good effort for a small trial. However, we would like to address one important issue directly. This concerns to their statements regarding treatment duration. They are unclear about the distinctions between clinically significant and statistically significant differences. While we agree that they identified a clinically significant difference in treatment duration, it is important to note that this difference was not statistically significant. This suggests that the difference could have occurred by chance, which is precisely why we use statistics!
We can draw two conclusions from this. First, the observed difference may have occurred by chance, regardless of its size. Alternatively, the sample size used in the study may have been too small to detect a statistically significant difference. The latter is more likely (compounded by the rather opaque sample size calculation). Therefore, we cannot conclude that the 7-day protocol demonstrated greater efficiency. A larger trial needs to be conducted to determine if this is the case. This could also apply to the other outcomes that were measured.
This also leads us to consider that as the sample size calculation was not explicit, there may be sample size issues with this study.
Number of aligners
When we closely looked at the data on the number of aligners. The authors stated that the number of aligners was not statistically significant between groups, ranging from n=92-101 for each arch and this is where it gets a little bit confusing. You would expect the number of aligners to be similar given the pre-treatment equivalence in malocclusion between randomised groups. If you calculate the expected treatment duration based upon the number of aligners required and the wear protocol, then the differences in total treatment times should be much bigger.
If we take the maxilla as an example, the 7-day group had an average of n=101 aligners, which should take >700 days to get through; whilst the 14-day group had an average of n=95 aligners, which should take >1300 days to get through. This is a difference of around 600 days (89 weeks). The differences in refinements may reflect the actual differences in treatment time seen in the study, but this is unclear. Perhaps we are missing something very obvious here, but with similar number of aligners for a course of treatment and wear protocols up to a week longer – surely the 14-day treatments would take way longer?
Final comments
It was good to see that the treatment results were good in the hands of an experienced operator. It was also great to see some long overdue research done on aligners.
We felt this small trial was useful as it indicated the way forward for larger studies. It would be more straightforward to confine future studies to evaluating the 7 and 14-day protocols. We hope the investigators can persist in working in this clinically significant area.
Emeritus Professor of Orthodontics, University of Manchester, UK.
You should also consider the type of aligner as concluded in our latest in vitro study published in the EJO
The objective of this study was to determine the effect of aging on tensile strength and surface condition of orthodontic aligners on days 0, 1, 5, 7, 10 and 14. A total sample of 80 aligners included five brands (Accusmile®, Angel®, GRAPHY®, Invisalign® and Suresmile®) were placed in a thermocycler to imitate the temperature variations of the oral cavity and accelerate aging for 50, 250, 350, 500 and 700 cycles. The mechanical tensile properties (Young’s modulus E, yield strength YS, maximum elastic stress MES, Ultimate Tensile Strength UTS and maximum stress MS) were measured by Universal Testing Machine at a rate of 5mm of deformation per minute for 4 minutes. Microscopic observations were made under a voltage of 10kV at magnifications times 50, 250, 500, 1000 and 2500 after cleaning with ethanol and ultrasound then metallization with gold. YS and MES of Angel® aligners are statistically reduced after five days of aging (p=0.003). Aligners from the most rigid to the most flexible are (decreasing E): Accusmile® > GRAPHY® > Suresmile® > Invisalign® > Angel®. Surface conditions also deteriorated with aging (appearance of scratches, porosity, cracks, etc.). GRAPHY® aligners are more heterogeneous and weaker than others. Mechanical properties of Accusmile®, GRAPHY®, Invisalign® and Suresmile® were not affected by ageing. YS and MES were reduced from day 5 for Angel® aligners. Surface conditions are also altered by aging.
Good start to a study long overdue. I think we all have to admit that aligner wear duration has been a guess at best. Our office, we use 7 day standard changes, however we do use weekly virtual at home monitoring and will hold patients in aligners longer if needed. So, basically a sliding scale of duration based on need. In this study, having a higher number of refinements in the 7 day group accounts for some of the longer treatment time despite the shorter wear duration and similar aligner numbers. I would imagine that they had 1-2 month ‘pauses’ every time they had to order new aligners, accounting for some of that treatment time. Also, did the patients have to check off or check in each week (or 10 days or 14 days depending on their group) to ensure they were strictly following the plan? Probably not, and we all know patients kind of end up changing their aligners when they feel like it not when we prescribe them to.
Also, if only seen in office every 2-3 months, with 7 day changes, they could be ‘off the rails’ for quite some time before it is detected, meaning that when the refinements are ordered, they are having to start back a ways from where they should have been at that time.
For those of us that use aligners extensively in their practices, it is obvious that there are so many factors that influence how the teeth move (with the number one factor being compliance of course). Given this, a ‘set’ standard for aligner duration just doesn’t really make sense. And, seeing patients every 2-3 months can really hinder treatment time as you just don’t pick up on those non tracking teeth early enough sometimes. My belief is that weekly monitoring is the best way to ensure you are having them change the aligners when they NEED to be changed. And, it also allows early detection of non tracking, pointing out non compliance early in treatment so that it can be addressed (or switched over to non compliant braces).
Clear aligners are a great way to move teeth and their technology has gotten to a level that we can treat just about anything with them now (a statement I thought I would never make in years past). However, patient compliance, figuring out when to see them, how to track them, etc., remains the biggest obstacle. As one of my professors said many years ago, ‘It works great when it works.’
Dr. Dischinger makes an excellent point about the complexities of determining an optimal aligner wear protocol. Beyond compliance, there’s a host of factors influencing the outcomes that are often overlooked. For instance, the type of mechanics used during treatment (attachments, their location, size, orientation, etc.), the velocity of movement programmed (degrees and mm of movements in each aligner); they all play a significant role. Why should we expect a lower incisor to behave the same as a lower molar during movement? And how do these variables change when factoring in different types of patients or tooth movements? It’s far from a one-size-fits-all scenario.
In practice, I’ve noticed that weekly aligner changes tend to work best most of the time, but I’ve also had cases (particularly with teenagers) where patients admitted post-treatment to wearing aligners part-time, yet their results were still excellent. This raises an important question: do we sometimes prescribe more than what is truly necessary for certain cases? Perhaps each tooth requires a unique amount of force and time for movement, which brings us back to the age-old question: what is the optimal force to move a tooth? Despite claims from some that they’ve cracked the code, the reality is undoubtedly more complex. Light, continuous forces often yield excellent results, but are these forces truly continuous, interrupted, or intermittent? And does that distinction hold significant clinical relevance in the long run? What we do know is that aligners seem to deliver forces in this optimal range. A study by a German team, published in AJODO in 2014, concluded: “The forces and moments generated by aligners are within the range of orthodontic forces” (AJODO 2014;145:728-36).
I’m also intrigued by emerging approaches, like trials evaluating daily or every-other-day aligner changes to achieve slower but more precise movements. While this would mean more aligners, it could potentially improve treatment predictability. On the flip side, we see innovations such as heat-activated aligners designed to last 3–4 weeks, which might be more ecological but could introduce new challenges.
Nothing is perfect, but it’s a fascinating time to witness and participate in these advancements. The key is to stay curious, remain critical, and embrace the changes. The future is a moving target: what seemed like science fiction a decade ago is our reality now, and what we’re experimenting with today could reshape the next decade of orthodontics. Keep reading, questioning, and learning. Because as we know, progress thrives on innovation and adaptation! 😊