Can patients wear their aligner for only 12 hours a day?
This is my final post of the year, and it examines a really interesting study on different wear-time protocols in aligner treatment.
As with all removable appliances, cooperation can be a challenge during aligner treatment. It is crucial that patients wear their aligners for 22 hours each day. Nevertheless, it remains uncertain whether shorter periods of wear are effective and improve patient cooperation. This question was explored in this very interesting randomised controlled trial.
A team from Baru, Brazil, did the study. The European Journal of Orthodontics published the paper.
What did they ask?
They did this study to:
“Evaluate whether a 12-hour daily wear protocol can achieve clinically comparable outcomes to the 22-hour standard protocol in Class I patients with mild crowding”.
What did they do?
The team conducted a single-centre randomised clinical trial with two parallel groups and a 1:1 allocation ratio.
The PICO was:
Participants
Male and female orthodontic patients aged between 18 and 60 years presenting with Class 1 malocclusion, a complete permanent dentition, good general health, and a maximum Little’s Irregularity Index (LLI) of 2 mm.
Interventions:
They treated the patients with Invisalign orthodontic aligners made of SmartTrack material and enrolled in a Comprehensive Protocol.
Group 1 included 25 patients, who were asked to wear their aligners for 22 hours a day.
Group 2, again, included 25 patients. They were asked to wear their aligners for 12 hours a day.
It is important to note that the intervention is a request asking patients to wear their aligners for a set period of time. We do not know the actual wear time. Nevertheless, this is a perfectly reasonable design as it reflects “real-world” clinical practice.
Outcomes
The primary outcome was the Little’s Irregularity Index.
The secondary outcomes were various dental measures such as intercanine distance, overjet, and overbite.
The team collected data at the start of treatment and at the end of the 10th aligner treatment. This corresponded to approximately 140 days of treatment.
All the aligners were fitted with compliance indicators. However, the study team preferred to collect compliance data by asking the patients to record the number of hours they wore their appliances.
Statistics
They conducted a thorough sample size calculation to determine the number of participants required. They powered the study to detect a 1.25 mm difference in the Littles Irregularity Index. This indicated that the required sample was 11 participants. However, to enhance statistical robustness, they recruited 25 participants per group.
They performed block randomisation before the study. They ensured allocation concealment by using sealed envelopes. A separate, independent operator from the clinical team generated the randomisation sequence and assigned participants.
They analysed the dat with an Intention-to-Treat Analysis. As with most orthodontic studies, it was not possible to blind the operator and the patient to the allocation of their treatment. However, the final scans and measurements were performed blinded.
They carried out a fairly simple statistical analysis using inter-group comparisons of the variables with an independent t-test.
What did they find?
50 participants took part in the study, and none were lost to follow-up. As a result, all the patients were included in the analysis. There were no differences between the groups at baseline.
At the end of treatment, both groups demonstrated a statistically significant improvement in the maxillary and mandibular Little’s Index. However, there was a greater correction (0.84mm) in the 22-hour group for the mandible (p=0.017) than for the 12 hour group.. The mean difference between the interventions was 0.84 mm. I have calculated the 95% CI as -1.5327 to -0.1473.
When I looked at the tables, I spotted some problems and raised these with the authors. They kindly got back to me to confirm that the data columns were transposed in the advanced access PDF. The authors have raised this with the journal, and they are addressing the problem. They also sent me the corrected data tables.
I have compiled this data into a table to present the LLI values at the end of the 10 aligner treatment stage. These are means with standard deviations in brackets.
| 12 hour | 22 hour | |||
| Pre-treatment | Post-treatment | Pre-treatment | Post-treatment | |
| Maxillary LLI | 5.3 (2.2) | 3.11 (1.76) | 4.47 (1.54) | 2.48 (1.26)) |
| Mandibular LLI | 4.43 (1.81) | 2.71 (1.35) | 4.32 (2.6) | 1.87 (1.07) |
They reported no harms in the study. I could not find any data on the number of hours of wear recorded by the participants.
Their most important conclusion was that,
“For the sample of patients included in this study and with outcomes assessed after the first 10 aligners (140 days), the 12-hour daily wear protocol produced clinically adequate alignment in the maxilla in cases of very mild crowding. However, correction of mandibular anterior irregularity was significantly more efficient with the traditional 22-hour protocol”.
What did I think?
This was another good randomised trial into the effectiveness of clear aligners. As I have previously stated, studies of this nature are long overdue. In this respect, this study provided us with helpful information.
The authors came to some clear conclusions. It was good to see that they compared values at the end of the treatment period rather than simply looking at treatment change. This is a far more valid way of analysing orthodontic data because it gives us a comparison at the end of the observation period.
Regular readers of my blog will now know that I am going to discuss the effect size and whether this was clinically significant. I know I have been writing about this for some time, but in studies of this nature, it is essential to consider. The only significant difference in alignment at the end of the study period was in 0.86 mm for the mandible.This is not clinically significant. As a result, we must bear this in mind when considering the conclusions of this study.
I cannot help feeling that there is no real difference between the two types of interventions, and it is perfectly feasible for patients to wear their aligners for the reduced 12 hours of daily wear. While this may be a bold conclusion, we also need to consider that this is only relevant to a group of patients with very mild malocclusions, as shown by their start-of-treatment Little’s index scores, which were in the region of 5 mm.
Concerns and confusion?
Another factor we need to consider is the length of follow-up. It was only for 140 days. As a result, we must consider these overall treatment results interim. It would be invaluable to see a follow-up paper outlining the end-of-treatment results for this group of patients.
Emeritus Professor of Orthodontics, University of Manchester, UK.
When all is said and done, I find myself asking if the research question is one rooted in an understanding of biology and a genuine desire to find out reality, or merely another delve into the “quest for easy street”. The biological phenomena related to forces generating tooth movement make me rather skeptical and cynical of studies like this
“The authors came to some clear conclusions.” No pun intended?
It’s striking how significant dental changes can occur in some adults wearing sleep apnea appliances for less than 7–8 hours per night. Despite long periods each day without the appliance, the changes still appear to compound over time.
In our practice the number of case refinements clearly correlates with poor wearing time.
And I agree with Dr. Wertheimer that one cannot beat biology. Most patients tend to wear their appliances less than recommended by doctors.
I agree to a certain extent; however, the “biological phenomena related to forces generating tooth movement” remains an enigma. Orthodontic theory was developed in an era dominated by Newtonian physics and Darwinian genetics, which have been partially supplanted by subsequent advances in clinical research techniques. Even tho’ many orthodontists still subscribe to Oppenheim’s original idea, molecular biology and molecular genetics have not supported the tension-pressure notion. For this reason, I posed questions in books on this topic, such as;
1. If a force is required to move a tooth, will a tooth move if a force is not applied? The answer is ‘yes’. As noted above, “unwanted” tooth movements are associated with mandibular advancements devices used to treat OSA in adults. No orthodontic force was used to inadvertently move these teeth – so why did they move?
2. When is a force not a force? Ostensibly, when it fails to reach a biologic threshold or elicit a biological response. Here I invoke the concept of signal transduction.
Thus, in my opinion, a trifecta of space, signal and guidance is needed for tooth movement, and the system will regress to homeostasis (colloquially referred to as ‘relapse’) unless proprioceptive faculty is monitored, partly due to the ageing process. Therefore, my definition of malocclusion is “a solution for a complex, adaptive system to remain in homeostasis” [1].
Furthermore, our understanding of orthodontic changes may also be hampered by outdated terminology. Thus, the classification of “removable” appliances, “fixed” appliances, “functional” appliances, “tooth-borne” appliances and “bone-borne” appliances does little to help the clinician understand the mode of action/developmental mechanisms by which these treatments effect the observed clinical outcomes.
I must admit, retirement allows me to be somewhat more philosophical than previously.
Singh GD. Outdated definition. Brit. Dent. J. 203(4), 174, 2007.
The inclusion criterion of ‘maximum Little Irregularity Index (LII) of 2 mm’ is surely incorrect – do they mean minimum? I believe Little’s Index to be a poor research outcome. Patients could be concerned about an irregularity in one lower incisor contact point of 2mm but might not notice slight irregularities of 0.7mm in three contact points, but according to Little’s index these outcomes are equivalent. I have 2mm irregularities in my lower incisors distributed across two contact points and it doesn’t bother me at all. A mean change in the incisor irregularity of 2mm over 140 days (4 to 5 months) is actually quite slow and the T2 outcomes are not that good. Brezniak argues convincedly (to me at least) that fixed appliances are much more efficient at moving teeth than aligners (Brezniak 2025 https://doi.org/10.1177/03015742241304500).
Also, as with so many of these aligner studies the participants have very mild malocclusions – how many have an objective need for orthodontic treatment? Treating patients with a low objective need for orthodontic treatment reinforces the view (certainly in the UK) that orthodontics no longer belongs within the remit of healthcare but is now part of the beauty and fashion industry – a view I have some sympathy with.
Yes, I think that they must mean the minimum, but it was not too clear.