February 18, 2019

Patients do not need to wear their Twin Block full time! A new trial.

Most of us ask their patients to wear their Twin Block full time.  This new study informs us that this is not necessary.

Occasionally a study team carry out a trial that changes practice, and I feel that this new trial is one of these studies.  Most of the time we ask our patients to wear their functional appliances full time.  However, we do not really know if our patients achieve this target. Fortunately, the development of intra-oral timers has allowed us to measure compliance with removable appliances, and these investigators used this new technology in this trial.

Effectiveness of part-time vs full-time wear protocols of Twin-block appliance on dental and skeletal changes: A randomized controlled trialOrthodontic East End

Jeet Parekh, Kate Counihan,  Padhraig S. Fleming,  Nikolaos Pandis,  and Pratik K. Sharma

Am J Orthod Dentofacial Orthop 2019;155:165-72https://doi.org/10.1016/j.ajodo.2018.07.016

A team from Barts and the London School of Dentistry and Bern did this study. The East End of London is a historic area of London and Bern is a sophisticated Swiss city. The AJO-DDO published the paper.

In their introduction, the authors point out that the Twin Block has been the subject of many clinical trials, but no-one has addressed the issues of compliance with this appliance.

What did they ask?

They did this study to;

“Compare the dental and skeletal effects of Twin Block wear when prescribed for either full or part-time wear”.

What did they do?

They did a parallel group randomised trial with a 1:1 allocation.  The PICO was:

Participants:   Class II Division 1 patients aged 11-13 years old with overjets greater than 7mm.

Intervention:    Advice to wear Twin Block part-time for a total of 12 hours a day.

Control:           Advice to wear Twin Block full-time, except for eating and sport. (22 hours a day).

Outcome:       Primary outcome was overjet reduction. Secondary outcomes cephalometric skeletal change.

They fitted each appliance with a Theramon microsensor into one of the blocks.  They did not let the patients know that their treatment was being timed.

Their sample size calculation showed that they needed to enrol 62 patients in the study. They used pre-prepared randomisation and concealed the allocation using sealed envelopes. They could not blind the clinicians. However, they collected all their data from anonymised cephalograms.  Their statistical analysis was an appropriate use of analysis of covariance.

What did they find?

62 patients enrolled in the trial. At the end of the 12 months, 55 patients completed the study. 6 of the full-time and 1 of the part-time patients dropped out.  The groups were similar at the start of treatment.

The mean overjet reduction in the full-time group was 7mm (SD=2.92) and 6.5mm (SD=2.62) in the part-time group. These differences were not clinically or statistically significant.  They also found no differences in the anteroposterior skeletal measurements. For example, ANB change for the part-time group was 1.5 degrees and 1.25 degrees for the full-time group.

There were more girls in the full-time group, but when they accounted for this in the statistical analysis, it did not have an effect.

Finally, when the looked at the actual time of appliance wear, they found that the full-time group only wore their Twin Blocks for 12.3 hours (51% of the suggested time) and the part-time group wore the appliances for only 8.7 hours (73% of the prescribed time).

Their overall conclusion was;

“There are no differences in skeletal and dental changes between full and part-time wear of a Twin Block”.

What did I think?

I thought that this was a great simple study that answered a clinically relevant question.  The methods were sound, and the paper was reported very well.  It was great to see a straightforward report that satisfied the CONSORT guidelines.  There was minimal bias in the study, and the stated confidence intervals were narrow, suggesting limited uncertainty.

There were several significant findings. Firstly, the patients did not wear their appliances as much as the orthodontists hoped. This has been reported in other studies that have used Theramon timers. In other words, we don’t get full co-operation.  One issue is that I think that the operators did not provide feedback on appliance wear to increase co-operation. However, they did input based on the overjet reduction at each visit.

They also reported that more of the patients who they asked to wear the appliance part-time completed the study. This suggests that perhaps there is better co-operation when we ask our patients to wear the Twin Block when they are at home or at night, as opposed to when they are at school.

The most important finding was that the duration of the appliance wear did not make a difference to the outcome. While we may be surprised, we should also think back to Proffit’s work on the duration of force required to move teeth.  He pointed out that this should be greater than 6 hours to enable tooth movement. As a result, the findings of this study do have some theoretical basis.

Finally, the results of this study suggest that we can ask our patients to wear their Twin Block part-time and still make good progress with their treatment. This is a clinically important finding.



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Have your say!

  1. Whilst I agree that this is potentially an important study is it not underpowered? There was a sample size calculation performed which demonstrated need for 56 participants. However there were only 55 included in the analysis as it was done per protocol.

  2. A simple trial and no doubt it satisfies the CONSORT guidelines. The Theramon chips however prove only that the comparison was between two groups of part-time wearers! Should the Theramon chip show a figure of over 18 hours at mouth temperature, I suspect you would find a difference.
    Many thanks for your Blog!

  3. Dear Prof.

    Many thanks for sharing this study with us. I am however interested in knowing how they have reached the conclusion that we can ask the patients to wear the appliance part time if the control group (full time group) did not wear it full time but rather an average of 12.3 hours/day! We would expect the results to be almost similar as they did not adhere to the instructions. It is as if they compared a (part time group) to another (part time group)!!
    Would it be a valid conclusion if the two groups adhered to the instructions and the results were similar?

  4. Interesting as ‘back in the day’ when i was training in the 1990’s functionals were worn 14hrs/day… so are we again going around in circles?
    I request full time wear with expectation that OJ will reduce within 6mths and we achieve this with >80% of our patients (audit data) so maybe we should offer the patients the option full time and shorter treatment or part-time wear and longer treatment?

  5. Hi Kevin,

    This is indeed a very interesting study. Now the only question remains is what would the treatment outcomes have been if you had a group which wore their appliances 20 hours?
    I am not surprised that there is little difference in outcome between the PT and FT group in this study as they are effectively both PT.
    It would however make all our patients lives easier if we can at least eliminate wear during school hours at least.

  6. Nice study. I think that the header ‘Patients do not need to wear their Twin Block full time’ is a bit inaccurate, Kevin? The positive reinforcement for device wear might be something like ‘Patients need to wear their Twin Block for prescribed the time’ or words to that effect. Maybe it’s just semantics, but this study appears to support the notion of a ‘biomimetic’ protocol that follows the circadian rhythm of human physiology, particularly peak growth hormone secretion during stage 3 sleep. Device wear during eating does not move teeth orthdontically, so 12-16hrs seems not only more appropriate more but might actually increase patient compliance. It my belief that the mode of correction with ‘functional’ appliances is ‘biomimetic’ in line with spatial matrix hypothesis.

  7. The study consisted of 2 part time groups in reality. We may never know if full time wear is better

  8. They only lost 6 out of 30 full time wearers? Fair play to them, they have great compliance from the start, never mind the hours of wear.

    Stephen Murray

  9. I have been using Twin Blocks after attending William Clark’s course in 1995. I routinely tell my young patients that if they can get over 20 hours wear each day, 6 months is all that is required to get the desired effects. They are great for adults patients with deep overbite.

  10. I agree with Ross that it would be more interesting to look at the rate of overjet reduction, rather than the outcome at 12 months. I will still be advising my patients to wear the functional full time (except meals and sport) and if they do I expect the overjet to be reduced within 9 months. Then they can just wear the functional in bed to allow the occlusion to settle. Also, the authors did not mention how often they asked the patients to turn the midline expansion screw in the upper block. I generally get the patients to turn it twice a week and if they are not wearing it full time then the upper block can quite quickly not fit properly.

    In regard to the accuracy of the TheraMon microsensor then this article might be of interest:

    Brierley CA, Benson PE, Sandler J. How accurate are TheraMon(R) microsensors at measuring intraoral wear-time? Recorded vs. actual wear times in five volunteers. J. Orthod. 2017; 44: 241-248. doi:10.1080/14653125.2017.1365220

  11. I think this blog could have been more appropriately titled by omitting the words “need to” in the title

  12. There have been a few comments on this post. These are concerned with the fact that the patients in the study did not wear their appliances full time and the investigators did not measure the effect of full-time wear. I think that I should have emphasised that the aim of the study was not to test the effect of different wearing times, but it was to evaluate the influence of the instructions that were given. If we consider that this was the case, I think that the results and discussion makes much more sense?

  13. What a waste of time comparing two groups of part time wearers. Anything less that 24 hours a day is part time. Bill Clark and I tell, not ask, patients to wear their appliance full time, including eating, sport and cleaning ( clean each plate one at a time ). This is easier for patients and results in perfect co-operation for 99.9% of my patients.
    The reference to Proffit is revealing as Kevin appears not to appreciate that functional appliances are NOT supposed to move teeth but effect other changes which, if you do not try to achieve them, will not occur. FULL TIME wear encourages postural changes of the mandible, reducing tooth moving forces on the teeth and gives fossa remodelling some chance

    • Thanks for the comment. I think that most of what you have said is not supported by current research. How do you know that your patients wear their Twin Block full time?

      • I know that my patients wear them full time because I talk to them and their parents. I also know because when I offer part time wear after correction, many patients prefer to continue FT wear and say they cannot eat without the plates in. I also know because when I remove both plates together to gauge changes, patients are very uncomfortable and initially experience pain.They are all more comfortable with plates in than out and find it much easier to wear them FT rather that part time.

  14. I am guilty of asking my twin block patients to wear their appliances less than full time so this post immediately caught my eye. I don’t think wearing with eating is practical and if they are removed for eating, cleaning and sport during the day at school, I think the potential for loss is high. I therefore ask for “not school hours and all weekend during term” and “full time in the school holidays”, again except for eating, cleaning and sport.

  15. This post recalls Larry Andrew’s “phantom activator” effect. When the activator comes out, the effect lingers on.

  16. For what it is worth, I started recommending part-time wear after seeing the partial results of this study presented at the British Orthodontic Conference’s session where trainees present their MSc research (always an interesting and varied session).

    Anecdotally, I haven’t noticed a difference in outcomes.

    Personally, I doubt if I would have ever worn a Twin Block at school. The early years of secondary school are difficult enough for young men and women.

  17. Yikes! Dr. Clark did not like nor he agrees with this paper at all. Then again, science and data over personal opinion anytime! (even from the inventor of the appliance)


  18. Hello, my daughter has just had a twin block fitted yesterday. She is finding this very uncomfortable and painful and feel if the option of part time use is there and just as beneficial, that could give a huge relief to her as I do not want having these braces to effect her childhood in a negative way as she is generally a happy and confident 11 year old. This blog seems to be very informative and gives less pressure for great results just as good as full time use, and honest too as even though patients are told to wear them full time you cannot monitor this fully or force them to be kept in. Thank you

  19. Dear Sir,

    Thank you for this blog and sharing widely what many of us are already seeing in practice. Prof Clark et al are very humble to refuse to accept that Twin Block is the most practical, cost efficient and dare I say patient-friendly functional appliance we have at our disposal. So they advocate the same instructions to every patient that the first-ever Twin Block patient received.

    We as assistants regularly see overjet reductions of 1mm per month and proportional improvements in canine and molar relationships with “Full-time wear except school hours”. It is fair to say that all improvements with Twin Block are attributable to either Dr Clark or the patients themselves, we just help with making their journey comfortable.

    With my limited experience of roughly 500 Twin Block cases so far, I would like to submit that this amazing appliance does not need to be worn full time including eating for pre-pubertal compliant patients.

    In the best interest of the patients, instructions should be “varied” depending on how much resistance we encounter from the patient (sometimes even the parent) or whether the patient is starting a bit later than the Orthodontist would have liked.

    The patient-clinician relationship is evolving rapidly post-Covid. Of course we need their consent, but more importantly we need their commitment to stay the course with this life-changing treatment, even as life happens around them. Where results are slower than expected, a discussion about ‘catching up the number of hours’ may be had at the second checkup.

    Just like other orthodontic appliances, gentler forces for longer period are more reliable in preventing damage/relapse than a 6-month stint with Twin Block for example in a boy with starting overjet of 10mm or more at age 12 years. The growth of a patient cannot be turbocharged.

    Thank you for your guidance.

    Yours sincerely,

    Mr Karun Sagar
    Orthodontic Assistant

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