January 13, 2020

What influences the stability of Class II correction with the Twin Block?

The most common method of treatment for Class II malocclusion is functional appliances. But what influences the stability of this form of therapy?  This new paper provides us with some useful information.

We do not really know what factors influence the stability of Class II correction.  The most useful studies have been the long term evaluations of the Herbst appliance. These suggest that poor interdigitation, habits or poor retention resulted in some instability.  

The Twin Block is a very popular functional appliance and has been extensively studied. However, there have been very few studies that have looked at the stability of treatment. As a result, this team of investigators based in fashionable East London decided to study this problem. The AJO-DDO published the paper.

What did they ask?

They asked the following questions:

“How stable is Class II correction with the Twin Block followed by fixed appliances”?  and

“What factors influence the stability”?

What did they do?

They did a prospective study of successfully completed Class II treatment.

The inclusion criteria were:

  • Treated Class II Division 1 malocclusion with Twin Blocks followed by fixed appliances.  
  • They treated all the patients at a dental hospital.
  • Recruited at debonding or review clinics. This was a convenience sample.

They collected cephalometric and study model data at 12 months after debonding.

The primary outcome was the stability of overjet correction at 12 months debond.

Secondary outcomes were the stability of molar and canine relationships.

They collected the following information from the models and ceps.

  • Occlusal interdigitation at debond
  • Pre-treatment sagittal skeletal discrepancy
  • Treatment-induced change in overjet
  • Retention regime.

They used logistic regression to evaluate the effect of any of these variables on relapse.

What did they find?

They collected information on 64 patients.  53% were male. The mean age of the start of treatment was 12.5 years, and 75% started treatment during peak growth.  They were treated with a Twin Block for a mean of 1.15 years, followed by a transition period of 0.24 years and final period of fixed appliances lasting a mean 1.88 years. The total mean treatment time was 3.26 years (SD=1.3), and the mean overjet reduction was 6.22mm (SD=2.46). They used a variety of retainers.

The mean overjet relapse over 12 months was 0.67mm. There was minimal change in molar and canine relationships.

The logistic regression did not identify any factors that significantly influenced the post-treatment changes. However, there was a weak association with change in overjet.

Their overall conclusion was:

“There were acceptable levels of stability, and there were no real predictors of relapse of Class II treatment with Twin Block treatment followed by fixed appliances”.

What did I think?

I thought that this study was ambitious and carried out well.  However, there are some issues with the methodology that we need to bear in mind when we interpret the results.  Firstly, while this was a prospective study,  I spotted that some patients were enrolled on review clinics.  This could lead to bias because the patients may have had a reason to attend the review.  Furthermore, they did not state how many patients who completed treatment did not attend for their review. Again, this is likely to lead to bias.  Importantly, we cannot identify or speculate on the direction of any bias.

Nevertheless, it does appear that for the patients who attended, there were no real predictors of relapse.  This is contrary to clinical experience, and the findings of some studies which have suggested that relapse is reduced if the buccal interdigitation is good.  I feel that this is an important finding.   However, this does not mean that we can finish cases poorly because we still need full overjet reduction, if only for aesthetics.


In summary, this was a good study providing useful clinical information.  We should all read this paper.

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Have your say!

  1. Is posturing of the mandible forward a TMJ compromise that is fooling all involved? Previous reviews have suggested there is no clinically significant inspiration of mandibular growth. Is this a fad that is reducing the use of actual strategies that change the mandible? Perhaps a decent dual bite is an acceptable alternative to no treatment or ideal but patients need to be informed of what is really happening.

  2. One wonders if a great deal of inherent bias re compliance etc.is possibly due to the dental school environment ,for many reasons.
    Why would dental interdigitation be considered a factor as studies have long shown that the teeth are only in full occlusion for 15 mins or so per day ? Discounting pts with major occlusal habits !!
    I doubt I could get compliance with a twin bloc for over 1 yr so very well done !
    Great study with a lot of clinical relevance.

  3. Just saw your announcement regarding your annual course.
    I think there would be a lot of support and attendance if you presented this in a central location in NAmerica eg.Toronto??
    I wonder if other blog followers would support this ????

  4. The selection of principles and criteria is the key to diagnosis, treatment and research.
    Unfortunately we have no unification in principles or criteria and each one of us do what thinks is right according to our philosophy, school of thought, training, etc.
    But that is not possible, it is not correct, if that is how we could perform a meta-analysis, take case series or any other study, with such varied philosophies, disciplines and systems it is not possible to obtain a discussion would be on the subject or obtain a conclusion worthwhile, here I do not believe that it applies that the variety of thought enriches the profession, on the contrary, such a variety of thought leads rather to confusion.
    Here the assessment is carried out during and after treatment by cephalometry and clinical examination, but in this study a prediction of growth was never made, therefore we do not really know how much of the changes was due to growth or really due to treatment.
    Another criterion that was overlooked is the diagnostic instrumentetion both before and after treatment, so we cannot categorically state whether it was growth or just a mandibular replacement.
    If we were exquisite prior to treatment, a splint should be placed as well as at the end the time necessary to measure possible displacements more accurately.
    This shows us the lack of standardization in the protocols of attention and research which negatively affects the thinking and way of acting in our specialty.
    Therefore, this study does not give us useful information about the effect of this treatment modality on growth.

  5. What definition of ‘functional’ appliance are you guys using?

  6. So it would appear a “Sunday Bite” is more stable when there is a socked-in buccal interdigitation occlusion finish. This can be accomplished in an average of 3 years. Very valuable information to know!

  7. “This is contrary to clinical experience, and the findings of some studies which have suggested that relapse is reduced if the buccal interdigitation is good. ”

    Dear Prof. O’Brien, what are these studies that say relapse is reduced if the interdigitation is good?
    I would like to read them, thank you.

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