May 24, 2021

What influences cooperation with functional appliance treatment?

The key to the success of functional appliances is patient cooperation.  This new study looked at factors that may predict cooperation with two types of functional appliance.

When I was using functional appliances to treat Class II problems, I always knew that cooperation was not ideal. Later in my career, when we did the UK Class II studies, I thought it was troubling to find that the non-cooperation of adolescent patients with the Twin Block was 33%.  In other studies, investigators have reported  Twin Block non-compliance rates of between 10-49%.  When they have looked at factors that may increase compliance, they have suggested that these include the relationship of the patient and orthodontist, parental support, treatment progress, reminders, and positive and negative influences of the patient’s social network.

This new paper adds to our knowledge.

A team from Croatia and Serbia did this trial. The Angle Orthodontist published it.

What did they ask?

They did the study to:

“Identify factors that could predict compliance in patients with Class II malocclusion during functional appliance treatment”.

What did they do?

They did a randomised trial with the following PICO.


Orthodontic patients aged 11-13 years old with Class II Division 1 malocclusion.


Twin Block appliance


Sander Bite Jumping appliance


The primary outcome was compliance with treatment. The authors defined this as:

  • No change in overjet at the first appointment, and the patient declined treatment.
  • No overjet improvement after 12 months of treatment.

They also collected data using the Child Perception Questionnaire, the Family Impact Scale and the Parent/Caregiver Impact Scale. These provide information on the patient’s emotional well-being, oral symptoms, functional limitations, parental emotions, family activities and financial burden.

They also recorded the severity of the malocclusion by measuring the start overjet.

Finally, they used these factors and the appliance used as explanatory variables in a regression analysis with cooperation as the dependent variable.

They described their method of randomisation. But I could not find any information on the sample size calculation, method of preparation of randomisation, allocation concealment or blinding.  Regular readers of this blog know that these are “red flags” regarding the potential risk of bias for the trial.

What did they find?

Their most significant finding was that 55% of the patients were not compliant with their treatment.  I could not find any information on the non-compliance rates of each of the appliances in the descriptions.  These are high levels of non-co0peration. I definitely would not use an appliance that was not going to work 55% of the time.

They did a precise outline of the findings of their univariate statistics.  When they looked at the regression, they found:

  • The initial overjet, type of appliance and parental perception of changes in the child’s emotional well-being (EW) were predictors of compliance.
  • The strongest predictor of compliance was the parent’s perception of their child’s EW, which increased compliance 3.4 times (95% CI 1.2-9.4).
  • An overjet of greater than 8mm increased the odds of compliance 3.1 times (95% CI=1.1-9.3)
  • The use of the Twin Block increased compliance by 3.2 times (95% CI, 1.1-9.3) compared with the Sander appliance.

Their overall conclusion was:

“Parents perception of their child’s improvement in emotional well-being, the severity of the malocclusion and the type of appliance are major predictors of compliance with these functional appliances”.

What did I think?

My feeling is that this study provides us with some helpful information on functional appliance cooperation. Indeed, the conclusions appear to have some clinical validity, particularly for the parent’s perception of their child’s emotional well-being. It was also interesting to see that cooperation was better with the Twin Block when compared to the Sander appliance. Unfortunately, the overall level of co-operation was low, and I wonder if this was due to the rather bulky nature of the two appliances?

These findings are important and are hopefully more useful than cephalometric and occlusal index studies. Nevertheless, more extensive studies are required,

However, we need to consider whether this study can add to our knowledge.  As with all papers, we must carefully carry out a critical appraisal.    In this respect, the study has several that problems that I highlighted previously.  Most importantly, the authors did not write the study according to the CONSORT guidelines. As a result, it was tricky to appraise.  The Angle Orthodontist has adopted the CONSORT guidelines, so I am a little confused that they do not seem to be applying them?

The biggest issue that I spotted was an imbalance between the sizes of the two study groups. This should not occur if the randomisation were ideal.

The other issues of lack of blinding, concealment and absence of sample size calculation are also relevant. This means that the trial is at a high risk of bias. Importantly, we do not know the direction of bias. As a result, the trial has a degree of uncertainty.

I want to emphasise that the application of CONSORT would have ensured that this information was included in the paper. Furthermore, if the journal had applied their policy on CONSORT, this would have helped the authors.

Final thoughts

We now have to consider the effects of bias on the results of the study. Most importantly, we need to consider if there was a possible bias in the treatment allocation. As a reader of this paper, it is up to you to consider whether this influences your interpretation of the findings.

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Have your say!

  1. Hi Kevin
    you will remember Jonathon Sandler and I had a lively discussion at one of the excellent ‘Art meets science’ courses… we concluded (totally unscientifically) that success was determined by operator personally or lack of… ie being able to “sell” the appliance was essential

  2. Health behavior is complex and influenced by individual strengths(eg. perseverance, self regulation) individual personality traits and values, intrinsic motivation, intention, self efficacy, resilience and support ( peers, parents and practitioners). Twin Block compliance and success in orthodontic treatment outcome relies on so many individual and treatment factors, I wonder why RCT’s like this that cannot measure or exclude many variables are still considered valuable or the most appropriate tool for understanding compliance?

  3. The idea that success depends on selling the appliance (ie operator brilliance) is an interesting one and appealing since it reinforces the idea that I may be a genius if my twin blocks work really well (or I think they do). This is similar to the business fallacy that my success is due to my innate brilliance and not due to luck.
    Also in the study lack of compliance was defined as no overjet change after 12 months. Could this not be just that the appliance didn’t work even though it was worn?

  4. Why not use functional appliances that are not removable,- e.g., Herbst, MPA, Forsus, Vektor, CS5, etc? Those have worked for me.

    • Many appliances that stimulate the patient to protrude the mandible will have a positive effect on mandibular growth when used in a growing patient; you have identified five of these that are non-removable. The problem is that these all have negative side effects. Some relate to adverse and excessive dento-alveolar changes and some to frequent breakages demanding emergency attention. Possibly, that is why clinicians utilise so many different types. For many practices, the cost is also a significant negative factor. Additionally, socialised healthcare systems may demand the cheapest possible approach, effectively precluding their routine use.

  5. Many of these papers on functional appliances do not consider the correct timing for such treatments. If the patients in the same group have different skeletal maturation stages (easily evaluated by taking an x-ray of the cervical vertebrae, hand or middle phalanx of the third finger), then they can´t be compared, as the effectiveness of the appliance will not be the same, due to different growth potentials. I feel that this is a crucial problem in such studies and we need to focus on the proper patient selection in order to have reliable results that can help us in the day-to-day practice.

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