July 31, 2023

Herbst vs Twin Block: A great new trial

One of the decisions we must consider when treating a Class II malocclusion is whether to use a fixed or removable appliance. The main advantage of a fixed appliance is the potentially more significant level of cooperation. However, are these appliances more effective and acceptable to our patients than removable appliances? There have only been a few studies that have evaluated this question.

I studied the effectiveness of the Herbst vs Twin Block many years ago in 2003. We showed that there were very few differences between the results of treatment. However, there was greater cooperation with the Herbst but more breakages than with the Twin Block. Our paper concentrated on cephalometrics and other important measures for orthodontists. Nevertheless, we attempted to capture the patient’s perceptions of their appliances using a questionnaire that we made up. This revealed that our patients did not have any preferences for either treatment.

Orthodontic research has now moved on, and it was interesting to see this new study incorporate patient values and a simple measure of treatment effectiveness.

I have already posted about an earlier publication from this team on their qualitative findings from this trial.

A multinational team based in London did this study. The AJO-DDO published the paper.

This paper was open access, so everyone can read it, which is excellent.

I want to declare a conflict of interest as I know the team of authors well, and I have collaborated with them on other research and educational projects.

What did they ask?

They did this study to

“Compare the effectiveness of the fixed Hanks Herbst and removable Twin Block appliance and look at treatment failure, complications and patients’ perceptions of their treatment”.

What did they do?

They did a parallel group 3 armed randomised controlled trial with a 1:1 allocation ratio. The study was done in one Dental School site. The PICO was


Orthodontic patients aged 10-14  with Class II Division 1 incisal relationship and an overjet greater than 7mm.


Standard modified Twin Block appliance. The clinician asked the patients to wear their appliances full time apart from meals and sports.


Hanks Herbst fixed functional appliance. With bands on the first molars, lingual and palatal arches (or RME). The operator cemented the appliance with glass ionomer cement.


The primary outcome was the time taken to reduce the overjet to less than 4mm. The team measured this from study casts taken at the start of the study and when the operator discontinued the treatment when the overjet had been reduced.

Secondary outcomes were the failure of treatment, number of attendances, chairside time, complications and impact of appliances on Oral Health Quality of Life.

One operator treated all the patients with appointment intervals of 6-8 weeks. The appliance was removed once the overjet was reduced and appeared stable. They classified a patient as non-co-operative if the overjet was not reduced by 10% in 6 months or if they did not get a normal overjet after 12 months.

The study statistician carried out both univariate and multivariate analyses. They also did a sample size calculation. This suggested they needed to enroll 40 patients into each intervention group.

The randomisation was pre-prepared, and they concealed the allocation in sealed envelopes.

What did they find?

They enrolled 80 participants, who they randomised between the two interventions. Two of the Twin Block group did not reduce their overjet after six months and lost to the study. 13 TB patients did not achieve overjet reduction. This resulted in a failure rate of 37.5%. In the Herbst group, 7 (17.5%) failed their treatment because of breakages and poor oral hygiene.

The team did an intention to treat analysis. As a result, all the patients were analysed.

The duration of the Herbst treatment was 8.8(SD=2.9) months, and for the Twin Block, it was 10.3(SD=3.7) months. This was a difference of  1.5 months (95% CI=-3.00 to -0.03) and was statistically significant.

The Herbst resulted in a more significant overjet reduction (-7.1mm, SD=2.5)) than the Twin Block(-5.8mm, SD=3.4)). The Herbst appliance resulted in more consistent and predictable overjet reduction than the Twin Block.

Chairside time was more with the Herbst (7.6, SD=2.5) hours than the TB (4.9 SD=1.3) months.

When they looked at breakages, the TB had fewer routine visits and emergency appointments.

There were no clinically effective differences in the quality of life measures. The participants in both groups reported difficulties in chewing, speaking, appliance pain, and problems with school work.

Their conclusion was:

“Treatment with the Herbst appliance resulted in more efficient and predictable overjet reduction than Twin Block”.

What did I think?

This was a good study that is an addition to the literature. Importantly, its results are similar to similar studies. These findings let us have robust information on the effectiveness of these appliances.

The team did the study well and should be congratulated on carrying out such a large study with no funding. This is an example of what can be done and provides a model for others wishing to carry out trials.

The results were logical, clinically relevant, and clearly set out. It was also great to see a study that reported on patient outcomes rather than vast numbers of cephalometric tables. The investigators mention that they will prepare another paper on cephalometrics. I hope they spare us from this, as it will only add a little to our knowledge!

I was only concerned about one matter. This was the fact that the operator discontinued the Herbst treatment after overjet reduction. In the paper, whether the clinician had placed multi-bracket fixed appliances on all the teeth was unclear. I contacted one of the authors for clarification.  He informed me that brackets were not placed until the Herbst had been removed at the end of the overjet reduction phase of treatment.  As a result, they compared the Herbst and Twin Block with the same treatment protocols. While this is logical to study the first phase of treatment, it does not necessarily reflect clinical practice, where brackets are placed while the Herbst treatment is ongoing.

I also hope they report data at the end of all the orthodontic care. We may see this in a few years because this is the most important information we require.

Final comments

Finally, we need to consider whether this paper will change practice. It would encourage me to use more fixed functional appliances because of the greater effectiveness of the Herbst appliance. But I also may wait until I see the final paper at the end of all treatment.

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Have your say!

  1. A major question to be answered is: was the overjet reduction dental alveolar or skeletal? If dental alveolar then the lower incisors are most likely proclined with partial loss of their boney support. As we know we cannot grow mandibles, I question the excitement for the results of this study and do look forward to the cephalometric report.

  2. My main question relating to studies like this is whether prescribing full time twin block wear still relates to contemporary clinical practice.

  3. Hi Kevin!
    What a crazy Ashes series! (that’s cricket for those not into the sport)

    Thank-you for your review. My thoughts revolve around the intriguing subject selection based mainly upon age and increased overjet, with no extra-oral or skeletal selection variable reported – apart from eliminating severe dolichofacial patients (perhaps the most needy of intervention of our Class II skeletal patients), as well as to the timing of the T1 record of overjet reduction (“immediately after the functional appliance withdrawal”), making possible / likely degree of relapse of mandibular position an unaccounted for variable between the interventions.
    I am also not clear on the pre-treatment records; the paper says “impressions and pre-treatment X-rays”.
    The authors (as you alluded) did say they will follow-up with reports on dental, skeletal and soft-tissue changes. To me, this is imperative as one of the primary indications for this kind of intervention is arguably not merely overjet reduction, but soft-tissue change – specifically profile, leading to overjet reduction, by default. Overjet reduction (particularly in light of likely relapse in days / weeks following removal of functional appliance) as a treatment goal alone may not require such “functional” intervention, and without assessing the degree of mandibular retrognathism (or not), incisor compensation at baseline; the ease and ability for decrease in overjet reduction introduces yet another variable that is often itself related to soft tissue position and function.
    Additionally, the TB cohort had a single “maximum protrusion” activation, whereas the HH cohort were activated incrementally via addition of shims. This difference in timing and degree of force application may be a significant variable in the rate of overjet reduction / relative stability / degree of incisor compensation / condylar change etc. between the groups.
    I suppose what I am looking forward to is the comprehensive report of baseline soft -tissue, skeletal and dental relationship within each group, including the all-important condylar position, and corresponding change at T1, so that we may better identify the contribution of these interventions as most frequently sought by clinicians using them – a profile, soft tissue balance change, and as a result, overjet reduction.
    Now the mechanism for these changes……hmmm….

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