Is a thermoplastic Twin Block better than an acrylic Twin Block?

The Twin block is the most widely used functional appliance. It is also one of the most extensively researched appliances, with many randomised trials evaluating its effectiveness. While it is an effective functional appliance, there are concerns about the high degree of cooperation required for successful treatment. This new study examined a recent modification of the Twin block, designed to improve cooperation.

Over the past 20 years, many studies have evaluated the effectiveness of the Twin Block compared with other functional appliances. Most of these have shown that, while the appliance is effective, the cooperation rate is not as high as we would hope. One recent development designed to improve cooperation is the modified clear Twin Block. Instead of being constructed from acrylic, this appliance is made from thermoplastic materials and cold-cure acrylic bite blocks. The aim of this design is to reduce bulk and weakness, improve aesthetics and comfort, and hopefully increase patient compliance. 

It has also been suggested that capping the appliance may reduce the side effect of extruding maxillary posterior teeth and proclining the lower incisors. The effectiveness of this appliance was examined in this new study. 

A team from Baghdad, Iraq, did this study. The European Journal of Orthodontics published the paper. 

What did they ask? 

They did this study to answer the following question: 

“What is the effectiveness of a modified clear twin block compared to a conventional Twin Block “?

What did they do? 

They carried out a multi-centre, single-blinded, randomised clinical trial with two-arm parallel groups. 

The PICO was 

Participants 

Orthodontic patients who were rapidly growing with a skeletal Class 2 malocclusion with an OVA jet greater than or equal to 8 mm 

Intervention 

A modified clear Twin Block made from thermoplastic material. With ramps made from cold cure ccrylic. An expansion screw is installed in the upper component of the appliance.

Comparison. 

Standard acrylic Twin Block. 

Outcome 

The primary outcome was a cephalometric evaluation using many cephalometric measurements. Secondary outcomes. 

These were linear measurements of the teeth made from scanned maxillary models aligned and superimposed on palatal rugae. In addition, the patients completed a questionnaire on their perceptions of the appliance, including pain and discomfort. 

They asked the patients to wear their appliances for 24 hours a day. The operator monitored patients’ compliance every 2 weeks and evaluated expansion progress and overjet changes every 4 weeks. The final record was taken after the overbite correction, and the buccal segment occlusal relationship was fully corrected. 

They conducted a clear sample size calculation to detect a 1.5 mm difference in overjet reduction between the two appliances. This indicated that they required 21 patients in each arm of the trial. To account for a potential dropout of 15%, they recruited a total of 48 patients. 

They used a pre-prepared randomisation, generated remotely by an independent person. Concealment was achieved using sealed envelopes.

 All data collection and measurement were done blind. The statistical analysis was rather simplistic, using univariate statistics across many variables. As a result, there is a risk of false positives due to the multiple related measurements. We need to bear this in mind when we look at the results.   

They conducted the study across several government-specialised dental centres and a private dental clinic. Although they did not specify the number of centres or operators involved in the study 

What did they find? 

They randomised 25 patients to each intervention, and all patients completed the study without any dropouts. The study began in September 2022 and was completed in June 2024. 

The mean age of patients at the start of treatment for the Twin Block group was 12.16 years, and for the modified Twin Block group, 11.76 years.

The active treatment duration was 9.14 ± 2.9 months for the Twin Block group and 8.8 ± 2.1 months for the modified Twin Block group.

 At the start of treatment, there were generally no differences between the two groups. However, there were statistically significant differences for SNA, SNB, and SN-Pog. They addressed this imbalance by carrying out an analysis of Covariance to adjust for these differences. However, they did not report this in any detail.

The team produced a large amount of data at the end of the treatment. Most of this concerned cephalometric measurements. As with most studies of this nature, we need to bear in mind the difference between statistical and clinical significance. In this respect, most of the differences were in the order of 1-2 degrees. Although they were statistically significant, I felt they were not clinically significant. However, the upper incisors were retroclined an additional 6 degrees with the conventional Twin Block. This suggests that the treatment may make many small changes that contribute to the overall treatment effect. This is similar to the results of many other studies into the effects of functional appliances and the results do not really add to our knowledge.

 There were no differences in treatment duration. The active treatment duration was 9.14 ± 2.9 months for the Twin Block group and 8.8 ± 2.1 months for the modified Twin Block group.

 When they evaluated patient perceptions of their appliances, however, they found several differences between the two treatments. The results showed that the modified twin block group of patients had a better treatment experience. This was particularly relevant when considering factors such as appliance breakage, difficulty in keeping the appliance clean, changes in speech during appliance wear, and improvement in teasing or bullying. The modified twin block group also had a less pronounced effect on their quality of life with respect to embarrassment, speech, and general appearance.  

 Their overall conclusion was 

“The modified Twin Block was effective in treating patients with Class II mandibular protrusion. It is comparable with slightly advantageous results to the standard Twin Block Appliance”.

What did I think?

This was another study that provides us with some useful information on the Twin Block appliance.  The trial was conducted well and reported in accordance with the CONSORT guidelines.  

One major strength of this study was that it was carried out in the real-world setting of government-run clinics with multiple operators. This is in contrast to many other studies conducted in university dental schools with resident operators. As a result, its findings are generalizable. However, I was unsure on the number of treatment centres and operators. This information was important and it should have been included in the paper.

The authors drew attention to several shortcomings in their study. The most important of these was that they only followed the patients until the completion of the functional appliance phase of treatment. As a result, they did not take into account the effect of any follow-up fixed appliance treatment or potential relapse. This is particularly important and is characteristic of many functional appliance trials. I hope the authors continue this study and report their end-of-treatment data.

 My other criticism is a favourite bugbear of mine. This was because they led on cephalometrics as their primary outcome measure. We know from many cephalometric studies that these measurements are only relevant to orthodontists and are clinically insignificant, yet statistically significant. In fact, in several parts of the discussion the authors mentioned that the MTB resulted in better treatment effects than the CTB. However, this was not supported by the data.

Nevertheless, it was great to see that they used a patient-based outcome measure that reflected their patients’ values. While I was not familiar with this measure, it certainly seemed to indicate considerable advantages of the modified Twin Block appliance. In general, patients appeared happier with the modified Twin Block than with the acrylic Twin Block. However, it was important to note that this was not reflected in the cooperation rates, as all patients completed the study. 

Final comments 

With all papers, we need to consider whether the findings would persuade us to change our practice. I certainly found this paper very interesting, and I would use this appliance on a few selected patients, as I believe it may have value in the future. 

I would like to congratulate the authors on this very useful and clinically relevant research project. Nevertheless, I think that this paper would have been improved by better refereeing, as there were several areas that were not entirely clear.