September 09, 2019

A new trial shows us how functional appliances work.

We all know that functional appliances are great at treating Class II malocclusion. This new study provides us with further information on how they “work”.

Many researchers have looked at the effects of functional appliances on Class II malocclusion.  The results of the trials have been consistent in their findings.  In short, they report that the main effects of this form of treatment are dento-alveolar with some small skeletal change.  These findings have been incorporated into systematic reviews, and the evidence for these treatment effects is becoming stronger.

This recently published trial adds to this evidence.  It is concerned with the treatment effects of carrying out a 15-month course of functional appliance treatment.  A team from the beautiful South of England did this trial. The EJO published the paper.

What did they ask?

They did this trial to answer this question.

“What are the treatment and post-treatment changes of Twin Block or Dynamax treatment over an extended period of appliance wear”?

What did they do?

They did a parallel-group two-arm randomised trial.  The PICO was

Participants: Caucasians aged 11-13 years old with overjets of greater than 7mm.

Intervention: Dynamax functional appliance

Control: Twin Block functional appliance

Outcome: Skeletal differences between the groups measured by cephalometric analysis.

They did pre-prepared randomisation that they concealed in sealed envelopes.  The sample size was based on changes in lower facial height. They recorded and analysed the data blind.

I thought that their treatment protocol was interesting, and I am going to describe it in detail.

  • At the start of treatment, they took a cephalogram (T1)
  • They asked the patients to wear their appliance full time for 15 months. This is longer than the “normal” functional phase of 9-12 months. At the end of this period, they took another cephalogram (T2). Importantly, they then stopped functional appliance treatment, and after three months, they reassessed the participants.
  • They then treated the participants according to standard protocols.
  • They took final stage records after another 12 months. This equated to 30 months from the original records and 15 months from the completion of the functional phase of treatment.

At this final stage of record collection, all the treatment had not been completed.  However, the authors argue that at this point, there is unlikely to be any additional skeletal change.

What did they find?

They asked 150 patients to take part in the study and 78 were allocated to Twin Block and 72 to the Dynamax groups.

68 (87%) Twin Block and 61 (84%) of the Dynamax patients completed the 15-month functional phase of treatment.

At the end of the study, there were 52 (66%) Twin Block and 48 (66%) Dynamax patients with full records.

I extracted relevant data from the start of the study to 15 months into this table.

Twin BlockDynamaxp
OJ reduction (mm)7.0 (6.-7.7)5.8 (5.2-6.5)0.0
Retroclination upper incisors (deg)5.8 (2.9-8.6)5.7 (2.4-8.9)0.97
Proclination lower incisors (deg)5.3 (4.0-6.7)5.4 (3.6-7.1)0.96
A point (mm)0.8 (0.3-1.2)0.2 (-0.3-0.6)0.06
Pogonion (mm)3.5 (2.8-4.2)1.7 (1.1-2.3)0.001

When they looked at the changes from 15 to 30 months, there were some small differences in upper incisor angulation. The lower incisor retroclination for the Twin Block group relapsed by 2.9 degrees for the Twin Block and 3.4 degrees in the Dynamax. Finally, when they looked at mandibular length, the Twin Block group had less mandibular growth (0.7mm) than the Dynamax group (1.8mm).

In summary, they concluded.

“During the first 15 months, the patients treated with the Twin Block showed greater reduction in overjet and skeletal change for all measurements apart from A point. However, at the final record collection, most differences disappeared except for a difference of 0.6mm in mandibular length”.

What did I think?

I thought that this was an ambitious and complex study. They carried out the trial well. As with all trials, there are some areas that we can criticise.  I will only mention the major ones. Firstly, the drop out rate at the end of the study is rather high.  This means that a fair amount of data has been lost. However, there are no differences in the number of participants between the groups, and this reduces risk of bias. Furthermore, we must also appreciate that this final response rate is good for an orthodontic trial with difficulties caused by the long term nature of the study.

I was also a little unclear why the final data collection was done at 30 months and not at the end of all treatment.  I wonder if the records had been collected at the end of treatment, whether this would have given us better information on the effects of the complete course of treatment?


However, if we bear these concerns in mind. This trial does add to the evidence. Importantly, this is another trial that illustrates that most of the effects of functional appliances are dentoalveolar with minimal skeletal change. When there are skeletal changes, these then “wash out” with growth. This reinforces the concept of a “mortgage” on skeletal growth, and at the end of treatment, minimal growth change has occurred.

In general, I thought that this was an excellent trial that added to our knowledge about the effects of functional appliances.

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Have your say!

  1. How was Twin Block treatment considered a control? Shouldn’t they have had an untreated control to compare the two interventions against?

  2. 0.6mm skeletal difference is not clinically significant, and doesn’t justify 15 months of functional therapy, specially considering the inherent inaccuracy of 2d cephalograms. This outcome, to me, indicates we can ditch the functional therapy and use elastics later. The title should change to ‘no clinically significant skeletal change’ IMHO. A big fan though doc 🙂

  3. Kevin, question about the wording on the table. Shouldn’t the treatment effect be A point (back) not “forward” as in the table, and B point (forward) not “back”. Otherwise, I am confused about how they measured.

    • Sorry and I agree, I have removed the “forwards and back” from the table. Both A point and Pg were moving in a positive direction forwards.

  4. I agree with Drs Bentele and Rahimi 🙂 This study (like so many others in the orthodontic literature) is essentially ‘descriptive statistics’ of cephalometric images, which do not fully represent what exists clinically. Since there were no control groups, and no correction for size, shape variation will mask some of the treatment-induced and natural changes expected at this age. I thought this study was going to provide new info on the underlying corrective mechanism (since this is the topic of my presentation at a conference in Orlando, FL this week).

  5. I have many problems with studies like these. Well intentioned study-after-study so often is comparing apples to oranges. For example, the picture at the top of this post shows a Cl I molar relationship yet overjet with Cl II in the cuspids due to a deep curve-of-Spee and a lower lip-sucking habit. Did the appliances even correct Cl II, or did they simply facilitate leveling the curve-of-Spee? I was taught the molar relationship tells the truth about the classification of a case. How many of these appliances disclude the teeth, enable distal growth of the condyles, posture the mandible forward, and then pray the occlusion is strong enough to hold the dento-alveolar advancement? How do they work in a non-growing patient? How many of these cases are actually “Sunday bite” postural correction that frequently relapse? How many of these cases work because of a functional shift that screams a mandible needs to come forward? I have seen a couple of studies on identical twins where one was given a functional appliance and the other not given a functional appliance. These completely killed the notion that we can grow mandibles. Yes we have all seen cases where a functional appliance worked. How about the cases where they don’t? The Ortho Specialty excels at getting a poor result only to then blame patient compliance. All to often functional appliances remain the “Hail Mary” tx for Cl II correction.

  6. Orthodontics is not adaptive. Over 60 years ago, Fränkel scientifically exhaustively proved that the monoblock and its derivatives are in conflict with biological laws. The devices used in this study also function only dynamically according to the principles of the monoblock. This does not make them any better.

    Ralph A Nicassio writes “I was taught that the molar relationship tells the truth about the classification of a case”.
    Another nonsense, the molar relation is completely useless to classify a case. For this you have to look at the children earlier. Then it becomes visible that there is still a skeletal class I, but the journey of misdevelopment is already visible in the molars.

    Read your Fränkel it is more modern than you think. There you will learn everything you need to know about biological growth and get the necessary understanding to be able to treat without systemic errors.

    Translated with

  7. Dear Kevin

    Dr DiBiase’s study on treating Class II cases while comparing 2 different appliances is, like so many such reports, fundamentally flawed in 2 respects, in my view.

    Firstly, the premise that such treatment is a simple 2-dimensional issue (A-P and vertical) is far from reality; many orthodontists are now taking a 3-dimensional approach to what is clearly a 3-dimensional problem. DiBiase’s failure to do this is shown by the fact that there is no mention in the report or its tables of any lateral measurements in the coronal plane. It is now generally understood that a major feature of a Class II malocclusion in many cases is a narrow maxilla preventing the mandible from developing forward into a normal Class I position due to inter-cuspation.

    The absence of lateral measurement is curious since the Twin Block appliance in the photograph shows the usual mid-line expansion screw, which apparently had not been activated. In fact, both models in the photo indicate strongly that reduced arch width is a major part in this case.

    Secondly, the multiple cephalometric measurements used in the study are taken entirely from reference points within the facial bones, and therefore it is highly likely that, since the members of the cohort are all in their teens or younger, any changes are enhanced as a result of cranio-facial growth. However, the full impact of these changes cannot be evaluated fully unless a more stable and fundamental reference point is used such as the occiput. Reference point Sella has always been viewed as stable, yet since it is located in the sphenoid bone, its position may be a part of the ‘problem’ in terms of growth and/or displacement.

    Dento-alveolar changes are constantly being referred to, rather dismissively, as the main result of functional treatment; this may be the case, but it is also entirely possible that these changes are a ‘return to nature’s intended norm’ following the release of the distorting pressures that caused the initial malocclusion, now facilitated by the functional appliances and any other interventions.

    If the mandible has indeed been held back in a retrusive position, there is a strong likelihood that the glenoid fossae of the temporal bones have also been driven posteriorly. Unless this is corrected, or self-corrects, at the same time as the functional treatment, relapse into the original Class II position is highly likely.

    I believe it is time we recognised that malocclusion is not the main issue, because many now understand it is a SIGN and a RESULT of what is going on in the skull. Unless the background issues are recognized and properly addressed, relapse is almost inevitable.

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