September 07, 2015

Does anchoring a fixed functional appliance to the mandible have an effect?

Does anchoring a fixed functional appliance to the mandible work?

After all the excitement of last week’s post I’m going to return to a simple review of a recent randomised trial on the effects of anchoring a fixed functional appliance to the mandible. It was published in the Angle orthodontist; so it is open access.

Three-dimensional effects of the mini implant anchored forces fatigue resistant device.

Elkordy et al

Angle Orthodontist: Advanced access

DOI: 10.2319/012515-55.1

I think that it is now well established that most of the changes that occur with fixed or removable functional are dento alveolar with some minor skeletal change.  Furthermore,  a large component of the overjet reduction is due to proclination of the lower incisors. This may be particularly marked with fixed functional appliances because they are tooth bourne.  This type of tooth movement is unwanted, as it may be unstable and attempts have been made to minimise this effect.  These investigators carried out a study to find out if fixing the mandibular component of the fixed function appliance to the mandible resulted in reduced lower incisor proclination and increased skeletal change.

What did they do?

They carried out a randomised clinical trial in which a total of 36 young people with Class II malocclusion were randomly allocated to receive treatment with either

  • Forsus appliance
  • Forsus appliance fixed to the mandible with mini screws
  • Untreated control (Delayed treatment for 6 months)

They carried out a sample size calculation, randomisation and allocation concealment were good. The patients were bonded up with MBT brackets until levelling and alignment were complete. They the fitted the two variants of the Forsus appliances.

They took Cone Beam CT images at the start of treatment and when the overjet was fully reduced for the treatment groups. While for the  control group they took CBCT at the start of the observation period and after 6 months. The study had full ethical committee approval.

The outcome measures were restricted to cephalometric measures derived from the cone beam CT images. Their statistics were very simple involving comparison of many cephalometric measurement with the high risk of false positive due the multiple comparisons of related measurements.

What did they find?

Their first finding was that the appliance corrected the incisor relationship for 88% of the patients. But then they  went into a massive mind numbing cephalometric festival comparing many measurements at different time points. I also thought that it was strange that they did not really compare the treatment with the control group. I was really disappointed to see this  because it was almost impossible to identify the effects of the treatment.  After a lot of work, I managed to come up with these values to summarise the effects of treatment within the treatment/control groups.

Difference (means, 95% CI)-1.08 (-2.16, 0.00)-0.52 (-0.73, -0.3)1.45 (0.43, 2.47)-4.82 (-5.83, -3.8)

So to summarise.

  • The upper incisors retroclined
  • The lower incisors proclined but less in the Forsus with mini implant anchorage by 4 degrees.
  • The upper molars were moved distally
  • There was no effect on ANB or mandibular growth.

What did I think?

I thought this was a fair study. However, I was very disappointed to find that there were so many cephalometric measurements. This made the paper very difficult to read and I really wish investigators would limit the number of cephalometric variables that they are measuring.  I am sure that no one looks at the massive tables of data.  In fact, I even wonder if the journal editors and referees look at them!  I have discussed this previously in this blog.

I was also confused on the discussion because the authors stated that there were skeletal changes with treatment but I think that they were comparing the before and after treatment values. When I looked at the data and tried to compare the treatment with the control, I could not find clinically significant differences.

Screen Shot 2015-09-07 at 13.51.18In their discussion, they did not really discuss why they did not detect greater growth modification with the functional appliances compared to the control, particularly with the Forsus retained by mini-implants. However, I did spot that the  implants were bonded to the mandibular canines ( see figure). This made me wonder whether some of the force was transmitted to the lower arch resulting in incisor proclination. Alternatively, this could also illustrate that no matter what you do you cannot increase mandibular growth?

In summary, this was an interesting trial.  However, the authors fell into the trap of presenting a large amount of cephalometric data which made the paper very difficult to interpret. It certainly is not an “easy read”.  Nevertheless, I think this still goes some way towards reinforcing other research that suggests we cannot really alter skeletal growth with our appliances.

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Have your say!

  1. Hello Dr Kevin
    Nice to read your blog and I really enjoy and admire the depth of study you do in an article.


  2. in class ii div 1 cases with severe overjet, when fixed functional appliances are anchored to skeletal implants,it leads to significant advancement of mandible,without protrusion of mandibular anterior teeth.

  3. Rather than a “respected dental specialty with scientific basis”, within some circles orthodontics has gained the image of an “assembly line trade”. Your critical analysis of the literature is essential for returning orthodontics to the status of a “learned calling”.
    With appreciation,

    • Hi Gerry, thanks for the comments. I am still enjoying doing the blog and the circulation is increasing, though last weeks post was the most popular and did not contain any science at all!

  4. Hello Dr Kevin
    Nice to read your blog .It is very nice
    And I like it the way the studies are evaluated .This topic is very interesting and most of the books emphasize overjet reduces but by lower incisor proclination .Practically I have read all the Books by Dr Mitchell ,Dr cobourne ,Dr gill to name a few .Special Thankyou regards kapil

  5. Kevin,

    Nice review; I would not argue with your interpretation. I would, however, like to comment on the application of miniscrews in the article reviewed herein. Presumably, the authors placed MSIs in attempt to prevent flaring the lower anteriors and “tie” the lower arch together as one unit. If I read the data correctly, the MSI group had 5.25 degrees of flaring, vs 9.05 degrees in the non-MSI group.

    Based on their MSI mechanics design, I would not anticipate seeing a significant reduction of lower incisor flaring between the MSI vs the non-MSi group. To explain, the authors did not take into account the mechanical construct of a screw, which is simply the application of an inclined plane relative to a long axis. It follows that a screw works very well at holding two objects together rigidly. Although not its original design function, a screw will also resist a force being applied through its long axis – the original rationale behind using MSIs in orthodontics. It appears that the authors assumed that placing an MSi perpendicular to the facial surface of the mandibular corpus and bonding a wire from the MSI to the canine would be rigid and therefore prevent the lower incisors from flaring. I.e., they assumed that they could rotate the screw into its final position for anchorage, and that it would resist rotating further when a clockwise force (right side) or count-clockwise force (left side) was applied by the Forsus appliance. In actuality, a screw rotates into and out of bone for placement and removal. Therefore, any force that has the net effect of rotating the screw further into or out of bone will likely not last long, and probably will not support the desired tooth movement, or prevention thereof. Further, as the force is applied, the screw can rotate, and the teeth can move; the distance being defined by the linear distance between the screw and the force, in this case from the MSI to the canine. For example, when looking at the patient’s right side, the canine (and incisors) can intrude, move forward, and tip forward relative to the linear distance of the wire from the MSI to the canine. Obviously, this may not always happen, but it can, and if it does, is not easy to correct. Other factors are also at play – duration of force, crestal bone height, crown to root ratio, etc. (I can send you 2 photos that demonstrate this point, if you want to post them.)

    A more practical way to prevent lower incisor flaring with a forsus and an MSI would be to place the MSI in the external oblique ridge or retromolar region (assuming space and lack of teeth/neurovascular structures) and ligating it to brackets as far anterior to the MSI as possible. In this way, the force is being applied through the long axis of the MSI (instead of around the long axis) and from a posterior MSI to an anterior attachment point instead of the vertical position that the authors used. Even then, the archwire should be stopped (RMO lock, Geurin lock, crimp stop, cinch back, etc) to tie the entire lower arch together as one unit. Even then, this is not absolute; but it does eliminate the force being applied around the long axis of the MSI.

    Miniscrews offer a great addition to our treatment armamentarium. However, their use should not be arbitrary. The basic biomechanics of orthodontic tooth movement as well at the application of MSIs should be understood prior to implementation.

    Jason B Cope, DDS, PhD
    Dallas, Texas

  6. Imagine that. Why do we keep beating a dead horse?

  7. I really enjoyed your Blog Kevin and both Gerry’s and Jason’s comments. As Jason pointed out there is a moment created which can either screw or unscrew the MSI/TSAD (depending upon the side). In addition there can also be flex in the wire or MSI/TSAD drift so Jason’s design sounds more suitable – so when’s the study coming Jason – no pressure 😉

    As Gerry alluded to we should be designing our treatment around our diagnosis and our treatment goal and not be appliance driven if we wish to remain relevant as a profession. Keep up the great work Kevin!

  8. I agree that the analysis used in this study seems to be seriously deficient. What is the value of 3D data if it’s reduced to 2D analysis? Why didn’t the authors do 3D geometric morphometrics instead of 20th century 2D cephalometrics? Without correcting for size, shape and directionality, the results make little sense. Thousands of landmarks can be analyzed using these robust methods in statistical shape-space without artificially fixing ‘reference’ planes. These elegant techniques might reveal skeletal changes may occur on an epigenetic basis.
    Moyers RE, Bookstein FL. The inappropriateness of conventional cephalometrics. Am J Orthod. 1979;75(6):599-617
    Singh GD and Clark WJ. Localization of mandibular changes in patients with Class II division 1 malocclusions treated using Twin Block appliances: finite-element modeling. Am J Orthod Dentofacial Orthop. 119(4), 419-425, 2001.
    Singh GD. 3-D photography combined with conventional and 3-D analysis. J Oral Maxillofac Surg. 63(8), [Suppl 1], 7, 2005.
    Singh GD, Levy-Bercowski D, Santiago PE. 3-D effects of nasoalveolar molding in patients with orofacial clefts: geometric morphometrics. Cleft Palate Craniofac. J. 42(4), 403-409, 2005.

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