Do fixed functional appliances change the temporomandibular joint?

It has been suggested that functional appliances may induce skeletal changes by altering the morphology of the temporomandibular joint (TMJ). This effect may be particularly significant in treatment involving fixed functional appliances, as they are permanently in place and necessitate less cooperation than removable appliances.

These theories primarily rely on retrospective investigations and have not been thoroughly tested using randomised trial methodology. This new trial addresses this gap in our knowledge and is valuable.

A team from Alberta, Canada did this study. The AJO-DDO published the paper.

This paper is open access, so we can all read it without being a member of a specialist society. 

What did they ask?

They did this study to

“Evaluate 3D positional changes of the condyle and glenoid fossa using different Class II fixed appliances compared to controls”.

What did they do?

This established team conducted a three-arm randomised controlled trial involving participants from a single graduate clinic.  The study had a 7-year duration from 2014 to 2021.  The PICO was

Participants

Orthodontic patients aged 10-14 years with a Class II malocclusion with an ANB angle of greater than 4 degrees in the late mixed or early permanent dentition stage.

Intervention 1

Herbst fixed a functional appliance with bands on the maxillary first permanent molars and mandibular first premolars or canines connected to telescopic arms.

Intervention 2

The X-bow. This is also a fixed functional appliance. It comprises a mandibular labial and lingual bow along with a hyrax appliance in the maxilla. A Forsus spring is attached to the first maxillary molar band and the mandibular labial bow in the canine area. 

Control group

This group were fitted with a fixed appliance for alignment alone. Importantly, they did not have any treatment mechanics specifically designed to correct the Class I relationship.

Outcomes.

3D analysis of the glenoid fossa and condyles was performed using CBCT images. These images were taken at the beginning of treatment. They were also taken after 12 months. Landmarks were identified and distances were measured to reference planes. I cannot describe this in detail here. The authors provide this information in their open-access paper.

They did a clear sample size calculation.

The method of randomisation was unclear. Which meants that the study was at risk of bias. However, they provided details regarding the allocation concealment, which was achieved using sealed envelopes. The team analysed the data employing the relevant statistical analysis.

What did they find?

Fifty-four participants took part in the study. Seventeen were allocated to the Herbst, eighteen to the Xbow, and the remaining to the control group. None dropped out of the study. There were no differences between the groups at the outset of the study.

They provided data on the position of the landmarks in the fossa and condyles. These were all under 1 mm. This was a clear, simple analysis, and it was good not to see complex superimpositions of multiple points.

The statistical analysis revealed no differences in the measurements between the Herbst, Xbow, and control groups.

Their conclusion was

“Fixed Class II correctors did not show condyle or fossa changes or change in position of the condyle relative to the glenoid fossa compared to a control group”.

What did I think?

This was a complex and ambitious randomised trial. The authors should be congratulated on completing this study. The strengths included the use of fixed functional appliances over a standardised treatment time. They also employed 3D imaging to analyse any complex changes in TMD morphology. 

I was unclear about the method of participant allocation and randomisation. This raised a bit of a red flag for me, and I am unsure why it was not clear. We need to keep this in mind when considering the findings.

Their selection of the control group was interesting. They fitted these patients with fixed appliances but did not apply any Class II mechanics. This seems logical. It may be the only way to establish an “untreated” control group in this type of study. Nevertheless, we must also consider whether the fixed appliance influenced growth. I cannot think of any reason for this to be the case. Unfortunately, the authors did not discuss this important point.

Final comments

Overall, this was a highly valuable study that contributed to our understanding. It provides further evidence that fixed functional appliances do not affect facial growth. So, can we cease making this claim?

Finally, I would like to apologise for not posting for a couple of weeks. We took a holiday in the Pembrokeshire National Park in Wales and we did not have good internet access. This was great.

I would also like to thank all those who made a donation to support this blog. We reached our target for funds to cover the increasing costs of running this surprisingly popular blog. We can continue for another year. I am very grateful for your support.