Orthodontic Expansion: Can we adapt our VFRs to prevent the inevitable?

I have reviewed several retention-related studies for this Blog over the past year. While this reflects my interest (or perhaps bias), I do think that it also reflects an encouraging increased emphasis on retention among orthodontic researchers. Some of these have involved more extended follow-up periods and have included both objective and patient-focused outcome measures. Studies of this nature are necessary as the retention period is effectively open-ended, and success depends on patient behaviour.

The use of vacuum-formed retainers (VFRs) has increased in recent years, with Hawley retainers often having more niche use. Among the indications for more rigid, Hawley-type retainers include posterior settling of the occlusion and demands for the maintenance of significant maxillary arch expansion. Theoretically, the more rigid plastic material may offer improved resistance to upper arch contraction during the retention phase. However, clinicians have developed modified VFRs to combine the aesthetic benefits of the clear retainer with enhanced rigidity.

In this study, the authors evaluated the potential use of a modification to the VFR involving palatal extension and coverage to enhance the retention of posterior expansion over 12 months.  A team from Malaysia did the study.

What did they do?

They conducted a two-group randomised controlled trial involving 35 participants as follows:

Participants:

Participants were 13 years or older at the time of debonding. They had undergone more than 3 mm of maxillary dentoalveolar expansion during treatment.

Intervention Groups:

• Vacuum-formed retainers (VFRs) (0.040-inch Essix, Dentsply)
• Hawley retainers with a labial bow from canine to canine and Adams clasps on first molars

Removable retainers were to be worn full-time (except for eating, drinking and cleaning) for the initial six months, followed by nights only wear after that.

They took impressions for dental casts after three months (T1) and at 12-month follow-up (T3).

What did they find?

During the trial period, the maxillary arch widths (represented by inter-canine, inter-premolar and inter-molar widths) decreased. However, the magnitude of the transverse decrease did not exceed 1mm for any dimension with either retainer over the 12 months. The authors found that the most significant reduction (just 0.7mm) was for inter-premolar width with the modified VFR.

The reduction in transverse dimensions was almost identical with the modified VFR and the Hawley retainer. Notably, a higher proportion of modified VFRs (22%) fractured over the study period, with the fracture of just one Hawley retainer.

What did I think?

This study was very interesting, and it addressed an important area. As we know, maxillary arch expansion seems to be undergoing yet another renaissance in contemporary orthodontics. I think we will need innovative solutions if we harbour hopes of preserving transverse correction in many of these patients in the short- to medium-term.

The methodology and writing are clear. It would, therefore, be straightforward to replicate this study or, indeed, to perform a similar study evaluating the potential merit of other modifications.

The sample size is small, and I note that the investigators based their calculation on the recurrence of incisor irregularity, while the primary outcome was transverse change. Furthermore, the small sample was compound by the pandemic, with some inevitable sample attrition. Finally, there is the risk that the study is underpowered, with potentially meaningful differences obscured by the low sample. Nevertheless, the magnitude of the changes in each group was small, with the difference between the groups not considered clinically significant.

It would also have been helpful to have more precise data on the amount of transverse expansion that arose during treatment. It would be intuitive to expect that cases that had more inter-molar increase during treatment, for example, would be more prone to instability in this respect over the study period. As such, an account would ideally be made for this in an adjusted statistical model.

Finally, as with any study involving retention, consideration of patient views and adherence levels would be of value.

What can we conclude?

The researchers described and tested the use of a modified Essix retainer with palatal coverage as a means of retaining significant transverse dimensional change. This adaptation is straightforward and, based on this analysis, appears to be a viable alternative to the use of a Hawley-type retainer. Nevertheless, it is noteworthy that neither retainer could mitigate the tendency for maxillary arch constriction over the relatively short follow-up period. We also have a range of plastic materials at our disposal, which may offer further potential to mitigate the risk of arch constriction during the retention phase. With our modern obsession with active expansion, additional VFR design work might be necessary.