Orthodontic Retention: A settling experience?

This post is about a new study that has looked at the amount of settling that occurs during retention.  This is a controversial area that needs studying. I thought that this paper was timely and potentially relevant to our clinical practice.

Recently, there has been a welcome uptick in retention-related research. In particular, investigators have done prospective studies with prolonged follow-up. They have also used both objective and patient-focused measures of outcome. These types of studies are imperative given the open-ended nature of retention and the centrality of patient behaviour to its success.

We know that posterior occlusal settling occurs during the retention phase.  Intuitively, we also know that more effective settling would happen in the absence of removable retainers. Moreover, hermetic coverage of the occlusal surfaces with acrylic is likely to impede settling. In my own practice, when I fail to achieve satisfactory interdigitation in adult patients, I tend to place bonded wires and fabricate removable retainers. Still, I recommend only very occasional (perhaps 1 or 2 nights per week) wear of these. I also follow these patients up more regularly during ‘active retention’. I sometimes adopt a more nuanced approach adjusting the extent and coverage of removable retainers based on the specific requirements. Fortunately, I have on-site technical support to facilitate these refinements. Of course, these approaches are mostly arbitrary with little evidence to support or indeed challenge them.

There is little research concerning the effects of retainer type on the extent, pattern and timing of settling.  A team from Turkey did this study.  The Journal of Oral Rehabilitation published it.

What did they ask?

The authors aimed to compare the changes in the surface area of contact and occlusal force distribution (settling) with vacuum-formed, Hawley and fixed retainers over 6 months.

What did they do?

They conducted a three-group randomised controlled trial with a 1:1:1 allocation (20 participants per group) as follows:

Participants:

Patients with a range of malocclusion treated with fixed appliances with Roth prescription (0.018-inch slot) in both arches.

Intervention Groups:

• Vacuum-formed retainers (VFRs) (0.040-inch Essix, Dentsply) covering all erupted teeth including second molars
• Hawley retainers with a labial bow from canine to canine and Adams clasps on first molars.
• Bonded retainers (0.0195-inch SS) from canine to canine. These participants did not have removable retainers.

Although the study is not labelled as a randomised controlled trial, they mention random allocation and measures to conceal group allocation.

They asked the patients to wear their removable retainers full-time (except for eating) for the initial 6 months followed by nights only wear.

They measured occlusal contact using T-scan III. This is a horseshoe-shaped sensor, bite-frame and hand-held device. It allows for chairside evaluation of the distribution of contacts as well as giving associated force levels. However, it is less adept at evaluating the number of contacts due to its bulk. Scans were taken 2 hours after debonding, after 6 months and at 12-month follow-up.

What did they find?

I thought that the results were a little confusing. They were occasionally contradictory being sub-divided into left and right sides, anterior and posterior regions. Generally, it appears that significant settling occurred throughout the 12-month observation period in all groups. Unsurprisingly, this happened more efficiently in the fixed retainer group as these participants did not have removable retainers. In the groups having removable retainers, the occlusion settled to a greater extent from 6 to 12 months. This coincided with the period following the transition to nights only wear.

They also reported local differences. For example, there was more contact established in the posterior region over time in the fixed retention group. Conversely, with Hawley retainers, a more positive anterior occlusion developed over the study period. Interestingly, the occlusal force distribution decreased in the anterior region with bonded retention, while increasing in the posterior areas. No significant change, however, concerning force distribution occurred in the removable retainer groups.

What did I think?

I thought that this was an interesting study. It may not change practice, but it could help our understanding of occlusal settling.  I think we now have better evidence that full-time wear of removable retainers does impede settling. However, most of us were already in little doubt that this is the case.

There are some limitations in terms of conduct and reporting. For example, there is no sample size calculation. This risks recruitment of insufficient participants and hence failure to demonstrate meaningful differences. The authors refer to randomisation procedures but label the study as a controlled clinical trial.

The statistics are also a little confusing. They did a  large number of tests (approx. 120). This risks false-positive findings. A mixed model which accounts for the repeated measurements could have been carried out as an alternative. The analyses were also focused mainly within each group rather than between the three groups. If this is a comparative study (evaluating how retainer type might affect settling), the statistical tests should ideally reflect this. Notwithstanding, we can learn a little more about patterns of settling with all three retention regimes.

Limitations?

The authors highlight limitations associated with the measuring technique. I have not seen or used the T-scan III.  It does appear to be a little bulky and therefore problematic when measuring the number of contacts. It may, therefore, have been helpful to augment this approach with further information to provide a more holistic appreciation of changes in occlusal contact. Furthermore, the comparative assessment of retainer type based on occlusal settling alone seems incomplete. I would struggle to make clinical decisions on the relative merits of a retention regime without also considering the associated effects on stability and patient reports of related experiences.

What can we conclude?

The researchers were able to shed some light on the pattern and timing of posterior settling with and without removable retainers. A ‘take-home message’ maybe that meaningful settling does not occur with removable retainers unless we instruct patients to wear these part-time only. These favourable changes seem to happen within 6 months of nights-only wear.