The stability of orthodontic treatment over time is still today one of the main challenges in orthodontics. Post-treatment assessment of treated malocclusions has been of interest for several decades, and several studies showed that transverse diameters tend to decrease during the post-retention period, especially if they had been expanded during treatment [13]. When possible, maxillary expansion represents the gold standard to correct skeletal transverse deficiency associated with posterior uni- or bilateral crossbite [14]. In growing patients with primary and mixed dentitions, it results in an increased transverse maxillary width and a prevention to impacted canines [15].
In the present study, we evaluated transverse effects of self-ligating appliances on virtual models. The analysis of the dental casts showed that during the active treatment there is an expansion in each sector, mainly at the premolar level in both arches, due to the arch form of the Damon system, which is more expanded at the level of the premolars to prevent black corridors [10]. Other studies [3, 16] found similar results in terms of the capability of STLs to increase dento-alveolar widths during active treatment by buccal tipping of the posterior teeth.
In our study, upper and lower premolars and lower first molars showed a significant reduction in their transverse diameter values one year (T2) and six years after treatment (T3), with respect to the end of treatment (T1). But, when comparing the couple of values at T3 and T2, no statistically different values can be found. This suggests that most of the relapse occurs in the first year post-treatment, and that it reaches a plateau of stability that is maintained up to 6 years post-treatment. This is in partial agreement with the results of a similar study by Lucchese, [12] where they found a tendency to transverse diameter restriction at premolars, even if non-statistically significant, in a 2-year follow-up. These results must be carefully compared, given the different protocols of retentions, which may play a crucial role in determining the amount of relapse.
In this sample, fixed canine-to-canine retainers were used in both arches, and it could explain the lack of transverse relapse at canine level; several studies have shown that fixed retainers could be the right approach to maintain the alignment of the anterior teeth, although there is a lack of high-quality evidence to endorse the use of one type of orthodontic retainer based on risk of failure [17, 18].
One study [2] analyzed a group of 24 patients who had received treatment with Damon3 appliances, assessing the stability of cast measurements and cephalometric values after six months and two years. The conclusion of the study was that, with regard to the cast evaluation, there was a significant relapse in the 2-year follow-up, especially at the upper and lower premolars and upper first molars (second molars were not taken into account). This has been confirmed by the current study which showed similar results regarding the stability of inter-canine diameters, even 6 years after treatment. They also proved a significant relapse in the inter-premolar and inter-molar measures, which was similarly observed in this dataset, with the main difference of upper first molars.
Atik and colleagues [13] aimed to compare the three-year stability out of two different expansion protocols (Damon SLB appliance vs. Quad Helix and Roth prescription-based brackets). All the patients had dentally constricted maxillary arches prior to treatment. Measures were performed on dental casts, measuring the distances between cusps of the same couple of teeth on the same arch. Both groups showed statistically significant increases in all transverse dental measurements during active treatment; in the Damon group, they observed a significant relapse in inter-canine width three years after debonding. It may be important to notice that the retention protocol for all patients contemplated upper and lower removable retainers Hawley type for one year (worn full time for six months and thereafter at night-time for the remaining 6 months). Retention in all the sample was solely based on fixed lingual bonded retainers, which apparently managed to maintain the inter-canine diameters unchanged.
Another paper [19] aimed to retrospectively evaluate the stability of various indexes, including inter-canine and inter-molar width, in a SLB group and a conventional brackets group. After a follow-up period of two years and another of 7.24 years, they found that the inter-canine and inter-molar expansion obtained during active treatment tended to stay stable in all the 30 SLB patients. These results also seem to be in agreement with those derived from this study, even if the different retention protocol must be considered (Hawley retainers were used in both arches for approximately 2 years in Yu and colleagues’ study).
While the use of anterior fixed retention from canine to canine is a well-established technique, there are still few indications whether a posterior retention is needed. In the present study, no removable retention (such as essix or Hawley-type retainer) was delivered to patients, so we managed to evaluate the transverse arch expansion and its stability. These results seem to booster that the absence of an adequate retention protocol, especially in the premolar sectors, after the important expansion obtainable with the SLB system, could lead to a relapse within the first year after debonding.