Findings and interpretations
The current trial was expanded to explore the clinical and patient-centered outcome of HR and mVFR in sustaining maxillary arch expansion for up to 24 months [7, 8]. ICW, IPMW, and IFMW were chosen as clinical outcome measures to reflect transarch stability, demonstrating the clinical effectiveness of retention techniques in preventing relapse, as previously demonstrated in numerous studies [15,16,17,18,19]. 100-mm VAS with six questions were used to evaluate patient-centered outcome, i.e., subjects’ perception on these orthodontic retainers.
Subjects from both retainer groups had mean ages of 22.58 years and 21.07 years for mVFRs and HRs, respectively. There were more females subjects compared to males in the trial, which is a frequent phenomenon in studies investigating orthodontic appliances [14, 19,20,21,22,23].
The average expansion for all measurement points was 4.35 ± 2.40 (ICW), 4.67 ± 1.74 (IPMW), and 3.05 ± 3.59 (IFMW). For the largest value among the measurement points, there was an average of 6.05 ± 2.73 mm arch width. Buccal inclinations of teeth, bone remodeling, and reduced bone thickness, notably in the buccal aspect, have all been described during dentoalveolar extension [16, 24]. The changes that occur after treatment have been attributed to relapse following orthodontic expansion [25,26,27,28], as well as growth changes. These differences were neither statistically or clinically significant between groups or time points from the 24-month post-retention period (Table 3). When the differences of the time points were calculated for ICW, IPMW, and IFMW (Table 5), changes occurred across the trial period with values below 1 mm, independent of the retention regime. A three-year follow-up of a randomized clinical trial on dentoalveolar expansion in the mixed dentition revealed a relapse of less than 1 mm , given that expansion is normally more stable in growing children.
Conversely, another study on relapse after dentoalveolar expansion in teenage patients discovered more than 1 mm of relapse over a year . The researchers concluded that this could be due to compliance issues, where subjects were sent text reminders monthly in the present trial. It is worth noting that, except for IFMW2, the HR group did exceptionally well from T1–T0 and for the total difference T4–T0 in the current study, despite the fact that these results were not statistically or clinically significant.
Over a 24-month retention period, the main outcomes of the current trial revealed no statistically significant differences between HR and mVFR in all mean arch width changes. This finding is comparable with previous investigations which compared HR’s stability and the conventional VFR without palatal coverage in non-expansion cases [17,18,19, 30, 31]. The findings showed that the HR and the mVFR are equally effective in sustaining maxillary transverse expansion after 24 months. The mVFR’s extended palatal coverage combined with the rigidity of the thermoplastic material  may have improved their physical qualities, allowing them to maintain an expanded arch akin to HRs, which have always been regarded as more rigid and better for transarch stability [26, 32]. Another reason for the mVFR’s effectiveness could be the three-dimensional coverage of the teeth, including palatal coverage, which, in principle, would better preserve dental inclination changes over HR . The findings of this study suggest that mVFRs would be a suitable option for expansion cases because it is easier to produce and does not necessitate any additional technical abilities. However, the mVFR group reported more retainer breakages than the HR group, with no further breakages after the one-year trial (6%—HR; 22%—mVFR) .
Subjects perceived the mVFRs as significantly more esthetically pleasing compared to HR. This finding is in agreement with the results of multiple studies and a systematic review [6, 14, 18, 34]. Several authors suggested that this was attributed to the transparent nature of VFRs as compared to metal showing in HRs [6, 34]. In addition, Hichens et al. found that VFRs caused less embarrassment when worn in public compared to HRs . The superior esthetics might be a factor for the increasing popularity of VFRs [2, 4, 6, 35, 36]. Nevertheless, Pratt et al. reported no differences in regard to the appearance of HRs and VFRs .
In the present study, no significant differences were found between the perception of the two retainers in terms of speech. This is inconsistent with multiple studies that found VFRs cause less disruption in speech [6, 38, 39]. Using acoustic analysis, both Wan et al. and Atik et al. found that the change in articulation was more obvious in patients wearing HRs compared to conventional VFRs [38, 39]. In the present study, the VFRs were modified with palatal coverage, therefore imparts greater speech disturbances compared to conventional horseshoe-shaped VFRs. As evidenced by Stratton and Burkland, retainers with palatal coverage tend to result in greater speech disturbances compared to those without palatal coverage . This may explain the insignificant differences in perception of speech disruption between mVFRs and HRs.
The results also found no significant differences in perception of comfort between both types of retainers. In the literature, VFRs without palatal coverage demonstrated superior comfort compared to retainers with palatal coverage [14, 40]. The mVFRs used in the present study had palatal coverage similar to HR, which could explain the insignificant differences. However, Hichens et al. found no difference in comfort level associated with VFRs and HRs, despite the VFRs used in their study did not have palatal coverage .
The subjects also reported no difference in perceived durability between the two retainers. This is inconsistent with the findings of Saleh et al. who found subjects perceived HRs to be significantly more durable . The inherent flexibility of the traditional horseshoe-shaped VFRs might come across as less durable to subjects . In the previous study, it is postulated that even though the material is in theory not as rigid as the acrylic in Hawley, the palatal coverage had increased the strength of mVFRs . This increased strength and reduced flexibility of the mVFRs might have been the reason for equal perception of durability between both the retainers. There was conflicting evidence in the survival times of the two retainers, possibly due to the varying thickness, material, design (amount of gingival coverage) of VFRs and the inconsistency in individual patient care and habits, e.g., bruxing. Note that these studies used VFRs without palatal coverage [6, 14, 41]. However, our trial reported a higher number of breakages in the mVFRs group compared to the HRs group (6%—HRs; 22%—mVFRs) within one year of retainer wear, which did not increase after the first year .
There was also no significant difference between the fitting and oral hygiene perception of HRs and mVFRs. This is consistent with result from a randomized trial conducted by Saleh et al. where they compared the fitting and oral hygiene perception between HRs and VFRs without palatal coverage .
The results for patient-centered outcomes suggest that mVFRs are comparable with HRs in aspects of fitting, speech, oral hygiene, durability, and comfort, with mVFRs being superior in terms of appearance.
By the time point of analysis, the relative dropout rates had risen (Fig. 3). Since January 2020, the main reason has been the COVID-19 pandemic . However, one subject in each HR and mVFR group refused to come due to COVID-19 concerns returned for their T4 visit when the situation improved, which increased the total number of subjects by two at the end of the trial. An ITT analysis was used to reduce the possibility of bias generated by comparing groups with different prognostic variables.
The compliance of retainer wear was not objectively measured in the study. However, monthly text reminders were sent, and the retainers were ensured to be well fitted at each T visit to mimic the real clinical scenario. The Hawthorne effect, which may change a certain aspect of the individuals’ behavior in reaction to the reminder in this trial, remains challenging to minimize. It has been demonstrated that compliance is most substantial during the early stages, where patient participation tends to fade over time .
This trial was conducted on subjects who have been wearing retainers for two years and may not represent patients in other phases of retention. Since the average amount of dentoalveolar expansion was minimal, the results of the study would not be applicable to other modalities of expansion such as skeletal expansion with RME or SARPE.
Long-term retention phase may harm teeth and gingival health. In addition, even the long-term wear of VFRs has demonstrated a significant premature occlusal contact in the posterior teeth and an anterior open bite [44, 45]. All these possible effects could be evaluated in future studies. Furthermore, the authors suggest that the questionnaire used in this study could be used in future studies investigating the patient-centered outcomes of various orthodontic retainers since this is the only validated questionnaire on patient acceptance of orthodontic retainers.
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