The effectiveness of Hawley and vacuum-formed retainer usage protocols on the stability of fixed orthodontic treatment results

An orthodontic challenge, beyond establishing an ideal occlusion and aesthetics through treatment, is the maintenance of achieved corrections. A broad range of different protocols has been recommended for retention and, in a recent systematic review, ¹ it was concluded that there is not enough high-quality evidence to recommend a single specific retention approach.

Either fixed or removable appliances are possible alternatives during the post-treatment retention phase. The advantages of removable retainers can be listed as the easier maintenance of oral hygiene by removing the appliance during tooth cleaning, and the possibility of night-time only wear. ² The two removable retainers most commonly used in orthodontic practice are the Hawley appliance and vacuum-formed retainers (VFRs). ³ Several studies have previously compared the provision of a Hawley retainer or VFR, related to stability, ^4,5 acceptability, ⁶ speech performance, ⁷ changes in occlusal contacts, ⁸ survival time, ⁹ and retainer wear and compliance. ¹⁰

A significant patient issue is the duration of the retention period. Joondeph et al. ¹¹ described retention as a 12-month healing phase, in which the newly completed tooth movements are stabilised. However, the usage protocol of retention appliances during this ‘healing period’ is a matter for debate. Proffit ¹² reported that teeth require essentially full-time retention for three to four months after orthodontic treatment and, later, night-time retention. Jäderberg et al., in a six-month follow-up study, ¹³ stated that night-time wear of an Essix retainer was sufficient to maintain the results after orthodontic treatment. Similarly, Shawesh et al. ¹⁴ concluded that night-only wear of a Hawley retainer for one year was as effective as six months full-time followed by six months night-only wear. When a Hawley retainer was used for six months full-time and six months night-only, a mean standard deviation of 1.8 ± 0.7 mm and 2.0 ± 0.8 mm increases in Little’s irregularity index were reported. It is therefore questionable whether this is the best protocol.

One reason for retention is to allow the gingival and periodontal tissues time to reorganise, and adapt to the soft tissue pressures that influence post-treatment change. It is known that, although the reorganisation of collagenous fibres can be completed in four to six months, the supracrestal fibres remodel more slowly, and can still exert forces that displace a tooth 12 months after the removal of orthodontic appliances. ¹² Several studies have previously been performed to compare different usage protocols for removable retainers; ^5,13-16 however, none of the studies evaluated if full-time usage (except for meals) for removable retainers provided better retention than alternative time periods. If the protocol to achieve a stable orthodontic treatment is to wear a removable appliance for 12 months full-time, patients should be encouraged to follow instructions. Therefore, the aim of this study was to evaluate and compare the clinical effectiveness of different usage protocols between Hawley and VFR appliances on post-treatment stability according to Little’s irregularity index, intercanine and intermolar widths, arch length, overjet and overbite measurements.

The null hypotheses of the study were: (1) that there is no difference in the effectiveness of Hawley and VFR appliances on stability following full-time usage for the first six-month period after debonding, (2) that there is no difference in the effectiveness of different usage protocols applied to Hawley and VFR appliances.

This single-centre prospective clinical trial was approved by Hacettepe University Interventional Clinical Research Ethics Committee (2017/02-53(KA17006)), and conducted in Hacettepe University, Faculty of Dentistry, Department of Orthodontics from May 2017 to May 2018.

All subjects were treated by two authors (H.G-C. or E.A.) and were invited to participate close to the removal of the fixed appliances. The inclusion criteria for the subjects were: (1) mild or moderate initial crowding, (2) a Class I or Class II malocclusion according to the ANB angle and molar relationship, (3) treated only with fixed appliances, (4) non-extraction treatment, (5) no missing teeth, (6) no syndromes, (7) older than 14 years of age, (8) compliance with the retention protocol prescribed by the orthodontist.

Of the 72 patients, 14 failed to complete the first six months of the trial, and six failed after the following six months. The patients with lost or broken appliances were excluded from the study. None of the patients who required replacement appliances were analysed in the study. A second exclusion reason was discontinued intervention. Finally, 58 patients were evaluated for six-month changes, and 52 patients were evaluated for 12-month changes.

The demographic variables, the changes in arch dimensions with active orthodontic treatment, and the comparisons between the groups are shown in Table I. None of the variables showed significant differences between the groups, except for gender (p = 0.009).

The present study aimed to compare the effectiveness of different wear protocols between Hawley and VFR retainers on arch stability during the first 12 months of retention. There are few studies that have evaluated the effects of different wear regimens on stability. ^{5,13,14,16,19,20} One study ²¹ compared the full-time usage of Hawley and VFR retainers for 12 months, and found no significant difference between the effectiveness of the appliances with regard to arch widths, arch length or Little’s irregularity scores. However, there is no study that has evaluated if 12 months full-time wear of removable retainers provides better retention than six months full-time followed by six months night-only usage. Therefore, the wear regimens of the groups were created as 12 months full-time Hawley, six months full-time followed by six months night-only Hawley wear, 12 months full-time VFR, and six months full-time followed by six months night-only VFR wear. In addition, it was aimed to compare the effectiveness of full-time wear of Hawley and VFR retainers during the first six months of retention.

The analysed sample consisted of four groups with comparable pretreatment properties related to the ANB angle and molar relationships. In addition, the age of the patients and the changes that occurred in Little’s Irregularity scores, intercanine and intermolar widths, arch lengths, overbite and overjet during active orthodontic treatment did not show any significant difference between the groups. To achieve better homogeneity, patients who had been treated by extractions were excluded, and only patients with mild and moderate crowding and Class I or Class II malocclusions were included in the study sample. With the aid of the strict inclusion criteria, it was aimed to create study groups with comparable characteristics, and to minimalise confounding factors.

Except for maxillary intermolar width stability associated with the six-month changes, the first null hypothesis was accepted. The intermolar width changes of the maxillary arch showed significant differences between the Hawley and VFR retainers. However, a difference of 0.32 mm in intermolar width may not be considered clinically significant. Except for this result, no significant differences were found related to other changes during the first six months of the retention period. Little’s irregularity scores increased significantly in both the Hawley and VFR groups, but the changes were not significantly different between the groups. Rowland et al. ⁴ assessed the effectiveness of Hawley and VFRs for six months and found greater increases in Little’s irregularity scores in the Hawley retainer group. The difference in the results may arise from different wear regimens, as the present study encouraged full-time wear for six months whereas in the study by Rowland et al. ⁴ Hawley retainers were worn for three months full-time, three months half-time, and the VFRs were worn one week full-time and then half-time. Barlin et al. ²¹ compared the effectiveness of full-time wear of Hawley and VFRs, and found no significant differences between arch widths, arch length or Little’s irregularity scores, in support of the present findings. According to the results of the present study, after six months of a fulltime wear protocol, either a Hawley retainer or VFRs may be chosen for clinical effectiveness.

In a consideration of the 12-month changes, the null hypothesis was partially rejected, as the changes in maxillary intercanine width showed a statistically significant difference between night-only Hawley and full-time VFR usage from the sixth to 12th month. The difference likely originated from a slight decrease in the Hawley group and a slight increase in the VFR group. This finding is in contrast to those of a systematic review, ³ which indicated that no difference existed to distinguish the value of Hawley retainers and VFRs with respect to changes in intercanine and intermolar widths after orthodontic retention. Nevertheless, it is recommended that additional high quality, randomised, controlled trials concerning these retainers are necessary to determine which retainer is better. The conflicting results between the studies may arise from different wear protocols. As an intercanine width decrease is one factor that affects relapse in anterior irregularity, better retention in the VFR group could be considered as an advantage when deciding between the removable retainers.

Little’s irregularity scores showed statistically significant increases in all groups from debonding to 12 months; however, from the sixth to the 12th month, the significant increases were only present in the night-only wear protocol groups for both retainers, in both arches. From the sixth to the 12th month, the mean changes in Little’s irregularity scores were in a non-significant decreasing pattern, from night-only wear of Hawley, night only wear of VFR, full-time wear of Hawley and full-time wear of VFR in both arches. The highest mean irregularity increase was in night-only wear of a Hawley retainer, with 0.80 ± 0.58 mm, and the least was in full-time wear of VFR, with a 0.27 ± 0.37 mm change in the mandible. However, the Little’s irregularity score changes did not reveal a statistically significant difference between the groups. Ramazanzadeh et al. ⁵ compared Hawley and VFRs and different retention protocols. The retention protocols were four months full-time followed by four months night-time Hawley wear, four months fulltime followed by four months night-time VFR wear, and one-week full-time followed by night-only VFR wear. It was concluded that VFRs were more effective than Hawley retainers for stable incisor alignment, after eight months of wear.

Although some intragroup changes were observed related to intermolar width, arch length and overjet during the retention period, no significant differences were observed between the groups. These findings were in accordance with the findings of Barlin et al. ²¹ and Demir et al. ²² When taking the results of the present study into consideration, it may be noted that six months full-time followed by six months part-time wear of Hawley retainers or VFRs can be retention alternatives rather than 12 months full-time wear. However, if better retention is needed, especially in the maxillary anterior region, 12 months full-time wear of VFRs may be encouraged, if retention is only provided by removable appliances.