Bonded versus vacuum-formed retainers: a randomized controlled trial. Part 1: stability, retainer survival, and patient satisfaction outcomes after 12 months

Bonded versus vacuum-formed retainers: a randomized controlled trial. Part 1: stability, retainer... Summary Background There is a shortage of evidence on the best type of retainer. Objectives Evaluate upper and lower bonded retainers (BRs) versus upper and lower vacuum-formed retainers (VFRs) over 12 months, in terms of stability, retainer survival, and patient satisfaction. Trial design Two-arm parallel group multi-centre randomized controlled clinical trial. Methods Sixty consecutive patients completing fixed appliance therapy and requiring retainers were recruited from 3 hospital departments. They were randomly allocated to either upper and lower labial segment BRs (n = 30) or upper and lower full-arch VFRs (n = 30). Primary outcome was stability. Secondary outcomes were retainer survival and patient satisfaction. A random sequence of treatment allocation was computer-generated and implemented by sealing in sequentially numbered opaque sealed envelopes independently prepared in advance. Patients, operators and outcome could not be blinded due to the nature of the intervention. Results Thirty patients received BRs (median [Mdn] age 16 years, inter-quartile range [IQR] = 2) and 30 received VFRs (Mdn age 17 years, IQR = 4). Baseline characteristics were similar between groups. At 12 months, there were no statistically significant inter-group differences in post-treatment change of maxillary labial segment alignment (BR = 1.1 mm, IQR = 1.56, VFR = 0.76 mm, IQR = 1.55, P = 0.61); however, there was greater post-treatment change in the mandibular VFR group (BR = 0.77 mm, IQR = 1.46, VFR = 1.69mm, IQR = 2.00, P = 0.008). The difference in maxillary retainer survival rates were statistically non-significant, P = 0.34 (BR = 63.6%, 239.3 days, 95% confidence interval [CI] = 191.1–287.5, VFR = 73.3%, 311.1 days, 95% CI = 278.3–344.29). The mandibular BR had a lower survival rate (P = 0.01) at 12 months (BR = 50%, 239.3 days 95% CI = 191.1–287.5, VFR = 80%, 324.9 days 95% CI = 295.4–354.4). More subjects with VFRs reported discomfort (P = 0.002) and speech difficulties (P = 0.004) but found them easier to clean than those with BRs (P = 0.001). Limitations Results are after 1 year and we do not know how much the removable retainers were worn. Conclusions After 1 year, there is no evidence of a significant difference in stability or retainer survival in the maxilla. In the mandible, BRs are more effective at maintaining mandibular labial segment alignment, but have a higher failure rate. In comparison with patients wearing VFRs, patients wearing BRs reported that they caused less interference with speech, required less compliance to wear them, and were more comfortable to wear than VFRs. Patients found the BRs harder to keep clean. Trial registration The trail was not registered. Introduction Retention is arguably the most challenging and unpredictable stage of orthodontic treatment and the most clinically effective form of retention remains unclear (1). In the UK, there has been a significant increase in the use of vacuum-formed retainers (VFRs) and bonded retainers (BRs). Ideally, the choice of retainer appropriate for each individual patient, whether fixed or removable, should be made considering not only the clinical effectiveness, but also the patient’s ability to, and willingness to comply with, the retention plan (2, 3).VFRs have been shown to be more clinically effective at maintaining the lower labial segment alignment than Hawley retainers in the initial 6 months of retention (4). The retentive capacity of the VFR appears to be equally effective when worn full-time or at night only, during the initial 6–12 months (5, 6). Furthermore, VFRs have been shown to be preferred by both clinicians and patients owing to their comfort, ease of fabrication and fit, reduced failure rate, and cost-effectiveness (7). BRs are commonly used as a means of permanent retention usually in the form of a wire attached to the lingual or palatal surfaces of the teeth within the labial segment (3, 8). They are particularly useful in cases of perceived instability, or high patient expectation. There are many different types of BR. The most common is fabricated from multi-strand twistflex wire, which is bonded to each of the labial segment teeth and is associated with a superior retentive capacity compared to the single-stranded wire bonded to the canines only (9–11). One of the clear advantages of BRs is that they are less reliant on patient compliance. However, notable disadvantages exist, leading to reports of reduced popularity amongst some orthodontists (12). Placement is time-consuming and technique sensitive (13), failure has been reported to be as high as 53% (10)), and bonding failures may progress unnoticed by the patient, leading to localized relapse, and the risk of caries or demineralisation (14). Wire detachment at the enamel–composite interface appears to be the most common reason for failure (14–16), particularly within the early stages of retention (17), with the canine region being the most common site for detachment failure (13, 18). Fracture of the wire is reportedly the least common failure type, and predominantly occurs mesial to the maxillary canines, probably due to occlusal trauma (13, 17). Post-treatment changes may even occur with BRs in situ (19). Spacing and rotation of teeth still firmly attached to BRs has also been reported when BRs have remained in place for several years and despite not breaking (20, 21), as has fraying of the multi-strand wire which can cause irritation to the tissues (21). At the time of starting this trial, the authors were aware of only one high-quality published trial directly compared the VFR to the BR. This prospective randomized controlled trial was conducted over a 12-month period and the outcomes were published in the form of conference abstracts (12, 22). More than three times as many problems, such as loss or fracture of the retainers, were noted in the VFR group. Labial segment alignment was statistically worse in the VFR group at 12 months and the authors related this to patient’s suboptimal compliance with removable retainers. There has been a growing emphasis on the importance of using qualitative research methods to gather data of patients’ views. As a result, focus groups, in-depth interviews, and questionnaires are increasingly used in research and audit to assess patient satisfaction (23). Several studies have investigated the attitudes and preferences of orthodontists towards various retention protocols but few have reported on the perceptions of patients (7, 24). The few studies which have explored patient satisfaction with orthodontic treatment have highlighted a nearly universal dislike of orthodontic retainers. Some find them to be more inconvenient than their fixed appliances and even headgear (25–27). The reasons cited include difficulty speaking and eating, extra salivation, smell, embarrassment, and the ease with which they could be lost (25, 26). A remarkably high degree of non-compliance was also noted, with most patients reporting little or no attention to dietary restrictions or flossing regimens (27). Hichens et al. compared levels of patient satisfaction of subjects with Hawley and VFRs during the first 6 months of the retention period. The results indicated that the majority of subjects showed a preference for VFRs compared with Hawley retainers. Wearing VFRs caused less embarrassment to subjects, especially in terms of speech and appearance, and broke less frequently than Hawley retainers. Interestingly, although more subjects in the VFR group wore their retainers as instructed compared with the Hawley group, there was no difference in complaints of discomfort between subjects in either group (28). Jaderberg et al. evaluated patient experiences of subjects wearing VFRs either full-time for 3 months followed by nights-only wear compared with 1 week full-time wear followed by nights-only wear, for the first 6 months of the retention period. Overall, the results of the questionnaire showed that the VFRs were well tolerated, easy to get used to, and most patients reported that they had no difficulties in remembering to wear the retainers at night. Soreness was reported in 13%, 22% had problems with speech, and 10% thought that they tasted bad (29). A recently updated Cochrane review looking at orthodontic retention emphasized the need for future research in this field to incorporate outcomes of stability, periodontal health, survival of retainers, and quality of life (1). Further well-designed prospective randomized controlled trials would therefore be desirable to strengthen the evidence comparing the most commonly used retainer types—VFRs and BRs. This is the first of two papers comparing upper and lower VFRs with upper and lower BRs. This paper aims to test the null hypothesis that there are no significant differences in clinical effectiveness between the two retainer types. The primary outcome is stability and the secondary outcomes are retainer survival, patient satisfaction, and affect on periodontal health. The second paper will report on periodontal outcomes. Methods Trial design and ethics This was a 2-arm parallel group randomized controlled clinical trial with a 1:1 allocation ratio. The study was approved by the National Research Ethics Service in December 2010 (REC number 10/H1306/79), and also the local ethical and research development departments of the three hospital trusts involved. Participants, eligibility criteria, and settings Consecutive patients nearing the completion of fixed appliance therapy who required retainers were invited to take part in the trial. Participants were recruited from the orthodontic departments of two district general hospitals (St Luke’s Hospital, Bradford and York Hospital) and one teaching hospital (Leeds Dental Institute) in the UK. The retainers were placed and reviewed by two clinicians (KF and MS), both of whom were Speciality Registrars in Orthodontics in the early stages of their training. Treatment was provided for free to patients under the UK’s National Health Service. The following inclusion criteria were applied: Completion of a course of fixed appliance therapy (involving both dental arches) with a satisfactory correction of the presenting malocclusion and dental alignment Good general health Demonstration of a good standard of oral hygiene (determined through questioning and clinical examination) Subjects had a full and normal complement of teeth in the upper and lower labial segments Teeth within the labial segments to be a regular size and shape Subjects willing to consent to the trial and comply with the trial regime Subjects may have presented with any malocclusion prior to orthodontic treatment and may have been managed on an extraction (premolar or molar) or a non-extraction basis. Subjects may have been treated with removable or functional appliances in conjunction with their orthodontic treatment, or may have had adjunctive surgery. The following exclusion criteria were applied: Any general medical health problems which may influence gingival health, such as those necessitating antibiotic cover as bacteraemia prophylaxis, diabetes mellitus, epilepsy, or physical or mental disability Poor periodontal health, including the presence of supra-gingival calculus or periodontal pocketing greater than 3 mm Periodontal problems with either a pending referral to the periodontal department or a history of periodontal management during the orthodontic treatment Gross or uncontrolled caries Absent or diminutive lateral incisors A starting malocclusion requiring extreme transverse correction (involving rapid maxillary expansion or surgically assisted rapid maxillary expansion) Cleft palate or severe facial deformities Interventions At the end of active treatment, two calibrated operators placed either upper and lower BRs or upper and lower VFRs using standardized procedures and materials. All patients had been seen for supra-gingival debridement prior to removal of the fixed appliances. Upper and lower BRs (Figure 1) were prepared using 0.0195 in (0.45 mm) 3-stranded twistflex stainless steel wire (Wildcat; GAC International, Bohemia, New York, USA). The wire was shaped by dental technicians against the dental casts to lie passively against the lingual surfaces of the upper and lower incisors and canines. The wire was not contoured interproximally. A silicone positioning jig was prepared to aid positioning of the retainer. This was moulded to engage the wire and rest on the central incisors. The wire was bonded using a low viscosity light-cured composite (Transbond™ LV; 3M Unitek, Monrovia, California, USA), following the separate application of etchant (37% phosphoric acid), and primer (Transbond™ XT adhesive primer; 3M Unitek). Care was taken not to leave any bonding substance in contact with the gingival tissues. Figure 1. View largeDownload slide Bonded retainers with silicone positioning jig. Figure 1. View largeDownload slide Bonded retainers with silicone positioning jig. Upper and lower VFRs (Essix™ C+; Figure 2) were constructed using the ‘Essix™’ machine and cooled rapidly using Arctic spray (Ortho-Care). Retainers were trimmed to cover all fully erupted teeth and extend half way across the occlusal surface of the most distal molar. Participants were instructed to wear their retainers only at night, every night. At the time of retainer insertion, both written and verbal oral hygiene instructions were given, including methods for interdental cleaning around BRs. Participants were advised to continue 6 monthly review appointments with their general dentist. Figure 2. View largeDownload slide Vacuum-formed retainers. Figure 2. View largeDownload slide Vacuum-formed retainers. Patients were advised to contact the department as soon as possible if they had any queries, concerns, or problems. Failed retainers were repaired on the same day adhering to the standardized protocol for materials, construction, and technique of placement. Outcome measures The primary outcome was stability. Secondary outcomes were: Retainer survival Patient satisfaction Periodontal health (reported in Paper 2) Stability The primary outcome was to evaluate the clinical effectiveness of both retainer types in preventing post-treatment changes over the initial 12-months of retention. Post-treatment changes were defined as change in labial segment alignment. This was measured from digitized study models (Figure 3) using Little’s Irregularity Index (LII). In addition, changes in arch dimensions, occlusal relationships, and re-opening of extraction spaces were also recorded. Table 1 provides details of these measurements. Figure 3. View largeDownload slide Demonstration of the mandibular arch dimensions (ICW, IMW, AL). Figure 3. View largeDownload slide Demonstration of the mandibular arch dimensions (ICW, IMW, AL). Table 1. Single-arch and occlusal measurements recorded at each milestone. Measurement Unit Description Little’s irregularity index of the upper and lower labial segments (LII) mm The sum of the five labial segment anatomical contact point displacements in a labiolingual direction Upper and lower inter-canine width (ICW) mm Distance between the cusp tips of right and left canines Upper and lower inter-molar width (IMW) mm Distance between the mesiobuccal cusp tip of the right first molar to the mesiobuccal cusp tip of the left first permanent molar. Where the first permanent molar had been extracted, the second permanent molar was used instead. Upper and lower arch length (AL) mm The sum of the right and left distances from the mesiobuccal cusp tip of the first permanent molars to the interproximal contact point of the central incisors Overjet (OJ) mm The maximum distance between the upper incisor edge and the lower incisal labial surface, horizontal to the occlusal plane Overbite (OB) mm The maximum vertical overlap between the upper and lower incisors with the models in maximal intercuspation. Measurement Unit Description Little’s irregularity index of the upper and lower labial segments (LII) mm The sum of the five labial segment anatomical contact point displacements in a labiolingual direction Upper and lower inter-canine width (ICW) mm Distance between the cusp tips of right and left canines Upper and lower inter-molar width (IMW) mm Distance between the mesiobuccal cusp tip of the right first molar to the mesiobuccal cusp tip of the left first permanent molar. Where the first permanent molar had been extracted, the second permanent molar was used instead. Upper and lower arch length (AL) mm The sum of the right and left distances from the mesiobuccal cusp tip of the first permanent molars to the interproximal contact point of the central incisors Overjet (OJ) mm The maximum distance between the upper incisor edge and the lower incisal labial surface, horizontal to the occlusal plane Overbite (OB) mm The maximum vertical overlap between the upper and lower incisors with the models in maximal intercuspation. View Large Table 1. Single-arch and occlusal measurements recorded at each milestone. Measurement Unit Description Little’s irregularity index of the upper and lower labial segments (LII) mm The sum of the five labial segment anatomical contact point displacements in a labiolingual direction Upper and lower inter-canine width (ICW) mm Distance between the cusp tips of right and left canines Upper and lower inter-molar width (IMW) mm Distance between the mesiobuccal cusp tip of the right first molar to the mesiobuccal cusp tip of the left first permanent molar. Where the first permanent molar had been extracted, the second permanent molar was used instead. Upper and lower arch length (AL) mm The sum of the right and left distances from the mesiobuccal cusp tip of the first permanent molars to the interproximal contact point of the central incisors Overjet (OJ) mm The maximum distance between the upper incisor edge and the lower incisal labial surface, horizontal to the occlusal plane Overbite (OB) mm The maximum vertical overlap between the upper and lower incisors with the models in maximal intercuspation. Measurement Unit Description Little’s irregularity index of the upper and lower labial segments (LII) mm The sum of the five labial segment anatomical contact point displacements in a labiolingual direction Upper and lower inter-canine width (ICW) mm Distance between the cusp tips of right and left canines Upper and lower inter-molar width (IMW) mm Distance between the mesiobuccal cusp tip of the right first molar to the mesiobuccal cusp tip of the left first permanent molar. Where the first permanent molar had been extracted, the second permanent molar was used instead. Upper and lower arch length (AL) mm The sum of the right and left distances from the mesiobuccal cusp tip of the first permanent molars to the interproximal contact point of the central incisors Overjet (OJ) mm The maximum distance between the upper incisor edge and the lower incisal labial surface, horizontal to the occlusal plane Overbite (OB) mm The maximum vertical overlap between the upper and lower incisors with the models in maximal intercuspation. View Large Alginate impressions were obtained at 4 time-points during the trial (T0–T3) and were measured by the same examiner (KF) to an accuracy of 0.01 mm. T0Debond T13 months post-debond T26 months post-debond T312 months post-debond Survival Retainer survival was recorded as the time to the first episode of failure. The date of failure was recorded as the day the patient became aware of the problem, or alternatively, the date the clinician noted the failure when participants were unaware of a failed retainer. The pattern of failure was also recorded: BRs could fail at: the wire–composite interface; the enamel–composite interface; by wire fracture; or by complete detachment from all teeth. VFRs were categorized as being lost, ill-fitting, or fractured. The occlusal wear was also noted, particularly if there was a breach in the full thickness of the retainer necessitating replacement. Patient satisfaction Patient satisfaction (secondary outcome measure) was determined by a questionnaire based on the most relevant questions in the patient satisfaction questionnaire from the Hichens et al. RCT assessing patient satisfaction with Hawley and VFRs (28). The main issues identified from their study were used to form the basis of this questionnaire (i.e. it was a modified version of the previously used questionnaire). A preliminary version of this questionnaire was piloted on a group of 10 patients attending the orthodontic department for retainer reviews. These patients had agreed to be involved in the pilot study and were not otherwise participating in the study. Any necessary modifications to ensure good readability and reproducibility were made before using the questionnaires in the main study. The decision was made to administer the questionnaire to the participants at their scheduled review appointments, rather than sending the questionnaire by post or e-mail to complete at home. It was hoped that this would ensure that the questionnaires were completed fully, increasing the response rate, and to allow the investigator to clarify any ambiguous questions. The data from the questionnaires was coded and assessed by one operator (MS). Sample size calculation The sample size was determined based on the primary objective of comparing the efficacy of each retainer group in minimising the 1-year post-orthodontic treatment change in the anterior arch alignment as measured by LII (30). Assuming a clinically relevant difference of 0.5 mm between the two randomized groups, a common standard deviation of 0.5 mm, a power of 90%, and a significance level of 5%, the study required 22 subjects in each group. To account for a potential dropout rate of 20% and to increase the sample size for the secondary outcome measures, the sample size was increased to 30 per group resulting in a total of 60 subjects. Randomisation Consecutive participants eligible for inclusion were approached by KF or MS and written informed consent was obtained from the participant (and parent if the participants were adolescents). Having consented to taking part in the study, the participant was allocated randomly to receive either upper and lower BRs or upper and lower VFRs using sequentially numbered, opaque, sealed envelopes, prepared in advance of the trial by a colleague independent of the study, using a computerized randomisation programme. Blinding It was not possible to blind the patient or clinician/operator in this study following opening of the envelope due to the nature of the intervention. Blinding of the outcome assessor looking at stability on the digital models was also not possible, but blinding of the patient satisfaction assessor was undertaken by removing patient-identifying information from the completed questionnaires. Patients leaving the study or refusing treatment To account for potential attrition bias, an ‘intention-to-treat’ analysis was employed for those patients who dropped-out of the study. Similarly, if a subject failed to co-operate with the retention regime, the data were still collected and an ‘intention-to-treat’ analysis was carried out. Average values for that particular retainer group were substituted for the missing data. Subsequent analyses were based on the new complete data set. The retainer survival time for subjects who had dropped-out at 12 months was based on the 6-month review data where the retainer had not failed previously. Statistical methods The SPSS software package was used for data analysis (version 20; SPSS, Chicago, Illinois, USA) with statistical significance set at the 5% level. The intra-class correlation coefficient (ICC) was used to assess the reliability of measurements made by the same operator (KF). Twenty randomly selected digital models were benchmarked against the ‘gold standard’ of digital callipers and plaster models. Results ranged from 0.882 (95% confidence interval [CI] = 0.728–0.951) for the arch length to 0.960 (95% CI = 0.805–0.987) for the inter-molar width. Excellent agreement was associated with LII (ICC = 0.951, 95% CI = 0.882–0.980). A further 20 randomly selected digital models were measured on two separate occasions. Excellent repeatability was demonstrated for the LII (ICC = 0.921, 95% CI = 0.841–0.961). Occlusal measurements, and the relapse data were found to be non-normally distributed, therefore non-parametric analyses were employed, using the median (Mdn) and inter-quartile range (IQR). Mann–Whitney U-tests were used to compare relapse values between retainer groups. Retainer survival was analysed through the Kaplan–Meier survival plot and the log–rank test. The patient satisfaction questionnaires were coded by the research team and all responses entered into a database. A descriptive analysis was used to compare the levels of satisfaction between VFRs and BRs, whilst Chi-squared tests were used for trend and significance. Results Participant flow and recruitment details The CONSORT flow diagram is shown in Figure 4. Figure 4. View largeDownload slide CONSORT flow diagram. Figure 4. View largeDownload slide CONSORT flow diagram. Of the 104 patients who were initially informed about the study, 44 were excluded as they either declined to be involved or did not satisfy the inclusion criteria. Of the 60 patients enrolled, 30 were provided with BRs (15 male, 15 female, Mdn age 16 years, IQR = 2) and 30 were provided with VFRs (12 male, 18 female, Mdn age 17 years, IQR = 4). All patients were recruited between March 2012 and September 2013. It was not possible to conduct a 3-month review for eight participants (BR = 6, VFR = 2) due to staff shortages in one of the hospitals. One participant (BR = 1) did not attend their 6-month review and 3 (VFR = 3) failed to attend their 12-month review. Furthermore, four sets of study models taken at the 3-month review were irretrievable (VFR = 2, BR = 2), and one participant attending the 12-month review declined impressions. Baseline data Baseline data were collected prior to the start of orthodontic treatment and were found to be similar in both groups (Table 2). Any changes in arch dimensions that occurred during active orthodontic treatment were not found to be statistically different between the retainer groups. Table 2. Baseline characteristics at T0 and treatment summary of the sample. Bonded retainer (N = 30) Vacuum-formed retainer (N = 30) Sex  Male 15 12  Female 15 18 Age (years)  Median (IQR) 16 (2) 17 (4) Study centre  Leeds 8 9  Bradford 16 14  York 6 7 Initial incisor classification  class I 5 4  class II Division 1 13 14  class II Division 2 3 8  class III 9 4 Initial skeletal classification  Class 1 7 6  Class 2 14 20  Class 3 9 4 Extractions  Non—extraction 11 8  Maxillary extractions only 10 9  Mandibular extractions only 2 3  Bimaxillary extractions 7 10 Treatment modality  Fixed appliances only 16 21  URA and fixed appliances 3 2  Functional and fixed appliances 10 4  Orthognathic surgery and fixed appliances 1 3 Bonded retainer (N = 30) Vacuum-formed retainer (N = 30) Sex  Male 15 12  Female 15 18 Age (years)  Median (IQR) 16 (2) 17 (4) Study centre  Leeds 8 9  Bradford 16 14  York 6 7 Initial incisor classification  class I 5 4  class II Division 1 13 14  class II Division 2 3 8  class III 9 4 Initial skeletal classification  Class 1 7 6  Class 2 14 20  Class 3 9 4 Extractions  Non—extraction 11 8  Maxillary extractions only 10 9  Mandibular extractions only 2 3  Bimaxillary extractions 7 10 Treatment modality  Fixed appliances only 16 21  URA and fixed appliances 3 2  Functional and fixed appliances 10 4  Orthognathic surgery and fixed appliances 1 3 View Large Table 2. Baseline characteristics at T0 and treatment summary of the sample. Bonded retainer (N = 30) Vacuum-formed retainer (N = 30) Sex  Male 15 12  Female 15 18 Age (years)  Median (IQR) 16 (2) 17 (4) Study centre  Leeds 8 9  Bradford 16 14  York 6 7 Initial incisor classification  class I 5 4  class II Division 1 13 14  class II Division 2 3 8  class III 9 4 Initial skeletal classification  Class 1 7 6  Class 2 14 20  Class 3 9 4 Extractions  Non—extraction 11 8  Maxillary extractions only 10 9  Mandibular extractions only 2 3  Bimaxillary extractions 7 10 Treatment modality  Fixed appliances only 16 21  URA and fixed appliances 3 2  Functional and fixed appliances 10 4  Orthognathic surgery and fixed appliances 1 3 Bonded retainer (N = 30) Vacuum-formed retainer (N = 30) Sex  Male 15 12  Female 15 18 Age (years)  Median (IQR) 16 (2) 17 (4) Study centre  Leeds 8 9  Bradford 16 14  York 6 7 Initial incisor classification  class I 5 4  class II Division 1 13 14  class II Division 2 3 8  class III 9 4 Initial skeletal classification  Class 1 7 6  Class 2 14 20  Class 3 9 4 Extractions  Non—extraction 11 8  Maxillary extractions only 10 9  Mandibular extractions only 2 3  Bimaxillary extractions 7 10 Treatment modality  Fixed appliances only 16 21  URA and fixed appliances 3 2  Functional and fixed appliances 10 4  Orthognathic surgery and fixed appliances 1 3 View Large Primary outcome: stability Occlusal measurements for each retainer were measured at debond (T0), 3 months (T1), 6 months (T2), and 1 year (T3; Table 3). Post-treatment changes are shown in Table 4. Table 3. Occlusal measurements (mm) at each time point using intention-to-treat analysis. Data are presented in the form of median (inter-quartile range) and Mann–Whitney P value due to its non-normal distribution. AL, arch length; BR, bonded retainer; ICW, inter-canine width; IMW, inter-molar width; LII, Little’s irregularity index; OB, overbite; OJ, overjet; VFR, vacuum-formed retainer. Bold indicates statistical significance P < 0.05. Occlusal measurements at each review Maxilla Mandible Inter-arch LII ICW IMW AL LII ICW IMW AL OJ OB T0 Debond BR 0.00 (0.93) 35.20 (2.83) 50.11 (3.96) 73.94 (12.74) 0.29 (1.02) 27.53 (1.68) 44.05 (4.64) 66.74 (6.00) 2.37 (0.70) 1.29 (1.22) VFR 0.23 (0.66) 34.09 (2.22) 48.46 (4.24) 71.23 (9.67) 0.06 (1.23) 26.17 (1.13) 41.34 (5.72) 65.53 (12.94) 2.38 (2.40) 2.00 (1.21) P value 0.31 0.003 0.01 0.101 0.382 0.003 0.071 0.188 0.203 0.083 T1 3 months BR 0.79 (0.91) 35.08 (1.92) 49.63 (2.92) 72.75 (10.57) 0.44 (0.63) 27.45 (2.11) 43.65 (3.67) 66.81 (8.68) 2.31 (1.04) 1.45 (0.71) VFR 0.30 (0.98) 33.69 (2.52) 47.90 (3.88) 69.81 (11.46) 0.44 (1.71) 25.69 (1.45) 41.70 (5.35) 63.43 (9.94) 2.62 (0.76) 1.87 (0.87) P value 0.05 0.001 0.018 0.147 0.432 0.000 0.055 0.104 0.064 0.001 T2 6 months BR 0.88 (1.07) 35.36 (2.41) 49.74 (3.46) 72.88 (10.35) 0.67 (1.47) 28.06 (1.65) 44.23 (2.34) 67.06 (8.83) 2.53 (1.09) 1.52 (0.92) VFR 0.64 (1.77) 34.21 (2.06) 48.84 (3.85) 70.19 (9.62) 1.30 (1.98) 26.26 (1.65) 41.84 (6.22) 65.17 (11.42) 2.74 (0.50) 1.92 (0.97) P value 0.22 0.000 0.015 0.051 0.335 0.002 0.174 0.055 0.181 0.036 T3 12 months BR 1.35 (1.98) 35.08 (2.31) 49.47 (3.88) 76.70 (10.81) 1.01 (1.28) 27.31 (2.21) 43.90 (4.32) 66.97 (8.21) 2.26 (1.07) 1.59 (0.78) VFR 0.97 (1.68) 33.21 (2.36) 47.70 (3.80) 68.86 (10.26) 1.73 (2.77) 25.56 (1.39) 41.32 (4.61) 62.57 (9.50) 2.59 (0.94) 2.01 (1.00) P value 0.24 0.001 0.064 0.117 0.053 0.001 0.068 0.071 0.128 0.022 Occlusal measurements at each review Maxilla Mandible Inter-arch LII ICW IMW AL LII ICW IMW AL OJ OB T0 Debond BR 0.00 (0.93) 35.20 (2.83) 50.11 (3.96) 73.94 (12.74) 0.29 (1.02) 27.53 (1.68) 44.05 (4.64) 66.74 (6.00) 2.37 (0.70) 1.29 (1.22) VFR 0.23 (0.66) 34.09 (2.22) 48.46 (4.24) 71.23 (9.67) 0.06 (1.23) 26.17 (1.13) 41.34 (5.72) 65.53 (12.94) 2.38 (2.40) 2.00 (1.21) P value 0.31 0.003 0.01 0.101 0.382 0.003 0.071 0.188 0.203 0.083 T1 3 months BR 0.79 (0.91) 35.08 (1.92) 49.63 (2.92) 72.75 (10.57) 0.44 (0.63) 27.45 (2.11) 43.65 (3.67) 66.81 (8.68) 2.31 (1.04) 1.45 (0.71) VFR 0.30 (0.98) 33.69 (2.52) 47.90 (3.88) 69.81 (11.46) 0.44 (1.71) 25.69 (1.45) 41.70 (5.35) 63.43 (9.94) 2.62 (0.76) 1.87 (0.87) P value 0.05 0.001 0.018 0.147 0.432 0.000 0.055 0.104 0.064 0.001 T2 6 months BR 0.88 (1.07) 35.36 (2.41) 49.74 (3.46) 72.88 (10.35) 0.67 (1.47) 28.06 (1.65) 44.23 (2.34) 67.06 (8.83) 2.53 (1.09) 1.52 (0.92) VFR 0.64 (1.77) 34.21 (2.06) 48.84 (3.85) 70.19 (9.62) 1.30 (1.98) 26.26 (1.65) 41.84 (6.22) 65.17 (11.42) 2.74 (0.50) 1.92 (0.97) P value 0.22 0.000 0.015 0.051 0.335 0.002 0.174 0.055 0.181 0.036 T3 12 months BR 1.35 (1.98) 35.08 (2.31) 49.47 (3.88) 76.70 (10.81) 1.01 (1.28) 27.31 (2.21) 43.90 (4.32) 66.97 (8.21) 2.26 (1.07) 1.59 (0.78) VFR 0.97 (1.68) 33.21 (2.36) 47.70 (3.80) 68.86 (10.26) 1.73 (2.77) 25.56 (1.39) 41.32 (4.61) 62.57 (9.50) 2.59 (0.94) 2.01 (1.00) P value 0.24 0.001 0.064 0.117 0.053 0.001 0.068 0.071 0.128 0.022 View Large Table 3. Occlusal measurements (mm) at each time point using intention-to-treat analysis. Data are presented in the form of median (inter-quartile range) and Mann–Whitney P value due to its non-normal distribution. AL, arch length; BR, bonded retainer; ICW, inter-canine width; IMW, inter-molar width; LII, Little’s irregularity index; OB, overbite; OJ, overjet; VFR, vacuum-formed retainer. Bold indicates statistical significance P < 0.05. Occlusal measurements at each review Maxilla Mandible Inter-arch LII ICW IMW AL LII ICW IMW AL OJ OB T0 Debond BR 0.00 (0.93) 35.20 (2.83) 50.11 (3.96) 73.94 (12.74) 0.29 (1.02) 27.53 (1.68) 44.05 (4.64) 66.74 (6.00) 2.37 (0.70) 1.29 (1.22) VFR 0.23 (0.66) 34.09 (2.22) 48.46 (4.24) 71.23 (9.67) 0.06 (1.23) 26.17 (1.13) 41.34 (5.72) 65.53 (12.94) 2.38 (2.40) 2.00 (1.21) P value 0.31 0.003 0.01 0.101 0.382 0.003 0.071 0.188 0.203 0.083 T1 3 months BR 0.79 (0.91) 35.08 (1.92) 49.63 (2.92) 72.75 (10.57) 0.44 (0.63) 27.45 (2.11) 43.65 (3.67) 66.81 (8.68) 2.31 (1.04) 1.45 (0.71) VFR 0.30 (0.98) 33.69 (2.52) 47.90 (3.88) 69.81 (11.46) 0.44 (1.71) 25.69 (1.45) 41.70 (5.35) 63.43 (9.94) 2.62 (0.76) 1.87 (0.87) P value 0.05 0.001 0.018 0.147 0.432 0.000 0.055 0.104 0.064 0.001 T2 6 months BR 0.88 (1.07) 35.36 (2.41) 49.74 (3.46) 72.88 (10.35) 0.67 (1.47) 28.06 (1.65) 44.23 (2.34) 67.06 (8.83) 2.53 (1.09) 1.52 (0.92) VFR 0.64 (1.77) 34.21 (2.06) 48.84 (3.85) 70.19 (9.62) 1.30 (1.98) 26.26 (1.65) 41.84 (6.22) 65.17 (11.42) 2.74 (0.50) 1.92 (0.97) P value 0.22 0.000 0.015 0.051 0.335 0.002 0.174 0.055 0.181 0.036 T3 12 months BR 1.35 (1.98) 35.08 (2.31) 49.47 (3.88) 76.70 (10.81) 1.01 (1.28) 27.31 (2.21) 43.90 (4.32) 66.97 (8.21) 2.26 (1.07) 1.59 (0.78) VFR 0.97 (1.68) 33.21 (2.36) 47.70 (3.80) 68.86 (10.26) 1.73 (2.77) 25.56 (1.39) 41.32 (4.61) 62.57 (9.50) 2.59 (0.94) 2.01 (1.00) P value 0.24 0.001 0.064 0.117 0.053 0.001 0.068 0.071 0.128 0.022 Occlusal measurements at each review Maxilla Mandible Inter-arch LII ICW IMW AL LII ICW IMW AL OJ OB T0 Debond BR 0.00 (0.93) 35.20 (2.83) 50.11 (3.96) 73.94 (12.74) 0.29 (1.02) 27.53 (1.68) 44.05 (4.64) 66.74 (6.00) 2.37 (0.70) 1.29 (1.22) VFR 0.23 (0.66) 34.09 (2.22) 48.46 (4.24) 71.23 (9.67) 0.06 (1.23) 26.17 (1.13) 41.34 (5.72) 65.53 (12.94) 2.38 (2.40) 2.00 (1.21) P value 0.31 0.003 0.01 0.101 0.382 0.003 0.071 0.188 0.203 0.083 T1 3 months BR 0.79 (0.91) 35.08 (1.92) 49.63 (2.92) 72.75 (10.57) 0.44 (0.63) 27.45 (2.11) 43.65 (3.67) 66.81 (8.68) 2.31 (1.04) 1.45 (0.71) VFR 0.30 (0.98) 33.69 (2.52) 47.90 (3.88) 69.81 (11.46) 0.44 (1.71) 25.69 (1.45) 41.70 (5.35) 63.43 (9.94) 2.62 (0.76) 1.87 (0.87) P value 0.05 0.001 0.018 0.147 0.432 0.000 0.055 0.104 0.064 0.001 T2 6 months BR 0.88 (1.07) 35.36 (2.41) 49.74 (3.46) 72.88 (10.35) 0.67 (1.47) 28.06 (1.65) 44.23 (2.34) 67.06 (8.83) 2.53 (1.09) 1.52 (0.92) VFR 0.64 (1.77) 34.21 (2.06) 48.84 (3.85) 70.19 (9.62) 1.30 (1.98) 26.26 (1.65) 41.84 (6.22) 65.17 (11.42) 2.74 (0.50) 1.92 (0.97) P value 0.22 0.000 0.015 0.051 0.335 0.002 0.174 0.055 0.181 0.036 T3 12 months BR 1.35 (1.98) 35.08 (2.31) 49.47 (3.88) 76.70 (10.81) 1.01 (1.28) 27.31 (2.21) 43.90 (4.32) 66.97 (8.21) 2.26 (1.07) 1.59 (0.78) VFR 0.97 (1.68) 33.21 (2.36) 47.70 (3.80) 68.86 (10.26) 1.73 (2.77) 25.56 (1.39) 41.32 (4.61) 62.57 (9.50) 2.59 (0.94) 2.01 (1.00) P value 0.24 0.001 0.064 0.117 0.053 0.001 0.068 0.071 0.128 0.022 View Large Table 4. Relapse values (measured in mm) at 3 months, 6 months, and 12 months. Data are presented in the form of median (inter-quartile range) and Mann–Whitney P value due to its non-normal distribution. AL, arch length; BR, bonded retainer; ICW, inter-canine width; IMW, inter-molar width; LII, Little’s irregularity index; OB, overbite; OJ, overjet; VFR= vacuum-formed retainer; PAR, peer assessment rating. Bold indicates statistical significance P < 0.05. Changes in measurements with time Maxilla Mandible Inter-arch LII ICW IMW AL LII ICW IMW AL OJ OB 3-Month relapse (T1–T0) BR 0.29 (0.55) 35.08 (1.92) 49.63 (2.92) 72.75 (10.57) 0.15 (0.32) 27.45 (2.11) 43.65 (3.67) 66.81 (8.68) 2.32 (1.04) 1.44 (0.71) VFR 0.04 (0.32) 33.69 (2.52) 47.90 (3.88) 69.81 (11.46) 0.33 (0.61) 25.69 (1.45) 41.70 (5.35) 63.43 (9.94) 2.62 (0.76) 1.87 (0.87) P value 0.010 0.562 0.712 0.304 0.048 0.036 0.712 0.802 0.34 0.26 6-Month relapse (T2–T0) BR 0.39 (1.08) 0.18 (0.81) −0.63 (1.79) 0.00 (1.42) 0.53 (0.72) 0.84 (0.75) 0.19 (1.40) 0.00 (0.87) 0.08 (0.53) 0.20 (0.50) VFR 0.30 (1.24) 0.01 (0.86) 0.15 (0.71) 0.35 (0.70) 0.70 (1.33) −0.18 (0.77) 0.49 (1.12) −0.49 (1.25) 0.14 (0.46) 0.02 (0.59) P value 0.30 0.13 0.734 0.515 0.116 0.031 0.188 0.268 0.308 0.652 12-Month relapse (T3–T0) BR 1.10 (1.56) 0.25 (1.16) −0.16 (2.60) −0.36 (1.77) 0.77 (1.46) −0.26 (0.90) 0.11 (2.07) 0.10 (1.19) −0.09 (0.74) 0.30 (0.71) VFR 0.76 (1.55) 0.19 (1.02) 0.02 (0.91) 0.10 (1.44) 1.69 (2.00) −0.45 (0.86) 0.54 (1.67) −0.70 (1.25) 0.12 (0.69) 0.15 (0.83) P value 0.61 0.124 0.802 0.918 0.008 0.071 0.544 0.040 0.408 0.965 Changes in measurements with time Maxilla Mandible Inter-arch LII ICW IMW AL LII ICW IMW AL OJ OB 3-Month relapse (T1–T0) BR 0.29 (0.55) 35.08 (1.92) 49.63 (2.92) 72.75 (10.57) 0.15 (0.32) 27.45 (2.11) 43.65 (3.67) 66.81 (8.68) 2.32 (1.04) 1.44 (0.71) VFR 0.04 (0.32) 33.69 (2.52) 47.90 (3.88) 69.81 (11.46) 0.33 (0.61) 25.69 (1.45) 41.70 (5.35) 63.43 (9.94) 2.62 (0.76) 1.87 (0.87) P value 0.010 0.562 0.712 0.304 0.048 0.036 0.712 0.802 0.34 0.26 6-Month relapse (T2–T0) BR 0.39 (1.08) 0.18 (0.81) −0.63 (1.79) 0.00 (1.42) 0.53 (0.72) 0.84 (0.75) 0.19 (1.40) 0.00 (0.87) 0.08 (0.53) 0.20 (0.50) VFR 0.30 (1.24) 0.01 (0.86) 0.15 (0.71) 0.35 (0.70) 0.70 (1.33) −0.18 (0.77) 0.49 (1.12) −0.49 (1.25) 0.14 (0.46) 0.02 (0.59) P value 0.30 0.13 0.734 0.515 0.116 0.031 0.188 0.268 0.308 0.652 12-Month relapse (T3–T0) BR 1.10 (1.56) 0.25 (1.16) −0.16 (2.60) −0.36 (1.77) 0.77 (1.46) −0.26 (0.90) 0.11 (2.07) 0.10 (1.19) −0.09 (0.74) 0.30 (0.71) VFR 0.76 (1.55) 0.19 (1.02) 0.02 (0.91) 0.10 (1.44) 1.69 (2.00) −0.45 (0.86) 0.54 (1.67) −0.70 (1.25) 0.12 (0.69) 0.15 (0.83) P value 0.61 0.124 0.802 0.918 0.008 0.071 0.544 0.040 0.408 0.965 View Large Table 4. Relapse values (measured in mm) at 3 months, 6 months, and 12 months. Data are presented in the form of median (inter-quartile range) and Mann–Whitney P value due to its non-normal distribution. AL, arch length; BR, bonded retainer; ICW, inter-canine width; IMW, inter-molar width; LII, Little’s irregularity index; OB, overbite; OJ, overjet; VFR= vacuum-formed retainer; PAR, peer assessment rating. Bold indicates statistical significance P < 0.05. Changes in measurements with time Maxilla Mandible Inter-arch LII ICW IMW AL LII ICW IMW AL OJ OB 3-Month relapse (T1–T0) BR 0.29 (0.55) 35.08 (1.92) 49.63 (2.92) 72.75 (10.57) 0.15 (0.32) 27.45 (2.11) 43.65 (3.67) 66.81 (8.68) 2.32 (1.04) 1.44 (0.71) VFR 0.04 (0.32) 33.69 (2.52) 47.90 (3.88) 69.81 (11.46) 0.33 (0.61) 25.69 (1.45) 41.70 (5.35) 63.43 (9.94) 2.62 (0.76) 1.87 (0.87) P value 0.010 0.562 0.712 0.304 0.048 0.036 0.712 0.802 0.34 0.26 6-Month relapse (T2–T0) BR 0.39 (1.08) 0.18 (0.81) −0.63 (1.79) 0.00 (1.42) 0.53 (0.72) 0.84 (0.75) 0.19 (1.40) 0.00 (0.87) 0.08 (0.53) 0.20 (0.50) VFR 0.30 (1.24) 0.01 (0.86) 0.15 (0.71) 0.35 (0.70) 0.70 (1.33) −0.18 (0.77) 0.49 (1.12) −0.49 (1.25) 0.14 (0.46) 0.02 (0.59) P value 0.30 0.13 0.734 0.515 0.116 0.031 0.188 0.268 0.308 0.652 12-Month relapse (T3–T0) BR 1.10 (1.56) 0.25 (1.16) −0.16 (2.60) −0.36 (1.77) 0.77 (1.46) −0.26 (0.90) 0.11 (2.07) 0.10 (1.19) −0.09 (0.74) 0.30 (0.71) VFR 0.76 (1.55) 0.19 (1.02) 0.02 (0.91) 0.10 (1.44) 1.69 (2.00) −0.45 (0.86) 0.54 (1.67) −0.70 (1.25) 0.12 (0.69) 0.15 (0.83) P value 0.61 0.124 0.802 0.918 0.008 0.071 0.544 0.040 0.408 0.965 Changes in measurements with time Maxilla Mandible Inter-arch LII ICW IMW AL LII ICW IMW AL OJ OB 3-Month relapse (T1–T0) BR 0.29 (0.55) 35.08 (1.92) 49.63 (2.92) 72.75 (10.57) 0.15 (0.32) 27.45 (2.11) 43.65 (3.67) 66.81 (8.68) 2.32 (1.04) 1.44 (0.71) VFR 0.04 (0.32) 33.69 (2.52) 47.90 (3.88) 69.81 (11.46) 0.33 (0.61) 25.69 (1.45) 41.70 (5.35) 63.43 (9.94) 2.62 (0.76) 1.87 (0.87) P value 0.010 0.562 0.712 0.304 0.048 0.036 0.712 0.802 0.34 0.26 6-Month relapse (T2–T0) BR 0.39 (1.08) 0.18 (0.81) −0.63 (1.79) 0.00 (1.42) 0.53 (0.72) 0.84 (0.75) 0.19 (1.40) 0.00 (0.87) 0.08 (0.53) 0.20 (0.50) VFR 0.30 (1.24) 0.01 (0.86) 0.15 (0.71) 0.35 (0.70) 0.70 (1.33) −0.18 (0.77) 0.49 (1.12) −0.49 (1.25) 0.14 (0.46) 0.02 (0.59) P value 0.30 0.13 0.734 0.515 0.116 0.031 0.188 0.268 0.308 0.652 12-Month relapse (T3–T0) BR 1.10 (1.56) 0.25 (1.16) −0.16 (2.60) −0.36 (1.77) 0.77 (1.46) −0.26 (0.90) 0.11 (2.07) 0.10 (1.19) −0.09 (0.74) 0.30 (0.71) VFR 0.76 (1.55) 0.19 (1.02) 0.02 (0.91) 0.10 (1.44) 1.69 (2.00) −0.45 (0.86) 0.54 (1.67) −0.70 (1.25) 0.12 (0.69) 0.15 (0.83) P value 0.61 0.124 0.802 0.918 0.008 0.071 0.544 0.040 0.408 0.965 View Large In the mandible, post-treatment changes tended to be greater in the VFR group, reaching statistical significance at both 3 and 12 months; mandibular LII values were BR = 0.77 mm IQR = 1.46, VFR = 1.69 mm IQR = 2.00, P = 0.008 at 12 months. In the maxilla, there was no significant difference in post-treatment changes between the retainer groups after 12 months (BR = 1.10 mm IQR = 1.56, VFR = 0.76 mm IQR = 1.55, P = 0.61). The only statistically significant relapse measurement in arch dimensions or occlusal relationships at 12 months was mandibular arch length (BR = +0.10 mm IQR = 1.19, VFR = −0.70 mm IQR = 1.25, P = 0.040). There was no difference in the re-opening of the extraction spaces during the initial 12 months of retention between BR and VFR groups. Mdn space opening within the maxilla was 0.00 mm (IQR = 0.28) with BRs and 0.00 mm (IQR = 0.00) with VFRs, 0.00 mm (IQR = 0.42) with BRs and 0.00mm (IQR = 0.25) with VFRs within the mandible. Secondary outcome: retainer survival There were no statistical differences in the survival rates of BRs and VFRs in the maxilla. The Kaplan–Meier plot for maxillary retainers shows 63.3% of BR and 73.3% of VFR survived the initial 12 months of retention, and this difference was statistically non-significant (P = 0.34). The mean survival time was 272.5 days (95% CI = 226.1–319.0) for the BR, and 311.3 days (95% CI = 278.3–344.29) for the VFR (Figure 5). Figure 5. View largeDownload slide Kaplan–Meier cumulative survival plots and log-rank P values for the time until the first episode of failure during 12 months of follow-up for (a) maxillary retainers and (b) mandibular retainers. Figure 5. View largeDownload slide Kaplan–Meier cumulative survival plots and log-rank P values for the time until the first episode of failure during 12 months of follow-up for (a) maxillary retainers and (b) mandibular retainers. This contrasted with the mandibular retainers, where there was a statistically significant difference in the 12-month survival of BR and VFR (P = 0.01) with respective rates of 50% and 80%. The Kaplan–Meier plot illustrates a mean survival time of 239.3 days (95% CI = 191.1–287.5) for the BR, and 324.9 days (95% CI = 295.4–354.4) for the VFR (Figure 5). Looking at VFR survival rates after 12 months, 8 patients (26.7%) with VFRs had experienced at least one episode of retainer failure; either retainer loss or poorly fitting retainers or not fitting. All eight patients exhibited failure of an upper retainer, whilst six of these patients also had a lower retainer failure. Of these eight patients three had repeat failures: two subjects experienced two episodes of failure, whilst one patient exhibited three episodes. No correlation was found between failure and patient gender, operator, or study centre (P = 0.21, 0.66, 0.17, respectively). After 12 months, 18 patients with BRs (60%) had experienced at least one episode of retainer failure; 11 maxillary BRs and 15 mandibular BRs. Adhesive and cohesive failures were prevalent in both arches. There was one episode involving a maxillary BR fracture mesial to the right canine, and there were three episodes of the mandibular BR completely detaching from all tooth surfaces simultaneously. Two episodes of maxillary BR failure occurred in three subjects, and involved detachment at the wire-composite interface. Three patients encountered repeated mandibular BR failures (two patient with two episodes and three patients with three episodes). Maxillary failure tended to occur between the wire and the composite, whereas, mandibular failure was more common at the enamel–composite interface, particularly within the initial 6 months. No statistical predictors were found for the survival of either upper or lower BRs. The confounders tested included the operator, hospital as well as patient factors such as gender incisor relationship and skeletal relationship. Secondary outcome: patient satisfaction A total of 52 questionnaires were completed at 3 months (BR = 24, VFR = 28), 60 at 6 months (BR = 30, VFR = 30), and 57 at 12 months (BR = 30, VFR = 27). Where subjects failed to attend review appointments, questionnaires were posted to patients to complete and return in an attempt to minimise missing data. Results are reported in Supplementary Table 1. There was no significant difference between patients’ opinions of wearing their retainers compared to their fixed appliances (P = 0.096) with both groups reporting that their retainers were better, or at least no worse, than their fixed appliances. There was no difference in the embarrassment felt by participants wearing either retainer type (P = 0.065) with only two subjects with VFRs reporting occasional embarrassment. More patients reported discomfort from VFRs than BRs (P = 0.002). More patients with VFRs reported that their speech was affected by their retainers than patients with BRs (P = 0.004). Subjects with VFRs found them easier to clean than those with BRs (P = 0.001). Harms No serious harm associated with the retainers was observed. Effects on periodontal health will be reported in Paper 2. Discussion Key findings and comparison to available literature Stability and retainer survival The study found no evidence of a difference in post-treatment changes, or retainer survival rate, between BRs and VFRs in the upper arch. In the lower arch, BRs reduced post-treatment mandibular labial segment alignment better than VFRs; however, there was greater failure rate with BRs. This randomized controlled trial was conducted by two operators and aimed to compare the clinical effectiveness of BRs and VFRs during the course of the initial 12 months of retention. Its prospective nature is in contrast to much of the early retrospective literature, which incorporates long-term follow-up along with the associated bias (13, 31–34). Within these studies, patients were treated by a number of operators with varying orthodontic appliances and treatment strategies, and using a variety of retainer designs (including the type of wire used, the number of teeth bonded, and the bonding agent). At the commencement of this trial the authors were aware of only one published trial within the literature, directly comparable to this trial (12, 22). This investigated the clinical effectiveness of mandibular BRs and VFRs and was published as a series of conference abstracts. Whilst the trend in relapse was similar, with the BR being more effective at retaining the labial segment alignment than the VFR, there was, however, a difference in the failure rates. McDermott reported greater failure of the VFR than the BR (62.5% and 15.6%, respectively), whilst this current trial found failure of the BR to be greater (VFR = 20%, BR = 50%). Since completion of the study, a further randomized controlled trial comparing BRs and VFRs has been published (35). In that study, BRs and VFRs were compared in the lower arch only. They found less relapse with the BR group after 6 months, but at 12 months and 18 months there was no difference between the two types of lower retainers. Interestingly, the amount of relapse was exceptionally low for both groups, which may be due to a low failure rate of the BRs and good adherence to wearing the VFRs. In this study, the BR failure rates for the maxilla and mandible were 36.7% and 50%, respectively, for the initial 12 months of retention. These figures may be regarded as high with failure rates of BRs constructed from multi-strand twistflex wire ranging from 7.8% (13) to 53% (10). The process of placing a BR is technique sensitive. The operators in this study were at the beginning of their orthodontic career and a general reduction in the survival rate has been associated with less experienced operators (36, 37). These factors may partially explain the higher failure rate in this study. An interesting finding of this research relates to the higher prevalence of BR failures occurring in the mandibular arch compared with the maxilla. This differs from many of the published trials, which demonstrate greater failure rates in the maxilla probably due to occlusal stress (10, 17, 20, 38). In fact, Dahl et al. found the maxillary failure rates to be almost twice as high as those found in the mandible (13, 38). The higher mandibular failure rate reported in this study was potentially influenced by the inexperience of the operators. This correlates with Scheibe’s retrospective analysis of 1062 retainers placed by operators of varying experience where the majority of mandibular failures were associated with less experienced operators (36). Published reports of BR survival tend to show most failures occurring within the initial 6 months of placement (9, 10, 16, 17, 39) and link these early failures to adhesive failure. This is similar to the findings in this study, which show that BR failures tend to occur early. The Kaplan–Meier plots within this study show failure of both maxillary and mandibular BRs occurring within the initial 6 months of retention. The majority of mandibular failures occurred at the enamel–composite interface, whereas, problems more commonly occurred at the composite–wire interface in the maxilla, perhaps in response to occlusal interference. The extent of relapse associated with VFRs within this current study is comparable to reports within the literature. Within our study, we noted similar failure rates between maxillary and mandibular VFRs and the majority of these were due to participants losing their retainers. In contrast, others have found retainer breakages to be the most common reason for failure, followed by lost retainers. Sun et al. instructed all participants to wear their retainers on a full-time basis, and perhaps this increased wear contributed to the higher breakage rates (40). Patient satisfaction There was no significant difference between patients’ opinions of wearing their retainer compared to their fixed appliances with both groups reporting that their retainers were better or at least no worse than their fixed appliances. This contrasts with the results of previous studies which have highlighted patients’ disappointment with retainers, noting that they found retainers very difficult to live with, more so than braces and headgear (25–27), mainly due to difficulties with speech, eating, increased salivation, smell, losing their retainers, and embarrassment over the appearance of the retainers (25, 26). Only 33% of subjects with VFRs reported that they always wore their retainers every night as instructed, though a further 37% stated they followed this advice most of the time. This finding is similar to Sawhney et al. who stated that self-reported compliance with removable retainers ranged between 75% and 85%, regardless of the regimen or type of retainer (41), and Wong and Freer who reported more than 50% of patients admitted that they did not wear retainers as instructed (24). Eighty-three percent of participants with VFRs and 100% with BRs stated that they were not embarrassed to wear their retainers, though VFRs were associated with more discomfort (37%) and difficulty speaking (47%) than BRs, though the relevance of difficult speaking with the retainer is questionable when VFRs are prescribed for night-time wear only. Jaderberg et al. also found similar problems with VFRs, albeit to a lesser extent, reporting soreness in 13%, and problems with speech in 22% of subjects (29). Similarly, Sawhney et al. found BRs affected speech less than both VFRs and Hawley retainers (41). Perhaps unsurprisingly, subjects in the VFR group found it easier to clean their retainers than those with BRs, supporting previous findings (41). Limitations In this study, it was not possible to blind the operators and patients to the treatment allocations. Although the minimum sample size was achieved, the power calculation was based on the primary outcome measure: the efficacy of each retainer group in minimising the 1-year post-orthodontic treatment change in arch alignment. As such, it is possible that the study is underpowered with respect to the secondary outcome measures, increasing the chance of making a type II error. The amount of VFR wear was not measured, so it is not clear whether the extra relapse in the mandible was due to the retainer itself, or a lack of patient compliance with the prescribed retention regimen. It could be argued that this is the ‘real-life’ situation. Post-treatment changes were measured in this study, rather than attempting to measure relapse towards the starting malocclusion. This is because it was felt that increases in incisor irregularity after treatment was more clinically relevant for patients. Post-treatment changes are unpredictable and do not always represent a move back towards the original malocclusion. This study therefore measures the ability of BRs and VFRs to reduce post-treatment changes in incisor irregularity, which may or may not reflect a relapse towards the presenting malocclusion. LII was chosen as the principle measurement of relapse as, anecdotally, incisor irregularity is often the patient’s main concern relating to post-orthodontic movement. This method is quick, simple, and widely used within research, therefore enabling direct comparison between studies. However, recognized limitations of this method include a disregard to spacing, or mutual rotations, where the contact point remained intact. Furthermore, the index is cumulative; therefore, a LII of 1.5 mm may have been awarded to a patient with evenly dispersed (but clinically insignificant) minor displacements, or equally to another patient with a single contact point displacement of the same value. It is likely that if the irregularity is limited to one larger displacement, then a patient will notice it more than if the same total amount of irregularity is evenly distributed across a number of teeth. A statistical difference in relapse of mandibular irregularity was found between the two groups. We considered whether to use a Bonferonni correction to allow for the possible affects of multiple testing, but in line with other similar studies, this was not used in this study for this outcome. This may be a statistical approach that could be considered in future research. Randomisation is undertaken to ensure equivalence between the two groups. It should be noted that there was statistical difference in the upper and lower inter-canine width at debond, although the differences were so small that it is unlikely to be clinically significant. The randomisation process did not enable exclusion of subjects where placement of the BR was deemed difficult, such as those with heavy, labial segment occlusal contacts. The retainer wire was contoured gingivally (where possible) to reduce, and hopefully avoid, traumatic occlusal contact. The influences of the incisor classification and overbite on failure could be investigated separately with a larger sample. All retainers were placed by orthodontic trainees. In an attempt to minimize the impact of operator inexperience, standardized positioning jigs were employed to aid placement of the retainers and reduce the potential for operator error and the risk of moisture contamination. In this study, the BRs were used in isolation to best assess the effects caused by them without the influence of an additional removable retainer. Some operators supplement a BR with a removable retainer in an attempt to maintain buccal segment alignment, retain space closed in extraction cases, and to act as a safeguard in case of breakage of the BR. A future study looking at this dual retention may be useful. Patient satisfaction is a complex area and is related to a number of different factors, which are not evenly weighted. The questionnaire used in the study was based on the most relevant questions in the patient satisfaction questionnaire from the Hichens et al. randomized controlled trial assessing patient satisfaction with Hawley and VFRs (28). To aid direct comparison of the data obtained with that from a previous study. Further research may be needed to devise a more appropriate and validated questionnaire for different types of retainers. This may involve the use of focus groups and patient interviews to identify specific concerns or problems. One of the challenges of orthodontic research is following up patients over a long period and this is particularly pertinent to research into retention. Due to the problem of large drop-outs in long-term orthodontic studies, this study only measured changes over 1 year. It is important to remember that the findings of this study only relate to 12 months of retention and it is possible that the findings may be different over a longer period. Generalisability The generalisability of these findings is limited to the retainer materials, design, and technique of placement, in addition to the operator experience. The operators were orthodontic trainees, therefore the clinical outcomes, in particular the survival rates, may be not be transferable to those with greater experience in the placement of BRs. Implications for clinical practice Within the initial 12 months of retention, BRs were more effective at retaining mandibular labial segment alignment than VFRs. However, there was a significantly higher failure rate relating to mandibular BRs, which had often progressed unnoticed by the patient. Therefore, it may be worthwhile considering whether a reduction of, on average, 0.92 mm of mandibular labial segment relapse warrants the additional clinical time spent during the supervision and amending of BR failures, particularly if the relapse is negligible once distributed across the full labial segment. If BRs are intended for the labial segment, it may be prudent to arrange more frequent recalls, particularly in the first 6 months after debond, to supervise BRs, especially if a less experienced operator had placed the BR. This would be beneficial not only in terms of minimising relapse, but also for the development of the operator’s awareness and technique. Prior to retainer placement, patients should also be counselled about the possibility of failure, and the importance of monitoring the integrity of the retainer. Within the maxilla, there was no evidence of a difference in relapse or retainer failure rates when comparing BRs and VFRs, so other outcomes may affect the choice of retainer. This will be explored further in Paper 2 Future research Future research should investigate the outcomes and implications of retainers over the longer term in terms of post-treatment changes, failure rates, adverse effects, and patient compliance. In addition, it would be useful to explore different types of BRs and VFRs. Conclusions After 12 months of retention, this randomized controlled trial comparing bonded with VFRs has shown that: - There is no evidence of a difference in post-treatment changes between patients who wear BRs or VFRs in the maxilla - The mandibular BR is more effective at reducing post-treatment changes in the labial segment alignment - There is no evidence of a difference in the survival rate of maxillary BRs and VFRs - BRs are more likely to fail than VFRs - Patients find VFRs easier to clean than BRs. However, patients report that BRs cause less discomfort and speech difficulties, and require less compliance Supplementary Material Supplemental data are available at the European Journal of Orthodontics online. Conflict of Interest None to declare. Acknowledgements We would like to acknowledge and thank Nadia Ahmed for her assistance during the early stages of the trial, and each of the laboratory technicians, reception staff, referring clinicians, and each patient and parent at the three hospitals. References 1. Littlewood , S.J. , Millett , D.T. , Doubleday , B. , Bearn , D.R. and Worthington , H.V . ( 2016 ) Retention procedures for stabilising tooth position after treatment with orthodontic braces . Cochrane Database of Systematic Reviews 2016, Issue 1. Art. No.: CD002283. DOI: 10.1002/14651858.CD002283.pub4. 2. Zachrisson , B.U . ( 2007 ) Long-term experience with direct-bonded retainers: update and clinical advice . Journal of Clinical Orthodontics , 41 , 728 . Google Scholar PubMed 3. Renkema , A.M. , Sips , E.T. , Bronkhorst , E. and Kuijpers-Jagtman , A.M . ( 2009 ) A survey on orthodontic retention procedures in The Netherlands . The European Journal of Orthodontics , 31 , 432 – 437 . 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( 1997 ) A 3-year follow-up study of various types of orthodontic canine-to-canine retainers . The European Journal of Orthodontics , 19 , 501 – 509 . Google Scholar CrossRef Search ADS PubMed 12. McDermott , P. , Field , D. , Erfida , I. and Millett , D.T . ( 2007 ) Operator and Patient Experiences With Fixed or Vacuum Formed Retainers . International Association of Dental Research , Cork , Vol. 17 . 13. Dahl , E.H. and Zachrisson , B.U . ( 1991 ) Long-term experience with direct-bonded lingual retainers . Journal of Clinical Orthodontics: JCO , 25 , 619 – 630 . Google Scholar PubMed 14. Bearn , D.R . ( 1995 ) Bonded orthodontic retainers: a review . American Journal of Orthodontics and Dentofacial Orthopedics , 108 , 207 – 213 . Google Scholar CrossRef Search ADS PubMed 15. Bearn , D.R. , McCabe , J.F. , Gordon , P.H. and Aird , J.C . ( 1997 ) Bonded orthodontic retainers: the wire-composite interface . American Journal of Orthodontics and Dentofacial Orthopedics , 111 , 67 – 74 . Google Scholar CrossRef Search ADS PubMed 16. Foek , D.L.S. , Ozcan , M. , Verkerke , G.J. , Sandham , A. and Dijkstra , P.U . ( 2008 ) Survival of flexible, braided, bonded stainless steel lingual retainers: a historic cohort study . The European Journal of Orthodontics , 30 , 199 – 204 . Google Scholar CrossRef Search ADS PubMed 17. Saidler , G. , McColl J. , and Stat , C . ( 1999 ) Breakage incidence with direct bonded lingual retainers . British Journal of Orthodontics , 26 , 191 – 194 . Google Scholar CrossRef Search ADS PubMed 18. Renkema , A.-M. , Renkema , A. , Bronkhorst , E. and Katsaros , C . ( 2011 ) Long-term effectiveness of canine-to-canine bonded flexible spiral wire lingual retainers . American Journal of Orthodontics and Dentofacial Orthopedics , 139 , 614 – 621 . Google Scholar CrossRef Search ADS PubMed 19. Abudiak , H. , Shelton , A. , Spencer , R.J. , Burns , L. and Littlewood , S.J . ( 2011 ) A complication with orthodontic fixed retainers: A case report . 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Australian Orthodontic Journal , 21 , 45 – 53 . Google Scholar PubMed 25. Bennett , M.E. and Tulloch , J . ( 1999 ) Understanding orthodontic treatment satisfaction from the patients’ perspective: a qualitative approach . Clinical Orthodontics and Research , 2 , 53 – 61 . Google Scholar PubMed 26. Travess , H.C. , Newton , J.T. , Sandy , J.R. and Williams , A.C . ( 2004 ) The development of a patient-centered measure of the process and outcome of combined orthodontic and orthognathic treatment . Journal of Orthodontics , 31 , 220 – 234 . Google Scholar CrossRef Search ADS PubMed 27. Vig , K.W. , Weyant , R. , O’Brien , K. and Bennett , E . ( 1999 ) Developing outcome measures in orthodontics that reflect patient and provider values . Seminars in Orthodontics , 5 , 85 – 95 . 28. Hichens , L. , Rowland , H. , Williams , A. , Hollinghurst , S. , Ewings , P. , Clark , S. , Ireland , A. and Sandy J . ( 2007 ) Cost-effectiveness and patient satisfaction: Hawley and vacuum-formed retainers . European Journal of Orthodontics , 29 , 372 – 378 . Google Scholar CrossRef Search ADS PubMed 29. Jäderberg , S. , Feldmann I. and Engström , C . ( 2012 ) Removable thermoplastic appliances as orthodontic retainers—a prospective study of different wear regimens . The European Journal of Orthodontics , 34 , 475 – 479 . Google Scholar CrossRef Search ADS PubMed 30. Little , R.M . ( 1975 ) The irregularity index: a quantitative score of mandibular anterior alignment . American Journal of Orthodontics , 68 , 554 – 563 . Google Scholar CrossRef Search ADS PubMed 31. Zachrisson , B.U . ( 1977 ) Clinical experience with direct-bonded orthodontic retainers . American Journal of Orthodontics , 71 , 440 – 448 . Google Scholar CrossRef Search ADS PubMed 32. Little , R.M. , Wallen , T.R. and Riedel , R.A . ( 1981 ) Stability and relapse of mandibular anterior alignment—first premolar extraction cases treated by traditional edgewise orthodontics . American Journal of Orthodontics , 80 , 349 – 365 . Google Scholar CrossRef Search ADS PubMed 33. Little , R.M. , Riedel , R.A. and Artun , J . ( 1988 ) An evaluation of changes in mandibular anterior alignment from 10 to 20 years postretention . American Journal of Orthodontics and Dentofacial Orthopedics , 93 , 423 – 428 . Google Scholar CrossRef Search ADS PubMed 34. Little , R . ( 2009 ) Clinical implications of the University of Washington post-retention studies . Journal of Clinical Orthodontics: JCO , 43 , 645 . Google Scholar PubMed 35. O’Rourke , N. , Albeedh , H. , Sharma , P. and Johal , A . ( 2016 ) Effectiveness of bonded and vacuum-formed retainers: a prospective randomized controlled clinical trial . American Journal of Orthodontics and Dentofacial Orthopedics , 150 , 406 – 415 . Google Scholar CrossRef Search ADS PubMed 36. Scheibe , K. and Ruf , S . ( 2010 ) Lower bonded retainers: survival and failure rates particularly considering operator experience . Journal of Orofacial Orthopedics/Fortschritte der Kieferorthopädie , 71 , 300 – 307 . Google Scholar CrossRef Search ADS 37. Schneider , E. and Ruf , S . ( 2011 ) Upper bonded retainers: survival and failure rates . The Angle Orthodontist , 81 , 1050 – 1056 . Google Scholar CrossRef Search ADS PubMed 38. Segner , D. and Heinrici , B . ( 2000 ) Bonded retainers–clinical reliability . Journal of Orofacial Orthopedics/Fortschritte der Kieferorthopädie , 61 , 352 – 358 . Google Scholar CrossRef Search ADS 39. Taner , T. and Aksu , M . ( 2012 ) A prospective clinical evaluation of mandibular lingual retainer survival . The European Journal of Orthodontics , 34 , 470 – 474 . Google Scholar CrossRef Search ADS PubMed 40. Sun , J. , Yu , Y.C. , Liu , M.Y. , Chen , L. , Li , H.W. , Zhang , L. , Zhou , Y. , Ao , D. , Tao , R. and Lai , W.L . ( 2011 ) Survival time comparison between Hawley and clear overlay retainers: a randomized trial . Journal of Dental Research , 90 , 1197 – 1201 . Google Scholar CrossRef Search ADS PubMed 41. Sawhney , B . ( 2013 ) Orthodontic retainers: a survey of patient compliance and satisfaction . Electronic Thesis and Dissertation Repository , The University of Western Ontario, Citeseer . © The Author 2017. Published by Oxford University Press on behalf of the European Orthodontic Society. All rights reserved. For permissions, please email: journals.permissions@oup.com http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png The European Journal of Orthodontics Oxford University Press

Bonded versus vacuum-formed retainers: a randomized controlled trial. Part 1: stability, retainer survival, and patient satisfaction outcomes after 12 months

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© The Author 2017. Published by Oxford University Press on behalf of the European Orthodontic Society. All rights reserved. For permissions, please email: journals.permissions@oup.com
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Abstract

Summary Background There is a shortage of evidence on the best type of retainer. Objectives Evaluate upper and lower bonded retainers (BRs) versus upper and lower vacuum-formed retainers (VFRs) over 12 months, in terms of stability, retainer survival, and patient satisfaction. Trial design Two-arm parallel group multi-centre randomized controlled clinical trial. Methods Sixty consecutive patients completing fixed appliance therapy and requiring retainers were recruited from 3 hospital departments. They were randomly allocated to either upper and lower labial segment BRs (n = 30) or upper and lower full-arch VFRs (n = 30). Primary outcome was stability. Secondary outcomes were retainer survival and patient satisfaction. A random sequence of treatment allocation was computer-generated and implemented by sealing in sequentially numbered opaque sealed envelopes independently prepared in advance. Patients, operators and outcome could not be blinded due to the nature of the intervention. Results Thirty patients received BRs (median [Mdn] age 16 years, inter-quartile range [IQR] = 2) and 30 received VFRs (Mdn age 17 years, IQR = 4). Baseline characteristics were similar between groups. At 12 months, there were no statistically significant inter-group differences in post-treatment change of maxillary labial segment alignment (BR = 1.1 mm, IQR = 1.56, VFR = 0.76 mm, IQR = 1.55, P = 0.61); however, there was greater post-treatment change in the mandibular VFR group (BR = 0.77 mm, IQR = 1.46, VFR = 1.69mm, IQR = 2.00, P = 0.008). The difference in maxillary retainer survival rates were statistically non-significant, P = 0.34 (BR = 63.6%, 239.3 days, 95% confidence interval [CI] = 191.1–287.5, VFR = 73.3%, 311.1 days, 95% CI = 278.3–344.29). The mandibular BR had a lower survival rate (P = 0.01) at 12 months (BR = 50%, 239.3 days 95% CI = 191.1–287.5, VFR = 80%, 324.9 days 95% CI = 295.4–354.4). More subjects with VFRs reported discomfort (P = 0.002) and speech difficulties (P = 0.004) but found them easier to clean than those with BRs (P = 0.001). Limitations Results are after 1 year and we do not know how much the removable retainers were worn. Conclusions After 1 year, there is no evidence of a significant difference in stability or retainer survival in the maxilla. In the mandible, BRs are more effective at maintaining mandibular labial segment alignment, but have a higher failure rate. In comparison with patients wearing VFRs, patients wearing BRs reported that they caused less interference with speech, required less compliance to wear them, and were more comfortable to wear than VFRs. Patients found the BRs harder to keep clean. Trial registration The trail was not registered. Introduction Retention is arguably the most challenging and unpredictable stage of orthodontic treatment and the most clinically effective form of retention remains unclear (1). In the UK, there has been a significant increase in the use of vacuum-formed retainers (VFRs) and bonded retainers (BRs). Ideally, the choice of retainer appropriate for each individual patient, whether fixed or removable, should be made considering not only the clinical effectiveness, but also the patient’s ability to, and willingness to comply with, the retention plan (2, 3).VFRs have been shown to be more clinically effective at maintaining the lower labial segment alignment than Hawley retainers in the initial 6 months of retention (4). The retentive capacity of the VFR appears to be equally effective when worn full-time or at night only, during the initial 6–12 months (5, 6). Furthermore, VFRs have been shown to be preferred by both clinicians and patients owing to their comfort, ease of fabrication and fit, reduced failure rate, and cost-effectiveness (7). BRs are commonly used as a means of permanent retention usually in the form of a wire attached to the lingual or palatal surfaces of the teeth within the labial segment (3, 8). They are particularly useful in cases of perceived instability, or high patient expectation. There are many different types of BR. The most common is fabricated from multi-strand twistflex wire, which is bonded to each of the labial segment teeth and is associated with a superior retentive capacity compared to the single-stranded wire bonded to the canines only (9–11). One of the clear advantages of BRs is that they are less reliant on patient compliance. However, notable disadvantages exist, leading to reports of reduced popularity amongst some orthodontists (12). Placement is time-consuming and technique sensitive (13), failure has been reported to be as high as 53% (10)), and bonding failures may progress unnoticed by the patient, leading to localized relapse, and the risk of caries or demineralisation (14). Wire detachment at the enamel–composite interface appears to be the most common reason for failure (14–16), particularly within the early stages of retention (17), with the canine region being the most common site for detachment failure (13, 18). Fracture of the wire is reportedly the least common failure type, and predominantly occurs mesial to the maxillary canines, probably due to occlusal trauma (13, 17). Post-treatment changes may even occur with BRs in situ (19). Spacing and rotation of teeth still firmly attached to BRs has also been reported when BRs have remained in place for several years and despite not breaking (20, 21), as has fraying of the multi-strand wire which can cause irritation to the tissues (21). At the time of starting this trial, the authors were aware of only one high-quality published trial directly compared the VFR to the BR. This prospective randomized controlled trial was conducted over a 12-month period and the outcomes were published in the form of conference abstracts (12, 22). More than three times as many problems, such as loss or fracture of the retainers, were noted in the VFR group. Labial segment alignment was statistically worse in the VFR group at 12 months and the authors related this to patient’s suboptimal compliance with removable retainers. There has been a growing emphasis on the importance of using qualitative research methods to gather data of patients’ views. As a result, focus groups, in-depth interviews, and questionnaires are increasingly used in research and audit to assess patient satisfaction (23). Several studies have investigated the attitudes and preferences of orthodontists towards various retention protocols but few have reported on the perceptions of patients (7, 24). The few studies which have explored patient satisfaction with orthodontic treatment have highlighted a nearly universal dislike of orthodontic retainers. Some find them to be more inconvenient than their fixed appliances and even headgear (25–27). The reasons cited include difficulty speaking and eating, extra salivation, smell, embarrassment, and the ease with which they could be lost (25, 26). A remarkably high degree of non-compliance was also noted, with most patients reporting little or no attention to dietary restrictions or flossing regimens (27). Hichens et al. compared levels of patient satisfaction of subjects with Hawley and VFRs during the first 6 months of the retention period. The results indicated that the majority of subjects showed a preference for VFRs compared with Hawley retainers. Wearing VFRs caused less embarrassment to subjects, especially in terms of speech and appearance, and broke less frequently than Hawley retainers. Interestingly, although more subjects in the VFR group wore their retainers as instructed compared with the Hawley group, there was no difference in complaints of discomfort between subjects in either group (28). Jaderberg et al. evaluated patient experiences of subjects wearing VFRs either full-time for 3 months followed by nights-only wear compared with 1 week full-time wear followed by nights-only wear, for the first 6 months of the retention period. Overall, the results of the questionnaire showed that the VFRs were well tolerated, easy to get used to, and most patients reported that they had no difficulties in remembering to wear the retainers at night. Soreness was reported in 13%, 22% had problems with speech, and 10% thought that they tasted bad (29). A recently updated Cochrane review looking at orthodontic retention emphasized the need for future research in this field to incorporate outcomes of stability, periodontal health, survival of retainers, and quality of life (1). Further well-designed prospective randomized controlled trials would therefore be desirable to strengthen the evidence comparing the most commonly used retainer types—VFRs and BRs. This is the first of two papers comparing upper and lower VFRs with upper and lower BRs. This paper aims to test the null hypothesis that there are no significant differences in clinical effectiveness between the two retainer types. The primary outcome is stability and the secondary outcomes are retainer survival, patient satisfaction, and affect on periodontal health. The second paper will report on periodontal outcomes. Methods Trial design and ethics This was a 2-arm parallel group randomized controlled clinical trial with a 1:1 allocation ratio. The study was approved by the National Research Ethics Service in December 2010 (REC number 10/H1306/79), and also the local ethical and research development departments of the three hospital trusts involved. Participants, eligibility criteria, and settings Consecutive patients nearing the completion of fixed appliance therapy who required retainers were invited to take part in the trial. Participants were recruited from the orthodontic departments of two district general hospitals (St Luke’s Hospital, Bradford and York Hospital) and one teaching hospital (Leeds Dental Institute) in the UK. The retainers were placed and reviewed by two clinicians (KF and MS), both of whom were Speciality Registrars in Orthodontics in the early stages of their training. Treatment was provided for free to patients under the UK’s National Health Service. The following inclusion criteria were applied: Completion of a course of fixed appliance therapy (involving both dental arches) with a satisfactory correction of the presenting malocclusion and dental alignment Good general health Demonstration of a good standard of oral hygiene (determined through questioning and clinical examination) Subjects had a full and normal complement of teeth in the upper and lower labial segments Teeth within the labial segments to be a regular size and shape Subjects willing to consent to the trial and comply with the trial regime Subjects may have presented with any malocclusion prior to orthodontic treatment and may have been managed on an extraction (premolar or molar) or a non-extraction basis. Subjects may have been treated with removable or functional appliances in conjunction with their orthodontic treatment, or may have had adjunctive surgery. The following exclusion criteria were applied: Any general medical health problems which may influence gingival health, such as those necessitating antibiotic cover as bacteraemia prophylaxis, diabetes mellitus, epilepsy, or physical or mental disability Poor periodontal health, including the presence of supra-gingival calculus or periodontal pocketing greater than 3 mm Periodontal problems with either a pending referral to the periodontal department or a history of periodontal management during the orthodontic treatment Gross or uncontrolled caries Absent or diminutive lateral incisors A starting malocclusion requiring extreme transverse correction (involving rapid maxillary expansion or surgically assisted rapid maxillary expansion) Cleft palate or severe facial deformities Interventions At the end of active treatment, two calibrated operators placed either upper and lower BRs or upper and lower VFRs using standardized procedures and materials. All patients had been seen for supra-gingival debridement prior to removal of the fixed appliances. Upper and lower BRs (Figure 1) were prepared using 0.0195 in (0.45 mm) 3-stranded twistflex stainless steel wire (Wildcat; GAC International, Bohemia, New York, USA). The wire was shaped by dental technicians against the dental casts to lie passively against the lingual surfaces of the upper and lower incisors and canines. The wire was not contoured interproximally. A silicone positioning jig was prepared to aid positioning of the retainer. This was moulded to engage the wire and rest on the central incisors. The wire was bonded using a low viscosity light-cured composite (Transbond™ LV; 3M Unitek, Monrovia, California, USA), following the separate application of etchant (37% phosphoric acid), and primer (Transbond™ XT adhesive primer; 3M Unitek). Care was taken not to leave any bonding substance in contact with the gingival tissues. Figure 1. View largeDownload slide Bonded retainers with silicone positioning jig. Figure 1. View largeDownload slide Bonded retainers with silicone positioning jig. Upper and lower VFRs (Essix™ C+; Figure 2) were constructed using the ‘Essix™’ machine and cooled rapidly using Arctic spray (Ortho-Care). Retainers were trimmed to cover all fully erupted teeth and extend half way across the occlusal surface of the most distal molar. Participants were instructed to wear their retainers only at night, every night. At the time of retainer insertion, both written and verbal oral hygiene instructions were given, including methods for interdental cleaning around BRs. Participants were advised to continue 6 monthly review appointments with their general dentist. Figure 2. View largeDownload slide Vacuum-formed retainers. Figure 2. View largeDownload slide Vacuum-formed retainers. Patients were advised to contact the department as soon as possible if they had any queries, concerns, or problems. Failed retainers were repaired on the same day adhering to the standardized protocol for materials, construction, and technique of placement. Outcome measures The primary outcome was stability. Secondary outcomes were: Retainer survival Patient satisfaction Periodontal health (reported in Paper 2) Stability The primary outcome was to evaluate the clinical effectiveness of both retainer types in preventing post-treatment changes over the initial 12-months of retention. Post-treatment changes were defined as change in labial segment alignment. This was measured from digitized study models (Figure 3) using Little’s Irregularity Index (LII). In addition, changes in arch dimensions, occlusal relationships, and re-opening of extraction spaces were also recorded. Table 1 provides details of these measurements. Figure 3. View largeDownload slide Demonstration of the mandibular arch dimensions (ICW, IMW, AL). Figure 3. View largeDownload slide Demonstration of the mandibular arch dimensions (ICW, IMW, AL). Table 1. Single-arch and occlusal measurements recorded at each milestone. Measurement Unit Description Little’s irregularity index of the upper and lower labial segments (LII) mm The sum of the five labial segment anatomical contact point displacements in a labiolingual direction Upper and lower inter-canine width (ICW) mm Distance between the cusp tips of right and left canines Upper and lower inter-molar width (IMW) mm Distance between the mesiobuccal cusp tip of the right first molar to the mesiobuccal cusp tip of the left first permanent molar. Where the first permanent molar had been extracted, the second permanent molar was used instead. Upper and lower arch length (AL) mm The sum of the right and left distances from the mesiobuccal cusp tip of the first permanent molars to the interproximal contact point of the central incisors Overjet (OJ) mm The maximum distance between the upper incisor edge and the lower incisal labial surface, horizontal to the occlusal plane Overbite (OB) mm The maximum vertical overlap between the upper and lower incisors with the models in maximal intercuspation. Measurement Unit Description Little’s irregularity index of the upper and lower labial segments (LII) mm The sum of the five labial segment anatomical contact point displacements in a labiolingual direction Upper and lower inter-canine width (ICW) mm Distance between the cusp tips of right and left canines Upper and lower inter-molar width (IMW) mm Distance between the mesiobuccal cusp tip of the right first molar to the mesiobuccal cusp tip of the left first permanent molar. Where the first permanent molar had been extracted, the second permanent molar was used instead. Upper and lower arch length (AL) mm The sum of the right and left distances from the mesiobuccal cusp tip of the first permanent molars to the interproximal contact point of the central incisors Overjet (OJ) mm The maximum distance between the upper incisor edge and the lower incisal labial surface, horizontal to the occlusal plane Overbite (OB) mm The maximum vertical overlap between the upper and lower incisors with the models in maximal intercuspation. View Large Table 1. Single-arch and occlusal measurements recorded at each milestone. Measurement Unit Description Little’s irregularity index of the upper and lower labial segments (LII) mm The sum of the five labial segment anatomical contact point displacements in a labiolingual direction Upper and lower inter-canine width (ICW) mm Distance between the cusp tips of right and left canines Upper and lower inter-molar width (IMW) mm Distance between the mesiobuccal cusp tip of the right first molar to the mesiobuccal cusp tip of the left first permanent molar. Where the first permanent molar had been extracted, the second permanent molar was used instead. Upper and lower arch length (AL) mm The sum of the right and left distances from the mesiobuccal cusp tip of the first permanent molars to the interproximal contact point of the central incisors Overjet (OJ) mm The maximum distance between the upper incisor edge and the lower incisal labial surface, horizontal to the occlusal plane Overbite (OB) mm The maximum vertical overlap between the upper and lower incisors with the models in maximal intercuspation. Measurement Unit Description Little’s irregularity index of the upper and lower labial segments (LII) mm The sum of the five labial segment anatomical contact point displacements in a labiolingual direction Upper and lower inter-canine width (ICW) mm Distance between the cusp tips of right and left canines Upper and lower inter-molar width (IMW) mm Distance between the mesiobuccal cusp tip of the right first molar to the mesiobuccal cusp tip of the left first permanent molar. Where the first permanent molar had been extracted, the second permanent molar was used instead. Upper and lower arch length (AL) mm The sum of the right and left distances from the mesiobuccal cusp tip of the first permanent molars to the interproximal contact point of the central incisors Overjet (OJ) mm The maximum distance between the upper incisor edge and the lower incisal labial surface, horizontal to the occlusal plane Overbite (OB) mm The maximum vertical overlap between the upper and lower incisors with the models in maximal intercuspation. View Large Alginate impressions were obtained at 4 time-points during the trial (T0–T3) and were measured by the same examiner (KF) to an accuracy of 0.01 mm. T0Debond T13 months post-debond T26 months post-debond T312 months post-debond Survival Retainer survival was recorded as the time to the first episode of failure. The date of failure was recorded as the day the patient became aware of the problem, or alternatively, the date the clinician noted the failure when participants were unaware of a failed retainer. The pattern of failure was also recorded: BRs could fail at: the wire–composite interface; the enamel–composite interface; by wire fracture; or by complete detachment from all teeth. VFRs were categorized as being lost, ill-fitting, or fractured. The occlusal wear was also noted, particularly if there was a breach in the full thickness of the retainer necessitating replacement. Patient satisfaction Patient satisfaction (secondary outcome measure) was determined by a questionnaire based on the most relevant questions in the patient satisfaction questionnaire from the Hichens et al. RCT assessing patient satisfaction with Hawley and VFRs (28). The main issues identified from their study were used to form the basis of this questionnaire (i.e. it was a modified version of the previously used questionnaire). A preliminary version of this questionnaire was piloted on a group of 10 patients attending the orthodontic department for retainer reviews. These patients had agreed to be involved in the pilot study and were not otherwise participating in the study. Any necessary modifications to ensure good readability and reproducibility were made before using the questionnaires in the main study. The decision was made to administer the questionnaire to the participants at their scheduled review appointments, rather than sending the questionnaire by post or e-mail to complete at home. It was hoped that this would ensure that the questionnaires were completed fully, increasing the response rate, and to allow the investigator to clarify any ambiguous questions. The data from the questionnaires was coded and assessed by one operator (MS). Sample size calculation The sample size was determined based on the primary objective of comparing the efficacy of each retainer group in minimising the 1-year post-orthodontic treatment change in the anterior arch alignment as measured by LII (30). Assuming a clinically relevant difference of 0.5 mm between the two randomized groups, a common standard deviation of 0.5 mm, a power of 90%, and a significance level of 5%, the study required 22 subjects in each group. To account for a potential dropout rate of 20% and to increase the sample size for the secondary outcome measures, the sample size was increased to 30 per group resulting in a total of 60 subjects. Randomisation Consecutive participants eligible for inclusion were approached by KF or MS and written informed consent was obtained from the participant (and parent if the participants were adolescents). Having consented to taking part in the study, the participant was allocated randomly to receive either upper and lower BRs or upper and lower VFRs using sequentially numbered, opaque, sealed envelopes, prepared in advance of the trial by a colleague independent of the study, using a computerized randomisation programme. Blinding It was not possible to blind the patient or clinician/operator in this study following opening of the envelope due to the nature of the intervention. Blinding of the outcome assessor looking at stability on the digital models was also not possible, but blinding of the patient satisfaction assessor was undertaken by removing patient-identifying information from the completed questionnaires. Patients leaving the study or refusing treatment To account for potential attrition bias, an ‘intention-to-treat’ analysis was employed for those patients who dropped-out of the study. Similarly, if a subject failed to co-operate with the retention regime, the data were still collected and an ‘intention-to-treat’ analysis was carried out. Average values for that particular retainer group were substituted for the missing data. Subsequent analyses were based on the new complete data set. The retainer survival time for subjects who had dropped-out at 12 months was based on the 6-month review data where the retainer had not failed previously. Statistical methods The SPSS software package was used for data analysis (version 20; SPSS, Chicago, Illinois, USA) with statistical significance set at the 5% level. The intra-class correlation coefficient (ICC) was used to assess the reliability of measurements made by the same operator (KF). Twenty randomly selected digital models were benchmarked against the ‘gold standard’ of digital callipers and plaster models. Results ranged from 0.882 (95% confidence interval [CI] = 0.728–0.951) for the arch length to 0.960 (95% CI = 0.805–0.987) for the inter-molar width. Excellent agreement was associated with LII (ICC = 0.951, 95% CI = 0.882–0.980). A further 20 randomly selected digital models were measured on two separate occasions. Excellent repeatability was demonstrated for the LII (ICC = 0.921, 95% CI = 0.841–0.961). Occlusal measurements, and the relapse data were found to be non-normally distributed, therefore non-parametric analyses were employed, using the median (Mdn) and inter-quartile range (IQR). Mann–Whitney U-tests were used to compare relapse values between retainer groups. Retainer survival was analysed through the Kaplan–Meier survival plot and the log–rank test. The patient satisfaction questionnaires were coded by the research team and all responses entered into a database. A descriptive analysis was used to compare the levels of satisfaction between VFRs and BRs, whilst Chi-squared tests were used for trend and significance. Results Participant flow and recruitment details The CONSORT flow diagram is shown in Figure 4. Figure 4. View largeDownload slide CONSORT flow diagram. Figure 4. View largeDownload slide CONSORT flow diagram. Of the 104 patients who were initially informed about the study, 44 were excluded as they either declined to be involved or did not satisfy the inclusion criteria. Of the 60 patients enrolled, 30 were provided with BRs (15 male, 15 female, Mdn age 16 years, IQR = 2) and 30 were provided with VFRs (12 male, 18 female, Mdn age 17 years, IQR = 4). All patients were recruited between March 2012 and September 2013. It was not possible to conduct a 3-month review for eight participants (BR = 6, VFR = 2) due to staff shortages in one of the hospitals. One participant (BR = 1) did not attend their 6-month review and 3 (VFR = 3) failed to attend their 12-month review. Furthermore, four sets of study models taken at the 3-month review were irretrievable (VFR = 2, BR = 2), and one participant attending the 12-month review declined impressions. Baseline data Baseline data were collected prior to the start of orthodontic treatment and were found to be similar in both groups (Table 2). Any changes in arch dimensions that occurred during active orthodontic treatment were not found to be statistically different between the retainer groups. Table 2. Baseline characteristics at T0 and treatment summary of the sample. Bonded retainer (N = 30) Vacuum-formed retainer (N = 30) Sex  Male 15 12  Female 15 18 Age (years)  Median (IQR) 16 (2) 17 (4) Study centre  Leeds 8 9  Bradford 16 14  York 6 7 Initial incisor classification  class I 5 4  class II Division 1 13 14  class II Division 2 3 8  class III 9 4 Initial skeletal classification  Class 1 7 6  Class 2 14 20  Class 3 9 4 Extractions  Non—extraction 11 8  Maxillary extractions only 10 9  Mandibular extractions only 2 3  Bimaxillary extractions 7 10 Treatment modality  Fixed appliances only 16 21  URA and fixed appliances 3 2  Functional and fixed appliances 10 4  Orthognathic surgery and fixed appliances 1 3 Bonded retainer (N = 30) Vacuum-formed retainer (N = 30) Sex  Male 15 12  Female 15 18 Age (years)  Median (IQR) 16 (2) 17 (4) Study centre  Leeds 8 9  Bradford 16 14  York 6 7 Initial incisor classification  class I 5 4  class II Division 1 13 14  class II Division 2 3 8  class III 9 4 Initial skeletal classification  Class 1 7 6  Class 2 14 20  Class 3 9 4 Extractions  Non—extraction 11 8  Maxillary extractions only 10 9  Mandibular extractions only 2 3  Bimaxillary extractions 7 10 Treatment modality  Fixed appliances only 16 21  URA and fixed appliances 3 2  Functional and fixed appliances 10 4  Orthognathic surgery and fixed appliances 1 3 View Large Table 2. Baseline characteristics at T0 and treatment summary of the sample. Bonded retainer (N = 30) Vacuum-formed retainer (N = 30) Sex  Male 15 12  Female 15 18 Age (years)  Median (IQR) 16 (2) 17 (4) Study centre  Leeds 8 9  Bradford 16 14  York 6 7 Initial incisor classification  class I 5 4  class II Division 1 13 14  class II Division 2 3 8  class III 9 4 Initial skeletal classification  Class 1 7 6  Class 2 14 20  Class 3 9 4 Extractions  Non—extraction 11 8  Maxillary extractions only 10 9  Mandibular extractions only 2 3  Bimaxillary extractions 7 10 Treatment modality  Fixed appliances only 16 21  URA and fixed appliances 3 2  Functional and fixed appliances 10 4  Orthognathic surgery and fixed appliances 1 3 Bonded retainer (N = 30) Vacuum-formed retainer (N = 30) Sex  Male 15 12  Female 15 18 Age (years)  Median (IQR) 16 (2) 17 (4) Study centre  Leeds 8 9  Bradford 16 14  York 6 7 Initial incisor classification  class I 5 4  class II Division 1 13 14  class II Division 2 3 8  class III 9 4 Initial skeletal classification  Class 1 7 6  Class 2 14 20  Class 3 9 4 Extractions  Non—extraction 11 8  Maxillary extractions only 10 9  Mandibular extractions only 2 3  Bimaxillary extractions 7 10 Treatment modality  Fixed appliances only 16 21  URA and fixed appliances 3 2  Functional and fixed appliances 10 4  Orthognathic surgery and fixed appliances 1 3 View Large Primary outcome: stability Occlusal measurements for each retainer were measured at debond (T0), 3 months (T1), 6 months (T2), and 1 year (T3; Table 3). Post-treatment changes are shown in Table 4. Table 3. Occlusal measurements (mm) at each time point using intention-to-treat analysis. Data are presented in the form of median (inter-quartile range) and Mann–Whitney P value due to its non-normal distribution. AL, arch length; BR, bonded retainer; ICW, inter-canine width; IMW, inter-molar width; LII, Little’s irregularity index; OB, overbite; OJ, overjet; VFR, vacuum-formed retainer. Bold indicates statistical significance P < 0.05. Occlusal measurements at each review Maxilla Mandible Inter-arch LII ICW IMW AL LII ICW IMW AL OJ OB T0 Debond BR 0.00 (0.93) 35.20 (2.83) 50.11 (3.96) 73.94 (12.74) 0.29 (1.02) 27.53 (1.68) 44.05 (4.64) 66.74 (6.00) 2.37 (0.70) 1.29 (1.22) VFR 0.23 (0.66) 34.09 (2.22) 48.46 (4.24) 71.23 (9.67) 0.06 (1.23) 26.17 (1.13) 41.34 (5.72) 65.53 (12.94) 2.38 (2.40) 2.00 (1.21) P value 0.31 0.003 0.01 0.101 0.382 0.003 0.071 0.188 0.203 0.083 T1 3 months BR 0.79 (0.91) 35.08 (1.92) 49.63 (2.92) 72.75 (10.57) 0.44 (0.63) 27.45 (2.11) 43.65 (3.67) 66.81 (8.68) 2.31 (1.04) 1.45 (0.71) VFR 0.30 (0.98) 33.69 (2.52) 47.90 (3.88) 69.81 (11.46) 0.44 (1.71) 25.69 (1.45) 41.70 (5.35) 63.43 (9.94) 2.62 (0.76) 1.87 (0.87) P value 0.05 0.001 0.018 0.147 0.432 0.000 0.055 0.104 0.064 0.001 T2 6 months BR 0.88 (1.07) 35.36 (2.41) 49.74 (3.46) 72.88 (10.35) 0.67 (1.47) 28.06 (1.65) 44.23 (2.34) 67.06 (8.83) 2.53 (1.09) 1.52 (0.92) VFR 0.64 (1.77) 34.21 (2.06) 48.84 (3.85) 70.19 (9.62) 1.30 (1.98) 26.26 (1.65) 41.84 (6.22) 65.17 (11.42) 2.74 (0.50) 1.92 (0.97) P value 0.22 0.000 0.015 0.051 0.335 0.002 0.174 0.055 0.181 0.036 T3 12 months BR 1.35 (1.98) 35.08 (2.31) 49.47 (3.88) 76.70 (10.81) 1.01 (1.28) 27.31 (2.21) 43.90 (4.32) 66.97 (8.21) 2.26 (1.07) 1.59 (0.78) VFR 0.97 (1.68) 33.21 (2.36) 47.70 (3.80) 68.86 (10.26) 1.73 (2.77) 25.56 (1.39) 41.32 (4.61) 62.57 (9.50) 2.59 (0.94) 2.01 (1.00) P value 0.24 0.001 0.064 0.117 0.053 0.001 0.068 0.071 0.128 0.022 Occlusal measurements at each review Maxilla Mandible Inter-arch LII ICW IMW AL LII ICW IMW AL OJ OB T0 Debond BR 0.00 (0.93) 35.20 (2.83) 50.11 (3.96) 73.94 (12.74) 0.29 (1.02) 27.53 (1.68) 44.05 (4.64) 66.74 (6.00) 2.37 (0.70) 1.29 (1.22) VFR 0.23 (0.66) 34.09 (2.22) 48.46 (4.24) 71.23 (9.67) 0.06 (1.23) 26.17 (1.13) 41.34 (5.72) 65.53 (12.94) 2.38 (2.40) 2.00 (1.21) P value 0.31 0.003 0.01 0.101 0.382 0.003 0.071 0.188 0.203 0.083 T1 3 months BR 0.79 (0.91) 35.08 (1.92) 49.63 (2.92) 72.75 (10.57) 0.44 (0.63) 27.45 (2.11) 43.65 (3.67) 66.81 (8.68) 2.31 (1.04) 1.45 (0.71) VFR 0.30 (0.98) 33.69 (2.52) 47.90 (3.88) 69.81 (11.46) 0.44 (1.71) 25.69 (1.45) 41.70 (5.35) 63.43 (9.94) 2.62 (0.76) 1.87 (0.87) P value 0.05 0.001 0.018 0.147 0.432 0.000 0.055 0.104 0.064 0.001 T2 6 months BR 0.88 (1.07) 35.36 (2.41) 49.74 (3.46) 72.88 (10.35) 0.67 (1.47) 28.06 (1.65) 44.23 (2.34) 67.06 (8.83) 2.53 (1.09) 1.52 (0.92) VFR 0.64 (1.77) 34.21 (2.06) 48.84 (3.85) 70.19 (9.62) 1.30 (1.98) 26.26 (1.65) 41.84 (6.22) 65.17 (11.42) 2.74 (0.50) 1.92 (0.97) P value 0.22 0.000 0.015 0.051 0.335 0.002 0.174 0.055 0.181 0.036 T3 12 months BR 1.35 (1.98) 35.08 (2.31) 49.47 (3.88) 76.70 (10.81) 1.01 (1.28) 27.31 (2.21) 43.90 (4.32) 66.97 (8.21) 2.26 (1.07) 1.59 (0.78) VFR 0.97 (1.68) 33.21 (2.36) 47.70 (3.80) 68.86 (10.26) 1.73 (2.77) 25.56 (1.39) 41.32 (4.61) 62.57 (9.50) 2.59 (0.94) 2.01 (1.00) P value 0.24 0.001 0.064 0.117 0.053 0.001 0.068 0.071 0.128 0.022 View Large Table 3. Occlusal measurements (mm) at each time point using intention-to-treat analysis. Data are presented in the form of median (inter-quartile range) and Mann–Whitney P value due to its non-normal distribution. AL, arch length; BR, bonded retainer; ICW, inter-canine width; IMW, inter-molar width; LII, Little’s irregularity index; OB, overbite; OJ, overjet; VFR, vacuum-formed retainer. Bold indicates statistical significance P < 0.05. Occlusal measurements at each review Maxilla Mandible Inter-arch LII ICW IMW AL LII ICW IMW AL OJ OB T0 Debond BR 0.00 (0.93) 35.20 (2.83) 50.11 (3.96) 73.94 (12.74) 0.29 (1.02) 27.53 (1.68) 44.05 (4.64) 66.74 (6.00) 2.37 (0.70) 1.29 (1.22) VFR 0.23 (0.66) 34.09 (2.22) 48.46 (4.24) 71.23 (9.67) 0.06 (1.23) 26.17 (1.13) 41.34 (5.72) 65.53 (12.94) 2.38 (2.40) 2.00 (1.21) P value 0.31 0.003 0.01 0.101 0.382 0.003 0.071 0.188 0.203 0.083 T1 3 months BR 0.79 (0.91) 35.08 (1.92) 49.63 (2.92) 72.75 (10.57) 0.44 (0.63) 27.45 (2.11) 43.65 (3.67) 66.81 (8.68) 2.31 (1.04) 1.45 (0.71) VFR 0.30 (0.98) 33.69 (2.52) 47.90 (3.88) 69.81 (11.46) 0.44 (1.71) 25.69 (1.45) 41.70 (5.35) 63.43 (9.94) 2.62 (0.76) 1.87 (0.87) P value 0.05 0.001 0.018 0.147 0.432 0.000 0.055 0.104 0.064 0.001 T2 6 months BR 0.88 (1.07) 35.36 (2.41) 49.74 (3.46) 72.88 (10.35) 0.67 (1.47) 28.06 (1.65) 44.23 (2.34) 67.06 (8.83) 2.53 (1.09) 1.52 (0.92) VFR 0.64 (1.77) 34.21 (2.06) 48.84 (3.85) 70.19 (9.62) 1.30 (1.98) 26.26 (1.65) 41.84 (6.22) 65.17 (11.42) 2.74 (0.50) 1.92 (0.97) P value 0.22 0.000 0.015 0.051 0.335 0.002 0.174 0.055 0.181 0.036 T3 12 months BR 1.35 (1.98) 35.08 (2.31) 49.47 (3.88) 76.70 (10.81) 1.01 (1.28) 27.31 (2.21) 43.90 (4.32) 66.97 (8.21) 2.26 (1.07) 1.59 (0.78) VFR 0.97 (1.68) 33.21 (2.36) 47.70 (3.80) 68.86 (10.26) 1.73 (2.77) 25.56 (1.39) 41.32 (4.61) 62.57 (9.50) 2.59 (0.94) 2.01 (1.00) P value 0.24 0.001 0.064 0.117 0.053 0.001 0.068 0.071 0.128 0.022 Occlusal measurements at each review Maxilla Mandible Inter-arch LII ICW IMW AL LII ICW IMW AL OJ OB T0 Debond BR 0.00 (0.93) 35.20 (2.83) 50.11 (3.96) 73.94 (12.74) 0.29 (1.02) 27.53 (1.68) 44.05 (4.64) 66.74 (6.00) 2.37 (0.70) 1.29 (1.22) VFR 0.23 (0.66) 34.09 (2.22) 48.46 (4.24) 71.23 (9.67) 0.06 (1.23) 26.17 (1.13) 41.34 (5.72) 65.53 (12.94) 2.38 (2.40) 2.00 (1.21) P value 0.31 0.003 0.01 0.101 0.382 0.003 0.071 0.188 0.203 0.083 T1 3 months BR 0.79 (0.91) 35.08 (1.92) 49.63 (2.92) 72.75 (10.57) 0.44 (0.63) 27.45 (2.11) 43.65 (3.67) 66.81 (8.68) 2.31 (1.04) 1.45 (0.71) VFR 0.30 (0.98) 33.69 (2.52) 47.90 (3.88) 69.81 (11.46) 0.44 (1.71) 25.69 (1.45) 41.70 (5.35) 63.43 (9.94) 2.62 (0.76) 1.87 (0.87) P value 0.05 0.001 0.018 0.147 0.432 0.000 0.055 0.104 0.064 0.001 T2 6 months BR 0.88 (1.07) 35.36 (2.41) 49.74 (3.46) 72.88 (10.35) 0.67 (1.47) 28.06 (1.65) 44.23 (2.34) 67.06 (8.83) 2.53 (1.09) 1.52 (0.92) VFR 0.64 (1.77) 34.21 (2.06) 48.84 (3.85) 70.19 (9.62) 1.30 (1.98) 26.26 (1.65) 41.84 (6.22) 65.17 (11.42) 2.74 (0.50) 1.92 (0.97) P value 0.22 0.000 0.015 0.051 0.335 0.002 0.174 0.055 0.181 0.036 T3 12 months BR 1.35 (1.98) 35.08 (2.31) 49.47 (3.88) 76.70 (10.81) 1.01 (1.28) 27.31 (2.21) 43.90 (4.32) 66.97 (8.21) 2.26 (1.07) 1.59 (0.78) VFR 0.97 (1.68) 33.21 (2.36) 47.70 (3.80) 68.86 (10.26) 1.73 (2.77) 25.56 (1.39) 41.32 (4.61) 62.57 (9.50) 2.59 (0.94) 2.01 (1.00) P value 0.24 0.001 0.064 0.117 0.053 0.001 0.068 0.071 0.128 0.022 View Large Table 4. Relapse values (measured in mm) at 3 months, 6 months, and 12 months. Data are presented in the form of median (inter-quartile range) and Mann–Whitney P value due to its non-normal distribution. AL, arch length; BR, bonded retainer; ICW, inter-canine width; IMW, inter-molar width; LII, Little’s irregularity index; OB, overbite; OJ, overjet; VFR= vacuum-formed retainer; PAR, peer assessment rating. Bold indicates statistical significance P < 0.05. Changes in measurements with time Maxilla Mandible Inter-arch LII ICW IMW AL LII ICW IMW AL OJ OB 3-Month relapse (T1–T0) BR 0.29 (0.55) 35.08 (1.92) 49.63 (2.92) 72.75 (10.57) 0.15 (0.32) 27.45 (2.11) 43.65 (3.67) 66.81 (8.68) 2.32 (1.04) 1.44 (0.71) VFR 0.04 (0.32) 33.69 (2.52) 47.90 (3.88) 69.81 (11.46) 0.33 (0.61) 25.69 (1.45) 41.70 (5.35) 63.43 (9.94) 2.62 (0.76) 1.87 (0.87) P value 0.010 0.562 0.712 0.304 0.048 0.036 0.712 0.802 0.34 0.26 6-Month relapse (T2–T0) BR 0.39 (1.08) 0.18 (0.81) −0.63 (1.79) 0.00 (1.42) 0.53 (0.72) 0.84 (0.75) 0.19 (1.40) 0.00 (0.87) 0.08 (0.53) 0.20 (0.50) VFR 0.30 (1.24) 0.01 (0.86) 0.15 (0.71) 0.35 (0.70) 0.70 (1.33) −0.18 (0.77) 0.49 (1.12) −0.49 (1.25) 0.14 (0.46) 0.02 (0.59) P value 0.30 0.13 0.734 0.515 0.116 0.031 0.188 0.268 0.308 0.652 12-Month relapse (T3–T0) BR 1.10 (1.56) 0.25 (1.16) −0.16 (2.60) −0.36 (1.77) 0.77 (1.46) −0.26 (0.90) 0.11 (2.07) 0.10 (1.19) −0.09 (0.74) 0.30 (0.71) VFR 0.76 (1.55) 0.19 (1.02) 0.02 (0.91) 0.10 (1.44) 1.69 (2.00) −0.45 (0.86) 0.54 (1.67) −0.70 (1.25) 0.12 (0.69) 0.15 (0.83) P value 0.61 0.124 0.802 0.918 0.008 0.071 0.544 0.040 0.408 0.965 Changes in measurements with time Maxilla Mandible Inter-arch LII ICW IMW AL LII ICW IMW AL OJ OB 3-Month relapse (T1–T0) BR 0.29 (0.55) 35.08 (1.92) 49.63 (2.92) 72.75 (10.57) 0.15 (0.32) 27.45 (2.11) 43.65 (3.67) 66.81 (8.68) 2.32 (1.04) 1.44 (0.71) VFR 0.04 (0.32) 33.69 (2.52) 47.90 (3.88) 69.81 (11.46) 0.33 (0.61) 25.69 (1.45) 41.70 (5.35) 63.43 (9.94) 2.62 (0.76) 1.87 (0.87) P value 0.010 0.562 0.712 0.304 0.048 0.036 0.712 0.802 0.34 0.26 6-Month relapse (T2–T0) BR 0.39 (1.08) 0.18 (0.81) −0.63 (1.79) 0.00 (1.42) 0.53 (0.72) 0.84 (0.75) 0.19 (1.40) 0.00 (0.87) 0.08 (0.53) 0.20 (0.50) VFR 0.30 (1.24) 0.01 (0.86) 0.15 (0.71) 0.35 (0.70) 0.70 (1.33) −0.18 (0.77) 0.49 (1.12) −0.49 (1.25) 0.14 (0.46) 0.02 (0.59) P value 0.30 0.13 0.734 0.515 0.116 0.031 0.188 0.268 0.308 0.652 12-Month relapse (T3–T0) BR 1.10 (1.56) 0.25 (1.16) −0.16 (2.60) −0.36 (1.77) 0.77 (1.46) −0.26 (0.90) 0.11 (2.07) 0.10 (1.19) −0.09 (0.74) 0.30 (0.71) VFR 0.76 (1.55) 0.19 (1.02) 0.02 (0.91) 0.10 (1.44) 1.69 (2.00) −0.45 (0.86) 0.54 (1.67) −0.70 (1.25) 0.12 (0.69) 0.15 (0.83) P value 0.61 0.124 0.802 0.918 0.008 0.071 0.544 0.040 0.408 0.965 View Large Table 4. Relapse values (measured in mm) at 3 months, 6 months, and 12 months. Data are presented in the form of median (inter-quartile range) and Mann–Whitney P value due to its non-normal distribution. AL, arch length; BR, bonded retainer; ICW, inter-canine width; IMW, inter-molar width; LII, Little’s irregularity index; OB, overbite; OJ, overjet; VFR= vacuum-formed retainer; PAR, peer assessment rating. Bold indicates statistical significance P < 0.05. Changes in measurements with time Maxilla Mandible Inter-arch LII ICW IMW AL LII ICW IMW AL OJ OB 3-Month relapse (T1–T0) BR 0.29 (0.55) 35.08 (1.92) 49.63 (2.92) 72.75 (10.57) 0.15 (0.32) 27.45 (2.11) 43.65 (3.67) 66.81 (8.68) 2.32 (1.04) 1.44 (0.71) VFR 0.04 (0.32) 33.69 (2.52) 47.90 (3.88) 69.81 (11.46) 0.33 (0.61) 25.69 (1.45) 41.70 (5.35) 63.43 (9.94) 2.62 (0.76) 1.87 (0.87) P value 0.010 0.562 0.712 0.304 0.048 0.036 0.712 0.802 0.34 0.26 6-Month relapse (T2–T0) BR 0.39 (1.08) 0.18 (0.81) −0.63 (1.79) 0.00 (1.42) 0.53 (0.72) 0.84 (0.75) 0.19 (1.40) 0.00 (0.87) 0.08 (0.53) 0.20 (0.50) VFR 0.30 (1.24) 0.01 (0.86) 0.15 (0.71) 0.35 (0.70) 0.70 (1.33) −0.18 (0.77) 0.49 (1.12) −0.49 (1.25) 0.14 (0.46) 0.02 (0.59) P value 0.30 0.13 0.734 0.515 0.116 0.031 0.188 0.268 0.308 0.652 12-Month relapse (T3–T0) BR 1.10 (1.56) 0.25 (1.16) −0.16 (2.60) −0.36 (1.77) 0.77 (1.46) −0.26 (0.90) 0.11 (2.07) 0.10 (1.19) −0.09 (0.74) 0.30 (0.71) VFR 0.76 (1.55) 0.19 (1.02) 0.02 (0.91) 0.10 (1.44) 1.69 (2.00) −0.45 (0.86) 0.54 (1.67) −0.70 (1.25) 0.12 (0.69) 0.15 (0.83) P value 0.61 0.124 0.802 0.918 0.008 0.071 0.544 0.040 0.408 0.965 Changes in measurements with time Maxilla Mandible Inter-arch LII ICW IMW AL LII ICW IMW AL OJ OB 3-Month relapse (T1–T0) BR 0.29 (0.55) 35.08 (1.92) 49.63 (2.92) 72.75 (10.57) 0.15 (0.32) 27.45 (2.11) 43.65 (3.67) 66.81 (8.68) 2.32 (1.04) 1.44 (0.71) VFR 0.04 (0.32) 33.69 (2.52) 47.90 (3.88) 69.81 (11.46) 0.33 (0.61) 25.69 (1.45) 41.70 (5.35) 63.43 (9.94) 2.62 (0.76) 1.87 (0.87) P value 0.010 0.562 0.712 0.304 0.048 0.036 0.712 0.802 0.34 0.26 6-Month relapse (T2–T0) BR 0.39 (1.08) 0.18 (0.81) −0.63 (1.79) 0.00 (1.42) 0.53 (0.72) 0.84 (0.75) 0.19 (1.40) 0.00 (0.87) 0.08 (0.53) 0.20 (0.50) VFR 0.30 (1.24) 0.01 (0.86) 0.15 (0.71) 0.35 (0.70) 0.70 (1.33) −0.18 (0.77) 0.49 (1.12) −0.49 (1.25) 0.14 (0.46) 0.02 (0.59) P value 0.30 0.13 0.734 0.515 0.116 0.031 0.188 0.268 0.308 0.652 12-Month relapse (T3–T0) BR 1.10 (1.56) 0.25 (1.16) −0.16 (2.60) −0.36 (1.77) 0.77 (1.46) −0.26 (0.90) 0.11 (2.07) 0.10 (1.19) −0.09 (0.74) 0.30 (0.71) VFR 0.76 (1.55) 0.19 (1.02) 0.02 (0.91) 0.10 (1.44) 1.69 (2.00) −0.45 (0.86) 0.54 (1.67) −0.70 (1.25) 0.12 (0.69) 0.15 (0.83) P value 0.61 0.124 0.802 0.918 0.008 0.071 0.544 0.040 0.408 0.965 View Large In the mandible, post-treatment changes tended to be greater in the VFR group, reaching statistical significance at both 3 and 12 months; mandibular LII values were BR = 0.77 mm IQR = 1.46, VFR = 1.69 mm IQR = 2.00, P = 0.008 at 12 months. In the maxilla, there was no significant difference in post-treatment changes between the retainer groups after 12 months (BR = 1.10 mm IQR = 1.56, VFR = 0.76 mm IQR = 1.55, P = 0.61). The only statistically significant relapse measurement in arch dimensions or occlusal relationships at 12 months was mandibular arch length (BR = +0.10 mm IQR = 1.19, VFR = −0.70 mm IQR = 1.25, P = 0.040). There was no difference in the re-opening of the extraction spaces during the initial 12 months of retention between BR and VFR groups. Mdn space opening within the maxilla was 0.00 mm (IQR = 0.28) with BRs and 0.00 mm (IQR = 0.00) with VFRs, 0.00 mm (IQR = 0.42) with BRs and 0.00mm (IQR = 0.25) with VFRs within the mandible. Secondary outcome: retainer survival There were no statistical differences in the survival rates of BRs and VFRs in the maxilla. The Kaplan–Meier plot for maxillary retainers shows 63.3% of BR and 73.3% of VFR survived the initial 12 months of retention, and this difference was statistically non-significant (P = 0.34). The mean survival time was 272.5 days (95% CI = 226.1–319.0) for the BR, and 311.3 days (95% CI = 278.3–344.29) for the VFR (Figure 5). Figure 5. View largeDownload slide Kaplan–Meier cumulative survival plots and log-rank P values for the time until the first episode of failure during 12 months of follow-up for (a) maxillary retainers and (b) mandibular retainers. Figure 5. View largeDownload slide Kaplan–Meier cumulative survival plots and log-rank P values for the time until the first episode of failure during 12 months of follow-up for (a) maxillary retainers and (b) mandibular retainers. This contrasted with the mandibular retainers, where there was a statistically significant difference in the 12-month survival of BR and VFR (P = 0.01) with respective rates of 50% and 80%. The Kaplan–Meier plot illustrates a mean survival time of 239.3 days (95% CI = 191.1–287.5) for the BR, and 324.9 days (95% CI = 295.4–354.4) for the VFR (Figure 5). Looking at VFR survival rates after 12 months, 8 patients (26.7%) with VFRs had experienced at least one episode of retainer failure; either retainer loss or poorly fitting retainers or not fitting. All eight patients exhibited failure of an upper retainer, whilst six of these patients also had a lower retainer failure. Of these eight patients three had repeat failures: two subjects experienced two episodes of failure, whilst one patient exhibited three episodes. No correlation was found between failure and patient gender, operator, or study centre (P = 0.21, 0.66, 0.17, respectively). After 12 months, 18 patients with BRs (60%) had experienced at least one episode of retainer failure; 11 maxillary BRs and 15 mandibular BRs. Adhesive and cohesive failures were prevalent in both arches. There was one episode involving a maxillary BR fracture mesial to the right canine, and there were three episodes of the mandibular BR completely detaching from all tooth surfaces simultaneously. Two episodes of maxillary BR failure occurred in three subjects, and involved detachment at the wire-composite interface. Three patients encountered repeated mandibular BR failures (two patient with two episodes and three patients with three episodes). Maxillary failure tended to occur between the wire and the composite, whereas, mandibular failure was more common at the enamel–composite interface, particularly within the initial 6 months. No statistical predictors were found for the survival of either upper or lower BRs. The confounders tested included the operator, hospital as well as patient factors such as gender incisor relationship and skeletal relationship. Secondary outcome: patient satisfaction A total of 52 questionnaires were completed at 3 months (BR = 24, VFR = 28), 60 at 6 months (BR = 30, VFR = 30), and 57 at 12 months (BR = 30, VFR = 27). Where subjects failed to attend review appointments, questionnaires were posted to patients to complete and return in an attempt to minimise missing data. Results are reported in Supplementary Table 1. There was no significant difference between patients’ opinions of wearing their retainers compared to their fixed appliances (P = 0.096) with both groups reporting that their retainers were better, or at least no worse, than their fixed appliances. There was no difference in the embarrassment felt by participants wearing either retainer type (P = 0.065) with only two subjects with VFRs reporting occasional embarrassment. More patients reported discomfort from VFRs than BRs (P = 0.002). More patients with VFRs reported that their speech was affected by their retainers than patients with BRs (P = 0.004). Subjects with VFRs found them easier to clean than those with BRs (P = 0.001). Harms No serious harm associated with the retainers was observed. Effects on periodontal health will be reported in Paper 2. Discussion Key findings and comparison to available literature Stability and retainer survival The study found no evidence of a difference in post-treatment changes, or retainer survival rate, between BRs and VFRs in the upper arch. In the lower arch, BRs reduced post-treatment mandibular labial segment alignment better than VFRs; however, there was greater failure rate with BRs. This randomized controlled trial was conducted by two operators and aimed to compare the clinical effectiveness of BRs and VFRs during the course of the initial 12 months of retention. Its prospective nature is in contrast to much of the early retrospective literature, which incorporates long-term follow-up along with the associated bias (13, 31–34). Within these studies, patients were treated by a number of operators with varying orthodontic appliances and treatment strategies, and using a variety of retainer designs (including the type of wire used, the number of teeth bonded, and the bonding agent). At the commencement of this trial the authors were aware of only one published trial within the literature, directly comparable to this trial (12, 22). This investigated the clinical effectiveness of mandibular BRs and VFRs and was published as a series of conference abstracts. Whilst the trend in relapse was similar, with the BR being more effective at retaining the labial segment alignment than the VFR, there was, however, a difference in the failure rates. McDermott reported greater failure of the VFR than the BR (62.5% and 15.6%, respectively), whilst this current trial found failure of the BR to be greater (VFR = 20%, BR = 50%). Since completion of the study, a further randomized controlled trial comparing BRs and VFRs has been published (35). In that study, BRs and VFRs were compared in the lower arch only. They found less relapse with the BR group after 6 months, but at 12 months and 18 months there was no difference between the two types of lower retainers. Interestingly, the amount of relapse was exceptionally low for both groups, which may be due to a low failure rate of the BRs and good adherence to wearing the VFRs. In this study, the BR failure rates for the maxilla and mandible were 36.7% and 50%, respectively, for the initial 12 months of retention. These figures may be regarded as high with failure rates of BRs constructed from multi-strand twistflex wire ranging from 7.8% (13) to 53% (10). The process of placing a BR is technique sensitive. The operators in this study were at the beginning of their orthodontic career and a general reduction in the survival rate has been associated with less experienced operators (36, 37). These factors may partially explain the higher failure rate in this study. An interesting finding of this research relates to the higher prevalence of BR failures occurring in the mandibular arch compared with the maxilla. This differs from many of the published trials, which demonstrate greater failure rates in the maxilla probably due to occlusal stress (10, 17, 20, 38). In fact, Dahl et al. found the maxillary failure rates to be almost twice as high as those found in the mandible (13, 38). The higher mandibular failure rate reported in this study was potentially influenced by the inexperience of the operators. This correlates with Scheibe’s retrospective analysis of 1062 retainers placed by operators of varying experience where the majority of mandibular failures were associated with less experienced operators (36). Published reports of BR survival tend to show most failures occurring within the initial 6 months of placement (9, 10, 16, 17, 39) and link these early failures to adhesive failure. This is similar to the findings in this study, which show that BR failures tend to occur early. The Kaplan–Meier plots within this study show failure of both maxillary and mandibular BRs occurring within the initial 6 months of retention. The majority of mandibular failures occurred at the enamel–composite interface, whereas, problems more commonly occurred at the composite–wire interface in the maxilla, perhaps in response to occlusal interference. The extent of relapse associated with VFRs within this current study is comparable to reports within the literature. Within our study, we noted similar failure rates between maxillary and mandibular VFRs and the majority of these were due to participants losing their retainers. In contrast, others have found retainer breakages to be the most common reason for failure, followed by lost retainers. Sun et al. instructed all participants to wear their retainers on a full-time basis, and perhaps this increased wear contributed to the higher breakage rates (40). Patient satisfaction There was no significant difference between patients’ opinions of wearing their retainer compared to their fixed appliances with both groups reporting that their retainers were better or at least no worse than their fixed appliances. This contrasts with the results of previous studies which have highlighted patients’ disappointment with retainers, noting that they found retainers very difficult to live with, more so than braces and headgear (25–27), mainly due to difficulties with speech, eating, increased salivation, smell, losing their retainers, and embarrassment over the appearance of the retainers (25, 26). Only 33% of subjects with VFRs reported that they always wore their retainers every night as instructed, though a further 37% stated they followed this advice most of the time. This finding is similar to Sawhney et al. who stated that self-reported compliance with removable retainers ranged between 75% and 85%, regardless of the regimen or type of retainer (41), and Wong and Freer who reported more than 50% of patients admitted that they did not wear retainers as instructed (24). Eighty-three percent of participants with VFRs and 100% with BRs stated that they were not embarrassed to wear their retainers, though VFRs were associated with more discomfort (37%) and difficulty speaking (47%) than BRs, though the relevance of difficult speaking with the retainer is questionable when VFRs are prescribed for night-time wear only. Jaderberg et al. also found similar problems with VFRs, albeit to a lesser extent, reporting soreness in 13%, and problems with speech in 22% of subjects (29). Similarly, Sawhney et al. found BRs affected speech less than both VFRs and Hawley retainers (41). Perhaps unsurprisingly, subjects in the VFR group found it easier to clean their retainers than those with BRs, supporting previous findings (41). Limitations In this study, it was not possible to blind the operators and patients to the treatment allocations. Although the minimum sample size was achieved, the power calculation was based on the primary outcome measure: the efficacy of each retainer group in minimising the 1-year post-orthodontic treatment change in arch alignment. As such, it is possible that the study is underpowered with respect to the secondary outcome measures, increasing the chance of making a type II error. The amount of VFR wear was not measured, so it is not clear whether the extra relapse in the mandible was due to the retainer itself, or a lack of patient compliance with the prescribed retention regimen. It could be argued that this is the ‘real-life’ situation. Post-treatment changes were measured in this study, rather than attempting to measure relapse towards the starting malocclusion. This is because it was felt that increases in incisor irregularity after treatment was more clinically relevant for patients. Post-treatment changes are unpredictable and do not always represent a move back towards the original malocclusion. This study therefore measures the ability of BRs and VFRs to reduce post-treatment changes in incisor irregularity, which may or may not reflect a relapse towards the presenting malocclusion. LII was chosen as the principle measurement of relapse as, anecdotally, incisor irregularity is often the patient’s main concern relating to post-orthodontic movement. This method is quick, simple, and widely used within research, therefore enabling direct comparison between studies. However, recognized limitations of this method include a disregard to spacing, or mutual rotations, where the contact point remained intact. Furthermore, the index is cumulative; therefore, a LII of 1.5 mm may have been awarded to a patient with evenly dispersed (but clinically insignificant) minor displacements, or equally to another patient with a single contact point displacement of the same value. It is likely that if the irregularity is limited to one larger displacement, then a patient will notice it more than if the same total amount of irregularity is evenly distributed across a number of teeth. A statistical difference in relapse of mandibular irregularity was found between the two groups. We considered whether to use a Bonferonni correction to allow for the possible affects of multiple testing, but in line with other similar studies, this was not used in this study for this outcome. This may be a statistical approach that could be considered in future research. Randomisation is undertaken to ensure equivalence between the two groups. It should be noted that there was statistical difference in the upper and lower inter-canine width at debond, although the differences were so small that it is unlikely to be clinically significant. The randomisation process did not enable exclusion of subjects where placement of the BR was deemed difficult, such as those with heavy, labial segment occlusal contacts. The retainer wire was contoured gingivally (where possible) to reduce, and hopefully avoid, traumatic occlusal contact. The influences of the incisor classification and overbite on failure could be investigated separately with a larger sample. All retainers were placed by orthodontic trainees. In an attempt to minimize the impact of operator inexperience, standardized positioning jigs were employed to aid placement of the retainers and reduce the potential for operator error and the risk of moisture contamination. In this study, the BRs were used in isolation to best assess the effects caused by them without the influence of an additional removable retainer. Some operators supplement a BR with a removable retainer in an attempt to maintain buccal segment alignment, retain space closed in extraction cases, and to act as a safeguard in case of breakage of the BR. A future study looking at this dual retention may be useful. Patient satisfaction is a complex area and is related to a number of different factors, which are not evenly weighted. The questionnaire used in the study was based on the most relevant questions in the patient satisfaction questionnaire from the Hichens et al. randomized controlled trial assessing patient satisfaction with Hawley and VFRs (28). To aid direct comparison of the data obtained with that from a previous study. Further research may be needed to devise a more appropriate and validated questionnaire for different types of retainers. This may involve the use of focus groups and patient interviews to identify specific concerns or problems. One of the challenges of orthodontic research is following up patients over a long period and this is particularly pertinent to research into retention. Due to the problem of large drop-outs in long-term orthodontic studies, this study only measured changes over 1 year. It is important to remember that the findings of this study only relate to 12 months of retention and it is possible that the findings may be different over a longer period. Generalisability The generalisability of these findings is limited to the retainer materials, design, and technique of placement, in addition to the operator experience. The operators were orthodontic trainees, therefore the clinical outcomes, in particular the survival rates, may be not be transferable to those with greater experience in the placement of BRs. Implications for clinical practice Within the initial 12 months of retention, BRs were more effective at retaining mandibular labial segment alignment than VFRs. However, there was a significantly higher failure rate relating to mandibular BRs, which had often progressed unnoticed by the patient. Therefore, it may be worthwhile considering whether a reduction of, on average, 0.92 mm of mandibular labial segment relapse warrants the additional clinical time spent during the supervision and amending of BR failures, particularly if the relapse is negligible once distributed across the full labial segment. If BRs are intended for the labial segment, it may be prudent to arrange more frequent recalls, particularly in the first 6 months after debond, to supervise BRs, especially if a less experienced operator had placed the BR. This would be beneficial not only in terms of minimising relapse, but also for the development of the operator’s awareness and technique. Prior to retainer placement, patients should also be counselled about the possibility of failure, and the importance of monitoring the integrity of the retainer. Within the maxilla, there was no evidence of a difference in relapse or retainer failure rates when comparing BRs and VFRs, so other outcomes may affect the choice of retainer. This will be explored further in Paper 2 Future research Future research should investigate the outcomes and implications of retainers over the longer term in terms of post-treatment changes, failure rates, adverse effects, and patient compliance. In addition, it would be useful to explore different types of BRs and VFRs. Conclusions After 12 months of retention, this randomized controlled trial comparing bonded with VFRs has shown that: - There is no evidence of a difference in post-treatment changes between patients who wear BRs or VFRs in the maxilla - The mandibular BR is more effective at reducing post-treatment changes in the labial segment alignment - There is no evidence of a difference in the survival rate of maxillary BRs and VFRs - BRs are more likely to fail than VFRs - Patients find VFRs easier to clean than BRs. However, patients report that BRs cause less discomfort and speech difficulties, and require less compliance Supplementary Material Supplemental data are available at the European Journal of Orthodontics online. Conflict of Interest None to declare. 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The European Journal of OrthodonticsOxford University Press

Published: Oct 20, 2017

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