Introduction
The objective of this study was to compare the survival rates and periodontal health in patients with 3-strand round twisted (RT) vs 8-strand rectangular braided (RB) fixed retainers bonded to all 6 anterior teeth in the mandible.
Methods
A total of 133 patients completing orthodontic treatment (median age, 24.6 years; 25th percentile, 17.2 years; 75th percentile, 32.4 years; minimum, 15.1 years; maximum, 49.8 years) were randomly allocated in a 1:1 ratio to receive either an RT or RB wire retainer. Inclusion criteria were all mandibular permanent incisors and canines present, no active caries, no restorations, no fractures on the mandibular incisors and canines, no periodontal disease. Patients with poor oral hygiene before debonding were excluded from the trial. The primary outcome was any first-time retainer failure. Secondary outcomes were periodontal index, bleeding on probing, plaque index, gingival index, and probing depth. Randomization was accomplished with random permuted blocks of size 4, 6, or 8 with allocation concealed in sequentially numbered, opaque, sealed envelopes. Blinding was not possible in this trial. Patients were evaluated at baseline, 3, 6, 12, 18, and 24 months after placement of the retainer. Retainer survival was assessed using Cox regression. Periodontal parameters were reported at each time point and generalised estimating equations were used to assess the effect of treatment, time, tooth and treatment X time interaction on the indices.
Results
Baseline characteristics were similar between groups; in 1 patient, the intervention was discontinued. During 2-year follow-up 37 of 66 (56.1%, RT group) and 32 of 66 (48.5%, RB group) retainers failed at least once (log-rank test, P = 0.55). The adjusted hazard ratio was 0.69 (95% confidence interval, 0.42-1.12; P = 0.13). Neither age nor gender was a predictor of failure. All periodontal parameters (periodontal index, bleeding on probing, plaque index, gingival index, and pocket depth) were comparable between groups and remained relatively stable during follow-up.
Conclusions
The overall risk for first-time failure was high and amounted to 52.3% (56.1% in the RT group and 48.5% in the RB group). There was no difference in terms of survival or periodontal health between the examined retainers.
Highlights
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Orthodontists use rectangular wire retainers to prevent the development of active retainers.
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This trial compares survival of round twisted vs rectangular braided wire retainers after 2 years.
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Both retainer types showed comparable survival rates.
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Periodontal health was good in patients wearing both retainer types.
Irrespective of the type and severity of malocclusion and treatment strategy, dental arch alterations are expected over time. Moreover, the degree of relapse is unpredictable, and no useful predictor for relapse has been identified. A recent systematic review reported that posttreatment changes in mandibular anterior dental alignment were limited with no association with pretreatment irregularity, the clinical setting, retention type, and adjunctive procedures. However, the limited increase in irregularity may be attributed to the adherence to retention protocols in the included studies.
Several randomized clinical trials (RCTs) evaluated the success , and cost of different retention strategies, patient compliance, and retainer acceptance. A comparison of the effectiveness among various combinations of vacuum-formed retainer, stripping, positioner, and retainers bonded to canines only, , , , despite some variability, on average revealed no significant differences in maintaining alignment. Fixed retainers (FR) bonded to all 6 anterior teeth seem to be more effective than FR bonded to 3’s only and are a sensible alternative when even small increases in incisor irregularity are unacceptable. Such FRs are more effective at maintaining mandibular labial segment alignment but have a higher failure rate in comparison with vacuum-formed retainers.
FRs bonded to all 6 anterior mandibular teeth come in variable cross-sectional dimensions, the number of strands, type of alloy or structure (twisted vs braided), and are widely used by orthodontists. Following the recommendations of Dahl and Zachrisson, it is plausible that many clinicians still use a round multistranded wire to fabricate FRs. There is evidence that in the Netherlands that a significant proportion of orthodontists use FRs made of square or rectangular wire, a choice that seems to be influenced by reports of dental arch alterations when round wires had been used. , Recently, Kocher et al , studied the long-term clinical performance of a 0.016-in × 0.022-in braided stainless-steel retainer bonded to all 6 anterior mandibular teeth and found that this type of retainer was effective in maintaining dental alignment. However, these studies were retrospective, and no direct comparison between the rectangular retainer and a round twisted (RT) retainer was made.
To our knowledge, the clinical performance of the FR made of rectangular wire has never been tested in a methodologically rigorous trial. Therefore, this study aimed to compare a 0.0215-in 3-strand RT with a 0.0265-in × 0.0106-in 8-strand rectangular braided (RB) in terms of their survival and periodontal health over 24 months. Our research hypothesis was that there were no differences between the 2 FRs.
Material and methods
The Ethical Committee of Warsaw Medical Chamber approved the study protocol (no. KB/956/14; October 23, 2014). Written informed consent was given by each participant (and legal guardian if a participant was underage). The trial was not registered.
Design, participants, eligibility criteria, and setting
It was a single-center 2-arm parallel-group randomized controlled trial with a 1:1 allocation ratio. Participants were recruited from December 2014 to April 2018 from a single orthodontic private practice of the last author (P.S.F). The following inclusion criteria were applied: aged 15-50 years at debonding; all mandibular permanent incisors and canines present; no active caries, no restorations, no fractures on the mandibular incisors and canines, no periodontal disease; and retention plan including only retainers bonded from 3 to 3. Exclusion criteria were inadequate hygiene, need for restorative or surgical treatment, active periodontal disease, or removable retainer as an adjunct to a bonded retainer. Each eligible patient was approached 2 months before planned debonding and provided information about the trial, its aims, and methodology. Then the patient was asked if he or she would be willing to participate in the trial. If extra time before making a decision was requested, several days for consideration were offered. Those patients who consented to participate underwent the procedure described in the Interventions section.
Sample size
Sample size was calculated with the following assumptions: ability to detect a clinically relevant 25% difference in the risk of first-time failure (primary outcome) between the 2 trial arms (50% vs 25%) with α = 0.05 and β = 0.8 (power = 80%). The assumptions were based on findings of Pandis et al, who detected an almost 50% first-time failure rate within 2 years after debonding. The minimum sample size was 58 participants per arm (group).
Randomization
Randomization was done using a random number generator provided by www.sealedenvelope.com online service. Random permuted blocks of 4, 6, or 8 patients were created to ensure equal allocation to the 2 arms. Allocation concealment was achieved with sequentially numbered, opaque, sealed envelopes prepared before the trial. The study coordinator (practice manager) was responsible for opening the next envelope in sequence and implementing the randomization process.
Interventions
One month before debonding, an alginate impression of the mandibular dental arch was taken and sent to the laboratory within 24 hours along with the information on the requested type of the retainer (ie, either 0.0215-in stainless-steel 3-strand RT wire retainer [Ortho Organizers, Lindenberg, Germany] or 0.0265-in × 0.0106-in 8-strand Bond-a-Braid wire retainer [Reliance Orthodontic Products, Itasca, Ill]). Then, approximately 2 weeks before debonding, a study participant was scheduled for scaling and tooth cleaning provided by an experienced hygienist.
The bonding procedure comprised the following steps: placement of cheek retractor, cleaning of lingual surfaces of the 6 anterior teeth, 37% phosphoric acid etching, rinsing and drying, placement of 3 pieces of dental floss to hold the retainer in place during bonding, application of primer (Transbond XT adhesive primer; 3M Unitek, 3M Dental Products, Monrovia, Calif) on etched surfaces of the teeth, application of light-cured composite (Transbond Supreme LV; 3M Unitek), setting the composite with light. All efforts were made to avoid moisturizing lingual surfaces of the teeth with saliva. All retainers were bonded by the same orthodontic assistant who had more than 15 years of experience with this procedure.
Outcomes
There were 2 types of outcomes assessed in this study: retainer survival (ie, any first-time failure of the retainer [primary outcome]) and periodontal outcomes (secondary outcomes) comprising periodontal index (PDI) by Russel, bleeding on probing (BOP), plaque index (PI) by Loe and Silness, gingival index (GI), and pocket depth (PD). The stability of the dental arch and patients’ satisfaction will be reported in a separate study.
Data collection
Data were collected at 6-time points: baseline, at debonding and retainer placement; 3 months after retainer placement; 6 months after retainer placement; 12 months after retainer placement; 18 months after retainer placement; and 24 months after retainer placement by the same assessor (E.W. -orthodontist with more than 10 years of clinical experience). In case of retainer failure, study participants were requested to contact the practice as soon as possible. At the end of the visit, the subsequent appointment was scheduled in advance to ensure regular follow-up.
During each data collection appointment, first, the retainer was checked visually for any sign of failure (eg, discoloration of the adhesive, loss of adhesive, etc), followed by manual control of bond quality between the wire and each of the 6 teeth. In case of failure, type (debonding, breakage, and complete loss of retainer) and location (ie, on which tooth [teeth]) of failure were noted. For example, when debonding occurred, the adhesive remnant index was used to categorize debonding into 1 of 4 categories: no bond on tooth surface, <50% bond on tooth surface, >50% bond on tooth surface, and 100% bond on the tooth surface.
After the assessment of the retainer integrity, the following periodontal indexes were recorded. The PDI was calculated as a mean score of individual scores of 6 anterior teeth using the following scale: 0, healthy periodontium; 8, advanced periodontal destruction. BOP was calculated as a proportion of points bleeding within 10 seconds after probing of dental pockets on 6 sites of each of 6 anterior teeth; PI was calculated as a mean score of individual scores of 6 anterior teeth on the following scale: 0, no dental plaque; 3, abundant dental plaque. GI was determined for each of the 6 teeth on the following scale: 0, no inflammation; 3, severe inflammation present. PD was measured with a periodontal probe on the lingual surfaces of 6 anterior teeth. All determinations were made by the same assessor who evaluated retainer integrity (E.W).
Blinding
Blinding was not possible because the retainer type—3-wire RT or 8-wire RB—could not be masked during clinical assessments.
Statistical analysis
Descriptive statistics were calculated at baseline per treatment group and for the different time points (3 months after retainer placement, 6 months after retainer placement, 12 months after retainer placement, 18 months after retainer placement, and 24 months after retainer placement).
For the survival analysis, the effect of retainer type on failure was examined using a Cox model adjusted for gender and age. In addition, the proportional hazard assumption was examined via the Schoenfeld residuals.
For the periodontal indexes population average generalized estimating equation models were fit. Because of the skewed distributions and the low number of observations for some outcome levels, some of the outcomes were converted to binary considering clinical relevance as follows: GI (0, >0), PD (≤1, >1), and PDI (0, >0). For those outcomes, logit models were fitted with treatment time, tooth, and treatment × time interaction as predictors using empirical standard errors and independent correlation structures. For PI, a Gaussian model was fitted with empirical standard errors and an exchangeable correlation structure. For BOP, the number of events was calculated, and a Poisson population average model was fitted with empirical standard errors and an independent correlation structure. Missing data analysis included the chained equations approach fully conditional specification with 20 burn-in iterations, and 40 imputations were applied using a logit, Gaussian, or a Poisson model depending on the outcome. The previous generalized estimating equation models were fitted in the complete dataset to assess the robustness of our results. All analyses were conducted in Stata (version 16.1; StataCorp, College Station, Tex), SAS (version 9.4; SAS, Cary, NC), and R software (version 3.6.1; R Foundation for Statistical Computing, Vienna, Austria).
Results
Participants
A total of 133 participants (42 males and 91 females), with a median age of 24.6 years (25th percentile, 17.2 years; 75th percentile, 32.4 years; minimum, 15.1 years; maximum, 49.8 years) were randomized to receive a 3-strand RT wire retainer (RT group; 65 participants: 22 males and 43 females) or 8-strand RB wire retainer (RB group; 66 participants: 19 males and 47 females) bonded to 6 anterior teeth in the mandible. The Consolidated Standards of Reporting Trials flowchart ( Fig 1 ) demonstrates participant flow. A total of 132 participants received the allocated treatment. One participant intervention was discontinued because the patient demanded retreatment for displaced premolar and was excluded from the analyses. Furthermore, 4 participants who missed 1-3 data collection appointments were included in the analysis.
Baseline data
Both groups were comparable regarding baseline characteristics, and periodontal health parameters at baseline (debonding) indicated good periodontal health at the start of the trial ( Table I ).
| Characteristics | RT | RB |
|---|---|---|
| Age | 23.9 (17.0-31.6) | 27.2 (17.5-33.2) |
| Sex, % males | 34.3% | 28.8% |
| PDI | 0 (0-1) | 0 (0-1) |
| BOP | 0.2 (0.1-0.3) | 0.2 (0.1-0.3) |
| PI | 0.3 (0.1-0.5) | 0.3 (0.2-0.5) |
| GI | 0 (0-0.5) | 0 (0-0.5) |
| PD | 1.5 (1-2) | 1.5 (1-2) |
Survival analysis
Thirty-seven out of 66 participants (56.1%) from the RT group and 32 out of 66 participants (48.5%) from the RB group had ≥1 retainer failure during 2 years after retainer placement.
The proportion of the number of teeth with failures to the number of teeth in subjects in whom the first-time failure occurred ( Table II )—25.2% in the RT group and 21.9% in the RB group—implies that retainers in some participants failed at multiple sites. In addition, 2 participants from the RT group lost their retainers completely, whereas no participants from the RB group lost the retainer. Nevertheless, most failures were limited to a single tooth, mainly the central or lateral incisor ( Supplementary Table I ). Failures on the canines were rare.
