The aim of this study was to evaluate and compare the clinical success of 2 lingual retainer wires.
The 120 patients included in the study were divided into 2 groups randomly. In group 1, 0.0175-in 6-strand stainless steel wire (Ortho Technology, Lutz, Fla) was used, the lingual retainers were fabricated on plaster models, and a silicon transfer key was used. In group 2, 0.0195-in dead-soft coaxial wire (Respond; Ormco, Orange, Calif) was used, and the lingual retainers were fabricated directly in the patient’s mandibular arch without a study model. Pretreatment, posttreatment, and posttreatment 3-month, 6-month, 9-month, and 12-month 3-dimensional orthodontic models were evaluated. Failure rates, mandibular arch irregularity values, intercanine distances, and arch lengths were compared.
The clinical bond failure rates were 13.2% for the 0.0175-in 6-strand stainless steel wire and 18.9% for the 0.0195-in dead-soft wire. The difference in bond failures between the 2 groups was not statistically significant. There was a statistically significant increase in mandibular arch irregularity in both groups during the 12-month follow-up. However, the increase was significantly higher in the second group than in the first one. Furthermore, the intercanine distance decreased over time in the second group.
Our findings regarding mandibular arch measurements indicate that fabrication of lingual retainers can be more safely accomplished with 0.0175-in 6-strand stainless steel wire than with 0.0195-in dead-soft coaxial wire.
Dead-soft coaxial wire is easily adaptable and may offer advantages for retainers.
We compared lingual retention with 6-strand stainless steel and dead-soft coaxial wires.
Six-stranded stainless steel was more effective at maintaining mandibular intercanine distance.
Retention of orthodontic treatment results and posttreatment stability are challenges for orthodontists. Although several factors affect stability, there are 3 major ones: time for reorganization of gingival and periodontal tissues, unstable position of the teeth after orthodontic treatment, and changes produced by growth. Many previous studies showed high relapse rates of mandibular anterior teeth after orthodontic treatment. Therefore, orthodontists prefer to use removable types of retainers or bonded fixed lingual retainers at the end of treatment routinely to prevent relapse of the mandibular anterior teeth, whose position is affected by factors that include continual growth and masticatory forces. Many types of removable retainers are available, but the most popular type for nearly a century has been the Hawley retainer. Alternative removable retainers are vacuum-formed retainers, which have become more popular in recemt decades.
Two different types of fixed lingual retainers bonded from canine to canine are commonly used in orthodontic practice. Whereas 1 type requires heavy-gauge (28-30 mil) steel wires bonded to the canines only, the second type uses a lighter, multistrand wire that is bonded to the incisors and the canines. The second type is more often used today.
Various types of retainers have been described with wires of differing material properties and diameters, different types of composites, and fiber reinforcement. A recent article also reported the fabrication of a custom lingual retainer cut from a nickel-titanium blank with CAD/CAM technology. The aim was to find a more accurate lingual retainer application for clinical practice. Manufacturers have also introduced dead-soft lingual retainer wires into the orthodontic market to facilitate the fabrication of fixed lingual retainers. They claimed that those wires have some advantages compared with 5-stranded stainless steel wires because they are easily adaptable and reduce undesirable tooth movements related to active force wires. To date, studies that have evaluated the performance of dead-soft wires have been limited. Long-term clinical studies provide more relevant data on the success and clinical failure of lingual retainers and may be a proper guide for clinicians. Therefore, the aims of this study were to investigate and compare the clinical success of 2 lingual retainer wires fabricated by different methods.
Material and methods
This clinical study was approved by the regional ethics committee (ethics number OMUKAEK2014/806) of Ondokuz Mayıs University. In total, 120 patients (60 patients treated with first premolar extractions, 60 treated without extractions) were included in the study. There were 60 patients in group 1 with a mean age of 15.7 years (17 boys, 43 girls) and 60 patients in group 2 with a mean age 16.2 years (20 boys, 40 girls). The sample size was calculated using data from a previous study. A sample size of 60 patients per group gave 80.07% power to detect significance differences at the 0.05 level. They were selected according to the following criteria: (1) no missing mandibular incisors, (2) no restorations on the mandibular incisors that might affect retainer bonding on lingual enamel surfaces, and (3) no morphologic crown anomalies.
Lingual surfaces of the mandibular anterior teeth were cleaned and polished before retainer bonding. To examine the influence of different lingual retainer wires, the patients were divided into 2 groups randomly with equal numbers of extraction and nonextraction patients. In the first group, 0.0175-in 6-stranded stainless steel wire (Ortho Technology, Lutz, Fla) was used as the lingual retainer; in the second group, 0.0195-in dead-soft coaxial wire (Respond; Ormco, Orange, Calif) was used.
In the first group, an impression was taken, and the lingual retainers were fabricated on plaster study models. The retainer wire was prepared in tight contact with the lingual surfaces of all teeth included in the retainer, and silicon transfer keys were used to transfer the retainer to the mandibular arch ( Fig 1 ). Lingual enamel surfaces of each tooth were acid-etched with 32% orthophosphoric acid for 15 seconds, rinsed, and dried. Transbond XT adhesive primer and Transbond LR (3M Unitek, Monrovia, Calif) adhesive resin paste were used according to the manufacturer’s instructions.
In the second group, the retainer was applied directly without a transfer key. First, the dead-soft retainer wire was bonded to the left canine with the same adhesives as in group 1, and then the wire was passively adapted to the lingual surfaces of the other canine and incisors. The dead-soft wire enabled the clinician to bend the retainer easily with light pressure using a hand instrument ( Fig 2 ). The mandibular incisors and the other canine were bonded with the same adhesive primer and paste. All retainers were bonded by the same investigator (F.G.), who has clinical experience of over 2 years. The retainer reached the first premolars in the extraction patients in both groups. Vacuum-formed retainers were used for the maxillary arches in both groups.
The patients were advised to come to our clinic without delay if they became aware of retainer failure. Otherwise, they were recalled at 3 months and followed up for 6, 9, and 12 months after the retainer was bonded. When retainer failure occurred, it was repaired with the same adhesives after removal of remnants. Pretreatment (T0), posttreatment (T1), posttreatment 3-month (T2), 6-month (T3), 9-month (T4), and 12-month (T5) orthodontic models were scanned and digitized with an orthodontic 3-dimensional (3D) scanner (R-700 desktop orthodontic scanner; 3Shape, Copenhagen, Denmark). Failure rates were recorded, and mandibular arch irregularity was measured as described by Little. Length of the mandibular arch was calculated with the sum of the distances from mesial contact point of the first molars to the midpoint of the incisal edges of the mandibular incisors. Mandibular arch irregularity, intercanine distance, and arch length were measured using Ortho Analyzer software (3Shape) by the same investigator (F.G.).
The 3D orthodontic model measurements at T0 were compared to establish any differences between the groups before orthodontic treatment. The angle between the mandibular incisors and mandibular plane and the angle between the maxillary and mandibular incisors were measured. The angular changes in the mandibular incisors (T1-T0) during orthodontic treatment were also investigated from lateral cephalometric radiographs in both groups. Patients with rotation of the mandibular anterior teeth were not included to the study because of the stronger tendency of relapse. Model and cephalometric measurements showed that both groups contained similar patients before orthodontic treatment ( Table I ). Seven patients in each group did not attend the appointments after the retainer was bonded; the final data were analyzed with 53 participants per group.
|Group 1||Group 2||P|
|Mean ± SD||Mean ± SD|
|Irregularity||7.6 ± 3.7||6.9 ± 3.9||0.306|
|Intercanine distance||26.4 ± 2.3||26.3 ± 1.7||0.614|
|Arch length||58.2 ± 3.8||58.2 ± 3.9||0.983|
|Mean (minimum-maximum)||Mean (minimum-maximum)|
|IMPA (T1-T0)||1.4 (−12.2 to 9.5)||0 (−10.1 to 9.8)||0.204|
|U1-L1 (T1-T0)||0.1 (−22 to 9.5)||−3 (−18 to 13)||0.909|
All statistical analyses were performed using the SPSS software package (version 23; IBM, Armonk, NY). Kolmogorov-Smirnov normality tests were conducted for quantitative data. Comparison of model measurements at T0 was done by independent sample t tests and cephalometric measurements with the Mann-Whitney U test to establish any difference between the groups before orthodontic treatment. Lingual retainer survival rates over 12 months were evaluated with the Kaplan-Meier test, and differences in retainer survival curves by retainer wire type were evaluated by the log-rank test. Chi-square tests were used to analyze the relationship between bonding failure and retainer type. The irregularity and intercanine distance at different times were compared using repeated-measures analysis of variance, whereas arch length measurements were compared using Friedman tests for intragroup comparisons in both groups. The irregularity change from the posttreatment period to the 12-month follow-up (T5-T1) was calculated for each variable, and the groups were then compared using the Mann-Whitney U test. The level of significance was set at P <0.05.
Total orthodontic treatment durations (T1-T0) were 17 months for group 1 and 19 months for group 2. After orthodontic treatment, 17 retainers failed over the 12 months ( Table II ). The failure rates were 13.2% for the 0.0175-in 6-stranded stainless steel wire (group 1) and 18.9% for the 0.0195-in dead-soft coaxial wire (group 2). The difference between the groups was not statistically significant ( P >0.05). The effects of type of wire on retainer survival rates are shown in Figure 3 . The distributions of the failures according to tooth type are given in Table III . In group 1, 2 retainers that included 2 teeth failed; in group 2, only 1 retainer that included 2 teeth failed.
|Group 1||Group 2||Test statistics||P|
|Failure||7 (13.2%)||10 (18.9%)||χ 2 = 0.280||0.597|
|No failure||46 (86.8%)||43 (81.1%)|