Indirect vs direct bonding of mandibular fixed retainers in orthodontic patients: Comparison of retainer failures and posttreatment stability. A 2-year follow-up of a single-center randomized controlled trial.

INTRODUCTION: The objectives of this 2-arm parallel trial were to compare the numbers of failures of mandibular fixed retainers bonded with indirect and direct methods and to investigate the posttreatment changes 2 years after placement.

METHODS: Sixty-four consecutive patients from the postgraduate orthodontic clinic of the University of Geneva in Switzerland were randomly allocated to either an indirect or a traditional direct bonding procedure of a mandibular fixed retainer at the end of their orthodontic treatment (T0). Eligibility criteria were the presence of the 4 mandibular incisors and the 2 mandibular canines, and no active caries, restorations, fractures, or periodontal disease of these teeth. The patients were randomized in blocks of 4 (using an online randomization service) with allocation concealment secured by contacting the sequence generator for assignment. The patients were recalled 12 months and 24 months (T3) after retainer bonding. The main outcome was any first-time failure of retainers (ie, at least 1 composite pad debonded or fractured); unexpected posttreatment changes of the mandibular incisors and canines were a secondary outcome. Impressions and lateral cephalograms were taken at T0 and T3: changes in mandibular intercanine and interpremolar distances and mandibular incisor inclination were assessed. Blinding was applicable for outcome assessment only. The chi-square test and Cox regression were used to compare the survival rates of the retainers bonded with direct and indirect methods. Paired t tests were used to assess differences in intercanine and interpremolar distances and mandibular incisor inclination at T0 and T3. Significance was set at P <0.05.

RESULTS: Sixty-four patients were randomized in a 1:1 ratio. One patient dropped out at baseline, and 3 patients did not reach the T3 recall. In 24 of 60 (40%) patients, the fixed retainer failed within 2 years: 13 of 30 (43%) in the indirect bonding group and 11 of 30 (37%) in the direct bonding group (log-rank test, P = 0.64). The hazard ratio was 1.26 (95% confidence interval, 0.56-2.81; P = 0.58). Bond failures occurred mainly during the first year. There were no clinically significant changes in mandibular intercanine distance, interpremolar distance, and incisor inclination between T0 and T3, or between groups. In 5 patients (17%), all in the direct bonding group, unexpected posttreatment changes, systematically consisting in a lingual inclination of the mandibular left canine, were observed. In 1 patient (3%), the change was considered clinically severe. No other serious harm was observed.

CONCLUSIONS: There was no difference in the risks of failure between mandibular retainers bonded with direct and indirect methods. Bonded retainers are effective in maintaining intercanine and interpremolar distances. There seem to be fewer unexpected posttreatment changes with retainers bonded with the indirect compared with the direct method.

REGISTRATION: The trial was not registered.

PROTOCOL: The protocol was not published before trial commencement.

FUNDING: No funding or conflict of interest to be declared.