Streptococcus mutans Biofilm in Orthodontic Retainers: In Vitro Comparative Study

Objectives: Orthodontic retention is an important final phase to maintain the teeth in correct position after the active treatment. However, retainers may represent a risk to the periodontal health. They should have comfortable designs that facilitate flossing and brushing. The objective of this study was to evaluate the accumulation of Streptococcus mutans (S. mutans) biofilm on a new design of V-loop retainer, comparing to V-loop retainer or straight retainer, in vitro.
Methods: Models of the lower-anterior dental arch were reproduced in thermo-activated acrylic resin and divided into 3 groups: New design of V-loop retainer (NVLR), V-loop retainer (VLR) and straight conventional retainer (SR) for comparative purposes. All retainers were made with same orthodontic wire and bonded to the lingual of anterior teeth using 7mg of resin in each fixation point. The orthodontic wire length and the retainer distances from the dental surface were measured. Thereafter, the samples were incubated at 37°C with S. mutans (ATCC 25175) in Brain Heart Infusion (BHI) medium, 2x10⁴ colony-forming units per milliliter (cfu/mL), for 48 hours. Biofilm analysis was performed by measuring S. mutans viability with MTT assay. The experiment was repeated three times independently (n=12/group). Statistics: Anova, Bonferroni post-test (p<0.05) and Pearson product-moment correlation.
Results: The NVLR group had lower levels of biofilm accumulation, statistically significant compared to the other groups. Biofilm accumulation was higher in VLR and SR, statistically similar to each other. The correlation between retainer distance from the dental surface and S. mutans biofilm was strongly negative (r=-0.7). The retainer length showed weak positive correlation with biofilm (r=0.4).
Conclusions: The data of this in vitro study will be relevant for a future randomized clinical trial design. NVLR group showed less biofilm, independent of flossing or brushing, which seems to be correlated with the distance from dental surface.