Effectiveness of experimental composites containing antimicrobial agents for orthodontic applications

This study evaluated the effectiveness of experimental composites containing antimicrobial agents such as nisin (N) and stannous fluoride (SnF₂) against S. mutans, for orthodontic applications. First, the antimicrobial agents were added at varying concentrations (%) into a base material consisting of methacrylate monomers and inorganic particles, forming the groups: control (C), N4% (N4), N7% (N7), N10% (N10), SnF₂0.6% (SnF₂0.6), SnF₂1.5% (SnF₂1.5), SnF₂ 3% (SnF₂3), and N4% + SnF₂ 0. 6% (N4SnF₂0.6). The degree of conversion (DC%) and colony-forming units (CFU) were evaluated. Brackets were bonded onto the enamel of bovine incisors using these composites, and the enamel hardness was measured at distances of 500, 550, and 600 µm from the bracket, before and after the cariogenic challenge. The shear bond strength was also investigated, and the composite remnant on the enamel was assessed using the ARI score. Data were analyzed using analysis of variance and Tukey's test (α = 0.05). Of all the groups, only N10 led to lower DC% (p < 0.05). CFU was significantly reduced for all groups when compared to C (p < 0.05), with SnF₂ showing the highest inhibition for all concentrations. After the cariogenic challenge, for all distances from the brackets, the decrease in hardness was lower for all experimental composites than for C (p < 0.05). SBS was lower only for the N4SnF₂0.6 when compared to C. Adhesive failure was more frequent, with the composite remnant covering less than 50% of the adhesive area. In conclusion, adding those antimicrobial agents effectively inhibited S. mutans and protected enamel around brackets without compromising most mechanical properties. However, the highest nisin concentration (10%) adversely affected the degree of conversion, and the combination of nisin and stannous fluoride reduced bond strength.