Assessing Strength of Bioactive and Orthodontic Adhesives After Aging Challenges

Objectives: A bioactive material, presumably advantageous for orthodontic application, showed good bracket bond strengths. Since cohesive strength also affects debonding behavior, diametral tensile strengths after aging and acid challenge were determined and compared to conventional resin-base and glass ionomer orthodontic adhesives
Methods: Cylindrical samples for diametral tensile testing (5mm diameter, 3.5mm length) were made with a bioactive (Activa Bioactive), conventional (Transbond XT), and glass ionomer (GlassLok) orthodontic adhesive. Samples were light cured for 20s from both sides with Valo curing light. The bioactive and glass ionomer materials were kept at 37^°C for 10 minutes before demolding to ensure setting. Samples were stored in water at 37^°C for 24h, thermocycled 5/55°C for 5,000 cycles, or thermocycled followed by three weeks in lactic acid gel (pH 5.1) before testing (N=10/group). The diametral tensile tests were carried out at 0.5mm/min crosshead speed. Tensile strength data were analyzed using two-way ANOVA and pairwise comparisons (significance level 0.05) and correlated with existing bracket bond strengths determined using the same experimental protocol.
Results: Diametral tensile strengths are shown in the table. Two-way ANOVA indicated that material, test condition, and material*test condition interaction significantly affected the diametral tensile strengths (p=0.0001, p=0.0146, and p=0.0135). The strength of the bioactive material was about 25% lower than the conventional adhesive, but was 3-times higher than the glass ionomer. Aging through thermocycling alone did not significantly decrease the tensile strength unless in combination with the acid challenge in the conventional and bioactive adhesives. Correlation coefficient between diametral tensile strengths and bond strengths was 0.67.
Conclusions: The diametral tensile strength of the bioactive adhesive was generally lower than the conventional adhesive and showed a positive correlation with bracket bond strengths. Unlike the conventional adhesive, the acidic environment did not significantly reduce tensile strength of the bioactive or glass ionomer materials compared to the initial values.