Polymerization characteristics of orthodontic adhesives: a study on degree of conversion

Relevance. Adhesive materials play a crucial role in orthodontic treatment. The introduction of universal bonding agents to the dental market offers greater flexibility in choosing adhesive protocols for clinical use. However, many studies evaluating the mechanical properties of adhesives, such as shear and tensile bond strength, fail to address the degree of conversion of polymerized bonding materials beneath brackets.

Objective. To compare the degree of conversion between a universal adhesive system and a total-etch system for orthodontic applications.

Materials and methods. The study compared the Transbond XT total-etch system (3M Unitek, USA) with the Tetric N-Bond Universal system (Vivadent Ivoclar, Liechtenstein). Four groups of five samples each (n = 5) were analyzed. Groups 1 and 2 involved polymerization without brackets, while Groups 3 and 4 involved polymerization under metal brackets (Gemini, 3M Unitek, USA). Spectra were recorded immediately after polymerization for 15 minutes using Fourier-transform infrared spectroscopy (FTIR). Statistical analysis included the Shapiro–Wilk test, analysis of variance (ANOVA), and Tukey's multiple comparison test, with a significance level of p < 0.05.

Results. The mean degree of conversion in the control groups was 55.9% ± 0.44 for Group 1 and 66.72% ± 0.15 for Group 2. A reduction in conversion was observed in specimens polymerized under metal brackets compared to the control groups. The mean conversion in Group 3 was 36.02% ± 0.19, which was 1.1% lower than in Group 4 (37.1% ± 0.16).

Conclusion. The universal adhesive Tetric N-Bond Universal demonstrated superior polymerization performance for orthodontic applications compared to the total-etch adhesive system.

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