Bond strength, degree of conversion, and microorganism adhesion using different bracket-to-enamel bonding protocols.

PURPOSE: This study aimed to evaluate the effect of bonding protocols and the type of orthodontic resin on the adhesion of microorganisms, degree of conversion (DC), and shear bond strength (SBS) of metallic brackets to enamel. METHODS: A total of 60 bovine incisors were prepared and randomly divided into 6 groups (n = 10): "bonding protocol" (A: phosphoric acid; AXT: A+Transbond™ XT primer adhesive [all Transbond™ products from 3M Unitek, Monrovia, CA, USA]; and SE: Transbond™ Plus Self Etching Primer) and "orthodontic resin" (XT: Transbond™ XT adhesive paste and CC: Transbond™ Plus Color Change). After bonding, the samples were subjected to thermocycling (5000 cycles) and to the SBS test. Bond failures were classified according to the adhesive remnant index (ARI). Next, 60 enamel blocks were sterilized in ethylene oxide in order to perform the CFU (Streptococcus mutans) assay in vitro to analyze the colony forming units (CFU/mL). Then, 60 discs of each orthodontic resin were made to measure the DC. The SBS (MPa), CFU/mL, and DC (%) data were statistically analyzed by two-way analysis of variance and Tukey's test (5%) was performed for the DC. RESULTS: CFU and SBS revealed no significance for all factors (P > 0.05). Tukey's test showed that A_XT (acid+Transbond™ XT adhesive paste) presented the highest DC (70.38% ± 10.5), while AXT_XT (acid+Transbond™ XT primer adhesive+Transbond™ XT adhesive paste) showed the lowest (23.47% ± 10.4). An ARI score of 2 was more frequent for the CC resin and an ARI score of 4 for the XT resin. CONCLUSION: The CC resin does not reduce adhesion of S. mutans around orthodontic brackets and the bonding protocol did not influence the SBS, although the SE and A_XT groups contributed to a better DC.