Does the Conditioning Mode and Duration of Universal Adhesives Affect the Bonding Effectiveness to Fluorotic Enamel?

Purpose: To compare the adhesive-enamel microshear bond strength (µSBS), in situ degree of conversion (DC), and the enamel-etching pattern of universal adhesives when applied for a prolonged period in the self-etch vs the etch-and-rinse mode in fluorotic enamel. Materials and Methods: Ninety-six human molars (48 with a Thylstrup and Fejerskov index [TFI] score of 0 and 48 with TFI score of 4) were sectioned into four parts and divided into 24 experimental groups based on enamel surface (sound enamel or fluorotic enamel), adhesive (Clearfil Universal Bond, Futurabond U, iBond Universal, or Scotchbond Universal), and enamel treatment/application time (etch-and-rinse mode [ER] or self-etch mode with application times of 20 s [20SE] and 40 s [40SE]). The specimens were stored for 24 h and tested in shear at 1.0 mm/min (μSBS). Adhesive-enamel interfaces were evaluated for DC using micro-Raman spectroscopy. The enamel-etching pattern was evaluated using SEM. For each adhesive, data from µSBS and DC were analyzed separately using two-way ANOVA and Tukey's post-hoc test at α = 0.05. Results: On sound enamel, 40SE usually resulted in statistically similar mean µSBS (p > 0.52) and statistically significantly improved mean DC values (p 0.001) compared with the ER mode. Moreover, significantly improved mean µSBS and DC values were obtained compared with 20SE (p 0.01). On fluorotic enamel, there was no statistically significant difference in µSBS between the experimental groups (p > 0.76). However, for each applied adhesive, 40SE resulted in improved mean DC values compared with 20SE or ER (p 0.001). The deepest enamel-etching pattern was obtained in ER mode, followed by 40SE in sound and fluorotic enamel. Conclusion: Compared with ER mode, the prolonged application time of universal adhesives in SE mode in fluorotic enamel increased the DC, enhanced enamel-etching pattern and promoted similar results in terms of adhesive-enamel bond strength. Keywords: adhesive bond strength, dental fluorosis, micro-Raman spectroscopy, scanning electron microscopy, universal adhesive