Universal Adhesives Applied to Dentine in Etch-and-Rinse or Self-Etch Mode

Objectives: This in-vitro study aimed at evaluating the microtensile bond strength and the resin-dentine interface morphology of modern universal adhesives applied onto dentine using different adhesive strategies (Self-Etch (SE) or the Etch-and-Rinse (ER)). Moreover, it was also evaluated the fractographic features after bond strength test.
Methods: Sixty-four molars were divided into four groups (16/group) based on the adhesive systems employed in this study: Zip bond Universal (ZIP: SDI Limited, Australia), Scotchbond Universal (SCH: 3M ESPE, USA), Clearfil Quick Bond (CQB: Kuraray, Japan), Prime & Bond Active (PBA: Dentsply, Germany). These specimens were further divided into two sub-groups (8/group) based on the bonding protocol employed (SE or ER). Incremental 1 mm build-ups (up to 5mm)were created using a universal (Filtek Universal Restorative, 3M ESPE, USA) composite (each layer was light-cured for 15 sec, and a final-light step of 60 sec). Specimens were stored in water for 24h, then cut into match-sticks(0.9 mm²) and submitted to microtensile bond strength test. The fractographic analysis was subsequently performed through scanning electron microscopy (FEG-SEM). Three resin-dentine slabs were selected from each sub-group, processed and analysed through dye-assisted confocal microscopy. The results were analysed by two-way ANOVA (adhesive system and bonding strategy as independent variables). Fisher's least significant difference (LSD) test was used to isolate and compare the significant differences between the groups. The level of significance was set at 5%.
Results: The lowest bond strength (MPa) was observed with the CQB quick application both in SE and ER mode, with resin-dentine interfaces characterised by gaps and porosities. Conversely, there was no significant difference in bond strength between the tested materials when applied in SE or ER mode (P < 0.05).For these latter adhesives, their interfaces were often characterised by the absence of defects or gap.
Conclusions: These innovative materials are able to achieve a reliable bonding performance due to their suitable interaction with dentine both when used in SE or ER mode. However, the use of “quick” bonding protocols should be avoided when using modern universal adhesives.