Contemporary self-etch and multi-mode adhesives were introduced to ensure a fast and reliable bonding procedure. Yet, in terms of bond strength and stability they failed to perform as well as two-bottle, etch-and-rinse adhesives, which remain the gold standard in terms of durability. The purpose of this study was to assess the shear bond strength (SBS) of dental adhesives to dentin with different application protocols. Two self-etch (Adper Easy One and Xeno V) and two multi-mode adhesives (Single Bond Universal and Prime&Bond One Select) were used in the study. The highest SBS was obtained for Single Bond Universal applied in three layers, while the lowest, for Xeno V applied in one layer. Other tested adhesives obtained the highest SBS when applied in three layers. For all tested adhesives, multi-layer application resulted in an increase in adhesive layer thickness, as observed in SEM. The increased thickness of the adhesive layer produced by triple application of unfilled adhesives corresponded with higher SBS values. The present study showed that using triple adhesive layers with simplified adhesive systems can be recommended to improve their performance. Due to differences in the composition of self-etch and universal adhesives, the exact application protocol is product dependent.

Contemporary adhesives use etch-and-rinse, self-etch, and multimode adhesive strategies. Simplified and universal adhesives present lower bond strength to dentin than conventional, two-bottle etch-and-rinse adhesives. The purpose of this study was to evaluate bonding performance of simplified and universal adhesives to dentin after modifying their application protocol (multiplying applications and extending application time). Adhesive layer thickness (ALT) and shear bond strength (SBS), as well as the correlation between these parameters was calculated. Two universal (Single Bond Universal and Prime & Bond One Select) and two self-etch adhesives (Adper Easy One and Xeno V) were tested. Significant differences in ALT were detected between the adhesives, as well as within the same adhesive between study groups. Tested adhesives presented the thinnest adhesive layer when applied 2 times in 20 s. Single Bond Universal obtained the highest SBS results of all adhesives. Most adhesives (except for Prime & Bond One Select) obtained the highest SBS, when applied two or three times in 40 or 60 s, respectively. No correlation between the ALT and SBS was found. The study showed that increasing the number of applications and extending the application time of self-etch and universal adhesives can be recommended to improve their performance.