Purpose: To examine whether dimethyl sulfoxide (DMSO) would affect resin-dentin bonding of a intermediately strong and HEMA free universal adhesive after long-term aging.

Methods: Sixty human molars were used in this study. Forty teeth were assigned to experimental groups using a HEMA free universal adhesive (iBond Universal, Kulzer, [IBU]), and twenty teeth were used for the positive control groups with a HEMA-containing universal adhesive (Scotchbond Universal [SBU]). The IBU groups were divided according to: 1) Treatment (untreated or DMSO pretreatment) and 2) Application mode (etch-and-rinse or self-etch). For storage time, specimens from the same tooth were randomly tested at 24 h or 4 years. The restoration was performed, and the specimens were sectioned into resin-dentin bonded beams. Resin-dentin bonded beams were tested (24 h and after 4 years of water storage) for microtensile bond strength (mu TBS) (0.5 mm/min) and to assess nanoleakage (NL). Data on mu TBS and NL were analyzed using three-way ANOVA and Tukey's test (α = 0.05).

Results: After 4 years of water storage, groups without DMSO pre-treatment showed significant reductions in mu TBS and increased NL, regardless of application mode (p < 0.001). In contrast, DMSO-treated samples showed no significant reductions after 4 years, regardless of the application mode (p > 0.05). When compared to the positive control group, the IBU untreated group exhibited significant reductions in mu TBS and increased NL, regardless of the application mode (p < 0.001). However, no significant differences were observed for IBU DMSO-pretreated samples compared to SBU, regardless of the application mode and evaluation times (p > 0.05).

Conclusion: DMSO-wet bonding effectively enhanced and preserved the quality of resin bonds over time when using an intermediately strong HEMA free universal adhesive.

Clinical significance: DMSO pretreatment is a simple, practical strategy that enhances adhesive restoration durability, making it highly applicable in routine clinical practice.