Abstract
This study investigated the effect of over-etching and prolonged application time of a universal adhesive on dentin bond strength. Ninety extracted human molars were ground to dentin and randomly allocated into nine groups (G1–9; n = 10 per group), according to the following acid etching and adhesive application times. In the control group (G1), phosphoric acid etching was performed for 15 s followed by application of the universal adhesive Scotchbond Universal (3M) for 20 s, as per manufacturer’s instructions. In groups G2–5, both the etching and adhesive application times were either halved, doubled, quadrupled, or increased eightfold. In groups G6–9, etching times remained the same as in G2–5 (7.5 s, 30 s, 60 s, and 120 s, respectively), but the adhesive application time was set at 20 s as in the control group (G1). Specimens were then restored with a nanofilled composite material and subjected to microtensile bond strength testing. Bond strength data were statistically analyzed by ANOVA and Tukey’s post-hoc tests (α = 0.05). The relationship of bond strength with etching and adhesive application time was examined using linear regression analysis. Treatment of dentin with halved phosphoric acid etching and adhesive application times (G2) resulted in a significant bond strength decrease compared to the control group (G1) and all other test groups, including the group with halved acid etching, but 20 s of adhesive application time (G6). No significant differences in bond strength were found for groups with multiplied etching times and an adhesive application time of 20 s or more, when compared to the control group (G1). In conclusion, a universal adhesive application time of at least 20 s is recommended when bonding to over-etched dentin.
Highlights
The achievement of a strong and solid hybrid layer plays an important role in bonding composite to dentin and is still a challenge during adhesive restorative procedures [1,2,3,4,5,6,7,8]
No significant differences in dentin bond strengths were observed between the groups with doubled (G3: 17.40 ± 2.33 MPa), quadrupled (G4: 18.84 ± 5.50 MPa) or eightfold increased etching and adhesive application times (G5: 15.88 ± 4.58) and the control group
Regression analysis showed no linear relationship between microtensile bond strength and etching time/adhesive application time (p > 0.05)
Summary
The achievement of a strong and solid hybrid layer plays an important role in bonding composite to dentin and is still a challenge during adhesive restorative procedures [1,2,3,4,5,6,7,8]. The subsequently applied polymer composite has to combine good physical, mechanical and biological properties, while matching the dental hard tissue to be restored [9,10]. For hybridization of dentin prior to the application of a resin composite, two adhesive approaches can be distinguished: (1) the etch-and-rinse approach and (2) the self-etch approach [2,7,11]. Polymers 2020, 12, 2902 etching step, followed by water rinsing and drying of the surfaces prior to application of the adhesive system [11,12,13]. The self-etch approach, relies on self-etching acidic monomers instead of phosphoric acid to rather integrate the smear-layer and to simultaneously demineralize and infiltrate dental hard tissue [2,14,15]
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