Effect of Double-layer Application on Bonding of Universal Adhesives to Dentin

Objectives: The aim of this in vitro study was to evaluate the effect of double-layer application of a mild (Futurabond U, VOCO GmbH) and an intermediate universal adhesive (G-Premio Bond, GC), in self-etch mode on microtensile bond strength (µTBS) to dentin.
Methods: Twenty extracted non-carious human third molars were ground flat with 180-grit SiC paper to expose mid-coronal dentin. After standardization of smear layer with 600-grit SiC paper, the roots were cut horizontally 2 mm below the cementoenamel junction. Then teeth were randomly assigned into four groups (n=4); 1) Futurabond U + Single layer (FBSL) 2) Futurabond U+ Double layer (FBDL) 3) G-Premio Bond + Single layer (GPSL) 4) G-Premio Bond + Double layer (GPDL). Adhesives were applied on the dentin surfaces however not light cured between the layers. Composite build-ups were performed with Filtek Z250 (3M ESPE). After storage in distilled water at 37°C for 24 h, teeth were sectioned into beams (1x1 mm) using a diamond saw (IsoMet, Buehler). Resin-dentin sticks (n=20) were subjected to µTBS test using universal testing machine (Instron). Data were analyzed using two-way ANOVA and independent t-test (p<0.05).
Results: DL significantly affected the bond strength to dentin (p=0.177), while the type of the universal adhesive did not (p=0.0001). The interaction between these two variables was also significant. DL significantly improved µTBS for FBU (p=0.001) and GP (p=0.001) universal adhesives to dentin. GP showed lower µTBS to dentin (20.28±2.87 MPa) than FBU (22.68±3.26 MPa) in SL, while higher µTBS was obtained with GP (31.95±10.54 MPa) than FBU (26.01±2.34 MPa) in DL and the differences were significant (p=0.018 and p=0.019 respectively).
Conclusions: Bonding of the mild and intermediate universal adhesives FBU and GP in self-etch mode to mid-coronal dentin are affected by double-layer application. However, the effectiveness of this technique should also be evaluated by long term bond strength tests.