Effect of smear layer thickness on bond strength mediated by three all-in-one self-etching priming adhesives

The aim of this study was to investigate the effect of dentin smear-layer thickness on the bond strength of three all-in-one adhesives of different acidity.

Peripheral dentin was prepared by wet grinding on SiC papers (80-, 180-, 240-, 320-, 400-, 600-, 4000-grit), and by turbine cutting with supercoarse, coarse, medium, fine, or extrafine diamond burs. The smear layer thickness (SLT) was measured microscopically. The relationship between SLT and average grain size of the abrasives used to prepare the dentin surface was described by regression analysis. Shear bond strength (SBS) was determined for the following adhesives: AC Bond, pH 2.1 (experimental, Heraeus); AQ Bond, pH 2.5 (Sun Medical); and Prompt L-Pop, pH 1.1 (3M-Espe). Six specimens were tested for each adhesive on dentin ground with each of the seven SiC grit sizes. The mode of failure was inspected by SEM. SBS data were analyzed by one-way ANOVA and Fisher's PLSD test at p < 0.05.

A logarithmic relationship between SLT and grain size of the abrasives was established. The coefficient of determination R2 for the regression line was 0.803. Shear bond strengths for the individual adhesives did not differ significantly by SiC grit size. SBS of AC Bond (18.3 MPa) was greater than the SBS of AQ Bond and Prompt L-Pop (16.9 MPa). Cohesive failures were consistently found in resin composite or adhesive.

Smear layer thickness increased with decreasing SiC grit numbers and increasing diamond bur roughness. In spite of the widely differing acidity, all three adhesives tested were equally effective over the range of SLT from 2.6 microm through 0.9 microm.