Effect of smear layer thickness and pH of self-etching adhesive systems on the bond strength and gap formation to dentin

Purpose: To measure the buffering capacity of self-etching adhesive systems and to evaluate the effect of smear layer thickness on the mean gap width (GW) and microtensile bond strength (mu BS) to dentin of these self-etching adhesives. Materials and Methods: Clearfil SE Bond (SE), Optibond Solo Plus Self-Etch Primer + Optibond Solo Plus (SO), Tyrian Self Priming Etchant + One Step Plus (TY) and as controls, Single Bond (SB) and ScotchBond Multi-Purpose Plus (SC) were used with Filtek Z250 Universal Restorative. The self-etching primers and 35% phosphoric acid (0.5 ml) were titrated by adding successive 0.05 ml drops of NaOH (1N). Thirty molars (n = 5) were sectioned in halves upon which thick (60-grit Sic) and thin (600-grit Sic) smear layer surfaces were produced. The adhesive application was randomized (n = 6) and a resin buildup was made. After 24 h, the resin-dentin sticks (0.8 mm(2)) were prepared. The GW on the sticks was measured at 400X magnification before the microtensile test (0.5 mm/min). The microtensile bond strength index was calculated including the relative failure type contribution. Data were analyzed by a two-way ANOVA and Tukey's multiple comparison. Simple linear regression was performed to determine whether correlation existed between mu BS and pH, GW and pH, and mu BS and GW. Results: Only the factor "adhesive" significantly effected mu BS (p = 0.001). TY had the lowest mu BS values. Regarding GW, both main factors adhesive (p = 0.0001) and smear layer thickness (p = 0.03) were significant. The highest mean gap width was observed for SE and under a thick smear layer. A linear relationship was observed between the pH and GW and pH and mu BS; however, a significant positive correlation was detected only between pH and GW (R = 0.99, p = 0.02). No relationship between mu BS and GW was found (R = 0.04, p = 0.77). Conclusion: Although the smear layer thickness had no effect on resin-dentin bond strength, thick smear layers adversely affected the mean gap width. The mu BS values were not affected by the acidity of self-etching primers; however, more aggressive self-etching primers showed the lowest mean gap width. The etch-and-rinse adhesive systems showed higher mu BS and lower mean gap widths.