Effect of smear layers on the bonding of a self-etching primer to dentin.

PURPOSE: The objective of this work was to evaluate the effect of the absence and presence of smear layers on bonds made to dentin using a self-etching primer system, Clearfil SE Bond.
MATERIALS AND METHODS: Dentin surfaces with different smear layer thickness were created from mid-coronal sound dentin in extracted, human third molars. The control group was cryofractured to create a bonding surface that was devoid of a smear layer. The experimental teeth were ground with wet 60-, 180- or 600-grit SiC paper. They were bonded using SE Bond, followed by resin composite buildups. After 1 day, bonded specimens were sectioned into multiple 1- x 1-mm beams. Microtensile bond strengths were determined and the results analyzed with ANOVA and the Student Neuman Keuls test. Fractographic study of cross sections of failed interfaces from the dentin side of representative beams was performed using both SEM and TEM.
RESULTS: SE Bond produced high bond strengths (ca 50 MPa) to both smear layer-free and smear layer-covered dentin. SEM examination was inadequate to define the exact nature of interfacial failures. TEM observations demonstrated a thin (ca 400 to 500 nm) hybrid layer in the fractured dentin and thicker (ca 1 to 4 microns) hybrid layers on smear layer-covered dentin. This included a thick, hybridized smear layer and a thin, underlying true hybrid layer in the intact dentin. Separation of the two hybrid layers was not evident in interfacial failures.
CONCLUSION: Self-etching primers create thin hybrid layers that incorporate the smear layer. This study shows that formation of true hybrid layers occurs irrespective of smear layer thickness and that both hybrid layers may function as a unit during loading without separation. identification of secondary cracks from TEM fractographic analysis exemplifies the complex reaction to tensile stresses in multilayered joint systems that comprise materials of variable compliance.