Potential smear interference with self-etch hybridization studied by TEM

Objectives: To evaluate whether hybridization of enamel/dentin by self-etch adhesives may be affected by smear. Methods: Two 2-step self-etch adhesives, the recently marketed Optibond XTR (O-XTR; Kerr) and Clearfil SE Bond (C-SE; Kuraray), were bonded strictly according to the manufacturer's instructions to either (1) bur-cut (100-ìm grit) enamel/dentin, (2) 600-grit SiC-paper ground enamel/dentin, or (3) smear-free un-cut enamel and fractured dentin (non-carious human third molars). The 3-step etch-and-rinse adhesive Optibond FL (O-FL, Kerr) served as control. After 1-day storage in water (37°C), non-demineralized/demineralized 70-90 nm sections were prepared following common TEM-specimen processing, and eventually examined by TEM (JEM-1200EX II, Jeol). Results: At enamel, a tight bond based on only superficial interaction was observed for both O-XTR and C-SE. Especially at uncut enamel, hydroxyapatite rods appeared hardly dissolved (in contrast to phosphoric-acid etched enamel in case of O-FL). At dentin, the hybrid layer varied from maximum 1 µm in thickness for C-SE to about 1.5 µm for O-XTR (again in contrast to the 3-5 µm hybrid layer produced by O-FL). For O-XTR, the hybrid layer appeared nearly completely demineralized, with only at the bottom part some hydroxyapatite remaining. For C-SE, residual hydroxyapatite could be found within the whole hybrid layer. Especially at bur-cut dentin, the hybrid layer of C-SE, somewhat in contrast to that of O-XTR, appeared to contain more minerals, most likely representing resin-encapsulated smear remnants. Conclusion: The obtained tight interface at both enamel and dentin indicates that both the two-step self-etch adhesives O-XTR and C-SE effectively bonded to tooth tissue. O-XTR etched slightly deeper, which should be attributed to it lower pH of 1.6 (vs. 1.9 for C-SE), and following the AD-concept of Yoshida et al. (2004) maybe also because of the more decalcification than adhesion effect of its functional monomer GPDM (vs. 10-MDP in C-SE).