TEM interfacial characterization of the resin-smear-complex at the adhesive-dentin/enamel interface

Objectives: The bonding potential of 'ultra-mild' one-step self-etch adhesives may be compromised due to smear interference, as they cannot dissolve/penetrate the smear layer effectively due to their relatively high pH. We characterized using TEM the interaction of such an ultra-mild self-etch adhesive with enamel/dentin covered with different smear layers.

Methods: Human dentin and enamel surfaces were prepared using either a medium-grit (100 μm) diamond bur or #600-grit SiC-paper. Fractured dentin and un-cut enamel served as smear-free control surfaces. Clearfil S³ Bond (Kuraray) was applied and non-demineralized/demineralized 40-90 nm sections were prepared following common TEM-specimen processing and diamond-knife ultra-microtomy.

Results: DENTIN: The thickness of the dentin smear layer differed depending on the substrate preparation methodology used. The adhesive did not dissolve the bur-, nor the SiC-smear, but impregnated them. Within this 'resin-smear complex', hydroxyapatite was abundantly present. Fractured dentin specimens lacked this complex so that the actual interaction effect could be observed. This interaction effect was limited to about 100 nm and showed hardly any sign of demineralization. In ultra-thin (50 nm) sections, a more electron dense band was consistently observed, which might could represent the interaction of the functional monomer 10-MDP with calcium. ENAMEL: The enamel smear layer differed not only in thickness, but also in crystal-density and -size depending on the preparation method used. Lab-demineralization of sections (1 N HCl, 5 sec) clearly disclosed that the different smear layers resulted in different infiltration patterns. Infiltration progressed preferentially along micro-cracks that were abundantly present in the bur-prepared specimens, but not in the non-prepared specimens; this pattern however substantially varied from region to region.

Conclusion: The nature of the smear layer significantly affects the interaction of ultra-mild adhesive. Especially at enamel, bur preparation did not only increase the surface roughness, but also induced micro-cracks that can provide additional micro-mechanical retention.