Limited Interaction of a Self-adhesive Flowable Composite with Dentin/enamel Characterized by TEM

Objectives: A new category of composite which does not require any acid etching or bonding protocol prior to application has been introduced. The purpose of this study was to characterize the interfacial ultra-structure at enamel and dentin surfaces by means of transmission electron microscopy (TEM).

Methods: Non-carious human third molars were randomly divided into 6 groups (bur-cut dentin, SiC-ground dentin, fractured dentin, bur-cut enamel, SiC-ground enamel, and un-cut enamel). After preparation of the respective surfaces, the self-adhesive flowable composite (Vertise Flow, Kerr) was applied. Non-demineralized/demineralized and non-stained/stained sections of 70-90nm were prepared following common TEM-specimen processing procedures. Additional specimens were immersed in an ammoniacal silver nitrate solution.

Results: The composite-dentin interface was free of voids and no de-bonding occurred during specimen preparation. For bur-cut and SiC-ground dentin, no surface demineralization was observed and the smear contained residual hydroxyapatite. On fractured dentin (i.e. without smear interference), a very thin interaction area of 100-200nm without apparent signs of surface demineralization was seen. When the composite was bonded to enamel, a distinct separation between the self-adhesive composite and enamel was present in all three groups. A tight interaction, yet without distinct dissolution of hydroxyapatite, could only be seen in some regions at bur-cut enamel where smear was absent or sparse. Silver nitrate infiltration was associated with the presence of smear.

Significance: The self-adhesive composite showed limited interaction with smear-covered substrates and aprismatic enamel, which explains its inferior diminished bonding capacity in comparison with current adhesives.