TEM Study on Acid-base Resistance of the Dentin-adhesive Interface

Objectives: To examine acid-base resistance of the dentin-adhesive interface with four different categorized dentin bonding systems by using transmission electron microscopy (TEM) and selected area electron diffraction (SAED). Methods: Human dentin disc specimens were bonded with one of the four bonding adhesive systems: Scotchbond Multi-Purpose (3M-ESPE: SMP), Clearfil Photo Bond (Kuraray Medical: CPB), Clearfil SE Bond (Kuraray Medical: CSE), and Adper Easy Bond (3M-ESPE: AEB). After overnight storage in water, the specimens were distributed into two groups. Those in the first one were left untouched (control), and the others were subjected to acid-base challenge with a demineralizing solution (pH 4.5) for 90 min and 5% NaClO for 30 min. The specimens were then processed for TEM observation. Results: Correlative TEM and SAED evidence from the control and challenged specimens disclosed an apatite-rich ABRZ beneath the partially demineralized hybrid layer of CSE, which was thicker and superior in quality compared to AEB. The etch-and-rinse adhesives (SMP and CPB) demonstrated a completely or dominantly demineralized hybrid layer, which were devoid of the ABRZ formation. Conclusions: The ABRZ observed in both ‘mild’ self-etch adhesive systems (CSE and AEB) should be ascribed to an effective diffusion of the reactive monomers beyond the hybrid layer, and also chemical bonding interaction potential with the remaining hydroxyapatite crystallites as the substrate.