Ultrastructure of acid-base resistant zone in a fluoride-releasing adhesive system

An acid-base resistant zone (ABRZ) has been reported at the adhesive interface in self-etching primer adhesive systems after acid-base challenge. Objectives: The purpose of this study was to observe the ABRZ using a fluoride-releasing adhesive system (Clearfil Protect Bond, PB) and a fluoride-free adhesive system (Clearfil SE Bond, SE) (Kuraray Medical, Japan). Methods: Dentin disks obtained from human molars were ground with #600-grit SiC and bonded with one of the adhesives according to the manufacturer's instructions. A flowable composite (Metafil Flow, Sun Medical) was placed between two dentin disks to make a dentin disk sandwich. The bonded specimens were stored in water for 1 day. They were then sectioned at the center perpendicular to the adhesive interface. The sectioned surfaces were then polished and subjected with a demineralizing solution (pH4.5) for 90 min and 5% sodium hypochlorite for 20 min. The surface was then vertically sectioned into two halves for SEM and TEM observations. Results: For the SEM observations, less than 1 µm of a hybrid layer distinguished by argon-ion etching (HA) was recognized in each group. The ABRZ created beneath the HA was observed in both PB and SE, however, thickness of the ABRZ was thicker in PB than that in SE. In the TEM observations of the adhesive interface, the ABRZ was recognized as a hydroxy apatite rich layer. At the mid point of the height of the outer lesion, the ABRZ in PB was thicker than that of SE. Conclusions: The ABRZ was observed in both PB and SE. The ABRZ was a mineral rich zone, which may be influenced by fluoride-release from the adhesive.