Evaluation of dentin bonding with a low-pH all-in-one adhesive resin

Objectives: To evaluate the bond efficacies of a low-pH all-in-one adhesives system, microtensile bond strength to dentin (MTBS) and ultra-structures of the bonding interface were investigated in the study. Methods: A low-pH all-in-one adhesives system, Adper Prompt L-Pop (APP, 3M ESPE, pH 0.9) and a reference adhesive (pH 2.7), Tri-S Bond (TSB, Kuraray Medical) were used. Middle dentin of intact human premolars was flattened with #600 SiC papers. The surface was treated with the adhesive systems according to manufacturers' instructions. After a resin composite was incrementally built up, the tooth was stored in water for 24 hours. Then, MTBS was measured. For ultra-morphological observation of the bonding interfaces, the dentin surface ground with #600 SiC papers was treated with the adhesives and covered with a flowable composite. The SEM specimens etched with argon-ion beams and TEM specimens without decalcification were produced routinely. Results: MTBS (SD) of APP was 11.6 (6.9) MPa for APP, which was significantly lower than that of TSB, 49.2 (9.1) MPa (p<0.05). For APP, the fracture occurred dominantly at the interface between the composite and adhesive resin while TSB showed the mixed failure. In the SEM and TEM observations, approximately 2 or 0.5 micron thick hybrid zone was found beneath the adhesive resin layer of APP or TSB, respectively. However, for APP, numerous small voids were seen along the interface between the composite and adhesive resin in SEM. In TEM figures of APP, an adhesive resin layer was composed of 2 layers of different electron density and the hybrid zone was apparently differenciated from the sound dentin. Conclusion: These results suggested that in a low-pH adhesive system APP, the resin monomers might not be well polymerized, despite of the deeper dentin demineralization and monomer penetration.