Improved bond performances of self-etch adhesives to enamel through increased MDP–Ca salt formation via phosphoric acid pre-etching

Objectives The chemical affinity between 10-methacryloyloxydecyl dihydrogen phosphate (MDP) and hydroxyapatite (HAp) is an important factor in the enamel bonding provided by MDP-based self-etch (SE) adhesives, besides microinterlocking mechanisms. This study aimed to investigate how phosphoric acid pre-etching affects MDP–Ca salt formation in the application of MDP-based SE adhesives. Methods Single Bond Universal (SBU), All Bond Universal (ABU), Clearfil Universal Bond Quick (CBQ), and a MDP-based all-in-one adhesive (EXP) were used in both SE and etch-and-rinse (ER) modes, along with Clearfil SE Bond and untreated enamel (UE) as controls. The MDP–Ca salts produced with or without etching were examined by nuclear magnetic resonance, Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and Fourier-transform infrared spectroscopy. Zeta potential , contact angle, and scanning electron microscopy measurements were employed to elucidate the mechanism behind the changes in MDP/HAp chemical affinity upon pre-etching. Results The percentage of MDP–Ca salt in EXP_ER (73.13%) was higher than that in EXP_SE (43.27%). Characteristic CH 2 (1130, 1441, 2853, and 2909 cm −1 ), C C (1641 cm −1 ), and C O (1718 cm −1 ) bands were observed in the Raman spectra of EXP_ER. Pre-etching increased the negative zeta potential of the enamel surface compared to that of UE ( P  < 0.001). The contact angles of MDP-based adhesives applied to pre-etched enamel were significantly lower than those of the self-etched surface ( P  < 0.05). Significance The increased MDP–Ca salt formation is a significant advantage of phosphoric acid pre-etching, improving the MDP/HAp chemical affinity in addition to increasing surface wettability.