Improved bond performances of self-etch adhesives to enamel through increased MDP-Ca salt formation via phosphoric...

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.

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.

The percentage of MDP-Ca salt in EXP_ER (73.13%) was higher than that in EXP_SE (43.27%). Characteristic CH₂ (1130, 1441, 2853, and 2909 cm^-1), CC (1641 cm^-1), and CO (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).

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.