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Mengke Qian,1 Zhicen Lu,1 Chen Chen,2 Huaiqin Zhang,1 Haifeng Xie1 1Department of Prosthodontics, 2Department of Endodontics, Jiangsu Key Laboratory of Oral Diseases, Affiliated Hospital of Stomatology, Nanjing Medical University, Nanjing, People’s Republic of China Abstract: Creating an alkaline environment prior to 10-methacryloyloxydecyldihydrogen­phosphate (MDP) conditioning improves the resin bonding of zirconia. The present study evaluated the effects of four alkaline coatings with different water solubilities and pH values on resin bonding of MDP-conditioned zirconia. Two alkaline nanoparticle coatings were studied in particular. Thermodynamics calculations were performed to evaluate the strengths of MDP-tetragonal phase zirconia chemical bonds at different pH values. Zirconia surfaces with and without alkaline coatings were characterized by scanning electron microscope (SEM)/energy dispersive spectrometer and Fourier transform infrared spectroscopy; alkaline coatings included NaOH, Ca(OH)2, nano-MgO, and nano-Zr(OH)4. A shear bond strength (SBS) test was performed to evaluate the effects of the four alkaline coatings on bonding; the alkaline coatings were applied to the surfaces prior to conditioning the zirconia with MDP-containing primers. Gibbs free energies of the MDP-tetragonal zirconia crystal model coordination reaction in different pH environments were -583.892 (NaOH), -569.048 [Ca(OH)2], -547.393 (MgO), and -530.279 kJ/mol [Zr(OH)4]. Thermodynamic calculations indicated that the alkaline coatings improved bonding in the following order: NaOH > Ca(OH)2 > MgO > Zr(OH)4. Statistical analysis of SBS tests showed a different result. SBSs were significantly different in groups that had different alkaline coatings, but it was not influenced by different primers. All four alkaline coatings increased SBS compared to control groups. Of the four coatings, nano-Zr(OH)4 and -MgO showed higher SBS. Therefore, preparing nano-Zr(OH)4 or -MgO coatings prior to conditioning with MDP-containing primers may potentially improve resin bonding of zirconia in the clinic. Keywords: adhesion, T-YZP, zirconia primer, bond strength, surface conditioning, MDP

Objectives Recently several zirconia primers have been introduced to mediate bonding of dimethacrylate resins to yttria-stabilized zirconia frameworks (Y-TZP). The aim of the study was to evaluate the changes in surface chemistry of Y-TZP frameworks induced by zirconia primer treatments. Methods Polished Y-TZP discs (Lava, 3M ESPE), ultrasonicated for 10 min in ethanol, water-rinsed and air-dried were treated as follows: A: Reference (no treatment), B: Treatment with Z Prime Plus (Bisco), and C: Treatment with Z-Bond (Danville Materials). The primer films formed on Y-TZP surfaces were air-dried, left intact for 5 days (dark storage, 37 °C, 40% RH), rinsed with 10 ml acetone to remove the loosely bound fractions, air-dried and studied by: (a) reflection optical microscopy, (b) reflection Fourier transform infrared microscopy (RFTIRM) and (c) scanning electron microscopy/energy dispersive X-ray microanalysis (SEM/EDX). Results An amorphous thick film was observed on primed and acetone rinsed Y-TZP surfaces after B treatment, whereas C treatment formed a thinner film with phase-separated aggregates. The RFTIRM study showed that both primers induced carboxylate salt formation on Y-TZP. Phosphate groups in a dissociative form have been identified on Y-TZP, as well, indicating formation of phosphate salts. EDX analysis showed increased C, O and P content on the films, which masked the substrate contributions. Significance The primers tested formed carboxylate and phosphate salts on Y-TZP, promoting thus chemical adhesion. However, the differences in the film forming properties and water solubility between the carboxylate and phosphate salts may affect the strength and the durability of adhesive resin interfaces with Y-TZP, as mediated by these primers. [ABSTRACT FROM AUTHOR]