Fracturing of Lithium-disilicate-reinforced glass-ceramic Crowns with Zirconia-abutment Bonding Conditions

Objectives: This study was aimed at evaluating the effects of bonding conditions on the fracture resistance of lithium disilicate-reinforced CAD/CAM crowns with a cylindrical, off-the-shelf zirconia core.
Methods: For the core material, 3Y-TZP discs of ZirTooth were used for CAD/CAM manufacturing; blocks of lithium disilicate-reinforced Amber Mill-Q were used as the veneer material. For heat bonding, a liner prepared through prior testing in a ratio of 5–15 Li₂O:55–65 SiO₂:5–25 other oxides and colorants was used to enable heat bonding at 700–800°C. For cement bonding, a dual-cure self-adhesive resin cement, Rely-XTM U200 was used. To test bond strength, microtensile bond test specimens of dimensions 1.0mm×1.0mm×10mm were soaked in distilled water at 37°C for 1day before measuring the tensile bond strength at a loading rate of 0.5mm/min. To evaluate the effects of bonding on crown chipping resistance, we bonded Amber Mill-Q veneers to 4-mm-diameter, 8-mm-long cylindrical ZirTooth cores using liner or resin cement, soaked the specimens in distilled water at 37°C for 1day, and measured initial chipping resistance at a loading rate of 0.5mm/min.
Results: At the interface between the zirconia and lithium disilicate liner, chemical bonds formed within the reaction and liner-treatment layers, where peaks were observed for Li₂SiO₃, ZrSiO₄, ZrO₂, and SiO₂. The liner-treated group exhibited 2.4-fold higher tensile bond strength than the cementation group, which was statistically significant (P<0.05). The liner group exhibited mixed fractures, whereas the resin cement group had interfacial fractures between the cement and zirconia.
Conclusions: When a ceramic restoration is bonded to a zirconia abutment, heat bonding with a liner is more effective than cementation.