Lithium disilicate and zirconia reinforced lithium silicate glass-ceramics for CAD/CAM dental restorations: biocompatibility, mechanical and microstructural properties after crystallization.

Objectives: The objective of this study was to define the impact of heating rate on the crystal growth, the mechanical properties, and the biocompatibility of three different kinds of CAD/CAM glass-ceramics treated with a conventional furnace.

Methods: Lithium disilicate (IPS EMax-CAD, Ivoclar Vivadent) (LS) and two zirconia reinforced lithium silicate (ZLS) ceramics (Vita Suprinity PC, VITA Zahnfabrik; Celtra Duo, Dentsply Sirona) (ZLSS; ZLSC) were used. The mechanical properties and the crystal growth were evaluated on 42 specimens (n = 14 per group). The thermal treatments recommended by the manufacturers were carried out. All groups were tested for fracture toughness (Ft) and Vickers hardness (Hv). Scanning electron microscope (SEM) images were taken after a slight surface etching with hydrofluoric acid solution (1% for 20 s). Differential Thermal Analysis (DTA) was performed and cellular adhesion with human periodontal ligament stem cells (hPDLSCs) culture was qualitatively assayed. Data were analyzed with Repeated Measurements ANOVA and ANOVA followed by Tukey post hoc test.

Results: The crystals' mean size (±SD) after heat treatment was 1650.0 (±340.0) nm for LS, 854.5 (±155.0) nm for ZLSS and 759.9 (±118.4) nm for ZLSC (p < 0.05 among the groups). As consequence of crystallization, the Hv was 6.1 ± 0.3 GPa for LS, 7.6 ± 0.7 GPa for ZLSS and 7.1 ± 0.5 GPa for ZLSC (p < 0.05 for LS vs ZLSS and ZLSC), while the Ft was 2.2 ± 0.1 MPa m for LS, 4.7 ± 0.8 MPa m for ZLSS and 3.8 ± 0.6 MPa m for ZLSC (p < 0.05 among the groups). The DTA curves showed a crystallization process for LS, ZLSS and ZLSC at a temperature range 810-840 °C. The amount of adherent hPDLSCs was superior on LS than on ZLS.

Conclusions: All the CAD/CAM materials can be properly crystallized if heat treated following the manufacturers' instructions. The crystallization process highly depends on temperature. ZLS glass ceramics show significantly inferior crystals dimensions and higher fracture toughness and Vickers hardness than LS ceramic. hPDLSCs cultured on LS have a superior adhesion than those cultured on ZLS.

Clinical Significance: The value of this study relies on the demonstration that a proper heat-treatment of CAD/CAM lithium disilicate and ZLS glass ceramics generates products that are suitable for clinical use . The differences highlightable in mechanical properties and biocompatibility behavior do not affect their successful clinical application.