Does glaze firing affect the strength of advanced lithium disilicate after simulated defects?

OBJECTIVE: To study the influence of glazing on strength repair of lithium disilicate glass–ceramics after defect incorporation in different production processing phases. MATERIALS AND METHODS: Bar-shaped specimens (1 × 1 × 12 mm, n = 280; 20/group) made from different lithium disilicate ceramics (IPS e.max CAD, Ivoclar, “LD” or advanced lithium disilicate CEREC Tessera, Dentsply Sirona, “ALD”) were exposed to 7 different protocols: crystallized without (c) and with glaze layer (cg), with a defect incorporated before crystallization without (ic) and with glaze layer (icg), with a defect after crystallization without (ci) or with glaze layer (cig), and defect incorporated after the glaze layer (cgi). The flexural strength was determined using the three-point bending test. Analysis of indented areas and fractured specimens was performed by scanning electron microscopy. Flexural strength data were evaluated by two-way ANOVA followed by Tukey tests (α = 5%). RESULTS: Two-way ANOVA revealed a significant influence of ceramic (p < 0.001; F = 55.45), protocol (p < 0.001; F = 56.94), and the interaction protocol*ceramic (p < 0.001; F = 13.86). Regardless of ceramics, defect incorporation as final step resulted in the worst strength, while defects introduced before crystallization did not reduce strength. Glaze firing after defect incorporation led to strength repair for ALD, whereas such an effect was not evident for LD. CONCLUSIONS: The advanced lithium disilicate must receive a glaze layer to achieve its highest strength. Defects incorporated in the pre-crystallized stage can be healed during crystallization. Defects should not be incorporated after glazing. CLINICAL RELEVANCE: Clinical adjustments should be performed on pre-crystallized or crystalized restorations that receive a glazer layer afterwards.