Effect of hydrofluoric acid etching and resin cement on the flexural strength of a leucite-reinforced glass ceramic with simulating CAD-CAM milling

Objectives: To evaluate the effect of hydrofluoric acid etching and resin cement coating on the surface roughness and biaxial flexural strength of a leucite-reinforced glass ceramic ground with a diamond bur, following a protocol simulating CAD-CAM milling. Methods: Leucite-reinforced glass ceramic (IPS Empress CAD, Ivoclar AG) discs were prepared and divided into four groups (n = 20): polishing (P); grinding (G); grinding followed by 5 % hydrofluoric acid application for 60s (GH); and grinding followed by 5 % hydrofluoric acid (HA) for 60s, silane, and resin cement application (GHRC). The discs were aged in distilled water at 37 C-degrees for three months and subjected to surface roughness analysis and biaxial flexural strength testing (ISO 6872:2015). Data were compared using the Kruskal-Wallis test followed by Bonferroni-corrected multiple comparisons (alpha = 0.05). The flexural strength data were also subjected to Weibull analysis to determine the Weibull modulus. Results: Grinding the ceramic surface with an extra-fine diamond bur increased the roughness and reduced the fracture strength when compared to the polishing group. Hydrofluoric acid (GH group) produced roughness values similar to those of the G group, without affecting the material's flexural strength (p > 0.05). Resin cement coating (GHRC group) was able to significantly increase the fracture strength, when compared to the GH group (p < 0.001), reaching similar values to the polishing group (p = 0.138). Weibull modulus of the P group was significantly lower than the G group. Conclusion: Resin cement reversed the damage from grinding, restoring flexural strength to levels similar to the polished group. Hydrofluoric acid did not affect flexural strength.