Aim: The purpose of this study was to evaluate the effect of grinding, over-glazing, regrinding, and polishing on flexural strength of monolithic zirconia.
Materials and methods: In this in vitro study, 50 bar-shaped zirconia specimens were fabricated and divided into 5 groups (n = 10) of no-treatment control group (C), grinding (G), grinding + glazing (GGl), grinding + glazing + grinding (GGlG), and grinding + glazing + grinding + polishing (GGlGP). A universal testing machine was used to measure the flexural strength. Data were analyzed using the Tukey\'s HSD test and ANOVA (p < 0.05).
Results: There were significant differences between group C and other groups (p < 0.001). Group C showed the highest mean flexural strength among all groups. There was also a significant difference between groups GGlG and GGlGP (p = 0.031), and the latter group showed higher mean flexural strength. There was no significant difference between other groups (p > 0.001).
Conclusion: Regrinding does not have a significant effect on flexural strength of monolithic zirconia compared with grinding. Glazing slightly decreased the flexural strength of ground zirconia surfaces, which was not statistically significant. Polishing improved the flexural strength of ground zirconia surfaces; however, their mean flexural strength was significantly lower than that of the control group.
Clinical significance: Grinding may lead to significant strength reduction and decrease the durability of zirconia restorations. Therefore, polishing of ground surfaces is suggested while glazing has an adverse effect on the strength.
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