Evaluation of Marginal Accuracy and Microstructure of Heat Pressed Lithium Disilicate and Zirconia Reinforced Lithium Silicate Crowns after Thermal Tempering

Abstract

Objectives: To evaluate the effect of two thermal tempering cycles on marginal accuracy of crowns fabricated from two heat-pressed glass ceramics. Materials and methods:Twenty-eight ceramic crowns were pressed from glass ceramic materials in two groups (n=14). Group (E): Lithium disilicate IPS e.max Press, Group (C): Zirconia reinforced lithium silicate (ZLS) Celtra Press. Each group was subdivided according to tempering temperature into (T1) 9% and (T2) 5% below pressing temperature of each ceramic. A stereomicroscope was used to measure the marginal adaptation before and after tempering and microstructure was analyzed. Three-way ANOVA test was used to study the effect of ceramic type, tempering, tempering procedure and their interactions on marginal gap distance. Bonferroni's posthoc test was used for pair-wise comparisons when ANOVA test is significant. The significance level was set at P ≤ 0.05. Results:The interaction between the three variables had statistically significant effect on mean marginal gap distance (P-value <0.001). Before tempering and after (T1) tempering, there was no statistically significant difference between ceramic types (P-value = 0.577). After (T2) tempering, there was a statistically significant difference between ceramic types (P-value <0.001). Celtra showed statistically significantly higher mean marginal gap distance compared to E.max. Conclusions:Mean marginal gap distance for lithium disilicate was not affected after tempering. ZLS after -5% tempering showed the highest mean marginal gap distance. All tested groups showed clinically acceptable marginal adaptation except ZLS with -5% tempering which showed clinically unacceptable marginal adaptation >120 µm.