Mechanical & physical properties of zirconia-based ceramic restorations

OBJECTIVE: The most critical issues facing restorations are strength, hardness, chemical stability and discoloration. Therefore, the aim of the in-vitro study was to evaluate and compare various full contour monolithic zirconia (FCZ) based restorations with the established lithium disilicate (LS2) in terms of flexural strength, hardness, solubility in acidic environment, critical surface tension and color stability. METHOD: Five different ceramic materials were tested in this study. Four materials were FCZ namely BruxZir (Glidewell Lab.), Zirlux (Zahn Dental), Zenostar (Ivoclar Vivadent) and Lava (3M ESPE). The fifth material was lithium disilicate (LS2) (Ivoclar Vivadent) that served as a control. Five parameters namely Flexural Strength, Critical Surface Tension (Surface energy), Hardness, Chemical Solubility and Color stability were determined. The groups were as follows: Group A had ten specimens from each group (n=10) of size 20 X 4 X 1.2 mm. Flexural strength followed by Critical Surface Tension (CST) were measured for this group. Load bearing capacities of the materials were evaluated by measuring flexural strength through Three- point Bending Test on the Universal Testing Machine as per the ISO 6872 (International Organization for Standardization). Consecutively, in order to see the effect of Radiofrequency Glow Discharge Treatment (RFGDT) on surface energy and wettability, the CST of two samples per group was determined before and after RFGDT using the Goniometer. Group B had ten specimens from each group (n=10) of size 12 X 6 X 4 mm. They were tested for hardness, chemical solubility and color stability as per American Dental Association (ADA) Spec. 69. In order to determine the hardness of the material, Vickers Hardness was measured by Tukon Microhardness tester under a 1kgf load over 15s. Thereafter the specimens were tested for their chemical solubility as per ISO 6872 by extracting the samples using 4% acetic acid in the soxhlet extractor. To determine the effect of acid on the materials, the hardness was re-measured. Besides that, to find the color stability of ceramic materials the baseline color (CIELAB) was measured using a surface colorimeter. Then the specimens were subjected to staining solution in a rotating apparatus for four days. Color was measured two times in a day after every 12 hrs. RESULTS: One way ANOVA results indicated a statistically significant difference in mean flexural strength among the groups. The follow up Tukey HSD suggests that LS2 has significantly lower flexural strength than all other groups. Among FCZ, Zirlux had lower strength than BruxZir. Furthermore, RFGDT increased the CST of the ceramic materials from 33mN/m to 39mN/m. Hardness results showed that LS2 has significantly lower hardness than FCZ. Among FCZ, Zenostar had lower hardness than Lava. However, there was no significant difference in color stability and chemical solubility among groups. CONCLUSION: Our study indicates superior load bearing capacity and hardness of FCZ as compared to LS2. Surface activation of all material after RFGDT is valuable in terms of cementation and bonding of FCZ and or LS2 to tooth. However the chemical solubility and color stability results did not show any difference among the groups. Therefore, it can be concluded that FCZ are significantly better than LS2 in terms of the flexural strength and hardness. Hence, FCZ have all the properties required for a restorative material. Further clinical studies are required to validate this.