Effect of veneering thickness and repeated firing on the flexural strength and translucency of lithium disilicate glass ceramic

Objective: To evaluate effect of veneering thickness and repeated firing on the flexural strength and translucency of lithium disilicate glass ceramic.
Materials and methods: Sixty-three-disc specimens were divided; each group have 21 discs specimens. First group was lithium disilicate core 1 mm with no veneering coverage, second group the lithium disilicate core was veneered by 0.5 mm nano flouroapatite glass ceramic. Third group was lithium disilicate core veneered by 1 mm nano flouroapatite glass ceramic. Each group was subjected to repeated firing cycles (3, 5, 7). Seven-disc specimens for each firing cycle were used to measure translucency then flexural strength. The degree of color difference between the compared colors was expressed in ∆E units. The total color difference, according to L*, H*, C* coordinates, was calculated. Then change in TP (∆TP) which is the difference between first and second sample regarding thickness and firing cycles was measured to determine the acceptability threshold. Then fracture load was applied to all specimens fixed on 3 balls fixture under static compressive loading using universal testing machine to determine the flexural strength. The results were tabulated and statistical analysis was performedusing repeated way ANOVA method.
Results: Regarding value, chroma and hue, in 3 firing cycles value means revealed a significant difference in the comparison between 1 mm, 1.5 mm (P=0.001) thickness and between 1 mm, 2 mm (P=0.001) but non-significant difference was shown between 1.5 mm and 2 mm (P=0.325). While chroma and hue showed significant difference in comparison between 1 mm, 1.5 mm and 2 mm (P=0.001). However, in 5 firing cycles, value showed significant difference between 1 mm, 1.5 mm and 1 mm, 2 mm (P=0.001) and also between 1.5 mm, 2 mm (P=0.016), and chroma also showed significant difference between 1 mm, 1.5 mm, 2 mm (P=0.001), and hue showed significant difference between 1 mm, 1.5 mm (P=0.027), and also between 1 mm, 2 mm and 1.5 mm, 2 mm. However, in 7 firing cycles, value showed significant difference in comparison between 1 mm, 1.5 mm (P=0.001) and 1 mm, 2 mm (P=0.001) but non-significant difference was revealed between 1.5 mm, 2 mm (P=0.053). While chroma showed significant difference between 1 mm, 1.5 mm and 2 mm (P=0.001), while hue showed significant difference in comparison between 1 mm, 1.5 mm (P=0.001) but non-significant difference was found between 1 mm, 2 mm (P=0.11) and 1.5 mm, 2 mm (P=0.148). Translucency parameters revealed a significant difference (P=0.001) between different firing cycles with different thickness. Flexural strength showed no significant decrease with increasing firing cycles from 3 to 5 firing cycles in the same 1 mm thickness (P=0.153) and showed significant decrease on increasing firing cycles from 3 to 7 cycles (P=0.001) and 5 to 7 cycles (P=0.005). While in 1.5 mm thickness showed significant decrease in flexural strength on increasing firing cycles from 3 to 5 cycles (P=0.013) and from 3 to 7 cycles (P=0.001) and showed no significant decrease on increasing firing cycles from 5 to 7 cycles (P=0.073). While in 2 mm thickness showed significant decrease in flexural strength on increasing firing cycles from 3 to 5 cycles (P=0.015), 3 to 7 cycles (P=0.001) and 5 to 7 cycles (P=0.03).
Conclusions:
1-       Translucency decreased on increasing the firing cycles and increasing the veneering thickness.
2-       Flexural strength increased on increasing veneering thickness and decreased on increasing the firing cycles.