Aim of the study Multilayer monolithic zirconia, which includes polychromatic layers that reflect natural tooth color gradients, gives an enhanced properties and advanced fabrication efficiency. Controversy, still present regarding the fracture resistance and marginal adaptation of multilayer monolithic zirconia restorations after application of variable sintering protocols. Thus, the aim of this study was to evaluate and compare the effect of sintering protocol on marginal adaptation and fracture resistance of zirconium oxide based three units FPDs. Materials and methods A total of 24 three-unit monolithic zirconia FDPs, with one premolar pontic and two retainers, were fabricated. The FDPs were divided into two groups (n=12) according to the material used for construction of FPDs Group I: White peak Copra supreme hyperion monolithic zirconia, Group II: White peak Copra supreme Symphony monolithic zirconia. Then each group was further divided into two equal subgroups according to the sintering protocol. Sub group 1: Regular (Normal) sintering protocol, sub group 2: Speed sintering protocol. The master cast was scanned with a laboratory scanner, the scanned data was used to design monolithic FDPs based on manufacturer instructions. The received CAD data were milled for each group of material. FPDs were sintered according to the manufacturers’ instructions. Vertical margin gap distance was measured using a stereomicroscope before cementation as control. The FPDs were cemented using self-adhesive resin cement (Rely X Unicem), and then samples were subjected to thermocycling and cyclic fatigue loading. The vertical marginal gap distance for all samples was subsequently re-measured. Fracture resistance test was done using universal testing machine. The fracture load was recorded in Newton. Data was collected, tabulated and statistically analyzed. Repeated measures ANOVA test was used to study the effect of variables and their interactions on mean marginal gap distance. Two-way ANOVA test was used to study the effect of variables and their interactions on mean fracture resistance. The significance level was set at P ≤ 0.05. Results In all tested groups and subgroups, regular sintering showed statistically significantly lower mean gap distance than speed sintering, while there was a statistically significant increase in mean gap distance after cementation, ceramic material regardless the sintering protocol had no statistically significant effect on mean fracture resistance (P-value = 0.821). Sintering regardless of ceramic type had a statistically significant effect on mean fracture resistance (P-value = 0.002). Conclusions The sintering protocol is shown to affect both the marginal adaptation and fracture resistance of multi-layered zirconia FPDs.
Emam et al. (Wed,) studied this question.