Influence of the yttrium content on the fracture strength of monolithic zirconia crowns after artificial aging

Objectives: The purpose of the study was to evaluate fracture strength of full-anatomical crowns manufactured from three different types of zirconia with different content of yttrium oxide. Method and materials: A total of 48 specimens were divided into three main groups (n = 16), according to the content of yttrium oxide in the crown material used, as follows: group A, DD Bio ZX² (3Y-TZP); group B, DD cubeX² HS (4Y-TZP); and group C, DD cubeX² (5Y-TZP) (Dental Direkt). After proper surface treatment of different materials, the crowns were adhesively bonded to metal abutments using a dual-curing composite resin (Multilink Automix, Ivoclar Vivadent). Eight specimens from each material group were randomly selected to undergo an artificial aging through dynamic loading and thermocycling in a chewing simulator (Willytec, SD-Mechatronic). After dynamic loading all survived specimens of the first subgroup together with all the specimens of the second subgroup were loaded in a universal testing machine (Zwick Z010, Zwick/Roell). The data were checked for normal distribution with the Shapiro-Wilk test and for homogeneity of the variances with the Levene test. The statistical analyses were done using the Kruskal-Wallis test and in subgroups using the Wilcoxon test. Results: The highest fracture strength median value was shown with the 3Y-TZP after artificial aging (median 7,530 N), while the lowest was shown with the 5Y-TZP after artificial aging as well (median 3,700 N). Conclusions: Increasing the content of yttrium oxide in attempt to improve the optical properties can reduce the mechanical properties after aging of the ceramic as shown in this study. However, even the more translucent 5Y-TZP showed very high fracture resistance, which exceeded 3,000 N. Based on the results of this study, care should be taken when using zirconia with high content of yttrium oxide in areas or situations with high stresses. Keywords: artificial aging, ceramics, yttrium oxide, zirconia