Changes in the flexural strength and surface characteristics of different zones of multilayered translucent zirconia during aging process

다국어 초록 (Multilingual Abstract)

Statement of problem
Multilayered translucent zirconia with different yttrium oxide content aims to balance translucency and strength. However, there is limited research on how hydrothermal aging affects the mechanical and surface characteristics of the transition zones of strength-gradient multilayered zirconia. Furthermore, while various flexural strength testing methods, such as 4-point bending test and biaxial flexural test exist, their correlation and reliability in assessing the mechanical properties of multilayered zirconia are not fully understood.

Purpose
The purpose of this in vitro study was to assess the changes in the mechanical and surface
characteristics of the transition zone of multilayered translucent monolithic zirconia compared to zones of different yttrium oxide content, after hydrothermal aging. Also, the 4-point flexural strength and biaxial flexural strength of zones of translucent zirconia with different yttrium oxide content were compared to find out the correlation between these two types of flexural strength, and to determine which type of flexural strength is more reliable for representing the mechanical properties of zirconia.

Materials and Methods
Two conventional translucent zirconia blocks (IPS e.max ZirCAD LT; L and IPS e.max ZirCAD MT; M) and three multilayered translucent zirconia blocks (IPS e.max ZirCAD MT Multi; T, IPS e.max ZirCAD Prime; P, and IPS e.max ZirCAD Prime Esthetic; E) were used in this study. L and M consisted of 3Y- and 4Y-TZP, respectively. T was designed to be located in the 5Y-TZP incisal zone, while P and E were fabricated from the 3Y/5Y-TZP and 4Y/5Y-TZP transition zone respectively. A total of 825 specimens were prepared, with disk-shaped specimens used for biaxial flexural strength (BFS), hardness, and surface characteristic analysis, and bar-shaped specimens used for 4-point bend test (4PB). Each group was divided into three subgroups: a control group (immersed in water at room temperature) and two aged groups (autoclaved at 134°C for 5 and 10 hours). 4PB, BFS, nanoindentation hardness, Young’s modulus and Vickers hardness were measured for mechanical property analysis. Surface characteristics including surface roughness, phase distribution, surface microstructure, and elemental composition were also analyzed. Data normality was assessed through the Shapiro-Wilk test. Pearson’s correlation analysis was used to find out the correlation between 4PB and BFS. One-way ANOVA and independent samples t-test were conducted, and two-way ANOVA was used to analyze the interaction between yttrium oxide content in each zone and aging, followed by Tukey's HSD test for post hoc analysis (α = .05).

Results
L demonstrated the highest 4PB and BFS both prior to and following aging. No significant decrease in flexural strength was observed across all groups after aging. Zones with varying yttrium oxide content exhibited a consistent correlation between 4PB and BFS, irrespective of aging. T exhibited the highest nanoindentation hardness and Young’s modulus values under all aging conditions. Vickers hardness was significantly higher in M, T, and E compared to L and P, both before and after aging. In terms of surface characteristics, L presented the highest surface roughness after aging, with the most pronounced increase observed as aging time progressed. Phase distribution analysis revealed an increase in the monoclinic phase in all groups following aging, with the most substantial changes observed in L and P. Additionally, L and P showed notable increases in grain pull-out, microcracks, and surface defects. Changes in the elemental composition of Zr, O, Y, Hf, and Al were observed before and after aging.

Conclusions
The transition zone of 3Y/5Y-TZP demonstrated flexural strength higher than that of 5Y-TZP but lower or comparable to that of 3Y-TZP, both before and after aging. Similarly, 4Y/5Y-TZP exhibited flexural strength comparable to that of 4Y-TZP and 5Y-TZP regardless of aging conditions. A consistent correlation between 4PB and BFS was identified in all groups, before and after aging. Surface roughness showed a marked increase, particularly in 3Y-TZP and 3Y/5Y-TZP, following aging. The phase composition and surface microstructure of 3Y/5Y-TZP closely resembled those of 3Y-TZP, while 4Y/5Y-TZP demonstrated similarities to 5Y-TZP. Furthermore, the interaction between yttria content and aging was evident across all mechanical and surface characteristics, with the exception of flexural strength. The mechanical and surface characteristics of different zones with varying yttrium oxide content must be taken into account when designing dental restorations made from multilayered translucent zirconia blocks.

목차 (Table of Contents)

• Ⅰ. Introduction 1
• Ⅱ. Materials and Methods 4
• Ⅲ. Results 14
• Figures 66
• Ⅳ. Discussion 71
• Ⅴ. Conclusions 76
• VI. References 77
• Korean Abstracts 81