The Effect of Artificial Aging (LTD) on The Mechanical and Optical Properties of Conventional and Translucent Zirconia for Fixed Prosthodontics

In the last two decades, zirconia has been of a great interest to dentists due to its superb mechanical and optical properties. At first, the use of zirconia was limited to fabrication of cores replacing the metal in crown construction. Nowadays, there is an increase in the trend of using monolithic ‘full contour’ translucent zirconia to overcome the problem of chipping of porcelain veneers and to overcome the limitation of using lithium disilicate in long span replacement. This has brought zirconia in direct contact with saliva and oral fluids and introduced the possibility of the material undergoing low temperature degradation. This study aimed to investigate the effect of accelerated hydrothermal aging on the mechanical and optical properties of two conventional core and two full contour translucent zirconia materials, expected to be used by 90% of zirconia manufacturers globally. Hydrothermal aging was carried out using an autoclave to simulate in vivo aging, using a specific protocol proposed by ISO 13356:2015 mimicking 15- 20 years of clinical service of the material. Each of the four materials were tested before and after aging, including structural analysis which was carried out using XRD, SEM, FIB-SEM and AFM. Mechanical property investigations were carried out by measuring BFS and Vickers hardness. Optical properties were thoroughly investigated through measuring a range of translucency parameters and changes in colour before and after aging. The results of this study showed that conventional core materials were less affected by hydrothermal aging in comparison to full contour translucent zirconia in terms of optical properties. All of the used materials showed clear colour changes after aging, however none of them showed significant changes in the mechanical properties even with more than 20% of t →m phase transformation in one of the translucent zirconia materials. Within the limitation of this in vitro study, it can be concluded that full contour translucent zirconia can be used clinically with no concern about its mechanical and optical properties, however, further studies on the perception and acceptability for changes in the optical properties would be highly recommended.