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.