The EADT informs: The classification of monolithic zirconium oxide
Monolithic zirconium oxide restorations are often used clinically when there is a lack of space. However, in order to use the material monolithically, certain requirements must be met. In addition to long-term stability, the material should be more translucent and therefore the restoration should appear more aesthetic. There are now three different ways to produce translucent zirconium oxide (AC). This results in the three generations of zirconium oxide.
A – Modification of the sintering temperatures for zirconium oxide 1. generation (3Y-TZP)
The conventional tetragonal zirconium oxide can become more translucent by changing the sintering temperature. In addition to the increased final sintering temperature, the duration of the holding time, the temperature increase and the cooling also have a positive effect on the translucency. However, this affects the strength of the material.
Caution: Sintering temperatures above 1.600 °C lead to a decrease in strength and long-term stability. Therefore, this option for improved aesthetic properties was not adopted in the first generation of monolithic zirconia. The sintering temperatures for all suppliers of zirconium oxide are now below 1.600 °C.
B – Modification at the molecular level in zirconium oxide 2. generation (3Y-TZP)
The second generation of tetragonal zirconium oxide has been available since 2012/2013. Here the number and grain size of the aluminum oxide grains (Al2O3) are reduced. In addition, the Al2O3 grains in the structure were repositioned onto the grain boundaries of the zirconium oxide. The result is a higher transmission of light with good long-term stability and high strength at the same time.
Background: The Al2O3 grains in the structure differ significantly from the refractive index of the zirconium oxide grains.
C – Modification of the crystal structure of zirconium oxide 3. generation (5Y-TZP)
Now the desire for an even more translucent zirconium oxide was awakened, because the second generation of zirconium oxides was still inferior to glass ceramics in terms of translucency. Zirconium oxide has been available since 2015, which contains not only the tetragonal phase but also a cubic phase of approx. 53%.
This fully stabilized zirconium oxide therefore has a cubic-tetragonal mixed structure. The cubic components - achieved through a higher doping (5 mol%) of yttrium oxide - show a larger volume compared to the tetragonal components. This means that the light scatters less strongly at the grain boundaries and the remaining porosities and causes a higher translucency of the material. In addition, the cubic crystal structures are more isotropic. This means that the incident light is emitted evenly in all directions. This property also significantly influences the translucency. The disadvantage of this generation of zirconium oxide is its significantly lower strength, although this is still higher than that of lithium (di)silicate ceramics.
Monolithic zirconium oxide restoration on prepared tooth stumps. The individual crowns have natural-looking light-optical properties. The occlusal area - especially the fissures - were somewhat characterized with paint. (Images: C. Fischer)
Monolithic zirconium oxide restoration on two implants (hybrid abutments made of zirconium oxide) and one tooth. The restorations were also slightly individualized using only stain. (Images: C. Fischer)
Author: A. Kieschnick (www.annettkieschnick.de)
Additional information
In a detailed publication in the journal Quintessenz Zahntechnik (6/2016), Bogna Stawarczyk and a team of authors discuss the classification of zirconium oxide [Stawarczyk B, Keul C, Einberger M, Figge D., Edelhoff D., Lümkemann N. Materials science update : Zirconium oxide and its generations – from veneered to monolithic. Quintessenz Zahnt 2016;42(6):740-765].