CAD/CAM Technology had made it possible to prepare restorations out of high strength ceramics like Alumina and Zirconium. Previously, ceramic restorations in the posterior region were limited to single units.
Now with the introduction of Zirconium as a dental material, clinicians are able to place all ceramic restorations in the anterior and posterior regions. This is due in part to the high flexural strength (almost two times higher compared to Alumina) and high fracture toughness of the Zirconium ceramic material.
Several companies are offering Zirconium materials in dentistry. These materials are chemically similar, consisting of 3% Yttrium oxide treated Tetragonal Zirconium Polycrystals.
In many cases they are also treated with a very small concentration of Alumina (< 0,25 %) to prevent leaching of the Yttrium oxide. This combination ensures the safety and longevity of Zirconium restorations. Even though Zirconium can be chemically similar it is not necessarily the same.
Many other factors outside of chemistry influence the final result including the order in which ingredients are mixed, the grain size or consistency of the flour, and time and temperature used for incubating the dough.
In addition, Zirconium can be baked at different temperatures. Aside from the ingredients and baking process, other differences such as the skill of the manufacturer can lead to a substantial difference in the final product.
Although the Zirconium ceramic is chemically similar, once processed, it can exhibit different mechanical and optical characteristics. Working with Zirconium, one can experience the differences in machinability (e.g. wet milling and dry milling) and in sintering (e.g. temperature for Vita™ YZ-Cube 1530°C; temperature for Lava™ frameworks1500°C; temperature for Cercon™ 1350°C).
What can be different
In principle, there is pre-sintered Zirconium and HIP (hot isostatic pressing) Zirconium available on the market. The pre-sintered Zirconium is milled, when the material still has a soft, chalk-like consistency. For full density, it is sintered again after milling. HIP material is milled in the fully sintered state.
Pre-sintered Zirconium is prepared by three main steps . The Zirconium powder is pressed and pre-sintered. This usually occurs by the manufacturer. The dental lab mills the pre-sintered blank and then sinters the coping or framework to achieve full density. The preparation of the pre-sintered blanks by the manufacturer differs depending on the Zirconium powder source and both the pressing and the pre-sintering conditions selected.
1. Powder
The available Zirconium powders can have different grain sizes, different distributions of the various grain sizes, and different additives (e.g. binder for the pressing step). The additives Yttrium oxide and Alumina can be distributed within the material in a variety of ways such as a homogeneous distribution throughout the whole material, higher concentration at grain borders, etc. The grain size has an effect on strength and transformation toughening, a special and key mechanical characteristic of Zirconium. Variations in grain size distribution affects the resulting porosity and hence the translucency of the material. The distribution of additives can affect the hydrothermal stability of the sintered material.
Differences in the zirconium powder effects the strength/ Long-term stability and translucency of the restoration.
2. Pressing conditions
The powder is first pressed, which can be accomplished by different procedures (e.g. isostatically or axially). The pressing conditions are adjusted to get an optimized blank for the pre-sintering step. The pressing methodology influences the homogeneity and the density distribution of the material and hence the marginal fit. The pressing conditions can lead to differences in strength, translucency and affect the final sintering temperature of the Zirconium.
The pressing condition and pressing method affect the marginal fit, strength and translucency of the restoration.
3. Pre-sintering
The pressed Zirconium powder is then pre-sintered in a furnace with an optimized temperature profile to generate a blank with suitable strength and millability,
Pre-sintering conditions affect the strength of the pre-sintered material and its millability.
4. Coloring
Some Zirconium materials can be colored in the pre-sintered state by immersing copings and frameworks in a dyeing liquid. This enables the absorption of coloring agents in the Zirconium material. Coloring can be achieved either by pigments (grains) or non-pigmented (ions) agents. It is important to control the effect of the dying liquid on the mechanical characteristics of the Zirconium material.
Coloring of the Zirconium can affect the marginal fit, strength and translucency of the material.
In summary, Zirconium in dentistry is chemically similar, but not necessarily alike.
References
1. 3M ESPE scientific facts page. Zirconia is not alike! Available at: http://multimedia.3m.com/mws/media/438262O/lavatm-zirconia-is-not-alike.pdf
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