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Rainer Haak MME
Pre-shaded dental zirconia typically consists of metal oxides, including zirconium oxide, yttrium oxide and aluminium oxide, as well as additives like binders and colour pigments or ions. Most manufacturers of dental zirconia obtain pre-fabricated powder from an external industry partner, the most popular option being Tosoh Corporation. In contrast, Kuraray Noritake Dental relies on an end-to-end in-house process. This includes the production and addition of the components forming the company’s innovative multilayered technology, which makes it possible to match the shades in the polychromatic blanks precisely to the colours of the VITA classical A1–D4 shade guide.
Since more powder production steps are carried out in-house, this gives the company full control of the quality of the raw materials, their grain size and the purity of the formulation. It also allows for a precise alignment of the mechanical and optical product properties. Properties of zirconia restorations that are affected by the powder quality and composition include translucency and shade appearance, flexural strength, ageing behaviour and sintering performance.
Zirconia discs and blocks used for CAD/CAM processing are usually produced by uniaxial and isostatic pressing. In the uniaxial compaction process, pressure is applied to the powder from one direction (uniaxial) or two directions (biaxial), whereas the isostatic compaction process involves virtually equal pressure applied from all sides. Hence, isostatic pressing typically results in a more uniform density distribution throughout the blank and a higher material homogeneity. These factors are prerequisites for a predictable processing and sintering behaviour and affect the fit of the final restoration. For optimal mechanical and optical properties of the zirconia material, it is essential to avoid large porosities, air pockets and impurities caused by airborne particles that are trapped during pressing.
At Kuraray Noritake Dental, a unique and extremely meticulous pressing process achieves a uniform pressure distribution and low risk of contamination by airborne particles. This specific procedure reduces gravitation forces and contributes to having as high as possible density of zirconia material. All the high-level preparation processes from raw material production to pressing are responsible for the high edge stability and surface quality of restorations milled from KATANA Zirconia.
The pre-sintering procedure is necessary in that it gives the pressed blanks the required stability to be machinable with milling tools. The selected temperature profile and duration of the pre-sintering cycle determine the material’s strength and processing properties and have an impact on the final sintering process.
The unique pre-sintering procedure carried out in the production facilities of Kuraray Noritake Dental results in blanks that are stable in their pre-sintered state. Although more stable, pre-sintered KATANA Zirconia is machinable with common diamond-coated milling tools without any increased risk of breakage or higher tool wear.
The unique procedure has a positive impact on the surface smoothness after milling and can significantly shorten sintering times. In fact, the speed sintering program offered for all variants of KATANA Zirconia is the fastest one on the market. In the dental laboratory, the sintering times may be reduced to 90 minutes* for single-tooth restorations and bridges of up to three units.
Using the KATANA Zirconia block with Dentsply Sirona’s CEREC system, it is possible to sinter single crowns up to three-unit bridges in 18–30 minutes without compromising the mechanical or optical properties.
Together, these efforts taken by Kuraray Noritake Dental to produce dental zirconia of exceptionally high quality make all the difference. The KATANA Zirconia series—KATANA Zirconia Ultra Translucent Multi Layered (UTML), Super Translucent Multi Layered (STML), High Translucent Multi Layered (HTML) and KATANA Zirconia High Translucent Mono Layered (HT)—have a homogeneous, high-density structure with low porosity and a high level of purity. This optimises the performance of the blanks during machining.
An optimised processing behaviour leads to regular restoration margins, smooth surfaces and a precise fit of the restorations. The latter is due to the fact that the milling behaviour and volumetric shrinkage during final sintering are highly predictable, so that a user designing a 20 µm cement gap will get what he or she desires. Owing to the great control over optical properties and precise match to the VITA classical A1–D4 shades, KATANA Zirconia is considered to be one of the most aesthetic dental zirconia options available on the market.
In order to ensure all the desired material properties, including aesthetics and strength, one thing is essential: the machining carried out in the dental laboratory—milling and sintering—needs to adhere to the recommended protocols. This means that the milling machine and furnace should be cleaned and calibrated on a regular basis, which provides the conditions for optimised zirconia processing from the powder to the final, true-to-life dental restoration.