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KATANA Zirconia Block (Fig. 1) has hardly any features in common with the zirconia that was so popular in dental laboratories two decades ago and was used as a framework material. In many cases, this innovative zirconia serves as a functionally and aesthetically sensible alternative to high-strength glass-ceramics. Its flexural strength is higher than that of lithium disilicate, its translucency on a level with the low-translucency variant of the ceramic and its surface—if smoothly polished—antagonist-friendly, causing low wear.
Apart from its natural translucency, the multilayered structure of KATANA Zirconia Block, with four gradient shades precisely imitating natural colour gradients found in anterior and posterior teeth, leads to highly aesthetic outcomes.
In order to exploit the full potential of the cubic zirconia KATANA Zirconia Block, users of the CEREC system need to adopt slightly different automatic and manual processing strategies than for glass-ceramic processing. The following answers to frequently asked questions provide information on the most important differences, as well as, tips and tricks that help achieve the best possible results.
What is the best position of the restoration in the block to leverage all benefits related to the multilayered structure?
Ideally, a restoration is positioned in the middle of the block (Fig. 2). In addition, the selected block should be one shade darker than the determined shade. This will lead to a natural shade appearance of the resulting restoration. If the restoration is placed at the upper margin of the block, parts of the body shade—and with it chroma—will be lost, which will make the restoration appear too light.
How are chipping effects in the area of the milling sprues (most often occurring in bridges) effectively prevented?
Marginal chipping in these areas might appear due to an uneven force distribution during milling. The problem is easily overcome by designing two milling sprues per crown or bridge unit.
Is wet or dry milling the best option for processing KATANA Zirconia Block?
The best results are obtained with dry milling. This procedure also leads to time-savings, as there is no need for drying after processing.
When opting for wet milling, are there any particularities that need to be respected for KATANA Zirconia Block?
Those users opting for wet milling and utilising the same machine for the processing of zirconia and glassceramics should implement a system utilising three water tanks (Fig. 3). One tank is filled with rinsing water, the tank used for processing of glass-based ceramicsis filled with water and additives (such as Dentatec, Dentsply Sirona) and the one used for processing of zirconia is filled with purified or distilled water without any additives.
Moreover, a prerequisite for a high quality of the manufactured restorations is that the machine has to be cleaned thoroughly every time before a different material is processed (Figs. 4 & 5). The following components need to be cleaned:
- the milling tools;
- the interior of the machine;
- the water tank(s); and
- all the filters.
Typically, zirconia reacts to milling additives (Dentatec) in the water with a reduced translucency, so their use cannot be recommended. Residue of glass-ceramic particles found on milling tools, in the water or in the interior of the machine might also have a negative impact on the aesthetic appearance of KATANA restorations (typically, white spots on the surface).
When is it necessary to replace milling tools used for the processing of KATANA Zirconia Block?
It is recommended to replace the toolsets every ten to 15 units. At this time, the degree of tool wear amounts to approximately 50%. Those who continue using the tools accept an increased risk of weakening the oxide ceramic’s microstructure: worn milling tools are likely to produce microcracks and microchipping. When these defects occur between the different units of a bridge, they increase the risk of chipping and fracture.
Are there any specific measures to be taken when removing the block from the machine?
When machining is completed, the restoration should be touched only by individuals wearing gloves. Even after thorough washing, bare hands will release fats, which may lead to smear formation and a reduced translucency of the oxide ceramic material.
How are the restorations cleaned after sprue removal?
Powder residue should be removed after processing with a gentle stream of air. Alternatively, a fine (and obviously clean) brush may be used for the removal of the zirconia dust. Powder residue remaining on the restoration surface might lead to the occurrence of white spots or to smear formation. Steam cleaning of the restoration should be avoided, as this measure dramatically increases the pores in the material and alters the material properties.
When is the right time to add fissures and texture to the surface of the restoration, and how is this task accomplished?
Ideally, all those details that cannot be incorporated by a milling machine (surface texture, fissures, etc.) are added in the restoration’s pre-sintered state, that is, between milling and final sintering. Diamond-coated milling tools for oxide ceramics should be used at a low speed (7,000–10,000 rpm) for this purpose.
Tank management: Cleaning procedure after using glass-ceramics
Ensure that the glass-ceramic cooling water tank is installed. Press the pump button on the touch panel to wash away any glass-ceramic powder that is inside the milling chamber and then thoroughly flush milling debris from the milling chamber filter and milling powder from beneath the milling chamber filter (Figs. 6 & 7).
What are the particularities to be respected during final sintering of restorations made of KATANA Zirconia Block?
Zirconia restorations are subject to volumetric shrinkage (approximately 20%) during sintering. Therefore, it is essential not to place the restoration on a rigid carrier during sintering. If the volume of the restoration is reduced while the volume of the carrier is not, the risk of fracture is high. Ideally, the restoration is placed on its largest side.
“Zirconia restorations are subject to volumetric shrinkage (approximately 20%) during sintering”
What measures should be taken to avoid discoloration of a restoration during sintering?
Discoloration (usually leading to a green-yellowish appearance) might be the result of contamination of the sintering furnace with metal oxides. These metal oxides originate from other materials processed in the furnace. They are released during sintering and absorbed by the restoration. Running a decontamination program on a regular basis is an effective measure to help prevent discoloration. For this purpose, white zirconia (either collected from odds and ends of non-shaded blanks or purchased) is placed in the furnace chamber and a sintering program is started. As the CEREC Speed-Fire furnace (Dentsply Sirona) does not offer a specific decontamination program, the user simply creates two separate jobs for the sintering of two single crowns, but sinters them together. The second job is used for the decontamination cycle. During this procedure, the white zirconia absorbs the metal oxides, which has a cleaning effect on the furnace.
Which techniques are best suited for optical refinement of monolithic restorations made of KATANA Zirconia Block?
A material designed for this kind of refinement is CERABIEN ZR FC Paste Stain (Kuraray Noritake Dental). The paste-like shading solutions are filled with ceramic particles. Therefore, the optical effects achievable with these pastes are much more durable than those obtained with classical stains. As all the pastes are fluorescent, it is possible to produce a natural long-term fluorescence effect without an additional work step. With just four shades of the liquid ceramic—A+, Grayish Blue, Value and Clear Glaze—it is possible to perfectly characterize 85% of all restorations made of KATANA Zirconia Block. The firing temperature is 750°C, and the coefficient of thermal expansion values of CERABIEN ZR FC Paste Stain and KATANA Zirconia Block are precisely adjusted to each other. The product is also very well suited for glass-ceramic characterisation (Fig. 8).
How can the restoration be held during characterisation?
The easiest option is the use of putty material on a carrier. The crown is simply placed on the unset putty and then positioned on the carrier. The surface of the restoration should be cleaned with a gentle stream of air before applying the pastes. Thanks to the ceramic particles in CERABIEN ZR FC Paste Stain, the pastes stay put during processing and do not flow away.
How does one ensure a low wear of the antagonist?
Zirconia stands out owing to its hardness, which makes careful polishing of the surface a prerequisite for clinical use. After characterisation with CERABIEN ZR FC Paste Stain, polishing of all occlusal contact areas is mandatory as well. The KATANA Twist DIA polishing set (Kuraray Noritake Dental) even allows for intraoral use.
What kind of resin cement is recommended for the definitive placement of restorations made of KATANA Zirconia Block?
The best possible results are obtained with PANAVIA V5 (Kuraray Noritake Dental; Fig. 9). This adhesive resin cement system is responsible for the development of a particularly strong bond between the tooth structure and the restoration, and this also has a strengthening effect on the restoration.
Editorial note: A list of references available from the publisher. This article was published in digital―international magazine of digital dentistry Vol. 1, Issue 4/2020.