News Feature

KATANA™ Zirconia Block: Useful tips for processing

KATANA™ Zirconia Block has hardly any common features with the zirconium oxide that captured dental laboratories two decades ago and was used as a framework material. In many cases, the 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 LT variant of the ceramic and its surface – if smoothly polished – antagonist-friendly causing low wear. Apart from its natural translucency, the multi-layered 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 and tips and tricks that help achieve the best possible results.

COMPUTER-AIDED DESIGN

What is the best position of the restoration in the block to leverage all benefits related to the multi-layered structure?

Ideally, a restoration is positioned in the middle of the block. 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 – are lost, which makes 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.

COMPUTER-MANUFACTURING

Is wet or dry milling the best option to process 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 utilizing the same machine for the processing of zirconia and glass ceramics should implement a system utilizing three water tanks. One tank is filled with wash water, the tank used for processing of glass-based ceramics is filled with water and additives (such as Dentatec, Dentsply Sirona) and the one used for processing of zirconia contains purified/distilled water without any additives.

Moreover, a prerequisite for a high quality of the manufactured restorations is that the machine is cleaned thoroughly every time before a different material is processed.

The following components need to be cleaned:

the milling tools
the interior of the machine
the water tank(s)
all filters

Typically, zirconia reacts to milling additives (Dentatec) in the water with a reduced translucency, so that their use cannot be recommended. Residues 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 (typical effect: 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 tool sets every 10 to 15 units. At this time, the degree of tool wear amounts to approximately 50 percent. Those who continue using the tools accept an increased risk of weakening the oxide ceramic’s microstructure: worn milling tools are likely to produce micro cracks and micro chipping. When these defects occur between the different units of a bridge, they increase the risk for chipping and fractures.

MANUAL POST-PROCESSING

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 residues 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 residues remaining on the restoration surface might lead to the occurrence or 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, i.e. between milling and final sintering. Diamond milling tools for oxide ceramics should be used at a low speed (7,000 to 10,000 rpm) for this purpose.

TANK MANAGEMENT: CLEANING PROCEDURE AFTER USING GLASS CERAMICS

Be sure 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. Then, thoroughly flush milling debris from the milling chamber filter and milling powder from beneath the milling chamber filter.

FINAL SINTERING

What are the particularities to be respected during final sintering of restorations made of KATANA™ Zirconia Block material?

Zirconia restorations are subject to volumetric shrinkage (approximately 20 percent) during sintering. Therefore, it is essential to not 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.

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 programme on a regular basis is an effective measure that helps prevent discoloration. For this purpose, white zirconium oxide (either collected from odds and ends of non-shaded blanks or purchased) is placed in the furnace chamber and a sintering programme is started. As the furnace CEREC® SpeedFire does not offer a specific decontamination programme, there is no way around working a fiddle: 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 zirconium oxide absorbs the metal oxides, which has a cleaning effect on the furnace.

CHARACTERISATION

Which techniques are best suited for an optical refinement of monolithic restorations made of KATANA™ Zirconia Block?

A material designed for this kind of refinement is CERABIEN™ ZR FC Paste Stain from 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 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 percent of all restorations made of KATANA™ Zirconia Block material. The firing temperature is 750 °C and the CTE 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 characterization.

How is it possible to hold the restoration during characterization?

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.

POLISHING

How to ensure a low wear of the antagonist?

Zirconium oxide stands out due to its hardness, which makes careful polishing of the surface a prerequisite for clinical use. After characterization with CERABIEN™ ZR FC Paste Stain, polishing of all occlusal contact areas is mandatory as well. The polishing set Polierset KATANA™ Twist DIA even allows for intraoral use.

PLACEMENT

What kind of resin cement is recommended for the definitive placement of restorations made of KATANA™ Zirconia Block material?

The best possible results are obtained with PANAVIA™ V5. This adhesive resin cement system is responsible for the development of a particularly strong bond between the tooth structure and the restoration, which also has a strengthening effect on the restoration.