Interview with Mathias Fernandez Y Lombardi, EU Scientific Manager, Kuraray Europe GmbH

Kuraray Noritake Dental Inc. reinvented its original zirconia multi-layer technology by integrating newly developed raw materials with different yttria content into the proven multi-layer colour structure. The material that is based on this innovative technology, KATANA™ Zirconia YML, offers a well-balanced flexural strength, chroma and translucency gradation throughout the blank.

According to pilot users of the new zirconia discs, the greatest benefit of using KATANA™ Zirconia YML is in the true empowerment of the laboratory. We asked Kuraray Noritake’s EU Scientific Manager Dental Ceramics & CAD/CAM Materials, Mathias Fernandez Lombardi, to explain how the new material is able to positively affect work routines in the dental laboratory.

The greatest benefit of using KATANA™ Zirconia YML is in the true empowerment of the laboratory.

Mathias Fernandez Lombardi, who should consider working with KATANA™ Zirconia YML?

KATANA™ Zirconia YML is designed for laboratories and milling centres with a strong focus on simplification. With its strong body and highly translucent enamel layer, it offers exactly the properties required for an unlimited indication range. Being perfectly suited for the production of durable monolithic long-span posterior bridges and of highly aesthetic anterior crowns or partial restorations, KATANA™ Zirconia YML is a true all-rounder that eliminates the need for another zirconia or even ceramic CAD/CAM material. At the same time, it supports highly automated production procedures and requires minimal hand work.

What are the benefits of using a single material for every indication?

Several benefits are related to the use of a single, all-round material. It leads to a reduced number of blanks to be stored, which simplifies inventory management and minimizes the storage space needed. Moreover, it facilitates standardization of laboratory workflows. Using the same material every time means that there are no differences in the basic design and milling parameters like minimum wall thicknesses and connector strengths, in the sintering protocols and finishing options to be taken into account. Hence, errors are less likely to occur and routines are easily established. Finally, the risk of cross-contamination is eliminated when a single material is processed with the available equipment (milling machine, milling and finishing tools, and sintering furnace). Predictable outcomes and flawless aesthetics are usually the result.

What are the benefits related to the use of KATANA™ Zirconia YML as the only ceramic CAD/CAM material?

I guess that the most important argument is that aesthetic outcomes are easily and efficiently achieved with KATANA™ Zirconia YML for all kinds of restorations, even for long-span bridges. It is usually sufficient to produce monolithic restorations for the posterior region, which are just polished or glazed. Anterior restorations may be designed monolithically or with minimal cut-back, and the user may choose between CERABIEN™ ZR (CZR) FC Paste Stains and CZR Internal Stains plus glaze to obtain a natural look. Another benefit of KATANA™ Zirconia YML is its seamless multi-layer structure combined with an extraordinary blank quality.

Why is this seamless structure and high blank quality important for users?

The seamless multi-layer structure, that is a smooth transition from one layer to the next, is the precondition for structural integrity and flawless aesthetics. It ensures that the material is free of transition lines, offers accurate dimensions after sintering (due to uniform shrinkage) and shows a reliable long-term performance not compromised by internal tensions. In KATANA™ Zirconia YML, a completely homogeneous composition, with carefully aligned CTE-values and shrinkage ratios in all parts of the blank, is responsible for this benefit.

Aesthetic outcomes are easily and efficiently achieved with KATANA™ Zirconia YML for all kinds of restorations, even for long-span bridges.

For its development, Kuraray Noritake Dental leveraged its long-standing expertise in dental ceramics and the advantages of an end-to-end in-house blank production process, without which it would have been impossible to reach the requested quality level. Due to the fact that the material is so well aligned, it was even possible to develop a 54-minute sintering programme that produces equally good results as the proven 90-minute or 7-hour programmes and becomes the go-to option for all rush cases. With this feature, KATANA™ Zirconia YML becomes the “jack of all trades” material: It offers highest comfort and security like an SUV, but when it comes to speed, it is in no way inferior to a sports car.

The above sintering recommendations represent only a guideline; depending on each individual furnace and condition, some adjustments might be necessary. If the 54 or 90-minute sintering program is not programmable in your furnace, it is not possible to set the furnace according to one of these schedules. *The material is removed from the furnace at 8000C. A furnace with a configurable YML sintering program is required.

Users who have already tested ceramic materials with a strength gradient know that positioning of long-span restorations in the blank can be tricky. What are the rules to be respected when positioning a long-span bridge in a YML blank?

Positioning of restorations in KATANA™ Zirconia YML discs is extraordinarily easy. The reason is that the gap between the lowest flexural strength found in the enamel area and the highest flexural strength found in the lowest body layer is comparatively small. Moreover, the Body Layer 1 that is found adjacent to the enamel layer already offers a flexural strength that is higher than the 800 MPa requested for bridges with four or more units. Consequently, users are on the safe side whenever they place their long-span restorations in the middle of the blank. More specifically, half of the connector cross section needs to be in the lower half of the disc, which is quite different from other materials with strength gradation that allow for connector positioning in the body area only. KATANA™ Zirconia YML restorations with a maximum of three units may be positioned in any layer. In this context, the fact that the thickness of each layer increases proportionally with the height of the disc comes as a benefit: The enamel layer is large enough to exploit its aesthetic potential in single-unit or small bridge cases.

Positioning of long-span restorations in the middle of the disc.

What would you recommend dental technicians who would like to empower their laboratory or milling centre?

I can truly recommend testing KATANA™ Zirconia YML in the laboratory environment to see how it handles and to assess its potential. The exceptional blank quality and seamless integration of the different layers, the ease of positioning and processing efficiency are factors potential users need to experience in real life to understand what they mean for their daily work. Similarly, the aesthetic outcomes need to be assessed in the clinical environment for an authentic impression of the natural look and feel obtained. I am sure that KATANA™ Zirconia YML will be able to convince virtually everyone who wants to empower the laboratory with an easy-to-use, efficient universal zirconia solution that fits virtually every patient’s and every dentist’s needs! Kuraray Noritake Dental knows that dental technicians and practitioners are in need of products that support their striving for simplification, standardization and increased efficiency so urgently needed in present times. Therefore, our product innovations are all focused on delivering more performance with less effort. KATANA™ Zirconia YML is the latest addition to a whole series of materials pursuing this goal.

Minimum connector cross-sections are to be respected during the computer-aided design of KATANA™ Zirconia YML restorations.

Mathias Fernandez Y Lombardi

EU Scientific Manager
Dental Ceramics & CAD/CAM Materials
Kuraray Europe GmbH

2013
Successful completion of an apprenticeship as a dental technician at Seuthe dental laboratory in Plettenberg, Germany

2018
Successful completion of dental technology studies at the University of Osnabrück, Germany. Graduation as an engineer in dental technology

Since March 2018
Employee of Kuraray Europe GmbH

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.