Users of dental zirconia are really spoilt for choice these days. Countless manufacturers offer zirconia materials that differ in their mechanical and optical properties and indication range. What is not apparent at first sight is that the available products also differ with respect to the quality of the CAD/CAM blanks offered. Blank quality is highly dependent on the quality of the raw materials and is affected by different aspects during raw material processing, pressing and pre-sintering. This has a huge impact on the surface quality, edge stability, fit and processing requirements of milled restorations.

Raw material production

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.

At Kuraray Noritake Dental, the powder is produced in-house.

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.

Blank pressing

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.”

Pre-sintering

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.

A KATANA™ Zirconia blank ready for milling.

Fast sintering for the laboratory

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.

^*The material is removed from the furnace at 800°C.

Overview of the recommended sintering protocols.

Fast sintering for chairside

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.

The KATANA™ Zirconia block displays superior optical properties after 18 minutes of sintering compared with representatives of major competitors’ raw material after 30-minute and 60-minute sintering programs designed by Kuraray Noritake Dental based on the manufacturer's recommendations.

Unique KATANA™ Zirconia 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.

Surface roughness

SEM image, magnification 33X, of competitor material surface roughness. Image courtesy of Dr Kunkela, Kunkela Research Academy.

SEM image, magnification 33X, of KATANA Zirconia surface roughness. Image courtesy of Dr Kunkela, Kunkela Research Academy.

SEM images of non-polished KATANA™ Zirconia and a competitor’s material at 33x magnification. The four images of each material show the surface structure at different areas of a molar crown. In all areas, the surface of the restoration made of the competitor’s material is rougher and shows more porosity than the surface of the KATANA™ Zirconia crown directly after milling, according to Dr Josef Kunkela’s research results. One of the contributing factors to this result is the more densely pressed blanks with smaller grain sizes of KATANA™ Zirconia.

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.

Excellent marginal fit

SEM images revealing the fit of restorations made of two different materials (lithium disilicate and KATANA™ Zirconia Block STML) on a tooth abutment. The KATANA™ Zirconia restoration shows a more regular margin and more precise fit (with a cement gap of 19-21 µm) than the lithium disilicate crown (cement gap 26-45 µm). Images courtesy of Dr Kunkela, Kunkela Research Academy.

Extremely regular margins of a KATANA™ Zirconia crown after milling, which is also a result of the favourable material structure.

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.

The use of highly translucent, gradient pre-shaded zirconia brings more efficiency into the dental laboratory. Due to the advanced properties of the materials, e.g. from the KATANA™ Zirconia Multi-Layered Series, true-to-life restorations may be created without any or with only a small vestibular layer of veneering porcelain. This saves a lot of time usually required for manual work around steps in the veneering of zirconia frameworks. At the same time, this also allows for a reduced wall thickness, beneficial in the context of minimally invasive dentistry.

In order to leverage the high aesthetic potential and balanced mechanical properties of these types of zirconia, however, it is essential that the restorations are processed under ideal conditions. The most advanced materials with the highest translucency are particularly sensitive to contamination during and after wet milling, contamination of the furnace chamber, and temperature variations during sintering. Possible undesirable effects include a grayish appearance of the restorations and low chroma, green, yellow, blue or gray traces in the restorations, white spots on the surface and variations in colour and translucency.

If carried out on a regular basis, the following measures will effectively eliminate these effects. Hence, they will support users in ensuring consistently beautiful results.

Optimizing the milling process

Blue or gray traces visible in the final restoration are usually the result of contamination of the cooling water with extrinsic particles in the context of wet milling, (which is usually conducted in chairside procedures). In most cases, silica particles left over from the processing of glass or silicate ceramics with the same milling unit are the root of the problem. The effect is easily avoided by thorough cleaning of the milling chamber, the water tank and the filter insert of the milling machine every time a different material needs to be processed. Another solution is dry instead of wet milling, which offers additional benefits such as shorter processing time and better quality edges and surfaces.

Decontamination of the furnace chamber

In general, the chamber of the sintering furnace should be cleaned before sintering. Important measures include the removal of dust inside the sintering chamber and cleaning of the heating elements, both done with a soft brush. The use of compressed air is contraindicated.

Unwanted optical effects that occur on restorations due to contamination of the sintering chamber include white spots on the restoration surface, a blue-grayish appearance and low chroma, and green or yellow traces in the material. White spots on a restoration surface are usually indicators of contaminated alumina sintering beads or the use of the wrong instruments for surface modification and sprue removal. The effect is avoidable through a monthly or even more frequent replacement of the sintering beads (as soon as they show any signs of discoloration) as well as the exclusive use of fine-grid diamond instruments for adjustments prior to sintering.

Alumina sintering beads may be the cause of white spots on a restoration surface if not replaced on a regular basis.

The blue-grayish appearance and low chroma may be attributed to mineral residues from dipping liquids in the chamber. They are effectively removed with the aid of a decontamination program to be selected in the furnace menu, which is run after inserting several residual pieces of a highly translucent, white zirconia blank. As soon as the decontamination cycle is completed, the chromatic intensity of the residual blank parts indicates whether a second cycle is required. In order to prevent the occurrence of a grayish appearance in new restorations, it is recommended to perform a decontamination program at least once per month.

Pieces of a white zirconia blank left over after milling.

MoSi₂ heating elements: Regeneration needed

If a restoration appears to be green or yellowish, it is most likely that the furnace is equipped with aging molybdenum disilicide (MoSi₂) heating elements in need of regeneration or replacement. The inner part of the elements is made of molybdenum (Mo), which is usually covered by a protective layer of silica (SiO₂). This layer is naturally built up during sintering at a temperature range between 1,000 and 1,600°C. As the thickness of the layer grows, its intrinsic residual compressive stress increases. This stress, as well as possible extrinsic influences, e.g. originating from acidic dipping liquids, may finally lead to cracks and a breakup of the protective layer. Once damaged, the molybdenum core is exposed. At a low temperature range of 400 to 600°C, the molybdenum reacts with oxygen in the sintering chamber, a process referred to as pest oxidation. The resulting molybdenum oxide (MoO₃), together with ions or metal oxides from colouring agents, is responsible for the green-yellowish discoloration on the surface of the restorations.

Restorations displaying greenish surface pigmentation.

Molybdenum disilicide heating element with a protective silica layer bursting off, leading to pest oxidation and the contamination of elements in the sintering chamber.

Regeneration firing, which involves a rapid heating rate and a long firing phase at approx. 1,450°C, aims at regenerating the layer of silica. This measure, however, works only a limited number of times, as a repeated process of pest oxidation and regeneration leads to aging of the heating element itself. Hence, it will ultimately lead to the need for replacement. The whole issue of pest oxidation may be effectively avoided by the use of a furnace with silicon carbide heating elements, which are highly aging-resistant and do not cause any discoloration. A positive side effect is that these types of heating elements deliver more constant temperatures.

Temperature control

Variations in translucency or chroma and pigmentation of restoration surfaces are often due to deviations of the actual sintering temperatures from the recommended temperature curve. The only way to solve this issue is temperature calibration. This measure is not only a prerequisite for aesthetic results, but also has a decisive impact on the mechanical properties of the restorations: if the maximum temperatures are too high, for example, the flexural strength of the zirconia materials may be expected to decrease¹.

Effect of temperature differences during sintering on restorations made of KATANA™ Zirconia UTML: The restorations were sintered at the same nominal temperatures in three different furnaces!

Temperature control is usually carried out with the aid of TempTABs or PTCRs (process temperature control rings). They are placed into the furnace on a sintering tray and typically processed by running a calibration cycle. After sintering, the tab or ring diameter is determined. As TempTABs and PCTRs exhibit controlled shrinkage, it is possible to calculate the actual sintering temperature based on the measured diameter. A conversion table supports the user in determining the deviation between the temperature actually reached and the temperature displayed on the furnace. Subsequently, the values displayed on the furnace are adjusted if necessary.

TempTAB on a sintering tray with restorations ready for sintering.

General recommendations

In order to set the stage for brilliant aesthetics and ideal properties of zirconia restorations, it is essential to ensure optimal processing conditions. Instead of troubleshooting carried out whenever discoloration appears after sintering, it is advisable to take the following actions on a regular basis as preventive measures:

• Cleaning of the milling machine’s water tank every time before starting to mill (wet milling only)
• Strict adherence to the sintering protocols recommended by the material manufacturer
• Removal of the dust from the sintering chamber and heating elements with a soft brush before each use
• Replacement of the alumina sintering beads whenever they show signs of discoloration (at least once per month)
• Exclusive use of fine-grid diamond instruments for sprue-removal and pre-sintering adjustments
• If possible: Use of furnace with silicon carbide heating elements
• Furnaces with molybdenum disilicide heating elements require constant visual control and regular regeneration cycles
• Running of a decontamination program with decontaminating powder or white zirconia residues (y-TZP) at least once per month
• Temperature control and calibration at least once per month

With these simple measures, it is possible to maximise the full potential of KATANA™ Zirconia Multi-Layered Series from Kuraray Noritake.

References
¹ Stawarczyk, B., Özcan, M., Hallmann, L. et al. The effect of zirconia sintering temperature on flexural strength, grain size, and contrast ratio. Clin Oral Invest 17, 269–274 (2013).

By Daniele Rondoni, RDT

Considering different criteria to select the ideal zirconia and frame design to meet the level of esthetics requested.

Step 1
Final Preparations.

Step 2
Zirconia Frame (KATANA Zirconia STML A2) cut-back designed to reproduce translucent incisal area.

Step 3
Application of 1st Internal Stain and firing.

Step 4
Application of 1st Luster, Clear Cervical and firing.

Step 5
Application of 2nd Internal Stain and firing.

Step 6
Application of 2nd Luster, and Opacious Body.

Step 7
Completion of firing.

Step 8
Completion of morphological correction.

Step 9
Post-operative view.

By D.T. Pier Francesco Golfarelli

Digital workflow and CAD/CAM shaping have now become a daily practice that helps to manage most cases, including the most extensive re-adaptations (rehabilitations).

KATANA™ HTML zirconia was selected, in consultation with the specialist, for the case presented here. It was principally chosen for its aesthetic and mechanical properties. One of our selection criteria was the advantageous lower abrasiveness level of zirconia. Because of its density, this material is less abrasive than the more traditional ceramics in combination with adequate mechanical polishing. Based on the initial situation, once the assembly in the articulator was completed, we designed a structure with anterior cutbacks for maximum aesthetics, while for the posterior teeth a monolithic solution was chosen for maximum strength and quality of the functional surfaces.

With the CORE & SHELL technique developed by the Noritake Italian Study Club, I can now fully exploit the optical properties of KATANA™ zirconia by integrating it, in the anterior area, with Noritake CERABIEN™ ZR ceramics. In the images here you can see the aesthetic results of the anterior and side areas, the mechanically polished monolithic surfaces and the special Noritake glaze.

Fig. 1 CAD Shaping - 3Shape Dental Designer

Fig. 2 Structure design with cutbacks

Fig. 3 KATANA™ HTML structure

Fig. 4 Occlusal surface – details

Fig. 5 Shade stain

Fig. 6 Shade Stain (SS)

Fig. 7 Core

Fig. 8 Internal Live Stain (ILS)

Fig. 9 Shell

Fig. 10 Shell Tissue

FINAL SITUATION

Fig. 11 Layering Details

Dentist:

D.T. Pier Francesco Golfarelli
Noritake Italian Study Club Teacher, Forlì

This case was conducted by Dr Davide Cortellini, owner of Studio Cortellini in Riccione in Italy, and dental technician Angelo Canale, owner of Canale dental laboratory in Rimini in Italy.

This patient came to the clinic to improve her chewing ability and aesthetic level. The physical examination revealed the presence of several endogenous erosive lesions that made chewing difficult, in addition to partly affecting the esthetics due to decrease in enamel thickness and the presence of dyschromic composite restorations. The possibility of using the new types of both tetragonal and cubic multilayer zirconia made it possible for us to plan the complete covering of all the elements with extremely conservative crowns with thicknesses between 0.5 and 1 mm in the axial and occlusal areas and up to 0.2 mm at the margin.

VERTICAL PREPARATIONS

Very conservative vertical preparations were carried out in the enamel without anesthesia. In the upper arch, the front group was prepared for full-veneer crowns, while the lower front group was treated with conventional lithium disilicate veneers without interproximal separation. In this case too, vertical preparations were carried out without finishing line. The impression was made using a 3Shape TRIOS intraoral scanner. The technician modeled the zirconia restorations that were then completed by the ceramist.

Knife Edge Preparation

SCANNING SEQUENCE

Digital DV models of temporary teeth

The three different materials were selected on the basis of the specific positions inside the mouth:

• UTML for the anterior teeth
• STML for the premolars
• HTML for the molars

The final result shows excellent integration between the 3 different types of zirconia and a good natural feeling.

MINIMAL PREP KATANA™ (KATANA™ MICRO LAYER)

HARMONIC OPTICAL INTEGRATION

The full-mouth rehabilitation procedure using three different types of zirconia led to a functional and beautiful treatment outcome. The optical integration between the materials is excellent and the high translucency especially in the anterior region creates a true-to-life appearance.

APPROACH WITH MINIMAL INVASIVE PREPARATIONS - REPORT ON THE SELECTION OF MULTI-LAYERED ZIRCONIA

Dental zirconia is no longer just the opaque framework material introduced two decades ago. Nowadays, it offers the high strength needed for long-span bridges, dentin-like translucency and strength perfect for thin-walled posterior crowns, or enamel-like optical properties for beautiful anterior restorations - depending on its composition and structure. Kuraray Noritake offers three types of dental zirconia - three with a multilayered structure created using patented powder coloring technology. They differ with regard to their optical and mechanical properties, which makes it possible to choose an ideal material for every clinical situation.

Dentists:

DR. DAVIDE CORTELLINI

Dr. Cortellini graduated with honours in Dentistry and Dental Prosthetics from the University of Siena in 1992. He won the scholarship of the Italian Society of Periodontology for the year 1994-95. He then received the opportunity to attend the Department of Periodontology and Fixed Prosthetics of Prof. NP. Lang at the University of Bern in Switzerland, carrying out clinical and research activities. He obtained the title of „Doctor Medicinae Dentium“ (D.M.D.) at the same university in 2000. Dr. Cortellini is the author of scientific publications in international journals of aesthetic dentistry. He is an active member and advisor to the Italian Academy of Prosthetic Dentistry, and a member of the International Academy for Digital Dental Medicine. He holds conferences in Italy and abroad and is dedicated to clinical activity in his own practice in Riccione, where he primarily focuses on aesthetic and digital prosthetics and complex prosthetic rehabilitation.

CDT. ANGELO CANALE

Mr. Canale is a dental technician graduated high school in 1981. He’s the owner of a dental laboratory in Rimini since 1986. He is specialized in fixed prosthesis on natural teeth and on implants with a metal free approach using different kind of materials. He is always interested about digital technique using CAD-CAM systems (in his laboratory the 80% of the work comes from digital impressions). He is co-author of international publications on PPAD, QDT, EJED and THE JOURNAL OF ADHESIVE DENTISTRY concerning the use of metal free in prosthesis. He is a speaker in national and international congress.

We are pleased to present the following short video series of MDT. Kazunobu Yamada using Katana Zirconia UTML for Stain & Glaze Wipeoff Technique.

Each step will be explained in the chapters below.

We are pleased to present the third episode of our short KATANA video series on how to check and manage your sintering.

Each step will be explained in the chapters below.

The full video can be viewed at the bottom of this page.

We are pleased to present the second episode of our short KATANA video series on how to improve your restoration before sintering.

Each step will be explained in the chapters below.

The full video can be viewed at the bottom of this page.

We are pleased to present “Katana Zirconia Design’ - a short video series on how to improve your design and milling.

Each step will be explained in the chapters below.

The full video can be viewed at the bottom of this page.

Though one is a master dental technician and the other a dentist, Daniele Rondoni and Dr Nicola Scotti share a passion for discovering new materials and technologies to better provide aesthetic, durable restorations for patients. Here, they discuss their dental journeys and how they have integrated Kuraray Noritake Dental’s KATANA™ Zirconia range into their workflows.

How did you get started in dentistry, and what was it about dentistry that first interested you?
Dr Nicola Scotti: Well, my professional interest goes back 15 or 20 years at this point, but I was first introduced to dentistry as a child through my father, who was a dentist himself. I grew up looking at teeth, looking at models, and it was something that was just in my blood. When I had to decide what to study, I followed my heart and decided to become a dentist, and ever since I have remained dedicated to learning new things about it.

Daniele Rondoni: I have a similar story to Nicola. My uncle was a dental technician and that influenced my decision to become a dental technician and open up my own dental laboratory. Though there were many things that interested me, I have always focused on the major possibility that this profession offers, the opportunity not only to use your hands but also to combine this skill with the newest dental technologies available.

Regarding this combination of manual and digital skills, do you still see a role for the dental professional as a craftsperson in this increasingly digital industry?
Scotti: Dentistry, even if the digital influence continues to grow, will surely continue to be a profession defined by what we can do with our hands. You cannot complete a restoration, for example, without having these manual skills. There is also the matter of personalisation. Though machines can create flawless dental restorations in great numbers, it is the artistry of the dental technician that allows our patients to have customised solutions made with the materials and the shapes that suit them best. If you want to provide natural-looking restorations for your patients, digital dentistry is a great tool to use, but you need to have manual skills and a passion for the work as well.

Rondoni: As a dental technician, it is important to remember that my work depends on technology, and so in this way, digital dentistry and the new-generation dental materials have helped my work greatly. Using this technology for individual situations is an essential part of my role, and excellent materials like Kuraray Noritake Dental’s KATANA™ Zirconia range make my artistic vision regarding these situations more easily achievable.

Scotti: In my opinion, even if 90 per cent of the workflow is digital in nature, you need that artistic input and vision for the remaining 10 per cent in order to really create a successful restoration. With KATANA™ Zirconia, these two aspects come together and allow you to create something really amazing.

Speaking of the KATANA™ Zirconia range from Kuraray Noritake Dental, is this material a part of your everyday workflow at this point? What do you see as its benefits?
Rondoni: It is definitely a part of my everyday workflow. I have worked with Noritake materials for many years, but when Kuraray released its newest generation of zirconia, it totally changed my approach in the dental laboratory. Over the last five years, KATANA™ Zirconia has greatly influenced my workflow; it allows me to use certain technologies without forgoing artistic needs, and since it’s available in different translucencies and shades and so on, I can create aesthetic restorations for individual cases with ease.

Scotti: In the past, I didn’t regard zirconia as a material for dentists. We used to take the impression and then ask the dental technician to use zirconia to make a restoration, but we didn’t use the material ourselves. What the KATANA™ Zirconia Block for chairside has introduced is a totally new concept of how zirconia can be used by dentists themselves. Having a material that can create monolithic full-contour restorations, even single crowns, is incredible. Its aesthetic qualities are fantastic as well, as Daniele mentioned, and it gives us a truly new option for treating patients.