Annett Kieschnick

Mon. 4. April 2022

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Differences within material classes in CAD/CAM ceramics are not obvious at first glance. A knowledge of materials science is required in order to classify the ceramics accurately, to use them according to the indication and to process them correctly. In this interview, Prof. Bogna Stawarczyk, whose team at the Ludwig-Maximilians-Universität München (LMU Munich) in Germany is renowned for its ground-breaking work in the field of zirconium dioxide and silicate ceramics, shares insights about lithium disilicates and the features of Initial LiSi Block (GC Europe).

Prof. Stawarczyk, could you please specify to which class of materials lithium disilicate ceramics belong?
In general, dental ceramics can be divided into two groups―oxide ceramics (for example, zirconia) and silicate ceramics. Lithium disilicate is a silicate ceramic that is additionally reinforced with lithium disilicate crystals. The reinforcement crystals result in higher mechanical properties, such as flexural strength or fracture toughness, compared with non-reinforced silicate ceramics (feldspar or leucite ceramics).

Prof. Bogna Stawarczyk is head of materials science research at the Department of Prosthetic Dentistry and a professor at LMU Munich. She is vice president of the European Association of Dental Technology. (Image: Bogna Stawarczyk)

The superordinate group of lithium disilicate ceramics is therefore lithium silicate. There are three subgroups here. Lithium disilicate ceramic has been available on the market for a long time. In addition, lithium metasilicate and lithium aluminosilicate ceramics have been around for a few years. The main components of these ceramics are lithium oxide and silicon oxide.

So, there are different lithium silicate ceramics and various products from several manufacturers. How can these be distinguished from a materials science perspective?
For us, the composition of the ceramics and the manufacturing process are interesting and ultimately decisive for the material’s properties. The glassy phase of all three lithium silicate ceramics is silicon oxide; the crystalline phase is lithium oxide.

Lithium disilicate and lithium metasilicate ceramics are formed by the crystallisation of lithium oxide and silicon oxide. The molar ratio between lithium oxide and silicon oxide in the glassy phase determines the formation of either lithium metasilicate or lithium disilicate crystals. In lithium aluminosilicate ceramics, a co-crystallisation of lithium disilicate and lithium aluminosilicate takes place.

That sounds very technical. What are the differences in processing in the practice and in the laboratory?
The industrial manufacturing process and the composition of the ceramics determine the application properties. Since the ceramics are reinforced differently, there are certainly deviations in certain properties. For example, all three lithium silicate ceramics are suitable for CAD/CAM milling but, at the moment, only lithium disilicate ceramic is suitable for the pressing technique.

CAD/CAM-manufactured veneers made of lithium disilicate ceramic Initial LiSi Block (GC Europe). (Image: Christian von Bukowski)

Furthermore, some ceramics are pre-crystallised and others are fully crystallised, which affects the manufacturing process. In addition, a lithium aluminosilicate ceramic cannot be individualised by glaze firing in the furnace with conventional ceramics owing to the low coefficient of thermal expansion (CTE). In contrast, lithium disilicate ceramics, for example, can be characterised with ceramic-based paints.

In general, lithium disilicate ceramics have a CTE comparable with that of zirconia. So, it is easy to remember that, if a ceramic’s CTE value is comparable with that of zirconia, it also bonds to a lithium disilicate ceramic. Hence, the practitioner should be aware of important differences between the different lithium silicate ceramics.

CAD/CAM-manufactured veneer for an anterior tooth made of lithium disilicate ceramic (Initial LiSi Block; GC Europe) after the grinding process. (Image: Christian von Bukowski)

Nowadays, new optimised ceramics are constantly coming onto the market. Some time ago, GC launched a fully crystallised CAD/CAM block, Initial LiSi Block. What is so particular about it?

Basically, Initial LiSi Block is a lithium disilicate ceramic. A special feature is that the material is already in the definitive crystallised state and thus already has its maximum density and final flexural strength. Therefore, the ceramic does not have to be recrystallised in the furnace after the grinding process.

Another positive aspect is that the Martens hardness parameters (Martens hardness and penetration modulus) are slightly lower compared with other lithium silicate ceramics, which means that the edge stability is very high. The ceramic is easy to mill. The risk of edge breakouts or brittleness is reduced because of these Martens hardness parameters. The light-optical properties also appear very good.

From a processing point of view, the fast production time should also be mentioned as a special feature; additional crystallisation firing is omitted. Nevertheless, individualisation is still possible on request. After a short time, ceramic-based paints can be used to individualise the restorations made from the monochromatic block. To individualise the milled restorations, a system could be used such as the paintable colour-and-form ceramic Initial IQ ONE SQIN (GC Europe).

You have subjected the ceramic to some laboratory tests for preliminary research. What were your first findings?
Compared with other lithium silicate ceramics for milling, there was a lower Martens hardness, which correlates with the good edge stability Initial LiSi Block has. In addition, the penetration modulus (indentation modulus/elasticity modulus) is slightly lower than that of comparable lithium silicate ceramics. Therefore, even finely tapered and sharp edges can be precisely implemented. We’ve tested various mechanical properties and found that the reliability (Weibull modulus) of Initial LiSi Block is high. From this it can be concluded that the material does not fracture spontaneously and unexpectedly.

In summary, a clear trend can be drawn from our first preliminary tests: the material has positive Martens hardness parameters, which indicate that the edge stability of the milled restoration is high.

Posterior crown milled from fully crystallised lithium disilicate (Initial LiSi Block; GC Europe). The picture illustrates the high edge stability, which is owing to a slightly reduced Martens hardness. (Image: Christian von Bukowski)

At LMU Munich, you do a great deal of research on CAD/CAM materials and pay attention to the very practical questions arising from practice and laboratory. Are there any questions that are brought to your attention repeatedly?
CAD/CAM materials play a key role in our research because they are the future. The material quality is high and standardised owing to industrial production. Questions from the practice and the laboratory usually concern processing: “How can the materials be ground and polished?”, “How much rework is necessary?”, “Are the materials compatible, for example, with paints?”, and so on.

Very often, the focus is also on questions relating to intra-oral luting. Here, I come back to the lithium disilicate ceramic Initial LiSi Block. From my point of view, restorations made of this ceramic should be cemented adhesively. The milled restoration is etched for 20 to  seconds and, after conditioning with a silane-containing primer, seated according to the protocol with a conventional resin composite cement or, according to the manufacturer's instructions, with a self-adhesive resin composite cement, such as G-CEM ONE from GC Europe. Since cementation is a sensitive step, materials science knowledge is especially important.

What material trends do you foresee for prosthetic dentistry in the near future and later?
In general, we try to copy the properties of the natural hard tooth substance in dental restorative materials. This is not possible with the currently available materials. For example, the elasticity modulus of ceramic materials is too high and that of polymer-based materials too low. Compromises lead to other disadvantages. So, it’s always a balancing act.

It is conceivable that thermoplastics will gain a higher priority in the future, but currently, the aesthetic properties are limited. As far as aesthetics are concerned, ceramic materials are convincing and will remain so for years to come. If these ceramic materials could then be implemented in the 3D-printing process at some point, we would work much more economically in terms of material consumption and quick achievement of long-lasting restorations. The printing of dental ceramics is certainly not foreseeable in the near future in prosthetic dentistry, but it is a very conceivable scenario.

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