e.max® vs. zirconia – We compare the materials based on critical success factors

It is better to speak of lithium (di)silicate instead of e.max®. e.max® has become established in the vernacular when speaking of lithium (di)silicates (we use the spelling “lithium (di)silicate” because both lithium silicates and lithium disilicates are available on the market). With its launch by Ivoclar (2005), this novel material was the first of its kind on the dental market – in the meantime, e.max® is spoken of with lithium (di)silicates as Kleenex are with tissues. With zirconia, on the other hand, the linguistic derivation is self-explanatory. It is simply the short form of zirconium dioxide, which is why we use the terms synonymously in this article.

Again and again, e.max® competitors try to enter the market, but to this day Ivoclar e.max® CAD maintains its position as the undisputed top dog. The latest products from Dentsply Sirona, GC and HASS Corp. no longer require a crystallization firing, unlike e.max® CAD, but it remains to be seen whether they will be able to establish themselves despite their advantages.

The zirconia market is a different story. Even industry experts quickly lose track of the situation. There is an almost unmanageable selection of manufacturers offering the most diverse variants in the most varied translucencies, coatings, shades and strengths. The prices also vary greatly. The portfolio has the ideal material for every workflow and every esthetic perception.

So when we ask ourselves the question e.max® or zirconia, zirconia has the edge on this point. More choice, more competition and therefore more options for the users.

Lithium (di)silicate vs. zirconia: 0:1

The esthetics of the natural tooth are unique and therefore still difficult to imitate today. However, lithium (di)silicates such as e.max® provide a good basis for this. The material is highly translucent, has a gloss close to that of its archetype, and has fluorescence like the natural tooth. Zirconia, on the other hand, is still more opaque than tooth enamel, even in the highly translucent variants with 5 Mol% yttrium oxide. Unlike lithium silicates and lithium disilicates, zirconia also lacks fluorescence and has higher light refraction than its natural archetype. Lithium (di)silicate consequently blends in better with the existing tooth structure.

In contrast, higher-quality zirconium dioxides, such as the industrially pre-shaded zirconia FAB Zirconia 3D ML, score points with a color and translucency gradient. In the development of these materials, the focus was on imitating the natural tooth. Nevertheless, lithium (di)silicates have the edge overall in terms of esthetics due to a more tooth-like play of light. According to some experts, the esthetics are even higher with lithium (di)silicate press ceramics – but we are focusing on the CAD/CAM workflow, which is why we are not including them in today’s comparison.

Lithium (di)silicate vs. zirconia: 1:1

Both classes of materials have a flexural strength that exceeds that of the natural tooth (enamel: approx. 300 MPa). Lithium (di)silicates reach up to 400 MPa, high-strength zirconium dioxides with 3 Mol% yttrium oxide are around 1200 MPa. Therefore, lithium (di)silicates are usually designed only for single-tooth restorations, but most zirconias are approved for up to 16-unit bridges. The strength comparison therefore clearly goes to zirconia. Nevertheless, the material does have one drawback: its high hardness. If it is not polished sufficiently, it causes abrasion on the antagonist.

From a purely mechanical point of view, however, e.max® is preferable to other lithium (di)silicates, especially since it is less brittle than glass ceramics. With them, splintering can occur quickly in a moment of carelessness or due to somewhat worn tools.

Lithium (di)silicate vs. zirconia: 1:2

Basically, the process is similar for both materials: CNC machining is followed by firing. In detail, however, the process differs considerably. Zirconia discs are supplied with a high porosity in the whiting state of up to 50% and can therefore be easily dry milled. Lithium (di)silicate blocks, on the other hand, are dense and therefore more difficult to machine. They have to be wet milled under cooling. While the machining of zirconia is part of the standard repertoire for dental milling machines on the market, not every machine is designed for the wet machining of lithium (di)silicate. It puts more stress on the tool and machine, and deterioration is higher. Due to the high brittleness of the material, the risk of frayed edges is higher with lithium (di)silicate. Slightly worn grinders can quickly become a problem here.

Following CNC machining, the material is given its final properties in firing. While lithium (di)silicate is only crystallized to completion, zirconia is completely sintered to close the pore space. Crystallization of lithium (di)silicate takes place at temperatures of 800 to 850 °C (depending on the manufacturer) and lasts about 30 minutes; for zirconia, a temperature of over 1450 °C is necessary. In this case, the sintering program often takes more than 7 hours.

CNC machining is clearly easier with zirconia. Lithium (di)silicate, however, has a significantly shorter and lower-energy post process, which is why this category ends in a draw.

Lithium (di)silicate vs. zirconia: 2:3

Lithium (di)silicate manufacturers like to advertise the speed of the overall process and thus the suitability of the material for one-visit treatment by dentists. One large company advertises total process times of less than 30 minutes for its lithium (di)silicate Cerec Tessera (CNC machining and crystallization firing). Whether these times are actually achieved in practice remains to be seen, but they underscore that lithium (di)silicate is clearly a chair-side material.

This was different for zirconia for a long time, but in recent years more and more “speed-sinterable” zirconia in block form – essentially for single-tooth restorations – have come onto the market. They can be sintered in well under an hour. However, this comes at a price: the esthetics suffer. The same zirconia is less color-intensive and translucent when speed-sintered than it would be after conventional sintering, which takes over 7 hours. This limits the range of applications for speed-sintered zirconia. Nevertheless, total process times of less than one hour are possible with both materials – something that seemed unthinkable just a few years ago. In combination with the higher esthetics, however, lithium (di)silicate wins the time comparison.

Lithium (di)silicate vs. zirconia: 3:3

Following machining and firing, the restorations need to be adjusted. Wet processing with the dental turbine, for example with the FAB Toolset DT Zirconia, is recommended for both materials. Materials such as glass-ceramics or oxide ceramics have very poor thermal conductivity, which is why we recommend cooling. Otherwise, cracks may occur due to the high temperature contrast in the material. The amount of care required is similar for both materials. As in machining, greater care is needed with lithium (di)silicate because of its brittleness. The tools must be really sharp, otherwise chipping will occur. With zirconia, on the other hand, it wears out more quickly due to the hardness of the material.

The finalization of lithium (di)silicate or esthetic multilayer zirconia is also comparable. Increasingly, stains or ceramics are suitable for both materials and therefore do not differ in workflow and ceramic firing. Due to the lower basic esthetics, a somewhat higher finalization effort must be calculated for zirconia, depending on the indication and requirements. Particularly in the occlusal surface, multilayer zirconia is often too white and has to be repainted. Nevertheless, there is no clear winner in this category.

Lithium (di)silicate vs. zirconia 4:4

Both lithium (di)silicate and zirconia can be conventionally and adhesively cemented in the mouth. Whether cementation of a highly esthetic anterior crown makes sense is another question. However, this category could not provide the decisive point for either the lithium (di)silicates or the zirconias.

Lithium (di)silicate vs. zirconia 5:5

So when it comes to the question of e.max® or zirconia neither material clearly prevails. Ultimately, it depends on the area of application and the indication, which overlap only to a limited extent for both materials. Lithium (di)silicate is preferred for esthetic restorations such as veneers, onlays, inlays or anterior crowns. Anything larger than a 3-unit bridge can only be fabricated monolithically from zirconia. So if strength is a priority, there is no way around zirconia.

Our professional dental technicians have summed it up like this: lithium (di)silicate is more beautiful – but it is also touchier. Zirconia is a more versatile material and more forgiving of errors.