Zirconia in dental technology
- processing and challenges

Zirconia has long been state of the art in dental technology. It has a multitude of advantages, making it popular with dental technicians, dentists and patients alike. In addition to the fact that zirconium oxide is metal-free, has high biocompatibility due to its good corrosion resistance and also has very good mechanical properties in the form of high flexural strength and fracture toughness, the material - especially the new generation - is particularly aesthetic. 

Processing of zirconia

The processing of zirconia – pressed in round form – in dental technology only became possible with the advent of digitization in the dental industry, because this is done mechanically using CAD / CAM milling facility units. When processed in the milling machines, zirconia is usually still in a “raw”, chalk-like state. Only when the frameworks, milled from solid material, have assumed the desired shape of the CAD construction are they sintered and thus achieve the necessary hardness and strength to be used later in the patient's mouth. Special furnaces are used for the sintering process. Certain kilns are even recommended for some zirconium oxide materials, as they require individual firing parameter settings with special requirements. 

Important to know: 
During the sintering process, the zirconia framework shrinks by around 20 to 30 %. The shrinkage factor must therefore always be taken into account when manufacturing milled zirconia frameworks.

The post-processing (i.e. the finishing and fitting of zirconium frameworks) then takes place in the sintered state. Since the material is very prone to cracking and chipping, finishing with a water-cooled turbine is recommended.

Hipped zirconia

With regard to zirconia, the term "hip zirconia" is used again and again in dental technology. What is meant here is fully sintered zirconia in round form, which is used for CAD / CAM processing and therefore no longer has to be sintered after the milling process. In addition to the fact that dental laboratories avoid having to purchase a sintering oven for processing hipped zirconia, the material, in its fully sintered form, is less sensitive due to its hardness and strength, and the source of error of considering a sintering factor is eliminated. However, since the tool wear is enormous when processing hipped zirconia and the processing time is also significantly longer than with zirconia in its raw state, this variant was not able to establish itself on the market. 

CADtools Zirkon Rohling zum Fräsen

etkon scanner and software

etkon offered a self-contained scanning system that made it possible to have data records produced exclusively on etkon milling machines in etkon's own production center. etkon scanners work with 3D laser technology. According to etkon advertising material, users of an etkon scanner would invest once in the hardware and then benefit from regular software updates of the 3D software etkon-visual.

Because veneering ceramic and zirconium oxide are two brittle materials that come together, stresses that result from different thermal expansion coefficients (CTE), among other things, can cause fractures in the ceramic, which can lead to breakouts, fissures, cracks in the veneering ceramic and later also to flaking. So-called "chipping" can also occur with metal frameworks, but it occurs more frequently with veneer frameworks made of zirconium oxide. 

Our tip:
It is advisable to choose a ceramic mass for the veneer that has an (almost) identical CTE value to the material of the framework to be veneered. In addition, frameworks made of zirconia should be anatomically supported and rounded in order to achieve a higher breaking load and to minimize the risk of chipping.
 

"Chipping" in veneer crowns and bridges made of zirconia

Because veneering ceramic and zirconium oxide are two brittle materials that come together, stresses that result from different thermal expansion coefficients (CTE), among other things, can cause fractures in the ceramic, which can lead to breakouts, fissures, cracks in the veneering ceramic and later also to flaking. So-called "chipping" can also occur with metal frameworks, but it occurs more frequently with veneer frameworks made of zirconium oxide. 

Our tip:
It is advisable to choose a ceramic mass for the veneer that has an (almost) identical CTE value to the material of the framework to be veneered. In addition, frameworks made of zirconia should be anatomically supported and rounded in order to achieve a higher breaking load and to minimize the risk of chipping.
 

Eliminate the risk of “chipping” with monolithic zirconia crowns and bridges

With the new zirconia materials, crown and bridge designs already have such a high level of aesthetics due to the particularly high translucency that they can be used on the patient without veneering, i.e. already in monolithic form. Without the need for a ceramic veneer, the risk of "chipping" can be avoided. But not only the "chipping" is avoided with monolithic solutions, monolithic work also requires much less space, since there are no facing layers. Full zirconia crowns are therefore ideal for oral situations that have little space.

Monochrome zirconia and multilayer zirconia

Many zirconia blanks (such as some Ivoclar zirconia materials from the Ivoclar IPS e.max® ZirCAD system) are already industrially pre-shaded. In contrast to monochrome zirconia blanks, which are dyed through in one color, with multilayer zirconia (e.g. KATANA™ Zirconia) different layers can be seen within a blank, which simulate a natural color gradient from dentin to incisal (from dentin - dark, to incisal - bright). As with natural teeth, the opacity is higher in the dentin area, while greater translucency is achieved in the incisal area. The dental technician who nests the virtual design in the blank can thus influence the translucency of the crown or bridge. For a natural color gradient, there is also the option of infiltrating color into the framework after milling zirconia. The most well-known material that works with this technique is Prettau® zirconium.

Our own brand CADtools covers the complete spectrum in the field of CADtools zirconia: from opaque zirconia to industrially colored monochrome and multilayer zirconia to customizable infiltratable zirconia. 

Challenges in the processing of zirconium oxyde

The more translucent the material being processed is, the thicker the wall and edge have to be, due to the decreasing flexural strength. The patient's oral situation often does not allow for this, which is why these requirements are often not met against better knowledge. If this is the case, that the recommendations regarding wall and edge thicknesses are not observed, fractures and cracks can occur in the structure.

The incorrect use of ceramic firing can also lead to cracks, fissures or, more generally, to “chipping”. Zirconia is a poor conductor of heat, which is why this material has different requirements for ceramic firing than metal frameworks.
 

OUR CONCLUSION

Zirconia is a real all-rounder in dental technology and can be used in many ways, even if the advantages and challenges are well balanced. For each patient case, it is necessary to decide individually which type of zirconia makes sense and whether it should be used as full zirconia or for veneering. Do you have a case where you are unsure which kind of zirconia to work with? Then contact us, our experts will be happy to advise you!