3rd generation zirconium oxide vs. lithium disilicate ceramic

In focus: flexural strength, fracture toughness and translucency

Zirconia for monolithic restorations is offered in various “configurations”. The materials can be divided into: B. by generation - from 1 to currently 4. Differences lie, among other things, in the mechanical and light-optical properties. For the user, this means that he has to deal with the materials in detail. In a study, the mechanical and optical properties (translucency) of 3rd generation zirconium oxide materials (oxide ceramics) were compared with a lithium disilicate ceramic (high-strength glass ceramic).

CAD/CAM blanks from 3rd generation zirconium materials

With the development of 3rd generation zirconium oxides, it has been possible to optimize the aesthetically limiting properties of conventional zirconium oxide. Compared to the 1st and 2nd generations, this material group has a significantly higher translucency due to the cubic-tetragonal mixed structure. The cubic crystals have a comparatively large volume, which is why the light is scattered less strongly at the grain boundaries. This results in the high translucency. In addition, the cubic crystal structures allow the incident light to radiate evenly in all spatial directions, which additionally influences the translucency. However, increasing the translucency reduces the flexural strength and fracture toughness. In some cases, 3rd generation zirconium oxide materials can be compared with the glass ceramic lithium disilicate in terms of their indications or their mechanical and light-optical properties.

Investigation setup

For the investigation, test specimens were made from six different 3rd generation zirconium oxide materials (uncolored):

• Ceramill Zolid FX (CZ),
• CopraSmile (CS),
• DD cubeX² (DD),
• NOVAZIR MaxT (NZ),
• priti multidisc ZrO & sub2 (PD) and
• StarCeram Z-Smile (SC).

Test specimens made of lithium disilicate ceramic served as a control group:

• IPS e.max Press LT A2 (CG).

Flexural strength and fracture toughness were tested in a four-point bending test and the translucency measurement was carried out in a UV/VIS spectrometer. The grain size of the examined zirconium oxide materials was examined using a scanning electron microscope (SEM). The data was statistically evaluated.

Illustrations: Milling test specimen production for the 4-point bending test made of zirconium oxide in (unsintered) and testing of the bending strength in the 4-point bending test

Results

All zirconium oxides examined showed higher mechanical and lower light-optical properties (translucency) than the lithium disilicate ceramic (CG). No differences were observed between the zirconium oxides tested in terms of flexural strength and fracture toughness. CG and CS showed significantly higher Weibull modulus than SC and PD. The lowest translucency values ​​were measured for NZ and SC, followed by CS, DD and PD. CZ showed the highest translucency values. The lowest grain sizes were NZ, DD and SC, the largest was CS.

Conclusion

A 3rd generation zirconium oxide (cubic-tetragonal zirconium oxide) shows higher mechanical properties than lithium disilicate ceramic. However, the light-optical properties (translucency) and the Weibull modulus/as well as the reliability of zirconium oxide are lower than those of lithium disilicate.

Conclusion for everyday practice and laboratory life

The user must be concerned with the material characteristics of a zirconium oxide material. It has long been true: “Zirconium oxide ≠ zirconium oxide”. As the variety of materials increases, processing and indications become even more restrictive; Only the categorization “zirconium oxide” does not allow a clear recommendation for action. For example, with a 3rd generation zirconium oxide there is no need for a long-span bridge.

Authors: Annett Kieschnick, Lisa Marie Schönhoff, Nina Lümkemann, Parissa Nassary Zadeh, Regina Pfefferle, Marlis Eichberger, Bogna Stawarczyk