The conventional zirconium oxide can be made more translucent by changing the sintering parameters. Although changing the sintering parameters has a positive influence on the optical properties, it can have a significantly negative impact on the mechanical ones. Studies show that zirconium oxide sintered at higher temperatures (1600 °C) is no longer resistant to aging. That is why the zirconium oxide (2nd generation) was optimized for the production of monolithic restorations on the molecular level and in their composition. This is accompanied by a change in the material properties.
In a study by the LMU Munich, the optical and mechanical material properties of four translucent zirconium oxides were compared with a conventional zirconium oxide.
Aim of the investigation
The mechanical and optical properties of four zirconia materials for monolithic restorations (2nd generation) were compared with a conventional zirconia (1st generation).
The zirconium oxide test specimens
Zirconia for monolithic restorations (2nd generation):
- DD Bio ZX2, Dental Direct (DD)
- Zenostar, Ivoclar Vivadent/Wieland (ZS)
- Ceramill Zolid, Amann Girrbach (CZ)
- InCoris TZI, Sirona (IC)
Control group: Conventional zirconium oxide (1st generation)
- Ceramill ZI, Amann Girrbach (CZI)
Structure of the investigation
The translucency of the materials was determined (ISO 2471:2008). In addition, the grain sizes of the zirconium oxides were examined using a scanning electron microscope. The bending strength was measured in a four-point bending test (ISO13356:2008) directly after the test specimens were manufactured and after aging in an autoclave or in a chewing simulator. In order to analyze the abrasion resistance of monolithically manufactured, polished and glazed or conventionally veneered zirconium oxide test specimens, a chewing simulation with natural teeth was used as antagonists. The data was statistically evaluated. The same sintering parameters were used for all test methods. All zirconium oxide test specimens were sintered in the same sintering furnace with the sintering parameters specified by the manufacturers. The final sintering temperatures were 1510°C for all materials with the exception of IC (1450 °C). The holding time at the final sintering temperature was always 2 hours.
Results
Translucency: The highest translucency values were determined for Zenostar (ZS), InCoris TZI (IC) and Ceramill Zolid (CZ). The control group showed the most opaque properties. SAVE: The 2nd generation zirconium oxide is more translucent than conventional zirconium oxide.
Grain size: IC showed the largest grain size - grain size of zirconia grains - followed by DD and CZI. The smallest grain size was observed for ZS, followed by CZ. There was no correlation between grain size and translucency. SAVE: Previous research shows that the higher the grain size of zirconium oxide, the more translucent the material is, but the lower the flexural strength!
Flexural strength: The aging of the zirconium oxides has no influence on the flexural strength. All unaged and autoclaved zirconia materials showed lower flexural strength than the control group CZI. After chewing simulation, ZS had significantly lower flexural strength than CZI. SAVE: Although the bending strengths of the 2nd generation are lower, they are still well above the value required by the standard. No aging of zirconium oxide was observed. However, studies report that incorrect sintering parameters can cause the material to age.
Material and antagonist wear: Higher material and antagonist wear was found for conventionally veneered CZI than for monolithic polished and glazed zirconias. The glazed test specimens showed higher material and antagonist wear than the polished materials. There was no correlation between surface roughness and wear. SAVE: Glass-based materials (glaze material and veneering ceramic) wear faster than polished zirconium oxide! The generation of zirconium oxide plays no role here.
Author: A. Kieschnick, Berlin (www.annettkieschnick.de)
Zirconia frameworks in an anatomically reduced crown shape. Illustration of the translucency Above: 1st generation zirconium oxide Below: Zirlux16x (Henry Schein) Image: C. Fischer
Veneered zirconium oxide bridge in the front tooth area. The labial surfaces were veneered for optimal aesthetics. The palatal parts are monolithic. Images: C. Fischer
ADDITIONAL INFORMATION
Full article published: J Mech Behav Biomed Mater. 2016 Jun;59:128-38. doi: 10.1016/j.jmbbm.2015.11.040. Stawarczyk B, Frevert K, Ender A, Roos M, Sener B, Wimmer T., Comparison of four monolithic zirconia materials with conventional ones: Contrast ratio, grain size, four-point flexural strength and two-body wear.