Zirconia All-Ceramic Teeth
At the moment, first preference of dental restoration materials are all-ceramic teeth. All-ceramic teeth not only have superior mechanical qualities but also excellent biocompatibility, won't irritate the gums, and there is no barrier to X-rays. It is categorized as follows: zirconia all-ceramic teeth, cast porcelain teeth, and alumina all-ceramic teeth. Zirconia is the most durable of these three materials, having a fracture toughness that is two or three times greater than that of alumina and is therefore difficult to crack. Additionally, zirconia is more attractive than other materials and can be colored and veneered, making it the most popular all-ceramic tooth material.
Although the performance of Zirconia can essentially satisfy people's daily needs, there are still some unsatisfactory areas. Zirconia is more attractive than other restorative materials, but it has a poor aesthetic effect when used as a crown and bridge restoration material due to its poor light transmission performance. Therefore, veneer is typically added to the surface in clinical applications to cover it. However, the strength difference between porcelain veneer and zirconia is significant, resulting in porcelain chipping, which has become one of the most common reasons for zirconia crown restoration failure. As material technology advanced, the industry began to study how to create ultra-transparent zirconia materials with greater transparency.
Factors Affecting Translucency of Zirconia Ceramics
Grain Diameter
The transmittance is influenced by the size of the crystal grain. Maximum light absorption occurs when the crystal grain size is closest to the wavelength of visible light, resulting in low light transmittance; maximum light transmittance occurs when the grain size is smaller than the wavelength of the incident light. Particles with small and uniform diameters can be densely arranged, increasing material density, reducing the number of pores, and decreasing pore size, all of which have a significant impact on the improvement of material's semi-transparency.
According to research, zirconia ceramics with a grain size of 82 nm and a thickness of 1.3 mm have good semi-transparency. However, as the thickness increases to 1.5 and 2.0 mm, the grain size must be decreased to 77 and 70 nm, respectively, in order to maintain the same semi-transparency of the material. Translucent polycrystalline ceramics can be created by reducing the grain size to the submicron or nanometer grade. It should be noted that the strength of zirconia ceramics will be impacted by both increase and decrease in grain size.
Type and Amount of Added Phase
The relative density and particle size of zirconia ceramics can change with the addition of phases, as can the composition of the ceramic microstructure, the optical uniformity, and the semi-transparency of the material. The 3Y-tetragonal zirconia polycrystal with yttrium oxide as a stabilizer has excellent stability and wear resistance in addition to good strength and toughness. A typical 3Y-TZP contains >90% tetragonal zirconia and 5.18% yttrium (3mol%). The cubic phase and semi-transparency of zirconia increase as yttrium content increases.
A translucent zirconia powder containing 7.10 wt% yttrium, an average grain size of 150 nm, 75% tetragonal zirconia, and 25% cubic zirconia was introduced in 2014 by the 3M Company of the United States. The experimental material can be made more transparent by increasing the cubic phase content and decreasing the grain size. The Tosoh Company of Japan used the same technique to produce translucent zirconia. However, in comparison to partially stabilized tetragonal zirconia, its bending strength and fracture toughness are decreased by 1/2 to 2/3 due to the reduction of the degree of phase transformation toughening. Zirconia products with an 8mol% yttrium content have been used in clinic recently.
Proportion of Cubic Zirconia
According to the percentage of cubic zirconia, zirconia products on the market can be roughly categorized as traditional zirconia, semi-transparent zirconia, high-transparent zirconia, and ultra-transparent zirconia. The higher the cubic phase ratio of zirconia, the more transparent of the product. This is due to the birefringent nature of tetragonal zirconia, which has an anisotropic refractive index in different crystallographic directions. As a result, light is reflected and refracted at the grain boundaries, lowering the light transmittance of tetragonal zirconia. Light scattering from grain boundaries is reduced by the isotropic cubic zirconia. Stabilizing the zirconia by increasing the amount of yttrium oxide will result in a higher percentage of the cubic phase.
Pores
Sintering should result in as little porosity as possible in translucent ceramics. Even if the density of the ordinary oxide ceramics produced by solid-state sintering method is high, they are typically not transparent. This results from the grains' rapid growth during the last stage of sintering, which creates dispersed closed pores. Air only has a refractive index of 1.00, whereas zirconia crystals have a refractive index of 2.20. If there are pores present in the zirconia material, the light will scatter when it encounters those pores, making them the center of scattering.
According to studies, reducing the amount of pores from 0.85% to 0.25% will result in a 33% increase in light transmittance. Porosity can be decreased successfully by optimizing the heat treatment procedure. To compact its internal structure, decrease the number of pores, and minimize light scattering, the zirconia all-ceramic material produced by the 3M Company in the United States uses the hot isostatic pressing sintering method. It's necessary to regulate the pores of ultra-transparent zirconia on a nanometer scale.
Sintering Temperature
A higher sintering temperature can reduce the number of pores, which in turn increases the transparency of the ceramic. Accordingly, with an increase in temperature, ceramics are gradually densified with expanded grain diameters, and the light transmittance will increase as well.
Nevertheless, the sintering temperature must be maintained within a reasonable range. If the temperature continues to rise as sintering nears completion, secondary recrystallization may occur at the grain boundary, and impurities such as pores will be encapsulated within the crystal. Simultaneously, the density is no longer increased, and the small pores are easily merged into the low-pressure large pores, causing the large pores to grow larger and the sintered body to expand.
In addition to the sintering temperature, it is also necessary to control the heating rate to ensure that the material is heated uniformly, and regulate the growth rate and size of the crystals, in order to achieve the goal of reducing pores. The final sintering temperature and holding time have a direct influence on the density and light transmittance of the material.
Applications of Ultra-Transparent Zirconia Ceramics
Ultra-transparent zirconia has optical characteristics akin to glass ceramics as well as the inherent corrosion resistance, high temperature resistance, and insulation properties of ceramic materials. However, the excellent semi-transparency of ultra-transparent zirconia comes at the expense of its strength, as its cubic phase is isotropic but relatively fragile. As the proportion of cubic phase increases, the capability of phase transformation toughening, as well as the strength and toughness of the material decrease.
For restorations with more than four units, the needed dental ceramics must have a minimum fracture toughness of 5 MPa m1/2, whereas ultra-transparent zirconia, which has an average fracture toughness of 4.82 MPa m1/2, can only be used for restorations with less than three units. Therefore, ultra-transparent zirconia is better suited as a material for minimally invasive restoration of anterior teeth rather than for the restoration of posterior teeth with more than three units. In comparison to glass-ceramic restorations, it can reduce the thickness of the veneer to between 0.1 and 0.3 mm while maintaining good mechanical properties.
While medium transparent zirconia can be used to replace dentin, it is not suggested that it be used to replace enamel. Zirconia that is ultra-transparent can replace enamel without increasing the restoration's volume, making it ideal for extremely thin restorations like partial crowns and veneers. Furthermore, ultra-transparent zirconia porcelain discs are available in a broader color range, reducing the need for surface staining. An excellent surface finish can be achieved by combining the application of fine, multi-step polishing process and diamond paste. This will result in a more natural and lifelike appearance.