Zirconia Hot Issues Gathering

All-ceramic restorations are widely used in oral restoration due to their good aesthetic effect, mechanical properties and biocompatibility. Among them, zirconia materials have attracted attention due to their high strength. However, due to various factors, zirconia restorations may not achieve optimal esthetic results.

Next, let's analyze the many factors that affect the color of zirconia from the operational side.

0 1. Staining solution

Dental zirconia block is a pre-sintered body after low-temperature sintering. Its structure is loose and its strength is low, which is convenient for cutting and processing by engraving and milling machines. After secondary high-temperature sintering, the grain growth increases, the structure is dense, and the strength increases. The immersion dyeing process is carried out before the secondary sintering. There are tiny gaps between the loose zirconia molecules. During dyeing, the zirconia will absorb the coloring ions in the dyeing solution, and finally reach the target color after high-temperature sintering.

The type of staining solution, soaking time, storage environment, etc. will all affect the final color of the restoration.

Dyeing solution category

There are many zirconia brands on the market, and the raw materials, production processes, and material properties of each brand are different. In order to ensure the accuracy of the color, the dyeing solutions of each brand and model should not be mixed .

Soaking time

The soaking time of different brands and series of dyeing solutions is also different depending on the raw materials and production process .

Example: For our JST and ST series products, the best soaking time is 40 seconds, while the HT-type products need to soak in the corresponding dyeing solution for about 2 minutes to achieve the best effect.

(2) Studies have shown that there is an interaction between the soaking time of the staining solution and the thickness of the restoration .

When the thickness of the restoration is the same, as the immersion time increases, the lightness of the surface color decreases, the saturation increases, and the color deepens. At the same soaking time, as the thickness increases, the lightness decreases, the saturation increases, and the color deepens. A restoration with a small thickness needs to be soaked for a long time to achieve the desired color, and a restoration with a large thickness takes a short time to achieve the desired color.

Example: In actual operation, when encountering thin restorations such as anterior crowns, the technician needs to flexibly extend the staining time according to experience. Similarly, the soaking time should be appropriately shortened for thicker restorations such as implant crowns.

storage environment

It should be stored at room temperature, avoid direct sunlight or leave the lid open for a long time, causing the water in the dyeing solution to evaporate or pollute.

02. Dyeing method

pre-dyeing

Transparent treatment solution T0 and diluent T1

(1) In order to simulate the effect of the incisal end of the natural tooth, it is necessary to brush T0 twice on the incisal 1/3 of the crown , and the second time can reduce the brushing range to reflect the natural transition of permeability; when the crown is thick, it can be properly brushed more 1-2 times. (Too much brushing can result in an unnaturally washed-out color.)

(2) T1 should be applied to thicker restorations such as fixed bridges and implant crowns before staining . The purpose is to control the absorption of staining solution in the later stage and reduce the color difference with adjacent teeth;

Control the dosage according to the thickness of the crown, brush more on thick parts, less brush or no brush on thin parts; do not apply T1 around the screw holes of implant crowns and the thinner parts of the pontic neck to avoid whitening.

dyeing process

(1) When soaking, the amount of teeth should not be too much at one time, so as to avoid stacking and incomplete soaking of some teeth with staining solution.

(2) When soaking, shake the restoration slightly to avoid bubbles inside, resulting in incomplete soaking.

(3) At the end of immersion, wipe off the excess staining solution on the surface of the restoration in time, and place it on a glass plate to avoid contact with toilet paper or other water-absorbing objects to cause the staining solution to be sucked out.

03. Sintering procedure

Heating rate

In the sintering process of zirconia, in order to obtain good permeability and strength, a lower heating rate should be adopted. It has been proved by research that with the acceleration of the heating rate, the crystallization time of zirconia crystals is shortened, the gas between the crystals cannot be discharged, and the internal porosity increases slightly, which affects the final strength of the restoration; The coexistence of large and small particles reduces the semipermeability of zirconia, and a slower heating rate is conducive to the formation of more uniform grains, thereby increasing the semipermeability .

Sintering temperature and holding time

With the increase of temperature and holding time, the average grain size of zirconia increases, the final density increases, the crystal arrangement is more uniform, and the semipermeability increases. Especially for restorations such as long bridges and half mouths, in order to ensure the permeability and strength and reduce the probability of deformation, it takes a longer time to heat up and cool down.

The final staining effect of the restoration depends on the degree of absorption of metal ions in the staining solution and the reflection of light at the grain boundaries. Usually the highest sintering temperature of zirconia block is 1530℃. When the temperature is higher than 1550°C, some color elements disappear and the color is lighter; when the temperature is lower than 1480°C, the zirconia is not completely sintered (uncooked), the color is whitish, and the permeability is poor.

Combined with the different conditions of the processing plant, the actual furnace temperature is normal at 1520-1540°C, and the influence on the color is within an acceptable range.

04. Other factors

Porcelain Thickness

Porcelain thickness has a crucial influence on its translucency and thus the color reproduction of the restoration. As the thickness increases, the brightness and transparency decrease, and the hue value increases.

Zirconia full crown thickness

As the thickness of the zirconia material increases, the number of grain boundaries increases, which hinders the passage of part of the light and reduces its translucency. While the transmittance decreases, the reflectance increases, so the lightness value increases. Furthermore, studies have shown that as the thickness of the zirconia increases, the yellow tone of the restoration increases.

Metamerism

Metamerism is a phenomenon in which two substances appear the same color under one light source, but appear different colors under another light source. The human eye's perception of the color of a restoration is altered by the surrounding light sources. Dental clinical colorimetry believes that the ideal natural light source is the sunlight reflected into the room through the north window from 10 am to 2 pm on a sunny day, and the natural light that is not directly exposed to the sun. The color temperature of the light at this time is about 5 500 ~ 6 000 K.

The color comparison of restorations in the processing room can be replaced by D65 artificial light source. Try not to compare colors under a desk lamp, it is best to use natural light.

Some scholars placed high-transparency layered zirconia under three common light sources: D65 light source (representing the average sunlight during the day), A light source (warm color light source, incandescent lamp with a color temperature of 2856K), and F2 light source (cool color light source with a color temperature of 4230K). Measurements of colorimetric parameters revealed that the restoration was reddish under light source A and yellowish under light source F2 compared to average daylight.

glazed

Studies have shown that glazing may reduce the brightness of the restoration while making it yellowish.