At present, the denture processing plant cuts the zirconia ceramic block through the CAD / CAM system to obtain the denture base, and then obtains the zirconia all ceramic restoration after surface porcelain decoration. Zirconia all ceramic restoration has been recognized by more and more dentists and patients in clinical application because of its unique aesthetic effect and good biocompatibility. However, a few clinical dentists and technicians have encountered some phenomena such as porcelain collapse and fracture.
There are many reasons for porcelain collapse and fracture of the restoration, such as design reasons, influence of final firing crystallization temperature and time, influence of grinding after crystallization, used porcelain powder, parameter setting of porcelain furnace, heating rate, cooling rate, etc., which may cause porcelain collapse and fracture of the restoration.
Design reasons
First, determine the location of the fracture, whether it is the weak surface of the inner crown or the connection between the crowns.
Individual dentists did not judge the feasibility of making zirconia restorations for patients, and they still followed the metal tooth preparation method, without the shoulders and rotator cuffs required by zirconia restorations, and the prepared abutments did not have zirconia reserved. The material must have a minimum thickness clearance of >0.5mm. It should be noted that zirconia substrates are not like metal substrates, they are tough and ductile, and can be made very thin.
In the scanning design process of the CAD system, in the above situation, in order to ensure the occlusion and installation, the technician can only thin the zirconia substrate. Too thin or uneven thickness of the substrate will cause stress concentration and leave hidden dangers of fracture.
The connection between the crowns is generally designed to be thinner on the labial side and thicker on the lingual side, which is easier to make better shapes. When designing the connection part, the recommended value in the design software should be followed as much as possible, and it should not be too small, exceeding the alarm size (arrow ②), and there should not be a sharp triangular area (arrow ①) that is prone to stress concentration.
2. Effect of sintering crystallization temperature and time
For the long bridge unit, the crystallization time is appropriately prolonged during final sintering to reduce the heating rate of the crystallization process.
The long bridge unit should be in the semi embedded state of zirconium ball in the crucible to better conduct temperature and reduce stress and deformation.
3. Grinding effect after crystallization
After final sintering, zirconia restoration will be treated by grinding and porcelain turning. Due to the poor thermal conductivity of zirconia, it will cause local overheating, stress and internal cracks, resulting in fracture risk.
It is recommended to spend more energy in the design process, try to achieve "zero adjustment" and avoid excessive grinding with grinding tools.
Recommended grinding tools: the processing plant can select the corresponding zirconia professional grinding kit according to experience. It is strictly prohibited to use metal grinding head for grinding.
4. Influence of heating rate and cooling rate
The used ceramic powder, the parameter setting of porcelain furnace, heating rate and cooling rate will make the cooling rate difference between the ceramic surface zirconia substrate, resulting in the change of internal stress and fracture risk.
The low thermal conductivity of zirconia is an important factor to be considered when sintering facing porcelain. There is a slow geothermal release between the zirconia substrate and the surface decorative porcelain. During the heating and cooling process, the heating and cooling rates of the decorative porcelain and zirconia are obviously different, which will affect the internal tension state of the decorative porcelain. If the sintering table of the porcelain furnace drops rapidly, the outer decorative porcelain can follow the rapid cooling, and the zirconia substrate cools relatively slowly. The inconsistent temperature changes inside and outside the repair will produce internal stress, which may lead to porcelain collapse and fracture.
For zirconia based all ceramic restorations, the heating and cooling rate of porcelain fused to metal teeth are slower than that of traditional metal porcelain fused to metal teeth. It is suggested that the heating rate should not exceed 35 ℃ / min, and the cooling time should be set ≥ 3min.
