Hello my dear friends. Welcome back to the channel and this is your friend Dr Suresh Shenvi. Gagan Yadhav suggested on my channel that I should make a video on dental ceramics and I do feel that this is one of the most difficult topic from dental materials, especially during undergraduation So today I am trying to make the basics of dental ceramics as simple as possible and I hope that you will like it. If you are new to the channel and if you like this video please consider subscribing. Treatment options have improved over a period of time and many cases which required crown today we have various options starting with the metal crowns then metal ceramic crowns and all ceramic. This all change happened because of one important reason, that you wanted a material which is strong but it should be aesthetic. We had a strong material that was metal crown but it was unaesthetic and today we have a strong material but highly aesthetic and that is ceramics . Before you understand today's types of ceramics it is very important that you understand the composition and the methods of how the ceramic were made strong over a period of time still maintaining its advantages. Ceramics are not only restricted to dentistry. It is also called as porcelain and used very commonly in day to day life but when it comes to dental ceramics we have lot of additions. But the silica is one of the most important component. Silica is basically available into two forms that is crystalline and non-crystalline and crystalline form is used mostly in investment materials and dental cements and composites. Tthe advantage of crystalline form for example quartz is it has low melting temperature but it has a very important. Disadvantage that it has less strength. Then comes the the second form of the silica, that is non the crystalline form which is actually used in the dental ceramics and the non crystalline form of dental ceramics is fused silica. The most important advantage of fused silica is that it has very high strength The fused silica is really strong because of its one important structure that is the Silica tetrahydrate and if you see the image, its is basically the silica ion surrounded by oxygen. The reason behind the strength of fused silica is not the ions but they are the bond between these ions. The bonds are really strong. But in spite of being so strong fused silica cannot be used in dentistry without any modification and why is that?. Thats because it has a very high fusion temperature. Suppose you have powder of the silica and you want to fuse it together so that you can form a restoration, in order to make that you will have to heat this fused silica to a very high temperature which will not be able to attain in dental labs. And also if you plan to use it for metal ceramic restoration where you have a coping of metal and ceramic over it, by the time you melt the ceramic powder the metal will be completely melted. Tat's why the fused silica was not used in a pure form in dentistry. The solution for this problem is very simple, you have to reduce the fusion temperature of the fused silica which can be attained into the dental labs. So the next question is obviously how do you reduce the fusion temperature of the fused silica and that is done by adding the materials named as Glass modifiers. Glass modifiers are basically carbonates of sodium potassium or calcium and how do they reduce the fusion temperature of the fused silica? that's very simple The fusion temperature of the fused silica was very high because the bonds were really strong between the ions and they came to know that if you can make these bonds flexible you can reduce the fusion temperature of the fused silica and also reduce the viscosity of the material. But I am still sure that if you have read the textbooks and you have seen what is the fusion temperature of the current generation of ceramics, you will see that there are somewhere around hundred to 1,500 to 2,000 degrees Celsius and you may ask me why not add a lot of glass modifiers and get the temperature as low as possible Unfortunately, that is not good for the ceramics because the more glass modifiers you add, it is bad for the ceramics because it reduces the strength of the ceramic and it makes the ceramic susceptible to oral fluids so the important components from ceramics may start leaching out. The other disadvantage of adding glass modifier is that with the increase in content of glass modifier you will also devitrification of the ceramic that is the increase in opacity which will eventually make the crown unesthetic. So based on the content of the glass modifier and the fusion temperature, the ceramics are basically classified into four categories.That is high fusing medium fusing and low fusing and ultra low fusing. The high fusing will have a fusion temperature which is more than 1300 degree Celsius and the ultra low fusing will have a fusion temperature which is less than 850 degree Celsius and please note that these temperatures will be asked in most of the exams including NBDE and Indian entrance exams. The important thing to remember in this classification is if you are fabricating a bridge which will have a lot of force in it you should be using low fusing ceramic rather than ultra low fusing because ultra low fusing although the fusion temperature will be less but the strength of that ceramic will not be sufficient to bear so much of force. The third important component of the ceramic is feldspar and feldspar is basically potassium aluminium silicate. Feldspar has two important functions in the ceramic. The first function is when you heat it to a fusion temperature it is going to soften and form a glass phase in which the silica particles are allowed to flow and fuse together. And this phase of fusion between multiple silica particles are called as liquid phase sintering The second important function of felspar is its ability to undergo a process called as incongruent melting. Now what is incongruent melting it basically means that a material when it melts it forms a liquid and it forms a crystalline material. We know that the liquid is basically the glass phase which helps in sintering and the other crystalline material which is formed because of incongruent melting, is called as lucite. Lucite is a very important component because it has high aesthetics and it also has high coefficient of thermal expansion. At this moment let us not confuse ourselves by discussing more lucite. So to just revise quickly we started with non crystalline silica and then we added glass modifiers to reduce the fusion temperature and then we added feldspar. Along with this three major components boric oxide was also added and boric oxide had two important functions, that is it acts as a glass modifier and it also formed a glass phase which facilitated the movement of the silica particles and apart from this pigments were also added because we will need ceramics with a different shade. So the world finally had a ceramic category known as Feldspathic ceramic and now everybody was hoping that will no longer make metal crowns and will only make crowns which are completely made up of full thick ceramic but very soon they realized that when they gave a crown which is made up of just felspar thick porcelain, the crown used to fracture very commonly. That's because the compressive strength of Feldspar was very good but unfortunately the tensile strength was very less, making the material very brittle and that's a very important point when it comes to ceramic and we'll try to understand it. So in this slide we can see the SEM image of the ceramic layer and I'm sure all of you can appreciate the presence of a large crack Of course it appears large at the SEM level but if you actually see the dimension it is so small that it passes between two atoms and these small cracks are present all over the ceramic surface and when you put a bending force over the surface of ceramic, this crack will start propagating and the ceramic will eventually fracture, So because of this the disadvantage of Feldspathic crown which is made completely by this material never succeeded. Although this was not a complete failure because they found a solution to overcome this problem. They took a layer of metal which is placed on the inside of the crown and the top portion was layered with the ceramic. So you got the aesthetics over the visible surface and the inner surface was able to prevent bending of the crown and you had sufficient amount of strength to prevent the fracture of the ceramic. But this also had two important disadvantages. One is that you needed a thick layer of metal and over that you require at least two millimeter of ceramic so to get so much of space you had to do more tooth reduction which is actually not good for the long-term strength of the tooth. The second disadvantage is the metal ceramic crown lacks the translucency that basically means when the light passes through the crown it will be stopped at the metal level giving the tooth slightly opaque appearance. Now at this point you should also know that the bond between the ceramic and the metal is not just mechanical. There is also a chemical bonding which happens between the oxide layer of the metal and the metal oxides of the ceramic. This oxide layer on the metal will form if the metal has a small percentage of base metal alloy for example iron indium and tin and this will be responsible eventually for the chemical bonding. Please note this is a very common question in your NBDE and any entrance exam. So the research continued in order to get a crown in which we don't have to use any metal coping and eventually giving a very good aesthetic to the crown and that was achieved by making four different changes in the ceramic. The first was to reduce the crack over the surface of the ceramic, Second was to introduce something called as residual compressive stress, the Third method was introduced to interrupt the crack propagation in the ceramic and then the last one was design modification and we will try to understand it in a very simple way.Coming to the first technique to reduce the surface crack, that process was named as glazing. There are two types of glazing, that is self glaze the second one is over glazed. In self glaze we don't apply any special material over the ceramic. This process basically involves a controlled heating of the ceramics so that the top layer selectively melts and fuses and this will eventually reduce the amount of cracks which are present over the surface. The second method is over glazing where you will apply a material which is made up of silica but with a lot of glass modifiers and when you add a lot of glass modifiers you will increase the flow of the material.This low viscosity material is now placed on the ceramic which will be able to close the gap. It is a very important question between the different types of glazing and which is commonly asked in the entrance exam and your viva and that question would be which is better between the or glaze or the self glaze? The answer is very simple , its self glaze since you are using the same material to make the glazing that has a longer duration. So between overglaze and self glaze, self glaze is actually better. The second major change which was done to improve the strength of the ceramic was introduction of residual compressive strength and I know that students really struggled to understand this change which was done in the cerami. I hope that the next few slides will really help you to understand it in a very simple way. So from our earlier slides we know that the ceramic fractures because the bending forces try to open up the crack.To overcome this theey introduced compressive stress into the ceramic at the end of its fusion. So these forces are basically trying to compress the crack and whenever you have a tensile force which is put on the crown the tensile force has to this counteracting force before opening up the crack. So this type of residual compressive stress which tries to close the crack are generated by three techniques. The first technique is ion exchange which is also called as chemical tempering and then you have the second method which is called as thermal tempering and the third method is thermal coefficient of mismatch. The first method of generating residual compressive stress is ion exchange in which we are trying to replace smaller size sodium ions with a 30% larger size of potassium. So when you do this, there is a tremendous amount of compression which occurs that leading to a force which is eventually trying to close the crack in the ceramic and that can be well appreciated in this image which is quite self-explanatory. In the second method to generate residual compressive stress, we use the thermal tempering process in which the ceramic top surface is rapidly cooled so it becomes rigid whereas the inner surfac, that is the core of the ceramic allowed to cool slowly. When the core is cooling it is trying to pull the outer surface towards the inside but because the outer surface is rigid at the end of the fusion of the ceramic you will have some amount of force which will eventually try to close the gap over the ceramic. The third method of generating residual compressive stress is called a thermal expansion coefficient mismatch. So in this technique we are using two different materials with two different coefficient of thermal expansion. For example you are using metal in the deeper layer which will shrink 50 percent per degree change in the temperature whereas the top layer will be ceramic which will probably shrink only thirty percent. So when the deeper portion is trying to shrink more but the top will not shrink as much as the deeper portion, the deeper portion will eventually end up leaving some amount of pulling force in this restoration assembly. So this pulling force is actually a residual force which will eventually try to close the crack in the ceramic. So I hope that you are understood three ways of generating residual compressive stress. Now we are coming to the third technique that is the interruption of crack propagation. So the technique of interruption of crack propagation doesn't stop the crack from opening but it just stops the crack from propagating into the deeper portion of ceramic, eventually reducing the chances of fracture and this can be done by two methods one the dispersion of crystalline phase and the second method is called as transformation toughening In the technique of dispersion of crystalline phase we add a ion called as alumina and the alumina basically stops the propagation of the crack because the crack can no longer penetrate the alumina particles, because of its strength. The new category of aluminous porcelain has then marketed. The lab started making the whole crown which is made up of a Aluminous porcelain, but they soon realized that this doesn't have sufficient strength to be used in the posterior teeth. But since the forces are less the anterior teeth , they were sufficient for the crown in the anterior region. Although the aesthetics were better compared to the metal ceramic or metal crown but still one should know that because of the alumina particles, the crown will be slightly opaque compared to feldspathic porcelain. Zirconia is also used in order to prevent the crack propagation, but the real use of zirconia was in the second type of method to prevent the crack propagation that is Transformation toughening transformation technique. Basically is a method which is similar to air bag, the air bags are really small they are not visible but under the impact they increase in size and they prevent injury to the people. So in transformation toughening you use this special form of zirconia that is partially stabilized zirconia and it undergoes something called a stress induced phase transformation.Basically at the beginning of manufacturing of zirconia based ceramics this zirconia it is there in metastable tetragonal crystals but once the tensile force is put on, this form of zirconia it converts from that dragon all crystal to a stable monoclinic form and this stable monoclinic form is five percent larger in the volume and hence this will try to compress and prevent the further propagation of the cracks into the ceramic. So he finally had this amazing ceramic which was very strong and we can now make the whole crown with just ceramic without the use of any metals But, unfortunately, zirconia-based ceramics also have some bit disadvantage zirconia based ceramics are opaque in nature, that's why it is generally not recommended in anterior teeth if the aesthetic expectations are very high Also they have high abrasion resistance so they can wear the natural teeth faster. Coming to the last method of strengthening the ceramics is to design the dental restoration. According to this step we have to design the restoration in such a way that the tensile stresses are minimized and the stress concentration can be avoided. This is achieved by preventing any sharp margin in the crown preparation and also by preventing any sudden change in the thickness of ceramic and the textbooks also say that you can use metal coping in order to reduce the impact of tensile stress on the ceramics. So my dear friends that's how the ceramics evolved over a period of time. If you want me to make more videos in this chapter then do mention in the comment section and I will surely consider i. If you are like the video don't forget to click the like button and subscribe to the channel and share this video with your friends. I will see you once again in my new video to learn dentistry in the simplified way.