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.