Hello, my dear friend. Welcome back to
the channel and this is your friend Dr
Suresh Shenvi. Thank you so much for
liking my earlier video on zinc
phosphate and today we are covering the
zinc polycarboxylate. Zinc poly
carboxylate is very important for
various exams and the obvious question
is why, even when we don't use it so much in our practice. That is because of one
important property and that properties
often asked in the exam. It was a
first cement to chemically bond with the
tooth structure and the other cement which
can bond chemically with the tooth
structure is glass ionomer cement and
that's often asked in your exam. The
major component of powder here too is
zinc oxide along with that they have
added magnesium oxide but the textbooks
also mentioned that the magnesium oxide
can be replaced partially with Magnesium
oxide and we also have the same Bismuth
dioxide which is present in the zinc
phosphate cement too. So what is the
major component which differentiates the
powder of zinc polycarboxylate from
zinc phosphate?. Its  is the presence of
stannous fluoride which has two
functions. It increases the
strength and it also modifies the
manipulative and working properties of
the cement. Keep watching this video and
you will know why this component is also
important for another major function and
that function is very routinely asked in
your example. Now let's talk about the
liquid component and this is the major
differentiating feature between the zinc
phosphate cement and zinc poly
carboxylate cement. The liquid here has
polyacrylic acid instead of phosphoric
acid and it needed some modification
with because the liquids tends to gel
over a period of time. So this was
modified by adding Itaconic acid and
tartaric acid. Along with this the second
the component is also water which is very
important for the setting reaction. There
is a special form of the cement that is
called as water settable or
freeze-dried in which the polyacrylic
acid is freeze-dried and made like
powder and that is incorporated into a
single bottle system which has that zinc
oxide component and the polyacrylic acid
powder. So when you get this in your
clinic you simply have to add water
and you get the same basic original
composition of the zinc phosphate powder
the liquid formulation. Is there any specific
classification of these in poly
carboxylate cement?. There is no classification as such. So let's talk
about the setting reaction. Setting
reaction here too is based on the same
acid-base reaction what you see in the
zinc phosphate and the powder is the
alkaline component and the liquid is all
acrylic acid. So it is the acidic
component. To make the setting reaction
simple, let us first look at the
structure of the polyacrylic acid. It is
basically made up of polymer of various
groups of acrylic acid,  but the point to
be noted here is the yellow arrow in this
picture indicating that there is a
special group which is very important
for the setting reaction and that
special group is called as carboxylic
group. Once you have understood this, we
basically know that the powder is also
made up of zinc oxide, magnesium and tin. So what happens is when you mix the
powder with the liquid the acrylic acid
will attack the powder particles and try
to dissolve it. So when this happens, the
zinc magnesium and tin ions will get
released into the matrix but these ions
will then bind to this free carboxylic
group. So the final structure is
basically the various combination
between the ions and the unreacted
powder and the acrylic acid. At this
point look at the picture once again and
you will realize that not all carboxylic
groups are involved in the reaction and
you still have some amount of free
carboxylic groups which are remaining.
Keep watching this video and will know
what is the importance of these free
carboxylic group into the properties of
this cement. So let us talk about the
properties of these in polycarboxylate
cement and will try to compare it with
the zinc phosphate. The first property
being the flow or the viscosity. It is
obvious that the viscosity of zinc poly
carboxylate cement is more than zinc
phosphate cement. Hence you may not get
the 25 micron film thickness which is
generally ideal for a
the luting. But surprisingly even
after being more viscous  the zing poly
carboxylate has the ability to give a
very thin film thickness of 25 micron
meter and that's because the poly
carboxilate is considered to be pseudo
plastic. Now, what does the pseudoplastic mean? It basically means that if you
apply more pressure,  the viscosity of the
polycarboxylate cement will reduce
although film thickness is less in zinc
phosphate but zinc phosphate has less
film thickness not because it has pseudo
plastic it is because zinc phosphate is
Newtonian fluid. So let's compare the
the strength between zinc poly carboxylate
and zinc phosphate. There are some
major differences. The compressive
strength is really less, that is almost
half of it compared to the zinc
phosphate but the tensile strength is
more and the major difference is also
there in elastic modulus which is
drastically reduced making the cement
less brittle and if cement is less
brittle than the removal excess of the
cement from the margin once the cement
set becomes difficult. So let's talk
about the major property of these in
poly carboxilate and that is how it
retains into the cavity preparation, that
is the bonding of the cement to the tooth
structure. We know that the zinc
phosphate bonds mechanically but zinc
polycarboxylate bonds chemically with
the tooth structure and if you remember
that in setting reaction, I told you that
we still have some amount of free
carboxylic group, these free carboxylic
group will bond with the calcium of the
tooth and will give you a chemical bond
with the tooth structure and this is a
very common question in your exam and
please remember it since the bonding
depends on the amount of calcium in the
tooth obviously the bond strength will
be better with the enamel, because it has
more inorganic component and the bond
strength will be slightly less with the
dentin. Please remember that if you have
smear layer on the tooth surface which
generally forms after cavity preparation
then the bond strength will not be
attained completely. In order to improve
it, you can always do something called as
conditioning where you
apply 10% of polyacrylic acid for 10 to
15 seconds and that removes the smear
layer giving a direct exposure of the
tooth surface so that you can have
better bond strength with the zing polycarboxylate cement. Effect of the type of
bonding is also seen the way the cement
will fail if you see the bond failure of
zinc phosphate cement then the failure
generally occurs between the cement and
the tooth interface and that is because
it's a mechanical bond. Whereas in the
zinc polycarboxylate, you will see the
failure into the cement and Crown
interface rather than cement and tooth
interface because there you have the
chemical bond. If clinically you want to know whether you can attain the chemical bond
with the tooth structure, that can be done
by following a test called a loss of
gloss test. This says that you should
always have some amount of gloss into
the cement mix indicating directly that
there is sufficient amount of free
carboxyl group to attain a chemical bond
with the tooth structure. Now let's talk
about the last major property of the
Zinc poly carboxylate cement. The major
reason for replacing phosphoric acid
with poly acrylic acid was to improve
the pulp response of the cement. As we
have learned in our last video that zinc
phosphate is highly acidic since its
initial pH is 2.14, However in zinc poly
carboxylate the initial pH is three
point four two, but that pH difference is
not great then the question arises that
why zinc poly carboxylate is better for
pulp when compared to zinc phosphate ?. And there are two reasons. First reason is
the molecular weight. The molecular
weight of polyacrylic acid is 25 to
50,000 whereas the phosphoric acid is
10,000. The bigger the molecular weight
of the liquid the lesser the chance of
entering into the dentin tubule.
So this basically prevents the liquid
from entering the dentinal tubules and
irritating the pulp which doesn't happen
with the zinc phosphate cement. And the
second reason is how much powder you can
add into the liquid in initial phases
which will definitely increase the pH
making it less
acidic.  In  Zinc phosphate we cannot add
a lot of powder in initial phases
because it is an exothermic reaction and
it affects the viscosity of this cement
however in the zinc polycarboxylate in
initial phases you can incorporate a lot
of powder, hence increasing the pH
drastically even in the initial phases
So that's why you can give zinc poly
carboxylate as a base even if the
remaining dentinal thickness is less
than 1.5 millimeters. What about the anti
cariogenic property of the Zinc poly
carboxylate cement in the composition. We
learned that this cement powder has
stainless fluoride which is not present
in the zinc phosphate cement powder and
this fluoride definitely gets released
over a period of time from the
restoration. Unfortunately it is only 15
to 20 percent when compared to the other
cement which has better anti-cariogenic
property. So it's the quiz time and it is
an important question for the exam - Can
you name all the cements which have the
fluoride releasing property making them
anti-corrosion? Please write the
answer in the comment section. Coming
to the last section that is manipulation
and I will highlight you the important
things here. First point is that you
don't have to use a large surface area
because the reaction is not greatly
exothermi. Second the viscosity will
greatly reduce if you mix faster and
apply pressure to the cement. Third point
is the powder liquid ratio that is 1.5
powder to one part of liquid and the
last point to remember is that when the
cement is in the rubbery stage while
setting do not pull the cement from the
margins because it may create gap into
the crown and the tooth structure. If you
liked this video do subscribe to the
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click the bell icon. I will see you once again very soon.
again very soon with another video on
dental cements.
