Dental material …
Lecture 3 …part 1
Today’s lecture is going to be about gypsum products … and we will talk
about investments next week with waxes …
So the midterm exam will include gypsum material, impression material,
and material properties!
We talked a little about gypsum and we said that it is the material that
we pour the impression in so we would have a study cast or model … to
work on it… so after I take my impression I need a +ve copy to
represent the oral cavity (teeth or soft tissue) this positive copy or
representation is given by gypsum material …
Gypsum itself occurs naturally in nature they get it out and
manufacture it in many ways so we get it and use it in the dental (or
laboratory) application that we need o.O” shut up O.o”
So it is a naturally accruing mineral … so manufacture it and process it
in many different ways so you get Gypsum products …
~~ It can be used (dental applications):
As an impression material … we said that there is an impression plaster
that is rarely used now
It can be used to make replicas it can be used to make casts or dies
We need certain properties to be available in this material (ideal
properties of die material) you want it to be dimensionally stable we
don’t want the shape or the size to change so finally we would have a
final result and it should be stable in the oral cavity (our final
restoration will fit probably in the oral cavity) we need it to be easy to
use, inexpensive, accurate, strong enough, not toxic or not irritant, to
have good abrasion resistance because we might curve things on it or
make restorations on it so we don’t want it to be soft or to be
destroyed easily
How is it produces? As we said it naturally occur as a di-hydrate
*which means it has 2 water molecules* it is called calcium sulfate
dihydrate it is present in rocks\sand outside … once they take it they
heat it (once we heat somthin that has water what will happen to the
water?) part of the water will evaporate and this will result in calcium
sulfate hemihydrates *one molecule of water* … this product when you
mix it again with water it will convert into di-hydrate again … but
different product…
Now this di-hydrate if you heat it at 115° c under normal atmospheric
pressure you get different product than if you heat it at higher
temperature or higher pressure so we say the way of manufacture
affects the product
So it is: Ca sulfate di-hydrate (natural product) they heat it under
different temperatures to remove some of the water and to get
different products
~~if it heated at 115° c under atmospheric pressure you get irregular
particles and porous material and this is called (beta) B hemihydrate
plaster
~~if it is heated under pressure the powder particles will be more
regular less porous this is called (alpha) a hemihydrates or dental stone
… stronger material, better material
If you even use extra pressure with heat you get even a stronger
material because the particles are even more regular and less pores are
present … so the material is more dense (denser) so it is stronger
Another way of changing the material they mix it with silica SiO2 … if
you mix stone with silica you get even a different product called
investment material (we will talk about it next week)
~~ Different processing methods different products different
applications
If you can see in one of the pictures the plaster is more irregular
compared to stones or high strength stone or what we call die stone …
because it is heated under pressure, this pressure will remove pores
and create more regular particles so no spaces inside them then you
will get a stronger cast or a stronger model
SLIDE 7
You get different products you get ur plaster ur stone ur die stone in a
form of a powder in bags
How do we use it in the lab? You simply mix it with water once you add
water gradually you get heat so it an exothermic reaction … so once you
add water and mix you will get heat and the products … and that is why
we call it exothermic reaction
Gypsum + heat gypsum powder + H2O
if you add accelerators it will force it quicker to one side which means
that the setting time will be less … if u add something that will slow
down the reaction (retarders) it will force the reaction to the other
side which will make the setting time slower (longer time) so it depends
on what you want
So when you add water to the di-hydrate there will be crystallization
and the crystals become larger and larger and the material become
more solid (which is the model or the cast) … so it sets by formation of
crystals that grow and become harder
SLIDE 8
Different products:
*plaster
*stone
*High strength\low expansion stone
And * high strength\high expansion stone
Ideally I don’t want my material to have high expansion; I want it to be
dimensionally stable … but in certain clinical applications you need your
gypsum material to be able to expand a little bit, in which cases when
make a bridge we use a metal this material is metal and covered with
porcelain (for esthetics) … now after we put it in the cast it will shrink
a little bit so we need something to resist this shrinkage … so the final
result of the crown shape won’t be damaged (wrong) … so the expansion
that happens in this crown or model will resist the shrinkage of the
material …
So once again when do we use high strength\high expansion stone?
When we are making cast restorations (crowns or bridges) they are
basically made of metal and on top of it we add porcelain … the metal
will provide strength and porcelain will provide esthetics … when you
work with metal and you melt it over a cast or die after some time
when it cools down it will become solid … metal when becomes solid
shrinks … we need to resist this shrinkage because we don’t need the
size of the restoration to change (and to differ from what you have
done inside the oral cavity) so you need the cast material to expand a
little t resist this shrink and maintain the original size of ur crown
Special gypsum material includes investment which is basically dental
stone mixed with silica … it has special properties we will talk about
them next week …
Slide 8 … you can see different casts or models and this is what we call
a die a representation of a single tooth … so cast or model
representation of a whole cavity or all of the teeth, and if you are
working with only one tooth it is called a die … the blue thing inside the
crown is wax! … These restorations crowns and bridges are first
shaped in wax then this wax is melted away and replaced by metal or
porcelain
Waxes are used to make the general shape of the crown or the bridge
then once it is made it is melted away and replaced by metal and later
on porcelain is added
SLIDE 9
Different products due to different processing methods
Now since each material is different from the other they have
different properties … plaster is more porous it needs more water so
the ratio W/P is higher compared to stone compared to high strength
stone to high strength high expansion stone … high strength high
expansion stone has the least amount of pores so it doesn’t need as
much water
The more pores you have in the product the more water you need to
add
So high strength high expansion stone is stronger it has fewer pores
less amount of water
Setting expansion is high in plaster which is not good that is why it is
not that accurate material so only use it as study model or treatment
planning while in stone the setting expansion is moderate so it is a
reasonably accurate material … so you can use it as a working cast to
make your restoration on … and high strength stone is good it has low
setting expansion … and in high strength high expansion stone it has
special application it needs to have high expansion
Low abrasion resistance in plaster so you can easily curve on the
surface which means it is soft and it is better as we go down in the
table slide 10 …
You use high strength high expansion stone to curve on it to make
crowns and bridges … plaster models only for study or treatment
planning … and stone could be used to make complete dentures or
partial dentures because it has moderate abrasion resistance and
moderate strength
SLIDE 11
We said before strength and hardness is affected by porosity …
amount of water … and shape of particles …
If you have regular particles they condense much better and gives you
a stronger model … If the porosity less the model or cast would be
stronger as well … less amount of water needs to be added it would be
a stronger model as well … which means the lesser the porosity the less
amount of water the more regular the particles are the stronger
(better) the cast or model you get … because they condense better to
each other
As you add more water the weaker the material becomes more soluble
and it can be easily abraded
Dimensional accuracy … if you got high setting expansion you got lesser
accuracy … so plaster less accurate than stone less accurate than high
stone less accurate than high strength stone
As we said expansion results from growth of crystals and joining which
causes some expansion … stone is dimensionally stable more than
plaster … stone expands less than plaster
The less water you put the less porosity you got better strength lower
solubility
Reproduction of details … the greater the porosity the lower the
surface details … so plaster doesn’t produce details accurately like
stone or high strength stone or die stone … so porosity determines how
accurate the surface details are
Now once you get your impression and want to produce your model or
cast you need gypsum material and the impression material to be
compatible if not you will not get good surface details
Best compatibility has been found between rubber impression material
and gypsum (poly ethers or silicon materials)
Impression plaster … which is called type one is not used anymore
Type two which we saw in the lab (grayish powder) it is used to make:
diagnostic casts … as you can see in the picture on the left is the
impression … and the other is after you pour the impression in gypsum
… it also can be used to make a base for this model … so the base of
the model or the cast can be made from plaster … for the articulation
of stone cast and flasking procedure should be explained in the lab
But in general when we do the complete or partial denture … we said
first we take the primary impression after that we take a secondary
impression we have a working cast or model now so we can do our
restoration on it … while doing this restoration of complete or partial
denture we should put it in a metallic mold … and heat it under high
temperatures so this material that we are making the denture of
become solid
In this mold that we are using to make the denture or model we need
something around our material to hold the model in its place (so this
material is going to be plaster)
A flask is a metal mold we use it when we make complete or partial
dentures … the cast is placed in this mold … but all around it we will
have an empty space … we can’t leave it just like that and put it in the
oven and heat it … we need something to hold it in its place so all
around it we pour plaster
So plaster is a part of the procedure … stone could be used for the
same purpose here
Dental stone … it is a stronger material more accurate called also type
three mainly they use it to make casts to produce complete or partial
dentures if you remember the yellow model or cast in the lab is made
of the yellow powder which is the dental stone … it is accurate enough
to be used as a working cast … and strong enough … but only for
complete or partial dentures simply because when I make partial
dentures I place the material during processing in hot water to allow it
to become hard stone is okay with moderate temperatures at higher
temperatures it will break of it will disintegrate so we don’t use it to
make crowns or bridges … in crown and bridges we melt metal we use
temperature more than 1000 dental stone will not be able to handle
such high temperature so you can’t use it for these applications
We can also use dental stone (type three) to make models in the
orthodontic treatment and it is also used in the flasking procedure
Both plaster and stone are used in flasking but in a little different way!
A model made of stone is stronger and more resistant to abrasion than
a model made of plaster
Now die stone or high strength stone …
Type four … is a stronger material it can be used to make crowns and
bridges … in the process of making implants … the precision
attachments they are just ways in maintaining partial dentures … as we
saw in the lab the wire was to keep the denture in place when we talked
about fatigue … now there is other ways to keep the denture inside the
patients mouth it is the precision attachment it is like a key and a lock
one part is present on the tooth (natural tooth), one part is present in
the partial denture once they are placed inside the patients mouth
they will lock … so precision attachment is just a way to make sure that
the partial dentures in the mouth is held good instead of using that
wire (as a retention) … how it is made … you need to take an impression
some sort of processing during this processing you need to use die
stone for pouring the impression
Usually it is colored pink or green … and it is two times stronger than
stone (type three) so its applications are different … you can use it for
crowns and bridges it can handle high temperatures
Again as we said high strength high expansion stone (type five) usually
blue or green in color it is very strong lowest W/P ratio it has high
expansion it compensate the metal shrinkage to prevent dimensional
changes in our metal crown or metal bridge and that is why it is mostly
used for crown and bridges work it can handle high temperatures and it
can compensate for metal shrinkage so we would have better results
Dental materials … 3rd lecture … part 2
Now we are done with the products and their uses …
Manipulation
How this material is manipulated? We basically mix it with water and u
need to maintain the W/P ratio once you mix it we said you have to
follow the manufacturer instructions you cannot add too much water or
too little water, because this will affect the property of a cast or a
model … you can either mix it by hand incorporating water with powder
mix it by hand until you get a creamy mix with n particles inside it … or
it can be mixed mechanically under vacuum which will suck all the air
bubbles so you minimize porosity inside the material … so either manual
mixing or mechanical mixing …
Now once you mix it with water it starts to set … how can we tell if it
is initial setting or final setting? There are two things can be used to
tell us whether the material has set or not … you need to follow the
instructions according to the working time or the setting time … usually
it tells you that you have to wait 10 mins to use the material … and once
you can see that the surface of the material is not shiny anymore this
what we call initial setting or loss of gloss which means that the
water is being taken by the powder and the setting is initiated
So loss of gloss indicates initial setting and usually it happens after 10
minutes … and this means that it has started to crystallize … particles
dissolved in water crystals are being formed and they gradually become
larger stronger … water will be taken by the powder and crystals will
precipitate gradually and you will get the solid model or cast
So crystallization will eliminate a part of water then precipitation and
then you will get the hard material
As you can see in Slides 23 and 24
We don’t just mix the powder with water and that is it has to be done
with a machine that is called a vibrator this machine is shaking the
impression is in contact with it and you are adding the gypsum material
bit by bit … why do we need the vibrator? Because of bubbles … so
they will leave the material go to the surface and burst … so by
eliminating the bubbles we eliminate porosity inside the material and
we get a stronger model
So I need to have my model on a vibrator to eliminate air bubbles and
get a stronger cast or model
You measure it properly you mix it in a bowel with a spatula and you
pour it in the impression
Now once the material has set you can feel that the model or the
impression is cold this is an indicator that the setting reaction is
almost done and you can safely handle the material and work with it
without it being broken
As long as it is hot it is still setting once it cools down the setting is
almost over and you can use the material and handle it without it being
damaged
The material can expand by setting expansion there are many methods
of getting even more expansion … by placing the model in water while it
is still setting … water molecules will go in and greater expansion will
happen
So we have expansion aided by water we call it hygroscopic expansion …
the water molecules will get between the crystals and pull them apart
When do we need to do that? We will talk about it when we talk about
investment … it is not a common practice to put it in water we only do
that in crown and bridges preparation when we are using investment
material
For now we have to know that setting expansion is for crystal growth
and hygroscopic expansion is when we add water
But you need to add water while the material is still setting … if it sets
and finished nothing would happen
Usually it is recommended to separate the cast from the impression to
use it after 1 hr
Strength will increase gradually and it will be double and triple after
24 hrs
So it is a gradual process and strength needs time to develop
So loss of gloss indicates the initial setting of the material but it is
still weak you can’t handle it … if you try to separate the impression
from the gypsum the gypsum will break off it is still very soft
There are many ways to test the setting of the material usually it is
used in research
How do they test the setting time of the material? And how do they
put the timing on the bags? Researchers when they develop this
material they test it they need to supply you with the working time and
the setting time so you know how to manipulate the material … how did
they came up with this information? They used certain machines one of
them is these needles Gillmore needle what they do is that they mix
the material and fix it in a mold … and then they allow this needle to
fall and touch the surface of the plaster or stone … as time passes the
material become harder and harder … so as long the material is still
setting this needle will go through and makes a mark … once the
material starts to set it becomes harder and resist the penetration of
the needle … so they keep allowing the needle to fall down over and
over again every second or so until it can’t penetrate anymore or it
can’t make a mark
At that point they say that this is the setting time of the material
So these needles have weights attached to them in order to fall on the
top of the material until it stops making a mark on the material which
indicates that the setting has started
Initial test is for initial setting and it is composed of stages not just
the loss of gloss … as you can see in the graph slide 29 … there is LG
loss of gloss then it is followed by a stage called initial Gilmore setting
(Gilmore to the needle used in the test) then another instrument called
vicat (this indicates a more advanced stage of setting) then Gilmore
final test or a test that indicates a different stage … and then you get
ready for use and this indicates final setting
So we have many stages of setting it is a gradual process and each
stage has a test to give us the timing of this stage
Now these stages differ with time and the strength will also be
different … the more advanced the stage is during setting the stronger
the material is
So you start with mixing the material with certain mixing time 7
minutes or sometimes up to 10 minutes and after the material is set
you can simply handle it and use it
Mixing time … working time … and then setting time
As we said the usually advice you to wait one hr before you handle the
material … some products are okay to be used after 30 minutes … and
with time you will be able to tell if the material is strong enough or not
Usually you can handle it after one hr because it would have reached
80% of its compressive strength
Nut right now most of the products are safe to be handled after 30
minutes
It is recommended that before you separate the impression from the
gypsum to remove any excess all around … and also you can soak it in
water just to soften the material a little bit for ease of separation
As we said W/P ratio has to be followed accurately … if you increase
the amount of water too much … you will get more water in the cast or
model … and you will get a weaker model … and the setting also will be
longer
If you decrease the amount of water the setting will be faster the
material will be difficult to accumulate because there is not enough
water and the model will be dry and you will get an inaccurate model so
both ways are not good
So you need to follow the instructions carefully
Spatulation … when you mix the product (powder) with water … the
quicker you mix the shorter the setting time because you are
accelerating the reaction
If you use hot water … working under higher temperatures you also
accelerate the setting but up to a certain degree … up to 100° F for
example after that the setting will become retarded
So you can use worm water to accelerate the setting it shouldn’t be too
hot so it has to be up to a certain degree
So spatulation increases the reaction shortens the setting time …. Hot
water up to a certain level will also accelerate the reaction and shorten
the setting time
So these are ways to accelerate the reaction instead of using too much
of water or too little water
Also accelerators can be used … certain materials like potassium
sulfate for example slurry water (which is basically water and with
some plasters ~gypsum~ powder in it) and this will accelerate the
reaction and it will be short
Other materials are called retarders if it is found on the impression it
makes retardation (slows the reaction): blood, saliva, and alginate
For example if you got blood or saliva on your impression they will make
the setting slower …
If you use a bowel that has remnants of alginate inside it this will also
retard the setting reaction
That is why you have to clean your bowel after using it and to use clean
instruments with your material
Once you have the impression you need to separate it and handle it
easily … that is why you need to make a base … this base is attached to
the gypsum that has been poured inside the impression
Even if the impression was made of stone you can make the base of
plaster … you don’t need the base to be extra strong that is why it can
be made of plaster or stone
It is just for easy handling (base)
So the part that represents the teeth or the soft tissues is called
anatomical portion
And the base part is called art portion
So this base is attached to the anatomical portion and allows us to
handle the model easily
Other way to make the base is to attach a strip of wax all around to
make like a mold … and then start to pour the impression inside … so
this is another way instead of doing the two ways separately to pour
the impression and aside make the base … there is another way we put
this strip of wax all around … we stuck it to the tray then we start to
fill and it works as a mold … so when we remove this wax we have the
base and the gypsum ready with each other … this way is called boxing
When we talk about storage we mean plaster and stone itself as a
powder before mixing it should be stored in an airtight place with no
moisture … if there is moisture around this will cause retardation of
the setting … and it will be even more difficult to mix the material with
water in order to use it … so storage needs to be done carefully
Any equipment that you are using when you are working with the
gypsum material should be very clean … and after 24 hrs if are not sure
that your impression is disinfected you can disinfect cast
We said that we should all the time disinfest impressions … if you are
not sure of that you can disinfect the cast … but only after 24 hrs of
pouring the impression so that the cast is strong enough … and will not
absorb disinfecting solution
Several disinfectants are used: sodium hypochloruite … iodophors …
chlorine dioxide
Whether impression or cast the same materials could be used
As we said the base could be made of plaster and it is easier to
trimming … to remove excess … and if your base is made of stone you
can make it easier to trimming it by soaking it in water just to soften
the surface a little bit
What do we mean by trimming? We use a machine … that when I make
the base there will be excess all around … I need a nice smooth regular
surface of my base … how to get it? We use a trimmer … which is a
machine that has a rough surface … you turn it on it starts rotating and
I place my model against this rough surface it makes it softer and
removes excess …
You need to be careful when you are trimming not to trim teeth … you
just need to get something smooth and remove excess …
You need to be careful how you do need to shape your model …
How you need to maintain the anatomical part …
We said that certain die materials are used when we are making crowns
and bridges …
When you first have these restorations you first handle them with
wax so you actually curving on the model itself and we said that it is
good to have a good abrasion resistance …
so to make it more resistant they add material on it for example Metal
plated die stone this will create a metal plated model which will create
a harder surface will resist abrasion one of the materials is called
epoxy as a hardener or they can mix it with a particle material again to
have better abrasion resistant …
So these are the methods to make the surface of the model harder
more abrasive resistant …. Such as resin reinforced die stone … the
whole process of the melted metal and wax is shown in slide 44 so this
is just to see what we mean of investment material … melting wax … and
adding metal … so we have the die (replica of one tooth) I want to make
what we call an onlay or even a crown so first we make wax and we
curve it on the model with wax … then I need to place it in a mold and
then place it in an oven …
This mold should be filled with something so it would hold my model
they fill it with investment material …
Because this material is strong and can handle high temperature … so
once you fill the mold with investment material … we put it in an oven
and the wax is melted away …
To fill the space they melt the metal and push it inside … so you get
your metal restoration … they call it lost wax technique … because the
wax is melted way
This is done under high temperature so you need good investment
material
Next week we will continue … the slides and references are all given to
one of your colleagues … so they all should be in the Jam3eyeh
So gypsum … impression material … material properties three lectures
are included in the exam … next week’s lecture is not included
Good luck to all … study smart … d3waaaaaatkom ^_^
Done by:nada nammas