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8/3/2019 Final Prosthodontics Lab 11 ,Denture Processing
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So you are probably proud of your creation you did during the entire lab, youve
done the upper and lower wax rims; youve inserted teeth in them. But the journey
of denture completion is not yet completed. This lab will be the last lab for
Prosthodontics subject- first semester and with this we finish this lab. I hope you
find this lecture enjoyable as much as we found it during writing. A few notesbefore we start off:
Amajor proportion of the questions from final exam will focus on thisparticular lab as the doctor said, so pay close attention to it.
Remember during the demonstration the doctor showed the steps on theupper denture only, but anything we disuses or say is for both the lower and
upper.
Use your imagination when reading this lab; remember every detailhappened in the demonstration, we tried our best to put every possible
image, but we cant add every single picturefor every single step of the
entire process.
What are the steps to convert the wax trial denture into the final acrylic denture?
Lost wax technique is the basic process, but the specific techniques for
dentures are (in order):
1.Flasking2.Dewaxing3.Packing4.Curing5.Deflasking6.Laboratory remount7.Finishing and Polishing
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You will not need the articulator duringsome of these procedures, so the first step
is that you have to separate the cast from the articulator with a blade, then use a
hammer to remove the cast from the plaster mounting, by tapping on it. They
will separate but they will go back to each other again, due to the indices that you
already have made it in that form, just like Lego.
By this you will end up with your cast that will be used during the next
procedures.
Is defined as the process of investing the cast with the
waxed denture in a flask to make a mold that is used to
make the acrylic denture.(see images 1,2)
There are different shapes for flasks (round, ovoid,squared). Each flask has two portions (a cover and a base)
and there is an opening between them (see image 2).
The upper flask is different from the lower flask:
In the upper they are parallel to each other, so when you
place the cast inside the base (lower portion of the flask),its
borders will almost leveled with the land area of the cast, so
that when you pour plaster you can remove the excess
quickly with no worries about the different heights. However; in the lower it's
oblique so that the retromolarpad is higher than the anterior part of the ridge, and
this oblique angle will ease the separation at the dewaxing stage, and it will
protect the stone in the back area of the cast.(see image 3)
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The plaster goes up to the retromolarpad area but it
doesn't cover the lingual area, so this layer will cover
everything in the cast except the lingual area and the
land area.(see image 3).
And of course it shouldn't cover the wax or the teethin both flasks (upper & lower) or beyond the land
area .It is written in the top of the cover of the flask
(Upper or Lower) to help you distinguish between
them.
1. In order to separate the gypsum from the flask, you should paint Vaseline onthe internal surface of the flask so that you can clean it up later on, also put a
thin layer of Vaseline on your cast to keep it safe so that you can re-mount it
on the articulatorafter denture processing to see if teeth occlusion has
changed or not.
2. If there is a space underneath the baseplate, when you pour stone it might getbelow it and stick to the cast! So you have to seal and close the space between
the baseplate and the cast all the way around the denture by usinghot wax so
that nothing can get under the denture .Usually do this step when the cast isstill mount on the articulator so it doesnt change the occlusal relationship.
3. This wax that you put around the denture will turn into acrylic, so you have tobe careful not to put wax on the teeth or their roots.
Place the cast in the bottom (base) of your flask; pourplaster around the lower portion so that the denture will
not move inside the flask
when the plaster sets you will be ready to pour the
second part of the mold, but before that you have to
separate between these layer of plaster as gypsum will
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stick to gypsum, otherwise if you try to open the flask the plaster will break
during dewaxing which is a big problem, so use a separating agent between them
which is Vaseline.
The mixing ratio for plaster: 100 g powder 50 ml of water, You should make
the posterior edge of the plaster tapered in order to make it opens and closes moreeasily. And always remember if any plaster gets to teeth you should remove it.
If there is an under cut in the plaster ,,when you
pour dental stone layer it will get inside this
undercut making the separation of the upper and
the lower parts harder which may break the cast .
You can't add plaster because it will not connect
with it, so you have to fill it with wax or a lot of
Vaseline. During the dewaxing process it will melt
away forming a space between the two portions
and make the dewaxing easier, but when
finishing the denture it will turn into acrylic thatwill need trimming . If the surface was rough I
can use sandpaper under water.
After applying the separating agent pour the
second layer which is consists of50% plaster and
50% dental stone. This layer is strong enough to
keep acrylic in place and weak enough to be
separated in deflask.
Next, after the second layer sets pour the third layer that is pure Dental Stone on
the top to prevent the movement of teeth .You just need to cover the cusp tip with
it. It also helps in deflasking procedure and to keep the shape of the denture.
To separate gypsum from gypsum use Vaseline To separate gypsum from new acrylic use tin foil substitute (sodium
alginate /cold mould seal)
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You don't need stone all the way around, you just need to provide sufficient
strength. It will go around the teeth to keep them in place, you can paint a layer of
the separating agent between the 2nd
and the 3rd
layers or not.
The mixing ratio for Gypsum type 3(dental Stone) is: 100 g powder to 30 ml ofwater.
You should mix it on the vacuum mixer to be more accurate and not to have any
bubbles between the flask and the teeth.
In private labs they just make two layers, they may use stone all the way, or 50/
50 (plaster and stone) all the way, in order to save time but it may cause
inaccuracy or difficulties in removing the cast.
You have to close all the undercuts that may stop the separation between the
upper portion and the lower portion of the mold (the polished surface and thefitting surface), because if there is any undercut you have to break and destroy the
mold, so everything should be nice and smooth.
Always be sure that the teeth are centered correctly, any irregularities may cause
difficulties in dewaxing.
With this we have finished our talk about Flasking , and now we will talk about
Dewaxing which is the 2nd step of this lab.
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After flasking you have to do dewaxing ,,,
where you should put the flask in a dewaxer
machine (water path with a timer and a
thermostat) .
Put it for 10 minutes in a 95 C it will soften the
wax inside. Once it is soften open the flask
through the provided opening (it will be so hot )
.Then put it again in the dewaxer machine while it is opened for another 10
minutes" total 20 minutes " ,then with a hot water jet wash away any remaining
wax until you have a clean surface .
After Dewaxing the baseplate is thrown away (it
has a temporary job during fabrication ) the wax is
washed away that were replaced by acrylic
At this point I am ready to do packing ,but before
packing you should paintTin Foil substitute over
everything in the mold except the teeth ,thus the
acrylic will NOT stick to gypsum either stone orplaster . If you painted on teeth, they will not
connect to acrylic.
This layer of tin foil substitute (cold mold seal)
should be very thin, and you should put more than
one thin layer. If it was so thick it will change the
depth of the sulcus. Then, you can use a brush to
remove the separator agent from teeth. Wait for it
to get dry otherwise it will not work as aseparating agent.
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Dont use Vaseline as separating agent between stone and acrylic; it will not workwell as a separating agent in packing.
A short summary of packing would be like this,
Mix the acrylic according to certain ratios, wait till the dough stage, put theacrylic inside the flask, close it so that the excess will get out ,you can't use your
hand to press it so use Denture Hydrolic Press
So starting off with the first thing which is mixing of acryl:
Now what Im going to do is that Im going to mix the acryl , useporcaline
container for mixing acrylic, you need 10 ml of the monomer for a denture then
add powder in (1:3) ratio, why?
The answer is when acryl reacts its reaction is exothermic, plus when
methylmethacrylate (monomer) react with itself there will be change in size;
contraction (reduction in size).
NOTE:
* Tin Foil Substitute is made of Na alginate *
Why sodium alginate is used as a separating agent?Because its reaction is similar to the reaction of alginate impression
material.
Na alginate + Ca sulfate Ca alginate + Na (in solution) + Sulfate (in
solution)
Ca alginate will make a network and precipitate.
The cast is made from Ca sulfate; the same material that react with Na
alginate
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Some of the mistakes that might occur duringmixing andpouring the acryl are:
Not enough momner during mixing will lead toporosity orLocalizedPorosity.
Adding too much momner will result in shrinkage.
If the mix is not homogenous will also lead to Localized Porosity.
As you remember from the previous labs we have five (or six) different stages of
the setting reaction of acryl. Ill write them just to refresh your memory:
1. Sandy stage or powder and liquid stage: you can clearly see in this stage that thepowder and liquid are not the same (two different distinguishable materials).
Here the shape of the mix is exactly like sand on beach.
2. Wet sandy stage3. Stringy or the fibrous stage : If you hold the material at this stage and stick it
between your fingers itll form fibers , just like holding a gum between your
fingers
4. Doughy stage: This is the perfect timing and form for acryl to be placed insidethe packing machine.
5. Rubber stage: Using the acryl at this stage is bad but why ?As you know indental materials we have two different properties,we have Elastic and Plastic , the difference
between these two properties is obvious that the
latter will deform its shape when execrating pressure
on it. If you shaped your acryl at this stage, what will
happen is that you will have some dimensional
changes and this is not good , so no further
manipulation can happen at this stage
6.
Finally is Harden stage
So we place the acryl in itsdoughy stage inside the
flasks, but to make sure that both flasks are pressed as
much as possible we are going to use the Hydrolic press
with the packing machine ( see image 10). A very
important note is that the maximum pressure can be
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made on the cast (which is made of gypsum type lll ) is 3000
Mp , above this pressure will causes fractures or cracks inside
the cast .
While the denture is inside the packing machine we dont rushthings because if we press too fast the excess will come out
and no details were recorded, we give the acryl enough time to
cover all the possible details.
After that Ill remove the excess in the flasks and then Ill put the upper and lower
denture attached to the Flaskclamp , the objective of the clamp is not to
excrete pressure and remove excess thats the objective of the packing machine ,
no its objective is simply tokeep the flask close during the processing .(See
image 11).
There is a special screw and springs in the bottom to keep the flasks close
together. (see image 11)
After setting them in the clamps Ill do something called bench curing Ill
simply leave them for half an hour to make sure that the momner in the acrylic mix
to wet the teeth and make bond with them .
So packing is a simple process, mix the acryl place the acryl inside the flasks in
its doughy form use packing machine to remove excess use flask clamps
bench curing.
We will take the flask clamps after leaving them on the bench
(bench curing), and put them inside the curing machine (a hot
water path that have a timer and a thermostat). The aim of this
process is toconvert the heat cured acrylic from the dough
stage to the final cured hard acrylic stage.(see image 12).
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Before we discuss the actual procedure of curing, I want you to know about the
curing cycles of acryl. We have long andshort curing cycles, each one leads to the
same thing which is converting the acryl to its harden form. In this lab the type of
acryl we used wasHeat Curedacryl, they are some diffrances between long and
short, lets talk more about them
Figure (13) is of great importance to help us understand the long curing cycle; youcan see we plotted time vs. temp. Know that the acrylic in this stage is present
within the water inside the curing machine. So what happens to the water (rising
temp. /lowering temp.) Will also happen to the acrylic.
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You also have to know that the initiator for the acrylic (chemical reaction ) starts at
74 degree , meaning that acrylic will start harden at 74 or slightly below .
With this reaction heat is generated so it is an exothermic reaction (but note that
inside the curing machine we have water and acrylic ,water also gives heat ).
Looking at the graph , Im heating up the water , rising the temperature until
reaching 74 degree the reaction of the acrylic will start (1 in the graph) , but then I
stopped Im not going to increase the temperature anymore (source of heat is fixed
to 74) . What will happen now , is the acryl reaction will start I have a new source
of heat this will lead to a peak as in (2 in the graph) , acryl peak means that the
acryl temperature went up until 95 and then went down back to 76 very fast. After
the peak is done, Ill fix the temperature for 9hrs (overnight) and this is (3 in the
graph).
The temperature of water boiling is 100C. The degree of acryl boiling is 100.8C ,
you can see that the long cycle is a safe cycle , because the temperature of water
never reached 100C , it reached only 74C.
What if I cant wait 9hrs for the long curing cycle to complete? What if I want tocure the denture in the shortest time possible? Ill probably use Short Acryl Curing
Cycle .
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Graph 14 is of importance to understand the short curing cycle of acryl,.
We start the cycle by increasing the water temperature up to 74C as in (1 in the
graph) , the heat now is fixed to 74C .But the acryl has reacted and itll cause a
peak as in (2 in the graph ) that was one hour of the 2hr for this stage. After thatwill leave the fixed temperature for another 1 hr as in (3 in the graph ) , after that
we will increase the temperature of water to 95C as in (4 in the graph ) , after that
the acryl will stay at 95C temperature for 1hr as in (5 in the graph).
Why did we increase the temperature up to 95 ( 5 in the graph ) ? Well we did this
because we are we are afraid of the free moner (the excess) that might cause
irritation to the person , but when we increase the temperature we make sure that
there is no free momner . Total duration of the short cycle is 3hrs, comparing with
the long cycle this is definitely shorter but the long cycle is safer.
The next talk is not about cycles, but rather a mistake that might be done during
curing:
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Graph 15 is of importance to help us understand the error Gaseous Prosoity, so
lets see what it is.
The mistake here is that we increased the temperature of water up to 100C as in (1
in the graph) , as you know by now acryl will have a peak as in (2 in the graph) thispeak reached a very dangerous degrees (for e.g. 110C ) , as you know by now the
boiling degree of acryl is 100.8 so the acryl in this case had boiled and it had
porosities within it , itll continue with water as in (3 in the graph ) .So the danger
is because the boiling temperature of water and acrylic are nearly the same 100 and
100.8 respectively
This type of porosity happens when we dont use a curing machine , in some labs
they just bring a dash and they boil it using a cooking oven the only way they know
is when the water boils . And sometimes and might be too late because the acrylhas already been deformed.
Just know this small note, during the gaseous porosity, the temperature is highest at
themiddle and not the corners .
If we to ask ourselves why Gaseous porosity happens? The answer could beone of
these things orcombination of them orallof them:
A sudden increase in temperature (too fast; didnt give enough time for thetemperature to escape) .
Rising the temperature beyond 100 degree . Having a thick denture: now some of you might ask where exactly the
thickest part of the denture is. Is it on the upper denture? Nope because the
palate has even thickness. It is on the lower denture under the teeth, I dont
see it on the surface because heat can escape, so usually I see it buried
inside the teeth in the lower denture.
If we ask ourselves how can we avoid having gaseous porosity mistake? Changethe cycle (spending money on expensive curing machine that has a fixed
temperature rates and thermostat)
Note that we have other types of porosities, as Sub-surface porosity and
Homogenous porosity (happens with bad mixing).
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After we get the dentures out of the curing machine, we have to give them
sometime (about 30min) before moving into the next step , which is deflasking.
After we let the dentures cool downfrom the curing machine , the first thing you notice is that tin foil substitute will
form a very thin dry layer, you can see slight reddish shade (theyve added this
reddish pigment so you can see where tin foil is on). Sometimes tin foil might
unfortunately get into the surface of the teeth, what you should do is wipe out the
surface of these contaminated teeth using a piece of cotton dipped in acryl momner
, If I dont take out this excess momner what are the results ? Teeth might fall off
the denture, why? Read below about how a tooth attaches to the acrylic to know
why:
1- One way is Chemical (between the artificial acrylic tooth and the momner of
the denture base)
2- The other way is mechanical, remember when you were making the denture
you did the ginval margins some of the facial gingival margins went over thetooth , and some of the lingual gingival margins went also over the tooth .
,, .
The curing cycles for Acrylic
Short 1st stage : 74 C in water for (1, 5-2) hours THEN2
ndstage : 95 C for 1 hour
Long 74 C for 9 hours (overnight) It is safer.
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selain itu terdapat juga cara mekanikal,sewaktu membuat gingival margin pada
denture,sebahagian gingival margin melepasi cervical gigi,dn sebahagian lingual
gingiva juga melepasi cervical gigi"
Keep in mind that, Chemical and mechanical are important for acrylic teeth.
But for proclien teeth it is only the mechanical; we dont have chemical in
proclien. They even have golden pins that hold the tooth in its firm place, another
way of proclien retention is that they make something similar to class l cavity in
the occlussal surface and then acryl can go inside them and cause retention.
So it is of importance to make sure that there is mechanical retention during your
work, cover the teeth probably, because everything in wax during this process will
turn into acryl. (Mechanical retention in wax = mechanical retention in the final
denture).
Reasons why we clean the tooth surfaces with acryl momner are:
A)Itll remove the excess tin foil, which might lead to teeth falling off due toinsufficient retention
B)Itll also help to make the bond between the teeth and the acryl denturestronger
C)We have a layer called The Glazed Layer which is that shinny layer you canspot on teeth , this layer will prevent bonding between the tooth and acryl ,
What techinechan do is that they trim the root surface (making it rough ) and
then apply the acryl momner on it , in that way accomplishing good adhesion.
Do techinechans trim the occlussal, labial, lingual and Buccal surfaces ? No ,
only the root surface inside.
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Open the flasks (using the plaster knife to open the flak, sometimes we might use a
hammer) but keep in mind, using hammer is not the correct way but most of the
labs in the Arab world do it. What are the things that help me to easily separate theflask now? Can you remember? Well:
If you put good amount of Vaseline around the borders This layer (look the arrow on image 5) will also help me to open the flask
more easily.
After getting the flask out of the different layers of gypsum .
There is still that gypsum surrounding the cast and the denture
(look image arrow on image 16) , now hitting with a hammer
around this area is something unacceptable and risky. What we
can do is place it in a special device Ultrasonic Water Bath
this device will send ultrasonic vibrations that we cant hear,
leaving it for half hourone hour thegypsum will dissolve and
any excess particles on the denture will fall off. In the Arab world, we use
something less expensive way;Lemon acid(citric acid).
While removing the plaster off the flasks do we care about having the cast safe?
The answer isyes, we need the cast to check the occlusion later. During this entire
lab process the teeth will definitely change their position a little bit, so we still
need the cast to mount them on the articulator and correct that error later, this step
is called Laboratory Remountas you know from the theory part. We also have
clinical remount which is different from the laboratory remount, in clinical the
correction procedure is done at clinic.
So , now that we know the aim of the laboratory remount it is very important to
know that while doing this step we are going to leave the denture inside the casts
and we are going to attach them to the articulator and check occlusion . Why dont
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we just remove the cast and put the denture on the articulator without the cast?
Well, when the denture is attached to the cast it is always stressed, if I remove the
cast there might be relaxation and this will lead to errors during laboratory
remount. So always remember laboratory remount:
Aim is to: correct the slight mispositoned teeth during the entire process. Is done: while the cast and denture are both attached to the articulator.
Alright, the denture now is clean and ready for the next
process which is Finishing and polishing . Before we
start off with this process keep in mind that we dont do
anything to the fitting surface (look image 17) during this
process , we leave it as it is , it is smooth enough and has
good details . The only case we try to polish it is if we
scratch it by mistake.
The tools we use during this stage are:
Acrylic bur Fissure bur : for clearing out the frenum area Sandpaper mandro : a long bur with a slight in the
middle to allow the insertion of the sandpaper
Wheel brushesThe doctor didnt talk about Acrylic bur , he immediately started with the
sandpaper mandro ( correct me if he actually talked about it on
Our facebook group) .
1. We firstly remove the large excess we use the sandpaper to remove theexcess acrylic around the borders but we dont use it in thepostdam area .
2. After that we start polishing the surface , well start with sandpaper on slowspeed (10k-15k rpm) , If I did good polishing for the wax I dont need to use
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this very much expect on the borders areas. (The more time you spend
waxing the less time you spend finishing).
3. After that well use a special rubberpolishing burs that has within the rubbera small particles that will give a shine surface.
4. After that Ill use a large trimming wheel ( look image 18 next page ) thesewheels are made out of cotton we dont usually use the normal plastic
brussures , during using these trimming wheels we dont use them
arbitrarily, but we do the following :
a. The first stage we use these wheels wet withpumice material, itcomes as powder so we add water to it, and it forms like dough. We
put it on the wheel and then the wheel goes around the denture and
itll give a very shiny nice appearance. (pre-final shine )
b. The final shine comes from :i. Using the wheel dry , at low speedsii. We use rouge or what is known as to give the ,
final shine (look image) . We have two types of rouge in
dentistry :
1. The red iron oxide ( picture ) is used to polish metal (crown , bridges and partial dentures made out of metal
..etc ) , we dont use this polish acrylic. This is called
Iron oxide
2. Triplen (not sure about the name) it is the white materialwe use to polish the acrylic denture. This is called
Aluminum oxide
When we have the final shine we leave it in water, we never let the denture dry out
why? Because the denture has very small pores in it, acrylic has these microscopic
pores in it. Imagine a sponge filled with water but then the water evaporates what
will happen to the sponge? The same thing with acrylic itll shrink. (Think of the
final denture as a fish, you always have to put it inside water. If it leaves water itlldie).
Just to recap with you on the finishing and polishing steps , we start off with the
acrylic bur then we used the sandpaper-mandro bur then rubber polishing bur then
we used the wet with pumice wheel bur then we used the wheel bur dry.
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** Due to polymerization shrinkage, acrylic
(self /heat /light cured) will shrink, so usually
you will see a space mostly below the palatalarea of the upper denture.
When processing the denture it will take
the shape of the cast, because in the mold you have the teeth and the ridge and the
fitting surface, in between them there is a space and the acrylic will fill this space.
** The baseplate just has an effect on the polished surface of the denture and it
doesn't have anything to do with the fitting surface.
That's mean acrylic will take place of the wax and the baseplate and the space
below them. I.e. if your baseplate is 2 mm thick and there was a 2 mm space
underneath the baseplate, your final denture will be 4 mm thick.
And if you accidently have an undercut in the cast and you filled with wax
before making the baseplate and the wax rims, the final acrylic denture will fill
the place that was occupied by the baseplate and the wax in addition to these
undercuts.
If you by mistake scratch the cast you will have a positive defect in the fitting
surface that will irritate the patient.
** Anything between the polished surface and the secondary cast will convert to
acrylic (wax, air) or any other things.
** If there is a very deep undercut and you don't want it to appear in the finaldenture, block it with something that will not melt with heat like plaster or stone.
** If some of the Vaseline gets on the wax it will turn into acrylic, that's why we
use Tin foil substitute for anything contact with acrylic, use Vaseline just between
gypsum and gypsum.
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