final prosthodontics lab 11 ,denture processing

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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final prosthodontics lab 11 ,denture processing

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