Labratory 4

7/28/2019 Labratory 4

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Prosthodontics Laboratory: 4, Cobalt chromium design.

Done by: Hasan Alblwa and Osama Yousef .

Important notes before you start reading this script:

In laboratory 3 , we said that the doctor didnt talk about Finishing andPolishing process , well turns out he did talk about it in this laboratory , so

please go back to your third lab and study it , it is included .

The topic of this lab is ahead of the theory part, some topics might not be veryclear because we didnt talk about them in many details. Butdoesnt worry just

take a general idea about them for now.

When reading this lab, any tile with a star is considered a main title and all ofthe processes that come after it are under that title.

General overview of the entire lab: you may skip if you want

In this laboratory we are going to learn about partial cobalt chromium denture

design. In short todays lab is:

Patient comes to your clinic for treatmentyoull make primary impression Primary impression primary cast ( diagnostic cast ) : well use the dental surveyor

Diagnostic survey (well use : Analayzing rod , undercut gauge and the marker )

Using elastomeric materials secondary impression pour stone secondary cast ( mastercast)

Another survey (verfytion survey ) everything is good we continue , something is wrong weredo

Drawing the design on the secondary cast then making blockout and relief then duplicatingthe secondary cast with either agar-agar or addition silicon by putting inside a mold then

making of refractory cast

Adding pattern wax that is supplied by companies to the refractory cast putting the refractorycast inside a mold adding another layer of investment ( sandwich ) putting the refractory

inside the oven the pattern wax has melted but we have tunnels now well put the cast

inside the casting machine with the casy alloys ( ) supplied

Cleaing and shaping of the metal Adding the acrylic and teeth conventional ( flasking , dewaxing , deflasking , packing and

curing ) now we have a metal framework with acrylic and teeth .


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.

Well talk about the steps of making cobalt chromium

dentures, and Dental Surveying.

Just a reminder so you wont forget: how many steps we

had in the making of acrylic complete denture? (Primary

cast, secondary cast, jaw relation record, try-in and insertion) , and as we said on

the first lab partial denture design is unique to each cast , so we might sometimes

do all these steps , other times we might only do a few of them . Also its obvious

but note that the partial denture differs from the complete denture in that it has

clasps for retention and metal framework.

When thinking of cobalt chromium design youll notice thatits different; we have

metal alloy framework and acrylic and teeth. The acrylic and the teeth in partial

dentures are processed as the usual method of complete dentures, but for the metal

part you might ask how do we get the metal in that shaped? The short answer

would be Lost wax techniquebut the process goes into much more details than

this, and youll learn it by the end of this laboratory.

So to make a cobalt chromium partial denture I need to have two processes , one

for the metal and one for the acrylic , so the steps are more involved and moretechnique sensitive.

The patient will come to your clinic asking for partial

denture treatment. The very first step is taking a

primary impression with a rectangular stock trays as

we said earlier we used Alginate for the demo as in

impression material, and after that we convert the

impression into the diagnostic cast using type 3 dental

stone. (Figure 1).

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The diagnostic cast is used to determine my treatment plan, and I examine the cast

to see what does the patient need to make him prepare to receive the denture. This

is very important because I need to know what Im going to do before taking the

final impression.

Usually all of the cases the teeth arent parallel to each other, we have undercuts

and the teeth are not disturbed in a balanced way inside the mouth. Usually the

partially dentate patient has had his teeth extracted at different times, and probably

he didnt have the perfect occlusion, all of this indicates that I need to make

Mouth Preparationfor the patient before my final definitive denture is made.

What I mean by mouth preparation is that I study the primary cast I have , and I see

what modifications to the patients mouth has to be done before undergoing the

making of the denture , if the patient needs to have some of his teeth excretedbefore starting Ill send him to get that done , other times he might need fillings ,

crowns and sometimes he might have a good tooth for support but that tooth is a

little tilted so I make sure I get that fixed . This is what we mean by mouth

preparation.

So, we determine which tooth needs modification by clinical conformation.

But how do I determine which tooth is good for

support for the clasps? We do this with an instrumentcalled Dental surveyor. Well talk a little bit about

this device first:

We use it to map out the general topography of the

cast, what the shape of the teeth where the undercut is

and variety of other things, the most important partsof the dental surveyor are shown in the adjacent

picture.(figure 2).

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Why do I do survey? you remember that a cast with natural teeth has undercuts, if

we were to draw a line around the maximum convexity of all the teeth we will

have an area above the survey line and an area below the survey line.

And as you already know all the area below the survey line is undercut , some ofthe undercuts are good because I use them for retention and these are called

favorable undercuts , some of them are bad because if my metal connecters goes

underneath it I cant get the metal in or out .

It becomes my objective when designing to change the tilt of the cast in such a way

that I have maximum number of parallel surfaces, to be more specific I need the

maximum number ofProximal surfaces to be parallel to each other so that when

the prosthesis goes in and out itll slide right into its place, and we do this by

shifting the angulations.

I have to make two pathways, the path of insertion and path of removal. And I

have to make it in such a way that the patient only has onepath of insertion and

not many, and onepath of removal and not many, we do this because we want to

make it very retentive and stable.

On the surveyor the path of insertion is synonym with the direction of the

surveying arm, so adjusting the arm will also adjust the path of insertion. Now

finding the right path of insertion is the most challenging part of using a surveyor,its almost like solving a Rubiks cube, so there must be a criteria or steps I do

when handling the surveyor , remember all of these steps are called Diagnostic

surveying ; diagnostic because its done on the diagnostic cast :

And they are four:

The guide planes (finding the path of inseretion/removal). Retentive Areas (retention).

Interferences. Esthetics.

Well talk about each one of these briefly, keep in mind were still using the

diagnostic cast and these steps are the essential keys to make my design.


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What do I mean by guide planes? They are planes that guide the denture into its

correct position.

When I first seat the partial prosthesis what is the

first thing that is going to touch? Its the guide

planes, they are the axial surfaces of the tooth on

the proximal and sometimeson the lingual

surfaces of the tooth which are parallel to each other.

To have an idea of what guide planes are, take a look at figure (3). And the two

lines are the guide planes (distal on the premolar, mesial on the molar), and as youcan see they are parallel.

How do I discover these guide planes? By looking at them

visually? No, Ill use something called the analyzing rod

(figure 4), which is a rod attached to the surveyor. With it

I try to find as many parallel surfaces as I can. When

using the analyzing rod I have to put the cast inside the

surveyor table at zero tilt, do you know why I do this?

Because the natural position of the mouth is at zero tilt

and its the position where it comes perpendicular with the occlussal force, thus

giving me support and retention.As we said most of the time the guide planes are the mesial and distal, so the first

access of rotationwill be? Its usually Anterior-Posteriorwhen we have mesio-

distal guide planes, and we have that most of the time.

So the main concept is that the first access of rotation (the first tilt move I make to

the cast in the surveyor) is usually if not always anterior-posterior, I do thisbecause Im looking for maximum parallelism of existing surfaces. If I tilted it and

this parallelism cant be found what do I do? Ill make what we said earlier: Mouth

preparation to have that parallelism.

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Our next brief talk will be about some of the cases where we failed to have

parallelism at the first rotation so we made some modifications, these cases are just

examples. The doctor talked about them in a hurry and he

said were going to take them in the theory part in much

more details, but we wrote it anywaybut dont worry toomuch about them:

1. Here you can find that the premolar is tilted and themolar is also tilted, at zero degree (figure 5) can I get my partial denture at

that position? No, we cant. So what we should do is we

should tilt it anterio-posterioly . And remember we

didnt tilt it laterally because the guide planes arent

facio-lingually.

2. Here both of the two teeth have undercuts (figure 6) ,what Ill do here is that Im going to do mouth

preparation as we said earlier well take for example 0.5 mm enamel from

each tooth , giving us two parallel surfaces.3. Here one of the teeth is tilted and one of them is upright? We tilt slightly and

we trim a little bit until we have parallelism.

So this step was about finding the correct path of insertion and removal.

The next step is finding our retentive areas, what do we mean by retentive areas?

They are the places where they show support for the clasps, so to know where I put

the clasps I have to look for retentive areas.

In contrast to guide planes; we look for retentive areas laterallyor facio-lingually

(most of the time the retentive areas are on the facial side and rarely they are on the

lingual side).

The doctor showed an example and he said that its still advanced to us , and he

just wanted us to memorize what he said just for now , so when we take this at the

theory well be exposed to the concept :

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The example that the doctor showed was about two teeth, one of them isseverely tilted with an undercut (premolar) the other one is fine. So here we

have to distribute the undercut, so we changed the original rotation degree

(which was zero) by tilting the cast laterally. Now I have the premolars

undercut distributed, and although it looks like the premolarisnt having anundercut but in truth its still an undercut, do you know why? Because as we

said earlier, what gives the retention is not the undercut itself but rather the

orientation of the undercut; is it perpendicular to the occlussal plane or not? On

this case I realized that after we tilted the premolar is not perpendicular to the

occlussal plane so Ill call the premolars undercut : Sudoorfalse undercut , if

it was perpendicular itll be true.

What if I have two deep undercuts on both sides? I can recontour the enamel tomake the undercut less or put a crown on the tooth.

So this step is all about identifying where the undercuts are, for the future claspsto be put in them, dont worry about the examples too much.

Consider this situation: after youve finished your measurements and designing ,

you discovered that there is a bony trabcule , this is called an interference , we

wont talk much about it here in the lab but we can deal with it in three different

ways :

1. Change my path of insertion ( redo the path of insertion so I could avoid thatinterference )

2. Change my design3. Blockout the interface, but blockout will turn into an empty space where

food and saliva might accumulate. So it is good for the design but not very

hygienic if the patient doesnt take good hygiene practice.

The last step in the sequence is esthetics, but well talk about it in the theory more.


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After I have determine where my retentive areas are (the favorable undercuts), a

new question rises, how exactly can I measure where to put the depth of the clasp

for the undercut? And this leads us to our next talk:

As you already know each type of undercuts (different depths) has

its own clasps, undercuts are divided into 0.25 mm undercut, 0.5

mm undercut and 0.75 mm undercut , sometimes its written ininches in the textbooks ( 0.1 inch , 0.2 inch and 0.3 ) . (Figures 7 and

8 respectively)

There are many type of clasps, the doctor only talked about two in

the lab: Cast clasps ( 0.25 mm ) and gingivaly approaching clasps (

0.5 mm ) . Cast clasp short, gingivally approaching is long and

flexible.

So lets say we have a good undercut and we want to put the castclasp (0.25 mm) on it, how do I measure the depth? Well use an

instrument called Undercut Gauge which is: an attachment

used in conjunction with a dental cast surveyor to measure the

amount of infrabulge of a tooth in a horizontal plane. (Figure 9)

So well bring the undercut gauge against the tooth, and a space

between the tooth and the gauge will be found. Lets say that the

undercut gauge is 0.25 mm, well touch the tooth at maximum

convexity at the survey line then Ill bring it up until it touches

the tooth, Ill mark where the 0.25 mm is. What do I know now? I know that from

here to here is 0.25 mm at this height there will be a 0.25 mm undercut is at that

point and a clasp will be put there. (Figure 10)

So from this step we concluded where the clasp depth should go.

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All what weve talked about and all the previous procedures were done on the

diagnostic cast when we did diagnostic surveying; the next talk will involve the

secondary cast that we make after the diagnostic cast.

In this stage well do the followingprocesses:

What well do to the patient is something called Rest preparation What well do on the secondary cast is: Verification surveying (to make

sure the first diagnostic survey was good) and Blockout and relief.

In the areas of the rests we said that I cant put a rest before I make a room for it.

Lets say we have a normal molar and on that molarthere is going to be clasp.

Using a slow headpiece Ill start trimming the tooth on the patient in the same way

as I determine on the diagnostic surveying

We call this rest preparation, and this is not as cavity preparation in conservativelaboratory, because in conservative laboratory the objective was to remove the

decay or carries and to prepare a retentive hole thatll keep the restorative material

inside the cavity so it wont come out. But in Prosthodontics the objective is:

A) I dont want to gobeyond enamel so that the patient doesnt become sensitive

B) the rest will come in and out of the patient mouth so it shouldnt be occlusaly

convergent but ocllusaly divergent (there must be undercuts here for the clasps to

attach ).

Now that the patient mouth is ready its time to make the final impression, which

will give me the secondary cast, remember with the final impression we have to

make custom trays. We take the final impression with any of the elastomeric

materials (poly-ether, poly-sulfide, addition silicon ...etc). Well then end up with

an impression that is going to be poured with stone and secondary cast is then


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formed. Some of the differences that should be noted between secondary and

primary:

Before making the secondary we trimmed the patient teeth, so we already havetrimmed teeth in secondary cast,but in primary thats not the case I didnt trimanything I just imagined it on the cast by drawing lines.

I have rests on the secondary cast but not on the primary cast. I have guide planes here ( on the primary I just noticed them ) I have modifications if present as we said earlier.

In order to fully understand what blockout and relief process

is, first well have to understand how the metal

relates to acrylic in the cobalt chromium denture.

This is an example of metal framework (figure 11,

12), youll see there are clasps on the inside andoutside of the denture, there is a major connector and

the meshwork or lattice work (minor connectors) ,

can we see the meshwork when we have our denture done ? No, because the

meshwork is the part where the acrylic binds to, and its hidden. So when I attach

acrylic to metal I need to have a junction and as you know acrylic cant be made as

a knife-edge junction.

You can see the acrylic tooth surrounded by acryl. Look at the meshwork and how

its filled with acryl, this gives mechanical support and retention. Look also to thejunction ( figure 12 : arrow ) between the metal and the acryl this is called finish

line and its a line where one material starts and another ends : in this cast where

the acrylic ended and metal started , youll see finish lines in crowns , bridges ,

cobalt chromium and many bridges , this line is very important in Prosthodontics .

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When I make the metal I have to imagine that there is acryl and I have to make the

finish line ahead of time , it doesnt make sense to put the metal and acryl and then

start scratching the metal to make that line ( impossible to do ) , it has to be done

before .

Does the metal touch the ridge in the endtoules areas? No, it should be hovering

above the ridge, how do I make this possible? We do this by getting a wax sheet

and adapted to form the shape of whatll be the metal, but still I dont want that

wax to touch the ridge (because again wax will turn into metal and metal will

touch), so from here the idea of blockout comes, I have to invent a space thatll

make sure the metal wont touch the ridge on some certain areas , blockout is also

not only to make free space later but also to know where my clasps will be by

adding a layer of wax and making a step . Because If I draw clasps location with a

pencil itll disappear when turning the secondary cast into refractory. Keep in mind

were still talking about the secondary cast, and this blockouts will turn into

refractory cast.

Parallel blockout (figure 13) : its a blockout that isparallel to the path of insertion, as you look in thepicture what about the area down? You can notice

that its undercut. Can metal go there? No, so I have to block this out so Ill

add wax below the guide plane and get my wax chisel and adapt it . Later on

when we have metal thisll be a space itll allow the partial denture metal

framework to go in and out without any interfering. Hence: it is like the

plaster blackout that we did for the acrylic partial denture. (Please note that

we do this on the dental surveyor) .

Shaped/Formed blockout (figure 14) : You have tounderstand that secondary cast or the master cast is

not the final denture , were just drawing designs on it so that I can transfer it

into another cast which is the refractory cast ( well talk about this shortly ) .

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Now I make the shaped blockout because its the only way I can send the

clasp position from the secondary cast to refractory cast , when I finish the

process of blockout and relief Ill make an impression for the secondary cast

and a step where the shaped blockout was will appear on the refractory cast ;

hence the name shape blackout comes from giving the shape of the step tothe refractory cast.

Arbitrary blackout (figue 15: notice this is aBuccal view): is just to make the copying

process easier. As I said in point 2 , the

secondary cast will be copied and turned into a refractory cast , I add the

Arbitrary blockout wax to locations that are not included in the design of thecobalt chromium denture they dont have any importance . I put the wax in

them because when I want to copy it , I dont want the impression material

to go into big undercuts and makes the process harder , but rather block it

now . Remember this type is not a part of the cobalt chromium design unlike

the first two types.

After weve finished the blockout we still have to do relief:

Relief wax: from its name you might wonder whats relief

wax relief used for? the answer as we said in the theory is

that the relief wax is used on the edentulous ridge is to

make a space between the metal and the edentulous ridge

in order when I make the metal framework partial denture

the frameworks ( e.g. Lattice ) will be raised a little above

.

There is another type of relief wax that is used on the sensitive areas (e.g. palate on

the upper denture) , so we use this type of wax in order for the metal framework

not to press the ginigiva . So relief wax either used to make room for acrylic under

the metal or relaxing the soft tissues ( gingiva ) .

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Looking at the relief wax (figure 16 ), you can see we made window(s) why did we

make that window? Its for stoppers. When I make the final metal partial denture

this stopper will become positive, the lattice work (or meshwork) will be above the

ridge everywhere expects here in the small window.

We do this because when I start adding the acrylic and I do packing the metal can

sometimes bend, so we make small leg (stopper) so that the metal cant move. This

is the same as when we made special tray with stopper on the inside.

So lets recap what we learned so far, a surveyor is a device that is used to

determine the correct path of insertions in partial removal prosthesis, so that we

disturbed the parallel surfaces anterio-posteriorly and retentive surfaces laterally,

taking into consideration interferences and esthetics.

We do surveying twice, one for the diagnostic cast to determine what the design

will be doing, and we survey again to verify and blockout on the master cast.

The instruments we used are the analyzing rod, carbon marker and the undercut

gauges. And in the master cast to remove the excess wax we used wax chisel.In the

lab you didnt use the chisel but you used the normal carver, this is not true but for

the educational purposes we did so.

Now that we have our secondary cast done, we can move to the next step which is

copying it to make the refractory cast, so far up to this point I didnt explain to you

why I copy the cast to refractory, so Ill first explain the idea behind duplicating

the cast and then well talk about the refractory cast preparation.

As the title says, why didnt I after making the blockout and relief adapted the

metal on the secondary cast? As you remember last semester when we were

working on the complete dentures we didnt have such a thing as refractory cast ,

because all of what I had to do was simply adapt acryl and then exposed it to the


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proper curing temperature of acrylic , which was great because the secondary cast

withstand the temperature and didnt break .

The case in partial cobalt chromium denture is different here we have metal , the

curing temperature for metal is very high ( 950- ~1050) , the secondary cast canttolerate that temperature and itll break , so what well do is that well take the

secondary cast and make an impression for and turn it into refractory cast .

So in short the advantages we had of duplicating this cast is:

Heat resistance : this was not achieved by the stone cast , metal curingoccurs in 950-1050

Blockout and Releif came achievable this is cant be achieved by the stonecast , because even if I add wax to make this space the entire cast will break

because of the high temperature of the metal .

What refractory cast is made of? Its made of a material

called Phosphate Bonded Investment Material will pour

this material on an impression inside a mold to form this

cast. This can tolerate high temperatures hence the namerefractory. (Figure 17).

What is the best suitable material to make the impression for the refractory cast?

(What is the material that we use to make an impression with after weve done

blockout and relief on the secondary cast , and then pour it using the phosphate

bonded investment material turning it into a refractory cast ? ) Ill use a

duplicating material, the choice I have is either Agar-agar or Addition silicon .

If we used Agar-agar , we have a container and well pour the agar-agar , it has

water content so when we heat it , itll become liquid . So well first mix it in a

special machine which is called and then heat and pour it inside the mold. Agar-

agar is humidity sensitive so itll become fragile if left.

The other option is addition silicon but its more expansive yet very accurate,

chemically set (cant be used more that once) .

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By now I think you fully understand that refractory

cast and the design we made on it will turn into thefinal shape of the metal including the clasps,

connectors and arms. But looking at the fine details of

the clasps and the other different components of the

chromium cast you might think how did we manage to mold

the metal in such an elegant way? What we actually do is

that we add a type of wax called pattern wax (figure 18 , 19

) ; the companies supply many different shapes and forms

for it, for many different components of the cast. Later onthis wax will be replaced by the metal and takes its shape.

So pattern wax is an essential step because without it itd be

very hard to mold the metal, and its done on the refractory

cast.

Weve come to the core step of making partial cobalt chromium dentures; metal is

the main charastics for partial dentures and now well learn how we mold it.

Well put the refractory cast in a mold and well pour another

layer of investment material ending with a shape like a

sandwich ( figure 20 next page ) , what well do next is were

going to put the cast inside a furnace and expose it to heat ,

when exposing it to heat the pattern wax will melt away , and

what will take its space now ? Nothing I have an empty tunnel

that is ready for the metal to get inside it and take its shape (an application of lost

wax technique).

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After that well use one of these cast alloys

(figure 21: arrow) inside the mold, well put it inside a

machine called casting machine(figure 22) that its

principal depends on the centrifugal force.

After we finished from the pouring process, well have

something like this (figure 23) well break it off and then

clean the metal, the cleaning process of metal is done by

using a special machine called Sand Blaster (figure 24 ),

this machine will inject aluminum oxide( last semester

we talked about some similar material ) slowly at the

metal and thus cleaning it. And the final polishing is

done by another machine which is called Electro-

Polishing; its simply a batch of water where we put the

metal and then electrical current will give the polish

shine appearance. Notice that our refractory cast is

destroyed while doing this process.

After I finished molding the metal how do I make sure

that my work is good ? Well simply you have to try the metal on the secondary cast

(remember refractory is destroyed) if it fits correctly move on, if not youll have to

make the entire work again.

The only part reaming in the journey of making the denture is the acrylic part,

which is the conventional method for preparing acrylic ( flasking , dewaxing ,

packing and curing ) . Read it from laboratory 3

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This page is a summary for the entire process:

patient comes in we make primary impression, pour up the primary impression and

youre going to end up with primary cast (diagnostic cast ) which we are going to

study to see which modifications we need to make to the patient before starting to

design this and in order to do this we have to use the dental surveyor , if there

were any modifications Ill go back to patient and make them ( remove a tooth ,

filling ..etc) , secondary impression ( custom rectangular trays ) , pouring the

impression well get a master cast ( secondary cast ) : secondary cast differs from

the primary is that we have rests and guide planes and modifications , we do

blackout and relief , duplicate using either agar-agar and addition silicon , using the

agar-agar or addition silicon impression we pour phosphate bonded investment

material into a mold to make the refractory cast , we draw our design on the

refractory cast ( we use the pattern waxes provided by the companies ) , then well

put another layer of investment inside a mold , well put the refractory cast inside a

furnace ; the wax now will melt creating tunnels for the later metal , after that Ill

use the casting machine to cast the metal , after that Ill add the wax and teeth ,

after that Ill do another lost wax process ( conventional flasking , dewaxing ,packing and curing for the acryl ) , finally Ill end up with a metal framework plus

acrylic and teeth .

The number of deaths in Syria to the date of writing this script is 11297people,

from that number802are kids, and676are women. My prayers go to you mycountry.

Done by : Hasan Alblwa and Osama Yousef .

Documents

Labratory 4