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A,

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Very important note: you should watch the video on the CD the CR gave

.There was a video about the procedures of fabrication of cobalt-

chromium partial denture.

So, we are going to do a quick review of the laboratory steps and then

you'll go the central lab to see the Casting machine and casting furnace.

-What are the steps for the fabrication of Co-Cr PD?

LaboratoryClinic

Primary cast (dental stone III)Primary impression

We do initial surveying and design

We fabricate custom tray

Tooth \mouth preparation

a-Secondary cast (Master )

b-Verification surveying and final design

c-Block out and relief

d-Duplicate master cast in Agar-Agar or

Addition silicon

- you get the REFRACTORY CAST made

of Phosphate bounded investment

material.

e-Fabricate wax pattern

f-Complete the investment process

:sprout and investment

g-Casting

h-De-investment and cut sprouts

i-Finishing and polishing metal

framework

Secondary impression (Alginate)

Mounting and teeth settingJaw Relationship record and Metal

framework try in

Wax try-in

Acrylic processing

Finishing and polishing

Insertion

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1- The initial survey and design are important to formulate the

treatment plan.

2- Custom trays are not always necessary, they are necessary for

distal extensions and marginal edentulous areas, short

bounded saddle areas do not require custom tray.3- Mouth preparation in general involving extraction, tooth rest,

guide planes (all discussed earlier). If we are to do border

molding we do border molding

4- The secondary impression is done using Alginate ,we need an

elastomeric material we can't use a rigid material unless

there's an edentulous area ,such as addition silicon

,dimensional silicon or Polysulphides.

5- Points from (e-f) are done on the refractory cast.

6-

We put a wax rim on the metal framework and we do Jaw

relationship record in the same visit.

7- The acrylic processing is done on the master cast, mounting

and teeth setting is on the master cast, the finish and polish is

on the master cast; so the master cast after we do duplication

we keep it.

Very quickly:

Edentulous patient, we take primary impression with stock trays

using alginate for primary impression, we can use utility wax to

modify alginate trays (different from edentulous ones). We pour

it we end up with primary cast made out of stone, we do initial

surveying and design, we use it have an idea about the treatment

procedure. In the lab, you do custom tray (two layers of spacers

near the teeth and one in the edentulous area).

We make final impression and we end up with final cast (mastercast) type 3 dental stone or type 4 di-stone.

the final cast will have rest and guide planes on them ; this cast

will have the right tissue contour to fit the PD ; the master cast I

can't pour the metal ,because won't stand the temp. in

Centigrade's it's very high necessary to do the casting ; So I do

block out and relief.

shaped block out for the clasps

arbitrary block out to simplify the duplication

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Relief to make room for the acrylic, which decides where

the internal finish line will be.

In order to duplicate we need a special material , we usually usea silicon type material , additional silicon ,it's flow able and we

use special molds to duplicate ; we end up with a mold which is

a copy of the cast ;or we can use Agar Agar (thermoplastic

setting ,while silicon chemical set).

*Agar Agar is thermoplastic and hydrocolloid; water sensitive and can be

re-used in the lab. While silicon can't be reused to do duplication but can

be report.

** Distinction: silicon can be used to make only one copy of one cast

but I can pour it many times. Agar Agar I can be use to make copy of

many casts I can mount it and use it many times but I can only pour it

ones.

Now I have the mold, copy of secondary cast, I am going to pour it in a

material stronger than master cast, which can withstand hightemperatures.

I will use investments materials (becausethe Co-Cr-Mo (Cobalt-

Chromium-Molybdenum)alloy has very high melting temperature, so I

use phosphate-bounded investment. I mix it up , I have special liquid

with high silica content ; I pour it up then sets and dries ;when I remove

it ,sometimes the surface is not very hard,

There is a thin layer of something that shines on the surface; what I do

to remove it? I have a hot bath of bees wax, when it's hot I soak cast in it

and I take it out again, the surface will absorb some of the bees wax , it

will make the surface harder ;technically can't scratch the cast even by

mistake during wax patter , so I do hardening of my refractory cast with

bees wax.

On the master cast, what am I going to do? I want to make the metal

framework, so what I do is the wax pattern, there are special waxpatterns fabricated by companies so we have:

http://www.springerlink.com/index/CK37284X6N60W584.pdfhttp://www.springerlink.com/index/CK37284X6N60W584.pdfhttp://www.springerlink.com/index/CK37284X6N60W584.pdfhttp://www.springerlink.com/index/CK37284X6N60W584.pdf

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Nice even thickness.

Steeled.

Sheet wax, for the palate.

Mesh work -latex work.

I-bar ginigivally approaching clasp -circumferential clasps.

Inlay wax to make the rest and meet everything together.

** In wax pattern we make the external finish line; the internal finish

line formed by the relief.

Everything that is wax will turn into metal.

The problem is , this is not like acrylic , I can't just make another mold

and open it again , so what I do is similar to acrylic fabrication ,I need

to make lost wax process but the technique which I use is the sprout

technique ; instead of making two-part mold I make one part mold

;what I do is I place a sprout ( a conical sprout) ;the objective of

sprout when it's completely enclosed in investment is to allow the

wax to escape ,to evaporate and burn out and for the metal to go

inside to fill the mold .

So how do I do the mold? I put it in a mold former, I seal it ,I mix myinvestment in a similar technique which I used with stone (in a

vacuum mixer )we pour it and we end up with something like a block

,but when I remove the funnel ,there'll be a space , then I put it in the

furnace at high temperature approximately 1000 C . For two things:

setting expansions, from chemical reaction of setting and

thermal expansion , because when I increase the temp it will

became larger

And when my mold is hot, the material (metal) will flow mucheasier, if my mold is cold and I pour metal inside it, it will cool

down before it goes to the details. So I warm it to allow

expansion to compensate for the contraction of metal when it

cools * I heat it up to allow metal to flow inside it.

I need the machines to do casting, I need a machine to heat up my

mold and another to melt the metal and pour the metal inside themold.

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** One is called the "CASTIING MACHINEand the other is "CASTING

FURNICE.

So I put the mold in the oven; I leave it for several hours, theresaspecial cycle to allow the wax to evaporate without destroying the

mold. When it reaches the right temperature, I take the mold out of

the oven and I put inside the casting machine, in the casting machine

either using a flame or using an electric coil. The metal inside this

container is heated and becomes liquid metal; I need to get this liquid

metal inside the mold (I don't hold it by hand, theresa special

machine). there are two basics techniques :

1- Centrifugal casting force: it starts rotating very fast so that the

metal inside will flow outside the opening of the machine and it'll go

inside the mold, inside the mold because its rotatingso fast, the

metal goes in the details of the mold .

2- vacuum casting ,(the air force will press on the metal pushing it

inside; the air is hot air ), of course the machine is different from

centrifugal.

The temperature source can be either flame or induction casting

(electric coil)

// The casting process is on the CD the Cr gave.\\

After casting we'll move the mold from the casting machine and let it

cool down, it usually changes in color, it becomes white. Inside it will be

all metal, so I'll tap it and inside I'll find something to indicate where the

sprout former were; I'll get the sprout and the metal casting. I will use a

special machine called "SAND BLASTER,it is a very simple design, it has

a tube with an opening at the top, with air pressure and Aluminum

Oxide Abrasive container.

The air will bring the Aluminum Oxide inside the tube and out of the tip,

it'll start shooting the aluminum oxide sand and we will use it to abrade.

the investment will become clean; if I use a bur maybe I'll hit the clasp or

destroy the machine ,so the way I'll get rid of this investment is I use the

sand blaster (it's like sand storm that will remove this investment which

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become soft after heating ).

Also the Aluminum oxide , if you take a look at casting ,because we need

the temp around 1000C , look at the surface of the metal it isntshiny ,sothe sand blasting of aluminum oxide will remove this strong oxide ,what

oxide? What type? Cr oxide, we said we place Cr specifically for a

purpose to protect co from other materials, so Cr oxide is more reactive

than other materials, this is called passivity .This is why doesnt burn up

in high temp ; the Cr oxide is protecting the rest of the alloy.

When we remove the layer of Cr. Oxide so we have a nice clean layer we

end with something with less investment; then I cut the sprouts. .

Carbide Lathe bits , carbide is a very strong material ,( it's silicon carbide)

;we have to be very careful when cutting sprouts and working on the

metal not to overheat the metal ; if you over heat the metal you will

change the molecular structure and grain structure of cast alloy ,it will

become softer, like annealing ,usually we cool it in cold water so it

doesn't overheat. So we trim this, we cut all of the sprouts we end up

with a bottom, this excess we are allowed to reuse it in about 1/3 one

metal to 2/3 two metals, Why? Because there are trace elements less

than 1%, which are essential for smooth casting process; so if I am going

to reuse this metal I need to give new trace elements.

There are bubbles on the surface? Because when I cooled my

investment I wasn't too careful , you see where sprouts where attached

you can see a very big piece of metal it doesn't look very nice ;it still

requires very much work, I we spend very long time waxing it up I won't

get a lot of problems. I start up using a carbide lathe bits. After sand

blasting you get rid of flasks and excess, then I'll start suing metal

stones...

++ I have a variety of trimming stones use to do polishing of metal

framework, they are a bit rough but they are not very expensive we can

use them to do initial finishing of the metal; we have green stones and

pink stones, they come in a variety of shapes, sizes.

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After doing that we can use special tungsten carbide bur with high-

speed hand piece, these are reusable, they last for a long period of time

but they are more expensive used for final finishing and polishing .come

in different shapes and sizes.

** Do I do any trimming of fitting surface? I can only sand blast but can't

do any finishing or polishing, I never touch this surface unless there's a

defect. I just work on the polished surface.

After that I use a variety of rubbery polishing tips, come in variety of

shapes and sizes, show bands and breeds. Ban material is what we

remove, the material that does the polishing is not the rubber as much

as the material impregnated within the rubber. we have a rouge

material and a green material ranging from medium green to very light

one .Essentially we go through the stages of polishing ,step by step to

get a brighter and brighter shine and the shine increases as the surface

layer becomes more regular and shinier . the metal is better to polish

than acrylic ,remember when in acrylic we used the cloth wheel , cotton

wheel ;with the cobalt chromium we can use something at higher speed

,so the brush of plastic bristles or natural bristles. very strong ones, weput a polishing material like Tripoli or rouge.

usually we don't do polishing on hand pieces, we do it on the trimming

wheel ,it has a larger brush .using to larger brushes at relatively high

speed we need to hold them, they'll cover a larger area but for details

we can use the smaller brush ;still it will not give us the highest shine

possible. we need a higher shine and we need to polish the internalsurface ; in order to do this we need to do something called "ELECTRO

POLISHING".

Its a container with concentrated acid solution, we put the metal

framework inside and there's an electric current and raw copper plates

;there'll be an electric current which will remove metal ions from the

surface of the metal . it will remove one or two layers of molecules from

the internal surface and external one and by removing those layers it will

give a high shine to the Co-Cr so it looks like it's nicely polished. The final

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shine needs an electric current to be achieved. If we overdo it we

destroy the framework, it dissolves. usually we leave for 5-15 min

depending on the size of metal framework.

The bead line at the edge is very shallow, it digs into the tissue but it

won't irritate.

At this stage :

*the refractory cast is destroyed during processing but I still have the

master cast with the block out.

I put the master cast in hot water, in a de-waxer, to remove the wax on

it and I end up with my master cast without any wax on it.

The metal framework is ready to fit master cast and I put wax rim to do

jaw relationship record and fitting in to the clinic, go back to the lab and

do mounting and then set teeth in wax.

Notice: The wax is flush with the top surface of the metal, the external

finish line, and in the inside, I'll place wax in the inside so it's flush with

the internal finish line. The internal finish line goes all the way adjacentto the tooth; you can see there's metal between the guide plane and

acrylic.

So in the final denture, the acrylic and the metal will be flush; and

between the guide plane and the metal is flush.

How do I get the acrylic on the metal? I need a second lost wax

technique, I have two choices for the process:

1- Conventional technique , essentially set the teeth ,place the metal

,put in flask , cover everything metal with plaster and stone ,just

expose the wax and teeth ,then pour a second layer which will go

over wax and teeth ,only the meshwork for acrylic will remain

exposed(as acrylic PD I want to cover everything except teeth ). so

after painting with Cold Mold seal-a variety of Foil substitute , I

mix my acrylic and at dough stage and I close my sandwich , I wait

for curing and I open the flask and get my acrylic denture. I do the

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finishing and polishing ,then I'll be ready to try in the prosthesis .

2- The second technique doesn't involve processing. we make a

quick mold ,not a flask mold; using the normal technique from

adding teeth , I add wax over the meshwork over and below, I dowax rim and I get the teeth and set them to seal them (done

mounted on articulator) ,as we did previously ,we do the

smoothing and polishing and I have a wax PD .

Quick mold: I make an index, I can use putty or plaster just to know

where teeth are.

Place separating agent on stone ,like Vaseline; I can make a mold in two

ways :

1- I can use putty (additional silicon putty, there are four

consistencies of silicon: light, medium, heavy, and putty. putty is

most usually mixed by hand). I place catalyst mix in it until it

becomes homogenous and then I will make an index on what I

have on facial aspect.

Essentially this is going to relate the teeth to the index so I canremove the index and teeth and wash out the wax, instead of doing

flasking, packing and de-waxing ; I just do this quick mold. how am I

going to get acrylic in ? I just place the acrylic by sprinkling method,

soltem powder technique.

I don't like to use putty because it's difficult to control.

2-

the other way to do this , which is probably cheaper and more

comfortable , is to make plaster index. we make the index out of

plaster (I'm going to mix slurry water- Terra Alba.)

So either I use putty or plaster, most of people use plaster because its

less expensive and easier to use. Usually the setting time for this

material is about 4-5 min in the mouth , since it's colder outside the

mouth it'll take a little bit longer. After setting, I remove it ,then I'll have

an index on it , it's like the second mold in flasking process ,like the top

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part of the flask .So I take teeth and put them in the index and wash

away the wax.

The doctor used plaster rather than putty.

So we put plaster on the facial surface; wait it to set; then I remove the

index, remove the wax, and attach the teeth to the index and wash away

the wax. Then we are going to put Tin foil substitute on the cast.

We use slurry water to make the plaster set faster. Two other methods

make setting faster: warming and mixing for a longer duration of time

and faster (being faster) (the more crystals you create at the begging the

faster it'll set).

The type of acrylic I'm going to add is not the conventional one; I can

add it with a brush as if I'm almost painting it on.

So I can add acrylic incrementally in 3 ways:

1- Brush it on

2- Sprinkle it on

3-

Mixing ,like heat cured acrylic

*Since the acrylic used is not heat cured but cold cured, so one method

allows me to add acrylic to it. it's called : bead brushing (acrylic

powder and cold cure acrylic called monomer), you pick up small beads

of the liquid ,touch the brush, touch the powder with the wet brush and

fix on the bead of acrylic and then you can paint it on and add it

incrementally to the acrylic.

This type of acrylic has the same stages of heat-cured acrylic, but herewe are using it in wet-sandy stage. To keep the brush clean I touch the

monomer, and I shape it to the shape I want. This is one technique to

make small additions.

*the other way is the sprinkle technique: it's a bit different, it's like a

salten paper. so the monomer is like the salt , I drop some monomer on

the surface ,which I'm going to add to ,and then I sprinkle on some

powder incrementally, I add some liquid and I add some powder layer

by layer until I get the final shape which I want .(this is the most

common technique to make orthodontic retainers).

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*The third technique is to mix it. I have the monomer: I add powder to

the monomer and it'll go through all the stages we talked about: wet-

sandy, sticky, I take it out when it reaches the dough stage fixed it and

do packing. We wait it until it becomes hard. Hard enough to remove it

without fracture.

So we remove the teeth; remove the wax and we use our index to

proceed back where we were. I push each tooth in its specific location;

on the cast I need to place a separating agent.

I mix acrylic, pour it and squeeze my index to create my PD .I remove the

excess.

Cold cured acrylic if I leave it exposed to air, not closed flask, the

monomer will evaporate, so to accelerate the set and to prevent the

monomer we put it inside a pressure bar at temp of 35-40 C and under

at least 2-2.5 atmospheric pressure .Like the one we used last semester.

2.5 Bar is enough to prevent the monomer from evaporating; normally

we wait about half an hour.

Cold cure even if you place it in pressure bar at temp of 35 C ,you usually

end up with some remnant monomer. so when you put it in the

patient's mouth it can irritate the mucosa ;also because packing is done

with hands the position of teeth is not going to be that accurate so voids

accumulate inside the fitting surface.

-We use 2.5 bar to make sure there are no voids in it but the problem

with cold cure acrylic is even if we do this there maybe some remnant

monomer, not like heat-cured acrylic in flask... Usually we wait for at

least half an hour but the initial set star after 5 minutes.

** When there's remaining monomer, this means it will leave some

collagen fibers from the denture base which will pick up stains and make

the material weaker.

So we get our PD and the index because teeth on the index, using putty,

might slightly move, I can put some wax to fix it. In this procedure

there's more polishing to do than packing method .I trim away theexcess and do polishing and I end up with an acrylic base .

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**So I can add acrylic using heat cured acrylic or cold cured acrylic;

Either using a brush technique, bead crush or I sprinkle it on or I can mix

it and pour it .

And you'll get the final Prosthesis.

THEN THE DOCTOR TOOK GROUPS TO THE CENTRAL LAB TO SHOW THE

CASTING MACHINE.

Done By: Sara Ibdiwi.