prostho final lec 1

8/23/2019 Prostho Final Lec 1

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Dr. Louie Hammad

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Over view of all laboratory procedure during

RPD production

-The laboratory procedures for metal frame work PD are very

similar to the procedure of acrylic PD with one major significant

difference which is the metal frame work!

*The steps for metal frame work fabrication:

1) Like every dental indirect procedure we need dental

impression, we start up with initial impression, we make it with

Alginate with stock try.

2) From initial impression we

pour our initial cast which is

made of:

*edentulousdental plaster.

*dentate..dental stone.

3) We make our custom try, in

complete denture we place one

layer of wax spacer, But in RPD

we place one layer over the

edentulous area in the palate

and two layer over the teethbecause we want it thicker

we want a greater thickness

of the impression material so

it doesn't tear so it's more

flexible when it comes out of

the undercut around the teeth,

so we have 2 layer of spacer

around the teeth.


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Dr. Louie Hammad

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-With the surveyor we draw our

initial design ,when we draw the

design we decide where the rest,

the GP are going to be, where themissing teeth are, and we design

what the frame work will going to

look like depending on the

surveying .but this is our initial

design!

- We still haven't apply it in the

patient's mouth ,So we took the

custom tray and we make our

final impression .After we

finish our final impression we

get our secondary cast


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Dr. Louie Hammad

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4) We make our Final impression with the custom tray and we

use any kind of elastomeric material (addition silicon,

condensation silicon, polyether, and sometime polysulfide).

- You can also use Alginate

which we usually use! Because

it's economical and it's easy to

work with but silicon more

accurate.

5) We pour our secondary cast

with type three or four stone

then we do surveying to know

the shape of the teeth and the

favorable &unfavorable undercut, is the way we decide how to

insert and remove the denture so there is good retention and

stability of the denture.

*What is the difference between the final cast (secondary cast,master cast or the working cast) and the primary cast?

6) Now we have the working cast, in the lab we have a problem

this cast is made of gypsum... Gypsum above a certain

temperature will break down!

-We said the gypsum is good when we want to make complete

acrylic denture, acrylic needs just to melt the wax in the boiling

water temperature and this temp will not destroy the cast.

-The basic procedures in the Dentistry is the Lost wax

technique, all the indirect procedure follow it, I shape

something out of wax, I invest it, make a mold out of it, I melt

the wax and then I replace the wax with acrylic or metal!


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Dr. Louie Hammad

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-If I want to replace it with acrylic all I need to do is to melt the

wax and put the acrylic inside, I need temp. Maximum 95-100C,

the stone could stand this temp.

-The problem is that Co-Cr alloy has melting temp. Which

exceed 1000C, titanium melting temp 2800C and some noble

alloys goes up and down so if you put a stone cast in such high

temp. It will turn into powder. And because of this we make a

copy of our stone cast with a harder and more heat resistance

material called refractory material.

7) We duplicate our cast with one of the duplicating material

which is either:

A) Agar-Agar:

-We use Agar- Agar in the lab

while gypsum we use it in the

clinic

- Agar-Agar is thermoplasticmaterial if we heat it will

become a liquid, jell, solid. It

can be jell like or liquid

according to the temperature

-While in the gypsum when we

mix the powder and the water

we have a chemical mix once itset we can't re-use it again, but

agar- agar can be re-used!

B) Addition silicon:

-We can re-pour the material

(the mold) but not re-use it.

-It's more expensive than agar-agar!


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Dr. Louie Hammad

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-So we make a copy, we had our final cast with the preparation

made out of stone

- now we have another cast which we DOUPLICATE it with the

duplicating material then we make our refractory cast" heat

resistance cast" that is made of investment material and the type

of investment which we use it's called phosphate bounded

investment or we can use sulfate bounded investment but it's

weaker!

8) Now we have two cast look exactly the same one is made out

of stone and the other made of phosphate bounded investment,

this cast additional to that it's a made of heat resistance material

also there is another advantage of it, in the master cast where

ever we want to block out we block out with wax where ever we

want a clasp we put a step, you placed arbitrary block -out,parallel ,shaped block out and reliefwaxwe put the wax in


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Dr. Louie Hammad

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the cast , all the wax that was in the secondary cast will also be

copied and will turns into investment , thats mean the stone +

wax block out turns into investment!

9) Now we have an investment cast, we don't go back to the

patient! We continue the procedure in the investment cast

-we started to make the metal frame

work and everything you shaped it

out of wax will turns into metal , So

you shape ,you place the major

connector ,the minor connector the

lattice ,the mesh work ,the clasps andeverything you need to and they

came in different color but in the end

all of them will turn into metal !

-We complete the mold by pouring more investment in the top

of it, we sprue, now I have investment mold like the flask of the

complete denture but it's made of investment not from stone.

-to make the refractory cast

stronger we apply a layer

above it ,bee's waxof hot

this wax make a very thin

layer on the surface of the

-anticast so it become

scratch and become very

strong on the surface!


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Dr. Louie Hammad

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Dr. Louie Hammad

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*Remember in the Co-Cr

everything should be on

the survey line or above it

except the tip of the

retentive arm of the clasp!


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Dr. Louie Hammad

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Dr. Louie Hammad

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Dr. Louie Hammad

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10) I get rid of the wax in the Co-Cr by putting the investment in

an oven and I raise the temp. Almost to 1000C, the wax not only

will become liquid but it will burn out!

-I end up with a very hot empty

mold, then we melt the Co-Cr alloy

and we pour it inside the mold .it

not that simple we have to raise the

Co-Cr temp. In a special casting

machine, so we place it and we

pour the metal inside the mold, it

will go into all the spaces that we

create it for the wax

-We will get a metal piece which

we will finish and polish just like

we did in the complete denture.

We have a metal frame work

what happened to the refractorycast? The refractory cast was

destroyed after we poured the

metal because of the high

temperature. And now our

reference if we want to change

anything we go back to our

secondary stone cast!


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Dr. Louie Hammad

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-This is dental frame work which as we know is made of Co-Cr

and the Cr is here for the passivity, it protects the cobalt from

oxidation, so here in the picture the black color is the cr-oxide

I have to clean it and finish and polish it using a nuzzle ( )

sandblasting machine, there is a fine sand which is aluminum

oxide particles 50-250 micron in diameter, it's very strong sand


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Dr. Louie Hammad

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and it will remove the investment which is attached to the frame

work

*on the secondary cast I use my metal frame work almost like

base plate I use it to do jaw relation record, I use it to set the

teeth , to make the waxing and the festooning in the area where

will be going teeth and now I process the metal frame work like

I do to the acrylic denture.

Then we use very strong metal

discs to do the final finishing,

and we cut away the sprues, we

do final polishing using

"electro polishing machine"

(which contain an acid and

electrical current which will go

throw the frame work and this

current will cause part of the

Co-Cr to be removed and leave

it very shiny) this machine will

remove one molecular layer

from the surface of the metal

and that will give the final

polishing and shine!


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Dr. Louie Hammad

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11) I will do for all the teeth and wax flasking, baking, curing

and I will add teeth to the metal frame work.


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Dr. Louie Hammad

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Dr. Louie Hammad

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*Additional notes:

-There are several methods of how we can pour the metal inside

the mold but the most common two methods are:

1) The centrifugal casting:

The mold will be on an arm and when we press the button the

metal will be poured inside the mold, the machine will make the

mold rotate (centrifuged) in a very high speed and because of

the high weight of the metal, this will make the metal get into

the details of the mold because it's too hot!

2) Suction:

Very strong suction or vacuum will cause the metal to be sucked

into the mold.

*During the fabrication procedure of cr-co partial denture, we

have two lost wax process:

1) For the metal frame work: which I made it on the refractory

cast2) To make the acrylic and the teeth which I used it with the

master cast.

prostho final lec 1

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