Download - 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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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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-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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-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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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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*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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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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-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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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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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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*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.