Prosthdontics Lab 5

7/28/2019 Prosthdontics Lab 5

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Prosthdontics laboratory 5.

Dony by: Anwar Druah & Osama yousef.

Part 1: Osama Yousef .

On the first part were going to talk about a few things that have been discussed in

laboratory 4 .

As you already know we have a diagnostic cast where we did the digonastic survey

on it , and then we said that if there is teeth modifications we have to do them first

in the patient mouth and then take an impression to create the master cast , so well

talk about these modifications in part 1 of the laboratory . Well also talk about

blockout and relief.

Well also talk about verification survey on the master cast.

This steps as we said in laboratory 4 is done on the patient mouth, after we do the

necessarypreparations well make an impression of it and create the master cast.

Please note we also talked about teeth modifications which is something different

than preparation, on modifications it involves ( filling , extraction etc ) but

preparation involves the following :


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After Ive done my diagnostic survey on the surveyor and marked my parallel

guide planes , its now time to make them inside the patient mouth . And alwaysremember this : you always have to start with the guide planes and then move to

making rests because guide planes can sometimes ruin the rests for us .

With the diamond bur were going to make occlusal rests for our case in the

laboratory the design should be like this:

mesial of upper first premolar just above the cingulum of upper left canine with a semi-linuar mesial of upper left second molar

Note that the first two are gingivally approaching and the last one is occlusally

approaching clasps. Also note that all of the rests are done on the lingual side of

the teeth.

Youll have to know the following piece of information although we havent yet

taken it in the theory : we have distal extension and we have bounded area . In the

bounded area we usually put the rests near the edentulous area and in the distal

extension area we usually put the rests away from the edentulous area, and this

also is going to be discussed in the theory part.

As we said , we always start with the guide planes and then move to make the rests

, because sometimes while trimming teeth to make the guide planes the rests might

be damaged ( the damage occur mainly on the molar rests , premolar and canine

wont change a lot ) .

In the lab well use a lacron-curver as a high speed bur, because the lab cant

supply each one of us with a high speed bur. Laterin the theory youll learn that

we have different types of burs; the inverted con bur and flame shaped bur are two

examples we can use to prepare the canine cingulum rest.


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As you already know by now , this type of survey is done on the master cast . So

we put the master cast on the surveyor ( in our laboratory design its zero tilt , ant-post guide planes are parallel and lateral undercuts are balanced between right and

left , if all of the these are checked then the position is correct ) .

After making sure we have the right path of insertion (which in the case of our lab

case is zero tilt ) . Now well use the carbon marker and we have to make sure that

its sharp , and well mark on the teeth lingually , you can mark it on the anterior if

you want to be more precise when doing the arbritery blockout .

And this marker marks the maximum convexity for each tooth, if there are parallelsurfaces , instead of a line Ill see it in area .

Once I prepared my maximum convexity and path of insertion Ill then need to

know where exactly I put my clasps ( remember that we havent yet take clasps

design so in the lab the dr is going to help us and cheat and give us where the

clasps are which are : the premolar and canine and the molar but in reality this is

not the case , youll have to do many things before you know where they are

exactly ) .

And as you know in the canine were going to use gingivaly approaching ( 0.5 mm

undercut ) but the molar has occlusaly approaching clasp so its a shallow one (

0.25 mm undercut ) , and I have three types of the undercut gauges ( 0.25 , 0.5 and

0.75 ) .

How do I use the undercut gauges?

As you already know I bring my undercut gauge to touch the maximum convexity

of the tooth ( marked earlier ) then I rise until it touches Ill know that this point

has a special depth ( depending on the undercut gauge ) . Remember we do this

inside the surveyor.

Where should the clasps be on the molar ? here we start above the survey line and

only the tip goes under the survey line ( not like the c-class in wrought wire ) . And

for the canine and premolars , if we divide the teeth into a mesial and distal halves


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I want the tip of my clasp to be at the junction (between the mesail and distal ) or

slightly in front .

Now were ready to make the blockout , we said there is different kinds of

blockouts and they are : A) parallel block ( all the unfavorable undercuts usually

on the lingual and proximal surfaces of the teeth ) . B) Shape blockout ( the

blockout that indicates where the claps should go , we see where the clasp location

is going to be in and then Ill make a step on the facial , and another step on the

lingual and this one should be at the survey line exactly.

Considering facial clasps as you already know they have reciprocating arm fromthe lingual so Ill need a step on the lingual that goes below the survey line and it

should be exactly on the survey line ( the reciprocating arm and the step ) .

Considering the canine case , its gingivaly approaching so I have to create some

parallel blockout underneath the arm that goes out but where exactly should the

shape blockout be ? above or bottom ? it should be from above , because the arm is

coming from below to the top , so logically the shape blockout should be from the

top because I want the end of the clasp ( which is the top ) , so Ill create the

blockout there by creating a step ( by trimming some of the wax ) on the roof ( orthe top part ) . Ill also make a window to show me where the end of the clasp is

and this window will be found on the refractory cast too.

The shape blockout will be on the Facial and lingual of the molar , facial window

on the canine ( the top part ) , facial window on the premolar . Notice that we

didnt put shape blockout on lingual surface of both canine and premolar , this is

because of the way the lab general design of the denture . But in reality there are

many designs and ways to put the blockout .

After that well make arbitery block ( using utility wax ) and were going to place

it on the deep undercuts that might ruin my duplicating process.

After that Ill do the relief , and its placed to make room for the acrylic under the

metal base , whats significant of relief wax ? A) thickness is 1 mm B) when its

finished on a distal extension the metal could bend down ( because there is nothing


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under it ) so Ill make a window there thatll prevent the metal from going down (

this windows is either called tissue stop or cast stop ) C)Ill usually put it at the end

about 2/3 the distance from the last standing tooth until the hummular notch in the

upper , and in the lower I put it 2/3 from the last standing tooth until the

reteromolar pad .

What else is important in the relief wax ? I dont really care if the relief wax

reached the depth of the sulcus or not , but it should cover the area where the metal

goes and the metal goes to the crest of the ridge any maybe 2mm but it doesnt go

out to the full depth of suclus , so block the full depth of suclus makes no

difference. Well talk about this in much details in the theory , but for the

laboratory its 1 cm . you have to go down 1 cm .

But everywhere else the location and shape of the relief wax is very important,when I cut between the relief wax and the tooth and when I cut between the relief

wax and the major connector . the junction between the relief wax ( going to be

acrylic ) and the internal surface (going to be metal ) should be 90 degree , so

when cutting the relief wax I have to make this edge 90 so I can have the internal

finish line which is in the final denture will be the flushed ( ) in the

final denture .

We also have 1 mm between the relief wax and the tooth, why ? because the metal

I want the metal to go into that depth and I want the metal to go down the gingiva

( in depth ) . the metal goes down all the way to gingiva because between the

acrylic and the major connector I need a finish line (posteriorly ) and also between

the acrylic and the metal I need a finish line ( antieroley ) , also there is a finish

line medially . So the bounded area has a finish line antierorly , posteriorly and

medially .

end of part 1


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Part 2 : Anwar Durah .

Cast Duplication

We use a special duplicating flask, it can be either metal or plastic, and

it has:

1-A base where the cast is placed.2-Holes on the top so the material can be poured inside the

flask.

Types of duplicating material:

1)Agar Agar2)Addition Silicone

It is an elastic, non-synthetic, thermoplastic, reversible aqueousmaterial in terms of classification.

1)Agar Agar:

Duplicating Flask


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It is a jelly-like material based on water 85% (hydrocolloid). It was used before in clinics but not anymore, nowadays it is used

in the lab as a duplicating material.

It is flexible but it has poor tear resistance. It can be used up to 20 times before it needs replacement.Very sensitive to humidity.Thermoplastic means that when I heat it, it becomes liquid.

(Transforms from being a gel to a liquid (the liquid state is

called the solstate)

Components:1-Water (80-85%)2-Agar (10-12%): Extracted from sea weeds, very complex

polysaccharide that absorbs a lot of water giving that jelly-like

consistency.

3-Borax.4-Filling material for color, smell and flavor.

As for the practical part:-

1-We put the cast we want to duplicate on the base of the flask andseal it with wax.

2-We pour agar agar from the top using the holes.Some labs heat agar agar in a pot and pour it inside the flask, but the

correct way to do it is using a special machine, because specific

temperatures are important, agar agar has a liquefying temperature

and a gelation temperature and they are NOT the same.

We usually heat the material up to (90-92) Co to make sure it's

completely sol, then we cool it down and we stabilize it atapproximately (40-45) Co, by that we make sure that the material is

stable.

The machine will keep the material liquid 24 hours a day for 7 days a

week. Inside the machine there is a rotor that will constantly move the


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material and distribute the temperature equally and keep it at (40-45)

Co.

When we open the machine it will pour out the material in to the

flask, and there, it starts to cool down and start gelation somewhere

between (30-40) Co. It should be poured slowly as not to incorporate

air bubbles, and a vibrater can be used but it's not necessary because

Agar Agar material is hydrophilic.

The agar agar should be relatively clear after its poured in the

flask, but air bubbles can cause it to have a milky appearance.

Cooling down of the agar agar takes a long time, and it should cool

down slowly on its own, otherwise it will distort. However in the lab

we used an ice bath to accelerate the cooling process, but we would

never do that for a patient because it will cause distortion.

This is the agar agar conditioning bath; it's called

Gelovit from a company called Bego.

Opening on the top where

agar agar is placed inside.

You can also see fans

rotating inside to keep the

material liquid.

Temperature stabilized

at 40-45 Co

.

The temperature cycles: At

first it's very hot (91 Co) then

it cools down 40-45 Co.An opening where the

material is poured out.


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DISADVANTAGES of Agar Agar:-

1) The main disadvantage of using agar agar is its sensitivity tohumidity. If it dries it will shrink (syneresis).

2) If I pour up my agar agar impression once, when I seperate it, itactually tears and will be destroyed, so it's usually a one-time use percase but I can reuse it for other cases; the mould can only be poured

out once and then it has to be reheated and reused for another

impression.

3) Agar agar can'tbe poured up with gypsum, unlike alginate (alginateis designed so that we can pour gypsum and it will set without thesurface becoming too powdery). So you can pour up investmentin

agar agar but you can't pour up stone, however if you pour up stone

in it you have to use a surface inhibitor (potassium nitrate ( ))

on the surface, in order to prevent the accelerate of the set of the

gypsum on the surface.

But what if I want a material that is stable and I can pour it up morethan once? We use an elastomeric impression material which is addition

or condensation silicone and we usually useAddition Silicone.

More stable and more expensive than condensation siliconeand agar agar.

There is a special machine used to mix it but in the lab we mixit by hand (we mix the base and the catalyst in a 1:1 ratio).

A vacuum mixer or a mechanical mixer will produce a muchmore even mix with less bubbles.

.

2) Addition Silicone


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A vibrator is used to pour it. It sets within 20 minutes to half an hour. In the lab we used

heat to accelerate the setting but never for a patient.

The result will be something like this (we always see it in thelab)

NOTE: On the mould (agar agar or Silicone) u can see the negative

where the positive of the wax was (block out and arbitrary and

shaped wax), every single little thing you did will be there. So every

positive on the cast will produce a negative in the mould.

Investment material:-

After I made the mould using agar agar or addition silicone, I want to

pour it up.Agar agar should be poured up right away, in the case of

silicone it is better to pour it right away but I can wait and pour it

later and even pour it multiple times.

The material used here is neither plaster nor gypsum, it is

Phosphate Bonded Investment. It is a high temperature resistant

material. We have other types like sulfate bonded investment but the

Dr doesnt have time to talk about it so refer to your dental material

to understand more.


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The investment usually comes in pre-packaged bags, and there is a

special liquid that we mix it with (for 400g powder we need 56-60

ml of liquid).

Why use this special liquid? Why not use water here? Remember

that I'm making an investment mould so that I can pour metal on to it.

The metal expands when I liquefy it and when it cools down it will

shrink, that means it will be smaller than the master cast, the

investment is designed to expand (setting, thermal and hygroscopicexpansion the hygroscopic happens with sulfate bonded not

phosphate bonded-), this special liquid will help the expansion in the

investment which compensates the shrinkage that occurred to the

metal.

I mix the powder and liquid under a vacuum mixer to an even mixwith no bubbles. Be careful that the way you mix and the number of

times you mix change the amount of expansion (the more you mix the

more the expansion and the faster the set).

The vacuum mixer has a timer and a pressure gauge, and the pressure

will be -1 Bar when I close the mixer. It has an automatic mixer (like aMoulinex). Dontinhale any of the powder because it has silica powder

which can be carcinogenic. The color of the mix tends to be brownish

to purplish. We time the mixing between 30-60 seconds depending on

the amount ofexpansion needed and how fastyou want it to set.


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After that I pour it in the mould (agar agar or silicone) using a

vibrator. For the addition silicone mould I need to use a surfactantas

a spray to make sure the material will flowevenly on all the areas. I

start pouring from one corner just like stone and then it will slowly

flow all over.

At last we wait for the material to set (half to one hour), and after it

sets it will become hot and ready to be separated, so we will end up

with our refractory castwhich has all the block out that we prepared.

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Prosthdontics Lab 5

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