Prosthodontics Lab 2,Making of special trays

8/3/2019 Prosthodontics Lab 2,Making of special trays

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Prosthodontics Lab Thursday Section 6/10/2011

Today we will learn how to make the 2nd step of making a denture which

is “The Special Tray “

So at first we had an “edentulous” Patient. We did first Impression, the

patient went home and the specialist made a cast using plaster of pairs

(Gypsum type ll ) , We called this a primary cast or diagnostic cast . The

aim of the primary cast is simply to make the special tray.

Why do we need to make another tray?

The stock tray we used on the first visit is made for average patients andthey do not give a proper extension for the different structures in the oral

cavity for each patient.

What are the materials we use to make the special tray?

We will talk in this section about the materials we can use to make the

special tray, keep in mind that each material has advantages anddisadvantages. Some are widely used some are very rare to use.

The most widely used are Acrylic materials and specifically: Light cured

and cold Cured materials.

Light Cured :

They look like sheets, and come in a box that has a black paper that actsas a shield. Remember that it is very important not to expose it to light

and not allow the setting rxn. to take place



every Acrylic material has two components Monomer and Polymer

(Monoetheal Crinite and Polymethal Crinite respectively ~ names are

not of importance ).

You have to know that the setting reaction in Acrylic materials is CrossLinking, this means the Monomers particles cross link with the Polymer

particles.

Advantages of the Light cured :

1. It is already made sheets; this means it has an equal thickness.

This is very useful for you when you want to fabricate and adapt.

2. Easy to manipulate

Cold cured / Self cured / Chemical Cured :

It comes as a Liquid and powder mixed together in an endothermic rxn.

by you. The initiator of this rxn is called Venzyol Paraxide .

**IMP** The Acrylic has 5 different stages in which they reach upon

setting and become ready:

1. Powder-Liquid stage (when mixed become 2nd stage)

2. Fibrous stage / Strength stage

3. Doughy stage ةنيجع / Operative stage: In this stage we make the

custom tray and we shape it the way we want.

4. Rubber stage: in this stage the temp increases and the setting

starts to occur. No further manipulation can be made when

reaching this stage

5. Hard / Setting stage : our work is done



Advantages of Self cured:

1. Less brittle than the Light cured but both of them show

considerable brittleness within them

2. Smooth surface: because you are the one who is mixing in self

cured. The light cured demands polishing and adding of Vaseline

to it to make it smooth.

3. Easy to manipulate

Main Advantages of Acrylic (both light and self cured):

1. Cheap

2. Easy to use3. Strong enough to act as a special tray

Main Disadvantages of Acrylic(both light and self cured):

1. Brittle (especially if thin, this is why the minimum thickness is

2mm). If the material is think it could break in three different

situations : 1- on the patients mouth 2-upon removing 3-on tray

2. Polymerization shrinkage, as a rule every polymerization reactionwill induce shrinkage. So it is very important to make good

adaptations before putting the work inside the curing machine, if

there are errors in your work you will end up with shrinkage and

you will have spaces, we don’t want spaces in our work.

Which one is less shrinkage the light or self? It is actually the

light because in light you don’t actually mix anything thus reducing

the chances of micro-spaces. You have to understand that

Shrinkage is absolutely a defect in the structure of the material, butwill it affect the making of tray? If we had spaces in our tray is this

very bad? It is always better to avoid them but if we did have

spaces this will not affect the function of the tray much.

3. Allergy of Acrylic: Acrylic is known for triggering Allergy reactions,

but in the case of special trays none of the patinas reported having



that. This simply due to the small amount of time the special tray is

being exposed to the oral cavity

4. Flexibility and undercuts: Some patinas have undercuts, you will

definitely see them if you designed your tray without taking into

account the undercuts what will happen? The tray won’t come out

and harden (not flexible). You either break the Acrylic or the

patinas head: D . The solution here is to make “Blocking for

undercuts” in this way the tray won’t go into the undercuts, the

same thing we do if we have irregular ridges.

What other materials can we use other than the Light /Self

cured?

The heat cured can be used occasionally ( has better accuracy and

strength )

Shellac is also used which is a thermostatic material meaning that

upon heating it becomes less hard and when it is cold it is hard, the

main disadvantage is that it is very brittle.

Metals can also be used but they are expensive

Final conclusion, Light cured and self cured are the most commonly

used special trays making materials.

Requirements for Special Trays:

What is Spacer?

From the name Spacer it implies that it is used to give space, sometimes

we need space when we have mobile organs. Is it only used in upper as

we see in the lab? NO the spacer can be used both on the upper and

lower, in the lab you see the spacer on the upper for educational

purposes only.



A very important thing you should know is that the type of wax we use in

spacer is called “Modeling wax” from the family “Utility wax “

What is the most important thing in making the Special

tray ?

It is the peripheral borders; the depth of the tray must cover the sulcus

(why?) We do this to have a peripheral seal good retention

What are the Retention forces we make in the denture?

We have physical, mechanical and chemical forces .Our talk will be on

the Physical.

1. Adhesion and cohesion: imagine having two slides of glass, in

situation A I want you to stick them together and then try to

separate them , very easy right ? In situation B I want you to put a

drop of water or oil and stick them, try to separate them, very hard

right? What happened is that the Oil ( Saliva in mouth ) stuck with

the Glass ( soft tissue in mouth ) and the other drop of oil (Saliva )

stuck with the other glass ( Denture in mouth ) , this is simply the

concept of Adhesion “ the sticking force between Dissimilar

objects ). Cohesion is the same but within the object like Saliva.

2. Atmospheric Pressure: We make a (-) pressure inside the denture

to have good retention, We do this by creating something called

peripheral seal which includes 1- The end of the Denture 2-Width

3-Depth of slucus, These three things we do something called

Border Molding which in turn will make peripheral seal this

seal will make the pressure inside the denture less than the oral

cavity (-) pressure good retention. If the image not yet

understood imagine the balloons you see in shopping centers and

how they stick to the root once they are free.



3. Denture extension: this is very important in keeping the (-)

pressure, look at the figure below.

What is the Vibrating line?

Most of students think that the Vibrating line is actually the line between

the hard and sot palatals, this is completely wrong. It is actually an

imaginary line between the moving and non-moving part of the soft

palate, this line has other names like AHH line (we ask the patient to

make this sound to see the where the vibrating ends and starts, we can’t

see a line though), it is also named the peripheral seal.

How to locate the Vibrating line?

The landmarks for this line are Fovea Palatine.

After disusing the retention forces we use, let’s see what are the

requirements we need to have a good special tray:

1. Spacer , we talked about this above

2. Retention factors , also explained

3. Handle ( in both the upper and lower trays )

4. Finger rests (exclusive only for lower): In the lower tray when you

want to make an adjustment there is nothing to rest your fingers

on, not like the upper where your fingers are rested on the palate

area , This is why we make them.

• ant : Labial Slculs

• post : Hammular notch

• must extends to the vibrating lineupper

• ant : Labial Slcus

• post : Retromolar area ( atleast 1/3

- 2/3 included )lower



• Canine

•6 or 7

upper

• Canine or Anteroir

• 6 or 7lower

5. Stoppers: We use them only if we have spacers. Their function

simply is two things A) to stop the tray from sinking even more

upon pressure and B) They give

even thickness to the material.

Now the locations of the

stoppers is in the figure below ,

Notice also in the upper we will

do a butterfly shape posteriorly

we have to insert a stopper there , the difference here is this

stopper function is different from the other stoppers , its function is

to make the vibrating line (peripheral seal ) more pronounced.

6. This might be known by now , but the material should be 2 mm

shorter than slculs

7. The final tray must be polished

This was the ending of the theory part, Now will go into the practical part.

Keep in mind this is only small points to refresh your memory and not

everything is written.

The doctor started the demo with the upper:

Holding a pencil insert it into the deepestpart of the sulcus , start drawing a line at

any desired point you like move slowly

and gently around the borderline of the

entire cast.

You will face frenulum pass above it and

create a V when you do so , do this with

all the frenum and hummalr notches

You just created the sulcus line Now do another line that is exactly like the first line but it is above it

by 2 mm and attach it with the first line posteriorly and release it

from it anteriorly, this is called tray line



Now do another one exactly like the two lines but it is above the

tray line by 2 mm , here you do the butterfly appearance , this is

called the spacer line.

Now get your wax piece and expose it to heat until you feel it is

ready to be used

Put it on the cast , start pressing with your thumbs not nails on the

palatal area and then into the rest of the cast , try to record the

depth of sulcus correctly

After the entire cast is covered by the wax , now remove the

excess by pressing the wax to the edge of the cast and by using a

carver

Now cut the wax according to the sulclus line using the carver (pen

grip here )

Now it is time to make stoppers , put them in the locations we

stated earlier , they should be something around 5*5 mm squares

Put it in water for 5 min

Fill the stoppers and the butterfly area with Acrylic , do the same

thing for Acrylic excess as we did earlier

It is time to make a handle , the trick here is to make a small

concavity with your carver and open it as if you are opening a bookthis will be the attachment of your handle to the object , you can

either make the handle all vertical or vertical and at the tip oblique

both are right.

Put the object in the machine for 3 min , remove it from the gypsum

cast and put it again for 1 min , by doing so you will be certain that

no soft material is at the core and all of it is harden.

The cast for the lower part is also similar only for the following things :

You only draw two lines the Sucls line and the Tray line , both of

them do not meet posteriorly

You put Vaseline to your work before inserting it into the machine



We come now into the other phase of making the special tray which is

“Finishing and polishing”, Dr.Essam Did the demo.

We do the polishing with an instrument called “Laboratory Hand piece “

the one you use in cons is called “ Contra angle hand piece “. We have

different labs here one is new and the other is old . The only difference

between the hand pieces in both labs is the motor, in the old it is hanging

on the desk and you can see it.

What is the correct way to hold the hand piece?

Here unlike cons lab , we avoid the pen grip instead we do the

Palm grip , The doctor showed many other grips but he didn’texplain what he was doing in the record at was ( I put my hand

here and I grip here , I put my hand there and do this .. so it wasn’t

clear )

How to attach the bur?

For the new lab simply rotate it put the bur rotate back , before

opreatiing make sure the bur is in its place by pulling it.

For the old lab you will see a lever, 12 clock means it is close 6

clock means it is open.

The doctor reviwed the types of bur , the one we didn’t have am

going to write about it : it is called Mandro حمال we use it to attach a

sandpaper or paper around it and run the bur to bolish

Rotation of the bur in this lab

Note that although cons lab had the slow bur run for something 15k

and the fast could reach up to 100k and in this lab you can reach

only up to 35k. But they are different in the diameter so in short



they both show strong powers and could damage if not used

properly

How far should I cut ?

If you did the lines correctly you should do the polishing up to the

third line for upper and up to the 2nd line for lower.

Done by :

Osama Emad Hsan Ahmaed Mohamed Salem Sayed Yousef

Prosthodontics Lab 2,Making of special trays

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