lab prostho 10 (1) (1)

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  • 7/30/2019 Lab Prostho 10 (1) (1)




  • 7/30/2019 Lab Prostho 10 (1) (1)



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

    us.. It helps a lot.. And sorry for any mistake.. It wasn't easy, the dr.

    explained a lot on his cast model.. I tried my best. ERRARE EST

    HUMANUM ! .

    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?


    Primary cast (dental stone III)Primary impressionWe do initial surveying and design

    We fabricate custom tray

    Tooth \mouth preparationa-Secondary cast (Master )b-Verification surveying and final designc-Block out and reliefd-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


    h-De-investment and cut sproutsi-Finishing and polishing metal


    Secondary impression (Alginate)

    Mounting and teeth settingJaw Relationship record and Metal

    framework try in

    Wax try-in

    Acrylic processing

    Finishing and polishing


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    1- The initial survey and design are important to formulate thetreatment plan.


    Custom trays are not always necessary, they are necessary fordistal extensions and marginal edentulous areas ,short

    bounded saddle areas don't 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 anelastomeric material we can't use a rigid material unless

    there's an edentulous area ,such as addition silicon

    ,dimensional silicon or Polysulphides5- 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, mountingand 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 pt , we take primary impression ,using stock traysusing 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 spacersnear 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 castwill 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

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    arbitrary block out to simplify the duplication 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 use

    a 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 butcan 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


    Now I have the mold , copy of secondary cast, I'm going to pour it in amaterial stronger than master cast which can withstand high


    I'll use investments materials ( because the Co-Cr-Mo (Cobalt-

    Chromium-Molybdenum) alloy has very high melting temp , 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's 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.
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    on the master cast , what am I going to do? I want to make the metal

    frame work , so what I do is the wax pattern ,there are special wax

    patterns fabricated by companies so we have : 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 which 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 thewax 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 my

    investment 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 2 things:

    setting expansions, from chemical reaction of setting andthermal 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 .

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

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

    mold .** one is called the "CASTIING MACHINE " and the other is "CASTING

    FURNICE" .

    So I put the mold in the oven ;I leave it for several hours ,there's a

    special cycle to allow the wax to evaporate without destroying the

    mold. When it reaches the right temperature ,I take the mold out 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 thiscontainer is heated and becomes liquid metal ; I need to get this

    liquid metal inside the mold (I don't hold it by hand ,there's a special

    machine) there are two basics techniques :

    1- centrifugal casting force: it start 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 it's rotating so 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


    The temperature source can be either flame or induction casting(electric


    // 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, usually changes in color, it becomes white; inside it will be all

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

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

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

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

    Abrasive container ..

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    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 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 I use the

    sand blaster (it's like sand storm that will remove this investment whichbecome 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 isn't shiny ,so

    the 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 upin 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 1/3one

    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'm 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


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    ++ 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.

    After doing that we can use special tungesten 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 ;is the material that does the polishing is not the rubber 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

    higher and higher shine and the shine increase as the surface layer

    become more regular and shiner; 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, we put a

    polishing material like Tripoli or rouge ;

    usually we don't do polishing on hand piece 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 will not give us the highest shine

    possible ,we need a higher shine and we need to polish the internal

    surface ; in order to do this we need to do something called "ELECTRO


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

    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 tissues but it won't


    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 dewaxer, 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 adjacent

    to the tooth ;you can see there's metal between the guide plane and


    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:


    Conventional technique , essentially set the teeth ,place the metal,put in flask , cover everything metal with plaster and stone ,just

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    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 waitfor curing and I open the flask and get my acrylic denture , I do

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

    2- The second technique doesn't involve processing ,we make aquick mold ,not a flask mold; using the normal technique from

    adding teeth , I add wax over the meshwork over and below, I do

    wax rim and I get teeth and set them seal them (done mounted on

    articulator) ,as we did previously ,we do 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're four consistencyof silicon light medium heavy and putty ; putty is most usually

    mixed by hand ) I place catalyst mix it until becomes homogenous

    and then I'll make an index on what I have on facial aspect.

    Essentially this is going to relate the teeth to the index so I can

    remove the index and teeth and wash out the wax, instead of doing

    flasking packing dewaxing ; I just do this quick mold , how am I going

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

    poweder 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 morecomfortable , is to make plaster index. ; we make the index out of

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

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

    expensive and easier to use. usually the setting time for this material is

    about 4-5 min in mouth , it's colder outside the mouth so it'll take a little

    bit longer. After setting I remove it I'll have an index on it , it's like the

    second mold in flasking process ,like the top part of the flask .So I taketeeth and put them in the index and wash away the wax..

    The dr. 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 methodsmake setting faster: warming and mix for a longer duration of time and

    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 I'm almost painting it on.

    So I can add acrylic incrementally in 3 ways:

    1- Brush it on2- Sprinkle it on3- Mixing ,like heat cured acrylic

    *Since the acrylic used is not heat cured it's cold cured, so one methods

    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 incrementallyto the acrylic.

    This type of acrylic has the same stages of heat cured acrylic , but here

    we 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 spinkle 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

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    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).

    *The third technique is to mix it ;I have the monomer : I add powder tothe 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 till it become 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 was. 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 removethe 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 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 mins.

    ** 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.

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    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 the

    excess and do polishing and I end up with an acrylic base .

    **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.



    Done By : Sara Ibdiwi .