prostho sheet #11-amani rabab3a

http://www.4shared.com/document/bSadOiyb/prostho.html



Production of metal framework

Steps of prosthesis of removable partial denture :
1- History and examination “ clinical step”
2- primary impression using alginate , this step take only from 5-6 minutes
3- production of study model , we do design for our prosthesis on this model , whether we have fixed or removable prosthesis
4- for removable prosthesis , we make analyzing and surveying for our model , the design might change during treatment
5- applying the design inside the patient’s mouth via teeth preparation ( rest preparation on the occlusal surfaces and axial recontouring for proximal surfaces
6- secondary impression using special tray
7- final working model on which we will construct our final prosthesis.
Working model: is made of stone,
→ for complete denture , the final prosthesis is made of acrylic material via direct construction on the model , so in this case the acrylic material is in contact with model (cast) and curing occurs at 100c
→ for partial denture final prosthesis is made of metal , and construction of the final prosthesis ,which is made of metal , is done at casting temperature : which is the temperature that is required for conversion of the metal from solid state to liquid with reasonable flow characteristics , means that we do not want to get a highly viscous material and do not take the form and the details of the mold .
Casting temperature is very high , so if you construct the final prosthesis with contact with cast , it will disintegrate and change in its dimensions , so you will not obtain a proper dimension for the prosthesis .
↓
The solution is :
Duplication of a “refractory cast” = another copy but more resistant for high temperature . On this refractory model or cast , we make modification for this model :
a- undercuts , the framework has some spaces between it and the teeth , and these spaces or undercut areas will be filled by metal during framework construction , these undercuts are undesirable and will interfere with insertion the framework inside the patient’s mouth ( the framework will not enter the patient’s mouth at all )

b- space for tissues , beyond the mesh where the acrylic material will enter so we have to create a space for it .
So we do modification on the master model to close the undesirable spaces so it will not be filled with acrylic , and get some relief or space between framework and desired areas . relief provides space between areas of the RPD
framework and oral structures. Done by placing sheet wax in certain areas of the master cast prior to duplication.
This process is called Block out using wax.
So block out is : Elimination of the undesirable undercuts on a cast to be used to fabricate the partial denture using wax , because it is reversible . using stones in block out is irreversible prosthesis

The type of refractory material being used :
A-Filler ( silica or quartz ) that has the following properties :
1- thermal expansion characteristics
2- Resistance to high temperature up to 1000 c

B- Binder : gypsum (CaSO4.H2O “calcium sulphate hemi hydrate ” that is converted to dihydrate.
There are another types of bonded materials as shown below .

The metal materials used for construction of final prosthesis :
1- gold alloy , with melting T. about 1000 c , using gypsum investment .

2- cobalt chromium , melting T. is about 1300-1400 c , so using gypsum bonded investment can not be used , in this case we use phosphate bonded investment

2- Titanium alloys , melting T. is the highest , we can not use phosphate bonded investment because it is very reactive when it is hot ( hot means when we put the model in the oven for wax elimination then inserting the melted metal on top of the hot model ) so phosphate that is reactive will react with titanium alloys , for this reason we use magnesia based investment that is compatable with titanium alloy and will not react with it .

3- aluminum alloys , but it is not highly used because it is not very strong .

Density “d” : gold = 2 x d. cobalt chromium = 4 x d. titanium alloy
( so titanium alloy has the lightest density )

8- after production of refractory cast , we do wax up and casting.
Means we cover the desirable areas, that will be covered with metal framework , with wax .
Now the inner surface of the wax will be determined by the fitting surface of the model . The outer surface is determined by secondary pour “another investment model” . so we have refractory model “primary investment” – wax “ that will give the shape of the final prosthesis” – then on top of it there is secondary investment.
Wax → in complete denture , wax will be replaced by acrylic material , and you can get the acrylic by opening the tow cast .
→ in RPD , wax will be replaced by metal, you can not open or close the two casts , you have to break them down to obtain the metal .
For metal production :
a- create wax channels that reaching the wax between the two models and opened from outside for metal injection .
b- wax elimination . we put the two investment with wax inside the oven , the heating to a temperature depending the type of the investment that has been used as mentioned above .
The aim for heating the model : - get an empty space inside
- mold expansion in a controlled manner to compensate the metal shrinkage when the metal is getting cool ( after casting )
Model expansion : is obtained from silica or quartz components ( SiO2) that is differ among difference in crystalline form . at the end we need the coefficient of thermal expansion for metal and model is equal . so that no space will be formed and the metal will get the proper desired dimensions .
To get the metal framework after processing , we break the model .
A-The metal looks black ; because the surface has been oxidized . so to have the silver and polished appearance of the metal framework :
1- use chemicals
2- special treatments
3- sand blasting : propelling a steam of abrasive material against the surface under high pressure to smooth the rough surface.
4- polishing .
B- Get the sprues away . sprue is a channel of wax that has been filled with metal

C- check the framework fitting on the original master cast ( what we break is the copy of the original cast ) . define the place of the interference and remove them so that the metal framework will fit properly.

9- after wax up and casting , we do metal try in inside the patient’s mouth .
In some case we make altered cast impression technique .
For your knowledge
Free end saddles are liable to be displaced under occlusal pressure (anteroposterior rocking around the abutment tooth, which acts as a pivot). This is as a result of the displaceability of the mucosa of the free end saddle. The altered cast technique was a popular technique that was introduced in an attempt to overcome the problem. The metal framework is constructed and light or self cured acrylic material is placed on the framework in the saddle areas creating a close fitting customised tray over the saddle areas. Zinc oxide eugenol or low viscosity silicone is then placed in the customised tray and the framework is placed into the mouth and the impression taken with pressure only on the support elements of the framework resting on teeth. The laboratory then alter the cast (remove the free end saddle), place the metal framework on the cast, invert the cast and framework and pour dental stone to complete the cast.

10- bite registration , because it was not registered in a decent way
11- teeth setting in the try in visit
12- production of acrylic material part by flasking , similar in complete denture with slight modification .
13- insertion and post insertion .


Now I will monition the laboratory steps again in harmony with pictures in the handout :
Slide 3
you can see the master model with : - sheet of wax on the edentulous areas = relief wax , it is needed to allow some spaces between the mesh and tissues to be filled with acrylic material
- undercuts : closed with wax, the trimming on the surveyor parallel to the path of insertion and removal ; block out wax


Slide 4
Beading of the upper denture only
Scoring a cast to provide a seal between the RPD and the soft tissues, with a depth about 0.5 mm . the aim of beading is to provide intimate contact between framework and soft tissues ; preventing the food accumulation beneath the framework which is annoying to the patient .
Site of the beading : on the upper denture at the borders where it communicates with out side , for example we do not make beading at the finish line since it is within the framework .

Types of the block out:
1- parallel block out : to block the undercuts , so if you are asked in the exam about the type of block out used for undercuts is : parallel block out . It is trimmed on the surveyor . ( done by specifically angled styli in the surveyor in block out wax . styli can be 0 , 2 , or 6 degree )
2- relief : used for - space for acrylic , beneath the mesh
- relief for mandibular tori , because movement of the prosthesis on these tissues will cause trauma .
The relief is done as either :
a- thin layer of wax
b- controlled layer of wax “dimensionally determined” beneath the mesh .
So relief is used in areas where there will be contact with thin soft tissues.
Tori.
Beneath lingual bars.
Beneath edentulous denture extensions.
relief block out is not carved of the surveyor , since it is not parallel to any thing . it is just a defined thickness of the material ))

3- arbitrary block out : for any undercuts has no relation with framework design . these undercuts are filled with wax with no controlled thickness or parallism to any thing ; as the name indicates ‘arbitrary’ .
why we do this type of relief : simply , after block out the model , we will take an impression by duplication material , it will enter in sever undercuts ( not part of the RPD design and during removing the duplication it might be teared or damaged . even the duplication model is not damaged , the refractory cast ,that is recorded inside the duplication model, might be broken down .
So these undercuts are not needed to be recorded since they will harm the following procedure .
Again arbitrary block out used to block less crucial areas.
Aid in removal of the blocked out cast from the duplicating material.
Prevent framework impingement on soft tissues.
See Slide 9 you see undercuts that are labially located and not engaged with partial denture design , so we do arbitrary block out .

4- shaped or ledging . on the master model , the wax in the block out region can be ledged in the area of the tip of clasp arm . so a ledge of the investment material aids in placement of the wax pattern of the clasp , and the clasp tip will be engaged in the determined undercuts . if you don’t do this type of block out you have to put the refractory cast on the surveyor and survey it again to determine the location of the clasp arm and it’s tip , also wax pattern clasp arm without ledging might remove from it’s position so ledging is better . Ledging is made for C- clasp not for I- bar ; I – bar is gingivally approaching clasp so the ledging will be located on the soft tissues, not on the tooth surface

Slide 12
You see the model with block out of (the undercuts , spaces between the teeth ) , if these areas are not block out then the duplication material might be teared , or the refractory cast might be broken .

■The type of the material used for duplication :
Agar -agar , which is a reversible hydrocolloids produced from sea molds .
Temperature ‘T.’ changing :
during heating it becomes in the liquid state , and cooling it down, it becomes in the solid state .
T needed to make it liquid is more than the T. needed to keep it in the liquid state
using this material during duplication , we do the following : heating to 100 c → then tempering to 60 c → then duplication . “cooling occurs by water path”
If we make the duplication at 100 c then the wax will melt , and we will lose the block out .
In the past they used to take impression for partially edentulous areas using agar-agar . now we use alginate .
Using agar-agar : - time consuming
- not convenient in the clinic
- more accurate than alginate
- cheap , because it is reversible , so it is reusable in the lab ( not between the patients ) .
Slide 13
You notice :
1- block out has been done
2- both upper and lower models have a central hole with a metal cone inside .
3- they are immersed in water , before duplication, so water closes the pores inside the models , as a result the tendency of the duplication material to stick to the model is less .
( don’t paint the models with Vaseline which is a hydrophobic and agar-agar is hydrophilic , the details will not be recorded by agar-agar )

Slide 14
- container with an opening and model located inside
- pour the agar-agar in the opening so it will take the shape of the model
- the container is surrounded with water for cooling the agar-agar and so solidification .

Slide 15
You can see agar-agar after solidification : beading has been recorded ; not for original master model , but recording the modified one “with block out” . space from the metal core , and we put inside it another metal core with the same dimensions .
Now we put the investment material in the duplication for refractory cast production , as seen in Slide 16 .
We will have a space in the refractory cast .
Refractory material is powder and liquid . liquid is a special liquid dissoluted with water , by the concentration of the special liquid we can control the expansion of the investment material . for example if you have a framework that is larger than required dimensions , we can solve this problem by using a new investment with less amount of special liquid .



Slide 17
Refractory model with a metal cone inside , that can be removed easily , the cast has already registed all the block out .
The processes that are done on the refractory cast :
1- pre-heated treatment in the oven at 200 c , so that the cast is completely dry
and undergoes slight expansion .
2- socking the heated cast in bee wax : “as seen in slide 18”
- to make the cast sticky to the wax pattern of the framework .
- abrasion resistance ; refractory material is like chalk and can be broken easily - to prevent damage of the refractory cast , that might occur by inappropriate contact with metal .

Slide 19
Another kind of duplication material “liquid silicone” :
It is irreversible , you can use it only once , very expensive , more accurate than agar-agar .
Then we put the duplication in pressure pot “special for silicone” → to get rid of air bubbles , so imperfection in the model is less . slide 21
Slide 22
You can see the duplication that is made of silicone , and the refractory model .
You can make silicone cheaper , by using less amount : we take parts from the ready made duplication ‘that is made of silicone’ and put it in the fresh material to reduce the amount of newly silicone .
( before insertion the refractory material , we apply a surface tension reducing agent , because silicone is hydrophobic and refractory material is hydrophilic , so we apply this agent to reduce the chance of bubbles production . we don’t use this agent in agar-agar duplication material since both agar-agar and refractory material are hydrophobic .




Slide 25
You can see the wax pattern , there some places need extra thickness of metal ‘during metal production’ for example at the site of the major connectors ,we put extra thickness of wax ‘free hand waxing is added , that means you don’t need a specific pattern of wax .
At the junctions between different parts of wax we melt the wax , so that the wax pattern becomes as a one piece of wax .
wrought wire clasp , we construct this clasp before metal framework production.
Common exam question :
Why we put Wrought wire clasp in this design ? because we have a free end saddle of the other side.
My explanation : we have fulcrum of movement between the occlusal rest seat ( mesio-occlusal rest seat on the lower left first premolar and mesio-occlusal rest seat on the lower right second molar , so there will be ‘ tissuer movement; toward the tissues’. There must be a balancing movement ‘ away from tissues movement’ and this movement will be provided by the clasps ‘indirect support’ anterior to fulcrum of movement . for this reason we will put a wrought wire clasp on the lower right first premolar and the clasp must be made from a material to provide movement for the clasp , so if it’s ‘clasp’ made of cobalt chromium , can not provide such free of movement and it will break .
How to stabilize the wrought wire clasp with the wax framework :
1- during cast on , the clasp will be part of wax design on the refractory model , and when we put the secondary investment , it will be hilled in the place .
2- soldering ‘nel7emha’ on the framework after casting
3- embedded in the acrylic material .

As seen in slide 26 , ledging has been replaced with investment material . we just put the wax pattern of C clasp .

Slide 26
Point A indicates tissue index : - no relief wax found , metal can enter
- prevent acrylic material from contact with undercut with abutment teeth , even though if the acrylic material enters this area it can be removed easily .

Slide 30 :
Bring wax pattern in the form of ropes extend from the central space ,find inside the refractory cast, to the other sides ; this pathway is called auxiliary spruces ( these spruces communicate the metal cone inserted in the central space ) . The central spruce will act the main entrance of the metal , after you have removed the metal cone from the space during casting .
Regarding the auxiliary spruces that are made of wax , the wax will melt and the space will be created and this space acts as channels where the metal can reach the whole designed framework .



Slide 31 :
I- bar : if it is crossing a mild undercuts , the undercut can be blocked out .
* if there is no undercut , but a thin layer of relief between I bar and tissues to prevent trauma of the tissues beneath .

Slide 33
Final framework after fitting to the master cast .
If there is a central hole in the master cast this indicates it will be used for duplication purposes :
- refractory model replication .
- production of any other models for any other purposes .

Slide 34 :
As you see we do ledging beneath the root wire clasp , we bent the wire using your hands .

Slide 35 :
wrought wire , fixed in its position by plaster , or by using piece of metal with low solidification T. With solder , heated using electric electrode , it will become part of my framework
To sum up : wrought wire clasp constructed either before casting , or after casting

Slide 43
You see the spruce comes superiorly relating to the cast , because we have filled the palatal area “ palatal coverage” , in a way the spruce not in contact with tissues .

Slide 47 :
Again after wax pattern → sprucing → mixing of the secondary investment in a metal ring where the model with wax pattern is impeded inside . the setting time , powder to liquid ratio , casting time all are defined by the manufacture .

Slide 49 :
Final assembly , the spruce entrance that is joined with auxiliary spruces that is bonded with wax pattern .

Slide 50 :
After proper setting has occurred , we turn the assembly up side down , and put it inside the oven which is programmed accordingly , until reaching the final casting T , now melt the metal then insert the metal inside the assembly ‘this will explained later’

Notes about fixed partial denture ‘slides from 52- 56’
1- wax up is done directly on the master cast , so there will be no refractory cast 2- we will have only one pour of investment to cover the whole wax pattern and the master cast .
3- same idea applied here for metal insertion , we have sprue for metal entrance .
4- slide 54 , cross section in wax pattern and surrounded by investment material . 5- we put the assembly with wax pattern in the oven for wax elimination , the metal insertion in the available space.
6- the after getting the final assembly , that is made of gold in this case, you have to cut the sprue , which is reusable since it is made of gold .
7- check the fitting on the cast then inside the patient’s mouth .

■Methods for melting the metal :
Using the torch, types of torch:
1- gas-air torch , this T. is enough to melt the gold
2- gas – O2 , to get higher T.
3- acetylene - O2 , to get higher T.
( second and third type are used for melting the cobalt chromium )
Parts of torch:
1- mixing zone .
2- combustion zone .
3- reducing zone , blue in colour with the highest T. and this zone is needed for melting
4- oxidizing zone , red in colour . if you expose this zone on metal for melting you will end up with black colour due to oxidation reaction .

Another melting technique :
High frequency induction melting :
It is prototype method for melting cobalt chromium alloys .
Mechanism of action : as seen in slide 59, we have the investment which is made of mold , and the opening inside the sprue . on the other side of the machine we have a cylinder where we insert the metal surrounded by coil .
To sum up : the machine is tow parts : one part where we put the cast assembly and the second part we have the metal in side the coil . tow parts communicate with each other by the opening in the primary sprue , the main entrance of the metal, facing the cylinder where the metal found.
Now an electric current with a high frequency will pass through the coli creating a magnetic field that is interchangeable in the polarity ‘south and north’ , so this change will move the metal inside and that will create a heat for melting the cobalt chromium .
We can use this method for melting the gold , but the problem here is overheating of the gold so boiling of the gold , and we will end up with framework with porosity due to air bubbles have been formed . so this technique is done specifically for cobalt-chromium alloy, because we can not reach overheating .

The metal will go inside the opened pathway ‘sprue’ that facing it by centrifugal forces ‘ these forces are the driver for metal entrance into the space and so filling it’ .






Gold IV Co-Cr alloy Ti-6Al-7Nb


Gypsum bonded Investment Electric resistance melting with centrifugal casting machine.
Phosphate bonded Investment. High frequency induction melting with centrifugal casting machine.
Magnesia-based Investment . Argon arc melting with centrifugal vacuum pressure casting machine .

Note: there names of machines found in the slides and not mentioned here in the table

Details :
1- gold :
Electric resistance melting : coil acts as resistance and during passing the current through this resistance it will heat and heating the metal inside . so the T. here is well controlled , there will be no overheating .

2- Co-Cr :
Using resistance method will not melt Co-Cr . so we use high frequency induction technique .

3- Titanium alloy , neither tow mentioned methods used for titanium , since we need the current to pass through the titanium directly , not indirectly by the coil, so we use Argon arc melting technique . so we have a vacuumed atmosphere for reaction , no air because the titanium is highly reactive at high T. so the reaction occurs in argon atmosphere which is a nobel inert gas .
The investment material may react with titanium so we have magnesia based investment ; to eliminate any unwanted reaction .







Slide 63
After casting we break the assembly and we will get the metal framework that needs modifications :
1- cutting the sprues
2- sand plasting to remove the plaster .
3- mechanical finishing using special kind of burs , as seen in slide 67
4- electro-chemical polishing , we insert the metal framework in acid solution and a current will pass . the reaction occurs will remove the surface layer and make it shiny . you can see bubbles ‘slide 72’ be careful : do not forget the framework in the solution, you may not find it –it has been consumed in the reaction- so there must be controlled time for reaction .
5- check fitting of the framework on the master cast .

Slide 74
Altered cast technique has been done . so we have another pour specifically for free end saddle with soft tissue loading , then beading for this part ‘slide75’ then construction of the acrylic base using sprinkle technique
For knowledge:
Autopolymerizing Resin: Sprinkle Method
This method involves applying the resin to segments of the arch, waiting for initial set, and then forming other segments. Because of the need for extended use of uncombined monomer liquid, this technique is most safely performed under a hood or in a well-ventilated area.


Now after metal try in → bite registration → teeth setting → denture processing and finishing.

و آخر دعوانا أن الحمد لله رب العالمين
إعداد : أماني اسماعيل الربابعة

now,, this is te sheet kamelatan

enjooy !!