PPAM laboratory steps
Introduction
The conditions for success in removable prosthesis depend on the close cooperation established between the prosthesis laboratory and the dental practitioner. The practitioner must have the theoretical knowledge related to the different laboratory stages.
p0 The prosthesis is made on the model, it is constructed according to the practitioner’s instructions, based on the lines indicated on the model and the additional information provided on the laboratory sheet.
In case of doubt or technical impossibility, the laboratory must contact the practitioner before continuing the work.
The different laboratory stages
- Study of the model on parallelizer
- Determination of the insertion axis
- Drawing the guide lines
- Determination of withdrawal zones.
- Preparation of the guide surfaces
- Analysis of models on articulator
From this examination he deduces the corrections to be made to the supporting teeth, as well as the preparations of the spaces for the occlusal stops and the transfer of these corrections to the mouth.
3-Tracing of the metal frame:
It is the result of specific reflection by the practitioner adapted to the clinical situation.
4-Production of the metal frame:
- Preparing the model
1-Scraping the model
2-The establishment of landfills
3-Installation of the hook sidewalks
1-The stripping of the model:
It can be used to fill in retentive areas and undercuts, both mucous and dental, to prevent the gelatin from tearing when removing the model.
Aims to:
Facilitate the disinsertion of the model
A- Filling the areas of mucosal undercut:
Using the pink wax at the level:
– Slopes of toothless ridges;
– The mandibular retro-incisive region;
– The vestibular faces of the dental alveoli in poor position
– The retromolar regions;
– The posterior part of the tuberosity.
B- Filling dental undercut areas:
-Below the guide line of all teeth;
-At the level of the embrasures under the future brackets of the hooks
– at the level of the interdental spaces
2. Setting up landfills:
- The role of discharge wax:
- To provide sufficient space between the future prosthesis and the mucosa to avoid compression.
- At the stool level, its presence allows resin to be placed between the metal frame and the fibromucosa.
The ridges, the bunoid papillae as well as the palatine torus are discharged with a calibrated wax sheet of 4/10 mm thickness.
3)- Highlight the path of the hooks with a wax shoulder (sidewalk of the hooks)
- Allows you to visualize the path of the hooks on the coating duplicate in order to more easily place the wax preforms.
- The shoulder is located 1mm below the exact location of the hook and is 1mm thick.
The wax constituting the shoulder extends to the gingival mucosa.
- Production of the coating duplicate:
Pouring the gelatin:
• Immersion of the plaster model for 20 minutes in water at 35°C: this allows deoxygenation of the model and avoids thermal shock.
Using a dry model causes the formation of bubbles (cavities within the gelatin), while a sudden change in temperature creates deformations
Fixing the model to the muffle base: using sticky wax
Mixing the gelatin:
Pouring the gelatin:
- Pouring gelatin into a duplicating bowl
- The casting temperature of the gelatin should not affect the waxes used on the model to be duplicated.
- Slow cooling: setting time is approximately 15 minutes at room temperature
Gelatin-plaster separation :
With a breath of air. All precautions must be taken so as not to tear the thin and fragile parts of gelatin .
Pouring the coating:
Vacuum casting reduces the risk of air bubble inclusion and increases mechanical strength.
The coating mixture will be slowly poured into the breeding form using a vibrator.
Setting time is 30 to 45 minutes.
After the coating has hardened the model can be carefully removed.
Preparation of the coating model:
The casting is placed for 1 hour in an oven at 220°-250°C to dehydrate it.
Surface hardening can be carried out immediately while hot by immersion in a hardening liquid for 05 to 10 seconds before returning to the oven for 10 minutes.
- Making the wax model of the chassis:
– Installation of saddles and plates
– Setting up the hooks
-Installation of the jibs and connections
Installation of saddles and plates:
In the maxilla:
Covers the toothless crest.
Covers the tuberosity.
Distance from the bottom of the vestibule of 3mm .
The plate:
We use a calibrated sheet of wax while following the contours and respecting the principles of turning.
In the mandible:
The saddle :
It covers the toothless crest.
-stops 1 to 2 mm in front of the trigon (which will be covered by the resin).
-3mm from the bottom of the vestibule
*The bar:
-distant from the lingual frenulum by 1 to 2 mm.
-4 mm tooth collar distance.
-height of 3.5 mm.
-thickness 1.8 to 2.5 mm .
The hooks:
The cleats: they are made using wax poured into the boxes.
The sculpture anatomically respects the dimple and the marginal crest of the supporting tooth.
They must not interfere with occlusion
The cleat-stem junction must be thick enough so as not to create a weak area in the metal at this point.
Hook arms: They are placed on the shoulders provided at the level of the covering model.
The arm-stem junction is thick enough to not create a weak area in the metal at this point.
- Setting up the casting rods, cone and casting vents:
- The rods have a diameter of 5 to 4 mm, 2 to 3 rods are enough for a frame.
- Are connected to the tank.
- Describe a loop and move towards the ends of the wax model.
- Setting up the casting cone
- Setting up the vents:
At the ends of the hook arms ; they are perpendicular to the hook arm and extend down to 3mm from the edge of the model.
At the level of the bar or plate ; there are 3 or 4 of them and they are placed in different places.
Remaining gases and impurities are forced out of the molten metal through the vents.
- Casting the alloy
- Cylinder heating
- Place the cylinder in the cold oven and heat to 250°c (30-60 mins) depending on the size of the cylinder
- Heat the oven to its final temperature 950°c to 1050°c (30-60 min)
- The induction slingshot:
- It is the passage of a high frequency current through a solenoid which causes an induced current in the melting mass.
- In practice, the solenoid is made up of a copper tube, in which water circulates to ensure its cooling.
- This system is normally coupled with a centrifuge. Induction melting allows for a precise melting temperature, and, moreover, it does not bring any external element to the alloy.
- Melting the alloy.
- The alloy weights are arranged in the crucible of the slingshot
- The cylinder is placed on the casting sling by induction
- The induction slingshot is in operation and an electronic device regulates the temperature, when the alloy is molten the slingshot is triggered.
- After casting, allow the muffles to cool to room temperature.
- Demolding and finishing of the metal frame.
Demolding
The coating is removed using pliers or hammers.
Sandblasting
The chassis is sandblasted using a sandblaster which projects particles at high speed which are either corundum, alumina or silica.
Roughing
The casting rods are cut with a separating disc.
The remains of the casting channels can be removed and smoothed using an abrasive disc.
- Polishing and finishing
Electrolytic polishing
Immersion of the prosthetic part in the electrolytic bath
Mechanical polishing
With tips mounted in hard rubber then increasingly softer
The finish is completed by polishing using a polishing paste or a silicone product.
- Base-resin polymerization
The most commonly used method is the classic pressed resin method using thermo-polymerizable resins. It begins with placing in a muffle. Sequences of boiling, preparation and stuffing of the resin, pressing and polymerization follow one another. The demoulding must be carried out delicately followed by the finishing of the prosthesis.
Conclusion :
The production of the chassis in the laboratory requires a complex technological chain. A good knowledge of the properties of the materials is essential, and above all their suitability with the techniques used, in particular at the level of the casting systems and the handling of refractory coatings.
PPAM laboratory steps
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