Production of Partial Prosthesis with Metal Frame in the Laboratory
Introduction
Plan
- Review of working models
- Preparation of working models
- Preparation of the replica in refractory material
- Preparing the chassis model
- Fixing the rods, casting cone and coating
- Preparatory heating
- Casting the alloy
- Finishes
- Articulating
- Selection and assembly of prosthetic teeth
- Polymerization of resin bases
Conclusion Bibliography
INTRODUCTION :
The production of a removable cast partial prosthesis requires a rigorous laboratory technological chain aimed at preserving all the information given by the clinician and developing a correct prosthesis that meets all the prosthetic requirements.
- REVIEW OF WORKING MODELS:
After making the preparations in the mouth and taking the impression, the practitioner must compare his working model obtained with the study model. It is desirable to reproduce on the working model the project highlighted on the study model (insertion axis, type of clasps and dental preparations).
It is essential to place the working model again on the parallelizer in order to check the insertion axis already chosen and to retrace the guide line on the hook support teeth in order to objectify the undercut areas necessary for the retention elements and the usable undercut areas for the chassis stabilization elements.
- PREPARATION OF WORKING MODELS:
It consists of removing all recessed areas except those reserved for the retentive ends of the hooks. It includes two phases:
-Removal of undercut areas at the level of the hook support teeth by pouring liquid wax (from the marginal edge of the gum to the lower gingival limit of the hook);
– Creation of discharge areas
- At the location of the lingual bar (allow a spacing of 0.4 mm to avoid any ulceration).
- At the level of certain sensitive areas (torus, retro-incisive papilla, strong undercut).
- At the level of the edentulous areas to allow the mechanical anchoring of the resin between the metal saddles and the model.
- At the level of certain hook arms spaced from the gum (example Roach T hook).
- PREPARATION OF THE REPLICA IN REFRACTORY MATERIAL:
A replica of the prepared model, called a “DUPLICATE”, must be prepared because it is on this that the wax model of the chassis and the casting of the
the alloy.
The making of this duplicate is done as follows:
-1- Taking an impression of the model prepared with gelatin which is a reversible hydrocolloid, kept in a warmer (gelatin) set to 50°.
This step requires the use of a gelatin muffle inside which the prepared model is placed after having hydrated it and then dried it with compressed air.
The gelatin is then poured into the muffle until saturation.
After the gelatin has set (1 hour of cooling), the starting model is carefully removed.
-2- Casting the impression into the refractory material:
Each coating marketed has its own characteristics and its own instructions for use, hence it is imperative to follow the manufacturer’s instructions regarding dosage and usage protocol.
Phosphate-bonded or silicate-bonded coatings are intended for the foundry of alloys with a melting range greater than 1100° (chromium-cobalt alloy).
The material is mixed under vacuum and poured on a vibrator into the gelatin impression.
After an hour, the “DUPLICATA” can be detached from the gelatin, which will then be dehydrated in an oven at 200°C for 45 minutes then quickly dipped in a hardening bath.
- PREPARATION OF THE CHASSIS MODEL :
The sculpture of the frame is done using prefabricated elements called “PREFORMS” marketed in wax or plastic and which will be glued to the surface of the covering.
The sculpting of the chassis model is done in five stages:
-Wax filling of the compartments intended for the occlusal stops,
-Installation of the various hooks,
-Installation of retention grids at the saddle level,
-Setting up the various connections (main and secondary),
– Finishing the model by adding wax. When finishing, care must be taken not to create weak points at the connection areas.
At the end of this step, the model is ready to be put into a cylinder.
- FIXING THE RODS, THE CASTING CONE AND COATING:
- Fixing the rods and the casting cone
After finishing the model sculpture, casting rods must be attached to this frame, their role is to conduct the molten alloy into the model to be cast. Their number varies depending on the area of the plate and the number of hooks.
All these rods must converge towards a main rod which is in turn connected to a rubber or wax casting cone which will be placed opposite the melting crucible.
- Coating
The model is secured to the base of a cylinder using non-brittle wax.
The same coating is used as for the “duplicate” while respecting the instructions for use.
After the investment has set and the cylinder has been removed, the casting cone is separated from the cylinder, the main rod appearing at the bottom of the impression. The cylinder is then placed in the furnace with the casting cone pointing downwards.
- PREPARATORY HEATING:
Slow heating is carried out in stages of 10 to 15 minutes, first at 300°C, then at 600°C, and finally a stage of 45 minutes at the melting temperature of the alloy which is 1050° for Chrome-Cobalt.
This heating has the following objectives:
- To remove all traces of wax and moisture,
- To eliminate unwanted gases formed,
- To develop the desired amount of expansion for the coating,
- To reduce the difference between the temperature of the cylinder and that of the molten alloy.
- CASTING THE ALLOY:
At the end of this stage, the actual metal casting can be initiated; for this, it is necessary to melt the alloy used in order to allow it to flow into the cylinder.
The most commonly used alloy in partial prosthesis with metal framework is Chrome-Cobalt (known in the industry as stellite) for its excellent rigidity under low thickness, its good biological tolerance and its cost price.
It is during the third stage that the casting takes place. This casting is done either by centrifugation or by pressure/vacuum.
- FINISHES:
- Roughing out the prosthetic part: After casting, the cylinder is cooled in open air. It is then fractured using a hammer by tapping on the third of the cylinder on the model side.
The prosthetic part is then recovered, sandblasted to remove coating debris and the oxide layer covering the metal surface.
At this stage, the casting rods are cut, then comes the electrolytic polishing (the chassis plays the role of a soluble anode on its surface) in an acid medium, a rinse with water to finish.
- Mechanical polishing: The entire part is deburred with a series of rotating instruments to remove all roughness on the surface of the casting, followed by polishing with horsehair brushes – Chamois Leather – to give the casting a shiny appearance.
At the end of this step, a final check is required to verify the correct adaptation of the prosthetic part to the initial model previously freed from the discharge wax.
- ARTICULATOR INSTALLATION:
The frame is then tried in the mouth, the occlusion is recorded using the frame as a support for the occlusion model.
The working models are mounted in the articulator and the assembly of the artificial teeth can begin.
- CHOICE AND ASSEMBLY OF PROSTHETIC TEETH:
The prosthetic teeth are chosen in material, shade, dimensions and shape.
The assembly of the teeth is done in accordance with the classic assembly rules, namely:
- The axes of the anterior teeth (in the three sagittal, horizontal and frontal planes),
- Integration of the proximal face of the prosthetic tooth into the shoulder of the hooks,
- Respecting the ridge line of the crest for the posterior teeth in order to locate the mesio-distal grooves on the same line.
- POLYMERIZATION OF RESIN BASES:
The polymerization of resin bases is significantly more complicated to implement due to the presence of the metal frame with numerous hooks.
The flask is placed on the working cast, which has the advantage of not losing the precision of the impression . The particularity of this flask is that the frame is entirely invested in the plaster. The classic steps of scalding, preparation of the thermopolymerizable resin, stuffing, pressing and finally polymerization are carried out. The deflask, given the fragility of the elements constituting the frame, must be carried out with caution. The use of very high viscosity silicone placed next to the fragile elements facilitates the recovery of the prosthesis invested in the flask.
CONCLUSION :
Respecting the technological chain allows us to develop a metal frame that will fit in the mouth with a minimum of possible alterations.
B IBLIOGRAPHY:
[1]- O’BRIEN, RYGE Translated and adapted by Pierre Desautels, Precise dental materials and selection guide,
Gaëtan Morin editor;
[2]- SKINNER and PHILLIPS,
Science of dental materials, Julien prélat, Paris 1971;
[3]- BEGO, The metallic removable prosthesis,
Official care provider to the German Olympic teams since 2002.
[4]- O. LAVIOLE, E. d’INCAU, J. BROUSSEAUD, V. ICHANE and I. QUEGUINER, D. BLONDEL and L. ARNOULD,
From impression to testing of the metal frame: laboratory steps and clinical implications, Prosthetic Strategy September 2005 • vol 5, n° 4;
[5]- P. Rocher, J.-J. Guyonnet, G. Grégoire, Work on dental alloys,
EMC Odontology 23-360-A-10, 2004;
Production of Partial Prosthesis with Metal Frame in the Laboratory
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