Treatment of impressions in fixed prosthesis
The working model, being the first laboratory step, is directly linked to the quality of the impression made by the practitioner, which is essential for obtaining a faithful replica of the clinical situation.
Many parameters come into play in the accuracy of the working model and the successive manipulations required are all sources of error or deformation. Knowledge of the properties and requirements of the materials used is essential to limit the risks of inaccuracy.
I / Casting materials:
- Material characteristics:
- Ability to reproduce details
- Very simple and easy handling.
- Compatibility with impression materials.
- Resistance to abrasion and fracture.
- Absence of toxicity.
- Dimensional stability.
- Dental casting materials:
- Dental plaster.
- Epoxy resin.
- Electroplated metals.
- Low melting alloys.
II/ Disinfection of the imprint:
Before casting the impressions, they must be rinsed and disinfected.
- Alginate: passage with water
disinfectant spray bleach at 0.5% print is enclosed in a waterproof bag rinsing with water poured from the print. - Silicone impression A : immersion in 2% glutaraldehyde solution.
- Polyethers : no prolonged immersion 2 to 3 minutes in 1% bleach or better by spraying with 3% glutaraldehyde.
- Reversible hydrocolloid : no immersion but 2% glutaraldehyde spray or 0.5% bleach.
III / Casting processes:
1- Treatment of the impressions by casting in stone plaster (extra hard plaster):
- Chemical characteristics :
It is an alpha hemihydrate of calcium sulfate, it is obtained by calcination of gypsum. It is the material most frequently used in fixed prosthetics.
In particular, type IV stone plasters, extra hard, while type III will be reservedto the casting of antagonistic models.
Easy and relatively quick to handle, the plaster is compatible with all impression materials.
The dimensional accuracy is excellent since the setting expansion inherent to this material ranges from 0.01% to 0.4%, the ability to reproduce details is quite good but the hardness and abrasion resistance are quite poor, these mechanical properties being very dependent on the preparation conditions and the composition specific to each brand
- Casting the prints:
-The elastomer prints are brushed with a wetting product in order to lower the surface tension.
-Strictly respect the water/plaster powder ratio recommended by the manufacturer.
-A mixture that is too liquid produces a fragile plaster, but with low setting expansion.
-A thick mix has a higher hardness, a faster setting but a greater expansion;
– Never add water during mixing, as this will reduce the hardness and cause abnormally high setting expansion.
- Hand mixing : using a flexible rubber bowl with a stainless steel spatula.
- Mechanical mixing : increases the homogenization of the mixture.
- The impression is filled using a vibrator.
- There are two methods for casting impressions:
- Casting with formwork : which consists of building a mold for the model using a strip of soft wax, the periphery of the impression is wrapped and then the mixture (powder + water) is poured into the impression under vibration.
- Casting without formwork : the mixture (powder + water) is poured into the impression; the remainder of the mixture is placed in a nut-like shape on a plate, the impression is poured onto this nut and the base is made.
The resulting model should be smooth and precise.
- Treatment of prints with epoxy resin:
- the mixture is carried out in a container allowing the obtaining of the creamy consistency, the epoxy resin is deposited in the impressions of the preparations using a spatula or a brush then the entire impression is poured on a vibrator, or better, centrifuged to limit the risk of air bubbles.
- They are incompatible with Hydrocolloids and have good reproduction of details, good resistance to abrasion and easy handling, however they present risks of allergies.
- Dismantling will be carried out 3 hours after the start of mixing.
3- Treatment of prints by electroplating .:
Treatment based on electrolysis which allows a deposit (Ag, or Cu+2, or Ni) to be applied in the impression.
- The Principle; transfer of metal ions from the anode (charge +) to the cathode (charge
-) thanks to a continuous electric current in a liquid medium (electrolysis).
The molecules unite, losing their positive charge and adhering strongly to each other, thus forming a very hard and resistant metallic surface.
The impression is then completed by pouring extra hard plaster or epoxy resin.
- Disadvantages:
Silver plating baths contain cyanide, while copper plating baths are acidic and release potentially dangerous fumes.
Implementation is complex and lengthy.
IV / Fractionation techniques:
These techniques aim to obtain the unitary positive model (UPM)
MPU: reproduces the prepared tooth with all its details
Various techniques are possible to obtain these fractions, whatever the technique used, the working models must have the following characteristics:
-MPUs should be removed and replaced without difficulty.
-The relationships between MPU and adjacent and antagonist teeth must be as accurate as if it were an unfractionated model.
- The working model must be able to be placed on an articulator.
- The MPUs must be parallel to each other to allow easy removal of the wax models.
- The manipulations carried out during the different working times must take place without moving the MPUs.
Obtaining removable MPUs integrated into a model involves 2 types of systems:
- First-line fractionation.
- Second-line fractionation.
- First-line fractionation:
“DOWEL PINS” system
Figure 1: The Dowel-Pins
Dowel pins are generally made of stainless metal and include:
- A retentive head which takes its place opposite the dental preparation.
- The body is tapered to facilitate insertion and removal of the MPU.
*The dowel pins must be placed parallel to the tooth preparation otherwise the MPU will not move easily in the working model.
- The dowel pin is held in place using a holding device consisting of a hairpin fixed by two pins (one placed on the vestibular edge of the impression, the other on the lingual edge).
*The impression thus prepared, the dowel pins will pass between the two heads of the hairpin, its retentive head must be at a distance of
1 to 2 mm from the internal face of the preparation impression.
- The impression is placed on a vibrator, extra hard plaster is poured up to 3mm above the teeth (the retentive head of the pin is covered with plaster).
When the plaster is still soft, paper clips are placed in the areas not affected by the splitting and which will serve as means of retention for the base which will be poured afterwards.
Figure 2: Placing the Dowel Pins and Pouring the Impression
*After the plaster has hardened:
- A ball of red wax is placed on the end of the pin.
- The body of the pine is coated with Vaseline, as is the plaster of the dental preparation.
- The impression is then poured into hard plaster of another color, thus creating the base of the working model: the added plaster must stop at the upper edge of the wax ball.
- After the base has hardened, the wax pellet is removed from the base, thus revealing the end of the pin body. The plaster is sawn on either side of the dental preparation until the upper part of the plaster base is reached.
- By simply pressing on the end of the pins, the MPU will be released.
Figure 3: Model splitting and MPU disinsertion
Figure 4: The split model and the MPUs
- Second-line fractionation:
- “ Monobloc ” System or Detachable Nesting System:
-The base is made using a removable plastic box called a die-lok.
- The impression will be cast in one piece, covering all of the teeth. The MPUs will be made in a second step.
- The one-piece model is ground so that it can be inserted into the mold (horseshoe-shaped)
- After filling the mold ¼ full with a hard plaster of a different color, the positive model (horseshoe-shaped) which has previously been in water is inserted into the mold.
- After hardening, the model is released from the mold and then cut according to the number of dental preparations. Cutting is carried out over ¾ of the height of the model.
- The MPUs will be made by fracturing the base of the model.
- All the MPUs obtained can be reconstituted in the mold (di-lok) thanks to the serrations which are engraved on its internal face.
Figure 5: MPUs reinserted into the Monobloc system
- Pindex system ( device that digs wells using a drill)
Principle: Drilling parallel wells at the base of the model obtained after casting the impression
-Horseshoe shaped model
-Determination of the location of wells by a light location system
-Drilling wells
-Installation of glued pins in the wells
– Splitting the model.
Figure 4: Pindex system: (the drill bit elevation lever is operated slowly, the wells are drilled successively to their maximum depth)
Conclusion :
The working model is the first link, at the laboratory level, in the prosthetic chain leading to the creation of a fixed reconstruction. The quality of this link is primarily dependent on the quality of the practitioner ‘s work , but the final quality of the work will largely depend on the rigor of its design.
Bibliography
- Laviole O, Bartala M Production of working models in joint prostheses EMC: 23-380-A-10 .2000
- Exbrayat J, Schittly, Borel JC. Manual of fixed single prosthesis.
- Herbert T, Shillingburg Fundamentals of Fixed Prosthetics
- R.BUGUGNANI, C.LANDEZ. Impressions in joint prosthesis.
Treatment of impressions in fixed prosthesis
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