Joint prosthesis in the laboratory

Joint prosthesis in the laboratory

The plan:

1. Introduction
2. The production of prosthetic parts in the laboratory:

1. The cast crown
2. The inlay core.
3. Crown and bridge with vestibular inlay:

1. Resin-metal crown and bridge
2. Metal-ceramic crown and bridge

4. Ceramic-ceramic
5. Inlays and onlays
6. Bonded bridges
7. Conclusion
8. Introduction:

The making of a joint prosthesis is the result of many actions that take place in the clinic and the laboratory, hence the importance of good communication between the practitioner and the prosthetist; especially with the evolution of dental products that better meet the aesthetic and functional requirements that the practitioner must be aware of.

2. The production of prosthetic parts in the laboratory:

• The articulator setting:

After having split the model and made the occlusion rims, we move on to placing it in the articulator. 

1. The cast crown :

It is made using the addition wax method : a wax model is sculpted on the MPU and then cast with the appropriate alloy.

It has 03 degrees of hardness:

• soft wax
• medium hardness wax
• hard wax
• Construction of the screed:

The screed must meet certain characteristics, namely:

• it must be stable on the MPU
• She must be a member
• It should not be removed before the edge finishing phase. 
• Its internal surface must perfectly reproduce the details of the preparation.

The thickness of the screed wax must be uniform.

PROTOCOL:

The development of the form is carried out in 4 stages which are: the preparatory stage, the occlusal stage, the axial stage and the finishing stage.

• Coating and casting:

• the metal to be cast: can be either:
  • a precious alloy
  • a non-precious alloy.
• The coating: there are three types:
  • silicate-based coating (maximum heating 1000°C for non-precious alloys)
  • Ammonium phosphate coating (maximum heating from 850°C to 950°C, this is the most used for precious and non-precious metals.
  • the gypsum-based coating (750°C) used only for casting precious metals.
• Preparation and coating

Preparation of the wax model:

The molten alloy fills the model, begins to solidify with a decrease in volume, and draws alloy that is still molten from the feeder.

• The casting rods: 

Model at 45° between the feed rod and the part to be cast; Their role is to lead the molten alloy as quickly as possible into the model to be cast, via the feed bar and the feed channels.

• The nourishing bar:
  Its advantages:

• stabilize the bridge frame
• serve as a reservoir for molten alloy

Make sure that the casting rods have the required dimensions.

In the case of single crowns, each of them must have its own supply channel.

The crowns should be outside the thermal center against the cylinder walls and will be the first to cool

• Coating:

A thickness of “liner” cylinder strip is applied in order to obtain surfaces that do not present any porosity.

The powder/liquid mixture of the coating is mixed under vacuum following the manufacturer’s recommendations (temperature and time) then the models are coated with the coating using a brush before filling the cylinder.

• Heating of refractory molds:

The preheating temperature of the refractory block is related to the melting range of the alloys to be cast. 

Low-melting precious alloys intended for inlays, inlay-cores, crowns, small bridges and low-melting metal-ceramics, their melting range is between 800 and 1,000 °C. Their recommended preheating temperature is 650 to 700 °C.

Joint prosthesis in the laboratory

Precious alloys intended for the metal-ceramic technique or non-precious nickel-chromium alloys, whose melting range is between 1,050 and 1,350 °C, must be cast in phosphate-bonded investments preheated to between 780 and 820 °C.

• Melting the alloy:

• By blowtorch 
• By compact induction 
• By Joule effect heating  
• Flows

• Centrifugal casting:
• Pressure/vacuum casting 
• Demolding the prosthetic part and polishing:

To ensure that the casting is not subjected to stress, allow the cylinder to cool slowly to room temperature. Then the part is sandblasted, the casting rods are sectioned with a disc; then the crown is adjusted on the MPU; roughing with tungsten carbide cutters;

Polishing with silicone polishers.

2. The inlay core

• MPU preparation:

Apply the spacer with a brush to all sides of the preparation, as well as at the entrance cone, and blow air to remove excess.

• Development of the model:

This manipulation includes three distinct times.

• Transcanal wax modeling:
• Modeling of the terminal prosthesis
• Wax subtraction

The wax removed from its MPU will represent the model of the future anchor.

• Casting the anchor:

• The casting will be done in a conventional manner.
• The anchor sites that will be responsible for ensuring a limit will be polished, the other sites will only be sanded.

3. Vestibular inlay crown

• Preparation of the frame:

A wax model of the armature is made, then cast according to the method already described; after roughing, the armature is sandblasted with alumina; finally the armature is cleaned with a steam jet 

The shape of the final prosthesis is sculpted in wax; a silicone key is made to allow control of the space provided for the cosmetic material.

The inlay can be made either in resin or in ceramic:

1. Inlay with resin:

For the resin and before casting, we apply an adhesive and beads to the model on the face to be inlaid to create retentions for the resin.

After finishing, the minimum thickness of the metal must be 0.3mm for single elements and 0.5mm for bridge abutments.

• Dentin stratification :

Joint prosthesis in the laboratory

Joint prosthesis in the laboratory

• Polymerization/heat treatment:

Joint prosthesis in the laboratory

Finishing is done using tungsten burs and fine diamond discs. It is recommended to work at low speed and under low pressure.

• The surface (convex area) and edges must be polished 

Joint prosthesis in the laboratory

2. Inlay with ceramic:

• Choice of alloy:

It can be precious or semi-precious. All these alloys must meet specific requirements:

• their melting temperature must be higher than the firing temperature of the ceramic, with 
• it must have a high modulus of elasticity to ensure high rigidity at low thickness 
• The ceramic supra structure:

Knowing that there are several types and classifications of dental ceramics:

• Classification according to chemical composition

• feldspathic ceramics
• glass ceramics
• Ceramics or rather hydrothermal glasses
• Aluminous ceramics
• Classification according to shaping processes.

• with metal frame
• without metal frame

The technique of placing the ceramic:

• Setting up the opaque:

It is used for:

• Hide the metal color
• It represents the base color of the finished tooth.
• It helps to strengthen the metal-porcelain bond.

After cooking, it should look like an eggshell.

• Mounting and layering of ceramics:

The ceramic powder is mixed with distilled water, or a modeling liquid to obtain the ceramic paste with which the tooth will be shaped.

The assembly of the paste is done with a brush. In this case, the ceramic paste is not condensed .

• Cooking ceramics:

After shaping, the raw dough is made up of solid materials, water and air.

Its cooking cycle begins at room temperature and ends with a return to this same temperature. 

• Finishing :

To adjust this surface condition, there is a thermal method, glazing, and a mechanical method, polishing, as well as the possibility of combining the two.

The icing : this consists of a final cooking at a temperature slightly lower than the cooking temperature of the biscuit. 

Polishing  : at the biscuit stage, a pre-polishing is carried out, intended to allow the fitting in the mouth. At the end of this test, the sculpture is finished with a diamond burr. 

4. Ceramic-ceramic:

There are several concepts in the production of ceramic-ceramic prostheses; these concepts are summarized in the following table:

5. Inlays and onlays

• In cast metal: 

The MPU is prepared, the limits visualized with a pencil, the spacer is applied avoiding the limits. The model is elaborated using a special wax for inlays which finely reproduces the details.

The casting is done in precious or semi-precious metal.

The inlay or onlay is then fitted onto the working model.

• In composite resins 
• In ceramic 

These restorations can be sealed (alumina or zirconia) or bonded (feldspathic ceramic veneers)

6. Bonded bridges

• Implementation of the infrastructure

The design of the infrastructure for a bonded bridge is important. It cannot be dissociated from that of the preparation because the extensions of the anchoring means will be determined by the preparation.

3. Conclusion :

A final result that stands out for its harmony is the fruit of the skill of the prosthetist and the good match between the different materials.

Good oral hygiene  Regular scaling at the dentist  Dental implant placement Dental x-rays  Teeth whitening  A visit to the dentist  The dentist uses local anesthesia to minimize pain  

Joint prosthesis in the laboratory