Impressions in joint prosthesis
1-Introduction:
2-Definitions: “An impression is a negative cast of all or part of the arch and surrounding tissues allowing a positive model to be obtained, a faithful replica of the recorded tissues” 1 .
“The reproduction in negative with exact dimensions of specific regions of the oral cavity or of preparations concerning dental organs.”
The purpose of the fingerprints is to establish:
- Documentary models, archival and forensic documents,
- Study models to aid diagnosis,
- Working models.
3-Physicochemical fingerprints
- Materials:
- properties
- Dimensional stability.
- Elastic behavior characterized by high elastic deformation and low plastic deformation.
- Safety for tissues.
- Setting curve allowing sufficient working time and short oral setting time.
- Compatibility with casting materials.
- High wetting power.
- Available in several viscosities.
- Easy to use.
- Materials used :
Plaster (for prints):
- Advantage :
- Very high rigidity.
- Dimensional stability (long processing time).
- Absence of secondary tissue reaction.
- Low cost.
- Disadvantages:
- No elasticity;
- Very brittle;
- Very low detail recording accuracy;
- Delicate treatment modalities.
Thermoplastic pastes: Commonly called Kerr paste
It comes in 3 forms:
- Red paste: 55° and 56°;
- Grey paste: 53 and 54°;
- Green paste: 50 and 51°
According to various authors, green Kerr paste would be the most recommended for joint prostheses.
- Used mainly for single prints.
- Disadvantages:
- Not used for preparations with an undercut;
- The imprint fractures easily;
- Easily deformable;
- Influences pulp physiology.
- Indications:
- Preparation on pulpless teeth;
- Coronal preparation of remains without marked cervical limit;
- Coronal-radicular preparation.
Hydro colloids:
- Advantage :
- Simplicity and speed of the fingerprint protocol.
- No bubbles, no mixing required.
- High precision.
- High wetting power, hydrophilic material.
- Disadvantages:
- Specific impression trays for water circulation.
- Low resistance to stress.
- Immediate print processing.
- Decontamination impossible.
Elastomers:
They belong to the family of synthetic rubbers, macromolecular substances with elastic properties.
The 03 families used are:
- Polysulfides;
- Polyethers;
- Silicones.
Polysulfides:
- Benefits :
- Long usage time;
- Acceptable dimensional stability;
- High elasticity.
- Disadvantages:
- Very long setting time;
- Unpleasant handling: odor, consistency;
- Low stress resistance;
- Hydrophobic material.
Polyethers:
- Benefits :
- High hardness;
- Low permanent deformation;
- Hydrophilic;
- Excellent dimensional stability.
- Disadvantages:
- Short usage time;
- Few different viscosities;
- Sensitive to imbibition;
- High hardness, difficult to remove the model, risk of plaster fracture.
Silicones:
Depending on their polymerization method, two types of silicone elastomers can be distinguished:
- First generation silicones, crosslinking by condensation reaction: poly-dimethyl siloxane base (silicone C).
- Second generation silicones, crosslinking by addition reaction: poly-divinyl siloxane base. (silicones A).
Condensation crosslinking silicones or C silicones
- Benefits :
- Many viscosities;
- Quick take;
- Compatible with all casting materials;
- Allows all imprint techniques.
- Disadvantages:
- Poor dimensional stability;
- Hydrophobic;
- Risk of bubble inclusion when mixing low viscosities.
Addition-crosslinking silicones or A-silicones
- Hydro compatible (for new generation).
3-2 Imprint techniques :
3-2-1 The single impression: this is the impression of the prepared tooth(s) only.
3-2-2-Situation footprints :
The purpose of the unitary positive model MPU obtained by the unitary imprint is to be integrated with the rest of the arcade, the situation imprint will allow this integration.
It is done by 2 different processes:
- Transfer imprint process.
- Transfer process.
3-2-3 Global impressions: Allows you to obtain both, and at the same time, the recording of the preparations and those of the teeth or adjacent sectors, these impressions can concern all or part of the dental arch.
With unitary guidance:
- The triple mix:
Single imprint in double mix + on imprint
- Materials to be used in copper tubing: Heavy material which is relined by a fluid material.
- The overprint is generally made with a medium viscosity material.
Indications:
Global impressions of coronal preparation and peripheral coronal-radicular with subgingival character.
Metal-resin formwork:
Take single impressions with a subgingival character followed by an over impression.
The impression screed:
Use a screed made of baked self-polymerizing resin covering the preparation and creating a spacer using
of an occlusal stop the latter allows:
- Limit PE sinking;
- To record the occlusal relationships of the opposing teeth by adding self-polymerizing resin;
- To condense the impression material at the periphery of the preparation, towards the bottom of the gingival sulcus.
- To reduce compressive stresses.
- To accommodate the impression material in uniform thickness
Without unitary guidance:
Reversible hydrocolloid prints:
Impression of choice particularly for patients with reduced periodontium. Indicated for:
- Impressions of peripheral coronary preparations;
- Impressions of preparations for inlay onlay veneer.
The technical wash: in 02 steps
The wash technique involves taking impressions in two stages and using two materials of different viscosity used one after the other.
Indication:
- For clinical situations inducing very little pulling at the time of removal (exclude poorly positioned teeth, exposed roots, unfilled caries cavities, etc.).
Clinical protocol:
- The impression tray is coated with adhesive.
- Access to the cervical margins is provided by a retractor cord or retractor material.
- The base-loaded impression tray (putty or soft putty) records the reliefs.
- The primary imprint is fitted to the blade holder to create vents.
- Low viscosity (light or super light) is deposited by syringe or gun in the impression tray and on the preparation.
- The cord or spacer material is removed.
- The primary imprint is repositioned.
- After the material has set, the secondary impression is removed.
Double Mix Technique: 1-Step Technique
The double mix technique corresponds to taking an impression in one go but with two materials of close viscosity. These two materials will oppose each other in their consistency, one having a higher viscosity (Putty, Soft), the other being of lower viscosity (Medium or Light).
-The impression is made with 2 elastomers of different viscosity and presenting the same state of plasticity.
Clinical protocol:
- The impression tray is coated with adhesive.
- A double cord or spacer material achieves the gingival separation.
- The impression tray is filled with medium viscosity.
- The self-mixing gun or syringe is loaded with low viscosity.
- The second retractor cord or spacer material is deposited.
- The preparations are coated with low viscosity material (Medium or light).
- Excess low viscosity material is distributed over the impression tray.
- The impression tray is inserted and held until the material sets .
- Unplug and check the recording, check for bubbles.
4-Optical prints:
For decades we have been making conventional physicochemical impressions, in recent years the intra-oral optical impression is emerging and trying to establish itself.
The intra-oral optical impression made using a camera is a fundamental step in Computer Aided Design and Manufacturing.
It intervenes during the direct and semi-direct CAD/CAM acquisition stage.
The intraoral optical impression is based on three separate recordings: the affected arch, the opposing arch, and a vestibular recording in the maximum intercuspation position allowing the two arches to be occluded.
The advantages of optical imprinting are based on precision, unalterability, the absence of drawing and
disinfection protocol.
This technique generates less stress and fewer tedious steps.
Deep cavities may require root canal treatment.
Interdental brushes effectively clean between teeth.
Misaligned teeth can cause chewing problems.
Untreated dental infections can spread to other parts of the body.
Whitening trays are used for gradual results.
Cracked teeth can be repaired with composite resins.
Proper hydration helps maintain a healthy mouth.