Basic concepts and impression classification
To create a high-quality restoration, the clinician interacts with the prosthetic bed — the complex of tissues with which the finished prosthesis will come into contact. The negative replication of these tissues and their boundaries is called an impression (or cast).
Impressions are classified according to several key criteria:
- By border molding method:
- Anatomical — taken with standard or individual trays. They reflect tissues in a state of physiological muscular rest. Used when creating any fixed prostheses.
- Functional — require the use of individual trays and special functional tests simulating the action of masticatory and mimic muscles. Necessary for complete edentulism or for single teeth.
- By tissue coverage: complete (entire dental arch) and partial.
- By pressure on the mucosa:
- Compression (pressure is exerted by the clinician's hands or the patient's masticatory muscle force).
- Decompression (relief, taken with perforated trays and flowable materials).
- With differential pressure.
Rigid impression materials: gypsum and zinc oxide pastes
Rigid materials include gypsum and zinc oxide-eugenol compositions.
Gypsum is the most common material used at all stages of work (from impressions to soldering). In dentistry, hemihydrate calcium sulfate (α-gypsum and β-gypsum) is used. Upon mixing with water, crystallization (setting) occurs. The rate of this process can be accelerated by heating, adding salts (sodium chloride, potassium sulfate) or slowed down by sugar, ethanol, and borax. Standard dental gypsum contains 99.7% hemihydrate gypsum, potassium sulfate as an accelerator, as well as colorants and peppermint oil. It is important to remember: the faster the gypsum sets, the lower its final strength.
Zinc oxide-eugenol pastes (e.g., "Dentol-M") consist of two pastes. Structuring occurs through the interaction of zinc oxide with eugenol. To eliminate undesirable effects, rosin (reduces stickiness), balsams (mask the eugenol odor), and oils are introduced into the composition. These pastes exhibit minimal shrinkage (0.1–0.15%) and are ideal for edentulous jaws.
Elastic materials
Elastic materials acquire elasticity after structuring. Historically, the first ones were compositions based on agar-agar (seaweed extracts).
Today, the following are actively used:
- Alginate materials (based on the sodium salt of alginic acid). Available as powders or paste-powder systems. Characterized by high elasticity, allowing easy removal of the impression in the presence of undercuts. Representatives: "Stomalgin-66", "Novalgin".
- Silicone materials. These are compositions of rubber, fillers, and a catalyst. Upon mixing, vulcanization occurs in 4–6 minutes. They possess excellent accuracy (capable of reproducing a groove 0.04 mm wide).
- Thiokol materials. Consist of polysulfide rubber and an accelerator paste. They have lower thermal shrinkage compared to silicones. They are characterized by high plasticity, but can be excessively sticky in their uncured state and have a specific odor.
Thermoplastic and modeling compounds
Thermoplastic materials soften upon heating and harden upon cooling. They are divided into reversible (withstanding multiple heating cycles) and irreversible. Their composition includes thermoplastic bases (wax, paraffin, gutta-percha), resins for hardness (rosin, shellac), and fillers (chalk, talc) that reduce tackiness.
A prominent representative is Orthocor. It does not harden completely in the oral cavity and can remain there for up to a day, which is ideal for obtaining functionally suctioned impressions in severe alveolar ridge atrophy. Disadvantages of such materials include poor replication of undercuts and soft tissues, as well as difficulty in sterilization.
Modeling materials are also used to create appliances. Strict requirements are imposed on them: absolute harmlessness, sufficient plasticity when heated, retention of elasticity after modeling, and minimal thermal shrinkage (no more than 0.1% per each degree of temperature drop).
Briefly: answers to topical questions
What general medical and technological requirements are placed on an ideal impression material?
An ideal impression material must meet a set of requirements ensuring patient safety and high tissue replication accuracy.
- Accuracy and stability — precise reproduction of relief, low shrinkage (up to 0.1%), absence of property changes during prolonged storage.
- Physicomechanical properties — high plasticity upon insertion, elasticity after setting, insolubility and absence of swelling in saliva, good separability from the model.
- Biocompatibility — absence of unpleasant odor, taste, and harmful effects, sterility to guarantee against infection introduction.
- Setting — rapid hardening under oral cavity humidity and temperature conditions without adverse effects on tissues.
What are the stages of layering and firing a porcelain-fused-to-metal crown?
The process of creating the ceramic coating of a porcelain-fused-to-metal crown includes four sequential stages of material application and firing.
- First firing — application of the first (opaque) ceramic layer onto the prepared metal framework.
- Second firing — modeling the shape of the crowns and the intermediate part of the prosthesis from dentin mass.
- Third firing — correction of the size, coating shape, occlusal contacts, and facets.
- Fourth (final) firing — color correction and glazing of the prosthesis after its clinical adjustment in the oral cavity.
The groups of elastic impression materials are easily remembered by the acronym AST: Alginate, Silicone, Thiokol.
Class and exam questions
Anatomical impressions reflect the tissue relief in a state of relative muscular rest, whereas functional ones are taken using special tests simulating muscle function.
Anatomical impressions reflect the tissue relief in a state of relative muscular rest, whereas functional ones are taken using special tests simulating muscle function.
How can the setting process of gypsum be accelerated?
Setting is accelerated by increasing the water temperature to 30–37 °C, intensive mixing, and the addition of catalyst salts such as sodium chloride or potassium sulfate.
Why are thiokol materials considered accurate?
They exhibit very low shrinkage (thermal expansion coefficient is 2 times lower than that of silicones) and allow casting multiple gypsum models from a single impression without loss of quality.
What functions does rosin perform in material compositions?
In zinc oxide-eugenol pastes, it reduces stickiness and imparts the necessary consistency to the mass, while in thermoplastic compositions, it increases overall hardness.
What else to study on this topic
- Mechanism of crystallization of α- and β-modifications of hemihydrate gypsum
- Functional tests during impression taking with an individual tray
- Chemical processes of silicone material vulcanization
- Composition and application of multi-component alginate powders
- Effect of inhibitory substances (sodium tetraborate, glycerin) on gypsum
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