Introduction

The purpose of my course work is to study impression materials, their use in dentistry, methods of making an impression, its use in work, as well as the use of some well-known modern Russian impression materials.

Definition of impression materials

Impression materials are used to obtain an accurate impression of teeth and oral tissues. From this imprint or impression a model can be cast on which designs for full or partial removable dentures, crowns, bridges and inlays are made.

Over the years, a wide variety of impression materials have been created and many methods have been developed for their practical application in order to obtain an impression material with the optimal combination of properties necessary for this.

Some impression materials do not have sufficient viscosity for use in a standard tray, these include zinc oxide eugenol, polyester and polysulfide elastomers. Others, such as impression compounds (thermoplastic impression materials), plaster, alginate and silicone materials of appropriate composition, can be used to take impressions using a standard impression tray. Although thermoplastic compounds can be used with a standard impression tray, the resulting impressions do not accurately reproduce surface detail unless they are enhanced by an additional impression using flowable zinc oxide eugenol material. Likewise, alginates, when used with a standard impression tray, do not always give the required degree of accuracy, in which case it is better to take the impression with a custom tray.

The choice of impression material and tray type depends on the required level of dimensional accuracy and reproducibility of surface detail.

Classification of impression materials

Plasticity is of great importance for obtaining an accurate impression, i.e. in relation to impression materials - the ability to fill all elements of the relief of the touching surface, and elasticity, i.e. the ability to maintain the given shape when removing the impression from the oral cavity without residual deformation.

All dental impression materials can be divided into:

ь hard;

b elastic;

ь thermoplastic.

Hard impression materials

In the work of dental institutions, it is important to follow the rules for storing plaster. Semi-aqueous dental plaster has significant hygroscopicity, absorbing atmospheric moisture, it deteriorates, and its setting becomes worse. Therefore, it is recommended to store the plaster in good packaging, preferably in a dry and warm place and not on the floor. This prevents it from getting damp. Long-term storage of gypsum, even in a well-closed container and without access to moisture, makes it unsuitable, since the gypsum cakes into lumps and sometimes does not set at all. This is explained by the fact that hemihydrate is an unstable compound and water is redistributed between its particles, resulting in the formation of a more stable compound - dihydrate and anhydride.

2(CaS04) x H20 -> CaS04 x 2H20 + CaS04

Depending on the heat treatment conditions, semi-aqueous gypsum can have two modifications - a- and beta-hemihydrates, which differ in physical and chemical properties:

A-gypsum is obtained by heating gypsum dihydrate under a pressure of 13 atm, which significantly increases its strength. This gypsum is called super gypsum, autoclaved gypsum, stone gypsum;

Beta gypsum is produced by heating gypsum dihydrate at atmospheric pressure.

After firing, gypsum is ground, sifted through special sieves and packaged in special paper bags or barrels. The setting of gypsum occurs very quickly. Immediately after mixing with water, thickening of the mass becomes noticeable, but during this period the gypsum is still easily molded. Further compaction no longer allows for molding. Freshly prepared gypsum and a previously hardened gypsum product are firmly connected to each other. This property is used in denture technology, for example, when plastering models in an articulator or cuvette.

Practice shows that the separation of two plaster products, for example an impression and a model, can be accomplished without the use of insulating substances. To weaken the connection between them, the print is first immersed in water until completely saturated, that is, until all the air is displaced from its pores. A print saturated with water can no longer absorb moisture from the freshly prepared plaster mass applied to its surface. However, along with its positive qualities, gypsum has a number of disadvantages, as a result of which in recent years it has been almost completely replaced by other materials. In particular, plaster is fragile, which often causes the impression to break when removed from the mouth. At the same time, its small details, filling the space between the teeth, are often lost. This lack of gypsum is especially evident in cases where there is divergence and convergence of teeth, their inclination towards the lingual or buccal sides, as well as in periodontal diseases, when the extra-alveolar part of the teeth increases.

In addition, the plaster impression is difficult to remove from the oral cavity by breaking into fragments, is difficult to separate from the model, and is not disinfected. Therefore, gypsum, especially super-hard varieties, is much more often used as an auxiliary material, mainly for obtaining models of jaws.

There are many varieties of gypsum produced for the needs of orthopedic dentistry. In accordance with the requirements of the international standard (ISO), there are 5 classes of gypsum according to the degree of hardness:

I -- soft, used for making impressions (occlusal impressions);

II - ordinary, used for applying plaster casts in general surgery (this type of plaster in the literature is sometimes referred to as “medical plaster”);

III - hard, used for the manufacture of diagnostic and working models of jaws in the technology of removable dentures;

IV - super-hard, used to obtain collapsible models of jaws;

V - extra hard, with the addition of synthetic components. This type of gypsum has increased surface strength. Mixing requires high precision in the ratio of powder and water.

Solid impression materials also include zinc oxide eugenol pastes, among which the most widely used is Czech Repin, which consists of 2 aluminum tubes with white (main) and yellow (catalyst) pastes. The composition of the catalyst paste includes:

Clove oil (eugenol) -- 15%;

Rosin and fir oil - 65%;

Filler (talc or white clay) - 16%;

Accelerator (magnesium chloride) - 4%.

Both pastes are mixed in equal proportions. The precipitation reaction leads to hardening of the material, which is accelerated by intensive mixing, adding moisture and increasing temperature. The material is intended for obtaining functional impressions, especially from edentulous jaws.

To manufacture any design, it is necessary to obtain an impression.

Imprint is called a negative reflection of the tissues of the prosthetic bed. In order to obtain an impression, it is necessary to have various impression materials. There is a close connection between the quality of the prosthesis and the quality of the impression from which it is made. No matter how carefully all other stages of prosthetics are carried out, the prosthesis will not satisfy the requirements placed on it if the impression from which it was made was defective. That is why methods for obtaining impressions are so carefully developed, which are different for different types of prostheses.

The quality of impression materials, their ability to give accurate impressions in various conditions of the oral cavity is also

are increasing. However, practice shows that the search for the ideal impression material has so far been fruitless. And one must think that the very formulation of this problem is unrealistic. For an attempt to create a universal impression material is made without taking into account the entire huge variety of prosthetic conditions: the general condition of the patient, his individual sensitivity to removing typos, age, the nature of the defects, deformation, the state of nasal breathing, the shape, position and relationship of the teeth, their statics, the degree of pliability of the mucous membrane the shell of the prosthetic bed in different people and in different areas of the same jaw, the nature of the folds, etc. These circumstances require the research and use of materials with different properties. Therefore, to obtain impressions under various conditions in the oral cavity, it is necessary to have a sufficient range of impression materials and, most importantly, to choose them correctly in each specific case and apply a technique that would ensure the desired result.

In recent years, the medical industry has been successfully working on the creation of new impression compounds. Some of them are already used in prosthetic practice, others have been prepared for production and are currently being tested in laboratories and clinics of orthopedic dentistry.

Chapter 16. Supporting materials

All impression materials can be divided into 3 groups:

I. Crystallizing:
Zinc oxide eugenol (dentol);

II. Elastic:

1. Hydrocolloid masses:
Kruglyakova,

"Double ha"

2. Alginate: Geltrey, Stomalgin-02,

3. Silicone: Sielast,

4. Thiokols: Tiodent;

III. Thermoplastic:
Thermoplastic masses N1, N2, N3,
Wall,

Akrodent,

Dentafol.

Crystallizing

Materials

Dentol. The history of the creation of cross-linking zinc oxide eugenol systems dates back to the 1880s. The first structuring material based on zinc oxide and clove oil was proposed for dental purposes in 1887. However, these materials were used for filling purposes. Zinc oxide eugenol impression material was described in 1934 by Ross, and in 1935 the dental company Kerr (USA) began producing impression material - Kelly paste.

In the USSR, zinc oxide eugenol material was developed by chemical engineers at the Kharkov Dental Materials Plant in 1962 and was named “Dentol”. Comes in a box containing two tubes of pink and white pastes, instructions for use and keys for squeezing pastes out of the tubes.

Zinc oxide eugenol impression materials are

developed compounds based on the zinc oxide-eugenol structuring system. The material contains the following main components: zinc oxide, eugenol, fillers, structuring accelerator, rosin, balm to reduce the irritating effect of eugenol, plasticizer and dyes.

Accelerators - zinc acetate (1.5-2%).

Fillers - talc, kaolin, chalk.

Rosin - provides the necessary consistency of the paste, reduces stickiness and is a structuring accelerator.

Plasticizers - the best plasticizer is petroleum jelly.

Corrective agents - peppermint oil.

Due to the fact that eugenol is quite expensive, it began to be replaced with guaiacol.

Dentol is a high-quality impression material. It has high ductility and is practically non-shrinking. Thanks to its properties, dentol makes it possible to take very accurate impressions not only from soft tissues, but also from teeth, and some of its elasticity allows you to avoid pulls and distortions when removing the impression from the oral cavity.

The main purpose of dentol is to obtain impressions from toothless jaws. High quality dental impressions can only be taken on hard individual trays with a small thickness (2-3 mm) of the impression material.

Dental impression mass is prepared by mixing white and pink pastes. Squeeze out equal volumes of pastes from both tubes onto a glass plate and mix them thoroughly with a flat spatula for 0.5-1 minutes until a uniform color is formed. The prepared paste is applied in a thin layer to the base spoon and fixed on the jaw. Its consistency allows you to remove compression and unload

Section II. Materials used for the manufacture of plate dentures for complete loss of teeth

The thickness of the impressions depends on the time elapsed from the beginning of mixing the mass to its introduction into the oral cavity.

Sometimes dentol causes a slight burning sensation on the surface of the mucous membrane in contact with it, but after removing the impression, these sensations disappear. The impression is structured in the oral cavity in 2-5 minutes, after which it is removed. Hardening time depends on the temperature of the paste and the environment, the amount of white paste and humidity. With increasing temperature, increasing the amount of white paste and humidity, the rate of structuring increases.

Dentol has one very good property. If you apply the newly mixed paste to the already hardened surface of a dento impression, then when it hardens it will bond well with the original layer. This quality is successfully used to obtain functionally suction impressions. To do this, a new layer of paste 2-5 mm wide and 1-3 mm thick is applied along the entire edge of the dental impression taken in the usual way. The impression is reintroduced into the oral cavity, pressed against the jaw, after which its edges are functionally shaped. With this technique, the newly applied layer of dentol slightly compresses the mucous membrane in the area of ​​the valve zone, as a result of which the effect of functional suction increases significantly.

The print can be stored for a long time without changing in volume or configuration. The plaster model is cast in the usual way. Removing the impression from the model is ensured by preheating it (2-3 minutes) in warm water. It should be noted that keeping a model with a print in warm water for a longer period of time is unacceptable, since the print becomes sticky and is difficult to separate from the model.

Currently, a large number of zinc oxide eugepol impression materials are produced in different countries: Kelly (Kerr, Italy), Realin (Stoma, Ukraine), Repin (Dental, Czech Republic), Dendia paste (Holland), Rapid Potty Soft "(Austria), "Koltex", "Las-tin" (Kolten, Germany), etc. However, when working with these materials, you must also adhere to the rules described above, as when working with dentol.

Currently, a variety of materials and technological processes are used for the manufacture of orthopedic devices and prostheses, the number of which is increasing every year. The dentist is required to have the ability to use various orthopedic dental materials, knowledge of their physical, chemical and medical-technical properties to control various technological processes in the manufacture of devices and prostheses.

Impression materials

Thermoplastic masses

Flaws:

Insufficient accuracy of the cast.

Poor shape retention during temperature changes.

Inability to remove the impression from the mouth if the setting point is missed.

Impossibility of sterilization.

Alginate masses

Representatives:

Ypeen,

Orthoprint,

Kromopan, etc.

Advantages:

1. Cheap.

2. Ease of use.

3. Sufficient accuracy in the case of manufacturing removable dentures, temporary crowns, diagnostic models, bite models

4. Ease of removing the finished model from the impression.

Flaws:

1. Insufficient precision for the manufacture of solid structures.

2. Large and rapid shrinkage.

3. The need to immediately make models to avoid drying out of the print.

4. Poor adhesion to the spoon.

Method of application:

When mixing alginate materials, it is necessary to strictly observe the proportions of powder and water, which may differ from each other for different materials. For this purpose, the manufacturer of the material supplies appropriate measuring tapes with it. Alginate masses are mixed in a rubber cup with a special spatula

To prevent medical errors when mixing alginate materials, it is recommended to use mechanical mixing devices like Alghamix (Zhermack®). When using such devices, it is much easier to achieve a uniform consistency of the material; the mixing time is reduced by 30%; in the process of mixing chromatic alginates, there are three stages (mixing, processing, placing in the oral cavity), which correspond to certain colors

The alginate impression model should be cast immediately. If this is not possible, the print is packaged in a sealed bag with a damp cloth.

C-silicones

Representatives of condensation silicones:

Oranwash, Zetaplus, Thixoflex (Zhermack®),

Speedex(Coltene Whaledent),

Exakt N,G, Viscoflex(KOHLER).

Chemical structure - polydimethylsiloxanes with hydroxyl end groups. They form a three-dimensional structure by polycondensation with the formation of a by-product - alcohol. Their main qualities are connected with this.

Advantages:

Low price

Sufficient precision for the manufacture of solid structures

Low shrinkage

Elasticity, but strength of both corrective and base mass

Possibility of disinfection

Flaws:

Not ideal quality when taking impressions with retraction cords

Requires thorough manual mixing of masses and catalysts of different consistency

Difficulty in accurately dosing the catalyst, everything is done by eye

Models cannot be cast from an impression multiple times.

Sensitivity to moisture - hygroscopicity

Low hydrophilicity

Insufficient adhesion to the spoon

The literature describes the possibility of a toxic effect

No automatic mixing

Somewhat excessive rigidity of the base mass

Method of application:

When mixing C-silicones, it is very important to adhere to the manufacturer's instructions, since an excess of activator leads to accelerated polymerization, and a lack of activator, as well as uneven mixing, can lead to incomplete polymerization of the material

Important! Restoration of the linear dimensions of the impression after removal from the oral cavity occurs within half an hour. Therefore, it is not recommended to cast the model before this time. At the same time, after about 1 hour, dimensional changes begin to occur due to the evaporation of the alcohol formed during the polycondensation process. This period of time is optimal for casting the model. The maximum period for casting a plaster model using a condensation silicone impression is 24 hours.

Impressions made from C-silicone materials are easily disinfected when exposed to a disinfectant solution for 30 minutes. Before casting the model, it is recommended to rinse the impression with liquid to reduce surface tension.

A-silicones

Advantages:

Nearly perfect reproduction of detail

Easy mixing and precise dosage of mass and catalyst due to their homogeneity

Variety of mass viscosities

Dimensional stability and accuracy, maintained during long-term storage (models can be cast 30 days after receiving the impression)

Resistance to deformation and ideal restoration of shape after it

High thixotropy, high hydrophilicity

Excellent adhesion between layers

Possibility of high-quality disinfection

Possibility of automatic mixing of both base and corrective mass

No unpleasant taste or odor

Optimal compatibility with mucous membrane and skin

Non-toxic, hypoallergenic

Flaws:

Do not knead with latex gloves

A-silicones are slightly more expensive than C-silicones

Polyester impression materials

Representatives:

Impregum, ESPE company

Advantages of polyester impression compounds:

Possibility of use for almost all types of work

High accuracy

Easy mixing when using an automatic kneading machine - Pentamix

High thixotropy High hydrophilicity

Possibility to use one impression to make several models

Increased working time due to reduced setting time

High strength

Possibility of sterilization and soaking in any solutions used to disinfect impressions

Prints can be stored, according to some sources, for more than a month without shrinking.

Disadvantages of polyester impression compounds:

In some cases, it is difficult to remove the impression from the mouth

Relatively high cost.

Basic polymers

Plastics– which are based on polymers that are in a viscous or highly elastic state during the formation of products, and during operation in a glassy or crystalline state

Classification (according to severity):

Rigid (plastics for removable dentures and their restoration)

- Ethacryl (AKR-15), Ftorax, Bakryl, Colorless plastic for prosthesis bases

Soft or elastic (boxing boots or as a soft lining under a hard base)

- MP-01

Self-hardening:

Individual spoons

Relining dentures

Repairing removable dentures

Orthodontic devices

- (redont, protacryl)

Cements

Classification by chemical composition

1) zinc phosphate

2) polycarboxylate

3) glass ionomer

4) polymer-modified glass ionomers

5) composite.

Classification according to the type of reaction on which the solidification process is based

Cements with acid-base hardening reaction (groups 1–3)

Cements with polymerization reaction (5th group)

Polymer-modified glass ionomer cements, cured through a combination of acid-base reaction and polymerization (4th group).

Indications

Structures on metal frames with a stump height of more than 5 mm – 3rd group of cements;

All-ceramic structures, structures on metal frames, with a stump height of less than 5 mm - the 5th group of cements.

Metal alloys

Requirements for metal alloys

Possess high mechanical properties - hardness, strength, elasticity

Possess high technological properties - malleability, fluidity, minimal shrinkage, good workability

Possess high chemical resistance to oral environments

Have the necessary physical properties - low specific gravity, the desired melting point and coefficient of thermal expansion.

Gold-based alloys

Alloy 900º:

Gold 90%

Silver 4%

Melting point 1000º

Alloy 750º:

Gold 75%

Silver 8.35%

Copper 12.5%

Platinum 4.14%

Melting point 1000º

Solder:

Gold 65-70%

Silver 8.35%

Copper 12.5%

Platinum 4.14%

Cadmium 5-10%

Melting point 800º

Alloys based on silver and palladium (SPS)

Silver 72% - the basis of the alloy, increases hardness

Palladium 22% - increases corrosion resistance due to the formation of a protective film on the surface of the alloy

Gold 6% - increases fluidity, eliminates the corrosive instability of silver in the oral cavity

Melting point 1100-1200º

Iron-based alloys (chromium-nickel alloys, stainless steel)

1Х18Н9Т:

Carbon 0.1%

Nickel 9%

Titanium 0.9%

Iron 72%

Casting shrinkage 3%

Alloys based on cobalt and chromium (CHS)

Cobalt 67% - the basis of the alloy, has high mechanical properties

Chromium 26% - increases corrosion resistance, adds hardness

Nickel 6% - increases viscosity

Molybdenum 0.5% - increases strength

Manganese 0.5% - lowers melting point, improves fluidity

Casting shrinkage 1.8%

Technological processes:

Pressure treatment

Heat treatment

Grinding

This group includes gypsum and zinc oxide eugenol pastes.

Gypsum occupies a leading place in the group of auxiliary materials used in orthopedic dentistry. It is used at almost all stages of prosthetics. It is used to obtain:

• imprint;
• jaw models;
• face masks;
• as a molding material;
• when soldering;
• for fixing models in an occluder (articulator) and a cuvette.

Natural gypsum is a widespread mineral of white, gray or yellowish color. Its deposits are found together with clays, limestones, and rock salt. The chemical composition of natural gypsum is determined by the formula CaS0 4 x 2H 2 O - calcium sulfate dihydrate. The formation of gypsum occurs as a result of its precipitation in lakes and lagoons from aqueous solutions rich in sulfate salts. Gypsum deposits usually contain admixtures of quartz, pyrite, carbonates, clayey and bituminous substances. The density of gypsum is 2.2-2.4 g/cm3. Its solubility in water is 2.05 g/l at 20° C.

Gypsum for dental practice is obtained by firing natural gypsum. In this case, calcium sulfate dihydrate loses part of the water of crystallization and turns into hemihydrate (hemihydrate) calcium sulfate. The dehydration process occurs most intensively in the temperature range from 120 to 190 ° C.

2(CaS0 4 x 2H 2 0) - (CaS0 2)2 x H 2 0 + 3H 2 0

Depending on the heat treatment conditions, semi-aqueous gypsum can have two modifications - ?-and?-hemihydrates, which differ in physical and chemical properties (Table 3):

• ?-gypsum is obtained by heating gypsum dihydrate under a pressure of 1.3 atm, which significantly increases its strength. This gypsum is called super gypsum, autoclaved gypsum, stone gypsum;
• ?-gypsum is obtained by heating gypsum dihydrate at atmospheric pressure.

After firing, gypsum is ground, sifted through special sieves and packaged in special paper bags or barrels. When gypsum hemihydrate is mixed with water, dihydrate is formed, and the entire mixture hardens.

(CaS0 4)2 x H 2 0 + 3H 2 0 2(CaS0 2 x2H 2 0)

This reaction is exothermic, that is, it is accompanied by the release of heat. Gypsum sets very quickly(see Table 4). Immediately after mixing with water, thickening of the mass becomes noticeable, but during this period the gypsum is still easily molded. Further compaction no longer allows for molding. The setting process is preceded by a short period of plasticity of the gypsum mixture. Mixed to the consistency of sour cream, the gypsum fills the molds well and gives clear imprints. The plasticity of gypsum and subsequent rapid hardening make it possible to use it to take impressions of jaws and teeth. However, the process of increasing the strength of gypsum still continues for some time, and the maximum strength of the gypsum impression and plaster model (see Table 4) is achieved when it is dried to a constant mass in the environment.

The setting speed of gypsum is influenced by a number of factors: temperature, degree of grinding (dispersity), mixing method, quality of gypsum and the presence of impurities in the gypsum. Increasing the temperature of the mixture to +30 - +37° C leads to a reduction in the setting time of the gypsum. When the temperature increases from +37 to + 50° C, the setting speed begins to drop noticeably, and at temperatures above 100° C, setting does not occur. The degree of grinding (grinding fineness) also affects the rate of hardening: the higher the dispersion of the gypsum, the larger its surface, and an increase in the surface of two chemically reacting substances leads to an acceleration of the process. The setting speed of the hemihydrate is also affected by the method of mixing it. The more vigorously the mixture is mixed, the more complete the contact between the plaster and the water will become and, therefore, the faster the setting. Damp gypsum hardens much more slowly than dry gypsum. It is best to dry such gypsum at a temperature of +150 - +170° C. During drying, it is necessary to constantly stir the gypsum, since due to its poor thermal conductivity, uneven heating is possible, which leads to the partial formation of products such as insoluble anhydride, etc.

Catalyst salts are of particular importance when working with dental plaster. They usually speed up the setting process of the plaster. The most effective accelerators are potassium or sodium sulfate, potassium or sodium chloride. When the concentration increases above 3%, they, on the contrary, slow down setting. Most often in dental offices a 2-3% solution of table salt is used as an accelerator. Inhibitors of gypsum hardening are sugar, starch, and glycerin.

Catalysts are substances that accelerate chemical reactions.

Inhibitors are substances that slow down chemical reactions or stop them.

When obtaining jaw models, accelerators should not be used, firstly, to slow down hardening, and secondly, to strengthen the plaster. There is, as a rule, an inverse relationship between the rate of hardening of gypsum and its strength: the faster the setting occurs, the lower the strength of the resulting product, and vice versa, the slower the mixture hardens, the stronger it is (see Table 5). For example, mixing gypsum with a borax solution results in a noticeable slowdown in hardening, resulting in a very durable product.

Strengthening of gypsum models is carried out using various techniques. After the plaster has been thoroughly dried (to remove any moisture remaining in the pores), the model is immersed in molten stearin or paraffin. The surface of the product acquires shine and the appearance of ivory. A similar treatment is used to prepare educational exhibits (dummies) in order to give plaster models a beautiful appearance and increase strength. Freshly prepared gypsum and a previously hardened gypsum product are firmly connected to each other. This property is used in denture technology, for example, when plastering models in an articulator or cuvette. In cases where a plaster model is obtained from a plaster impression, this property serves as an obstacle to their subsequent separation. In order to avoid this phenomenon, sometimes a fat layer is applied to the surface of the mold.

However, the use of fat or petroleum jelly can lead to distortion of the model, so a more suitable material for separating the surfaces of the print and the model may be a soap solution or a solution of liquid glass, in which the print is immersed for 5-10 minutes. These solutions form a thin film and distort the relief of the model less. Practice shows that the separation of two plaster products, for example an impression and a model, can be accomplished without the use of insulating substances. To weaken the connection between them, the print is first immersed in water until completely saturated, that is, until all the air is displaced from its pores. A print saturated with water can no longer absorb moisture from the freshly prepared plaster mass applied to its surface. Thus, the surface of the model will fit tightly to the surface of the print without the penetration of particles of one into the thickness of the other, and they can be easily separated by chipping.

In the work of dental institutions, it is important to observe rules for storing gypsum. Semi-aqueous dental plaster has significant hygroscopicity, absorbing atmospheric moisture, it deteriorates, and its setting becomes worse. Therefore, it is recommended to store plaster in good packaging (metal barrels, thick paper bags), preferably in a dry and warm place and not on the floor. This prevents it from getting damp. Long-term storage of gypsum, even in a well-closed container and without access to moisture, makes it unsuitable, since the gypsum cakes into lumps and sometimes does not set at all. This is explained by the fact that hemihydrate is an unstable compound and water is redistributed between its particles, resulting in the formation of a more stable compound - dihydrate and anhydride.

2(CaS0 4) x H 2 0 CaS0 4 x 2H 2 0 + CaS0 4

The fact that gypsum has long been the main material for impressions is explained, firstly, by the lack of alternative masses. Secondly, it was accessible and cheap. In addition, the advantages of plaster include the fact that it allows you to obtain a clear imprint of the surface of the tissues of the prosthetic bed, is harmless, does not have an unpleasant taste or odor, practically does not shrink, does not dissolve in saliva, does not swell when wetted with water and is easily separated from the model when using the simplest release agents (water, soap solution, etc.). However, along with the positive qualities of gypsum has a number of disadvantages, as a result, in recent years it has been almost completely replaced by other materials. In particular, plaster is fragile, which often causes the impression to break when removed from the mouth. At the same time, its small details, filling the space between the teeth, are often lost. This lack of gypsum is especially evident in cases where there is divergence and convergence of teeth, their inclination towards the lingual or buccal sides, as well as in periodontal diseases, when the extra-alveolar part of the teeth increases. In addition, the plaster impression is difficult to remove from the oral cavity by breaking into fragments, is difficult to separate from the model, and is not disinfected. Therefore, gypsum, especially super-hard varieties, is much more often used as an auxiliary material, mainly for obtaining models of jaws (see Table 5).

There are many varieties of gypsum produced for the needs of orthopedic dentistry. In accordance with the requirements of the international standard (ISO), there are 5 classes of gypsum based on the degree of hardness (see Table 4):

• I - soft, used for making impressions (occlusal impressions);
• II - ordinary, used for applying plaster bandages in general surgery (this type of plaster in the literature is sometimes referred to as “medical plaster”), for example Taliplaster (Galenika, Yugoslavia), which contains α-calcium sulfate hemihydrate;
• III - hard, used for the manufacture of diagnostic and working models of jaws in the technology of removable dentures, for example Plaston-L (GC, Japan), Gypsogal (Galenika, Yugoslavia), which contains β-calcium sulfate hemihydrate ;
• IV - super-hard, used to obtain dismountable jaw models, for example Fujirok-EP (GC company, Japan), Galigranit (Galenika company, Yugoslavia), which contains β-calcium sulfate hemihydrate;
• V - extra hard, with the addition of synthetic components. This type of gypsum has increased surface strength. Mixing requires high precision in the ratio of powder and water. For example, Duralit-S, a material based on synthetic β-calcium sulfate hemihydrate, is characterized by very low expansion during solidification, which ensures accurate working models.

High fluidity provides good mold filling ability, as well as high compressive strength and hardness. The ratio of powder and water when mixing is 100:19-21. Setting time is 7-10 minutes; expansion after setting< 0,12%; прочность на сжатие >50 N/mm; Brinell hardness > 15 MPa.

Superhard gypsum (?-hemihydrates)- Supergypsum (Russia), Begodur, Begostone, Herastone-M, Vel-Mix Stone and Supra Stone (Germany) - have a hardening time of 8-10 minutes, while expansion during hardening does not exceed 0.07% -0, 09%, strength under pressure 1 hour after hardening is 30 N/mm2, after 1 day - 35-60 N/mm2. The strength of some grades of gypsum from a number of companies is presented for comparison in Tables 3, 4, 5.

These materials are used in the manufacture of dismountable models of jaws combined with conventional plaster. The ratio of powder and water when mixing is 100 g per 22-24 ml of water. Synthetic extra hard plaster, for example, Herarok, Moldasint (Germany), are characterized by an expansion coefficient of approximately 0.1% 2 hours after mixing. In this case, the compression resistance reaches a level of 48 N/mm2. Superhard gypsum powders are strictly dosed with water and mixed in vacuum mixers. To mix extra-hard synthetic plasters, Hereus Kulzer (Germany) recommends using a special liquid - Gypsum-Brilliant-liquid. Thanks to the use of this liquid, the powder is evenly distributed in the liquid and the gypsum sets. The resulting plaster model is characterized by high homogeneous density, strength and accuracy of reproduction of the original. The tendency for pores to form on gypsum surfaces upon contact with water is reduced to a minimum when this liquid is used. The liquid is supplied in 1 liter bottles as a concentrate and is diluted with 19 liters of distilled water, for a total weight of 20 liters.

The Dutch company Euro-Dental produces an electronic gypsum mixer that operates completely automatically. The gypsum tank has a volume of 25-30 kg. Mixing takes place in a vacuum, and you can choose the time. After mixing, the inside of the device is automatically cleaned. If necessary, you can heat the water. Mixing devices are standard equipment even in small laboratories. The Bego company (Germany) has developed the Mottawa-SL vacuum mixer. With the help of a strong motor, it ensures intensive mixing and produces up to 98% of the mixed mass. The device uses 2 motors: one serves as a drive for the mixing device, the other drives the vacuum pump. The mixing bowl is made of hard rubber and makes cleaning easy. After completing the mixing programs, the magnetic valve automatically switches off the vacuum pump.

The company "Hereus Kulzer" (Germany) produces a vacuum mixing device CL-VMR-W for mixing molding mass and gypsum, which allows you to obtain a material free of air bubbles. After setting the mixing time (maximum 90 s), the process proceeds automatically. The forms are filled with plaster on vibrating tables (Vibromister, Vibroboy, Vibrobaby, KV-16, KV-36, KV-56 - all made in Germany). This eliminates the appearance of pores and cavities in the model.

Solid impression materials also include zincoxy-deugenol pastes, among which the most widespread is the Czech Repin, which consists of 2 aluminum tubes with white (main) and yellow (catalyst) pastes. The composition of the catalyst paste includes:

• clove oil (eugenol) - 15%;
• rosin and fir oil - 65%;
• filler (talc or white clay) - 16%;
• accelerator (magnesium chloride) - 4%.

Both pastes are mixed in equal proportions. reaction, precipitation, occurring between eugenol and zinc oxide, leads to hardening of the material (zinc eugenolate), which is accelerated by vigorous mixing, adding moisture and increasing temperature. The material is intended for obtaining functional impressions, especially with toothless jaws. It gives a clear, detailed imprint of the mucous membrane, adheres well to an individual spoon, and is quite easily separated from the model.

Eugenol mass Neogenate (Septodont, France) includes a white paste based on zinc oxide and a red paste based on eugenol (15%). Designed for obtaining functional impressions from edentulous jaws, relining dentures, fixing the wax base while determining the central relationship of the jaws. To prepare the material, approximately 10 cm of paste is squeezed out from each tube onto a glass plate or a block of thick coated paper. Using a hard wide spatula, mix both pastes thoroughly for 30 seconds until a fluid, homogeneous mass of pink color is obtained. The latter is applied to an individual spoon, which is inserted into the oral cavity, slightly shaken to evenly distribute the material, pressed against the jaw and held for about 1 minute, after which the patient makes the necessary functional movements with the lips, cheeks, tongue, floor of the mouth, and soft palate. The impression is taken out 2.5-3 minutes after insertion of the spoon. If the print has defects, then a layer of mass 1 mm deep is removed in their area and along the periphery. This place is filled with freshly prepared paste, and the spoon is reinserted into the oral cavity. The material does not shrink, so receipt of the model can be delayed.

Vikopres - zinc oxide eugenol paste Galenika company (Yugoslavia) for functional impressions. Due to its water-absorbing properties, it absorbs water from the surface of the oral tissues when taking an impression and ensures an accurate impression.

Additional components included with the paste:

• Viko-1 is an antiseptic skin cream designed to protect the patient’s lips and the dentist’s hands;
• Viko-2 is a liquid for removing paste from tools and models.

However, for all their advantages, zinc oxide eugenol pastes can become deformed or crumble when removed from the oral cavity. Therefore, they are being replaced by elastic impression materials and are mainly used as temporary fixation material for fixed dentures.

Initially, the classification of impression materials by I.M. Oksman became most widespread in Russia.

1. Crystallizing (gypsum and zinc oxide eugenol).
2. Thermoplastic.
3. Elastic (agar).
4. Polymerizing.

Then it was modified and proposed his own classification of impression materials by M.A. Napadov. .

Classification of impression materials according to M.A. Napadov. (1980).

I. Solid crystalline impression materials.

1. Plaster.

2. Zincoxide eugenols.

3. Zinc oxide guaiacol.

II. Elastic impression materials.

1. Hydrocolloid (agar).
2. Alginate.
3. Thiokol.
4. Silicone.
5. Polyester.

III . Thermoplastic impression materials.

1. Epoxy: Dentafol.

2. Based on rosin esters: MST-02, 03; Stens; Akrodent.

All impression materials, according to their properties, content of components and method of application, can be divided into two groups - reversible and irreversible.

Materials of the first group are characterized by the fact that from a solid or elastic state under the influence of temperature or other chemical reactions they pass into a plastic state, and then upon cooling or the end of the reaction they return to their previous state.

The materials of the second group are characterized by the fact that, being in a plastic state during the period of receiving the impression, as a result of chemical reactions they pass into an elastic state and remain in this state for a long time. The transition to the elastic state of these materials is irreversible.

Currently, there is an international classification of impression materials, proposed Nurt in 2002. This classification is based on the state of the impression material (the presence or absence of elasticity) after it hardens.

Classification of impression materials according to Nurt.

I. Hard impression materials.

1. Gypsum.
2. Thermoplastic compounds.
3. Zinc oxide eugenols.

II. Elastic (hydrocolloid).

1. Reversible - agar.
2. Irreversible - alginate.

III. Elastomeric.

1. Polysulfide.
2. Polyester.
3. Silicone, cured in a polycondensation reaction (C-silicones).
4. Silicone, cured in a polyaddition reaction (A-silicones).

Ryakhovsky A.N. and Muradov M.A. (2006) believe that the most complete, simple, logical and easy to use is the following classification of impression materials:

I. Inelastic.

Nbsp; 1. Zinc oxide eugenol pastes.

Nbsp; 2. Thermoplastic materials.

Nbsp; 3. Plasters.

Nbsp; 4. Bis-acrylates.

II. Elastic.

Nbsp; 1. Hydrocolloid: a) alginates; b) agars.

Nbsp; 2. Anhydrous elastomers: a) polysulfides; b) polyesters; c) silicones (A- and K-types).

Ibragimov T.I., Markov B.P., Tsalikova N.A. (2007) classify impression materials as follows:

I. Hardening.

Nbsp; 1. Plaster.

Nbsp; 2. Zincoxide eugenols.

II . Thermoplastic.

III. Elastic.

Nbsp; 1. Hydrocolloids: a) agar; b) alginate.

Nbsp; 2.Elastomers: a) polysulfide; b) polyester; c) silicone (A- and K-types).

Poyurovskaya I.Ya. (2008) proposes the following classification of impression materials:

I. Solid.

Nbsp; 1. Chemical hardening (irreversible): a) gypsum; b) zinc oxide eugenols.

Nbsp; 2. Thermal hardening (reversible) - thermoplastic compounds.

II. Elastic.

Nbsp; 1. Hydrocolloid: a) reversible - agar; b) irreversible - alginate.

Nbsp; 2. Elastomeric: a) thiokol; b) polyester; c) silicone - type K - polycondensation and type A - additive.