Articulating dental plaster, dental super-plaster: composition, use

Despite the development of the dental industry, the emergence clasp and metal-ceramic prosthesesThe dental technician's need for plaster powder has not decreased.

The material is among the auxiliary and is widely used for making high-precision impressions, creating models (copies of hard and / or soft tissues of the oral cavity), is a part of metal dental prostheses as the main or additional molding materials.

Composition and specifications

In terms of its composition, dental gypsum is calcium sulfate dihydrate (CaS04 - 2H20). It is a sedimentary rock with a layered crystal lattice formed by precipitated sulfate salts.

Gypsum is traditionally extracted from lakes and lagoons from aqueous solutions as a result of drying. Also deposits of natural gypsum are found in mountainous areas along with limestone, rock salt, clays.

Heat treatment (roasting or calcination) converts the substance into calcium sulfate hemihydrate (CaSO4) 2 - H20, further heating - into anhydrite.

In dental practice, a semi-aquatic modification of gypsum is used, which has a number of necessary characteristics, namely:

• dimensional stability and accuracy;
• excellent color contrast;
• environmental safety;
• lack of taste and smell;
• insolubility under the influence of saliva;
• low shrinkage rates;
• affordable cost.

When choosing an auxiliary material, it is necessary to pay attention to the strength indicators, the degree water absorption, the absence of metallic impurities, the proportion of hydrated water and the volumetric extensions.

Manufacturers supply gypsum powder in bags made of paper or cellophane impregnated with a water-repellent substance, packaged in jars equipped with ground-in lids. The package must contain information about the trade name of the product, information about the manufacturer and supplier, the class of gypsum, the scope of use, color characteristics, and net indicators. Also on the packaging are recommendations on the rules and shelf life, batch number.

How is the material obtained?

Crystals are colorless and transparent. However, all sorts of impurities, such as clay, pyrite, quartz or carbonate, paint them in different shades - from pink to black. To obtain dental plaster, the material at the first stage is cleaned of impurities and crushed to a powdery state.

Next, the natural material is heated to a temperature sufficient to remove some of the water. There are several manufacturing methods. As a result of their application, medical, modeling and super gypsum are obtained. The composition of medical gypsum of all varieties is identical - (CaS04) 2 - H20.

The material differs in structure and particle shape:

1. The hemihydrate obtained by firing is medical gypsum (β-hemihydrate). The process of its manufacture is heating in an open digester until some of the moisture evaporates. The material is porous and loose. Before use in dental practice, the powder is mixed with water in a 2: 1 ratio.
2. Model gypsum (a-hemihydrate) is made by autoclaving. When mixing non-porous powder with water, the following proportions are used - 5 parts of gypsum particles to 1-1.5 parts of water.
3. To obtain super dental gypsum, the boiling method with the addition of chloride (magnesium or calcium) is used. Chlorides act as deflocculants, preventing floc formation and promoting particle separation. When adding water, proportions of 5: 1 are traditionally used.

Video from a specialist:

Types of plaster in dentistry

There is a classification of dental plaster, according to which this material is subdivided according to its hardness and purpose.

1. Low harddesigned for impressions. The softest and most pliable material used in the dental industry. Differs in small expansion and high speed of hardening. It is used to create full or partial impressions, including when working with jaws that are completely devoid of teeth.
2. Medical alabaster. It is an auxiliary material often used to recreate anatomical models used in various prosthetic constructions. This class is not suitable for working models, as it has low strength indicators, but is indispensable for technical work.
3. High strength solid. Class 3 materials are used for partial and complete dentures. They are also suitable for the manufacture of the base of collapsible non-removable structures. Differs in decent strength indicators.
4. Extra strong dental plaster 4 classes with reduced expansion. High strength and almost complete absence of expansion of the material when working with it makes such a variety gypsum is indispensable both for creating collapsible models and for carrying out combined dental work.
5. Heavy-duty with adjustable expansion rate. A rare class of material with impurities of various synthetic components. Articulating plaster is designed to work with high-precision models.

For complex structures, they work with several types of raw materials at once. For example, the basement is made of class 3 plaster, and super plaster is used to create the alveolar ridge and teeth. If the model is to be resistant to high temperatures (up to +1000 degrees), quartz sand is added to the mixture. If it is necessary not to lose properties at +1500 degrees, special refractory compounds should be used.

Work rules

When working with dental material, it is necessary to adhere to a number of rules, which determine how high-quality and durable plaster teeth will be.

1. Store the material in a place protected from moisture. If the powder is damp, it must be dried at a temperature of + 150-170 degrees. The restoration of the working properties of the material can be judged after control mixing.
2. All devices, storage containers, mixing accessories must be thoroughly cleaned of previously used material.
3. The use of curing catalysts is in most cases unacceptable. If necessary, you should opt for a quick-hardening material or adjust the mixing time.
4. Do not mix large portions of the mixture. It is enough to combine powder and water in the amount necessary to fill 2-3 impressions.
5. It is important to accurately observe the proportions of powder and distilled or well-settled tap water, following the manufacturer's recommendations regarding work with a particular class of material. This will provide the expected expansion and strength of the future product.
6. The optimum temperature of water and gypsum powder is + 19-21 degrees. If the temperature of the mixture is increased to + 30-37 degrees, the setting time of the material will be reduced. Further heating (+ 37-50 degrees) will not affect the solidification rate. After +50 the mixture begins to solidify very slowly, and at a temperature of +100 - the setting process stops altogether.
7. The machine kneading time in a vacuum device is 30 seconds. When manually connecting the components, it doubles.
8. The mixture should be poured into the mold immediately after preparation.
9. The beginning of the hardening process can be judged by the lack of gloss on the surface of the future cast. You can start modeling and cropping (60 seconds).
10. The model is removed from the mold only after its temperature has dropped.
11. The finished casts must not be exposed to harsh influences. Damage during steam building can be avoided by wetting the model with water. It is not recommended to clean the product with a steam jet. You can take care of it with a soft brush and a special detergent.

Hand kneading technology

You can get a high-quality plaster cast only if you follow the mixing sequence:

1. Distilled water is poured into a special bowl, the amount of which is determined by the class of powder.
2. Gypsum powder is slowly poured into the liquid. According to standards, it should take about 10 seconds from the beginning of contact of the first particles with water until the end of falling asleep.
3. After that, you should wait until the particles are completely submerged in the water and only then start mixing with a metal or plastic spatula.
4. Movements should be as vigorous as possible. The consistency of a full-fledged mixture is creamy, homogeneous.

If too much water is added, the gypsum will only absorb the amount it needs. Remaining water will quickly loosen the structure of the material and reduce the accuracy of the impression. Adding less liquid than required by the technology for producing the mixture would be not the best solution. Thick gypsum does not make it possible to obtain an accurate imprint due to the formation of air bubbles, which simply do not have time to come to the surface due to too fast solidification.

Taking an impression of the upper jaw on video:

Application of inhibitors and catalysts

The strength characteristics of prints and models are influenced by the solidification rate. However, dental technicians prefer not to use special accelerators and process retarders. Additives can degrade the quality of products.

The optimal solution is the choice of high-quality raw materials and adherence to the production technology (choice of temperature regime, powder dispersion, intensity of mixing of components).

But in rare cases, the use of solidification optimizers is also permissible, in the role of which are:

• sodium chloride solution, potassium nitrate (catalysts);
• ethyl alcohol (5%), sugar solution (5-6%), borax solution (2-3%), joiner's glue (inhibitors).

Accelerators (catalysts) reduce the strength and hardness of future casts. In contrast, inhibitors increase these characteristics. Additives cannot be used when creating jaw models, as the likelihood of errors and inaccuracies increases. Inhibitors and catalysts are added both to water and directly to gypsum powder.

Diverse prosthetic material is widely used in dentistry due to its ease of use, affordable price, excellent aesthetic and functional characteristics. Observing the technique of making impressions and models, you can create high-quality impressions that facilitate the diagnosis and measures for the prosthetics of the dentition.