Silicone impression materials

Preparation technique for silicate cement

To obtain a high-quality filling mass, the mixing protocol must be strictly followed. The process is carried out on an absolutely clean and dry glass slab, which must be free of even minimal traces of moisture. Only a plastic spatula is used as an instrument.

The use of metal instruments is a strict contraindication. This is because silicate cement powder has high abrasiveness. During friction, it can abrade microscopic metal particles from the spatula, which will inevitably lead to contamination of the cement mass and a change in its properties.

The main working principle is the gradual addition of powder to the liquid. The entire process must fit into exactly one minute. Before starting work, the powder on the glass is divided into specific portions: first, exactly half of the total volume is separated, and the remaining part is divided into two equal quarters.

1. First half (1/2): Incorporated into the liquid during the first 30 seconds. Mixing is performed with light, wave-like movements of the plastic spatula.
2. First quarter (1/4): Added over the next 15 seconds. At this stage, the spatula movements change to circular.
3. Second quarter (1/4): Introduced in the final 15 seconds. Rotational movements continue until a completely homogenous, uniform dough-like mass is obtained.

Properly mixed cement, upon two light pressures with the spatula, should acquire a moist and shiny appearance, while the mass should not string after the instrument for more than two millimeters. If the mass turns out excessively thick, it is strictly forbidden to add extra liquid to it—such a portion must be discarded and the process started over.

Setting chemistry and clinical stages of work

The setting process of silicate cements is a complex two-stage chemical reaction. At the primary stage, the phosphoric acid contained in the liquid actively interacts with the glass particles from the powder. As a result of this reaction, silicic acid and aluminum phosphate are synthesized.

The secondary reaction is characterized by the formation of long chains of silicic acid gel, as well as colloidal aluminum phosphate. The final structure of the hardened material is a fibrous network of silicic acid gel and phosphates. Grains of unreacted particles of the original powder are firmly embedded in this spatial lattice.

It is important to note that during the chemical setting process, a significant portion of the acids remains in an unbound state for a long time. It is this factor that causes the pronounced toxic effect of silicate cement on the tooth pulp. The acidic reaction of the medium (pH about 4.0) gradually shifts toward neutral (pH 7.0) during the first 24 hours, however, a weakly acidic environment can persist within the bulk of the material for up to thirty days.

Clinical stages of work are strictly regulated in time. From one to one and a half minutes are allotted for condensation and final finishing of the filling in the tooth cavity. Complete setting of the restoration directly in the patient's oral cavity occurs three to four minutes after placement.

Features of filling with silicate cements

Silicate cements possess a number of specific mechanical properties, the main one being the pronounced brittleness and fragility of the finished restoration. This dictates strict indications for their use.

The main indications are filling of class III and V carious cavities, which are localized on the visible surfaces of teeth where aesthetics is more important than resistance to masticatory pressure. The use of these cements for class IV cavities is extremely limited and allowed only in exceptional cases when more modern filling materials are not available. The risk of fracture of the restored tooth crown angles in such situations is extremely high.

The method of material placement is of critical importance for the longevity of the filling. The cement mass must be introduced into the pre-formed cavity strictly in a single portion. Layer-by-layer placement of the material in separate fragments is strictly unacceptable. This will inevitably lead to a loss of structural monolithicity and a catastrophic decrease in the quality of the restoration.

After placing the material, it must be firmly pressed using a celluloid matrix (a special strip). To prevent sticking, the matrix is pre-lubricated slightly with petroleum jelly. Removal of the celluloid strip after setting begins is performed with a careful sliding motion, which additionally ensures high-quality smoothing of the filling surface.

Silicophosphate cements: an intermediate link

Silicophosphate cements were developed as a hybrid material representing a combination of classical zinc-phosphate and silicate cements. Due to this composition, they occupy an intermediate position between these two groups in both their physical and chemical characteristics.

The powder composition usually includes 60 to 95 percent silicate cement and 5 to 40 percent phosphate cement. The liquid is an aqueous solution (40-58% water) containing phosphorus oxides (35-45%), zinc (4-9%), and aluminum (3-6%). A typical domestic representative of this group is the powder-liquid system "Silidont-2".

This group of materials has a number of significant advantages over pure silicates:

• Increased mechanical strength and lower brittleness.
• Improved adhesion to hard tooth tissues.
• Higher plasticity during modeling.
• Radiopacity, which makes it possible to monitor the quality of filling on radiographs.
• Coefficient of thermal expansion as close as possible to natural tooth tissues.

However, silicophosphates also have disadvantages, including low aesthetics (mismatch with enamel color), solubility under the influence of saliva, and persistent toxicity. Due to their toxicity, their application strictly requires the placement of a base/lining to protect the pulp.

The mixing technique for silicophosphates is based on the same rules as for silicate cements, but requires the application of slight physical effort to overcome the increased viscosity of the dough. The powder should be introduced into the liquid in smaller portions, which guarantees the complete reaction of all chemical ingredients with each other.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Specifics of using Infantid cement in pediatric dentistry
• Comparative characteristics of modern group representatives (Silicin-2, Fritex, Silicap)
• Influence of the proportions of silicate and phosphate components on the properties of silicophosphate cements
• Detailed analysis of the physicochemical properties of the hardened material and the pH change process

Other topics in "Chapter 8. Dental Materials"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.