Dental impressions are essentially negative imprints of the alveolus, gums, and teeth. An important step in the creation of dental casts for orthodontics and prostheses is the development of these impressions. Dental impressions are crucial for accurate diagnosis and exact construction of well-fitting oral appliances because they make it possible to create copies of patients' teeth and mouth structures (1). Dental scans, a promising way to take impressions, nonetheless have limits even though digital dentistry is rapidly developing and replacing traditional techniques (2). The inferior quality of these scans if the abutment is sloped or the span is too long is one of the problems fixed in newer scanners. In addition, materials used for restorations in the oral cavity have the potential to reflect light and distort three-dimensional appearances. Consequently, full-arch impressions-which are very useful impressions for full denture fabrication-are often obtained using the traditional impression procedure (3). The choice of materials used to create dental impressions can have a big impact on the end product's correctness and precision. Dental imprints have traditionally been made using both elastic (agar, alginate, polyether, condensation silicone, addition silicone, and polysulfide) and stiff (impression plaster and zinc oxide-eugenol) materials (4). Sodium alginate is now the primary substance used to take imprints prior to the creation of splints, individual trays, orthodontic equipment, and diagnostic gypsum castings. However, because they constrict owing to syneresis and show dimensional instability as they absorb water and inflate, sodium alginate-based impressions are not advised for more accurate applications(5). High-quality dental restorations can be produced with casts created from elastomeric impressions. Dentists frequently like silicones because of their rapid recovery and flexibility after being removed from the mouth canal, as well as their capacity to be poured for up to a week with very minor alterations to their estimated 0.3%-dimensional stability (6). The optimum imprint material should also have high dimensional stability and satisfy other requirements such as the right setting time, flow characteristics, mechanical strength, accuracy, compatibility with cast materials, safety, simplicity of manipulation, cheap cost, and disinfectant ability (7). The accuracy with which a dental prosthesis fits is a critical component of prosthodontic treatment success, and it depends on the perfect replication of intraoral structures. The impression material should not deform during the course of removal from the mouth (8). 

The overall tensile force is more than the overall compressive force during the process of extraction from undercuts, interproximal spaces, and so on. The susceptibility of impression materials to tearing is also increased in gingival crevices (9, 10). The sterilization and disinfection procedures should not deteriorate the mechanical properties of the impression material. Silicone, an elastomeric impression material, is the preferred material for all impression procedures due to its excellent physical, mechanical, and biological properties. Approximately 50% share of the impression material market has been captured by silicone materials (1). Different test methods are available to test the tear resistance and tensile strength of impression materials, which makes it difficult to compare different materials (11).The comparison of elastomeric impression materials' maximum elongation and tear energy-a measure of their tensile strength and elongation-has been the subject of several investigations. Dentists, along with paramedical personnel, are very susceptible to several illnesses, such as hepatitis B, the human immunodeficiency virus, and TB, because of their extensive exposure to a diverse spectrum of pathogens (12). Through infection control standards, the American Dental Association and the Centers for Disease Control have recommended several steps to potentially stop the spread of infectious illnesses. According to certain research, frequent bacteria discovered in up to 67% of impressions include Escherichia coli, Enterobacter cloacae, and Klebsiella oxytoca (13). The risk of microbial contamination increases with blood stains in the impressions. Attempts were made to produce sterile impression materials by adding disinfectants that inhibit the growth of microbes. However, it met with limited success. This was followed by the current practice of “immersion disinfection or spray disinfection” of the impressions after removal from the mouth, although studies have shown incomplete removal of potential contaminants (14). Immersion of the replica in a disinfection liquid is preferred as spray may not cover the entire impression surface evenly and adequately, but it can distort the hydrophobic impression materials. Certain researchers have proposed disinfectants and immersion times for various imprint materials (15). The imprint should be autoclaved as soon as it is prepared to prevent the spread of infections, but not all impression materials are suitable for this process (16). 

Materials used in the study

A hydrorise, addition-type silicone impression material (Zhermach, Germany) was evaluated. Material was disinfected using autoclave sterilization (Raypa, Spain), and microwave sterilization (microwave oven model GUM 2S, GUM, Italy) to determine the effects on tear and tensile strength. 

Mold preparation 

Custom portioned plastic molds were used to fabricate 15 samples of each material. Specimens were molded into dumbbell shapes for tensile testing measuring 2×75×12.5 mm. 

A plastic mold representing the rectangular‐shaped specimen with dimensions 100mm in length, 19mm in width, and 2mm in thickness was prepared to measure tear strength. An indentation was created in the middle of the mold along its length and a triangular notch (90°) was pushed in this indentation along the width. 

Mixing

After measuring each silicone substance by weight and mixing it by hand for 30 seconds as directed by the manufacturer, the mold was filled and placed in a water bath with a thermostat set to 37 °C. After that, samples were carefully taken out, tagged, and kept at room temperature in plastic bags with zip tops until testing. 

Sterilization

The specimens were chosen at random and placed into three study groups, each with ten samples: Group I specimens were the control group and were not sterilized or disinfected; Group II specimens were microwaved for five minutes at high power; and Group III specimens were autoclaved for five minutes at a pressure of twenty pressure pumps at 134°C. To protect the thyratron tube, water ballast was stored in an oven with the samples. Following the disinfection and sterilizing process, the specimens were examined under dry, clean circumstances for tensile and tear strength. 

Tensile strength determination 

An Instron universal testing equipment linked to a computer was used to measure the tensile strength at a constant cross-head speed of 1 mm/min. Using a capacity-load shell, the specimens were pushed apart while being held at the grip portion. Consideration was given to specimens failing at gage length to standardize the experimental data. Specimens that broke at the reduced section and where the grip and reduced portions joined were thrown away. The Instron machine's integrated software recorded the tensile strength (Figure 1). 

Figure 1: Tensile strength test.

Tearing strength determination 

The rectangular specimen strip was mounted on the Instron machine, clamping both its ends. The machine was calibrated to move at a uniform crosshead speed of 1mm/ min. The movement was continued till the tear commenced and was retained. The force required to commence the tear was recorded electronically (Figure 2). 

Figure 2: tear strength test.

Tensile Strength 

All specimens' tensile strength data are shown in (Table 1). When compared to the specimens in the control group, microwave sterilization dramatically decreased the tensile strength of every elastomer, but autoclave sterilization had no discernible influence on this property. 

[Table 1]: Results of Tensile strength. 

Tearing Strength 

All specimens' ripping strength data are shown in [Table 2]. When compared to the specimens in the control group, microwave sterilization considerably decreased the tensile strength of every silicone, although autoclave sterilization had no discernible influence on the tearing strength of silicone. 

[Table 2]: Results of Tearing strength. 

The ability of the imprint materials to perform is crucial for the exact fit of maxillofacial and oral prosthesis. Following its extraction from the oral cavity, the imprint 

is sterilized to get rid of food particles and pathogenic microbes (17). Concerns about the spread of harmful bacteria among dental offices and laboratory staff have prompted researchers to suggest potential methods for disinfecting the impression (18). Generally speaking, impressions are sprayed with a suitable disinfection solution to kill all microbiological forms, however, this is not a 100% effective method. It is possible to eradicate all pathogenic microbes by subjecting the imprint to extremely high temperatures (19). The present study employs autoclave and microwave sterilization procedures to observe and evaluate potential impacts on many crucial and therapeutically significant characteristics of silicon. Because they have a major impact on the final prosthesis's fit and quality, tensile strength and tearing strength were deemed crucial variables in this investigation (20). The current study's results demonstrate that after autoclave sterilization, none of the materials exhibited a statistically significant decrease in tensile strength. Tensile strength was greatly decreased by microwave sterilization; this might be due to the dry heat produced by the radiation, which causes elastomers' chemical components to be lost and alter their characteristics (21).

Autoclave sterilization involves the exposure of the elastomers to steam under pressure at about 121°C which had generally minimum effect on these materials. The key reason is the inherent chemical stability of silicone elastomers (22). The molecular structure involves silicon-oxygen (Si-O) bonds that have high resistance to thermal degradation (23, 24). Steam, while hot, does not significantly break such strong (Si-O) bonds or penetrate deeply into the hydrophobic matrix. As a result, the tensile and tear strength of the silicone impression material remains mostly intact because the high-pressure steam does not modify the vital properties of the material or cause significant thermal damage.

On the other hand, microwave sterilization involves heating materials via dielectric heating in which it induces rotation in the polar molecules (such as water) to create heat. Silicone impressions used in the study are non-polar and do not interact strongly with microwaves (25). Such an effect can led to localized heating when there is any residual moisture or uneven material distribution. This localized heating can cause surface damage and initiate thermal gradients inside the material (26). These high-temperature areas can lead to degradation, microcracking, and structural weak points in the polymer, which are responsible for the reduction in the tensile and tear strength of the material (27). Thus, while autoclaving generally preserves the integrity of silicone elastomers, microwave sterilization can compromise their mechanical properties.

The ideal tensile strength of the elastomeric impression materials is unknown, as it has not been reported, but a recent study found that the force required to remove the impression from the oral cavity ranges from 224 N to 514 N (28). Given this, the impression materials should at least withstand forces in that range without permanent deformation. As is known, the accuracy of fit and intraoral structures determines the outcome of the prosthetic treatment. When the impression material is removed from the mouth, especially from interproximal spaces, sharp line angles, and gingival crevices, both compressive and tensile forces act on it. The tolerance to these forces and ability to withstand deformation define the quality of the impression material (29). Any defect in the impression compromises the accuracy and quality of the final restoration. 

Within the confines of the current investigation, it was discovered that microwave sterilization somewhat decreased the tensile and tearing strength of the silicon elastomers, but autoclave sterilization had no discernible effect on either. When elastomers are exposed to microwave radiation, their tensile and tearing strengths are decreased in addition to the surface details being damaged. 

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