Impression Materials and Digital Impressions - 2.1
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Questions and Answers

What is the primary purpose of using inert fillers like silica in silicone addition curing?

  • To control viscosity (correct)
  • To initiate the curing reaction
  • To increase the catalyst activity
  • To enhance the rubber's elasticity

    • Which component in the base paste of silicone condensation curing aids in mixing?

  • Ester derivative of aromatic sulphonic acid
  • Inert filler
  • Phthalate (correct)
  • Imine-terminated prepolymer

    • What is the function of the dibutyl phthalate in the base paste of polysulphides?

  • Provides color stability
  • Enhances elasticity
  • Controls viscosity (correct)
  • Initiates cross linking

    • Which consistency classification corresponds to a material with a very high consistency in synthetic elastomers?

    <p>0</p> Signup and view all the answers

    What is the purpose of the chloroplatinic acid in the addition curing of silicone?

    <p>Acts as a catalyst</p> Signup and view all the answers

    What effect does increasing filler content have on synthetic elastomers?

    <p>Increases viscosity and dimensional stability</p> Signup and view all the answers

    Which component in the catalyst paste of silicone addition curing is responsible for polymerization?

    <p>Divinylpolysiloxane</p> Signup and view all the answers

    What initiates the cross-linking process in the catalyst paste for polysulphides?

    <p>Ester derivative of aromatic sulphonic acid</p> Signup and view all the answers

    Which material is primarily involved in the gelation stage of alginate?

    <p>Calcium sulphate dihydrate</p> Signup and view all the answers

    What is the main function of trisodium phosphate in the composition of alginate?

    <p>Acts as a retarder to control setting time</p> Signup and view all the answers

    Which of the following materials is classified as non-elastic?

    <p>Impression plaster</p> Signup and view all the answers

    At what temperature does agar transition from sol to gel?

    <p>37-45 degrees</p> Signup and view all the answers

    Which component of alginate serves as a filler to increase thickness and strength?

    <p>Diatomaceous earth</p> Signup and view all the answers

    What is the purpose of potassium sulphate in the formulation of agar?

    <p>Ensures set of gypsum materials</p> Signup and view all the answers

    What is the function of borax in the composition of agar?

    <p>Strengthens the gel</p> Signup and view all the answers

    Which impression material has a physical reaction and is reversible?

    <p>Impression compounds</p> Signup and view all the answers

    Study Notes

    Accuracy

    • Accuracy of impression materials is vital for accurate reproduction of tooth preparation and soft tissues in the mouth
    • Impression materials vary in accuracy based on their material properties, manipulation technique, and the clinical scenario

    Clinical scenarios

    • Alginate is commonly used for preliminary impressions for diagnostic casts
    • Polyether can be used for final impressions in crowns and bridges
    • Silicone is used for impressions in implant dentistry due to its high accuracy and dimensional stability

    Synthetic elastomers

    • They are widely used in dentistry for taking impressions
    • They include materials like silicone, polyether, polysulfide, and agar-agar

    Silicone - Addition curing

    • Silicone is a versatile elastomer used for dental impressions
    • It is available in two types: addition curing and condensation curing
    • Addition curing silicone is more popular due to its superior properties like improved dimensional stability and less odor

    Composition:

    • It consists of two components: a base paste and a catalyst paste

    Reaction:

    • The addition curing reaction occurs when the base paste and catalyst paste are mixed
    • This sets the material into a rubber-like impression

    Material

    • The base paste consists of:

      • Polymethylhydrosiloxane (liquid silicone prepolymer) - this is the component that is polymerised and cross-linked to form the rubber
      • Inert fillers - silica is a common filler that controls the viscosity of the paste
      • Platinum containing catalyst - these are usually chloroplatinic acid
      • These components are combined in the base paste to facilitate the polymerization reaction

    • The catalyst paste consists of:

      • Divinylpolysiloxane (liquid silicone prepolymer) - this component polymerises and cross-links to form the rubber
      • Inert fillers - silica is used to control viscosity
      • These components are combined in the catalyst paste

    Silicone - Condensation curing

    • This type of silicone is less commonly used than addition curing silicone because it produces a by-product (water) which can compromise the accuracy of the impression

    Composition:

    • It contains a base paste and a catalyst paste
    • The base paste typically consists of a silicone prepolymer, pigments, fillers, and a plasticizer
    • The catalyst paste typically consists of a silane coupling agent (like an alcohol) which enhances the bond between the filler and the silicone polymer

    Reaction

    • Mixing the two components initiates a condensation reaction which involves the release of water as a by-product
    • As the reaction progresses, the silicone polymer chains link together, forming a cross-linked network (solid rubber)

    Polyethers

    • These materials are known for their excellent dimensional stability, tear strength, and accuracy
    • They have a strong affinity for water which makes them suitable for impressions of areas with moisture

    Composition:

    • Consists of a base paste and a catalyst paste

    Reaction:

    • When the two components are mixed, a chemical reaction occurs that forms a very stable and strong rubber-like material

    Material

    • The base paste typically contains an imine-terminated prepolymer, inert fillers (like silica), and a plasticizer to improve workability
    • The catalyst paste typically contains an ester derivative of aromatic sulphonic acid which acts as a catalyst, to initiate crosslinking
    • Inert fillers (like silica) are also added to control viscosity

    Polysulphides

    • These materials are characterized by a long working time, and high tear strength
    • They are known for their strong odors

    Composition:

    • Consists of two components: a base paste and a catalyst paste

    Reaction

    • When the two components are mixed, a chemical reaction occurs where the polysulfide polymer reacts with the catalyst, forming a strong rubber-like material

    Material

    • The base paste typically contains a polysulfide prepolymer, dibutyl phthalate (controls viscosity), and inert fillers (like chalk or titanium dioxide)

    • The catalyst paste typically contains oxidizing agents (like lead dioxide) which react with the polysulfide polymer, forming a cross-linked network

    Impression materials and digital impressions

    • These impression materials are used to create an impression of the teeth and surrounding tissues
    • The impression is used to create a model of the teeth which will be used for dental treatment planning and fabrication of dental restorations

    Mode of Setting

    • Impression materials can be categorized based on their mode of setting
    • Non-Elastic: these materials set into a rigid material, typically used for creating models
    • The reaction is usually chemical and irreversible (Impression plaster)
    • Elastic: these materials are flexible, allowing for easy removal from the mouth and accurate reproduction of the oral tissues
    • The reaction is usually chemical and irreversible (Alginate, Polysulfide, Polyether)
    • Some can be reversible ( Agar) - These materials are heated to a liquid state and then cooled to set into a semi-solid state

    Hydrocolloids

    • They are based on a colloidal suspension of particles that bind to a liquid
    • Their properties are based on the interaction between the solid particles and the liquid medium

    Sol

    • The particles are dispersed evenly in the liquid, creating a flowing suspension

    Gel

    • The particles form a network that traps the liquid, giving the material a semi-solid consistency

    Alginate

    • A highly popular impression material due to its efficiency and low cost

    Composition:

    • Sodium alginate - this material is the basis of the gel and constitutes 15-20% of the material
    • Calcium sulfate dihydrate - This material reacts with the sodium alginate to form a more stable calcium alginate gel (represents 14-20 % of the material)
    • Potassium Sulfate - This material ensures that the gypsum material sets properly
    • Trisodium phosphate - This component acts as a retarder to control the setting time, it contributes to 2% of the material
    • Diatomaceous earth - This component is a filler material that adds strength and thickness to the alginate, representing 55-60% of the material

    Reaction:

    • The setting reaction of alginate is a chemical reaction, irreversible
    • The sodium alginate dissolves in water, forming a sol
    • Then, the calcium ions from the calcium sulfate react with the sodium alginate to form a calcium alginate gel

    Properties:

    • Dimensional Stability: Alginate is known for its good dimensional stability when properly handled
    • Tear Strength: This refers to the amount of force that is required to tear the alginate impression material, it is relatively low
    • Handling:
      • Mixing: The powder and water are mixed quickly and thoroughly to ensure a consistent mix and avoid air bubbles
      • Setting time: The setting time is the amount of time it takes for the alginate to set from a liquid to a solid state. Proper mixing and the water to powder ration is crucial
    • Accuracy: Alginate is considered an accurate material when handled correctly
    • Clinical Scenarios: Alginate is primarily used for preliminary impressions, because it has a low tear strength and can be more prone to distortion than other materials
    • The impression can be affected by humidity, temperature, and handling

    Agar

    • This material is a reversible hydrocolloid commonly used due to its accuracy and versatility
    • This material has a long working time and provides precise reproduction of the oral tissues, especially when a good impression tray is used

    Reaction:

    • Agar undergoes a physical change which is reversible
    • The material is heated (above 70 degrees) to a liquid state (sol)
    • The sol is then cooled (37-45 degrees) back to a gel state. The sol can also be heated again, turning it back to a liquid state

    Composition

    • Agar: This is the main component (12-15%) of the material. It is a seaweed extract and the basis for gel formation.
    • ** Potassium sulfate**: This material is added to control the setting time (1%)
    • Borax: An additional ingredient that strengthens the agar gel (0.2%)
    • ** Alkyl Benzoate**: This is an antifungal agent that helps prevent the material from growing mold (0.1%)
    • Water: This is the dispersion medium for the agar, representing 85% of total composition

    Properties

    • Dimensional Stability: Agar is highly dimensionally stable when properly handled
    • Tear Strength: Agar has a relatively high tear strength
    • Handling:
      • Equipment: This material requires special equipment (agar bath) to heat and cool the impression material properly
      • Working time: This material has a high working time, which allows for more time to take the impression
      • Setting time: The setting time is relatively rapid once the agar impression is cooled (37-45 degrees Celsius).
      • As it is a reversible material, it can be reheated to a liquid state and cooled again to produce multiple impressions, making it a good material for crown and bridge impressions.

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