Lecture 5- Impression material

DESIRABLE CHARACTERISTICS OF IMPRESSION MATERIALS

  1. Fluidity

    • Must be fluid enough to spread and cover the region of interest.

  2. Viscosity

    • Should be viscous enough to be retained in the tray.

  3. Setting Time

    • Must convert into a rubbery solid in a short time in the mouth (less than 7 minutes).

  4. Distortion Resistance

    • Should resist distortion and tearing after setting or during removal from the mouth.

  5. Dimensional Stability

    • Must maintain dimensional stability until the casting process.

    • Capability of maintaining stability for more than one impression.

  6. Biocompatibility

    • Should be biocompatible with oral tissues.

  7. Cost-Benefit Relationship

    • Good cost-effectiveness is necessary, which is typically not found in a single material.

Reference: Phillips – Dental Materials, 12th ed, 2013.

ACCURACY

  • Definition: The ability to duplicate the anatomical features of oral tissues.

  • Influenced by:

    • Viscosity: Higher viscosity can affect the material’s ability to replicate fine details.

    • Wettability: Relates to contact angle/surface energy; defined as:

    • \theta = 0^{\circ}

      • High Wettability: High surface energy and low surface tension.

    • Low Wettability: Low surface energy and high surface tension.
      Reference: Phillips – Dental Materials, 12th ed, 2013.

TEAR RESISTANCE

  • Definition: The ability of a material to resist tearing during removal from the mouth.
    Reference: Phillips – Dental Materials, 12th ed, 2013.

CLASSIFICATION OF IMPRESSION MATERIALS

  • By Setting Mechanism

    • Mechanical Characteristics

    • Inelastic Materials:

      • Examples: Plaster, Zinc oxide-eugenol paste

    • Elastic Materials:

      • Examples: Alginate, Polysulfide, Polyether, Condensation silicone, Addition silicone, Agar

  • By the reaction type:

    • Chemical Reaction: (Irreversible) - common in elastomers

    • Physically-induced Reaction: (Reversible) - like agar
      Reference: Phillips – Dental Materials, 12th ed, 2013.

HYDROCOLLOIDS

  • Definition: Colloids dissolved in water or containing a water component.

  • Types:

    • Agar: Physically-induced plastification (reversible).

    • Alginate: Chemical reaction leading to irreversible setting.
      Reference: Phillips – Dental Materials, 12th ed, 2013.

AGAR

  • Composition: Derived from seaweed.

  • Liquefaction Temperature: Between 71 °C and 100 °C.

  • Handling Characteristics:

    • Requires a three-compartment conditioning unit (liquefaction, storage, tempering).

    • Syringe used to load the tray and requires complex tray designs.
      Reference: Phillips – Dental Materials, 12th ed, 2013.

ALGINATE

  • History: Developed during WWII due to reduced agar supply. Derived from Alginic acid from brown algae.

  • Characteristics: Easy handling, comfortable for patients, low cost, no complex equipment needed.

  • Applications:

    • Diagnostic casts, orthodontic study models, preliminary impressions for custom tray or mouthguard preparation, with limited accuracy for fine anatomical details.
      Reference: Phillips – Dental Materials, 12th ed, 2013.

SETTING REACTION OF ALGINATE

  • Process:

    1. Mix alginate powder with water.

    2. Soluble alginate reacts with calcium sulfate to form calcium alginate gel (fast process).

    3. Retardation (using Na3PO4) may be necessary to slow the reaction down.
      Reference: Phillips – Dental Materials, 12th ed, 2013.

MIXTURE PROPORTIONS

  • Powder/Liquid Ratio:

    • 16 g of powder to 38 ml of water or a 1:1 ratio.

    • Manufacturer’s instructions should be followed closely to reduce the risk of clustering such as dispensing water first.
      Reference: Phillips – Dental Materials, 12th ed, 2013.

WORKING AND SETTING TIME

  • Regular Set: 3 to 4.5 minutes.

  • Fast Set: 1.5 to 3 minutes.

  • Temperature Implications in Reaction: Change in water temperature affects reaction time by 1 minute for each 10 °C change.

    • Using slight amounts of potassium fluoride and titanium prevents interference of alginate with gypsum reaction; however, blood and saliva can contaminate the reaction.

    • Essential to avoid extended contact with casts post-setting stage.
      Reference: Phillips – Dental Materials, 12th ed, 2013.

ELASTOMERS

  • Polysulfide: Composition includes polymer of polysulfide, titanium dioxide, dibutyl phthalate, and sulfur.

    • Setting involves polymerization through a condensation reaction with released by-products.

  • Condensation Silicone: Composed of polydimethyl siloxane for the base and stannous octoate as a catalyst with by-products released during polymerization.

  • Addition Silicone: Involves polyvinyl siloxane and polymethyl hydrosiloxane; noted for not releasing by-products during the reaction.

  • Polyether: Specific to dental applications and does not release by-products, known for fast setting times and dimensional accuracy.

COMPARATIVE PROPERTIES OF ELASTOMERS

  • Working Time and Setting Times by elastomer type:

    • Polysulfide: Working 4 to 7 minutes; Setting 7 to 10 minutes.

    • Polyether: Working 3 minutes, Setting 6 minutes.

    • Condensation Silicone: Working 2.5 to 4 minutes, Setting 6 to 8 minutes.

    • Addition Silicone: Working 2 to 4 minutes, Setting 4 to 6.5 minutes.
      Reference: Phillips – Dental Materials, 12th ed, 2013.

DIMENSIONAL STABILITY

  • Importance highlighted in that most elastomers need to go through specific handling and must be poured immediately after disinfection (typically after 30 minutes) for optimal results.

ZINC-OXIDE EUGENOL PASTE

  • Composition:

    • Base/catalyst and a mixture where Eugenol, Caulim, rosin, zinc oxide, and calcium chloride are utilized.

  • Indications: Primarily used for final impressions for complete dentures and bite registrations, showcasing a specific setting reaction denoted