Revised Hydrocolloids (alginate) 2025

Impression Materials Overview

  • Aqueous Hydrocolloids: Main materials used in making impressions.

    • Commonly referenced brands: KIMAX Products (e.g., KIMAX -100).

Duplicating the Patient's Teeth

  • Negative Replica: Refers to the impression made from the patient's teeth.

  • Positive Replica: The stone model created from the impression.

Properties of Impression Materials

  • Elastic vs Inelastic:

    • Elastic: Springs back to shape after distortion, required for dentition impressions with undercuts.

      • Example: Alginate

    • Inelastic: Rigid, used typically for edentulous ridges.

      • Example: Wax Plaster

  • Chemical Reaction vs Physical Change:

    • Chemical Reaction: Produces stable rubber materials that form when heated or cured.

      • Thermoset: Hardens upon heating and curing.

    • Physical Change: Sets by cooling, often unstable.

      • Thermoplastic: Changes with temperature like jello.

  • Accuracy:

    • High accuracy needed for inlays, onlays, crowns, bridges.

    • Alginate is not acceptable for high-accuracy requirements, but suitable for models and mouthguards.

Flow and Detail Reproduction

  • Viscosity: Affects how well the material captures details.

    • Low viscosity materials capture higher detail around crown preparations.

Chemistry Review

  • Mixtures:

    • Homogeneous Mixture: Uniform composition; completely dissolved (e.g., salt in water).

    • Heterogeneous Mixture: Non-uniform, particles not evenly mixed.

    • Colloids: Microscopic particles suspended in a medium.

    • SOL Colloid: Viscous liquid state; often diluted solutions.

    • Gel Colloid: Solid or semi-solid state, jelly-like consistency (e.g., jello).

ADA Classification of Impression Materials

  • Inelastic Materials: e.g., Plaster.

  • Elastic Materials:

    • Aqueous Hydrocolloids: Hydrophilic (likes water).

    • Nonaqueous Elastomeric: Hydrophobic (hates water).

Aqueous Hydrocolloid

  • Gel formation through physical or chemical reactions.

Reversible Hydrocolloid

  • Example: Agar-Agar.

    • Sets by temperature change; generally accurate and economical but rarely used.

Advantages and Disadvantages of Reversible Hydrocolloids

  • Advantages:

    • Hydrophilic, accurate, inexpensive, easy to pour, and does not stain or have unpleasant taste.

  • Disadvantages:

    • Requires specialized equipment, complicated preparation, sensitive to evaporation, syneresis, imbibition, and must be poured immediately due to instability.

Irreversible Hydrocolloids

  • Example: Alginate.

    • Sets via chemical reaction; widely used but less accurate than reversible options.

Ingredients in Alginate Powder

  • Key Components:

    • Potassium alginate, calcium sulfate, sodium phosphate, diatomaceous earth, color/flavor enhancements, and antimicrobial agents.

Advantages and Disadvantages of Alginate

  • Advantages:

    • Inexpensive, easy to use, good patient acceptance, elastic, hydrophilic, compatible with gypsum.

  • Disadvantages:

    • Accuracy issues due to evaporation, syneresis and imbibition, easily torn, deformed, and limits to single pour of stone.

Setting of Alginate

  • Transitions from sol to gel via chemical reaction.

    • Setting Times: Varies by temperature; fast-set (30-45 seconds working time) vs. regular-set (1 minute working time).

Types of Trays for Impressions

  • Selection Criteria: Length, width, retromolar area coverage, tuberosities coverage for patient comfort.

Tray Preparation

  • Steps: Application of adhesive, addition of wax if necessary.

Measuring Alginate

  • Standard Amounts:

    • Large upper: 3 scoops powder + 2/3 measure water.

    • Medium upper: 3 scoops powder + 1 measure water.

    • Partial: 1 scoop powder + 1/3 measure water.

Manipulation Techniques

  • Process:

    • Fluff powder for 30 seconds, measure accurately, mix in rubber bowl until homogeneous, place tray, and allow time for gelation (2-3 minutes).

Mixing and Setting Time

  • Type I (Fast Set): Quick to work with but requires timing about 30-45 seconds.

  • Type II (Regular Set): More working time but slower to set (3-4 minutes).

  • Temperature matters: Warm water speeds up the setting process while cool water slows it down.

Tray Removal Technique

  • Use SNAP technique to avoid damaging the impression by rocking.

Inspecting Impressions

  • Steps: Check for accuracy, rinse gently, disinfect, and should ideally be poured immediately or stored for up to 24 hours for humidity control.