Dental Impression Materials Overview

Impression Materials

• Inelastic Impression Materials: Less common now, prone to breakage during removal, suitable for edentulous cases (minimal undercuts). Examples include Plaster of Paris (type I gypsum) and impression compounds.

Elastic Impression Materials

• Reversible Hydrocolloids: Agar hydrocolloid (seaweed-based). Changes from gel to sol at high temperatures (90°C), and sol to gel at low temperatures (below 50°C).

  • Advantages: Hydrophilic, low cost, reusable, no custom tray needed.
  • Disadvantages: Requires heating/cooling equipment, poor tear strength, low stability (must be poured rapidly).

• Irreversible Hydrocolloids: Alginate impressions form an irreversible gel when mixed with water. Comes in various setting speeds.

  • Advantages: Rapid setting, hydrophilic, easy technique, low cost.
  • Disadvantages: Only suitable for stone casts, less detailed than other materials, some permanent deformation.

• Elastomeric Materials: More modern and versatile.

  • Polysulfides: Oldest elastomeric type, two-part paste system (base & catalyst), available in different viscosities (light, regular, heavy), brown color, requires lead peroxides for polymerization.
    • Advantages: High tear strength, high elasticity, easy removal, easy pouring, better stability than hydrocolloids (still needs fast pouring).
    • Disadvantages: Slight contraction during polymerization (custom tray helps), long setting time (10 minutes), affected by humidity/temperature, patient dislike (color/odor), messy, staining potential.
  • Condensation Silicones: Two-part base and catalyst, dimethylsiloxane with reactive -OH groups, higher molecular weight = heavier consistency, available as putty, heavy body, light body.
    • Advantages: Short setting time (6-8 minutes), less affected by high temperatures, odorless.
    • Disadvantages: Hydrophobic (poor wetting), needs moisture-free impressions, needs surfactant before pouring to avoid air bubbles, putty contamination can prevent light-body adherence, potential dimensional instability over time if water/alcohol is lost.
  • Addition Silicones (Polyvinyl Siloxane): Similar to condensation silicones but more stable (no water/alcohol byproducts), after setting, more rigid than polysulfides. Available in various consistencies (putty, heavy body, regular body, light body).
    • Advantages: Dimensionally stable (some up to a week), pleasant to use, short setting time, automix available, less material waste, better mixing, fewer air bubbles.
    • Disadvantages: Hydrophobic nature, hydrophilic materials can absorb water, setting inhibited by latex gloves (dithiocarbamates), some release H₂ gas during polymerization (air bubbles in cast - wait 1 hour before pouring).