Impression Materials and Trays Quiz

Impression Materials

• Definition: Substance used to capture the form of oral structures.
• Types: Primary (initial impression) and Secondary (final impression)
• Cast (Model): Positive replica of oral structures.
• Tray: Instrument used to support and control impression material.
  • Metallic Stock Tray: Aluminum or stainless steel.
  • Acrylic Special Tray: Custom-made for specific patients.
  • Plastic Stock Tray: Disposable for primary impressions.

Functions of an Impression Tray

• Supports impression material in contact with oral tissues.
• Supports impression material during removal from the mouth, allowing for cast pouring.

Types of Trays

• Material:
  • Metallic
  • Non-metallic
• Impression Material:
  • Stock Tray: Used for primary impressions, metallic or plastic.
    • Perforated: Used with alginate, provides retention.
      • Without Rim Lock: Offers flexibility.
      • With Rim Lock: Offers stronger retention.
    • Non-Perforated (Plain): Used with impression compound.
      • Without Rim Lock: Best for single teeth.
      • With Rim Lock: Offers stronger retention and allows compound to stick to the tray.
  • Special (Custom) Tray: Used for secondary impressions (final), made from shellac, acrylic, or compound.

Applications for Impressions

• Study teeth alignment.
• Treatment planning for dental procedures.
• Fabrication of indirect restorations.
• Creation of special (custom) trays.

Ideal Requirements for Impression Materials

• Rigidity: Necessary for edentulous patients without undercuts.
• Elasticity: Required for dentulous or edentulous patients with undercuts.

Material Classification by Setting Reaction

• Physical (Reversible): Softened by heating and hardened by cooling.
  • Impression Compound
  • Agar-Agar
• Chemical (Irreversible): Sets chemically, cannot be reversed.
  • Zinc Oxide Eugenol
  • Alginate
  • Rubber Base

Rigid Impression Materials

• Impression Compound:
  • Thermoplastic material softened by heat and hardened by cooling.
  • Also known as “modeling compound”.
  • Forms: Sheets and sticks of different colors.

Impression Compound Composition

• Thermoplastic Materials: Provide thermoplasticity, flow, and cohesion (e.g., natural resins and waxes).
• Fillers: Provide body and working consistency (e.g., talc powder).
• Plasticizers: Act as lubricants, control consistency (with fillers), and improve workability (e.g., stearic acid or stearin).
• Coloring Agents: Provide characteristic color (e.g., rouge).

Types and Uses of Impression Compound

• Type I: Lower Fusing Compound (70°C): Used for the wash impression technique, where a thin layer of zinc oxide eugenol (ZOE) is applied on top.

Setting Reaction of Impression Compound

• Thermoplastic, physically reversible reaction.
• Hardens at 45-55°C, softens at 37°C.

Manipulation for Primary Impressions with Impression Compound

1. Softening: Heat compound over a flame or in a water bath.
2. Kneading: Immerse and knead under water until fully softened but without leaching out ingredients. Use thermostatically controlled water bath.
3. Tray Selection: Use a plain stock tray (non-perforated) because compound sticks to it.
4. Cooling and Removal: Use water spray to cool and harden the impression. Remove after hardening to prevent distortion.
5. Sterilization: Can be autoclaved for 10 minutes.

Properties of Impression Compound

• Accuracy: Not sufficiently fluid to record fine details.
• Dimensional Stability: High coefficient of thermal expansion and contraction, shows considerable shrinkage on cooling.
• Elasticity: Non-elastic, cannot be used in undercut areas.
• Compatibility: Compatible with cast and die materials, no separating medium required. Use warm water to separate the cast.
• Reusability: Can be reused after sterilization.
• Correction: Compound impression can be added to or corrected.
• Other: Non-toxic, irritant-free, suitable setting time, long shelf life.

Hydrocolloid Impression Materials

• Definition: Colloids dispersed in water. Used for dental impressions.
• States: Exists in sol and gel states. Inserted in sol state (fluid) for detail capture, removed after gel formation (elastic).
• Types:
  • Agar-Agar (Reversible): Sol state achieved by heating, gel state by cooling.
  • Alginate (Irreversible): Sol state transformed to gel state through a chemical reaction.

Irreversible Hydrocolloid (Alginate) Impression

• Viscoelastic material that transforms from sol to gel chemically, making it irreversible.
• Indicated for primary impressions for both dentulous and edentulous patients.

Presentation Forms of Alginate

• Powder: Traditional form available in:
  • Bulk containers with measures (spoons and cylinders) for dispensing powder and water.
  • Small sealed packets.

Alginate Composition

1. Potassium (or Sodium) Alginate (15%): Main reactive ingredient; forms a sol with water.
2. Calcium Sulfate Dihydrate (CaSO4.2H2O) (16%): Reacts with soluble alginate to form insoluble alginate, setting the material.
3. Sodium Phosphate (Na3(Po4)2) (2%): Acts as a retarder, extending working time by reacting with Ca+ ions before cross-linking occurs.
4. Potassium Sulfate (3%): Accelerates the setting time of gypsum materials used for pouring models.
5. Fillers (Zinc Oxide): Strengthens the gel.
6. Flavoring, Coloring, and Disinfecting Agents (Chlorhexidine): Provide flavor, color, and disinfecting properties.
7. Chemical Indicator (Some Products): Changes color with pH change, indicating different manipulation stages (chromatic alginate).

Setting Reaction of Alginate

• Gelation Reaction: Chemical reaction occurring in several steps.
  • Calcium Sulfate Dissociation: CaSO4 + 2H2O → Ca + SO4
  • Sodium Phosphate Dissociation: Na4P2O7 → 4Na + P2O7
  • Calcium Phosphate Formation: Calcium reacts with phosphate, forming insoluble Calcium Phosphate.
  • Calcium Alginate Formation: After all phosphate is exhausted, calcium ions react with soluble alginate (sodium or potassium alginate) to form insoluble alginate (calcium alginate). This creates the final elastic gel.

Manipulation of Alginate Impressions

• Equipment: Alginate powder, mixing bowl, metal spatula, perforated or plain rim-locked stock tray.
• Proportioning and Mixing: Follow manufacturer-specified powder-to-water ratio. Combine water and powder in a bowl with vigorous mixing using a spatula. Mixing time is 45 seconds to 1 minute, setting time is 1.5 to 5 minutes from start of mix. Aim for a smooth, creamy consistency.
• Tray Loading and Impression Taking: Avoid tray movement during setting.
• Impression Removal: Remove the tray with a snap motion to increase tear strength and elastic recovery.
• Disinfection: Disinfect promptly to prevent dimensional changes.
  • Methods:
    • Spraying: Rinse with water, spray disinfectant, wrap in a disinfectant-soaked paper towel and seal in a plastic bag for 10 minutes.
    • Immersion: Immerse in disinfectant for 10 minutes.
  • Disinfectants: Iodophors, sodium hypochlorite, glutaraldehyde.
• Impression Pouring: No need for a separating medium. The impression is ready for pouring.

Properties of Alginate Impressions

• Flexibility: Easily removed from undercuts with minimal stress.
• Tear Strength: Poor tear strength; requires snap removal.
• Elasticity: Viscoelastic, meaning it has both viscous and elastic properties.
  • Elastic Recovery: Approximately 97.3%.
  • Importance: Remove with a snap motion to minimize permanent deformation.
• Compatibility with Gypsum: No separating medium required. However, alginate can affect the setting and surface properties of gypsum materials.
  • Causes:
    • Water content inhibits gypsum setting, leading to a soft surface.
    • Hydrophilic alginate absorbs water from gypsum, causing a chalky appearance.
  • How to Avoid:
    • Immerse the impression in 2% K2SO4 before pouring (gypsum accelerator).
    • Incorporate gypsum accelerators (potassium titanium fluoride or potassium sulfate) into the alginate material.
• Dimensional Stability: Alginate is dimensionally unstable on storage.
  • Causes:
    • Water Loss: Occurs through syneresis (exuding fluids onto the surface) or evaporation, leading to shrinkage.
    • Water Uptake: Occurs through imbibition (absorbing water), resulting in swelling and expansion.
  • How to Reduce Dimensional Changes:
    • Pour the impression immediately (within 10 minutes).
    • If delayed pouring is necessary, wrap the impression in a wet towel (saturated with water) and place it in a sealed plastic bag or humidor (100% humidity).

Advantages of Alginate Impression Material

• Inexpensive
• Easy to use
• Hydrophilic (attracts water)

Test your knowledge on impression materials and trays used in dentistry. This quiz covers types of impression materials, their functions, and various types of trays including metallic and non-metallic. Enhance your understanding of how these components are crucial in capturing oral structures.