Impression notes

Impression Materials Overview

  • Impression materials used in partial denture fabrication are categorized into four groups:

    • Rigid

    • Thermoplastic

    • Elastic

    • Digital

Common Issues with Impression Materials

Tray Selection Issues

  • Problems Utilized with Small Trays:

    • Inadequate impressions due to incorrect tray size.

Plastic vs. Metal Trays

  • Plastic Trays:

    • Not recommended unless the impression material is expressed through all holes.

    • Risk of material pulling away from the tray.

    • Requires careful pouring to avoid distortion during handling.

  • Metal Trays:

    • Similar issues as plastic, must ensure appropriate handling to avoid inaccuracies.

Classification of Impression Materials

Categories Defined

  • Rigid Materials (Non-Elastic):

    • Primarily used historically in dentistry.

    • Examples: Plaster, Zinc Oxide-Eugenol (ZnOE).

  • Thermoplastic Materials:

    • Temporary distortion while maintaining functional details, e.g., modeling plastics.

  • Elastic Materials:

    • Widely used today for their ability to avoid permanent deformation.

    • Include types like hydrocolloids, rubber bases, silicons.

  • Digital Materials:

    • Advanced materials with clinical uses.

Specific Impression Materials and Their Uses

Rigid Materials

  • Plaster:

    • Historical significance; mucostatic properties useful for capturing soft tissues.

    • Cannot be reused after it sets, requires custom trays.

  • Zinc Oxide and Eugenol (ZnOE):

    • Preferred for occlusal registrations due to rigidity and temperature stability.

Thermoplastic Materials

  • Compound/Modeling Plastic:

    • Characteristics depend on temperature; common errors arise from excessive heat.

    • Used for tray extensions and functional impressions.

  • Impression Waxes:

    • Allows for tissue equalization and prevents displacement.

    • Best for small voids in elastic impressions.

Elastic Materials

  • Reversible Hydrocolloids:

    • Most hydrophilic; requires detailed handling and immediate pouring after use.

  • Irreversible Hydrocolloids (e.g. Alginate):

    • Easy manipulation; accuracy decreases with time post-removal.

    • Ideal for various dental casts; thickness should be between 3-5 mm.

Precautions in Handling Alginate Impressions

  • Do not allow to dehydrate or immerse in water.

  • Hold steady during setting to prevent stress in the material.

Examination of Impressions

  • Ensure impressions capture all necessary anatomical landmarks without voids or tears.

  • Avoid distortion during removal by using proper techniques.

Handling of Casts

  • Rinse all saliva and blood post-impression.

  • Correct water/powder ratio is critical for accuracy.

  • Avoid inverting casts during the setting of stone.

Advantages of Digital Impressions

  • Time-efficient with enhanced problem-solving capabilities post-capture.

  • Less material waste; more environmentally friendly.

  • Greater accuracy with fewer chances of errors in technical specifications.

Common Causes for Inaccurate Casts

  • Inadvertently inverted impressions.

  • Distortion or improper water/powder ratios.

  • Premature or delayed separation affecting impression integrity.