Chapter 15: Impression Materials

Objectives

• Describe the purpose of dental impressions.
• Identify three basic types of impressions.
• Explain key properties of impression materials.
• Define sol and gel states in hydrocolloids.
• Discuss why alginate is an irreversible hydrocolloid.
• Differentiate between hydrophobic and hydrophilic materials.
• Explore elastomers and their properties.
• Discuss the popularity of PVS (polyvinyl siloxane).
• Compare PVS and polyether materials.
• Examine polye impression materials for crown impressions.
• Discuss gingival retraction and cord placement.
• Explain the use of ferric sulfate astringent.
• Define digital impressions and their advantages/disadvantages.

Purpose of Dental Impressions

• Reproduce the form of teeth, arches, and oral tissues.
• Aid in restorative treatments and fabrication of dental appliances (e.g., dentures, retainers).
• Used in maxillofacial reconstruction.

Types of Impression Materials

• Elastic Materials: Moldable, adapt to teeth.
  • Setting States: Semi-solid elastic or rigid.
  • Negative Reproduction: Impression
  • Positive Reproduction: Cast or model from dental stone.
• Preliminary Impressions: Initial stage for diagnostics.
• Final Impressions: High detail for restorations.
• Bite Registration Impressions: Capture occlusion.

Key Properties of Impression Materials

• Accuracy: Detailed reproduction of tooth surface.
• Tear Resistance: Strong enough to avoid tearing.
• Dimensional Stability: Consistent size after removal.

Impression Trays

• Types: Metal (reusable), plastic (disposable), perforated, solid.
• Special trays: Sectional, triple trays.

Hydrocolloids

• Agar: Reversible, not commonly used today.
• Alginate: Irreversible, widely used.
  • Made from seaweed derivatives.

Alginate Impression Process

• Fast and regular set options.
• Proper mixing crucial for accuracy and tear strength.
• Permanent Deformation: Needs careful removal.
• Dimensional Stability: Sensitive to moisture loss.

Elastomers

• Polyvinyl Siloxane (PVS): Popular, detailed, expensive.
• Polyether: Good flow, hydrophilic.
• Hybrid: Vinyl polyether silicone combines best features.

Comparison: PVS vs. Alginate

• PVS: More accurate, stable, expensive.
• Alginate: Less detailed, more affordable.

Conclusion

• Importance of impression materials in dental treatments.
• Differences between various materials and their applications.