Dental Impression Materials

Overview of Impression Materials in Dentistry

Impression materials play a crucial role in restorative dentistry, serving various applications from creating models for metal castings to designing complete dentures. The materials are categorized into elastic and non-elastic types, each having unique properties and applications.

Categories of Impression Materials

• Non-elastic Materials

  • Includes Plaster and Compound Waxes

• Elastic Materials

  • Aqueous Hydrocolloids

  • Agar (reversible)

  • Alginate (irreversible)

  • Non-aqueous Elastomers

  • Polysulfide

  • Silicones

  • Polyether

    • Condensation

    • Addition

• Key Types of Elastomers

  • Polysulfides

  • Polyethers

  • Condensation Silicones

  • Addition Silicones

Elastomers

• Definition:

  • Highly accurate elastic impression materials often referred to as "rubber base" materials.

• Applications:

  • Extensively used in restorative dentistry including

  • Construction of metal castings

  • Ceramic restorations

  • Bridges

  • Implant restorations

  • Partial denture frameworks

  • Complete dentures

Types of Elastomers

1. Polysulfides

• Characteristics:

  • Oldest of the elastomers, termed "rubber base"

  • Greater tear strength compared to alginate

  • More accurate than alginate

  • Requires hand mixing, cannot be used in automixing cartridges

  • Notable for its messy application and unpleasant sulfur odor

• Uses:

  • Still used for impressions for complete dentures

• Precautions:

  • Wear vinyl gloves during mixing

• Setting Characteristics:

  • Long setting time

  • Produces water as a by-product during polymerization

2. Silicone Rubber Impression Materials

• Types:

  • Condensation Silicone

  • More desirable than polysulfides

    • Ease of mixing

    • Pleasant taste and no odor

    • Shorter working and setting time

    • Sets through a condensation reaction producing ethyl alcohol as by-product

  • Addition Silicone

  • Improvement over condensation silicone

  • Greater dimensional stability and accuracy

  • Clean and easy to use with no foul odor or taste

  • Most popular for crown and bridge procedures

  • Most expensive of impression materials

    • Available in putty, automix systems, and two-paste configurations

    • Contraindications for latex gloves during mixing

    • Smallest dimensional change upon setting

    • Must be poured immediately to prevent hydrogen formation

3. Polyvinyl Siloxane (PVS) or Vinyl Polysiloxanes (VPS)

• Characteristics:

  • Works through polymerization that lengthens chains and creates cross-links with reactive vinyl groups, resulting in stable silicone rubber

  • No evaporative by-products that can induce shrinkage

  • Smallest dimensional change during setting (0.05%)

  • High elastic recovery post-removal from undercuts

  • Highly resistant to tearing

• Practical Application:

  • PVS impressions can be poured multiple times in stone and remain dimensionally stable for a week without distortion

  • Should be sent to dental laboratories with a prescription for pouring several days later

4. Polyethers

• Characteristics:

  • Elastic impression materials that are very accurate

  • Notable for good flow and tear strength

  • Excellent for crown and bridge work

  • Highly hydrophilic, displaying good wetting properties even in the presence of moisture

  • High stiffness among elastomeric materials

  • Available in a single viscosity

• Popular Product:

  • 3M Impregum is the most known polyether brand

  • Note that the catalyst used can cause skin irritation

Viscosities of PVS and VPS Impression Materials

• Variants available include light, extra light, regular (or monophase), and heavy viscosities

Sample Viscosity Data

• Example of different viscosities with their respective properties:

  • Light Body Regular Set: Work Time 2:30, Set Time 3:30, Color: Yellow

  • Medium Body: Work Time 1:00, Set Time 2:00, Color: Buff

  • Heavy Body: Work Time 2:30, Set Time 3:30, Color: Purple

  • Putty: Regular Set, Fast Set

Conclusion

The choice of impression material depends on the specific requirements of the dental procedure, including the need for accuracy, ease of use, and environmental factors such as moisture. Understanding the various types of materials, their properties, and appropriate handling provides crucial knowledge for optimal outcomes in dental practices.