Impression Materials (Ch. 15)

Chapter 15: Impression Materials

Objectives

  • Describe the purpose of an impression.

  • Describe the types of impressions.

  • Explain the key properties of an impression.

  • Clarify why alginate is an irreversible hydrocolloid.

  • List supplies needed for taking an alginate impression.

  • Outline steps in taking an impression.

  • Define acceptable impression criteria.

  • Detail disinfection protocol for impressions.

  • Compare Polyvinyl siloxane and Polyether.

  • Explain the difference between hydrophobic and hydrophilic impression materials.

  • Record a patient's bite in centric occlusion.

  • Describe digital impressions and their uses.

Purpose of Impressions

  • Replicate oral structures of the patient's mouth.

  • Negative Reproduction: Impression captures the shape and detail of teeth and oral tissues.

    • Positive reproduction results in diagnostic casts (study models) or dies.

Types of Impressions

  • Preliminary Impression:

    • Serves as a precursor for treatment planning, models, custom trays, or temporary restorations.

    • Alginate is a commonly used material.

  • Final Impression:

    • Very accurate; used for crowns, bridges, implants, partials, complete dentures.

    • Common materials: Polyvinyl siloxane (PVS) and Polyether.

  • Bite Registration:

    • Captures the relationship between restoration/prosthesis and opposing teeth.

    • Facilitates proper mounting of study models in occlusion.

Types of Impression Materials

  • Elastic Materials:

    • Hydrocolloids (agar, alginate)

    • Polysulfides

    • Silicone rubber (PVS)

    • Polyethers

    • Hybrid polyether and PVS

  • Inelastic Materials:

    • Dental compound, impression plaster, zinc oxide-eugenol, impression wax (seldom used).

Key Properties of Impressions

  • Accuracy: Ability to adapt and flow over teeth/tissues, capturing fine details.

  • Tear Resistance: Material should resist tearing when thin.

  • Dimensional Stability: Maintain shape/size over time.

Impression Trays

  • Carry impression materials and support them until set.

  • Characteristics:

    • Rigid to prevent distortion.

    • Available for dentulous arches or edentulous ridges.

Stock Trays

  • Metal Trays:

    • Reusable, less likely to distort, can be sterilized.

  • Plastic Trays:

    • Inexpensive, disposable, but does not support impression material as well.

    • Available in various sizes and shapes (solid/perforated).

Custom Trays

  • Used if stock trays do not fit.

  • Highly accurate and usually constructed in lab with special materials.

Hydrocolloids

  • Types:

    • Reversible Hydrocolloid (Agar): Can change from gel to liquid and back.

    • Irreversible Hydrocolloid (Alginate): Most commonly used, benefits include:

      • Inexpensive, easy to manipulate, no special equipment, reasonably accurate.

Composition of Alginate Impression Material

  • Key components include Sodium or potassium alginate, calcium sulfate dihydrate, potassium sulfate, trisodium phosphate, diatomaceous earth, and various additives (flavoring, coloring, disinfectants).

Working and Setting Time for Alginate

  • Working Time: Regular set (2-3 min), Fast set (1-2 min).

  • Setting Time: Regular set (2-5 min), Fast set (1-2 min).

  • Affected by water temperature (cold lengthens, warm shortens).

Steps in Taking Impressions

  1. Wear PPE (eyewear, gloves, mask, lab coat).

  2. Prepare and mix alginate material.

  3. Load into impression tray and smooth surface.

  4. Position tray in patient's mouth and wait for gelation (1 min).

  5. Remove impression carefully (snap technique) and rinse.

  6. Disinfect impression and prepare for storage.

Properties of Alginate

  • Deformation: Safety thickness (2-4mm) needed to prevent tearing; rapid removal is crucial.

  • Dimensional Stability: Sensitive to moisture loss; rinse and wrap for protection.

  • Tear Strength: Too much water can increase tearing risk.

Disinfection of Impressions

  • Pass alginates through compatible disinfectants and observe immersion times (between 10-30 minutes).

Elastomers

  • Highly accurate, elastic materials that resemble rubber.

  • Types include Polyvinyl Siloxane and Polyether, used for various restorations.

Polyvinyl Siloxane (PVS)

  • Known for stability and accuracy; costs more than alternatives.

  • Available in multiple viscosities (light, medium, heavy, putty).

Digital Impressions

  • Utilize CAD/CAM technology for precise imaging of dental structures.

  • Advantages include reusability, accuracy, elimination of mess, and patient comfort.

  • Disadvantages include high cost and potential size limitations.

Summary Questions

  • Examples of positive and negative reproductions.

  • Types of impressions.

  • Most popular material for preliminary impressions.

  • Effects of water temperature on setting time.

  • Evaluation criteria for impressions.