Impression Materials

Impression Materials

• Medium used to record oral tissues and spatial relationships

• Negative reproduction

• Must be an accurate duplication for restoration and fabrication

• Used to pour a positive reproduction-model cast or die

The Ideal Impression Material

• Semi-liquid then sets

• Appropriate working & setting time

• Strength upon removal

• accuracy and stability

• Biocompatible and compatible with gypsum products

• Easy to use

• reasonable cost

Impression Material Systems

Forms include:

• Powder mixed w/ water

• Two-paste systems

• Materials that are heated or melted

  • Stock or custom-made trays

Bite Registration

• Records the occlusal surface of both arches

• Used to relate upper and lower casts

Classification of Impression Materials

• By setting reaction

  • Physical change or chemical reaction

• By use

  • Inelastic or elastic (rigid or flexible)

Types of Impression Materials

• Inelastic or rigid

  • Impression plaster

  • Impression compound

  • Zinc oxide-eugenol

Classification of Impression Materials

• non-elastic (rigid)

  • plaster

  • compound

  • waxes

  • ZnO (eugenol)

OR

• elastic (flexible)

  • aqueous hydrocolloids

    • agar (reversible hydrocolloid)

    • alginate (irreversible hydrocolloid)

  • non-aqueous elastomers

    • polysulfide

    • silicone

      • condensation

      • addition

    • polyether

Nonelastic or Rigid Materials

NOTE: rigid material cannot be used where undercuts exist

• Impression Plaster

  • Similar to gypsum

  • edentulous mouths only

  • Inexpensive

  • Unpleasant taste

• Impression Compound

  • Wax with fillers added

  • Thermoplastic

    • Softened in warm water

    • Placed in tray and in mouth

    • Cools and sets to rigid state

    • preliminary impression for full dentures

• Zinc oxide-eugenol

  • Two-paste system

  • edentulous impressions

  • Excellent reproduction

  • Inexpensive and easy

  • Largely replaced by rubber-based materials

Aqueous Elastomeric Impression Material

• Hydrocolloids

  • Hydro: water-based

  • Colloid: particles suspended in a medium

  • 2 phases of a colloid: liquid (sol) and semi-solid (gel)

    • Sol→liquid state

    • Gel→ semi-solid state

  • Setting reaction is called GELATION

  • Setting reaction results from either:

    • Thermal reaction

      • Sol ↔ Gel

      • Reversible hydrocolloid (agar)

    • Chemical reaction

      • Sol →Gel

      • Irreversible hydrocolloid (alginate)

Hydrocolloids: Advantages

• mainly water

• Wets tooth surfaces

• Easy to pour

  • Compatible with gypsum products

  • Gypsum wets impression surface well

Hydrocolloids: Disadvantages

• Water content affects stability of the impression

• Syneresis: loss of water; impression will shrink

• Imbibition: absorption of water; the impression will swell

  • Hydrocolloid impressions must be poured immediately!

Reversible Hydrocolloid/Agar

• Indications

  • crown and bridge

    • high accuracy

• Composition

  • Agar-agar 8-15%

  • Water 80-85%

  • Introduced in 1925, used until 1950s

Equipment for Hydrocolloid/Agar

• 3-compartment water bath

• Water-cooled trays

Preparing Agar

• 3-chamber conditioning water bath needed

• 1. Boiling bath changes material to sol (liquefies material)

  • 212o F for at least 10 minutes

• 2. Storage bath

  • 150°F→keeps material until ready for use

• 3. Conditioning bath

  • 110o F → fill tray and place in bath for 15 minutes (gelation begins)

  • Tempering: lowers temperature for pt’s mouth

  • seat tray

  • Cool with water at 13°C for 3 minutes

    • Converts sol to gel

Alginate/ Irreversible Hydrocolloid

• One of the most widely used

• Changes from sol to gel and cannot be reversed

• NOT as accurate as agar →rounds off sharp angles

  • Cannot be used where detail is needed, such as impressions for crowns, bridges, and in-lays/on-lays

Indications

• study models, impressions for dentures, partial dentures, night guards, whitening trays, fluoride trays

• Examples

  • Jeltrate (Dentsply/Caulk)

  • Coe Alginate (GC America)

Composition

• Sodium alginate: extracted from seaweed (algae)

• Calcium sulfate: reactor

• Sodium phosphate: retarder

• Potassium fluoride: improves gypsum surface

• Filler

• Supplied as powder that is mixed w/ water

• Fluff container before use

Mixing Alginate

• Place water in a flexible bowl FIRST then add powder

• Wet all particles, then mix against the side of bowl (stropping)

• Mix until smooth for 1 minute

• Load tray and seat in mouth

• Remove 2 to 4 minutes after gelation (loss of tackiness)

Taking Impression

• TAKE MANDIBULAR IMPRESSIONS FIRST

• Seat impression and allow material to set

• Delay removal for 2-3 additional minutes for added strength and elasticity

• Remove with a snap to avoid permanent deformation (distortion)

• Disinfect according to manufacturer’s directions

Controlling Setting Time

• Regular set: gels in 3-4 minutes

• Fast set: gels in 1-2 minutes

• Water temperature controls the rate of the setting reaction

  • Warmer water→ sets faster (gagger)

  • Cooler water→ sets slower (inexperienced user)

Criteria for Evaluation

• Lack of voids or air bubbles

• No distortion

• Extended to all structures

• Free of debris

• Stable material attached to tray

Pouring impression

• Due to imbibition and syneresis, alginate impressions MUST be poured immediately

• If unable to pour immediately, store at 100% humidity for less than 1 hour

  • Humidor→best

  • Damp towel or plastic bag → next best

Important Considerations

• must be disinfected according to the manufacturer’s recommendations before pouring

  • blood or debris on impression → rinse under water, then disinfect

• models should NOT include frenum, uvula, genial tubercle, or other oral structures beyond periodontium

• RIGHT handed clinician should presume 11 o’clock (12 only if 11 is not available)

causes of permanent deformation:

• improper handing

• temperature and humidity

• expired impression material

• movement during impression

Comparison of Aqueous Hydrocolloids

Feature	Agar (Reversible)	Alginate (Irreversible)
Accuracy	High accuracy; suitable for crowns and bridges.	Lower detail; rounds off sharp angles; unacceptable for fixed restorative work.
Equipment	Specialized & Expensive: Requires 3-chamber water bath, water-cooled trays, and tubing.	Minimal & Inexpensive: Uses standard stock trays and basic mixing tools.
Preparation	Time-Consuming: Must be prepared in advance (boiling, storing, tempering); SUBJECTED to imbibition and syneresis	Fast & Easy: Simple powder/water mix; sets in 1–4 minutes.
Handling	No mixing required; pleasant flavor.	Requires aggressive mixing; can be messy or difficult for beginners.
Cost	Inexpensive after initial equipment investment.	Very inexpensive overall.
Tear Strength	Poor tear strength.	Tears easily; High permanent deformation
Patient comfort	Pleasant taste

Shared Disadvantages and Stability

• Dimensional Instability: Both are subject to syneresis (shrinkage) and imbibition (swelling).

• Pouring Requirement: both must be poured with gypsum immediately

• Hydrophilic Nature: Both are hydrophilic, meaning they wet tooth surfaces and gypsum well.