Types of questions

• Categories commonly used in practical/quiz assessments:

  • Multiple choice

  • Give reason(s)

  • Give a short account on / Enumerate

  • Answer by ONE word (scientific term)

  • Compare

Impression materials: overview

• Two broad groups by use in a single patient:

  • Primary impression material

  • Characteristics: Low flow; cannot record fine details well

  • Examples listed: impression compound, alginate

  • Key concepts: definition, requirements, classifications, setting reaction, properties, uses

  • Secondary impression material

  • Characteristics: High flow; can record fine details better; high elastic recovery

  • Examples listed: impression plaster (ZnO/Eugenol), elastomeric materials

  • For edentulous patients with no undercut and for edentulous areas including crowns/bridges

• Important material types referenced in slides/notes:

  • Impression compound

  • Alginate (irreversible hydrocolloid)

  • Agar (referred to as hydrocolloid used in primary impressions)

  • ZnO/eugenol (used in impression plaster as secondary impression material)

  • Elastomers: polyether, addition silicone (PVS), polysulfide, others

Key properties and concepts in impression materials

• Imbibition vs syneresis (water movement in hydrophilic systems)

  • Imbibition: gain of water by hydrocolloid

  • Syneresis: expulsion of liquid from a gel

  • Wetting agents and surface-active agents can influence tear strength and permanent deformation

  • Hydrophilicity helps wet oral tissues and record fine details; also influences how materials interact with dies/model surfaces

• Dimensional stability and storage

  • Among impression materials, the most dimensionally stable on storage is typically an elastomer (e.g., addition silicone)

• Undesirable properties to avoid in impression materials

  • High viscosity at time of insertion is undesirable (reduces flow into fine details, undercuts, and comfort)

• Agar vs alginate (hydrocolloids) vs elastomers

  • Hydrocolloids (agar, alginate) are used for impressions with specific thickness and wetting properties

  • Elastomers (polyether, polyvinyl siloxane) offer high dimensional stability and tear strength

Specifics on hydrocolloids and elastomers

• Hydrocolloids used for impressions:

  • Agar (agar-agar): primary impression material in some settings

  • Alginate: common primary impression material with low flow and recording limitations; needs immediate pouring due to dimensional changes

• Elastomers used for impressions:

  • Addition silicone (polyvinyl siloxane, PVS)

  • Polyether

  • Polysulfide (older elastomer)

  • Elastomers typically offer better dimensional stability and elastic recovery than hydrocolloids

Practical implications and quick references

• Thickness recommendations (as per slides):

  • Hydrocolloids: typically used in 4–6 mm thickness to balance dimensional stability and detail capture

  • Elastomers: typically 2–3 mm thickness for adequate details while maintaining manageability

• Wetting and hydrophilicity

  • Hydrophilic impression materials wet tissues better and record fine details more reliably

  • Wetting and surface interaction affect how well a impression can be poured and how accurately the model/die captures details

• Tray considerations for edentulous patients

  • For edentulous patients with undercuts, careful selection of impression materials and technique is essential to avoid tearing and undercut escape issues

Materials and their manipulation (selected highlights)

• Alginate (irreversible hydrocolloid)

  • Uses: common for preliminary impressions due to ease of use and cost

  • Handling cues: must be poured promptly to avoid dimensional changes

• Agar/gel hydrocolloid systems

  • Requires controlled heating and cooling bath to transition between sol and gel states

  • Main disadvantage: sensitivity to temperature and storage affecting dimensional stability

• Impression plaster (ZnO/ZnO-eugenol)

  • Used for secondary impressions in specific edentulous situations

• Impression compound

  • A thermoplastic material used in some primary impression workflows; heat-softened and adapted

• Elastomers (overview)

  • Addition silicone (PVS): high dimensional stability, excellent tear strength, hydrophobic but hydrophilic variants exist

  • Polyether: good wettability, stiffer handling

  • Polysulfide: flexible, longer working time, worse dimensional stability than PVS/polyether

Cements, liners, bases, and bases of preparation

• Calcium hydroxide cement (Dycal and similar)

  • Uses: subbase, pulp capping materials; stimulates secondary dentin formation in deep cavities

  • Liner/bases: often used to protect pulp under restorations and to provide a remedial barrier

• ZOE (Zinc oxide eugenol) cement

  • Uses: temporary luting and provisional bases; scent and handling well-known; avoid near resin composites due to inhibition of polymerization

• Zinc phosphate cement

  • Historical luting cement; very acidic on mixing; used with careful handling to manage film thickness and pulpal protection

  • Setting reaction and working times depend on powder-to-liquid ratio and cooling, e.g., mixing over a large area on a cooled glass slab

  • Some slides emphasize the effect of water in setting and handling specifics

• Zinc polycarboxylate cement

  • Chemical bonding to tooth structure (adhesive bonding) vs purely mechanical; offers chemical adhesion

  • Considered an advance over zinc phosphate due to bonding properties

• Glass ionomer cement (GIC)

  • Composition: calcium fluoro-alumino-silicate glass + polyacrylic acids (and related acids) with a water-based liquid

  • Manipulation steps (from slide content):

  • Powder and liquid are dispensed on a glass slab or disposable pad

  • Powder divided into two equal portions; first portion mixed with the liquid to wet the liquid before adding the second portion

  • Working time: approximately $2 ext{ min}$ at $22^ ext{o}C$

  • Avoid cement use once a surface