Q40. Prosthetic field impression. Segment impression:

whats in it

impression

field impression

imrpression trays

preliminary impression

partial/segmental dental impression and their features

impression

a negative likeness or copy in reverse of the surface of an object; an imprint of the teeth and adjacent structures for use in dentistry.

field impression

impression tray

preliminary impression

materials for individual impression trays

Field impressions: a field impression refers to a preliminary or provisional dental impression taken outside of a traditional dental office setting

• Usually quick and simple

• Uses portable materials (like alginate or silicone)

• May not be as precise as lab-quality impressions

• Often used for diagnostic, provisional, or triage purposes

impression trays: Object that is put ibn the mouth to take the impression.
Preminary impression: a negative likeliness made for the purpose of diagnosis or the fabrication of the impression tray
Materials for individual impression trays: shellac baseplates, acrylic resins, photo-cured baseplates

Partial or segmental dental impression

features and applications

The partial impression captures only a part (segment) of the oral cavity, namely the part where the prosthetic reconstruction is designed.

Features:

• -  Partial impressions can be taken in partial coverage trays, designed to fit over several teeth, or without trays. In the second variation, the upper teeth, the lower teeth and bite registration are captured on the same impression.

• -  A partial impression may utilize the one-step or the two-step technique.

• -  Partials do not provide complete details, but they are quicker and easier to make.

Application :

• -  Dental crowns Small

• -  Small Dental bridges

what is this one step two step method

Feature	One-Step	Two-Step
Speed	Faster	Slower
Detail Capture	Moderate	High
Materials Used	Heavy + Light-body together	Putty/heavy first, then light-body
Technique Sensitivity	High	Moderate to High
Best For	Routine impressions	High-precision (e.g., crowns, implants)

TWO STEP:

step 1

A preliminary impression is taken using putty or heavy-body material and allowed to set.

Step 2:

• The set impression is relined with light-body material, which captures fine details.

• The tray with light-body material is re-seated over the teeth.

ONE STEP:

1. Heavy-body (thicker material) is loaded into the tray.

2. Light-body (thin, more flowable material) is syringed around the prepared teeth.

3. The tray is immediately seated in the mouth while the light-body is still unset, so they cure simultaneously.

NON ELASTIC MATERIALS (1) dental impression compound (2) plaster of parid

dental impression compound:

theres 2 types:

type 1 = for impressions (eg edentulous jaws etc)

type 2 = tray prep

properties:

NOT ELASTIC = NO UNDERCUTS

HIGH THERMAL EXPANSION = dimensional inaccuracy

thermoplasstic

composed of: stearic acid, waxes, fillers etc

2: plaster of paris

• Used as a non-elastic impression material

• Primarily for edentulous impressions

• Fractures easily in undercuts

• Requires careful handling

• (CaSO4)2. H2O + 3 H2O ―› 2 (CaSO4 . 2 H2O) + T°C

non elastic impression material - ZOE (zinc oxide eugenol)

• Uses: Wash impressions over compound, edentulous ridge impressions, bite registration.

• Form: Powder-liquid or two-paste system.

• Reaction: Zinc oxide + eugenol → zinc eugenolate (sets in moist environment).

• Setting: Affected by temperature and humidity; ~0.1% shrinkage.

• Composition:

  • Base: Zinc oxide, oil, hydrogenated rosin.

  • Accelerator: Eugenol (12–15%), oil, rosin, filler (e.g., talc/kaolin).

  • Pastes have contrasting colors for easier mixing.

Advantages:

• High accuracy and surface detail.

• Stable after setting.

• Inexpensive.

• Good adhesion to trays.

Disadvantages:

• Nonelastic – not suitable for undercuts.

• Messy.

• Variable setting time (temperature/humidity dependent).

• Eugenol may irritate soft tissues.

Non-Eugenol Alternatives:

• Contain carboxylic acids instead of eugenol for sensitive patients.

• Same handling and usage but less irritation.

thermoplastic materials

🔹 Impression Waxes

• Used for:

  • Edentulous ridges

  • Direct impressions for micro-restorations, inlays, post-and-core

• Tray: Individual tray

• Setting: One phase, one step

• ✅ Pros: Easy to handle, good for small areas

• ❌ Cons: Poor dimensional stability, warps with heat

non elastic impression materials - self cured and photocured

Feature	Self-Cured Plastics	Photo-Cured Plastics
Also Known As	Cold-cure, chemically cured, auto-polymerizing	Light-cured resins
Polymerization Trigger	Chemical reaction (initiator + monomer)	Exposure to visible light (blue light, ~400–500 nm)
Control Over Working Time	Limited (sets soon after mixing)	Excellent (only sets when exposed to light)
Form	Usually powder + liquid	Light-sensitive sheets or pastes
Use in Dentistry	Custom trays, record bases, denture repairs	Custom trays, record bases
Setting Reaction	Exothermic (heat released)	Minimal heat generation
Shrinkage	Moderate (polymerization shrinkage)	Less shrinkage compared to self-cured
Surface Finish	May need finishing/polishing	Generally smoother finish
Equipment Needed	Mixing tools	Curing light/unit
Elasticity	Non-elastic	Non-elastic
Suitability for Undercuts	❌ Not suitable	❌ Not suitable

Impressions with Thermoplastic materials: stenz/gutta percha

stenz gutta-percha: • for edentulous alveolar ridges • individual tray • one phase, one step • Nowadays: root canals filling

Impressions with Thermoplastic materials - impression wax

• Type: Thermoplastic material, softens with heat and hardens upon cooling.

• Uses: Primarily for border molding and preliminary impressions in edentulous cases.

• Properties:

  • Non-elastic (not suitable for undercuts).

  • Low viscosity and easy to manipulate.

  • Limited detail reproduction.

• Composition: Primarily paraffin and microcrystalline wax.

• Advantages:

  • Easy to use, comfortable for patients, cost-effective.

• Disadvantages:

  • Inaccurate for fine detail, may distort when cooling, and can fracture easily.

elastic impression materials - agar and alginate

• Advantages:

  • No unpleasant odor.

  • Can be poured easily into stone.

  • Easy to remove casts from impressions.

• Disadvantages:

  • Requires expensive equipment.

  • Potential for thermal shock discomfort for patients with metallic restorations.

  • Tears easily and has limited detail reproduction.

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2B) Non-Reversible Alginate Hydrocolloid

• Uses: Preliminary impressions for complete dentures, orthodontic models, study models, removable partial dentures.

• Composition: Sodium/potassium alginate (12-15%) and calcium sulfate dihydrate (8-12%).

• Properties:

  • Mixed with water to form a gel.

  • Hydrophilic, easy to manipulate.

  • Dimensionally unstable, requires immediate pouring after removal.

  • Elastic recovery of about 97.3%.

• Advantages:

  • Inexpensive, pleasant taste, easy to manipulate.

  • Compatible with stock trays.

  • Can be easily poured into stone.

• Disadvantages:

  • Limited detail reproduction, tears easily.

  • Not suitable for fixed partial dentures.

  • Incompatible with some die materials.

elastomer - polysulfide/polyether

) Polysulfide Rubber (Mercaptan)

• Type: Elastomer (rubber-based material)

• Uses: Ideal for fixed partial denture impressions, especially in difficult areas (e.g., deep subgingival regions).

• Properties:

  • High tear strength and good elasticity (96% elastic recovery).

  • Condensation polymerization with lead dioxide as a catalyst.

  • Requires mixing of base paste and catalyst paste.

• Advantages:

  • High accuracy and good for multiple impressions.

  • Relatively low cost.

• Disadvantages:

  • Strong odor.

  • Long setting time and requires extra time for use.

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3B) Polyether

• Type: Elastomer (rubber-based material)

• Uses: Accurate impressions for a few teeth without severe undercuts.

• Properties:

  • Elastic recovery of 98.5%.

  • Low shrinkage (0.3% in 24 hours).

  • High stiffness after setting, making it less flexible.

  • Available in low, medium, high viscosities.

• Advantages:

  • Higher accuracy compared to polysulfides.

  • Dimensionally stable for up to a week (if kept dry).

  • Good surface detail reproduction.

• Disadvantages:

  • Expensive.

  • Short working and setting times.

  • Objectionable taste for some patients.

  • Can tear if thickness is inadequate.

elastomer (silicone based) - condensation silicone and vinyl

Condensation Silicone Rubber

• Type: Elastomer (silicone-based material)

• Uses: Fixed partial denture impressions, especially for single-unit inlays.

• Properties:

  • Elastic recovery of 99%.

  • Shrinkage between 0.2% to 1.0% in 24 hours.

  • Uses poly(dimethylsiloxane) with tin octoate catalyst for polymerization.

  • Hydrophobic, requires a dry field for accurate impressions.

• Advantages:

  • Pleasant handling and clean.

  • Highly elastic and controllable setting time.

  • Putty-wash system improves accuracy.

• Disadvantages:

  • Hydrophobic, so difficult to pour in stone.

  • Requires dry field for optimal results.

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3D) Addition Vinyl Silicones

• Type: Elastomer (silicone-based material)

• Uses: Ideal for high-accuracy impressions where dimensional stability is critical.

• Properties:

  • No shrinkage upon setting, better dimensional stability compared to condensation silicones.

  • Elastic recovery is high (similar to condensation silicones).

  • Uses addition polymerization and platinum catalyst.

  • Rarely releases hydrogen gas, which may cause porosity.

• Advantages:

  • Excellent accuracy and detail reproduction.

  • No shrinkage and high dimensional stability.

• Disadvantages:

  • Sensitive to mixing with latex gloves (can cause contamination).

  • Expensive and requires careful handling to avoid bubbles.