5- Impression Materials

Impression Materials

• Definition: Materials used to take impressions of teeth and supportive soft tissues in dentistry.

Ideal Properties of Impression Material

• Non-toxic and non-irritant: Biocompatibility is essential.

• Good flow: Ensures an accurate impression.

• Easy manipulation: User-friendly during application.

• Dimensionally stable after setting: Maintains shape over time.

• Acceptable price and availability: Should be affordable and easily sourced.

• Good shelf life: Durability is important for storage.

• Acceptable setting time: Should not set too quickly or slowly.

• Pleasant taste, odor, and color: Enhances patient acceptance.

Types of Impression Materials

Non-Elastic Impression Materials

• Examples:

  • Impression Plaster

  • Impression Compound

  • Zinc Oxide Eugenol (ZOE)

  • Impression Waxes

Elastic Impression Materials

• Examples:

  • Agar Hydrocolloid (reversible)

  • Alginate Hydrocolloid (irreversible)

  • Polysulfide Elastomer

  • Polyether Elastomer

  • Silicone Elastomer (addition and condensation)

Non-Elastic Impression Materials

1. Impression Plaster

• Characteristics:

  • Earliest dental impression material; rigid with limited flow.

  • Recommended for edentulous patients only, specifically for final impressions without undercuts.

• Uses:

  • Final impression for edentulous patients.

  • Bite registration.

• Properties:

  • Setting time: 3–5 minutes.

  • Brittle; handle carefully to avoid fractures.

2. Impression Compound

• Characteristics:

  • One of the oldest impression materials; rigid and thermoplastic (sets through physical changes).

• Uses:

  • Preliminary impressions for edentulous patients.

  • Special trays and border molding.

• Composition:

  • Resin (30%), Copal resin (30%), Carnauba wax (10%), Stearic acid (5%), Talc (25%), Coloring agents.

Types of Impression Compound

• Type I: Brown cake; used for primary impressions, softened with heat.

• Type II: Green stick; used for border molding, softened with flame.

• impression compound component: wax, resin, filler and pigments(fillers give viscosity which makes it more rigid), stearic acid as a lubricant

• imp. comp. becomes soft at 45-60 max and hard at 37 degrees.

3. Zinc Oxide Eugenol (ZOE)

• Characteristics:

  • Composed of two tubes: Base (zinc oxide)(white) and Catalyst (eugenol)(coloured).

  • Thin and brittle; needs careful handling.

• Uses:

  • Secondary/final impressions for edentulous patients.

  • sedative effect

  • very shin and brittle

  • no undercuts or will break

  • poured with stone not plaster

  • Temporary fillings and bite registration.

  • advantages: -cheap - accurate surface detail - dimensionally stable

  • disadvantages: -may cause irritation -thin, brittle, sticky, long setting time (5-15 mins)

• non eugenol pastes are developed to overcome the burning sensation caused by eugenol

• bactericides and other medicaments are also incorporated

• used in eugenol irritant patients

4. Impression Waxes

• Characteristics:

  • Thermoplastic; flows at mouth temperature, softens at room temperature.

  • Used to correct small imperfections in other impressions.

  • do not set by chemical reactions

  • consist of a combination of low melting paraffin wax and beeswax in a ratio of approximately 3:1 so it flows at room temp

• Properties:

  • Should be poured immediately to avoid distortion.

Elastic Impression Materials

1. Agar Hydrocolloid

• Characteristics:

  • Organic hydrocolloid; reversible material allowing reuse with heat.

• Uses:

  • Final impressions; supplied as syringes or tray material.

• Composition:

  • Agar, Borax, Potassium sulphate, Benzoate, Additives.

2. Alginate Hydrocolloid

• Characteristics:

  • Irreversible; used for primary impressions(poor detail) with undercuts.

• Types:

  • Normal-setting (2-5 minutes) and fast-setting (1-2 minutes).

• Composition: potassium or sodium triethanolamine alginate(15%), calcium sulphate dihydrate(16%), zinc oxide(4%), potassium titanium fluoride(3%)<accelerator that counteracts the inhibiting effect of hydrocolloid in the setting of stone, ensures good surface quality of cast>, diatomaceous earth(60%), trisodium phosphate(2%)<retarder to produce regular or fast set alginate>, colouring agents, flavouring agents

• 2 types:

  fast setting alginate - setting time: 1-2 mins, working time: 1 min, mixing time: 45 secs

  normal setting alginate - setting time: 2-5 mins, working time: 2 mins, mixing time 60 secs

Control of Setting Time

• Powder/Water Ratio:

  • More powder decreases setting time; less powder increases setting time.

• Water Temperature:

  • Cold water increases setting time; warm water decreases setting time.

• Indicators:

  • Alginate is set when it becomes rubbery or changes color.

  • Impressions should be poured within 30 minutes to avoid distortion.

Advantages and Disadvantages of Alginate

• Advantages:

  • Elastic, cost-effective, good taste, easy to mix/manipulate, hygienic.

• Disadvantages:

  • Poor detail, tears easily, poor dimensional stability.

• Disinfection Methods:

  • Iodophores, Sodium hydrochloride, Glutaraldehyde, Phenyl phenol.

Polysulfide Elastomer

• Characteristics:

  • Synthetic elastomer; offered as a two-paste system (base and accelerator).

• Properties:

  • Available in 3 viscosities (light medium and heavy bodies); working time: 5-7 minutes; setting time: 8-12 minutes. impression pouring 30-60 mins

• Advantages:

  • Good flexibility, high detail, tear resistance.

• Disadvantages:

  • Long setting time, unpleasant odor.

Polyether Elastomer

• Properties:

  • Used for secondary impressions; comes in 3 viscosities.

  • Working time: 2-5 minutes; mixing: 4-5 minutes; can be stored for up to a week.

• Advantages:

  • High accuracy, dimensionally stable, non-toxic, good on undercuts, good surface details, easy to mix and handle

• Disadvantages:

  • Allergic reactions, expensive, high stiffness.

Silicone Elastomer

used with both stock and special trays to take final impression

supplied as 2 pastes or paste and liquid

Condensation Silicone

• Characteristics:

  • Comes as light, medium, heavy, and very heavy body.

  • Work and set times vary (3 minutes working, 6-8 minutes setting, impression poured within 30 mins).

• Advantages: excellent surface detail, dimensionally stable, non toxic and non irritant, easy to use, used with severe undercuts

• Disadvantages: hydrophobic, irritant, bad odour very thick

Addition Silicone

• Characteristics:

  • Available in low to very high consistencies, with specific usage instructions.

  • supplied as: jars, cartridge, syringes and sausages

  • 2-5 min working time, 2-7 setting time, impression poured within a week

• Advantages:

  • Dimensionally stable, excellent detail, fast setting, non toxic and non irritant.

• Disadvantages:

  • Short working time; cannot use latex gloves during mixing, expensive.

1. light body additional silicone elastomer elastic impression material

   -used in combination with another additional silicone

   -used to take final impressions for crowns and bridges

2. medium body additional silicone elastomer elastic impression material

   -can be used alone or in combination to take final impressions

3. heavy body additional silicone elastomer elastic impression material

   -used alone or in combination

4. putty additional silicone elastic impression material

   -used to take primary impression or in combination for final impression

   -used for border molding in complete denture patients instead of greenstick

   -latex in gloves effects the polymerization of the material

Techniques of Impression Taking

One Step Technique

1. Mix putty (base & catalyst) and adapt to the tray.

2. Inject light body around the prepared tooth/teeth

3. Take the impression.(both materials polymerize simultaneously)

Two Step Technique

1. Mix putty (base and catalyst)

2. Adapt on a tray and place a separator on top

3. Take impression

4. After impression is set, remove it and remove separator

5. Inject light body over the set putty impression and retake the impression again

6. Wait until the impression sets and remove it