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Impression materials

الحسين العزيز عبد أحمداألردنية الصحة وزارة في سنيه تركيبات أخصائي

األردنية والتكنولوجيا العلوم جامعة في متفرغ غير محاضرAhmad Abdul Aziz Alhussien Prosthodontist at M.O.HPart time lecturer at J.U.S.TBSc , MSc, Jor. Board/ prosthodontics

• The function of an impression material is to make a negative copy of the oral tissue … that is poured to produce a positive replica for these tissues

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Requirement of an impression material

• Must be a semi-liquid material that will flow and adapt itself around the structure of interest.

• It must set and harden into a solid ?? that is rigid enough “not too rigid” to be removed from the mouth without becoming deformed.

• Copy details accurately• Dimensional stability after removal from the mouth• Appropriate working time (from the start of mix)• Appropriate time to harden in the mouth (setting time)• Bio-compatibility/Aesthetic, odor/taste• Chemically compatible with material used to pour cast or

dies

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Impression trays

• Used to carry the impression material into the oral cavity• Can be

– metal or plastic– Perforated or non perforated– for edentulous or dentate – pre-manufactured (stock trays) or custom-made (made in the

lab. From resin).• Trays need to be rigid enough to support impression

material.• For impression material to be retained in tray, tray

maybe perforated, and adhesive spray can be used to enhance retention.

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Variety of trays

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• Castreplica of dental arch

• Diereplica of a single tooth (cut from a cast)

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Classification of dental impression materials

• Inelastic / Elastic

• Mechanism of hardening

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Impression material (rigid)

Inelastic materials Setting mechanismImpression compounds ‘Dental compounds’

Cooling to mouth temperature

Zinc-oxide eugenol (ZOE)

Chemical reaction

Impression plasterImpression wax

Chemical reactionCooling to mouth temperature

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Impression materials (aqueous)

Elastic material (hydro-colloid)

Setting mechanism

Reversible (Agar) Cooling

Irreversible (Alginate)

Chemical reaction

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Impression materials (rubber)

Elastic material (non-aqueous)

Setting mechanism

Polysulfides Polymerization reaction

Condensation silicon

Polymerization reaction

Polyether Polymerization reaction

Addition silicon Polymerization reaction

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Inelastic impression materials (rigid)• Plaster of Paris (Type I gypsum)• Dental compound/impression compounds

– Softened by heat and pressed against tissue before it hardens. Thermoplastic softens at 60°C, remains plastic at 45°C and becomes firm at mouth temperature. Used for primary impression

• ZOE (Zinc oxide eugenol)– Other uses ???

• Impression wax

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Elastic impression materials (non-rigid)• Hydrocolloids• Elastomers (Rubber)

Hydrocolloids

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Hydrocolloids • The colloid state represents a highly dispersed phase of

fine particles within another phase, some where between solution and a suspension.

• The colloid can exist as a viscous liquid known as a sol or a solid known as gel.

• If the particles are suspended in water, the suspension is called a hydrocolloid (hydrosol, hydrogel

• Hydrocolloids come in two forms:– Reversible agar– Irreversible alginate

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AgarAgarComposition of agar:

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Material Percentage

purpose

Agar (seaweed extract)

12-15% Colloidal particles as basis of the gel

Potassium sulfate

1% Ensures set of gypsum materials

Borax 0.2% Strengthens gel

Alkyl benzoate

0.1% Antifungal agent

water 85% Dispersing medium for the colloidal suspension

Properties • Flows well and adapts readily to hard and soft

tissue contours• Accurate reproduction (hydrophilic nature),

however, the impression need to be poured immediately.

• Need to be stored in 100% humidity for short period of time if not poured immediately

• Distortion is more likely to occur if impression not poured within an hour.

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• Tear strength is not high.• If compressed it might rebound if compression is not too

sever otherwise deformation occurs.• Material suffers from loss or gain of water:– Syneresis: a process by which the gel contracts and

some of the liquid is squeezed out, forming an exudate on the surface.

– Evaporation: loss of water which causes the material to shrink, and impression is distorted.

– Imbibition: uptake of water, this will swell and distort the impression.

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Clinical application

• The use of agar requires special equipment consisting of hydrocolloid conditioner (has 3 water bath chambers) and water -cooled tray connected to a rubber hose that delivers water.

• Usually used in lab for cast duplication • Agar is supplied in two viscosities, thick and thin

(depending on the amount of agar).

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• Preparing the material for impression:– Gel heated to 71-100°C, becomes liquid (sol)– Tray and agar syringe are liquefied in 8-12

minutes in boiling water in 1st chamber– 2nd chamber is used to store the agar at 60-66°C.– 3rd camber kept at 45-47°C to temper agar before

use to prevent burns.

• Agar exhibits hysteresis, melting at 85 °C and solidifying from 32-40 °C

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AlginateAlginate • Inexpensive• Easy to manipulate• Requires no special equipment• Reasonably accurate

Common uses:• Diagnostic cast (study model)• Preliminary impression for complete denture• Partial denture framework• Custom trays for fluoride or bleaching

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Composition/setting rxn/working time

Material Percentage

purpose

Na, K alginate

15-20% Basis of the gel

Ca sulfate dihydrate

14-20% Creates irreversible gel with alginate

Potassium sulfate

10% ensures set of gypsum

Trisodium sulfate

2% Retarder to control setting

Diatomaceous earth

55-60% Filler to increase thickness and strength

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Setting rxn/working time• Setting rxn occurs when the powder is mixed with water:

Ca sulfate dihydrate + NaAlginate Ca alginate• Working time: total time from start of mixing to the final time at

which an impression tray can be fully seated without distortion– Regular set: 2-3 minutes– Fast-set: 1.25-2 minutes

• Setting time: elapsed time from the start of mixing until impression material becomes firm enough to resist permanent deformation.Regular set: 2-5 minutes, Fast set: 1-2 minutes

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Setting rxn/working time• Controlling water temperature shortens or

lengthens setting time• Changing P:L has adverse effect on

materials property and strength• Allow extra 1-2 minutes, tear strength

increases, and rebound from undercuts w/out deformation improves

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Important considerations to ensure accurate impression

• 2-4 mm bulk material in tray• Snap action removal from mouth• Allow extra 1-2 minutes after setting to improve tear

strength• Stored in a moist environment to avoid loss of water and

deformation• Disinfect in less than 10 minutes to avoid dimensional

instability

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Impression making• Fluff powder, wait 30 seconds for the dust to

settle• Measure powder and water according to

manufacturer instruction• Use spatula to stir water into powder to mix• Stir vigorously to create homogenous mix (45

seconds for regular, 30 seconds for fast setting alginate)

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• Tray should cover all anatomical features• Utility wax can be added on tray borders to

extend its coverage• Adhesive spray is used to prevent separation of

alginate from tray.• Tray is loaded from posterior towards anterior• Moisten and smooth alginate• Posterior part of tray is seated first then anterior.• *tips to control gag reflex*

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Alginate impression

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Elastomers

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Elastomers

• Often called rubber materials since they have properties similar to rubber.

• Clinical uses:– Bridges– Implants– Partial dentures complete dentures– Indirect esthetic restorations

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Polymerization reaction• Involves formation of long-chain polymers and

cross-linking of chains• General properties:– Not as sensitive to water as hydrocolloids– Have a certain amount of rebound, the highest is for

addition silicon and polyethers, medium for condensation silicon and poorest for Polysulfides

– Not wet well by water (hydrophobic) possible solution: surfactants

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PolysulphidesDispensing & Composition

• Supplied in two tubes as base and catalyst, equal lengths are mixed. Light, regular, heavy viscosities

• Chemical composition of base:– 80% low-molecular-weight organic polymer containing

mercaptan reactive groups (-SH), and 20% reinforcing agents: titanium dioxide, silica, zinc sulfide

• Chemical composition of catalyst:– Lead dioxide or copper hydroxide

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Setting reaction

Mercaptan + lead dioxide polysulfide+H2O

• The reaction is sensitive to temperature and moisture so increase in any will accelerate the setting.

• Also sensitive to correct mixing ratio

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Uses • Crown and bridge impressions• Partial and complete denture impressionsClinical considerations when used• Used with custom trays• Allow 2mm thickness• Use tray adhesive• Dry field

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Properties of clinical interest

• Setting time: 8-14 minutes• Higher tear strength than hydrocolloids• Accuracy improves if impression is poured within 30

minutes• very unpleasant taste and odor• Messy (orange solvent to remove stains)• Can be irritant to oral mucosa

For all these reasons, Polysulfides have been largely replaced by other rubber materials

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Silicon rubber

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Addition (A) siliconCondensation (C) silicon

Condensation silicon• Developed as alternative to Polysulfides• Has more desirable qualities in

comparison:– Easy mix– Better taste and odorless– Shorter setting time (5-7 minutes)

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Dispensing & Composition

• Two pastes, base and catalyst.• Comes as light, medium, or heavy viscosity• Base: dimethylsiloxane + filler (silica)• Catalyst: suspension of stannous octoate +

alkyl silicate

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Setting rxn

• Condensation reaction that produces ethyl alcohol as by product. The alcohol evaporates which causes dimensional instability.

• The material continues to contract with time, so needs to be poured within minutes.

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Addition silicon (PVS)

• Desirable clinical qualities:– Dimensional stability– Accuracy– Clean – Easy to mix– No foul odor or taste

the most expensive

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Dispensing & composition• Light, regular and heavy viscosities and also putty • Dispensed as cartridge with 2 chambers (pastes),

or two putty- system – Base: low-molecular-weight silicon with vinyl

groups (paste system), or low-molecular-weight silicon with silane hydrogens (putty) + silica filler

– Catalyst: chloroplatinic acid

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Setting rxn• Polymerization rxn of chain lengthening and

cross-linking with reactive vinyl groups, producing a stable silicon rubber.

• No ethyl-alcohol by product• Some addition silicons produce hydrogen as by

product, manufacturers incorporated palladium powder that absorbs hydrogen

• setting time: 3-7 minutes

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Impression making techniques

• Putty/wash technique– One step– Two step

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Polyethers

• Also used for crown and bridge work since they are very accurate and also more hydrophilic than other silicons

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Dispensing & composition• Dispensing same as other rubber materials (2 tubes)• In addition it’s supplied in pouches of base and catalyst

placed in mechanical mixer.• Composition:– Base: low- molecular-weight polyether with cation

reactive group– Catalyst: aromatic sulfonic acid *clinical tip: mix well to avoid irritation from unmixed

catalyst.

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Properties • Stiff, difficult to remove from undercuts• Short working and setting times• Setting time 3-5 minutes• Sensitive to moisture and temperature• More hydrophilic (must not be stored in water or

disinfectant) • accurate

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Inelastic impression materials

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Impression compound:

• Impression compound: softens with heat, hardens in the mouth.

• 2 main forms:– Cakes (sheets) /low fusing – Sticks /high fusing

• Clinical uses: – Sheets:

• Primary impression in metal trays

• To make custom trays– Sticks are used for border

molding

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Composition & properties• Composition

– Thermoplastic resin and waxes– Fillers to reduce flow– Plasticizers– Organic acids or oils – Pigments

• Properties– Softened at 60°C, remains plastic at 45°C, firm at 37°C– Heated in water not by flame– Should be poured ASAP to avoid distortion – Flame used to heat sticks

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Impression plaster

• Impression plaster: seldom used– Composed of plaster gypsum (Ca hemihydrae)– Used for primary impression (high P:L)– Scored with a knife in the mouth then removed and

reassembled in the lab (distortion?)– Wash impression – Easy to use– Inexpensive

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ZNO eugenol

• Secondary impression for complete dentures, or wash impression (mucostatic impression)

• 2 tube paste system, different colorsZinc oxide (80%) and fillers, eugenol (15%) with oils, resin, fillers. In addition to chemical accelerator (zinc acetate)

• Dispensed in 2 equal lengths and mixed to homogenous color forming zinc eugenolate

• Initial set:3-6 minutes, final set:10 minutes• To accelerate the setting, a drop of water or zinc acetate

is added.

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Properties • Brittle, so not suitable for areas with

undercut• Flows readily• Accurate• Eugenol can be irritant (burning sensation) • Once set, dimensionally stable

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Impression wax• Clinical uses:– Preliminary impression for edentulous

patients– Bite registration– Baseplate wax used to be used for provisional

crown and bridge work– Melted to correct voids in gypsum casts

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Disinfection of impressions

• The disinfectant should be compatible with the impression materials.

• After taking the impression, it should be rinsed with water, excess water shaken off, and disinfectant sprayed or impression immersed in disinfectant

• Protective gloves should be worn • Rinse after disinfection is complete

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Disinfecting casts• Maybe necessary if impression was not

properly disinfected, or if immersion of impression adversely affects the impression

• Casts should be set and stored for 24 hours before disinfection.

• Solution used: Na hypochlorite, iodophors.

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Sterilizing trays

• Trays should be properly sterilized before use

• Disposable trays are recommended if appropriate

• Sterilization can be achieved by heated steam, dry heat, chemical vapors

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