impression material lecture3student 2

7/31/2019 Impression Material Lecture3student 2

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The function of an impression material is to

make a ve copy, which records the

dimensions of the oral tissue and their

relationships.

The impression must be accurate so that the

model, cast, or die will be accurate. That is

the positive copy of the oral structure will be

accurate.

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Initial 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 to be removed from the mouth without

becoming deformed.

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Other characteristics of animpression material

Copy details accurately

Dimensional stability after removal from the mouth

Appropriate working time Appropriate time to harden in the mouth

Bio-compatibility/odor/taste

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Impression trays

Trays need to be rigid enough to support impressionmaterial.

For impression material to be retained in tray, tray

maybe perforated, and adhesive spray can be usedto enhance retention.

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Study model

a physical model of the oral tissue for study.Used in diagnosis and treatment planning.

Cast

a model requiring very exact replication ofsize and shape

Die

replica of a single tooth (cut from a cast)

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Classification of dental

impression materials Inelastic / Elastic

Mechanism of hardening

Thermal behavior

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Impression material (rigid)

Inelastic materials Setting mechanism

Impression compounds

Dental compounds

Cooling to mouth

temperature

Zinc-oxide eugenol (ZOE) Chemical reaction

Impression plaster Chemical reaction

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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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Hydrocolloids

The colloid state represents a highly dispersedphase 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, thesuspension is called a hydrocolloid (hydrosol,hydrogel

Hydrocolloids come in two forms:

Reversible agar Irreversible alginate

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Composition of Agar

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

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Properties

Flows well

Accurate reproduction , however, theimpression need to be poured

immediately or stored in 100%

humidity.

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, formingan 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 Agar is supplied in two viscosities, thick and thin

(depending on the amount of agar).

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Alginate

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 ensureaccurate impression

2-4 mm bulk material in tray Snap action removal from mouth

Stored in a moist environment to avoid loss of water

and deformation

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Impression making

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Examples of packaging for alginate.

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Scoop and water measure for alginate.

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An alginate impression

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Troubleshooting alginate

impression Premature set

Slow set

Voids

Distortion

Excess alginate at back of tray

*cause & solution*

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Criteria for acceptable alginateimpression

All teeth and relevant soft tissue

recorded

No large voids

Free of debris

No distortion

Etc etc etc (refer to text book)

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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 polymersand 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 forcondensation silicon and poorest for Polysulfides

Not wet well by water (hydrophobic) possiblesolution: surfactants

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Polysulfides

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Dispensing & Composition

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

Chemical composition of base:

80% low-molecular-weight organic polymercontaining mercaptan reactive groups (-SH), and20% 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 30minutes

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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Classification according to

polymerization rxn

Condensation silicon Addition silicon

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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 alcoholevaporates 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

However, they are among the most

expensive

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Dispensing & composition

Light, regular and heavy viscosities and alsoputty

Dispensed as cartridge with 2 chambers

(pastes), or two putty- system

Base: low-molecular-weight silicon withvinyl groups (paste system), or low-molecular-weight silicon with silanehydrogens (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

Pros and cons for each technique and

possible solution

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Clinical use

Crown and bridge

work

Bite registration (rapidsetting 1-2 minutes)

Indirect composite

inlays (silicon die technique)

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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 its supplied in pouches of base and

catalyst placed in mechanical mixer. Composition:

Base: low- molecular-weight polyether with cationreactive group

Catalyst: aromatic sulfonic acid

*clinical tip: mix well to avoid irritation from unmixedcatalyst.

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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

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

Cakes (sheets) and sticks

Clinical uses:

Sheets: Primary impression in

metal trays

To make custom trays

Sticks are used forborder molding

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Composition & properties Thermoplastic resin and waxes

Fillers to reduce flow

Plasticizers

Organic acids or oils

Pigments

Properties

Softened at 60C, remains plastic at 45C, firm at

37C

Heated in water not by flame Should be poured ASAP to avoid distortion

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Other impression materials

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 colors

Zinc 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

Initial set:3-6 minutes, final set:10 minutes

To accelerate the setting?

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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 berinsed with water, excess water shaken off,

and disinfectant sprayed (2 disadvantages?)

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 ofimpression adversely affects theimpression

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

Solution used: Na hypochlorite,iodophors.

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Sterilizing trays

Trays should be properly sterilized

before use

Disposable trays are recommended ifappropriate

Sterilization can be achieved by heated

steam, dry heat, chemical vapors

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Reference: dental materials, clinicalapplications for dental assistants and dentalhygienists

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impression material lecture3student 2

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