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Dental Impression Materials

Silicone Reversible hydrocolloids

Irreversible hydrocolloids

Purpose of Impression Materials

• To make an accurate replica of the oral tissues

• To give a negative reproduction of the tissues

• To be filled with model material to make a positive cast

Positive cast

Desirable properties1. A pleasant odor, taste, and

esthetic color2. No toxic or irritant constituents3. Adequate shelf life 4. Economically commensurate with

the results obtained5. Easy to use with the minimum of

equipment

Desirable properties (con’t)6. Setting characteristics that meet clinical

requirements7. Satisfactory consistency and texture8. Readily wets oral tissues9. Elastic properties with no permanent

deformation after strain10. Adequate strength so that it will not

break or tear on removal from the mouth

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Desirable properties (con’t)11. Dimensional stability over temperature

and humidity ranges met in clinic and lab12. Compatibility with cast and die materials13. Accuracy in clinical use14. Readily disinfected without loss of

accuracy15. No release of gas during the setting of the

impression or cast and die materials

“ No impression material fulfills all the requirements”

� the selection of the material best suited for a particular clinical situation and technique rests with the dentist.

Classification

Agar hydrocolloid

CompoundWax

Temperature changes

(reversible)

Alginate hydrocolloidElastomers

Plaster of ParisZOE

Chemical reaction

(irreversible)

Elasticnon-elastic

By Application or Mechanical PropertiesBy Setting

Mechanism

Classification

1. Elastic1.1 Hydrocolloid• Reversible hydrocolloid (Agar)• Irreversible hydrocolloid

(Alginate)1.2 Elastomeric

� Polysulfide� Silicone � Polyether

2. Non-elastic• Zinc oxide eugenol• Impression wax

(ZOE)• Impression

compound• Impression plaster

� by application or mechanical properties

Hydrocolloid

•Colloid has two phase: dispersed phase + dispersion phase

Colloidal Solution (Sol.)Solution Suspension

(emulsion)

Adequate concentration

Semisolid material = Gel

Sol-Gel transformation

Hydrocolloid

• Reversible hydrocolloid (Agar)

• Irreversible hydrocolloid(Alginate)

Dipersion phase = water � Hydrocolloid

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Reversible Hydrocolloid(Agar)

Overview• Setting = “Gelation” ( sol�gel states)• Phase change is induced by Temp.

change• A colloid forms with water and the

material liquefies between 70 and 100°C (liquefaction temp) and sets to a gel-like state between 37 to 50°C (gelation temp).

Composition• Agar (8-15%)

– Polysaccharide from seaweed– Provides matrix phase which

forms fibrils in the “gel” stage.• Water (>80%)

– Reaction medium, controls flow properties

• Borax– to strengthen the gel– retarder for the setting of

gypsum

• Potassium sulfate– to accelerate the setting of

gypsum• Fillers

– to control the strength, viscosity, and rigidity

• Thymol and glycerine– bactericidal and plasticizer

• Alkyl benzoate– anti-fungal

• Pigments and flavors

Agar manipulation• Three stationary tempering• Boiling temperature• Storing temperature 65 ο C• Tempering in water-filled

container (~45 ο C )�Final temperature < 55 ο C

MManipulationanipulation�� HeatHeat inin waterwater atat 100100oo CC forfor 88--1212 mm�� StoreStore inin waterwater atat 6565oo CC�� PlacePlace inin traytray atat 6565oo CC�� TemperTemper inin 4455oo CC inin waterwater forfor 22 mm�� ImpressionImpression makingmaking:: waterwater coolingcooling

untiluntil gelationgelation occursoccurs ((Water circulated~ 2020oo CC))

Impression with reversiblehydrocolloid

• Accuracy - exellent impression of highaccuracy and fine detail

• Require careful control and moreexpensive appartus than alginate

• Dimensional unstable on standing:model should be made as soon aspossible

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• Agar lose moisture and shrink in air• Should be poured immediatly• Store in 100%relative humidity not

more than 1 hour.• After initial setting of model material

should be store in humidor

Impression with reversiblehydrocolloid Viscoelastic Properties

• The stress-strain relationship of the hydrocolloid impression materials changes as the rate of loading changes.

Clinical significance• To reduce the amount of

permanent deformationattributed to dashpot D1, the impression should not be removed by a teasing or weaving method, but it should be removed suddenly in a direction as nearly parallel as possible to the long axes of the teeth.

Permanent Deformation• Deformation should be < 1.5%

following a 10% total compression on the material and a 30 second recovery.

• It is important to lower the amount of compressive loading on the material in undercuts; therefore, (again!) one should remove the material as fast as possible and have more thickness in areas of severe undercuts.

Irreversible Hydrocolloid (Alginate)

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Overview

• Substance is alginic acid, which was extracted from seaweed.

• Potassium salt form of alginic acid (Potassium alginate) react with calcium sulfate dihydrate and water to form a calcium alginic gel.

Composition• Potassium alginate (15%)

– Soluble alginate• Calcium sulfate dihydrate (16%)

– Reactor• Diatomaceous earth (60%)

– Fillers– Strength, Stiffness, Texture

• Potassium titanium fluoride (3%)– Accelerator for the setting of

the stone• Zinc Oxide

– Filler

• Sodium phosphate (2%)– Retartder - reacts

with calcium ions to slow the working time

• Glycols – Decrease dust

formation• Quaternary ammonium

salts – Disinfectants

• Pigments, taste

Gelation Process(Sol) (Gel)

Soluble Alg + CaSO4 Insoluble Alg(rapid reaction)

Sodium or Potassium alginate

Add water-soluble salt e.g. sodium phosphate to react with CaSO4 --> prolong the working time.

Calcium alginate

Gelation Process (con’t)Calcium sulfate

Potassium alginateTrisodium phosphate

+ Water

2Na3PO4 + 3 CaSO4 --> Ca3(PO4)2 + 3 Na2SO4

K2nAlg + nCaSO4 --> nK2SO4 + CanAlg

Na3PO4 used up

Gel Structure

• The final structure Sodium alginate cross-

linked with calcium ions.

Cross-linking of Sodium alginate

Gelation Time• Once gelation starts, the impression

material must not be disturbed because the growing fibrils will fracture and the impression would be significantly weakened.– Fast setting = 1.5 -3 mins.– Normal setting = 3 – 4.5 mins

• Best regulated by the amount of retarder added during manufacturing.

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Gelation Time (con’t)• Water Temperature

– 1 min. of reduction in gelation time for each 10°C of temp. decrease

Strength• Gel strength doubled during the first

4-min period.• Water amount

– Weakened, less elastic• Spatulation

– Insufficient spatulation --> failure of the ingredients to dissolve sufficiently

• Overmixing– Breaks the CaAlg gel network --> decrease strength

Viscoelasticity• Strain-rate dependent• Avoid torquing or twisting when

remove the impression.

Accuracy

• Not as good as elastomeric impression materials– Reproduction of fine detail– Dimension stability– Roughness of the impression surface

• Can be used for Removeable Partial Denture (RPD)

Dimensional Stability• Syneresis, evaporation,

imbibition• The impression should be

exposed to air for as short a time as possible.

• Immediately constructing the stone cast

air water

Dimensional Stability

• 100% relative humidity is the best storage environment to preserve the normal water content of the impression.

Percentage of change in water content by weight

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Compatibility with Gypsum• The surface of the gypsum cast may be too

soft for waxing procedures.– Immerse the impression in the accelerator for

gypsum product– Incorporate a plaster hardener or accelerator in

the impression material (fluoride)

Disinfection• Certain disinfectants my result in gypsum casts

that have a lower surface hardness or diminished surface detail

• Current protocol– Impression --> thoroughly rinsed --> spray

disinfectant --> wrap in a disinfectant-soaked paper towel --> place in a sealed plastic bag for 10 minutes

– Iodophor, Sodium hypochlorite, Glutaraldehyde

Shelf Life• Storage temperature and moisture

contamination• Materials stored for 1 month at 65°C

are unsuitable to use, either failing to set or setting too rapidly.

• Should not stock more than one year’s supply.

Effects of Mishandling• Examples of common causes for

failures encountered with hydrocolloid impression materials– Distortion– Tearing– Rough stone model

Distortion

• Impression not poured immediately.• Movement of tray during gelation• Premature removal form mouth• Improper removal from mouth• Tray held in mouth too long (only with

certain brands)

Tearing

• Inadequate bulk• Moisture contamination• Premature removal from mouth• Prolonged mixing

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Rough stone model

• Inadequate cleaning of impression• Excess water left in impression• Premature removal of model• Leaving model in impression too long• Improper manipulation of stone