Impression materials and techniques

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  • IntroductionDefinitionHistoryIdeal RequirementsClassificationImpression materials - Elastomeric materials - Hydrocolloids Latest advances Impression Techniques

  • Introduction

  • Impression materials are used to produce the accurate replicas of intraoral tissues.

    There are a wide variety of impression materials available each with their own properties, advantages and disadvantages.

    Materials in common use can be classified as elastic or non-elastic according to the ability of the set material to be withdrawn over undercuts.

  • Definition

  • Any substance or combination of substances used for making an impression or negative reproduction. - GPT

  • An impression is an imprint or negative likeness of the teeth and/or edentulous areas, made in plastic material which becomes hardened or set while in contact with the tissue. -Hartwell 1992

  • An impression is the perpetual preservation of what already exists and not the meticulous replacement of what is missing. -M.M.Devan 1995

  • 1782 : William Rae used wax with Plaster of Paris.1940s: American dentists used Plaster OF Paris for impression & the technique was presented to the profession at large by Chaplin Harris in 1953.1925 : Alphous Poller of Vienna was granted a British Patent for a totally different type of impression material which was later described by Skinner as Colloidal Sols of Emulsoid type. The possibility of using colloidal substance for dental impression became apparent when Pollers Negacoll was modified & introduced into the dental profession as Dentacol in 1928. Agar hydrocolloid was introduced to the dental impression.1930 : JD Hart of Oklahoma began to use Agar for fabrication of cast restoration.

  • 1930 : AW Ward and EB Kelly introduced ZnO Eugenol.1890s : A chemist from Scotland noticed that Brown Seaweed yielded peculiar mucous extraction. He named it Algin.1936-40 : S William Wilding used Algin as a dental impression material1950s : Development of Rubber base impression materials (Polysulphides and Condensation Silicones)1960s : Polyether impression material developed in Germany1970s : Addition silicone was introduced as a dental impression material1988 : Latest addition and light cure elastomers1990-2000 : New auto devices and delivery systems

  • Ideal Requirements

  • Fluid enough to adapt to the oral tissuesViscous enough to be contained in the traySetting time - less than 7 minutesAdequate tear resistanceDimensionally stableBiocompatible Cost effective

  • Classification

  • I. Based on the degree of tissue compression/ amount of pressure applied

    Mucostatic Ex: Impression plaster Mucocompressive Ex: Impression Compound

  • II. Based on Mode Of Setting & Elasticity

    A. Chemical reaction (irreversible)By elasticity and use Inelastic or rigid Elastic Materials Use Material Use 1.Plaster of Paris 2.Zinc-oxide eugenol Edentulous ridge Interocclusal records 1. Alginate 2. Non-aqueous elastomers Polysulfide Polyether Condensation silicone - Addition silicone Teeth and soft tissue B. Physical reaction (reversible)3.ImpressionCompoundPreliminary impression 3.Agar hydrocolloid Teeth and soft tissue

  • III. Based on the chemical composition

  • IV. Based on type of impression & area of use

  • V. According to the use of the materials in dentistry

    A) Materials used for obtaining impression of dentulous mouth

    AlginateAgarNon-aqueous Elastomers

    B) Materials used for obtaining Impression of edentulous mouth:

    Impression Compound Impression Plaster Zinc Oxide eugenol Wax

  • VI. Based on Manipulation

  • VII According to their method of use

  • VIII. Based on the tray used for impression

  • Hydrocolloids

  • Colloids often classed as fourth state of matter as colloidal state. can be considered as a compromise between the very small molecules in solution & very large particles in suspension. Two phases Dispersed phase or dispersed particles Dispersion phase or dispersion medium All colloidal dispersions are termed as sols.

  • Types Of ColloidsThe only exception is the gaseous state, i.e. two gases.

  • Hydrocolloids If the dispersion medium is water, the material is known as hydrocolloid. Particles larger than those in solution. Particle size ranges from 1-200 nm. There is no clear line of demarcation among the solutions, colloids & suspensions.

    Gels Colloids with a liquid as a dispersion medium can exist in two different forms known as Sol & Gel.

  • Sol- has a appearance & many characteristics of a viscous liquid. Gel- is a semisolid & produced from a sol by the process of gelation.

    GelationIt is a process of conversion of sol to gel, to form fibrils, micelles of the dispersed phase which become interlocked to give characteristic jelly like consistency.

    Within the gel, the fibrils branch & intermesh to give a brush heap structure.

  • micellesbrush-heap structure

  • Gelation Temperature The temperature at which gelation occurs is known as gelation temperature. Agar - secondary bonds (weak) hold the fibrils together- break at slightly elevated temperatures and become re-established as it cools to room temperature - reversible

    Alginate - the fibrils are formed by chemical action - irreversible

  • The Gel may lose water by evaporation from its surface or by exuding fluid onto the surface by a process known as SYNERESIS. The gel shrinks as a result of evaporation & syneresis.

    If a gel is placed in water, it absorbs water by a process known as IMBIBITION. The gel swells during imbibition, thereby altering the original dimensions.

    The effects of syneresis, evaporation & imbibition on the dimensional changes are of considerable importance in dentistry, since any change in dimension that occurs after the impressions are removed from the mouth will lead to inaccurate casts & models.


  • Reversible hydrocolloids undergo transition from gel to sol on heating.

    On cooling they return back to the original state.

    The hydrocolloid supplied as the gel is heated above its liquefaction temperature, cooled to a temperature compatible with the oral tissues & placed in the mouth. Gelation occurs as the tray continues to cool, after which the impression is removed & poured.

  • Agar It is a sulphuric ester of a linear polymer of galactose. Gelation Temperature of agar is approx- 37-50c. The temperature at which the gel changes to sol i.e. liquefaction temperature is 70-100c.

    Although it is an excellent impression material & yields accurate impressions, it has been largely replaced by alginates & rubber impressions because- Of the minimum equipment required Possibility of obtaining metal dies from rubber impression.

  • ComponentFunctionComposition AgarBrush heap structure13 17% BoratesStrength0.2 0.5% SulfatesGypsum hardener1.0 2.0% WaxFiller0.5 1.0%Thixotropic materialsThickener0.3 0.5% WaterReaction mediumBalance 84%Alkyl BenzoatesPreservatives0.1% Color and FlavorsTaste & appearanceTrace

  • Mode of supply :Gel in collapsible tubes (for impressions)A number of cylinders in a glass jar (syringe material)In bulk containers (for duplication)

  • Full mouth impressions without deep undercutsQuadrant impressions without deep undercutsSingle impressionsCan be used for crown and bridge impressions because of their accuracyCast duplicationTissue conditioner

  • ManipulationPreparation, conditioning and tempering of the agar material:

    Before use the material is subjected to a controlled regimen in three water baths.

    Liquefying bath -100C for 10 min.Storage bath 63-66C Tempering bath- 44-46C for 3-10 min.

  • Conditioning unit

  • Making the Agar impression

  • Syringe material, directly taken from storage compartment - applied on to the prepared tooth.Water-soaked outer layer of tray hydrocolloid is removed from the tempering basin, outer layer removed and impression made. Gelation is accelerated by circulating cool water (18-21oC) through the tray for 3-5 min.

  • According to ADA specification no.11 :-Compressive strength : 8000 gm/cm2Tear strength : 700 gm/cm2Viscosity : Sufficient fluid to allow detailed reproduction of hard and soft tissuesFlexibility: 11% when a stress of 14.2 psi appliedElastic recovery : 98.8%Accuracy : 25mWorking time : 7-15 min.Setting time : 5 min.Gelation temperature : 37-50CLiquefaction temperature : 70-100C

  • HydrophilicGood elastic propertiesCan be re-used as a duplicating materialLong working time and low material costNo mixing technique, the potential for errors are eliminatedPalatable and well tolerated by patientsHigh accuracy and fine detail recordingCompatible with die and cast materials

  • Low tear resistanceLow dimensional stabilityOnly one model can be usedExtensive and expensive equipment requiredIt can not be elecroplatedImpossible to sterilize for re-use