Non elastic impression materials

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Non elastic impression materialsASST PROFESSORDr Mumtaz ul IslamB.Sc. BDS. MHR. M.Phil.

Rigid impression materials and typesIncapable to record undercut area Impression plaster fractures on recovery others distortsImpression plasterImpression compound Impression waxesZinc oxide eugenol pastes system

Impression plasterSimilar in composition of dental plasterBeta calcium sulphate hemihydrate Reacts with water form calcium sulphate di hydrateWater powder ratio is very high (0.60)Mucostatic Fluid consistency

Control of propertiesAccelerator potassium sulphateRetarder borex Antiexpansion agent Alizarine red dye Accelerator, retarder and dye mixed in a solution can be used Too fluid stock tray cant hold it

Impression recordingTwo methods direct and wash Direct with special tray Puddling the impression tray Wash in a preliminary (primary) compound impression Simply seat the impression homeGentle movement from side to side and anterio posterior

Removal of impression Very fluid records fine detailsHemihydrates absorb moisture a more closer contactDimensionally stable after setting Removal is more difficult often fractures in undercut areaDry sensation after impression and bad taste complaintsDisinfection with sodium hypochlorite for 10 minutes

Casting the impressionIf impression of an undercut area taken it fracturesFractured pieces can be joined with each otherImpression must be coated with a separating media Impossible to separate the impression


Properties Contd,Rapid settingLow strength Excellent for recording flabby ridge (mobile soft tissues)Requires a special technique Impression of sound ridges taken in ZnO Eugenol A window is framed over the flabby ridge Impression plaster is painted over the exposed tissue

Impression compoundThermoplastic resembling waxesTwo types Lower fusing (type I) impressionHigher fusing (type II) construction of impression traysPresentation Type I sheet (cake) or stick formsType II Uses Sheets for impression of edentulous ridgesStick for border extension of impression and trays etc for single tooth impression with copper ring technique


Manipulation Softened in a water bath (conditioning)Type I at 47 C flows well 37 C or below minimum flowStick direct on flame and tempered in warm waterEffects of time of immersion Less not softensMore material becomes sticky leaching of lubricating substanceIdeal softening temperature is 55- 60 C Poor conductor takes more time to soften from the centerCake or sheet should be broken in to pieces

Copper ring technique

Difference between both typesType I flows above 37C (mouth temperature)Sheet shows 20% and stick shows 6% or less flowType II not flows at or near mouth temperature2% or less flowType I should record the impression of groves 0.02 4 mm

Viscosity The most viscous impression materialFine details can not be recordedMakes it muco compressiveRecords full depth of sulcusDisplace lingual and buccal soft tissue

Internal stressesHigh coefficient of thermal expansionPoor thermal conductivityLarge temperature drop from mouth to room Distortion occors gradually Model should be poured as soon as possible

Clinical considerations Border seal & dynamic shape of tissuesLocalized mucocompression for post dam area markingSurvival is questionableChemical disinfection is uncertainIf not tempered can burn the soft tissue (common)

Impression waxesLow melting paraffin wax and bees wax in 3:1Readily flow at mouth temperature and relatively soft at room temperatureRarely used to record complete impressionCorrective impressions especially for ZnO Eugenol Small imperfections of impression are corrected by paintingImpression of lower free end saddle partial denture

Applegate technique Melted wax applied to the faulty or short impression Where needed loaded in the tryTray seated in to the mouth with firm pressureAllow the wax to flow well at mouth temperature under pressure

Zinc oxide eugenol impression materialPresentation is in the form of pastes - base and catalystZinc oxide base paste usually white Contrast in color facilitates thorough mixing streak freePhenolic OH of the eugenol weak acid acid base reaction 2C10H12O2 + ZnO Zn(C10H11O2)2 + H2OTwo molecules of Eugenol reacts with zinc oxide Ionic bond and two co-ordinate bonds donation of pairs of electronsMethoxy oxygens to zinc


Setting reaction

Chelate compound (claws)

Ionic reactionNeeds an ionic medium & ionizable salts Water acts as ionic medium and acceleratorSome manufacturers not add water Water added materials starts reaction soon after mixing

Properties Thickness around 1 mm insignificant dimensional changeLow initial viscosity pseudoplastic fine detailsDefects can be corrected with impression -------------Major disadvantage lack of elasticityConsidered as non-irritantOrtho ethoxy benzoic acid is used in sensitive patients