BY DR KHAWAJA RASHID HASSAN HEAD SCIENCE OF DENTAL .

BYDR KHAWAJA RASHID HASSANHEAD SCIENCE OF DENTAL MATERIALSDEPARTMENTRAWAL COLLEGE OF DENTISTRYISLAMABAD1

CAST / MODEL:A replica of several teeth and their associated softtissues or an edentulous arch.DIE:Replica of a single prepared tooth.MODEL PLASTER:Weakest and most porous form of gypsum productused in dentistry.DENTAL STONE:A stronger and less porous form of gypsum productused in dentistry.DIE STONE:The densest form of gypsum product used indentistry.2

DIAGNOSTIC CASTS:The casts generally used for patient education,treatment planning or tracking the progress oftreatment. (e.g. orthodontic models)WORKING CAST:The cast that is strong enough to resist thestresses of fabricating an indirect restoration orprosthesis.POURING:The process of vibrating the flowable gypsumproduct into the impression.3

GYPSUM DENTAL INVESTMENT:A refractory material, consisting of silica andgypsum as a binder. Used for taking moulds formetal casting process.HYGROSCOPIC SETTING EXPANSION:The amount of setting expansion that occurswhen the gypsum bonded casting investmentis immersed in water.NORMAL SETTING EXPANSION:The amount of setting expansion that occurswhen a gypsum bonded casting investment isallowed to set in air.4

Gypsum is a white powdery mineralwidely found in nature. Used for making dental casts since 1756. Chemical name Calcium sulphatedihydrate.(CaSO.2H2O) In dentistry, gypsum is used in the form ofCalcium sulphate hemihydrate.(CaSO.½H2O) 5

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Plaster of ParisDental stoneDie stone7

Current I.S.O standard for dental gypsumproducts identifies 5 types .Type 1Type 2Type 3Type 4Type 5Dental plaster, Impression.Dental plaster, model.Dental stone, die andmodel.Dental stone, die, highstrength, lowexpansion.Dental stone, die, highstrength, highexpansion.8

The main requirements of model anddie materials are,1. Dimensional accuracy.2. Adequate mechanical properties.9

Should ideally be fluid at the time it ispoured into the impression.Set material should be sufficiently strong toresist accidental fracture.Set material must be hard enough to resistthe abrassion during carving of wax pattern.Should be compatible with all the materialsit comes in contact.Should have good colour contrast withvarious waxes.10

Plaster and stone products are formedby calcining gypsum. (calcium sulphatedihydrate) Gypsum is grounded and subjected totemperatures of 110º - 120 ºc to drive offpart of water of crystallization. As the temperature is further raised theremaining water of crystallization isremoved. 11

110 - 130ºCCaSO4 2H2O 130 - 200ºC CaSO4 ½H2OCaSO4 ½H2OCaSO4200 - 1000ºCCaSO4CaSO4Gypsum110 - 130ºCCalcium sulphatehemihydrateCalcium sulphate 130 - 200ºCHexagonalhemihydrateHexagonal200 - 1000ºCanhydrateanhydrateOrthorhombicanhydrate12

Gypsum is heated to atemperature of 120ºC in anopen vessel. Produces irregular, porousparticles. Also referred as βhemihydrate. Overheating causes calciumsulphate anhydrate. 13

May be produced by one of 2 methods1. Gypsum heated to about 125ºC understeam pressure in an autoclave.This produces more regular and lessporous hemihydrate.Referred as ά-hemihydrate.2. May be boiled in a solution of CaCl2.Gives material similar as produced by autoclave but with lessporosity.14

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Calciumsulphate hemihydrate powderreacts with water to form gypsum.(CaS04)2 . H2O 3 H2O2CaSO4.2H2O unreacted(CaSO4)2.½H2O Heat The product formed is gypsum. The exothermic heat evolved is equivalent tothe heat provided originally. The conversion to dihydrate form is never100%.17

The ratio of water to hemihydratepowder is always expressed as W/P ratio. Obtained when weight (volume) ofwater is divided by the weight of thepowder. E.g. if 100g of plaster is mixed with 60mlof water, the w/p ratio will be 0.6. W/P ratio is important for determining thephysical an l set is of concern. 34

Setting expansion can be a disadvantage.Lower W/P ratio and longer mixing time increases thesetting expansion.Lower W/P ratio more nuclei of crystallization lessspace between the nuclei growth interaction. (thickermix) more outward thrust more expansion.Chemicals are added by manufacturers.Potassium sulfate, sodium chloride and borax.35

Setting expansion in water.Magnitude is more than double as compared to the settingexpansion in air.Accepted reason is that the crystals grow more freely inwater as compared to air.Basic mechanism of crystal growth is the same.Reduced W/P ratio increases the hygroscopic settingexpansion.Increased spatulation increases the hygroscopic settingexpansion.36

Hygroscopic setting expansion of dentalstone or plaster is generally small inmagnitude.E.g. dental stone for making cast may exhibita normal linear setting expansion of 0.15%with maximum hygroscopic setting expansionof 0.30%.37

PlasterPlasterFlask38

IMPRESSION1)2)3)PLASTER (Type I):Composed of plaster of Paris.Modifiers are added for the regulation ofsetting time and setting expansion.Rarely used now a days for takingimpression.39

MODEL1.2.3.4.PLASTER (Type II):Also called as laboratory type IIplaster.Marketed in white colour.Used principally to fill the flask indenture construction.Relatively weak.Compressive strength 9 MPa.Tensile strength 0.6 MPa.40

DENTAL1.2.3.4.5.STONE (Type III):Introduced in dentistry in 1930.Improved hardness.Type III stone preferred for casts used toprocess dentures and dies of prepared tooth.Denture is easier to remove afterprocess.2 methods are used for cast production.41

DENTALSTONE, HIGH STRENGTH(Type IV):1.2.3.Particles are cuboidal shaped.Hard surface is necessary for a die stone.Surface dries more rapidly so the surfacehardness increases more rapidly too ascompared to compressive strength ( 92Rockwell Hardness ).42

DENTALSTONE, HIGH STRENGTH,HIGH EXPANSION (Type V):1.Higher compressive strength than type IV.2.Improved strength is attained by lower W/P ratio.3.High setting expansion is an advantage.4.Type V stones should be avoided for making diesfor inlays.43

Bi products or waste products of phosphoricacid production can also make ά-hemihydrateand β-hemihydrate. Expansive than natural gypsum. Properties are either equal or better than thenatural gypsum.44

PROPORTIONING:Strength of stone is inversely proportional tothe W/P ratio.Important to keep the amount of water aslow as possible.Water and powder should be measuredaccurately by a graduated cylinder forwater and weight balance for powder.Pre weighed envelops have become morepopular these days.45

MIXING: Bowl must be parabolic, smooth and resistant to abrasion.Spatula must have a stiff blade and comfortable handle.Air entrapment must be avoided to prevent porosity.Automatic vibrator of high frequency and low amplitude ishelpful.Repeated addition of water or powder is the main cause ofinaccuracy in the gypsum products.46

CARING FOR GYPSUM PRODUCTS:Gypsum products are sensitive to the change inhumidity.Gypsum surface made with high W/P ratio are effectedmore.Hemihydrate of gypsum takes up water from air.The best way is to seal the product in moisture proofmetal container.47

CARING OF CAST: Cast should be an accurate reproduction of the oro-dentaltissues. Cast should be allowed to harden completely beforeremoval. On completion of setting reaction the dimensions will beconstant. Sometimes required to be immersed in water. Linear dimension may decrease 0.1% for every 20 min incase of stone cast.48

Plaster bases are recommended sincetrimming them is easier than dentalstone. If base is made from stone, it should besoaked in water for 5-10 minutes tosoften it before trimming. Importantconsiderations when trimming?49

Proportion of base toanatomical partParallelismUse of wax biteregistrationOuter border of castShaping of anteriorpart of upper andlower arches50

Increased chances for the spread of infection. Disinfection of both impression and the model or diematerial is important. Disinfectant solutions can be used. Alternatively, dental stone containing disinfectant canbe used. May lead to slight change in physical andchemical properties. Spray disinfectants, hypochlorites and iodophores areuseful. 51

Casts should have set for 24 hours beforebeing disinfected if necessary. Spray rather than immerse Disinfectants commonly used: › Sodium hypochlorite› iodophors› Chlorine dioxide52

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Plaster of Paris Dental stone Die stone . Current I.S.O standard for dental gypsum products identifies 5 types . Type 1 Dental plaster, Impression. Type 2 Dental plaster, model. Type 3 Dental stone, die and model. . mix) more outward thrust more expansion.