impression materials - stav lkařsk chemie a (tear strength) of impression materials compromised...

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  • 1

    Impression materials

    Pavel Bradna

    Institute of Dental Research

    To prepare accurate and true replicas of oralstructures (teeth, mucosa)

    Purpose

    Step 1. Making a negative (impression)

    Step 2. Prepare a model or a cast -accurate positive

    The replica is prepared in 2 steps:

    (C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague

  • 2

    Requirements1. Capable of plastic to rigid body transformation2. Acceptable for a patient

    - non-toxic, non-irritant, tasteless, reasonablesetting time up to 5-7 min

    3. Good handling properties easy to prepare/mixplastic before set, but viscous enough not to flowout of a tray (thixotropic), adequate working andsetting times

    After being set:4. Accuracy and detail reproduction (25-50m),5. Dimensionally stable6. Resistant to mechanical stress - elastic and rigid7. Compatible with model materials8. Resistant to disinfectant solutions9. Cost effective

    Important terms

    Elastic/plastic deformations Strength Pseudoplastic/thixotropic Hydrophilic/hydrophobic Working time Setting time

    (C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague

  • 3

    Elasticity/plasticityelastic/plastic deformation

    stre

    ss (f

    orce

    /m2 )

    Describes behaviour of material under load

    Region of elasticdeformations,

    Hooks law applies

    Region of plasticdeformations

    Slope Youngs modulus,modulus of elasticity

    Deformation

    Maximum load atfracture ultimate

    strength

    Proportional limit

    5 N

    When unloaded!fully recovers!

    No load

    Mechanical model of elastic behaviourA spring ideally elastic behaviour (Hooks law)

    Def

    orm

    a tio

    n

    t1loaded

    Def

    orm

    atio

    n

    t2unloaded

    Loaded

    Unloaded

    (C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague

  • 4

    A dashpot ideally plastic behaviour

    Permanentdeformation

    Def

    orm

    atio

    n

    t1loaded

    t2unloaded

    Mechanical model of plastic behaviour

    When unloaded!No recovery!

    Plastic =permanent/irreversibledeformation

    defo

    rmat

    ion

    Timet1 t2

    Plastic/permanentdeformation

    90o)

    unable to wet humidsurfaces

    Hydrophilic materialable to wet humid surfaces

    Contactangle

    Does not copy the wet surface

    Examples of clinical significance of pseudoplasticproperties, hydrophilicity and high mechanicalresistance (tear strength) of impression materials

    Compromised flow in smalldetails - too high viscosity

    and hydrophobicity

    Preparedtooth

    Impressionmaterial

    Fracture and lost detail dueto low strength

    (C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague

  • 7

    Working time a period from the start of mixing tothe final time at which the impression can be seatedin the mouth without its distorsion

    Setting time a period from the start of mixingtill the impression becomes elastic enough toresist deformation during its withdrawal from themouth

    Impression com-pounds

    ZnO-eugenol (ZOE)Impression plaster

    Inelastic/rigid

    Agar hydrocolloidAlginateElastomeric: Polysulfide Polyether Silicone

    Elastic

    ReversibleIrreversible

    Classification of impression materials

    (C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague

  • 8

    1. Zinc-oxide eugenol impressionpastes (ZOE)

    Inelastic/rigid impressionmaterials

    Main indications: impression of edentulous ridges,surgical dressing

    A. Irreversible

    Setting reaction:

    chelate structure

    H2O, acetic acid, Zn acetate

    A two-paste system composed of:Paste A ZnO/mineral or vegetable oil as a

    plasticizerPaste B oil of cloves with app. 85 % of eugenol or

    pure eugenol, resins and fillers, accelerators

    -OH and methoxy groups

    (C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague

  • 9

    Silicone impression materials

    Low viscosity of ZOEimpression pastes enablesmucostatic impressions

    Comparison of viscosity changes during ZOE setting

    Edentulous ridges - lowviscosity is favourable to

    avoid displacement oftissues

    rigid

    ity

    ZOE

    Silicone

    Advantages:1. Low viscosity no compression of soft tissues

    2. Dimensional stability (shrinkage less than 0.1 %) 3. Good surface detail reproduction

    4. Low price

    Disadvantages:

    1. Cannot be used in deep undercuts

    2. Eugenol allergy in some patients (o-ethoxy benzoic acid [EBA] to replace eugenol)

    (C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague

  • 10

    CaSO4.0.5H2O + 1.5H2O CaSO4.2H2O + heat +expansion

    2. Impression plaster

    CaSO4 . 0.5H2O -hemihydratePotassium sulfate to reduce expansion but accelerates

    settingBorax a retarder to prolonge setting timeDiatomaceous earth, quartz, lime - to make the plaster

    more brittle

    Composition:

    Setting reaction:

    app. 0.1 lin %

    Main indication: impression of edentulous ridges

    Advantages:

    1. Cheap and long shelf life, easy to prepare 2. Very good surface detail reproduction

    3. Excellent dimensional stability

    Disadvantages: 1. Very rigid often need to be fractured when

    removed from the mouth 2. Fractures if undercuts are present 3. Non-toxic but may dry soft tissues - unpleasant to

    patients

    Old fashioned not frequently used

    (C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague

  • 11

    3. Impression compounds(Kerrs, Stents impression compounds)

    Thermoplastic material (softens when heated to 50oCand hardens on cooling) for making impressions of

    edentulous ridge, tooth impressions in a copper band

    Composition:1. Resins (wax, shellac, guttapercha)2. Filler (talc)3. Lubricants (stearic acid, stearin)

    B. Reversible

    Advantages:1. Can be reused, easy to use

    2. Non-irritant and non-toxic

    Disadvantages

    1. Poor dimensional stability2. Easy to distort when withdrawn from the mouth

    Old fashioned not frequently used

    (C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague

  • 12

    Elastic impression materialsA. Hydrocolloid impression materials

    B. Elastomeric impression materials

    Silicone - condensation

    - addition

    Polyethers

    Elastomeric (non-aqueous)(irreversible)

    HydrocolloidIrreversible

    ReversiblePolysulfide

    A. Hydrocolloid impression materialsHydrocolloid a colloidal system (particle size up to

    app. 0.5 m) with water as a dispersion medium so called HYDROCOLLOID SOL that can be

    tranformed to a semisolid GEL

    Cooling

    Heating

    Coagulation

    reversible/agar sol gel

    irreversible/alginate sol gel

    Setting reaction

    (C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague

  • 13

    Alginate impression materials(Irreversible hydrocolloid impression material,

    preliminary, orthodontic impressions etc.)

    Alginate chains M = 30 150 000

    Form viscous sols and gels afteraddition of Ca+2 ions

    Constituent units

    B. Irreversible hydrocolloid

    Based on natural polysaccharide Na+, K+, triethanol aminealginate salts (isolated from brown seaweeds)

    Setting/gelation reaction:

    NanAlg + CaSO4 nNa2SO4 + CanAlg2H2O

    powder gel

    Egg-box structureCross-linked structure of alginate gels

    (C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague

  • 14

    1. Na/K alginate2. Calcium sulfate (CaSO4.2H2O, CaSO4.1/2 H2O)3. Diatomaceous earth (amorphous SiO2)4. Retarder phosphates prolonge working time

    2Na3PO4 + 3CaSO4 3Na2SO4 + Ca3(PO4)25. Setting accelerator - K2TiF6 (makes also stone surface

    hard)6. Additives glycol, paraffin oils to agglomerate particles

    and make material dustless

    Composition:

    Mixing ratio powder/water app. 10 g/20 mL

    Properties:1. Set after mixing with water2. Shrink due to loss of water by Syneresis

    expression of water from the surface ofimpression (if contains Na2SO4 decreasedquality of stone surface); by Evaporation ofwater from the surface

    3. Imbibition sorption of water causing adimensional change

    4. Chromatic phase indicator may be incorporated tosignal the impression setting

    (C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague

  • 15

    Dimensional changes of a cast prepared fromalginate impression stored in various environments

    Phillips Science of Dental materials

    Store in a closedcontainer with atowel saturated

    with water

    0.00

    0.20

    0.40

    0.60

    0.80

    1.00

    0 5 10 15 20 25

    Ypeen PremiumYpeenPhaseHydrogumElastic

    Time /hours

    Dim

    ensi

    onal

    cha

    nges

    lin

    %

    Dimensional changes of a cast pre