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Impression Materials BDS 3. Dr M Slabbert Dept Prosthodontics Wits. First Impressions count. Classification of Impressions. Preliminary impressions Taken either by the dentist or an expanded-function dental assistant. Used to make a reproduction of the teeth and surrounding tissues. - PowerPoint PPT Presentation

Text of Impression Materials BDS 3

  • Dr M Slabbert Dept Prosthodontics Wits

  • First Impressions count

  • Classification of ImpressionsPreliminary impressions Taken either by the dentist or an expanded-function dental assistant.Used to make a reproduction of the teeth and surrounding tissues. Used to make (1) diagnostic models, (2) custom trays, (3) provisional coverage, (4) orthodontic appliances, and (5) pretreatment and posttreatment records.

  • Classification of Impressions- contdFinal impressionsTaken by the dentist.Used to make the most accurate reproduction of the teeth and surrounding tissues.Used to make indirect restorations, partial or full dentures, and implants.

  • Classification of Impressions- contdBite registrationsTaken by the dentist or dental assistant.Make a reproduction of the occlusal relationship between the maxillary and mandibular teeth.Provide an accurate registration of the patients centric relationship between the maxillary and mandibular arches.

  • Oral ExaminationPrimary ImpressionStudy model (cast)Treatment Planning duplicate modelMouth and tooth preparationFinal ImpressionMaster modelLaboratory procedure investment cast refractory castDentureDelivery

  • An impression is a negative record of the tissues of the oral cavity which constitutes the basal seat of the denture. An impression is made in a material which has plasticity and which hardens or sets while in contact with the tissue.

    Prof Owen Fundamentals of Removable partial dentures 2nd Ed pg. 124 -131Wits Pros Book Vol 2 Pg. 14Study pages 14- 31 Vol 2 Primary impressions in alginate Definition

  • The Ideal Impression MaterialEasy to mix and handle. Suitable working time. Suitable setting time. Compatible with die and stone Not toxic or allergenic to the patient. Dimensionally stable on setting. Accurate to record the fine details of the prepared tooth Has acceptable odor and taste. Adequate strength. Adequate shelf life.

  • The Ideal Impression Material cont. Economical Ready to disinfected without loss of accuracy. Fluid or plastic when inserted into the mouth. It must be an exact record of all the aspects of the prepared tooth and sufficient unprepared tooth structure immediately adjacent to margins, to allow the dentist and the technician to be certain of the location and configuration of the finish line. Other teeth and tissue surrounding the abutment tooth must be accurately reproduced to permit proper articulation of the cast and contouring of the restoration. It must be free from air bubbles especially in the finish line area.

  • High accuracy Biocompatibility(very small contraction
  • Elastic recoveryThe amount of rebound after a cylinder of material is strained 10% for 30 seconds.


  • Hydrophilic VS Hydrophobic

  • Which one of the impression materials we choose? Whats the job you do?primary impressionfinal impressionHow accurate do you want?removable denturefixed prosthesesWhat technique do you use?


  • PROBLEM ANALYSISWhat are tolerable limits for error in indirect procedures?a.Impressions= b.Casts, Dies=c.Waxing=d.Investing=e.Casting=f.Finishing, Polishing=g.Cementation=+/- 0+/- 0+/- 0+ 1.5% 1.5%+/- 0+/- 08,000 m x 0.5% = 40 m = 20 m/sideTypical clinical error = >100 m/side

  • Properties:Reaction:Set:Rigid:1.Impression PlasterRigidIrrev(Chem)2.Impression CompoundRigidRev(Phys)Zinc Oxide/ EugenolRigidIrrev(Chem)CLASSIFICATION SYSTEMBased on Properties of Set MaterialsWater-Based Gel:4.Alginate (Irreversible Hydrocolloid)FlexibleIrrev(Chem)5.Agar-Agar (Reversible Hydrocolloid)FlexibleRev(Phys)Elastomers:6.Polysulfide (Rubber Base, Thiokol)FlexibleIrrev(Chem)7.Silicone (Conventional, Condensation)FlexibleIrrev(Chem)8.PolyetherFlexibleIrrev(Chem)9.Polyvinyl Siloxane (Addition Silicone)FlexibleIrrev(Chem)

  • Impression materialsImpression materialElasticNonelasticZinc oxide EugenolImpression CompoundPlasterNon-aqueouselastomersPolyethersAdditionsiliconeCondensationsiliconePolysulfidesImpression waxes HydrocolloidsAgarReversible AlginateIrreversible

  • NonelasticZinc oxide eugenolImpression CompoundGypsum (Plaster)Impression waxType I

  • Dental Plaster Type I impression plaster Type II model (laboratory) plaster (used for mounting casts)plaster is composed of the form of calcium sulfate hemihydrate Crystalsplaster is weaker than dental stone due to: 1.) porosity of the particles, requiring more water for a plaster mix2.) irregular shapes of particles prevent them from fitting together tightly

    Dental Stone Type III - dental stone(diagnostic casts) Type IV - high strengthdental stone (workingmodels) Type V - high-strength, high-expansion dental stone

    Stone is the hemihydrate form Types of Gypsum ProductsDental Plaster

  • Calcination Calcination H or other meansMineral gypsum ---------------- > Model plaster + Water(CaSO4 . 2H2O) Dental stoneHigh-strength dental stone (CaSO4 . 1/2H2O)

    Reverse Reaction When calcium sulphate hemihydrate (dental plaster, stone, etc.) is mixed with water, the reverse reaction takes place, and the hemihydrate is converted back to the dihydrate:

    CaSO4 . 1/2H2O + 11/2H2O ---> CaSO4 . 2H2O + 3900 cal/g mol

  • Elastomeric Impression Materials A material that is used when an extremely accurate impression is essential. The term elastomeric means having elastic or rubberlike qualities.

  • ElasticHydrocolloidsAgarAlginate

  • Hydrocolloids

    Reversible and Irreversible Introduced by Sears 1939 First elastic Sears AW. Hydrocolloid impression technique for inlays and fixed bridges. Dent Digest 1937; 43: 230-234.Lin C, Zeiber G J. Accuracy of impression materials for complete arch fixed partial dentures. J Prosthet Dent 1988; 59: 288-291Philips Science of Dental Materials 11th Ed. Part 2 Pg. 231

  • Irreversible Hydrocolloid Material that cannot return to a solution state after it becomes a gel.Alginate is the irreversible hydrocolloid most widely used for preliminary impressions.

  • Makeup of AlginatePotassium alginate (Alginic Acid) (12-15%)Comes from seaweed; is also used in foods such as ice cream as a thickening agent. Calcium sulfate (8-12%)Reacts with the potassium alginate to form the gel.Trisodium phosphateAdded to slow down the reaction time for mixing.

  • Makeup of Alginate- contdDiatomaceous earth (70%)A filler that adds bulk to the material. Controls the stiffness of the set gelZinc oxideAdds bulk to the material.Potassium titanium fluoride (~10%)Added so as not to interfere with the setting and surface strength. Sodium Phosphate (retarder) (2%) Coloring and flavouring agents (traces)

  • Physical Phases of AlginateThe first phase is a sol (as in solution). In the sol phase, the material is in a liquid or semiliquid form. (sol: resembles a solution, but is made up of colloidal particles dispersed in a liquid)The second phase is a gel. In the gel phase, the material is semisolid, similar to a gelatin dessert. gel entangled framework of solid colloidal particles in which liquid is trapped in the interstices and held by capillary forces (Jello)

  • Packaging and Storing of AlginateContainers about the size of a coffee can are the most commonly used form of packaging. Premeasured packages are more expensive, but save time by eliminating the need for measurement of the powder.Shelf life of alginate is approximately 1 year.

  • Causes for Distortion and Dimensional Change of AlginateIf an alginate impression is stored in water or in a very wet paper towel, the alginate will absorb additional water and expand. This condition is called imbibition. If an alginate impression remains in the open air, moisture will evaporate from the material, causing it to shrink and distort. This condition is called syneresis.

  • ADA Specifications
  • Altering the Setting Times of AlginateCooler water can increase the setting time if additional time is needed for the procedure. Warmer water can reduce or shorten the setting time of the procedure.

  • Water-to-Powder Ratio An adult mandibular impression generally requires two scoops of powder and two measures of water.An adult maxillary impression generally requires three scoops of powder and three measures of water.

  • Taking an Alginate ImpressionExplain the procedure to the patient:The material will feel cold, there is no unpleasant taste, and the material will set quickly.Breathe deeply through your nose to help you relax and be more comfortable.Use hand signals to communicate any discomfort.

    Mouth Preparation

    Rinse and dry the patient's teeth If teeth are too dry, alginate will stick

  • An Acceptable Alginate Impression The impression tray is centered over the central and lateral incisors. There is a complete "peripheral roll," which includes all of the vestibular areas. The tray is not "overseated," which would result in exposure of areas of the impression tray.The impression is free from tears or voids.There is sharp anatomic detail of all teeth and soft tissues.The retromolar area, lingual frenum, tongue space, and mylohyoid ridge are reproduced in the mandibular impression.The hard palate and tuberosities are recorded in the maxillary impression.

  • Trouble Shooting

    Inadequate working or setting time: temperature of the water, incomplete spatulationW/P too lowimproper storage of alginate powder

    Distortion:Tray movement during gelation or removed from mouth prematurelyweight of tray compressing or distorting alginateimpression not poured up immediately Tearing:removing impression from mouth before adequat

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