dental impression


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Dental Inpression Materials

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dental impression

  1. 1. 1 INTRODUCTION A dental impression is a negative imprint of hard (teeth) and soft tissues in the mouth from which a positive reproduction (or cast) can be formed. They are made by using a container which is designed to roughly fit over the dental arches ("trays"). Impression materials are designed to be liquid or semi-solid when first mixed and placed in the tray, and then quickly set to a solid (usually a few minutes depending upon the material), leaving an imprint of the structures in the mouth. Impressions, and the study models which are cast from them, are used in several areas of dentistry including prosthodontics (such as making dentures, inlays and plastic casts), orthodontics, restorative dentistry (e.g. to make impressions of teeth which have been prepared to receive indirect extracoronal restorations such as crowns or bridges), maxillofacial prosthetics (prosthetic rehabilitation of intra-oral and extra-oral defects due to trauma, congenital defects, and surgical resection of tumors) restorative, diagnosis and oral and maxillofacial surgery for both intra oral and or extra-oral aims (e.g. dental implants). The required type of material for taking an impression and the area that it covers will depend on the clinical indication. A correctly made dental impression will capture a part or all of a person's dentition and surrounding structures of oral cavity. The dental impression forms an imprint (i.e. a 'negative' mould) of teeth and soft tissues, which can then be used to make a cast of the dentition. Casts are used for diagnostics, patient record, treatment planning, fabrication of custom trays, fabrication of dentures, crowns or other prostheses and orthodontics. An impression is made by placing a viscous, thixotropic impression material into the mouth via a custom or stock dental impression tray. The material, then sets to become an elastic solid, and, when removed from the mouth, provides a detailed and stable negative of teeth. Common materials used for dental impressions are sodium alginate, polyether and silicones - both condensation-cured silicones and addition-cured silicones, such as polyvinyl siloxane. Historically plaster of Paris, zinc oxide eugenol and agar have been used. Impression materials can ceas Rigid, Elastic ( Hydrocolloid and Elastomeric)
  2. 2. 2 IMPRESSION MATERIALS  DEFINITION: A dental impression is defined as the negative record of the tissues of the mouth. It is used to reproduce the form of the teeth and surrounding tissues. DESIRABLE PROPERTIES OF AN IMPRESSION MATERIAL:  Have pleasant taste, odor and esthetic color.  Not contain any toxic or irritating ingredients.  Be economical.  Have adequate shelf life for storage and distribution.  Be easy to use with minimum equipments.  Exhibit dimensional stability.  Have adequate strength so that it will not break or tear while removing from the mouth.  Posses’ elastic properties with freedom from permanent deformation after strain.  Be able to be electroplated  Be compatible with die and cast materials CLASSIFICATON: There is several classifications. According to philips science of dental materials impression materials are classified on the basis of setting and elasticity. Mode of setting rigid Elastic Set by chemical reaction(irreversible) Impression plaster, zinc oxide eugenol. Alginate, polysulfide, polyether, silicone, Set by temperature Compound, waxes Agar hydrocolloid.
  3. 3. 3 change(reversible) IMPRESSION COMPOUND:  Impression compound is rigid, reversible impression material sets by physical change.  It softens on heating and hardens on cooling.  ADA specification no 3  Supplied as sheets, sticks, cakes and cones. Applications:  Making preliminary impression of edentulous mouth.  For individual tooth impression.  To make a special tray.  Border molding.  To check undercuts in inlay preparations. Composition: INGREDIENTS PARTS Resin 30 Copal resin 30 Carnauba wax 10 Stearic acid 5 Talc 75 Properties:  Glass transitional temperature =39 degree centigrade.  Fusin temperature=43.5 degree centigrade.
  4. 4. 4  They are poor conductors of heat.  Coefficient of Linear Expansion is high.  Good dimensional stability.  Good flow.  Fine reproduction of detail. Advantages:  Material can be reused several times.  Inaccurate portions can be remade without having to remake the entire impression.  Accuracy can be improved by flaming surface material. Disadvantages:  Distortion due to poor dimensional stability.  Compress soft tissue while making impression.  Difficult to remove if there are undercuts.  Difficult to record details because of high viscosity. ZINC OXIDE EUGENOL IMPRESSION PASTE: Applications:  Cementing and insulating medium.  Temporary filling.  Root canal filling material.  Bite registration paste.  Impression material for edentulous patients.  Temporary relining material for dentures.
  5. 5. 5 Available as: base paste(white in color) Accelerator or reactor or catalyst paste(red in color) Setting reaction:  Setting reaction is an acid base reaction to form a chelate.  This reaction is also known as chelation and the product is called zinc eugenolate.  Zno + H2O Zn(OH)2  Zn(OH)2 +2HE ZnE2 +2H2O (Base) (Acid) ( Salt) Setting time; Initial setting time Final setting time Type 1 3-6 min 10 min Type 11 3-6 min 15 min Factors controlling setting time:  Particle size of zinc oxide powder.  Longer mixing time shortens setting time.  High atmospheric temperature and humidity accelerate setting.  Setting can be delayed by cooling mixing slab, spatula or adding small amount of retarder or oils or waxes. Properties:  Good consistency and flow.  Detail reproduction.  Rigid and having good strength.  Good dimensional stability.
  6. 6. 6 Manipulation:  Mixing is done on an oil-impervious paper or glass slab.  Two ropes of paste of same length and width, are squeezed on mixing slab.  A flexible stainless steel spatula is used for mixing.  Two ropes are combined until a uniform colour is obtained.  Mechanical rotary mixing are also used.  Mixing time is one minute Advantages:  It has enough working time to complete border moulding.  It can be checked in mouth repeatedly without deforming.  It registers accurate surface details.  It is dimensionally stable.  Does not require separating media since it does not stick to cast material.  Minor defects can be corrected. Disadvantages:  Requires special tray for impression making.  Sticky in nature and adheres to tissues.  Burning sensation of eugenol causes tissue irritation.  Highly inelastic in nature. ELASTIC IMPRESSION MATERIALS; ARAG;  Agar hydrocolloid was the first successful impression material to be used in dentistry.  It is an organic hydrocolloid extracted from certain types of seaweeds.
  7. 7. 7  Agar is a reversible hydrocolloid.  Since it is reversible can be reused. Uses  As tissue conditioner.  For full mouth impression without deep undercuts.  Used extensively for crown and bridge impression before elastomers came to the market.  Widely used at present fore cast duplication.  Supplied as;  Gel in collapsible tube(for impression)  A number of cylinders in a glass jar(syringe material).  In bulk containers(for duplication) Composition; INGREDIENTS PERCENTAGE BY WEIGHT Agar 13-17% Borates 0.2-0.5% Potassium sulphate 1-2% wax, hard 0.5-1% Thixotropic material 0.3-0.5% Alkylbenzoates 0.1% water Around 84% Coloring and flavoring agents Traces Function of each ingredient;  Agar-basic constituent.  Potassium sulphate -It counters retarding effects of borates.
  8. 8. 8  Wax-filler.  Thixotropic material-plasticizer.  Alkylbenzoates-preservative.  Coloring agents-for patients comfort  Water-dispersion medium. Manipulation; Agar hydrocolloid requires special equipments;  Hydrocolloid conditioner.  Water cooled rim lock tray. IMPRESSION DISINFECTION  Main disinfectants used include iodophor, bleach or glutaraldehyde. Advantages  Accurate die can be prepared, if material is properly handled  Has good elastic properties.  Good recovery from distortion  Palatable and well tolerated by patient  Cheap Disadvantages not flow well compared to newly available material  It cannot be electroplated  Tears relatively easily  Only one model can be poured ALGINATE  Its an irreversible elastic hydrocolloid
  9. 9. 9  It is a mucous extract yielded from certain brown sea weeds  Types  type 1 – fast setting  type 2- normal setting  Applications  Used for impression making;  when there are undercuts  in mouth with excessive flow of saliva  For impression to make study models and working casts  For making preliminary impression Composition INGREDIENTS FUNCTIONS Sodium or pottasium alginate-15% React with calcium ion Calcium sulphate(reactor) - 16% React with potassium alginate Zinc oxide - 4% Acts as filler Potasium titanium flouride -3% Gypsum hardener Diatomacous earth -60% Acts as filler Sodium phosphate(retarder) -2% React with calcium sulphate Colouring and flavouring agents Wintergreen, orange etc. Setting Reaction  Two main reaction occur  2Na3Po4 + 3CaSo4 Ca3 (PO4)2 +3Na2So4  Here firstly the retarder is completely getting reacted to delay the reaction time.
  10. 10. 10  Sodium alginate + CaSo4+H2O calcium alginate +Na2So4 Properties of Alginate  Alginate has pleasant taste and smell  It is highly flexible  Good elasticity and elastic recovery  Fine reproduction of tissue detail  Compressive strength – ranges from 0343-70 mpa  Tear strength- 350-700gm/cm2  Dimensionally stable  Poorly adheres to the tray  Biological properties- inhalation of silica particles cause health hazards. Manipulation  steps in manipulation  Fluff or aerate the powder by investing can several times.  For mixing a clean flexible plastic bowl and clean wide bladed stiff metal spatula is being used.  Proper water powder ratio specified by the manufacturer should be used(1 measure water with two level scoops of powder)  Mixing is started with a stirring motion. vigorous figure 8 motion can also be used. Properties:  Mixing time  Fast setting - 45 sec  Normal setting- 60sec  Working time –
  11. 11. 11  Fast setting – 1min  Normal setting – 2 min  Gelation time  Fast setting - 1 to 2min  Normal - 2 to 4.5 min Advantages  Easy to mix and manipulate  Low cost  Comfortable to patient  Hygienic  Good surface detail even in presence of saliva  Accuracy if properly handled  Minimum requirement of equipment Disadvantages  Cannot be electroplated  It cannot be corrected  Poor dimension stability  Poor tear strength  Distortion may occur if not properly handled ELASTOMERIC IMPRESSION MATERIAL Classification A) According to chemistry 1) Polysulfide 2) Condensation polymerizing silicones
  12. 12. 12 3) Addition polymerizing silicon 4) Polyether B) According to viscosity 1) Light bodied (syringe consistency) 2) Medium (regular) 3) Heavy body (tray consistency) 4) Very heavy (putty consistency) Composition A) Base paste  liquid polysulfide polymer – 80 to 85%  Inert fillers – 16 to 18% B) Reactor paste  lead dioxide -60 to 68%  Dibutyl phthalate -30 to 35%  Sulphur-3%  retarders like magnesium steerages Setting reaction  Base and acceleration paste are mixed together which undergoes chemical reaction to form polymer  The reaction is exothermic – 3-4 degree Celsius rise in temperature  Mercaptan + lead oxide polysulfide +water Properties  Unpleasant order and colour  Material is extremely viscous and sticky and exhibits pseudo plasticity
  13. 13. 13  Long setting time of 12.5min  High tear strength  Good flexibility  Excellent reproduction of surface detail SILICON RUBBER IMPRESSION MATERIAL  Types;  1-condensation silicones  2-addition silicones  Condensation silicones;  Also known as conventional silicones.  Available in three viscosities namely light, medium bodied and heavy. Composition; Base  1-polydimethyl siloxane.  2-colloidal silica  Color pigments. Accelerator Stannous octoate Setting reaction;  Dimethyl siloxane+orthoethyl silicate silicone in presence of stannous octoate. Composition; Base paste and catalyst paste; Setting reaction;
  14. 14. 14 Vinyl siloxane+silane siloxane silicone rubber Polyether impression materials; ◦ Good mechanical and compressive strength comparing other elastomers. ◦ Available in three viscosities. ◦ Consist of base paste and catalyst.