Published on April 27, 2014
IDEAL REQUIREMENTS TO OBTAIN AN ACCURATE IMPRESSION. Should be fluid enough to adapt to the oral tissues. Should be viscous enough to be contained in the tray that is seated in the mouth. While in the mouth, they should transform (set) into a rubbery or rigid solid in less than 7 minutes. The set impression should not distort or tear when removed from the mouth. Impressions made should remain dimensionally stable until the cost can be poured. Should maintain its dimensional stability after removal of a cost so that a second or third cost can be made. Should be biocompatible The materials, associated processing equipment and processing time should be cost effective. Classification I. Based on the degree of tissue compression a. Mucostatic : Example : Impression plaster, ZOE impression paste. b. Mucocompressive : Example : Elastomers. - mucostatic impressions materials do not compress the tissue during seating of impression tray -> ideal for edentulous jaw structures. - Muco compressive – ideal for dentulous impressions. II. By setting mechanisms A. Chemical reaction (irreversible) By elasticity and use Inelastic or rigid Elastic Materials Use Material Use 1. Plaster of paris 2. Zinc oxide- eugenol Edentulous ridge Interocclusal records Alginate Non-equcous elastomres - Polysulfide - Polyether - Condensation silicone - Addition silicone Teeth and soft tissue B. Thermally induced physical reaction (reversible) Compound wax Preliminary impression Agar hydrocolloid Teeth and soft tissu III. Based on the chemical composition 1. Hydrocolloids - Reversible -> agar – agar - Irreversible -> alginate
2. Non- squeous elastomeric impression materials - Polysulfides - Condensation silicone. - Addition silicone - Polyether 3. Impression compound 4. Impression plaster 5. Zinc / Oxide eugenol pastes 6. Impression waxes. IV. Based on the viscosity of elastomers (ISO 4823) Type 3 - Low viscosity (light bodies) 2 - Medium viscosity (medium bodied) 1 - High viscosity (heavy bodied) 0 - Very high viscosity (Putty) V. Based on the physical properties categorized by ADA specification No.19 for elastomers - Compression set, flow, 24 hour dimensions change. Type Maximum/ Maximum % Maximum % Compression set flow Shrinkage (24 hours) I 2.5 0.5 0.5 II 2.5 0.5 1.0 III 5.5 2.0 0.5 The significance of this classification stems from the fact that impression materials of the some chemistry may be different depending or critical physical properties. Example polysulfide are type I or III depending on compression set. If large deformation are anticipated on withdrawal over exaggerated undercuts, a type I material experiences less distortion. * HYDROCOLLOIDS Hydrocolloid was patented in 1925, and it was introduced to the United States in the late 1920s. Credit for its first use in the United States for fabricating cast restorations is given to J.D. Hart of Oklahoma, who began using it for the purpose in 1930. Hydrocolloids was initially introduced to make impressions of hard tissues in place of in elastic materials. After World War II, advances in polymer technology brought to the
dental profession a group of synthetic rubbery materials called ealstomers, capable of making impressions of both soft and hard tissues. Colloids are often classified as a fourth state of matter, “The colloidal State”. True solutions exist as a single phase. However, both colloid and the suspension have two phases. • Dispersed. • Dispersion phases. Colloid as substances may be combinations of any States of matter, with exception of gaseous states. • Liquids or solids in air are called aerosols • Gases, liquids or solids in a liquid are called forms, solids evulsions and solid suspensions respectively. All colloidal dispersions are termed sols. The colloidal materials used for making impressions are either agar or align, dissolved in water, hence the name hydrocolloids. Sol-gel transformation – When the concentration of the dispersed phase in the hydrocolloid is sufficient, the sol-gel. In the gel state, the dispersed phase aggromerates, forming chains or fibrils called micelles. The fibrils may branch and intermesh to form a brush-heap structure (Pic McCake 134-Page ) For agar, secondary bonds (week) hold the fibrils together. These bonds break at slightly elevated temperatures and become re-established as it walls to room temperatures. The process is reversible. For alginate, the fibrils are formed by chemical action because the transformation is not reversible. Gel strength For the reversible gel, the lower the temperature, the stronger the gel, and vice versa. When gel is heated, resulting in greater inter fibriller distances and reduction of their cohesions. On the other hand, strength of irreversible gel, not affected by normal temperature changes. The strength of gels can be increased by addition of certain modifiers such as fillers and chemicals. Dimensions effects • The gel may lose water by evaporation from its surface or by exudation of fluid on its surface by a process known as synthesis. • Whenever water or fluid is removed from the micelles of the gel by synthesis, the gel shrinks. • If gel is placed in water, it will absorb water by a process known as inhibition. • The effects of synthesis, evaporation and inhibition must be limited to ensure the proper dimensions of impressions -> pouring cost immediately can reduce distortion. AGAR (REVERSIBLE HYDROCOLLOIDS)
- Was introduced to dental progression in 1925 mainly used in the area of removable partial denture prosthesis. - Until early 1950’s, it was the principal and only elastic impression material used in crown and bridge and partial denture prosthesis. Compression Component Function Composition Agar Brush – heep structure 13 – 17% Borates Strength 0.2 – 0.5% Sulfates Gypsum hardener 1.0 – 2.0% Wax hard Filler 0.5 – 1.0% Thixotrosic materials Thickness 0.3 – 0.5% Water Reaction Balance Alkyl Benzuate Prevent growth of mold in impression material during 0.1% storage. Color and Flavors Taste appearance Trace Cylation process - Setting of reversible hydrocolloid, called gelation - Physical change from sol – gel induced by temperature changes - Gel converts to sol when heated to 70 – 100oC -> liquefaction temperature. - When cooled sol transforms to get at 37o – 50oC -> gelation temperature. - Temperature leg between gelation temperature and liquefaction temperature of the gel makes it possible to use agar as dental impression material. • Sol - Gel • Gel -> sol at 70 – 100o C (Liquefaction temperature) • Sol -> gel at 37 –50o C (Gelation temperature) Manipulation Includes liquefying the gel, placing it in impression tray, tempering it to a lower temperature, and maintaining it in its fluid state to capture the details of the oral structures. Once in the mouth, the material is cooled below mouth temperature to ensure gelation. Conditioning unit for the agar hydrocolloid impression materials consists of water baths for: • Liquefying the material : Placed in boiling water for 10 min (at high attitudes water (Colorado) boils at below 100oC, propylene glycol can be added to obtain a temperature of 100oC Agar hydrocolloid frequently supplied in two forms – syringe and tray materials (gel) because first step is to convert gel -> sol by liquefying.
• Storage after boiling : Stored in sol condition at 65oC until need for injection into cavity preparation or for filling a tray. • tempering the tray hydrocolloid. : Since 55oc is the maximum tolerable temperatures with storage temperature of 65oC could be too hot. • For the oral tissues -> therefore tempered at 45oC. For 3-10 minutes. Tube of hydrocolloid sol removed from storage, tray is filled, gauze ped placed over the tip of tray (prevent inhibition) and placed in tempering basin. Syringe materials is not tempered, temperature is lowered as the material extrudes out of the syringe. (Pic of conditioning Page 31) Making the Agar impression First syringe material directly taken from storage compartment applied to the base of the preparation, then remainder of the prepared tooth is covered. Tray hydrocolloid is removed. From the tempering basin, outer layer removed and impression made. Gelation is accelerated by circulating cool water (approx. 18-21oC) through the tray for 3-5 min. Sol is a poor ocnductor of heat because rapid cooling may cause concentration of stress near the tray. Hydrocolloid exhibit viscoelastic behavior therefore, it is necessary to remove impression with a snap rather then tease it out. Time required to re-liquify is 2-4 min longer than previous time -> because more firmer -> can the re used 4 times (Pic tray – water cooled Page 32 Rahul Tyagi) Properties ANSI / ADA Spec. No. 11 1. Permanent deformation : ANSI / ADA specification requires recovery from deformation to be greater than 96.5% after material is compressed 20% for 1 seconds tray type -> 99%. 2. Flexibility - ANSI / ADA specification requires 14-15% Agar impression material meets this requirements. 3. Strength - Compressive strength – 8000 g/em2. Tear strength – 800 – 900 g/cm ANSI / ADA specification requires 765 g /cm of fear strength. ALGINATE (IRREVERSIBLE HYDROCOLLOID) Was developed as a substitute for the agar impression material when its supply become scarce during World War II, as they were imported from Japan. A chemist from Scollend perceived that certain brown seaweed (Elgae) succumbs on atypical mucous extraction. The substance is called anhydro-B-d-mannuronic acid or alginic acid (insoluble in water) COMPOSITION Component Function Weight percentage Potassium alginate Soluble alginate 15 Calcium alginate Reacter 16 Potassium titanium Fluoride Accelerator 3
Zinc oxide Filler particles 4 Diatomaccous earth Filler particles 60 Sodium phosphate Retarder 2 When powder in the alginate can is fluffed to break loose the particles, fine silica particles will become airborne from the can when lid is removed. Inhalation of these silica particles is a possible health hazard. (silicosis and pulmonary hypersensitivity) Because Dustless alginate Introduced -> incorporated glycerine on the alginate powder to agglomerate the particles. Cylation process Soluble alginate reacts with calcium sulfate to form an insoluble calcium alginate gel. The production of calcium alginate is so rapid that it does not allow sufficient working time because a third water soluble salt, trisodium phosphate is added to prolong working time -> calcium sulfate will react with this salt in preference to soluble alginate until trisodium phosphate is exhausted. Classification of alginate (Robert C. Craig) I. According to concentration of sodium phosphate • Fast set – 1.25 – 2min. ) • Regular set – 3 – 4.5 min ) working time II. According to concentration of filler • Soft set • Hard set Controlling setting time • By the amount of retarder added (Manufacturer) • By altering the temperature of water -> Higher the temperature, shorter is the setting time. • Rate of setting reaction doubled by temperature increase of 10oC because water cooler than 18oC or warmer than 24oC not advisable. Manipulation The measured powder is sifted into pre-measured water that has already been poured into a clean rubber bowl. A vigorous figure – 8 motion is best, with the mix being stropped against the sides of rubber mixing bowl with intermittent rotation of the spature to press out air bubbler Mixing time between 45 sex – 1 min -> resulting in a smooth cream mixture that does not readily drip off the spatule when raised from the bowl. Perforated tray normally used, in case of plastic or metal vin – lock tray, a thin layer of alginate tray adhesive is used.
Alginate is very week, therefore sufficient bulk of material between tray and tissue of 3 mm is required. Tear strength of increased when the impression is removed with a snap. Properties Permanent deformation : ANSI / ADA specification requires recovery from deformation to be more than 95% when material is compressed 20% for 5 seconds. Flexibility : ANSI / ADA specification permits a range of 5 – 10% at a stress of 1000 g /cm2 and most alginate have 14% Strength : ANSI / ADA specification requires compressive strength -> at least 3570 g/cm2. Alginates -> compressive strength = 5000 – 9000 g.cm2 Tear strength = 380 – 700 g / cm Compatibility with zypsum : Free water accumulation, dilute the model material yielding a soft chalky surface. Dimensional stability : Alginate impression lose or inbibe water by evaporation, syneresis or inhibition because it should be poured immediately. Disinfection : 1% Na. Hypochlorite. 2% glutaraldehyde - 10 – 30 min. showed 0.1% dimensional change -> insignificant when used for study models and working cost. Laminate technique (Alginate – Agar Method) - The tray hydrocolloid is replaced with a mix of chilled alginate that bonds to the syringe agar. - The alginate gels by a chemical reaction, whereas the agent gels by means of contact with the cool hydrocolloid. - Maximum detail is produced. Short comings 1. Bond between agar and alginate is not always strong. 2. Higher viscosity alginate displaces the agar during seating. 3. Dimensional accuracy of alginate limits the use to single units. Modified alginates - Alginate in the form of sol, containing the water but no source of calcium ions a reactor of plaster of paris can then be added to the sol. - Two component system -> paste form -> one containing alginate sol and second calcium reactor. - Alginates modified by the incorporation of silicone polymers -> supplied as two pastes show better fine – detail reproduction and tear resistance but poor dimensions stability (Alginates) hybrids of alginates and silicone elastomers but properties closely related to alginates.
Disinfection - Current protocol for disinfecting hydrocolloid impression recommended by center for disease control is the use household bleach (1:10 dilution) iodophors or synthetic phenols as disinfectants - Can be sprayed on exposed surface and then wrapped in disinfectant soaked paper – towel and placed in sealed plastic bag for 10 min. - Alternative method is immersion, but should not exceed 10 min. Shelf life - two major factors 1. Storage temperature 2. Moisture contamination from ambient air. - Sealed pouches and can available - Best if used before 1 year. - Store in cool, dry environment. Effects of Mishandling (Table 9-11) Annusavice Page 249) NONAQUCOUS ELASTOMERIC IMPRESSION MATERIALS - ANSI / ADA specification No.19. - An elastomeric material consists of large molecules with week interaction among them. They are tied together at certain points to form a three dimensional net work. - The chains of these materials uncoil on stretching and upon removal of the stress they bounce back or snop back to their relaxed entangled state. - Setting occurs through a combination of chain lengthening polymerization and chemical cross linking by either a condensation reaction or addition reaction. I. Chemically four kinds of elastomers a. Polysulfide b. Condensation polymerizing silicone c. Addition polymerizing silicone d. Polyether II. The current ANSI/ADA specification No. 19 recognizes three types based on selected and the dimensional change of the set materials rather than on chemistry. - type I - Type II - Type III III. Each type is further divided into four viscosity classes. 0 = very high consistency (putty like) 1 = High Consistency (Heavy bodied) 2 = Medium consistency (medium bodied) 3 = Low consistency (light bodied)
Polysulfides (Synonyms : Rubber base, morception, Thiokol rubber) Component Function Base Paste 1. Polysulfide This is further polymerized and Prepolymer with cross linked to form rubber Terminal and Pendent thiol (-SH) groups. 2. Dibutyl pthelate Platicizer to control viscosity 3. Inert filler To provide the required (lithopone or Strength titanium dioxide) 4. Sulfur (0.5%) To accelerate the reaction (Reactor paste) Note : (Introduced in the early to middle 1950s). Catalyst paste Lead dioxide (Brown) on To react with thiol groups alternative oxidizing causing setting agents hydrated copper Oxide (Greens) Oleic or stearic acid Reterder to control the rate of setting reactions Some amount of Plasticizer and fillers Setting reaction The terminal and pendant perception group of each molecule is oxidized by the addition of oxidizing agent present in the reaction paste, which in turn causes both lengthening of polymer chain by oxidation of terminal merception groups and cross linking by oxidation of pendant metception groups. Reaction is of condensation type since water is produced as a by produced – dimensional changes. Polymerization is exothermic and is affected by moisture and temperature -> decreases working time. Unpleasant subjects odor and long setting time in the mouth. Lead dioxide -> sticky and stains clothing permanently alternatives -> copper hydroxide organic hydroperoxides (Evaporate after cure). Available in three consistencies
- Low - Medium - High Packaged as two pastes base (whitie) catalyst (Brown) Advantages Disadvantages - Extensive shelf life - Odor lipid patient - Less hydrophobic - Must be poured in stone instantaneously - Established precision - Unitidy and stains clothing - High tear strength cheep to use - Potential for distortion - Customary tray mandatory - Subsequent pours are less accurate. CONDENSATION SILICONE (SYNONYMS, SILICONE, POLYSILOXANE) ORGANAIZATION OF SILICONE) Composition Components Function Base paste Hydroxyl – terminated undergoes cross linking Polydimethyl to form rubber Siloxne Propolymer Colloidal Silica Filler Catalyst Poly Vinyl Silicone Prepolymer Colloidal silica filler Chloroplatinic Cat-alyer Acid Setting reaction The polymerization reaction if of addition type, the cross linking is taking place between the vinyl terminal group of accelerator paste and the siloxone group of base paste and chloroplatinic acid activates the reaction. No by product formed If proper balance between base and reactor is not maintained, hydrogen gas is produced .Hence platinum or palladium gas is produced manufacturer to act as a scavenger for the released hydrogen gas. As inherently hydrophobic -> nonionic surfactant added that migrates towards the surface of the impression material and has its hydrophilic segment oriented towards the surface -> allows impression material to readily wet soft tissue and enhance the ability of gypsum to optimum maximum detail. Sulfur contamination from natural latex groves inhibits the setting of addition silicones.
Available in 4 consistencies – Light body Medium body Heavy body Putting Advantages Disadvantages - Excellent dimensional - Hydrophobic accuracy - Long term dimensional - Expensive Stability - Hydrogen gas evaluation in some Materials - Pleasant to use - Poor wetting - Short setting time - Hydrophilic formulations - Auto mix available inhibe moisture. - If hydrophilic, good compatibility with gypsum - Sulfur contamination by latex glove Note : Incorporated surfactant makes electroplating more difficult and also makes impression material more sensitive to the retardant action of sulfur. Newer Formulations 1. Hydrophilic – addition of surfactant 2. Hydrogen absorbs – Palladium 3. Monophase – Sheer thinning 4. Made shiffer – Bite registration POLYETHER (Synonymic epimine) First developed in Germany in Mid 1960’s primarily to function as on impression. Composition Base Paste -> Immune terminated Becomes cross lined Prepolymer to form rubber Inert filler – Silica Filler – to control viscosity Plasticizer -> To aid mixing Pthelate Catalyst paste Alkyl – cromotic Initiates cross linking Sulfonate Silica Filler Base Catalyst -> 8:1 Pthalets Plastilizer
Setting reaction The polyethers curve through cross linking of a difunctional epimine terminated prepolymer catalyzed by on alkyl benzenes sulfonate catalyst. The reaction involves ring opening without formation of volatile by products. - Modified to greatly reduce the stiffness -> identified by additon of F following brand name. - Stable if stored dry, otherwise absorbs moisture and undergo significant dimensional change. - Available in 3 consistence -> light body Medium body Heavy body. Advantages Disadvantages Dimensions stability - Set material very stiff Accuracy - Inhibition Shorter setting time - Short working time. Automix available - Allergic hyper sensitivity in some Cases. POLYETHER URETHANE DIMETHACRYLATE This material appeared on the market briefly in late 1980’s, discontinued because of problems with surface polymerization. Composed of - Polyether urethane dimethacrylate resin. - Diketone initiator - Amine accelerator - 40 – 60% silica filler - Used in a clear tray, photo initiated by 400 – 500 mm blur light. - Most useful traits – unlimited working time coupled with short setting times. MANIPULATION Paste form -> equal lengths of base and reactor paste taken over the mixing pad, a stiff stainless steel spatula is used for mixing, reactor paste is first spread over the base paste, mixing continued until a smooth, homogenous, streak free mix is obtained. When reactor supplied in liquid form -> number of drops per unit length are recommended by manufacturer. Two putty system -> kneaded between the finger, in case of liquid reactor initial mixing with spatula is accomplished. * Automatic dispensing and Mixing devices. - Used for light and medicine viscosity meteride of addition silicone, condensation silicone and polyethers, not polysulfides (Sticky).
- Material is supplied is prepackaged cartridges with a disposable mixing tip. - The cartridge inserted in a gun like device, and base and catalysis are extruded into the spiral mixing tip, where mixing occurs as they progress to the end of the tube. Advantages 1. Greater uniformity in proportioning and in mixing. 2. Less air incorporated into the mix. 3. Mixing time reduced. 4. Less chance of mix getting contaminated 5. Less wastage of material. Dynamic mechanical mixer The device uses a motor to drive parallel plungers that force the materials into a mixing tip, and the spiral inside the mixing tip rotates as the material are extruded through the tip -> higher viscosity material can be mixed. Example pentamix, ESPE America. Properties Viscoelastic Properties Viscoelesticity describes the dependence of an impression materials response to the speed of removal. Visco elastic behavior is intermediate between that of an elastic solid and a viscous liquid. Elastic solid -> Spring, which deforms instantly to a certain extent when one applies a specific load -> deformation reversed completely or removal of the load. Viscous liquid -> oil dashpot, which does not respond instantly but deforms as the load is applied over time -> deformation is permanent. Maxwell – Voigt model – Fig 9-3, Page 21, American A -> Stress Free state B -> As force is applied, S1 responds instantaneously with definite amount of deformation, D1, D2 and S2 no deformation, D2 prevents S2 deformation because of dash pot inertia. C -> As force applied over time, both deshpots are activated and continue to deform as 10 mg as force applied, S2 deforms a long with D2 D -> As force is released, S1 recovers instantly others remain unchanged. E -> As time posses, S2 over comes inertia of D2 and recovers along with D2 D1 remains unchanged. Clinical importance The amount of permanent deformation is attributed to either deshpot is dictated by the duration of tension or compression exerted on the materials therefore a teasing or rocking method should not be used to remove the impression -> snap removal.
WORKING AND SETTING TIMES Working time measured at room temperature setting time measured at mouth temperature. The end of working time might be defined as the time when a blunt needle of a certain diameter and weight fails to penetrate a volume of impression material to a specified depth. Setting time is defined as the transitional time at which plastic properties which permit molding and impression taking at lost and elastic properties permitting removal of the impression material are acquired or when a blunt instrument fails to permanently indent the set impression materials. Effects of temperature. Increase -> decreases both working and setting time by accelerating the curing rate. Decrease -> Increase working time, can be accomplished by refrigerating the materials or mixing on a chilled dry glass slab. Effect of viscosity Increase -> decrease working time and setting time. Altering base / catalyst ratio can alter the working and setting time -> but adversely affects the mechanical properties. Impression materials Mean working time (min) Mean setting time (min) 23o C 37o C 23o C 37o C Polysulfide 6.0 4.3 16.0 12.5 Condensation silicone 3.3 2.5 11.0 8.9 Addition silicone 3.1 1.8 8.9 5.9 Polyetter 3.3 2.3 9.0 8.3 Clinical importance • Duel arch technique for single crown preparation Quick set material – short working time. • Fill arch impression with multiple prepared teeth. Longer working time. Dimensional stability - Five major sources of dimensional changes. 1. Polymerization shrinkage. 2. Loss of by product (water or alchohol) during condensation reaction. 3. Thermal contraction from oral temperature to room temperature. 4. Inhibition when exposed to water, disinfectant or high humidity environment over a period of time. 5. Incomplete recovery of deformation because of viscoelastic behavior. - Addition silicone -> most stable -> costs produced between 24 hours and 1 week as accurate as a cast made in first hour.
- Polyether -> can absorb water from the atmosphere -> poured within 1 hour for maximum accuracy. - Condensation silicone and polysulfides -> loss of by product -> poured within 30 minutes. ACCURACY - ANSI/ADA specification No.19 -> impression materials must be able to reproduce fine detail of 25 μm or less. - Lower the viscosity better it records fine detail, putty can record only upto 75 μm. - Corresponding specification for gypsum die materials is 50μm - Most accurate -> addition silicone reversible hydrocolloid. Elastic Recovery Impression materials needs to be able to flow readily into undercut areas in the mouth, set in that position, and to be able to “rebound” back to its original shape, when the set impression is removed from the mouth -> elastic recovery. PVS impression material – 99% elastic recovery condensation silicone. Polyetter Polysulfide Elastic recovery can be maximized by blocking the undercuts. RHEOLOGIC PROPERTIES (FLOW AND FLEXIBILITY) Viscosity of material is most important factor in controlling the flow. Light body materials flow readily into minute details, while the tray or heavy body material provide more rigidity to the impression and help force the lower viscosity material into the gingival sulcus. Early versions of light body tended to flow off the prepared tooth with time, now the newer pus and polyettersare thioxtrophic and stay where they are syringed but flow readily when the heavier body tray materials are placed over them. Monophesic material -> shear thinning effect when “false body” permits the material to stay in the tray without segging or dripping, and yet the same material can be used in a syringe. A material that exhibits this property of becoming more fluid when shear rate is increased (Shaking, spatulating or injecting through a syringe) is described thioxtrophic. Stiffness -> polyetter (Decreasing order) addition silicone, condensation silicone, polysulfide. Polyetler -> most rigid -> 1. Problem when dealing with long, thin preparations of periodontally involved teeth 2. Also can result in fracture of dies. Advantageous -> duel etch impressions -> where double bite trays are flexible -> thixotrophic materials best used. Tear strength Important property when dealing with impression materials used in interproximal and sub gingival areas. Ranking from lowest to highest
Hydrocolloids > Silicones > polyether > polysulfide. Cager and Alginate (Addition and Condensation) Influenced by (1) consistency -> increased viscosity increases tear strength. (2) Manner of removal -> rapid rate of force during removal increases tear strength. HYDROPHILICITY - Hydrocolloids -> hydrophilic - Polyetter -> hydrophilic but require dry preparation for making impression. - PVS and condensation silicones -> hydrophobic - Newer materials -> classified as hydrophilic Including non-ionic surfactants. Grafting the surfactant to silicone polymer. - Improves wet ability and reduces contact angle -> 95 to 53o. - Hydrophilic materials -> 35o contact angle. - Technically classified as hydrophilic -> PVS material easier to pour die materials, but do not make it possible to make acceptable impression in a wet environment. Biocompatibility - Cell cyto toxicity -> polysulfide results in lowest cell death event and set polyetter highest. - Elastomer – induced biocompatibility problem occurs when a segment is lodged in patients gingival sulcus -> gingival inflammation. - Radiopacity of polysulfides advantageous here. - Contact dermatitis from polyether catalyst to dental assistant has been reported, but no cyto toxic effects has been reported. Shelf life. - Showed in a dry, cool environment. - Tubes should be kept tightly closed, reseal the alginate container after dispensing the powder. - Arbitrarily suggested that no more than 6 months supply should be kept on hand at any time. Economic factors - Reversible hydrocolloid less expensive than elastomers, but equipments associated with it compensated for the cost. - Polyether and PVS most expensive. - Reduce cost by using auto-mix devices, dual and technique when indicated. Effects of mishandling elastomers (Tab 9.5 Page 230 Anusavice Tab 9.6) DUPLICATING MATERIALS (ANSI/ADA Specification No. 20) Both types of hydrocolloid are used in the dental laboratory to duplicate dental casts or models used in the construction of prosthetic appliances and orthodontic products.
Reversible (Agar) Hydrocolloid most commonly used because it can be used many times. Has same composition as impression material, water content is higher. Classified into two types according to ANSI/ADM specification No.20. – Thermoreversible – Type I Non reversible – Type II Silicones and polythers can be used but costly. INELASTIC IMPRESSION MATERIALS - Impression plaster - Impression compound. - Zinc oxide eugenol paste. Impression plaster - Composition similar to dental plaster, consists of β-calcium sulphate hemihydrate which when mixed with water reacts to form calcium sulphate dehydrate. - Mucostatic -> for edentulous flabby ridge. - Differences from dental plaster. • Rapid setting. • Smaller setting expansion. • Lower strength. - Rarely used because of their inability to sustain elastic deformation without fracture. Impression compound (ANSI/ADA specification No.3) composition. Waxes, thermoplastic resins, filler and coloring agent, shellac, stearic acid and gutta percha are added to improve plasticity and workability. Classified into : Type I – low fusing Type II – high fusing Low fusing -> impression materials -> sheet or stick form -> used for making impressions of edentulous ridges using stock trays. Sticks are used for border moulding or for recording impressions of single crowns using the upper ring technique. Compound may be softened over a flame or by immersion in warm water bath. Dimensional stability Allow thorough cooling before removal and pour within 1 hr ZINC OXIDE EUGENOL IMPRESSION PASTES (ANSI / ADA SPECIFICATION No.16) Composition Base Percentage Function Zinc Oxide 87 Reactive ingredient Fixed vegetable or mineral oil 13 Plasticizer, off sets action of eugenol as an irritant. Paste Oil of cloves or eugenol 12 Oil of cloves contain 70% - 85%, eugenol less irritating
Gum or polymerized rosin 50 To accelerate setting Filler 20 Lanolin 3 Resinous balsam 10 To increase flow and improve mixing properties. Acceler solution (CaCl2) and color 5 Classified as Type I – hard paste -> 10 min setting time Type II – soft paste -> 15 min. Setting time Shortened -> adding small amount of acceleration Drop of water Prolonged -> cool mixing slab -> plasticizer – inert oil or wax. Thick consistency -> compress the tissue Thin consistency -> little or no compression. Dimensional stability Less than 0.1% shrinkage can be preserved indignity Non-eugenol pastes - Orthoethoxybenzoic acid (EBA) -> valuable substitute for eugenol. - Bactericidal agens and other medicaments can be incorporated without interfering with the reaction. Surgical pastes - After gingivectomy, ZOE paste may be placed over the wound to aid in the retention of a medicament and to promote healing. - Softer and slower in their setting reaction. Bite registration pastes. - Occlusal relationships recorded include impression plaster, compound wax, resin and rectal oxide paste. - ZOE paste as recording material in construction of complete dentures and fixed or RPD. - ZOE interocclusal record more stable than one made in wax. TRAYS All impression materials shrink upon setting. Reversible hydrocolloid -> shrinkage occurs as a thermoplastic event. Elastomeric impression -> polymerization shrinkage materials. To obtain accurate impression a relatively uniform amount of bulk in the impression is imperative, so that there is uniform shrinkage throughout the body of the impression. Water based impression materials, such as reversible or irreversible hydrocolloid provide maximum accuracy with a cross sectional thickness of 4-6 mm-> best achieved with stock trays.
Elastomeric impressions, most accurate when used with a cross sectional thickness of 2 mm -> best achieved with custom tray. STOCK TRAYS - Stock plastic trays - Stock metal trays : perforated Non-perforated Rim lock trays Denticular - Clear trays Edentulous Stock plastic trays (Pic. BDJ. Article) - Can be used for impression of one or two single units. - Trays do not have sufficient rigidity - Low cost - Contra indicated with fixed bridge work because its flexibility adversely affects inter preparation, cross arch and anteroposterior dimensions Stock metal trays (Pictures) - Coated steel or stainless steel trays combine the convenience of stock plastic trays with the rigidity of custom trays. - Perforated or rim lock trays indicated for putty wash impressions for mechanical retention in addition to tray adhesive (adhesion of putty to its impression adhesives is not as reliable as with other viscosities). Custom trays - Custom tray improves the accuracy of an elastomeric impression by limiting the volume of the material, thus reducing two sources of error: • Stresses during removal. • Thermal contraction - Custom trays, constructed on the diagnostic cast using one layer of bae plate wax as a spaces - Trays can be fabricated because (Picture page 364 – 68) Roseinsteil - Pollymethylmethecrylate - Photo-cure bisacryl materials (Triad) - PVS putty materials - Thermoplastic trays - PMM trays should be fabricated at least 24 hours in advance to in sure stability. - Tray should extend 3-5 cm for gingival margin, wax spaces covered with tin foil. - Thermoplastic trays with elastomers not recommended as the plasticizers in elastomers attack and soften the compound. - Occlusal stops – critical for proper orientation of tray in the mouth. - Three stops ideal, with atleast one posterior to prepared teeth. - Should be placed on non-functioning cusps so that distortion will not interfere with inter cuspal relationship.
- 45 degree slope helps to center the tray. STOCK TRAY VERSUS CUSTOM TRAY - The difference in cross sectional thickness of material in a stock tray is only about 1.5 – 2mm thicker than that in a custom tray. - Numerous articles have compared accuracy of impression made with a custom tray with the accuracy of impressions made with stock tray. - Gorden et al, has reported that the inter preparation distance in cast made from polysulfide, poly vinyl siloxane and polyether impression was 45- 100 um, greater when stock trays were used instead of custom acrylic resin or thermoplastic trays. - However, Bomberg et al found no significant difference in the marginal fit of single – tooth restorations on case made from poly vinyl siloxane impressions in stock and custom trays. DUAL ARCH TRAYS Trays are available in both anterior and posterior designs and consist of an outer rim that is spanned by a mesh fabric. Both plastic and metal design are produced. Any elastic impression material can be used, but more rigid- bodied materials are preferred, also developed to be used with reversible hydrocolloid impression materials. Duel arch tray may be used to make impressions for single unit crowns where as well established inter cuspal position is present and lateral contacts are not a factor (anterior guided occlusion). If bucco-lingual width of the elevator ridge is wider then the width of the tray, the plastic outer rims will be wedged apart when the patient closed into the impression material – resulting is distorted impression -> tray can be modified by cutting the mesh and spreading the tray. Tray adhesive Adhesion of impression material to the tray is achieved through the use of specific chemical tray adhesives. Tray adhesives are usually liquid rubber (eg. Butyl rubber) dissolved in a volatile solvent such as chloroform or ketone -> note that these adhesives should not be inter changed. Roughening the surface of custom trays increases the adhesion. PVS putty does not adhere well to its adhesive therefore mechanism retention in the tray (perforation / rim lock) mandatory. Pointed in a thin layer on the internal of the tray and the tray borders atleast 7-15 minutes before making the impression. Gingival displacement Indirect restorations – including cast gold inlays, onlays, partial veneer restorations and complete crowns – metal ceramic and all ceramic, and bonded ceramic inlays and onlays are routinely used to restore defective teeth. These restorations frequently have cervical margins that are intentionally placed in the gingival sulcus for esthetic or functional reasons.
The procedure used to facilitate effective impression making with intra crevicular margins procedure is the irreversibly displace the gingival tissue in a lateral direction, so that a bulk of low-viscosity impression material can be introduced into the widened sulcus and capture the marginal detail. A critical sulcular width of 0.2 mm is required for maximum accuracy of the impression, and to improve the tear strength of the material so that it can be removed from the mouth intact with no tearing. According to Bensen Bomberg (1986) gingival tissue can be displaced laterally or vertically lateral retrection. It displaced the tissues so that an adequate bulk of impression material can be interfaced with the prepared tooth (gingival displacement) Vertical retraction : Exposes the uncut portion of the tooth apical to the finish bone. One of the prime requisites to successful tissue management is the begin the restorative procedure only after the gingival tissues are deemed healthy This is not always possible in the clinical setting, but nonetheless it should be a constant goal. Classification of techniques - Mechanical - Chemical - Surgical - Combination of the three Biologic width – defined as the dimension of the soft tissue, which is attached to the portion of the tooth coronal to crest of the alveolar bone (Gargiulo et al 1961). Biologic width = Junctional + Connective tissue 2.04 mm 0.97 mm 1.07 mm Tissue displacement must be done gently but with sufficient firmness to place the cord just apical to the margin. Over packing -> cause tearing of gingival attachment, leading to irreversible gingival recession. Violation of biologic width Biologic width forms a “Biologic seal” around the neck of the tooth. Prevents micro organisms an their by products from migrating to the underlying tissue. Violation of biologic width Leads to migration of micro organisms and toxins. Inflammatory reaction Loss of attachment
Apical migration of marginal attachment Periodontal pocket -> bone loss - Location of prepared cervical margin is 0.5 mm from the healthy free gingival margin or 2-3mm from the crest of the alveolar bone and must follow the natural scalloped form of the attachment and alveolar housing. Techniques for gingival displacement. Majority clinicians use a combination of mechanical – chemical displacement, using retraction cords along with specific hemostatic made cements. 3 main variations of mechanical chemical method a. Single cord technique b. Double cord technique c. Infusion method of gingival displacement. Retraction cords are supplied in three basic designs, twisted cord, knitted cord and braided cord. The key to effective displacement is to use a cord of sufficient diameter to provide adequate displacement is largest cord that can be automatically placed in the sulcus. Hemostatic medicaments ; Aluminium potassium sulfate Aluminium sulfate Aluminium chloride Epinephrine - Use of epinephrine as a gingival displacement medicament has the potential to cause systemic side effects if used in appropriately not used for routine gingival displacement cord packing instruments -> roseinsteil figure 14.3 page 358. Single cord technique (Picture) Indicated when making impressions of one or three prepared teeth with healthy gingival tissues The largest diameter braided or knitted cord that fits in the sulcus is selected. Socked in the medicament of choice Excess is blotted from the socket cord with sterile cotton sponge. The cord is packed into the sulcus in a counter clockwise direction -> starting from the inter proximal area. The instrument should be angled toward the tooth so the cord is pushed directly into the area. Should also be angulated towards the cord already packed, to avoid it being displaced. The cord needs 8-10 minutes to effect adequate lateral displacement. The cords should then be soaked in water to allow it to be easily removed from the sulcus.
Double cord technique (Picture) - This technique is used • When making impressions of multiple prepared teeth. • When tissue heath is compromised and it is impossible to delay the procedure. - With the double cord technique - A small diameter cord with no medicament is first placed in the depth of the sulcus. - A larger – diameter cord with the medicament is placed above the small diameter cord. - After waiting for 8-10 minutes, the larger diameter cord is socked in water and removed. - The small – diameter cord is left in the sulcus during impression making. The infusion technique • Haemorrhage is controlled using a specifically designed dentoinfusor with a ferric sulfate medicament (15% or 20%) ->20% preferred because it is less acidic and does not remove smear layer of dentin from prepared tooth. • Infusion is used with a burnishing motion in the sulcus and is carried circumferentially 360o around the sulcus. • When hemostasis is verified, a knitted retention cord is soaked in the febric sulfate solution and packed into the sulcus -> left in place for 1-3 min and then removed. • Effective ancillary technique for control of haemorrhage when using single cord technique. Every other tooth technique Placing retraction cord simultaneously around all prepared teeth (especially anterior) may result in strangulation of gingival papillae and eventual loss of papillae -> resulting in unesthetic black triangles in the gingival embressures. Single or double cord technique canbe used, retraction cord placed around the alternate prepared tooth -> impression mode -> then gingival accomplished on remaining teeth and record impression made -> finally a pick up impression allows fabrication of a master cast with diet for all the prepared teeth. NEWER MATERIALS Merocel : Synthetic material that is chemically extracted from a bio-compatible polymer (hdyroxylate polyvinyl acetate) that creates a net like strip. -> capable of straumatic gingival retraction. Used in strips of 2mm thick that expand with absorption of selected oral fluids -> gingival retraction.
Expa- 3 year is a paste used for gingival retraction that opens the sulcus. It is supplied in a syringe, that is designed to be injected into the unretracted sulcus which then becomes rigid and creates space between the tooth and the tissue. Takes about 2 min 30 seconds to achieve sulcular exposure. It contains haemostatic astringent -> kaolin, aluminium chloride. Size, exerts moderate and calculated pressure on gingival margin, 0.1 N/mm2, attachment safe guarded. Retroc : Gingival retraction putty is a condensation silicone formula with potassium aluminium sulfate. Surgical methods - Rotary curettage - Electro surgery Rotary curettage (Pic. 16-23, Page 268, Shillingburg) Is a “Troughing” technique, the purpose of which is to produce limited removal of epithelial tissue in the sulculs while a change finish line is being created in tooth structure. (Described by Ansterdam 1954). The technique is also called “Gingettage” used with the sub gingival placement of restoration margins should always be done on healthy, inflammation free tissue to avoid the tissue shrinkage that occurs when diseased tissue heals. (Note : Periodontal curettage -> debride diseased tissue from the sulcus to allow re- epithelization and healing). Electro surgery :Picture Page 269, 270, Shillinburg, …) D. Arsonval 1891 – responsible for the development Electro surgery has been described for the removal of irritated tissue that the has proliferated over preparation finish lines. For enlargement of the gingival sulcus and control of haemorrhage to facilitates impression making Current flows from a small cutting electrode that produces a high current density and a rapid temperature vise at its point of contact with the tissue. The cells directly adjacent to the electrode are destroyed by this temperature increased. Commonly used electrodes. - Coagulating - Biomandibular loop -> crown lengthening - Round loop -> gingivectomy - Round loop -> gingivectomy - Small, straight -> gingival sulcus enlargement - Small loop - Contra indications -> cardiac pacemakers not be used in presence of flammable agents In the presence of nitrous – oxide – oxygen analgesia
TECHNIQUES OF IMPRESSIONS MAKING Double mix technique (Multiple mix technique) The light body material is injected from the filled syringe within and around the tooth preparation. The tray filled with heavy body material in inserted in the mouth and seated over the syringe material. The tray material will fore the syringe material to adapt to the prepared tissues The two materials bond together on setting, most community used with custom tray. Advantages • Over comes the polymerization shrinkage of the light body materials. • Margins duplicated in light body -> finer details are reproduced. Disadvantages • Use of custom tray • An assistant required for mixing the material tray/syringe simultaneously • Margins duplicated in heavy body in case of excess pressure. Single mix technique (Single viscosity technique, monophase technique) Medium viscosity of polyether and addition silicones are often used in a stock tray while making impression with this technique. Only one mix is made, part of the material placed in the tray and another portion placed in the syringe for injection in the cavity preparation or on prepared teeth. Success of this technique depends on the pseudoplastic behavior of these two materials. Viscosity of a monophase material is a compromise between a low viscosity material that can flow well and record surface detail with the need for a higher viscosity to prevent slumping and placement in an impression stock tray. Advantages • Reduced mixing -> reduce wastage of the materials • Less time consumptions • Avoids the time involved in fabrication of custom tray. Disadvantages * Relatively high viscosity and reduced flow of the monophase materials, makes their injection onto the preparation more difficult to control -> increased incidence of surface voids (Strephen M. Dunne et al 1998). PUTTY – WASH TECHNIQUE Developed is over come the polymerization shrinkage associated with condensation silicons. 1. Putty wash simultaneously technique (One stage) light body materials syringed on to the preparation while the putty material loaded in a stock tray is simultaneously inserted into the mouth and sealed over the light body materials. Advantages Reduced chair side time saving of impression materials. Disadvantages • Absolute lock of control the bulk of wash materials (Chee and Dunovan 1992).
• Posibility of margins duplicated in putty medicine. • Tendency of bubbles to be formed and occluded in the set impression. • By mixing putty, syringe material simultaneously, setting distortion of putty included in over all distortion of impression. Chee and Donovan 1992) 2. Putty wash relief channel technique - Pre-operative putty impression is made intra orally. - In the area where the teeth are to be prepared impression material is removed or channels prepared using putty cutter instrument. - The impression is then washed on relined with low viscosity material. Advantages • Impression can be captured with the wash materials. Disadvantages • To confine the wash material to area of relieved impression. • If entire area is washed -> creates hydraulic displacement of putty impression resulting in smaller dies. (Bonovan TE 2004) 3. Putty wash 2mm spacer technique - 2 mm thickness wax spacer is prepared on a diagnostic cast, occlusal steps are provided on non-functional cusps. - A putty is made with a stock tray resulting in a putty custom tray with 2mm space for the wash materials. Advantages • wash stage cervical out after the putty has set and contracted. • Controlled wash bulk compensates for this contraction with minimal dimensional change. Disadvantages • Extra chair side time • Extra a materials. Copper band technique - This technique is used to sehvage on impression of multiple preparations when there are only vague margins on one or two preparations that are not adequately replicated in the impression. - Copper tube is prepared that extends 1 mm beyond the finish line. - Top one – third of copper tube filled with impression compound end sealed on the preparation. - Remove 2 mm of compound from the impressed occlusal surface for the rubber base materials. Disadvantages Can cause damage to the attachment apparatus • Time consuming
DUAL ARCH IMPRESSION TECHNIQUE (TRIPLE TRAY TECHNIQUE CLOSED MOUTH TECHNIQUE) - This technique captures the prepared teeth, the opposing arch and the occlusal articulation in maximum intercuspation (MIP) simultaneously. Indications • Used with a maximum of two prepared teeth. • Unprepared stops anterior and posterior to the prepared teeth should be present. • Used only with patients that have existing anterior guidance. • Must be able to close completely in maximum intercuspation position with the impression tray in place. Contra Indications • Presence of third molars. • Rapidly ascending ramus • Excess soft tissue distal to the molars - Rigid metal trays, (Qued trays) as well as rigid PVS or polyether material should be used. - Flexible trays (Plastic) should be avoided -> often bucco-lingual width of the arch is wider than the trays. The resilient tray flexes outward when the impression is made and rebounds when the impression is removed from the mouth, thus permanently distorting the impression. Advantages • Accurate recording of the MIP. • Eliminated any mandibular flexure that might be associated with opening. Disadvantages • Complicated laboratory procedure. Segmental impression technique To make impressions of multiple prepared teeth due to inherent limits in working time end difficulties maintaining moisture control. Arch to be impressed is broken down into segments of two prepared teeth. Custom tray prepared for each segment with 1mm of wax relief, trays should extend 3 mm post the gingival margin, as there are no occlusal stops and gingival tissue must prevent over seating of the trays. Low viscosity material is loaded into the syringe as well as the segmental tray, and then are impression made. Procedure repeated with each segment. Finally on over impression is made using a stock tray. Uses When moisture control is difficult in specific patients. When making simultaneously impressions of implants and prepared teeth.
Tooth / Implant impressions Implant copings can reduce access to the prepared teeth and impede the extrusion of impression material to the margins of the prepared teeth. In such cases, a combination of custom impression trays to impress the prepared teeth and then place the implant impression copings and make an over impressions of the arch. Indirect dowel, cores Indirect technique of obtaining dowel patterns is indicated when multiple dowel cores are required or when redicular attachments are to be used. A 25 gauge local anaesthetic needles is used as a vent to allow cir to escape as the impression materials is injected into the canal space. Needle is gradually removed while the low viscosity materials is injected into the canal. An appropriately sized plastic impression dowel coated with adhesive is inserted into the canal. Procedure is repeated with multiple dowels and finally an over impression is made. Pin-retained restorations (Picture Rosei…) Elastomers are strong enough to reproduce a pinable without tearing. However to ovoid bubbles, they must be introduced corgully cement tube. Cement tube : fill the tube and squeeze a small amount of material into each pinhole make sure no air is trapped by inserting and removing an explorer into the materials. Centulo : Spiral the material into the pinholes – rotating slowly while moving the lentulo along the side of the pin-hole. With reversible hydrocolloid -> special nylon bristles are used to register the pin hole (should extend 2 mm above the opening of pin hole). DISINFECTION OF IMPRESSIONS The dental impression is one of the ways by which pathogens can leave the operatory and spread their risk abroad. The impression must be rendered harmless before being passed on to other people who will work with it or with the gypsum cast made from it, outside the dental operatory. Chemical disinfectants used for this purpose : - Chlorine compounds. - Synthetic phenolic compounds - Glutaraldehydes - Iodophors - Phenolic/ alchoholic combinations Table 9.4, Page 226 (Annusavica) Polyethers are susceptible to dimensional changes if immersed for a long time (> 10 min) -> hydrophilic nature. Disinfectant can be sprayed on the impression, wrapped in a disinfectant soaked paper towel and placed in sealed plastic bag for 10 min.
Long immersion time may cause the surfactant in hydrophilic PVS to reach out and vender the impression less hydrophilic. Ultra violet treatment was effective against candida organisms, and there was not adverse effect on either dimensional change or surface roughness of impression materials. Disadvantages with this method, cannot kill micro organisms that are shadowed from the emission, UV light with should be designed in such a way that the impression, rotates on the taste and unit surrounded by miwon (Hiroshi Ishiode 1991, JPD) Concepts for transporting impressions to ……. Laboratory. 1. Send it well cleaned (Rinsed) and undisinfected in a biohazerd – labeled, heat sealed plastic bag. OR 2. Debride, clean (rinse) and adequately disinfect it, place it in a sealed transport bag labeled with the precautions taken.
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