2 Impression Materials: Used to make replicas of oral structures“Negative” replica made by impressionCast material is placed into impression to yield a “positive” replica of the oral tissueAllows for the indirect restoration or dental appliance to be fabricated without the patient presentVariety of materials for a variety of applications
3 Uses of Impression Materials: Used in the fabrication of:Indirect restorations:Crowns & bridgesDental prostheticsTemporary crownsOrthodontic & Dental appliancesAcrylic traysBite registrationStudy modelsImpression materials have so many uses since so much of dentistry relies on an exact replication of the oral cavity to create a dental prosthetic or appliance.Some appliances may include ortho retainers or tooth movement appliances. Other appliances may include the fabrication of a space maintainer. Acrylic trays such as whitening trays or custom-fit mouthguards require an exact duplication of the oral tissues involved. Custom trays are also used to carry final impression materials into the mouth for the most accuracy – especially used in edentulous areas.These mentioned above, are cast in a stone or plaster model upon which the restoration or appliance can be fabricated.Here is a molar prepped for a crown. Before an impression is taken a “gingival retraction cord” is packed into the sulcus to retract the gingiva from the prep so that impression material can fit down into the sulcus and around the cervical area of the tooth. Sometimes these are impregnated with an astringent that allows for a decrease in bleeding also – so as not to harm the integrity of the impression in the area.This impression material is put into a Triple tray and inserted around the prep-site – the pt occludes into the tray and material and stays until it is set. You can see with this tray, the opposing teeth are also being “impressed” so that will give an accurate bite registration for the occlusal relationship to be accurately duplicated. If a triple tray is not used, the registration will need to be taken as a separate step.the impression removed from the prep – the dr. will check for accuracy – DA’s MAY pack cord and mix dental impression materials but may NOT take final impressions – these must be taken by the dentist!
4 Uses of Impression Materials: 4) The lab receives the impression and will pour-up the impression in gypsum stone (this is plaster in this picture –labs use a high strength stone for fabricating crowns) From this stone cast, dies are cut upon which the individual crown will be made. (This is NOT the cast from this series)5) The crown is finished by the lab and sent back to the office for cementation.6) Here is the crown seated into the patients mouth.
5 Characteristics of Impression Materials: Fluid enough to flow around area of interestMust “set” in reasonable amount of timeCan remove from mouth without distortionNo harmful effects on tissuesRelatively tasteless & odorlessDimensionally stable until a cast is createdGive detailed reproductionCompatible with cast materialsThe materials we will talk about are available in varying VISCOSITIES – thicknesses. The material must have a LOW enough viscosity to FLOW into the detailed areas of a prep site – like a Class II inlay – to replicate the detail for the lab.Materials “Set” in a variety of ways – through a chemical or physical reaction – this process has to occur in a reasonable amount of time.If the replica is to be ACCURATE there must not be distortion of the impression material. It must be ELASTIC enough to be removed from the mouth and then RESUME the shape that it held in the mouth.Materials must be compatible with the body and not too unpleasant for the patient.Stability of the material is also critical for ACCURATE replication of detail – some materials have low stability and a cast must be created immediately, vs. others that are stable for hours and days.
6 Impression Trays: Used to carry the material to pt’s mouth Must be sturdy enough to support the materialNeed to be disposed of or sterilized after useMetallic trays are autoclavableStock trays available: full arch, quadrantCustom trays made of acrylic for most accuracyTriple trays takes impression of opposing teeth, occlusal registration, and prep sitePerforated trays have holes for retention of material
7 Categories of Impression Materials: INELASTICRigidCannot be used with undercutsFor edentulous or bite registrationELASTICFlexible & “rubbery”Used with undercutsGenerally used todayTwo subcategories:Hydrocolloids (Aqueous)ElastomersINELASTIC materials do NOT have the elastic ability we just discussed – to resume the shape the material had in the mouth – therefore they cannot be used where there are “undercuts” For example: the cervical portion of a molar is more constricted than the height of contour of the buccal and lingual surfaces therefore what might happen when the tray is removed from a pt’s mouth if a RIGID material is used to take an impression? The impression material fractures. (Plaster used to be used for impressions – altho’ it was rigid and broke when it was removed from the mouth, the pieces would be glued back together to make a negative replication.)
9 Inelastic Impression Materials: Impression Compound:Resins + waxThermoplastic, solid materialHeat to soften materialCool to set materialPhysical change only; NO chemical reactionForms: plates and sticksSoften in water bath (10° warmer than body)Applied to tray and inserted in mouth to coolUsed as a custom trayWe will use other materials during the semester that are “thermoplastic” in nature. Think of how candle wax is – as it is heated it melts, when the heat is removed, the wax solidifies again – that is thermoplastic – the chemical make-up of the materials does not undergo change.Since the compound is slightly warmer than the human body temp, it will “harden” in the pt’s mouth is a few minutes and then can be removed. Obviously, this impression is now very heat-sensitive and would need to be handled accordingly. These compounds are rarely used in private practices today since materials have improved in this area – there are other materials that can be easily used to create a custom tray.
10 Inelastic Impression Materials: ZOE Impression Paste:Similar formulation as ZOE cementsPastes mixed together – chemical reactionBASE + CATALYSTApplied in thin layer in custom tray for edentulous arch; or to occlusal surfacesSets in 4 – 5 minutesZOE is rarely used in this application. As the other inelastic impression materials; they have been replaced with more convenient methods and more versatile impression materials. We will not be using the INELASTIC materials in our lab.
12 Elastic Impression Materials: HYDROCOLLOIDS –“water-based” impression materialsColloid – liquid suspension of particlesParticles derived from seaweed & kelpLiquid = waterGEL – the rubber-like form of the materialSOL – the material in liquid solutionNot dimensionally stable – must be “poured” immediatelyThe GEL form of the hydrocolloids is the material as it is “Set”. The SOL form is when it has flow and is in a liquid solution. This is important to remember as we discuss how hydrocolloid materials are used.Since hydrocolloids are mostly comprised of water, they are not dimensionally stable – they MUST be poured-up immediately in stone or plaster before the water has evaporated from the impression and changed the dimension.
13 Hydrocolloids: Reversible Hydrocolloid: Agar – a gelatinous material + waterThermoplastic – NO chemical reaction“Reversible” –Goes from gel, to sol, and back to gel,by raising and lowering temperatureDispensed in tubes & syringesVery accurate final impression materialThe tubes of the material are used to dispense material into the impression tray. The syringe carpules are used to dispense the material in areas of the most detail – for example, the prep site. The syringe material will be extruded all around the prep; the tray will be filled from the tube; then the tray will be placed over the arch or quadrant. The syringe material will be imbedded into the tray material when it is set.
14 Hydrocolloids: Handling Reversible Hydrocolloid: Special water baths with 3 chambers are used1) Material is immersed in boiling tank (212°F)for minutes; turns from gel to sol2) Place in storage bath (150°F);10 minutes to few days; remains sol3) Load tray; immerse in the tempering bath (110°F); 5-10 minutes; lowers temp for pt comfortThe material boils for about 10 minutes if new. IF the tube has been previously boiled, it may require a longer boiling time to turn to sol (20 minutes).
15 Hydrocolloids: Handling Reversible Hydrocolloid: 4) Load syringe; extrude onto prep site5) Seat tray in mouth6) Attach cooling hoses to tray7) Hold firmly in pt’s mouth until cooled and transformed to gel state8) Remove from pt’s mouth and pour immediately
16 Reversible Hydrocolloid: Advantages:Very accurateExcellent compatibility with gypsumDisadvantages:Labor-intensivePoor tear resistancePoor stabilityNeed equipment space and very organized staff
17 Hydrocolloids: Irreversible Hydrocolloid: ALGINATE impression material Formed by chemical reactionPowder + water; mixedNOT reversible; SOL to GEL onlyProtect from inhalationDispensed in cans or bulk packagesUsed when less detail is required
18 Hydrocolloids: “What are indications for alginate impressions?” Study modelsOpposing models of final castsCasts for mouthguards, whitening trays, custom trays, orthodontic appliances, provisional crowns, etc.To fabricate a direct provisional crown
19 Hydrocolloids: Handling Alginate: Fluff powder and measure accurately Measure water – temp!Mix in a flexible,rubber bowlStir to wet powderStrop mixture against side of bowl to eliminate air bubblesMix until creamy & homogeneous; 60 sec.Water temperature is a factor in setting time!!Cooler water will INCREASE the working time (Slows down the setting time)Warmer water will DECREASE the working time (Faster setting time)Can purchase regular or FAST set alginate.
20 Hydrocolloids: Handling Alginate: Load alginate into tray from posteriorPress material into tray to eliminate voidsSmooth & indent alginate with wet fingerUse extra alginate to wipe onto occlusal surfaces of teethLoad the mandibular tray from a posterior and lingual direction. Press alginate into tray to make sure no air is trapped.
21 Taking an Alginate Impression: Bead the tray as necessaryStand behind patient for maxillary (11:00)Center tray above teeth and seat posteriorPress tray down & toward anterior and allow lip to cover tray; material should fill vestibuleHold tray in patient’s mouth until set (2 - 3 min)Loosen cheeks and lips with fingerRemove occlusally with a firm snapYou can check if material is set by feeling material left in bowl or probing into material on outside of impression – it should feel firm and not change shape when touched.
22 Evaluating an Alginate Impression: Full coverage, including retromolar areaTray centeredClear & sharp impressionNo voids, air bubbles, or tearsHas a “peripheral roll” (vestibular area)If all is OK, rinse & disinfect impression,pour-up in stone/plaster, or store in humid bag.IF the impression is NOT okay, it must be re-taken.
23 Irreversible Hydrocolloid: Advantages:Easy & economicalMany applicationsPatient comfortExcellent wetting by gypsumDisposable trayDisadvantages:Not enough detail for final impressionMust be poured immediately
24 Hydrocolloids:SYNERESIS – shrinkage in impression due to loss of water from heat or exposure to air.IMBIBITION – swelling of impression due to taking up moistureTo avoid these conditions (dimensional distortion), pour-up immediately!
25 Elastomers: Two-paste systems: Set by chemical reaction Two tubes – pastes mixed by handTwo putties – mixed/kneaded by handCartridge & Extruder Gun – automix tips usedCartridge & Motor-driven Mixer – automix tips usedSet by chemical reactionWarmth and moisture may slow settingMust use a tray adhesive (if tray not perforated)Latex may inhibit set of polyvinylsiloxanesTray adhesives vary per each elastomer – if a tray has good retention holes or perforations, then an adhesive may not be necessary.Latex gloves and rubber dams should not contact impression material – it will inhibit the set. Use of vinyl gloves are recommended when kneading putty or handling materials.
26 Elastomers: Elastomers are used in a two-step process: Preliminary Impression - the material used to form the base or the “tray” material used in an impression; usually more viscous or heavy-bodySecondary or “Wash” Impression – the material applied through a syringe around the prep site for detail; usually less viscous;light or medium -body.
27 Elastomers: POLYSULFIDE: Oldest elastomeric used in dentistry Two pastes – mix base with catalystLiquid polymer with “sulfhydryl” group – contains sulfur“rubber-base” impression materialUsed best with custom traysOther materials have replaced this one
28 Elastomers: Handling Polysulfides: Equal lengths of pastes on pad Pastes are “swirled” together, then stroppedMaterial placed in custom trayTake impression – hold in place for up to 15 min.Remove from pt’s mouth slow & steady forceRinse & disinfectPour-up impression within several hours
29 Polysulfides: Advantages: Economical Good tear resistance Good compatibility with gypsumDisadvantages:MalodorStains clothingLong setting timeFair stabilityLess accurate
30 Elastomers: Condensation Silicones: Similar to silicone rubber products (but non-toxic)Setting by-product of alcohol“Hydrophobic” – results in voids in stoneShrinkage occurs as it setsMust be poured immediatelyAccurate, but slow setting timeReplaced by improved productsThe better materials are hydrophilic – water loving and have better wettability for the gypsum. This material resulted in bubbles and voids in the stone, since it is hydrophobic – not attracted to water
31 Elastomers: POLYETHERS: Developed in 1960’s No reaction by-product producedShorter working and setting timeOnly come in a single viscosityStiff material – can use a triple trayVery popular – “Impregum”No need to pour-up immediatelyThe polymerization of the polyether does not yield a by-product as it sets.
32 Elastomers: Handling Polyether: Mix equal lengths of paste; or extrude through the automix cartridge tipLoad syringe and apply to tooth thru tipLoad tray and invert over area for impressionAllow to set; 4 – 5 minutesRemove from mouthRinse & disinfect
34 Polyether: Advantages: Short setting time Single viscosity Good stabilityGood tear strengthClean & easy to useDisadvantages:Bad tasteMost difficult to remove from mouth
35 Elastomers: ADDITION SILICONES: “Polyvinylsiloxanes” – silicone polymerTwo pastes or two puttiesHydrophobic by nature – manufacturers adding components to increase wettabilityVery accurate & fast settingAvoid contact with latex (gloves, rubber dams)Low setting shrinkage & very stable
36 Elastomers: Handling Addition Silicone: Mix equal lengths of pastes, or automixApply light-body material to tooth thru syringeLoad tray with heavy-bodySet tray over prep siteSet in 4 – 5 minutesRinse & disinfect
37 Addition Silicone: Polyvinylsiloxane Advantages:Very stableShort setting timeGood tear resistanceGreat accuracyNo bad tasteDisadvantages:May have poor wettabilityTwo pastes to mixCan be poured up days after the Impression is taken.