Impression Making Part 2 PDF

IMPRESSION MAKING PART 2  Impression Materials IMPRESSION MATERIALS Elastic materials...

IMPRESSION MAKING PART 2  Impression Materials IMPRESSION MATERIALS Elastic materials Non-elastic materials Non-elastic materials are mainly used for removable parts of prosthodontic Hydrocolloids - Plaster of Paris - Impression compounds Hydrocolloid either Reversible -Reversible Synthetic elastomers - Impression waxes -Irreversible (Agar) and Irreversible (Alginate) - Zinc oxide eugenol The addition silicone is the main type we’re going to use for crown & bridge impression material. Polysulphides Polyether Silicones Condensation silicone Addition silicone  Synthetic elastomers Polysulphides Polyether Silicones Condensation Addition silicone silicone  REVERSIBLE HYDROCOLLOIDS Introduced in 1925 Made of: Agar, borax, and water. Reaction can be reversed and the material reused by heating the material. Initial cost for apparatus consisting of conditioning unit and water cooled impression trays. Less comfortable to patients due to bulky cooling apparatus, and long setting time. Immediate pouring is required to avoid distortion. REVERSIBLE HYDROCOLLOIDS Good recording of fine details due to fluidity of the material. Low tear resistance The accuracy is improved with higher bulk of material unlike elastomeric materials The agar-agar is found in liquid form between 50-100 C⁰ Below 50 C⁰ it starts changing to gel form We start at the conditioning unit, liquefy, store, then temper the agar to avoid burning the patient. Next, we load the agar on the tray & insert it in the patient mouth. The cooling water will drain & circulate through out the tray for 5-6min until the material is set. IRREVERSIBLE HYDROCOLLOIDS Introduced in 1941 Made of: Soluble salt of alginic acid Irreversible reaction which gives calcium alginate gel Cheaper than elastomers Flavours are added to make it more acceptable to patients Immediate pouring is required to avoid distortion Excellent recording of fine details due to fluidity. Easily distorted and have low tear resistance mainly used to make study models In Cr&Br, it’s used for the opposing arch only  Here an alginate impression, all the occlusal surfaces and neck of the teeth are evident with no show through of the tray POLYSULFIDE First elastomeric material introduced in 1950s Made of: Polysulfide, sulphur, and lead dioxide (gives it the brown colour) Unpleasant smell due to sulphur content. Reaction gives water as by product which leads to shrinkage (0.5% -1.5%) Should be poured within 1 hour to avoid distortion Setting time long, around 10 min (less time is required with higher moisture and temperature) the less bulk used, the more accurate it is (use custom tray) Least expensive between all elastomeric materials Commonly available in 2 paste system (base & catalyst) and has a characteristic brown color POLYETHER Introduced in 1960s Made of: Unsaturated polyether, plasticiser, and filler Setting reaction is fast 3-6 min Expensive No by products but could absorb moisture from the air and swells Should be poured within 1 hour to avoid distortion High accuracy and tear resistance Have bitter taste POLYETHER Snap set property where the transition to set state is abrupt while other materials the transition is gradual This graph shows the setting rxn of polyether: silicone. The PE suddenly increase in viscosity, while the silicone gradually increase in viscosity over time Polyether is commonly used for implants impression due to its high rigidity CONDENSATION SILICONE First introduced in the 1950s Made of: Silicone polymer, filler, cross linking agent, and activator Cheaper than the polyether and addition silicone Setting reaction takes 4-6 min, but could have variations ( depends on amount of catalyst added to base) Less affected by temperature and humidity Polymerisation shrinkage is 0.3-0.6% Gives Ethyl alcohol as by product No smell or taste CONDENSATION SILICONE Know as the lab putty, more used in the lab than clinically. It’s commonly used for making copy dentures Very hydrophobic Dry field is a must Pouring with gypsum stone is more difficult, surfactant must be used Must be poured within 1 hour to avoid dimensional changes due to evaporation of by products Excellent elastic recovery and reproduction of details Good tear resistance Safely mixed with gloves (unlike addition silicone) ADDITION SILICONE (PVS) Introduced in 1970s Also know as Polyvinyl siloxane The material of choice for fixed work impressions Most dimensionally stable Setting reaction takes 4-6 min, but could have variations Expensive Hydrogen might be released which causes pitting of the stone cast pouring can be delayed for 1hr. Lowest polymerization shrinkage 0.05% -1.6% Pouring can be delayed up to a week No smell or taste but could be flavoured sometimes ADDITION SILICONE (PVS) Hydrophobic: surfactants are added to make them hydrophilic The higher the temperature the faster the setting reaction Cooling increases working and setting time by 25% High tear strength Excellent surface details Sulfur found in latex gloves inhibits the polymerization reaction Available as heavy, medium, and light body ADDITION SILICONE Can be found in different consistency. The light & medium silicone can be applied using a gun, While heavy putty silicone come as base & catalyst and mixed together in equal amounts IMPRESSION MAKING – TRAY SELECTION Stock tray Custom made tray Cost Low cost. Higher cost. Availability Readily available in the clinic. Requires laboratory procedures for fabrication. Impression material Larger quantity is required. Smaller quantity is required. Rigidity Could be rigid as in metal, or Rigid, made from cold or light cured partially rigid as in plastic. acrylic material. Size Available in standard sizes. Shape and size are based on the patients’ primary cast. Adjustment Hard to adjust. Can be easily adjusted with acrylic bur. Features of a good custom-made tray Written on the pic IMPRESSION TECHNIQUES Two stage technique (putty and wash) One stage technique (double mix) Monophase technique (single mix) TWO STAGE TECHNIQUE Also know as the putty and wash technique Impression taken with heavy body (putty) followed by a light body (wash) impression The putty material is used to modify the stock tray into close fitting one the next step is using the lighter and more flowable material to capture the details in the form of wash, which is placed inside the tray and then re-seated intraorally.  TWO STAGE TECHNIQUE Steps: Select the stock tray and verify intraorally Apply the tray adhesive extending few mm into the external surface of the tray Isolate the prepared teeth and apply the retraction cord TWO STAGE TECHNIQUE Take the first impression using the heavy body Mix equal amounts of the base and catalyst Mix into a homogenous color Form a cylinder form and place in the tray Avoid overfilling the tray since it would expand the tray walls and cause rebound once removed Seat intraorally and hold still until fully polymerised Once set remove from mouth TWO STAGE TECHNIQUE Trim excess materials and remove interdental collets with scalpel or blade Remove undercuts which would make it difficult to reseat the tray Create venting channels palatally and lingually to allow excess light body to escape Straight low handpiece can be used to remove the undercuts Try the tray intraorally to make sure it can be reseated with ease TWO- STAGE TECHNIQUE Some variation for 2 stage tech. to avoid making modifications after the 1st step: Polyethylene sheet could be used as a spacer to avoid trimming Or the first impression could be taken prior to preparing the teeth which will leave space for the light body once the teeth are prepared TWO STAGE TECHNIQUE The first impression would act as a special tray for the light body A mark can be placed on the tray to help reseating The tray is dried thoroughly before the light putty impression Retraction cord is removed The field is dried and isolated TWO STAGE TECHNIQUE Taking the second impression Light body could be mixed with spatula and loaded into a syringe or it comes in automixing devices which gives homogenous bubble free mix Light body is injected over the prepared teeth and the remaining material into the indentations of all teeth in the tray  TWO STAGE TECHNIQUE Gently seat the tray intraorally Apply little pressure to make sure it goes back to the original position Hold tray still without movements to avoid distortion Impression is removed from the mouth by applying pressure on the border of the tray away from the teeth on one side then the other This should be done very rapidly to avoid tearing and distortion of the material Inspect the impression for accuracy.  ONE STAGE TECHNIQUE Both heavy and light body materials are placed in the tray at the same time Can be used in stock tray and custom-made tray Retraction cord is removed just before taking the impression as single shot Less time consuming than the two-stage technique This photo shows part of the prepared tooth is captured by the heavy putty ONE STAGE TECHNIQUE STEPS Tray selection & applying the tray adhesive Mix both heavy and light body material with the help of an assistant at same time Inject the light body over the prepared teeth while the assistant is loading the tray with the heavy body Gently seat the tray into the mouth and hold still until it sets Inspect the impression for accuracy MONOPHASE TECHNIQUE One material is used (medium body) Impression is taken as one step where the same material is injected around the teeth and used to load the tray Custom made tray is used Impression is seated and held until fully set After removal the impression is inspected for accuracy  FEATURES OF ACCEPTABLE IMPRESSION 1. Finish line is reproduced continuously with no interferences from the gingiva 2. No tears, voids or bubbles 3. At least 1 mm of uninterrupted cuff (black arrow) of impression material should be present beyond finish line 4. No streaks of material indicating improper mixing 5. No separation between the impression and the tray 6. No exposure of the tray surface ERRORS (2 stage technique) The light body didn’t cover the whole impression in addition to improper seating which resulted in a step which will be reproduced in the master model which will result in inaccurate occlusion of final prosthesis  ERRORS (1 stage technique) Improper blending of the putty and wash resulting in horizontal crease lines due to difference of viscosity between heavy and light putty OR because the wash has set on the prepared teeth before inserting the loaded tray which will cause folding of the impression creating horizontal crease lines  Shinny mesial margin of Second attempt after applying preparation due to poor moisture double retraction cord, the control finish line was captured properly ERRORS Improper injection of the light body which creates voids Make sure to extrude a small amount outside before starting to inject to remove any air bubbles in the tip  Impression tearing due to inadequate retraction or removing the tray before the material sets completely ERRORS Drag marks commonly occur near undercut areas, happen due to lack of fluidity of the material either because high temperature or delay in inserting the tray. We can avoid that be refrigerate the impression material to improve the flow ERRORS Tray separation due to inadequate application of tray adhesive, or using too much force while removing the tray from the patient mouth.  Repeat the impression DISINFECTION Proper disinfection must be done to prevent cross infection All dental impressions must be rinsed under running water to remove saliva, blood, and debris Then immersed in the appropriate disinfectant for the recommended time period by the manufacturer Spray disinfectants are not recommended, since they weaken the effect of the disinfectant, and create an inhalation risk A final rinse is done before sending the impression to the dental laboratory.  DISINFECTION Alginate and polyether impressions are dipped in the solution then covered with damp gauze instead of immersion to avoid distortion of the impression Elastomeric impression can be immersed in the solution safely for the recommended time Many disinfectant materials are available commercially such as chlorine, chlorhexidine, glutaraldehyde, and phenolic compounds REFERENCE Contemporary Fixed Prosthodontics book by Rosentiel, Land and Fujimoto ch14