Assalamu-alaikum DR. TAJAMMUL AHMED a.tajammul@gmail.com

IMPRESSIONS & TECHNIQUES IN FIXED PARTIAL DENTURE

Impression GPT-8 Impression : a negative likeness or copy in reverse of the surface of an object; an imprint of the teeth and adjacent structures for use in dentistry.

Considerations for Precision Impression Taking Before Impression Making (Extra-oral): - Easy to mix - Long working time During Impression Making (Intra-oral): - Hydrophilic - Ability to flow around preparation without slumping - Detail-capturing - Snap set - Good taste & odor

Upon Removal from Mouth: - Easy to remove - High tear strength - Good recovery from deformation After Impression Making (Storage): - Dimensional stability - Easy to read - Can be disinfected

During Pouring: - Compatibility with gypsum - Reproduction of details (ADA Specification 19: 20 microns) - Ability for multiple pouring

CLASSIFICATION OF IMPRESSION MATERIAL Nonelastic Elastic Hydrocolloids Synthetic elastomers Waxes, gums, resins Polysulphides Plaster of paris Agar-Agar Condensation-silicone (Type I silicone) Zinc oxide eugenol Alginate Impression compound Polyether Addition silicone (Type II silicone) Eugenol-free paste

ELASTOMERIC IMPRESSION MATERIALS Depending on viscosity Light body Medium body / regular body Heavy body Putty

Reversible hydrocolloid (Agar-Agar) : Introduced by Sears 1937 First elastic impression material Longest continuous use Available as : Tray material Syringe material

Tubes and cartridges of hydrocolloid placed in liquefying bath Hydrocolloid conditioning with three baths : liquefying, storage and tempering Tubes and cartridges of hydrocolloid placed in liquefying bath Tubes and cartridges of hydrocolloid transferred from liquefying bath to storage bath Tray filled with impression material placed in tempering bath

Cartridge of liquefied hydrocolloid is removed from storage bath Cartridge of impression material is loaded into syringe Blunt needle is attached to impression syringe

Irreversible hydrocolloid (Alginate) : During World War II Salt of alginic acid (anhydro d mannuronic acid) Available as : Dust free alginate Chromatic indicator

ELASTOMERIC IMPRESSION MATERIALS Accuracy / Detail capture : ADA No. 19 fine detail of 25m PVS impression material  best Reversible hydrocolloid  least Reproduction of fine detail of gypsum die materials  50 m Lower the viscosity  Increased fine detail Putty material  reported fine detail of 75m

Point of consideration  Interproximal, subgingival areas Tear strength: Measures the resistance to fracture of an elastomeric material subjected to tensile force acting perpendicular to a surface flaw. Point of consideration  Interproximal, subgingival areas  Slot and groove Polymerized material in the gingival crevice Torn material during removal

Hydrophilic group Hydrocolloids Polyether Hydrophobic group Polysuphide C – silicone A – silicone (PVS) Works in wet environment (blood and saliva) Readily wettable by the gypsum Resistant to wetting (repelled by hemorrhage or tissue fluids) Not readily wettable by the gypsum

Limited to small isolated areas CONTAMINATION Inhibition of polymerization of PVS impression material. Direct inhibition Direct contact with latex gloves Impression are made with rubber dam in place Mechanism of inhibition  Element sulphur react with chloroplatinic acid catalyst present in PVS impression material. Indirect inhibition Indirect contact of tooth and periodontal tissues during tooth preparation and gingival displacement procedure. Limited to small isolated areas

Unpolymerized material remain adherent to prepared tooth surface Contaminated surface will be slippery and sticky to touch Clinical consideration : Use of non latex gloves – Vinyl gloves – Polyethene gloves Avoid touching the tooth preparation and gingival areas Avoid handling retraction cord with gloved hands.

TRAY SELECTION AND PREPARATION OF THE TRAY Tray should be rigid. Dimensionally stable. Resist the deformation (loading heavy body material). Provides the space for optimal thickness. Plastic vs metal stock trays

Tray should be 2-3mm thick to ensure sufficient rigidity. Incorporation of stops Guide the clinician for positive seating Three widely spaced stops Non functional cusp of unprepared teeth, edentulous areas or the palate.

Tin foil or aluminium foil should be used over the wax spacer to prevent decrease in adhesive bond

Adhesion of the impression material to the tray For accurate impression  proper adherence to the tray Tray adhesive composition – poly dimethyl siloxane and ethyl silicate.

Adequate mixing and loading : Earlier materials  base and catalyst tubes Present materials  cartridges with mixing tips  Gun like dispenser Automixing system : Lesser inherent voids Extend the essential working time Reduced waste Uniformity in proportioning

ELECTRIC MACHINE DEVICE (Dynamic Mechanical Mixer) Provides uniform mix with touch of a button Far fewer voids in the set elastomers

IMPRESSION TECHNIQUES

AGAR IMPRESSION TECHNIQUE Armamentarium : Tray and syringe material Water cooled trays Conditioning unit Procedure : Tray material Syringe material Loaded tray in tempering bath

Dentate arch is flooded with water Water cooling tubes connected to seated tray Completed impression Disadvantages – Dimensionally unstable / distortion during gelation Initial expenditure for instruments Multiple pouring is not possible

ALGINATE IMPRESSION TECHNIQUE Clinical interest : For the study cast / diagnostic cast – diagnostic wax up – mock preparations For interim restoration (temporization) Clinical considerations Tray selection Perforated metal / rigid plastic trays Tray adhesive Perforations – insufficient To prevent detachment of impression, Extend 2-3 mm outer surface

Loading the tray Making the impression Sectional placement Wet gloved finger Making the impression

HYDROCOLLOID LAMINATE TECHNIQUE ALGINATE – AGAR METHOD Prepared tooth surfaces Injection of reversible hydrocolloid Stock tray with chilled alginate Combined alginate –agar impression Master cast

Critical to success  bond between agar and alginate Advantages : Satisfactory surface reproduction Good wettability (smooth, dense model) Simple conditioning (no tempering bath / water cooled trays) More economical Disadvantages : The bond between agar and alginate is not sound Higher viscosity alginate displaces agar hydrocolloid

PUTTY WASH IMPRESSION TECHNIQUES (Stock tray impression technique) One stage or one step putty wash impression (Simultaneous technique / squash technique) In this technique putty and wash are recorded simultaneously Disadvantage : Difficult to control thickness of wash material Higher viscosity material displaces the wash material - Critical areas captured in putty rather than light body Setting distortion of the putty add to over all distortion of impression

TWO STAGE OR TWO STEP PUTTY WASH TECHNIQUE Two step unspaced impression technique Preoperative putty impression is recorded Preset putty impression is relined with thin layer of wash Two step spaced impression technique Preoperative putty impression is made Space for the light body wash material is provided to reline the putty

DUAL PHASE IMPRESSION TECHNIQUE (Multiple mix technique, laminate single impression technique, custom tray impression technique) Two consistencies Light body syringe material (wash type) Heavy body – tray material Tray material will force the light body material into the gingival sulci and preparation detail Custom tray with 2–3mm space

ADVANTAGES Uniform thickness of impression material (2-3 mm) Viscosities of 2 materials reduces drags and improves the blending of 2 phases. Good marginal definition. Less impression material is required. DISADVANTAGES Construction of custom tray (time / cost) Differential shrinkage

Monophase impression technique (Single viscosity technique) Medium viscosity – polyether or addition silicone Used as both tray material and syringe material Custom tray with 3 mm spacer Depends on pseudoplastic property DISADVANTAGES Reduced ability to flow into intra coronal features and gingival crevice Greater amount of polymerization shrinkage compared to heavy body Surface reproduction may not be as good as light body material

Copper band or tube method The copper band is used to salvage an impression of multiple preparation where there is only vague margins on one or two preparation that are not adequately replicated in the impression. Copper band with orentation holes Copper band contoured Evaluating the fit of the copper band Copper band with the final impression

Close bite double arch method Close bite double arch method . synonyms: dual quad tray, double arch ,triple arch, accu-bite, closed mouth impression. Advantages The physical deformation of the mandible is minimal The shifting of teeth occurring during maximum intercuspation is captured. Less impression material is needed. Less gagging may occur. Disadvantages The tray is not rigid . Limited to one casting per quadrant . The distribution of impression material is not uniform

Close bite double arch method Close bite double arch method . synonyms: dual quad tray, double arch ,triple arch, accu-bite, closed mouth impression. Technique Evaluate the fit of the tray in the patients mouth & observe the complete bilateral closure and the patients comfort. The high viscosity material is used in the tray & then the low viscosity material is syringed over the high viscosity in the tray & positioned on the arch. Instruct the patient to close the mouth and observe for the interdigitation on the opposite arch. Wait for the material to set . Instruct the patient to open the mouth & remove it by applying equal pressure bilaterally. Evaluate the impression.

Closed mouth impression Close bite double arch method . synonyms: dual quad tray, double arch ,triple arch, accu-bite, closed mouth impression. Closed mouth impression

FLAWS RELATED TO IMPRESSION TECHNIQUE Visible flaws : Finish line not visible  Inadequate gingival displacement Gingival inflammation and bleeding Crevicular fluid and heamorrhage Poor moisture control Dry field

Air bubbles / voids in critical areas Mixing Syringing Keep syringe tip in expressed material Start syringing from distal interproximal area (distoligual) Fill the base of grooves, boxes first Tray loading and tray seating

Drags Reason : Poor flow of putty Premature syringe of impression material Poor bond between the tray and syringe material Unset / unpolymerized impression material  contamination INVISIBLE FLAWS Tray and impression recoil Permanent deformation Detachment of impression from the trays

DISINFECTIONS OF IMPRESSIONS To prevent exposure and cross infection Disinfection before poring the cast. Disinfection methods : Spray-on disinfectants Immersion of impression in chemical disinfectant (Submerged / soaking) Types of chemical disinfectant used : Chlorine compounds Synthetic phenolic compounds Glutaraldehyde Idophores Combination of phenols and alcohols Hydrophilic impression material  hydrocolloids, polyether should be sprayed rather than immersion. Idophore sprays, sodium hypochlorite sprays (1:10 ratio). Agar (reversible hydrocolloid)  10 min immersion in 2% alakaline glutaraldehyde  more than 20 min affects the dimensional stability and surface detail. Alginate (irreversible hydrocolloid)  Westerholm et al (1992) full strength (5.25%) sodium hypochlorite spray. Polysulphide and condensation silicone immersion in sodium hypochlorite, glutaraldehyde 10-30min.

ADA recommends the use of 1: 2.13 dilution of iodophor If an impression has not been disinfected, or if the laboratory has no assurance that an appropriate disinfection protocol was followed, it is necessary to disinfect the stone cast. ADA recommends the use of 1: 2.13 dilution of iodophor 1: 10 dilution of sodium hypochlorite Stone cast be disinfected by Spraying until wet Immersing for the time recommended for tuberculocidal disinfection. Use of disinfecting solutions, spray disinfectants (containing iodophor or hypochlorite) Disinfectant containing dental stone When a patient with known cases of infections are being treated, overnight gas sterilization is an option Cast to be disinfected should be fully set (i.e. stored for atleast 24 hours). A disinfectant stone is marketed now and has been shown to have bactericidal properties. However, this product has not yet EPA registered as a disinfectant.

CaviCide A broad-spectrum tuberculocidal disinfectant decontaminant cleaner that effectively kills TB (in 5 minutes) and HBV (in 3 minutes). CaviWipes Durable, non-woven, nonabrasive towels pre-saturated with CaviCide.

Conclusion

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