Impressions in Fixed Prosthodontics PDF

Impressions in Fixed Prosthodontics Dr. Mahinour Ahmed  Accurate impressions are critical in fixed prosthodontics. Dental impressions are essential in: Creating accurate models(casts) Give precise dental restoration. After finishing preparation and starting to take...

Impressions in Fixed Prosthodontics Dr. Mahinour Ahmed  Accurate impressions are critical in fixed prosthodontics. Dental impressions are essential in: Creating accurate models(casts) Give precise dental restoration. After finishing preparation and starting to take impression……  A good impression not only captures the necessary details but also fosters a positive relationship between the dentist and the patient. Positive Experience. Trust Building. Bad Experience  Impression: A negative replica of the teeth and surrounding structures that used to obtain a cast, on which the planned restoration is fabricated. Positive replica (positive copy) A negative imprint of an oral structure used to produce a positive replica of the structure to be used in the production of a dental restoration. Gypsum or stone is poured into the impression to create a cast of the oral structure that is a positive copy of the original structure.  1. Primary (diagnostic) Types of impression in impression. fixed prosthodontics: 2. Secondary impression. 1. Primary (diagnostic) impression. Purpose: Diagnostic Construction of tray. Secondary impression Purpose: Obtain master (definitive) cast.  Tray A device that holds impression material. Types of Impression Trays  1.Stock trays (prefabricated): available in various sizes. Plastic/ aluminium trays Stainless steel trays (non-rigid) cause distortion (Rigid): avoid distortion  Stock trays (prefabricated) Metallic or Plastic. Perforated or Non-perforated. Full or sectional/ inter-arch tray (dual tray). Types of Trays Dual-Arch Trays: Capture both arches simultaneously.  For retention of impression material within the tray use: Perforated trays (to expand through the holes and so be held securely in place). Rim Lock Tray adhesive Types of Trays 2. Custom/ Special Trays: Individualized tray. Made on a diagnostic cast. More accurate adaptation. Custom tray - Fabrication Line is drawn to mark end of spacer. 3-5 mm from gingival margins. 2 layers of base plate wax used. Trimmed until the marked line. Custom tray - Fabrication Wax is removed to form tray stops. Best to make tray stops on non functional cusps Tin foil is adapted on the wax. Custom tray - Fabrication Resin is adapted to the cast. Excess material trimmed. Attach a handle. Trimming of the custom tray. Proper tray selection Ensure tray covers all surfaces of teeth. Tray Selection  Tray Selection Sufficient distance to the tray wall on both sides. Tray Selection  A tray that is too Tray narrow may prevent Selection adequate seating. Impression Materials Requirement for an Ideal Impression Material Biocompatible material. Patient Comfort. Have acceptable taste, odor and appearance. Ease of Use. adequate working and setting time. Can be disinfected without changing properties. Requirement for an Ideal Impression Material Have: Dimensional stability Accuracy and details reproduction. Tear resistance. Wettability.  Dimensional Stability: ability to retain its absolute dimensional size over time. Accuracy: ability to replicate the intraoral surface details. Tear Resistance ability to resist tearing in thin sections (such as the feather-edged material within the gingival sulcus.  ability of the liquid to cover the surface Wettability completely.  Hydrophilic VS Hydrophobic materials Hydrophilic materials have a high affinity for moisture (low contact angle), provide good surface wetting, and allow for a high degree of surface detail. Hydrophobic materials have a low affinity for moisture (high contact angle), provide poor surface wetting, and a lower degree of surface detail. Classification of Impression Materials 1. Non-elastic (Rigid). 2. Elastic. Non-elastic Impression materials 1- Plaster of Paris (type I gypsum). 2- Impression compound. 3- Zinc oxide eugenol impression. 4- Waxes. Not used so much today. Can be brittle and break during removal. Best for edentulous cases (minimal undercuts). Types of Elastomeric Impression Materials Agar Aqueous (reversible) Hydrocolloids Alginate (irreversible). polysulfide Non-aqueous - Condensation Silicone Elastomers - Addition Silicone Polyether Impression Materials Gel Aqueous Hydrocolloids: Agar (reversible) Heating Changes between sol Sol and gel based on temperature. Cooling Gel  Irreversible Hydrocolloid (Alginate) Advantage: Material of choice for diagnostic impression (primary impression). Easy of manipulation Rapid set. Hydrophilic material.  Imbibition: Syneresis: absorption of loss of water. water. Hydrocolloids Properties Factors affecting setting time of Alginate Hot water, cold water, water amount.  Irreversible Hydrocolloid (Alginate) Disadvantage: Poor accuracy and surface details. Low tear resistance. Not suitable for definitive casts. Should pour immediately (within 30 minutes). Limited shelf life.  Modified alginate Alginate modified by in corroborating silicone polymer (paste form) Advantage: Fine details production. Better tear resistance. better dimensional stability. Non-aqueous Elastomers  Polysulfide Brown in color — polymerized aided by lead peroxides. Different viscosity (light - regular - heavy). Supplied as a two-tube paste system (base and catalyst). Polysulfide Advantages: higher tear strength. least expensive elastomer. Disadvantage: unpleasant sulfide odor, Messy, long setting time in the mouth (about 10 minutes). Hydrophobic.  Condensation Silicone Supplied as putty, heavy body, light body. Advantages. Odorless, shorter setting time (6-8 minutes). Adequate tear resistance. Disadvantages. Hydrophobic so the prepared teeth and gingival sulci must be completely dried. Alcohol byproduct causes shrinkage. Should be poured immediately. Polyvinyl siloxane (PVS) Addition Silicone Supplied as (Putty, Heavy body, Regular body, Light body). Automix is available so, less material waste, better mixing and less air bubbles.  Polyvinyl siloxane (PVS) Addition Silicone Disadvantage: NO byproduct. High cost. the most accurate material, Hydrophobic. offering excellent dimensional Material setting can be inhibited by stability and tear resistance. sulfur in latex gloves N.B. vinyl or nitrile gloves can be worn for mixing.  Polyvinyl siloxane (PVS) Addition Silicone (now hydrophilic type is available).  Polyether Hydrophilic (best detail reproduction). no byproduct. Excellent dimensional stability. short setting time in the mouth (about 5 minutes). Automix available.  Polyether Disadvantages: The stiffness ------causes broken of stone cast when separated from the impression. Expensive. Should be stored dry as it absorbs moisture by imbibition. Polyvinyl ether silicone (PVES) A new elastomeric impression material which is a combination of vinyl polysiloxane (VPS) and polyether (PE) elastomers. Hydrophilic the highest accuracy. Great flow. High tear strength flexibility. Clinical steps Laboratory steps Examination and diagnosis, Primary impression Pouring of Diagnostic casts and mounting on (alginate impression) articulator. Sectional tray & index (with putty silicon impression). Tooth preparation abutment A. Provisional / temporary crown restoration for abutment A. Tooth preparation abutment B. Alginate impression. Temporary bridge. Provisional / temporary crown restoration for abutment A, B. Finalization of teeth preparations, gingival Pouring of master cast retraction and secondary impression. Die preparation and die trimming Shade selection. Fabrication of Wax pattern, Spruing, Investing and casting Trimming and Finishing Metal try in Porcelain build-up and glazing Insertion and cementation Thank you