Impression Materials & Techniques PDF

Impression materials & Impression techniques Dr. Hanaa Nassar Lecturer of fixed prosthodontics Definition: An impression reproduces the negative form of the prepared tooth or teeth and their relation to the surrounding oral structures. Requirements of an i...

Impression materials & Impression techniques Dr. Hanaa Nassar Lecturer of fixed prosthodontics Definition: An impression reproduces the negative form of the prepared tooth or teeth and their relation to the surrounding oral structures. Requirements of an ideal impression material: 1- Fluid or plastic when inserted in the patient mouth. 2- Accurate to record fine details of the prepared tooth. 3- No distortion upon removal from undercuts. 4- Easy to manipulate. 5- Reasonable working time. 6- Reasonable setting time. 7- Compatible with die and stone material. 8- Acceptable for the patient. 9- Non toxic and non irritant. 10- Reasonable cost. Classification of impression materials: according to its behavior after setting: 1- Inelastic Impression Materials: a- Plaster impression. b-Impression compound c- Zinc oxide and eugenol d- Some waxes  2- Elastic impression materials Hydrocolloids Rubber bases Polysulphide Reversible : Agar- Ag Polyether Irreversible : Alginate Silicone rubber -Condensation - Addition 1- Impression plaster: Presentation : Powder to be mixed with water. Uses : as an accurate template for soldering Advantages: - Has high flow so can register fine details. - Easily assembled when fractured. 2- Impression compound Presentation : Thermoplastic material presented in the form of cakes, sticks or sheets. Uses : In fixed prosthodontics , used in single tooth (copper band ) impression technique. Advantages : It does not need gingival retraction as the copper band causes tissue displacement 3- Zinc oxide eugenol Presentation : In the form of pastes. Uses : In bite registration in conjunction with bite registration frame 4- Waxes Uses in fixed prosthodontics: - Blue inlay wax: used for wax pattern construction - Sticky wax : used during soldering. - Modeling wax: used as a bite registration material. 2- Elastic impression materials 1- Hydrocolloids: Colloids are suspensions of groups of molecules, their particle size is between the small particle size of molecules of true solutions and the larger size of suspension. When the dispersing medium is water it is called a hydrocolloid. )A ( Reversible hydrocolloid Composition: 8-15% by weight Agar 80% water Presentation : It is presented in the form of a semisolid packaged in polyethylene tube. Setting reaction : SOL by heating GEL  Advantages : - It does not require mixing. - It is accurate. - It is hydrophilic Disadvantages: Low tear strength - It needs special conditioning equipment: 1- The liquefying bath: Boiling water (100C) for 10 min. 2- The storage bath: the material stored at ( 63- 66 C) and it can be stored for 5 days. 3- The tempering bath: where the material is tempered for a temperature suitable for the patient mouth after being loaded in a special tray with a circulating tube for cooling. Loading of the tray with the heavy body followed by the light body The tray inside the tempering device The teeth is sprayed by water or surfactant The cooling system is attached to the tray The final impression )B ( Irreversible hydrocolloid ( Alginate) Presentation : It comes in bags or jars to be mixed with water. Uses : Study model. Impression of opposing arch. Advantage : -Easy to use - Low cost Disadvantages : - Low tear strength. - Possibility of syneresis or imbibition - Low flow. 2- Elastrometric impression materials: Classification of rubber bases: a- According to chemistry 1- Polysulfide 2- Condensation Silicone 3- Addition Silicone 4- Polyether b- According to viscosity - Very high viscosity ( putty) - High - Medium - Low ( light) Putty materials contain high amounts of fillers (75%) therefore they are more dimensionally stable than light body consistency It is recommended to use light body in thin sections (2-4 mm) to minimize their polymerization shrinkage Polysulfide Rubber Composition and setting reaction: Base :80% polysulfide liquid polymer with terminal mercaptan group (SH) with 20% silica as filler. Accelerator : paste containing lead dioxide accelerator. Polysulfide liquid polymer + Lead dioxide Polysulfide rubber + H2O Presentation : two pastes and all consistencies ( heavy, medium and light ) except putty Advantages: 1- High tear strength. 2- Improved elastic behavior (Facilitate the intra sulcular impression). Disadvantages: 1- Prolonged setting time (10-15 min.). 2- Bad sulfur odour and can stain patient cloths. 3- Dimensionally unstable due to loss of water. 4- Hydrophobic. Silicone Rubber Condensation silicone Composition and setting reaction: Base :80% silicone rubber liquid polymer with terminal OH group with 20% silica as filler. Accelerator : paste containing Tin Octoate accelerator. Siloxane liquid polymer with terminal (oH) + Tin Octoate ( condensation polymerization) silicone rubber + alcohol  Advantages: 1- available in in all consistencies Disadvantages: 1- Dimensionally unstable due to the loss of the reaction by-product, so the impression must be poured immediately.  Advantages: 1- available in in all consistencies Disadvantages: 1- Dimensionally unstable due to the loss of the reaction by-product, so the impression must be poured immediately. Addition silicone Composition and setting reaction: Base :80% polyvinyl silicone rubber liquid polymer with terminal Vinyl group with 20% silica as filler. Accelerator : paste containing Chloroplatinc acid accelerator. Polyvinyl Siloxanes liquid polymer with terminal vinyl group + Chloroplatinc acid ( addition polymerization) silicone rubber + Hydrogen gas In products evolving hydrogen pouring of the impression was delayed for 24 hours but new products containing palladium to absorb hydrogen so it can be poured within 15 min. also hydrophilic brands are available Presentation : It is presented in two paste tubes Advantages: 1- Excellent dimensional stability as no by- product. so delayed pouring up to 7 days is possible and multiple pouring is accepted. 2- Hydrophilic types are available. Disadvantages: 1- Sulfur in latex gloves inhibits polymerization. 2- Moderate stiffness. Polyether Rubber Composition and setting reaction: Base :80% polyether liquid polymer with terminal imine group with 20% silica as filler. Accelerator : paste containing Sulfonate ester. Polyether liquid polymer + Sulfonic acid ester (addition polymerization) Polyether rubber Presentation : two pastes and all consistencies ( heavy, medium and light ) except putty Advantages: 1- Excellent dimensional stability as no by- product. 2- Short working time. Disadvantages: 1- Allergy. 2- High stiffness so it is difficult to remove from the patient mouth. 3- Can absorb water. Impression trays: Requirements of impression trays: 1- It must be rigid and does not flex to avoid impression distortion. 2- It should be dimensionally stable and provide an even space for impression. 3- It should extend to include all prepared teeth and adjacent teeth. 4- It should provide retention for the impression materials ( either through perforations or adhesive Types of impression trays : 1- Stock trays: These are either metallic or plastic trays and usually perforated and supplied either for full arch or sectional arch impression. 2- Personalized special trays: Usually it is constructed from acrylic resin. It provide a thin equal section for the impression material Impression techniques  Impression techniques According to the extension of the impression : 1- Single tooth impression technique. 2- sectional arch impression technique. 3- full arch impression technique  Impression techniques According to the technique of impression making: 1. Single tooth impression technique. 2. One step ( monophase) impression. 3. Double mix two step technique. 4. Double mix one step technique. 5. Digital impression Single tooth impression using copper band technique Double mix two step technique Double mix single step technique Medium body / light body double impression technique Auto mixing tip attached to the syringe of the impression material Machine mixing Open bite registration technique Open bite registration technique Some properties of clinical significance: 1- Wetability : Ease of pouring with gypsum varies among the different impression materials. Irreversible hydrocolloids , reversible hydrocolloids and polyether are the easiest to pour (hydrophilic). Polysulfide, polyvinyl siloxanes and condensation silicone are the most hydrophobic and more difficult to pour in ascending order this property not only affect the wetting ability by the die materials but also these materials are more repelled by hemorrhage or other liquids in the sulcus. Surfactant may be used to enhance casts pouring 2- Viscosity : The viscosities of the impression materials vary with the type of impression material. Light body polysulfide and condensation silicone are the least viscous and the heavy body polysulfide is the most viscous. Viscosity of these materials increase as the time elapse after the start of mixing. The viscosity of these materials decrease as the shear rate increases ( the speed at which a liquid flows under external force ) as when the materials are expressed through a syringe and this called shear thinning and the material is called thixotropic. Impression for cast metal post and core  Impression Errors 1. Inadequate margin 2. Voids (Bubbles) 3. Margin tear 4. Pulls and drags 5. Separation from the Tray 6. Impression contamination  1. Inadequate margin Problem: inadequate margin details, voids at margin Cause: fluid contamination - improper gingival retraction How to avoid: use advanced retraction materials (ex. Expasyl™) X √  2. Voids (Bubbles) Problem: internal voids (walls and line angles) Cause: fluid contamination, air entrapment How to avoid: proper air dryness of teeth, bleed the syringe of light body before injection  3. Margin tear Problem: inadequate tear of impression at margin Cause: inadequate tear strength of impression material, removal of impression before setting How to avoid: use advanced retraction materials (ex. Expasyl™)  4. Pulls and drags Problem: pulls & drags = folds in impression Cause: -using ↑↑viscosity impression material (heavy, putty). - Improper tray seating (improper pressure) - material is partially set before seating. How to avoid: -using ↓viscosity material (light, medium). -proper tray seating  5. Separation from the Tray Problem: impression separated from tray Cause: no tray adhesive How to avoid: - apply tray adhesive. - Create Mechanical retention (perforations)  6. Impression contamination Problem: contamination → impression material not set Cause: latex (gloves), provisional material (O2 inhibited layer) How to avoid: - use powder free gloves - Clean teeth before impression Digital Impression Thank You