Impression Materials PDF

 The set impression is a negative reproduction of the oral tissues. To obtain a positive reproduction; the set impression is poured by a suitable model or die material.  The impression material is introduced to the mouth in an unset (plastic) condition and is maintained in positio...

 The set impression is a negative reproduction of the oral tissues. To obtain a positive reproduction; the set impression is poured by a suitable model or die material.  The impression material is introduced to the mouth in an unset (plastic) condition and is maintained in position against the oral tissues until it set, and then it is removed from the mouth. 1. Adequate shelf life 2. Acceptable odor and taste. 3. Biologically acceptable: No toxic or irritant constituents 4. Easily manipulated: Easy to use with the minimum of equipment 5. Setting characteristics that meet clinical requirements:  Suitable working time.  Suitable setting time.  Readily wets oral tissues.  Not affected by the oral fluids. Does not release gas during setting. 6. Properties of the set material:  Sufficient mechanical strength so that it will not break or tear on removal from the mouth.  Elastic properties with no permanent deformation after strain.  Good dimensional accuracy.  Dimensional stability over temperature and humidity ranges met in clinic and lab.  “No impression material fulfills all the requirements” The selection of the material best suited for a particular clinical situation and technique rests with the dentist 1- Method of setting: a. Hardening by chemical reaction: (thermoset) e.g. Plaster of Paris, Alginates, and Rubber impression materials.(irreversible) b. Softening and hardening by heat: (thermoplastic) These are reversible substances. 2- Behavior after setting: a. Non-elastic impression materials: They are used only when no undercuts exist. e.g. Plaster of Paris, and Zinc oxide Eugenol. b. Elastic impression materials: They can be used when undercuts exist.e.g. Hydrocolloids, and Rubber impression materials.  Waxes Plaster Non-Elastic Zno-Eugenol Compound Impression Alginate Material Hydrocolloid Agar Agar/Alginate Elastic Polysulfide Condensation Rubber base Silicone (Elastomers) Addition Polyether A. Wax B. Impression Plaster (Plaster of Paris) C. Impression compound D. Zinc oxide-Eugenol impression material Definitions:  Solution  Suspension  Colloid Reduction in Chemical temperature Reaction Agar Alginate impression impression material material  Dimensional effects by Syneresis Imbibition Loss of water Absorption of water Shrinkage Expansion 1. Alginate Advantages: 1. Easy of mixing and manipulation. 2. Minimum equipment. 3. Flexible. 4. Accuracy. 5. Low cost. Disadvantages: 1. Restrict the model & die material to only gypsum. 2. Not accurate in reproducing fine details.  Uses Preparing Preparing gypsum study cast models Removable Athletic partial mouth denture protector NOTE: Not recommended for impression of cavity preparations 3. Storage: It should be stored in 100% relative humidity to avoid syneresis/imbibitions and thus avoid shrinkage/expansion. Manipulation:  Equipment needed: a. A powder-dispensing cup. b. A water-dispensing cup. c. A rubber mixing bowel. d. A spatula with 2. Agar  Excellent Advantages Highly accurate Elastic  Difficult manipulation Dimensional unstable Disadvantages Patient discomfort of thermal shock 3 chamber conditioning unit Definition: Non aqueous elastomeric impression materials, that are rubber-like, and are made-up from long chain coiled molecules. Main advantage of elastomers are:  High accuracy.  More dimensional stability than Agar & alginate.  Ease of manipulation.  Can be used with undercuts.  Can be electroplated. Consistencies: 1. Low (Syringe/wash). 2. Medium (Regular). 3. High (Tray). 4. Putty (Extra high). Mixing systems: 3 types of mixing systems are available:  Hand mixing  Auto-mixing  Dynamic mechanical mixing Impression techniques:  Simultaneous dual viscosity technique.  Single viscosity (mono-phase) technique.  Putty wash technique. Types of Elastomeric impression materials: A. Polysulfides. B. Silicone Condensation. Addition. C. Polyether. A. Polysulfides B. Silicone: a. Addition b. Condensation C. Polyether 1-Polysulfide rubber impression material (Cont.) C. Properties:  Working time: Ranges from 5-7 minutes.  Final setting time: 8-12 minutes.  Permanent deformation: 2-3%.  Dimensional accuracy: Highly accurate.  Dimensional stability: Polymerization shrinkage of 0.25% takes place within the 1st 24 hours.  Flow: An average flow of 0.3-0.9% occurs within the 1st 15 mintues.  Flexibility: Most flexible of all Rubbers.  Elasticity: Viscoelastic with elastic recovery of 98%.  Compatibility with die materials: Compatible.  Tear strength: Has the highest resistance to tear.  Can be disinfected by most disinfectants. 1-Polysulfide rubber impression material (Cont.) D) Advantages:  High dimensional stability.  High strength & resistance to tear.  Can be electroplated. E) Disadvantages:  Dimensional instability (water by-product).  Hydrophobic.  Bad odor (Sulfide odor).  Staining of clothes.  Relatively long setting time.  Condensation silicone: Solve many problems of polysulphides Advantages: 1-moderate working time 2-better odour and taste 3-less permnant deformation Disadvantages: 1-hydrophobic 2-dimensional instability (alcohol by-product) B. Addation Silicone (Cont). C. Properties: Accuracy: Highly accurate. Dimensional stability: Addition is more stable. Flow: Is low. Flexibility: Less flexible than Polysulfides. Elasticity: More elastic than Polysulfides (elastic recovery is 99.5%). Compatibility with die materials: Compatible with most types. Tear strength: Relatively low, but rapid rate of removal gives high tear strength. Can be electroplated. Can be disinfected by most disinfectants.  Advantages:  1-High accuracy  2-Flexibility  3-can be electroplated  Disadvantages:  1-hydrophobic  2-senstivity to latex gloves  3-release of hydrogen gas N.B:Used with auto mixing 3. Polyether impression material (Cont). C. Properties :  Accuracy: Highly accurate.  Dimensional stability: Polymerization shrinkage of 0.25% within 24 hours.  Flexibility: Least flexibility of all rubbers.  Elasticity: Viscoelastic with elastic recovery 98.9%.  Compatibility with die materials: Excellent  Tear strength: Better than Silicone, less than Polysulfide.  Can be disinfected by various antimicrobials. 3. Polyether impression material (Cont). D) Advantages:  Better mechanical properties than polysulfides.  Less dimensional changes than condensation Silicone. E) Disadvantages:  Increased working time.  High stiffness.  High cost. Optical impression Advantages:  No need for impression materials or trays.  Impression can be transmitted & stored electronically.  No mess.  3D, 360˚ view of preparation that can be moved & turned to every angle.  Adjustments can be made to the virtual impression, no need for second impression.  No disinfection required. Disadvantages: Start up cost. Technique sensitive. More equipment required.