Primary Impression Materials PDF

Primary Impression Materials Dr. Neven Aref Assistant Professor of Dental Biomaterials, Faculty of Dentistry, Qassim University 1. Define the impression. 2. Classify impression materials and indication of each. 3. Recognize setting reaction of impres...

Primary Impression Materials Dr. Neven Aref Assistant Professor of Dental Biomaterials, Faculty of Dentistry, Qassim University 1. Define the impression. 2. Classify impression materials and indication of each. 3. Recognize setting reaction of impression materials. 4. Recognize types of hydrophilic impression materials 5. Recognize types of plastic impression materials. 6. Recognize composition, types of alginate impression, manipulation and properties. 7. Describe the composition, types, manipulation and properties of impression compound. 8. Recognize types of materials used for pouring impression study casts.  Impression material: the substance that is used to take an impression of the patient’s oral structure.  Impression: Negative replica of the oral hard (teeth) or soft (gingiva, mucous membrane) structures. (Types: Primary & Secondary) Cast (model): positive replica of the oral hard (teeth) or soft (gingiva, mucous membrane) structures. 5  Tray: An instrument used to carry and control the impression material. Metallic stock tray Acrylic special tray Plastic stock tray 6 Functions of the impression tray: To support the impression material in a planned contact with oral tissues. To support the impression material when removed from the mouth so that a cast can be poured Types of the tray according to the tray material 1. Metallic trays Aluminum or stainless steel 2. Non-metallic trays Plastic, acrylic resin, shellac or compound. Types according to the impression material: 1. Stock tray>> for primary impression >> metallic or plastic a. Perforated stock tray: i. Perforated stock tray without rim lock ii. Perforated stock tray with rim lock (rim lock stock tray). Both are used with alginate impression material ii i b. Non-perforated (plain) stock tray i. Plain stock tray without rim lock used with impression compound. ii. Non perforated stock tray with rim lock used with alginate impression 2. Special (custom) tray >> for secondary (Final) impression >>Shellac, acrylic, compound. Purpose of taking impression 1. To study alignment of teeth 2. Treatment planning 3. Fabrication of indirect restorations 4. Construction of special (custom) tray 12 Ideal requirements of impression material 1. Accuracy (dimensionally accurate) = ability to record fine details and dimensions of the oral structures accurately (by high flow of the unset impression material). 2. Dimensional Stability = ability to retain its dimensional size over time (minimal dimensional changes during and after setting). 3. Should adhere to the tray during removal from the mouth. 13 4. Adequate elasticity on removal from the mouth (after setting), so that undercuts can be recorded without distortion of the impression. (Undercut = area under inclined plane) 14 5. Should be compatible with gypsum products (cast material): a. Not need separating medium, which may occupy space and affect accuracy. b. Not react with the cast material or release by products which affect the model surface. 6. Biologically acceptable: non-toxic, non-irritant and of acceptable taste & odor. 15 7. Easily disinfected without loss of accuracy. 8. Easily manipulated with minimal equipment. 9. Have suitable working and setting time. 10. Have suitable shelf life. 11. Not expensive. 16 Classification of impression materials I. According to elasticity after setting 17  Plaster Non-elastic (rigid) Compound ZnO - Eugenol Impression Materials Agar-Agar (reversible) Aqueous Hydrocolloids Alginate (irreversible) Elastic Polysulfide Condensation Non-aqueous Elastomers Silicones (rubber base) Additional Polyether 18  What is the importance of classifying impression materials according to their elasticity after setting? Rigid >>>> edentulous patient without undercut Elastic >>>>>dentulous or edentulous patient 19  Another classification: According to setting reaction Physical Chemical (reversible) (irreversible) Impression Zinc Oxide Agar- Agar Alginate Rubber base compound Eugenol 20  A. Rigid impression materials 1. Impression Compound ❖ Thermoplastic impression material that is softened by heating & hardened by cooling. ❖ Also known as “modeling compound”. ❖ Forms: sheets & sticks of different colors 21 * Composition: 1. Thermoplastic materials: To give thermoplasticity, flow and cohesion. e.g. natural resins & waxes 2. Fillers: To give body and suitable working consistency. e.g. talc powder 3. Plasticizers: To act as lubricants & control consistency (with fillers) and improve workability. e.g. stearic acid or stearin. 4. Coloring agents: to give characteristic color. e.g. rouge 22  Types and uses: Type I: “Lower fusing compound” (70°C): Used as special tray material for wash impression technique (in which thin wash of ZOE is applied. 23  Setting reaction: Thermoplastic physical reversible reaction 45-55° C Hard Soft (Set) 37° C (unset) 24  Manipulation (Primary impression): 1. Compound is softened by heating over a flame or in a water bath. 2. As the material has low thermal conductivity, it must be immersed and & kneaded under water for sufficient, but not Thermostatically-controlled water bath too long, time to ensure complete softening but without leaching out of its ingredients. 3. Type of tray used Plain stock tray (non-perforated) because it will stick to the tray. 25 26 4.Cooling & removal - Use water spray to cool the impression and to accelerate hardening of the material. - Remove impression from the mouth after complete hardening to avoid its distortion. 5. Sterilization: Can be autoclaved for 10 minutes.  Properties 1. Accuracy Compound is not sufficiently fluid to record fine details. 2. Dimensional stability It has high coefficient of thermal expansion and contraction, so it shows considerable amount of shrinkage on cooling (during hardening & cooling to room temperature). 3. Elasticity the material is non-elastic … so cannot be used in undercut areas.  Properties (continued) 4. Compatible with cast & die materials & does not need separating medium. To separate cast from the impression, use warm water. 5. As the material is reversible, it can be re-used, but after sterilization. 6. Compound impression can be added or corrected. 7. Other properties: * Non-toxic or irritant. * Suitable setting time. * Long shelf life. 29 Hydrocolloid impression materials Colloids: Heterogeneous two phase system like suspension but the particle size of the dispersed phase is smaller (1-200 nm) than that of the suspension. Hydrocolloids Colloids are called hydrocolloids when the dispersion medium is water. Used for dental impression. It exists in the sol and gel state. It is inserted in the mouth in sol state, when it is sufficiently fluid to record details. Then, it is removed after the gel is formed and exhibited elastic properties.  Sol Cooling (physical reaction) Gel I. (Agar-Agar) Heating Types of hydrocolloids Sol Chemical reaction Gel II. (Alginate)  Irreversible hydrocolloid (Alginate) impression It is a viscoelastic impression material which changed from a sol to a gel state through a chemical reaction, so it is irreversible It is indicated as a primary impression that is used for both dentulous and edentulous patients Presentation forms →Traditional powder form packed either in A- bulk form in container with suitable measures for dispensing the correct quantity of powder and water (spoon and plastic measuring cylinders). B- Small sealed packets  Composition 1. Potassium (or sodium) alginate 15 % Main Rx ingredient because it forms sol with water. 2. Calcium sulfate dihydrate ( CaSO4.2H2O ) 16 % It is the reactor of the material, it reacts with the soluble alginate form to produce insoluble form. 3. Sodium phosphate Na3(Po4)2 2% It acts as a retarder, as it allow longer working time where Ca+ react with Po4+ before the cross linking happens. 4. Potassium sulfate 3 % It is accelerator for the setting time of the gypsum products used in pouring models 5. Fillers As zinc oxide which strengthens the gel 6. Flavoring, coloring and disinfecting agents (chlorhexidine) 7. Chemical indicator (present in some products) Changes color with pH change, to indicate different stages in manipulation (chromatic alginate) Setting reaction Gelation reaction The reaction of alginate is chemical, done through several steps When the powder is mixed with water, calcium sulfate is dissociated into calcium ions and sulfate ions CaSo4 + 2H2O →Ca + So4 Sodium phosphate is also dissociated Na4P2O7 → 4Na + P2O7  Then, the Calcium reacts with Phosphate to form insoluble Calcium phosphate. This process continuous until the whole amount of phosphate is exhausted , then the calcium ions start to react with the soluble alginate (Sodium or Potassium alginate) to form insoluble alginate (Calcium alginate) Ca + Sodium alginate → Calcium alginate (final elastic gel) Manipulation І- Equipment Alginate powder Mixing rubber bowel Metal spatula Perforated stock tray or plain stock tray rim locked for retention of the impression on removal. ІІ- Proportioning and mixing - Proportioning of powder/water ratio according to the manufacturer instructions.  Water is inserted into a clean rubber bowel, and then the powder is incorporated into the water by vigorous mixing with a metal spatula against the wall of the bowel. Mixing time is 45 sec-1 min & setting time 1.5-5 min from start of mix. The resulting mix should be creamy and smooth. III. Tray loading and impression taking Avoid movement of the tray during gelatin. Do not clean the mixing bowl immediately. Use the alginate remaining in the bowl as an indicator for setting time. If the alginate in the bowl is set, then the material in the mouth is also set. IV- Impression removal The impression should be removed with a snap movement as this increases the tear strength and elastic recovery. V- Disinfection The disinfection procedure should be relatively rapid to prevent dimensional changes. Disinfectant agents: Iodophors, sodium hypochlorite and glutaraldehyde Technique: Spraying technique The impression is rinsed in water. The disinfectant is sprayed on the surface. The impression is wrapped in a disinfectant soaked paper towel and placed in a sealed plastic bag for 10 minutes.  The impression is removed from the bag, unwrapped, rinsed, and shaken to remove excess disinfectant then pouring. Immersion technique An alternative disinfection method is by immersion for 10 minutes. VI. Impression pouring without need for separating medium Alginate impression 45 Properties of alginate Flexibility Flexible → easy to be removed from undercut with minimum stresses. Tear strength Poor tear strength, so should be removed with snap movement. Elasticity Viscoelastic Elastic recovery: 97.3% Clinical importance The impression should be removed suddenly to reduce the amount of permanent deformation. Compatibility with gypsum Don't need separating medium. Affect the setting and surface properties of the gypsum materials. Causes 1. The water content of the impressions inhibits the setting of the gypsum at the surface and results in soft surface. 2. The hydrocolloids are hydrophilic and so absorb water from the gypsum mix which results in chalky appearance. How to avoid? 1. Immersion of the impression in 2 % K2SO4 before pouring (gypsum accelerator). 2. Incorporation of the gypsum accelerator in the alginate impression material itself, such as potassium titanium fluoride or potassium sulphate. Dimensional stability Dimensionally unstable on storage Causes A. Water loss Occurs either by phenomenon called syneresis (exuding of fluids onto surface) or evaporation. Results in shrinkage. B. Water uptake Occurs by phenomenon called imbibition Results in swelling and expansion. How to reduce dimensional changes? Pouring the impression immediately (within 10 min). If the pouring must be delayed, the impression is then wrapped in a wet towel (saturated with water) and placed in a sealed plastic bag or a humidor (stored in 100% humidity) Advantages of alginate impression material Inexpensive Easy to use Hydrophilic. So, good wetting for oral tissues Stock trays with no need for special tray Disadvantages Dimensionally unstable - Immediate pour or stored in 100 % humidity - Single cast Lower detail reproduction Difficult to disinfect Low tear strength  Materials used for pouring impressions Gypsum products are used in dentistry for preparation of study models. The most common gypsum materials used for pouring study casts are: Dental plaster or model plaster (Type II) Dental stone (Type III) – It has higher strength than Dental plaster. 1. Kenneth J. Anusavice. Philips science of dental materials, Impression materials (Chapter 9); eleventh ed. 2003: Page 207:251.