Impression materials By 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. Recognize setting reaction of impression materials. 3. 4. Recognize types of hydrophilic impression materials 5. Recognize types of plastic impression materials. 6. Recognize composition, types of alginate impressions and their properties and manipulation. 7. Describe the composition, types, manipulation and properties of impression compound. 8. Recognize types of materials used for pouring impression for study casts. • Impression material: the substance that is used to take an impression of the patient’s oral structure. 4 • 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 the impression material. *Types: 1. Stock tray >> for 1ry imp. >> metallic or Metallic stock tray plastic 2. Special (custom) tray >> for 2ry imp. >>Shellac, acrylic, compound. Acrylic special tray Plastic stock tray 6 Purpose of taking impression: 1. To study alignment of teeth 2. Treatment planning 3. Fabrication of indirect restorations 4. Construction of special (custom) tray 7 Ideal requirements of impression material 1. Accuracy (dimensionally accurate) = ability to record fine details of oral structures (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. 8 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) 9 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. 10 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. 11 Classification of impression materials I. According to elasticity after setting 12 Plaster Non-elastic (rigid) Compound Impression Materials ZnO - Eugenol Aqueous Hydrocolloids Elastic Agar-Agar (reversible) Alginate (irreversible) Polysulfide Non-aqueous Elastomers (rubber base) Condensation Silicones Polyether 13 Additional • What is the importance of classifying impression materials according to their elasticity after setting? Rigid >>>> edentulous patient without undercut Elastic >>>>>dentulous or edentulous patient 14 Another classification: According to setting reaction Physical (reversible) Impression compound Agar- Agar Chemical (irreversible) Zinc Oxide Eugenol Alginate 15 Rubber base 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 16 * 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 17 • Types and uses: Type I: “Lower fusing compound” (<70°C): a. Sheet form for recording primary imp. of edentulous pts. without undercut…. Why? b. Stick form for border tracing of special tray. Type II: “Higher fusing compound” (>70°C): Used as tray material for wash impression technique. 18 • Setting reaction: Thermoplastic physical reversible reaction Hard (Set) 45-55° C 37° C Soft (unset) 19 • Manipulation (for 1ry 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). 20 1. Impression compound 21 Manipulation (continued) 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. 22 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. 23 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. * Can be copper plated. 24 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) I. (Agar-Agar) Gel Heating Types of hydrocolloids • Sol II. (Alginate) Chemical reaction Gel 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 (scoop and plastic measuring cylinders). B- Small sealed packets/ individually sealed pouches → Modified forms • Two pastes system Composition 1. Potassium (or sodium) alginate 15 % • Soluble salt of alginic acid. • 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 occurs. 4. Potassium sulfate 3 % • It is accelerator for the setting time of the gypsum products used in pouring models 5. Fillers As zinc oxide or diatomaceous earth 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 depleted , 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 (bulk container or reweighed package) • Mixing rubber bowel • Metal spatula • Perforated or rim locked stock tray ІІ- Proportioning and mixing - Proportioning of powder/water manufacturer instructions. ratio acc. to • 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. • Avoid movement of the tray during gelation. 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 water then pouring. Immersion technique • An alternative disinfection method is by immersion for 10 minutes. VI. Impression pouring without need for separating medium Alginate impression 40 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. 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 • Inexpensive • Easy to use • Hydrophilic. So, good wetting for oral tissues • Stock trays with no need for special tray Disadvantages • Dimensionally unstable • • • • - Immediate pour - Single cast Lower detail reproduction Difficult to disinfect Can not be electroplated 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. Sakaguchi RL, Power JM. Craig's Restorative Dental Materials. Mosby CO., St. Louis; 12th ed. (2006) and 13thed. (2012).