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Impression materials A brief introduction Dr saransh malot
Impression materials Elastic Chemical reactions Irreversible Alginate Elastomers Polysulphides Polyethers Condensation silicon Addition silicon Temperature change Reversible Agar hydrocolloid Chemical reactions Irreversible Plaster of Paris ZnO Eugenol Temperature change Reversible Impression compound Waxes Rigid / Inelastic
ELASTIC Material susceptible to being stretched compressed or distorted and then tending to resume the original shape. Elastic impression materials are capable of accurately reproducing both the hard and the soft structures of the mouth including the undercut areas and interproximal spaces
ADVANTAGE OF ELASTIC IMPRESSION MATERIAL OVER RIGID IMPRESSION MATERIAL
Elastic impression material can be used in both dentulous and edentulous cases with undercuts.
The distortion of elastic impression material when removed from an undercut is minimal.
COLLOIDS A solid, liquid or gaseous substance made up of large molecules or masses of smaller molecules that remain in suspension in a surrounding continuous medium of different matter. TYPES OF COLLOIDS Aerosols----– liquids or solids in air Lysosols ------gas or liquid or solid in liquid. Foams---------gases in solid Solid emulsion – liquids in solid Solid suspension -- solids in solid
HYDROCOLLOID A colloid that contains water as the dispersion phase
GEL A network of fibrils that form a weak slightly elastic brush heap structure of hydrocolloid
SOL-GEL TRANSFORMATION If a hydrocolloid contains an adequate concentration of dispersed phase, a sol under certain conditions, may change to a semisolid material known as gel. In the gel state the dispersed phase agglomerates to form a chain of fibrils called micelles. These fibrils may branch and intermesh to form a brush heap structure. The dispersion medium is held in the interstices between the fibrils by capillary attraction or adhesion .
For agar secondary bonds hold the fibrils together. These bonds break at slightly elevated temperatures and become re-established as the hydrocolloid cools to room temperature. This process is reversible. In case of alginate the fibrils are formed by chemical action and the transformation is not reversible.
GELATION Setting of reversible hydrocolloid THERE IS A PHASE CHANGE FROM SOL GEL
LIQUIFACTION TEMPERATURE Temperature at which gel changes to sol. (70 -100°c) GELATION TEMPERATURE Temperature at which sol changes to gel.(37-50°c) HYSTERESIS The temperature lag between the liquefaction temperature and the gelation temperature.
IMBIBITION The process of water sorption i.e. the gel swells when placed in water. SYNERESIS Expression of fluid on to the surface of gel structure.
AGAR Agar is an organic hydrophilic colloid (polysaccharide) extracted from a certain type of sea weed. It is a sulphuric ester of a linear polymer of galactose.
AGAR It forms the main constituent but not by weight. Provides the dispersed phase of the sol and the continuous fibril structure to the gel. WATER It provides the continuous phase in the sol and the second continuous phase in the gel. The amount controls the flow property of the sol and the physical properties of the gel. BORAX Is added to increase the strength or density of the gel by intermolecular attraction.
SULPHATES Acts as a plaster hardener i.e. counteracts the inhibiting effect of borax and agar on the setting of gypsum material. FILLERS Added to control the strength, viscosity and rigidity of the material. e.g. diatomaceous earth, silica, wax, rubber and similar inert powder. COLOR AND FLAVOUR To improve the appearance and taste.
GELATION TEMPERATURE: The gelation temperature of agar is approximately 37°C if the gelation temperature is too high it is possible that injury may result to the oral tissues involved , a sever surface stress may also develop . if the gelation temperature is too low it will be difficult or even impossible to chill the material to a temperature sufficiently low to obtain a firm gel adjacent to the oral tissues . According to ADA specification no. 11 gelation temperature must not be less then 37°C or more then 45°C.
GELATION TIME: Gelation of the reversible hydrocolloids is a function of both temperature and time. Available forms: Syringe material Tray material The only difference between the syringe and the tray material is colour and the greater fluidity of the syringe material.
Manipulation: Agar hydrocolloid requires special equipment. Hydrocolloid conditioning unit Water cooled rim lock trays. PREPARATION OF THE MATERIAL : First step is to reverse the hydrocolloid gel to the sol form. Usually done at 100 C for 10min 3 min should be added to this time whenever the material is being reused. As it becomes difficult to break down the agar brush heap structure. After liquefaction material may be stored in sol condition. Storage temperature: 65 C to 68 C.
CONDITIONING OF THE MATERIAL: This refers to the cooling of the material (also called as ‘tempering’ of the material) Tempering is usually done at 43 C for 7min. Although tempering time and temperature varies according to different product specifications and also operator choice but in any case tempering of the material should not exceed 10min since the gelation may proceed too far. Uses of tempering : It increases the viscosity of the material so that the hydrocolloid does not flow out. It reduces the temperature of the material so that it is not uncomfortable for the patient.
IMPRESSION : The syringe material is first filled in the prepared cavity or the desired area of impression. The tempered tray material in a tray is then seated in the oral cavity covering the already placed syringe material with passive pressure. Excess water from the surface of the tray material should be removed to facilitate proper union of the tray and the syringe material. Gelation is accomplished by circulating cool water at 18 C to 21 C through the tray for at least 5min. Care should be taken to prevent the movement of the tray during gelation. After complete gelation the impression is removed from the mouth with a single jerk to prevent the tearing of the impression .
Disinfection of the impression : Disinfection of agar is very important to prevent cross infection since the material can be re-used. Disinfectant choice varies with the product according to the specifications of the manufacturer. Commonly used ones are iodophor, bleach, or glutaraldehyde
Dimensional Stability of the impression : Gels are invariably subject to changes in dimension by syneresis and imbibition. Syneresis is the loss of water by evaporation from the surface of the gel or by exuding of fluids. Imbibition is the sorption of water which results in swelling up of the gel. Dimensional changes begin as soon as the impression is removed from the oral cavity. Storage of the impression in 100% relative humidity is suggested to prevent dimensional changes.
According to the ADA specification number:- 11 the compressive strength should not be less the 0.245 MPa
1) Tear strength ----------- 800 to 900 gms/cm2
2) Flexibility ------------ 4 to 15
3) Working time is 7-15 min
4) Setting time is approximately 5 min
Elasticity and elastic recovery : recovery occurs upto 98.8%
Advantages: Accurate dies can be prepared. Due to good elasticity reproduction of undercuts is accurate. It gives good model surface as it is not hydrophobic. It is palatable and well tolerated by patients. It can be re-used
Disadvantages: It cannot be electroplated. Material tears relatively easily. Only one model can be prepared. Extensive instrumentation is required to use agar. A soft surface of the gypsum cast results unless plaster hardener is used.