Plaster of Paris is seldom used as an impression material now that elastomeric materials are available, but it can be used as a wash impression material (a thin lining material placed over a stiffer base material or tray) for edentulous impressions. The main component of impression plaster is calcium sulfate hemihydrate, which reacts with water to form calcium sulfate dihydrate. Manufacturers incorporate additives to adjust the setting time and setting expansion. The water/powder (W/P) ratio recommended by the manufacturer should be measured out carefully. The powder should be sprinkled into the water, allowed to sit for 30 seconds to wet the powder, and then mixed for the minimum time necessary to obtain a homogeneous mix. Impression plaster is rigid and will break rather than bend. The plaster must be stored in an airtight container, because it will absorb water from the air which may adversely affect its setting time.
Dental impression compound (types I and II)
Although dental compound has fallen into disuse, it can be used for full-crown impressions (type I), impressions of partially or completely edentulous jaws (type I), and impression trays in which a final impression is taken with another material (type II). Dental compound cannot be used to record undercuts because it is not elastic. Impression compound is available in either cakes or sticks in various colors from a number of manufacturers.
Natural resins give the compound its thermoplastic character and make up about 40% of the formulation. Shellac is often used. Waxes (about 7%) also have thermoplastic properties, and stearic acid (about 3%) acts as a lubricant and plasticizer. Fillers (eg, diatomaceous earth, soapstone, and talc) and inorganic pigments account for the remaining 50% of the formulation.
Thermal and mechanical properties
Dental compound’s thermoplastic property allows it to be used warm (45°C) and then cooled to oral temperature (37°C), at which it is fairly rigid. The setting mechanism is therefore a reversible physical process rather than a chemical reaction. Dental compound is limited by its thermal properties. Type I materials have a flow of at least 85% at 45°C and less than 6% at 37°C. Type II materials flow about 70% at 45°C but less than 2% at 37°C. Both types become quite plastic with only an 8°C rise in temperature. The thermal conductivity of dental impression compounds is very low. These materials require heat soaking to attain a uniform temperature throughout the mass. When heated or cooled, they soften or harden quickly on the outside, but time is needed for the temperature to become uniform throughout the entire mass. If the impression is removed from the mouth before it has cooled completely, severe distortion may occur. Since these materials contain resins and waxes, they have high thermal expansion and contraction coefficients. Contraction from oral temperature to room temperature may be as high as 0.3%. Therefore, the dimensions of the resulting impression could be significantly different from those of the mouth. Since the compound has such a high viscosity, it is difficult to record details.
Impression compound is softened by heating over a flame or in a water bath. Care must be taken to prevent volatilizing ingredients over a direct flame. Kneading in water also may cause changes in composition and flow properties. The aim, therefore, is thorough heating without excessive temperatures or long periods of storage in water. A room-temperature water spray is used to cool the impression in the mouth. Cooling must be continued until the entire mass is rigid to reduce plastic flow. Care must also be taken to prevent overheating and burning of the tissues being replicated. Also, cooling water should not be too cold, to prevent thermal shock.
To ease separation of the die stone, the impression should first be softened by immersion in warm water.
Dental impression compound is compatible with die and cast materials and is easily electroplated to form accurate and abrasion-resistant dies.
The handling of dental impression compound is very technique sensitive. If it is not prepared properly, volatiles can be lost on heating or low-molecular-weight ingredients can be lost during immersion in a water bath. Also, excessive wet kneading can incorporate water into the mix and change the flow properties of the compound. Due to a high coefficient of thermal expansion, the dimensions of the impression are not likely to be the same as the dimensions in the mouth. These materials are nonelastic and may distort on removal from the mouth. The casts should be poured within 1 hour.
1. Distortion. If the material is not completely cooled, the inner portions of the impression will still be soft when the impression is removed, resulting in distortion. Also, if water has been incorporated as the result of wet kneading, the material could have excessive flow at mouth temperature, producing distortion during removal from the mouth. If the tray used to carry the compound to the mouth is too flexible, distortion can result. It is important to select a tray that is strong and rigid. A delay in preparing the stone cast also may cause distortion. The cast should be poured as soon as possible after the impression has been removed from the mouth.
2. Compound is too brittle or grainy. Prolonged immersion in the water bath will cause low-molecular-weight components to leach out.
Dental impression compound can be disinfected by immersion in sodium hypochlorite, iodophors, or phenolic glutaraldehydes. The manufacturer’s recommendations for proper disinfection should be followed.
The main use of zinc oxide–eugenol is for dentures on edentulous ridges with minor or no undercuts. It can also be used as a wash impression over the compound in a tray or in a custom acrylic tray. Zinc oxide–eugenol is also used as a bite registration material.
This material is commercially available in a powder and liquid form and as two pastes. One paste, called the base or catalyst paste, contains zinc oxide (ZnO), oil, and hydrogenated rosin. The second paste, the accelerator, contains about 12% to 15% eugenol, oils, rosin, and a filler such as talc or Kaolin. These two pastes have contrasting colors so it can be determined when the pastes are thoroughly mixed.
These materials are supplied as a soft- or hard-set type. Equal lengths of the two pastes, or properly proportioned amounts of the powder and liquid, are mixed with a stiff spatula on a special oil-resistant paper pad or a glass slab. The mixed material is placed in a preliminary impression made from tray compound or tray acrylic. The setting time is shortened by increases in temperature and/or humidity. The set material does not adhere to set dental plaster or stone.
Zinc oxide, in the presence of moisture, reacts with eugenol to form zinc eugenolate, which acts as a matrix holding together the unreacted zinc oxide:
The setting reaction is accelerated by the presence of water, high humidity, or heat. A dimensional change of only about 0.1% shrinkage accompanies the setting.
These impression materials are classified as hard-and soft-set. The hard-set material sets faster (in about 10 minutes, compared with 15 minutes for the soft-set material), although the hard- and soft-set materials both begin to set in about 5 minutes. The hard-set material is more fluid before setting than the soft-set material; after setting, it is harder and more brittle.
Noneugenol pastes containing carboxylic acids (eg, lauric or ortho-ethoxybenzoic acid) in place of eugenol are available to avoid the stinging and burning sensation experienced by some patients.
The hardness of zinc oxide–eugenol impression materials is determined using a Krebs penetrometer with a load of 100 g for 10 seconds. The hardness for type I (hard-set) materials should be no greater than 0.5 mm, and the hardness for type II (soft-set) materials should be between 0.8 and 1.5 mm. The shrinkage of these materials during the hardening process is approximately 0.1%. Subsequently, no additional dimensional change should occur.
These materials are usually mixed on a mixing pad with a spatula. Equal lengths of base and catalyst are extruded on the mixing pad. The components are mixed thoroughly with a stiff stainless steel spatula. Adequate mixing time is 45 to 60 seconds, after which the mix should appear streak free. The pastes have an initial set time of 3 to 5 minutes, with the setting time decreasing as the temperature and/or humidity increases. The cast should only be made from gypsum-type plaster or stone. After the stone has set, the impression is immersed in warm water (60°C) to ease its removal from the cast. The spatula may be cleaned by warming or by wiping with available solvents.
Zinc oxide–eugenol gives high accuracy of soft tissue impressions due to its low viscosity. The material is stable after setting, has good surface detail reproduction, and is inexpensive. It also adheres well to dental impression compound.
This material is messy and has a variable setting time due to temperature and humidity. Eugenol is irritating to soft tissues. This material is nonelastic and may fracture if undercuts are present.
1. Inadequate working or setting time. An increase in humidity and/or temperature results in decreased working and setting time. It is important to select a material that provides the required setting time.
2. Distortion. If the tray warps on standing, the impression will also become distorted. It is important to select a stable tray material.
3. Loss of detail. If there is loss of detail, the impression material may not be compatible with the stone used to prepare the cast, and/or there may be adhesion between the impression and the stone.
Zinc oxide–eugenol impressions can be disinfected by immersion in 2% glutaraldehyde or 1:213 iodophor solutions at room temperature. The manufacturer’s recommendations for proper disinfection should be followed.
Agar (reversible) hydrocolloid
Agar hydrocolloids have been largely replaced by rubber impression materials but are still used for full-mouth impressions without deep undercuts, quadrant impressions without deep undercuts, and single impressions (less frequently). Because of their high accuracy, they can be used for fixed partial denture impressions.
Agar hydrocolloids are available in both tray and syringe consistencies. The material is supplied as a gel in plastic tubes and contains agar (12% to 15%) as a gelling agent, borax (0.2%) to improve strength, potassium sulfate (1% to 2%) to provide good surfaces on gypsum models or dies, alkylbenzoates (0.1%) as preservatives, and coloring and flavoring agents (traces) for ease of “reading” the impression and esthetics. The balance of the formulation (~85%) is water. The syringe consistency is prepared by increasing the water content and decreasing the agar content.
The gel material can be converted to a sol (liquid) by heating; cooling the sol will return the material to the gel state:
The gel-to-sol and sol-to-gel transformations depend on time and temperature. The liquefaction and gelation temperatures are different (the latter being lower), and the effect is called hysteresis. A typical value of the gelation temperature is 43°C (109°F).
The mechanical properties of agar hydrocolloids are presented in Table 7-1. They are highly elastic (98.8%) and sufficiently flexible (11%) to give accurate impressions of teeth with undercuts. They are stronger when stressed quickly; therefore, rapid removal is recommended.
Agar requires a special water bath with three chambers for heating and water-cooled trays. The following sequence is used:
1. Heat in water at 100°C (212°F) for 8 to 12 minutes.
2. Store in water at 65°C (149°F).
3. Place in a tray (containing cooling coils) at 65°C (149°F).
4. Temper in 46°C (115°F) water for 2 minutes before taking the impression.
5. After seating the tray, cool it with water at no less than 13°C (55°F) until gelation occurs.
6. After the impression is removed from the mouth, wash it to remove saliva, which will interfere with the setting of the gypsum.
7. Shake off excess water and lightly blow off remaining excess with air.
8. Disinfect the impression.
9. Pour mixed dental stone into the impression. If the impression is stored for a short time in 100% relative humidity, it should be washed as described in steps 6 and 7 to remove any exudate on the surface caused by syneresis (the exudation of water, accompanied by contraction) before pouring the cast.
10. After the initial setting of the stone, store the gypsum cast and impression in a humidor.
Agar impressions become less accurate during storage, so prompt pouring of gypsum casts is necessary. Table 7-2 lists the dimensional changes that occur during storage under different conditions. If agar impressions must be stored, the minimum changes in dimensions occur in 100% relative humidity for no longer than 1 hour. However, the gel structure can absorb water, a process called imbibition, which is usually accompanied by expansion.
As the values in Table 7-3 indicate, agar materials have a long working time. Handling, however, offsets this convenience because of the need for storage tanks. Gelation, produced by circulating cool water through the special trays, also requires special equipment. Thermal shock produced by suddenly cooling the warm colloid may be painful to patients who have metallic restorations.
Contact with agar retards the setting of gypsum, resulting in dies and casts with poor surface finish. With older products, soaking the impression in a 2% potassium sulfate solution was necessary to achieve a smooth surface finish. Most agar products now contain potassium sulfate, which acts as an accelerator for the gypsum setting reaction, and soaking is no longer necessary.
Agar impression materials are inexpensive, have no unpleasant odors, and are nontoxic and nonstaining. They do not require a custom tray or adhesives, and the components do not require mixing. These materials are hydrophilic and can be used in the presence of moisture and are able to displace blood and body fluids. In addition, they are easily poured in stone, and the stone casts are easily removed from the hydrocolloid impressions.
These materials require the use of expensive equipment and must be prepared in advance. They tear easily, must be poured immediately, are dimensionally unstable, can only be used for single casts, and cannot be electroplated. The surface of stone casts will be weakened by compositions containing borax.
Sometimes problems of distorted impressions or loss of detail may be encountered when using agar hydrocolloids. The following are factors that could lead to distortion:
1. Slow removal from the mouth. To avoid permanent deformation, the impression should be removed with a quick jerk.
2. Removal from the mouth before the gel reaches a temperature of 37°C (98.6°F) or less. Above this temperature the impression material will still be plastic. The cooling rate of hydrocolloid depends on the temperature of water circulating through the tray.
3. Cooling water that is too cold (< 13°C). Rapid cooling of the impression may cause a concentration of internal stresses that may be subsequently released.
4. Application of force on the tray during gelation. After the load is removed, relaxation of stresses will occur.
5. Delay in pouring the cast. Waiting any length of time to pour the cast will result in shrinkage of the impression due to the loss of water.
6. Instability of tray. If there is loss of detail, it may be caused by movement of the tray before gelation is complete. Failure to keep the impression stabilized will result in a multiple impression of the oral structures.
|STORAGE CONDITIONS||DIMENSIONAL CHANGE||CAUSES|
|Air||Shrinkage||Evaporation of water from gel|
|H2O||Expansion||Imbibition and absorption of water|
|100% relative humidity||Shrinkage||Syneresis|
|Inorganic salt solutions||Expansion or shrinkage||Depends on relationship of electrolyte in gel and in solution|
Agar hydrocolloids can be disinfected by immersion in sodium hypochlorite, iodophors, or phenolic glutaraldehydes. The manufacturer’s recommendations for proper disinfection should be followed.
Alginate (irreversible) hydrocolloid
Alginates are the most widely used impression materials in dentistry. They are used for making impressions for removable partial dentures with clasps, preliminary impressions for complete dentures, and orthodontic and study casts. They are not accurate enough for fixed partial denture impressions.
Alginates are supplied as a powder containing sodium or potassium alginate (12% to 15%) and calcium sulfate dihydrate (8% to 12%) as reactants; sodium phosphate (2%) as a retarder; a reinforcing filler (70%), such as diatomaceous earth, to control the stiffness of the set gel; potassium sulfate or alkali zinc fluorides (~10%) to provide good surfaces on gypsum dies; and coloring and flavoring agents (traces) for esthetics. The sodium phosphate content is adjusted by the manufacturer to produce either regular- or fast-set alginates.
The powder is mixed with water to obtain a paste. Two main reactions occur when the powder reacts with water during setting. First, the sodium phosphate reacts with the calcium sulfate to provide adequate working time:
2Na3PO4 + 3CaSO4 → Ca3(PO4)2 + 3Na2SO4
Second, after the sodium phosphate has reacted, the remaining calcium sulfate reacts with the sodium alginate to form an insoluble calcium alginate, which forms a gel with the water:
To avoid the inhalation of alginate dust, some materials have been introduced in a dustless version in which the powder is coated with a glycol (eg, Identic Dust Free, Cadco; Jeltrate Plus, Dentsply Caulk).
Some products contain a chemical disinfectant in the alginate powder to control infection (eg, Coe Hydrophilic Gel, GC America; Identic Dust Free). Two examples of these disinfectants are didecyl-dimethyl ammonium chloride and chlorhexidine acetate. When the quaternary ammonium compound is used, the detail reproduction and gypsum compatibility of the alginate improve. However, the impressions made from these materials should still be disinfected on removal from the mouth.
Table 7-1 gives an elastic recovery value of 97.3% for alginates, which indicates less elasticity and therefore less accuracy than agar hydrocolloids and silicone and polyether impression materials. The compressive and tear strengths increase with increasing rates of deformation. The limit of reproduction is also lower, thus, less fine detail will be obtained. Figure 7-2 compares the elasticity of alginates with the more accurate agar materials. Alginates have a higher permanent deformation on stretching to pass over undercuts.
Although easy to use, care is required in handling alginate hydrocolloids. The powder, supplied in a can, should be shaken up for aeration, and one scoop of powder should be used for one measure of water. A powder scoop and a graduated cylinder for water are usually supplied with the product. With predispensed powder products, one packet of powder is used with the amount of water specified by the manufacturer. A lower W/P ratio increases strength, tear resistance, and consistency, and decreases working and setting times and flexibility. Also, cooling the water increases the working and setting times. Insufficient mixing results in a grainy mix and poor recording of detail. Adequate spatulation gives a smooth, creamy mix with a minimum of voids.
One minute of thorough mixing for the regular-set material and 45 seconds for the fast-set material are generally recommended. Alginates have a relatively short working time of about 2.5 minutes (see Table 7-3) and set about 3.5 minutes after mixing. They are as unstable as agar hydrocolloids because both are gels, and they undergo shrinkage or expansion when water is lost or gained. Storage in either air or water results in significant dimensional change; however, storage at 100% humidity results in the least dimensional change. Therefore, the cast should be poured soon after removal of the impression and cleaning (see Table 7-3). Alginates, like agar, retard the setting of the gypsum cast and die materials when in contact. Potassium sulfate is added by the manufacturer to accelerate the setting of the gypsum and to obtain smooth cast and die surfaces.
An alginate tray material can be combined with an agar syringe material to prepare impressions. These impressions take advantage of the agar hydrocolloid’s detail reproduction and compatibility with gypsum qualities and at the same time minimize equipment needs. A simple heater can be used to prepare the syringe material, and the water-cooled trays are no longer necessary. The alginate is placed in a tray, the agar is dispensed around the preparation, and then the alginate is seated on top of the agar. Care must be taken to select an agar-alginate impression pair with suitable bond strengths. It is best to select combinations recommended by the manufacturers. Best results are obtained when single-unit impressions are made by this technique.
Alginate impression materials are inexpensive, easy to manipulate, pleasant tasting, able to displace blood and body fluids, hydrophilic, and easily poured in stone. They can be used with stock trays.
Alginates tear easily, must be poured immediately after removal from the mouth, have limited detail reproduction, are dimensionally unstable, and can only be used for single casts. The gypsum compatibility varies with the brands of alginates and dental stones used. They are incompatible with many epoxy resin die materials.
Problems may sometimes be encountered when using alginate hydrocolloids. The following should serve as a guide for troubleshooting problems with these materials:
1. Inadequate working or setting time. The temperature of the mixing water may be too high. Generally, the temperature of the water should range between 18°C and 24°C (65°F and 75°F). If the mixture is incompletely spatulated, it may be inhomogeneous and may set prematurely. Under normal conditions, adequate spatulation requires 45 to 60 seconds. If the W/P ratio is too low as the result of incorrect dispensing, the setting time could be too fast. Improper storage of the alginate powder can result in deterioration of the material and shorter setting times.
2. Distortion. If the tray moves during gelation or if the impression is removed prematurely, the result will be a distorted impression. The amount and duration of compression should be considered. It is important, therefore, to remove the impression from the mouth rapidly. Since the weight of the tray can compress or distort the alginate, the impression should not be placed face down on the bench surface. If the impression is not poured immediately, distortion could occur.
3. Tearing. If the impression tears, it is possible that it was removed from the mouth before it was adequately set. Wait 2 to 3 minutes after loss of tackiness to remove the impression for development of adequate tear strength. Also, the rate of removal from the mouth may be a factor. Because the tear strength of alginate increases with the rate at which a stress is applied, it is desirable to rapidly remove the impression from the mouth. In addition, thin mixes are more prone to tearing than those with lower W/P ratios. The presence of undercuts also can produce tearing. Blocking out these areas will place less stress on the impression material during removal. It is also possible that there is not enough impression material; there should always be at least 3 mm of material between the tray and the oral tissues.
4. Loss of detail. If there is loss of detail, the impression may have been removed from the mouth prematurely. Multiple impressions of the oral structure will result if the material is still in the plastic state when removed.
5. Consistency. If the preset mix does not have the proper consistency (either too thick or too thin), the W/P ratio is incorrect. Care must be taken to fluff the powder before measuring and not to overfill the powder dispenser. Vigorous spatulation and mixing for the full recommended time is required to avoid consistency problems caused by inadequate mixing. If hot water is used, the mix may become grainy and prematurely thick.
6. Dimensional change. If dimensional change is a problem, a delay in pouring the impression might be the cause. Such delays will result in a cast that is distorted as well as undersized, because alginate impressions lose water when stored in air.
7. Porosity. Whipping air into the mix during spatulation can cause the impression to be porous. After the powder has been wetted, the alginate should be mixed so as to squeeze the material between the spatula blade and the side of the rubber bowl.
8. Poor stone surface. If the set gypsum remains in contact with the alginate for too long, the quality of the stone surface will suffer.
Alginate hydrocolloids can be disinfected by immersion in sodium hypochlorite or iodophors. The manufacturer’s recommendations for proper disinfection should be followed.
Polysulfide rubber (mercaptan)
Due to their high accuracy and relatively low cost, polysulfide rubbers are widely used for fixed partial denture application. These materials are useful for multiple impressions when extra time is needed. Polysulfides are supplied in tubes of base paste and catalyst paste, which are mixed together. They are available in low, medium, and high viscosities.
The base paste contains the polysulfide polymer, fillers, and plasticizers. Low-molecular-weight (~4,000 MW) polysulfide polymer, having both terminal and pendant (near the center of the polymer) mercaptan groups (-SH), is used:
The content of the reinforcing fillers (eg, zinc oxide, titanium dioxide, zinc sulfide, and silica) varies from 12% to 50% depending on the consistency (ie, low, medium or high viscosity). The accelerator or catalyst paste contains lead dioxide (30%), hydrated copper oxide or organic peroxide as a catalyst, sulfur (1% to 4%) as a promoter, and dibutyl phthalate or other nonreactive oils (17%) to form the paste. The balance of the catalyst