Making impressions of oral structures is an almost everyday occurrence in a busy dental practice. Selection of the impression material will be influenced by what the impression will be used for. To replicate oral structures, the impression materials must be in a moldable or plastic state that can adapt to the teeth and tissues. Usually, the impression material in its plastic state is loaded into a tray for carrying it to the mouth and supporting it so that it does not slump and distort. Within a specified period of time, the impression material must set to a semisolid, elastic, or rigid state. Elastic impression materials are used more extensively than rigid materials, because elastic materials flex from tissue undercuts when removed from the mouth, whereas rigid materials cannot. The completed impression forms a negative reproduction of the teeth and tissues. When plaster or stone is poured into the impression and hardened, the replica that is formed is a positive reproduction of the teeth and tissues (see Chapter 16 [Gypsum and Wax Products]). The replica is called a cast or model. In the initial diagnosis and treatment planning phase, the dentist may request that the dental assistant or hygienist make impressions of the teeth and surrounding structures, so that diagnostic casts, commonly called study models, can be made for further study when the patient is no longer present. When an impression is made of a tooth that has been prepared for a restoration, the replica of the prepared tooth is called a die and is used for fabrication of the restoration in the dental laboratory. Figure 15-1 shows an impression and the mold and die made from that impression. Use of the die allows the dentist or laboratory technician to perform the procedure by the indirect technique. With the indirect technique, the restoration is not made directly on the tooth, as with the direct placement of amalgam, but is constructed in the laboratory (indirectly) and later is cemented on the tooth.

Dental impressions can be categorized into three basic types based on how they will be used. These types include the following:

Preliminary impressions, as the name implies, are made as a precursor to other treatment. Casts made from them are often used for planning purposes such as for diagnostic casts (study models). Preliminary impressions may be used to make working casts from which custom trays or provisional restorations can be made or to create casts for pre- and post-treatment records. On occasion, what starts out to be a preliminary impression can also be used as the final impression. For example, a cast made from an alginate impression to design a removable orthodontic appliance may be accurate enough to send to the laboratory for fabrication of the appliance. Alginate is a useful, inexpensive material that is excellent for preliminary impressions but lacks the detail and accuracy to be used for a crown impression.

Final impressions are impressions that are more accurate in their replication of the oral structures. To provide a good fit and marginal integrity for a crown, bridge, or implant a very detailed and accurate impression of the preparation and surrounding structures is needed. A detailed replication of the oral tissues is needed to fabricate well-fitting partial and complete dentures as well. Polyvinyl siloxane and polyether are the two most commonly used materials for final impressions.

A replication of the patient’s bite is needed to establish the proper relation between a restoration or prosthesis and the opposing teeth. An impression is made that captures this relationship (see Procedure 15-3), so it can be used in the office or sent to the dental laboratory where the restoration will be fabricated. Bite registrations are also used to help mount diagnostic casts in their proper relationship (often used in conjunction with a facebow) on an articulator. Although wax has been used for bite registration for decades, polyvinyl siloxane is currently more popular for this purpose. A wax bite registration is easily distorted.

Impression materials can be categorized into two major groups:

Although impression materials must have a degree of strength, their key properties are as follows:

Impression trays are used to carry the impression material to the mouth and to support it until it sets, is removed from the mouth, and is poured into dental plaster or stone. They should be rigid to prevent distortion of the impression. They can be made for arches with teeth or for edentulous ridges.

Impression trays can be premanufactured trays, called stock trays, which are purchased in a variety of sizes (small, medium, large, and extra large) for both adults and children (Figure 15-2). Stock trays can be metal or plastic, and each of these can be solid or perforated. Perforated trays have holes in their sides and bottom to help retain impression material as it extrudes through the holes and locks into place. Solid trays often have raised borders on the internal surfaces that help lock in the impression material. These are called “rim-lock” trays. Materials used in solid trays require the application of an adhesive to further retain them and prevent distortion of the impression if they should partially pull out of the tray. Plastic trays are inexpensive and disposable, whereas metal trays are more expensive and must be cleaned and sterilized between uses.

Impression trays can also be custom made. Custom trays are usually constructed in the laboratory with chemical-cured, light-cured, or thermoplastic resins on casts of the teeth (see Chapter 17 [Polymers for Prosthetic Dentistry]). They are custom fit to the mouth of the individual. They can also be made by lining a stock tray with a putty material that is adapted to the dental arch of the individual, and then an impression is made in this arch-adapted custom tray.

Bite registration trays are typically U-shaped plastic frames with a thin fiber mesh stretched between the sides of the frame. The mesh retains the impression material (called bite registration material) and is thin enough so as not to interfere with closure of the upper and lower teeth in proper bite relationship. Bite registration material is placed on both sides of the mesh, the frame is positioned over the teeth to be recorded, and the patient closes into the normal bite relationship until the material sets (see Figures 15-33 through 15-36 in Procedure 15-3).

A colloid is a glue-like material composed of two or more substances in which one substance does not go into solution but is suspended within another substance. Hydrocolloids are water-based colloids that function as elastic impression materials. The two hydrocolloids used in dentistry are agar hydrocolloid (or reversible hydrocolloid) and alginate hydrocolloid (or irreversible hydrocolloid). Much like gelatin, when agar powder is mixed with water, it forms a glue-like suspension that entraps the water, making a colloidal suspension called a sol. Heating it will disperse the agar in the water faster. When the agar sol is chilled, it will gel, becoming semisolid or jelly-like (like Jell-O). When the agar gel is heated, it will reverse its state back into a liquid suspension (sol). Therefore, it is a reversible hydrocolloid. Alginate powder will also form a sol that gels. However, with alginate, a chemical reaction occurs that prevents it from reversing back to a gel when heated. Therefore, it is an irreversible hydrocolloid.

Reversible hydrocolloid was introduced into dentistry in 1925 and was the first elastic material to gain popularity. It overcame many of the problems with inelastic materials (see the section “Inelastic Impression Materials,” below) in that it could take accurate impressions of teeth and arches with tissue undercuts and could be removed from the mouth without injuring the patient or breaking. Its main clinical use is for impressions of operative and crown and bridge procedures. It also has uses in the laboratory for the duplication of casts (models). Its use has declined over the years as elastic (rubber) impression materials have been introduced. Detailed information about reversible (agar) hydrocolloid can be found on the Evolve website at www.evolve.elsevier.com/Hatrick/materials.

Alginate, also called alginate hydrocolloid or irreversible hydrocolloid, is by far the most widely used impression material. It is inexpensive, easy to manipulate, requires no special equipment, and is reasonably accurate for many dental procedures. Alginate is used for making impressions for diagnostic casts, partial denture frameworks, and repairs of broken partial or complete dentures, as well as for fabrication of provisional restorations, fluoride and bleaching trays, sports protectors, preliminary impressions for edentulous arches, removable orthodontic appliances, and a multitude of other uses. However, it is not accurate enough for the final impressions for inlay, onlay, crown, and bridge preparations. It does not capture the fine detail of the preparation needed for a precise fit of such restorations. Also, it is thick and does not flow well into embrasures or occlusal surfaces. Final impressions are made with more accurate materials such as one of the elastomers (polyvinyl siloxane or polyether). Final impressions are used to make detailed replicas of the prepared teeth. It is from these detailed replicas that precisely fitting restorations will be made.

The main active ingredient in alginate is potassium or sodium alginate, which makes up 15% to 20% of the powder. Proportions of ingredients vary from manufacturer to manufacturer and with fast-, regular-, and slow-set materials. It is produced from derivatives of seaweed. See Table 15-1 for the components of alginate and their functions. The “dustless” alginate powders have organic glycols or glycerin added to keep powder from becoming airborne when it is dispensed. The dust contains silica particles, and they are a potential health hazard if inhaled.

Regular-set alginates have a working time (from start of mix to seating in the mouth) of 2 to 3 minutes, and fast-set alginate has a working time of 1.25 to 2 minutes (American Dental Association [ADA] specification no. 18 sets the minimum at 1.25 minutes). The longer the time used to mix the alginate, the faster it must be loaded into the tray and seated in the mouth.

Regular-set alginates set in 2 to 5 minutes, and fast-set alginates set in 1 to 2 minutes. Setting time can be lengthened by using cold water or shortened by using warm water. Adjusting the powder-to-water ratio can affect the set but also adversely affects the physical and mechanical properties and therefore is not recommended. It is advisable to leave the impression in the mouth for an additional minute after it appears set, because the tear strength and the ability to rebound from undercuts without permanent deformation increase during this time.

Alginate will be compressed when it is removed from undercuts in the mouth. The greater the compression, the more likely it is that the alginate will be permanently deformed to some degree. A certain thickness of alginate (2 to 4 mm) is needed between the impression tray and the teeth or tissue undercut; alginate that is too thin will deform more and will tear more easily. As with reversible hydrocolloid, when an alginate impression is removed it should be done with a rapid “snap” to prevent the deformation of critical surfaces. If 8 to 10 minutes are allowed to elapse from the time an alginate impression is removed from the mouth until pouring the model, some recovery or rebound will occur from the deformation. That deformation which does not recover is the permanent deformation, and it will be recorded in the poured gypsum cast as a distortion. As long as the distortion is small, it may not be clinically significant. Usually, the time needed for disinfecting the impression is at least 10 minutes, and most of the rebound will have occurred by then.

Alginate is very sensitive to moisture loss and will shrink as a result. Once the impression is removed from the mouth, it should be rinsed and disinfected (Procedure 15-4), wrapped in a damp (not dripping wet) paper towel, and sealed in a zippered plastic bag. (An alternative to wrapping in a damp paper towel would be to put a few drops of water in the plastic bag. Alginate could imbibe water from the towel and swell.) Enclosing the impression this way will create an environment with 100% humidity to minimize water loss from the alginate. Some moisture will be lost from the impression even in 100% humidity from syneresis. Syneresis occurs when many gels are left standing, whereby they contract and some of the liquid is squeezed out of the gel, forming an exudate on the surface. This loss of water changes the properties of the material. Ideally, the impression is poured after it is disinfected. If the impression must be stored until it can be poured a few hours later, it must be kept at 100% humidity (as with the zippered plastic bag and a few drops of water). The longer the delay in pouring the alginate impression, the more likely that some distortion will occur.

The tear strength of alginate is more important than its compressive strength, because most commercial alginates far exceed the minimal allowable value for compressive strength. Alginate mixed with too much water will be weaker and more likely to tear on removal from the mouth. Thin sections of alginate are also prone to tearing. In addition, slow removal of the alginate from the mouth will contribute to tearing. If the impression can be left in the mouth for an additional minute beyond the point when it is set, it will increase in tear strength. When properly handled, alginate has adequate tear strength for most purposes for which it is used. Some alginates have silicone polymers added to increase the strength.

The objective of impression making is to reproduce the oral structures with acceptable accuracy while practicing good infection control and maintaining patient comfort. The dental assistant and the dental hygienist can make alginate impressions. They will need to prepare the patient for the impressions and to dispense, mix, and load alginate into trays. After removal of the impression, the assistant or the hygienist disinfects and properly handles the impression until it is poured with the appropriate gypsum material. The assistant or hygienist also may be responsible for clearing residual alginate from the mouth and face of the patient.

Stock trays work well with alginate, because they leave plenty of room for an adequate thickness of alginate. Alginate must be tightly adapted to the tray to be accurate. If the alginate pulls loose from the tray, a distortion will occur. If a tray is set on the bench top, unsupported alginate extending from the back of the tray may lift a portion of the impression and dislodge it from the tray. A perforated tray can be used because the alginate oozes through the perforations and locks into place. A solid tray can also be used if an adhesive made for alginate is applied to the inside of the tray before the alginate is loaded. Solid rim-lock trays should also have adhesive applied because alginate will occasionally pull free from the rim-lock on removal of the impression. If disposable plastic trays are used, they should be rigid. Flexible plastic trays have the potential to distort under the weight of the wet gypsum during pouring or when used in areas of undercuts in the mouth.

Manufacturers supply measures for powder and water for their alginates. Use the appropriate ones and adhere to the recommended proportions of powder and water to maintain the desired physical properties of the alginate. Powder measures (also called scoops) will vary among manufacturers, so do not interchange them with other manufacturers’ scoops. The same principle applies to water measures.

For a moderate to large upper adult arch, three scoops of alginate powder are usually required; a small upper arch requires two scoops. Most adult lower arches require two scoops. One unit of water is required per scoop. Typical water measures are marked to indicate up to three units. Room temperature tap water is placed in the rubber bowl, and the powder is added to it. Cold water retards the set, and warm water accelerates it. The powder is stirred into the water so that the powder becomes wet. Next, the wet powder is aggressively mixed against the sides of the bowl with a wide-bladed spatula. Some operators prefer to rotate the bowl in one hand while mixing with the other. Some offices use mechanical mixing machines for rapid mixing, ease of use, and a consistent mix. The rubber bowl is attached to the mechanical mixer that spins the bowl. With both mechanical and hand-mixing, the water-powder mixture is forced against the sides of the bowl to further incorporate the powder into the water and to force out entrapped air. Mixing should be completed within 45 seconds for regular-set alginate and within 30 seconds for fast-set alginate. The completed mix should have a creamy consistency (see Figure 15-19 in Procedure 15-1). If it appears grainy, it has not been mixed thoroughly.

Mixed alginate is picked up on the spatula and pushed into the depth of the tray. This action forces out air, thus preventing large voids in the impression. The alginate should be loaded in large increments as quickly as possible. The greater the number of small increments added to the tray, the greater is the chance for entrapped air. The tray should be loaded until the alginate is even with the tops of the sides of the tray. A wet, gloved finger is used to smooth the surface of the alginate and to create a shallow trough over the ridge area of the alginate (see Figure 15-20 in Procedure 15-1) that reduces the chance for entrapped air and helps to orient the tray over the teeth when it is seated.

Once the tray is loaded, the operator should take some alginate from the bowl on the gloved index finger and wipe it on the occlusal surfaces and embrasures of the teeth to force air out from the grooves and embrasure spaces. If regular-set alginate has a 2-minute working time and the alginate was mixed for 45 seconds, the operator has 75 seconds to load the tray, wipe the alginate on the teeth, and seat the tray. For fast-set alginate, the operator has about 45 seconds for the same process after mixing for 30 seconds. On warm days, the tap water may be warmer than usual and may accelerate the set. Conversely, on cold days, cooler tap water may retard the set.

Alginate left in the mixing bowl can be checked for completeness of set. The impression should be left in the mouth for approximately 1 minute after the set, because it gains in tear strength during this time. This may not be possible with patients who gag easily. When you are ready to remove the tray, use a finger at the side of the tray to apply pressure to break the seal while pulling the tray quickly away from the teeth with a snap. Protect the teeth in the opposing arch with fingers placed on top of the tray.

The impression should be rinsed thoroughly under running water to remove adherent saliva. Next, it should be evaluated to determine whether the impression is acceptable for its anticipated use. If determined to be acceptable, the impression is held in a plastic bag (to prevent the inhalation of disinfectant spray) and sprayed with a suitable disinfecting solution. An alternative to spraying the impression is to immerse it for 10 minutes in a suitable disinfectant. Immersion for up to 30 minutes in 1% sodium hypochlorite or 2% glutaraldehyde has been shown not to significantly affect the dimensions (by swelling) or surface detail of alginate.

Alginate impressions should be evaluated immediately after they are removed from the mouth and rinsed. The determination should be made at this point as to whether or not the impression should be repeated, so it can be done while the patient is still seated and the operatory is set up for it. An acceptable impression will cover all areas of interest (teeth, ridge form, muscle attachments, palate, etc.). The structures should be recorded with sufficient detail to be clearly identified and should not have a grainy surface, which is usually the result of inadequate mixing. There should be minimal voids caused by entrapped air, especially in areas critical to the use of the impression (e.g., occlusal surfaces if a night guard will be made). The alginate should be fully seated in the tray and should not have pulled free or distorted. The impression should be free of debris. Table 15-2 lists criteria used to assess an alginate impression for clinical acceptability. When problems are found with an impression, refer to Table 15-3 for a troubleshooting guide that suggests possible causes and solutions for a variety of problems.

Elastomers are highly accurate elastic impression materials that have qualities similar to rubber and, hence, are often called rubber materials. They are used extensively in restorative dentistry for construction of metal castings, ceramic restorations, bridges, implant restorations, partial denture frameworks, and complete dentures. The four types of elastomers are as follows:

The rubbery nature of elastomers means that they do not adhere well to solid metal or custom acrylic impression trays. An adhesive is placed in the tray to prevent the material from separating from the tray and causing distortion. Each type of elastomer has its own adhesive with which it is compatible; therefore adhesives should not be interchanged among different types of materials. The tray adhesive should be applied in a thin layer and allowed to dry. If the adhesive is not applied well in advance of use of the tray, then an air syringe can be used to accelerate the drying process.

Because of their rubbery nature, elastomers have a certain amount of elastic recovery, or “rebound,” from deformation. Rebound reduces distortion in the cast that is poured from the impression. PVS impression material has the best elastic recovery of the elastomers.

Elastomers generally are not wet well by water (and are therefore called hydrophobic), because water forms a high contact angle with them (see Figure 5-1 in Chapter 5 [Principles of Bonding]). In other words, water beads on their surface much like raindrops on a newly waxed car. Of the elastomers, the polyethers are the most hydrophilic, or wettable. Wettability can be seen clinically when impression materials are able to capture the detail of a tooth preparation when the surface is moist (but not submerged in water or saliva). It also means that wet gypsum materials will flow better into the fine details of the preparation when the impression is poured.

Polysulfides are the oldest of the elastomers and are commonly referred to as “rubber base.” They are more dimensionally stable and have greater tear strength than alginate or agar hydrocolloids. They are more accurate than alginate but not as accurate as the other elastomers. Polysulfides have been used successfully for crown and bridge impressions and for partial and complete denture impressions. They cannot be used in automixing cartridges and must be hand mixed. They are messy and have an unpleasant sulfur odor. When polyethers and polyvinyl siloxanes came on the market most practitioners abandoned the polysulfides for these more accurate, dimensionally stable, and pleasant materials. Polysulfides are still used by some for impressions for complete dentures. Detailed information for the polysulfides can be found at the Evolve website at www.evolve.elsevier.com/Hatrick/materials.

Two types of silicone impression materials have been developed and are named according to the type of polymerization reaction they undergo during setting. Condensation silicones were introduced in the 1960s and were useful for crown and bridge procedures. Addition silicone in the form of polyvinyl siloxane was introduced in the 1970s and because of its superior properties it soon replaced condensation silicone.

Condensation silicone was developed as an alternative to the messy, smelly polysulfide and was first used in the 1960s. It has more desirable characteristics than polysulfide, such as ease of mixing, pleasant taste, no odor, and shorter working and setting times. The material sets through a condensation reaction that produces ethyl alcohol as a by-product. The ethyl alcohol is rapidly lost by evaporation, leading to a relatively high dimensional instability from shrinkage. Condensation silicones have been replaced by the more accurate and stable addition silicones.

The addition silicone impression materials are an improvement over the condensation silicone materials. Their properties provide greater dimensional stability and accuracy. They are clean and easy to use, with no foul odor or taste. As a result of these improvements, they have become the most popular materials for crown and bridge procedures. They are also among the most expensive of the impression materials.

Addition silicone impression material, commonly known as polyvinyl siloxane (PVS) or vinyl polysiloxane (VPS), undergoes a polymerization reaction of chain lengthening (called an addition reaction) and cross-linking with reactive vinyl groups that produces a stable silicone rubber. The addition reaction does not produce a by-product that can evaporate and cause shrinkage as with the condensation silicones. PVS has the smallest dimensional change (0.05%) on setting of the elastomers and hydrocolloids. PVS materials have high elastic recovery after removal from undercuts, and they resist tearing (high tear strength).

PVS impression materials are hydrophobic by nature and must be used in a dry field. A little moisture on the prepared tooth will result in loss of surface detail in the impression, because the impression material cannot displace the moisture and establish close contact with the surface (it has a high contact angle and low wetting of the surface).

PVS is manufactured in light, extra light, regular (or monophase), and heavy viscosities. A monophase viscosity is available from most manufacturers that is used as both a tray material and a syringe material. It is not as viscous as the tray material and is thicker than the light body material, yet it flows well enough to be used in a syringe to be injected around a tooth preparation. Some PVS materials are also available in putty form, consisting of base and catalyst putty. Powdered silica is added as a filler to give thickness to the base or catalyst pastes or putties.

The accuracy of an impression material is measured by how well it captures the surface detail of a structure. To capture the surface detail the material must wet (have a low contact angle) and flow over the surface well. Low-viscosity materials (wash/syringe materials) wet and flow better than high-viscosity (tray/heavy body) materials and, therefore, capture more detail.

The most popular dispensing system for the light, extra light, regular, monophase, and heavy materials involves a cartridge with two chambers—one with base and one with catalyst. A mixing tip fits on the end of the cartridge (Figure 15-3). A hand-operated gun-type dispenser or a motor-driven dispenser (see Figure 15-6) pushes both the base and the catalyst through the mixing tip at the same time. They pass through an intertwined spiral that mixes appropriate amounts of each material together thoroughly by the time they exit the end of the tip. These mixing devices ensure the proper ratio of the two materials without the creation of bubbles or voids, which are common with hand-mixing. It is important for the operator to make certain that the orifices of the cartridges are cleaned of any residual set material that might block the flow of base or catalyst before applying the mixing tip. Otherwise, proper proportions of base and catalyst may not be mixed.

Because of the popularity of the PVS materials, manufacturers have put much effort into improving their properties to make their products more appealing than those of their competitors. The working time of elastomers (from start of mix until it can no longer flow) is approximately 2 minutes. The setting time is the time measured from the start of mix to the time the material is hard and can be removed from the mouth. Setting times have been adjusted so that the practitioner has an assortment of materials with fast or regular set. Setting times vary among manufacturers and range from approximately 2 minutes (fast set) to 6 minutes (regular set). The working and setting times are affected by temperature. On a warm day the materials may set faster. Working and setting times can be increased by refrigerating the material before use.