Impression Materials and Laboratory Procedures

Learning Objectives

1 Pronounce, define, and spell the Key Terms.

2 Describe the types of impression trays and their characteristics of use.

3 Discuss hydrocolloid impression materials and their uses, mixing techniques, and application.

4 List the uses of a diagnostic cast.

5 Discuss elastomeric impression materials and their uses, mixing techniques, and application.

6 Describe the importance of a bite registration and its uses in a procedure.

Key Terms

Alginate

Automix

Centric

Diagnostic Cast

Extrude

Impression

Model Trimmer

Perforated

Plaster

Polyether

Polysulfide

Registration

Silicone

Stone

Trays

Viscosity

In many dental procedures, an impression will be taken. Impressions in dentistry are used for creating a model for diagnosing conditions, making custom trays, making a temporary crown, or for the laboratory technician to create a permanent indirect casting. Because of the many uses of an impression, many types of impression materials are available.

The dental assistant is responsible for knowing the different types of impression materials used in the office, the setup for the procedure, the mixing of the material, and either assisting in the procedure or the actual taking of the impression.

Impression Trays

Impression trays are designed for obtaining an accurate impression of the area required. The type of tray selected for a procedure will depend on (1) the dentist’s preference and (2) what will provide the most accurate result for the type of impression material being used.

Impression trays are supplied as quadrant trays, which cover half of the arch; section trays, which are suited for the anterior teeth; and full arch trays, which cover the complete arch. Table 22-1 reviews the different types of trays available (Figure 22-1).

TABLE 22-1

Types of Impression Trays

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Types	Characteristics	Use
Stock trays (see Figure 22-1)	Pre-formed tray made from a hard plastic, metal, or styrofoam. Supplied in standard sizes for children through adult. They can be perforated to allow the impression material to form a mechanical lock with the tray, or smooth, which would require an adhesive to be applied to hold the impression material securely in the tray.	Preliminary impressions
		Fabricating provisional coverage
		Final impressions
Custom trays (see Figure 22-1)	Tray that is customized to fit a patient’s mouth. Made from light-cured resin, acrylic resin, or thermoplastic resin.	Final impressions
		Fabricating provisional coverage
		Vital bleaching

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Figure 22-1 Types of stock trays. A, Metal perforated tray used for preliminary impressions. B, Metal water coolant tray used with reversible hydrocolloid impressions. C, Plastic perforated tray used for preliminary and final impressions. D, Gauze tray used for bite registrations. E, Triple tray used to take final impression and bite registration at the same time. F, Acrylic custom tray used to take final impressions for fixed and removable prostheses. G, Vacuum-formed custom tray, used for tooth whitening and for fabricating provisionals. (E, From Boyd LB: Dental instruments: a pocket guide, ed 4, St Louis, 2012, Saunders. F and G, From Bird DL, Robinson DS: Modern dental assisting, ed 10, St Louis, 2012, Saunders.)

Impression Materials

Impression materials are selected because of their unique qualities that allow the dentist to get the most accurate reproduction of the tooth and surrounding tissues. An impression is a negative reproduction of the teeth and their surrounding structures. When an impression is poured in stone or plaster to make a model, it creates a positive reproduction of the teeth and their surrounding structures. This model is referred to as a cast.

There are three types of impressions that can be obtained (Box 22-1):

Preliminary impression (Figure 22-2)

Final impression (Figure 22-3)

Occlusal (bite) registration (Figure 22-4)

Box 22-1

Types of Impressions Obtained in Dental Procedures

• Preliminary impressions—used for the making of (1) diagnostic models, (2) custom trays, (3) provisional coverage, (4) dental and orthodontic appliances, and (5) pretreatment and posttreatment records.

• Final impressions—show more accurate tissue and tooth structure detail for the laboratory technician to make a cast restoration.

• Occlusal (bite) registrations—reproduce the occlusal relationship of the maxillary and mandibular teeth when occluded.

Figure 22-2 Example of a preliminary impression.

Figure 22-3 Example of a final impression. (From Hatrick CD, Eakle WS, Bird WF: Dental materials: clinical application for dental assistants and dental hygienists, ed 2, St Louis, 2011, Saunders.)

Figure 22-4 Example of a bite registration. (Courtesy 3M Dental Products, St Paul, Minnesota.)

Preliminary Impressions

Preliminary impressions are used to create a reproduction of the teeth and surrounding tissues. A preliminary impression can be taken by the dentist or by the expanded-functions dental assistant.

Alginate

Alginate is an irreversible hydrocolloid (“hydro” meaning water and “colloid” meaning a gelatinous substance) and is the material of choice when taking preliminary impressions. Alginate consists of potassium alginate, which is derived from seaweed; calcium sulfate; trisodium phosphate; diatomaceous earth; zinc oxide; and potassium titanium fluoride. This material has two physical phases that it goes through when setting: a sol (solution) phase, where the material is in a liquid or semiliquid form; and a gel (solid) phase, where the material becomes semisolid.

The material is available in two settings: normal set, which has a working time of 2 minutes and a setting time of up to minutes, and fast set, which has a working time of minutes and a setting time of 1 to 2 minutes.

Alginate is supplied as a powder, packaged in premeasured packages or in bulk canisters. A plastic scoop is provided for dispensing the powder, and a plastic cylinder is supplied for measuring the water (Figure 22-5). The water-to-powder ratio for mixing alginate is 1 scoop of powder to 1 “measure line” of water. When mixing the material for a mandibular impression, generally 2 scoops of powder and 2 measure lines of water are required. For a maxillary impression, 3 scoops of powder and 3 measure lines of water are necessary.

See Procedure 22-1: Mixing Alginate Impression Material.

Taking Alginate Impressions

It is important for the clinical assistant to be ready to mix the alginate, load the tray, and help keep the patient comfortable while the dentist takes the impression. If this procedure is a legal function in the state in which the dental assistant works, then he or she would proceed with taking the impression. When taking an impression, there are specific steps to be followed to gain an accurate impression (Box 22-2). Before taking the impression, the procedure should be explained to the patient to ensure his or her comfort. The patient should be informed of the following:

• That the material will feel cold, there is no unpleasant taste, and the material will set quickly

• That he or she should breathe deeply through the nose. This will help the patient relax and be more comfortable.

• That the patient should not talk after the tray has been placed. If the patient needs to communicate, he or she should do so through hand gestures.

Box 22-2

Steps in Taking an Accurate Alginate Impression

• Position yourself for easier insertion of the tray and better control of the patient.

• Turn the tray slightly to insert a corner of the tray into the patient’s mouth.

• Retract the patient’s cheek when sliding the tray into the patient’s mouth.

• Make sure the tray is centered over the teeth.

• Seat the posterior portion of the tray first, followed by the anterior portion. This keeps a lot of the material from escaping toward the back of the throat.

• Try not to “overseat” or push the tray too hard.

• Gently pull the patient’s lips out around the tray.

• Hold the tray firmly in place while the material sets.

See Procedures 22-2: Taking a Mandibular Preliminary Impression (Expanded Function), and 22-3: Taking a Maxillary Preliminary Impression (Expanded Function).

Diagnostic Casts

A diagnostic cast is the model that is made from the impression. The materials most commonly used to create diagnostic casts are model plaster and dental stone.

Model plaster, which is a derivative of plaster of Paris, is used when strength is not essential and dimensional accuracy is not critical. Model plaster is easy to trim and is excellent for diagnostic casts because of its clean appearance (Figure 22-6).

Figure 22-6 Dental models made from plaster.

Dental stone, which is a form of gypsum, is stronger than model plaster and is commonly used when a more durable diagnostic cast is required, for example, when used as a working model to make a retainer, a custom tray, or a casting by the laboratory technician.

Water-to-Powder Ratios

Although plaster and dental stone have the same chemical formulas, their physical structures are different, and it is necessary to use specific water-to-powder ratios for each. The water-to-powder ratio has a dramatic effect on the setting time and strength of any gypsum product. Water is measured by volume (cc), and powder is measured by weight (g).

Each type of cast has an optimal water-to-powder ratio, which has been specified by the manufacturer. These ratios should be carefully followed. The following are the recommended water-to-powder ratios for one impression, including the base:

• Model plaster: 45 to 50 ml of water to 100 g of powder

• Dental stone: 30 to 32 ml of water to 100 g of powder

• High-strength stone: 19 to 24 ml of water to 100 g of powder

Influences on Setting Time

It is important to have enough working time to mix the material and transfer it into the impression. Setting time is the length of time it takes for the mixture of stone or plaster to turn into a rigid solid. The setting time of gypsum products is influenced by the factors described in Table 22-2.

TABLE 22-2

Factors Influencing the Setting of Gypsum

Factors	Reaction
Type of gypsum	Model plaster sets faster than dental stone.
Water-to-powder ratio	The less water used, the faster the set.
Mixing	The longer and faster the mixing, the faster the set.
Temperature of water	For best results, water should be at room temperature and no warmer than 70° F (21.1° C). The warmer the water, the faster the set.
Humidity	On a humid day, the gypsum can absorb water, which will slow the set.

See Procedure 22-4: Mixing Dental Plaster.

Pouring Diagnostic Casts

Three methods are commonly used for pouring diagnostic casts: the double-pour method, the box-and-pour method, and the inverted-pour method. These differ only in the way the base portion is formed (Figure 22-7).

Figure 22-7 Examples of pouring methods. *Upper left,* boxed; *upper right,* inverted; *lower,* double pour.

See Procedure 22-5: Pouring Dental Models Using Inverted-Pour Method.

Trimming and Finishing Diagnostic Casts

When diagnostic casts are to be used for a case presentation or as part of the patient’s permanent record, they should have an esthetic appearance. This is accomplished by trimming the casts to a geometric standard.

Casts are trimmed by using a model trimmer (Figure 22-8). A model trimmer is a device that is set up in the lab. The trimmer has a circular abrasive wheel that is set at 90 degrees to the cast. The wax bite registration is used to articulate the casts during the trimming process.

The cast consists of anatomic and art portions (Figure 22-9).

Figure 22-9 Anatomic and art portions of a dental model.

The anatomic portion includes the teeth, oral mucosa, and muscle attachments. This portion should make up two thirds of the overall cast.

The art portion forms the base. This section should be no more than one-half inch thick and should make up one third of the overall cast.

See Procedure 22-6: Trimming and Finishing Dental Models.

Final Impression Materials

As discussed earlier in the chapter, final impression materials are designed to provide the dentist and laboratory technician a more accurate definition of a prepared tooth and/or surrounding gingival structures.

Elastomeric Impression Materials

Elastomeric impression materials have an elastic or rubber-like quality after setting. These materials are supplied as a base and a catalyst and are self-curing. Impression material is packaged several ways for preference of use: tubes of paste, cartridge, or putty system (Figures 22-10 and 22-11).

Figure 22-10 Example of a two-paste final impression material.

Figure 22-11 Example of a cartridge or putty system of final impression material.

Final impression materials are supplied in three forms or viscosities (viscosity is the ability of the material to flow):

1 Light body is the thinnest material of the three. This material is expressed from a syringe around the tooth and into the sulcus of the prepared tooth or teeth. This gives the detail of the margin that was created by the dentist.

2 Regular body is slightly thicker than light body. This material is used as a tray material but has the ability to flow more easily, thus requiring improved control from the tray.

3 Heavy body is the thickest of the three. This material is most commonly used as a tray material and has the ability to force the light-bodied material into close contact with the prepared tooth and surrounding tissue to ensure a more accurate impression.

Types of Impression Materials

The types of final impression materials commonly used in dental practice are polysulfide, polyether, condensation silicone, and addition silicone. Each type of material has different properties and characteristics. Table 22-3 provides a summary of the comparative properties of these materials.

TABLE 22-3

Properties of Final Impression Materials

Type	Characteristics	Working Ability
Polysulfide	A paste material supplied in two tubes. A strong odor and taste are associated with the material. The stiffness and stability of the material are poor.	A harder material to mix. It has a longer mixing time of 60 seconds and a longer setting time of 10-20 minutes.
Polyether	A paste material supplied in tubes or cartridges that has an acceptable odor and taste. The stiffness and stability of the material are very good.	An easy material to mix, with a short mixing time of 30-45 seconds, and a fast setting time of 6-7 minutes.
Condensation silicone	Material supplied as a paste and a liquid. It has an acceptable odor and taste. The stiffness and stability are average.	A fairly easy material to mix, with an average mixing time of 30-60 seconds, and an average setting time of 6-10 minutes.
Addition silicone	A paste material supplied as a two-paste system, as a putty material, or in cartridges. This material has an acceptable odor and taste. The stiffness and stability are excellent.	An easy material to mix, with a short mixing time of 30-45 seconds, and an average setting time of 6-8/> Only gold members can continue reading. Log In or Register to continue

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22. Impression Materials and Laboratory Procedures

Impression Materials and Laboratory Procedures

Learning Objectives

Key Terms

Impression Trays