CHAPTER 35 Impressions, Study Casts, and Oral Stents

Esther K. Andrews

Competencies

Use alginate impression material to make dental impressions for the construction of study casts and baseplate wax to register a bite.

Mix model plaster to make study models and dental stone to make diagnostic casts from a dental impression.

Trim and finish study models and/or diagnostic casts.

Differentiate between the various stents and splints used for dental procedures.

Construct an ethylene vinyl acetate mouth guard, following safety procedures.

A dental impression is a negative imprint of the teeth and surrounding tissues. A dental impression is used to create an accurate three-dimensional reproduction of the teeth and surrounding tissues. This positive reproduction is called a study model or a diagnostic cast. There are three main types of dental impressions used in dentistry: a preliminary impression, a final impression, and a bite registration. Preliminary impressions are taken to construct study models for diagnosing, documenting clients' dental arches as part of permanent records, and enhancing client education as a visual aid. Final impressions are used to make casts with more accurate detail of the tooth structures and surrounding tissues that are used to make oral stents (molds or devices), temporary crowns, or removable dental appliances. Final impressions also are used by dental laboratory technicians to construct crowns, bridges, partial or full dentures, and restorations placed on titanium implants. The dental hygienist often has the responsibility to take preliminary dental impressions, construct casts, trim models, and fabricate custom-made oral stents and splints.

Stents are molds or devices used for the target delivery of materials (e.g., tooth whitening, home fluoride application) and protection of oral structures. Splints are orthopedic devices used for immobilization, restraint, or support of the teeth and/or jaws. This chapter provides an overview of concepts related to making dental impressions, diagnostic casts, bite registrations, and custom-made stents and splints. The reader is advised to consult a text on dental materials for complete information on properties and manipulation of the different types of dental materials.

DENTAL IMPRESSIONS

Impression Trays

Impression material is placed into the mouth in an impression tray. Because dental impressions are used in oral healthcare for many purposes, there are various types of impression trays. Impression trays are designed for different areas of the mouth and include quadrant trays, which cover half an arch, and full trays, which cover the complete maxillary or mandibular arch (Figure 35-1). Tray selection depends on the purpose of the impression. Metal, plastic, or Styrofoam trays are available in standard small, medium, and large sizes for children and adults. They may be perforated to promote a mechanical lock with the impression material (Box 35-1). Procedure 35-1 describes the details of selecting the correct size tray and preparing it for use.

Figure 35-1 Types of impressions trays.

(From Bird DL, Robinson DS: Torres and Ehrlich modern dental assisting, ed 9, St Louis, 2009, Saunders.)

BOX 35-1 Guidelines for Proper Impression Tray Selections

• Tray should feel comfortable to the client.

• Tray should extend slightly beyond the facial surfaces of the teeth to enclose all teeth, musculature, and vestibule.

• Tray should extend 2 to 3 mm beyond the most posterior tooth in the arch and include the retromolar or maxillary tuberosity area.

• Tray should allow for a 2- to 3-mm depth of material beyond the biting surfaces or edges of the teeth.

• Tray should be comfortable and minimize tissue trauma during insertion and removal.

Procedure 35-1 SELECTING THE CORRECT TRAY SIZE AND PREPARING IT FOR USE

EQUIPMENT (FIGURE 35-3)

Personal protective equipment

Antimicrobial mouth rinse

Lubricating gel

Maxillary and mandibular impression trays

Mouth mirror

Utility wax

STEPS

Preparation

1. Gather all necessary supplies.

2. Position self at side and in front of client, and seat the client in an upright position.

3. Explain the procedure to the client. Have client remove any removable oral appliances.

4. Don personal protective equipment. Disinfect hands and don gloves.

5. Place protective eyewear on the client.

6. Provide preprocedural antimicrobial mouth rinse.

7. Lubricate the client's lips with a small amount of lubricating gel.

Mandibular Tray Selection

8. Inspect client's mouth to estimate tray size. Note teeth out of alignment, tori, and length of dental arch that may require additional tray adaptation for client comfort.

9. Instruct client to tilt chin down. Retract the client's lip and cheek with index and middle fingers of nondominant hand and at the same time turn the tray sideways and distend the lip and cheek on the opposite side of the mouth with the side of the tray to gain entry into the client's mouth. Insert the tray with a rotary motion (Figure 35-4).

10. Make sure the tray is centered over the lower teeth by placing the handle at the midline, usually between the central incisors and in line with the center of the chin.

11. Instruct client to raise tongue. Lower the tray and at the same time retract the cheek to make certain the buccal mucosa is not caught under the rim of the tray.

12. Check to be sure that the tray covers the teeth and soft tissue. Lift the front of the tray to make certain that the area posterior to the retromolar pad is covered and that there is enough room to allow for ¼ inch of impression material in the facial and lingual surfaces of the teeth. If necessary, adapt the tray borders with utility (beading) wax to extend into the depth of the vestibule or extend the posterior length of the tray (Figure 35-5).

13. Reselect larger or smaller tray as needed.

Maxillary Tray Selection

14. Repeat steps 8 and 9.

15. Center the tray by placing the handle between the central incisors in line with the center of the nose.

16. Bring the front of the tray about ¼ inch anterior to the incisors.

17. Seat the tray first by lowering the handle toward the mandibular teeth.

18. Make certain all the posterior teeth and soft tissue, including the maxillary tuberosity, are covered. Check that laterally there is enough room to allow for ¼ inch space between the inside of the tray and the facial and lingual surfaces of the teeth.

19. Retract the lip and raise the anterior portion of the tray into place. The tray should fit to the depth of the vestibule and not impinge on soft tissue.

20. Reselect larger or smaller tray as needed.

Tray Preparation

21. Spray smooth trays with adhesive. Wait 15 minutes before use.

22. Record service in “Services Rendered” section of the client's dental chart, and date the entry.

Alginate Impression Material

The impression material used most often in dentistry for preliminary impressions is hydrocolloid (hydro meaning water and colloid meaning gelatin). Because hydrocolloid is in a water suspension, the product is hydrophilic, meaning “loves water.” Hydrocolloids can exist in a sol (solution) or a gel (solid) state. Depending on the type of hydrocolloid used, the physical change from sol to gel is either reversible (changed by thermal factors) or irreversible (changed by chemical factors).¹

Gelation is the transformation from sol to gel. An irreversible hydrocolloid does not change its physical state after gelation. Alginate is an irreversible hydrocolloid powder impression material that changes from a sol to a gel state by means of a chemical reaction. Cool water is added to the potassium alginate powder and mixed to produce a sol. The impression material reaches the gel state after the chemical setting reaction in the client's mouth. At this time it is removed from the mouth.¹

Packaging and Storage

Alginate impression material is available either in premeasured packages or in bulk canisters. The premeasured packages are more expensive, but they save time by eliminating the need to measure the powder. Because the powder deteriorates when exposed to elevated temperatures or water, it is important to store the powder in a tightly closed container in a cool, nonrefrigerated place. The individual premeasured packages should be used immediately on opening to avoid water condensing on the powder from the humidity in the air. Properly stored alginate impression material has a shelf life of about 1 year.

Although an alginate impression does not change its physical state after gelation, it is subject to distortion as a result of slight changes in its physical surroundings. As a result, alginate impressions must be “poured up” within an hour of having been made. The potential for dimensional change is due to the fact that so much of the material is made of water. For example, if left exposed to the room environment uncovered, the impression may shrink owing to syneresis (the loss of water) from evaporation. Imbibition is the uptake of water in the presence of moisture. It is recommended that a disinfected impression be wrapped in a slightly moistened towel and placed in a plastic biohazard bag or unwrapped in a humidor (a covered container with a moist environment) to cause the least amount of distortion.¹

Water/Powder Ratio

When alginate powder stored in canisters is used, a plastic scoop for dispensing the powder and a calibrated plastic cylinder for measuring the water are supplied (Figure 35-2). Mandibular alginate impressions generally require two scoops of powder and two measure lines of water. A large mandible may require three scoops of powder to three measure lines of water. A maxillary impression generally takes three scoops of powder and three measure lines of water.¹

Figure 35-2 A plastic scoop and plastic cylinder are supplied with alginate.

(From Bird DL, Robinson DS: Torres and Ehrlich modern dental assisting, ed 9, St Louis, 2009, Saunders.)

Figure 35-3 Equipment for selecting and preparing an impression tray.

(Courtesy Gwen Essex.)

Figure 35-4 Inserting impression tray.

(Courtesy Gwen Essex.)

Figure 35-5 Extending impression tray with utility wax.

(From Bird DL, Robinson DS: Torres and Ehrlich modern dental assisting, ed 9, St Louis, 2009, Saunders.)

Figure 35-6 Equipment for mixing alginate impression material.

(From Bird DL, Robinson DS: Torres and Ehrlich modern dental assisting, ed 9, St Louis, 2009, Saunders.)

Figure 35-7 Proper consistency of mixed alginate impression material.

(From Bird DL, Robinson DS: Torres and Ehrlich modern dental assisting, ed 9, St Louis, 2009, Saunders.)

Figure 35-8 Scoop for alginate powder, water dispensers, mixing bowl, spatula, and stock impression tray.

(From Bird DL, Robinson DS: Torres and Ehrlich modern dental assisting, ed 9, St Louis, 2009, Saunders.)

Figure 35-9 The mandibular impression tray is filled with alginate and is smoothed.

(From Bird DL, Robinson DS: Torres and Ehrlich modern dental assisting, ed 9, St Louis, 2009, Saunders.)

Figure 35-10 The mandibular impression tray is seated in the arch with the tongue out of the way.

(Courtesy Gwen Essex.)

Figure 35-11 Holding the mandibular tray.

(Courtesy Gwen Essex.)

Figure 35-12 How a mandibular impression must look.

(Courtesy Gwen Essex.)

Figure 35-13 Rinsing the impression.

(From Bird DL, Robinson DS: Torres and Ehrlich modern dental assisting, ed 9, St Louis, 2009, Saunders.)

Figure 35-14 Spraying the impression.

(Courtesy Gwen Essex.)

Figure 35-15 Impression in precaution bag with client name.

(From Bird DL, Robinson DS: Torres and Ehrlich modern dental assisting, ed 9, St Louis, 2009, Saunders.)

Figure 35-16 The maxillary impression tray is filled with alginate. The filled tray is smoothed on the alginate surface.

(Courtesy Gwen Essex.)

Figure 35-17 A maxillary alginate impression is placed in the arch. The maxillary lip is lifted and positioned outside of the tray.

(Courtesy Gwen Essex.)

Figure 35-18 How a maxillary impression must look.

(From Bird DL, Robinson DS: Torres and Ehrlich modern dental assisting, ed 9, St Louis, 2009, Saunders.)

Figure 35-19 Supplies for taking a wax-bite registration.

(From Bird DL, Robinson DS: Torres and Ehrlich modern dental assisting, ed 9, St Louis, 2009, Saunders.)

Figure 35-20 Wax-bite registration in client's mouth.

(From Bird DL, Robinson DS: Torres and Ehrlich modern dental assisting, ed 9, St Louis, 2009, Saunders.)

Figure 35-21 Wax-bite registration on a wax wafer.

(From Bird DL, Robinson DS: Torres and Ehrlich modern dental assisting, ed 9, St Louis, 2009, Saunders.)

Setting Time

The time needed to mix the impression material, load the tray, and seat it in the client's mouth is called the working time. The time required for gelation, after which the impression tray is removed from the client's mouth, is called setting time. Alginate impression material is available in normal-set powder (a working time of 2 minutes and a setting time of up to 4.5 minutes) and fast-set powder (a working time of 1.25 minutes and a setting time of 1 to 2 minutes). Currently there are several brands of color-changing alginate available. When the alginate powder is mixed with water, it changes color from purple to pink or peach when it is time to place it into the mouth, and it changes to white at gelation.

The alginate powder is mixed with room-temperature water (68° to 70° F or 20° to 21° C). The temperature of the water is measured with a thermometer before the powder is introduced into the water. Powder is incorporated into the water to wet the powder completely and then vigorously mixed until a smooth, creamy consistency is achieved. To allow the chemical reaction to proceed effectively, manufacturer's instructions with regard to mixing time should be followed. Loading the tray and insertion should take no more than 1 minute. The objective is for the impression material to reach the gel state in the client's mouth. Procedure 35-2 describes the details for mixing alginate impression material. Table 35-1 presents factors that affect gelation and therefore the success of the impression.¹

Procedure 35-2 MIXING ALGINATE

EQUIPMENT (FIGURE 35-6)

Personal protective equipment

Alginate powder

Water

Measuring scoop

Vial for measuring water

Wide-blade spatula

Rubber mixing bowl

Timer

Thermometer

STEPS

1. Read the manufacturer's directions for the dispensing and manipulation of the alginate.

2. Place one measure of room-temperature water into the mixing bowl for each scoop of alginate. Check temperature of water with thermometer.

3. Shake or fluff the alginate by tipping the container two or three times.

4. Overfill the correct scoop with powder; tap the scoop with the side of the spatula. Scrape the excess from the scoop with the spatula.

5. Sift the powder into the water, and stir with the spatula until all the powder has been moistened.

6. Cup the rubber bowl in your hand with the mouth of the bowl next to the wrist. Firmly spread the alginate between the spatula and the side of the rubber bowl. Spatulate the mixture vigorously using a back-and-forth hand motion, spreading the material against the sides of the bowl. Use both sides of the spatula, and turn the bowl with your fingers during spatulation (Figure 35-7).

7. Spatulate vigorously for 30 seconds and gather the material together. Use the spatula to crush the mixture and spread it out again. Repeat until a smooth, creamy consistency is achieved within the designated mixing time for either the normal-set or fast-set alginate.

8. Gather the material into one mass, and wipe on the inside edge of the mixing bowl.

TABLE 35-1 Factors That Affect Gelation

Factor	Comments
Ratio of water to alginate powder	Manufacturer's directions must be followed carefully. The water/powder ratio must be exact. Too much water in the mix will make a weak impression that will tear easily during removal from the mouth owing to tension. Too little water creates a grainy impression that will cause an inaccurate reproduction of the hard and soft tissues. If the container holding the alginate is not fluffed before measuring, too much powder will be dispensed, causing a grainy impression.
Water temperature	Water temperature affects gelation time. If the water is too warm, the product will gel at a faster rate, resulting in poor detail in the impression. If the water is cool, the product will gel at a slower rate and the final impression will be more accurate in detail. In hot, humid climates, it is recommended to use cooler water and to refrigerate the bowl and spatula.
Spatulation technique	Proper mixing (spatulation) will determine setting time. A mechanical device that automatically mixes the material can be used. Too much spatulation will decrease the strength of the impression material because the gel is broken as it forms. Too little spatulation decreases strength up to 50% and will cause a grainy impression that is inaccurate in details of the mouth.
Tray movement	Movement of the tray during gelation causes an inaccurate impression. It is important to hold the impression tray steady in the mouth during gelation.
Removal of impression	Premature removal of impression from the mouth creates an inaccurate impression because the material has not fully gelled. The most frequent result of premature removal is inaccuracy of the incisor teeth. The elasticity of alginate increases with time. A better impression results from being patient. Rocking the tray back and forth to release it from the client's mouth may cause distortion of the impression.
Improper storage of impression	The impression should be poured within 1 hour. Leaving the impression unprotected to the environment can result in imbibitions or syneresis. After disinfection, maintain the integrity of the impression, if it cannot be poured with a gypsum product, immediately by wrapping the impression in a wet towel or storing it in a humidor. A humidor is a closed plastic container with a moist bottom layer of paper towels that create a humid environment. Before taking the maxillary impression, wrap the mandibular impression in a wet towel.

Impression Taking

Before the impression is taken, the procedure is explained to the client to enhance the client's comfort and cooperation. Specifically, clients should be informed of the following:

That the material will feel cool, what the specific flavor is, and that the material will gel quickly

That breathing deeply through the nose during the procedure will help them to relax

That refraining from talking after the tray has been placed will help to produce a good impression

That if the client needs to communicate during the procedure, raising a hand is best

Box 35-2 lists areas to precoat with alginate. Box 35-3 lists criteria for a quality impression. Procedures 35-3 and 35-4 present details for making mandibular and maxillary dental impressions, respectively. The impression of mandibular teeth should be taken first, because gagging is less likely in the mandibular area and enhances trust in the clinician (see Box 35-4 for additional suggestions to minimize gagging).

BOX 35-2 Mouth Areas to Precoat with Alginate

• Occlusal surfaces

• Tooth surfaces adjacent to edentulous areas

• Areas of erosion, abfraction, or abrasion

• Vestibular areas

BOX 35-3 Criteria for a High-Quality Alginate Impression

• No visible voids, tears, or debris

• Clear and distinct detail of desired structures

• Retromolar area or maxillary tuberosity present

• Alginate material firmly attached to tray

• Adequate peripheral roll

Procedure 35-3 MAKING A MANDIBULAR PRELIMINARY IMPRESSION

EQUIPMENT (FIGURE 35-8)

Personal protective equipment

Antimicrobial rinse

Occupational Safety and Health Administration (OSHA)–approved disinfectant

Alginate powder

Water

Measuring scoop

Vial for measuring water

Wide-blade spatula

Utility wax

Rubber mixing bowl

Selected mandibular impression tray

Saliva ejector

STEPS

Preparation

1. Gather all necessary supplies. Seat the client upright and explain the procedure. Have client remove any removable oral appliances.

2. Check the client's health history to determine any risk factor that may complicate the procedure.

3. Don personal protective equipment, safety glasses, mask, and bonnet. Disinfect hands and don gloves.

4. Place protective eyewear on the client.

5. Provide preprocedural antimicrobial mouth rinse.

6. Lubricate the client's lips with a small amount of moisturizer.

7. Dry the teeth with compressed air.

8. Measure two measures of room-temperature water with two scoops of alginate, and mix the alginate.

Loading the Tray

9. Quickly gather half the alginate in the bowl onto the spatula. Wipe the alginate into one side of the tray from the lingual side, working from the posterior toward the anterior. Fill to an area just below the rim. Quickly press the material down to the base of the tray.

10. Gather the remaining half of the alginate in the bowl onto the spatula and load the other side of the tray in the same way.

11. Moisten fingers with cold water and smooth over alginate. Make a slight indentation where teeth will insert (Figure 35-9).

12. Take a small amount of impression mixture from the spatula and quickly apply to the occlusal surfaces of the teeth, undercut areas, and vestibular areas.

Seating the Tray

13. Place yourself at the 8-o'clock position (4-o'clock position if left-handed), and ask the client to tilt the chin down.

14. Turn the impression tray sideways.

15. Retract the client's lip and cheek with fingers of nondominant hand. Turn the tray sideways when placing it in the mouth, distending the lip and cheek on the opposite side of the mouth with the side of the tray.

16. Center the tray over the teeth, and center the handle in line with the center of the client's chin.

17. Align the tray ¼ inch anterior to the incisors. Press down the posterior portion of the tray first and then seat the anterior portion of the tray directly down. Instruct client to raise the tongue (Figure 35-10).

18. Instruct client to move the lips and to breathe normally.

19. Hold the tray steady in place until the material has gelled. Apply firm bilateral pressure with the middle fingers, and use the thumbs to support the jaw (Figure 35-11).

Removing the Impression

20. Place fingers of nondominant hand on top of the tray. The index finger of the nondominant hand rests on the incisal surface of the maxillary anterior teeth.

21. Move index finger of other hand along the buccal mucosa posteriorly between the impression and the peripheral tissues. The index finger is placed under the posterior facial portion of the tray to lift the tray and break the seal between the impression and the teeth. Grasp the handle of the tray with the thumb and index finger of the dominant hand, and use a firm lifting motion.

22. Remove the tray by turning it sideways to take it out of the client's mouth.

23. Evaluate the impression for accuracy (Figure 35-12).

Postimpression Care

24. Give the client water to rinse the mouth.

25. Gently rinse debris from the impression under a stream of cold water (Figure 35-13).

26. Spray the impression with an approved disinfectant (e.g., 1:213 iodophor or 1:10 sodium hypochlorite) within 10 to 15 minutes. Follow the manufacturer's recommended procedure (Figure 35-14).

27. Wrap the impression in a moist paper towel and place it in a biohazard bag before pouring it up (Figure 35-15) or in a humidor; label with client's name. Prepare the laboratory prescription if sending the impressions to the dental laboratory.

28. Remove any remaining alginate from client's mouth with floss, scaler, or explorer.

29. Remove any alginate from client's face and lips with a warm cloth.

30. Return any removable oral appliances to client.

31. Document in ink the completion of this service in the client's record under “Services Rendered,” and date the entry. For example, “9-1-09 Alginate impression for diagnostic casts.”

Procedure 35-4 MAKING A MAXILLARY PRELIMINARY IMPRESSION

EQUIPMENT

Personal protective equipment

Antimicrobial rinse

Occupational Safety and Health Administration (OSHA)–approved disinfecting solution

Alginate powder

Water

Measuring scoop

Vial for measuring water

Wide-blade spatula^∗

Bite registration wax (baseplate utility wax or wax wafer)

Rubber mixing bowl

Maxillary and mandibular impression trays

Saliva ejector

STEPS

Preparation

1. Gather all necessary supplies. Seat and prepare the client.

2. Measure three units of room-temperature water and three scoops of alginate, and mix the alginate.

Loading the Tray

3. Load the maxillary tray in one large increment. Load from the posterior end of tray. Use a wiping motion to bring the material forward with the spatula, being careful to place the bulk of the material in the anterior palatal area of the tray. Fill to an area just below the edge of the wax rim.

4. Be careful not to overfill the posterior portion of the tray that rests against the palate.

5. Moisten fingers with water, and smooth surface of the alginate (Figure 35-16).

Seating the Tray

6. Position yourself at the 11-o'clock position (1-o'clock position if left-handed), and instruct the client to tilt head forward and chin down.

7. Retract the client's lips and cheek with fingers of nondominant hand. With the dominant hand, turn the impression tray sideways and at the same time distend the lip and cheek on the opposite side of the mouth with the side of the tray.

8. Center the tray over the client's teeth, and center the handle at the midline in line with the center of the client's nose.

9. Seat the back of the tray against the posterior border of the hard palate to form a seal. Place the tray ¼ inch or 6 mm anterior to incisors, and seat posterior to anterior direction with a slight vibratory motion.

10. Gently move the client's lips out of the way as the tray is seated, and instruct the client to move the lips (Figure 35-17).

11. Place middle fingers over the premolar areas, and hold the lip out with the index finger and the thumb.

12. Instruct the client to breathe slowly through the nose and form an O with his or her lips.

13. Hold the tray in place until the material has gelled.

Removing the Impression

14. Place an index finger under the posterior facial portion of the tray to break the seal between the impression and the teeth.

15. Place the index finger of the nondominant hand on the incisal surface of the mandibular anterior teeth.

16. Move index finger of other hand along the buccal mucosa posteriorly between the impression and the peripheral tissues. The index finger is placed under the rim of the tray to lift and break the seal between the impression and the teeth. Grasp the handle of the tray with the thumb and index finger of the dominant hand to lower it from the maxillary teeth.

17. Remove the tray by turning it sideways to take it out of the client's mouth.

Postimpression Care

18. Evaluate the impression for accuracy (Figure 35-18).

^∗ If the same bowl and spatula are used that were used for the mandibular impression, they must be thoroughly cleaned and dried to prevent contamination of the maxillary mix (see Figure 35-8).

BOX 35-4 Guidelines to Minimize Gagging during Impression Taking

• Seat maxillary tray from posterior to anterior to direct flow of impression material anteriorly away from soft palate.

• Avoid overfilling the tray with impression material. Fill to the level just below wax beading of tray rim.

• Seat client upright. During insertion of tray, instruct client to bend head forward with chin tilting down.

• Avoid using too large a tray.

• Use a calm, confident, yet gentle approach; work efficiently.

• Instruct clients to breathe slowly and deeply through the nose, to point toes, and/or to hold skin between index finger and thumb.

Wax-Bite Registration

When an impression is made of both dental arches, the dentist and laboratory technician also need an accurate registration of the normal centric occlusion. Centric occlusion is the maximal stable contact between the occluding surfaces of the maxillary and mandibular teeth when the jaws are closed. This relationship is recorded with a wax-bite registration. The wax-bite registration is made at the time of the impression taking (Procedure 35-5). Wax-bite registration is used to articulate the models or diagnostic casts after the client has left the office. Articulated models can then be trimmed to ensure accurate articulation.

Procedure 35-5 MAKING A WAX-BITE REGISTRATION

EQUIPMENT (FIGURE 35-19)

Protective barriers (safety glasses, mask, gloves, hair bonnet)

Antimicrobial rinse

Bite registration wax (baseplate wax or wax wafer)

Wide-blade laboratory knife

Heat source (warm water, Bunsen burner, or torch)

Occupational Safety and Health Administration (OSHA)–approved disinfectant

STEPS

Preparation

1. Gather all necessary supplies. Seat the client upright. Explain the procedure.

2. Reassure the client that the wax will be warm, not hot.

3. Measure the length of the wax needed by placing the wax over the biting surfaces of the teeth. If the wax extends past the last tooth, use the laboratory knife to shorten its length after removing the wax from the client's mouth.

4. Soften the bite registration wax in hot water or with another heat source (e.g., Bunsen burner or torch).

Seating

5. Place the softened warm wax over the maxillary occlusal surfaces and instruct the client to bite together on posterior teeth gently and naturally into the wax (Figure 35-20).

6. Allow the wax bite to cool in the mouth. If necessary, air from the air-water syringe can cool the wax.

Removal

7. Remove the wax carefully when it has cooled.

Post–Wax-Bite Care

8. Inspect the wax to be sure it represents the client's bite (Figure 35-21). Chill in cold water until firm.

9. Write the client's name on a piece of paper and keep it with the wax-bite registration.

10. Disinfect wax-bite registration with an OSHA-approved disinfectant.

11. Store the wax-bite registration with the impressions or casts until it is needed for the trimming of the casts.

12. Record service in “Services Rendered” section of the client's record, and date the entry.

Baseplate wax and wax wafers are used to record the client's bite registration. Baseplate wax and wax wafers are pliable waxes at room temperature. Baseplate wax is supplied in 1- to 2-mm–thick red or pink sheets, and wax bite wafers are horseshoe shaped. The most common technique used for obtaining a bite registration is to have clients close the teeth into softened wax.

DIAGNOSTIC CASTS

A diagnostic cast is an accurate three-dimensional model of the teeth and surrounding tissues of the client's maxillary and mandibular arches created from a dental impression. Gypsum products are used to make diagnostic casts.

Gypsum Products

Gypsum is a powdered hemihydrate, which means there is one-half part water to one part of calcium sulfate. When mixed with water, the hemihydrate crystals grow to form clusters that grow during the setting process. The interlinking of the crystals results in a stronger and harder final product. There are three types of gypsum products used in pouring casts: model plaster, dental stone, and high-strength stone. These materials consist of hemihydrate crystals that vary in terms of size, shape, and porosity. These differences determine the characteristics of the product.

Plaster, a beta calcium sulfate hemihydrate (plaster of Paris), has very porous crystals that vary in shape. Because of its porosity, it requires the most water when mixing, compared with the other types of gypsum products. It is used for pouring preliminary impressions to construct models when strength is not critical but a detailed reproduction of the mouth is required (e.g., orthodontic models).¹

Dental stone, an alpha calcium sulfate hemihydrate, is stronger than plaster. Its crystals are uniform in shape and less porous than plaster. Dental stone is used when a stronger working diagnostic cast is needed to make dentures, orthodontic retainers, custom trays, nightguards, or cast restorations. High-strength stone has very dense crystals and requires the least amount of water for mixing. High-strength stone has a hardness that makes it ideal to create casts used in the production of crowns, bridges, and indirect restoration. Both plaster and stone are mixed by hand with a spatula or mechanically with a vacuum mixer, which eliminates trapping air into the mix.¹

Water/Powder Ratio

Each gypsum product has an optimal water/powder ratio specified by the manufacturer. The water/powder ratio affects the setting time and product strength. For one preliminary impression and its base, the commonly used water/powder ratio for plaster is 50 mL of water to 100 g of powder; for dental stone, it is 30 mL water to 100 g of powder; and for high-strength stone it is 24 mL of water to 1000 g of powder.¹

Setting Time

Time is another critical factor in the setting reaction of plaster and stone. There is working time immediately after mixing when the mixture will flow into the alginate impression. Initial setting time occurs when a semihard mass forms and is the point at which the mixture can no longer be poured into the impressions. For plaster, the initial setting time is 12 to 14 minutes; final setting time is 45 to 60 minutes. For stone, the initial setting time is 8 to 10 minutes; final setting time is 45 to 60 minutes. Final setting time occurs after the exothermic reaction (a chemical change accompanied by the liberation of heat) and the final product can no longer be manipulated without fracture. After final setting time (45 to 60 minutes) the preliminary alginate impression can be removed from the model or cast. Table 35-2 presents factors that affect setting time of plaster and stone.¹

TABLE 35-2 Factors That Affect Setting Time and Quality of Cast

Factor	Comment
*Setting Time*
Type of gypsum	Dental stone sets more slowly than dental plaster.
Water/powder ratio	The less water used, the faster the set. Follow the manufacturer's proportions exactly. Too much water increases setting time and decreases strength. The resultant cast is smooth in appearance. Too little water decreases setting time and decreases strength. The resultant cast is grainy in appearance.
Water temperature	The warmer the water, the faster the set. Water that is too warm creates a cast that sets too fast, and water that is too cool creates a cast that sets too slowly. In general, water should be at room temperature and no warmer than 70° F (21.1° C). Cool water increases setting time, and warm water decreases setting time. On a humid day the powder can absorb water, causing a slower set.
Mixing time	The longer and faster the spatulation, the faster the set. Prolonged and very rapid mixing shortens the setting time by increasing the chemical reaction and decreases the strength of the study model owing to breakage of the crystals that are forming. Too little mixing also decreases strength and makes the study model grainy.
Improper storage of gypsum	Stone and scoop dispenser need to be kept clean and dry. Stone should be stored in a tight container that is closed immediately after use to eliminate exposure to humidity and problems with moisture (see Procedure 35-2). Purchase gypsum products only as needed, because of possible contamination with moisture.
*Quality of Cast*
Factors that affect setting time	See above.
Removal of the impression	Premature or improper removal of the alginate impression from the cast will break teeth or crack the cast.
Movement of tray or cast	Movement of the alginate impression or cast during the setting process will create a thin, flat base.

Pouring the Cast

The cast consists of the anatomic portion and the art portion. The anatomic portion includes the teeth, oral mucosa, and muscle attachments. The art portion forms the base (Figure 35-22). The procedure for pouring the cast from a preliminary impression consists of two components: filling the impression with the mixed gypsum material to form the anatomic portion of the cast, and forming its base by mounding mixed gypsum on a smooth, nonabsorbent surface. These two parts are then connected by inverting the poured impression and seating it on the base. Procedure 35-6 provides details for pouring the cast from a preliminary alginate impression using dental plaster.

Figure 35-22 Anatomic and art portion of the diagnostic cast.

(From Bird DL, Robinson DS: Torres and Ehrlich modern dental assisting, ed 9, St Louis, 2009, Saunders.)

Figure 35-23 Supplies needed for pouring dental casts.

(From Bird DL, Robinson DS: Torres and Ehrlich modern dental assisting, ed 9, St Louis, 2009, Saunders.)

Figure 35-24 Smooth, creamy mix.

(From Bird DL, Robinson DS: Torres and Ehrlich modern dental assisting, ed 9, St Louis, 2009, Saunders.)

Figure 35-25 Mixing bowl on vibrator.

(Courtesy Gwen Essex.)

Figure 35-26 Initial placement of material in distal of most posterior tooth.

(From Bird DL, Robinson DS: Torres and Ehrlich modern dental assisting, ed 9, St Louis, 2009, Saunders.)

Figure 35-27 Impression filled with large amounts of gypsum.

(From Bird DL, Robinson DS: Torres and Ehrlich modern dental assisting, ed 9, St Louis, 2009, Saunders.)

Figure 35-28 Filled impression tray and base on Plexiglas.

(From Bird DL, Robinson DS: Torres and Ehrlich modern dental assisting, ed 9, St Louis, 2009, Saunders.)

Figure 35-29 Smoothing the plaster base mix up into the margins of the tray.

(From Bird DL, Robinson DS: Torres and Ehrlich modern dental assisting, ed 9, St Louis, 2009, Saunders.)

Figure 35-30 Plaster knife used to free tray from stone.

(Courtesy Gwen Essex.)

Figure 35-31 Removing impression from cast.

(Courtesy Gwen Essex.)

Procedure 35-6 POURING THE CAST AND THE BASE

EQUIPMENT (FIGURE 35-23)

Personal protective equipment

Rubber bowl

No. 7 wax spatula

Scale

Room-temperature water

Dental plaster

Two Plexiglas squares

Laboratory spatula

Disinfected alginate impressions

Water measuring device

Plaster knife

Vibrator covered with plastic

Occupational Safety and Health Administration (OSHA)–approved disinfectant

STEPS

Preparation

1. Don personal protective equipment.

2. Disinfect the alginate impression following the manufacturer's instructions. Rinse with cool, running water, shake the excess water off in the sink, and gently dry with compressed air.

3. Use the laboratory knife to remove any excess impression material that will interfere with the pouring of the model, i.e., impression material past the end of the tray or excess material from the tongue area.

Pouring the Mandibular Impression

4. Measure 50 mL of room-temperature water, and place it into a clean mixing bowl.

5. Place a paper towel on a scale, and make necessary adjustments. Obtain a measure of 100 g of dental plaster.

6. Add the powder into the water in steady increments to allow the powder to settle.

7. Use the spatula to incorporate the powder into the water. Use a wiping motion against the sides of the bowl. Spatulate 20 seconds to achieve a smooth, creamy mix (Figure 35-24).

8. Set the vibrator at low speed. Place the bowl on a vibrator and vibrate the material for 10 to 15 seconds. Lightly press and rotate the bowl on the vibrator.

9. Gather the gypsum as a mass in the bowl. Remove bowl from vibrator (Figure 35-25).

10. Hold the impression tray by the handle, and press handle against the vibrator.

11. Use the end of a wax spatula or laboratory knife to pick up about ½ teaspoon of mixed material. Allow mix to flow into the impression at the distal of the most posterior tooth while the impression is vibrated so that the material flows toward the anterior teeth. Turn the tray on its side to provide the continuous flow of material forward into each tooth. Tip the impression forward to make the gypsum mixture flow into the bottom of the alginate impression. Continue to add the gypsum product in small increments at the same place until the occlusal and incisal surfaces are filled. Vibrate continually (Figure 35-26).

12. When all tooth indentations are filled, use the laboratory spatula to add larger amounts of gypsum to fill the impression. Continue to vibrate until the entire impression is filled (Figure 35-27). Then set the poured impression aside.

Pouring the Base for the Mandibular Cast

13. Gather the remaining amount of mixed material together in the bowl.

14. Place the mix in a mound on a Plexiglas square or tile. Shape the base to approximately 2 × 2 inches wide and 1 inch thick (Figure 35-28).

15. Invert the firm, poured impression onto the firm base. Do not push the impression into the base.

16. Position impression tray on the center of the mound to provide a uniform thickness all around it. Position the occlusal plane of posterior teeth parallel with the table top as judged by the handle of the tray.

17. Hold the tray steady, and with the laboratory spatula or a moistened finger, smooth the sides around the base onto the margins of the impression tray (Figure 35-29).

18. Remove excess stone or plaster above the edge of the tray rim.

19. Allow the gypsum to reach the initial set before moving the Plexiglas square.

Pouring the Maxillary Impression and Base

20. Repeat steps 2 through 19 above for the maxillary impression to create an anatomic and art portion of a dental cast. Use clean equipment for the fresh mix of plaster.

Separating the Impressions from the Casts

21. Wait 45 to 60 minutes after the base has been poured before attempting to separate the impression from the cast.

22. Use a plaster knife to remove excess material from the edges of the impression tray and to gently separate the margins of the tray from the cast (Figure 35-30).

23. If the teeth are in good alignment, remove tray and impression material together. First release the anterior portion by gently pulling downward and forward one time. Then make a firm, straight pull upward. Do not apply lateral pressure or rock the tray (Figure 35-31).

24. If the tray does not separate, check to see where the tray may be locked by the gypsum. Use the plaster knife to free the tray from the gypsum.

25. If teeth are misaligned, remove the tray first, then cut the impression material carefully along the occlusal line and gently peel off.

Postseparation Procedures

26. Use a pencil or permanent marker to label the base (bottom) of the model or cast with the client's name. Keep the wax bite with the gypsum models and casts.

27. Store the casts until they can be trimmed.

28. Remove gypsum material from the vibrator, spatula, and mixing bowl, and clean with cool water.

Trimming and Finishing Casts

After the casts have set and the impression trays have been separated from them, the casts are trimmed to a geometric standard by using a model trimmer. The purpose of trimming is to produce attractive diagnostic casts for case presentation or for use as part of the client's permanent record. A model trimmer is a laboratory machine that has a circular abrasive wheel that is set to 90 degrees to the cast (Figure 35-32). The wax-bite registration is used to ensure that the teeth of the casts are in proper occlusion during the trimming process. The desired trimming outcome is to make the bases parallel to themselves and the occlusal plane and to make the base one third and the anatomic portion two thirds of the cast height (see Figure 35-22 and Box 35-5). The reader is advised to refer to a standard dental materials textbook for specific information on the trimming procedure and care of the trimming equipment.

Figure 35-32 Model trimmer.

(From Bird DL, Robinson DS: Torres and Ehrlich modern dental assisting, ed 9, St Louis, 2009, Saunders.)

Figure 35-33 Supplies for constructing a custom-made stent.

(From Bird DL, Robinson DS: Torres and Ehrlich modern dental assisting, ed 9, St Louis, 2009, Saunders.)

Figure 35-34 Trimmed diagnostic cast.

(From Bird DL, Robinson DS: Torres and Ehrlich modern dental assisting, ed 9, St Louis, 2009, Saunders.)

Figure 35-35 Opening hinge and placing mouth guard material.

(Courtesy Gwen Essex.)

Figure 35-36 Sagging mouth guard material.

(Courtesy Gwen Essex.)

Figure 35-37 Hinged frame pulled over vacuum plate.

(Courtesy Gwen Essex.)

Figure 35-38 Cutting away gross excess material.

(From Bird DL, Robinson DS: Torres and Ehrlich modern dental assisting, ed 9, St Louis, 2009, Saunders.)

Figure 35-39 Trimming material away from the gingival margin.

(Courtesy Gwen Essex.)

Figure 35-40 Trimming mouth guard with an acrylic burr.

(Courtesy Gwen Essex.)

BOX 35-5 Criteria for Evaluation of Study Models and Diagnostic Casts

• No visible voids, air bubbles, excess material, or fractures are present.

• Surface is smooth and hard.

• Anatomic structures are visible and account for two thirds of the cast.

• Base accounts for one third of the cast; top and bottom of bases are parallel to the floor.

• Angles and cuts of base are accurate and symmetric.

• Cast retains occlusion when placed on all sides.

• All oral landmarks are present.

CUSTOM-MADE STENTS

Oral Stents

A stent is a mold or device used for a variety of dental and medical procedures. In dentistry, stents are placed intraorally for the targeted delivery of materials and protection of oral structures. Wearing a stent serves the following purposes:

Application of fluoride and tooth-whitening products

Protection of the teeth, mucosa, and bone

Containment of radon seeds in the radiation treatment of head and neck cancer

Protection of teeth during endotracheal anesthesia, bronchoscopy, tonsillectomy, and electric shock therapy

Protection of palate during long-term intubation of premature infants and palatal skin graft donor sites

Home Fluoride Trays

Home fluoride trays are constructed for the target delivery of topical fluoride for clients with high caries risk, such as individuals who have xerostomia.

Tooth-Whitening Stents

Clients often seek oral healthcare to improve their appearance. A focus on sexuality and self-image is predominant in the media. Because the smile speaks a thousand words, the appearance of the teeth dominates our image-conscious society and is an important factor in meeting a person's human need for a wholesome facial image. Consequently, tooth discoloration and its removal are often the reasons individuals seek oral healthcare.

Many tooth-whitening procedures require the use of a stent by the client. (See Chapter 27 for dentist-dispensed home-use and in-office tooth whiteners that carry the American Dental Association [ADA] Seal of Acceptance.) This tooth-whitening stent is a custom-made device constructed in the same manner as a mouth guard using an acrylic material. Tooth-whitening trays are filled with carbamide or hydrogen peroxide gels for tooth bleaching.

Not all teeth, however, will respond similarly to whitening procedures. Yellow teeth whiten the best, brown teeth less well, and gray teeth may not bleach well at all. Tetracycline stain and fluorosis may be treated with a combination of tooth whitening, microabrasion and restorative dentistry cosmetic procedures.

Some tooth-whitening systems on the market do not require a stent for delivery. Tooth-whitening strips are a viable option for clients with temporomandibular joint (TMJ) disorder who cannot tolerate stents. Procedure 35-7 describes construction of a custom-made stent. See Chapter 27 for methods of in-office tooth whitening.

Procedure 35-7 CONSTRUCTING A CUSTOM-MADE STENT (A SINGLE-LAYER MOUTH GUARD, FLUORIDE TRAY, OR TOOTH-WHITENING TRAY)

EQUIPMENT (FIGURE 35-33)

Personal protective equipment

Petrolatum lubricant, silicone lubricant

Polyurethane

Mouth guard 4 × 4 square

Long-shank acrylic burr in a laboratory engine

Matches

Diagnostic casts

Crown and collar scissors

Hanau torch

Vacuum forming machine

Laboratory knife

STEPS

1. Don personal protective equipment.

2. Trim the diagnostic cast so that the base extends 3 to 4 mm past the gingival border and the vertical height is minimal. Spray the cast with silicone lubricant (Figure 35-34).

3. Place the vacuum forming machine under a hood fan for control of organic emissions.

4. Prepare the machine. The perforated vacuum plate and the sides of the hinged frame must be lightly sprayed with silicone lubricant.

5. Open the hinged frame, and center the polyurethane material onto the lower frame (Figure 35-35).

6. Close the frame and secure the frame with the latch knob.

7. Grasp both handles of the locked, hinged frame and lift it until it clicks into position approximately 3 inches above the vacuum plate.

8. Swing the heating unit to the center position and turn on the heating element switch at the base of the unit.

9. Center cast on the vacuum plate. Some units have extra holes at the front and back of the machine; place the cast between these holes.

10. Do not leave the machine unattended. Watch the material as it heats for 1 to 2 minutes until it sags ½ inch below the hinged frame (Figure 35-36).

11. Grasp both handles of the hinged frame and pull it down over the vacuum plate. The material will be draped over the cast (Figure 35-37).

12. Turn on the vacuum motor for 10 seconds.

13. Swing the heating unit out of the way and turn the switch off.

14. Turn off the vacuum switch. Release the hinged frame knob and open the frame and hold by the edges to remove it from the vacuum plate.

15. Hold the splint and cast under running, cold water for at least 30 seconds.

16. Cut excess material just below the depth of the periphery to remove it from the cast (Figure 35-38).

17. Use small, sharp crown and collar scissors to trim approximately 0.5 mm away from the gingival margin (Figure 35-39).

18. Place the mouth guard back on the cast.

19. If necessary, place a thin coat of petroleum jelly on the facial surfaces. Use a low flame to gently readapt the margins so that they cover the entire tooth, but do not overlap the gingivae.

20. Wearing a mask and safety goggles, trim the mouth guard with an acrylic burr in a laboratory engine (Figure 35-40).

21. Use the Hanau torch to smooth the edges from the peripheral border of the mouth guard.

Oral Splints

A splint is an orthopedic device used for immobilization, restraint, or support of the teeth and/or jaws. Splints are used to stabilize periodontally involved teeth, immobilize the mandible, restrain the teeth, and advance the mandible. In the dental office the most common splints include athletic mouth guards, nightguards, dayguards, periodontal splints, sleepguards, and surgical splints.

Athletic Mouth Guards

A mouth guard is a protective appliance that covers the teeth and palate and fits to the depth of the vestibule to stabilize the mandible. Mouth guards help meet the client's human need for protection from health risks. Wearing a mouth guard serves the following purposes:

Protection of teeth and oral tissues, the maxilla and mandible, the TMJ, and the head and neck against intracranial pressure changes and bone deformation during contact sports.2 Blows transmitted to the TMJ during athletic competition can be absorbed and distributed throughout the mouth guard.

Protection of the brainstem against shear stress in contact sports. A properly fitting mouth guard separates the mandible from the maxilla and thus reduces the force transmitted to the base of the brain at the TMJ.

Prevention of tooth avulsion. Over 5 million teeth are avulsed yearly in the United States; the most frequent injuries occur in 8- to 15-year-olds.

Prevention of facial bone fractures, TMJ injury, and head injuries.

Provision of an occlusal cushion against bruxism and clenching.

The ADA and the Academy for Sports Dentistry suggest a properly fitted mouth guard be worn while individuals play contact sports or participate in nonimpact sports. Nonimpact sports (e.g., weightlifting) require mouth protectors owing to the clenching of teeth involved in these strenuous sports (see Box 35-6 for sports in which participants should wear mouth protection).^2,³

BOX 35-6 Sports in Which Participants Should Wear Mouth Protection

From Academy for Sports Dentistry: Frequently asked questions. Available at: http://www.sportsdentistry-asd.org/faqs.asp. Accessed December 11, 2008.

• Acrobatics

• Baseball

• Basketball

• Boxing

• Cycling

• Discus

• Equestrian sports

• Field hockey

• Football

• Gymnastics

• Handball

• Horseback riding

• Ice hockey

• In-line skating

• Lacrosse

• Martial arts

• Motocross

• Racquetball

• Rugby

• Shot-put

• Skateboarding

• Skiing

• Skydiving

• Soccer

• Squash

• Surfing

• Tennis

• Trampoline

• Volleyball

• Water polo

• Weight lifting

• Wrestling

Box 35-7 summarizes the types of athletic mouth guards available. The following are mouth guard guidelines:

Adequate thickness in all areas to reduce impact forces

A fit that is retentive so the device does not dislodge on impact

Full palatal coverage that equals the demands of the playing status of the athlete

Construction with U.S. Food and Drug Administration (FDA)–approved materials

Life of mouth guard limited to one season of play

BOX 35-7 Types of Athletic Mouth Guards Available

Stock Type

“One size fits all,” purchased over the counter, inexpensive, offers the least protection

Boil and Bite

User formed, purchased over the counter, inexpensive, offers little protection

Custom Vacuum-Formed Single-Layer Mouth Guard

Fabricated in the dental office, custom fit, moderately expensive, provides good protection

Pressure-Laminated Multiple-Layer Mouth Guard

Fabricated in the dental laboratory, custom fit, expensive, provides the best protection

In addition to preventing tooth avulsion, a full palatal coverage mouth guard worn during sports participation helps to prevent or moderate a variety of head injuries. Concussion is the alteration of consciousness and/or disturbance in vision and equilibrium caused by a direct blow to the head, rapid acceleration and/or deceleration of the head, or direct blow to the base of the skull from a vertical impact to the mandible. Symptoms of concussion include headaches, earaches, facial pain, photophobia, vertigo, and impaired speech.

Single-Layer Mouth Guards

Vacuum-formed thermoplastic polymers are used to make mouth guards called single-layer mouth guards. They are fabricated from one sheet of polyurethane or soft ethylene vinyl acetate (EVA) acrylic, are custom fitted, allow for breathing and speech, and do not deform over time and use. They are the most commonly prescribed mouth guards in dental offices. These vacuum-formed mouth guards, custom fabricated by dental staff or laboratory technicians, are superior to the store-bought stock or boil-and-bite mouth guards. Strap attachments for helmets are easily adapted to the custom-made single-layer mouth guard, and the client can select the color and decorative pattern of the mouth guard (see Procedure 35-7).

Multiple-Layer Mouth Guards (Pressure Laminated)

Multiple-layer mouth guards made of EVA are recommended for full-contact sports. Dental technicians in a dental laboratory setting fabricate pressure-laminated mouth guards because they maintain their shape better than single-layer mouth guards. Two or three layers of EVA material are fused to achieve the necessary thickness. At a minimum, mouth guards should have a labial thickness of 3 mm, palatal thickness of 2 mm, and occlusal thickness of 3 mm. The mouth guard material should be biocompatible and last 1 to 2 years; however, mouth guard replacement is recommended after one season of play.

Several commercial laboratories fabricate pressure-laminated mouth guards in the United States and Canada. Playsafe Mouthguards from Glidewell Laboratories are pressure-laminated mouth guards made from EVA material. One of the following four types of pressure-laminated mouth guards is recommended based on the client's degree of risk:

Light (two layers approximately 2 mm thick)

Medium (two layers approximately 5 mm thick)

Heavy (two layers approximately 5 mm thick with power dispersion strips)

Heavy Pro (three layers, approximately 5 mm thick, with a hard support)

Bruxism

Bruxism is a subconscious habit of grinding, clenching, rubbing, or gritting of the teeth that usually occurs during sleep. Occlusion discrepancy between centric occlusion and centric relation is thought to be a cause of nocturnal bruxism. Stress, a major contributing factor in bruxism, may increase the frequency and intensity of bruxism. Central nervous system disorders such as Parkinson's disease and Huntington's disease can contribute to nocturnal bruxism. Many individuals experience bruxism, but the degree of bruxism determines the sequelae. Box 35-8 lists the possible sequelae of bruxism.

BOX 35-8 Sequelae of Bruxism

• Abfraction lesions

• Exostosis to support the teeth

• Gingival recession

• Headaches

• Impaired hearing

• Limited range of motion of mandible

• Linea alba

• Pain

• Periodontal pockets

• Tenderness of muscles of mastication

• Tinnitus

• Temporomandibular joint disorders

• TMJ noise with movement

• Tooth fracture

• Tooth mobility

• Tooth sensitivity

• Tooth wear facets (attrition)

Treatment of Bruxism

Treatment for bruxism is categorized as reversible or irreversible. Reversible treatment is conservative, is noninvasive, and causes no permanent changes in the structure or position of the jaw or teeth. Most therapy for bruxism starts with reversible interventions, but treatment may progress to irreversible procedures. Irreversible treatment is invasive and causes permanent changes in the structure or position of the jaw or teeth.

Reversible Procedures

Appliance therapy is the use of dental stents in a variety of situations. Dental splints such as nightguards or dayguards treat bruxism. Evidence suggests that wearing a nightguard during sleep to reduce bruxism will decrease damage to the teeth and relax strained muscles.⁴

Biofeedback is the use of electromyography (EMG) to record muscle activity to help the patient recognize when he or she is overusing the muscles of the head and neck. During treatment the client wears pairs of electrodes attached to the surface of the skin in contact with the muscles of mastication. The electrodes transmit muscle activity information to a computer monitor the client can see, to allow the client to consciously reduce muscle tension. Sleep biofeedback therapy involves EMG-activated alarms, which sound during sleep to awaken the client to stop the bruxism. Sleep biofeedback can also use electrical stimulation to produce pain during nocturnal bruxism to awaken the patient, thus stopping bruxism.

Drug therapy is the use of antianxiety, sedative, anti-inflammatory, and muscle relaxant drugs to control bruxism. Moreover, when the overuse of the muscles of mastication produces muscle fiber changes, methocarbamol (Robaxin) can be injected into muscle trigger points to provide pain relief. Over-the-counter medications such as ibuprofen and naproxen are recommended as nonsteroidal anti-inflammatory agents.

Exercise therapy using isokinetics and stretching of the muscles of mastication can be performed by the client. Reflex and relaxation exercises may also be useful.

Physical therapy procedures may be effective in restoring normal muscle function. Heat and cold therapy using moist heat or ethylene chloride spray may end the muscle spasm and pain cycle. Mandibular relearning therapy helps the client to work on the opening and closing of the mouth.

Psychotherapy from an appropriate mental health professional may be indicated to reduce stress via relaxation, medication, and guided imagery. The antidepressant amitriptyline can be prescribed by the mental health professional. Other stress reduction strategies also may be recommended for the client to perform before sleep.

Irreversible Procedures

Equilibration therapy is used to adjust the occlusion of the teeth by recontouring occlusal enamel with dental burrs. The objective is to create a centric occlusion coinciding with centric relation.

Orthognathic surgery can be employed to improve a skeletal malocclusion. Splint therapy must first be used to reconfirm the effectiveness of equilibration therapy in relieving the client's symptoms. Surgery is recommended only after preliminary orthodontics has eliminated the dental compensations of the malocclusion.

Nightguards and Dayguards

A nightguard or dayguard is a hard acrylic appliance that fits over the maxillary or mandibular teeth to protect the teeth from clenching and grinding. If this appliance is worn during sleep it is referred to as a nightguard. If the appliance is to be worn throughout the day, it is called a dayguard. The purpose of these appliances is to do the following:

Reduce clenching or grinding of the teeth (bruxism)

Minimize loss of tooth structure (attrition)

Ease muscle hyperactivity

Reduce pressure on the TMJ

There are several nightguard and dayguard designs.

Partial coverage nightguards or dayguards are maxillary appliances that cover the palate and the lingual surfaces of the maxillary anterior teeth. The disadvantage of such appliances is that they cover only some of the teeth, and the occlusal force may overstress those teeth covered by the acrylic. Subsequently, teeth that are not covered by the nightguard or dayguard may hypererupt or drift.

Full occlusal coverage nightguards or dayguards, the appliances of choice, are fabricated in acrylic to cover the occlusal surfaces. A nightguard or dayguard that fully covers either the maxillary or mandibular teeth is the safest design because the dentist can adjust the occlusion on the appliance. The lifespan for full occlusal coverage nightguards and dayguards varies from 3 to 10 years.

A single-layer athletic mouth guard may be used as an inexpensive nightguard or dayguard to temporarily change the neuromuscular behavior and produce muscle relaxation in mild clenching and bruxism. This appliance usually lasts less than 1 year. However, the occlusion cannot be adjusted, and the mouth guard can create increased parafunctional clenching and related temporomandibular disorders.

The fabrication of nightguards and dayguards usually requires two dental appointments. At the first appointment, accurate alginate impressions are made. The alginate impressions are poured in Class I dental stone to create diagnostic casts. A centric relation bite registration is made to enable placement of the casts on an anatomic articulator that simulates tooth occlusion and TMJ positioning. The articulated models are sent to the dental laboratory for fabrication of the appliance by a dental technician.

Periodontal Splints

Periodontal splints are used for the stabilization of mobile teeth to prolong their presence in the mouth.

Sleepguards

Sleep disorders comprise a group of medical conditions. Snoring and obstructive sleep apnea (OSA) are two sleep disorders recognized by the dental hygienist for referral. Snoring and OSA are treated intraorally with a sleepguard oral splint, also called a snoreguard. Snoring is successfully managed by the wearing of a snoreguard appliance. Mild to moderate OSA patients have the choice of a snoreguard appliance as one of several treatment options. The snoreguard is constructed of hard acrylic resin and is designed to fit over the maxillary dentition to reposition the mandible. It repositions the mandible to prevent the tongue from obstructing the oropharynx, thereby making it a splint because it stabilizes the mandible.

Surgical Splints

Surgical splints are orthopedic devices used to cover the graft donor site for protection of the palate after the harvesting of soft tissue for periodontal soft-tissue graft surgery.

CLIENT EDUCATION tIPS

Explain the purposes of study casts and models and why they are necessary.

Explain the purpose of mouth guards and why they are recommended for use during contact sports.

Describe the differences between custom-made and over-the-counter mouth guards.

Explain the use of a multiple-layer mouth guard to reduce concussion injury.

Describe the contributing factors to bruxism and the difference between reversible and irreversible treatment of bruxism.

Describe sleeping disorder treatment and appliance care.

LEGAL, ETHICAL, AND SAFETY ISSUES

In some states, dental hygienists can legally make preliminary dental impressions. It is the legal responsibility of the dental hygienist to practice within the scope authorized by state law.

Impression trays can be a source of cross-contamination. They are classified as semicritical instruments because they become contaminated by saliva and must be either discarded if disposable or sterilized for reuse.

Impression trays trial-sized in the client's mouth but not used for the impression are either discarded if disposable or sterilized for reuse.

Anything that comes into contact with contaminated impression trays must be sterilized (e.g., spatulas, bowls, and measuring devices used for mixing alginate).

Countertops are disinfected using an Occupational Safety and Health Administration (OSHA)–approved surface disinfectant after the impression procedure has been completed and the client has been dismissed (see Chapter 7).

Product manufacturers are required to provide written handling instructions called Material Safety Data Sheets (MSDSs) according to OSHA laws. MSDS warnings for alginate include eye irritation, congestion, and irritation of throat, nasal passages, and upper respiratory system. Unnecessary exposure to alginate powder should be avoided.

Health conditions aggravated by exposure to alginate powder include the lung diseases bronchitis, emphysema, asthma, and silicosis. Long-term exposure to alginate may produce silicosis because of the crystalline silica element of the diatomaceous earth ingredient. Dustless alginate powder is now available.

After an alginate impression is removed from the mouth, it is a biohazardous material. Because they are contaminated with the blood and saliva of the client, alginate impressions must be disinfected according to the manufacturer's recommendations and labeled as biohazardous material before being sent to a dental laboratory.

The protocol for disinfection of a dental impression based on the manufacturer's recommendations needs to be followed.

The hygienist is advised to always read the manufacturer's directions and MSDS on gypsum materials.

The dental hygienist must not inhale gypsum powder because it may be hazardous to health. Gypsum contains free crystalline silica (cristobalite quartz) and may cause delayed lung disease such as silicosis and pneumoconiosis.

Study models and diagnostic casts are retained as part of the client's permanent record.

KEY CONCEPTS

A dental impression is used to create an accurate three-dimensional reproduction of the teeth and surrounding tissues called a diagnostic cast, study model, or study cast.

There are three main types of dental impressions used in dentistry: a preliminary impression, a final impression, and a bite registration.

Loading and inserting the tray should take no more than 1 minute. The objective is for the impression material to reach the gel state in the client's mouth.

A maxillary impression tray is seated in a posterior to anterior direction to avoid triggering the gag reflex, prevent the excess material from going toward the back of the mouth, and move the impression material forward, ensuring complete coverage of the oral structures with alginate.

Stone casts and plaster models are made of gypsum products; stone casts are stronger than plaster models.

Safety precautions must be used when handling alginate and gypsum materials.

Baseplate wax and wax wafers are used for the bite registration procedures (interocclusal record).

Mouth guards should be recommended to clients at risk for sports-related dentofacial injury. Tooth avulsion, facial bone fractures, temporomandibular joint (TMJ) injuries, and concussions can be reduced by using properly fitted professionally manufactured mouth guards.

Signs and symptoms of bruxism include abfraction lesions, exostosis, gingival recession, headaches, impaired hearing, TMJ noise with movement, limited range of motion of jaw, linea alba, tenderness of muscles of mastication, pain, periodontal pockets, tinnitus, TMJ disorders, tooth fracture, tooth mobility, tooth sensitivity, and tooth wear facets (attrition).

Bruxism is treated with reversible and irreversible therapies. Reversible therapies include biofeedback, drug therapy, appliance therapy, exercise therapy, physical therapy, heat and cold therapy, mandibular relearning therapy, and psychotherapy. Irreversible therapies include orthognathic surgery and occlusal equilibration.

Sleep disorders are treated in the dental practice using a snoreguard. Snoring is reduced while a snoreguard is worn owing to the repositioning of the mandible and tongue. Mild to moderate obstructive sleep apnea can be treated with the use of a snoreguard, which repositions the mandible and tongue.

CRITICAL THINKING EXERCISES

1. Visit the local pharmacy and athletic store. Review the types of athletic mouth protectors available over the counter. What would you say to a client about these products?

2. Review the factors that affect alginate and gypsum materials. Manipulate the alginate and gypsum materials by purposefully using the factors in your procedure. What is the outcome in terms of quality?

3. What signs and symptoms are commonly associated with bruxism? Compare these signs and symptoms with those of students in your class who experience bruxism.

4. Joseph is a healthy 27-year-old man who works in an urban setting as an attorney. He has just been diagnosed with bruxism. To prevent any further tooth damage, he was informed regarding tooth attrition, tooth breakage, and tooth loss. He has opted to have a custom-made nightguard constructed. During procedure setup, he informs you that he is a “gagger.” Clinical evaluation of this client reveals an average size mouth with maxillary and mandibular exostoses. How would you factor this information into your dental hygiene care plan? Write a care plan for Joseph including diagnosis, client goals, interventions, and methods of evaluation.

Refer to the Procedures Manual where rationales are provided for the steps outlined in the procedures presented in this chapter.

REFERENCES

1. Anusavice K.J. Phillips' science of dental materials, ed 11. St Louis: Saunders, 2003.

2. American Dental Association (ADA): ADA website. Available at: www.ada.org. Accessed July 2007.

3. Academy for Sports Dentistry: Welcome to the Academy for Sports Dentistry. Available at: www.sportsdentistry-asd.org. Accessed January 2007.

Visit the website at http://evolve.elsevier.com/Darby/Hygiene for competency forms, suggested readings, glossary, and related websites..