For additional ancillary materials related to this chapter, please visit thePoint.
After performing the laboratory/clinical exercises in this chapter, the student will be able to do the following:
1. Select the appropriate tray for the patient by using the criteria listed in this chapter.
2. Prepare the impression trays with rope wax, including the palatal area of the maxillary tray.
3. Discuss the aspects of patient preparation for alginate impressions.
4. Demonstrate proper mixing of the alginate as well as loading, seating, and removal of the tray to obtain acceptable alginate impressions.
5. Evaluate the impression by using the criteria listed in this chapter, and determine if the impression is acceptable.
Use of alginate impression material is the most common method for obtaining diagnostic casts. It is pleasant tasting, inexpensive, and easy to manipulate. It also adequately records details of the patient’s oral structures. Study models or study casts are positive reproductions of the patient’s dental arches and surrounding tissues. They are important visual aids in diagnosis and treatment planning for a patient, and they also serve as an important part of the patient’s permanent record. In addition, diagnostic study models are useful as pretreatment and posttreatment records for patients who undergo extensive restorative or orthodontic treatment, fabrication of vital bleach trays, and bruxism splints. Taking (or making) alginate impressions is a procedure that is included in the scope of practice in most states for both dental hygienists and dental assistants.
The purpose of taking an alginate impression is to make an accurate reproduction of the maxillary and mandibular arches and adjacent tissues. Study models constructed from alginate impressions can be used for planning patient treatment and can serve as part of the patient’s permanent record.
II. Selection and Preparation of the Tray
Impression trays are usually perforated so that the alginate will flow through the holes when the tray is seated on the dental arch. These perforations act as a retentive mechanism to keep the set alginate from separating from the tray upon removal.
B. Available Varieties
Impression trays are available in a variety of sizes and materials. Disposable (or single-use) trays are usually made of plastic, and autoclavable (permanent) trays are made of stainless steel.
C. Proper Tray Selection
Before taking the impression, one should conduct an oral exam to assess the maxillary and mandibular arch for its width, length, or presence of mandibular tori or maxillary exostosis (harmless overgrowth of bone). If tori/exostosis is present, a wider or larger tray will be necessary for patient comfort. The empty tray should be seated in the patient’s mouth to determine the correct fit. Each tray should
1. Extend facially to include all teeth as well as the musculature and vestibule.
2. Extend distally approximately 2 to 3 mm beyond the last tooth in the arch to include the retromolar area.
3. Provide a 2- to 3-mm depth of alginate beyond the occlusal surface and incisal edge.
4. Be comfortable for the patient.
5. Some metal trays can be adapted to the patient by bending the sides of the tray.
Trays that have been “tried” or “used” for taking the impression should be sterilized before they are returned to storage.
E. Adding Wax to the Tray
After the impression trays have been selected, rope (beading) wax can be molded around the periphery of the tray. This protects the tissues from injury, extends the tray, and may aid in the retention of alginate material within the tray.
F. Adding Wax to the Palatal Area
The maxillary tray may also have rope wax placed in the palatal area of the tray to support the alginate in a high vault and to avoid trapping air. Figure 27.1 illustrates rope wax placed on tray peripheries and the palatal area.
FIGURE 27.1. Beading wax placed around tray peripheries and in the maxillary palatal area.
III. Preparation of the Patient
A. Explain the Procedure
Ask the patient if he or she has had impressions taken in the past. If not, explain the procedure. Advise the patient that the material has the consistency of mashed potatoes or pudding when first placed in the mouth and will then change to a rubberized material, much like silly putty. The patient should follow the directions of the clinician during the procedure. Instruct the patient that the setting time is 3 to 4 minutes depending on the use of fast- or regular-set material and that the tray cannot be removed before this time period. Instruct the patient not to talk but, rather, to raise his or her hand to signal any needs or discomfort.
B. Guard against Spills
Place a napkin on the patient to protect his or her clothing. Give the patient a tissue or paper towel should excessive salivation occur.
C. Minimize Gagging
To minimize the chance of gagging or vomiting, have the patient breathe through his or her nose while seated in an upright position. The gag reflex is stimulated when material touches the posterior third of the tongue, so avoid touching the tongue with either the tray or material overflow. Material overflow can possibly be removed using the end of the mirror and a high-speed evacuation tip.
D. Dental Appliances
Have the patient take out all removable dental appliances such as full or partial dentures, orthodontic retainers, and so on. Cover the gingival areas of fixed bridges and other fixed appliances with rope wax to prevent alginate from locking under crowns and pontics, which causes tearing and distortion of the impression on removal.
Ask the patient to rinse with mouthwash to reduce the amount of bacteria, debris, and saliva. The result is a more accurate impression.
It is recommended that the mandibular impression be taken first to familiarize the patient with the procedure as well as with the taste and consistency of the alginate material.
IV. The Alginate Impression
Alginate material is available in regular-set and fast-set products. The regular-set material has a setting time of approximately 3 to 4 minutes, whereas the fast-set material will set (or gel) in approximately 1 to 2 minutes. The working time, or the time that it should take to mix the alginate, is 1 minute.
A. Mixing the Alginate
1. You will need the necessary equipment listed in Table 27.1.
2. If the bulk form of alginate is used, shake or “fluff” the can, as discussed in Chapter 8, Impression Materials. Both bulk and unit packaging are illustrated in Figure 27.2. Wait 30 seconds to allow the “dust” to settle before opening the lid.
3. Using the calibrated water measure, which is marked and provided by the manufacturer, place one measure of water for each scoop of alginate powder into the flexible rubber bowl. Use three “measures” of water for most preweighed alginate packets (three scoops). The cooler the water, the more working time you will have; however, the consistency of the mix will be thinner. With warmer water, the working time is decreased.
4. For bulk alginate, use the measuring scoop provided. Measure the amount of alginate powder needed for the impression as directed by the manufacturer’s instructions. Add the alginate to the bowl containing the water. For preweighted packets, tear or cut open the packet and empty into the bowl.
5. Holding the bowl in one hand and the spatula in the other, gently stir to wet the alginate powder with the water.
6. Once the powder and water have been incorporated, start to rotate the bowl in the palm of one hand, and vigorously press the spatula flat against the bowl with the other. This spatulation method is pictured in Figure 27.3.
Incorporate the powder and water thoroughly to create a homogeneous mix—one that has a smooth consistency. At the completion of spatulation, the alginate will become “creamy,” similar to the consistency of peanut butter.
7. With the spatula, gather the alginate material into one mass in the bowl.