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After studying this chapter, the student will be able to do the following:
1. Demonstrate the following laboratory procedures:
- Use a gypsum product to pour a study model from an impression.
- Trim the study model on the model trimmer.
2. Differentiate between the two methods of diagnostic cast/study model fabrication.
3. Describe, in order, the cast cuts that are used to trim a study model for patient consultation and dental treatment planning.
4. Explain the laboratory safety procedures and necessary equipment for use in the laboratory mandated by the Occupational Safety and Health Administration.
5. Critique the completed study model for acceptable cuts.
The composition, properties, and uses of gypsum materials are discussed in Chapter 9, Gypsum Materials. Study models (diagnostic models or diagnostic casts) are defined as “a positive replica of the dentition and surrounding structures used as a diagnostic aid and/or base for construction of orthodontic appliances or prosthetic devices.”
Because a dental hygienist is legally permitted to perform these procedures, competency is expected in the associated knowledge and skills. These procedures are commonly performed in the practice of dentistry to gather information concerning the patient’s dentition and to establish an accurate diagnosis. As with all skills, however, “practice makes perfect.” Most people must continue to practice to be consistently competent. Therefore, do not be discouraged if your first attempt is not exact.
During any process, each step is important to the quality of the final outcome. If a flaw is present in step one, that flaw will be reproduced throughout the entire procedure and be present in the final product. The ability to self-critique to make adjustments or repeat the step is critical for obtaining satisfactory results. The procedural steps for these techniques are detailed in this chapter.
The purpose of this chapter is to apply previously learned information and acquire the necessary skills to prepare esthetically pleasing and useful study models. The process of pouring an alginate impression, preparing a base for the model, separating the model from the impression, and trimming the model, is described in the following pages. It is also important that these skills are practiced both safely and effectively; therefore, compliance with Occupational Safety and Health Administration (OSHA) guidelines is mandatory.
II. Construction of a Study Model
Several laboratory techniques for fabricating study models are acceptable. The armamentarium is listed in Table 28.1. This chapter presents two of these techniques: one using a “double-pour” and another using a “single-pour” of the impression.
TABLE 28.1. Armamentarium for Constructing Study Models
A. Using a Double-Pour Technique
A two-stage pour will prevent distortion and provide an adequate base for reproduction of the oral structures. The double-pour technique involves two separate mixes and, thus, two separate setting times.
1. First-Stage Pour
a. Measure 100 to 150 g of the dental stone or other gypsum depending on the size of the impression. Regardless of the material, follow the manufacturer’s directions.
b. Measure the correct quantity of the corresponding water, and pour it into your plaster mixing bowl. Pour the powder into the liquid.
c. Spatulate (stir) the mixture for 30 seconds with a stropping motion. Pressing the spatula against the side of the bowl helps to remove air bubbles from the mixture.
d. Place the mixing bowl on the vibrator, and compress the sides of the bowl. Bubbles should rise to the surface and break. Remove the mixing bowl from vibrator.
e. Place the impression on the vibrator. Starting at one corner and using a wax spatula, “deliver” a small amount (pea size) of the mix into the mold distal to the last molar (Fig. 28.1A). Allow this material to flow into the impression under vibration, filling the impression of the most distal tooth in the quadrant.
f. Repeat the previous step until the impression is filled, adding the new material at the same location as the initial increment. The material should flow around the arch, filling one tooth at a time. The size of the increment may be increased when the material has a greater distance to flow around the arch. The plaster spatula may be used to add larger amounts of material for these final additions (Fig. 28.1B).
g. After all the teeth are filled, pour the remainder of the mix from the bowl into the impression (Fig. 28.1C). Overfill the impression with gypsum material sufficiently to cover all areas by approximately 0.25 inch.
h. Add small amounts of the gypsum material to the surface of the pour to assist in retention of the second pour (Fig. 28.2). Allow the impression to set facing up.
i. Allow the mix to reach the initial set (see the manufacturer’s instructions). This will be several minutes after the “loss of gloss,” or approximately 10 minutes after the initial pour.
FIGURE 28.1. Pouring an impression. A. A small amount of gypsum material is added distal to the most posterior molar. B. Larger increments are added with the plaster spatula. C. After the teeth are filled, the remaining gypsum material is poured to fill the impression.
FIGURE 28.2. Alginate impression poured with dental stone. The paddy serves as a base for the alginate impression.
2. Second-Stage Pour
a. Make a thicker mix of gypsum product (100–150 g) to serve as a base to support the first pour.
b. Make a paddy of gypsum material approximately 1 inch thick in a shape resembling the size of the impression tray, and place it on a glass or acrylic slab or a surface that will not absorb water or stick to the mix (Fig. 28.2).
c. Invert the poured impression onto the paddy. Do not apply pressure to the inverted impression tray or allow the impression to sink closer than 0.5 inch to the surface. Do not allow the gypsum material to overlap outside the tray (Fig. 28.3).
d. Blend the two gypsum mixes together with the plaster spatula to create a continuous connection of the two mixes (Fig. 28.4). While holding the handle of the impression tray, use the spatula to shape the sides of the stone around the first pour. Avoid placing the wet gypsum material from the base onto the impression tray because this would create a mechanical lock that is difficult to separate when set. Be certain that the handle of the impression tray and the occlusal surface of the teeth are parallel with the slab to promote a base of uniform thickness. A cross section of the base and impression is illustrated in Figure 28.5. Note how the additional small mounds of gypsum added to the first pour “interlock” into the base to provide a stronger finished product.
e. After loss of gloss, trim the excess stone base with a laboratory knife to approximately ¼ inch from the outer border of the impression (Fig. 28.6). This will develop the sides and base area of the cast, thus requiring minimal use of the model trimmer.
f. Wait 45 to 60 minutes after the beginning of the second pour to separate the cast from the impression. Carve away gypsum material and alginate exposing the edge of the tray.
g. To begin tray removal from the cast, place the laboratory knife on the edges of the periphery of the tray, and slightly twist the knife to pry apart the adhering alginate and gypsum materials (Fig. 28.7). Repeat this procedure around the periphery of the tray.
h. When the margins of the tray are free of the cast, use an upward, pulling motion to facilitate the separation (Fig. 28.8).
i. If the tray does not freely separate from the cast, determine the location of the obstruction on the periphery, and carefully remove it with a laboratory knife.
j. Avoid rocking the impression tray back and forth or side to side. This may cause the teeth to fracture.
k. Trim the base and sides of the cast on the model trimmer. Use of the model trimmer is discussed later in this chapter.
FIGURE 28.3. Inverted set impression on the dental stone paddy.
FIGURE 28.4. Adapting the dental stone base material to the set stone in the impression tray.
FIGURE 28.5. Cross section of the poured alginate impression inverted onto the gypsum paddy (base).
FIGURE 28.6. Trimming the excess dental stone with a laboratory knife after loss of gloss.
FIGURE 28.7. Separating the cast from the impression tray with the laboratory knife.
FIGURE 28.8. Using an upward, pulling motion to separate the cast from the impression.
B. Using a Single-Pour Technique
After developing skill with the double-pour technique, a single-pour technique may be used for some models or casts. In the single-pour technique, a larger amount of material is initially mixed. The impression is filled and the paddy prepared as described above. The filled impression is inverted and placed on the paddy before the material sets. Greater care is needed to prevent the impression tray from being locked into the setting gypsum material.
C. Using Boxing Wax
Boxing wax can be used to form a mold for the base of the study model. The boxing wax completely encircles the impression tray and creates a temporary “extension” of the impression. This extension forms the mold for the base of the study model. Use of boxing wax is more common for pouring casts for dentures and other restorations.
Tips for the Clinician—Pouring an Impression
- Properly proportion and mix the gypsum product.
- Fill one tooth at a time.
- Do not lock the impression tray into the cast.
- Do not allow the material to vibrate out of the impression at the opposite end of the arch.