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After performing the laboratory/clinical exercises in this chapter, the student will be able to do the following:
1. Describe the use or purpose of the following materials:
- Calcium hydroxide
- Zinc phosphate
- Glass ionomer
- Zinc oxide–eugenol (ZOE)
- Temporary cement
2. Demonstrate the proper mixing technique for the materials listed above, and then, evaluate the mix according to the criteria stated in this chapter.
3. Recall the approximate mixing and setting times for the liners, bases, and cements discussed in this chapter.
4. Clean the cement spatula or mixing instrument and slab with the appropriate cleaning agent before the material sets.
The various cements, bases, and liners used in restorative dentistry are discussed in detail in Chapter 7, Dental Cements. The discussion covers the use, composition, and physical as well as chemical properties of each material. As stated earlier, these materials can be used as base or lining agents, luting materials, or permanent or temporary restorations. This chapter focuses on the manipulation of some of these commonly used materials.
Mixing of cements, bases, and liners is not a procedure that the dental hygienist typically performs on a routine basis. However, the hygienist may be asked by the dentist to mix these materials if the dental assistant is absent. This situation would include using them as a temporary restoration. Step-by-step instructions for measuring and mixing some of the commonly used cements, bases, and liners serve as the basis for this chapter.
II. Calcium Hydroxide Base/Liner Material
Calcium hydroxide may be used for pulp capping and as a base/liner under other dental restorative materials in deep preparations.
B. Protective Properties
Calcium hydroxide serves as a protective barrier between tooth tissues (dentin and pulp) and acid-containing cements and restorative materials.
1. The items necessary for mixing calcium hydroxide base/liner material are listed in Table 23.1.
2. Dispense small but equal amounts on a paper mixing pad, as shown in Figure 23.1.
TABLE 23.1. Armamentarium for Mixing Calcium Hydroxide Base/Liner Material
FIGURE 23.1. Calcium hydroxide dispensed on a paper mixing pad.
1. Mix thoroughly with a cement spatula or the crook of a small, ball-pointed instrument until a uniform color is achieved. A completed mix of calcium hydroxide is shown in Figure 23.2.
2. Mixing should be completed within 10 seconds.
3. The criteria for a correct mix of calcium hydroxide are listed in Table 23.2.
FIGURE 23.2. Completed mix of calcium hydroxide.
TABLE 23.2. Evaluation Criteria for Mixing Calcium Hydroxide Material
Use the tip of the ball-pointed instrument to place the mixed material on the floor of the cavity preparation. Avoid placing the mixed material on walls and margins, and avoid placing it in large amounts.
1. The setting time of mixed calcium hydroxide is 2 to 3 minutes on the mixing pad at normal room temperature.
2. The setting time for a pulp capping or base/liner will be greatly decreased in the mouth because of the moisture of dentin.
III. Zinc Phosphate Cement
As discussed in Chapter 7, Dental Cements, zinc phosphate cement is used to lute inlays, crowns, bridges, orthodontic bands, and other appliances to tooth structure. It can also be used as a base material under restorations.
B. Related Information
Remember that when the powder and liquid are mixed, an exothermic (heat-releasing) reaction occurs, as discussed in Chapter 7. To dissipate the heat of this reaction:
1. A large portion of the glass slab must be used during mixing.
2. The powder must be added in small increments.
3. The mixing time must extend to 1.5 to 2 minutes.
C. Measuring, Mixing, and Application for Luting Consistency
1. The items necessary to mix zinc phosphate cement are listed in Table 23.3.
2. This technique is described in detail, along with illustrations, in Chapter 7.
3. The number of drops of liquid and scoops of powder that are required can be found in the directions provided by the manufacturer.
4. Forcibly mix the powder into the liquid by increments. A figure-8 motion is frequently used, as shown in Figure 23.3. Sometimes, a back-and-forth stropping motion is used.
5. The mixing time is usually 15 seconds for each increment.
6. Remember that for correct luting consistency, the cement should form a “1-inch string” between the spatula and the slab, as shown in Figure 7.6E.
7. The Skill Performance Evaluation sheet for mixing zinc phosphate cement to a luting consistency is located in Appendix 2.
TABLE 23.3. Armamentarium for Mixing Zinc Phosphate Cement
FIGURE 23.3. The Figure-8 mixing technique for a dental cement.
D. Measuring, Mixing, and Application for Base Consistency
1. The difference between luting consistency and base consistency is that a higher powder/liquid ratio is used for base consistency. This means that fewer drops of liquid will be needed to mix the cement for a base consistency. The specific amounts to be used are provided by the manufacturer.
2. The powder is also divided into increments for base consistency, but this may include the division of one powder increment into yet smaller increments.
3. The mixing time is approximately the same as that for luting consistency (1.5–2 minutes).
4. The completed mix should be thick, putty-like, and able to be rolled into a ball.
5. The base consistency for zinc phosphate is shown in Figure 7.6F.
6. The Skill Performance Evaluation sheet for mixing zinc phosphate cement to a base consistency is located in Appendix 2.
The setting time for both consistencies of zinc phosphate cement is between 5 and 9 minutes.
Tap water is used to clean the spatula and glass slab before the cement sets. If the mix has set on the spatula and slab, the instruments can be cleaned by soaking them in a solution of bicarbonate of soda (baking soda) and water.
Tips for the Clinician
Calcium Hydroxide Liner/Base
- Mix until a homogeneous color is obtained.
- Wipe the mixing instrument clean with a gauze before applying material to the cavity preparation.
Zinc Phosphate Cement
- Mix using a large area of the slab to dissipate the heat.
- When finished, clean the slab and spatula immediately with soap and water.
Glass Ionomer Cement
- When dispensing the liquid, take the necessary time to make precise, uniform drops.
- Mixing time is short, usually 45 seconds or less.
Zinc Oxide–Eugenol (ZOE) Cement
- Use firm pressure with the flat face of the spatula when incorporating powder.
- Enough powder has been incorporated for a base consistency when the mix “flakes” from the spatula when pressing on the cement.
- Mix until a homogeneous color is obtained.
IV. Glass Ionomer
Glass ionomer cement may be used as a base, a luting agent, or a restorative material. As a luting agent, it is used to permanently cement crowns, bridges, and orthodontic bands. As a base, it is used as a thermal insulating material in deep cavity preparations.
B. Related Information
As discussed in Chapter 7, Dental Cements, glass ionomer cements are high in strength and low in solubility. Compared to other cements, they are also relatively kind to the pulp. They chemically bond to tooth structure, and they release fluoride ions into enamel and dentin, which is believed to reduce recurrent decay. It is for these reasons that this cement has become so popular.
C. Dispensing Systems
Glass ionomer may be dispensed in three ways:
1. Powder/Liquid Systems
FIGURE 23.4. Glass ionomer cement: powder, liquid, dispensing scoop, and paper pad.