Because these types of restorations are seated on a prepared tooth structure, the hollow, shell-like structure of the indirect restoration requires that a luting agent should have qualities that do not interfere with a proper fit (Fig. 45-1).

Temporary Cements

Temporary cementation of an indirect restoration may be completed if the dentist needs to remove the indirect restoration at a later date; if the tooth is sensitive or is exhibiting other symptoms that might require removal of the cast restoration; or when temporary cementation of provisional coverage is required while the patient waits until the laboratory technician completes the cast restoration. Provisional coverage is a temporary coverage that is placed on a tooth to protect the tooth until the permanent restoration is ready to be cemented (see Chapter 51).

Variables Affecting Final Cementation

A number of factors can influence actual cementation with a luting cement. Cementing errors can be the result of improper mixing technique, time, humidity, or incorrect temperature of the glass slab.

Mixing Time

One of the first and foremost rules in the preparation of a cement material is to follow the manufacturer’s directions regarding a material’s exact mixing time, working time, and delivery time. Any delay between completion of the mix and seating of the cast restoration will result in the initial setting process, which could cause the casting not to seat properly.

Regardless of the type of dental cement used, basic guidelines must be followed.

Guidelines for Mixing Dental Cements

• Before mixing, read and carefully follow the manufacturer’s directions for the type of cement being mixed.

• Determine the use of the cement (restorative, liner, base, cement); then measure out the powder and liquid according to the manufacturer’s instructions.

• Place the powder toward one end of the glass slab or paper pad and the liquid toward the opposite end (the space between allows room for mixing).

• Divide the powder into increments. Each manufacturer uses a slightly different system for sectioning the powder. Some manufacturers divide the powder into equal parts, whereas others divide the powder into progressively smaller increments. When increment sizes vary, use the smaller increments first.

• Incorporate each powder increment into the liquid; then mix thoroughly. The mixing time per increment will vary according to the type of material and its use.

Humidity

If the clinical area is warm, or if it is a humid day, premature exposure of the cement to these environmental conditions can create loss of water from the liquid or addition of moisture to the powder. Always dispense the powder first, then the liquid, to minimize the loss of water from evaporation. Wait until it is time to mix the material before placing it on the pad. Do not set it out at the beginning of the procedure.

Powder-to-Liquid Ratio

Incorporating too much or too little powder will alter the consistency of the cement. Be sure to fluff the powder in the bottle before dispensing the powder in the measuring scoop. Always hold the bottle or vial upright to ensure drops of consistent size when dispensing the liquid (Fig. 45-2).

FIG. 45-2 Hold the bottle upright when dispensing the liquid of a cement.

Temperature

Specific types of cement will have a chemical reaction during the setting stage. An example of this is seen when the material zinc phosphate generates heat; this is referred to as an exothermic reaction. In such cases, it is beneficial to cool a glass slab in the refrigerator before mixing the cement. If the glass slab is kept in the refrigerator, make sure to thoroughly wipe the slab dry before dispensing the material, so as not to incorporate any moisture condensation into the material.

1 What is another name used for permanent cement?

2 When is a temporary cement used instead of a permanent cement?

3 What variable affects the addition or loss of water in a dental cement?

Types of Cement

The cements described in this chapter are designed to be versatile in their properties, uses, and mixing techniques. Many of the cements that were traditionally used for routine cementation through the 1990s have been replaced by newer makeup and mixing techniques. The selection of cement for a specific procedure requires knowledge of the chemical and physical properties of each particular cement type for the specific restorative process to be performed.