For additional ancillary materials related to this chapter, please visit thePoint.
Objectives
After studying this chapter, the student will be able to do the following:
1. Describe the use of dental cements as a:
- Luting agent
- Base/liner
- Filling material
- Temporary restoration
- Intermediate restoration
- Periodontal pack
- Temporary cement
2. Explain the importance of adhesion and microleakage of dental cements.
3. Describe the use of a cavity varnish or cavity sealer.
4. Describe the differences between the two cement powders and three cement liquids.
5. Explain the setting reaction of a typical dental cement.
6. Based on the properties of the liquid and the powder, discuss the properties of:
- Zinc oxide–eugenol (ZOE) cement
- Zinc phosphate cement
- Polycarboxylate cement
- Glass ionomer cement
- Composite cement
- Calcium hydroxide base
7. Summarize the mixing process for the first four cements in objective #6 and how it relates to the setting reaction.
8. Describe the use and advantages of a (resin) composite cement.
Key Words/Phrases
base
composite cements
direct pulp cap
eugenol
glass ionomer cements
liner
luting agent
obtundent
polycarboxylate cement
resin cements
silicate cement
temporary cements
varnish
zinc oxide–eugenol (ZOE) cement
zinc phosphate cement
Introduction
Dental cements are used to lute (glue or cement) inlays, crowns, bridges, and other restorations in place, as shown in Figure 7.1. These are used similar to the cements and glues of everyday life. In addition, dental cements are used for a wide variety of other dental procedures depending on the material.
FIGURE 7.1. A. When luting a crown, the clinician paints the inside with cement. B.The crown is then seated on the prepared tooth, and the excess cement is forced out at the margins. (Courtesy of GC America, Inc., Alsip, IL.)
I. Use of Dental Cements
Each use of dental cement requires specific characteristics. Luting materials need to be very fluid when being used to cement a crown and should become very strong after they set. Other cements are mixed to a putty-like consistency to restore missing tooth structure or to help protect the pulp under a filling. Some cements are used for a variety of procedures; others have limited use. Chapter 23, Mixing Liners, Bases, and Cements, presents the laboratory and clinical application of five of the materials and may be used to supplement this chapter.
A. Luting Agent
The term “cement” implies that the material will be used to lute or glue things together. Although other uses are common, use as a luting agent has given this group of materials their name, cements. Dental cements hold appliances and restorations in place with micromechanical and macromechanical retention. Some dental cements are adhesive through chemical bonds, but most are not. Adhesion is discussed in Chapter 4.
When a crown is luted to the preparation, the cement is mixed and then painted inside the crown (or filled), as shown in Figure 7.1A. The crown is seated in place on the preparation, causing the excess cement to ooze out at the margins, as shown in Figure 7.1B. The cement is allowed to set either partially or completely, and the excess is removed, much like scaling calculus from teeth. It is critical that all the excess cement is removed because any excess cement left behind will become a plaque trap and cause gingival irritation.
Luting cements have the most demanding requirements of any dental material. They must set in the mouth, changing from a fluid liquid to a rock-hard solid in a matter of minutes. The resulting material must be biocompatible and insoluble in oral fluids. Because cements are much more soluble than the overlying restorative material, accurate fit of the restoration to be luted is critical. Crown margins should precisely fit the preparation to minimize the amount of cement that is exposed to the oral environment. Poorly fitting margins increase the solubility of the cement and the likelihood of recurrent decay.
A patient’s oral hygiene and diet can also affect the longevity of a luted restoration. As bacteria ferment sugars, the pH in the mouth drops. Dental cements are much more soluble in an acidic environment. As the cement dissolves, a gap is created between the tooth and the restoration. The tooth structure at the gap will have a high risk for caries. Plaque control and proper diet will reduce the probability of recurrent decay and help maintain a less destructive environment for dental cements and other materials.
B. Pulp Protection
Dental cements are also used as an intermediate base or liner when the remaining dentin is believed to be less than 2 mm thick. A base or liner is placed on the dentin between the pulp and the restorative material. A base and a liner are illustrated in Figure 7.2E and F. Because the solubility of dental cements is much greater than is that of the overlying restorative material, bases and liners must not be placed on margins.
FIGURE 7.2. Several uses of dental cements for pulpal protection. A. Carious lesions are present. B. The tooth is prepared. C. A temporary filling is placed. D. At a later appointment, the temporary filling is cut back, leaving a cement base. E. The base is covered with a permanent restoration. F. A liner is much thinner than a base.
1. Liners
A liner is used to protect the pulp from chemical irritation, as illustrated in Figure 7.2F. A liner may stimulate secondary dentin formation (bioactive) or release fluoride. Liners are too thin (<0.5 mm) to provide thermal insulation, and they may be too weak to support the restorative material or to resist amalgam condensation forces.
2. Bases
A base is stronger and thicker than a liner, as illustrated in Figure 7.2E. A base provides thermal insulation. Some support the restorative material and may release fluoride, and some are irritating to the pulp before the setting reaction has completed. Such a base may be used in conjunction with a liner. Previously, the distinction between bases and liners was clear. Today, liner materials are much stronger, and the distinction between bases and liners is quite blurred.
C. Temporary Restoration
Some of the same dental cements that are used to lute crowns and that serve as a base may also be used as a temporary restorative material (Fig.7.2A–C). For some cements, the material is mixed to a thicker consistency than is used for luting. Other cements have formulations that are designed for use as temporary and permanent restorations.
1. Temporary Restorative Materials
Many dental cements are used as a temporary restorative material. The specific cement is chosen based on the particular clinical requirements of the situation. A temporary restoration (filling) may be placed as an emergency procedure when time restraints prevent a more complex treatment. Also, when pulpal pain and other symptoms do not result in a definitive diagnosis, such as reversible versus irreversible pulpitis, a temporary restoration might be placed.
2. Temporary Material as a Base
At times, a temporary filling is placed, and at a later appointment, part of the temporary filling material is removed. The remaining temporary material then becomes a base, which is covered with a permanent restoration. The advantage to such procedures is that the pulp is less irritated since the overlying dentin is not exposed a second time. These procedures are illustrated in Figure 7.2A–E.
3. Caries Control
When patients have a high number of carious lesions (>10), caries control procedures may be implemented. The goal of caries control is to change the oral environment from cariogenic to noncariogenic. It is hoped that caries control procedures change the oral flora from acid-producing and acid-loving bacteria to other nonpathogenic species. Caries control includes quick, efficient removal of as much decay as possible in the shortest time possible, placement of temporary restorations, improved oral hygiene, diet changes, and fluoride supplements. Two dental cements that are frequently used for caries control are zinc oxide–eugenol (ZOE) and glass ionomer cement. If all the decay is removed from a lesion, the temporary restoration can later function as a base, as previously described. Light-cure glass ionomer materials will bond to composite materials to a certain extent. Light-cure glass ionomers may be used for caries control and then veneered with composite material to improve esthetics and surface roughness. The ZOE materials are snow white in appearance and, for many patients, are not suitable for use in the anterior teeth.
D. Other Formulations of Dental Cements
Formulations of dental cements are also used as endodontic sealers and surgical/periodontal packs; these formulations are discussed later in this and in other chapters.
Still other cement formulations include impression materials and bite registration materials. These formulations are discussed in the following chapter.
E. Cavity Sealers
Cavity sealers are discussed in this chapter because both these and dental cements are used to protect the pulp.
1. Varnish
In dentistry, copal varnish and other varnish formulations are used much like varnishes are used to protect wood. Varnish is composed of resins dissolved in a solvent. Figure 7.3 shows an example of a cavity varnish. The varnish is painted onto the entire cavity preparation, including the margins. The solvent then evaporates and leaves behind a very thin layer of resin. Varnish is frequently used to seal preparations for amalgam restorations. It can also function as a chemical barrier, protecting the pulp from an irritating base or a luting cement. It decreases the initial microleakage of the amalgam restoration until corrosion products form at the interface. Varnish is not thick enough to affect thermal sensitivity, and it is not used with composite materials because it would interfere with adhesion.
FIGURE 7.3. Cavity varnish.
2. Dentinal Bonding Agents
Some dentists are substituting dentinal bonding agents for varnish. Several of these products will bond the amalgam to the tooth structure. Clinical studies of postoperative sensitivity, however, do not support the routine use of dentinal bonding systems to seal all amalgam restorations. Some dentists are using Gluma primer as a substitute for varnish. Gluma is a brand name for one of the original dentinal bonding systems. The original Gluma system has been completely reformulated, but the original primer is still available.
3. Order of Placement
If more than one base, liner, varnish, or dentinal bonding system is used, the properties of the materials determine their order of placement. Bioactive materials are placed first and then adhesive materials. Irritating materials are placed last.
II. Chemistry of Dental Cements
With the exception of composite cements, dental cements are brittle, ceramic materials. For many cements, the chemistry is a simple acid–base reaction. The resulting product is insoluble in water and oral fluids.
A. Formulation
1. Dental cements are often a powder/liquid system.
2. The liquid is an acid.
3. The powder is a base. The powder must be insoluble in oral fluids but reactive with the acid.
4. If one understands the properties of the components of a dental cement, then one will be able to predict the properties of the resulting set material.
B. The Reaction
1. The reaction is
2. In terms of the components of the dental cement, the reaction is
3. The residual powder and the matrix must be insoluble in oral fluids.
4. The end result is a “cored structure,” much like that of set amalgam.
C. Composite Cements
The chemistry of composite cements is the same as that of acrylics and composites. This chemistry was presented in Chapter 5, Direct Polymeric Restorative Materials.
III. Powders Used in Dental Cements
Two materials are used to make powders for dental cements: zinc oxide and glass. The manufacturing process grinds and sieves the powders to obtain the proper particle size. The size of the particles determines the film thickness of the resulting mixed cements. Film thickness determines how well a casting or other restoration can be seated on a preparation. Excessively large particles result in high film thickness, open margins, and recurrent decay.
A. Zinc Oxide
1. Zinc oxide is the only insoluble, nontoxic, reactive oxide or hydroxide that is available to react with an acid. Common additives to the zinc oxide powder are aluminum oxide (alumina) to strengthen and magnesium oxide to control the setting rate.
2. Zinc oxide has some antibacterial effects and is included in diaper rash, sunscreen, and foot powder products.
B. Powdered Glass
1. Silicon oxide, the chemical formula of glass, is very unreactive. However, if oxides of sodium, calcium, and potassium are added in sufficient quantity, the glass will react with a strong acid. As expected, the powder is white because it is made of small, translucent glass particles.
2. The glass formulation also contains fluoride. Fluoride is a common glass additive because it reduces the melting temperature and improves the flow of the molten glass. Fluoride in the glass powder gives the resulting dental cement the ability to release fluoride and inhibit recurrent caries.
C. Reactivity of Powders
The reactivity of the powder components is controlled by the manufacturer and is matched to the reactivity of the liquid component.
IV. Liquids Used in Dental Cements
The composition or strength of the acid determines the reactivity of the cement liquid. The manufacturer controls this.
A. Eugenol
Eugenol is an organic liquid that is also a weak acid. Eugenol is a major component of oil of cloves. As a result, eugenol has the distinctive smell and taste of cloves.
1. Eugenol is a phenol derivative that is antibacterial and also obtundent to the pulp. Obtundent means that it reduces irritation.
2. Eugenol inhibits free radical polymerization. This limits the use of eugenol-containing cements because they will inhibit the set of composite restorative materials.
3. Other organic liquids have been added to eugenol to formulate dental cements. The most notable of these added organic liquids is ethoxybenzoic acid.
B. Phosphoric Acid
The phosphoric acid used in dental cements is approximately two-thirds phosphoric acid and one-third water by weight. This formulation is very acidic and can be quite irritating to biologic tissues in or out of the oral cavity. The amount of water present affects the reactivity of the liquid by changing the ionization of phosphoric acid. Therefore, it is important to keep the cap on the bottle and not dispense the liquid until one is ready to mix the cement. High or low humidity will affect the water content and, therefore, the pH, reactivity, and properties of the resulting cement. If the liquid appears cloudy, it has outlived its shelf life and should be discarded.
C. Polyacrylic Acid
Several dental cements use an aqueous solution of polyacrylic acid. These solutions are 30% to 50% polyacrylic acid by weight and are very viscous liquids.
1. Dispensing
Dispensing these liquids requires more attention than does dispensing other cement liquids. If one is not careful, the viscous liquid does not form independent drops. Instead, the drops can “run together,” and the amount of liquid dispensed will not be accurate and will vary greatly with each mix. Like phosphoric acid, the liquid should not be dispensed until one is ready to mix the cement because water can evaporate, changing the reactivity and the cement properties. These cement liquids should not be stored in the refrigerator because some will gel and become unusable.
2. Bonding
The carboxyl groups of polyacrylic acid bond to calcium in tooth structure. This bond is believed to be relatively stable in a wet environment. Adhesion of glass ionomer materials was presented in Chapter 4, Adhesive Materials.
3. Water-Hardening Cements
“Water-hardening” or “water-setting” cements use anhydrous, freeze-dried polyacrylic acid. The manufacturer mixes zinc oxide or glass powder with the powdered anhydrous polyacrylic acid. This combined powder is mixed with a companion liquid that is predominately water. When mixed, the polyacrylic acid first dissolves in the water and then reacts with the zinc oxide or powdered glass.
V. Powder/Liquid Ratios and Systems of Dental Cements
Dental cements combine these three liquids and two powders. Table 7.1 lists the resulting cements. The properties of the resulting cements are based on the properties of the components involved. The manufacturer adjusts the reactivity of both the liquid and powder components to obtain proper setting characteristics and other properties. Do not mix powders and liquids of different cements or different products of the same type.
TABLE 7.1. Components and the Resulting Cements
A. Components Dictate Handling and Mixing
1. The ZOE cement and zinc phosphate cement are mixed with a powder/liquid ratio that depends on the intended use. A base or temporary restoration mix is thicker than a luting mix. The higher the powder/liquid ratio, the greater the strength, the lower the solubility, and, in general, the better the cement. On the other hand, working time decreases, and viscosity increases. If a luting mix becomes too thick, the restoration may not seat adequately. In this case, the marginal gap is increased, as is the likelihood of caries. In addition, micromechanical retention is reduced.
2. The powder/liquid ratio is limited in that the liquid must wet all the powder for the cement to function adequately.
3. Glass ionomer and polycarboxylate cements have powder/liquid ratios that are determined by the manufacturer; it is important to follow the manufacturer’s directions. Mixing time is also important. If the mixing procedure is too slow, two problems can occur. The first involves the resulting mix becoming too thick. In this case, the restoration may not adequately seat when luted, which increases the marginal gap. The second involves adhesion. The mixed cement must be fluid enough to wet the tooth for both micromechanical and chemical adhesion. If the mixing process is too slow, the carboxylic acid groups react with the powder and are not available to react with the tooth structure. Chemical adhesion is then reduced or even eliminated.
4. Paper pads are available for mixing many dental cements and other dental materials. One must be careful, however, if a paper-mixing pad is used for mixing dental cement. Cement liquids may weaken the paper surface, causing it to become abraded. The abraded paper particles are then incorporated into and weaken the resulting cement. Some mixing pads use paper that has been coated with a thin layer of plastic. This type of pad is commonly used with glass ionomer and polycarboxylate cements. A thick glass slab is favored for mixing zinc phosphate cement; typically, the slab is cooled to improve the resulting mix.
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