Сraig. Dental Materials

canal reamer. Apply thin layers of the varnish with a partially saturated pledget. Use a gentle stream of air for drying, but take care to avoid forming ridges. Add a new layer only to a previously dried one. Two thin layers have been found more protective than one heavy layer. To prevent contamination of the cavity varnish, use a new cotton pledget for each application. Tightly cap varnish solutions immediately after use to minimize loss of solvent. Most varnishes are supplied with a separate bottle of pure solvent. This solvent may be used to keep the varnish from becoming too thick. Replace the loss from evaporation by adding solvent to keep the bottle at least half full by diluting the contents with the solvent. Eventually the solvent will be exhausted and a new supply should be purchased. The solvent is also useful for removing varnish from external tooth surfaces.

PROPERTIES

Cavity varnishes reduce but do not prevent the passage of constituents of the phosphoric acid cements into underlying dentin. Variations in results and the mere reduction rather than the prevention of the passage of acid appear to be a result of pinpoint holes in the varnish film formed during volatilization of the organic solvent. Greater continuity of the dried varnish film is ~ossiblebv the use of successive lavers of thin t,arnish; t h ~ stechnique is more effectitre than using just one layer of thicker varnish

~ h i ' nfilms of iesinous cavity varnishes significantly reduce leakage around the margins and walls of metallic restorations. Although the effect of the cavity varnishes is not completely known, it can be hypothesized that this reduction of fluid penetration around cavity margins would minimize postoperative sensitivity. These varnishes are applied to prepared cavity walls, including the margins. The integrity of the resin film is destroyed when composite restorative materials are placed in contact with them. The monomer contained in these resin materials dissolves the film.

Varnishes neither possess mechanical strength nor provide thermal insulation because of inad-

equate film thickness. Values of film thickness have been measured at between 1 and 40 ym for different commercial varnishes. Contact angles of varnishes on dentin range from 53 to 106 degrees. Improved integrity of a varnish film might be achieved by improvement in the spreading of the varnish on the tooth surface.

APPLICATIONS

Cavity varnishes are indicated for use (1) on dentinal surfaces to minimize the penetration of acid from zinc phosphate cements, and (2) on enamel and dentinal walls to reduce the penetration of oral fluids around metallic restorations. Cavity varnishes appear also to retard the penetration of discolored corrosion products from dental amalgam into dentin. Varnishes are not used under composite restorations, because bonding agents effectively seal the dentin tubules. A cavity varnish is applied to the dentinal walls of those tubules in direct contact with the pulp when a base of zinc phosphate cement is used. When therapeutic action is expected from a low-strength base or liner or when the cement base material itself is bland in its action on the pulp, a cavity varnish is not used on the underlying dentin. The varnish may be applied over the cement base in these situations.

CAVITY LINERS

A cavity liner is used like a cavity varnish to provide a barrier against the passage of irritants from cements or other restorative materials and to reduce the sensitivity of freshly cut dentin. Unlike a varnish, a liner may provide some therapeutic benefits to the tooth. Liners, however, do not set as the calcium hydroxide pulp-capping agents do.

COMPOSITION

Cavity liners are suspensions of calcium hydroxide in an organic liquid such as methyl ethyl ketone or ethyl alcohol or in an aqueous solution

of methyl cellulose. The methyl cellulose functions as a thickening agent. Liners also may contain acrylic polymer beads or barium sulfate. Fluoride compounds such as calcium monofluorophosphate have been added to some liners. On evaporation of the volatile solvent, the liner forms a thin film on the prepared tooth surface.

MANIPULATION

Cavity liners are fluid in consistency and are easily flowed or painted over dentinal surfaces. The solvents evaporate to leave a thin film residue that protects the underlying pulp. Certain products are claimed to have better integrity and pulpal protection when used with composite restorative materials.

PROPERTIES

Like varnishes, cavity liners neither possess mechanical strength nor provide any significant thermal insulation. The calcium hydroxide liners are soluble and should not be applied at the margins of restorations.

Fluoride compounds have been added to some cavity liners to reduce the possibility of secondary caries around permanent restorations or to reduce sensitivity. Effectiveness of the fluoride for either purpose would depend on its availability to enamel and dentin through its solubility. Although in vitro studies with one material have shown reduced solubility of tooth tissue, clinical studies have not yet demonstrated its efficacy. However, such an investigation has shown an absence of bacteria at the resin composite-tooth tissue interface when a cavity liner containing calcium monofluorophosphate is used.

Low-strength (low-rigidity) bases are two-paste calcium hydroxide or ZOE cements that set to a hard mass when mixed. These cements are commonly referred to as liners, intermediay bases, or pulp-capping agents (calcium hydroxide prod-

ucts only). Glass ionorner, hylxid ionorner, and compoiner bases are discussed in the section on high-strength bases.

COMPOSITION AND CHEMISTRY

OF SETTING

Calcium Hydroxide Bases The base paste of a typical product contains calcium tungstate, tribasic calcium phosphate, and zinc oxide in glycol salicylate. The catalyst paste contains calcium hydroxide, zinc oxide, and zinc stearate in ethylene toluene sulfonamide. The ingredients responsible for setting are calcium hydroxide and a salicylate, which react to form an amorphous calcium disalicylate. Fillers such as calcium tungstate or barium sulfate provide radiopacity.

A light-cured calcium hydroxide base consists of calcium hydroxide and barium sulfate dispersed in a urethane dimethacrylate resin.

Zinc Oxide-Eugenol Bases These bases are non-modified (Type IV) ZOE cements as described in ANSI/ADA Specification No. 30 (IS0 3107). They are typically two-paste systems in which the zinc oxide and eugenol are formulated with inert oils and fillers. The cement sets to a hard mass when mixed. The setting reaction is accelerated by moisture and an increase in temperature.

MANIPULATION

Calcium hydroxide bases and ZOE low-strength bases are supplied as two-paste systems. Equal lengths of the different-colored pastes are dispensed on a paper pad and then mixed to a uniform color.

PROPERTIES

Calcium hydroxide cements are used for lining deep cavities or for direct pulp capping. The antibacterial action of calcium hydroxide makes these cements useful in indirect pulp-capping procedures involving carious dentin. ZOE cements are used in deep cavities to retard pene-

tration of acids and reduce possible discomfort to the pulp. Both calcium hydroxide and ZOE lowstrength bases are often used with a highstrength base in restoring a tooth. Root canal sealers containing calcium hydroxide have been developed.

Calcium Hydroxide Bases The important properties of these bases are mechanical and thermal properties, solubility, and pH. Calcium hydroxide (self-curedl bases have low values of tensile strength and compressive strength, or elastic modulus, compared with high-strength bases (Table 20-15). Although setting times vary between 2.5 and 5.5 minutes, compressive strengths of these cements continue to increase over a 24-hour period. For a group of five commercial products, compressive strengths ranged from 6.5 to 14.3MPa at 10 minutes to from 9.8 to 26.8 MPa at 24 hours. The low elastic modulus of calcium hydroxide bases restricts their usage to areas not critical to the support of restorations. Mechanical support should be provided by sound dentin or by a high-strength base. Calcium hydroxide bases are, however, considered strong enough to support the forces of condensation of amalgam.

Calcium hydroxide bases may provide some thermal insulation to the pulp if used in sufficiently thick layers. A thickness greater than 0.5 mm is not suggested. Practically, thermal protection should be provided by the overlying high-strength base.

The solubility of calcium hydroxide bases has been measured in several solvents for various periods of immersion. For a group of five commercial products, values ranged from 0.4% to 7.8%in distilled water at 37' C for 24 hours, from 0.1% to 6.2% in 35% phosphoric acid for 60 seconds, and from 0.3% to 1% in ether for 10 seconds. One product that was resistant to dissolution in water and in acid disintegrated when exposed to ether. Some solubility of the calcium hydroxide is necessary to achieve its therapeutic properties, although an optimum value is not known. Clearly the use of acid-etching procedures and varnish in the presence of calcium hydroxide bases must be done with care. Over a long term, some calcium hydroxide products seem to "disappear" from the cavity. The cause of this dissolution is unclear, but some products have been reformulated in an attempt to minimize the problem.

The pH of commercial products has been

measured at between 9.2 and 11.7.Free calcium hydroxide in excess of that necessary to form the calcium disalicylate stimulates secondary dentin in proximity to the pulp and shows antibacterial activity.

One light-cured calcium hydroxide base is reported to have a surface pH of 11.9, low dissolution in acid (<O.5%),low 24-hour solubility in water (<1.0%), and high compressive strength (80 MPa). Whereas two-paste calcium hydroxide bases show antibacterial activity, a light-cured type did not.

ZINC OXIDE-EUGENOL BASES

ANSI/ADA Specification No. 30 (IS0 3107) requirements for zinc oxide-eugenol liners are listed in Table 20-8. The mechanical properties of these cements are compared with those of calcium hydroxide bases and high-strength bases (see Table 20-15). ZOE products tend to be weaker and less rigid than calcium hydroxide pastes that set. Because the base is used in thin layers, it provides little thermal insulation. The eugenol has a sedative (obtundent) effect on the pulp. The base should not be used when a composite is to be placed, because eugenol can inhibit polymerization of the bonding agent and composite.

HIGH-STRENGTH BASES i ',

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,'

High-strength bases are used to provide thermal protection for the pulp and mechanical support for a restoration. Bases are usually prepared from a base or secondary consistency (higher powder/ liquid ratio) of zinc phosphate, zinc polyacrylate, glass ionomer, hybrid ionomer, or compomer cements. A polymer-reinforced ZOE base is also available.

Recently, self-cured and light-cured glass ionomer and hybrid ionomer cements have become available as lowand high-strength bases. The low-strength bases (liners) typically flow more readily than the high-strength bases and are less rigid, as shown in Table 20-15. Light-cured

glass ionomers are water-based cements but have a unique chemistry of setting that involves both an acid-base reaction between carboxylate ions and the glass particles and a lightaccelerated polymerization of dimethacrylate oligomers. Most of the light-cured liners are premixed pastes or powder-liquid systems mixed by hand, but one recent light-cured liner is encapsulated. The composition and setting reaction of the hybrid ionomers used as bases are identical to those of the luting cements discussed earlier in this chapter.

PROPERTIES

The tensile strength, compressive strength, and elastic modulus of five types of high-strength bases are compared in Table 20-15. The secondary consistencies of these cements result in higher values of strength and elastic modulus than values obtained from mixes of a primary (luting) consistency (see Table 20-3). Zinc phosphate cements are the most rigid of the five types, whereas the polymer-modified ZOE cements (Type 111) have the lowest properties. ANSI/ ADA Specification No. 96 specifies the net setting time, minimum compressive strength, and maximum values of acid erosion and acid-soluble arsenic and lead content for zinc phosphate, zinc polyacrylate and glass ionomer bases, as listed in Table 20-2.

The ability of a cement base to support a restoration during function has been studied by stress analysis techniques, as described in Chapter 4. The thickness and elastic modulus of the base affect the deflection of the base and restoration. Mismatches in the moduli of the base and restorative material can cause tensile stresses at the cement-restoration interface that can lead to failure of either material. Studies have recommended that a zinc phosphate base be used to support an amalgam restoration, whereas zinc phosphate, glass ionomer, or zinc polyacrylate may be used to support a Class 1resin composite restoration.

The glass ionomer lining cements and bases are unique in their ability to bond to dentin. Bond

strengths measured in tension vary from 2.0 to 4.9 MPa. These cements also can be etched to provide additional retention to a composite restoration (so-called sandwich technique). Glass ionomer lining cements and bases release fluoride ions and are radiopaque.

The thermal conductivity and diffusivity of dental cements (see Tables 3-4 and 3-6) are similar to those of enamel and dentin. Thus cement bases provide thermal protection to the pulp if used in a sufficiently thick layer

(>0.5 mm).

I SELECTED PROBLEMS

Problem 1

A provisional acrylic crown was difficult to remove, and upon removal was found to be soft. What caused these problems, and how can they be resolved?

Solution

The use of reinforced ZOE cement makes the removal of a provisional acrylic crown difficult, and the eugenol softens the acrylic. Therefore a low-strength, non-eugenol-zinc oxide or provisional resin cement is recommended for use with acrylic crowns.

Problem 2

A low-gold alloy bridge with normal retention is to be temporarily cemented. Which cement is recommended for this application?

Solution

Low-strength ZOE cement is suggested to allow for easy removal and cleanup of the bridge for final cementation.

Problem 3

A low-gold content MOD inlay with normal retention is to be cemented permanently. Which cements are recommended for this application? Which are not?

Solution a

Glass ionomer, hybrid ionomer, zinc phosphate, and zinc polyacrylate cements are suf-

ficiently strong and insoluble in oral fluids for this application.

Solution b

EBA ZOE cements are weaker and more soluble in oral fluids than are other cements and are generally not recommended for final cementation.

Problem 4

A gold alloy casting seated properly before cementation but failed to seat when cemented with zinc phosphate cement. What factors might have caused this difficulty, and how can it be corrected?

Solution

An excessive film thickness of zinc phosphate cement may result from (1) too high a mixing temperature, (2) too high a powder/liquid ratio, (3) water contamination during mixing,

(4) a glass slab temperature below the dew point, (5) too long a time between completion of mixing and cementation, or (6) cement being placed on the tooth before being placed on the casting.

Problem 5

A clinician experienced difficulties in reproducing the consistency of mixes of a zinc polyacrylate cement. The mixes were too thick. What were the probable causes of this problem, and how can they be corrected?

Solution a

The liquid of a zinc polyacrylate cement (excluding the anhydrous type) is very viscous and difficult to dispense accurately. The dropper bottle should be held perpendicular to the mixing pad when the liquid is dispensed. Try to dispense drops of uniform size.

Solution b

The loss of water by evaporation will increase the powder/liquid ratio. The liquid should be dispensed just before mixing, and a waterresistant mixing pad should be used.

Solution c

A mixing temperature higher than 23" C or overmixing will result in a mix that is too thick. Use a cool slab for mixing, and carefully follow the recommended mixing time for more consistent mixes.

Problem 6

During the insertion of a zinc phosphate cement base into an extensive cavity preparation, the mix became dry and friable and did not adhere well to the cavity walls. What factors might be related to such a change in consistency, and how can the consistency be improved?

Solution a

The heat of reaction accelerates the setting reaction of zinc phosphate cement, thereby causing the working time to decrease. Mixing the cement with incremental additions of powder and over a large area of a cool slab will reduce the temperature rise and allow a longer working time.

Solution b

Too much powder incorporated into a mix as it approaches secondary consistency decreases the working time and causes the cement base to be weak and friable because of insufficient matrix to bind the powder together. The base should be slightly tacky after it is rolled in powder at the completion of mixing.

Problem 7

A properly mixed zinc phosphate cement base became contaminated with saliva during placement. Should there be any deleterious effect expected from the contamination?

Solution

Yes, a soft, friable surface results from the dilution of acid and leaching of the matrix of a setting cement base by saliva.

Problem 8

A calcium hydroxide base appeared to disintegrate when acid came into contact with it during an acid-etching procedure. Are there bases that can be used in conjunction with acid-etching procedures for a composite?

Solution a

Yes, acid-resistant calcium hydroxide bases containing disalicylates are available.

Solution b

Glass ionomer bases can be etched to enhance the bond between the base and the composite.

Solution c

Use hybrid ionomer cement. Its bond to composite is not affected by etching.

Problem 9

When an amalgam is placed over a ZOE base, the cement sometimes crumbles and becomes incorporated into the amalgam mass. Should a thicker layer of ZOE cement or a different cement base be placed to provide better support?

Solution a

The ZOE base is a material with a low rigidity. Increasing its thickness will result in less rather than more support. Maintain the thickness of a low-strength base at less than 0.5 mm.

Solution b

A more rigid cement base, such as zinc phosphate cement, should be placed over the ZOE

base to provide additional support for condensation and for the amalgam from occlusal forces.

Problem 10

A paste-primer (one-step) composite orthodontic cement failed shortly after the appliance was activated. Composite remained on the tooth and the metal bracket. What factors might explain these observations, and how can the problem be corrected?

Solution a

The bond strength of a one-step composite cement can be very low if too thick a layer of the paste is placed on the bracket. The primer cannot diffuse through the thick layer of paste, causing insufficient polymerization of the cement. Keep the thickness of the cement paste to less than 0.25 mm.

Solution b

The enamel primer of a one-step cement remaining in the warm, humid environment of the mouth too long before the bracket is placed will cause a decrease in the cohesive strength of the cement. Apply the brackets to the teeth within 1 minute after applying the primer.

General

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Zinc Phosphate Cements

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