Сraig. Dental Materials

Setting time (min)

Fig. 20-4 Effect of powderiliquid ratio on setting time.

Several factors influence the rate of set of zinc phosphate cement. Those factors under the control of the manufacturer and the operator are listed in Tables 20-5 and 20-6. Although the manufacturer initially adjusts the setting time, improper handling of the powder and liquid can greatly modify the setting time. Anything that tends to speed the rate of reaction will shorten the setting time.

Strength The strength of zinc phosphate cement is influenced by the initial powder and liquid composition, the powder/liquid ratio and manner of mixing, and the handling of the cement during its placement.

ANSVADA Specification No. 96 (IS0 9917) stipulates that a standard inlay-seating consistency must exhibit a minimum 24-hour compressive strength of 70 MPa. The compressive strength of the zinc phosphate cement develops rapidly, with the inlay-seating consistency reach-

ing at least two thirds of its final strength within 1 hour.

A proper mixing technique ensures a higher powder/liquid ratio for the consistency of cement desired, increasing the compressive strength of the cement mass. A cement base consistency, when properly achieved, offers greater strength than the inlay-seating consistency does. The maximum powder/liquid ratio is reached, however, when further amounts of powder do not increase the strength but may in fact decrease it because of the presence of excess unattached powder. For this reason a mass of mixed cement should evenly incorporate the powder throughout the mix.

Solubility and Disintegration The premature contact of the incompletely set cement with water results in the dissolution and leaching of that surface. Prolonged contact, even of the well-hardened cement, with moisture demonstrates that some erosion and extraction of soluble material does occur from the cement. ANSI/ ADA Specification No. 96 allows a maximum rate of erosion of 0.1 mm/hr when the cement is subjected to lactic acid erosion by an impinging jet technique. Even the filling cement mixes show considerable loss of material in the mouth over a period of time, indicating that zinc phosphate cement can be regarded as only a temporary filling material. Wear, abrasion, and attack of food decomposition products accelerate the dis-

integration of zinc phosphate cements within the oral cavity. Greater resistance to solution and disintegration is obtained by increasing the powder/liquid ratio. A thicker mix of cement therefore exhibits less solubility and disintegration than a thinner mix.

The difference between the resistance to abrasive and chemical attack intraorally and the passive resistance to solution and disintegration in distilled water causes a different clinical observation between zinc phosphate and other cements. This lack of correlation has been demonstrated in a clinical study in which the order of solubility of cements tested in the mouth was, from least to most soluble, glass ionomer, zinc phosphate, ZOE reinforced with ethoxybenzoic acid, and polyacrylate cements. Laboratory tests in distilled water indicate that glass ionomer cements are the most soluble and that polyacrylate, ZOE, and zinc phosphate cements are the least soluble.

ANSI/ADA Specification No. 96 specifies the maximum acid-soluble arsenic and lead contents as 2 and 100 mg/kg, respectively.

Dimensional Stability Zinc phosphate cement exhibits shrinkage on hardening. The normal dimensional change when properly mixed cement is brought into contact with water after it has set is that of slight initial expansion, apparently from water absorption. This expan-

Fig. 20-5 Direct pH values determined on external surface of cements stored in humidor. Note: Zinc silicophosphate cements are no longer used.

(Adapted from Norman RD, Swartz ML, Phillips RW: J Dent Res 45136, 1966.)

sion is then followed by a slight shrinkage on the order of 0.04% to 0.06% in 7 days.

Acidity During the formation of zinc phosphate cement, the union of the zinc oxide powder with the phosphoric acid liquid is accompanied by a change in pH, as shown in Fig. 20-5. In the early manipulative stage this increase in pH is relatively rapid, with a standard mix reach-

ing a pH of 4.2 within 3 minutes after mixing is started. At the end of 1 hour this value increases to about 6 and is nearly neutral at 48 hours.

Investigation has shown that the initial acidity of zinc phosphate cement at the time of placement into a tooth may excite a pulpal response, especially where only a thin layer of dentin exists between the cement and the pulp. In a normal, healthy tooth this response may be entirely reversible, whereas in a tooth whose pulp has already been put under stress from other trauma the response may be irreversible, with pulp death ensuing. When zinc phosphate cement is to be used, exercise precaution in a deep cavity to protect the nearby pulp tissue from further trauma by the initial acidity of the cement. Such precautions include the use of resinous, filmforming, cavity varnishes; calcium hydroxide and zinc oxide suspensions; ZOE or calcium hydroxide bases; and, more recently, dentin bonding agents.

Gross decalcifications occasionally observed after the removal of orthodontic bands that have been retained in place with zinc phosphate cement are most probably attributable to the loss of the luting material between the band and the tooth, resulting in a favorable environment for bacterial action.

Thermal and Electrical Conductivity

One of the primary uses of zinc phosphate cement is as an insulating base beneath metallic restorations. Recent studies have shown that as a base material this cement is an effective thermal insulator, although not more effective than dentin itself. Although the presence of moisture does not have a significant effect on the thermal conductivity of the cement, the moisture present under clinical conditions greatly reduces the potentially good electrical insulating properties of the material.

APPLICATIONS

Zinc phosphate cement is used most commonly for luting permanent metal restorations and as a base. Other applications include cementation of

orthodontic bands and the use of the cement as a provisional restoration.

Cementation of Orthodontic Bands

Zinc phosphate cement has been used for the cementation of orthodontic bands to the teeth for many years. The orthodontic consistency lies somewhere between the luting and base consistencies. Apparently the most important requirement is working time. Longer working times have been achieved by using the frozen slab method.

When certain types of zinc oxide are mixed with eugenol, the mix sets to a hard cement that is compatible with both the hard and soft tissues of the mouth. Cements of this type have been used extensively since the 1890s. Simple mixtures of these two materials do not have great strength when compared with zinc phosphate cements, and their use has been limited to situations in which strength is not important. Quite early it was found that they had a sedative effect on exposed dentin. For many years, ZOE cements have been used as provisional restorations, soft tissue packs in oral surgery and periodontics, and root canal sealers. Because eugenol acts as an inhibitor for free-radical polymerized materials, select other materials for provisional restorations when bonding of the permanent restoration is anticipated.

Non-eugenol-zinc oxide cements are also available for temporary cementation. These cements are suitable for patients sensitive to eugenol.

COMPOSITION

Atypical formula for a ZOE cement compounded as a provisional filling material is shown in Table 20-7. The powder is mainly zinc oxide, with added white rosin to reduce the brittleness of the set cement, zinc stearate as a plasticizer, and zinc

acetate to improve the strength of the cement. The liquid is eugenol with olive oil as a plasticizer. Two compositional changes have been used to increase the strength of the cement for luting purposes. In one, methyl methacrylate polymer is added to the powder, and in the other, alumina (A1,03) is added to the powder and ethoxybenzoic acid (EBA) to the liquid.

A typical polymer-reinforced cement has 80% zinc oxide and 20% poly(methy1 methacrylate) in the powder and eugenol in the liquid. These cements are sufficiently strong for final cementation of fixed prostheses and are used also as cement bases and provisional restora-

Adapted from Wallace DA, Hansen HL:J A m DentAssoc 26: 1536, 1933.

Eugenol

tions. A typical EBA-alumina-reinforced ZOE cement contains 70% zinc oxide and 30% alumina by weight in the powder. In some cases, rosin and copolymers may be added to reduce the brittleness and film thickness and improve the mixing qualities. The liquid of the EBA-alumina- reinforced cements contains 62.5% ortho-EBA by weight and 37.5% eugenol by weight. The compressive strength of a typical EBA cement is shown in Table 20-3.

The non-eugenol-zinc oxide cements typically contain an aromatic oil and zinc oxide. Other ingredients may include olive oil, petroleum jelly, oleic acid, and beeswax.

SETTING REACTION

The setting of ZOE cements is a chelation reaction in which an amorphous, zinc eugenolate is formed. The setting reaction is shown below, where two molecules of eugenol react with ZnO in the presence of water to form the chelate, zinc eugenolate. Excess zinc oxide is always used, so the set material consists of a matrix of amorphous zinc eugenolate that binds the unreacted zinc oxide particles together. The setting reaction is accelerated by increases in temperature or humidity. EBA also forms a chelate with zinc oxide, and its presence allows some crystalline zinc eugenolate to form, which provides additional strength. The reaction is not measurably exothermic, and the presence of moisture is essential for setting to occur.

Zinc eugenolate

MANIPULATION

Dispensing The unmodified ZOE and non-eugenol-zinc oxide cements are typically two-paste systems. Equal lengths of each paste are dispensed and mixed to a uniform color.

For some cements used for temporary cementation or for provisional restorations, powder is often incorporated into a dispensed amount of liquid until a suitable consistency is achieved for the operation at hand. The dentist makes this determination from experience. A considerable amount of powder can be incorporated into the liquid by heavy spatulation with a stiff spatula. In general, the more powder incorporated, the stronger the cement and the more viscous the mixed cement.

Cements intended for final cementation of restorations carry manufacturers' directions and measuring devices that are important to use. Because of the deceptive flow qualities of these cements, adding powder until the operator feels the mix is of suitable consistency for cementing a restoration will lead to a cement deficient in powder and a lowered strength in the set cement. To those accustomed to the handling qualities of zinc phosphate cement as a luting medium, a correctly proportioned and mixed ZOE luting cement appears far too viscous.

Mixing Procedures Because the setting reaction is not significantly exothermic, a cooled mixing slab is not required. Usually a paper mixing pad with disposable sheets is used, which facilitates the cleanup procedures after cementation. A glass slab is recommended for mixing the EBA-alumina-modified cements. There is no need to incorporate the powder in small increments; usually the bulk of the powder is incorporated in the initial step, the mix is thoroughly spatulated, and a series of smaller amounts is then added until the mix is complete. The mix is thoroughly kneaded with the spatula (a stiffbladed steel spatula is the most effective type). The EBA-alumina-modified cement is dispensed according to the instructions, kneaded for 30 seconds, and then stropped for 60 seconds to develop a creamy consistency. Oil of orange can be used to clean eugenol cements from instruments.

CHARACTERISTIC PROPERTIES

The variety of compositions of the ZOE cements and the many uses to which they are applied make it difficult to write a specification for these cements. ANSVADA Specification No. 30 (IS0 3107) for dental zinc oxide-eugenol cements and zinc oxide non-eugenol cements gives standards for temporary cements, permanent cements, filling materials and bases, and cavity liners. It sets requirements for the general characteristicsof the powders, liquids, and pastes used in these cements and for the important physical properties of setting time, compressive strength, disintegration, film thickness, and acid-soluble arsenic content, where these are applicable. The limiting values established for these properties for each type of cement are shown in Table 20-8.

Film Thickness Film thickness is an important factor in the complete seating of restorations at the time of cementation. The film thickness should be not more than 25 pm for cements used for permanent cementation and not more than 40 pm for cements used for temporary cementation, as determined by the specification test. This requirement is not applied to cements used for purposes other than cementation.

Setting Time For all except two of the cements, a range of setting times from 4 to 10 minutes is required. For cements intended for filling materials and bases, the preference of some operators for a faster setting cement is recognized in the specification by extending the lower end of the range to 2 minutes.

Compressive Strength A maximum value of 35 MPa is required for cements intended for temporary cementation. A minimum of 35 MPa is required for cements intended for permanent cementation and 25 MPa for filling materials and bases. Lining materials are required to have a minimum compressive strength of 5 MPa. Clinical studies have shown that for temporary cementing of restorations a variety of cements with compressive strengths varying from 1.4 to 21 MPa is desirable. The strength of a cement for temporary cementation is selected in

Cement

Type I. Temporary Cement

Class 1. Powder-liquid

Class 2a. Paste-paste (eugenol)

Class 2b. Paste-paste (non-eugenol)

Class 3. Paste-paste (nonsetting)

v p e II. Permanent cement Class 1. Powder-liquid

Type HI. Filling materials and bases Class 1. Powder-liquid

Class 2. Paste-paste

Type IV Cavity liners

Class 1. Powder-liquid

Class 2. Paste-paste

608 Chapter 20 CEMENTS

relation to the retentive characteristics of the restoration and the expected problems of removing the restoration when the time arrives.

For permanent cementation the strongest cement possible is preferable. Most ZOE cements for permanent cementation have compressive strength values considerably higher than the 35 MPa required by the specification. The strength of the non-eugenol-zinc oxide cements is similar to that of the unmodified ZOE cements intended for temporary cementation. A comparison of the compressive strengths of several types of cements is given in Table 20-3.

Disintegration The disintegration of cement is generally regarded as less critical for cements used as provisional restorations or for temporary cementation. This is reflected in the maximum specification values for disintegration in 24 hours. A maximum value of 2.5% is acceptable for provisional cementing materials, but a value of 1.5% is required for the other cements. The test used is the amount of disintegration, measured by weight loss, which occurs in a disk of the cement immersed in distilled water for 24 hours. There is no close correlation between this test and the clinical behavior of these cements, and the test serves only to compare cements of known clinical acceptability with other products. It is a useful monitoring method for evaluating new products and ensuring quality control of manufacturing processes.

APPLICATIONS

A range of ZOE and modified ZOE cements are suitable for many uses in restorative dentistry, and the practitioner should become familiar with each type and its application.

Base Materials having a compressive strength of 5.5 to 39 MPa are used as a cement base, and the strength reaches a maximum in about 12 to 15 minutes. They are normally used under zinc phosphate cement, which acquires about three times the strength in the same time. The ZOE cements have the advantage that the

thermal insulating properties of the cements are excellent and are approximately the same as those for human dentin. Bases are discussed later in this chapter.

Temporary Cementation Unmodified ZOE cements are also used as luting materials for provisional crowns and temporary cementation of metal restorations in crown and bridge prosthodontics. Laboratory studies have shown that the retention of metal restorations with unmodified cements is proportional to the compressive strength of the cements. Unmodified ZOE cements are available in compressive strengths varying from 1.4 MPa to 21 MPa. A clinical study of the use of various unmodified cements for luting provisional crowns indicated that a cement with a compressive strength of 15 to 24 MPa was the most appropriate cement, based on: ( 1 ) retention, ( 2 ) taste, ( 3 ) ease of removal, and ( 4 ) ease of cleaning. Another clinical study indicated that an unmodified ZOE cement with a compressive strength of 6.9 MPa was the most commonly used material for the temporary cementation of complete crown and bridge restorations, although a range of cements with compressive strengths from 1.4 to 21 MPa was found to be desirable.

The non-eugenol-zinc oxide cements d o not adhere as well to preformed metal crowns as the eugenol-containing cements, and they are slower setting. The non-eugenol cements, however, do not soften provisional acrylic crowns.

Provisional Restorations The EBA- alumina-modified cements have been tried as provisional restorations based on their physical properties. Clinical studies showed these cements were handled easily and had improved carvability, which prevented chipping during trimming, and that symptomatic teeth without pulp exposure showed no symptoms. The EBA- alumina-modified cements, despite their low solubility in water, disintegrated and wore excessively in the mouth. A thick mix, 2.6 g/0.4 ml of polymer-modified ZOE, was more serviceable than the EBA-alumina-modified type, and al-

though some chipping was observed at the margins, all provisional restorations of ZOE were serviceable for 2 to 10 months of observation.

Permanent Cementation The use of EBA-alumina-modified ZOE cements has been clinically successful for the permanent cementation of crowns and bridges. The film thickness of these cements is therefore important, and, as indicated in Table 20-4, a film thickness of 25 to 35 pm is readily obtained. Other properties are shown in Tables 20-3 and 20-4.

Endodontic Sealers Endondontic ZOE preparations have been used as a root canal sealer alone and with gutta-percha. There are two major groups of products based on ZOE cements-conventional and therapeutic sealers.

Composition and Setting Conventional sealers are generally based on the formulas of Grossman or Rickert, as summarized in Table 20-9. The setting reaction occurs between zinc oxide and eugenol. Resins improve the mixing characteristics and retard setting. Radiopacity is improved by adding barium or bismuth salts or silver powder. The conventional sealers are used with gutta-percha points.

Therapeutic sealers are usually used without a core material and are formulated with ingredients

such as iodoform, paraformaldehyde, or trioxymethylene, which may have therapeutic value. The use of these sealers is controversial.

ANSI/ADA Specification No. 57 (IS0 6876) covers materials used in endodontics within the tooth to seal the root canal space. The physical properties specified include working time, flow, film thickness, setting time, dimensional change following setting, solubility, and radiopacity. A summary of these requirements is given in Table 20-10.

Viscosity The ability of a sealer to penetrate into irregularities and accessory canals has been termed ,flow, though viscosity is a more correct term. Viscosity has been shown by several different tests to decrease during the mixing procedure (an example of shear thinning). Values of viscosity range from 8 to 680,000 cp (centipoise).

Setting Time The setting time of cements is measured by a penetration test and ranges from 15 minutes to 12 hours at mouth temperature. Sealers set much more rapidly at mouth temperature than at room temperature.

Film Thickness Film thickness, measured as directed by ANSVADA Specification No. 57, is influenced by viscosity, setting time, and the size

of filler particles in the cement. Lower values of film thickness are considered desirable for condensation of gutta-percha. Values range from 80 to 500 pm, depending on the testing load.

Compressive Strength The strength of a sealer is an indication of its ability to support tooth structure weakened by the cleaning of the canal and its durability. Values range from 8 to 50 MPa.

Solubility Solubility is an undesirable characteristic in a root canal sealer, because the process of dissolution can cause the sealer to release components that may be biologically incompatible. Solubility has been measured in water, and typically values range from 0.1% to 3.5%.

Radiopacity Radiopacity is desirable, and a minimum value has been established at the equivalent of 3 mm of aluminum. Values of radiolucency range from 0.10 to 0.98 among various sealers and 0.78 for gutta percha. These are relative values with lower numbers for more radiopaque materials.

Dimensional Change Most root canal sealers shrink as a result of setting. This shrinkage affects the integrity of the bond between the sealer and the tooth or core material. Values of volume loss after 90 days in a capillary tube range from -0.7% to -5.0%.

Biological Properties These properties have been studied by in vivo and in vitro tests of HeLa cells, human skin fibroblasts, and bovine pulp tissue; endodontic fillings in dogs, monkeys, and rats; and implants in tibias and in subcutaneous connective tissues of animals. Conventional ZOE sealers generally elicit mild to moderate reactions, whereas several of the therapeutic sealers elicit severe reactions.

Tissue Management Another variation of the ZOE cements has been necessitated by the special requirements imposed in the management of gingival tissues. This group of cements is used in two ways: (1) to mechanically displace

soft tissue, and (2) to dress soft tissues after surgery. When these cements are used as a mechanical tissue pack, a thin consistency is incorporated into cotton fibers that are placed into the gingival sulcus. As a surgical dressing, this preparation affords greater comfort to the patient during eating, obtunds the surgerized tissue, promotes epithelial growth, and helps prevent the overgrowth of granulation tissue.

The setting times of these ZOE surgical cements must be quite long to facilitate the mixing of rather large quantities and to permit the proper placement and contouring of the dressing. Usually no accelerator is added to these materials. On placement of the packs in the mouth, the moisture and increased temperature tend to hasten the setting reaction. When mixed to a proper consistency, the cement must be soft enough to permit placement and contouring with gentle pressures, and yet firm enough to maintain the desired form.

These formulations generally have greater quantities of mineral, peanut, or almond oil present to increase plasticity over that of filling cements. Cotton fibers are often added to increase the strength and durability. In addition to the normal ingredients (zinc oxide, rosin, and eugenol), tannic acid is often added as a hemostatic agent and to decelerate the setting reaction. Aromatic oils and coloring agents may be incorporated to improve the taste and color of the dressing. Chlorhexidine has also been added as an antibacterial agent.

COMPOSITION

Zinc polyacrylate cements (or zinc polycarboxylate) are supplied as a powder and a liquid or as a powder that is mixed with water. The liquid is a water solution of polyacrylic acid, the formula being:

-CH2-CH-CH2-CH-

I I