Сraig. Dental Materials

Ohno H, Kanzawa I, Kawashima I et al: Structure of high-temperature oxidation zones of gold alloys for metal-porcelain bonding containing small amounts of In and Sn,

J Dent Res 62:774, 1983.

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58:1987, 1979.

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Shell JS, Nielsen JP: Study of the bond between gold alloys and porcelain, J Dent Res

41:1424, 1962.

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for use with porcelain, J Dent Res

49:283, 1970.

Valega TM, editor: Alternatives to gold alloys in dentistry, proceedings of a conference held at NIH, January 1977, Bethesda, MD,

DHEW Publication No (NIH) 77-1227. Vermilyea SG, Huget EF, Vilca JM: Observations

on gold-palladium-silver and gold-palladium alloys, J Prosthet Dent 44:294, 1980.

purposes: as restorative filling materials, either alone or with other materials, and to retain restorations or appliances in a fixed position within the mouth. In addition, certain cements are used for specialized purposes in the restorative, endodontic, orthodontic, periodontic, and surgical fields of dentistry.

When the properties of dental cements are compared with those of other restorative materials, such as amalgams, resin composites, gold, or ceramics, cements exhibit less favorable strength, solubility, and resistance to oral conditions. As a result, the general use of cements for restorations exposed to the oral environment is quite limited.

Zinc phosphate, glass ionomer, and zinc oxide-eugenol (ZOE) cements can be applied as a base in deep cavities to insulate the pulp from possible chemical and thermal trauma. A metallic, ceramic, or composite filling material may then be placed over the cement base in sufficient bulk and in proper adaptation to the cavity walls to form the final restoration. The sedative effect of ZOE mixtures has made them valuable for a variety of applications. The ability of the glass and hybrid ionomer and compomer cements to release fluoride and to bond chemically to tooth structure has resulted in their uses as bases and for cementation. Resin cements are used for retention of orthodontic brackets, all-ceramic veneers, crowns and inlays, and resin-bonded bridges because of their strength and ability to bond to acid-etched enamel and dentin treated with a dentin bonding agent.

The following is a classification of dental cements, based on their chief chemical ingredients and application:

GLASS AND HYBRID IONOMERS

Class 5 restorations (see Chapter 8)

Retention of alloy restorations

Retention of orthodontic bands

High-strength bases

Provisional restorations

ZlNC POLYACRYLATE

Retention of alloy restorations

Retention of orthodontic bands

High-strength bases

ZlNC PHOSPHATE

Retention of alloy restorations

Retention of orthodontic bands

High-strength bases

Provisional restorations

ZlNC OXIDE-EUGENOL

Lowand high-strength bases Provisional restorations

Temporary and permanent retention of restorations

NON-EUGENOL-ZINC OXIDE

Temporary retention of restorations Root canal sealers

Gingival tissue packs Surgical dressings

CALCIUM HYDROXIDE

Low-strength bases

COMPOMERS

Bonded conventional crowns and bridges Retention of orthodontic brackets High-strength bases

COMPOSITES AND ADHESIVE RESINS

Bonded conventional crowns and bridges Bonded ceramic veneers, inlays, and onlays* Bonded laboratory composites*

Bonded posts and cores* Bonded Maryland bridges*

Retention of provisional restorations

--

"Used with bonding agents

Retention of orthodontic brackets

High-strength bases

Ten types of cements are currently available. Although members of the profession are not in unanimous agreement regarding the purposes of each cement or the necessity for all the types, they are available to the dental profession and have been employed principally in the ways listed.

Zinc phosphate cement is a traditional crown and bridge cement used for alloy restorations. It is supplied as a powder and a liquid, both of which are carefully compounded to react with one another during mixing to develop a mass of cement possessing desirable physical characteristics.

COMPOSITION

Powder The principal ingredient of the zinc phosphate cement powder is zinc oxide. Magnesium oxide, silicon dioxide, bismuth trioxide, and other minor ingredients are used in some products to alter the working characteristics and final properties of the mixed cement. A typical formulation of a zinc phosphate cement powder and liquid is shown in Table 20-1. The magnesium oxide, usually in quantities of about lo%, is added to the zinc oxide to reduce the temperature of the calcination process. The silicon dioxide is an inactive filler in the powder and during manufacture aids in the calcination process. Although the bismuth trioxide is believed to impart a smoothness to the freshly mixed cement mass, in large amounts it may also lengthen the setting time. Tannin fluoride may be added to provide a source of fluoride ions in some products.

The ingredients of the powder are heated together at temperatures ranging from 1000" to 1300" C for 4 to 8 hours or longer, depending on the temperature. Calcination results in a fused or sintered mass. The mass is then ground and

Adapted from Paffenbarger GC, Sweeney WT, Issacs A:

J A m Dent Assoc 20:1960, 1933.

pulverized to a fine powder, which is sieved to recover selected particle sizes. The degree of calcination, fineness of particle size, and composition determine the reactivity of the powder with the liquid.

Liquid Zinc phosphate cement liquids are produced by adding aluminum and sometimes zinc, or their compounds, to a solution of orthophosphoric acid. Although the original acid solution contains about 85% phosphoric acid and is a syrupy fluid, the resulting cement liquid usually contains about one-third water, as shown in Table 20-1. The partial neutralization of the phosphoric acid by the aluminum and zinc tempers the reactivity of the liquid and is described as buffering. This reduced rate of reaction helps establish a smooth, nongranular, workable cement mass during the mixing procedure. The zinc phosphate cement liquid is adjusted by both partial neutralizing or buffering and dilution so it reacts with its powder to produce a cement mass with proper setting time and mechanical qualities.

596 Chapter 20 CEMENTS

SETTING REACTION

When an excess of zinc phosphate cement powder is brought into contact with the liquid to begin the cement mix, wetting occurs and a chemical reaction is initiated. The surface of the alkaline powder is dissolved by the acid liquid, resulting in an exothermic reaction.

The set zinc phosphate cement is essentially a hydrated amorphous network of zinc phosphate that surrounds incompletely dissolved particles of zinc oxide. This amorphous phase is extremely porous. There is no evidence that the magnesium oxide present in the powder reacts with the phosphoric acid. Although no crystalline phosphate is involved in the setting process of the cement, there can be subsequent growth of crystalline hopeite, Zn,(PO,), . 4H,O, in the presence of excess moisture during setting.

MANIPULATION

The manner in which the reaction between the zinc phosphate cement powder and liquid is permitted to occur determines to a large extent the working characteristics and properties of the cement mass. Incorporate the proper amount of powder into the liquid slowly on a cool slab (about 21" C) to attain the desired consistency of cement. Certain requirements must be met to carry out this type of manipulation.

Mixin g Slab A properly cooled, thick glass slab will dissipate the heat of the reaction. Should a rapid reaction occur, ample working time would not be available for proper manipulation of the cement before hardening or setting occurs. The mixing slab temperature should be low enough to effectively cool the cement mass but must not be below the dew point unless the frozen slab technique is used; this method is described subsequently. A temperature of 18" to 24" C is indicated when room humidity permits. The moisture condensation on a slab cooled below the dew point contaminates the mix, diluting the liquid and shortening the setting time. The ability of the mixing slab to be cooled and

yet be free of moisture greatly influences proper control of the reaction rate of the zinc phosphate cement.

PowdedLiquid Ratio The amount of powder that can be incorporated into a given quantity of liquid greatly determines the properties of the mixed mass of cement. Because an increase in the ratio of powder to liquid generally provides more desirable properties, incorporate as much powder as possible to obtain a particular consistency.

Care of the Liquid When zinc phosphate cement liquid is exposed to a humid atmosphere, it will absorb water, whereas exposure to dry air tends to result in a loss of water. The addition of water causes a more rapid reaction with the powder, resulting in a shorter setting time. A loss of water from the liquid results in a lengthened setting time. Therefore keep the bottle tightly closed when not dispensing the material. Polyethylene squeeze bottles do not require removal of a dropper and therefore eliminate the tendency for gain or loss of water from the liquid.

Mixin g Procedure By initially incorporating small portions of powder into the liquid, minimal heat is liberated and easily dissipated. The heat of the reaction is most effectively dissipated when the cement is mixed over a large area of the cooled slab. Use a relatively long, narrow-bladed stainless steel spatula to spread the cement across this large area to control the temperature of the mass and its setting time.

During the neutralization of the liquid by the powder, the temperature of the mixing site is inversely proportional to the time consumed in mixing. Thus if a large volume of powder is carried to the liquid all at once rather than spatulated over a large area of the slab for a sufficient time, the temperature at the site of the reaction becomes higher. This temperature rise speeds the reaction and hinders control over the consistency. In this case the consistency of the mass is achieved by the rapid approach of the initial setting rather than by the establishment of a

higher powder/liquid ratio under more ideal mixing conditions.

During the middle of the mixing period, larger amounts of powder may be incorporated to further saturate the liquid with the newly forming complex zinc phosphates. The quantity of unreacted acid is less at this time because of the prior neutralization gained from initially adding small increments of powder. The amount of heat liberated will likewise be less, and it can be dissipated adequately by the cooled slab.

Finally, smaller increments of powder are again incorporated, so the desired ultimate consistency of the cement is not exceeded. Thus the mixing procedure begins and ends with small increments, first to achieve slow neutralization of the liquid with the attendant control of the reaction and last to gain a critical consistency.

Depending on the product, 60 to 90 seconds of mixing appears adequate to accomplish a proper zinc phosphate cementing mass. When the mixing time is unduly long, the cementing mass may be ultimately weakened by the breaking down of the matrix because it tends to form and bind the undissolved powder particles together.

Frozen Slab Method The mixes used in the normal mixing procedure have adequate working and setting times for the cementation of inlays and crowns. However, in the cementation of orthodontic bands, the short working time of normal mixes allows the cementation of only a few bands with one mix, and the setting times are too long for clinical convenience. The frozen slab method has been developed to overcome these difficulties.

In this method, a glass slab is cooled in a refrigerator at 6"C or a freezer at -10' C.No attempt is made to prevent moisture from condensing on the slab when it is brought to room conditions. A mix of cement is made on the cold slab by adding the powder until the correct consistency is reached. The amount of powder incorporated with the frozen slab method is 50% to 75% more than with the normal procedures. The compressive and tensile strengths of

cements prepared by the frozen slab method are not significantly different from those prepared from normal mixes, however, because incorporation of condensed moisture into the mix in the frozen slab method counteracts the higher powder/liquid ratio. No difference exists in the solubility of frozen slab and normal mixes.

The advantages of the frozen slab method are a substantial increase in the working time ( 4 to 11 minutes) of the mix on the slab and a shorter setting time (20% to 40% less) of the mix after placement into the mouth. This method has also been advocated for cementation of bridges with multiple pins.

CHARACTERISTIC PROPERTIES

Zinc phosphate cements exhibit certain properties during setting and when hardened. The more important of the properties are included in ANSI/ ADA Specification No. 96 ( I S 0 9917) for dental water-based cements. A summary of these requirements is given in Table 20-2.

Selected properties of a typical zinc phosphate cement and of other luting materials are listed in Tables 20-3 and 20-4.

The desired consistency of the zinc phosphate cement mix depends on the particular purpose of the material and the working convenience needed, as expressed by the setting time. Two arbitrary consistencies, termed inlay seating, or luting, and cement base, or filling, are in general use. A third consistency of zinc phosphate cement, which lies midway between the inlay seating and the cement base, is used for the retention of orthodontic bands and has been termed a band-seating consistency.

The inlay-seating consistency of zinc phosphate cement is used to retain alloy restorations. Although the unhardened zinc phosphate cement is somewhat tenacious, the retaining action in its hardened state is one of mechanical interlocking between the surface irregularities of the tooth and the restoration. This interlocking is illustrated in the photomicrograph of the inter-

face of a gold alloy casting and the opposing dentin and enamel wall shown in Fig. 20-1. The gold alloy casting and the dentin are separated slightly and mechanically locked by cement.

The film thickness of the zinc phosphate cement greatly determines the adaptation of the

casting to the tooth. The strength of the retention bond may also be influenced by the film thickness. ANSVADA Specification No. 96 has requirements for cements designed for the seating of precision appliances. The maximum film thickness is 25 ym. The heavier the consistency, the

greater the film thickness (as shown in Fig. 20-2) and the less complete the seating of the restoration. The ultimate film thickness that a wellmixed, nongranular cement attains depends first on the particle size of the powder and second on the concentration of the powder in the liquid, or the consistency of the cement. The film thickness also varies with the amount of force and the manner in which this force is applied to a casting during the cementation. The type of restoration being cemented influences the ease of the cement escaping from around the margins of the restoration. A full-crown casting presents the greatest problem in obtaining maximum displacement of the cement.

Obviously, the consistency of the zinc phosphate cement to be used in the cementation of a casting is critical. An increased amount of powder incorporated into the liquid will increase the consistency of the cement mass. Heavier-than- normal inlay-seating consistencies of cement are more difficult to express from under a casting, and incomplete seating of the inlay or crown may result from their use. The operator must frequently test each mass as the end of the mixing time is approached. The final consistency will be fluid, yet the cement will string up from the slab

Film thickness (ym)

Fig. 20-2 Relation of powdertliquid ratio to film thickness.

on the spatula about 2 to 3 cm as the spatula is lifted away from the mass.

A heavy, puttylike consistency of zinc phosphate cement is used as a thermal and chemical insulating barrier over thin dentin and as a highstrength base. This same consistency may also

serve as a fairly durable provisional filling material. As such, the cement is exposed to dissolution in saliva, abrasion of mastication, and other oral conditions for an extended period of time. The cement base or filling consistency is achieved when using a powder/liquid ratio higher than that used for the inlay-seating or band-seating consistency.

Viscosity The consistency of cements can be quantified by measuring viscosity. The initial viscosity of mixes of zinc phosphate cements of inlay consistency is shown in Fig. 20-3 for mixes made at 18",20•‹,and 25" C. A small but significant increase in viscosity is seen at the higher temperatures. The viscosities of the mixes 2 minutes after completion of mixing are also shown. In all instances, pronounced increases in viscosity occurred during this time interval, with greater increases at the higher temperatures. Values for mixes of zinc polyacrylate cements are also listed. The rapid increase in viscosity demonstrates that restorations should be cemented promptly after completion of the mixing to take advantage of the lower viscosity of the cement. Delays in cementation can result in considerably

larger values of film thickness and insufficient seating of the restoration.

Setting Time Of equal importance to the consistency of the cement is its setting time. A sufficient period of time must be available after the mixing to seat and finally adapt the margins of a casting, to seat and adjust a series of orthodontic bands, or to properly contour a base or provisional restoration. Adequate working time is expressed by proper net setting time, which, as determined by ANSI/ADA Specification No. 96 and based on an inlay-setting consistency, is between 2.5 and 8 minutes at a body temperature of 37" C. The first 60 to 90 seconds are consumed by mixing the powder and liquid, so the net setting time is the time elapsed between the end of mixing and the time of setting, as measured by resistance to a standard indentor. The setting time at mouth temperature of one brand of zinc phosphate cement, as the powder/liquid ratio is increased, is shown in Fig. 20-4.

The setting time for the band-seating or cement-base consistency is only slightly shorter because of the greater quantity of powder used to establish the heavier consistency.