The phosphate materials have adequate thermal insulating properties when used under metallic restorations. The value of thermal conductivity is approximately 1.17 Wm−1 ºC−1 compared with 0.63 Wm−1 ºC−1 for dentine and 23 Wm−1 ºC−1 for dental amalgam. The value of thermal diffusivity does not differ markedly from that of tooth substance. They are not able to form an effective chemical barrier, however, due to their inherent acidity.

The set material is opaque due to a high concentration of unreacted zinc oxide. This may detract from the aesthetic appeal of a porcelain crown having a zinc phosphate lute, particularly if the cement lute margin is visible. Consequently crown margins should be placed within the gingival crevice if they are going to be visible in order to hide any exposed cement lute.

Zinc phosphate cements are widely used for all types of lute applications, and as cavity linings under amalgam fillings.

28.3 Silicophosphate cements

Silicophosphate cements are, essentially, hybrids of zinc phosphate and silicate materials. They are supplied as a powder and liquid. The liquid is an aqueous solution of phosphoric acid, containing buffers, whilst the powder is, essentially, a mixture of zinc oxide and aluminosilicate glass. The setting reaction produces a matrix of zinc and aluminium phosphates enclosing unreacted cores of zinc oxide and glass particles.

As seen in Table 28.2, the silicophosphate cements are stronger and less soluble than the phosphate cements. Their properties are more akin to those of the silicates (Chapter 20). The aluminosilicate glass particles also contain a significant amount of fluoride. The leaching of fluo-

ride ions imparts a significant anticariogenic influence on the surrounding tooth substance.

These materials are used, primarily, as temporary filling materials.

28.4 Copper cements

These materials are closely related to the zinc phosphate cements. They are supplied as a powder and liquid. The liquid is an aqueous solution of phosphoric acid whilst the powder is a mixture of zinc oxide and black copper oxide. The setting reaction is similar to that for zinc phosphate materials.

The two properties which distinguish these products from simple zinc phosphate materials are their black appearance and their bactericidal effects, produced by the presence of copper.

The materials are not widely used nowadays, although they can be used as filling materials in deciduous teeth where it has not been possible to remove all caries. Their durability is not good but they are capable of preserving such teeth until they exfoliate. Another application is the cementation of orthodontic appliances, although they have been largely superseded by other materials for the latter application. Finally they can be used for the cementation of cast silver cap splints that are used occasionally during the management of facial fractures.

28.5 Suggested further reading

Going, R.E. & Mitchem, J.C. (1975) Cements for permanent luting: a summarizing review. J. Am. Dent. Assoc. 91, 107.

Øilo, G. (1991) Luting cements: a review and comparison. Int. Dent. J. 41, 81.

Smith, D.C. (1983) Dental cements. Current status and future prospects. Dent. Clin. North Am. 27, 763.

29.1 Introduction

Many cements used in dentistry can be characterised by setting reactions which involve the formation of chelate compounds between divalent metallic ions such as zinc ions and ortho-disubsti- tuted aromatic compounds. Three types of aromatic compound are commonly used, delineating the three groups of materials.

(1)Zinc oxide/eugenol cements.

(2)Ortho-ethoxybenzoic acid (EBA) cements.

(3)Calcium hydroxide cements, in which the aromatic ligands are silicylates.

29.2 Zinc oxide/eugenol cements

Composition and setting: These products may be supplied as a powder and liquid or as two pastes (Figs. 29.1 and 29.2). The composition of a typical powder/liquid material is given in Table 29.1. The small quantity of zinc acetate in the powder acts as an accelerator by helping to create an ionic medium in which the setting reaction can occur. Some commercial products contain hydrogenated rosin or polystyrene and are known as resin-rein- forced zinc oxide/eugenol cements. The relative proportions of powder and liquid are not normally measured accurately, although some manufacturers provide a scoop which gives a known volume of powder to which a given number of drops of liquid are added. Thin mixes, having a low powder/liquid ratio, should be avoided since they produce inferior properties – lower strength and higher solubility. The paste/paste materials are similar to the impression pastes described on p. 151. These have the advantage of easier proportioning and mixing.

The setting reaction involves chelation of two eugenol molecules (see Fig. 17.5) with one zinc

ion to form zinc eugenolate (see Fig. 17.6). This reaction proceeds very slowly in the absence of moisture. When the mixed material contacts water, however, setting is often completed within a few seconds.

Properties: The setting characteristics of the zinc oxide/eugenol cements are, to some extent, ideal. They offer a combination of adequate working time, during which very little increase in viscosity occurs, coupled with rapid setting after placing into the cavity. The latter is caused by residual moisture in the cavity and the higher temperature of the mouth compared with room temperature. The effect of cavity moisture is noteworthy, particularly since efforts are generally made to dry the cavity before placement of a lining. Only very small amounts of water are required to cause the accelerating effect.

The ultimate compressive strength values for the zinc oxide/eugenol cements are somewhat lower than those recorded for zinc phosphate materials, typically 20 MPa for unreinforced materials and 40 MPa for reinforced materials, The nature of the setting reaction is such, however, that the materials develop their strength rapidly and the reinforced materials in particular are unlikely to flow or fracture during amalgam condensation, providing correct technique is used.

Zinc oxide/eugenol cements may be used as linings in deep cavities without causing harm to the pulp. Unconsumed eugenol is able to leach from the set material and although this substance has been shown to be irritant under certain conditions, it appears to have an obtundant effect on the pulp. The free eugenol is also bacteriocidal which helps to minimize the effects of bacterial ingress and the production of exotoxins causing pulpal damage as a consequence of microleakage.

Fig. 29.2 A zinc oxide eugenol cement provided in the form of two pastes. The active ingredients are very similar to those in the powder/liquid system shown in Fig. 29.1. Two pastes are mixed together on a mixing pad.

The ease with which eugenol can gain egress from the material is responsible for its relatively high solubility. Leached eugenol is replaced by water which, under certain conditions, can cause hydrolysis of the zinc eugenolate and disintegration of the cement structure. The materials are,

Fig. 29.3 A zinc oxide eugenol paste used in a temporary cement.

therefore, not suitable for luting applications except on a temporary basis.

Free eugenol may also have an effect on resinbased filling materials, interfering with the polymerisation process and sometimes causing discoloration. Clinically this can also result in softening of the surface of the composite. In addition there can be problems with resin-based adhesive luting systems that are being used increasingly, if a provisional restoration is luted using a zinc oxide eugenol temporary cement.

The materials form an effective thermal barrier under metallic restorations having a value of thermal diffusivity similar to that for dentine.

The main uses of these cements are for linings under amalgam restorations, either used alone or as a sublining overlaid with a zinc phosphate material. They are also used as temporary luting cements and as temporary filling materials. (Fig. 29.3)

Root-canal pastes: The uses of these materials in endodontics are covered in Chapter 31.

}

}

Primary reactive ingredient Reinforcing agents

Reactive ingredients

Fig. 29.4 Structural formula of ortho-ethoxybenzoic acid.

29.3 Ortho-ethoxybenzoic acid (EBA) cements

for zinc oxide/eugenol products. Thermal characteristics are similar to those of the zinc oxide/ eugenol materials.

The materials form a suitable cavity lining for amalgam condensation due to their adequate strength and resistance to flow, although they are primarily used as luting cements.

29.4 Calcium hydroxide cements

Composition: These cements are generally supplied as a powder and liquid. The composition of a typical material is given in Table 29.2. The ratio of o-ethoxybenzoic acid to eugenol in the liquid may vary from one product to another but is usually about 2 : 1. The structural formula of o- ethoxybenzoic acid is given in Fig. 29.4. Comparison with Fig. 17.6 shows the similarity between the structure of o-ethoxybenzoic acid and eugenol. Both compounds are able to form chelate compounds with zinc ions.

The structural formula for zinc eugenolate is shown in Fig. 17.7. The structure of zinc o-eth- oxybenzoate is very similar.

Properties and applications: The setting characteristics are similar to those of the zinc oxide/eugenol materials and are similarly affected by moisture. It is possible to achieve a higher powder/liquid ratio with these products since much of the powder consists of inert, reinforcing filler.

The set material is significantly stronger than even the reinforced zinc oxide/eugenol products due to the high powder/liquid ratio and the presence of fillers. A typical commercial product has an ultimate compressive strength of around 85 MPa. In addition, the lower level of residual eugenol, coupled with a greater resistance to hydrolysis of the zinc o-ethoxybenzoate, produces a cement with lower solubility than that observed

Composition: Some calcium hydroxide preparations consist simply of a suspension of calcium hydroxide in water. This is applied to the base of the cavity and dries out to give a layer of calcium hydroxide. These materials are both difficult to manipulate and form a very friable cavity lining which is easily fractured. A solution of methyl cellulose in water or of a synthetic polymer in a volatile organic solvent can be used instead of water. These additives produce a more cohesive cement but the compressive strength remains very low at about 8 MPa. This is well below the value of strength required to withstand amalgam condensation and when this filling material is to be used the calcium hydroxide preparation must be overlaid with a layer of a stronger cement. Most calcium hydroxide products in current use are supplied in the form of two components, normally pastes, which set following mixing to form a more substantial cavity lining (Fig. 29.5). The composition of a typical commercial product is given in Table 29.3.

The structural formula of butylene glycol disalicylate, a glycol salicylate commonly used in one of the pastes, is given in Fig. 29.6. This is a difunctional chelating agent having two aromatic groups with reactive groups in ortho positions. On mixing this with a paste containing zinc oxide and calcium hydroxide, chelate compounds with structures similar to that shown in Fig. 17.7 are formed. It

is thought that zinc ions are primarily responsible for chelation, with calcium being largely unreacted.

The sulphonamide compound used in the zinc oxide/calcium hydroxide paste is present merely as a carrier and is not thought to have any therapeutic effect.

Some cements contain paraffinic oils instead of sulphonamides. These cements are more hydrophobic and release their calcium hydroxide more slowly. This may have an adverse effect on the antibacterial properties of the cement.

Light-activated calcium hydroxide cements are available. They carry brand names which suggest that they are chemically similar to the two-paste products. They are, however, based on a totally different setting reaction involving light-activated polymerisation of a modified methacrylate mo-

nomer of the type used in resin based filling materials (see Chapter 22). A typical material contains dimethacrylate (e.g. Bis GMA) hydroxyethylmethacrylate (HEMA), polymerisation activators and calcium hydroxide. The purpose of the HEMA is to produce a relatively hydrophilic polymer which can absorb water and release calcium hydroxide to create an alkaline environment.

Properties: The mixed materials have very low viscosity and setting can be relatively slow for some products. Moisture has a dramatic effect on the rate of setting however, and the materials set within a few seconds of being placed in the cavity, even when the cavity has been ‘dried’. Setting of the light-activated materials is more under the control of the operator and residual moisture in the cavity does not have the same influence on setting time. An exposure to activating light for only a few seconds is required to activate polymerisation of the thin layer of cement. One characteristic of these cements which has been largely ignored is the relatively high temperature rise produced on setting. This results from the heating effect of the light source and the exothermic setting reaction.

The set material is relatively weak compared to other cements, having a compressive strength of

Primary reactive ingredients

Accelerator

Oil compound, acts as carrier Primary reactive ingredient

Inert fillers, pigments and radiopacifiers

about 20 MPa. This strength is gained rapidly, however, and under the constrained conditions of a cavity the material is able to resist flow and fracture during amalgam condensation, providing correct technique is used. The light-activated materials in which the set material has a crosslinked resin matrix are less brittle than the two paste products.

The consistency of the materials makes them difficult to apply to cavities in thick section. In deep cavities, therefore, a commonly used technique is to apply a thin sublining of a calcium hydroxide cement and then to build up the base of the cavity with a zinc phosphate material prior to amalgam condensation, as illustrated in Fig. 29.7.

The set materials have a relatively high solubility in aqueous media. Calcium hydroxide is readily leached out, generating an alkaline environment in the area surrounding the cement. This is thought to be responsible for the proven antibacterial properties of these materials. This characteristic is utilized in very deep carious lesions, sometimes involving exposure of the pulp, or occasionally in cases of traumatic exposure of the pulp during cavity preparation. The calcium hydroxide cement is used as a pulp capping agent in such situations. It is sufficiently biocompatible to be placed adjacent to the pulp and capable of destroying any remaining bacteria. The material is also able to initiate calcification and formation of a secondary dentine layer at the base of the cavity. This calcification process is a product of irritation of the pulp tissues by the cement, possibly mediated by the activation of TGFβ, a cellular growth factor. Calcium from the cement does not become bound into the mineralized tissues of the calcific barrier/ secondary dentine. In pulp capping procedures the calcium hydroxide material is generally overlaid with a strong cement base material such as zinc phosphate cement before completing the restoration of the tooth with amalgam.

The resin-based calcium hydroxide materials are far less soluble than the conventional products. This is advantageous providing that the rate of calcium hydroxide release remains great enough to maintain the antibacterial and dentine regeneration properties of the material. One problem with the resin-based materials is that the unreacted methacrylate groups can become attached to a freshly placed composite resin restoration. The composite shrinks during its setting reaction

Fig. 29.7 Diagram showing the use of a calcium hydroxide cement as a sublining beneath a zinc phosphate cement.

and this can pull the calcium hydroxide material away from the tooth, leaving a void.

Calcium hydroxide cements are routinely used as lining materials beneath silicate and resin-based filling materials. Unlike the eugenol-containing cements they have no adverse effect on these filling materials and form an effective chemical barrier against acids and monomers.

The need for a lining material beneath a composite is controversial. As stated previously, some authorities would suggest that with modern dentine adhesives there is no need for a lining as the cavity margins are sealed by the adhesive, making microleakage unlikely. Obviously any lining that is placed will act as a barrier between the bonding agent and the dentine, reducing the potential area available for bonding. Indeed, it has been suggested that even exposed pulps can simply be etched and then coated with a resin adhesive to give a primary seal.

The converse of this argument is that pulp perfusion studies (a technique whereby fluid is pumped into the pulp chamber at physiological pressures during in vitro microleakage studies to mimic the effects of dentinal tubule fluid flow) have failed to demonstrate complete sealing of the dentine tubules, even under ideal laboratory conditions (i.e. despite bonding composite to a cut dentine surface there is continued egress of fluid from the pulp measured by monitoring fluid flow into the pulp chamber). This suggests that some of the tubules remain patent and hence the potential for microleakage remains.