At the time of writing the consensus view is

•That exposed pulps should be capped with a proprietary calcium hydroxide material before attempting to bond composite to the adjacent dentine.

•That where the dentine at the base of the cavity is superficial (judged to be some distance from the pulp) then a lining is not required beneath an adhesive restoration.

•That where a cavity is very deep and the dentine at the base of the cavity is judged to be ‘close to the pulp’, then it may be prudent to place a lining over that area alone.

This final point gives problems clinically as it can only be based on a dentist’s subjective assessment of the depth of the cavity.

Calcium hydroxide preparations, similar to those used for cavity lining and pulp capping but containing retarders, are now available as rootcanal sealing pastes. The retarders are required to extend the working times of the materials whilst they are being manipulated in the warm, humid environment of the root canal. The advantage of the calcium hydroxide products over some of the alternative pastes is that they have effective antibacterial properties without being irritant to apical tissues.

Non-setting calcium hydroxide pastes are also widely used during root-canal therapy either as a dressing for the canal between visits or as a temporary root filling when managing problems with teeth in very young patients. Teeth erupt into the mouth before the end of the root is fully formed. If such a tooth with an ‘open apex’ requires a root filling it is very difficult to achieve an adequate apical seal due to the canal morphology. However, if non-setting calcium hydroxide is packed into the canal then often apical development will continue providing there is no infection. The mechanism for this is probably similar to that outlined above for dentine bridge formation. Once this process is complete (apexification) then a conventional condensed gutta percha root filling can be placed.

The high solubility and low strength of the calcium hydroxide cements render them unsuitable for luting purposes.

29.5 Suggested further reading

Fisher, F.J. (1977) The effect of three proprietary lining materials on micro-organisms in carious dentine. Br. Dent. J. 143, 231.

Milosevic, A. (1991) Calcium hydroxide in restorative dentistry. J. Dent. 19, 3.

30.1 Introduction

Before reading this chapter, the attention of the reader is drawn to Chapters 24 and 25. Here, the glass ionomers and resin-modified glass ionomers used for restorative purposes are discussed in some detail. Much of the information given in these previous chapters is relevant to our discussion of luting and lining cements.

Two types of cement based on polyacids are in common use for both luting and cavity lining applications. The first products to be developed were the polycarboxylate cements which rely on the reaction between zinc oxide and a polyacid. The second group of products are described as glass ionomer or polyalkenoate cements. The setting reaction takes place between a polyacid molecule and cations released from an ionleachable glass.

A new family of cements has been developed by producing cements which have a nature which lies somewhere between that of the purely salt matrix glass ionomers and that of the purely resin matrix composite systems. These newer cements may be viewed as hybrids of the two parent groups from which they are derived.

30.2 Polycarboxylate cements

These materials may be supplied as a powder and liquid or as a powder which is mixed with water (Fig. 30.1). For powder/liquid materials, the powder is finely ground zinc oxide which sometimes contains minor quantities of other oxides such as magnesium oxide. The liquid is an aqueous solution of polyacrylic acid of about 40% concentration. In the powder/water materials the powder contains zinc oxide and freeze-dried polyacrylic acid. On mixing the powder with water, the polyacrylic acid dissolves and starts to react with zinc

oxide. The setting reaction is similar to that reported in Chapter 24 for glass ionomers. Zinc oxide behaves as a basic oxide and undergoes an acid–base reaction with acid groups in the polyacid to form a reaction product which consists of cores of unreacted zinc oxide bound together by a salt matrix in which polyacrylic acid chains are cross-linked through divalent zinc ions (Fig. 30.2). Many recently developed products contain fluoride salts which may exert an anticariogenic effect on surrounding tooth substance.

The cements have sufficiently early strength to resist amalgam condensation and, allowing for product variations, have an ultimate compressive strength of about 80 MPa, a similar value to that recorded for the EBA materials.

The polycarboxylate materials are acidic, though not as irritant as phosphate cements, for two reasons. Polyacrylic acid is a weaker acid than phosphoric acid and the polyacid chains are too large and lack the mobility required to penetrate dentinal tubules. Despite the more biocompatible nature of these materials they are not widely used as linings in very deep cavities unless a sublining of a calcium hydroxide or zinc oxide/eugenol material is used. One reason for this is that they are difficult to handle well in a clinical setting. They tend to be rubbery during their setting reaction and they also adhere to stainless steel instruments, making their placement complex.

Laboratory tests show that the solubility values of polycarboxylate cements are greater than those for the zinc phosphate, silicophosphate and glass ionomer materials (Section 28.2). Despite this apparent disadvantage, the materials are widely used for luting without appearing to display an unduly high failure rate.

The materials form an adhesive bond with enamel and dentine but only a weak bond with

gold and no perceptible bond with porcelain. Hence, the adhesive nature of the materials when used for the luting of gold or porcelain crowns is only utilized to a limited degree and cannot be considered an overwhelming advantage for such applications. They will, however, bond to nonprecious metal alloys that are being used increasingly for porcelain fused-to-metal crowns. The materials form a strong bond with stainless steel which makes them useful for attaching orthodontic bands. Care must be taken when using steel instruments for mixing and placing. Excess material should be removed from such instruments before it sets, otherwise a tenacious bond will form.

Fig. 30.1 A polycarboxylate cement. The cement contains two main reactive ingredients, zinc oxide and polyacrylic acid and both are in the powder; the bottle is filled with water by the dentist. Powder and water are dispensed onto the mixing pad and mixed with a spatula. In other products the powder contains only the zinc oxide and the liquid is an aqueous solution of polyacid.

As for most other cements, strength and solubility are optimized by achieving a high powder/ liquid ratio. For polycarboxylate materials however, two restricting factors should be remembered. First, some free polyacid is required to form an adhesive bond and this will not be possible if a very dry mix is used. Secondly, a relatively low viscosity is required to allow seating of restorations during luting.

The set materials are opaque due to a high concentration of unreacted zinc oxide cores. This may detract from the appearance of porcelain crowns, particularly if the cement lute margin is visible.

The materials are primarily used as luting cements for attaching crowns, bridges and inlays or as cavity base materials. The adhesive properties of the materials are occasionally utilized for the attachment of orthodontic bands, since the material forms a strong bond with stainless steel as well as with enamel and dentine.

30.3 Glass ionomer cements

The use of glass-ionomer cements as adhesive restorative materials is discussed in Chapter 24. The cavity lining and luting cements are of broadly similar composition to that given in Table 24.1. One difference is that the luting and cavity lining cements contain glass of smaller particle size to allow the formation of a thinner film thickness during luting. Powder/liquid and powder/water materials are both available and widely used (Fig. 30.3).

The set materials are stronger than the polycarboxylate products having a compressive strength value of about 130 MPa, although there may be

Fig. 30.3 A glass ionomer lining and luting cement. The material is provided as a powder and liquid which are mixed together on a mixing pad or on a glass slab as shown. The material is chemically similar to the glass ionomer cements used for restorative purposes. However, the luting cements are formulated to produce a cement with lower viscosity which will flow to form a thin film.

wide variations from one product to another. The materials can withstand amalgam condensation and are occasionally used as cavity linings for amalgam restorations. Their biological properties are akin to those of the polycarboxylate cements which are covered in the previous section. Although they are considered relatively bland they are rarely used as linings in very deep cavities. The glass ionomer cements are now widely advocated as lining materials beneath posterior composite filling materials. They are claimed to give more rigid support than the calcium hydroxide cements. Their success in this situation has been further enhanced by the introduction of radiopaque glass ionomer cements containing either barium salts or metallic silver as the radiopacifying agents. Materials containing silver are grey in colour and may detract from the appearance of the composite restoration, although this is not very noticeable in molar and premolar teeth. Specialized techniques in which glass ionomers are used to form an adhesive ‘sandwich’ between dentine and restorative resins are described in Chapter 24.

The glass ionomer cements are less soluble than the polycarboxylates, and most of the other cement products, when measured under ideal laboratory conditions. The solubility can be adversely affected by early moisture contamination, however, as discussed in Chapter 24 for the

glass ionomer filling materials. It is essential that cement lute margins are covered with a layer of a protective varnish immediately after seating the restoration. This can often be a difficult procedure, particularly if the margins lie subgingivally. Although the glass ionomers are theoretically capable of producing an insoluble cement lute this ideal may not be as easy to achieve in practice. There have been reports of pulpal damage associated with the use of some glass ionomer cement luting agents, particularly those based on poly(acrylic/maleic)acidasopposedtopoly(acrylic) acid alone. It is thought that the acrylic/maleic copolymers maintain a low pH for a relatively long time after mixing. Associated with the solubility of the cements is a slow and maintained release of fluoride which may help to protect adjacent tooth substance from attack by the acids of plaque.

The materials display the same adhesive properties as the polycarboxylates and are more translucent due to the presence of unreacted cores of glass rather than zinc oxide. The extra translucency is considered an advantage for luting porcelain crowns, although further improvements in appearance are required if the cement is to be truly able to match porcelain.

Glass ionomer cements are now widely used in orthodontics for the attachment of bands. The adhesive seal between the cement and tooth enamel, coupled with fluoride release, helps to maintain the banded teeth in good condition throughout a course of treatment. Several trials have been performed to evaluate the use of glass ionomers for bonding orthodontic brackets. It is felt that they could theoretically provide the ideal characteristics of a cement for this purpose, i.e. adequate bonding to ensure that debonding does not occur during treatment, coupled with easy debonding and rapid clean up of the teeth after treatment, combined with the benefits of fluoride release. In practice, however, the cements have not proved suitable for bonding brackets due to the unacceptable rate of debonding during treatment.

30.4 Resin-modified glass ionomers and compomers

In Chapter 25 the potential benefits of incorporating a significant resin component in a glass ionomer are explained. A new family of restorative

Fig. 30.4 A resin modified glass ionomer used for luting and lining purposes. The material consists of a powder and liquid which are mixed together on the pad shown. Setting takes place by a combination of both an acid-base reaction and chemically activated polymerisation.

Fig. 30.6 A compomer-type luting cement. The material is provided in the form of a powder and liquid which are mixed together on a mixing pad. The initial setting reaction involves polymerisation and for this type of cement the reaction is strongly inhibited by air; therefore the manufacturer provides an air-block gel which can be seen in the syringe at the front of the photograph. The air-block gel is applied over the exposed surfaces of the cement to ensure that the cement can polymerise correctly.

Fig. 30.5 A resin modified glass ionomer designed specifically for use in orthodontics. Chemically, the material is very similar to the luting cement shown in Fig. 30.4 but in this product the resin polymerisation is light activated and the cement has characteristics developed specifically to aid bonding in orthodontics where there is a need to position a bracket on the tooth and for the bracket to remain in place without sliding during setting of the cement.

materials has grown out of these findings. For each restorative material the manufacturers invariably produce a similar material for lining and/or luting purposes. Just as for those products described previously in Chapter 25, the materials available have a composition which lies on a continuum between the ‘true’ glass ionomers and the

resin–matrix composites. Hence, some are adequately described as resin-modified glass ionomers whilst the term acid-modified composite better describes other products.

Whereas most restorative cements have a lightactivated polymerisation setting reaction as an important part of the setting procedure, the lining and luting cements may be chemically activated. In this case the material is likely to be provided as a powder and liquid containing the necessary monomers and polymerisation initiators/ activators (Fig. 30.4). Setting begins when the components are mixed. The main advantages of resin-containing materials compared with conventional acid–base cements is that they are less soluble and less brittle. Many of the other potential benefits are outlined in Chapter 25.

Products based on a hybrid of GIC and resin chemistry have been produced for a variety of luting applications. Some, such as the lightactivated resin modified glass ionomer shown in figure 30.5, are designed specifically for use in a specialised luting application such as orthodontics bonding. Others, such as the chemically activated compomer system shown in figure 30.6, are advocated for general luting applications.