Fig. 27.6 The problem highlighted in Fig. 27.5 is overcome by condensing amalgam into the interproximal box first. This then provides support for the cement lining of the axial wall and the occlusal part of the cavity can be filled without causing fracture.

Radiopacity and compatibility

Cavity linings should, ideally, be radiopaque in order that the dentist can observe them on radiographs, thereby aiding the diagnosis of caries around the filling. A radiolucent lining material may hinder the early detection of such lesions.

Finally, the cavity lining material chosen for any particular application must be compatible with the filling material which is placed above it. The constituents of the lining should not have any effect on the setting characteristics or properties of the filling.

A technique to increase the retention of amalgam restorations using zinc phosphate cement has been described in which a thin lining is placed over the tooth and the amalgam packed immediately, whilst the lining is still fluid. This may result in some ‘bonding’ of the amalgam to the tooth. This approach has been ‘rediscovered’ with the advent of materials that can bond to the amalgam and

Fig. 27.5 When condensing amalgam into a class II cavity, the cement lining on the axial wall is most vulnerable to fracture.

tooth tissue. These materials are either glassionomer cements (Chapter 24) or chemicallyactive methacrylate-based luting agents (Chapter 23). Again, the amalgam is condensed against freshly mixed cement/resin with the deliberate intention of achieving a chemical union between the tooth and the amalgam to assist with the retention and in improving marginal seal.

27.3 Requirements of Iuting materials

Some of the requirements of dental cements are covered in the appropriate ISO standards (e.g. ISO 9917-1: Dental water-based cements or ISO 9917-2: Dental water-based cements, lightactivated cements).

Many dental appliances and restorations are constructed outside the patient’s mouth and then fixed into place using a cement luting material. Examples include the fixing of porcelain and metal crowns, bridges, inlays and metal posts.

Many of the requirements of luting materials are similar to those of cavity lining materials, for example, the material should, ideally, be nonirritant. This requirement is not as critical for luting as for cavity lining since the luting cement is normally applied to a thicker residual layer of dentine than would exist in a deep cavity.

The setting characteristics should allow sufficient time for mixing the material, applying to the restoration and/or tooth preparation and for seating the restoration in place in the mouth. The material should, ideally, be of low initial viscosity or be pseudoplastic, to allow flow of the cement lute so that proper seating can occur. If the viscosity of the cement is high at the time of insertion there is a danger of the restoration being incompletely seated. For crowns, a thick layer of cement may be produced at the margins as illustrated in

Fig. 27.7. An indication of the ability of a cement to flow during seating is sometimes obtained by measurements of film thickness. A given volume of mixed cement is placed on a flat surface and at a predetermined time is compressed under a constant load. The thickness of the film of cement produced gives an indication of the flow during seating of a crown or other restoration.

Luting cements should, ideally, give thermal and electrical insulation since many of the restorations commonly cemented to teeth are based on alloys, for example gold crowns.

The retention of restorations depends on the correct design and accuracy of the restoration and on the strength of the cement. Figure 27.8 illustrates how the cement enters microscopic interstices on the rough tooth and restoration surface. On setting, the cement gives mechanical resistance to the displacement of the restoration, and must be strong enough to resist fracture when loads are applied to the restoration. Retention may be further improved if the luting cement adheres to the tooth surface and restoration.

The solubility of a luting cement should be low because cement margins are often exposed to oral fluids as illustrated in Fig. 27.7. Dissolution or erosion of the cement may lead to failure by loss of retention or by the initiation of caries in the tooth substance adjacent to the eroded lute.

Fig. 27.8 Diagram illustrating how the cement lute gives mechanical retention of the restoration by entering small interstices in the tooth and restoration surface.

27.4 Requirements of endodontic cements

The reader is encouraged to read Chapter 31 for a more complete consideration of these materials.

Certain forms of treatment require removal of the pulp followed by preparation and disinfection of the root canal. The cleaned canal is then filled

using a combination of an inert filler (most commonly gutta percha) and a cement as a sealant. The root canal system is a complex three-dimen- sional structure and the fluidity of both the filler and the sealant needs to be adequate to flow or to be pressed into such a complex structure. Alternative forms of treatment involve the sealing of the canal with cement sealer alone, but this is not a widely used or recommended procedure.

The requirements of the cement sealer are mainly concerned with its biocompatibility and its ability to resist dissolution and to form a good seal, both along the walls and at the apex of the canal. The material should also be radiopaque in order that the dentist can confirm the attainment of a satisfactory root filling radiographically.

Cements used for sealing root canals must have setting characteristics which enable them to be forced into the warm, moist environment of the root canal before setting. This would not be possible with some conventional cements which set almost instantaneously in the presence of moisture. Hence, specialist products have been developed for endodontic applications; these tend to have a very prolonged working time to allow the procedure to be completed (which may take 20– 30 minutes for a multi-rooted tooth) and then for a check radiograph to be taken to ensure that the root filling is satisfactory. If it is not satisfactory then it can be removed immediately if the sealant has not set (see also Chapter 31).

27.5 Requirements of orthodontic cements

Cements are used extensively in orthodontics for attaching bands and brackets. Materials used for this purpose have special requirements which are based on the fact that the brackets and bands must be held securely in place during the course of treatment but must be quite readily removed at the end of treatment without damaging the teeth. Furthermore, after removing the brackets and bands there should ideally be a minimal amount of residual cement attached to the teeth so that only a short time is required for ‘clean up’. It would be an added advantage if the cement were able to exert a protective influence on the surrounding tooth substance during treatment as these areas are prone to plaque accumulations and decalcification of the enamel around brackets can be a problem.

The rheological properties of orthodontic cements are important. The mixed cement should have a viscosity which is low enough to enable positioning of the bracket and the creation of a low film thickness of cement. Likewise, it should be possible to make minor adjustments to the position of the bracket before cement setting occurs. Once positioning is complete there should be no drift in the position of the bracket and this will be most easily achieved if the cement has pseudoplastic characteristics or a significant yield stress (Bingham characteristics). Rapid setting (e.g. through light activation) will also help to ensure that the position of the bracket does not change.

In order to create an effective bond between bracket/band and tooth, the bond strength to enamel and to the material of the band or bracket (alloy, ceramic or plastic) must be adequate. The bonding surfaces of bands or brackets are often provided with mechanical undercuts (e.g. in the form of a mesh) or are treated with adhesion promoters (e.g. silane for ceramics), in order to give an adequate bond. Ideally, bond failure should not occur during treatment, but on completion of treatment debonding should occur readily. If debonding occurs at the cement/enamel interface leaving the enamel surface ‘clean’ a minimal clean up is required. One danger of trying to encourage debonding at the interface is that fracture can occasionally occur partly within the tooth. Fracture within the cement may require some clean up of the enamel but will help to ensure that fracture of the enamel does not occur. Many cements used for orthodontic bonding are of the resin type described in Chapter 23. Acid–base type cements and resin-modified cements are attracting more attention as it is recognized that some of these products are capable of matching the ideal characteristics more closely. See also Section 23.9.

27.6 Suggested further reading

Diaz-Arnold, A.M., Vargas, M.A. & Haselton, D.R. (1999) Current status of luting agents for fixed prosthodontics. J. Prosthet. Dent. 81, 135.

Kramer, N., Lohbauer, U. & Frankenberger, R. (2000) Adhesive luting of indirect restorations. Am. J. Dent. 13, 60.

Rosenstiel, S.F., Land, M.F. & Crispin, B.J. (1998) Dental luting agents: a review of the current literature. J. Prosthet. Dent. 80, 280.

28.1 Introduction

One group of widely used cements is based on the vigorous reaction which occurs between certain basic oxides and phosphoric acid to form phosphate salts of low solubility. The three products considered in this section are the zinc phosphate cements, silicophosphate cements and copper phosphate cements. The silicate filling materials described in Chapter 20 are closely related, having a liquid component which is essentially an aqueous solution of phosphoric acid and a powder which is a glass derived from amphoteric or basic oxides, but these are considered separately due to their different applications.

28.2 Zinc phosphate cements

Composition: These materials are generally supplied as a powder and liquid which are mixed together by hand (Fig. 28.1). Encapsulated products are available but are rarely used due to the extra cost involved (Fig. 28.2). They do, however, give both a greater speed and consistency of mix which has the dual benefits of more reliable performance and an increased working time. This is of particular benefit when using these cements as luting agents.

The composition of powder and liquid in a typical cement are given in Table 28.1. The major reactive component of the powder is zinc oxide. Small quantities of other oxides such as magnesium oxide may also be present. The liquid is essentially an aqueous solution of phosphoric acid buffered by adding small quantities of zinc oxide or aluminium oxide. These compounds form phosphates which stabilize the pH of the acid and reduce its reactivity.

Setting reaction: On mixing the powder and liquid together a vigorous reaction occurs, resulting in the formation of a relatively insoluble zinc phosphate as follows:

3ZnO + 2H3PO4 + H2O → Zn3(PO4)2 · 4H2O

Only the surface layers of the zinc oxide particles react, leaving unconsumed cores bound together by the phosphate matrix.

The reaction is rapid and exothermic although its rate is tempered somewhat by the presence of buffers in the acid and a special process of deactivation of the zinc oxide powder involving heating and sintering with other, less reactive, oxides.

Manipulative variables: The powder/liquid ratio depends on the application. For cavity lining a putty-like consistency having a powder/liquid ratio of about 3.5 : 1 is used. For luting, a more fluid mix, with lower powder/liquid ratio, is employed to ensure flow of the cement during seating of the restoration. It is not normal practice to measure the proportions of powder or liquid but rather to assess the suitability of the mix ‘by experience’. When proportioning, it is important to remember that lowering the powder/liquid ratio produces a weaker, more soluble and more irritant material.

The powder is best incorporated into the liquid in small increments until the desired consistency is reached. This method has the effect of delaying the set slightly and creating more working time, since the concentrations of zinc phosphate produced during the early stages of setting are not sufficient to cause a noticeable increase in viscosity. When all the powder has been incorporated mixing should be quickly discontinued, since continuing to mix after zinc phosphate has started to

form can significantly weaken the cement. Mixing is easier if carried out on a cooled glass mixing slab. Care must be taken, however, not to cool the mixing slab below the dew point, since water may

Fig. 28.1 A zinc phosphate cement supplied as a white powder containing primarily zinc oxide and a colourless liquid which is an aqueous solution of phosphoric acid.

Mixing typically is performed on a glass mixing slab of the type shown.

Fig. 28.2 An encapsulated type zinc phosphate cement. The capsule contains powder and liquid similar to that shown in the powder liquid system in Fig. 28.1, but here mixing takes place automatically on a device similar to that shown in Fig. 24.3. The mixed material can then be extruded from the syringe type capsule.

Table 28.1 Composition of zinc phosphate cements.

condense from the atmosphere into the mix of cement below this temperature. Excess water affects both the setting characteristics and the physical properties of the set material. Proportioning and mixing are greatly simplified by using pre-encapsulated materials. Some products are supplied pre-encapsulated in syringes which enable the mixed material to be syringed into place following mechanical mixing for 5 or 10 seconds.

For hand-mixed powder/liquid systems it is necessary to take precautions over the handling of the liquid. The cap should be removed from the bottle only long enough to dispense sufficient liquid for one mix, and then replaced immediately. If the liquid is left open to the atmosphere for extended periods, water will either be lost or gained depending upon the ambient humidity. Such changes in the water content of the liquid may alter the setting characteristics and physical properties of the material. In a hot, dry atmosphere crystallization of phosphates may be observed on the sides of the bottle as water evaporates. If this occurs the remaining liquid is useless and should be discarded. Only a small change in the water content of the liquid is required to produce a large and unacceptable change in the properties of the cement.

When using the materials for luting, the working time is optimized by adding the cement to the fitting surface of the restoration, which is initially at room temperature, and not to the tooth preparations which are at mouth temperature (37ºC). If the cement is added to the tooth preparations first, there is every chance that its viscosity will have increased considerably before the restoration can be seated. In extreme cases the cement may even be completely set. This demonstrates the marked effect of temperature on the rate of reaction for these products. Cast or all-porcelain restorations are usually designed to have a reasonable space between the casting and the preparation for the cement lute (approximately 30–40 μm). This space is generated for castings by painting the surface of the working die with a varnish or die spacer which

dries in a single coat to give a reliable film thickness. Multiple coats are often required to achieve the desired spacing. The platinum foil achieves the same purpose for porcelain jacket crowns. For those crowns that are made on a refractory die the dental stone model of the prepared tooth is coated in die space before the preparation is replicated. The luting agent should be loaded into/onto the restoration in sufficient quantity to fill this ‘defect’, but not in gross excess otherwise there may be problems seating the restoration.

Properties: Some of the important properties of zinc phosphate cements, as required in the ISO Standard (ISO 9917), are outlined in Table 28.2.

Providing the materials are manipulated correctly, the phosphate cements have sufficient working time to allow placement of a cavity lining or cement lute before the viscosity has increased markedly. The ISO Standard (ISO 9917) carries no requirement for working time. However, there is a minimum setting time of 2.5 minutes for luting cements and 2 minutes for lining cements which is designed to ensure that there is sufficient working time. The evaluation is made by determining the resistance to penetration of 1 mm diameter probe under a load of 400 g. The material is considered set when it can fully support the loaded probe. At the lower powder/liquid ratio used for luting, the material is sufficiently fluid to allow seating of the restoration and formation of a thin film of lute.

Film thickness, which is an important property for luting cements, is not only controlled by powder/liquid ratio but also by particle size of the zinc oxide powder. Large grains of powder tend to form a thick layer of cement and prevent proper seating of a crown. In order to achieve adequate seating it is important that cements with fine grain

zinc oxide powders are used. Such powders have relatively high exposed surface areas and react rapidly with phosphoric acid. Adequate working time is achieved by manufacturers reducing the reactivity of the acid with buffers. Proper material selection and manipulation should result in a cement film thickness of less than 25 μm as required by the ISO Standard. This is determined in the standard by the application of a force of 150 N across two circular glass plates (area 200 mm2) with a sample of mixed cement sandwiched between them. Another factor which can seriously affect the seating of crowns is an incorrect convergence angle of the axial walls of the prepared tooth. This should ideally be 10º (5º taper on each side); any angle which is considerably smaller than this will make proper seating of the crown difficult due to no fault of the cement.

Initial hardening of the material normally occurs within 4–7 minutes, although the strength continues to increase for some time after that. The ISO Standard requires a maximum setting time of 6 minutes for lining cements and 8 minutes for luting cements. The compressive strength ultimately reaches a value of about 80 MPa for luting cement and 140 MPa for lining material, reflecting the differing powder/liquid ratios used. As seen in Table 28.2, the ISO Standard requirement for compressive strength (at 24 hours) is only 70 MPa and the difference between this and some typical values given above reflects the variation in properties which can be seen when comparing different brands. For lining materials used beneath amalgam restorations, the mechanical properties at between 3 and 6 minutes after placing are important, since this is the time at which amalgam is normally condensed. This time has been reduced over the years by the increased use of mechanically mixed amalgams requiring only a few seconds tritura-

tion. At about 5 minutes after placement a typical zinc phosphate lining cement has a compressive strength of only 30 MPa. This value is comparable with the stress used by some practitioners during amalgam condensation with lathe-cut alloys. Obviously the packing pressures required for spherical particle alloys are lower. Fracture of lining is unlikely since it is almost totally constrained in both class I and class II cavities providing a correct technique is used (see Figs 27.5 and 27.6). The materials have generally achieved a sufficient degree of set at 5 minutes to resist flow during amalgam condensation.

The mechanical properties of the cement play a part in determining the retentive properties of a cement luting agent. Phosphate cements are not adhesive to tooth substance or to restorative materials. The cement flows into the myriad of fine undercuts naturally occurring on the fitting surface of the restoration and on the cut tooth surface. The importance of an element of surface roughness on these two surfaces which are to be joined is clear. Polished surfaces are not easily joined by cement luting agents. Tags of set cement, along with a correct design of taper to the preparation, prevent displacement of the restoration. It is important therefore that the tags should resist fracture in function. Experience suggests that phosphate cements are sufficiently strong to resist such fracture. As mentioned earlier, the strength increases with increasing powder/liquid ratio up to a maximum, then reaches a plateau before falling again if insufficient acid is present to bind all the zinc oxide particles together. Maximum strength can only be achieved therefore at high powder/liquid ratios but these cannot be used for luting cements because of the adverse effect on film thickness. As a general rule it follows that the cement should be as thick as practicable for the particular work being carried out.

The set material has a small, though significant, solubility in water and cement lute margins may erode slowly in the mouth by a combination of dissolution and abrasion. The cement lute is, potentially, the weak link of any indirect restoration since it normally joins two resistant materials, for example, gold and enamel or dentine, or porcelain and dentine. Erosion, leading to loss of the cement lute and failure of the restoration, is not, however, a problem particularly associated with zinc phosphate cement. Loss of restorations is more likely to occur due to a poorly retentive

design of the preparation. An important consequence of cement lute erosion is that the crevice formed is a potential site for caries development. One problem with evaluating cement erosion and solubility is that it is notoriously difficult to reproduce in laboratory experiments. The simplest of tests for solubility involve suspending discs of cement in water for 24 hours, at which time the weight loss of the cement is recorded. In such a test a phosphate cement would be expected to give a solubility of about 0.1% whereas in a similar test a solubility of around 0.3% would be normal for a silicate cement. It is known, however, that in the clinical situation the relative rankings of solubility for the two cements is reversed. A better indication of solubility and erosion is achieved by testing the materials in acidic media (e.g. pH 2.7 lactate buffer) in the presence of mildly abrasive conditions. The method currently used in the ISO Standard employs the use of impinging jets of buffered lactic acid which can both dissolve material at the surface and at the same time wash away the solubilized material to expose a fresh surface. The maximum values of material loss (Table 28.2) allowed in the standard are determined using this method. It can be seen that whereas the erosion rate is expected to be smaller than that for a polycarboxylate cement it is significantly greater than that for a glass-ionomer cement.

Zinc phosphate cements may have an irritant effect on the dental pulp, particularly when used as cavity lining materials. The pH value of the cement at the time of application to the tooth is between 2 and 4 depending on the particular brand and the powder/liquid ratio. The degree of irritation depends on the depth of the cavity and the thickness of residual dentine. It is necessary to remember that attitudes towards the acid tolerance of the dentine and pulp are changing (Chapters 23, 24 and 27), although many experts still feel that zinc phosphate cements are unsuitable for use as linings in deep cavities unless a sublining of a less irritant material such as calcium hydroxide or zinc oxide/eugenol cement is used (see Fig. 29.6). The powders of zinc phosphate cements often contain small quantities of a fluoride salt. This is thought to leach out from within the set cement and to afford some protection to the surrounding tooth substance. The quantity of fluoride leached is generally much less than that leached from silicate or glass-ionomer cements.