Composite resin attached to glass polyalkenoate (ionomer) cement — the laminate technique

J. Dent. 1993;

21: 31-38

31

Review

Composite resin attached to glass polyalkenoate (ionomer) cement-the laminate technique M. Woolford Department of Conservative Dentistry, Dental School, The University, Dundee, UK

ABSTRACT Since the introduction of the laminate technique where composite resin is placed over glass polyalkenoate cement in an endeavour to obtain a marginal seal to tooth substance, it has been advocated and used by many clinicians. This review examines the available literature on this apparently commonly used technique. Awide-

ranging evaluation of the technique is allowed by the extensive nature of the literature. The conclusion of the review is that more work is required to evaluate the procedure, but the results of in vitro and, in particular, in vivo studies suggest that the technique

may not be as successful

clinically

as the theory may support.

KEY WORDS: Review, Dental materials, Laminate techniques J. Dent. 1993; 21: 31-38 1992)

(Received 8 May 1992;

reviewed 20 July 1992; accepted 25 September

Correspondence should be addressedto: Mr M. Woolford, Department of Conservative Dentistry, Dental School, Park Place, Dundee DDl 4HN. UK.

INTRODUCTION There are a number of problems in using composite resin, one of the major being its inability to bond to dentine as strongly as to enamel, even with the use of intermediary bonding systems. Frequently, composite resin cannot be placed in a cavity where the periphery is totally of enamel. Cervical ‘abrasion’ cavities have one margin in dentine and in others, caries may extend beneath the amelocemental junction. Attempts to bond resin-based materials to dentine have not been wholly successful and do not counteract the inherent polymerization contraction of the resin. Consequently there is no resin-based material which can provide a perfect seal in a cavity with margins on both enamel and dentine. Glass polyalkenoate (ionomer) cements were developed in the early 1970s as a logical extension of the technology used in dental silicate cement and zinc polycarboxylate cement (Wilson and Kent, 1971). The new material was originally referred to as a glass ionomer cement but is, however, more correctly termed a glass polyalkenoate cement due to the differing formulations of the parent acids used in modem versions of the material. The cement has two fundamental advantages applicable to dental use. The first is that it is able to bond, by chemical means, to @ 1993 Butterworth-Heinemann 0300-5712/93/010031-08

Ltd

both enamel and dentine (Wilson et al., 1983). The second is the ability of the material to release fluoride over a significant period of time (Wilson et al., 1985). The material has been shown to have a number of potential clinical uses (McLean and Wilson, 1977). The major drawback to using the material in non-stress-bearing situations in the anterior region of the mouth is the poor aesthetic proper@ of some formulations and their inability to obtain a surface polish. An indication that glass polyalkenoate cement could be used as a base material beneath composite resin, but without any means of uniting the two materials, was given by McLean and Wilson (1977). The bonding of glass polyalkenoate cement to a composite resin would combine beneficial aspects of both groups of material, in particular the aesthetic properties of composite resin with the ability of the glass polyalkenoate cement to bond reliably to dentine and release fluoride. This was the basis of the technique proposed by McLean et al. (1985). In essence, McLean et al. (1985) showed that by etching, with a phosphoric acid solution, the surface of a glass polyalkenoate cement placed as a base in a cavity, it was possible to achieve a mechanical bond between the cement and composite resin. Thus the composite resin

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may be sealed to enamel using the acid-etch technique (Buonocore, 1955) while being bound to dentine via the intermediate layer of glass polyalkenoate cement. The glass polyalkenoate is, in turn, chemically bound to the dentine. This procedure then came to be known as the glass polyalkenoate-composite resin laminate, or sandwich, technique. Reviews of the potential application of glass polyalkenoate cements with composite resin were given by McLean (1987) and by Mount (1989a, 1990).

STRENGTH OF THE BOND BETWEEN GLASS POLYALKENOATE CEMENT AND COMPOSITE RESIN After its first description in the literature it was soon established that the potential bond between glass polyalkenoate cement and composite resin could be greater than the tensile strength of the glass polyalkenoate cement itself @need and Looper, 1985; Hinoura et al., 1987; Wexler and Beech, 1988). The cohesive failure seen in such specimens suggests that it is the cohesive strength of the cement itself which is of paramount importance (Hinoura et al., 1990). Indeed, few studies have actually measured the bond strength, rather they have tended to assess the tensile strength of the cement itself, unless failure of the test specimens always occurred at the interface of the two materials. One of the highest bond strengths found between a composite resin and glass polyalkenoate cement was reported by Shimizu et al. (1986). They examined an adhesive resin composite cement which could bond to various materials in its own right. The glass polyalkenoate cement was in a very thin layer (200 urn) over dentine and was etched for 60 s. The findings of this study highlighted the importance of the thickness of glass polyalkenoate cement required for optimum strength of the union to composite resin. Very few studies have been able to quote the thickness of glass polyalkenoate cement used and the relative importance this factor may have to the clinical success of the procedure. For example, Fukuda and Katsuyama (1989) were able to show the importance of the thickness of base material beneath composite resin with regard to the adaptation of the composite resin to the base. In a series of experiments, Mount (1989b) studied the tensile strength of a number of combinations of glass polyalkenoate cement, intermediate resins and composite resins. He felt able to conclude that there were four main factors which controlled the strength of the bond between the materials: 1. The tensile bond strength of the polyalkenoate cement. 2. The wettability of the resin. 3. The polymerization contraction forces of the composite resin. 4. The adaptation of the composite resin to the underlying cement.

ETCHING THE GLASS POLYALKENOATE CEMENT The increase in the use of gel-based phosphoric acid induced a study that compared the use of an etchant liquid compared with a gel and concluded that the vehicle nature made no difference to the bond strength of the cement to composite resin (Andreus, 1987). The importance of washing the surface of the cement after etching to remove the residual acid was highlighted by Hinouraet al. (1987). Acids other than 37% phosphoric acid have been tested. The use of 50% citric acid was found to be ineffective as a surface etchant, for the cement, compared to phosphoric acid (Causton et al., 1987).The importance of maintaining a clean cement surface after etching was stressed in the work by Suliman et al. (1989). They showed that salivary contamination significantly reduced the bond strength of glass polyalkenoate cement to composite resin. This has important implications not only regarding contamination of the surface, but also with regard to the actual mechanism of effect of the acid. A study by Welbury et al. (1988), using tensile bond strength measurement with analysis by means of the Weibull distribution function (Weibull, 1951; McCabe and Cat-rick, 1986) showed that omitting the etching stage produced a less reliable bond, but not one that was statistically weaker. Premature etching, that is etching prior to the initial set time of the cement, and failure to use an unfilled intermediate resin increased the probability of failure. The importance of an intermediate resin layer between the glass polyalkenoate cement and the composite resin was also demonstrated by Causton etal. (1987), Hinouraet al. (1989) Mount (1989b), Subrata and Davidson (1989) and Meyers et al. (1990). Mount (1989~) investigated the viscosity of the intermediate resins and showed the importance of the wetting ability of this resin to the surface of the glass polyalkenoate cement. The clinical problem of evaporation of the vehicle used to enable these resins to be more easily applied was highlighted by the same author (Mount, 1988, 1989b).

EFFECT OF ACID ETCHING GLASS POLYALKENOATE CEMENT There have been a number of studies using the scanning electron microscope to study the topographical changes to the cement surface after the application of phosphoric acid. Garcia-Godoy and Malone (1986) demonstrated, in a simple study, the surface roughening effect of this procedure. Kingsford-Smith and Martin (1990) in a more thorough investigation showed the effect of increasing the time of application of the acid on change of surface topography. On a simple topographic criterion it has been shown that cements do not all behave in a similar fashion (Garcia-Godoy et al., 1988; Fuss et al., 1990), some cements requiring a greater period of etching than others to produce a similar etched appearance.

Woolford: The laminate technique

The study of Fuss et al. (1990) underlined the problems of the numerous systems marketed under the one generic name, as glass polyalkenoate cements. The determination of an optimum time period for etching as attempted by Joynt et al. (1989). Van der Merwe and De Wet (1989) and Mangumetal. (1990). therefore, does not take into account variations within the material types. The differing behaviour of the cements with varying glasses and polyelectrolytes has been outlined with regard to surface erosion, of which this is an aggressive example (Setchell et al., 1985;Walls et al., 1988).Tjan and Glancy(1989) clearly demonstrated the difference in acid resistance and acid seepage of a range of glass polyalkenoate cements. They concluded that a thickness of 0.5 mm of cement was necessary to prevent penetration of the phos’phoric acid during etching for any of the materials used in their study. A major study by Papagiannoulis ef al. (1990) produced evidence that applying acid to the surface of a fairly mature cement for 20 s created a surface with excessive porosity and matrix dissolution. Analysis of the surface showed that there was a significant change in the surface chemistry of the material. They supported, therefore, the work of Taggart and Pearson (1988) who, using the scanning electron microscope, demonstrated the destructive effect of the phosphoric acid to a considerable depth in an immature cement. Later work by the same authors (Taggart and Pearson, 1991) showed that etching for longer than 10 s was detrimental to the strength of the cement. However, with a similar technique using a scanning electron microscope, Joynt et al. (1989) were unable to find any subsurface changes, possibly because they etched the glass polyalkenoate cement at a later stage in the maturation period. The degree of maturity of the cement and the strength of bond was demonstrated in a simple study by Chin and Tyas (1988). This study explained the early weakness of the cement being due to the immature surface of the cement etching, or eroding, in a manner different to that which may occur with a mature cement. This was supported in a study by Wexler and Beech (1988). Auger and Tay (1987) used X-ray microanalysis techniques to demonstrate that etching glass polyalkenoate cement selectively removed calcium from the surface, increasing the proportions of aluminium, silica and phosphorus. Mature cement, which had been allowed to set for several days, demonstrated a different etch pattern, whereby more calcium was retained. It is also apparent from this study that on a microanalytical basis different proprietary brands of glass polyalkenoate cement behave in a manner unique to themselves when subjected to phosphoric acid. Etching not only alters the surface topography of glass polyalkenoate cement as shown by the numerous studies quoted above, but it will also alter the surface tension of the cement (Wieczkowski etal., 1990b). This has important implications for the use of this technique and in the application of intermediate bonding resins as suggested by Mount (1989a, b).

33

THE NEED FOR ETCHING? ALTERNATIVES TO ETCHING Garcia-Godoy (1988) had already begun to question the need for acid etching of the surface of the base material. Krejci ef al. (1988) in the conclusion of their report, assessing the marginal quality in vitro of Class II composite resin restorations, also questioned the validity of the etching stage of the laminate technique. Subrata and Davidson (1989) were able to show that a strong bond could be achieved by mechanically roughening the surface of the glass polyalkenoate cement with a bur, as opposed to the application of phosphoric acid, and the use of an intermediate resin. Sheth eta]. (1989) were unable to show any difference in the bond of composite resin to one type of glass polyalkenoate cement, whether it was etched, or not. Allowing the glass polyalkenoate cement to set in air appeared to create a surface that was sufficiently rough to give an adequate bond to composite resin, supporting the conclusion of Subrata and Davidson (1989). The extensive study of Papagiannoulis et al. (1990) seriously questioned the necessity for acid etching the glass polyalkenoate cement in the laminate technique. The best results were obtained by the use of a dentine bonding agent on an unaffected surface of glass polyalkenoate cement. This contradicts the initial conclusion of Subrata and Davidson (1989) and Tzoutzas et al. (1989) that some form of surface treatment is required to obtain a significant bond using the laminate technique. A microleakage study by Woolford (1990) also demonstrated that some form of surface modification was necessary to get a reliable seal between glass polyalkenoate cement and composite resin. Causton and Sefton (1989) concluded that the use of an adhesion promoter based upon hydroxy-ethyl-methacrylate (HEMA) and maleic acid negated the need for acid etching the glass polyalkenoate cement to gain a stable, reliable bond with composite resin. However, the study did not demonstrate the effect of this priming agent on the surface of the cement. In a long review, Brannstrometal. (1991) stated that one of the desirable properties for a cavity liner beneath composite resin was that it specifically did not bond to the composite resin. It should adhere well, however, to the dentine. In fact in this paper they questioned the use of glass polyalkenoate cement as a liner beneath composite resin, stating, that in their opinion, it did not meet many of the criteria required for a material used in this context. The criteria they felt were important were: 1. The liner should be able to cover the dentine out to the cervical margin, should not adhere to the composite resin and therefore not be dislodged from the dentine when the composite resin contracts. 2. The liner should be applied in a very thin layer (lo20 urn). 3. The liner should be insoluble to oral or dentinal fluids. 4. It should be simple to apply and set very rapidly so as not to seep onto the cavity surface.

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5. It should be biologically acceptable and not acidic, thus potentially removing smear plugs from dentinal tubules. 6. It should have antibacterial properties. 7. It should not be damaged by the acid-etching procedure and the liner should have no deleterious effect on composite resin.

The evidence contained in this review highlights the fact that glass polyalkenoate cement may not meet all the above stated criteria.

POTENTIAL INTERACTION BETWEEN COMPOSITE RESIN AND GLASS POLYALKENOATE CEMENT Marshall et al. (1982) clearly demonstrated a reduction in hardness of the upper surface of a composite resin when a glass polyalkenoate cement was used as a base beneath it. It should be stated that they used a slow setting glass polyalkenoate cement, not one of the modern formulations now available. Berrong et al. (1989) did use one of the rapid setting base materials and found a significant reduction in hardness for the upper surface of a composite resin, in a layer up to 1 mm thick, overlying the glass polyalkenoate cement. These studies suggest that glass polyalkenoate cement can have a detrimental effect on the hardness of a composite cement some distance from the cement itself. The effect of the cement on thin layers of composite resin and composite resin adjacent to the cement must, therefore, be considered with regard to the clinical use of the material. If the technique is to be used the cement needs to be as mature as possible to prevent potential interaction.

ASSESSMENT OF THE SEAL BETWEEN GLASS POLYALKENOATE CEMENTS AND COMPOSITE RESIN Studies of microleakage invariably use a tracer such as a dye to demonstrate leakage. These tracers usually have a low molecular weight. An interesting study by Heys and Fitzgerald (1991) used bacteria as a means of assessing microleakage. In this case the presence of a glass polyalkenoate base beneath an unetched and unbonded composite resin Class V restoration minimized bacterial penetration at the material-tooth interface. The antibacterial properties of glass polyalkenoate cement may offer some advantage when the composite laminate technique is used clinically. The significance of the penetration of a marker dye between the materials is, therefore, unknown. There is much conflicting evidence regarding the potential microleakage of restorations using the laminate technique. The studies, invariably, are laboratory based. Fayyad and Shortall (1987) demonstrated a significantly reduced degree of leakage for composite resin restorations when lined with a glass polyalkenoate cement, which was

etched for 60 s prior to composite resin application. This was supported by the studies of Mathis et al. (1990) who used an etch time of 20 s for the glass polyalkenoate cement, Kanca (1987) who used an etch time of 30 s, and Gordon et al. (1991) who used an etch time of 60 s. Peutzfeldt and Asmussen (1989) were unable to show any microleakage between glass polyalkenoate cement and composite resin, although, paradoxically, there was leakage between dentine and glass polyalkenoate cement and between composite resin and enamel. This result is supported by the study of Kingsford-Smith et al. (1988). Prati (1989) demonstrated the benefit of the laminate technique in reducing early microleakage of Class II restorations when glass polyalkenoate was etched for 20 s and used in combination with a dentine primer and bonding resin compared to composite resin restorations placed without a glass polyalkenoate base. A study by Hembree (1989) showed that after 1 year glass polyalkenoate cement used as a liner could reduce the degree of microleakage in Class II cavities restored with composite resin. The cement was etched for 30 s and the acid applied before the material had reached its initial set stage. Gordon et al. (1985) for Class V cavities, and GarciaGodoy and Malone (1988) for Class III cavities, could find no difference in microleakage with, or without, a glass polyalkenoate base, when the cavities were restored with composite resin. This was echoed in the work of Prati and Montanari (1989) and Holtan et al. (1990) in Class V cavities and Cheung (1990) in Class II cavities. Crim and Shay (1987) found that microleakage was worse in Class V cavities when glass polyalkenoate cement used in the laminate technique was compared with composite resin used with dentine bonding agents. The cement was applied to the edge of the cervical margin of the cavity, which was in dentine, and etched for 30 s. The conclusion of a study by Shortall et al. (1988) was that the sandwich/laminate type restoration was unreliable with regard to its ability to prevent microleakage at any margin beneath the cemento-enamel junction. The results of the study by Pleunik et al. (1989) and Phair et al. (1988) also suggest that this is a valid conclusion for the materials used in these studies.Wieczkowski et al. (1990a) in a microleakage study demonstrated that preventing the bond between composite resin and glass polyalkenoate cement, using copalite as a separator, gave better marginal seal to the restorations in vitro than for restorations where the cement had been bonded to the composite resin. Their conclusion being that doubt was raised regarding the use of glass polyalkenoate cement as a base beneath composite resin. Microleakage may be reduced by conditioning the dentine to enhance the bond of the polyalkenoate cement to it prior to its use in the laminate technique. Complete removal of the smear layer from the surface of the dentine was not shown to be advantageous with regard to reduction of microleakage for laminate restorations (Prati et al., 1989). No studies have ever shown that microleakage is totally

Woolford: The laminate technique

eliminated by the use of an etched glass polyalkenoate base beneath composite resin. One very important complicating factor which may be detrimental to the results of these studies is the polymerization contraction of the composite resin. The force of this contraction may tear the composite resin from the glass polyalkenoate cement, a failure at the union of the two materials (Mount, 1989b; Meyers et al., 1990).Alternatively failure may occur cohesively within the bulk of the glass polyalkenoate cement (Shortall and Asmussen, 1988).The third potential mode of failure would be that of tearing the glass polyalkenoate cement away from the dentine, an adhesive failure. All these potential modes of failure would lead to the creation of a gap at the margin of this type of restoration, with consequent marginal leakage. It has been demonstrated, by direct visual examination, that the use of a glass polyalkenoate base will reduce the gap at the cervical margin when composite resin is used and so potentially reduce the degree of microleakage (Peutzfeldt and Asmussen, 1989). The adaptation of composite resin to the cavity margins was shown to be improved by the use of a glass polyalkenoate liner in one study using direct visual examination of the materials (McConnell et al., 1986). This is, perhaps, not unexpected since the bulk of composite resin is reduced by the presence of the base cement, especially if applied in a thick layer. The amount of polymerization contraction which can occur, therefore, must be reduced. Thermal and load cycling made little difference to the ability of glass polyalkenoate cement to reduce, but not prevent, microleakage in Class II cavities restored with composite resin (Darbyshire et al., 1988). This study also demonstrated that the use of a dentine bonding agent and a glass polyalkenoate together may not produce a statistically significantly better seal than these materials used separately. No information was contained in this report with reference to any etching of the glass polyalkenoate cement. It can be concluded from the plethora of information available regarding microleakage and the composite laminate technique that studies have to be specific with regard to the area examined and means used to detect microleakage in this type of restoration. Specific criticism of this technique can only be justified if leakage can be detected between the cement and the composite resin. Microleakage at other interfaces may define limitations of a specific material, combination of materials, or technique.

CLINICAL BEHAVIOUR OF THE LAMINATE TECHNIQUE RESTORATIONS Despite the large quantity of laboratory-based research on the laminate technique there has been little published on the clinical behaviour of this type of restoration. Suzuki and Jordan (1990) produced an article on the clinical technique, but did not provide any evidence of the clinical success of the procedure. Bitter (1986) and Quiroz and

35

Swift (1986) published articles of a similar type. In a short review of treatment modalities for cervical restorations Brackett and Robinson (1990) using in vitro data, were unable to show that the laminate technique offered any advantages over conventional tilling techniques. Welbury and Murray (1990) published a report on a 2year in viva study of the technique used in Class II cavities. The results were very disappointing, showing that the failure rate was unacceptably high. It should be pointed out, however, that they used a very thick layer of cement, over 1 mm, at the base of the approximal box of the cavity. The failure of the restorations was due to the dissolution of the material from this area. They also used a poly(maleic/acrylic acid) copolymer based cement which has been shown to be more susceptible to erosion than some other types of the cement (Walls et al., 1988). Knibbs (1992) supported the conclusion that the ‘open’ technique for laminate restorations is unsatisfactory in the Class II situation in a 2-year clinical study using a different glass polyalkenoate cement based on poly(acrylic acid). The restorations that were placed by totally enclosing the cement with composite resin in the approximal box area did perform satisfactorily for the period of this study. Results of another 2-year clinical study of laminate restorations in the Class II situation showed no statistical difference between laminated and non-laminated restorations used with posterior composite resins (Grogonoet al., 1989). Four-year results by the same principal author showed that all the restorations were performing similarly, 90% still remaining in service (Grogono et al., 1991). Tyas et al. (1989) reported another clinical trial carried out over a 2-year period on abrasion lesions restored by this technique. The failure rate in this study was higher than for the simple glass polyalkenoate cement restorations assessed in a previous study by the same principal author (Tyas and Beech, 1985). Clinically, the laminate technique was inadequate to retain the restorative materials, unless the adjacent enamel was also acid etched, the etched enamel presumably providing the greatest degree of retention. The laminate technique was shown to perform similarly to composite resin with dentine bonding agent in a non-undercut cavity. In another trial of restored cervical abrasion cavities, laminate technique restorations performed as well as glass polyalkenoate cement and composite resin used individually (Neo and Chew, 1991). In an in viva study carried out over a period of 6 months, Krejci et al. (1991) assessed Class V restorations using standardized criteria and by direct examination of the margins of the restorations using replicas and scanning electron microscopy. They concluded that in this comparatively short study there was no difference in terms of marginal adaptation, between laminate type restorations and conventionally placed composite resin with dentine bonding agent. Their final statement was that the laminate technique is still the method of choice for mixed Class V restorations as glass polyalkenoate cement has cariostatic potential and may seal dentine. However, there

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is no evidence in the literature to totally support this statement. A 2-year clinical study reported by Reich (1989) concluded that laminate restorations used in Class V cavities were more prone to failure than simple glass polyalkenoate cement restorations.

RESIN-MODIFIED CEMENTS

GLASS POLYALKENOATE

A recent development in the field of glass polyalkenoate cements is the production of light-curing versions of the material. A review of the chemistry and development of these materials has been given by Wilson (1990). In their simplest form they are glass polyalkenoate cements with the addition of resin components (Mathis and Ferracane, 1989). More complex examples have been developed by modification of the poly(alkenoic acid) with side chains which can polymerize by light-curing mechanisms, but they remain glass polyalkenoate cements by their ability to set without light curing, although this reaction takes place much more slowly than for the traditional cements. They also release fluoride (Horsted-Bindslev and Larsen, 1991; Mitra, 1991a) and retain the ability to bond to tooth substance (McCaghren et al., 1990; Mitra, 1991b). These materials are all produced from opaque glasses and so have been marketed as lining/base materials. They lend themselves, therefore, to use in the laminate technique with the potential for chemical bonding between the resin component of the cement and the composite resin. One microleakage study reported significantly better results using light-cured compared to conventional types of glass polyalkenoate cement (Tjan and Dunn, 1990) but not all studies have shown such a conclusive result (Prati et al., 1990).The adaptation of one material to dentine has been shown to be susceptible to contraction shrinkage of the overlying composite resin, considerable polymerization shrinkage in its own right, and dehydration (Watson, 1990). The future for this group of materials is uncertain. The currently available materials do not appear to offer any major advantages for use in the laminate technique with the exception of the rapid set by light curing, which may be of advantage to the clinician. The success, or otherwise, of these materials when used in the laminate technique will be borne out only by long-term clinical trials, the results of which are awaited with interest. SUMMARY It can be seen, therefore, that doubt remains over the effectiveness of the seal between glass polyalkenoate cement and composite resin. The precise role of etching the polyalkenoate cement in relation to retention and seal between these two materials is also unclear as it appears that one group of studies point to its importance, while others indicate that it is not only unnecessary, but also undesirable. Clearly, therefore, further elucidation of

these points in relation to a range of available materials is required. The precise technique of the composite resinglass polyalkenoate cement laminate technique must be open to debate. The paucity of carefully controlled clinical trials and the poor results for the laminate technique in those trials which have been carried out must open the debate still further with regard to the actual concept of this restorative technique. References

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Woolford: The laminate technique

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